Heavy Duty Hot Mix Asphalt Intersections

While Hot Mix Asphalt (HMA) has continually proven to have superior lifecycle cost benefits, special attention, in some cases, should be focused on intersections to ensure the same outstanding performance. Some mixes that have a history of good performance in posted-speed applications may not perform well in intersections, climbing lanes, truck weigh stations and other slow-speed areas.
Slow moving or standing loads occurring at these sites subject the pavement to higher stress conditions, which may be enough to induce rutting and shoving. In addition, the increase in the number of trucks and heavier wheel loads can also play a significant role in the premature failure of some pavements.
Pavement engineers often do not consider the fact that these slow-moving load applications are much more severe. Consequently, there has not been a plan for addressing the situation.
Recognizing that some traditional mixes may not always be successful at meeting this challenge, the opportunity exists to implement a strategy for ensuring good performance at slow-speed applications. Specifically, the pavement must be designed and constructed to withstand the more severe conditions.
The AASHTO Joint Task Force on Rutting (1987) identified four types of rutting:
Mechanical deformation is the result of insufficient structural capacity. Alligator cracking usually accompanies it.
Plastic flow can result for various reasons: high pavement temperatures; materials and mixture design (rounded aggregate, too much binder and/or filler, insufficient VMA); or asphalt production.
Consolidation in the wheel paths occurs because of insufficient compaction of the pavement. A number of factors can contribute to lack of compaction: too few roller passes; material cooling prior to achieving target density; or high fluid content (asphalt, moisture, dust) contributes to tender mixes.
Surface wear is a result of loss of surface aggregate. Moisture damage or raveling will also occur if proper drainage is not obtained.
To perform well, an intersection pavement must first have adequate thickness to provide the structural capacity to meet traffic needs. For new pavements, thickness design must account for normal factors such as subgrade strength, drainage and traffic.
For existing pavements, it is critical that the structural adequacy of the in-place material be evaluated. Any failed or weak layers must be removed. Simply paving over existing failed material will likely result in recurring failure.
The materials available today offer the designer many choices in attaining improved performance. Careful selection of the asphalt binder will go a long way toward providing desirable performance.
Recognizing that a more rutting-resistant binder is needed at intersections, the use of Superpave’s Performance Graded (PG) asphalt binder system is highly recommended. The advantage of using this system is that the binder is characterized based on its ability to perform in the climate in which it will serve.
Like the asphalt binder, aggregates used in intersection mixtures must be carefully selected. The aggregate structure has to be capable of carrying the load and developing a high degree of stone-to-stone interlock that will resist shearing.
Certain aggregate properties are critical to delivering this performance. Both coarse and fine aggregate must be angular to provide the aggregate interlock required for rutting resistance. Limiting the use of rounded particles and uncrushed sands is essential. Superpave criteria for selecting aggregates are recommended to produce high performance mixtures.
The goal of the mix design process is to select and proportion appropriate materials that resist rutting. Mixtures meeting Superpave volumetric criteria, or stone mastic asphalt (SMA) mixtures, are good candidates to use at intersections because of their ability to resist rutting.
Experience has shown that the voids-in-the-mineral-aggregate (VMA) property of the mixture is particularly important. Mixes with marginally low VMA can be sensitive to relatively small changes in the total fluids content (asphalt binder, moisture and fine filler). Small increases in fluids can then cause these mixes to be subject to in-service rutting or shoving. On the other hand, mixes with high VMA will produce thick asphalt coatings on the aggregate particles (assuming four percent air voids), which could serve as a lubricant allowing the particles to reorient themselves under traffic. Quality control of the asphalt mixture is critical and initial trial runs are necessary to identify any needed volumetric adjustments.
The secret to intersection construction is no secret: Practice proper construction techniques, and pay attention to details.
Avoid the use of diesel fuel in truck beds.
Do not overheat the mixture.
Thoroughly clean milled areas.
Avoid segregation during production, transportation and placement.
Proper joint construction is important to prevent the entrance of water.
Achieve target density.
Implementation of an intersection strategy will result in a dramatic increase in the performance life of the pavement, thus reducing lifecycle costs. Evaluating the performance history of intersections and other similar high-stress areas should play a key role in deciding whether changes should be made to the normal procedures. Although the initial unit cost of building these improved asphalt intersections may be higher than the cost of regular asphalt pavements, they will still be more economical than portland cement concrete (pcc).
These improved asphalt intersections can be constructed much quicker, avoiding lengthy lane closures and rerouting of traffic, which is necessary with pcc construction and curing. Intersections can be opened to traffic within minutes after the rollers have finished. Motorists and businesses benefit greatly from the fast construction. The final product will be a long-lasting, cost-effective, smooth asphalt intersection.
Asphalt intersections can be economically maintained to preserve superior ride quality over long periods, and smooth roads keep the motoring public happy.
Utility repairs are also much quicker and less costly to make in asphalt pavements. Street cuts can damage electronic traffic detour loops, which are reinstalled quicker in asphalt pavements, thus reducing traffic flow disruptions.
Asphalt provides superior driver visibility of pavement markings. This enhances safety for drivers, bicyclists and pedestrians. White and yellow stripes show up best against smooth black surfaces, helping drivers to be more aware of cross walks, stop bars, turn lanes and offset center line stripes at intersections. Also, asphalt provides superior skid resistance without special grooving.
Article Courtesy of Texas Asphalt Pavement Association