SUBBASE FRICTION EFFECTS ON CONCRETE PAVEMENTS
Authors: Wimsatt Andrew J. , McCullough B. Frank 
Published: January 2025

Designing, constructing, and maintammg concrete pavements under rigorous and unpredictable traffic and environmental loads is a rather formidable challenge. Several effects of the environment on concrete pavements, including the effects of moisture and temperature, have been studied. However, one very important factor, the frictional characteristics between concrete pavements and stabilized subbases, has not been adequately researched. This paper presents findings of a study researching the frictional effects of stabilized and unstabilized subbases. The study was funded by the Texas State Department of Highways and Public Transportation and the Federal Highway Administration and conducted by the Center for Transportation Research at The University of Texas at Austin. The paper first discusses the concepts of subbase friction, including defining that it consists of three components - an adhesion, or gluing, component between the concrete pavement and the subbase; a bearing component that is influenced by the surface texture of the subbase; and a shearing component which is induced by the movement of the slab across the subbase. The paper then covers the results of a literature search on the topic of subbase friction, which found, among other facts, that the frictional effects of stabilized subbases have not been adequately pursued until this study, and that, according to one report, the frictional effects of cement stabilized subbases are very large, on the order of 28 psi and higher. Then, experiments on several subbases, in the form of push off tests, are discussed. These experiments were performed on cement stabilized, asphalt stabilized, lime treated clay, flexible, sand-stabilized shell, and untreated clay subbase materials and resulted in frictional restraint - movement graphs. The experimentation found that, on all but the cement stabilized subbases, the maximum frictional restraints ranged between 0.6 psi on an untreated clay subbase to 3.4 psi on a flexible subbase. However, a peak frictional restraint on the cement stabilized subbase could not be found without exceeding the testing equipment's maximum load capacity, and, as a result, it was conservatively estimated that the maximum frictional restraint of the subbbase was 15.4 psi, 4-1/2 times larger than the flexible subbase's maximum frictional restraint. Results and implications from the experiments are then presented in the paper. Discussion will include the observation that, for stabilized subbases, the failure planes resulting from the push-off tests were within the subbases itself, not at the slab-subbase interfaces. From the experimentation, it was also found that slab thickness, or overburden pressure, was not a significant factor in the frictional effects of stabilized subbases, the flexible subbase, and the sand-stabilized shell subbase. Since the 1986 AASHTO Guide for Design of Pavement Structures assumes that slab thickness does affect the frictional properties of all subbases, as shown in its use of coefficients of friction, the Guide should be modified to account for this fact. In addition, results of testing on two asphalt stabilized subbase layers indicated that the subbase's thickness and internal temperature do have substantial effects on the subbase's frictional properties. It was also found that the asphalt stablized subbase's surface texcture does not significantly affect the frictional restraint. Results of using the indirect tensile testing of subbase cores to estimate subbase friction are also presented. The paper concludes with a discussion of how subbase friction could affect design of concrete pavements.