If you understand the
fundamentals, you will be better able to keep
costs down, improve the
strength of your concrete,
and minimize cracking.
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Depending on the needs of the project, ready-mix suppliers can provide hundreds of different concrete mixes. In general, it’s a good idea to tell your supplier what the concrete will be used for, and follow the supplier’s recommendations for the appropriate mix. However, a builder should understand the way mix adjustments affect the concrete’s properties.
Figure A: Percentages of cement, water, air, sand, and gravel
At a minimum, concrete specifications will usually call out the compressive strength and water/cement ratio (Figure B), as well as the slump (Figure E). Concrete mixes can vary in the type and quantity of cement, the ratio of water to cement, the percentage of entrained air, and the size and grading of aggregates. You may also want to order concrete with various admixtures for special circumstances (see Admixtures, below).
Figure C shows the five standard types of cement in use today.
Sand and gravel are the strongest and cheapest ingredients in concrete. It is most economical to use aggregate that is large and well-graded (containing a good proportion of various sizes from large to small), because this reduces the required volume of cement paste. If reinforcing steel will be spaced close together, or if concrete must be pumped, maximum gravel sizes may have to be reduced.
Using more fine sand makes a concrete mix “creamier” and makes it easier to achieve a smooth finish; however, the mix will require more water, and therefore should have more cement added for adequate strength.
Water used to mix concrete should be clean enough to drink. Adding water to a concrete mix can weaken concrete. Follow water/cement ratio guidelines below.
The ratio of water to cement should be strictly controlled to ensure that the concrete reaches the specified strength (Figure D).
On site, add the minimum amount of water needed to make the concrete workable. More water makes the concrete easier to handle, but also makes it much weaker and more prone to shrinkage and cracking (Figure D).
Figure E: Concrete Slump
Air entraining creates billions of microscopic air voids in hardened concrete, which serve to absorb the pressures caused by expanding ice or de-icing salts. Most ready-mix suppliers today add an air-entraining admixture to a standard cement mix.
Air entrainment is crucial for exposed concrete in cold climates, but it is recommended for almost all concrete, even in mild climates, because it reduces water demand, improves workability, reduces segregation of aggregate, and reduces bleeding of excess water. Recommended entrained-air percentages for different weather exposures are shown in Figure F. Refer to the map in Figure G for exposure regions throughout the continental United States.
Finishing air-entrained concrete. A concrete finisher may wait for bleed water to evaporate before starting to trowel the surface, but when concrete is air-entrained, bleed water may not appear. Excess water should still be allowed to evaporate from beneath the surface for a time before troweling begins — otherwise, water may be trapped just below a hard surface skin and cause later scaling or flaking.