ACI Fresh Concrete Field Testing Questions and Answers 1 — Questions and Answers
Question 1: An ACI Certified Field Testing Technician is performing a slump test (ASTM C143) on a sample of fresh concrete. The concrete slumps and falls away to one side. What is the correct action for the technician to take?
- Record the slump at the lowest point of the sheared surface.
- Average the highest and lowest points and record that value.
- Disregard the test and immediately perform a retest on another portion of the same sample. (Correct answer)
- Reject the entire batch of concrete as it is not cohesive.
Correct answer: Disregard the test and immediately perform a retest on another portion of the same sample.
According to ASTM C143/C143M, if the concrete shears off or slips sideways, the test is considered invalid. The technician must disregard the result and conduct a new test on a different portion of the composite sample. If a similar result occurs on the retest, the concrete is likely not cohesive enough for the slump test to be applicable, and this should be noted in the report.
Question 2: According to ASTM C172, Standard Practice for Sampling Freshly Mixed Concrete, what is the maximum allowable elapsed time between obtaining the first and final portions of a composite sample?
- 5 minutes
- 10 minutes
- 15 minutes (Correct answer)
- 20 minutes
Correct answer: 15 minutes
ASTM C172 specifies that the process of collecting a composite sample, which involves taking portions from the middle part of the batch, must be completed within a 15-minute timeframe from the start of sampling.
Question 3: A technician is measuring the density (unit weight) of freshly mixed concrete following ASTM C138. After filling the measure and striking off the excess concrete, what is the immediate next step?
- Weigh the measure with the concrete and subtract the weight of the measure.
- Clean all excess concrete from the exterior of the measure. (Correct answer)
- Take the temperature of the concrete in the measure.
- Cover the measure with a glass plate to check for voids.
Correct answer: Clean all excess concrete from the exterior of the measure.
The ASTM C138 procedure requires that after striking off the top surface, the technician must clean any excess concrete from the outside of the measure. This ensures an accurate weight measurement of only the measure and the concrete inside it.
Question 4: When measuring the temperature of freshly mixed hydraulic-cement concrete (ASTM C1064), how much concrete cover should surround the sensing portion of the temperature measuring device?
- At least 1 inch (25 mm) in all directions.
- At least 2 inches (50 mm) in all directions.
- At least 3 inches (75 mm) in all directions. (Correct answer)
- The device only needs to be submerged halfway.
Correct answer: At least 3 inches (75 mm) in all directions.
ASTM C1064/C1064M requires the temperature sensing portion of the thermometer to be submerged a minimum of 3 inches (75 mm) into the fresh concrete. Additionally, there must be at least 3 inches (75 mm) of concrete cover in all directions around the sensing portion to ensure the reading reflects the internal temperature of the concrete and is not influenced by ambient air temperature.
Question 5: Which of the following is the correct tamping procedure for filling a 0.5 ft³ unit weight measure when performing the test for Density (Unit Weight) of Concrete (ASTM C138)?
- Fill in 2 equal layers, rodding each layer 25 times.
- Fill in 3 equal layers, rodding each layer 25 times.
- Fill in 3 equal layers, rodding each layer 50 times. (Correct answer)
- Fill in 2 equal layers, rodding each layer 50 times.
Correct answer: Fill in 3 equal layers, rodding each layer 50 times.
For a measure with a volume of 0.5 ft³, ASTM C138 requires the concrete to be placed in three approximately equal layers. Each layer must be rodded 50 times with the tamping rod, distributing the strokes evenly over the cross-section.
Question 6: A field test for air content is being performed on lightweight concrete using the volumetric method (ASTM C173). Why is this method specified for lightweight concrete instead of the pressure method (ASTM C231)?
- The pressure method takes too long for lightweight concrete.
- The aggregate in lightweight concrete can absorb a significant amount of water, affecting the results.
- The aggregate in lightweight concrete is porous and contains its own air, which would be measured incorrectly by the pressure method. (Correct answer)
- The volumetric method requires a smaller, more manageable sample size.
Correct answer: The aggregate in lightweight concrete is porous and contains its own air, which would be measured incorrectly by the pressure method.
The pressure method (ASTM C231) is not recommended for concrete made with lightweight aggregates because the aggregate itself is porous and contains internal air voids. The pressure applied during the test can compress this air within the aggregate, leading to a false high reading for the entrained air in the cement paste. The volumetric method (ASTM C173) is not affected by aggregate porosity and thus provides an accurate measurement.
ACI Fresh Concrete Field Testing Questions and Answers 1
An ACI Certified Field Testing Technician is performing a slump test (ASTM C143) on a sample of fresh concrete.
The concrete slumps and falls away to one side.
What is the correct action for the technician to take?