7.1 Concrete Materials, Mix Design, and Water-Cement Ratio
Key Takeaways
- Water-cement ratio = weight of water / weight of cement; lower w/c means higher strength and lower permeability (typical structural 0.40-0.50).
- ASTM C150 cement types: I normal, II moderate sulfate, III high early strength, IV low heat, V high sulfate resistance.
- f'c is the 28-day compressive strength in psi; common structural values run 3,000-5,000 psi.
- 1 cubic yard = 27 cubic feet; always convert slab thickness from inches to feet before takeoff and add 5-10% waste.
- Air-entrainment (ASTM C260) gives freeze-thaw resistance; never use chloride accelerators near reinforcing or prestressing steel.
Concrete is a composite of portland cement, coarse aggregate (gravel/crushed stone), fine aggregate (sand), water, and optional admixtures. On the NASCLA Accredited Commercial General Building Contractor exam, this is an open-book topic anchored in ACI 318, ASTM C150 (cement), ASTM C33 (aggregates), and the carpentry/masonry/concrete reference texts. Know where to find mix proportions and the water-cement ratio (w/c) fast.
Portland cement types (ASTM C150) are a recurring trap. Memorize the function, not just the number:
| Type | Name | Use |
|---|---|---|
| I | Normal | General construction |
| II | Moderate | Moderate sulfate, moderate heat |
| III | High early strength | Fast strength, cold-weather, fast-track |
| IV | Low heat | Mass concrete (dams) |
| V | High sulfate | Severe sulfate soils/water |
Trap: Type III gains strength fast but does not yield higher ultimate strength than Type I; it is finer-ground.
The water-cement ratio governs strength and durability. It is the weight of water divided by the weight of cement in the mix. Lower w/c = higher strength and lower permeability. Typical structural concrete uses w/c near 0.40 to 0.50.
Worked numeric: A mix has 564 lb of cement and 282 lb of water per cubic yard. w/c = 282 / 564 = 0.50. Adding 1 gallon of water (8.33 lb) raises it: (282 + 8.33) / 564 = 0.515 — strength drops. The exam trap: adding water for workability weakens concrete; use a water-reducing admixture instead.
Strength is specified as f'c, the 28-day compressive strength in pounds per square inch (psi). Common values: 2,500 psi (footings/residential), 3,000-4,000 psi (slabs, walls), 4,000-5,000 psi (columns, commercial structural).
Aggregate rule (ACI 318): maximum aggregate size must not exceed the smallest of: 1/5 the narrowest form dimension, 1/3 the slab depth, or 3/4 the clear spacing between reinforcing bars. Trap: oversized aggregate causes honeycombing and bridging between bars.
Admixtures modify fresh or hardened properties:
- Air-entraining (ASTM C260): adds 4-7% microscopic air bubbles for freeze-thaw resistance; reduces strength ~5% per 1% air.
- Water-reducing/plasticizer: maintains workability at lower w/c.
- Accelerator (e.g., calcium chloride): speeds set in cold weather — never use chloride accelerators near reinforcing steel or in prestressed work (corrosion). Trap question.
- Retarder: slows set in hot weather or for long hauls.
- Pozzolan (fly ash, slag): partial cement replacement, improves durability.
Yield and takeoff numerics appear often. Concrete is ordered by the cubic yard (CY) = 27 cubic feet.
Worked takeoff: A slab 40 ft x 60 ft x 4 in thick. Thickness = 4/12 = 0.333 ft. Volume = 40 x 60 x 0.333 = 800 ft^3. CY = 800 / 27 = 29.6 CY. Add ~5-10% waste -> order ~32 CY.
Trap: forgetting to convert inches to feet, or forgetting the 27 ft^3 per CY divisor, is the most common takeoff error on the exam.
A concrete mix contains 600 lb of cement and 270 lb of water per cubic yard. What is the water-cement ratio?
Which portland cement type (ASTM C150) is specified for concrete exposed to severe sulfate soils?
The Four Ingredients and Cement Types
Concrete = portland cement + water + fine aggregate (sand) + coarse aggregate (gravel/stone), often with admixtures. ASTM C150 cement types: Type I general, Type II moderate sulfate resistance, Type III high early strength, Type IV low heat (mass concrete), Type V high sulfate resistance. Aggregates should be clean, well-graded, and durable; the maximum aggregate size must not exceed 3/4 of the clear spacing between bars nor 1/3 of slab depth.
Water-Cement Ratio — The Master Variable
The water-cement (w/c) ratio controls strength and durability: lower w/c = higher strength and lower permeability. Typical structural concrete uses w/c ≈ 0.40–0.50. Worked example: a mix with 600 lb cement and a target w/c of 0.45 needs water = 0.45 × 600 = 270 lb (≈ 32.4 gallons, since water weighs 8.34 lb/gal). Adding water on site to ease placement raises w/c and destroys strength — the cardinal concrete sin.
Admixtures and Air Entrainment
Admixtures modify behavior: air-entraining agents create microscopic bubbles for freeze-thaw durability (target ~4–7% air for exterior exposure); water-reducers/superplasticizers boost workability without adding water; accelerators (e.g., non-chloride) speed set in cold weather; retarders slow set in hot weather. Chloride accelerators (CaCl₂) must be avoided in reinforced concrete — they corrode the rebar. Specified strength f'c (e.g., 3,000–4,000 psi) is verified at 28 days.
Common Exam Traps
- Trap: Adding water at the truck improves the pour. It raises w/c and cuts strength.
- Trap: More cement always means more strength regardless of water. w/c ratio governs.
- Trap: Using calcium chloride accelerator in reinforced concrete — it corrodes steel.
- Trap: Forgetting air entrainment for freeze-thaw exposure.
A concrete mix contains 560 lb of cement and is designed for a water-cement ratio of 0.50. How much water (by weight) is required?
Yield, Slump, and Ordering a Worked Quantity
Concrete is ordered by the cubic yard (27 ft³) and the slump (Ch. 7.3) is specified for the placement. Worked yield example: a footing 2 ft × 2 ft × 50 ft = 200 ft³ = 200/27 = 7.4 CY; with a 5–10% waste allowance, order 8 CY. Mix designs also balance strength, durability, and economy — a leaner mix saves cement cost but a too-lean mix or excess water fails the f'c acceptance test, so the supplier issues a batch ticket documenting the actual proportions for the inspector.