Mechanical and Microstructural Investigation of the Aggregate and Cement Interface
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ZONGJIN LI*, SURENDRA P. SHAH**, AND MATTHEW J. AQUINO*** "Hong Kong University of Science and Technology, Department of Civil and Structural Engineering, Clear Water Bay, Kowloon, Hong Kong. **Northwestern University, NSF ACBM Center, 2145 Sheridan Rd., Evanston, IL 60208. "*sSkidmore, Owings and Morrill (S.O.M.), 224 s. Michigan Ave., Chicago, IL 60604.
ABSTRACT The understanding of the interface between aggregate and cement is crucial in determining the properties of concrete. Recently, a pushout experimental technique and a theoretical model have been developed to determine the stiffness, strength and surface energy of the interface layer. The validity of these material parameters was verified in the present study by examining the effect of diameter and embedment length of aggregate. In addition, the effect of the pretreatment of aggregate surface and various admixtures was investigated. All pushout tests were performed in a closed-loop manner to obtain the load vs. displacement relationship. The interfacial zone was further investigated by using backscattered electron imaging and energy dispersive analysis of x-rays (EDAX) to characterize the microstructure of the interface. The relationship between mechanical properties and microstructure of interfacial zone was studied. It was shown that the microstructure of the interface plays a substantial role in the mechanical behavior of the aggregate/cement bond. INTRODUCTION The interfacial zone between cement paste and aggregate is an area of particular interest associated with the strength and the durability of concrete. Inevitably, the mechanical and physical properties of the interface should be determined[I][2]. In a study reported earlier[l] a pushout test was developed to obtain the load-slip relationship of the interface. Based on the observed load-slip relationship and a one-dimensional shear-lag model, material parameters to characterize the interface can be determined. These parameters include the stiffness of the interfacial layer, the shear and frictional bond strengths and the interfacial fracture energy. A good correlation was indicated between the values of above mentioned bond parameters and the microstructure of the interface. The above cited experiments were performed on a single size cylindrical aggregate which was pushed out from a surrounding annulus of a cementitious matrix. If the bond parameters determined from the load vs slip curves are to be considered as valid material parameters then they should be independent of the dimension of the test specimens. One of the goals of the study reported in this paper is to examine the influence of the diameter and the embedment length of the aggregate core on the values of the bond parameters. Of much importance is to identify the influence of various factors on the properties of the aggregate-cement interface. These factors include the addition of polymer latex, and silica fume to the cement paste mixture as well as pretreatment of the aggregate surface with silica fume plus cement paste. 319 Mat. Res
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