POTHOLES: - CSIR

POTHOLES:Technical guide to their causes,identification and repair

POTHOLES: A technical guide to their causes,identification and repairCSIR Project Team:Dr Phil Paige-Green (project leader)Amrita MaharajJulius KombaNote: This comprehensive technical guide on potholes, theircauses, identification and repair is accompanied by a short,non-technical guide. The short document summarises the maincauses of typical potholes and ways of limiting their formation.The non-technical guide is aimed at people not directlyinvolved in the technical aspects of infrastructure engineeringof roads.An e-book published by CSIR Built EnvironmentDecember 2010Copyright © CSIR 2010ISBN 978-0-7988-5594-5Both publications are available free of charge:Download the documents in PDF format at www.csir.co.za.pothole_guidesor contact CSIR Built Environment:Tel 012 841 2871 • Email lsibanyoni@csir.co.za

forewordWe have great pleasure in presenting this technical guide on potholes,their causes, identification and repair.Over the past few years, the development of potholes in South African roads hasaccelerated considerably, leading to serious concern and wide media coverage.The increase in pothole damage can be attributed primarily to reduced preventativemaintenance being applied to many roads, combined with particularly wet periodsduring rainy seasons and rapidly increasing numbers of heavy vehicles.This guide describes the causes of typical potholes and uses a decision key system toidentify the appropriate repair methods. Various methods are described to ensure that repairwork is appropriate for the specific type of pothole and that the pothole will thus not formrepeatedly due to failure to address the cause. Mechanisms for quality control of potholerepairs are presented. A standard form for use by inspectors during the field rating ofpotholes and identification of repair methods is included.A short companion brief on potholes, their causes and prevention has also been produced.This is targeted at administrators and non-technical management officials.Given the extent of the pothole situation countrywide, we believe the CSIR has anobligation, and is also ideally positioned, to produce such a guideline document. Bothdocuments are available freely for use by the various authorities and interested parties.In this way the CSIR wants to ensure all those responsible for roads have access to theguidelines. We hope that the application of the information presented here will result inthe formation of fewer potholes and more effective corrective actions.Hans W IttmannExecutive Director: CSIR Built Environment1

TABLE OF CONTENTChapter 1 – INTRODUCTION............................................................................................... 4Chapter 2 – CAUSES OF POTHOLES..................................................................................... 62.1 Asphalt...................................................................................................... 62.2 Thin bituminous-surfacing seals...................................................................... 102.3 Unusual forms of pothole development........................................................... 192.4 Reinstated service trenches........................................................................... 192.5 Failure of existing patches............................................................................ 192.6 Mechanisms of pothole development............................................................. 202.7 Location of potholes................................................................................... 202.8 Non-labour-based repair............................................................................. 23Chapter 3 – CLASSIFICATION AND MANAGEMENT............................................................. 243.1 Classification............................................................................................ 243.2 Management............................................................................................ 26Chapter 4 – REPAIR/CORRECTION..................................................................................... 274.1 Background.............................................................................................. 274.2 Safety and traffic control............................................................................. 284.3 Preliminary treatment................................................................................... 284.4 Materials for filling potholes......................................................................... 314.5 Pothole repair methods................................................................................ 33Chapter 5 – QUALITY CONTROL......................................................................................... 395.1 General................................................................................................... 395.2 Control.................................................................................................... 395.3 Field test equipment.................................................................................... 41Chapter 6 – CONCLUSIONS............................................................................................. 42REFERENCES..................................................................................................................... 43APPENDIX A: Standard form for the field assessmentAND categorisation of potholes............................................................. 44APPENDIX B: Key to the identification of pothole repair measures............................. 462

TABLE OF FIGURESFigure 1:Localised ponding of water from seepage in the adjacent area during the dry season(note the presence of hydrophilic plants near the road).................................................................................... 7Figure 2: Typical fatigue-cracking of asphalt leading to a pothole................................................................................... 7Figure 3: Cracking of asphalt resulting from ageing..................................................................................................... 8Figure 4:Typical reflection-cracking of stabilised material through a G1 base and 30 mm of asphalt(mostly sealed)......................................................................................................................................... 8Figure 5: Typical shallow failure of asphalt at the interface with older asphalt.................................................................... 9Figure 6:Figure 7:Initiation of potholes as a result of overloading of a stabilised base – initial crackingfollowed by pumping and then potholes..................................................................................................... 10Development of potholes on a low-volume road with thin bituminous surfacingand insufficient preventative maintenance.................................................................................................... 11Figure 8: A typical pothole formed under a thin surfacing with a granular base............................................................... 12Figure 9: A pothole resulting from large stones in the base beneath the seal.................................................................... 12Figure 10: A typical pothole resulting from a weak upper-layer of stabilised material.......................................................... 13Figure 11: A pothole resulting from lamination, carbonation and weak layers in the top of a stabilised base material............... 13Figure 12: Fatigue cracking of a thin bituminous seal with pothole formation..................................................................... 14Figure 13: A pothole (filled with roadside material) resulting from unsealed cracks in a thin bituminous surfacing...................... 15Figure 14: A typical pothole formed as a result of carbonation of a stabilised base............................................................ 16Figure 15: The initiation of potholes due to a lack of prime............................................................................................ 16Figure 16: A pothole developing as a result of excessive soluble salts in the pavement base layer........................................ 17Figure 17: Repaired potholes caused by mole-rats....................................................................................................... 17Figure 18: A typical pothole formed adjacent to a service trench.................................................................................... 17Figure 19: Failure of a patch within one year of repair – note standing water adjacent to the road in this area....................... 18Figure 20: Strongly-cemented gravel patches.............................................................................................................. 18Figure 21: A patch using strongly-cemented gravel (almost concrete).Note cracking in the road due to differential stiffness between the patch and the road......................................... 18Figure 22: Shear failure in the base (note heaving of the asphalt adjacent to the yellow line)and a patched pothole that formed............................................................................................................ 21Figure 23: Enlargement of an unrepaired pothole through ravelling and whip-off............................................................... 21Figure 24: Moisture/traffic-associated cracking in the outer wheel track leading to potholing and extensive patching............... 22Figure 25: Potholes resulting from construction deficiencies............................................................................................ 22Figure 26: Long, linear development of potholes, resulting from inappropriate layer materials............................................... 23Figure 27: Repeated repair of potholes due to the fundamental problem (subsurface drainage) not being addressed................ 27Figure 28: The effective use of a geosynthetics crack-sealing strip to seal the edges of a patch............................................. 28Figure 29: A typical pothole-patching operation on a rural road..................................................................................... 37Figure 30: A poorly-finished and untidy patch............................................................................................................. 40Figure 31: A well-executed and aesthetically-pleasing rectangular patch next to a less effective ’circular’ patch....................... 40LIST OF ABBREVIATIONSCMADCPHMASAMIRCCDCold-mix asphaltDynamic Cone PenetrometerHot-mix asphaltStress Absorbing Membrane InterlayerRapid Compaction Control Device3

CHAPTER 1INTRODUCTIONThe unprecedented development of potholes during the 2009/2010 summer rainfallseason on particularly the South African provincial and metropolitan roads withbituminous surfacings led to widespread concern among road users and significantmedia reporting.Numerous claims were laid against roadauthorities for vehicle damage caused bypotholes and even for serious vehicle accidentsresulting from excessively large potholes.Potholes have always been a problem onsealed/paved roads, but never to the extentexperienced during the summer of 2009/2010.The causes of the large increase in the degreeand extent of potholes during this period weremany, but can probably be attributed mainly tothe following:• Insufficient routine, periodic or preventativemaintenance leading up to the summer;• Unusually wet conditions for sustained periods;• Ineffective or no repair of existing potholes.The actual costs of potholes in South Africa interms of damage to vehicles and accidentscaused directly by potholes and other road-usereffects have not been quantified, but probablyrun into many millions of rand. It should, however,be noted that this problem is not unique to SouthAfrica. The costs of similar problems over thewet period in the United Kingdom have beenestimated as in excess of £10 billion (aboutR120 billion) (World Highways, June 2010).This is four times more than the entire annual roadbudget for South Africa. It is anticipated that itmight take 15 years to ‘fix’ the pothole problemin the United Kingdom.The internet has literally millions of referencesto potholes, mainly from the United States andgenerally associated with the spring thaw thatoccurs in cold regions. In these regions, moisturein the pavement freezes during winter with anincrease in volume and a consequent decreasein material density. As the frozen pavement layersthaw out in spring, the moisture content of thematerial increases (often to saturation), whichdecreases the pavement support. Under trafficloading, high pore water pressures develop inthe wet materials with subsequent failure of thematerial. The climate in South Africa, however, issuch that this problem can be considered minimaland not an important contributor to the overallpothole problem.There is no doubt that water is the primary causeof potholes, with the access of water into theroad structure to cause the potholes being mostlya function of the surface condition. A lack ofperiodic and/or preventative maintenance ofroads often leads to the development of surfacecracks, which allow rapid ingress of water intothe structural layers during rainfall.As road budgets are constrained andpreventative maintenance is reduced orprolonged, the potential for the developmentof potholes during wet weather increasessignificantly. The main objective in reducingpothole formation is thus to ensure that4

CHAPTER 2CAUSES OF POTHOLESAlthough the presence of water is the primary cause of potholes, their formation differssomewhat depending on the road pavement structure and materials used. Potholes can,of course, also result from diverse, non-structural causes such as diesel (or other chemical)spillages; mechanical damage to surfacings from vehicle rims and/or accidents andfires; damage caused by falling rocks in cuttings; animal hooves on road surfaces in hotweather; and poor road design over certain subgrades, e.g. expansive, collapsible anddispersive soils.The majority of potholes form in the wet or rainyseason, but it is not uncommon for potholes todevelop and deteriorate during the dry seasondue to not only the action of traffic, but alsotemporary wet conditions resulting from localisedirrigation, ponding and/or seepage of water, etc.(Figure 1). The latter can usually be identified bythe presence of water-loving (hydrophilic) plants inthe area.Potholes also occur commonly as a result ofpoor reinstatement of service trenches that areexcavated through bituminous-surfaced roads.These are dealt with separately in this document.Potholes may be accompanied by severe crackingand deformation or distortion of the surfacingaround the pothole, indicating a deeper-seatedcause for the pothole formation. Where littledeformation is observed in the vicinity of thepothole, the cause is more likely to be the entryof water through surficial cracks in the roadpavement and deterioration of only the surfacingand upper structural layers of the pavement.Primary differences in pothole formation arise fromwhether the bituminous pavement surfacing isasphalt or a thin bituminous-surfacing seal (locallycalled ‘chip and spray’, surface dressing, surfacetreatment or chip seal). These differences arediscussed here.2.1 AsphaltWhere asphalt is used on roads in South Africa,it is typically between 25 and 50 mm thick, unlikethe thick asphalt surfacings (and bases) usedcommonly in many northern hemisphere countries(100 mm plus).Potholes in asphalt originate in two ways. Theyare caused either by cracking of the asphalt asa result of fatigue or ageing (binder shrinkage)that allows water into the support, or by thepenetration of water to a less permeable interfacewithin the asphalt layer, resulting in stripping of theasphalt.2.1.1 Cracking of asphalt surfacingThe cracking of asphalt surfacings is typicallythe result of poor support (unsuitable materialtypes or thicknesses, or excessive water), resultingin fatigue-cracking of the asphalt (Figure 2).In addition, environmental cracking can occurdue to ultraviolet light from the sun, heat, oxidationor some other cause that has resulted in shrinkageof the asphalt (Figure 3). Furthermore, reflectioncracking due to the shrinkage of underlying6

Figure 1: Localised ponding of water from seepage in the adjacent area during the dry season (note the presence ofhydrophilic plants near the road)stabilised materials as the cementitious stabiliserhydrates also leads to cracking (Figure 4).An underlying weakness in the support layersusually results in high surface-deflections undertraffic loading, particularly if the materialbecomes wet. Moisture may penetrate thepavement layers through incipient cracks fromabove, or build up beneath the pavement asa result of poor drainage in the area.Potholes associated with environmental and trafficcracking usually start as spalling of the asphaltadjacent to the crack, which then enlarges withtime and traffic to develop a pothole. If the cracksare sealed or the spalling is repaired in time, noFigure 2: Typical fatigue-cracking of asphalt leading to a pothole7

Figure 3: Cracking of asphalt resulting from ageing8Figure 4: Typical reflection-cracking of stabilised material through a G1 base and 30 mm of asphalt (mostly sealed)

significant damage will occur to the pavement, butif left open, the access of water through the cracksresults in deeper deterioration of the road.Irrespective of the cause of the cracking, waterentering the road through the surfacing can leadto the rapid development of potholes during wetweather, as seen frequently in recent years.2.1.2 Water penetration into the asphaltThe separation of asphalt overlays from underlyingasphalt (or other bituminous seals) as a result ofpermeability inversions and/or moisture effects atthe interface or possibly the presence of a stressabsorbingmembrane interlayer (SAMI), results inthe stripping of the asphalt and the development oftypically shallow potholes (Figure 5).It should be noted that the underlying materialexposed in the pothole is frequently old and dryasphalt, which is more susceptible to ravellingthan the newer asphalt at the surface. If this typeof pothole is not sealed or repaired quickly, thesurface area enlarges and the underlying asphaltis abraded, leading to the development ofdeeper potholes that are more difficult to repair.2.1.3 Traffic loadingHeavy traffic loading (in excess of thepavement design loading) causes excessiveroad deflections that result in fatigue failures.Repeated high deflections, or even a few passesby overloaded vehicles, cause the road surfaceto crack, allowing water to flow through thesecracks into underplaying layers (base/sub-base),which causes loosening of the material.This loose material can then be pumped out ofthe road leaving the upper layers unsupported,which eventually collapse to form a pothole(Figure 6). Overloading control is thus anessential part of preserving road functionality andreducing general pothole formation. This entailsthe effective control of traffic loading to ensurethat it does not exceed the design loading, e.g.by means of signage and enforcement.Figure 5: Typical shallow failure of asphalt at the interface with older asphalt9

Figure 6: Initiation of potholes as a result of overloading of a stabilised base – initial cracking followed by pumpingand then potholes2.2 Thin bituminous-surfacing sealsThin bituminous-surfacing seals such as single anddouble seals, Cape seals and slurry seals are themore common types of bituminous surfacing usedin South Africa. They are generally durable sealsbut their performance depends on the underlyingmaterial, which is often moisture-sensitive andsusceptible to rapid deterioration in the presenceof water. Slurry and single (or even double)sand seals on their own are very thin and moreprone to irregularities in the top of the basepunching through the seal than other seals andforming potholes (Figure 7). These need carefulpreventative maintenance to retain the integrity ofthe seal over the design period.In nearly all cases, the propagation of potholesin thin surfacings progresses from the top, oncewater is allowed to access the underlying material,whether it is crushed stone, natural gravel orcemented gravel.10Thin bituminous-surfacing seals (possibly apartfrom slurry seals) are considered to be quiteflexible if maintained properly. Such seals canabsorb relatively high deflections for sometime before cracking, especially when they areconstructed using modified binders. However,once cracking starts, the access of water intothe pavement structure is rapid and failure (lossof surfacing and upper structural layers) occursover a much wider extent than when subjected tolocalised ‘breakthrough’. Greater depths of thepavement are affected much quicker. Potholescan also develop in thin bituminous seals as aresult of extreme loss of aggregate.2.2.1 Loss of surfacing (‘surface breakthrough’)The localised loss of surfacing, usually leading torelatively small potholes that are not very deepinitially, is often caused by mechanical damageto the surfacing. However, surfacings that containimpediments such as stone loss, localised weak

Figure 7: Development of potholes on a low-volume road with thin bituminous surfacing and insufficient preventativemaintenanceaggregate, aggregate containing sulphidesor large stones may develop small potholes atthese sites. Thin bituminous seals such as sandand slurry seals, which are usually less than5 mm thick – which is similar to the typicalirregularity permitted in newly-constructed bases(COLTO, 1998) – can be broken through easilyunder traffic as they may consist solely of a thinbitumen layer if a 5 mm irregularity (a smallprotruding stone, for instance) exists in the base.Where this happens, both wear-and-tear under trafficand the entrance of water will result in the formationof potholes, in a similar manner to those developedfrom mechanical damage to the surfacing (Figure 7).Other potholes in thin surfacings can result fromlocalised weaknesses in the base material orconstruction (Figure 8), large stones beneath the seal(Figure 9), underlying weak layers (Figure 10) orsurface laminations in stabilised bases (Figure 11).11

Figure 8: A typical pothole formed under a thin surfacing with a granular base12Figure 9: A pothole resulting from large stones in the base beneath the seal

Figure 10: A typical pothole resulting from a weak upper-layer of stabilised materialFigure 11: A pothole resulting from lamination, carbonation and weak layers in the top of a stabilised base material13

14Figure 12: Fatigue cracking of a thin bituminous seal with pothole formation

2.2.2 CrackingWith time, bituminous surfacings will crackwithout ongoing preventative maintenance,primarily as a result of oxidation and drying ofthe bitumen binder, but also through fatigue asthe pavement deflects under traffic (Figure 12).Routine preventative maintenance such as theperiodic application of fog sprays, timeousresealing and the sealing of cracks will avoid this.At present this is generally not practiced inSouth Africa.2.2.3 Poor repairsAlthough highly undesirable, it is not uncommonfor potholes to be repaired with material obtainedfrom the roadside (Figure 13). This should neverbe done, as subsequent sealing of the potholeoften involves removal of some of the upper (poor)material and replacement with asphalt. This newasphalt is directly underlain by a weak, watersensitivematerial which will fail rapidly when wet.Only appropriate materials should be used forpothole filling, even when done as a temporarymeasure.2.2.4 Poor adhesion between base and sealThe development of potholes commonly resultsfrom a lack of adhesion between thin surfacingsand the base course, particularly stabilisedones. This is demonstrated clearly in Figure 14,where carbonation of the upper portion of thebase resulted in a thin layer of loose materialbetween the base and the seal. The passageof heavy traffic caused lateral movement of theseal, extension cracking and the development ofpotholes.Sometimes the poor adhesion between thebase and seal can be the result of a localisedloss of prime prior to sealing. This too leads tostripping of the surfacing in these areas and thedevelopment of potholes (Figure 15).Figure 13: A pothole (filled with roadside material) resulting from unsealed cracks in a thin bituminous surfacing15

16Figure 14: A typical pothole formed as a result ofcarbonation of a stabilised baseFigure 15: The initiation of potholes due to a lack ofprime

Figure 16: A pothole developing as a result ofexcessive soluble salts in the pavement base layerFigure 17: Repaired potholes caused by mole-ratsFigure 18: A typical pothole formed adjacent to a service trench17

Figure 19: Failure of a patch within one year of repair – note standing water adjacent to the road in this area18Figure 20: Strongly-cemented gravel patchesFigure 21: A patch using strongly-cemented gravel(almost concrete). Note cracking in the road due todifferential stiffness between the patch and the road

2.3 Unusual forms of potholedevelopmentPotholes can be formed by a number of othercauses. An unusual cause of loss of bondbetween the base and a bituminous surfacingis the presence of excessive soluble salts inthe pavement. If water evaporates through thesurfacing (either as a result of high permeabilityor the presence of any defects – cracks,excessive voids, etc.), soluble salts can bedeposited between the seal and the base atthese points. This will cause a loss of bond(Netterberg, 1979) and lead to potholes. Thesepotholes are not considered part of routinemaintenance and cannot be repaired followingthe methods described in section 4. Specialtreatments are usually required – these are notdescribed in this document.In certain areas of South Africa, potholes havealso been caused widely by mole-rats. Theseanimals tunnel under roads (particularly insandy areas), causing the collapse of the roadsurface above the tunnels and particularly wherecavities are excavated and where the excavatedmaterial is ejected at the surface (molehills)(Figure 17). These potholes can be quite deepand often need specialised attention, which isalso not covered in this document.2.4 Reinstated service trenchesTrenches are regularly excavated across manyurban and occasionally some rural roads toinstall or repair various underground servicessuch as electric cables, water reticulationsystems, sewage pipes, etc. These works arenormally carried out by the relevant municipalauthority or contractors appointed specificallyfor this. In either case, the quality of repair ofthese trenches is often not done by road repair‘specialists’ and thus seldom meets the requiredstandards. This results in settlement of thesurfacing (leading to ponding of water, crackingand potholes), cracking or opening of the joinsbetween the new trench and the adjacentexisting seal and the development of potholes(Figure 18).Repair of these problems should follow thesame process as for any other similar potholesdescribed later in this document.2.5 Failure of existing patchesMany instances are observed where potholepatches have failed. This is normally the resultof either the original cause of the problem notbeing addressed, or the effect of poor patchingworkmanship.Where patches fail (as illustrated in Figure19), the full classification and repair processdescribed later should be followed. In mostcases, the size of the new patch is likely toexceed the size of the original patch in order toeliminate the primary cause of the problem.The repair of patches using strongly-cementedmaterials is not recommended (Figure 20).The patch is usually significantly stiffer than thesurrounding material, starts to ‘rock’ under trafficand results in failure of the surrounding contactareas (Figure 21).19

2.6 Mechanisms of pothole developmentA well-constructed and maintained, intact,flexible bituminous seal should not developpotholes. It may deform as the underlying layerfails, but without the development of cracks inthe seal or actual loss of the seal, potholes willnot develop. Preservation of the seal in a goodcondition (with regular application of fog spraysand reseals) will thus avoid the formation ofpotholes.The majority of potholes in roads are associatedwith wet conditions, while water in thepavement structure will seldom cause distress orpotholes without the application of loads fromvehicles. Once the pavement is loaded, shearfailure of the material in contact or close to theloaded tyre will occur. This is caused by theapplied stresses exceeding the shear strengthof the material, which at this stage is usuallysaturated and subjected to effective stresseswith high pore-water pressures (a phenomenonexacerbated by rapid loading).If the material is only moist and not saturated,but the total stress exerted by the wheel loadexceeds the strength of the material, shearfailure may also occur – the depression formedin this way usually leads to a pothole over time(Figure 22). It is sometimes seen that the exposedmaterial in potholes starts to ravel (especiallywhen the plasticity is low) and de-densify asthe pothole enlarges. This material, apart fromhaving a low shear-strength, is also subject towhip-off and loss of material from the potholeunder rapidly moving traffic (Figure 23).In addition to the direct stress/loading effects,it is also possible that the seal/base interfacebecomes weakened and more prone to abrasionunder wheel movement, resulting in enlargementand deepening of the pothole. As the potholebecomes deeper, the impact of vehicle tyreson the weakly-supported edge of the potholeresults in collapse or disintegration of the sealand accelerated enlargement of the pothole topotentially dangerous conditions.2.7 Location of potholesThe actual location of potholes within the roadcarriageway can be a useful indication of theorigin or cause of the pothole. The majority ofpotholes seem to occur in the outer wheel pathsof single carriageway roads leading to extensivepatching (Figure 24). These can usually beattributed to the effects of increased moisture inthe subgrade and pavement layers during thewet season. This results from seasonal moisturefluctuations within the outer 600 to 1 200 mm ofthe carriageway, as described by Emery (1992).This effect is probably exacerbated by the extraload on the outer wheels of heavy vehicles as aresult of the road camber.20

Figure 22: Shear failure in the base (note heaving of the asphalt adjacent to the yellow line) and a patchedpothole that formedFigure 23: Enlargement of an unrepaired pothole through ravelling and whip-off21

Figure 24: Moisture/traffic-associated cracking in the outer wheel track leading to potholing and extensive patching22Figure 25: Potholes resulting from construction deficiencies

Potholes that develop along the centre-line of theroad are frequently the result of poor sealing/bonding of joins between successive runsduring the sealing process. Water can enterthe pavement along these joins, weakeningthe material and forming potholes. Otherconstruction deficiencies (e.g. localised areasof poor compaction because they are too wetduring construction, material segregation, lackof bond between base and seal, etc.) canalso lead to potholes not directly relating to theseasonal moisture movements (Figure 25). Thewidth of the road and whether the shoulders aresealed can also affect the location of potholes.On narrower roads, the inner wheel paths inthe two directions may overlap, resulting in aneffective doubling of the load in this area andpossible development of potholes.Potholes between the centre-line and the outerwheel track, in the zone of moisture equilibrium,are generally the result of unsealed cracks, poorbonding between the base and seal, or localisedareas of poor construction or material quality.2.8 Non-labour-based repairThe majority of pothole repairs will normallybe undertaken using teams of road workers,usually with a mechanical compactor. However,when potholing becomes extensive or affectslong, linear sections of road (Figure 26), it maybe more effective to use a small recycler ormilling machine to remove the affected areaand replace the material with an appropriaterepair material. This process is not discussed inthis document and should follow conventionalrecycling or large repair procedures.Figure 26: Long, linear development of potholes, resulting from inappropriate layer materials23

CHAPTER 3CLASSIFICATION ANDMANAGEMENT3.1 ClassificationPrior to any successful pothole repair, it isessential to have identified and classified thecause of the problem. The surficial repair ofpotholes, without attending to the fundamentalcauses, is normally a complete waste of timeand resources – incorrectly repaired potholes arelikely to fail again soon after repair (Figure 19).To assist with the identification of the causesof potholes, the decision key process shown inTable 1 can be used. Table 1 shows that theclassification process is purely visual, exceptfor Key point 10, where it is necessary todetermine whether the top of a cementitiouslystabilisedbase has carbonated. This requiresthe application of a small quantity of dilute(5N) hydrochloric acid. If the acid effervesces(or fizzes) vigorously, this is a strong indicationof excessive carbonation. The use ofphenolphthalein to confirm the absence of severecarbonation may also be necessary.Similarly, the determination of whether thesubgrade is excessively wet requires somejudgement, which is gained with experience.An assessment of whether water-loving vegetationor evidence of ponding is present in the area(see Figure 1) will usually be an initial guideto whether excessive moisture (not necessarilyseasonal moisture fluctuations) is the cause ofproblems. Seasonal moisture fluctuations in theouter zone of the road adjacent to unsealedshoulders are a necessary and expected partof road performance. Failure in this zone isusually attributed to marginal or poor materialquality, resulting in excessive moisture sensitivityof the material strength. This key is followednumerically with reference to various correctionor repair techniques described in Section 4.It is important to classify asphalt pavements withcare. If the asphalt has been resealed with a thinbituminous surfacing (slurry and chip seals arecommon applications over asphalt), the potholeshould be classified as an asphalt pothole forrepair-identification purposes. The applicationof asphalt overlays on top of old surfacingseals is rare, but could occur. In these cases,the surfacing would be classified as asphalt forpothole cause-identification purposes. It is alsocommonly seen on older roads that numerouschip seals have been applied over the years,resulting in an effective seal of 25 mm thicknessor greater. These seals should still be classifiedas thin bituminous seals and not as asphalt forrepair purposes described in Section 4.Potholes are seldom deeper than 150 or200 mm, as they then become serious safetyhazards and are generally repaired or filledwith local materials at that stage. The failureof layers beneath (or even within) the subbaseis thus very seldom visible from thesurface. Potholes associated with failures ofdeeper materials tend to be manifested byvisible surface deformation (rutting, shearing,mounding, depressions, etc.). Evidence of thisindicates the need for deep repairs, as discussedin the following sections.24

Table 1: Key to the decision process for the repair of potholesKey Defect Repair action Go to1Surfacing is asphalt 2Surfacing is thin bituminous seal 42Pothole is deeper than asphalt wearingcourseBottom of pothole is within asphalt wearingcourseShallow asphalt (HMA or cold mix)33Pothole caused by cracking due to fatigue ofasphaltPothole caused by localised surface wateringress with no associated crocodile crackingDeep repair after sub-soil drainageinstallationMedium-depth asphalt repair45678Pothole has exposed an unstabilised base 5Pothole has exposed a stabilised base 10Pothole is not associated with cracks 6Pothole is associated with cracks 8Pothole affects seal and top of baseonly (< 50 mm)Shallow surface repairPothole extends > 50 mm into base 7Pothole affects only the baseMedium-depth repairPothole extends below the base 8Pothole does not affect entire pavementstructure (only base and sub-base)Medium-depth or deep repairPothole affects entire pavement structure 99Pothole is the result of saturated subgrade orsupportPothole is the result of poor material – noevidence of excessive subsoil waterDeep repair after sub-soil drainageinstallationDeep repair10Top of base has carbonated and is weak 11Top of base has not carbonated excessivelyand is still strongShallow surface repair11Pothole is associated with crocodile crackingPothole is not associated with crocodilecrackingDeep repairMedium-depth repair25

3.2 ManagementFor effective management and control ofpotholes, particularly during the wet, rainyseason when large potholes can form literallyovernight and a rapid response is necessary,it is essential that a coordinated reportingsystem be implemented.Probably the most effective system is the onecurrently in use by the South African NationalRoads Agency Ltd (SANRAL), making useof appropriate notice or sign boards with atelephone number that is operational 24/7.This does, however, require that the public ismade aware of the system and has confidencethat reports will be responded to timeously. Italso requires trained stand-by maintenancecrews to repair potholes immediately afterinformation has reached the specific roadauthority.A more arduous and costly system wouldnormally involve regular inspections bysuitably-trained staff of the road authority,particularly during and after periods of heavyor prolonged rainfall. The assessor should beable to classify the nature and cause of thepothole on site and make a recommendation(using Table 1) as to how it should be repaired.For effective repairs of potholes, it is essentialthat the correct resources are readily available.Equipment such as picks and shovels,compactors, a diamond-saw for cutting ofsurfacing, drums of bitumen emulsion andpockets of approved cold-mix asphalt (asdiscussed in section 4.4.1) must be readilyavailable to the repair teams. A range ofappropriate compaction equipment (handrammers, small plate compactors and/orpedestrian rollers, as necessary) should alsobe available.Appendix A provides a standard form forthe field assessment of pothole types andrecommendations of the repair actions. Thiscan be used by field evaluators to assess eachof the parameters necessary for the decisionprocess for pothole repair required in Table 1.26

CHAPTER 4REPAIR/CORRECTIONPrior to any pothole repair, it is essential to have identified the cause of the problemas described in the previous section.4.1 BackgroundOther than for shallow potholes affecting thesurfacing only, without repairing or attendingto the fundamental problem, pothole repairswill rarely be successful and will requirerepeated repair (Figure 27). During this time thedegree and extent of the potholes will increasesignificantly. The overall riding-quality of the roadgenerally decreases simultaneously. Aspectsrelating to attending to the fundamental problemsare included in this section of the document underthe different repair techniques.The fundamental principle relating to potholerepair is to produce a patch with a deflectionunder traffic similar to the adjacent road.Significant differences in deflection will leadto cracking at the interface of the patch,as frequently seen at the junctions betweenmanholes in the road (essentially deep and stiffenough to avoid any deflection under traffic)and the adjacent road. Any cracking thatdevelops between patches and the road undertraffic loading will lead to ingress of water andadditional potholing.Figure 27: Repeated repair of potholes due to the fundamental problem (subsurface drainage) not being addressed27

4.3 Preliminary treatmentThe area to be patched must be marked usingchalk, spray paint or crayons ensuring that theentire ’failed’ area together with some soundarea is included. It is essential that the markedarea includes the whole area affected by thepothole and any associated distress. This isoften the primary distress (cracking, spalling,etc.) that led to the formation of the pothole inthe first instance.28Figure 28: The effective use of a geosynthetic cracksealingstrip to seal the edges of a patchWhen the extent of the potholes is large or thedegree of failure is severe, a decision needs tobe made whether the repair of the potholes isjustifiable economically and what the options are.Typically these would be to either implement aholding action (e.g. patch and place a bitumenrubber seal) or whether to rehabilitate the roador parts of the road (e.g. in situ recycling). Thisdecision is best made by a specialist pavementengineer, who should be consulted.4.2 Safety and traffic controlPotholes can occur in all areas of all types of road.Their repair thus poses significant safety issues. Thenormal safety precautions relating to the provisionand visibility of safety apparel, appropriate safetyvests, safety training of patch-repair teams andthe use of appropriate traffic-control measuresare absolutely essential. Details in this regard arenot dealt with in this document and should followconventional safety requirements.The patch should be marked using straight linesas these can be cut more neatly than roundor oval patches. A diamond saw should beused to cut through the surfacing and to givea neat and clean, well-shaped patch. Oncethe surfacing has been cut, the material to bereplaced can be excavated using a pick andspade or jackhammer, if necessary.The majority of patches will normally berectangular with sides parallel to the edge ofthe road. This is certainly recommended forlarge patches of more than 1 m in dimension.Experience has shown that for smaller areas,a diamond-shaped patch with the upper andlower apices in the direction of traffic movementtends to be more effective and durable. Theimpact of tyres on the edge of such patchesis not uniform and the stresses appear to beabsorbed differentially, reducing the tendencyof the patch to crack at the edge or deformlongitudinally. Round patches would probablyhave similar benefits, but are more difficult toconstruct with a reasonable aesthetic quality.The join between the patch and the existingroad is the area that fails most frequently, withan open crack developing that allows theaccess of water and premature failure of anotherwise satisfactory patch. The applicationof a geosynthetic crack-sealing strip over these

joins, using a layer of bitumen emulsion tostick the strip and a second layer on top ofthe strip to ‘waterproof’ the geosynthetic, hasbeen shown to be a good means of minimisingdamage at this interface (Figure 28). Blindingwith some coarse sand ensures that the bitumendoes not stick to vehicle tyres.The excavated hole will need to be wellcleanedprior to any repair. The depth of thematerial that needs to be removed depends onthe cause of the pothole (see section 4.5) andthe material in which it occurs. It is necessaryto remove all loose and unbonded material aswell as material that has been affected by thepothole, whether it is de-densified, sheared orexcessively moist. Once this is complete, thehole should be cleaned of all loose material bysweeping it. Without ensuring that all unsoundmaterial is removed, even the best repair ofthe pothole will soon be affected by continuedfailure of material outside the pothole patch.4.4 Materials for filling potholesSmall potholes are usually repaired by fillingthem with cold-mix asphalt material (CMA).Large ones can be repaired more effectivelyusing hot-mix asphalt (HMA), but the location ofthe potholes needs to be within a suitable hauldistance from an HMA source. The materialordered needs to be of significantly largequantity to make the delivery cost-effective andwithin a suitable proximity so that the materialcan be delivered at a temperature that allowssuccessful placement and compaction withoutexcessive cooling.Asphalt can, however, be rather costly. It is bestto replace the bulk of the material removedfrom deep potholes with natural, cemented orbitumen-treated materials and restricting the useof asphalt to the upper portion of the repair.4.4.1 Cold-mix asphalt (CMA)This is the material mostly used for small tomedium potholes. The material is usuallyavailable in bags (25 to 40 kg) from commercialsuppliers. Most cold-mix materials cannot besupplied in bulk, as the majority make use ofbitumen emulsion or cut-back bitumen as thebinder. The binder needs to be sealed fromdrying out (loss of cut-back) or the ‘breaking’ ofthe emulsion. Even bags that are torn or brokenmay result in unsuitable materials for patchingpotholes after a relatively limited time. Mostproducts have a specified shelf life and careshould be taken to ensure that the material is stillin a workable condition.There are numerous commercial suppliers ofCMA in South Africa. The products have a widevariety of mixes, components and properties andthus potential uses. No general recommendationsregarding their use can therefore be made.No standard specification for the requirementsor properties of CMA is available, but AgrémentSouth Africa compiled a document recentlyfor the certification of cold-mix materials(Agrément South Africa, 2010). This outlinescertain performance levels for a range of tests(aggregate polishing value, resistance againstmoisture-induced stripping and permanentdeformation, aggregate strength, voids content,permeability, etc.). Materials complying withthese requirements and which are Agrémentcertificated are expected to perform satisfactorilyfor pothole patching.The permeability (or voids content) of theasphalt is one of the major factors affectingits suitability. All asphalt for patching shouldbe as impermeable as possible, this being afunction of the grading of the aggregate usedfor manufacture of the mix (as well as the degree31

of compaction after placement). To ensure a lowpermeability, a grading that is continuous andbalanced (Fuller type) should be used.As commercial suppliers may change thecomposition and formulation of their productsperiodically, it is recommended that specificproducts are tested and identified for use. Closecontact should be maintained with suppliers sothat any changes will be noted and additionaltesting and proving done. The ad hoc procurementof cold mix should be avoided and any tenderprocesses should ensure that a suitable product,and not necessarily the lowest-priced one, ispurchased. Large stocks should not be kept toavoid deterioration of the material over time.CMAs that make use of cut-back bitumen can bedifficult to work with when cold and often require‘warming up’ in the sun before use. Emulsion-basedproducts do not usually present this problem.With CMA, it is necessary to ensure that theentire patched area that will be in contact withthe CMA is covered with a tack coat, usuallybitumen emulsion, to ensure good bonding with theexcavated hole.4.4.2 Hot-mix asphalt (HMA)Where potholes occur within a reasonable hauldistance of an HMA plant and the quantity ofasphalt required to patch the potholes warrants anadequate supply (usually in urban areas), HMA isprobably the best material for patching potholes.The HMA should comply with the necessaryspecification for continuously-graded asphalt asspecified for a project, or in COLTO Section 4203(COLTO, 1998). The minimum working temperatureis a function of the viscosity of the material (whichdepends on the binder grade) determined in thelaboratory and will be identified for each mix.In most cases this should not be less than about135 °C. It is, however, essential that the materialis not too cold as compaction will then not beeffective.Most continuously-graded HMAs will have a lowpermeability, an essential requirement for successfulpothole patching. Despite certain current practices,it is not recommended that HMA be used to patchpotholes with a thickness of more than 75 mm,as it is difficult to ensure adequate compactionof thicker layers.With HMA, it is necessary to ensure that theentire patched area that will be in contact with theHMA is covered with a tack coat, usually bitumenemulsion, to ensure good bonding between theHMA and the excavated hole.32

4.4.3 Natural gravelIf the material originally used for the specificlayer is still available or a nearby borrow-pitwith essentially the same material exists, thiswould probably be the best material to use forthe repair of deep potholes. Use of a similarmaterial compacted to the same density willensure that the material strength is similar,that there are no significant permeabilitydifferences, and settlement or furthercompaction of the patch under traffic will beminimised.Where the same materials are not available,similar materials complying with the TRH 4(1996) and TRH 14 (1985) requirements forthe specific layer should be used.4.4.4 Cemented materialNatural gravel treated with a small percentageof cement is commonly used to fill the bulkof the pothole. This can be very effective ifproportioned correctly, but too frequently,the percentage of cement is excessive. Thematerial placed in the pothole then acts asa single mass of lean concrete in the roadstructure, rather than a continuous material withapproximately uniform properties. This resultsin the material deflecting under traffic in amore rigid manner than the flexible pavementsurrounding it. The effect of this is cracking atthe interface between the cemented materialand the surrounding material, possible rockingof the stiffer ‘block’ and ultimately access ofwater into the pavement as the cracks widen.Where stabilised materials are to be usedto repair potholes which contain a stabilisedlayer, a stabiliser type and content similar tothat of the original structure should be usedas far as possible. The material should becompacted to 96 or 97% Mod AASHTOdensity, as this would probably have been thetypical target density for the stabilised layerduring the original construction.4.4.5 Bitumen-stabilised materialNatural or processed gravel treated with a smallquantity of bitumen emulsion can also be usedto replace material removed from a pothole.Bitumen-treated materials are usually morewater resistant than natural materials and theaddition of bitumen emulsion usually improvesthe properties, particularly the strength of thematerials significantly.Recommendations on the type of material andpossible application rates and properties ofbitumen-stabilised materials are provided inManual TG2 (Asphalt Academy, 2009). Notethat foamed bitumen, unless produced at alocal batch plant, is seldom used for potholepatching.4.4.6 Material from the roadsideThe repair of potholes with material extractedfrom the road shoulders or road reserve is veryseldom successful (Figure 13). This materialshould never be used to repair potholes, eventemporarily, as it is not adequately durableand results in rapid deterioration of the entirepotholed area. Drivers are also given a falsesense of security when they see that the potholeshave been repaired, although significantdepressions may still be present, often causedby compaction and/or whip-off by traffic.4.5 Pothole repair methods4.5.1 Shallow asphaltThis type of repair is generally restricted topotholes that occur entirely in an asphalt layerand will seldom require more than a 75 or100 mm-thick asphalt layer.33

MethodOnce the area to be patched has beenidentified, marked, cut, excavated and cleaned(section 4.3), the entire exposed area (verticaland horizontal exposure) of the patch must becovered (painted) with an appropriate bitumenemulsion tack coat 1 . The asphalt (HMA orCMA) should then be placed into the hole andraked level, normally in a single layer. If thehole is more than about 75 mm deep, it shouldbe filled in layers not exceeding 75 mm, eachone compacted separately. The asphalt shouldthen be compacted using hand tampers orplate compactors, although small pedestrianrollers can be used for larger patches.All loose material should be swept from thepatch area. It is useful to blind the patch withsome fine sand or gravel to avoid adhesionto tyres in the period immediately followingcompaction. The patch can then be opened totraffic.The surface finish of the final patch should bechecked for level, using a straight edge (section5). No depressions or unevenness should beallowed. In fact, it is recommended that thefinal surface should be between 5 and 10 mmabove the surrounding road, depending onthe thickness of the fill, to allow for a nominalamount of traffic compaction with time.4.5.2 Medium-depth asphalt repairIf the pothole is deeper than the asphaltthickness and passes into the base course,but is no deeper than the bottom of the baseor the upper part of the sub-base, the potholeshould be patched as described for shallowasphalt in section 4.5.1. If the pothole is,however, larger than 0.5 m 2 in area, it maybe more economical to fill the pothole partiallyto within 75 mm of the surface with crushedstone, natural gravel or treated gravel (cementor bitumen emulsion). This needs to be assimilar to the surrounding material as possibleand compacted to the same density as thesurrounding material.MethodOnce the area to be patched has beenidentified, marked, cut, excavated and cleaned(section 4.3), the entire exposed area (verticaland horizontal exposure) of the patch must becovered (painted) with an appropriate bitumenemulsion, if a full-depth asphalt patch is tobe used. The asphalt (HMA or CMA) shouldthen be placed into the hole and raked level,normally in a single layer. If the hole is morethan about 75 mm deep, it should be filledin layers not exceeding 75 mm, each onecompacted separately. The asphalt should thenbe compacted using hand tampers or platecompactors, although small pedestrian rollerscan be used for larger patches.All loose material should be swept from thepatch area. It is useful to blind the patch withsome fine sand or gravel to avoid adhesion totyres. The patch can then be opened to traffic.The surface finish of the final patch shouldbe checked for level, using a straight edge(section 5). No depressions or unevennessshould be allowed. In fact, it is recommendedthat the final surface should be between 5 and10 mm above the surrounding road to allowfor a nominal amount of traffic compactionwith time.If crushed stone, gravel or cemented gravel isto be used as partial filler, this must be placedin the prepared hole to the required depth(taking into account the bulking factor, typicallyof about 30%) at optimum moisture contentfor the material. The edges of the cleaned341 The suppliers of certain cold-mix asphalts indicate that tack coats are not necessary – this may possibly apply to those that are emulsionbased, but it is recommended that a tack coat be used where cold mix and asphalt are in contact and on the top of the base layer.

hole should be moistened – not soaked – toimprove adhesion of the material to the edgeof the hole. The optimum compaction moisturecontent can be estimated by moulding thematerial into a ‘sausage’, which should justfracture when squeezed between the thumband forefinger – if the material compresses, itis too wet and if it crumbles, it is too dry.This material must then be compacted usinghand tamping (or a plate compactor if thehole is large enough), preferably in layers notexceeding 100 mm. It is important that theedges of the material in contact with the in situmaterial are compacted to the same degreeas the rest of the material to eliminate anypermeability differences.The upper surface of the compacted materialand the edges of the hole above this levelmust then be ‘painted’ with bitumen emulsionbefore the asphalt is placed and compacted asdescribed above.4.5.3 Deep repair (asphalt surfacings)Deep patches in asphalt-surfaced roads arenecessary when the pothole is the result ofstructural failure at depth. This is usuallyindicated by localised crocodile cracking andis primarily the result of excessive water inthe lower portion of the pavement, or poorquality materials. In theory, the source of thewater should be eliminated prior to patching,otherwise there is a strong probability that thepatched and/or adjacent area will fail again.Elimination of the water will often require theuse of sub-soil drains adjacent to the road(see TRH 15, 1984) and will usually requiresignificant investigation and drainage design.If the water is permeating into the pavementfrom beneath the surface, failure to take careof this will certainly lead to recurring problems.MethodPatching in such areas will require markingand cutting of the surfacing and the removalof all failed material as well as any materialthat contains excessive moisture. If thedrainage has not been attended to, this mustbe replaced with material that has preferablybeen stabilised with cement or bitumenemulsion to improve its moisture resistance andstructural capacity. It is, however, probable thatthe surrounding material will fail before thereplaced material.The stabilised material needs to be compactedas densely as possible in layers not exceeding100 mm-thick up to between 75 and 100 mmfrom the existing pavement surface. Placementof asphalt in this upper area will follow theprocedure described earlier.4.5.4 Shallow-surface repair (thin bituminousseals)These failures are usually the result of a loss ofbonding between the seal and the underlyinglayer and are common particularly with thinbituminous seals on stabilised bases. These areseldom more than 50 mm deep.MethodIn these cases the pothole should be marked,cut and excavated to at least 75 mm deep. Thehole must be cleaned carefully, painted withbitumen emulsion and the pothole repaired asdescribed in section 4.5.1. It is recommendedthat the join between the bituminous seal andthe newly-placed asphalt is sealed with ageosynthetic crack-sealing strip and wellcoveredwith bitumen emulsion. The totalpatched area should then be blinded with agraded, coarse sand (maximum size 4.75 mm).35

4.5.5 Medium-depth repair (thin bituminous sub-base, when necessary) should be constructedseals)in the moistened hole to provide continuity of theWhere potholes have formed in thin bituminoussealedpavements as a result of seal damage, loss must be roughened lightly before placement oflayers as far as possible. The top of each layerof bonding with the base or some other surficial the following layer.defect, failure to patch the pothole quickly results inThe asphalt patch at the top of the reconstructeda rapid deterioration of the surface. Enlargementpothole area should not be less than 50 mmand deepening of the pothole will follow, usuallythick and should be constructed as describedinto the base or even sub-base. In these casespreviously with the joins sealed using a wellbondedgeosynthetic strip.the underlying support layers are normally stilladequate, unless the pothole is not patched for anextended period, and do not need deep repair.This type of failure is usually indicated by a lack of4.5.6 Deep repair (thin bituminous seals)severe crocodile cracking of the seal in the failedWhen the pavement is structurally unsoundarea and affects the full base-depth and possiblyor water has entered the pavement structurethe sub-base.at depth, the road surface will usually form adepression, crocodile cracks will appear in theMethodsurfacing and this will start to ravel and spall,In these cases, the damaged volume (and a little forming large potholes. Although these usuallybeyond) must be marked, cut and removed totally affect the base and sub-base, failure to repairand the edges and bottom cleaned to leave a the underlying support material will result in anhard, compacted surface. Layers of crushed stone, inadequate repair.natural gravel or cement-treated material (75 to100 mm thick) to match the existing base (andMethodIn these cases, the damaged volume (and a littlebeyond) must be marked, cut and removed totallyand the edges and base cleaned to leave ahard, compacted surface. Layers of crushedstone, natural gravel or cement-treatedmaterial should be constructed in thePotholessame locations (and to the same60densities) as the existing pavementlayer materials to provide continuityof the layers as far as possible.The top of each layer must beroughened lightly before placementof the following layer.36

The asphalt patch at the top of the reconstructedpothole area should not be less than 50 mmthick and should be constructed as describedpreviously.4.5.7 GeneralStabilised layers in the road structure willcarbonate over time. This often makes it difficultto use a comparable stabilised material forthat layer in deep patches. In these cases, itis probably best to try to use a material that issimilar to the stabilised material, once it reachesthe equivalent granular state (TRH 13, 1986) orto use a bitumen emulsion-stabilised material.To achieve similar densities as the originalconstruction, it is unfeasible to determine actualdensities in the field. The following densitiesshould thus be assumed:Base98% Mod AASHTOSub-base95% mod AASHTOCemented materials 97% Mod AASHTOUpper selected subgrade 93% Mod AASHTOOther layers93% Mod AASHTOPothole patching is essentially a labour-intensiveoperation (Figure 29). A full method-statementfor pothole patching has been produced bySabita (2010) which can be used to supplementthis document. It is, however, essential that thepothole patching teams are properly trained toensure effective and durable repairs.Figure 29: A typical pothole-patching operation on a rural road37

CHAPTER 5QUALITY CONTROL5.1 GeneralFor the repair of potholes to be effective, acontrolled quality-assurance programme mustbe followed. Each stage of the process shouldbe checked to ensure that it conforms to therequirements for that stage.For pothole repairs, the primary requirementsare that:• The fill materials are suitable for the specificlayers• Each layer is properly and adequatelycompacted to meet density requirements• If HMA is used, the compaction temperaturesshould not be too low• The final riding quality is acceptable• The pothole surface does not form a depressionafter traffic compaction but rather be slightlyraised compared with the rest of the road• All joins are properly sealed• The patch must be aesthetically pleasing.5.2 Control5.2.1 Materials must be suitable for thespecific layersThe crushed stone, natural gravel or stabilisedgravel materials used for filling the potholes shouldbe tested to ensure that they are suitable. It isunlikely that materials from the same source as thoseoriginally used in the construction will be available.The materials should then comply with typicalrequirements for the appropriate material for thatlayer, as identified in TRH 4 (1996), e.g. a G1 toG4 for base course, a G5 or C4 for sub-base, etc.Locally-available materials to be used should betested and their properties checked to conform tothe necessary requirements. Stockpiles should bekept expressly for pothole patching.5.2.2 The layers should be compacted properlyTo minimise the potential for the materials used torepair potholes to further compact under traffic,it is essential that they are compacted properlyduring patching and the density is controlled.This is usually done using nuclear-density methods(with gravimetric moisture correction), but thisis not effective within smaller holes or for verythin, localised surface layers. It is often betterto use portable in situ test equipment such as aRapid Compaction Control Device (RCCD) orDynamic Cone Penetrometer (DCP) calibrated onthe material used for the patch under controlledlaboratory conditions. Any holes left in thepatched material by the in situ testing should befilled as well as possible.The actual control of compaction of materialadded to a small patch can be approximated bymeasuring the quantity (mass) of material and thechange in volume in the hole during compaction.The density (and relative compaction) can thenbe determined. Correlation with laboratorytesting using properties such as the bulking factorand refusal density can assist in making thenecessary estimates.39

Figure 30: A poorly-finished and untidy patch40Figure 31: A well-executed and aesthetically-pleasing rectangular patch next to a less effective ’circular’ patch

5.2.3 HMA compaction temperatures must notbe too lowA remote digital thermometer should be usedto ensure that the application and compactiontemperatures of any HMA are above the lowerlimit recommended for that particular mix.5.2.4 Final riding quality should be acceptableA straight edge should be used to ensure that thefinal patch is not too high (> 5-10 mm) above thesurrounding pavement. A 5 to 10 mm ‘hump’ willallow for some traffic compaction without causinga significant effect on riding quality in the longerterm.5.2.5 The pothole surface must not form adepressionNo patch should exhibit any depression underthe straight edge before traffic compactionoccurs, as any additional compaction will havea detrimental effect on the riding quality of theroad.5.2.6 Joins should be sealed properlyAll patches should be inspected to ensure thatthere are no open joins between the patchand the surrounding material. It is stronglyrecommended that a geosynthetic strip be ‘glued’with bitumen emulsion over the joins and then‘painted’ with bitumen emulsion and blinded withcoarse sand.5.2.7 The patch must be aesthetically pleasingThe final patch should blend into the road as faras possible and should have the appearance ofa well-engineered product, e.g. straight edgesand smooth finish.5.3 Field test equipmentTo carry out appropriate quality control, varioussimple pieces of equipment can be used.These include:• A straight edge and wedge• A digital thermometer for measuring asphalttemperature• A simple device for assessing the compactionof gravels and stabilised materials (RCCD orDCP)• A method for checking the adhesion of cracksealingmaterial and geosynthetic strips• A camber board to ensure that pothole patchesnear the edge of the road are constructed tothe correct camber and do not interfere withroad surface drainage.These controls are mostly visual although anumber of the properties can be tested. The CSIRhas developed a field kit for quality assuranceof general sealed-road maintenance, whichincludes all the apparatus necessary to monitorand quantify the measurable properties identifiedabove. Full instructions are provided with the kit.Figure 30 shows a recent patch that, althougheffective, is untidy and poorly finished. It is alsoclear that the problems leading to the originalpothole have not been attended to (e.g. opencracks and deeper fatigue) and the area is likelyto require repeated patching. The height of thepatch is also such that the riding quality of theroad would be affected. A well-constructed andmore pleasing rectangular patch is shown inFigure 31, together with a less-pleasing, smaller‘circular’ patch that required additional work.The rectangular patch has performed well thusfar, even without the geosynthetic sealing strip,although the join was well sealed with bitumenemulsion.41

CHAPTER 6CONCLUSIONSBudget constraints reduce preventative road maintenance, with the prevalence of potholes likely toincrease significantly. Potholes are typically ’repaired’ by mostly unskilled or badly-trained teams on anad hoc basis, quite some time after formation, thus leading to additional deterioration. The patches areseldom sufficient to address the basic cause of the problem and this usually results in the need to return tothe site repeatedly for ongoing repairs.This document provides a procedure to identify the causes of the problems and summarises repairtechniques relevant to the specific type and cause of each problem. The aim is to minimise the needto return continually for additional repairs. A standard field rating form is provided to assist in theclassification of the type of pothole and associated repair requirements.42

APPENDIX A – Standard form for the field assessmentInspectorWeatherDateROAD NO. LOCATION SURFACING TYPE LAYERS AFFECTEDcrocodilecrackingsurfacedeformationsubgradesaturatedCODES:Surfacing type: A – Asphalt TS – Thin sealLayers affected: S – Surfacing only B – Surfacing and base SB – Surfacing, base and sub-baseCrocodile cracking, surface deformation (mounding, rutting, shearing, etc.), subgrade saturated,stabilised base, carbonation of base: Yes/No44

and categorisation of potholesstabilised baseCarbonationof baseActionComments45

APPENDIX BKey to THE identification of pothole repair measuresThe following key to the identification of pothole repair measures is provided to illustrate the potholetypes. It will require enlargement of some of the photographs to identify finer details.To use the key, one must start at the top left-hand box and sequentially follow the vertical numbering andhorizontal boxes until a repair technique is identified. These then refer to the process described in themain text (Section 4.5).1Surfacing isasphaltSurfacing is thinbituminous seal234....………. > 2Pothole isdeeper thanasphalt wearingcourse....………….> 3Pothole causedby cracking dueto fatigue ofasphaltDeep repair aftersub-soil drainageinstallationPothole hasexposed anunstabilised base....…………. > 4Bottom of potholeis within asphaltwearing-courseShallow asphalt(HMA or CMA)Pothole caused bylocalised surfacewater ingress withno associatedcrocodile crackingMedium-depthasphalt repairPothole hasexposed astabilised base5....………….> 5Pothole is notassociated withcracks..…………. > 10Pothole isassociated withcracks....………. > 6…....………. > 846

67891011Pothole affectsseal and topof base only(< 50 mm)Shallow-surfacerepairPothole affectsonly the baseMedium-depthrepairPothole doesnot affect entirepavementstructure (onlybase and subbase)Medium-depthor deep repairPothole isthe result ofsaturatedsubgrade orsupportDeep repairafter sub-soildrainageinstallationTop of base hascarbonated andis weak.…………. > 11Pothole isassociatedwith crocodilecrackingDeep repairPothole extends> 50 mm into base.......…………. > 7Pothole extendsbelow the base.......…………. > 8Pothole affectsentire pavementstructure.......…………. > 9Pothole is the resultof poor material– no evidence ofexcessive sub-soilwaterDeep repairTop of base hasnot carbonatedexcessively and isstill strongShallow-surfacerepairPothole is notassociated withcrocodile crackingMedium-depthrepair47

Notes

Download both these documents in PDF format at:www.csir.co.za/pothole_guides

www.csir.co.zaSee www.csir.co.za/pothole_guides to downloadnon-technical and technical guides