Microstructural Study of Aggregate/Hydrated Paste Interface in Very High Strength River Gravel Concretes
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MICROSTRUCTURAL STUDY OF AGGREGATE/HYDRATED PASTE INTERFACE IN VERY HIGH STRENGTH RIVER GRAVEL CONCRETES Shondeep L. Sarkar, Yaya Diatta and Pierre-Claude ATtcin, Department of Civil Engineering, Universit6 de Sherbrooke, Sherbrooke (Quebec) JIK 2RI
ABSTRACT The aggregate/hydrated paste interface represents the weakest link in very high strength river gravel concrete, due to the surface smoothness of these aggregates. Microstructural examination of the aggregate/hydrated paste interface in four different (very low W/C ratio) very high strength concretes with and without silica fume shows major differences in the nature of the In the non-silica fume transition zone at the interface level. concretes, hydrated lime and ettringite are found quite exclusively at the interface, while in silica fume concretes, only C-S-H is observed. The modulus of elasticity can be correlated to the compressive strength by the equation, E' = 2.3 ,/c + 15.8, 42.6 < fV < 83.4 MPa, with a low correlation index (70%) for non-silica fume concrete, whereas in silica fume concrete it becomes E' 3.6 '/T + 3.9 for 65.9 < fc < 106.7 MPa, with excellent correlation in ex of 95%. These results can be explained by the nature of the aggregate/ hydrated paste interface, which is stronger in silica fume concrete.
INTRODUCTION River gravel as an aggregate in concrete has been used since historical times. The material, in some respects, still poses a challenge to concrete technologists in the sense that its mineralogical composition tends to be inhomogenous, and therefore, its reactivity in It is, however, the service life of concrete is difficult to predict. cost effective, since it can be used in the as-received form whereas most other types of rock require crushing. Moreover, due to its rounded shape and smooth surface river gravel is known to require less mixing water to obtain a given workability. This reduced water content is also advantageous with respect to compressive strength due to its clear correlation with water to cement ratio. Equally it is feasible to have lower cement content without adversly affecting the compressive strength. From a mechanical point of view the smoothness of the subrounded to rounded river gravel aggregates is not favourable characteristic because it tends to reduce the adhesion between the aggregate and the paste. On the other hand river gravel particles not having been obtained through a crushing process should be individually stronger than crushed coarse aggregates that have been subjected to very brutal treatment. As a result of these conflicting advantages and disadvantages it was decided to initiate a research program on the use of river gravel aggregates to make very high strength concrete with and without silica fume.
Mat. Res. Soc. Symp. Proc. Vol. 114. '1988 Materials Research Society
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