Workability of Hot Mix Asphalt
An increasingly important issue with regards to hot mix asphalt is the ability to compact and/or place mixes with ease. Some of the factors that contribute to stiff mixes are: increased use of coarse-graded mixes, increased use of polymer-modified binders, aggregate shape, binder content, and placement of asphalt in colder temperatures. These are only a few of the many factors that could cause a mix to be less workable and in turn make compaction achievement harder.
Workability is defined as the ease at which pavements can be constructed. Coarser graded mixes are more difficult for the paving crews to achieve compaction with. Crews have to work much harder to achieve the same level of compaction with a coarser-graded mix than they would have to with a fine-graded mix. The workability of asphalt is directly related to the Nominal Maximum Aggregate Size (NMAS) of the mix. As the NMAS increases for a given aggregate type, gradation, shape and binder type, the workability decreases. For example, a ½″ maximum mix will tend to be more workable and easier to compact than a ¾″ maximum mix. The actual gradation within the mix will lend itself towards being less or more workable. A ¾″ Maximum Medium graded mix will tend to be easier for the paving crew to achieve compaction than a ¾″ Maximum Coarse graded mix. Mixes with a high percentage of fines are more easily compacted because the fine size material allows for the coarse and fine rock to blend together within the mix.
The type of asphalt binder used in Hot Mix Asphalt (HMA) contributes to the workability and/or achievement of compaction. Binder type significantly affects the workability of mixes. Mixes containing stiffer binders, such as an AR 16000, are significantly less workable than mixes containing less stiff, unmodified binder for a given temperature. Using polymer-modified binders also lessens the workability of a mix. The polymer-modifiers tend to stiffen up the binder, hence resulting in a less workable mix. Certain applicationscall for these polymer-modified binders, however the gradation of the mix could be adjusted to help the workability. For example, a specification might call for the use of a polymer-modified binder, however the gradation of the mix could be adjusted to the fine side to help make the mix more workable.
Workability of HMA is also affected by aggregate particle shape and angularity. Mixes prepared with cubical, angular granite tend to be less workable than mixes prepared with semi-angular, crushed gravel. The sharp edges act as hindrances when trying to compact the mix together. The edges interlock together and add more friction to the mix. The less angular rock allows for the material to blend together and sort of roll into place, allowing for compaction to be attained. The use of rounded aggregate and river sands improves the mix workability, as does the choice of a mix that leaves out some of the intermediate size aggregates known as a “gap-graded” mix. A more easily compacted mix may be achieved with attention to aggregate shape, aggregate grading and bitumen content with the aim at producing a more workable mix than that which would be more suitable for highways. See Table 1 below for a list of the differences in a residential or commercial asphalt mix and a structural highway mix.
The binder content of HMA has a great deal to do with the workability of the mix. In light duty or residential pavements, the prime cause of distress of asphalt is not deformation or fatigue cracking but oxidation of the binder, causing hardening of the binder and raveling of the pavement surface. The oxidation of the bitumen can be significantly reduced if the asphalt is designed and compacted to produce low air voids. This requires that the mix is easily compactable even in thin layers. The most significant measure however is to increase binder content with the use of an additional 0.5%–1.0% by mass of bitumen in the mix. The additional bitumen takes the place of some voids making the mix denser, lubricates the aggregates, thus allowing higher compaction to be achieved and adds to total bitumen content, thus making loss by oxidation less significant. A more workable mix is, however, more susceptible to surface scuffing from vehicles with power steering making turns during hot weather in the first summer after placing.
The temperature of the HMA also significantly affects its workability. There is a relationship between workability and temperature that shows increased workability at higher temperatures. Even with mixes designed purposely for easier compaction the effect of temperature on compaction achievement remains of paramount importance and must always be taken into account. The colder the day and the colder the mix, the harder compaction will be to achieve. Workability goes hand in hand with the ease of compaction. You will almost always have a hard time compacting a less workable mix.