ACI Concrete Strength & Testing Methods 3 — Questions and Answers
Question 1: The splitting tensile test (ASTM C496) applies load to a concrete cylinder in which direction?
- Along the vertical axis (end-to-end)
- Along the horizontal diameter (side-to-side) (Correct answer)
- At a 45-degree angle
- Uniformly around the circumference
Correct answer: Along the horizontal diameter (side-to-side)
ASTM C496 loads the cylinder diametrically (horizontally along its length), creating indirect tensile stress that splits the cylinder.
Question 2: Which curing method produces the highest 28-day compressive strength in concrete cylinders?
- Air drying at room temperature
- Moist curing at 73°F (23°C) (Correct answer)
- Storing in direct sunlight
- Covering with black plastic without moisture
Correct answer: Moist curing at 73°F (23°C)
Moist curing at 73°F maintains the moisture needed for continued cement hydration, maximizing strength development.
Question 3: What is the purpose of the 7-day compressive strength test on a concrete cylinder?
- It is the only accepted strength test under ACI standards
- It provides an early indication of whether 28-day strength will be achieved (Correct answer)
- It replaces the need for a 28-day test
- It measures only the surface hardness
Correct answer: It provides an early indication of whether 28-day strength will be achieved
The 7-day test gives an early estimate — typically 65–70% of 28-day strength — so corrective action can be taken if needed.
Question 4: When taking a concrete sample for cylinder fabrication from a truck, at what point in the discharge should the sample be taken?
- From the very first portion discharged
- After the first 10% and before the last 10% of the load is discharged (Correct answer)
- After the entire load has been discharged
- From the chute after adding water on site
Correct answer: After the first 10% and before the last 10% of the load is discharged
ASTM C172 requires sampling after discarding the first and last 10% of the load to obtain a representative sample.
Question 5: Cores drilled from hardened concrete are typically accepted if their compressive strength averages at least what percentage of the specified f'c?
- 50%
- 75%
- 85% (Correct answer)
- 100%
Correct answer: 85%
ACI 318 generally accepts drilled cores when the average of three cores is at least 85% of f'c and no single core is below 75% of f'c.
Question 6: What ASTM standard governs the making and curing of concrete test specimens in the field?
- ASTM C31 (Correct answer)
- ASTM C39
- ASTM C78
- ASTM C138
Correct answer: ASTM C31
ASTM C31 covers the standard practice for making and curing concrete test specimens in the field.
Question 7: How does increasing the water-to-cementitious material (w/cm) ratio affect concrete compressive strength?
- Increases strength by improving workability
- Has no effect on strength
- Decreases strength due to higher porosity (Correct answer)
- Increases strength only at early ages
Correct answer: Decreases strength due to higher porosity
A higher w/cm ratio creates more capillary voids after excess water evaporates, directly reducing compressive strength.
ACI Concrete Strength & Testing Methods 3
The splitting tensile test (ASTM C496) applies load to a concrete cylinder in which direction?