Concrete mix design

Published on 30 January 2008 (updated on 14 February 2008)

Concrete is a mixture of several natural constiuents.These include:

* A binder: cement, which hardens in the presence of water;

* Water: it is required for the cement to harden and in order to lay it. However, care is necessary as too much water reduces the strength and durability of the concrete.

* Aggregate: it varies in size from sand to gravel, which forms the "skeleton" of the concrete.

* And, when required, additives: they modify the properties of the concrete depending on their nature.

To formulate a concrete, it is firstly necessary to select:

* the type of aggregate (the stones) and the size of the particles

* then the type of cement (there is a large variety of types of cement with different characteristics and performances - in general 350 Kg of cement for 1m3 of concrete)

* then the amount of water (generally between 130 and 150 Litres for 1m3 of concrete)

* and lastly, if required, the amount of additives (a few Kilograms for 1m3 of concrete).

One cubic metre of concrete weighs 2.5 Tonnes.
Typically, 1m3 of concrete is made up of 350Kg of cement, 700Kg of sand, 1,200Kg of chippings and 150 Litres of water.

The mix design process can either be conducted on the basis of charts or experimentally. These methods are based on achieving the maximum density of concrete..

Approximate proportions of constituents in a conventional concrete

Constituent Water Air Aggregate Cement
Volume (%) 14-22 1-6 7-14 60-78
Weight (%) 5-9 - 9-18 63-85

This composition is then optimized and adjusted on the basis of compression tests conducted on test specimens.

The water content must be kept as low as possible, compatible with the workability and hydration of the concrete. The proportion of cement has a direct influence on the mechanical strength. The water/cement (w/c) ratio must be between 0.35 and 0.5.

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Mix design must take account of:

* project data : mechanical characteristics, dimensions of the structure, reinforcement...

* worksite data: laying equipment, climatic conditions...

* data relating to the desired properties of the cement: workability, density, durability, aesthetic appearance,...