Pavement structure designers generally assume that pavement materials will be constructed in such a manner that the assumed design performance period will be achievedthis is particularly so for asphalt concrete (AC) mixtures. Various field studies have shown that this is not necessarily the case. Reliability concepts modify that assumption but generally result in thicker layerswhich do not ensure adequate pavement performance. It appears that the greatest uncertainty associated with the final layer material properties in a flexible pavement is the AC layer. Recent studies have also shown that a major factor in flexible pavement network-wide performance relates to construction factors (as opposed to structural design). Much of the national research emphasis for the last ten years has been on pavement structural design factors. Though such research is well intended and appropriate, insufficient focus has been placed on construction factors with a few notable exceptions (such as WesTrack).
For AC materials, it is critical that adequate compaction be achieved to fulfill its role in pavement performance. Interestingly, AC mixes are continuing to evolve particularly so with the advent of the Superpave mixture design system. That system uses the PG binder grading system which will result in the expanded use of modified binders. Further, aggregates will have a greater percentage of crushed material and, in some cases, a larger maximum size. The net effect will be mixes that are more temperature sensitive during construction and that will require more compactive effort.
The picture that evolves is that the pavement designer may be somewhat "removed" from these construction related issues. This should not be the case.
The objectives for the proposed research are:
Identify the construction related factors that are the most critical for flexible pavement designers to consider.
Quantity the effect that AC mix (specifically) and other flexible pavement layer parameters (broadly) affect pavement performance.
Incorporate relationships that the pavement designer can use to assess the critical nature of construction variability.
Develop a process that pavement designers can use to "localize" estimates of pavement performance variability.
The first objective is intended to be a straightforward but important first step in the proposed study. It is assumed that construction related factors include air voids, binder
content, and aggregate gradation for AC mixes. Other flexible pavement layers (such as crushed stone base courses, etc.) should be evaluated as well. Presumably, density and gradation are critical construction factors for unstabilized layers.
The second objective is to collect available information and, possibly, through a survey/interview process with state DOTs, to quantify the effect each of the major factors identified in the first objective have on pavement performance. It is recognized that these relationships would be approximate; however, it would lead to a better awareness of the critical nature of construction to pavement performance.
The third objective requires that the relationships developed in the second objective be "packaged" in such a way that they can be used in various flexible pavement design procedures. At a minimum they should be directly applicable to any current or near future mechanistic-empirical design approach.
The fourth objective would allow "local" pavement designers to calibrate the performance relationships to local conditions.
Estimated of Problem Funding and Research Period
The cost will be modest since the intent is to use existing data supplemented with state DOT surveys. The cost is estimated to be $150,000 to 200,000.
Estimated time of two yearsURGENCY , PAYOFF POTENTIAL, AND IMPLEMENTATIONThis study should be a high priority for the improvements that will result from enhancing the interaction of pavement design and construction.