11.2 Concrete: Formwork, Reinforcement & Placing

Concrete Materials

Concrete is a composite of four ingredients. For commercial construction, ACI (American Concrete Institute) 318, Building Code Requirements for Structural Concrete, is the governing standard, referenced by the 2018 IBC.

• Cement — typically Portland cement (ASTM C150), the binder that reacts with water (hydration) to glue the mix together. Types I, II, and V are common; Type II is moderate sulfate resistant, useful in Arizona soils.
• Aggregates — fine (sand, ASTM C33) and coarse (gravel or crushed stone). Aggregates occupy 60–80% of the volume and dominate strength, shrinkage, and cost.
• Water — clean and potable; the water-cement ratio (w/c) is the single biggest strength driver. A lower w/c (0.40–0.45) yields higher strength and durability.
• Admixtures — chemicals or minerals added to modify properties: air-entraining (freeze-thaw resistance, not typically needed in Phoenix but used in northern AZ), water reducers (plasticizers), accelerators (calcium chloride for cold weather), retarders (hot weather), and fly ash or slag (supplementary cementitious materials).

Slump and Strength

Slump (ASTM C143) measures consistency — how wet or workable the concrete is, not how strong it will be. A 4-inch slump is typical for slabs; a 2–3 inch slump is used for footings. Adding water on site to increase slump is the most common (and most damaging) field error — every extra gallon of water per yard lowers strength and durability. Use a water-reducing admixture instead.

Compressive strength (f'c) is measured in psi (pounds per square inch) at 28 days by breaking cylindrical specimens (ASTM C39). Typical concrete for commercial construction ranges from 3,000 to 5,000+ psi, depending on the structural element.

Reinforcement

Reinforcing steel (rebar) resists tension — concrete itself is strong in compression but weak in tension. Rebar is graded by minimum yield strength per ASTM A615:

• Grade 40 — 40,000 psi yield (older, less common)
• Grade 60 — 60,000 psi yield (the modern standard, marked with a "60")
• Grade 75 — 75,000 psi yield (commercial / heavy structural)

Bar sizes run from #3 (3/8 inch diameter) to #18. Common bar sizes include #4 and #5 for walls and slabs, with #3 for light slabs and larger bars (#6 and up) for heavy structural members. Spacing typically ranges from 12 to 24 inches on center.

Cover — the distance from the rebar surface to the concrete face — protects steel from corrosion and fire. ACI 318 specifies minimum cover:

Formwork Design and Removal

Formwork holds plastic concrete in shape until it cures. ACI 347, Guide to Formwork for Concrete, provides industry guidance on formwork design and practices. Forms must be:

• Strong — to carry the hydrostatic pressure of wet concrete (full liquid head at the bottom).
• Braced — to resist wind, construction loads, and vibration from placing.
• Tight — to prevent mortar leakage that causes honeycombing.
• Coated — with a release agent so forms strip cleanly without spalling the surface.

Form removal timing depends on strength gain, member type, and load. Non-load-bearing forms (wall sides, slabs under 10 feet span) can usually be removed in 24–48 hours. Load-bearing forms (beams, slabs supporting loads) must stay until concrete reaches a specified percentage of f'c — often 70% — verified by field-cured cylinders. Forms should not be removed until the concrete is sufficiently strong to carry its own weight and any construction loads placed on it.

Placing and Consolidation

Placing deposits concrete without segregation (separation of paste from aggregate). Drop concrete as close to its final position as possible; vertical free-fall should not exceed 5 feet to avoid segregation. Place in layers 12–18 inches deep and vibrate each layer.

Consolidation (vibration) removes trapped air. An internal vibrator (stinger) is the standard tool. Insert vertically, penetrate 1–2 inches into the previous layer, withdraw slowly, and never use the vibrator to move concrete laterally — that causes segregation.

Curing Methods and Timing

Curing maintains moisture and favorable temperature so hydration continues. Concrete gains strength rapidly in the first 7 days and continues for 28 days and beyond. Curing should begin as soon as the surface will not be damaged. Minimum 7 days of moist curing is standard for normal concrete.

Quick Answer: Cure concrete for at least 7 days by keeping it moist; strength is specified at 28 days.

Hot-Weather Concreting (Arizona-Critical)

Hot weather (ACI 305R) is defined by high ambient temperature, low humidity, and wind — the default Arizona condition from April through October. Hot weather accelerates set, increases water demand, and promotes plastic shrinkage cracking before final set. Controls include:

• Chilled water or ice to keep concrete temperature below 90°F at placement.
• Sun shades and windbreaks over the placement area.
• Evaporation retarder (monomolecular film) on the fresh surface during finishing.
• Retarding admixtures to extend workability.
• Rapid sequence — place, finish, and cure quickly; have enough crew and tools on hand.
• Night placement — common for large Arizona pours.

Common Defects

• Honeycombing — voids left by inadequate consolidation or mortar leakage. Fix by vibrating properly and sealing forms.
• Cracking — from plastic shrinkage (hot weather), drying shrinkage (inadequate curing), settlement, or thermal movement. Control joints manage shrinkage cracks.
• Spalling — surface flakes off, usually from freeze-thaw or corroding rebar expanding. Adequate cover and air entrainment (where needed) prevent it.
• Scaling — surface loses thin layers, often from over-finishing or deicing salts.