ACI Concrete Strength Testing Procedures Questions and Answers 1 — Questions and Answers
Question 1: According to ASTM C39, what is the specified loading rate for applying a compressive force to a standard 6x12 inch concrete cylinder?
- 20 ± 5 psi/s
- 50 ± 10 psi/s
- 35 ± 7 psi/s (Correct answer)
- 40 ± 8 psi/s
Correct answer: 35 ± 7 psi/s
ASTM C39/C39M specifies that the load shall be applied continuously and without shock at a constant rate within the range of 20 to 50 psi/s (0.14 to 0.34 MPa/s). A loading rate of 35 ± 7 psi/s (0.25 ± 0.05 MPa/s) falls within this required range and is the standard rate to be maintained during the latter half of the loading phase.
Question 2: A technician is preparing to perform a flexural strength test (Modulus of Rupture) on a concrete beam specimen in accordance with ASTM C78. Which of the following loading configurations is specified by this standard?
- Center-point loading
- Third-point loading (Correct answer)
- Cantilever loading
- Uniformly distributed loading
Correct answer: Third-point loading
ASTM C78/C78M, "Standard Test Method for Flexural Strength of Concrete (Using Simple Beam with Third-Point Loading)," explicitly requires the use of a third-point loading setup. This method applies the load at two points, each one-third of the span length from a support, which creates a region of maximum bending moment in the middle third of the beam.
Question 3: When making standard-cured test specimens in the field per ASTM C31, what is the primary purpose of this type of curing?
- To determine the earliest time for form removal.
- To estimate the in-place strength of the concrete in the structure.
- To check the adequacy of mixture proportions for strength and for acceptance testing. (Correct answer)
- To evaluate the effectiveness of the curing methods used on the actual structure.
Correct answer: To check the adequacy of mixture proportions for strength and for acceptance testing.
ASTM C31/C31M outlines that standard-cured specimens are used for acceptance testing, checking the adequacy of mixture proportions for strength, and for quality control. Field-cured specimens, in contrast, are used to determine form removal times or the adequacy of in-place curing.
Question 4: A 4x8 inch concrete cylinder is being cast for strength testing. According to ASTM C31, how many layers should the concrete be placed in, and how many times should each layer be rodded?
- 3 layers, 25 times each
- 2 layers, 50 times each
- 3 layers, 50 times each
- 2 layers, 25 times each (Correct answer)
Correct answer: 2 layers, 25 times each
ASTM C31 specifies that for cylinder molds with a diameter of 4 inches [100 mm], the concrete shall be placed in two approximately equal layers. Each layer must be rodded 25 times uniformly over the cross-section.
Question 5: During the initial curing period for standard-cured concrete test cylinders on a job site, which of the following conditions is required by ASTM C31?
- Specimens must be stored in direct sunlight to accelerate hydration.
- Specimens must be stored at a temperature between 40°F and 60°F.
- Specimens must be stored in an environment with a temperature between 60°F and 80°F and prevented from moisture loss. (Correct answer)
- Specimens should be left uncovered and exposed to the ambient job site conditions.
Correct answer: Specimens must be stored in an environment with a temperature between 60°F and 80°F and prevented from moisture loss.
ASTM C31/C31M requires that during the initial curing period (up to 48 hours), standard-cured specimens must be stored in a temperature range of 60°F to 80°F (16°C to 27°C). Furthermore, measures must be taken to prevent moisture loss, such as using tight-fitting plastic lids or storing them in a curing box.
Question 6: Which of the following must be reported after performing a compressive strength test on a concrete cylinder according to ASTM C39?
- Only the maximum load applied and the calculated strength.
- The cylinder's color and the time of day the test was performed.
- The average measured diameter, maximum load, compressive strength, and type of fracture. (Correct answer)
- The name of the technician and the brand of the compression machine.
Correct answer: The average measured diameter, maximum load, compressive strength, and type of fracture.
ASTM C39 requires a detailed report of the test results. This includes the specimen ID, average measured diameter, test age, maximum load applied, the calculated compressive strength, the type of fracture, and any defects in the cylinder or its caps.
ACI Concrete Strength Testing Procedures Questions and Answers 1
According to ASTM C39, what is the specified loading rate for applying a compressive force to a standard 6x12 inch concrete cylinder?