403A Concrete Formwork 4 — Questions and Answers
Question 1: What is the function of a sleeper (or mud sill) under formwork shoring?
- To distribute shoring loads over a larger area of the ground or supporting surface
- To provide a level base for setting forms to grade
- To prevent shoring from sinking in wet soil
- All of the above — distribution, leveling, and preventing sinking (Correct answer)
Correct answer: All of the above — distribution, leveling, and preventing sinking
Sleepers serve all three functions: they spread the concentrated point load from shoring posts over a larger bearing area, provide a level surface to plumb shores from, and prevent individual post bases from punching into soft or wet ground.
Question 2: What is the maximum water-cement ratio (W/C) permitted for concrete exposed to freeze-thaw cycles and deicing salts per CSA A23.1 Exposure Class C-XL?
- 0.40 (Correct answer)
- 0.50
- 0.45
- 0.35
Correct answer: 0.40
CSA A23.1 Class C-XL (severe exposure to chlorides and freeze-thaw) requires a maximum W/C ratio of 0.40. Lower W/C ratios produce denser, less permeable concrete that resists chloride penetration and freeze-thaw damage.
Question 3: What is the purpose of air entrainment in concrete exposed to Canadian winters?
- Microscopic air bubbles provide space for water expansion during freezing, preventing internal cracking (Correct answer)
- Entrained air reduces the concrete's weight for easier pumping
- Air bubbles improve the workability and slump of the mix
- Entrained air increases compressive strength
Correct answer: Microscopic air bubbles provide space for water expansion during freezing, preventing internal cracking
Water expands approximately 9% when it freezes. In non-air-entrained concrete, this expansion causes internal tensile stresses that crack the paste. Microscopic entrained air bubbles (5-8% by volume) provide relief spaces for the expanding water, preventing surface scaling and internal damage.
Question 4: What is the purpose of re-shoring after a concrete slab is poured on upper floors of a multi-storey building?
- To transfer the load of the fresh upper-floor slab back through lower floors to the foundation until the concrete gains strength (Correct answer)
- To support formwork during stripping on the upper floor
- To provide access scaffolding for finishing workers
- To prevent differential settlement between columns
Correct answer: To transfer the load of the fresh upper-floor slab back through lower floors to the foundation until the concrete gains strength
Young concrete gains strength over 28 days. Re-shoring extends the load path from fresh upper-floor pours through multiple lower floors to the foundation, preventing any single slab from being overloaded before it reaches adequate strength.
Question 5: What is a bulkhead (or end stop) in concrete formwork?
- A temporary form closure that defines the end of a concrete pour at a construction joint (Correct answer)
- The bottom plate of a wall form that anchors to the footing
- A solid concrete block used as a form support
- The vertical stiffening ribs on a prefabricated form panel
Correct answer: A temporary form closure that defines the end of a concrete pour at a construction joint
A bulkhead is a temporary form panel placed perpendicular to the formwork run to stop the concrete pour at a planned construction joint. After stripping, the bulkhead face defines the joint surface that must be prepared for the next pour.
Question 6: What PPE is mandatory when working with concrete formwork on a construction site in Canada?
- Hard hat, safety boots (CSA Grade 1), safety glasses, and appropriate gloves (concrete is caustic) (Correct answer)
- Only a hard hat and safety boots
- Hard hat and high-visibility vest only
- Gloves and safety glasses only when placing concrete
Correct answer: Hard hat, safety boots (CSA Grade 1), safety glasses, and appropriate gloves (concrete is caustic)
Provincial OH&S regulations require hard hats, CSA Grade 1 (or higher) safety footwear, and safety glasses for all construction site workers. Gloves are essential when handling concrete because fresh concrete is highly alkaline (pH ~12) and causes chemical burns to skin on prolonged contact.
403A Concrete Formwork 4
What is the function of a sleeper (or mud sill) under formwork shoring?