Major Changes in Hot Mix Asphalt: Caltrans Adds More "Superpave" Concepts to Improve Performance
Posted by Mike Cook on Mar 18, 2015
In recent years the California Hot Mix Asphalt (HMA) industry has seen major changes in technology and state specifications. Warm mix asphalt, recycled asphalt pavement and anti-stripping agents have already been implemented, and the use of intelligent compaction and asphalt shingles are hitting the street soon. Further changes in specifications include a new binder grading system (PG grades), a shift in responsibility for quality control from owner to contractor (QC/QA) and tighter tolerances for materials used in HMA Caltrans Section 39.
Now, after more than two years of development and refinement, a new Caltrans specification is being introduced into state projects. The so-called “Superpave” specification is actually a modification of the existing Section 39, which governs asphalt use on state roads and highways. These changes are required for new project designs and have been spurred by pressure from the Federal Highway Administration to extend infrastructure life through new standards and practices believed to extend the life of pavement.
Changes in Total Void Content During Actual Production
First and foremost is a requirement for a minimum specified volume of voids (VMA) to accommodate asphalt oil while providing space for a prescribed air content. For the past few years, a VMA requirement existed for pre-qualification of mix designs but there was not an acceptance level during actual production. The purpose of the new requirement is to ensure sufficient asphalt in the mix to prevent fatigue cracking and extend the life of pavement.
Introduction of New Design and Testing Equipment
The second major change is in equipment used for design and testing new mixes. For over a half-century, California relied on research and equipment developed by engineer Francis Hveem and his colleagues. The “Hveem method” equipment has been used to develop mixes and test plant-produced mixes to ensure quality, but is now being replaced by new equipment developed in the U.S. and Europe.
One of the new machines is a gyratory compactor to replace the Hveem compactor to better replicate field compaction and allow different labs to produce more consistent results. This should reduce disputes between producer labs and Caltrans labs over differing results, and in turn avoid costly delays, re-work and other waste associated with testing variability.
Another new machine of note is the Hamburg wheel tracker. The wheel tracker is designed to “torture test” pavement to measure durability. It is substantially different from performance testing done in the Hveem system. It incorporates testing under moist conditions so it not only determines dry durability, but also the likelihood of failure due to water in the pavement.
Tighter Mix Tolerances
The third significant change is tighter tolerances for some aspects of the mix. Increasing the consistency of a product and requiring oil content and aggregate grading be closer to the design values are expected to deliver performance closer to the design expectation.
The final major change now in the works is restoring the “QC/QA” incentive system that had been removed from Section 39. This statistically driven system applied bonuses and deductions to projects based on material and construction performances. Caltrans has promised to integrate the QC/QA system back into the new specification, recognizing that incentives can produce substantial improvement in pavement quality. The challenge is to create a new scoring system that includes results from the new requirements (VMA, Hamburg Wheel Tracker) and the long held requirements for oil content and air voids. The industry hopes the new QC/QA platform is in place by 2015.
As with any new system specification, the key to understanding its value will be in the results. This is an area of improvement for Caltrans. Implementation of the “Superpave” specification is a perfect time to reinvigorate that effort so we can truly know whether the new design and testing path is right for the state.
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