Concrete Materials, Mixture Proportions, and Specifications
Concrete materials, mixture proportions, and specifications is the domain where the exam asks you to link what is batched to what the project documents require. Questions center on cement and supplementary cementitious material (SCM) identification, aggregate and admixture conformance, water-to-cementitious materials ratio (w/cm), and exposure-related limits in ACI 318 and ACI 301.
Materials and standards
- ASTM C150 portland cement types: I (general), II (moderate sulfate), III (high early), V (high sulfate resistance). Do not substitute Type I for Type V on a sulfate exposure without approval.
- SCMs: ASTM C618 fly ash (Class F, Class C), C989 slag cement, C1240 silica fume. All count toward total cementitious content.
- Aggregates to ASTM C33; chemical admixtures to C494; verify certificates and mill test reports against the specification.
Proportions
w/cm = total water ÷ total cementitious material. Total water includes batch water, free moisture on aggregate, and water contributed by admixtures - not water absorbed inside aggregate. ACI 211.1 provides the proportioning procedure; ACI 318 exposure classes F, S, W, and C set maximum w/cm and minimum f'c.
Worked example
A mix contains 564 lb cement and 141 lb fly ash, with 300 lb of batch water. Total cementitious = 705 lb, so w/cm = 300 ÷ 705 = 0.426. If the specification caps w/cm at 0.45, the mix complies. Excluding the fly ash gives 300 ÷ 564 = 0.532 and a false failure; the SCM is cementitious for this calculation.
Traps
Ignoring SCM in the cementitious total; mixing up absorption and free moisture; forgetting that a project specification can be stricter than ACI 318; assuming all portland cements have the same sulfate resistance; and accepting a batch ticket that does not show aggregate moisture corrections.