Concrete Materials, Mix Design, and Prestressed Concrete Properties
Domain 3 tests concrete that can carry high compression at transfer and then behave predictably over time. Expect mix-design, curing, cylinder testing, and camber questions.
Strength at transfer and at 28 days
ACI 318 sets a floor of 3,000 psi for f'c in prestressed members and 3,500 psi for f'ci at transfer. Design strengths are commonly 5,000–8,000 psi, with release at roughly 0.70–0.80 f'c. The release strength is a design value, not a suggestion.
Mix design
Low water/cementitious ratio, high-range water reducer, well-graded aggregate, and air entrainment where exposed. Steam curing must follow a controlled temperature rise and limit; rapid heating or cooling damages the surface.
Time-dependent behavior
Elastic shortening, creep, shrinkage, and steel relaxation change camber and prestress over time. Low-relaxation strand reduces relaxation; matched curing and realistic release strengths reduce creep and camber surprises.
Worked example
A beam is designed for f'c = 6,000 psi with release at 0.75 f'c = 4,500 psi. Cylinders cured beside the member break 4,200 psi at the planned detensioning time. That is below the required f'ci, so detensioning is held, curing continues, and the member is retested. A standard-cured 28-day cylinder is irrelevant at this stage.
Traps
- Using standard-cured instead of field-cured cylinders for release.
- Confusing f'c with f'ci.
- Sampling concrete from a location that does not represent the member.
- Measuring camber before the member is fully detensioned and stable.