Mixing concrete

Concrete is a mixture of paste and aggregates. The paste, composed of cement and water, coats the surface of the fine and coarse aggregates. Through a chemical reaction called hydration, the paste hardens and gains strength to enclose and fix the aggregate to form the rock-like mass known as concrete. Good quality concrete in its fresh state should be flowable enough to be transported and placed in the formwork with good cohesiveness. The properties of fresh concrete should be of high interest as they directly influence the handling, placing, and consolidation, as well as the properties of hardened concrete.

The key to achieving a strong, durable concrete rests in the careful proportioning and mixing of the ingredients. The ingredients to be batched should follow the specified mix proportions and the moisture content of the aggregates should be determined with reference to their Saturated Surface Dry (SSD) conditions so that the specified water/cement (w/c) ratio is not altered. In general, the higher the w/c ratio the higher the workability of fresh concrete as more mixing water can lubricate the mixture, however, the strength decreases with increasing w/c ratio. All the ingredients should be added and mixed properly and thoroughly in the concrete mixer for an adequate mixing time.

The figure below demonstrates the mixing procedure according to AS 1012.2:2014.

Figure: Mixing procedures