Concrete Formwork Flashcards

What is the cure time for structural concrete to reach its standard design strength (f'c) per CSA A23.1?

Answer: 28 days under standard curing conditions

Concrete compressive strength is specified at 28 days (f'c at 28 days) under standard moist curing at 23°C. This is the reference design strength used in structural calculations. Concrete continues to gain strength beyond 28 days but more slowly.

What is the purpose of a fog cure or wet burlap cover applied to a concrete slab after finishing?

Answer: To maintain moisture so the cement can continue hydrating and gain strength

Curing maintains moisture in the concrete so the cement hydration reaction continues. If the slab dries out too quickly, hydration stops prematurely and the concrete does not reach its design strength. Wet burlap or curing compounds retain moisture during the critical early curing period.

What is the minimum compressive strength (f'c) commonly specified for residential foundation concrete in Canada?

Answer: 25 MPa

The NBC and CSA A23.1 require a minimum f'c of 25 MPa for concrete in residential foundations exposed to soil. Higher strengths (30-35 MPa) are required for more severe exposures such as high sulfate soils or exposure to deicing chemicals.

What is the purpose of a void (slip) form in concrete construction?

Answer: A continuously moving form that allows concrete to be placed and set while the form moves vertically or horizontally

Slip forming is a continuous concreting method where the form is slowly jacked or pulled while concrete is continuously placed. Used for tall structures (silos, towers, bridge piers), it allows continuous pours without construction joints by advancing the form as concrete gains initial set.

When calculating the lateral pressure on wall forms, which factor does NOT significantly affect the maximum pressure?

Answer: The colour of the concrete

Concrete colour has no effect on lateral pressure. The key factors are pour rate (faster = higher pressure), temperature (colder concrete sets more slowly = higher pressure accumulates), unit weight of concrete (heavier = more pressure), and form height (more height = more hydrostatic head).

Under provincial OH&S regulations in Canada, what must be done before workers enter an excavation deeper than 1.2 m?

Answer: The excavation must be shored, sloped, or otherwise protected against collapse, and inspected by a competent person

All Canadian provincial OH&S regulations require excavations over 1.2 m deep to be protected against cave-in by shoring, sloping, benching, or a combination. A competent person must inspect the excavation before workers enter and after any event (rain, vibration) that could affect stability.