ACI Certification Concrete Strength Testing Procedures 2 — Questions and Answers
Question 1: According to ASTM C39, what is the required rate of load application during compressive strength testing of a concrete cylinder?
- 20 to 50 psi/s (0.14 to 0.34 MPa/s) (Correct answer)
- 100 to 150 psi/s (0.69 to 1.03 MPa/s)
- 5 to 10 psi/s (0.03 to 0.07 MPa/s)
- 150 to 250 psi/s (1.03 to 1.72 MPa/s)
Correct answer: 20 to 50 psi/s (0.14 to 0.34 MPa/s)
ASTM C39 specifies a loading rate of 20 to 50 psi/s (0.14 to 0.34 MPa/s) during the final phase of loading; rates outside this range can produce inaccurate results.
Loading rate affects measured compressive strength; loading too fast gives artificially high results while loading too slowly allows stress relaxation. ASTM C39 requires maintaining the specified rate during at least the final half of the loading phase. Deviations from the 20–50 psi/s range must be reported as a departure from standard test conditions.
Question 2: What is the maximum permissible departure from perpendicularity of the end surface of a concrete test cylinder when measured with a square?
- 0.5° (Correct answer)
- 1.0°
- 2.0°
- 3.0°
Correct answer: 0.5°
ASTM C39 and C617 require that the ends of cylinders be perpendicular to the cylinder axis within 0.5°; non-perpendicular ends introduce eccentric loading and reduce measured strength.
When a cylinder's end is not perpendicular to its axis, load is applied non-uniformly, creating bending stresses in addition to compression. This reduces the measured compressive strength and increases variability. Cylinders are ground or capped with sulfur mortar (ASTM C617) or neoprene pad systems (ASTM C1231) to achieve flat, perpendicular ends.
Question 3: What is a Type B fracture pattern in a compressive strength test per ASTM C39, and is it considered acceptable?
- A well-formed cone with slight cracking through one base; it is an acceptable fracture pattern (Correct answer)
- Columnar vertical cracking through both ends; it is not acceptable
- Diagonal fracture with no cracking through either end; it is acceptable
- Side fracture at mid-height without any cone formation; it is not acceptable
Correct answer: A well-formed cone with slight cracking through one base; it is an acceptable fracture pattern
ASTM C39 Figure 2 defines Type B as a well-formed cone on one end with some vertical cracking toward the other end; it is a normal, acceptable fracture pattern indicating good end conditions.
ASTM C39 classifies fracture patterns to help identify testing anomalies. Type B (cone on one end, shear on the other) indicates proper load distribution and is acceptable. Columnar fracture (Type E) suggests poor end preparation. A side fracture (Type F) at mid-height may indicate a defective test. The fracture type is recorded on the test report.
Question 4: If a compressive strength test result falls below f'c by more than 500 psi, what condition must be checked first according to ACI 318?
- Whether the strength test is the average of at least two cylinders (or three for 4×8-inch cylinders) from the same sample (Correct answer)
- Whether the concrete truck was overloaded at delivery
- Whether the aggregate used was from a pre-approved quarry
- Whether the batch plant was certified within the past year
Correct answer: Whether the strength test is the average of at least two cylinders (or three for 4×8-inch cylinders) from the same sample
ACI 318 defines a strength test as the average of cylinders from one sample; if averaging was not done correctly, that is the first thing to verify before escalating the investigation.
Per ACI 318 Section 26.12.2, a strength test is defined as the average of at least two 6×12-inch cylinders or at least three 4×8-inch cylinders made from the same concrete sample. If a lab reports a single cylinder break as a 'test,' it is not compliant with ACI 318. Before initiating a full investigation, the inspector should verify that the test was performed and averaged correctly.
Question 5: Why must concrete test cylinders be stored in a standardized moist curing environment (73 ± 3°F / 23 ± 2°C) after the initial 24-hour field period?
- To simulate ideal hydration conditions and produce strength results that reflect the concrete's potential strength, not the field curing quality (Correct answer)
- To accelerate the hydration reaction and obtain results faster
- To prevent carbonation of the surface mortar
- To allow cylinder molds to be reused immediately after stripping
Correct answer: To simulate ideal hydration conditions and produce strength results that reflect the concrete's potential strength, not the field curing quality
Standard laboratory curing conditions eliminate variability from field temperature and moisture, so all cylinders are tested under the same reference conditions, making results comparable and representative of the mix's potential.
Temperature significantly affects hydration rate and ultimate strength. ASTM C31 requires initial curing within 60–80°F for the first 24 hours in the field, then transportation to the lab without shock, and final curing at 73 ± 3°F in a moist room or water tank. This protocol ensures that test results reflect concrete quality rather than field curing variability.
Question 6: What is the maximum permitted length-to-diameter ratio (L/D) for a concrete test cylinder per ASTM C39?
- Between 1.8D and 2.2D (where D is diameter), with 2.0D being the standard (Correct answer)
- Exactly 3.0D for all cylinder sizes
- Between 1.5D and 1.8D
- Between 2.5D and 3.0D
Correct answer: Between 1.8D and 2.2D (where D is diameter), with 2.0D being the standard
ASTM C39 requires cylinders with a length-to-diameter ratio between 1.8 and 2.2; cylinders outside this range require strength correction factors.
The L/D ratio of 2.0 is critical because it allows the formation of the standard failure cone without interference from end friction. Cylinders with L/D less than 1.8 test artificially high; cylinders with L/D greater than 2.2 test artificially low. ASTM C39 Table 3 provides correction factors for L/D ratios between 1.75 and 1.50.
ACI Certification Concrete Strength Testing Procedures 2
According to ASTM C39, what is the required rate of load application during compressive strength testing of a concrete cylinder?