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Mix Design

WMA Mix Design

My WMA publications

            http://www.springer.com/energy/renewable+and+green+energy/book/978-3-662-44718-5

Zaumanis. Warm Mix Asphalt., K. Gopalakrishnan et al. (eds.), Climate Change, Energy, Sustainability and Pavements, Green Energy and Technology, Springer-Verlag, (2014), DOI: 10.1007/978-3-662-44719-2_10

Zaumanis, Martins, and Juris Smirnovs. 
Analysis of Possibilities for Use of Warm Mix Asphalt in Latvia.” 
In Civil Engineering. International Scientific Conference. Proceedings. Vol. 3. Jelgava, Latvia, 2011.
 
Laboratory Evaluation of Organic and Chemical Warm Mix Asphalt Technologies for SMA Asphalt.” 
The Baltic Journal of Road and Bridge Engineering 7, no. 3 (2012): 191–197. doi:10.3846/bjrbe.2012.26.
 
“Laboratory Evaluation of Warm Mix Asphalt Properties.”
 In 
5th International Conference “Bituminous Mixtures and Pavements.” Thessaloniki, Greece, 2011.
  
Development of Calculation Tool for Assessing the Energy Demand of Warm Mix Asphalt.”
Procedia - Social and Behavioral Sciences 48 (January 2012): 163–172. doi:10.1016/j.sbspro.2012.06.997.
 
“Laboratory Testing of Organic and Chemical Warm Mix Asphalt Technologies.” 
St. Louis, MO, USA: National Asphalt Pavement Association, 2011


My book:                                          My Masters thesis:  
http://www.amazon.com/Asphalt-Going-Green-Overview-technologies/dp/384430018X
                 

References

Warm Mix Asphalt has been used in all types of asphalt materials, including dense graded, stone mastic, porous, and mastic asphalt. It has been used with different aggregates and all grades of binder as well as polymer modified bitumens RAP and RAS. A variety of layer thicknesses and traffic levels have been applied to WMA. Based on these findings, there are generally no restrictions on WMA implementation. There are, however, some considerations regarding WMA design procedures that may be different from HMA and should be taken into account to ensure performance equal to that of Hot Mix Asphalt (HMA).


Binder Content


Mixture designed as WMA, may exhibit less binder absorption due to lower temperatures. Together with increased workability this may result in a lower amount of air voids and, according to mix design practices, require a reduction in binder content. However, there is consensus in the industry that this would result in a stiffer mix that is susceptible to cracking, raveling (Jones et al., 2012), accelerated oxidative aging, and moisture susceptibility. Therefore, the current practice is to use an approved HMA mix design binder content and substitute the WMA process without changes in the binder content. For these reasons the WMA is often designed using the "drop in" method. That is, the mixture is designed according to HMA standard mix design practices and WMA technology is introduced without changes in other mix design parameters.

Binder Grade

As described above, bitumen exhibits less aging in the WMA production process. If the difference in production temperature is very large it may be necessary to bump the high temperature PG in order to compensate for the less aged WMA binder and avoid plastic deformations of the pavement soon after opening to traffic.

NCHRP report No.691 (Bonaquist, 2011a) recommends considering an increased high performance grade if the difference between the HMA and the resultant WMA PG exceeds 3°C. Since various binders exhibit different susceptibilities to aging, a fixed reduction in temperature for which the binder grade should be increased cannot be determined. For a typical binder with average aging susceptibility, a temperature difference between HMA and WMA production temperatures of more than 30°C would require a change in the high binder grade. The same report suggests that the low temperature grade should not be altered, since changes between HMA and WMA in resultant low PG temperature are relatively small.


Mixing in Laboratory

Laboratory mixing of WMA with organic and chemical technologies does not require modifications, other than the addition of the right amount of additives. There are two methods of introducing additives into the mixture:

·  Addition to binder before mixing with aggregates simulates pre-blended binder.

·  Adding additives together with binder simulates the in-line addition process.

Production of WMA with foaming technologies in the laboratory is rather challenging and three production technologies can be distinguished:

·  Foaming additives can be introduced to aggregates together with binder and will offer a limited time of improved workability. Precaution must be used because of the water steam in the process.

·  Water injection with nozzles is technology dependent and will vary by type of nozzle, addition rate, water pressure, etc. There are several commercially available foamers for simulation of this process (see Figure).

·  Sequential addition of materials to simulate processes of WAM-Foam and LEA may be impossible in the laboratory because of an inability to simulate the strict full-scale technological operations in the laboratory.


Production and Compaction Temperature

For normal paving grade binders the production technology can be determined based on the required binder viscosity. WMA technologies use various methods to increase the binder workability, improve “wettability” of aggregates and “lubricity” of binders. Therefore evaluation of the binder alone will not enable determination of the optimum compaction temperature. Moreover, WMA processes are often hard to replicate in the laboratory which means the optimum mixing and compaction temperature should preferably be determined in field conditions.

One method for determining the optimum temperature is by comparing the bulk density of reference HMA with that of WMA at different temperatures but equal compaction forces. The temperature at which both densities are the same can be determined. This can be defined as the optimum compaction temperature. This is illustrated in Figure below. However, the Superpave gyratory compactor has been recognized as relatively insensitive to temperature changes and therefore may not be suitable for this purpose (Hurley and Prowell, 2006).



If RAP or RAS is used in the mix design the minimum production temperature must also be adjusted to facilitate melting of the hard binder. To ensure sufficient mixing of virgin and aged binders NCHRP project 9-43 (Bonaquist, 2011a) suggests to limit the minimum WMA paving temperature above the high temperature Performance Grade (PG) of RAP or RAS. The same project showed that PG of RAP recovered binder typically does not exceed 94°C, meaning that most warm mix technologies allow incorporation of RAP at the producer specified WMA temperature. However, the recovered high PG temperature for tear off shingles can exceed 130°C, thus limiting the potential reduction of temperature.

RAP and RAS Content

Due to the binder viscosity reduction in WMA, stiff mixes, such as those containing a high percentage of Reclaimed Asphalt Pavements (RAP), can be made easier to work with. The reduced viscosity is beneficial to the workability of the mixture and the decreased aging of the binder compensates for the stiffer RAP binder, reducing the cracking potential. According to the NCHRP research project No.691 (Bonaquist, 2011a) the RAP amount in an asphalt mixture can be increased by 10% if, due to the reduced oxidation, the low performance grade is 0.6°C lower than that of conventional asphalt. For a typical asphalt binder this can be achieved through a reduction of the production temperature by 28°C.

It was reported by NAPA research (Gandhi, 2008) that for mixes containing high percentages of RAP, the compaction effort was reduced by 40% when using Sasobit. In Germany, a study (D’Angelo et al., 2008) was presented in which 45% RAP was used in the base course containing Aspha-min WMA. In the Netherlands LEAB Warm Mix Asphalt is routinely produced with 50% unfractioned RAP (D’Angelo et al., 2008). A study in Maryland (Kristjansdottir, 2007) concluded that the use of Sasobit allowed an increase in the amount of RAP from 25% for HMA to 45% for WMA. A course of 5 km was placed and it was estimated that the financial benefits of a higher RAP content can compensate for the additional costs of the WMA technology.

Laboratory Aging

It is recognized that while the physical properties of binders may initially be the same, their properties change when exposed to heat and other external factors.  Short term aging occurs during mixing with aggregates, transportation and the laying processes. If the WMA mixtures are produced at significantly lower temperatures the aging may be reduced. Moreover, there are evidences (Lee, 2008) showing that short term laboratory aging is more critical for WMA as compared to HMA and can influence the testing results to a large extent.

The NCHRP report 691 (Bonaquist, 2011a) suggests using two hour aging (instead of the four used for HMA) at planned field compaction temperature before running performance tests, which is supported by other research (Hurley and Prowell, 2006). These conditions are believed to more closely simulate actual aging during production than the conventional HMA aging of four hours at 135°C according to AASHTO R30.

For the products that involve foaming actions to reduce bitumen viscosity and allow better workability of the mix, additional aging time may be necessary to allow dissipation of the added moisture before performing the tests.

Adhesion Additives

Most foaming technology suppliers advise the use of antistripping additives to improve adhesion that is compromised due to water presence. However, lower temperatures used for WMA production may reduce the effectiveness of some antistripping additives used in the process (Perkins, 2009). This should be considered when choosing a specific anti-stripping product for WMA production. Many WMA chemical technologies already use antistripping additives and therefore eliminate the need for introduction of an additional adhesion agent. 

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