403A Concrete Formwork 1 — Questions and Answers
Question 1: What is the primary cause of formwork failure during concrete placement?
- Inadequate design for lateral fluid pressure of fresh concrete (Correct answer)
- Improper curing of the concrete
- Using too many form ties
- Incorrect placement of reinforcing steel
Correct answer: Inadequate design for lateral fluid pressure of fresh concrete
Fresh concrete behaves as a fluid and exerts lateral (hydrostatic) pressure on forms. If forms are not designed to resist this pressure — accounting for pour rate, concrete temperature, and height — they can fail catastrophically, causing injury and concrete waste.
Question 2: How does the rate of concrete placement affect lateral form pressure?
- Faster pour rates increase lateral pressure because the concrete doesn't set before more is added above (Correct answer)
- Faster pour rates decrease pressure because vibration dissipates energy
- Pour rate has no effect — only height determines pressure
- Slower pour rates increase pressure because the concrete has time to slump
Correct answer: Faster pour rates increase lateral pressure because the concrete doesn't set before more is added above
When concrete is poured rapidly, the weight of fresh concrete above adds to the fluid pressure on the forms before the lower concrete has time to stiffen and gain strength. Slow pour rates allow partial stiffening, reducing peak pressure.
Question 3: What is the purpose of form release agent (form oil) applied to formwork before a concrete pour?
- To prevent concrete from bonding to the form so it can be stripped cleanly (Correct answer)
- To waterproof the form for reuse in wet conditions
- To reduce concrete shrinkage during curing
- To lubricate the form ties for easier removal
Correct answer: To prevent concrete from bonding to the form so it can be stripped cleanly
Form release agents create a barrier between the concrete and the form surface. Without them, concrete bonds to the form — especially plywood — making stripping difficult and damaging both the concrete surface and the form for reuse.
Question 4: What is the function of snap ties (form ties) in wall formwork?
- To hold the two faces of the form apart at the correct wall thickness and resist the spreading force of concrete pressure (Correct answer)
- To connect form panels vertically for wall height
- To attach walers to the form panels
- To anchor the form to the ground against uplift
Correct answer: To hold the two faces of the form apart at the correct wall thickness and resist the spreading force of concrete pressure
Snap ties span between the two form faces and are held in tension by the lateral concrete pressure. They maintain the exact wall thickness while preventing the forms from spreading. After stripping, the tie ends are snapped off flush with the wall surface.
Question 5: What is the purpose of chamfer strips in formwork?
- To create beveled edges on concrete corners that resist chipping and improve appearance (Correct answer)
- To provide drainage channels for water during curing
- To separate concrete pours at construction joints
- To mark stud spacing on form panels
Correct answer: To create beveled edges on concrete corners that resist chipping and improve appearance
Chamfer strips are triangular wood inserts placed in form corners. They produce beveled (chamfered) edges on the finished concrete, replacing sharp 90° corners that would chip easily during stripping and service.
Question 6: At what minimum temperature must fresh concrete be maintained during cold weather concreting in Canada?
- 10°C for the first 3 days after placement (Correct answer)
- 5°C at time of placement only
- 15°C throughout the curing period
- 0°C — freezing is only harmful before initial set
Correct answer: 10°C for the first 3 days after placement
The CSA A23.1 standard and cold weather concreting guidelines require fresh concrete to be maintained at a minimum of 10°C for the first 3 days (72 hours) to ensure adequate hydration and strength gain. Freezing before the concrete reaches 3.5 MPa can permanently damage it.
403A Concrete Formwork 1
What is the primary cause of formwork failure during concrete placement?