To be successful, one must learn from past experiences. These can be your experiences or the experiences of others. Not all experiences are positive, so we use a softer term," lessons learned". For the use of high polymer asphalt, VDOT and industry have been learning lessons since the initial project in Northern Virginia. Haul distance and temperature are very critical to success. Minimal handwork should be performed given the viscosity or stiffness of the mix. Binder production, testing, transport, and storage can all impact the success of a project.
The most recent project to utilize highly polymer modified asphalt mixes is the I-64 Phase I widening project in the Hampton Roads District. This project’s scope included the addition of a new travel lane and inside shoulder along with incorporating the existing jointed reinforced concrete pavement (JRCP) to increase highway capacity. The JRCP lanes were extensively patched and repaired before being overlaid with asphalt concrete. Originally, the pavement design called for the overlay of the JRCP with THMACO (Thin Hot Mix Asphalt Concrete Overlay) with a standard polymer modified binder and then that layer being overlaid with SMA using PG 64E-22 (i.e., PG 76-22). However, based on the initial performance of the high polymer asphalt mixes and the need to delay reflective cracking due to the underlying JRCP, the binder for the THMACO and SMA was changed to high polymer [PG 76E-28(HP)]. Additionally, the SUPERPAVE Mix (SM-12.5) was modified to use the high polymer binder as well. In all, the paving contractor, Branscome, Inc., laid over 18,000 ton of THMACO, 35,000 tons of SMA and 14,000 tons of SM-12.5.
Overall, this was a successful use of highly-polymer modified binder, but it was not without some challenges. In reviewing the project, Branscome offered some lessons learned for other contractors and project designers/specifiers to consider.
1. Temperatures are key. Do not keep the binder at a high temperature in the storage tank. The chemistry of the binder is sensitive to heat and the hotter the binder, the more stiffening occurs. This creates problems with asphalt pumps, asphalt meters and mixing of the asphalt. Mix production and placement temperatures are critical for placement and compaction. The asphalt should be at least 320F behind the screed, but the hotter the mix, the stiffer it becomes. In the summer when the air and paving platform is hotter, there is more time for compaction. However, early and late season paving presents a challenge due to heat transfer from the mix to the air and paving platform. The hotter the mix becomes, the less time is available for compaction.
2. Not all asphalt mixes behave the same way. During this project, binder was used in three mixes. The most challenging mix was the SMA. Due to the higher binder content, placement and compaction were more difficult. Overall the THMACO and SM-12.5 were easier to handle and place.
3. Watch your transfer vehicle. Branscome utilized three different transfer vehicles on this project. The build-up of the asphalt material in the MTV caused problems. The MTV must be hot before receiving the mix. The cold surfaces will lead to sticking of the mix and clogging of the equipment.
4. Forget handwork. The nature of the mixes with the high polymer asphalt binder make scheduling and paving pulls critical. Paving pulls need to be long and continuous, preferably from one structure to the next. Short segments lead to numerous transverse joints that are very difficult to construct and repair. This impacts the final ride quality. Asphalt surfaces with high polymer should be placed as the last operation of a project and not placed intermittently.
While other lessons were shared, these four stand out as top considerations when developing future contracts using high polymer asphalt binder.
