Concrete Constituent Materials
Concrete consists of cementitious materials, water, fine aggregate, coarse aggregate, and admixtures. Portland cement is manufactured by heating limestone and clay to 1,450 C. Supplementary materials including fly ash and slag can replace a portion of Portland cement, reducing cost and environmental impact.
Aggregates occupy 60 to 80 percent of concrete volume. Coarse aggregate provides the structural framework. Maximum aggregate size is typically 3/4 inch for reinforced concrete. Fine aggregate fills voids between coarse particles.
Mix Proportioning
The water-cementitious materials ratio is the primary determinant of concrete strength. A w/cm ratio of 0.40 produces high strength while 0.60 is for general use. truss geometry and member sizes. corner bead protects outside corners. house wrap with drainage channels. The absolute volume method calculates the volume occupied by each ingredient based on specific gravity.
| w/cm | 28-Day Strength | Application |
|---|---|---|
| 0.40 | 5,500-6,500 psi | High-strength columns |
| 0.45 | 4,500-5,500 psi | Structural beams |
| 0.50 | 3,500-4,500 psi | General structural |
| 0.55 | 2,500-3,500 psi | Slabs, footings |
Chemical Admixtures
Water reducers decrease water content for a given workability. Superplasticizers can reduce water by 12 to 30 percent. Retarders slow setting in hot weather. Accelerators speed early strength development. Air-entraining admixtures create microscopic bubbles for freeze-thaw resistance.