ACI Certification Field Testing Technician 3 — Questions and Answers
Question 1: The aggregate correction factor (A) used in the pressure method air content test (ASTM C231) corrects for:
- Temperature effects on the air meter gauge
- Air voids within aggregate particles that are not part of the paste air void system (Correct answer)
- The difference between entrained and entrapped air
- Calibration error in the pressure gauge
Correct answer: Air voids within aggregate particles that are not part of the paste air void system
The aggregate correction factor accounts for air absorbed within aggregate pores that the pressure meter would otherwise count as entrained air.
Question 2: The volumetric method (ASTM C173) is preferred over the pressure method (ASTM C231) when testing:
- High-strength normal weight concrete
- Lightweight aggregate concrete or concrete made with porous aggregate (Correct answer)
- Concrete containing silica fume or fly ash
- All concrete with design strength above 5,000 psi
Correct answer: Lightweight aggregate concrete or concrete made with porous aggregate
The volumetric method is used for lightweight and porous aggregate concrete because the pressure method gives inaccurate readings due to aggregate porosity.
Question 3: When striking off excess concrete from the top of a unit weight measure per ASTM C138, the preferred instrument is:
- A wooden straight edge
- The tamping rod used for rodding
- A flat plate or piece of plate glass (Correct answer)
- A hand float
Correct answer: A flat plate or piece of plate glass
A flat plate or piece of plate glass is used in a sawing and rolling motion to create a smooth, level surface on the unit weight container.
Question 4: Air content by the gravimetric method (ASTM C138) is calculated as:
- (Theoretical unit weight minus Actual unit weight) divided by Theoretical unit weight times 100 (Correct answer)
- (Actual unit weight minus Theoretical unit weight) divided by Actual unit weight times 100
- Pressure meter reading minus aggregate correction factor
- Slump in inches divided by unit weight times 100
Correct answer: (Theoretical unit weight minus Actual unit weight) divided by Theoretical unit weight times 100
Gravimetric air content = (D minus M) divided by D times 100, where D is the theoretical air-free unit weight and M is the measured unit weight.
Question 5: Yield (volume of concrete produced per batch) per ASTM C138 is calculated as:
- Total mass of all batch materials divided by unit weight of fresh concrete (Correct answer)
- Unit weight of fresh concrete divided by total mass of cement and water
- Volume of coarse aggregate plus volume of cement paste
- Slump divided by the water-cement ratio
Correct answer: Total mass of all batch materials divided by unit weight of fresh concrete
Yield = total mass of all batch materials (lb or kg) divided by the unit weight of freshly mixed concrete.
Question 6: Per ASTM C138, when the nominal maximum aggregate size exceeds 1.5 inches (37.5 mm), consolidation of the unit weight sample must use:
- Hand rodding only
- Internal vibration (Correct answer)
- External vibration only
- No consolidation — the aggregate settles on its own
Correct answer: Internal vibration
ASTM C138 requires the vibration method when nominal maximum aggregate size exceeds 1.5 inches because hand rodding cannot adequately consolidate such mixes.
Question 7: To calculate theoretical (air-free) unit weight for the gravimetric air content method, a technician needs:
- Slump and temperature of the fresh concrete only
- Specific gravities and batch masses of all materials in the mix (Correct answer)
- Water-cement ratio and design compressive strength only
- Aggregate nominal maximum size and cement content only
Correct answer: Specific gravities and batch masses of all materials in the mix
Theoretical unit weight requires the specific gravity and batch mass of every ingredient (cement, aggregates, water, admixtures) to calculate.
ACI Certification Field Testing Technician 3
The aggregate correction factor (A) used in the pressure method air content test (ASTM C231) corrects for: