Concrete Materials and Placement
Concrete Materials and Placement
Questions in this domain cover sampling and testing, acceptance of strength results, environmental limits, and verification of placement and curing. This is the part of the job where numbers, not opinions, decide acceptance.
Sampling and test frequency (ACI 318)
- A strength test set is required at least once per day of placement, once per 110 m³ of concrete, and once per 460 m² of slab or wall surface area.
- Sample per ASTM C172; slump per C143; air content per C231 (pressure) or C173 (volumetric).
- Standard-cured cylinders govern acceptance; field-cured cylinders are for form and shore removal only.
Acceptance criteria
- Average of any three consecutive strength tests must equal or exceed f'c.
- No single strength test may fall below f'c − 3.5 MPa.
- A failing result triggers an investigation; cores may follow, but only under the code's investigation procedure.
Environment and curing
- Hot weather (ACI 305): concrete temperature at placement is typically limited to 35 °C; cool aggregates, use ice, avoid long waits. Discharge is generally within 90 minutes or 300 drum revolutions.
- Cold weather (ACI 306): protect against freezing and keep concrete above about 5 °C until it reaches 3.5 MPa.
- Curing (ACI 308): moisture and temperature maintained for the duration the specification requires.
Worked example
f'c = 30 MPa. Three consecutive 28-day tests give 33.5, 31.0 and 26.0 MPa. The average is 30.17 MPa and passes the first criterion. The second criterion is f'c − 3.5 = 26.5 MPa; 26.0 MPa is below it, so the results do not comply even though the average passes, and an investigation is required.
Traps
- Using field-cured cylinders as acceptance results.
- Averaging individual cylinders instead of averaging three test results.
- Accepting a low slump by adding water at the site; that changes the water-cementitious ratio and was not approved.
- Overlooking air content where the member has a freeze-thaw exposure class.