Concrete and Structural Systems
Core concepts
Concrete is strong in compression, weak in tension; reinforcement carries tension. Understand water-cement ratio as the primary driver of strength and durability - lower ratio, higher strength, less permeability. Know slump as a workability test, air entrainment for freeze-thaw resistance, curing as moisture and temperature control, and consolidation by vibration to remove voids.
Structural systems questions cover formwork (pressure, shoring, reshoring), placement (avoid cold joints, limit free fall), finishing (floating, troweling, joints), and inspection (cylinder breaks, rebar cover, embedment). Know control joints (planned cracks) versus construction joints (placement boundaries).
Traps to watch
- Adding water on site to improve workability - this silently raises the w/c ratio and lowers strength.
- Confusing curing with drying; curing keeps moisture in for hydration.
- Forgetting cover requirements over rebar for corrosion and fire protection.
- Treating cylinder break results as valid when curing or handling was improper.
- Assuming a control joint stops all cracking - it only localizes it.
Rule of thumb
If slump is low, fix it with admixtures or mix redesign, never with a hose. A rough check: each additional gallon of water per cubic yard adds roughly a few points of slump but costs strength - treat field-added water as a red flag.
Worked example
A pour is scheduled at 4,000 psi with a max w/c of 0.45. The crew wants to add 5 gallons per yard to speed finishing. That raises the effective w/c and reduces strength and durability. Correct response: reject the addition, contact the supplier for a plasticizer, and document the decision.