15221Coarse Aggregate

For HPC, the coarse aggregate particles themselves must be strong. A number of different rock types used to make HPC include: limestone, dolomite, granite, andesite, and diabase. It has been suggested that in most cases the bond strength between the cement matrix and the aggregate is not usually the limiting factor in HPC; rather, it is the aggregate strength itself. In addition, aggregates that may be susceptible to alkali-aggregate reaction or D-cracking, should be avoided if possible, even though the low w/c ratios used will tend to reduce the severity of these types of reaction.

From both concrete strength and fresh concrete mixture rheological considerations, the coarse aggregate particles should be roughly equidimensional: either crushed rock or natural gravels are suitable. Flat or elongated particles must be avoided at all costs. They are inherently weak, and lead to harsh mixes. In addition, it is important to ensure that the aggregate is clean, since a layer of silt or clay may reduce the cement-aggregate bond strength and increase the water demand. Thus, the coarse aggregate should be washed if there is too much fine powder. Finally, the aggregates should not be highly polished (as is sometimes the case with river-run gravels), because this will reduce the cement-aggregate bond.

Not much work has been carried out on the effects of aggregate mineralogy on the properties of HPC. However, a detailed study by A'itcin and Methta [10], involving four hard strong aggregates (diabase, limestone, granite, natural siliceous gravel), revealed that the granite and the gravel yielded much lower strengths and E-values than the other two aggregates. This phenomenon appeared to be related both to aggregate strength and to the strength of the cement-aggregate transition zone. Cook [11] had also pointed out the effect of the elasticity modulus of the coarse aggregate on that of the concrete. However, much work remains to be done to establish the relationship between the mechanical and mineralogical properties of the aggregate and those of the resulting HPC.

It is commonly assumed that a smaller maximum size of coarse aggregate will lead to a more homogenous concrete and higher strengths. The maximum particle size of 20 mm coarse aggregate is commonly recommended for the production of HPC.

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