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Concrete Construction matter
T. Pauly, M. J. N. Priestley
Abstract
Viewed in terms of accepted practices, concrete construction operations leave much to be desired with respect to the quality, serviceability, and safety of completed structures. The shortcomings of these operations became abundantly clear when a magnitude 7.6 earthquake struck northern Paki-stan on October 8, 2005, destroying thousands of buildings, damaging bridges, and killing an esti-mated 79,000 people. The unusually low quality of construction operations prevalent was a major cause of the immense devastation.
Keywords: Concrete Placing Curing Construction Technology
Placing Concrete
If concrete is placed in the surface, the sur-face should be filled with water sufficiently to prevent it from absorbing the concrete of its water. If fresh concrete is to be placed on or nearby to concrete that has solidified, the surface of the placed concrete should be cleaned absolutely, preferably with a high-pressure air or water jet or steel-wire brushes. The surface should be wet, but there should be no much water. A little quantity of cement grout should be brushed over the whole area, and then followed immediately with the application of a 1/2-in Layer of mortar. The fresh concrete should be placed on or against the mortar.
In order to decrease the disintegration re-sulting from carriage after it is placed. The con-crete should be placed as nearly as probably in its
final point. It should be placed in layers to permit uniform compaction. The time interval between the placing of layers should be limited to assure perfect bond between the fresh and previously placed concrete.
In placing concrete in deeper patters, a ves-sel should be used to limit the free fall to not over 3 or 4 ft, in order to prevent concrete disintegra-tion. The vessel is a pipe made of lightweight metal, having adjustable lengths and attached to the bottom of a hopper into which the concrete is deposited. As the patters are filled, sections of the pipe may be removed.
Immediately after the concrete is placed, it should be compacted by hand pudding or a me-chanical vibrator to eliminate voids. The vibrator should be left in one position only long enough to reduce the concrete around it to a plastic mass; then the vibrator should be moved, or disintegra-
tion of the aggregate will occur. In general, the vibrator should not be permitted to penetrate concrete in the prior lift.
The mainly advantage of vibrating is that it permits the use of a drier concrete, which has a higher strength because of the reduced water content. Among the advantages of vibrating con-crete are the following:
1.The decreased water permits a reduction in the cement and fine aggregate because less cement paste is needed.
2.The lower water content decreases shrinkage and voids.
3.The drier concrete decreases the cost of finishing the surface.
4.Mechanical vibration may replace three to eight hand puddles.
5.The lower water content increases the strength of the concrete.
6.The drier mixture permits theremoval of some patters more quickly, which may reduce the cost of patters.
Curing Concrete
If concrete is to gain its maximum strength and other desirable properties, it should be cured with adequate moisture and at a favorable tem-perature. Failure to provide these conditions may result in an inferior concrete.
The initial moisture in concrete is adequate to hydrate all the cement, provided it is not should replace the moisture that does evaporate. This may be accomplished by many methods, such as leaving the patters in place, keeping the surface wet, or covering the surface with a liquid curing compound, which comes being to a water-tight membrane that prevents the escape of the initial water. Curing compounds may be applied by brushes or pressure sprayers. A gallon will cover 200 to 300 sq ft.
Concrete should be placed at a temperature not less than 40 or more than 80°F.A lower tem-perature will decrease the rate of setting, while a
higher temperature will decrease the ultimate strength.
Placing Concrete in Cold Weather
When the concrete is placed during cold weather, it is usually necessary to preheat the water, the aggregate, or both in order that the ini-tial temperature will assure an initial set and gain in strength .Preheating the water is the most ef-fective method of providing the necessary tem-perature. For this purpose a water reservoir should be equipped with pipe coils through which steam can be passed, or steam may bedischarged directly into the water, several outlets being used to given better distribution of the heat.
When the temperatures of the mixtures are known, some specific charts may be used to cal-culate the temperature of concrete. A straight line pass all three scales, passing through every two known temperatures, will assure the determina-tion of the third temperature. If the surface of sand is
