Effect of Bond Strength Between Aggregate and Cement Paste on the Mechanical Behaviour of Concrete
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EFFECT OF BOND STRENGTH BETWEEN AGGREGATE AND CEMENT PASTE ON THE MECHANICAL BEHAVIOUR OF CONCRETE
CHEN ZHI YUAN and WANG JIAN GUO Department of Materials Science Shanghai, China
and
Engineering,
Tongji
University,
ABSTRACT Concrete is a composite material, consisting of hardened cement paste (HCP), aggregate and pores. The interfacial bond between HCP and aggregate is very important in determining some of the properties of concrete, such as the mechanical behaviour and durability. Recently, the microstructure of the transition zones between different aggregates and HCP has been studied and a mechanism for the formation of the transitional zone has been proposed by Maso. The objective of the present work has been to study the relationship between interfacial bond and mechanical behaviour of concrete in the light of both theoretical and experimental research on the microstructure of the interfacial zone.
MICROSTRUCTURE WITHIN THE INTERFACIAL ZONE AND ITS IMPROVEMENT The microstructure of the interfacial zone between aggregate and cement paste is characterized mainly by coarser crystals, more pores, and preferred orientation of portlandite (1,2). The preferred orientation of portlandite within the interfacial zone of a composite sample can be detected by XRD analysis, layer by layer. AFt enrichment in the vicinity of the interface was found in our earlier work [3]. The crystal size can be estimated by Schiller's
equation
based
on the
XRD
data.
The water
film hypothesis
was
proposed by Maso (2) to interpret the mechanism of formation of the microstructure within the interfacial zone. It is an idealized model, but it encompasses the key points of the formation mechanism of the weak interfacial zone. The authors of this paper consider the high local w/c ratio at the cement paste-aggregate interface, caused by the wetting properties of different constituents in the fresh concrete, as the main reason for the weak interfacial bond. The process of formation of the interfacial microstructure is a transformation process, from a liquid-solid interface to a solid-solid one. The microstructural character of the solid-solid interface is determined by the characterization of the liquid-solid interface under specific conditions. In order to improve the interfacial bond, the liquidsolid interface must be improved. If we take it for granted that the local high w/c ratio at the interface is proportional to the free water in the system which is added to maintain plasticity, any method which can increase the solid-liquid ratio at the interface will improve the interfacial bond. Three methods were evaluated in order to improve the microstructure within the interfacial zone: low w/c ratio, cement with a high specific surface area, and cement mixed with silica fume (SF). Table I shows the influence of silica fume on the cleavage strength (see Ref. 5 for details of the test method), crystal size and volume of CH at the interface. The data show that the microstructure of the interfacial zone can be changed significantly by us
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