Concrete

As you know concrete is prepared by thoroughly mixing aggregate, sand, cement or lime mortar and water. Concrete is designed for specified compressive strength by suitable proportioning of the constituent elements, grading of the aggregate and quantity of water. Proper compaction  and curing are also very important to attain the specified strength. 

Cement Concrete 

It is prepared by mixing graded stone or brick aggregate with fine aggregate and cement in specified proportions with required quantity of water. The proportion gradmg and quality of materials should be such that the specified strength IS attuned by the concrete. One sample of six cubes 15 cm x 15cm x 15 cm should he taken for every 15 cum of concrete and tested after 7/28 days. 

i) Proportioning: It should be done by weight or volume as specified. Boxes of suitable size should he used. A hag of 50 kg. cement should he taken as 0.035 cubic metre. For wet sand bulkage should he considered. 

ii) Preparation: Materials of specified proportion should be mixed in a mechanical batch type concrete mixer fitted with power loader conforming to IS 170 1- 1985. Hand mixing may be allowed in special circumstances with addition of 10%- extra cement. Before mixing brick aggregate should be well soaked with water for at least two hours. Mixing time in a mixer may be 1.5 to 2 minutes. 

iii) WorkabiIity: The quantity of water to be used for each mix should be such that the concrete is of adequate workability for the placing conditions of the concrete and could properly be compacted with the means specified. Regular slump tests should be carried out and water quantity should be regulated accordingly. For concreting of lightly reinforced section with vibrator slump upto 25 mm should he attained.  For heavily reinforced section slump of 25 mm-75 mm should be provided. 

iv) Transportation: Concrete should be transported from the mixer to the place of laying as quickly as possible avoiding segregation or loss of any of the ingredients 

and maintaining the required workability. 

v) Placing:  Concrete should be deposited as nearly as practicable in its final position to avoid re handling. It should be gently laid, not thrown. It should be thoroughly vibrated and compacted before setting commences and should not be sequentially disturbed. Segregate should be avoided. 

vi) Compaction:Concrete should be thoroughly compacted and fully worked around embedded fixtures and into corners of form work. Compaction should he done by mechanical vibrator of appropriate type till a dense concert is obtained. The vibrator should conform to IS 2505-1980. Over vibration should be avoided to prevent seggregation. Hand compaction should be done by tapping rods only In exceptional cases. Compaction should be completed before starling to initial setting. After compaction the top surface should be finished even and smooth with wooden trowel before the concrete begins to set. 

vii) Construction Joints: Concreting should he carried out upto construction joints as shown in the drawing or as directed by the engineer-in-charge. Number of such joints should be kept minimum. In case concreting has to be resumed on a surface that has hardened, then it should be roughened, swept clean and thoroughly wetted. For vertical joints cement slurry using 2 kg. of cement per sq. metre should be applied on the surface before concreting. For horizontal joints, the surface should be covered with a layer of kept continuously damp by pounding or by covering with a moist layer of material at least for 7 days. Curing should be extended for ten days where portland pozzolona cement is used. 

Approved curing compounds may be used in lieu of moist curing with the permission of the engineer-in-charge. Such compounds should be applied to all exposed surfaces of the concert as soon as possible after the concrete has set. 

Freshly laid concrete should be protected again rain by suitable covering. Over the foundation concrete masonry work may be started after 48 hours of its compaction but curing should he continued at least for 7 days. Similarly over the base concrete floor concrete may be laid early but full curing should be done.