Concrete

Concrete

Concrete is a rock-like solid mass,formed from the mixture of cement, fine and coarse aggregates and water, in a particular proportion, with a particular strength at a particular age. The cement and water reacttogether chemically (in hydration process) to form a paste, which binds the aggregate particles together.

The paste could be poured into a mould and compacted by vibration or ramming to entrapped air. After setting, the mould or formwork is removed, for the concrete to have its shape.

The qualities of the constituent proportions in which they are mixed determine the strength and properties of the resulting products.

In ordinary structural concretes, the aggregates occupy about 70 to 75% of the volume of the hardened mass while cement, water, and air constitute the remaining volume as stated by Neville and Brooks, (1987).

Hence, a good knowledge of the properties of cement, aggregates and water is required in understanding the behaviour of concretes.

The concretes mass, though, have a considerable compressive strength but little resistance in tension and as a result of this,steel bars are introduced at appropriate places to increase its resistance to tensile force, thus, increasing the strength qualities and the resulting composite structure is called Reinforced concrete.

Sometimes, external stabilizers are added to concrete to give it a desired characteristic and this explains how the current mixtures of concrete has catered for various engineering requirements with the developments in science and the innovations in engineering, as argued by Chandra et al; (2002).

In their argument above, concretes was described as not being a component in the construction fields only, but also as a design ingredient in deciding upon the strength and other physical properties that govern the stability of a given building.

History of concretes

According to (Mark and Hutchison, 1986), in (Wikipedia, 2009), in Roman Empire, roman concrete was produced from quicklime, aggregate of pumice and pozzolanic materials. Many Roman structure use it. The main events in Architectural history termed Revolution of concretes, freed Roman construction from the restrictions of brick material and atone and allow for new design revolution in terms of both dimension and structural complexity.

However due to the absence of reinforced steel, the mode of application was different tensile strength was far lower. (Mark and Hutchison, 1986). The concrete secrete was lost for 13 centuries until 1756, when the British Engineer John Smeaton, first to make the use of hydraulic lime in concrete using powered brick as aggregate. In the Early 1840s Portland cement was used first. However the version of history has been challenged, as Canal Dumidi was constructed using concrete in 1670 (Donald, 1984).

Recently, the use of materials as concretes ingredient, because of increasingly enforced environmental legislation is gaining popularity.

 

Properties of concrete

The properties of concretes are too many. Few discussed include compressive strength, cracking and shrinkage.

 

Compressive and tensile strength of concretes

Concretes has relatively high compressive strength, but significantly lower tensile strength. According (ACI 318 – 08, 2008) it is fair to assume that a concretes tensile strength sample is about 10% to 15% of its compressive strength.

Expansion and shrinkage

Concrete has a very low coefficient of thermal expansion. However if no provision is made for expansion, forces causing cracks in parts of the structure that are not capable of resisting the forces or expansion and contraction repeated cycle. (Wikipedia, 2009)

 

Cracking of concretes

Crack will result to some extent in all structure of concrete due to tensile stress concrete cracks induced by shrinkage or stresses occur during setting or use. Water tank and highways are examples of structure requiring crack control. (Wikipedia, 2009)

 

Creep of concretes

This creep is used to describe permanent deformation and movement of a material in order to reduce stress within the material. Concrete which is subjected to forces of long time is prone to creep. Short time forced (such as wind or earth quakes) do not cause creep. Creep sometimes relieves the amount of crack that occurs in structure of concretes. (Wikipedia, 2009).

 

Composition of concrete

Concrete is made up of cement, water, aggregate (sand and gravels) and in some cases reinforcement (Wikipedia, 2009).