Roads Department - Government of Botswana

Roads Department 

The Prevention and Repair of Salt 

Damage to Roads and Runways 

Guide to the Prevention and Repair of Salt Damage to Roads and Runways 

1


Ministry of Works, Transport & Communications, 


Private Bag 0026 

Gaborone, Botswana 

Phone + 267 - 313511 

Fax + 267 - 314278 

JULY 2001 

ISBN 99912 - 0 - 380 - X 

Reproduction of extracts from this Guideline may be made subject to due acknowledgement of the source. 

Although this Guideline is believed to be correct at the time of printing, Roads Department does not accept any 

contractual, tortious or other form of liability for its contents or for any consequences arising from its use. Anyone 

using the information contained in the Guideline should apply their own skill and judgement to the particular issue 

under consideration. 

2 Guide to the Prevention and Repair of Salt Damage to Roads and Runways


ROADS DEPARTMENT 

Under the policy direction of the Ministry of Works, Transport & Communications, Roads Department is responsible 

for providing an adequate, safe, cost-effective and efficient road infrastructure within the borders of Botswana as well 

as for facilitating cross-border road communications with neighbouring countries. Implied in these far-ranging responsibilities 

is the obligation to: 

1. ensure that existing roads are adequately maintained in order to provide appropriate level of service for road users; 

2. improve existing roads to required standards to enable them to carry prevailing levels of traffic with the required 

degree of safety; 

3. provide new roads to the required geometric, pavement design and safety standards. 

The Department has been vested with the strategic responsibility for overall management of the Public Highway Network 

(PHN) of some 18, 300 km of roads. This confers authority for setting of national specifications and standards and 

sheared responsibility with the District Councils and Department of Wildlife and National Parks for the co-ordinated 

planning of the PHN. 

Roads Department is also responsible for administering the relevant sections of the Public Roads Act, assisting local 

road authorities on technical matters and providing assistance in the national effort to promote citizen contractors in the 

road construction industry by giving technical advice wherever possible. This task is facilitated by the publication of a 

series of Technical Guidelines dealing with standards, general procedures and best practice on a variety of aspects of 

the planning, design, construction and maintenance of roads in Botswana that take full account of local conditions. 

Guideline No. 1 The Design, Construction and Maintenance of Otta Seals (1999) 

Workshop Proceedings, September 2000, Addendum with reference to 

Guideline No. 1 The Design, Construction and Maintenance of Otta Seals (1999) 

Guideline No. 2 Pavement Testing, Analysis and Interpretation of Test Data (2000) 

Guideline No. 3 Methods and Procedures for Prospecting for Road Construction Materials (2000) 

Guideline No. 4 Axle Load Surveys (2000) 

Guideline No. 5 Planning and Environmental Impact Assessment of Road Infrastructure (2001) 

Guideline No. 6 The Prevention and Repair of Salt Damage to Roads and Runways (2001) 


3


TABLE OF CONTENTS 

1. INTRODUCTION ...............................................................................................................................................9 

1.1 Background .................................................................................................................................................9 

1.2 Purpose and Scope of Guideline .................................................................................................................9 

1.3 Structure of the Guideline ...........................................................................................................................9 

2. OCCURRENCE AND CHARACTERISTICS................................................................................................11 

2.1 General ......................................................................................................................................................11 

2.2 Salt Damage Occurrence in Botswana .....................................................................................................12 

2.3 Salt Damage World-wide ..........................................................................................................................14 

2.3.1 India ................................................................................................................................................14 

2.3.2 Australia..........................................................................................................................................14 

2.3.3 Southern Africa ...............................................................................................................................15 

2.3.4 Middle East .....................................................................................................................................16 

2.3.5 North Africa ....................................................................................................................................16 

2.3.6 North America.................................................................................................................................16 

2.4 International Experience on Salt Damage, Limits and Preventative Measures.........................................16 

2.4.1 Thickness of Surfacing/Permeability Ratio ....................................................................................17 

2.4.2 Bituminous Surfacing Layers..........................................................................................................17 

2.4.3 Brooming ........................................................................................................................................17 

2.4.4 Immediate cover..............................................................................................................................17 

2.4.5 Prevention of Moisture Rise - Cut Off............................................................................................18 

2.4.6 Relevance of Published Literature to Botswana .............................................................................18 

2.5 Factors Influencing Salt Damage................................................................................................................18 

2.5.1 Climate............................................................................................................................................18 

2.5.2 Geology and Hydrogeology............................................................................................................18 

2.5.3 Materials Characteristics.................................................................................................................18 

2.5.4 Pavement Surfacing Design ............................................................................................................21 

2.5.5 Construction Practice ......................................................................................................................21 

2.6 Appearance and Identification...................................................................................................................22 

2.6.1 Damage to Primes ...........................................................................................................................22 

2.6.2 Damage to Permanent Surfacings...................................................................................................22 

2.7 Summary of Physico-chemical Influences on Salt Damage ........................................................................22 

3. LABORATORY AND FIELD TESTING FOR SALT ...................................................................................25 

3.1 General ......................................................................................................................................................25 

3.2 Soil and Water Sampling...........................................................................................................................25 

3.2.1 Water Samples.................................................................................................................................26 

3.2.2 Soil Samples....................................................................................................................................26 

3.3 Field Salt Content Tests.............................................................................................................................26 

3.3.1 Field Electrical Conductivity test....................................................................................................26 

3.3.2 Oral Testing (Taste).........................................................................................................................27 



3.3.3 Field Determination of TDS ..........................................................................................................27 

3.3.4 Field Chloride content determination .............................................................................................27 

3.4 Laboratory Tests........................................................................................................................................27 

3.4.1 Electrical Conductivity ...................................................................................................................27 

3.4.2 Total Dissolved Salts.......................................................................................................................27 

3.4.3 Ionic Composition...........................................................................................................................28 

3.4.4 Mineralogic Analysis ......................................................................................................................28 

3.5 Presentation of Salt Content Analysis Results.............................................................................................28 

4. RISK EVALUATION IN SALINE ENVIRONMENTS .................................................................................29 

4.1 General ......................................................................................................................................................29 

4.2 Salinity Levels of Materials and Water .....................................................................................................29 

4.3 Climatic Conditions ..................................................................................................................................31 

4.4 Combined Risk Evaluation (M x C) .........................................................................................................31 

5. PREVENTATIVE DESIGN PROCEDURES FOR MC > 20 ........................................................................33 

5.1 Types of Bituminous Surfacing.................................................................................................................33 

5.2 Selection of Bituminous Primes................................................................................................................33 

5.3 Selection of Permanent Surfacings ...........................................................................................................34 

5.4 Control of Salt Movement.........................................................................................................................34 

6. REPAIR OF DAMAGED SURFACING..........................................................................................................37 

6.1 General ......................................................................................................................................................37 

6.2 Prime Surfaces ..........................................................................................................................................37 

6.3 Final Bituminous Surfacings.....................................................................................................................37 

6.4 Resealing with Bitumen Rubber ...............................................................................................................37 

6.5 Damage caused by Sulphate Salts.............................................................................................................37 

7. SUMMARY ........................................................................................................................................................38 

8. REFERENCES ..................................................................................................................................................39 

9. APPENDICES…...............…………………………………………………………………………………….40 


5


LIST OF TABLES 

Table 2.1 Salt Content of pavement layers (Sekoma-Kang road).............................................................................14 

Table 2.2 Some salts which can contribute to salt damage of pavements.................................................................19 

Table 3.1 Salt Analysis tests for materials and water in Botswana...........................................................................25 

Table 4.1 Climate risk evaluation..............................................................................................................................32 

Table 5.1 Suggested Maximum salt content limits for Botswana.............................................................................36 

LIST OF FIGURES 

Figure 1.1 Flowchart of guideline ..............................................................................................................................10 

Figure 2.1 World Distribution of dry climates and occurrence of Salt Damage ........................................................11 

Figure 2.2 Areas of Botswana most susceptible to salt damage.................................................................................12 

Figure 2.3 Salt damage process in relation to pavement and water table and salinity ...............................................20 

Figure 2.4 Solubility of some natural salts in relation to temperature .......................................................................23 

Figure 4.1 Risk analysis for salt damage to bituminous surfacings ...........................................................................29 

Figure 4.2 Materials risk rating-salt damage to bituminous surfacings .....................................................................30 

Figure 5.1 Permissible intervals between prime and final surfacing in relation to subgrade salinity and 

pavement surface salinity ......................................................................................................................... 34 

Figure 5.2 Electrical conductivity readings at the surface (0-50mm) of a layer with time for various levels of 

initial conductivity of the bulk material ....................................................................................................35 

REFERENCES 

LIST OF APPENDICES 

Appendix A Maximum salt content limits worldwide..............................................................................................41 

Appendix B Field Electrical Conductivity measurements of soils by the quick conductivity method. ..................43 

Appendix C Risk Evaluation and determination of required preventative measures- Worked Example..................44 

Appendix D Glossary of Terms.................................................................................................................................46 

Appendix E Abbreviations .......................................................................................................................................47 


FOREWORD 


The prevention of soluble salt damage to bituminous roads and runways plays a vital role in reducing the cost of 

both construction and maintenance of roads in Botswana. 

Without adequate prevention measures and monitoring of soluble salt during construction, the road may deteriorate 

and will often result in extensive rehabilitation. There is, therefore, a need to draw the attention of designers and 

engineers to the dangers of soluble salts in the early stages of road construction. 

Because of Botswana’s environment, construction materials and construction water contain soluble salts which 

when used in construction has resulted in severe damage to roads and runway surfaces. These problems have 

been encountered in several projects including Sua Pan airstrip, Nata-Gweta road, Orapa-Mopipi-Rakops road, 

Selibe-Phikwe runway, the trans-Kgalagadi road and many others. 

It is exorbitantly expensive to avoid the use of saline materials and water by importing non-saline alternatives. 

By providing guidelines for the use of available saline materials and water where technically feasible, significant 

cost savings can be achieved. 

The guidelines discuss the occurrences of salt damage in Botswana and elsewhere worldwide, detailed design and 

construction procedures are provided for the prevention of salt damage. Methods of testing and measurement of salts 

are also given together with repair methods where damage has already occurred. 

The user, whether a technician or an experienced engineer will find the guideline useful in identifying whether 

there is likelihood of damage occurring and in determining the design and construction measures required to 

prevent damage. 

The guideline will also be useful to those working in other semi-aid environments of the SADC region where much 

of the available materials and contain soluble salts. 

Gaborone 

July 2001 

A. Nkaro 

Director of Roads 


Ministry of Works, Transport and Communications 


7


ACKNOWLEDGEMENTS 

This Guideline is one of a series that has been produced by Botswana Roads Department under the Institutional Cooperation 

with the Norwegian Public Roads Administration (NPRA). This agreement falls under a NORAD Technical 

Assistance Programme to Roads Department, which is co-funded by the Government of the Republic of Botswana and 

the Kingdom of Norway. 

The guideline was written by Dr. Bernard Obika. 

Significant contributions and comments on various drafts of the guideline were made by the following: 

Mr. Barry Kemsley, Roads Department 

Mr. Charles Overby, NPRA 

Mr. B. Sharma, Roads Department 

Mr. B. Kowa, Roads Department 

Mr. E. Maswikiti, Roads Department 

Mr. M. Segokgo 

Mr. Charles Overby co-ordinated production of the guideline including final editing and formatting. 

The photographs were provided by: 

Dr. Bernard Obika, 

Ms. M.T. Keganne, Diwi Consult 

Mr. Charles Overby, NPRA 



1. INTRODUCTION 

1.1 Background 

Due to the semi-arid environment of Botswana much of the available construction 

materials and water contain soluble salts. In some areas the subgrade upon 

which the roads are constructed are also saline. As a result road and runway 

surfacings have been severely damaged by salts. Roads Department has been 

engaged in a 12-year research programme in collaboration with the UK Transport 

Research Laboratory and others. The results of this research and other 

works undertaken both in Southern Africa and elsewhere in other countries are 

embodied in these guidelines. 

Soluble salt damage to bituminous road surfacings occurs when dissolved salts 

contained in either the pavement layer materials, construction water or subgrade 

migrate to the road surface. 

Blistered bituminous prime due to salt crystallisation 

(Nata-Maun road). 

This migration, through capillary action, is mainly caused by evaporation at 

the surface. At or near the surface, the salts in solution become supersaturated 

and crystallise. This creates pressures with associated volume change, which 

can lift and physically degrade the bituminous surfacing and break the adhesion 

with the underlying pavement layer. 

1.2 Purpose and Scope of Guideline 

This guideline provides design and construction methods for the prevention of 

salt damage to road and runway surfacings. Details are also provided for the 

repair of damaged surfacings. 

The damage may appear in the form of ‘blistering’, 

‘doming’, ‘heaving’ and ‘fl uffing’, of the 

bituminous layer. 

The guideline is intended to assist Engineers and Senior Technicians within 

Roads Department and Consultants and Contractors engaged by the Roads 

Department or other Government bodies to: 

Assess the likelihood of salt damage occurring in a particular road project 

given the environment and salinity levels; 

Identify, measure and interpret levels of salinity in water, construction 

materials and subgrade including field and laboratory methods for determination 

of salt contents; 

Design and specify appropriate preventative measures, where necessary, 

for a particular project; 

Undertake quality control and monitoring of salt levels and prevention 

during and after construction; 

Design and implement repairs for damaged bituminous surfacings. 

1.3 Structure of the Guideline 

The guideline is divided into seven chapters comprising: 

Chapters 1, 2 and 3 (Occurrence and Testing for Salts) 

These chapters introduce the problem of salt damage, where it occurs and how 

to identify damaged surfacings. Techniques and methodology for determina- 

Guide to the Prevention and Repair of Salt Damage to Roads and Runways Chapter 1 

Introduction 

9


tion of salt content are described together with an introduction to interpretation, 

comparison and presentation of test results. 

Chapters 4 and 5 (Risk Evaluation and Design of Preventative Measures). 

A sequential procedure for assessment of the likelihood of damage occurring is 

provided in chapter 4. The risk depends on climate, materials and salt contents 

for a particular project. Once it is established that there is a high probability 

that salt damage will occur, the preventative measures to be considered are also 

detailed in chapter 5. 

Chapter 6 (Repair of Damaged Surfacing). 

This chapter provides currently used methods for repair of damaged surfacings, 

both for temporary and permanent surfacings. 

Chapter 7 (Summary). 

This chapter includes the summary. The list of References and Appendices are 

given at the end of this chapter. 

Details of international practise and salt limits are at Appendix A and is 

intended only as background literature. Appendix B details methodology for 

salt content analysis using the Electrical conductivity whilst Appendix C provides 

an example of risk evaluation and worked example for selection of preventative 

measures for a given project following the procedures described in 

chapters 4 and 5 of this guideline. Appendix D contains a glossary of terms to 

assist the reader. Appendix E contains a list of abbreviations. 

The general layout of the guideline is shown in the flowchart below. 

1. Introduction 2. Occurrence 

and characteris- 

3. Testing for Salt 

and Interpretation 

4. Risk 

Evaluation 

5. Damage Prevention 

methods 

Field 

Laboratory 

Materials 

Climate 

Subgrade 

6. Repair 

techniques 

Figure 1.1 Flowchart of guideline. 

7. Summary 

10 Chapter 1 


Introduction


2. OCCURRENCE AND 

CHARACTERISTICS 

2.1 General 

Soluble salt damage to bituminous surfacings occurs in many countries 

with semi-arid, arid or warm dry climates such as exists in many parts of 

Botswana. 

In these countries the annual evaporation exceeds the annual rainfall and there 

is a net upward migration of soil moisture. If soluble salts are present in this 

moisture, they will be precipitated (crystallise) at or near the surface. 

In such environments there is also a large variation between day and night temperatures 

and humidity. This results in some salts dissolving and recrystallising 

more than once in a day, thereby creating disruptive pressures which can 

damage road surfacings. 

Zoroga Salt Pans. Roads cross highly saline 

pans in some parts of Botswana. 

Figure 2.1 shows areas of the world with arid and semi-arid climates. Also 

shown are some locations where salt damage has been reported in published 

literature. 

The purpose of this section of the guideline is to provide a general understanding 

of the damage process. Preventative measures suggested in general literature 

should not be used in Botswana as they lack detailed information and 

applicability. 

The semi-arid climate to Botswana creates conditions 

conducive to salt damage. 

q 

Extremely arid 

Arid 

Semiarid 

Reported occurrence 

of salt damage to 

highway pavements 

0 3000 km 

Figure 2.1 World Distribution of dry climates and occurrence of Salt Damage. 


Occurrence and Characteristics 

11


2.2 Salt Damage Occurrence in Botswana 

Figure 2.2 shows areas of Botswana (lighter shaded) most susceptible to the 

occurrence of salt damage to bituminous surfacings. In these areas the dry 

climate combined with the presence of saline materials (often calcrete) and 

saline groundwater or surface water (such as Sua Pan) create conditions necessary 

for salt damage to occur. Reported locations of salt damage in Botswana 

include: 

Shakawe 

Maun 

Nata 

Ghanzi 

Orapa 

Francistown 

Maron grass, a tough, coarse grass not liked by 

cattle, found on some road verges can indicate 

presentce of saline ground. Western part of the 

Kalahari desert. 

Mamuno 

Kang 

Bobonong 

Sekoma 

Gaborone 

Tsabong 

Bokspits 

Figure 2.2 Dark areas are less susceptible to occurrence of salt damage 

Salt damage during the construction of 

- Mopipi-Rakops 

- Sekoma-Makopong 

- Kang -Hukuntsi 

were prevented by judicious materials selection 

using conductivity tests. 

Sua Pan Airstrip 

Nata - Maun Road (km 0-45) 

Phikwe Runway 

Sekoma - Kang road (Trans-Kalahari road) 

Sekoma - Makopong 

Kang - Hukuntsi 

Tsabong - Makopong road 

Orapa - Mopipi road 

Rakops-Motopi 

Maun Runway 

The first four above cases are described below together with the preventative 

measures adopted. 

Domed cape seal surfacing due to salt attack 

Sua Pan Airstrip. 

Sua Pan Airstrip 

The Sua Pan airstrip was constructed in 1988. The pavement comprised calcrete 

subbase and base with a Cape Seal bituminous surfacing. Within six 

months after construction, the surfacing developed star shaped blisters and 

domes ranging in size from a few millimetres to 15 mm in diametre and 1 to 

5 mm in height. The damage occurred initially in the untrafficked edges of the 

runway and progresed towards the centre of the runway. Some of the domes 

had opened up to reveal clusters of white salt crystals. 

12 Chapter 2 


Occurrence and Characteristics


Preliminary field investigations and X-ray studies at University of Birmingham 

showed that the salts originated from the saline subgrade and comprised of 

sodium chloride and Trona salt (sodium hydrogen carbonate or “soda ash”). 

A number of remedial measures were considered including bitumen rubber 

reseal to increase impermeability. It was finally decided to remove and reconstruct 

the damaged parts of the runway with concrete pavement slabs. An alternative 

solution would have been to reconstruct with an impermeable plastic 

layer placed at the top of subgrade to prevent upward salt migration. 

Nata - Maun Road 

Sections of the Nata to Maun road between Nata and 20 km past Zoroga cross 

the northern extensions of the Makgadikgadi Pans. As a result, these sections 

contain saline subgrade soils with salinities ranging from 0.1% Total Dissolved 

Salts (TDS) to 7% TDS. During design trials, damage occurred in the form of 

blister and powdering of both bituminous cutback (MC 30) and emulsion (KR 

60) primes. The damage generally occurred within 48 hours to several days 

after priming depending on the salinity of the top 50 mm of the base course. 

Damage to single and double surface seals also ocurred in areas of saline subgrade 

or where the salinity of the calcrete basecourse exceeded 0.4% TDS. 

To prevent damage to the main road, impermeable plastic sheets were placed 

at the top of subbase along the saline subgrade sections of the road. In areas of 

non saline subgrade, damage was prevented by careful timing of the duration 

between priming and placement of the permanent surfacing. The shoulders 

were also double sealed to minimise evaporation and upward salt migration. 

These preventative measures have performed well to date, 11 years after construction. 

Phikwe Runway 

Extensive blistering of the Phikwe runway was reported several months after 

construction. The damage occurred in the form of star shaped small blisters 

of the double surface treatment surfacing. Recent investigations (Maswikiti 

and Obika, 2000) indicate that the damage is attributable to pyritic oxidation 

which forms soluble sulphides. The pyrites probably originated from the mine 

waste used for the pavement construction. As the source of the salt is finite, 

the damage has not progressed substantially since it’s initial identification 

Salt attack Sua Pan Airstrip. 

Powdered cutback prime and top of base course 

due to salt attack. Nata - Maun road. 

Sekoma - Kang Road 

Salt damage in the form of blistering, doming and powdering occurred to the 

single seal before construction of the second seal of the carriageway along sections 

of the first 12 km of the Sekoma - Kang road. Within twelve months after 

construction further damage to the single seal shoulders occurred over several 

sections extending to Km 250. The damage on the shoulders were notably 

worse where there were imperfections on the surfacing and salts could migrate 

due to evaporation. 

The salts had originated from the saline water used for compaction of lower 

pavement layers. The salt had migrated to upper pavement layers. Table 2.1 

shows the distribution of salt within the pavement approximately 13 months 

after construction. There had been a general increase in salt content under the 

surfacing (0-50 mm) indicating a need for timely reseal to prevent evaporation 

and consequent salt damage. 

Salt blistering on the Sekoma - Kang road. 



13


Table 2.1 Salt content of pavement layers (Sekoma - Kang road). 

Average salt content % TDS (from E.C measurements) 

Depth, 

mm 

Edge of 

shoulder 

Centre of 

shoulder 

Shoulder/ 

carriageway 

interface 

Centre of 

carriageway 

0-50 

0.23 

0.38 

0.45 

0.31 

50-100 

0.18 

0.39 

0.43 

0.30 

100-150 

0.13 

0.33 

0.33 

0.28 

150-200 

0.11 

0.26 

0.28 

0.12 

200-250 

0.10 

0.19 

0.24 

0.17 

Remedial measures included removal of the damaged single seal and reseal of 

the carriageway. Severely damaged sections of the shoulder were removed and 

reconstructed. In areas of less severe damage material in the area surrounding 

the dome was dug out approximately to 20 mm diameter and 50-100 mm 

depth) and replaced with an emulsion based premix. As shown in table 2.1, salt 

is still present in the pavement and it will be necessary to maintain impermeability 

by timely reseal in order to avoid further salt damage. 

The most extensive work to date on the mechanism 

of salt damage has been undertaken by 

Obika et al (1989). 

2.3 Salt Damage World-wide 

There is a paucity of published work on salt damage to bituminous pavements 

which probably does not reflect the scale of occurrence. The published papers 

deal with local environments and materials, and this has resulted in a variety 

of recommendations for damage prevention and repair. These recommendations 

have not always been used successfully in other environments, and have 

resulted in delays to construction or damage to the bituminous surfacing. 

2.3.1 India 

Uppal & Kapur (1957) and Mehra et al (1955) reported on the detrimental role 

of soluble salts on stabilised and unstabilised soils in India. In these cases the 

damage was due to soluble sulphate salt attack on asphaltic surfacings. 

2.3.2 Australia 

There is clear Documentation of physical salt damage to bituminous surfaced 

pavements in Australia (Cole & Lewis 1960). The deterioration often took the 

form of ‘fluffing’ and ‘powdering’ of sandy loam soils immediately beneath 

the bitumen surface. Failed and sound areas linked the deterioration to high 

sodium chloride (NaCl) content. This salt was believed to have migrated from 

a saline water table varying from 4.5 to 24 metres below surface level. Most 

of the salt damage in Australia is limited to high sodium chloride (NaCl) in the 

subgrade and construction materials. 

Filamentous (whisker) crystals lifting a road 

surfacing. In this case the whiskers are visible 

to the naked eye. In other cases a magnifying 

glass or microscope is required. 

Simple laboratory tests were performed on samples of soil taken from sound 

and failed sections of the pavement. The samples were compacted and allowed 

to stand under laboratory conditions. After 9 months the samples that con- 

14 Chapter 2 




tained up to 0.5% NaCl did not show any deterioration but the observed deterioration 

on the specimens containing over 0.5% NaCl was due to the growth 

of white ‘hair like’ crystals. An upper limit of 0.2% NaCl content in sandy clay 

soils was adopted providing a factor of safety of 2.5. 

Januszke & Booth (1984) have documented severe blistering of sprayed seals 

in Western Australia where highly saline water (6.3 to 13.6% NaCl) and natural 

gravel (0.27 to 0.45% NaCl) are used in the pavement construction. Deterioration 

of the surfacing generally occurred in the form of blisters up to 40 to 

100mm in diameter rising up to 10mm in height. 

2.3.3 Southern Africa 

A detailed examination of soluble salt damage to bituminous sealed roads in 

several regions of South Africa was undertaken by Weinert & Clauss (1967) 

following the widespread occurrence of blistered surfacings. Early occurrence 

of salt damage, sodium and magnesium sulphates present in mine waste 

material used for pavement construction were identified to be responsible for 

the salt damage problem. The pavement material was a quartzite waste from 

industrial mine processes. 

By analogy with permissible levels of sulphate 

in building stones an upper limit of 0.05% sulphate 

content was recommended for highway 

materials, whilst accepting the chloride limits of 

0.2% NaCl suggested by Cole & Lewis (1960). 

Extensive work relating to the Southern African experience was undertaken by 

Netterberg (1970, 1979, 1984), Netterberg et al (1974), Netterberg & Maton 

(1975), Netterberg & Loudon (1980) and Blight et al (1974). 

Netterberg (1970) discussed the various types of soluble salts present in 

highway construction materials and concluded that sodium chloride (NaCl), 

sodium sulphate (Na 2 

SO 4 

), sodium carbonate (Na 2 

CO 3 

), magnesium sulphate 

(MgSO 4 

) and calcium sulphate (CaSO 4 

) were likely to be the deleterious salts 

most commonly encountered. A simple conductivity test was proposed as a 

preliminary test for materials used in highway construction in arid and semiarid 

zones. Netterberg discussed some possible sources of error in the salt 

limits suggested by Cole & Lewis (1960) and by Weinert & Clauss (1967). 

In particular he noted that the maximum limits of 0.2% NaCl by Cole & 

Lewis and 0.05% for sulphates by Weinert & Clauss were determined by analysing 

the top few centimetres of the base after upward migration of salt had 

occurred. 

Problems have also been reported in Zimbabwe (Netterberg 1984) where blistering 

and cracking of bituminous surfacing was linked to the formation of salt 

within the sub-base and subgrade layers. These acidic sulphates were derived 

from the oxidation of sulphides in industrial waste material which was used for 

pavement construction. Although present in the material, gypsum was not contributory 

to the degradation of the surfacing, possibly due to its low solubility. 

It is important to take into account not only 

the initial salt content of the material but also 

the salt content near the surface (0-50mm) 

after evaporation and migration may have taken 

place. The latter is more indicative of potential 

for damage to occur. 

The limits suggested were, therefore, not necessarily 

the initial salt content of the bulk material. 

The electrical conductivity limits previously suggested 

by Netterberg (1970) were found to be 

inadequate for prediction of the potential for 

damage. 

Whilst other authors looked at the effect of salt in bases and at the base surfacing 

interface, Blight et al (1974) investigated the properties of rolled asphalt 

made with quartzite mine waste sands with various soluble salt contents. The 

asphalt layer constructed with sand of up to 2.0% soluble salt had no deleterious 

effect on the asphalt after 6 years. The authors demonstrated that soluble 

salts can have significant effect on the flow and stability properties of asphalt 

mixes. 

Cases of salt damage have been reported in Namibia, particularly near Swakopmund. 

However these are not documented in detail. 

Road near Swakopmund, Nambia. 



15


2.3.4 Middle East 

Fookes & French (1977) with considerable experience of soluble salt damage 

in the Middle East, produced a paper which considered in detail pavement 

damage from natural saline materials. They differentiated clearly between soluble 

salt damage due to a high saline water table and that due to saline materials. 

The authors defined four relevant zones of moisture associated with the 

groundwater table, groundwater fluctuations, capillary rise and transient moisture 

which can be used to locate pavements away from hazardous ground. A 

range of soluble salt limits for various types of pavement construction, local 

moisture regimes and materials was presented. Attention was also drawn to 

the possible role of various salt combinations. 

Tomlinson (1978) also reported the blistering of sealed aircraft pavement surfaces 

in the Middle East without explanation of how it occurred. 

Non crystalline salt causes little damage. 

2.3.5 North Africa 

Following observations of salt damaged roads and runway pavements in the 

Algerian Sahara, Horta (1985) produced an interesting physico-chemical analysis 

of the salt damage mechanisms. He ascribed the damage of 50mm 

thick wearing coarse to the crystallisation of halite (NaCl) whiskers or filamentous 

crystals and identified some physico-chemical parameters relevant to the 

damage mechanism. Attempts to repair a salt damaged surface by recompaction 

of the blisters completely failed. A new airport was finally built at a different 

location. Horta’s observation drew attention to some critical crystal growth 

factors, which had not been considered previously in highway work. 

2.3.6 North America 

Soluble salt damage to bituminous surfaces has also been reported in other 

areas. For example, Dunn (1984) reported on the development of small domes, 

50mm to 100mm in diameter on bituminous road pavements in Virginia, North 

America, due to growth of pickeringite (magnesium alum) crystals. 

These above cases have provided a background to understanding the salt 

damage process. 

Microscopic sized salt whiskers breaking through 

road surfacing. This type of crystal causes maximum 

damage due to high pressures. 

The existing salt limits do not account for upward 

migration of salts and should, in general, 

not be used. 

2.4 International Experience on Salt 

Damage, Limits and Preventative 

Measures 

Various recommendations of maximum salt content in highway materials have 

emerged from the above studies. These recommendations are detailed in 

Appendix A. These salt limits are generally based on local experience of salt 

types and pavement design in other parts of the world. They also lack any 

detailed understanding of salt migration and other influencing factors. 

These salt limits should not be applied in Botswana. They are given in this 

guideline only for general understanding of the levels of salt content that can 

cause damage. 

16 Chapter 2 




In addition to the recommended maximum salt contents, various other preventative 

and remedial measures have been suggested. 

2.4.1 Thickness of Surfacing/Permeability Ratio 

With a few exceptions (Horta 1985, Januszke & Booth 1984) pavement 

damage from soluble salts is confined to thin bituminous surfaces such as 

double surface treatment. Relative impermeability can be achieved by using a 

minimum of 30mm dense asphalt concrete. The essential function of a thick 

surfacing is to stop evaporation and hence migration and crystallisation of salt 

at the surface. If salt is kept in solution or in a totally dry state, damage will not 

occur. Damage will occur once the salts are allowed to re-crystallise. 

Microscopic cracks in binder allows evaporation, 

hence salt crystallisation. 

Although a thick impermeable surface can be effective in preventing shortterm 

damage particularly at the bitumen-base interface, in practice, complete 

impermeability is difficult to achieve. As bitumen dries micro-cracks develop 

which allows evaporation (see photograph). Furthermore, salt may continue to 

accumulate beneath relatively impermeable surfaces as a result of temperature 

changes. Degradation of the base and sub-base material may result in loss of 

density with rutting and pot holing in the longer term 

2.4.2 Bituminous Surfacing Layers 

Unsealed roads generally perform well in saline ground. For unsealed roads 

salt helps to bind the surface and suppress dust. Salt efflorescence is commonly 

observed on unsealed surfaces in arid areas and there is no evidence of physical 

degradation of the surface. When a thin seal is applied it may blister and 

crack within 36 hours. 

The sequence and type of damage appears to depend on the thickness of the 

bituminous layer, salt content in the pavement material, the climate and other 

factors discussed in section 2.5. 

Emulsions primes are less susceptible to salt damage than cutback or tar 

primes. 

Salt crystal braking through a bituminous surface 

(viewed through electron microscope). 

Calcrete gravel road, western part of 

Botswana. 

Emulsion primes tend to sit on the surface rather than penetrate into the pavement 

layer. This provides lower permeability and hence reduce the damage 

potential. Road sections with emulsion prime will generally not suffer damage 

despite high soluble salt contents, provided the prime are left no longer than 48 

hours before application of a double seal. 

2.4.3 Brooming 

In cases where there is no source of salt replenishment from a saline water 

table, careful surface brooming of the base can be carried out to remove the 

salts before the bituminous surfacing is applied (Horta 1985). This process 

had not been studied but the effectiveness of such a process would depend 

on the reduction of salt content achieved. However, depending on the circumstances, 

further salt migration to the surface may occur. 

2.4.4 Immediate cover 

Immediate sealing can prevent the accumulation of the salt at the surface after 

compaction with a relatively impermeable surfacing. This reduces evaporation 

and ensures that salt does not migrate rapidly and crystallise at the surface. 

Gravel road on saline ground with additon of 

salt water sprinkle. Road near Swakopmond, 

Namibia. 

Brooming was used successfully on parts 

of Sehitwa-Tsau, Tsau-Gumare and Tsabong- 

Makopong roads. 

It is important to maintain impemability of the 

surfacing when salts are present in underlying 

pavement layers. 



17


This approach was used commonly on Rural Roads projects where salts were 

identified to be problematic. 

2.4.5 Prevention of Moisture Rise - Cut Off 

Salt migration by upward capillary rise of soil moisture can be prevented by 

placing an impermeable or semi-impermeable membrane in the base course 

(French et al 1982). A granular layer in the base course may also reduce capillary 

rise (Horta 1985). French et al (1982) experimented with ‘filtram’, a commercial 

geofabric, and concluded that neither salt nor groundwater will pass 

through the geofabric if placed outside the zone of near saturation in the soil. 

The provision of a coarse grain uniform layer between the sub-base and subgrade 

has been shown to be ineffective. 

2.4.6 Relevance of Published Literature to Botswana 

The published work does not explain the fundamental mechanism of the 

damage. Existing methods of prevention and repair are based mainly on experience 

of local materials and conditions and should not be used blindly for 

Botswana conditions. 

2.5 Factors Influencing Salt Damage 

2.5.1 Climate 

Temperature, relative humidity, wind-speed and rainfall all influence salt 

damage. They affect evaporation significantly and hence the potential for 

upward salt migration. Temperature and relative humidity also determine 

whether salt crystallisation thresholds are crossed. This is discussed by Obika 

et al (1989). Precipitation influences the net water balance at a given location 

and also whether there is a seasonal or perennial moisture deficiency which 

would provide the conditions for a net upward saline moisture migration. 

Where rainfall is insufficient to leach out minerals from weathering rocks, insitu 

accumulation of mineral salts generally occurs. 

2.5.2 Geology and Hydrogeology 

The depth and quality of groundwater contributes significantly towards creating 

bituminous surfacing damage from salts. Saline groundwater may result 

from the solution of minerals present in sediments or from the ingress of seawater 

to the host material. The predominant type of salt depends on a variety 

of geochemical processes, the source of the salt and the local climatic environment. 

The most commonly encountered salt in many arid and semi-arid zones 

is sodium chloride, known as halite. 

Weather station measurement of local climate 

is useful. 

In arid and semi-arid zones the capillary moisture rise can be more than ten 

metres. The height of capillary moisture rise depends on a variety of factors 

including porosity and temperature gradients. 

2.5.3 Materials Characteristics 

The various salt types which can contribute to the damage of pavements in dry 

lands include but are not limited to sodium chloride (NaCl), sodium sulphate 

(Na 2 

SO 4 

), sodium carbonate (Na 2 

CO 3 

), and magnesium sulphate (MgSO 4 

). 

18 Chapter 2 




The various salt types that contribute to the damage of pavements are presented 

in Table 2.2. 

Table 2.2 Some salts which can contribute to salt damage of pavements . 

Name 

Sodium Chloride 

Magnesium sulphate 

Formula 

NaCl 

MgSO4 

Common Name 

Halite, common salt 

Magnesium Sulphate 

Hydrates 

MgSO 

. xH 

O e.g Epsomite 

4 2 

Sodium sulphate 

Na 

2 

O 

S 4 

Thenardite 

Sodium sulphate 

Sodium Hydrogen 

carbonate 

hydrate 

NaSO. 10H 

O Mirabilite 

4 2 

NaHCO 

Glaubers salt (Soda Ash) 

3 

Fine-grained porous materials can encourage deleterious filamentous crystal 

growth and also the pore characteristics of the individual particles can influence 

the movement of saline moisture in the pavement layers. Obika et al 

(1991) have discussed the nature and magnitude of salt crystal pressures. For 

materials of equal mechanical strength, those which contain large pores, separated 

from each other by micropores, are the most liable to salt weathering. 

This is analogus to frost susceptibility criteria. Thus fine grained pavement 

materials are more likely to encourage higher capillary rise of saline moisture. 

The resulting salt crystal pressures are also higher. See Obika et al (1992) for 

more detailed description. 

To mitigate salt damage it is better to avoid fine graded basecourse finish where 

practical. Slushing, for example, should be avoided where other considerations 

permit. 

Figure 2.3 illustrates the salt damage process in relation to pavement salinity 

and water table. 



19


MOVEMENT OF SOLUBLE SALTS FROM SALINE 

GROUND WATER TO TOP OF CAPILLARY FRINGE 

1st STAGE BLISTER FORMS 

2nd STAGE BLISTER FORMS 

ROAD BASE 

SUB-BASE 

CAPILLARY FRINGE 

ZONE OF INTERMITTENT SATURATION 

1. PAVEMENT BUILT WITHIN THE CAPILLARY FRINGE 

SALT CAN MOVE IN SOLUTION FROM THE SALINE WATER TABLE TO THE UNDERSIDE OF A 

RELATIVELY IMPERMABLE BITUMINOUS SURFACE. CRYSTALS MAY FORM IN SUCH A 

PATTERN TO CREATE FORCES SUFFICIENT TO LIFT THE SURFACING. 


GROUND WATER TO TOP OF CAPILLARY FRINGE 



ROAD BASE 

SUBBASE 



2. PAVEMENT BUILT BELOW A CAPILLARY FRINGE BUT WITH NO SALT IN MATERIAL 

SALT CAN MOVE IN SOLUTION FROM THE SALINE WATER TABLE TO THE SURFACE OF A 

RELATIVELY IMPERMEABLE SURFACING. CRYSTALS FORM AT THE SURFACE CAUSING A 

PHYSICAL DEGRADATION OF THE BITUMINOUS SURFACING. 


ROAD MATERIAL 



ROAD BASE 

SUBBASE 



3. PAVEMENT BUILT ABOVE A CAPILLARY FRINGE 

SALT IN THE PAVEMENT MATERIAL CAN MOVE IN SOLUTION EVEN IF ABOVE THE 

CAPILLARY FRINGE. THEY CRYSTALLIZE AT THE UNDERSIDE DEPENDING ON THEIR FORM 

MAY LIFT THE SURFACE. 

Figure 2.3 Salt damage process in 

the relation to pavement salinity and 

water table. 

20 Chapter 2 




2.5.4 Pavement Surfacing Design 

The type of bituminous surfacing and its application rate influences the rate of 

evaporation from the pavement surface and therefore the rate of upward salt 

migration. Pavement damage from soluble salts appears to be confined to thin 

bituminous surfaces, generally less than 50 mm thick. However, a few exceptions 

have been recorded in Algeria and Western Australia (Stuart Highway). 

In southern Africa, Netterberg, Blight, Theron and Marais discovered that 

damage from sulphates in mine waste pavement material could be prevented 

by applying a bituminous surface seal, which had permeability to thickness 

ratio not exceeding 30 (permeability in mm/sec, surfacing thickness in mm). 

Thick surfacings minimise evaporation and hence reduce migration and crystallisation 

of salts at the surface. 

Obika and Freer-Hewish and Woodbridge et al have shown that bitumen emulsion 

primes perform slightly better than bitumen cutback primes in reducing 

salt damage. The emulsion ‘sits’ on the surface rather than penetrating into the 

base, thereby forming a less permeable surface than cutback primes. However, 

emulsion generally gives a poorer bond to the underlying pavement layer. 

Bitumen rubber or polymer modified binders used for sealing have been shown 

to retain impermeability for longer periods than conventional binders. Where 

there is a high risk of salt damage Rubber bitumen should be considered. 

2.5.5 Construction Practice 

The intervals between the construction of the pavement layers, a water bound 

or cemented material, a bituminous prime coat and a final surfacing, such as a 

surface dressing, can be critical if evaporation is high and when salts are present 

in the pavement material and/or a shallow groundwater. Substantial salt accumulation 

may occur at the exposed surface in periods longer than 24 hours. 

Brackish water is often used for compaction and/or curing of pavement layers. 

This can lead to a significant precipitation of salt on the surface of the compacted 

layer. Also, there is evidence, from laboratory studies and field observations, 

to suggest a high risk of surfacing damage when salts in a pavement 

layer are subjected to repeated wetting and drying (solution and re-crystallisation). 

Construction practises which involve repeated wetting of the pavement during 

construction should be avoided. In Botswana, damage is often observed within 

days after a rainy period due to the re-crystallisation of salts. 

Salt damage to shoulders. Damage will normally 

start at the shoulders and progress to the 

centre of the carriageway. This is due to high 

evaporation and less traffic at the shoulders. 

Close-up of salt blirsters above. 



21


2.6 Appearance and Identification 

Depending on the environment, salt levels and type of surfacing salt damage 

may occur within days after priming or up to 2 to 3 years after surfacing. 

Surfacing Type 

Bituminous cut back primes 

Bituminous emulsion prime 

Single surface dressing 

Slurry seal 

Cape seal 

Double surface dressing 

Otta Seal -Single with sand cover seal 

Otta Seal -Double 

Typical duration before first 

signs of damage 

1 day to 7 days 

2 days to 14 days 

7 days to 6 months 



3 months to 3 years 



2.6.1 Damage to Primes 

Bituminous prime coats are the most susceptible to salt damage because of 

their thickness and high permeability to evaporation. Damage to prime coats 

typically occurs in the form of small blisters which, when opened, reveal white 

salt powders. This can often be mistaken for vapour blisters, which result from 

vapour pressure differentials following rainfall on a freshly primed road surface. 

Salt damage is often observed within days after 

priming or up to 2-3 days after surfacing. 

Tsabong - Makopong road. 

In other cases damage to primed surfaces occurs in the form of powdering 

of the surface such that it becomes completely loose and has a brown ‘dead’ 

appearance instead of black. Damage typically starts at the edge of the road 

where evaporation occurs most or where there has been disturbance to the 

surface texture such as along the overlap of spray applications or along construction 

vehicle wheel tracks. The top of the underlying base layer may also 

appear loose. In many cases hair-like (whiskers) crystals can be observed with 

land lens and in severe cases of damage, with the naked eye. Initial signs of 

damage to prime coats can be observed within 24 hours after surfacing. 

2.6.2 Damage to Permanent Surfacings 

Salt damage to more permanent surfacings such as double surface treatments 

and slurry seals may take several days to a few years to manifest at the surface. 

This will generally appear as star shaped domes that open at the top to reveal 

clusters of white salt powder. The domes can range from a few centimetres to 

20 cm in diameter with a typical height of 2 to 6 cm. 

2.7 Summary of Physico-chemical 

Influences on Salt Damage 

The main factors governing the mechanism of salt damage are salt solubility, 

migration, crystallisation, crystal growth habit and crystal pressures. 

Highly soluble salts can re-crystallise several 

times a day as temperature changes causing 

physical damage to road surfacings. 

Solubility 

Only those salts that are soluble in water can migrate to the surface of a pavement. 

The solubility of some natural salts is shown in Figure 2.4 in relation 

22 Chapter 2 




to temperature. The solubility of sodium chloride, the most ‘common’ salt, is 

only slightly temperature-dependant, whereas other salts show rapid changes 

in solubility with change in temperature. In practice this means that these salts 

can re-crystallise several times a day with disruptive crystal growth pressures 

at the road surface. 

60 

50 

Na 2 

SO 4 

Solubility of salt per 100g. H 2 O. 

40 

30 

20 

10 

Na 2 

SO 4 

7H 2 

O 

Mg SO 4 

6H 2 

O 

Mg SO 4 

7H 2 

O 

Na 2 

CO 3 

10H 2 

O 

Na 2 

SO 4 

10H 2 

O 

TRANSITION POINTS 

Mg SO 4 

Na 2 

CO 3 

10 H 2 

O + Na 2 

CO 3 

H 2 

O 

Na 2 

CO 3 

10 H 2 

O + Na 2 

CO 3 

7H 2 

O 

Mg SO 4 

6H 2 

O 

Na 2 

CO 3 

H 2 

O 

Na C1 

Na 2 

SO 4 

Mg 2 

CO 3 

Mg SO 4 

H 2 

O 

Na 2 

CO 3 

0 10 20 30 40 50 60 70 80 90 10 

TEMPERATURE ( O C ) 

Figure 2.4 Solubility of some natural salts in relation to temperature 

Sodium chloride (NaCl) crystals are stable at relative humidity below 76%. 

Above this humidity (typically at night) the crystals will attract moisture from 

the air (hygroscopic) and go into solution. As the humidity drops during 

midday they re-crystallise creating disruptive pressures sufficient to disintegrate 

road surfacings. 

The magnitude of salt crystal pressures generated by crystal growth is sufficient 

to heave over one metre thick concrete slabs and buildings. Prevention of 

salt damage to roads must, therefore, rely solely on methods of stopping migration 

and crystallisation of salts. See Pincher and Hawkins, 1986. for more 

detail on the magnitude of salt crystal pressures. 

An alternative is to ensure that supersaturation does not occur. High supersaturation 

results in the formation of the most deleterious type of salt crystals with 

high growth pressures. These crystals are known as filamentous crystals or salt 

‘whiskers’. 

Obika (1989, 1992) provides a detailed description of the physico-chemical 

mechanisms of salt damage including the magnitude of crystal pressures which 

are outside the scope of this guideline. 



23


Cubic NaCl crystals cause little damage. 

Whisker NaCl crystals cause maximum damage. 

24 Chapter 2 




3. LABORATORY AND FIELD 

TESTING FOR SALT 

3.1 General 

All natural gravels and water used for road and runway construction in 

Botswana should be tested for soluble salt content. Once it is established that 

soluble salts are present in significant concentration (e.g. >0.02% for materials, 

>2000 PPM for construction water), additional tests may be necessary to determine 

the main type(s) of salt present by ionic content analysis and/or X-Ray 

diffraction. Extra caution is required in obtaining and packing of samples for 

salt content tests as described below. 

The broad procedures for salt content assessment and analysis to be followed 

are shown in the following Table: 

Table 3.1 Salt Analysis Tests for materials and water in Botswana. 

Natural 

gravel 

Subgrade 

soil 

Construction 

water 

Applicability 

Field 

Tests 

Laboratory 

Stage 1 

Laboratory 

Stage 2 Stage 3 

Electrical conductivity 

(2:1 soil paste) 

Electrical conductivity (2:1 

soil paste) 

Mouth taste 

Electrical Conductivity pH 

Mandatory for all Projects 

(Mouth taste is optional) 

● Electrical Conductivity 

(TMH - paste method) 

● Total Dissolved Salts (TDS) 

● Electrical Conductivity 

(TMH - paste method) 

● Total Dissolved Salts 

● 

● 

● 

Total Dissolved Salts 

pH 

Electrical Conductivity (EC) 

Mandatory (Extent of EC testing 

depends on extent of field EC 

carried out). 

● Ionic composition 

Cl-, 

SO HCO 

, Na 

+ 

, 

3 3 

K + 2+ 

, Mg 

● 

● 

● 

X-Ray diffraction 

EPM 

Scanning Electron 

Microscopy 

- ditto - 

- ditto - 

- ditto - 

Where no existing 

information on salt 

types exists. 

Essential for salt 

damage risk/design 

assessment. 

- ditto - 

on residue 

Only necessary for 

detailed research 

purposes or if Stage 2 

tests are not able to 

identify salt types 

3.2 Soil and Water Sampling 

Sampling for Salt Content Analysis 

Extra caution is required when taking and handling water or soil samples for 

salt content analysis. This is required to take into account the variable distribution 

of salt in soil horizons and the potential consumption of salts in water by 

algae and bacteria. 


Laboratory and Field Testing for Salt 

25


3.2.1 Water Samples: 

Water samples should be taken from near the surface of water sources to be 

used for construction. 

Approximately 0.5 litres of water is normally adequate for most routine salt 

content analysis. Water samples should be taken and tightly sealed in a nontransparent 

glass or plastic bottle. Water samples exposed to the sun may 

attract algal growth. Such algae may feed on the salt and reduce the salt content 

prior to laboratory testing. All water samples should ideally not be stored 

for more than four weeks before testing. 

Water samples should be taken from near 

the surface of water sources to be used for 

construction. 

Some aquifers with salinity levels exceeding 

that of sea water have been encountered along 

the Jwaneng - Ghanzi road. These are often 

overlain or underlain with fresh (non-saline) or 

brackish water aquifer. 

When samples are taken from boreholes during drilling, it is important to 

ensure that samples are taken from each water strike level. The samples should 

be taken immediately the water is struck to avoid contamination from other 

aquifers. Electrical conductivity tests should be carried out on site using 

portable conductivity metres. Much of the groundwater present in western 

Botswana is ancient groundwater with variable levels of salinity. 

3.2.2 Soil Samples: 

Natural gravel from Borrow Pits 

Approximately 50 to 100 gram samples are adequate for routine conductivity 

and TDS testing depending on grain size of selected gravels. 

Samples used for salt testing should be taken from exploratory holes. A separate 

sample should be taken from the top 50mm of the upper most horizon 

encountered in the soil profile. 

It would normally be adequate to assess the salinity level of a borrow pit on the 

basis of 3 to 8 full salt content tests (depending on size of borrow pit and local 

environment). 

3.3 Field Salt Content Tests 

3.3.1 Field Electrical Conductivity test 

Electrical conductivity tests can be carried out very quickly in the field using 

the quick conductivity test method. This provides a good measure of salt content 

in water or soil which correlates well with more elaborate laboratory tests. 

The methodology for field conductivity tests is relatively simple and is given 

in Appendix B. The equipment required can be carried in a small wooden case 

or brief case and comprises: 

Hand held conductivity metre, is an accurate 

and quick method to measure salt content. 

Hand held conductivity metre, 

Porcelain conductivity cup, 

Stirrer, spatula, bottle of distilled water and a wash bottle. 

Electrical conductivity may be related directly to salt damage potential because 

it measures only those salts which are soluble and can migrate in solution 

to the road surface where crystallisation occurs. E.C can therefore be used 

directly as an indicator of salt damage potential. As such, it can be used to 

assess whether preventative measures are required or not. 

26 Chapter 3 


Laboratory and Field Testing for Salt


E.C can also be correlated to TDS or Chloride ion content once the relationship 

between these are determined for a particular material/soil type or water body. 

The relationship between E.C and TDS for soils along the Nata to Maun road 

is TDS=0.04+0.16* E.C and this relationship is probably applicable for many 

of the saline soils in areas adjacent to Makgadikgadi Pan. In other areas of 

Botswana the relationship may vary due to the predominance of chlorides with 

less carbonate salts. 

E.C test results should be expressed in millisiemens per centimetre (mS/cm) 

or Siemens per metre (S/M) at 25 o C. A Siemen is the reciprocal of electrical 

resistance in ohms. The TDS and other salt content determinations are normally 

expressed as the % weight of the dry soil. In the case of water as milligrams 

per litre or parts per million (PPM). 

3.3.2 Oral Testing (Taste) 

A sweet taste similar to the taste of cooking salt will often indicate NaCl salt 

content of over 0.5%. A sharpe tangy taste may indicate the presence of carbonate 

salts or sulphates. Tongue tasting is particularly useful when trying to 

establish whether damage to bituminous surface is due to salt attack. A moistened 

finger touched to the underside of blistered or domed surfacings may be 

used. 

3.3.3 Field Determination of TDS 

The TDS of water samples may be determined in the field but it is usually 

better to do this under laboratory conditions because of the need for accurate 

weighting. A quick indication of levels of TDS can be obtained in the field by 

correlation with field EC, as described above, with upto 90% accuracy. 

Caution is required when tasting soil and the 

mouth must be fully washed immediately after 

tasting. 

Most field balance will only weigh to an accuracy 

of 0.2 g. and will therefore limit the accuracy. 

3.3.4 Field Chloride content determination 

A rapid indication of PPM Chloride content of water samles may be obtained 

in the field with indicator strips or Tabs. These are chemically active strips 

which when dipped in water, will indicate the chloride content in a graduated 

column to a limited accuracy. 

3.4 Laboratory Tests 

3.4.1 Electrical Conductivity 

Laboratory Electrical conductivity test may be determined either by the quick 

2:1 soil paste method (Obika et at 1989) or the more lengthy saturated soil 

paste method. Details of the latter are given in TMH1 (1986) Method A21T. 

The 2:1 soil paste is based on agricultural practise and has been found to correlate 

well for engineering purposes. The methodology is given in Appendix B. 

3.4.2 Total Dissolved Salts 

The total dissolved salt (or sometimes termed total soluble salt, TSS) should 

be determined in accordance with SABS Method 849. For soils this involves 

shaking the ground soil sample with distilled water for a period of 24 hours and 

determining the mass of salt in a filtered aliquot by evaporating to complete 

dryness in an oven. 

Electrical conductivity test carried out at the 

CML. 

TDS is expressed in milligrams per litre (or 

PPM) for water samples or percentage of the 

dry mass for soil samples. 


Laboratory and Field Testing for Salt 

27


Various methods are available for the determination 

of various Anions. Water soluble sulphates 

should be determined in accordance 

with THM 1 ( 1986) Method B17T. Chlorides 

can be determined by titration with barium sulphate 

and can be carried out in accordance with 

BS 812 Part 4 (1975). 

3.4.3 Ionic Composition 

In order to determine the predominant types of salt present in a soil or water 

sample it is usually necessary to carry out an ionic content analysis. This information 

may be required particularly where there are no existing records of salt 

types. 

Cations such as Na+ and Mg++ may be determined by flame photometry or 

atomic absorption methods using specialist apparatus. 

A knowledge of the salt type provides an indication of the likely levels of salt 

content that will cause damage. Sulphate levels exceeding 0.05% TDS can 

cause damage whereas Chloride levels need to exceed 0.15% before damage 

occurs. 

Quantitative X-ray analysis is unlikely to be 

used for routine highway engineering purposes. 

3.4.4 Mineralogic Analysis 

The determination of the mineralogy of salts is not normally required for routine 

engineering purposes. The precise mineralogy of salts present in soils can 

be determined by qualitative X-Ray diffraction. Quantitaive X-Ray diffraction 

is more rigorous and costly but will also provide information on the quantity 

of salt present in a given soil sample. 

3.5 Presentation of Salt Content Analysis 

Results 

When presenting the results of salt content tests it is important to employ convention. 

Test results are frequently mis-interpreted due to use of differing terminology 

and units of measurement. For example 0.2% Cl- is quite different 

from 0.2% NaCl although both are frequently referred to as chloride. Sulphate 

content may be calculated as SO 3 

or as SO 4 

. (to relate SO 3 

to SO 4 

multiply 

SO 3 

by 1.2). 

28 Chapter 3 


Laboratory and Field Testing for Salt


4. RISK EVALUATION IN 

SALINE ENVIRONMENTS 

4.1 General 

Salt damage risk evaluation is recommended whenever a bituminous surfacing 

is proposed for a pavement in Botswana. Clearly, the damage process is 

dependent on a complex interaction of many variables, but the proposed design 

method is based on two significant parameters, salt content and climate, which 

can be measured relatively easily. Further design parameters can be added as 

other variables can be linked to the damage process in qualitative terms. The 

procedure is illustrated in Figure 4 .1. 

SALINITY LEVELS: 

Material: 

Pavement/subgrade 

M (Fig. 4.2) 

Compaction water 

MC VALUE 

CLIMATE: 

= M x C 

Regional Climate 

Precipitation 

Temperature 

C 1 

(Table 4.1a) 

C 2 

(Table 4.1b 

C 3 

(Table 4.1c) 

C 

C=C 1 

(C 2 

+C 3 

) 

Figure 4.1 Risk Analysis for salt damage to bituminous surfacings. 

Firstly, a ‘M’ value is obtained by allocating scores depending on the salt 

levels in the pavement and subgrade, compaction water, and in some situations, 

ground water. 

Secondly, a ‘C’ value is obtained by allocating scores to climatic conditions. 

Thirdly, the values (M and C) are combined to provide an overall rating which 

indicates the damage risk for bituminous surfacings for the given project. A 

worked example is given in Appendix C. 

4.2 Salinity Levels of Materials and Water 

Salinity values are required for pavement and subgrade materials, imported 

and/or in-situ, and compaction and/or ground water. Methods for determining 

salt content contained in this guideline should be followed and it is important 

to adhere to these methods for consistency and comparability of results. 


Risk Evaluation in Saline Environments 

29


Salt measurement 

In the first instance, total dissolved salts (TDS) will normally be measured, 

however it is useful to have some indication of the dominant salt type for more 

detailed design and construction control particularly if salt levels are significant. 

The Electrical Conductivity (EC) should be measured. The correlation between 

the quick EC and the TDS measurements, for the Botswana field trials, where 

the TDS was measured according to the method given in BS 1377:Part 3, was: 

TDS = 0.04 + 0.16 EC. 

A correlation coefficient of 0.9 was obtained for this relationship. All determinations 

were carried out on the minus 2 mm fraction of the samples, corresponding 

to about 75% by mass of the borrow pit material. The major 

proportion of the salt is contained in the fines. 

The salt content of water and materials should each be determined separately. 

The risk assessment in this guideline takes into account the combined influence 

of each. 

Salt from compaction water is potentially more 

harmful than salt contained in the materials 

due to the ability of salt in water to rise more 

rapidly to the road surface. 

Obtaining Materials and Water rating ( ‘M’ Value) 

Using the appropriate salt levels for materials and water determine the weighting 

value M from Figure 4.2. A M value of 10 should be adopted if the pavement 

or subgrade salinity exceeds 0.8% TDS irrespective of compaction water 

salinity. 

1.0 

Pavement or subgrade material salinity 

Total Soluble Salts TSS (%) 

0.8 

0.6 

0.4 

0.2 

Compaction water salinity 

(see key) 

5.0 

4.0 

3.0 

2.0 

1.0 

Pavement or subgrade material salinity 

Electrial Conductivity E.C (mS/cm) 

(For Botswana Conditions) 

0 

0 2 4 6 8 10 

M value 

0 

Key: 

Compaction water salinity 

Fresh 0 - 0.5% 

Brackish 0.5 - 1.0 % 

Saline >1.0 

Notes: 

For Pavement/subgrade material, 

with TSS levels in excess of 0.8% 

use an M value of 10, irrespective of 

compaction water salinity. 

Figure 4.2 Materials risk rating-salt damge to bituminous surfacings. 

30 Chapter 4 


Risk Evaluation in Saline Environments


The salt content value to be used should be the maximum value obtained from 

the pavement or subgrade materials and may be measured either in terms of 

TDS, or EC if calibrated locally for the materials used. At this stage, the salt 

content of the bulk material or water is used (not the 0-50 mm surface sample). 

The bulk material EC or TDS is normally provided in routine laboratory results 

of potential borrow pit materials. 

4.3 Climatic Conditions 

Characteristics of the regional climate, seasonal precipitation pattern and seasonal 

temperature pattern are required. 

Obtaining Climate rating (‘C’ Value) 

The project site can be classified regionally, as extremely arid to other in Table 

4.1, A value (C 1 

) is assigned appropriate to the regional climate. 

Similarly, using Table 4.1 (b) assign a C 2 

score depending on the season of precipitation. 

In Botswana the rainy season is during summer therefore C 2 

value 

of 3 is appropriate for all cases in Botswana. 

50 cm 

10 cm 

Fibre glass insulation 

Perspex tube 

Compacted Material 

Aluminium sheet sealing 

bottom container 

Coarse granular Limstone 

Soil compacted into mould, primed or surfaced 

and allowed to stand in an oven for a number of 

days can provide a good indication of whether 

damage will occur. 

Assign the C 3 

value for the project location by referring to Table 4.1 (c). The 

temperature range refers to typical daily range. For example the temperature at 

Kang, Kgalagadi District, can drop to 10 o C or lower at night but rises above 

35 o C at day time therefore the range is typically between 20 o C and 30 o C (i.e C 3 

value of 2). 

The overall rating for the climate (‘C’) is obtained by multiplying the sum of 

C 2 

and C 3 

by C1. 

4.4 Combined Risk Evaluation (M x C) 

The combined risk value (MC) is obtained by multiplying M by C. 

MC Value > 30 = Very High Salt damage potential 

MC Value >20


>30 Very High 

>20


5. PREVENTATIVE DESIGN 

PROCEDURES FOR 

MC > 20 

5.1 Types of Bituminous Surfacing 

For reasons discussed earlier, only surfacings less than 50 mm thick are normally 

damaged by salts, and the degree and rate of damage varies according to 

the type of bituminous surfacing. 

5.2 Selection of Bituminous Primes 

The prime surfacings are the most susceptible to damage from salt crystallisation, 

primarily because they are the thinnest surfacings and the least effective 

in reducing evaporation from the underlying pavement. Damage can occur 

within two days of application. 

Prime Type and Application rate 

Primes made from penetration grade bitumen cutback with a highly volatile 

fluid such as kerosene are more susceptible to salt damage than primes made 

from an emulsion. Whilst the use of emulsion is useful to alleviate the onset 

of salt damage, it can create a ‘tacky’ surface, which cannot be trafficked prior 

to final surfacing, unless dusted with fine aggregate. Increasing the prime 

application rate and hence providing a thicker barrier to reduce evaporation 

from the pavement may also create a “tacky” surface. The salt threshold values 

showen in Figure 5.1 provide guidelines for the use of either cutback or emulsion 

primes. 

For long term performance of final surfacings, the maximum salt content 

thresholds recommended for Botswana are shown in Table 5.1. which refer to 

the surface (0-50 mm) of the pavement just before sealing. Figure 5.1. Provides 

a relationship, obtained from Botswana field trials, between initial salt 

content within the pavement material at the time of construction and salt content 

at certain time intervals after construction. Ideally, trials on site to check 

this relationship are recommended. The salt content thresholds and time intervals 

between surfacing operations, as given in figure 5.1, have been designed 

for protection of primes. Note that the values presented in Figure 5.2. (used for 

initial risk assessment) are initial bulk salt values. 

Cattle will congregate and lick salt from the 

road shoulders if salts are present in materials. 

This causes a traffic hazard as well as erosion 

of the road shoulder. 

Caution Salt values at Surface (0-50mm) are 

required for ascertaining appropriate preventative 

measures whereas bulk salt content (i.e 

from sample of whole material) is used for the 

initial risk assessment. 

Construction - Time Intervals between bituminous surfacing and 

construction of pavement layers 

From the above, it is clear that the time intervals between compaction of the 

road base and the prime and the surface dressing are important. Ideally, primes 

should be covered immediately if salts are present in the pavement. 

In the situation when the pavement materials have a negligible salt content and 

there is the possibility of ingress of salt from the water table and/or subgrade 

through capillary action, then vulnerable primes and primer seals should be 

covered within a week by a more substantial surfacing. Figure 5.1 incorporates 

the time constraints for various conditions. Control of salt movement is 

another option and is considered below. 

Actual rates of saline moisture rise in pavement 

from the Botswana field trials appeared to be of 

the order of 5-mm/day. 


Preventative Risk Evaluation Design in Procdures Saline Environments 

for MC > 20 

33


Double Otta Seals (between 30 - 40 mm in 

thickness) are relatively impermeable. In areas 

where there is a potential for salt damage, Otta 

seals should be considered as an alternative to 

other seals 

5.3 Selection of Permanent Surfacings 

Damage to thin permanent bituminous surfacings takes considerably longer to 

develop than damage to primes. This period can vary from one week to several 

years and may depend on the condition of the prime when covered by the permanent 

surfacing, the type, and position of harmful salts in or below the pavement 

and trafficking of the surface. 

The importance of the impermeability of the bituminous surfacing as a means 

of retarding the upward rise of salt was mentioned earlier. Surface dressings 

appear to be impermeable, however, upward movement of moisture has 

occurred on roads in Botswana field with double surface dressing. Cracking 

of a surface caused by shrinkage, oxidation and/or traffic encourages localised 

evaporation and salt crystallisation. 

Stricter limits are required for untrafficked surfaces. The kneading action of 

traffic on surfacings is very important in preventing damage, and increases 

the resistance of surface dressings to salt damage. Cases of salt damage in 

Botswana show detachment of single sealed shoulders alongside intact double 

seal trafficked carriageway. 

5.4 Control of Salt Movement 

When the salts are inherent in the subgrade and/or groundwater an impermeable 

plastic fabric can be introduced at the subgrade/pavement interface. This 

has been found to be effective in preventing saline water rising to the pavement 

surface, thus preventing surface damage. A thick bitumen layer placed in the 

same position is usually not successful in preventing damage. 

BITUMINOUS PRIME TYPE 

CUTBACK (MC 30) 

Shoulder deterioration can also be caused by 

other factors such as insufficient binder spray 

rate. 

SUBGRADE SALINITY 

SALT CONTENT AT 

PAVEMENT SURFACE 

BEFORE PRIMING. 

m S/cm at 25 o C 

(% TSS) 

PERMISSIBLE 

DURATION 

TO SEAL 

SALINE SUBGRADE 

NON SALINE SUBGRADE 

5.0 5.0 

(0.36) (0.36 - 0.84) (0.84) (0.36) (0.36 - 0.84) (0.84) 

30 days 2 days seal immediately no limit 2 days seal immediately 

See note 2 

Bituminous surfacings constructed on saline 

subgrades without an impermeable plastic 

fabric have a high probability that salt damage 

will occur to the surfacing. 

BITUMINOUS PRIME TYPE 

SUBGRADE SALINITY 

SALT CONTENT AT 

PAVEMENT SURFACE 

BEFORE PRIMING. 

m S/cm at 25 o C 

(% TSS) 

PERMISSIBLE 

DURATION 

TO SEAL 

SALINE SUBGRADE 

EMULSION (Cationic) 

NON SALINE SUBGRADE 

8.0 8.0 

(0.60) (0.60 - 1.32) (1.32) (0.60) (0.60 - 1.32) (1.32) 

30 days 5 days seal immediately no limit 10 days seal immediately 

Figure 5.1 Permissible intervals between prime and surfacing in relation to subgrade salinity 

and pavement surface salinity. 

34 Guide to the Prevention and Repair of Salt Damage to Roads and Runways 

34 Chapter 45 

Risk Preventative Evaluation Design in Saline Procdures Environments for MC > 20


20 

19 

12 11 10 9 8,5 

Electrical conductivity at pavement surface (0-50 mm) in mS/cm at 25 o C) 

18 

17 

16 

15 

14 

13 

12 

11 

10 

9 

8 

7 

6 

5 

4 

3 

2 

Initial salt levels in 

bulk material 

(mS/cm at 25 o ) 

8 

7 

6 

5 

4 

3 

2 

1,5 

1 

0,5 

1 

0 

0 2 4 6 8 10 12 14 

Time after compaction (days) 

Figure 5.2 Electrical conductivity readings at the surface (0-50 mm) of a layer with time for various 

levels of initial conductivity of the bulk material. 


Preventative Risk Evaluation Design in Procdures Saline Environments 

for MC > 20 

35


Table 5.1 Suggested maximum salt content limits for Botswana. 

primin 

Surface Traffic Status Subgrade Maximum total soluble salt 

Type Condition content at surface prior to 

g (E.C mS/cm) 

(0-50 mm sample depth) 

Emulsion 

Prime 

Cutback 

Prime 

Prime - - 1.60 1.0 

Single Untrafficked Saline 1.60 0.70 

Surface (Shoulders 

treatment & airstrips) Non Saline - - 

Single Trafficked Saline 5.40 4.10 

Surface (>50 vpd) Non Saline >7.25 >4.75 

Double Trafficked Saline >12.25 >6.0 

Surface (>50 vpd) 

treatment Non Saline - - 

Single Otta Trafficked Saline >12.25 >6.0 

with sand (>50 vpd) 

cover seal Non Saline - - 

Double Trafficked Saline >15 >8 

Otta 

(>50 vpd) 

Surfacing Saline - - 

Notes: 

1. A factor of safety of 2 has been applied. Values refer to salt content at surface (0-50 mm) 

prior to priming. If initial salt contents are only known, obtain an estimate of surface salt 

content for the appropriate time delay using Fig. 5.2. 

2. For Botswana Total Dissolved Salt, TDS % = 0.04 + 0.16 x Electrical Conductivity (E.C.). 

3. Salts can be inherent in pavement materials or introduced with brackish/saline compaction 

water. The values refer to total salt content at surface (0-50 mm) irrespective of source. 

4 Values for Otta seal have been estimated from known relative impermeability of Otta seals. 


36 Chapter 45 

Risk Preventative Evaluation Design in Saline Procdures Environments for MC > 20


6. REPAIR OF DAMAGED 

SURFACING 

6.1 General 

There are not many methods available for cost effective repair of damaged surfacings. 

The repair technique adopted will depend on the severity of damage 

and project specific considerations. Those which have been used succesfuly in 

Botswana and elsewhere are described below. 

6.2 Prime Surfaces 

Damage detected in its early stages can be arrested by rolling which may control 

further ‘blistering’ until more layers can be added and adhesion may be 

regained with the underlying base. 

For severe damage, rolling will not be successful and the surface has to be 

broomed to remove the prime. In situations when the underlying base is still 

sound a bitumen rubber reseal can be used, but if the base consists of soft 

aggregates brooming can damage the base surface. It may then be difficult to 

regain the same surface level and smoothness without scarifying to at least 100 

mm. 

6.3 Final Bituminous Surfacings 

Small failures should be treated locally by removing the surface and hand 

spraying a new surfacing, possibly replacing cutback bitumens with emulsions 

and increasing the application rate without causing the risk of severe bleeding. 

A base course layer with high salt content. The 

prime has been exposed too long before the surfacing 

is applied and deterioration of the prime 

is the result. 

6.4 Resealing with Bitumen Rubber 

Where economically feasible, damaged surfacings should be replaced with 

bitumen rubber seal. The top 50 mm of the existing base course should be 

removed and replaced with either asphaltic material or gravel before resealing 

with bitumen rubber double surface treatment. This remedial measure is not 

applicable when salts are present in the subgrade, providing a source of 

continuous replenishment of salt. Under the latter situation a cut -membrane 

should be inserted at the top of subbase or subgrade prior to sealing with a 

conventional binder or bitumen rubber. 

6.5 Damage caused by Sulphate Salts 

In the infrequent cases where damage is caused by acidic sulphate salts it is 

possible to neutralise the salts by mixing lime into the saline pavement material. 

This forms insoluble salts which do not cause damage. 

Acidic sulphate salts caused damage to Phikwe 

runway. 


Repair of Damage Surface 

37


7. SUMMARY 

In the semi-arid climate of Botswana evaporation exceeds precipitation, soluble 

salts accumulate in the upper layers of the road pavement and can damage 

bituminous surfacings such as prime coats and surface dressings. 

Single salt blister, exposing the base layer. 

Studies have identified the importance of climatic factors and intervals between 

the construction of each pavement layer, surfacing types and trafficking. The 

design procedure show that single values of salt limits, as suggested in other 

reports, are not appropriate for all surfacing types and construction procedures. 

A procedure for risk evaluation of potential salt damage has been developed 

based on the laboratory and field trials in Botswana. Risk ratings are assessed 

for materials, compaction and ground water, and climatic conditions for different 

surfacing types. 

Bituminous prime coats are very sensitive to salt damage and can be damaged 

if the soluble salt content exceeds 0.3% TDS in the road base material as a 

whole. Cutback prime is more sensitive to damage than emulsion prime. 

Salt dome, which will breake under traffi c and 

- resulting in an exposed base. 

Surface dressings and Otta seals are more resistant than bitumen primes to 

salt damage due to their increased bitumen thickness. In Botswana trafficked 

single and double surface dressings will generally not be damaged by roadbase 

TDS contents up to 0.5% and 1.0% respectively. Trafficking appears to 

increase the resistance of surface dressing to salt damage. Surface sealed road 

shoulders and large areas of runway are especially vulnerable to salt attack at 

lower salt content levels. 

For salt levels at the upper acceptable limits, a prime coat should preferably 

be excluded where other engineering considerations, such as adhesion to roadbase, 

allows. Alternatively, the prime coat could be surface dressed within two 

days of application but this is sometimes impractical in contract situations. 

Interestingly, on withdrawal of traffic from hitherto sound sections, the surfacing 

became damaged. 

When the subgrade is saline and pavement layers comprise non saline materials, 

impermeable fabric (plastic) placed at the bottom of the roadbase prevents 

upward rise of salt and protects the bituminous surfacing from salt damage. 

If placed with care, drainage and long term performance is not compromised. 

A thick bitumen layer placed in the same position has not been successful in 

this respect. The technique of a cut-off membrane has been applied to roads 

in Botswana. 

38 Chapter 7 


Summary


8. REFERENCES 

References 

1. Netterberg, F. and Maton, L.J. (1975) Soluble Salt and pH determinations on highway materials. 6th Regional 

Conference for Africa on Soil Mechanics and Foundation Engineering, Durban. 

2. Meigs, P. (1953) World distribution of arid and semi-arid homoclimates. Arid Zone Hydrology. UNESCP pp 

203-209. 

3. Obika, B., Freer-Hewish, R. J., and Fookes, P.G. (1989) soluble salt damage to thin bituminous road and 

runaway surfaces. Quarterly Journal of Engineering Geology, Volume 22, pp 59 - 73. 

4. Weinert, H.H. and Claus, M.A. (1967) Soluble sales in road foundation. Proceedings of the 4th Reg. Conference 

or Africa on Soil Mechanics & Foundation Engineering. Cape Town, pp 213- 218. 

5. Fookes, P.G. and French, W. J. (1977) Soluble salt damage to surfaced roads in the Middle East, J Institution. 

Highway Engineers, 24 (12). 

6. Netterberg, F., Blight, G. E., Theron, P.F. and Marais, G.P. (1974) Salt damage to roads with bases of crusher-run 

Witwatersand quartzite. Proceeding of the 2nd Conference on Asphalt Pavements for Southern Africa, Durban, 

pp 34 - 35. 

7. Netterberg, F. (1979) Salt damage to roads - an interim guide to its diagnosis, prevention and repair. Institution 

of Municipal Engineers of South Africa. NITRR, CSIR, 4. 

8. Cole, D.C.H. and Lewis, J. G. (1960) Progress report on the effect of soluble salts on stability of compacted 

soils. Proceedings of the 3rd Australia-New Zealand Conference. Soil mechanics and foundation engineering, 

Sydney, pp 29-31. 

9. Obika, B. and Freer-Hewish, R. J. (1988) Salt damage to bituminous surfaces for highway and airfield pavements. 

Final report to the Overseas Development Administration, University of Birmingham. 

10. Obika, B. and Freer-Hewish, R.J. (1991) The control of soluble sale damage to bituminous road surfaces in 

tropical environments. Final report to Overseas Unit of the Transport and Road Research Laboratory. University 

of Birmingham. 

11. Obika, B. Ghataora,G and Freer-Hewish, R. (1992). Heave of Lime Stabilised Capping Layer-M4o Motorway. 

A report for the Dept. of Transport, UK. 

12. Woodbridge, M., Obika, B., Newill, D. and Freer-Hewish, R.J. (1994) Proceedings of the 6th Conference on 

Asphalt Pavements for Southern Africa, Cape Town. 

13. Obika, B., Freer-Hewish, R.J. and Newill, D. (1992) Physico-chemical aspects of soluble salt damage to thin 

bituminous road surfacing. International Conference on the Influence of Ground Chemistry in Construction. 

IGCC ‘92’, Bristol University. 

14. Cooke, R.U., Brunsden, D., Doornkamp, J.C., and Jones, D.K.C. (1982) Urban geomorphology in dry lands. 

Oxford University Press. 

15. Horta, J.C. de O.S. (1985) Salt heaving in the Sahara. Geotechnique, 35(3) pp 329-337. 

16. Januszke, R.M., and Booth, E.H.S. (1984) Soluble salt damage to sprayed seals on the Stuart Highway. 

Proceedings of the 12th Australian Road Research Board Conference (ARRB), Part 3, pp 18-30. 

17. Doornkamp, J.C. and Ibrahim, H.A.M. (1986) Electrical conductivity and saline concentrations in arid land 

groundwaters. Quarterly Journal of Engineering Geology, Volume 19, pp 249-250. 

18. British Standards Institution (1990) British Standard Methods of Test for Soils for Civil Engineering purposes, 

Part 4 Compaction-related tests. 

19. Tomlinson, M.J. (1978) Engineering problems associated with ground conditions in the Middle East. 


39

Appendices 

APPENDICES 


Appendix A - Maximum Salt Content Limits worldwide 

Appendix B - Field Electrical Conductivity measurement of soils by the quick conductivity method 

Appendix C - Risk Evaluation and Determination of required preventative measures - worked example 

Appendix D - Glossary of Terms 

Appendix E - Abbreviations 


41 



Appendices 

uthor 

A 

t 

al 

S 

t 

aximum Limi 

M 

n 

Locatio 

aterial 

M 

s 

Criteria/Remark 

Lewis 

& 

Cole 

(1960) 

as 

Chloride 

NaCl 

be 

0.2% (0.5% may 

safe) 

and 

soils 

clay 

Sandy 

semibase 

gravel 

lateritic 

Australia 

Western 

- 

arid 

base 

damaged 

salt 

soluble 

of 

Observations 

compacted 

with 

tests 

laboratory 

and 

courses 

salt 


amounts 

various 

which 

to 

gravels 

was 

damage 

No 

added. 

been 

had 

solution 

to 

up 

containing 

materials 

with 

encountered 

warn 

0.2% and 

suggest 

authors 

0.5% but 

tests. 

their 


scope 

limited 

the 

against 

Soluble 

given. 

not 

analysis 

salt 


Method 

results 

Australia 

Western 

in 

damage 

salt 

in 

chloride 


rise 

from capillary 

mainly 

groundwater. 

& 

Weinert 

(1967) 

Clauss 

Chloride 

as 

Sulphate 

O 

S 3 

y 

(mostl 

- 

gSO 

M 4 ) % 

0.2 

0.05% 

foundations 

road 

For 

but 

generally 

materials 

quartzite 

for 

principally 

Africa 

South 

waste 

mine 

Lewis 

& 

from Cole 

adopted 

limit 

Chloride 

from 

suggested 

limit 

Sulphate 

(1960). 

0.05%) 

(ie 

limit 

critical 

the 


considerations 


content 

sulphate 

and 

stones 

building 

for 

occurred. 

had 

damage 

where 

materials 

base 

5:1 


analysis 

by 

obtained 

as 

contents 

Salt 

carried 

was 

Analysis 

extracts. 

soil 

water: 

few 

top 

from the 

obtained 

material 

on 

out 

upward 

after 

probably 

base 


inches 

this 

in 

Damage 

place. 

took 

salt 


migration 

saline 


use 

from the 

resulted 

case, 

material. 

construction 

Netterberg 

(1970) 

an 

As 


indication 

total 

sulphate 

if 

material 

Reject 

conductivity 

electrical 

>1.5mmhos/cm at 

is 

0.6-1.5 

If 

15.8ºC. 

for 

mmhos/cm test 

if 

reject 

and 

sulphates 

0.06% 

If 

>0.05%. 

mhos/ 

m 

t 

cm accep 

material 

sub-base 

and 

base 

any 

For 

Africa. 

South 

material. 

limits 

salt 

to 

correspond 

limits 

Conductivity 

and 

(1960) 

Lewis 

& 

Cole 

by 

suggested 

Electrical 

(1967). 

Clauss 

& 

Weinert 

indirapid 

a 

as 

used 

be 

can 

conductivity 

major 

a 

has 

it 

however 

salinity, 


cation 

the 

identify 

not 

does 

it 

that 

in 

drawback 

to 

only 

Applicable 

present. 

salt 


type 

material 

construction 

waste 

mine 

quartzite 

varies 

relationship 

conductivity/salinity 

as 

material. 

the 

on 

depending 

Netterberg, 

& 

Theron 

Blight, 

(1974) 

Marais 

soluble 

Total 

salt 

soluble 

Total 

sulphate 

0.2% 

0.15% 

sub-base 

and 

Base 

Witwatersand 

materials. 

see 

- 

waste 

mine 

quartzite 

Clauss. 

& 

Weinert 

Africa 

South 

the 


nature 

peculiar 

the 

stress 

Authors 

upon 

material) 

waste 

(mine 

experience 

based. 

are 

limits 

suggested 

the 

which 

water 

on 

out 

carried 

analysis 

Sulphate 

& 

Fookes 

(eg 

authors 

some 

Note: 

extract. 

acid 

to 

refer 

1979) 

Netterberg 

1977), 

French 

are 

considered 

salts 

The 

sulphate. 

soluble 

construction 

waste 

mine 

from quartzite 

materials. 

Stewart 

Blight, 

(1974) 

Theron 

& 

soluble 

Total 

(mostly 

salt 

sulphate) 

safe) 

be 

% (3% may 

2 c 

asphalti 

for 

used 

sand 

For 

Africa 

South 

mixes. 

various 


surfaces 

asphaltic 


Observations 

bearing 

sulphate 

with 

made 

ges 

a 

o 

Als 

sands. 

specimens 

on 

tests 

aboratory 

l 

g 

containin 

salt. 

soluble 

added 


quantities 

various 

APPENDIX A: MAXIMUM SALT CONTENT 

LIMITS WORLDWIDE



Appendices 

uthor 

A 

t 

al 

S 

t 

aximum Limi 

M 

n 

Locatio 

aterial 

M 

s 

Criteria/Remark 

- 

1083 

SABS 

(South 

1976 

Bureau 

African 

Standards) 


soluble 

Total 

salt 

.5% 

0 a 

Afric 

outh 

S 

n 

o 

Based 

(1979). 

Netterberg 

in 

uoted 

Q 

n 

a 

when 

salts 

soluble 

total 

that 

understanding 

salts 

harmless 

other 

include 

will 

determined 

0.2% 

Therefore 

material. 

the 

n 

i 

g 

Netterber 


too 

considered 

(1974) 

al 

t 

e 

e 

Solubl 

stringent. 

extracts. 

water 

on 

determined 

content 

salt 

(1976) 

light 

B 

e 

solubl 

Total 

salt 

.2% 

0 e 

sub-bas 

and 

Base 

materials 

preof 

sulphates 

are 

discussed 

salts 

The 

Author 

iron. 

magnesium and 

dominantly 

be 

also 

0.2% can 

that 

uggests 

s 

d 

applie 

predomi 

is 

salt 

the 

here 

w - y 

antl 

n . 

chlorides 

that 

acknowledge 

not 

does 

limit 

The 

ul 

s . 

chlorides 

than 

deleterious 

more 

are 

phates 

& 

Fookes 

(1977) 

French 

- d 

an 

traffic 

with 

Varies 

stipulated 

other 

conditions 

Middle 

the 

in 

experience 

field 

on 

Primarily 

the 


assessment 

on 

and 

ast 

E 

f 

o 

type 

found 

conditions 

and 

aterials 

m 

t 

tha 

in 

possible 

the 

to 

point 

Authors 

egion. 

r 

t 

effec 

specified 

limits 

the 

on 

mixtures 

salt 

f 

o 

d 

an 


type 

predominant 

the 

account 

into 

take 

region. 

the 

in 

salts 

Netterberg 

(1979) 

as 

Sulphate 

O 

S 3 

o 

t 

according 

BS1377 

to 

According 

BS1377 

soluble 

Total 

salt 

0.3% 

0.5% 

2.0% 

treated 

cement 

and 

Lime 

cohesive 

if 

materials 

cohesive 

not 

If 

fines 

- 

material 

Untreated 

discussed. 

not 

limits 

suggested 

for 

Criteria 

water 

on 

out 

carried 

are 

Determinations 

discussed 

that 

to 

similar 

Materials 

extracts. 

(1974). 

al 

et 

Netterberg 

in 

William 

Sir 

& 

Halcrow 

(date 

Partners 

unknown) 

soluble 

Acid 

sulphate 

soluble 

Acid 

sulphate 

soluble 

Acid 

sulphate 

0.3% 

0.5% 

2.0% 

base 

and 

course 

Wearing 

material 

course 

hard 

and 

Roadbase 

shoulder 

Sub-base 

Middle 

the 

in 

construction 

road 


Experience 

East.


APPENDIX B 

Appendices 

FIELD ELECTRICAL CONDUCTIVITY MEASUREMENT OF SOILS BY 

THE QUICK CONDUCTIVITY METHOD 

EQUIPMENT: 

Electrical conductivity meter such as the Portec PI 8140 

Porcelain conductivity cup or similar 

Spatula, 1 x 5 litre bottle of distilled water 

Wash bottle, 1 x 50 ml graduated beaker and glass stirring rod 

Box of tissues 

SAMPLING: 

Using the spatula, scoop the fine grained (

Appendices 

APPENDIX C 

RISK EVALUATION AND DETERMINATION OF REQUIRED 

PREVENTATIVE MEASURES-WORKED EXAMPLE 


INTRODUCTION 

This example follows the Procedure described in Chapters 4 and 5 of the guideline. There are two steps. First step is 

to establish whether the condition for salt damage exists. This is done by looking at the climatic and salt content data 

to obtain a risk rating. Step 2 involves determining the prevention measures required. The following hypothetical case 

is used for illustration. 

PROJECT DATA 

Project location: Village of Kang, Western Botswana. 

Road Construction: 150 mm subbase, 150 mm base. Assumed OMC: 10% moisture content. 

Surfacing: carriageway: Double surface treatment (trafficked) 

Shoulder: Single seal with sand cover seal (untrafficked). 

Climatic Conditions 

Regional climate: Semi-arid 

Annual mean temperature: 26°C 

Rainy season (precipitation): During Summer months 

Typical daily temperature range: Winter.......... 0 - 30°C - Range: 30 

Summer....... 15 - 42°C - Range: 27 

Salinity Conditions 

Sub-grade: non saline 

Calcrete Sub-base material: 0.4% TDS (or 2.25 E.C) 

Base Course material: 0.4% TDS (or 2.25 E.C) 

Compaction water: 5000 mg/l 

A- OBTAINING THE RISK RATING 

Stage 1: 

Obtaining M-Value 

From Fig. 4.2 

Read Pavement Materials Salinity on vertical axis (i.e. 0.4% TSS). 

Move horizontal to Brackish water (middle diagonal line) since the compaction water is brackish. 

Move downwards from this point to intercept M-Value. 

This is the Materials Risk Rating M-Value (M = 4) 

Stage 2: 

Obtaining Climatic Risk Rating 

From Table 4.1 

Since the Kang region is semiarid, select C 1 

value of 6, C 1 

= 6 

a) Since the rain falls during the Summer months select C 2 

value of 3 (i.e. Summer precipitation), C 2 

= 3 

b) Since the typical daily temperature variation is between 20° and 30° (I.e. 27 and 30 from our data for Kang), select 

C 3 

value of 2, C 3 

= 2 



Appendices 

Stage 3 Obtaining the combined C value 

Obtain combined climatic risk rating (C) by multiplying sum of C 2 

and C 3 

by C1, i.e. C = (3+2) x 6 = 30 

Stage 4: Obtain an overall risk value (MC) for Kang 

By multiplying M-Value by combined C-Value, i.e. MC-Value = M x C = 4 x 30 = 120 

As the MC-Value for this particular project in Kang is greater than 20, there is a potential for salt damage to occur. 

Therefore design of Preventative Measures are required. An example of this is given below: 

B - DESIGN PREVENTATIVE MEASURES 

Stage 1: 

Determine total salt content level in pavement materials as follows: 

1. Approximate quantity of salt to be added through construction water: 

From the data, the OMC is 10% (add 3% for evaporation): = 13 x 0.5 = 0.065% TSS 

100 

2. Salt content already present in materials = 0.4% TSS 

Therefore total salt content in pavement materials = 0.065 + 0.4 = 0.465 or 2.7 mS/cm 

(Note conversion to E.C using the following formula: TSS = 0.04 + 0.16 EC (See section 4)) 

Stage 2: 

Estimate the likely increase in salt content at the pavement surface (0-50 mm) that will result from 

evaporation after compaction 

From Fig. 5.2 

a) Select the number of days constructed base is likely to be exposed before priming (horizontal axis). (10 days is 

used for this example). 

b) Draw a vertical line to intercept the 2.7 mS/cm diagonal lines (i.e. the initial built materials salt content). 

c) Draw a horizontal line from this intercept and read off the likely salt content at the pavement surface on the vertical 

axis = 5.5 mS/cm. 

Stage 3 

Determine the allowable time delay between priming and final surfacing 

From fig 5.1 

The permanent surfacing must be placed to cover the prime immediately (within 24 hours) if Cutback Prime is used. If 

Emulsion prime is used, the permanent surfacing can be placed up to 10 days after priming. 

Specifications: 

For the example provided, a possible specification could be as follows: 

There is potential for salt damage to occur if the saline pavement materials and construction water are used. 

In order to avoid salt damage: 

- The constructed base should not be exposed for periods exceeding 10 days. (or if exposed for more than 10 days 

more stringent (shorter) time delays between priming and sealing must be adopted). 

- Bituminous Cutback Prime, if used, should be sealed with permanent surfacing within 24 hours. 

It is recommended that Bituminous Emulsion Prime is used instead of Cutback Prime. The Emulsion Prime should not 

be exposed for more than 10 days before the permanent sealing is constructed. 


45

Appendices 

APPENDIX D - GLOSSARY OF TERMS 


Soluble Salts 

Solubility 

Crystallise 

Saturation 

Supersaturation 

Crystal Pressures 

Capillary action 

Electrical conductivity 

Crystal habit 

Halite 

Whiskers 

Hygroscopic 

Hydration 

Salt damage 

Chloride 

Total Soluble Salt (TSS) 

Total Dissolved Salt 

Maximum Salt Limits 

Salts that have high solubility in water at normal temperature and pressure. 

The process of solid “dissolving” in a liquid solvent to form solutes. 

Formation of solid crystals from a solution. Sometimes termed “precipitation”. 

The point achieved when the concentration of solutes (salts) in the solvent (water) 

are at equilibrium at the prevailing temperature and pressure conditions. 

The point reached when the concentration exceeds the equilibrium concentration 

resulting in crystallisation of the solute. 

The forces generated during crystallisation and the growth of crystals. 

The process of moisture movement inside small interconnecting pores of soil 

brought about by attractive forces between the moisture and walls of the pores. 

The reciprocal of electrical resistance measured in ohms and provides a measure 

of the concentration of solutes (salts) in the solution. 

Refers to the shape and form of the crystal. 

Sodium chloride solid crystals, major component of common salt used for cooking. 

Crystals that have “hair like” shapes which tend to form at high supersaturation and 

have high disruptive crystal pressures. 

Attracts moisture. 

The process of incorporation of moisture molecules in the crystal during 

crystallisation. 

The process of physical degradation of road surfacings due to pressure exerted 

during crystallisation and crystal growth. 

Compounds with chlorine as the dominant anion, such as NaCl. 

The quantity or concentration of soluble salts in a given quantity of solvent (water). 

The quantity or concentration of soluble salts in a given quantity of solvent (water). 

The concentration of Total Dissolved Salts below which salt damage is unlikely to 

occur. 



APPENDIX E - ABBREVIATIONS 

Appendices 

BS 

C Value 

EC 

EPM 

M C Value 

M Value 

MC 

MgSO 4 

mS/cm 

Na 2 

CO 3 

Na 2 

SO 4 

NaCl 

NORAD 

NPRA 

OMC 

pH 

PHN 

PPM 

S/M 

TDS 

TMH 1 

TSS 

UK 

- British Standards 

- Risk of Salt damage occurring in due to Climatic conditions 

- Electrical Conductivity 

- Electron Probe Microanalysis 

- Risk of Salt damage occurring due to combination of materials and climatic factors 

- Risk of Salt damage occurring due to Materials salinity. 

- Medium Curing 

- Magnesium Sulphate 

- milli siemens per centimetre 

- Sodium carbonate 

- Sodium Sulphate 

- Sodium Chloride 

- Norwegian Agency for Development Cooperation 

- Norwegian Public Roads Administration 

- Optimum Moisture Content 

- Hydrogen ion concentration 

- Public Highway Network 

- Parts per million 

- Siemens per metre 

- Total Dissolved Salt 

- Technical Methods for Highways 

- Total Soluble Salt 

- United Kingdom 


47

Appendices

Roads Department - Government of Botswana

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