CONCRETE MIX AND FACTORS TO BE CONSIDERED

         

         The process of selecting suitable ingredients of concrete and determining their relative amounts with the objective of producing a concrete of the required, strength, durability, and workability as economically as possible, is termed the concrete mix design. The proportioning of ingredient of concrete is governed by the required performance of concrete in 2 states, namely the plastic and the hardened states. If the plastic concrete is not workable, it cannot be properly placed and compacted. The property of workability, therefore, becomes of vital importance.

         The compressive strength of hardened concrete which is generally considered to be an index of its other properties, depends upon many factors, e.g. quality and quantity of cement, water and aggregates; batching and mixing; placing, compaction and curing. The cost of concrete is made up of the cost of materials, plant and labour. The variations in the cost of materials arise from the fact that the cement is several times costly than the aggregate, thus the aim is to produce as lean a mix as possible. From technical point of view the rich mixes may lead to high shrinkage and cracking in the structural concrete, and to evolution of high heat of hydration in mass concrete which may cause cracking.

Concrete mix design is of two types:

1. Nominal concrete mix
2. Designed concrete mix
Nominal concrete mixes are those specified by standard codes for common construction works. These mix takes into consideration the margin for quality control, material quality and workmanship in concrete construction.
Designed mix concrete suggests proportions of cement, sand, aggregates and water (and sometimes admixtures) based on actual material quality, degree of quality control, quality of materials and their moisture content for given concrete compressive strength required for the project. Designed mix concrete are carried out in laboratory and based on various tests and revisions in mix designs, the final mix proportions are suggested.
Properties desired from concrete in plastic stage: –
• Workability – Suitable workability for proper placement of concrete in structural member.
• Cohesiveness – better cohesiveness between cement and aggregates to prevent segregation of concrete.
• Initial set retardation – to control the initial setting time of concrete based on requirements.
Properties desired from concrete in hardened stage:-
• Strength – Strength of concrete is the main objective of the concrete mix design.
• Imperviousness – Better mix proportions to improve imperviousness for protection of reinforcement form corrosion and enhanced durability of concrete.
• Durability – To increase the durability of concrete.

Mix Proportion designations

The common method of expressing the proportions of ingredients of a concrete mix is in the terms of parts or ratios of cement, fine and coarse aggregates. For e.g., a concrete mix of proportions 1:2:4 means that cement, fine and coarse aggregate are in the ratio 1:2:4 or the mix contains one part of cement, two parts of fine aggregate and four parts of coarse aggregate. The proportions are either by volume or by mass. The water-cement ratio is usually expressed in mass

Factors to be considered for mix design

The grade designation giving the characteristic strength requirement of concrete.

 The type of cement influences the rate of development of compressive strength of concrete.

 Maximum nominal size of aggregates to be used in concrete may be as large as possible within the limits prescribed by IS 456:2000.

 The cement content is to be limited from shrinkage, cracking and creep.

 The workability of concrete for satisfactory placing and compaction is related to the size and shape of section, quantity and spacing of reinforcement and technique used for transportation, placing and compaction.