Monday, September 21, 2020


The Warm Alternative


ABHISHEK MITTAL, Scientist, and DR SUNIL BOSE, Scientist (retired) of the Central Road Research Institute, New Delhi weigh the pros and cons of warm mix asphalt (WMA) in an untested Indian market.


Hot mix asphalt (HMA) is used as the primary paving material, with over 90 per cent of the Indian paved roads made of it. HMA consists of aggregates and asphalt which are heated and mixed together at an elevated temperature of about 150°C – 155°C. This results in high energy (fuel) costs and production of greenhouse gases. However, with the environmental concerns increasing rapidly over the past years and comprehensive measures like air pollution reduction targets set by the countries signatory to Kyoto Protocol (including India) have encouraged the efforts to reduce air pollution. The asphalt industry is constantly exploring technological improvements that will enhance the material’s performance, increase construction efficiency, conserve resources and improve environmental stewardship.

All these have led to the development of Warm Mix asphalt (WMA). Warm Mix Asphalt (WMA) refers to asphalt concrete mixtures that are produced at temperatures approximately 30°C lower (or more) than temperatures typically used in the production of conventional hot mix asphalt (HMA). The goal with WMA is to produce mixtures with similar strength, durability and performance characteristics.


Advantages of WMA


The most important benefit of WMA is the possibility to reduce the greenhouse gases in the atmosphere. This is realised through reduced temperatures for production and paving of asphalt. The ranges of possible energy reduction in the production process are reportedly as follows: WAM Foam – 30 to 40 per cent; Asphamin® – 30 per cent; Sasobit® – 20 per cent; Evotherm® – 50 to 70 per cent.

It is also reported that this gives a plant stack emission reduction of

• CO2 in the range of 15 to 40 per cent

• SO2 in the range of 20 to 35 per cent

• Volatile Organic Compounds (VOC)

up to 50 per cent

• Carbon Monoxide (CO) in the range

of  10 to 30 per cent

• Nitrous Oxides (NOX) in the range of  60 to 70 per cent

Some other potential benefits of WMA include cold weather paving, reduced thermal segregation of material, extended paving window, improved workability, earlier traffic opening after construction, reduced worker exposure to asphalt fumes and slow binder aging potential.

Hot asphalt fumes generated during HMA mixing process contain polycyclic aromatic hydrocarbon (PAH) compounds. PAH compounds are of concern regarding exposure to workers because some of these compounds have been identified as carcinogenic, mutagenic and teratogenic. Studies have reported that a distinct relationship exists between production temperatures and asphalt fume generation. The use of WMA can effectively reduce the production of these fumes, thus reducing exposure to workers. It has been reported in the literature that the use of WMA had a reduction of 30 to 50 per cent in PAHs.    

WMA also promises several benefits that are indirectly related to the reduction of atmospheric pollution. Lower mixing temperatures and the modification of bitumen results in different viscoelastic behaviour of binder in the WMA pavements. Less aging during production and paving process tends to improve pavement flexibility, which reduces susceptibility to fatigue and temperature cracking. This results in the improvement of pavement longevity (life cycle), further reducing the potential costs for restoring the asphalt overlay. The lowering of bitumen viscosity in the production process allows incorporating a higher percentage of reclaimed asphalt pavement (RAP). The use of up to 90 per cent of RAP has been reported in literature and WMA still results in less effort needed for compaction, which means an additional energy saving realized in the paving process. The overall benefit of RAP usage is the resolving of the problem of RAP utilisation, saving of landfill space, reduction of virgin aggregates and energy used for mining.



WMA mechanism


There are three WMA mechanisms by which reduction in production temperatures is achieved, these are foaming, viscosity reduction and use of chemical additive to change the surface tension or internal friction properties of binder. Table 1 provides a summary of some of the widely used warm mix additives (products), the mechanisms by which they are known or claimed to operate and the production temperatures recommended by their suppliers. However, the actual production temperatures will be based on a number of factors including mix type and weather conditions among others. These WMA technologies can be easily used with the available hot mix plants with only few of them requiring minor modifications to the plant.   



Tested Worldwide


The WMA technology has been quite successful in the US and European countries. Huge quantities of warm mixes have been laid successfully using the different WMA technologies worldwide. More than 3,00,000 tonne of WMA have been produced worldwide with zeolite mainly in Germany, France and the US. Over 10 million tonne of asphalt mix has been produced worldwide with Sasobit, most of it was used to produce WMA. In the US, over 250,000 tonne of WMA have been produced with Sasobit. Over 60,000 tonne of WMA have been produced with WAM-Foam primarily in Europe. At least eight projects totaling over 38,000 tonne of WMA have been produced with LEA so far in the US with over 100,000 total tonne produced worldwide. At least nine projects totalling over 5,600 tonne of WMA have been constructed with REVIX technology in the US.



Mix Design criteria for WMA


At present, there are no Indian specifications detailing the design criteria for dense grade bituminous mixes with WMA. Based on the laboratory studies and performance evaluation of trial pavements constructed with WMA technology, Indian Roads Congress (IRC) is in the process of formulating the guidelines for mix design and construction of WMA. It is expected that such a guideline would be soon brought out.

In  general, the  current  IRC  specifications for dense graded bituminous mixes (IRC: 111-2009) shall be followed for mix design for WMA with certain additional requirements to suit WMA technology adopted. Since WMA is produced at relatively lower temperatures, which may cause incomplete drying of aggregates, rendering it susceptible to moisture damage. Therefore, WMA mix must be tested and must satisfy the requirements of moisture susceptibility test as per AASHTO T 283. Plant trials should be carried out to establish that WMA can be produced at temperatures significantly lower than HMA. Plant produced WMA mix shall be tested for bitumen content, gradation, air voids and tensile strength ratio (TSR) to check for moisture susceptibility. Laying trials should also be carried out to establish that WMA can be compacted to the required density at significantly lower temperatures than required for compacting conventional HMA.


Concerns regarding WMA Performance


The most significant advantage of the use of WMA is of course the possibility to reduce the use of fuel and thus cut the carbon footprint of asphalt industry. However, there are still some concerns about the mechanical properties and longevity of WMA, which are listed in the subsequent sections.


Permanent Deformation


There is a general concern for WMA rutting performance that is connected with the decreased mixing temperature which may lead to incomplete drying of aggregates and insufficient coating with bitumen. Another aspect that may influence decreased resistance to permanent deformations is the decreased oxidative hardening of bitumen due to the lower production and compaction temperature. These problems might be treated with adding active adhesion agents or initially choosing harder bitumen grade. The WMA that is produced by treating bitumen with wax usually shows better resistance to deformations than conventional HMA. This is due to the formation of the lattice structure in bitumen below the crystallisation point of wax, which stiffens the asphalt at in-service temperatures.



Moisture sensitivity


Due to low mixing temperatures, it is possible that aggregates are not completely dried and the retained water in the aggregates could lead to increased susceptibility to moisture damage. Because of residual moisture left behind by the microscopic foaming process, this is even more critical for WMA technologies that involve foaming as a binder viscosity lowering action. These problems, if they occur, can be successfully treated with active adhesion agents.





WMA is reported to have better compaction potential due to decreased viscosity and less bitumen ageing in the production process. This can allow saving compaction energy and reducing the time necessary for compaction which may be especially important in low temperature paving. However, if wax technologies are used, they require additional attention regarding the temperature conditions for rolling. The compaction must be finished before the wax starts to crystallis e; after this temperature the wax forms lattice structure in the asphalt that may be damaged if the compaction is continued. This means that compaction window is shorter than for HMA and additional rollers may be required to reach the necessary density in the given time window.


Cost Effectiveness


Different WMA technologies offer various energy savings for production – this depends mostly on the lower production temperature and the kind of fuel used.  Potential savings may also be realised during the paving process as well. However, the savings with reduced energy consumption may be offset by the additional costs of WMA technologies. Potential increases depend on production techniques as different WMA technologies require different additional costs. Increase in costs may arise from

- the investment and the depreciation of plant modification

- the costs of the additives

- possible costs for technology licensing 

Because the WMA technologies are just approximately 15 years old, there are still concerns over the long term performance of the pavements. If it is not proved that WMA has the same or better longevity than HMA, the economical increases for the life-cycle of WMA may far exceed the benefits of this technology. So, the actual cost effectiveness of WMA technology can be established once the long term performance/longevity of WMA pavements compared to HMA is proven successfully. 



Need for Trials


WMA is a promising technology that promises huge environmental benefits and better working conditions for the construction workers. It must be noted that some of the environmental benefits may be offset with the carbon footprint embodied for producing additives and/or any additional equipment supporting the production of WMA. This must be taken into account while calculating the environmental benefits of WMA technology. The mechanical properties of WMA show that it has a potential to replace conventional HMA and in special circumstances it even has advantages over HMA. Environmental and other benefits alone are not sufficient for a widespread implementation of this technology. More data regarding the laboratory testing and performance evaluation of trials sections of WMA under different climatic and traffic conditions is needed to overcome the caution in the road industry for widespread implementation of WMA. It must be established whether the reduced energy consumption also reduces the overall costs of WMA production. Only then, it will get the required impetus for widespread implementation amongst the contractors.





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