One hundred years ago, engineers were debating whether density, aggregate gradation, or water cement ratio controls the strength of concrete. Today, those debates have been settled, but the factors that control the behavior of concrete in compression remain controversial. The debate has now shifted to the roles played by cement paste, the interfacial transition zone between paste and aggregate, and the relative stiffness of the components. While all three ingredients play significant roles, the properties of cement paste and the heterogeneous nature of the material appear to be the key factors in the response of concrete in compression. This article highlights some of the research that demonstrates the roles played by the various constituents, with emphasis on microcracking, interfacial bond strength, and models of concrete. The reader is directed to Reference 1 for a more complete discussion of the subject.
Darwin, D., "Concrete in Compression," Concrete International, Vol. 21, No.8, Aug., 1999, pp. 82-85.