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
CEC July 2018- 19
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