CSLB General Building (B)Site Work, Foundations and ConcreteHard
A contractor is designing formwork for a concrete wall pour and increases the rate of concrete placement significantly to speed up the job. What effect does this faster placement rate have on the formwork design requirements?
- AIt has no effect on lateral pressure since concrete weight is constant
- BIt increases the lateral pressure on the forms, requiring stronger bracing and tie spacing
- CIt only affects the top of the form, not the base
- DIt decreases the lateral pressure on the forms, allowing lighter bracing
Show answer & explanationAnswer & explanation
Correct answer: B. It increases the lateral pressure on the forms, requiring stronger bracing and tie spacing
Faster placement rates cause concrete to remain in a fluid, unset state over a greater height of formwork before the lower layers begin to set, increasing the lateral (hydrostatic-like) pressure exerted on the forms. This requires stronger form panels, more closely spaced wall ties, and additional bracing to prevent blowouts.
Why the other options are wrong
- A. Placement rate is a major factor in lateral pressure calculations, not a negligible one.
- C. Lateral pressure increases throughout the height of unset concrete, not just at the top.
- D. Faster pours increase, not decrease, pressure on the forms.
Formwork Lateral Pressure
The pressure fresh concrete exerts on formwork increases with faster placement rates, higher slump, and lower temperatures, requiring adequately designed bracing and ties.
- Faster pour rate = greater unset concrete height = higher pressure
- High slump and cold temperatures also increase lateral pressure
- Formwork must be designed for peak lateral pressure, not just concrete weight
Memory trick: Pour faster, push harder — the forms feel more fluid force.