CO Structural Concrete Contractor
Exam Questions And Correct Answers
(Verified Answers) Plus Rationale2026
Q&A| Instant Download Pdf
1. What is the primary purpose of reinforcing steel (rebar) in structural
concrete?
A. To increase thermal resistance
B. To reduce concrete curing time
C. To carry tensile forces
D. To prevent chemical attack
Rationale: Rebar provides tensile strength because concrete is weak
in tension but strong in compression; reinforcement handles tensile
and flexural stresses.
2. Which factor most directly controls concrete strength in a mix design?
A. Aggregate shape
B. Curing method
C. Water-cement ratio
D. Formwork type
Rationale: Water-cement ratio governs the porosity of hardened
cement paste and is the primary determinant of compressive strength.
3. What slump value (ASTM C143) would typically indicate workable
concrete for a normal reinforced slab placed by shovel and screed?
A. 0–1 inch (0–25 mm)
B. 3–4 inches (75–100 mm)
C. 8–10 inches (200–250 mm)
, D. 12–14 inches (300–350 mm)
Rationale: A 3–4 inch slump is common for placeable, workable
concrete for slabs without excessive segregation; higher slump may
indicate admixture use or pump placement.
4. Which test specimen and age are standard for measuring compressive
strength of concrete? A. 3-inch cube at 14 days
B. 6x12 in cylinder at 28 days
C. 4x8 in cylinder at 7 days
D. 150-mm cube at 56 days
Rationale: The 6x12 in (or 150x300 mm) cylinder tested at 28 days is
the common standard for reporting compressive strength in many
codes (some regions use 150-mm cubes). The 28-day strength is a
standard maturity reference.
5. What is the main reason for air entrainment in exterior concrete
exposed to freeze–thaw cycles?
A. To reduce weight
B. To increase strength
C. To provide microscopic voids that relieve hydraulic pressure
D. To speed curing
Rationale: Entrained air creates tiny discontinuous voids that allow
water to expand on freezing without generating internal pressures
that cause scaling and spalling.
6. Which admixture is commonly used to delay the setting time of
concrete in hot weather?
A. Air-entraining agent
B. Retarder
C. Accelerating admixture
D. Water-reducing admixture only
Rationale: Retarders slow the hydration process, delaying initial set
and allowing placement and finishing during high temperatures.
7. What is the primary purpose of consolidation (vibration) of
freshly placed concrete? A. To speed curing
B. To remove entrapped air and help concrete achieve full contact
around reinforcement and forms
C. To increase slump
, D. To reduce water content after placement
Rationale: Proper vibration eliminates entrapped air and reduces
voids, improving bond, strength, and durability.
8. Minimum concrete cover for reinforcing bars in a beam exposed
to weather (per common codes) is typically:
A. 5 mm (¼ in)
B. 10 mm (3/8 in)
C. 25 mm (1 in)
D. 75 mm (3 in)
Rationale: Typical required cover for beams exposed to weather is
about 25 mm (1 in) depending on exposure and bar size; cover
ensures durability and protection against corrosion.
9. What does the term "bleeding" refer to in fresh concrete?
A. Rusting of rebar
B. Separation of water to the surface
C. Excessive air entrainment
D. Over-vibration
Rationale: Bleeding is the upward movement of water in freshly
placed concrete, which can affect finishing and surface quality if
excessive.
10. Which aggregate property has the greatest effect on concrete
workability and finishability?
A. Specific gravity
B. Particle shape and grading
C. Chemical composition
D. Thermal conductivity
Rationale: The shape and grading (distribution of sizes) determine
how well the aggregate packs and how much paste is needed—key to
workability and finishing.
11. What type of cement is commonly specified for structural
concrete with higher early strength requirements?
A. Low-heat cement
B. Sulfate-resisting cement
C. Type III (high early strength) Portland cement
D. White cement
Exam Questions And Correct Answers
(Verified Answers) Plus Rationale2026
Q&A| Instant Download Pdf
1. What is the primary purpose of reinforcing steel (rebar) in structural
concrete?
A. To increase thermal resistance
B. To reduce concrete curing time
C. To carry tensile forces
D. To prevent chemical attack
Rationale: Rebar provides tensile strength because concrete is weak
in tension but strong in compression; reinforcement handles tensile
and flexural stresses.
2. Which factor most directly controls concrete strength in a mix design?
A. Aggregate shape
B. Curing method
C. Water-cement ratio
D. Formwork type
Rationale: Water-cement ratio governs the porosity of hardened
cement paste and is the primary determinant of compressive strength.
3. What slump value (ASTM C143) would typically indicate workable
concrete for a normal reinforced slab placed by shovel and screed?
A. 0–1 inch (0–25 mm)
B. 3–4 inches (75–100 mm)
C. 8–10 inches (200–250 mm)
, D. 12–14 inches (300–350 mm)
Rationale: A 3–4 inch slump is common for placeable, workable
concrete for slabs without excessive segregation; higher slump may
indicate admixture use or pump placement.
4. Which test specimen and age are standard for measuring compressive
strength of concrete? A. 3-inch cube at 14 days
B. 6x12 in cylinder at 28 days
C. 4x8 in cylinder at 7 days
D. 150-mm cube at 56 days
Rationale: The 6x12 in (or 150x300 mm) cylinder tested at 28 days is
the common standard for reporting compressive strength in many
codes (some regions use 150-mm cubes). The 28-day strength is a
standard maturity reference.
5. What is the main reason for air entrainment in exterior concrete
exposed to freeze–thaw cycles?
A. To reduce weight
B. To increase strength
C. To provide microscopic voids that relieve hydraulic pressure
D. To speed curing
Rationale: Entrained air creates tiny discontinuous voids that allow
water to expand on freezing without generating internal pressures
that cause scaling and spalling.
6. Which admixture is commonly used to delay the setting time of
concrete in hot weather?
A. Air-entraining agent
B. Retarder
C. Accelerating admixture
D. Water-reducing admixture only
Rationale: Retarders slow the hydration process, delaying initial set
and allowing placement and finishing during high temperatures.
7. What is the primary purpose of consolidation (vibration) of
freshly placed concrete? A. To speed curing
B. To remove entrapped air and help concrete achieve full contact
around reinforcement and forms
C. To increase slump
, D. To reduce water content after placement
Rationale: Proper vibration eliminates entrapped air and reduces
voids, improving bond, strength, and durability.
8. Minimum concrete cover for reinforcing bars in a beam exposed
to weather (per common codes) is typically:
A. 5 mm (¼ in)
B. 10 mm (3/8 in)
C. 25 mm (1 in)
D. 75 mm (3 in)
Rationale: Typical required cover for beams exposed to weather is
about 25 mm (1 in) depending on exposure and bar size; cover
ensures durability and protection against corrosion.
9. What does the term "bleeding" refer to in fresh concrete?
A. Rusting of rebar
B. Separation of water to the surface
C. Excessive air entrainment
D. Over-vibration
Rationale: Bleeding is the upward movement of water in freshly
placed concrete, which can affect finishing and surface quality if
excessive.
10. Which aggregate property has the greatest effect on concrete
workability and finishability?
A. Specific gravity
B. Particle shape and grading
C. Chemical composition
D. Thermal conductivity
Rationale: The shape and grading (distribution of sizes) determine
how well the aggregate packs and how much paste is needed—key to
workability and finishing.
11. What type of cement is commonly specified for structural
concrete with higher early strength requirements?
A. Low-heat cement
B. Sulfate-resisting cement
C. Type III (high early strength) Portland cement
D. White cement
