Fracture Surfaces of Cement Pastes, Rocks, and Cement/Rock Interfaces
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ABSTRACT Stereo pair imaging in a scanning electron microscope was used to document the details of the topography of fracture surfaces at various degrees of magnification. The fracture surfaces had been produced by fracture testing of chevron-notched beam specimens according to ISRM Method I. The surfaces examined included the fracture surfaces of dolomite and andesite rocks, cement pastes with and without silica fume, the fracture surfaces from specimen prepared by casting cement paste on previously fractured rock specimen faces, and the fracture surfaces of mortars prepared using the same rock types and cement pastes. The principal topographic features of the various fracture surfaces are described.
INTRODUCTION The cement-aggregate interface in concrete has long been the subject of study. Since this has often been assumed to be the "weak link" in concrete, the recent development of very high strength concretes has re-awakened interest in the interface and the related transition zone. Traditionally, methods of studying the interface have included direct measurement of the bond strength between paste and sawn or polished rock surfaces, measurements of the fracture properties of composite specimens using naturally fractured rock surfaces, microstructural studies incorporating SEM observations and measurements of, for example, porosity gradients, and X-ray diffraction studies to try to identify the hydration products in the vicinity of the interface. The work reported here is part of a much larger study to determine the mechanical effects associated with interfaces involving fractured rock surfaces [1-7]. In particular, it describes the use of stereo pair imaging in a scanning electron microscope to document details of the topography of fracture surfaces in cementitious systems at various degrees of magnification. This technique can provide unique information regarding the nature of the fracture process itself. However, stereo pairs have rarely been published for concrete materials. To the knowledge of the authors, the work summarized here [5-7] represents the first systematic use of stereo pair imaging in a scanning electron microscope to describe fracture surfaces in cementitious systems. Materials and Experimental Methods The materials and experimental methods are described in detail in Refs. 5-7. Briefly, the rock materials used in these investigations were hard, durable rocks used commercially for crushed rock aggregates in South Africa. They included a dolomite rock from the Olifantsfontein Quarry, an andesite rock from the Eikenhof Quarry and, for the mortar studies, a granite rock from the Jukskei Quarry. The cement used was an ordinary portland cement of 295
Mat. Res. Soc. Symp. Proc. Vol. 370 0 1995 Materials Research Society
South African origin. The condensed silica fume, also from South Africa, was supplied as a dry powder. A commercial naphthalene sulfonate superplasticizer was used with all of the cement pastes. Initially, the rock specimens were cored to a diameter of 42 mm and a length of 168 mm
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