WHAT IS A CEMENT CONCRETE
Cement concrete is a mixture of cement coarse aggregate, fine aggregate (non coherent materials) and water to form a hard coherent mass as a result of the chemical reaction which takes place between the water and the cement (hydration). It is a man made composite: the major constituent of which natural aggregate such as gravel is and sand or crushed rock. Alternatively, artificial aggregate for example: blast-furnace slag, expanded clay, broken brick and steel shot may be used where appropriate.
The fine aggregate (sand) forms the filling agent in the concrete mix called the filler while the cement reacting with water (hydration) binds the aggregates together. The cement is therefore called the binding agent or binder.
There are principally three types of cement concrete namely: lean concrete, mass concrete and reinforced concrete.
Lean concrete is the weakest in strength of all concrete mix, it is used as a binding material to prepare formation level for major concrete works in weak soil or water logged trenches. It is regarded as a grade strength 10N/mm2 concrete with a mix ratio of 1:4:8.
Mass concrete is the most widely used cement concrete mix: used for floors and walls without any reinforcement. It has a grade strength of 15 to 20N/mm2 and the mix ratio normally adopted for its mix is 1:3:6.
Reinforced concrete is a cement concrete in which reinforcement bars are inserted. It has the highest strength and it is used for suspended floors, beams, pillars and retaining walls. It has a grade strength of 25N/mm2 and above: depending on the nature of work being done. Its mix ratio is in the order of 1:2:4.
PROPERTIES OF MATERIALS USED IN CEMENT CONCRETE
The basic ingredients used for making cement concrete are aggregates, cement and water. The choice and quality of these materials greatly affects the finished concrete strength. Their qualities are described below:
Cement- this is the binding agent in a cement concrete mix. There are various types of cements and the ones used for a mix depends on the nature of work, and the environment. The most commonly used for most engineering work is the ordinary Portland cement. Where a high early strength is required, so that a road can be opened to traffic earlier than usual (>7 days), a rapid hardening cement may be used. In cold weather or when very rapid cement is required, a high alumina cement may be used. Similarly, when working in harsh dry weather, a slow setting maybe used.
Aggregates- this is divided into 2 groups namely; fine aggregates and coarse aggregates. As a rough rule, the largest size of aggregate used in a mix should not exceed one quarter of the thickness of the slab if the combination of strength and work ability is to be obtained.