Roads Department - Government of Botswana

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Roads Department 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. Guide to the Prevention and Repair of Salt Damage to Roads and Runways Chapter 2 Occurrence and Characteristics 19
Roads Department 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. MOVEMENT OF SOLUBLE SALTS FROM SALINE GROUND WATER TO TOP OF CAPILLARY FRINGE 1st STAGE BLISTER FORMS 2nd STAGE BLISTER FORMS ROAD BASE SUBBASE CAPILLARY FRINGE ZONE OF INTERMITTENT SATURATION 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. MOVEMENT OF SOLUBLE SALTS FROM SALINE ROAD MATERIAL 1st STAGE BLISTER FORMS 2nd STAGE BLISTER FORMS ROAD BASE SUBBASE CAPILLARY FRINGE ZONE OF INTERMITTENT SATURATION 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 Guide to the Prevention and Repair of Salt Damage to Roads and Runways Occurrence and Characteristics
Page 2 and 3: Roads Department The Prevention and
Page 4 and 5: Roads Department ROADS DEPARTMENT U
Page 6 and 7: Roads Department 3.3.3 Field Determ
Page 8 and 9: FOREWORD Roads Department The preve
Page 10 and 11: Roads Department 1. INTRODUCTION 1.
Page 12 and 13: Roads Department 2. OCCURRENCE AND
Page 14 and 15: Roads Department Preliminary field
Page 16 and 17: Roads Department tained up to 0.5%
Page 18 and 19: Roads Department In addition to the
Page 22 and 23: Roads Department 2.5.4 Pavement Sur
Page 24 and 25: Roads Department to temperature. Th
Page 26 and 27: Roads Department 3. LABORATORY AND
Page 28 and 29: Roads Department E.C can also be co
Page 30 and 31: Roads Department 4. RISK EVALUATION
Page 32 and 33: Roads Department The salt content v
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Page 40 and 41: Roads Department 8. REFERENCES Refe
Page 42 and 43: 41 Guide to the Prevention and Repa
Page 44 and 45: Roads Department APPENDIX B Appendi
Page 46 and 47: Roads Department Appendices Stage 3
Page 48 and 49: Roads Department APPENDIX E - ABBRE

Roads Department - Government of Botswana

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