There are, of course, many considerations to take into account when placing HMA. Many are dependent upon local materials, weather, crew knowledge and training, and individual experience. This subsection presents a few of the basic considerations that apply in virtually all situations:
- Lift thickness. A “lift” refers to a layer of pavement as placed by the asphalt paver. In order to avoid mat tearing (which generally shows up as a series of longitudinal streaks) a good rule-of-thumb is that the depth of the compacted lift should be at least twice the maximum aggregate size or three times the nominal maximum aggregate size (TRB, 2000[1]).
- Longitudinal joints. The interface between two adjacent and parallel HMA mats. Improperly constructed longitudinal joints can cause premature deterioration of multilane HMA pavements in the form of cracking and raveling.
- Handwork. HMA can be placed by hand in situations where the paver cannot place it adequately. This can often occur around utilities, around intersection corners and in other tight spaces. Hand-placing should be minimized because it is prone to aggregate segregation and results in a slightly rough surface texture. If hand placement is necessary the following precautions should be taken (Asphalt Institute, 2001[2]):
- Place the HMA in a pile far enough away from the placement area that the whole pile must be moved. If the pile is located in the placement area its appearance, density or aggregate distribution may be slightly different than the surrounding handworked mat.
- Carefully deposit the material with shovels and then spread with lutes. Do not broadcast (scoop and pitch) the HMA with shovels – this is likely to cause aggregate segregation.
- All material should be thoroughly loosened and evenly distributed. Chunks of HMA that do not easily break apart should be removed and discarded.
- Check the handworked surface with a straightedge or template before rolling to ensure uniformity.
- SMA. SMA mixes behave differently than dense-graded mixes during placement and compaction. Experience and understanding of dense-graded mix placement should be augmented with specific training and precautions before attempting to place an SMA mix for the first time. SMAs are generally stickier and more difficult to work with than dense-graded mixes because (1) they have more asphalt binder, (2) the asphalt binder is modified, and (3) the binder and filler combination creates a viscous mastic. Also, it is not uncommon for large amounts of mastic (the combination of asphalt binder and mineral filler) to collect on paving equipment. If not carefully monitored, this mastic will release from the equipment into the mat leaving an over-asphalted area – commonly referred to as a “fat spot“. These considerations only scratch the surface of SMA construction. A more thorough treatment can be found in:
- National Asphalt Pavement Association (NAPA). (1999). Designing and Constructing SMA Mixtures – State-of-the-Practice, Quality Improvement Series 122. National Asphalt Pavement Association. Landham, MD.
- Mat problems. The asphalt paver, MTV, rollers, mix design and manufacturing introduce many variables into HMA pavement construction. A familiarity with common causes of the more typical mat problems can help improve construction quality. Some common mat problems are microcracking, fat spots, joint problems, non-uniform texture, roller marks, shoving, surface waves, tearing (streaks) and transverse screed marks.