Warm-Mix Asphalt

In terms of making a change to reduce the industry’s carbon footprint, switching to warm-mix asphalt, instead of hot-mix asphalt, is probably the easiest transition. By simply adding an additive, such as CWM™ into the bitumen, mixing temperatures at the hot-mix plant can be reduced by at least 30 °C. Temperature reductions of up to 40 °C have been achieved, even with the very dense EME mixtures used in France.

Needless to say, the further the reduction in mixing temperatures, the greater the savings in energy use at the asphalt mixing plant and, as a consequence, the greater the reduction in greenhouse gas emissions. It has been widely reported in the literature that energy savings of the order 15 to 20 % can be achieved when the mixing temperatures are reduced by 30 to 40 °C. In Ireland’s case, as we manufacture approximately 3 million tonnes of hot-mix asphalt per year, this would equate to a reduction in carbon emissions of the order 15,000 tonnes of CO2 eq. per annum.

In terms of performance, during the past ten years it has been well established that warm-mix asphalts behave in the same way as conventional hot-mix asphalts. Since 2008, over 7 million tonnes of warm-mix asphalt have been made using the CWM additive alone. Many more millions of tonnes have been produced using other additives. Warm-mix asphalt has been used in every bituminous layer of the pavement from base to surface course, including mixes that contain high percentages of RAP, and under all traffic levels.

The road construction/maintenance industry and the asphalt industry, in particular, has within its grasp the ability to play a significant role in achieving Ireland’s carbon reduction targets by promoting and/or even insisting on the use of both warm-mix and cold-mix asphalt. The benefits are obvious and can be implemented immediately.

Warm-mix trial with TII and Roadstone in 2008

A Proven Technology

The Colas group of companies has been using CWM to produce warm-mix asphalt since 2008. At this stage, over 8 million tonnes of warm-mix asphalt have been produced by Colas subsidiaries worldwide, using this technology. And, touch wood, there has been no negative feedback from the end clients.

Of course, the majority of this warm-mix has been paved in France, as it’s asphalt industry was one of the first in Europe to implement warm-mix asphalt. As recently as May 2021, IDDRIM (the French Institute for Roads, a professional association that includes road administrations, laboratories, engineers and contractors) published an “information note” on warm-mix asphalt. It reflects the state of the art and insists on the fact that long term performances of warm asphalts, whichever manufacturing process is used, are equivalent to traditional HMA. It sets a target of 80 % of WMA by 2030. A copy of that report can be found here.

The full conclusion is translated to English below:

"Lowering the manufacturing temperatures of asphalt mixes significantly reduces the energy consumed, greenhouse gas emissions and significantly reduces the risk of exposure to bitumen fumes. Various warm-mix technologies, whether based on either an adapted production process or the use of a specifically modified bituminous binder, are now available throughout France.

Studies, research and monitoring of operations carried out in this field have shown that warm-mix asphalts, when manufactured and used under normal conditions, behave identically to conventional hot-mix asphalts, including when they contain recycled materials (i.e. RAP) and have displayed equivalent durability.

There is no technical or operational barrier to the use of warm-mix asphalts on current pavement construction or maintenance jobsites.

In the context of an ecological and energy transition, as public policies are geared towards reducing our carbon footprint, it is necessary to make the use of warm-mix asphalts commonplace.

Under the site conditions specified in this information note, IDRRIM recommends considering the use of warm-mix asphalt as the reference solution, for the supply and implementation of common asphalt mixtures."

Please also note that a document published by the UK government’s All Part Parliamentary Group on Highways in 2019 states the following:

"WMA complies with all current UK asphalt composition and performance criteria, with the exception that it is compacted at lower temperatures than covered by some existing requirements."


"WMA can provide solutions to delivering long-lasting roads to help meet the country’s low-carbon objectives, while also improving conditions for the workforce, road users and the wider community."

The full report is available here.

For further evidence that a warm-mix asphalt made using CWM technology will perform as well as a conventional hot-mix, please follow below link to a paper that describes a trial that was conducted in Dublin in 2012. In this trial, the performance of the warm-mix SMA surfacing was directly compared to that of the warm-mix.

Please follow this link to download a paper that published in the 2012 Eurobitume congress proceedings. It describes a lab study that was performed by Atlantic Bitumen to compare the performance of CWM-modified warm-mix to the hot-mix versions of SMA 10 surf, AC 20 dense bin and AC 14 EME bin. Eurobitume Paper.

CWM Warm-Mix Asphalt trial

A safer, cleaner work environment

For every 10 °C degree reduction in mixing temperatures, the quantity of VOCs emitted to the environment is reduced by 50 % - Reference. Colas have confirmed this on several CWM warm-mix sites by engaging the services of a specialist laboratory who conduct such measurements. Their measurements were performed by attaching air quality probes to the laying crew and found an 80 % reduction in VOC exposure levels. These finding were reported in the IAT paper referred above.- Reference,.

This can only be good news for the residents and road users located close to the jobsite.

No smoke behind the paver screed

CWM also works as an adhesion agent

There are many test methods for assessing the degree of adhesion between an aggregate and the bitumen in the laboratory. EN 12697-11 (Rolling Bottle) describes a very good method for assessing the degree of adhesion when only the coating of a single size aggregate is of concern. EN 12697-12, Method B (Resistance to Water Sensitivity or “Duriez Test”) is a good method for assessing the degree of adhesion or “resistance to stripping” of a full asphalt mixture, as it involves measuring the tensile strength of asphalt mixtures in both a dry condition and wet, i.e. soaked in water, condition. The American standard AASHTO T283 describes a very similar test method.

A similar test was performed by the Colas Solution laboratory in the USA. It was based on specimens taken from a 10,000 warm-mix trial that was performed using CWM in 2016. The figure below shows the obtained TSR result, compared to the conventional hot-mix asphalt.

There has been a lot of research work done in recent years to show that the TSR (AASHTO T283) and the Wet/Dry ratio (EN 12697-12) of warm mixes can sometimes be lower than that of the hot-mix asphalt because, due to the lower mixing temperatures, the bitumen is less oxidized, less age hardened, so tends to have lower early life compressive strengths. (This could be considered a good thing, as less oxidative hardening should result in a longer fatigue life – but nobody has been able to show this yet.)

The Eurobitume Paper describes research work that found that the Dry Strength and Wet/Dry ratio (as measured on lab specimens) for three CWM-modified warm-mix asphalts (i.e. for three commonly used asphalt mixes in Ireland) were very similar to that of the normal hot-mixes (see Figures 7 and 8). Significantly though, when additional tests were performed on lab specimens cured for a further 7 days and on specimens that were prepared from loose samples of the warm-mix obtained on-site, the Wet/Dry ratio was found to have increased (see Figure 10). This indicates that the ageing that happens in the asphalt plant and during transport to the jobsite is under-estimated by the ageing that occurs in the mixing bowl and sample preparation in the laboratory.

To date, there has been no evidence of a reduced resistance to moisture sensitivity or durability of warm-mix asphalts in general.

It should also be noted, when CWM removes the need to use an adhesion agent/anti-strip such as TPH or XTX(R) in the asphalt mix (for mixes that normally require their use), the increased cost to the asphalt manufacturer to use CWM will be minimal. As stated above, the use of CWM will almost certainly be cost neutral when the cost of fuel is raised with increased carbon taxes! #climateemergency

Laying CWM modified warm-mix EME in France

Other benefits of CWM-modified Warm-mix asphalt

  • CWM has also been shown to be such a good compaction aid that the asphalt mat can be laid and compacted without the use of vibration on the compaction rollers. The attached Colas TechNews describes one project where CWM was used for this purpose – because compaction was forbidden on the bridge deck - Reference

  • Needless to say, the lower the mixing and laying temperature of the asphalt, the quicker the compacted mat will cool, so can be reopened to traffic. Therefore, CWM-modified warm-mix enables higher productivity rates to be achieved and less delays for the road users. This also applies to airport work where more demanding time pressures exist.

  • It is very easy to manufacture and evaluate CWM-modified warm-mixes in the laboratory. This is not the case for foamed-bitumen warm-mix.

  • CWM does not increase the stiffness of the asphalt – unlike when organic waxes are used in the manufacture of warm-mix. Such stiffness increases can reduce the workability of the mixture, during the laying process, and can also result in an asphalt layer that is not flexible enough to withstand the movements and deflections in the underlying layers, for example, in the overlay of legacy roads.

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