8 - The effects of natural aggregates on the properties of concrete

Abstract

This chapter explains how the properties of natural aggregates influence the properties of fresh and hardened concrete. It describes British and European Standard tests for aggregates and emphasizes the importance of correct sampling and describes the interpretation of aggregate test results. Aggregate is the main constituent of concrete and the properties of aggregate influence the properties of concrete. Petrological classification alone is an inadequate guide to aggregate performance. Any determination of aggregate properties requires samples to be taken. Correct sampling, sample reduction, and sample preparation are essential if test results are to be meaningful. Consistency of particle size distribution is important to the properties of fresh concrete. There is a more minor influence on hardened concrete properties. Larger maximum particle size benefits lean concrete but the optimum size is around 20 mm for normal strength concrete. Equidimensional particle shapes are best. Shape and texture affect water demand and aggregate-cement paste bond. In practice, the aggregate strength usually exceeds concrete strength considerably. In addition, bulk density is used as a check on the consistency of aggregate grading and shape. Particle density and absorption is known for mix design purposes. Most aggregates restrain concrete shrinkage. A few aggregates undergo large volume changes on wetting and drying, and produces concrete with poor volume stability. Some aggregates are less able to cope than others with stresses induced by environmental conditions.