Validation of Fatigue Cracking Prediction Models for Structural Design of Flexible Pavements
I. Research Problem Statement
Fatigue cracking is an important deterioration mechanism of asphalt concrete-surfaced pavements, because of the detrimental effect these cracks have on the overall pavement life, reducing its durability and strength. Top-down fatigue cracks provide a path for moisture to readily infiltrate the underlying layers and subgrade soils of flexible pavements. Research has shown that fatigue cracking is caused by a combination of repeated wheel loadings over time and aging of the pavement’s surface. The pavement structure, mixture composition and construction are major factors that have an effect both on the initiation and propagation of fatigue cracks with load repetitions. Environmental and climatic factors also play an influential role on the development of fatigue cracks with time. The documented mechanism for top-down fatigue cracking cites that stress concentrations increase at the crack tip and propagate the cracks at an accelerated rate through the thickness of the asphalt concrete. NCHRP project 1-42A has identified the structural-mixture property combinations, in addition to the wheel load characteristics (type of tire, tire pressure and load) for which the fatigue cracks initiate at the surface, rather than at the bottom of the asphalt concrete layers. Finite element analysis programs have been used to evaluate and predict the location of where fatigue cracking initiates throughout the pavement structure. Thus, the tools and mixture characterization procedures are now available to develop conditions and/or combinations of properties that can be developed for determining when fatigue cracks will likely initiate at the surface, as compared to the underlying layers. Results of NCHRP 1-42A were crafted specifically for direct application into the mechanistic-empirical design procedure (MEPDG) being developed for draft AASHTO MEPDG and to support the future and ongoing work of the Superpave distress prediction software. A critical need exists to test the predictive equations and laboratory results from NCHRP 1-42A through extensive laboratory and field validation of the models.
II. Research Objective
Test and validate the structural conditions and mixture property interactions in draft fatigue cracking models through laboratory validation of pavement structures in the field.
The development of the fatigue cracking prediction models in the SHRP Program and in NCHRP 1-42A have developed fatigue cracking test procedures for measuring the fatigue properties of different asphalt concrete mixtures. These tests and mixture property characterization procedures and the use of finite element analysis techniques customized to pavements, have made it possible to effectively and efficiently determine the structural conditions, climatic conditions and mixture property interactions to determine those conditions at which fatigue cracks would occur or develop at the top of an asphalt concrete layer. The next step is to evaluate materials from field pavements with a variety of materials, climates, and loading conditions. Accelerated load facilities should also be included.
III. Estimate of Problem Funding and Research Period
The cost of such a study based on the analytical requirements for gathering field data and materials, along with associated laboratory testing to test the predictive models for fatigue characteristics of asphalt concrete mixtures is believed to be reasonable based on the availability of existing test protocols, laboratory equipment, and customized finite element programs currently available. A reasonable cost to conduct this program is approximately $1,000,000 which is believed to be reasonable considering the complex logistics involved in obtaining field materials from State DOT pavements, along with sophisticated laboratory testing. A reasonable timeframe in which to accomplish this would be approximately 36 months.
IV. Urgency, Payoff Potential and Implementation
The availability of the analytical equipment test procedures and performance modeling procedures have made it possible to in a more accurate manner, predict the location of fatigue crack initiation in an asphalt concrete pavement structure. This study is critically needed in support of the draft AASHTO MEPDG to provide pavement designers and mix designers for accurately predicting conditions most conducive to the development of fatigue cracks at the surface of the pavement. Knowing these conditions, decisions can be made during the pavement design phase to minimize the occurrence of surface initiated fatigue cracks. This research and/or work needs to be done aggressively to support the current revisions being completed in the draft AASHTO MEPDG, specifically in the establishment of calibration factors for fatigue cracking.