Cement and Concrete Concrete is an artificial conglomerate,with the following constituents: Cement Aggregate Water Cement replacements and admixtures
Properties Fluid when wet Initial setting in approx 12 hours Strength increases with time Strong in compression, weak in tension Fire resistance Durability Chemical resistance
Standards There are a bewildering number of British and European standards covering the design, specification, use and testing of concrete. Two key standards are: BS 5328 (Parts 1 to 4) and BS EN 206 which will ultimately replace it.
Reacts with water to harden and bind the other constituents. Cement Reacts with water to harden and bind the other constituents. May be replaced in part by other pozzolanic materials. The cement content has a significant bearing on the strength of the final product. A pozzolan is a material which, when combined with calcium hydroxide, exhibits cementitious properties. Pozzolans are commonly used as an addition (the technical term is "admixture") to Portland cement concrete mixtures to increase the long-term strength and other material properties of Portland cement concrete. Pozzolans are primarily vitreous siliceous materials which react with calcium hydroxide to form calcium silicates; other cementitious materials may also be formed depending on the constituents of the pozzolan.
Types of Cement Ordinary Portland Cement OPC Rapid Hardening Portland Cement RHPC Sulphate Resisting Portland Cement SRPC White Portland Cement Low Heat Portland Cement Masonry Cement Portland Pulverised Fuel Ash Cement
Aggregates The major bulk of the concrete Usually separate coarse and fine aggregate used Large bearing on final concrete properties. Lightweight aggregates may be used to produce low-density concrete.
Water Should be clean and free from impurities Water content will affect workability Too much water – weak concrete Lightweight aggregates may be used to produce low-density concrete.
Admixtures Accelerators Retarders Water reducers (Plasticisers) Air entrainers Superplasticisers
Testing (BS 1881) The Slump Test: A site test to determine the workability of the fresh concrete. The Concrete Cube Test: A laboratory test to determine the compressive strength of hardened concrete. BS 1881 will be superseded by BS EN 12350 by December 2003.
Specification BS 5328 describes four ways of specifying Designed Mix Prescribed Mix Standard Mix Designated Mix BS EN 206 has just three ways.