Important points & characteristics of good concrete.

 

• The volume of one 50 kg bag of cement is 34.5 litres

•  About 19 litres (38%) of water is required to hydrate 50 kg cement in sealed container.

•  The ordinary Portland cement when tested for its compressive strength at 28 days   according to the method describe by the standard specification, yields a minimum compressive strength of 43N/mm2 is called 43 grade  cement .

• Too fine cement is susceptible to air set and deteriorates earlier.

•  The setting time of cement can be controlled by varying the quantity of gypsum in cement.

• Sometime with the addition of water to cement, a premature set occurs within 5 minutes. This is called false set an is due to  the presence of anhydrous gypsum which is formed due to grinding of gypsum with too hot clinker .this is not to be worried .continuous mixing aggregates , cement and water will break the false set without  harming any property of the concrete .

•  The ability of cement to maintain a constant volume is known as soundness of cement.

• The ability of aggregate to resist excessive change in volume due to change in physical conditions is known as soundness of aggregate. To disintegration by saturated solution of sodium sulphate or manganese sulphate.

• Concrete with increase workability without adding more water is called flowing concrete.

• M20 is the designation of concrete letter M refer to the mix and number 20 refer to the characteristic strength of 15 cm cube after 28 days equal to 20N/mm. 

•  Bleeding of concrete   is said to occur when unreacted water in the mixed tends to rise to the surface of freshly placed concrete due to sedimentation of constituents of concrete. This produces continuous capillary pores which provides a clear straight access to chemicals and deleterious material in concrete and lowers the strength and workability of concrete.

•    Compaction factor is defined as the ratio of mass of partially compacted concrete to the mass of the fully compacted concrete.

• Concreting shall not be done during when the temperature falls below 4.5⁰ C. In cold weather, the concrete placed shall be protected against frost. During hot weather, It shall be ensured that the temperature of wet concrete does not exceed 38⁰ C.

•  The nominal tensile stress in concrete when cracking occurs in a flexure test measured by performing a flexure test on un-reinforced concrete beam of specified size and span considering concrete to be homogeneous.