Concrete: NASCLA Commercial General Building Practice Questions
Mixes, testing (slump, cylinders), placing and curing, hot and cold weather, reinforcing steel and cover, formwork and shoring, joints, post-tensioning and precast, OSHA Subpart Q, and concrete and rebar takeoffs.
Often missed
Question 1
Which factor has the greatest effect on the strength of hardened concrete?
Correct answer: The water-cement ratio
Trade practice (concrete mixes)
Less water for the same cement gives stronger, less permeable concrete; adding water at the site to make it flow weakens it.
Question 2
What does a slump test measure in fresh concrete?
Correct answer: Its consistency or workability
Trade practice (concrete testing)
The slump cone is filled and lifted, and the drop of the concrete is measured; it checks consistency from batch to batch, not strength.
Question 3
At what age are standard concrete test cylinders usually broken to check specified compressive strength (f'c)?
Correct answer: 28 days
Trade practice (concrete testing)
Specified strength is normally based on 28-day tests; 7-day breaks give an early indication, and some mixes specify 56 days.
Question 4
Why is air-entraining admixture added to concrete exposed to freezing and thawing?
Correct answer: Air bubbles relieve freezing pressure
Trade practice (concrete mixes)
Entrained air gives freezing water room to expand; it slightly lowers strength, so mixes are designed with it in mind.
Question 5
On a hot day, which admixture helps keep concrete workable longer during placement?
Correct answer: A set retarder
Trade practice (concrete admixtures)
Retarders slow setting in hot weather; accelerators such as calcium chloride speed it up and are used in cold weather (calcium chloride is avoided around reinforcing because of corrosion).
Question 6
What is the purpose of curing concrete?
Correct answer: To keep it moist and warm so it keeps gaining strength
Trade practice (curing)
Hydration needs water and warmth; concrete that dries early is weaker and cracks more. Methods include wet burlap, curing compounds and covers.
Question 7
In cold weather concreting, what is the main reason to protect freshly placed concrete from freezing?
Correct answer: Freezing before about 500 psi can permanently damage it
Trade practice (cold weather)
Young concrete holds a lot of free water; if it freezes early the expanding ice breaks it. Blankets, heated enclosures and warm mix water are typical protection.
Question 8
What is the nominal diameter of a #6 reinforcing bar?
Correct answer: 3/4 inch
Trade practice (rebar)
In inch-pound bar sizes the number is the diameter in eighths of an inch: #6 = 6/8 = 3/4 in.
Question 9
Concrete cast against and permanently exposed to earth (such as the bottom of a footing) typically needs about how much cover over the reinforcing steel?
Correct answer: 3 inches
Trade practice (ACI 318 cover)
Cover protects the steel from corrosion and fire; cast-against-earth concrete gets the most (3 in), formed surfaces exposed to weather less, and interior slabs least.
Question 10
What are rebar chairs and bolsters used for?
Correct answer: Holding bars at the right height and cover
Trade practice (rebar)
Supports keep bars from being stepped down to the bottom of the slab, which would destroy the designed depth and cover.
Question 11
Why are control (contraction) joints cut or tooled in a concrete slab on grade?
Correct answer: To make shrinkage cracking happen in straight, planned lines
Trade practice (joints)
Concrete shrinks as it dries and will crack somewhere; a weakened plane cut about one-quarter of the slab depth makes it crack there. Isolation joints separate the slab from columns and walls.
Question 12
Calculation
A slab on grade is 5 inches thick. Using a joint depth of one-quarter of the slab thickness (given), how deep should the saw cut be?
Correct answer: 1.25 inches
Trade practice (joints)
One-quarter of 5 inches is 1-1/4 inches. Cuts that are too shallow let random cracks form beside the joint.
Worked solution
1
Formula
Cut depth = thickness × 1/4
2
Values
Cut depth = 5 in × 0.25
3
Result
Cut depth = 1.25 inches
Question 13
Calculation
A slab is 40 ft by 30 ft by 4 inches thick. How many cubic yards of concrete does it need, before waste?
Correct answer: 14.81 CY
Estimating math (concrete)
Convert the thickness to feet, multiply, then divide by 27. Dividing by 9 instead of 27 gives 44.44.
Worked solution
1
Formula
Thickness = 4 in ÷ 12 = 0.333 ft
2
Values
Volume = 40 ft × 30 ft × 0.333 ft = 400 cu ft
3
Result
400 cu ft ÷ 27 = 14.81 CY
Question 14
Calculation
A continuous footing is 180 ft long, 24 inches wide and 12 inches deep. With 5% waste (given), how many cubic yards should be ordered?
Correct answer: 14.00 CY
Estimating math (concrete)
Net volume is 13.33 CY; adding 5% for waste gives 14.00 CY. Subtracting the waste gives 12.67.
Worked solution
1
Formula
Volume = 180 ft × 2 ft × 1 ft = 360 cu ft
2
Values
360 ÷ 27 = 13.33 CY
3
Result
With waste: 13.33 × 1.05 = 14.00 CY
Question 15
Calculation
Twelve square column footings are each 4 ft by 4 ft by 18 inches deep. How many cubic yards of concrete do they need, before waste?
Correct answer: 10.67 CY
Estimating math (concrete)
18 inches is 1.5 ft. Forgetting to multiply by 12 footings gives 0.89 CY; using 1 ft depth gives 7.11 CY.
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Practice questions are written in our own words and cite code sections by number. Always study from your own copy of the code and the official candidate bulletin.