Plain Concrete

  Cement concrete is an artificial material obtained by mixing together cemen , coarse aggregte and water , in suitable proportions . All these ingredients , when freshly mixed , produce a plastic mass which can be poired into suitable forms or moulds , to give a desire shape to the rtesultingh solid mass . Plastic mass gets converted innto solid stone like hard mass with passage of time , due to chemical action taking place between cement and water . Aggregate ( btoh coarse as well as fine ) which are mineral materials like sand , gravel , crushed stone etc . do not undergo any chemical change . They simply provide mass volume to the concrete and reduce shrinkage effect in it . Hardened cement concrete resembles stone , in weight , strength and hardness .

  A chemical reaction between cement and water is responsible for setting and hardening of cement concrete. Setting of cement concrete is the stage or phase when concrete mass changes from plastic stage to a non-flowing mass . Process of gaining strenbgth by the mass of cement concret3 is known as hardening of cement concrete . In short , the process of loosing plasticity is known as setting ,whereas process of gaining strength is nown as hardening of the cement concrete . The process of setting of cement concrete can be divided into three distinct stages . The first stage of setting is known as initial setting of the cement concrete . . The first stage of setting is known as initial setting of the cement concrete . This pohase of setting may take from 45 minmuits to about 8-10 hours in completion . In this stage of setting , plasticity of concrete is dexreased markedly and concrete develops considerable resistance to flow . The second stage or phase of setting is known as final setting of the concrete . This phase of setting is known as final setting of the concrtete . The phase may last 5 to 20 hour after mixing of water , depending upon the type of cement used and other weather conditions . During this period concrete looses all its plasticity and gets converted into relatively soft solid mass , whose surface is not very hard . The last or third stage envolves progressive hardening and increase in strength of the concrete . The process of setting of concrete is almost completed within 24 hours of miux ing water , but process of hardening or gaining strength contuinues for years . In the initia stage of hardening , rate of gaining strength by the concrete is quite rapid , but with passage of time it goes on slowing down . it is seen that cement concrete  . The process of setting of concrete is almost completed within 24 hours of mixing water , but process of hardening or gaining strength by the concrete is quite rapid , but with passage of time goes on slowing down . It is seen that cement concrete acquire almost 80-90 percent of its potential strength within first 30 days after mixinh water in it .

  The prperties of cement concrete are dependent upon the quantity and proportions of the ingredients use in th mix . The controls exercised during various operations starting from mixing of aggregates to the placement in the forms or moulds , also greatly influence the proprties of the concrete . Plain cement concrete is very strong in compression but has little strength in tension . Due to weakness in tension , plain concrete is also weak in tortision , shear and bending . Hence use of plain cement concrete is advocated only.Howecer it is seen that in most of the cases , structural members are subjected to shear , bending and torsion . Concrete is quite strong in compression and its weakness in tenion can be made good by embedding steel bars in the tensile zones of the structural members . Cement concrete , in which steel rods are embedded is called reinforced cement concrete or R.C.C.