The Civil Engineering Contractor July 2018 | Page 19

The international roughness index ( IRI ) is the roughness index most commonly obtained from measured longitudinal road profiles , which determines both smoothness of road surface and ride .
The bitumen emulsion turns from brown to black within 30 minutes , indicating that the surface has become ‘ tacky ’ ( hence ‘ tack coat ’) and is ready to receive the asphalt . It forms a seal between the old , milled asphalt and the new layer .
The versatile Vögele feeder with pivoting conveyor ensures that the asphalt can be precisely fed to the desired place .
6.5 %,” assures Breyten Dirks , regional laboratory manager . Also included is the cellulose fibres that prevent drain down of the binder . Full Marshall testing on the final product ensures adherence to specifications . Further to National Asphalt implementing process control to produce a product that meets the specifications , an independent SANAS-accredited laboratory was appointed on this project and is responsible for acceptance control on the A-E2 and SMA . Statistical acceptance procedures are used to decide whether work should be accepted , rejected , or accepted at a reduced payment . The maximum voidless density ( SANS 3001 — AS11 ), often referred to as the Rice test method , is used to calculate air voids in the compacted sample and the amount of bitumen absorbed aggregate . The bulk density ( SANS 3001-AS10 ) is also determined , whereby the asphalt is compacted ( at 142 ° C ) to determine the unit weight of the compacted asphalt briquettes and to obtain the percentage air voids . “ The target on this particular mix is 4.7 %,” Dirks adds . Binder extraction involves extracting the binder from the mix with a solvent . The binder content is calculated as the difference of the mass of the mix and that of the extracted aggregate . SMA is a stone skeleton mix and for this project is manufactured with an AE2 binder . This binder is also subjected to various testing to ensure it provides sufficient deformation resistance . It should not be too rigid and prone to cracking otherwise the mix can start to deform , creating ruts in the road , for example in designated truck lanes . “ Before offloading the AE2 into the working tanks , on every load of AE2 , the softening point is checked ,” Dirks says , “ to ensure it is within its range of 65 to 85 ° C and complies with its relevant specification .” “ With conventional bitumen , the softening point is 46 to 56 ° C ,” Smith adds . “ Over and above the Marshall tests , the asphalt is also subjected to performance testing , such as wheel tracking , gyratory , and Modified Lottman tests .” After the contractor has completed a base production , cores are drilled from the road to determine density . If the density has not been achieved , the section is pulled up . “ Paving 1 000 tons a day means nothing if the density is wrong ,” says Smith and adds , “ If the mix does not meet specification , it is rejected and needs to come out .”
Resealing process
Riaan van Rensburg , construction manager at KPMM , describes the resealing process : “ We need to prepare the road , concrete side drains and any joins . The old asphalt is initially milled , or scoured , to create a roughened surface to which ultimately , the asphalt will adhere . This surface is then sprayed with SS60 , a low viscosity anionic slow set bitumen emulsion , which relies on the evaporation of the water component to cure , acting as a tack coat for an asphalt overlay . This is diluted with water and reduced to SS30 .” The initially brown in colour tack coat is left to dry , turning black within a brief period — depending on ambient conditions — undergoing a process called ‘ breaking ’. It is now ready to receive the asphalt . The asphalt-loaded trucks tip the asphalt into an asphalt transfer
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