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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Specifying Ready-Mix
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
Percentages of cement, water, air, sand, and gravel in concrete mixes. Rich mixes contain higher percentages of cement. Air-entrainment introduces tiny air bubbles that allow the concrete to flow easier with less water, which makes for a stronger mix. The trapped bubbles also absorb minor expansion and contraction, allowing the concrete to better resist freeze/thaw damage.
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 B
Cement Types
Figure C shows the five standard types of cement in use today.
Figure C
Concrete Types I, II, and III will meet most residential needs. For freeze-thaw durability, order air-entraining cement or an air-entraining admixture.
Aggregate
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
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.
Water/Cement Ratio
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 D
The more water added to a concrete mix on site, the weaker it will become, and the more it will shrink and crack. Adding 2 gallons per yard will cut compressive strength by around 10% and will increase shrinkage by close to 20%.
Figure E: Concrete Slump
A slump test uses a standard cone — 12 in. high, 8 in. wide at the base, and 4 in. wide at the top. To perform the test, fill the cone in one-third lifts and “rod it” (churning by moving a piece of rebar up and down) 25 times between each lift. Remove the cone and measure the distance from the height of the cone to the height of the slumped concrete. Residential concrete should slump no more than 4 in.
Air Entrainment
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.
Figure F
*Air content is specified as a percentage by volume of concrete. For severe exposure conditions, air content of the mortar alone (cement paste and sand) should be about 9%. Lower air content percentages for concrete with large aggregate reflect the fact that less mortar is needed for mixes that contain large gravel.
These levels of air-entrainment shown for different climate exposures are minimums. Higher air amounts are permissible as long as the design strength is maintained. Refer to Figure G for exposure locations.
Figure G
“Severe” and “Moderate” exposures are determined based on the likelihood that de-icing salts will be used at a given location. Check local climate data because icing conditions may vary locally with altitude.
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.
