Reflecting on Cracking

Highways England’s strategic road network has over 1,200km of concrete pavement throughout its system, with the local authority network containing even more. Bridges, flyovers and other concrete structures also require special consideration when asphalt surfacing is applied, due to even greater movement from thermal and dynamic forces. Overlaying these pavements with an asphalt thin surface course improves ride quality, reduces noise and maintains a deteriorating surface. This combination of concrete and asphalt brings its own challenges. The overlay may suffer from reflective cracking due to thermal expansion and contraction cycles in the underlying concrete as well as the effects of traffic loading. Any cracking in a pavement is undesirable, as this leads to water ingress and ultimately a pavement failure.

One effective method to assess a materials resistance to these thermally induced strains is the Texas Overlay Tester (TOT). The TOT allows us to compare the performance of asphalt overlays and their constituents in a controlled environment. It simulates the effect of a temperature change, in a concrete construction with set distance between joints in an accelerated test.

A sample, cut from a 150mm core, to dimensions 150x76x38mm creates a lozenge which is fixed to two base plates with epoxy resin. The sample is conditioned to temperature for a minimum of two hours, then the machine cycles a load on the sample, opening and closing the gap between the two plates by 0.635mm returning to its start position within 10 seconds. The load taken to complete this cycle is recorded and the cycle is repeated. The test is completed when either 1000 cycles have been completed or the load needed to complete the cycle has been reduced by 93% from the initial load. Once the test is completed the sample is studied for visible cracks and the result is recorded. The ASTM test is run at a standard 25°C, but an adjustment in temperature can give us a greater understanding of the material.

An initial assessment was performed at the Total Bitumen UK Technical Centre using a sand asphalt design, comparing a 40/60 pen binder and Total Styrelf® eXtreme 100 to demonstrate how a highly polymerised binder can contribute to the resistance of reflective cracking. The conditioning and testing was implemented at a reduced temperature of 10°C to amplify the performance of the material. On this limited comparison the performance improvement is readily demonstrated. The 40/60 pen material reached the 93% load reduction within an average of 76 cycles. There was a visible crack on each sample and when the sample was removed from the machine the two halves were easily separated. The Total Styrelf® eXtreme 100, in contrast, ran the full 1000 cycles and the final load reduction was an average of 76%, there were no visible cracks and the sample remained intact when removed from the machine.

The assessment is continuing on a standard thin surfacing design comparing the performance of a range of different polymeric binders at a variety of temperatures. Early indications suggest this innovative protocol is very useful to understand the behaviour of the asphalt materials under these varying conditions. The TOT protocol is being developed by Total Bitumen into a powerfully convincing benchmarking tool discriminating between asphalt designs and bitumen specifications, being particularly useful in optimizing the performance of SAMI layers. It also illustrates that even though the bitumen forms only a relatively small part of the asphalt, a generic specification of “polymer modified bitumen” or PMB is completely inadequate; as the performance of the different grades are very different in the same asphalt design.

Author: Rebecca Haigh - Asphalt Manager - Total UK Ltd.

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