Concrete is composed of three main materials:
1.) Aggregate (Coarse & Fine)
2.) Water
3.) Cement
The Chemical Reaction:
The three materials (aggregate, water, and cement) are mixed together to form concrete. A chemical reaction between the cement and water cause the concrete to harden through a process called hydration. This reaction starts immediately and continues over the life of the concrete. It may only take the concrete a few hours to harden, but it typically takes seven days for concrete to reach its designed strength.
How is Concrete Strength Measured?
Concrete strength is measured by the amount of compressive force that can be placed on the slab before it breaks. Concrete mix designs range from 2,500psi – 4,500 pounds per square inch (psi) for residential and commercial projects. The average driveway typically uses 3,500psi concrete, however, it is worth noting that a mix designed at 3,500psi will usually reach closer to 4,000psi after 28 days due to overdesign by the concrete supplier. Engineers have been known to specify concrete mix designs all the way up to 10,000psi for commercial and industrial projects. The concrete strength varies depending on the amount of cement that is mixed in the concrete.
Mix Design Variables:
There are an infinite number of concrete mix designs one can create, resulting in a concrete with the desired characteristics of the project. The differences in a mix depends mainly on the water/cement ratio, aggregate size, and included additives. Additives are materials such as accelerators, retarders, water reducers, air entrainers, and fly ash among others.
Concrete Mix Design (Data Sheet):
Below is an example of a concrete mix design data sheet. The mix design data sheet lays out exactly how the concrete was made and what materials are included in the concrete.
| 3,500psi Interior Slab on Grade | |||||
|---|---|---|---|---|---|
| Material | Source | Description | ASTM | Spec. Gravity | Weight (lb) |
| Sand | TriStar Industries | TriStar Valley Sand | C-33 | 2.63 | 1530 |
| Coarse Aggregate | TriStar Industries | TriStar Valley Aggregate 57 | C-33 | 2.68 | 1840 |
| Cement | TriStar Cement | TriStar Cement | ASTM C | 3.15 | 510 |
| Water | TriStar Industries | Water | C-94 | 1.00 | 250 |
| Specified f’c: | 3,500 psi | Designed Unit Weight: | 148.6 lbs./cu.ft. | ||
| Specified Slump: | 3 to 5 inches | Designed W/C + P Ratio | 0.49 | ||
| Specified Air: | 1 to 3 % | Designed Volume | 27.47 cu.ft. | ||
| Designed Air: | 2% | ||||
