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Concrete Composition

Concrete is a composite construction material. which consists mainly of relatively inert mineral matter in the form of particles or fragments held together by a binding medium which gives concrete its solidity and strength. This is effectively an artificial stone or rock. The most of the load bearing elements of the site are done using concrete. Its primary property is that workable before hardening, strong in compression and stays strong for extremely long time scale.

There are many types of concrete available, created by varying the proportions of the main ingredients below. In this way or by substitution for the cemetitious and aggregate phases, the finished product can be tailored to its application with varying strength, density, or chemical and thermal resistance properties.

Materials Used For Concrete:-

  • Cement
  • Additives
  • Water
  • Coarse aggregate
  • Fine aggregate
  • Admixture


Cement

In my site ordinary Portland cement was used. Ordinary Portland cement made complying with the standards. That has setting time between 45 minutes to 10 hours suitable the site conditions, is used for concrete. A cement bag containing 50 kg is used and the brand generally used is Mitsui cement.

Aggregates

The most part of the concrete is representing by aggregates.  Aggregates occupy 60 to 80 percent of the volume of concrete. Sand, gravel and crushed stone are the primary aggregates used. All aggregates must be essentially free of silt and/or organic matter.

Concrete properties are influenced by aggregate characteristics:-

  • Size and grading
  • Shape and surface texture
  • Strength
  • Specific gravity and bulk density
  • Water absorption and surface moisture
  • Bulking of sand
  • Soundness and durability including alkali aggregate reaction
  • Coarse Aggregates

Generally crushed stones from a quarry of gneiss are used as coarse aggregates. Normal sizes of coarse aggregate used are ¾” and 1”. As a general rule the largest particle should not be larger than a quarter of the least dimension of the member being cast and be smaller than the least clear spacing between single bars. We generally used ¾” size particles other than screed concrete.

Fine aggregates

River sand is the most commonly used fine aggregate in Sri Lanka. The standard size for fine aggregate is that it should pass through the 4.75mm sieve and be retained on 1mm sieve. Sand should be cleaned and the presence of clay, silica, mica and fine dust would reduce the strength of concrete.