New Experimental Techniques for Characterizing Cement-Based Materials
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NEW EXPERIMENTAL TECHNIQUES FOR CHARACTERIZING CEMENT-BASED MATERIALS. HAMLIN M. JENNINGSt AND K. SUJATA*t *Center for Advanced Cement-Based Materials, Northwestern University, Evanston, Illinois. *National Cement and Ceramics Laboratories, Evanston, Illinois.
INTRODUCTION Materials science is the discipline of studying the relationships between structures and properties of materials. In the case of concrete as an engineering material most research has focussed on properties or on microstructure, but only modest progress has been made on establishing relationships between them. In other words a lot of work has been done in the area of strength, fracture toughness [1], shrinkage [2,3], creep and microcracking [2,4], and microstructure each as somewhat isolated subjects [5]. Microstructure has not been related to properties in a quantitative manner. The properties of cement paste depend critically on the porosity. Size, shape, volume fraction and connectedness of pores control the severity of flaws and diffusion of water and other ions which, in time, control all the important properties. Cement paste is the matrix material and it hardens with time. The products that cause the material to harden incorporate water and their structures are sensitive to the relative humidity of the environment. Most techniques for observing the microstructure involve sample preparation treatments that include severe drying and/or other damaging processes that result in incomplete interpretation. Recently several techniques have become available that can examine the microstructure of cement paste without severely drying the specimen. Two of these techniques will be reviewed in this paper. Another characteristic of microstructure that must be considered when relating microstructure to properties is that the materials are three-dimensional. Three-dimensional aspects of the microstructure such as volume fraction, size and shape of a phase can be obtained from a two dimensional image but only under the condition of isotropy. Perhaps the most important characteristic of a three dimensional structure, that is not evident in a two dimensional "slice" or image, is the connectedness of any phase including porosity. Microscopes cannot see a three dimensional structure, and other techniques must be used to infer connectedness (or other three dimensional characteristics) and this information can then be combined with information deduced from two dimensional images. Of the non-drying techniques mentioned, two probe the three dimensional character of the material. Mat. Res. Soc. Symp. Proc. Vol. 245. @1992 Materials Research Society
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In order to relate microstructure to properties, such as strength, toughness and fatigue behavior, mechanisms of damage must be understood. The location and severity of damage associated with crack propagation, as well as a complete description of the crack path must be documented and related to other characteristics of the material, for for assessing damage example porosity. Three new techniques are reviewed here.
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