5.11 : Bonding and Strength of Aggregate

The bond between aggregate particles and the cement matrix is significantly influenced by the shape and surface texture of the aggregates. High-strength concretes benefit from a rougher texture, which leads to stronger bonding due to greater adhesion. Angular aggregates with larger surface areas also enhance this bond. The bonding quality, however, is complex to assess as no universally accepted test exists. Good bonding is indicated when a crushed concrete specimen shows some aggregate particles broken throughout and most of them separated from the matrix.

The strength of concrete is predominantly determined by the aggregate strength, as concrete's compressive strength cannot surpass the strength of the majority of the aggregate it contains. Aggregate strength information is typically derived from indirect tests such as the crushing value of bulk aggregate and the crushing strength of rock samples from which the aggregates are derived. It's observed that while high-strength aggregates contribute to overall strength, aggregates of moderate or low strength are beneficial for the structural integrity of concrete, as they can compress under thermal or hygral changes, reducing stress on the cement paste and thus preventing cracking.