CCIL - Canadian Council of Independent Laboratories Concrete Compressive Strength Testing Questions and Answers — Questions and Answers
Question 1: A technician tests a standard 100 mm x 200 mm concrete cylinder and applies the load much faster than the rate specified in CSA A23.2-9C. What is the most likely effect on the measured compressive strength?
- The measured strength will be artificially low.
- The loading rate has no significant effect on the final strength.
- The measured strength will be artificially high. (Correct answer)
- The cylinder will most likely exhibit a columnar fracture.
Correct answer: The measured strength will be artificially high.
Applying the load too quickly does not allow for the natural progression of micro-cracking before failure, leading to a higher-than-actual failure load. CSA A23.2-9C specifies a controlled stress rate (0.15 to 0.35 MPa/s) to ensure consistent and accurate results.
Question 2: According to CSA A23.2-9C, what is the primary purpose of capping concrete cylinders with sulphur mortar or using unbonded neoprene pads before testing?
- To protect the testing machine platens from damage.
- To ensure the load is applied uniformly over the entire cross-section. (Correct answer)
- To increase the overall strength of the cylinder.
- To absorb moisture from the cylinder ends before testing.
Correct answer: To ensure the load is applied uniformly over the entire cross-section.
The ends of concrete cylinders are rarely perfectly plane and parallel. Capping creates smooth, parallel bearing surfaces that are perpendicular to the cylinder's axis, which is crucial for uniform load distribution. Non-uniform loading creates stress concentrations that can cause premature failure and inaccurate results.
Question 3: As per CSA A23.2-9C, what is the required moisture condition of a standard-cured concrete cylinder immediately before compressive strength testing?
- It must be submerged in lime-saturated water until the moment of testing.
- It must be allowed to air-dry for at least 4 hours to remove surface water.
- It must be dried to a constant mass in an oven at 110 °C.
- It must be tested in a moist condition, immediately after removal from curing. (Correct answer)
Correct answer: It must be tested in a moist condition, immediately after removal from curing.
CSA A23.2-9C and related standards require that specimens be tested in a moist condition after being removed from the standard curing environment (moist room or lime-saturated water). Allowing the cylinder surface to dry can result in a significant, artificial increase in measured compressive strength.
Question 4: A technician needs to test a cored concrete specimen with a diameter of 100 mm and a length of 175 mm. According to CSA A23.2-14C, what must be done before reporting the final compressive strength?
- The calculated compressive strength must be multiplied by a correction factor. (Correct answer)
- The result must be discarded as the L/D ratio is outside the acceptable range.
- The test should be performed at a slower loading rate to compensate.
- The core must be trimmed to a length of 200 mm before testing.
Correct answer: The calculated compressive strength must be multiplied by a correction factor.
The standard compressive strength test is based on a length-to-diameter (L/D) ratio of 2.0. When this ratio is not met (in this case, 1.75), confining stresses affect the failure load. CSA A23.2-14C provides a table of correction factors for L/D ratios between 1.00 and 1.99 to normalize the result to an equivalent L/D of 2.0.
Question 5: After testing a concrete cylinder, a technician observes a fracture pattern that consists of well-formed cones on both ends of the specimen. According to CSA A23.2-9C and referenced standards like ASTM C39, what does this type of fracture typically indicate?
- The concrete had insufficient air entrainment.
- The cylinder was not properly centered in the machine.
- A valid and acceptable test result. (Correct answer)
- The loading rate was applied too slowly.
Correct answer: A valid and acceptable test result.
The 'Cone' or 'Cone and Shear' fracture is considered the standard, ideal fracture for a compressive strength test. It indicates that the failure was not unduly influenced by testing machine issues, improper capping, or eccentric loading. While other fracture types can be acceptable, this type is the most desirable.
Question 6: What is the critical function of the spherically seated bearing block (upper platen) in a concrete compression testing machine?
- To apply the load at the precise rate specified by the standard.
- To allow the block to tilt and align itself with the top surface of the specimen. (Correct answer)
- To absorb the shock of the cylinder fracturing, protecting the load cell.
- To measure the diameter of the concrete cylinder automatically.
Correct answer: To allow the block to tilt and align itself with the top surface of the specimen.
The spherically seated block is designed to tilt freely to ensure the load is applied perpendicular to the cylinder's top surface, even if there is a slight lack of perpendicularity. This ensures uniform compression and prevents eccentric loading, which would invalidate the test.
CCIL - Canadian Council of Independent Laboratories Concrete Compressive Strength Testing Questions and Answers
A technician tests a standard 100 mm x 200 mm concrete cylinder and applies the load much faster than the rate specified in CSA A23.2-9C.
What is the most likely effect on the measured compressive strength?