The process of selecting suitable concrete ingredients and determining their relative quantities in order to produce a concrete of the required strength, strength, durability and workability as economically as possible is called a concrete mix design. The proportion of the concrete ingredient is governed by the required yield of the concrete in 2 states, namely, plastic and hardened states. If plastic concrete is not workable, it can not be placed and compacted properly. The property of workability, therefore, acquires a vital importance.
The compressive strength of the hardened concrete which is generally regarded as an index of its other properties depends on many factors, quality and quantity of cement, water and aggregates; batching and blending; placement, compaction and curing. The cost of concrete is made up of the cost of materials, plant and labor. Variations in the cost of materials arise from the fact that the cement is several times expensive than the aggregate, therefore the aim is to produce the thinnest possible mixture. From the technical point of view, rich mixtures can lead to a high contraction and cracking in the structural concrete and the evolution of a high heat of hydration in concrete of mass that can cause cracks.
The actual cost of the concrete is related to the cost of the materials required to produce a minimum average strength called characteristic strength that is specified by the designer of the structure. This depends on the quality control measures, but there is no doubt that the quality control is added to the cost of the concrete. The scope of quality control is usually an economic commitment and depends on the size and type of work. The cost of labor depends on the viability of the mixture, for example, a mixture of unsuitable workability concrete can lead to a high labor cost to obtain a degree of compaction with the available equipment.
Concrete Mixing Design Requirements
The requirements that form the basis of the selection and dosage of the ingredients of the mixture are:
a) The minimum compressive strength required by the structural consideration
b) The appropriate workability required for complete compaction with the available compaction equipment.
c) Maximum water-cement ratio and / or maximum cement content to give adequate durability for the particular conditions of the site
d) Maximum cement content to avoid shrinkage cracking due to temperature cycling in bulk concrete.