403A Concrete Formwork 2 — Questions and Answers
Question 1: What is a waler (wailer) in wall formwork and what does it do?
- A horizontal member running along the form face to distribute tie loads and stiffen the form panels (Correct answer)
- A vertical structural post supporting the form from the outside
- The bottom plate of a wall form anchored to the footing
- A diagonal brace preventing the form from tipping
Correct answer: A horizontal member running along the form face to distribute tie loads and stiffen the form panels
Walers are horizontal members (typically doubled 38×89 mm lumber) running along the outside face of form panels. They span between form ties, distribute the concentrated tie loads across the panel face, and prevent the panels from bowing between ties.
Question 2: What is the minimum concrete cover required over steel reinforcing bars for a foundation wall exposed to soil in Canada per CSA A23.1?
- 60 mm (Correct answer)
- 40 mm
- 25 mm
- 75 mm
Correct answer: 60 mm
CSA A23.1 requires a minimum cover of 60 mm for concrete cast against and permanently exposed to earth (such as foundation walls). This protects the steel from corrosion-causing moisture and soil chemicals.
Question 3: What is the purpose of vibrating fresh concrete after it is placed in formwork?
- To consolidate the concrete, eliminate voids and air pockets, and ensure it flows around reinforcing steel (Correct answer)
- To speed up the setting time
- To reduce the water-cement ratio after placement
- To level the top surface before screeding
Correct answer: To consolidate the concrete, eliminate voids and air pockets, and ensure it flows around reinforcing steel
Internal vibrators consolidate concrete by temporarily liquefying it, allowing air pockets and voids to escape and ensuring complete contact with form surfaces and encapsulation of reinforcing steel. Over-vibration can cause segregation.
Question 4: What is bleed water in fresh concrete and why is it a concern for surface finishing?
- Water that migrates to the surface as concrete settles — finishing too early traps it and weakens the surface (Correct answer)
- Water added at the job site to improve workability
- Excess water leaking through form joints
- Water used to cure the concrete surface
Correct answer: Water that migrates to the surface as concrete settles — finishing too early traps it and weakens the surface
Bleed water rises to the surface as the denser concrete particles settle. If finishers work the surface before bleed water evaporates, they drive water back into the top layer, increasing the water-cement ratio and creating a weak, dusty surface layer (surface scaling).
Question 5: What is the purpose of a construction joint in a concrete pour?
- A planned stopping point in a pour where fresh concrete will later be bonded to the hardened face (Correct answer)
- A joint that allows thermal movement between concrete sections
- A saw-cut joint to control cracking
- A joint filled with expansion material between independent slabs
Correct answer: A planned stopping point in a pour where fresh concrete will later be bonded to the hardened face
Construction joints are planned stoppage points where a pour ends and a subsequent pour begins. The hardened surface must be prepared (cleaned, roughened, and often treated with bonding agent or fresh concrete) to ensure proper bond with the next pour.
Question 6: When stripping (removing) wall forms after a concrete pour, what is the minimum concrete strength typically required before formwork is removed?
- Approximately 3.5 MPa (sufficient to resist damage from stripping forces) (Correct answer)
- Full design strength of 25 MPa
- Concrete must cure for exactly 24 hours regardless of strength
- No minimum — forms may be stripped as soon as the concrete is solid
Correct answer: Approximately 3.5 MPa (sufficient to resist damage from stripping forces)
Forms may be stripped once the concrete can support its own weight and resist damage from the stripping operation — typically around 3.5 MPa. This usually occurs within 24-48 hours depending on temperature and mix. Full design strength takes 28 days.
403A Concrete Formwork 2
What is a waler (wailer) in wall formwork and what does it do?