Effects of Repeated Loading on Gravel and Crushed Stone Base Course Materials Used in the AASHO Road Test

Abstract

This paper presents the results of a laboratory investigation of the behavior of the AASHO Road Test gravel and crushed stone mixtures subjected to repeated loading. Cylindrical specimens were stressed trivially to levels approximating those which would be found in the base course of a highway pavement. The repetitive loadings were applied with equipment developed in the laboratories of the School of Civil Engineering at Purdue University.The materials used in the study were obtained from the site of the AASHO Road Test. The gravel and crushed stone with grain size distribution equal to that used in the Road Test and compacted to the mean density levels found in the field were the subject of primary interest. Variation in percent fines and degree of saturation were considered to have a great effect on these basic gradations. Therefore, the percent passing the number 200 siesh sieve was varied so that three mixtures of each material were obtained. These three mixtures were tested at three levels of saturation.The effects of these variables on the deformation-rebound characteristics of the materials under repeated loads were studied. From these studies, the advantages and disadvantages of each material, concerning their value as base course materials, were found and are reported in detail. Also, a comparison between the laboratory and field performance of the two AASHO materials is presented. Oil GRAVEL AIID CRUSHED STOIIE EASE COURSE MATERIALS USED II! THE AASHO ROAD TEST IIITRODUCTI01I The variable of base course type was included at several locations in the flexible test sections of the AASHO Road Test at Ottawa, Illinois. These special test sections were in the form of wedges and each was l60 feet in length. Base thiclaiess and type were the only variables in the various sections. EFFECTS OF REPEATED LOADINGMaterials used in the special base course studies included cement treated gravel, bituminous treated gravel, untreated crushed stone and untreated gravel.The test sections were divided into^0 foot sub-sections for purposes of analysis; performance was related to the mean thicloiess of base. A three inch surface was used in the special sections in which stone and gravel were compared. No sub-base was used in loop 3 whereas in loops h and 5,^i nches of sub-base was included in the design. The typical section as outlined in the AASHO Road Test Report (5) included a shoulder of crushed stone materials.During the winter of i960, personnel from the Road Test contacted Purdue University to determine if tests could be performed to evaluate the special base course materials. It was hoped that a test would be devised wherein appropriate strength parameters could be assigned to the materials which in turn could be related to relative performance. It soon became apparent that it would be extremely difficult to devise a single test which would describe the properties of all of the materials since tensile strength was of importance when considering the bituminous and cement treated gravels whereas the propert...

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