The physical properties of concrete depend upon the mix ratio of three key ingredients. Builders proportion, measure and combine water, cement and aggregate are by volume or by weight to produce a workable cement. A basic mortar mixture comes from using one part water, two parts cement, and three parts sand. Mixing concrete involves combining one part cement, two parts sand and three parts gravel into a dry mix. You slowly add water until the concrete is neither too stiff nor too sloppy for its intended use.
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Water is Key
In any concrete mix, the key ingredient is water. Too much water produces weak concrete that may fail to withstand the forces that will be placed upon it. Too little water in the mix makes concrete sluggish and difficult to spread. Ready-mix companies add water reducing agents to the mix water to improve workability in low water-to-cement ratios. The chemicals allow concrete to be mixed with less water, thereby strengthening the final product.
The water-to-cement ratio in a concrete mix is calculated by dividing the water in one cubic yard of the mix (in lbs.) by the cement in the mix (in pounds). If one cubic yard of the mix contains 107kg. of water and 213kg. of cement, the mix has a .50 water to cement ratio. Water permeability increases exponentially in water cement ratios that exceed .50. Lower water-cement ratios improve both durability and strength.
1997 Uniform Building Code
Water-to-cement ratios change with how and where the concrete will be used. If concrete will be exposed to repetitive freezing and thawing when it is wet or if deicing chemicals will be used on the concrete surface, the 1997 Uniform Building Code mandates a mixture with a water-to-cement ratio no greater than .50. Strength improves with lower ratios. A .45 ratio is able to withstand 2041kg./square inch of load while a .50 ratio supports only 4000 psi.
One cubic yard of concrete will fill an area of 80 square feet to 4 inches in thickness. The same cubic yard will provide a five-inch thick base to 65 square feet or a six-inch base to 52 square feet. One cubic yard of concrete will require 5 1/2 bags of cement, 17 cubic feet of sand and 18 cubic feet of gravel or forty 36.3kg. bags of ready-mix. For smaller batches, substitute buckets for cubic feet. ( i.e. 1 bucket of cement, 3 buckets of sand, 3 buckets of stone, and 1/2 bucket of water.)
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