General Contractor License Site Work and Concrete 4 — Questions and Answers
Question 1: What is the purpose of vapor retarder under a concrete slab-on-grade?
- To improve concrete bond to the subbase
- To prevent soil moisture from migrating up through the slab (Correct answer)
- To provide thermal insulation below the slab
- To distribute slab loads to the subgrade
Correct answer: To prevent soil moisture from migrating up through the slab
A vapor retarder (typically polyethylene sheeting) prevents ground moisture from wicking up through the concrete slab.
Question 2: Reinforcing steel cover in concrete refers to:
- The total diameter of the rebar bar
- The minimum distance from the rebar surface to the concrete surface (Correct answer)
- The lap splice length between bars
- The spacing between parallel bars
Correct answer: The minimum distance from the rebar surface to the concrete surface
Cover is the minimum thickness of concrete between the surface and the nearest reinforcing bar, protecting steel from corrosion and fire.
Question 3: What is 'blowout' in the context of concrete formwork?
- Air pockets trapped in the concrete during vibration
- Sudden failure of formwork under hydrostatic concrete pressure (Correct answer)
- Excess water bleeding to the top of fresh concrete
- Cracking due to thermal expansion during curing
Correct answer: Sudden failure of formwork under hydrostatic concrete pressure
Formwork blowout is a sudden structural failure caused by the lateral hydrostatic pressure of fresh concrete exceeding the formwork's capacity.
Question 4: In soil classification, 'cohesive' soils primarily include:
- Sand and gravel
- Clay and silt (Correct answer)
- Decomposed rock
- Well-graded crushed stone
Correct answer: Clay and silt
Cohesive soils such as clay and silt have particles that stick together due to electrochemical forces rather than friction.
Question 5: What does 'compaction factor' refer to in earthwork calculations?
- The ratio of wet to dry unit weight of soil
- The ratio of compacted fill volume to the bank (in-place) volume from which it came (Correct answer)
- The percentage of air voids eliminated during compaction
- The proctor density as a percentage of maximum dry density
Correct answer: The ratio of compacted fill volume to the bank (in-place) volume from which it came
The compaction factor (or swell/shrinkage factor) accounts for volume changes when soil is excavated and recompacted in a new location.
Question 6: Which type of pile is driven into the ground and derives its capacity primarily from skin friction along its length?
- End-bearing pile
- Friction pile (Correct answer)
- Sheet pile
- Helical pile
Correct answer: Friction pile
Friction piles transfer structural loads to surrounding soil through friction and adhesion along the pile shaft rather than bearing on a hard layer.
Question 7: What is the primary purpose of a subdrain (French drain) installed in a retaining wall?
- To anchor the wall to the retained soil
- To relieve hydrostatic water pressure behind the wall (Correct answer)
- To increase the wall's bearing capacity
- To provide a conduit for utility lines
Correct answer: To relieve hydrostatic water pressure behind the wall
A subdrain behind a retaining wall removes accumulated groundwater to prevent dangerous hydrostatic pressure buildup that could cause wall failure.
General Contractor License Site Work and Concrete 4
What is the purpose of vapor retarder under a concrete slab-on-grade?