Talking RAP
- robball6
- 16 hours ago
- 2 min read
Transport for London (TfL) has revealed the results of a nine-year trial which shows that a road surface containing 50% RAP has performed just as well as a control section made from a standard specification mix.
This is a high proportion for the UK. Up until this summer, National Highways’ specification, the Manual of Contract Documents for Highways Works (MCHW), allowed up to 10% of RAP in the surface course of a road and 50% in the lower layers, although this is being updated to allow up to 20% in the surface course.
TfL’s successful trial doesn’t mean that every road authority should rush to incorporate 50% recycled materials in its surfacing. Any road authority that wants to put higher proportion of RAP into an asphalt mix first needs to check what state the bitumen and the aggregates in that RAP are in. And that takes time and money.
FM Conway, which laid the trial section on the A1 at Mill Hill, had carefully sourced and tested the RAP and extensively tested the proposed mix before putting it on the road. It is also worth noting that both the high-RAP mix and the control mix contained polymer modified bitumen (PMB), used to improve the asphalt’s performance for the busy A1.
Depending on its age, the environment it’s been laid in, and how much traffic loading it has experienced, the bitumen in RAP may have oxidised and become quite brittle. Or it may still be quite fresh and flexible.
The aggregate in the asphalt may have had its corners worn down, giving it a lower polished stone value (PSV) meaning the road surface will have lower skid resistance. For RAP used in road surfaces such as the TfL’s A1 section, the PSV must be high enough for safety reasons.
There’s another reason why most authorities can’t go above 20% RAP in a mix: this is the maximum proportion that can be achieved using a standard asphalt mixing plant. It is possible to get to around 40% with some modifications but above 40%, different asphalt plant technology is needed to ensure that the RAP is heated sufficiently for it to be mixed with the new material. In the UK, there only seven such plants which can do this.
It's also important not to overheat RAP because that could cause the remaining ‘good’ bitumen to oxidise and become brittle. That’s something we are very aware of at Thermal Road Repairs and the reason why our heaters are computer controlled so that they don’t overheat the existing asphalt around a pothole during the repair process. National Highways reports that RAP works better with warm mix than hot mix for the same reason.
In conclusion, it’s important to realise that RAP cannot be thought of as one material. Creating long-lasting roads requires an understanding of the chemistry of the materials being used. TfL’s trial results are good news because they show what is possible with high proportions of RAP, as long as there is careful testing and mix design. But it isn’t the way forward for every road authority.
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