Over 500 million tons of HMA are placed in the United States each year so it's important that the material be properly sampled and tested during construction to ensure satisfactory performance. When adequate testing is not conducted the performance of the HMA normally suffers.
There are several tests used during construction to help ensure adequate quality is produced. However, prior to beginning work in the field a mix design has to be developed and approved. The mix design provides a job mix formula that is used as the target for material and mixture properties during mix production. The tests discussed in this report include: aggregate gradation, asphalt content, volumetrics and in-place density.
The mix design is developed before the project begins. The mix design evaluates the materials and determines the mixture proportions for optimum performance. As a result of the mix design, a job mix formula (JMF) is produced. The JMF tells how much of each aggregate source to use in the mixture and the optimum asphalt content. It also provides a target for the percent of the aggregate passing each size range. The mix design is really a starting point, and it is important to understand that during production the mix design will likely have to be adjusted.
It's also possible that performance testing will be done during the mix design stage but not during production. For example, some states conduct a rutting test on the mix during mix design but do not conduct additional tests during production. Also some states will conduct a moisture susceptibility test during mix design but not conduct these tests during production.
Most mix designs are now conducted in accordance with the Superpave procedures and criteria. The primary component of these tests is to compact the HMA using some specified compaction effort and determining the volumetric properties. These same tests are typically conducted during the construction process.
There is disagreement about the importance of the aggregate grading on performance. But there is no disagreement on the importance of uniformity in the produced grading. Most specifications will have a gradation band that the contractor will have to meet as the HMA is produced. For HMA, a dense graded mixture is typically used. This plots as a straight line on 0.45 power plot that most laboratories have. The densest grading will be a gradation that falls on top of the maximum density line. When the grading is on top or near the maximum density line, there may not be sufficient voids in between the aggregate properties (VMA) in the compacted mixture to allow for sufficient asphalt to be added to the mixture. The grading will often need to be above the line or below the line to provide sufficient VMA to allow for some minimum desired asphalt content.
Sufficient gradation tests have to be conducted to ensure the gradation is uniform and meets the specification requirements. This requires gradation tests be conducted on samples from the stockpiles, as well as the plant produced HMA. Of course the gradation of the aggregate in the HMA is what really matters, but the stockpiles must be controlled to ensure proper gradation in the HMA.
There are two types of gradations that are conducted: washed and dry. The dry gradation is quicker and easier to conduct but it does not give a true measure of the grading, especially the very fine material. The percent passing the 75 micron size greatly affects the mixture properties and this can only be accurately measured using the washed gradation method. Many times technicians will conduct dry gradations for the most part during construction and simply conduct enough washed gradations to provide a correction factor that can be used with the dry gradation to estimate the washed gradation results.