Road Pavement Crack Sealing: Experiences in the Republic of ...

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8 th CONFERENCE ON ASPHALT PAVEMENTS FOR SOUTHERN AFRICACracks wider than 3mm are readily sealed by the infill methods described in this paper.However, when incidence of wide cracking is too high then infill cracking becomes tooexpensive. In general, when the crack incidence is more than 3 linear metres of cracks per m 2 ofpavement, then surface sealing is more economical. In such cases wide open cracks can beblown clean by compressed air prior to surface sealing to let some of the surface spray rundown into the cracks.The cost break-even point between infill crack sealing vs. surface sealing depends on the unitrates and should be calculated for each contract.2.3 Methodology2.3.1 Cleaning of cracksThe cracks are blown out with a mobile compressor capable of discharging 3m 3 compressed airper metre at 650 kPa pressure. The compressed air is heated by a heat exchange apparatus sothat compressed air is used to blow out and clean the crack. The compressed air is heatedusing liquid petroleum gas ignited by a spark plug. The temperature of the hot air shall beadjustable up to 300 o C. The main advantage of hot air blasting is that all of the old and agedasphalt on the crack sidewalls is removed, thus leaving a fresh face for the prime and sealant toadhere to.2.3.2 Vegetation killerWeed killer, dissolved in water should be sprayed into all cracks with vegetation using rucksackpressure type spray cans. Shoulder cracks harbour more vegetation and deserve specialattention. A total herbicide, ‘Hyvar X’, as weed killer two days before cleaning the cracks isrecommended.2.3.3 Priming of cracksPrime should be jetted into the open cracks using compressed air propulsion. An inverted prime,such as MSP 1, is recommended. A schematic drawing of the equipment is shown in Figure 1.Essentially, the process consists of a blowpipe with a nozzle to direct the jet of compressed airmixed with prime into the crack. A venturi device is fitted inside the blowpipe to create apressure differential for sucking in prime from a storage vessel.Figure 1. Prime injector.A closed pressure vessel prime tank, where the air space above the prime was pressurisedusing a tap-off from the same compressed air source, can be used. This arrangement generatessimilar head losses across the venturi as across the prime, and with the prime at a slightlyhigher pressure than the downstream end of the venturi, a constant rate of prime feed can beobtained. A needle and seed valve can be installed in the prime line for the fine adjustment ofthe air to prime ratio. A complete shut-off valve should be added to isolate the injector from thecompressed air. This is most important when public traffic occupies the same carriageway, asthe prime injectors have a considerable ‘shooting range’ of up to 10m.Paper 016
8 th CONFERENCE ON ASPHALT PAVEMENTS FOR SOUTHERN AFRICAWhen jetting the cracks with prime, the sides of the cracks must be thoroughly wetted butwithout flooding the cracks. The prime must then be allowed to soak and dry before the firstapplication of crack sealant is applied. The prime largely assists in creating a proper bondbetween the crack sidewall and sealant.2.3.4 SealantsThree types of sealant can be used:• warm bitumen-rubber sealant or cold emulsion sealant (main seal)• final seal (rubber crumb slurry)• wide crack infillsWarm bitumen rubber sealant:The bitumen rubber is heated in a crack sealer tanker to the specified temperature andcirculated continuously. A hand-operated lance is used to inject the bitumen-rubber into thecrack by pump force. Directly after the crack has been sealed a scraper is used to removesurplus bitumen rubber from the surface. The workmen get trained to strike the said bitumenalmost level with the surface so that this bandage can act as a SAMI.The bitumen rubber consisted of the following mix by volume:• Bitumen (60/70 penetration grade) 76%• Rubber (finely ground) 22%• Extender oil 2%Table 2. Properties of bitumen rubber.Property Min Max Test methodRing and ball softening point ( o C) 65 80 ASTM D36Compression recovery:BR3T5 min1 day4 days8570251009055Resilience 10% 35% BR2TFlow 0mm 70mm BR4TOther commercial types of warm sealant can be used (e.g. Viaflex) which is a highly elastic, hotapplied penetration grade bitumen, modified with a high percentage of an electrometric polymer.This product must be applied at a temperature of 130 o C with an applicator or pour directly intothe cracks with a small container.Table 3. Properties of Viaflex.Property Test value Test methodRing and ball softening point ( o C) 80 min ASTM D36Elastic recovery @ 10 o C (%) 80 min ASTM D113 (mod)Flow at 60 o C (mm) 0 BR4TDuctility @ 10 o C (mm) 500 ASTM D113Final seal (Rubber crumb slurry):A slurry seal, using rubber crumbs instead of aggregate can be used to top up and bandageover the wider active cracks. The slurry then serves as a SAMI. Hand tools are used to mix andapply this slurry seal.Paper 016
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