CCIL Concrete 4 — Questions and Answers
Question 1: The rebound hammer test (Schmidt hammer) per ASTM C805 is best used for:
- Determining exact compressive strength of concrete
- Assessing uniformity and relative strength differences in-place (Correct answer)
- Measuring tensile splitting strength
- Determining modulus of elasticity
Correct answer: Assessing uniformity and relative strength differences in-place
The rebound hammer is an in-situ non-destructive test for assessing concrete uniformity and identifying relative variations in strength, not an absolute strength measurement.
Question 2: What is the term for the phenomenon where concrete continues to gain strength beyond 28 days due to continued cement hydration?
- Creep
- Long-term strength gain (Correct answer)
- Carbonation hardening
- Autogenous healing
Correct answer: Long-term strength gain
Concrete continues to hydrate and gain strength well beyond 28 days, a process known as long-term strength gain, especially with SCMs present.
Question 3: In mix design, the term 'nominal maximum aggregate size' refers to:
- The largest sieve on which any aggregate is retained
- The smallest sieve through which 100% of aggregate passes but larger than the sieve retaining more than 15% (Correct answer)
- The sieve size that retains 50% of aggregate
- The size of aggregate at the D60 point of the gradation curve
Correct answer: The smallest sieve through which 100% of aggregate passes but larger than the sieve retaining more than 15%
Nominal maximum aggregate size is defined as the smallest sieve size through which 100% of aggregate passes, with the next larger sieve retaining more than 15%.
Question 4: Alkali-silica reaction (ASR) in concrete is characterized by:
- Rapid early strength loss and surface scaling
- Expansive gel formation causing cracking and map cracking (crazing) (Correct answer)
- Corrosion of embedded steel reinforcement
- Delamination of the concrete surface in horizontal layers
Correct answer: Expansive gel formation causing cracking and map cracking (crazing)
ASR occurs when alkalis in cement react with reactive silica in aggregates, forming a hygroscopic gel that absorbs water and expands, causing internal cracking and map cracking.
Question 5: When performing the splitting tensile strength test (ASTM C496), the load is applied along:
- The full flat end surfaces of a cylinder
- A diametral plane along the length of the cylinder via bearing strips (Correct answer)
- The circumference of the cylinder evenly
- A single point at mid-height of the cylinder
Correct answer: A diametral plane along the length of the cylinder via bearing strips
In the splitting tensile test, load is applied through two narrow bearing strips along a diametral plane of the cylinder, inducing tensile stresses perpendicular to the load.
Question 6: According to CSA A23.2, concrete test cylinders must be transported to the laboratory within how many hours of casting?
- 12 hours
- 24 hours (Correct answer)
- 48 hours
- 72 hours
Correct answer: 24 hours
CSA A23.2 requires cylinders to be transported to the laboratory within 48 hours after casting to minimize damage from handling and temperature variation.
Question 7: The ratio of flexural strength to compressive strength for normal concrete is approximately:
- 5–10%
- 10–20% (Correct answer)
- 25–35%
- 40–50%
Correct answer: 10–20%
Concrete flexural (modulus of rupture) strength is typically 10–20% of its compressive strength, reflecting concrete's inherently greater strength in compression.
CCIL Concrete 4
The rebound hammer test (Schmidt hammer) per ASTM C805 is best used for: