| Topic : Residential construction & remodelling |
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Construction Planning & Management
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Activity:
67 views;
last activity : 07 06 2010 20:18:09 +0000
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Cement
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Calcium Chloride
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Concrete on Frozen Ground
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I have heard in many places that You should specify a concrete mix by number of bags of cement. Some in the concrete business still call out concrete mixes based on the number of bags of cement (a 6-bag mix, or a 7-bag mix), but bag counts don't accurately describe the desired properties of the concrete. The traditional standard, in accordance with ASTM C 150 “Standard Specification for Portland Cement.” But how much cement is necessary to get high-quality concrete? It depends on the intended use. To maintain economy and avoid adverse effects on workability, shrinkage, and internal temperature rise, high cement contents should be avoided. Minimum cement contents are often specified to improve durability, finishability, wear resistance, or appearance (of vertical surfaces). The most important property of concrete starts with the water-to-cementitious materials ratio.
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The concrete technology is under going a lot of R & D with the result that the focus has shifted in favour of durability and cost consideration.The use of FLY ASH /Granulated blast furnace slag is being considered to produce a densar concrete enhancing durability by reducing porosity resulting in a more cohesive /homogenous mix as well in effecting economy of concrete production with reduced cement content.Polymar modified concrete and glass fiber concrete is also being specified for higher grade concrete with tangible advantages over conventional concrete production .
Making cocnrete with 6 bag and 7 bag is an outdated thing. IT MAY BE DONE FOR SMALL JOBS. FOR BIG PROJECTS WHERE HUGE QUANTITY OF CONCRETING IS INVOLVED DESIGN MIX COCNCRETE IS TO BE USED. DESIGN MIX CONCRETE WILL TAKE CARE OF ALL PARAMETERES OF STRENGTH REQUIRED, WORKABLITY ETC. IN DESIGN MIX CONCRETE QUANTITIES OF EACH MATERIALS I.E CEMENT SAND, AGGREAGTE AND WATER ARE FIXED AND THESE ARE MIXED THROUGH AUTOMATIC BATCHING PLANTS. THIS WAY WE ENSURE ECONOMY AND QUALTY CONTROL AND DEFINELTLY CONCRETE IS MUCH MORE DURABLE.
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Many of them think that Calcium chloride is an antifreeze agent, but it is an accelerator only and not an antifreeze. Accelerators speed up the rate of hydration and strength development of concrete at early ages. Fresh concrete, however, still needs protection from freezing at least until the concrete reaches a minimum strength of 500 psi. Without that initial protection, concrete that freezes will have significantly reduced strength. To avoid cold weather placement problems, make sure the concrete is properly protected and that it's temperature is maintained....
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We think that Concrete can be placed on frozen ground without any precautions but Precautions must be taken to protect concrete and prevent future soil problems in adverse weather conditions. Concrete placed on a frozen subgrade may settle unevenly when the subgrade thaws, leading to cracking. The difference in temperature between the frozen soil and the warm concrete can also cause rapid cooling of the concrete and retard the rate of hardening. Ideally, the soil temperature should be as close as possible to the concrete temperature when placed. There are methods available to thaw the ground and allow the placement of concrete, including curing blankets and hydronic heating systems.
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