ACI Concrete Strength & Testing Methods 2 — Questions and Answers
Question 1: According to ASTM C39, how many days after casting is the standard compressive strength test for concrete cylinders performed?
- 7 days
- 14 days
- 28 days (Correct answer)
- 56 days
Correct answer: 28 days
ASTM C39 specifies 28 days as the standard age for compressive strength testing of concrete cylinders.
Question 2: What is the standard size of a concrete cylinder used for compressive strength testing per ASTM C39?
- 3 in × 6 in
- 4 in × 8 in
- 6 in × 12 in (Correct answer)
- 8 in × 16 in
Correct answer: 6 in × 12 in
ASTM C39 specifies a 6-inch diameter by 12-inch height cylinder as the standard specimen for compressive strength testing.
Question 3: The Schmidt hammer (rebound hammer) test is used to estimate which concrete property?
- Tensile strength
- Flexural strength
- Surface hardness related to compressive strength (Correct answer)
- Permeability
Correct answer: Surface hardness related to compressive strength
The Schmidt hammer measures surface rebound to estimate surface hardness, which correlates with compressive strength.
Question 4: When capping concrete cylinders for testing, what material is typically used to create a flat bearing surface?
- Portland cement paste
- Sulfur mortar or neoprene pad (Correct answer)
- Epoxy resin
- Sand-cement grout
Correct answer: Sulfur mortar or neoprene pad
Sulfur mortar caps or neoprene pads are used to provide uniform, plane bearing surfaces on concrete test cylinders.
Question 5: A concrete mix has a specified 28-day compressive strength (f'c) of 4,000 psi. What is the minimum acceptable average strength (f'cr) the mix design must target?
- 4,000 psi
- Greater than 4,000 psi with a safety margin (Correct answer)
- Exactly 3,500 psi
- 2,500 psi
Correct answer: Greater than 4,000 psi with a safety margin
ACI 318 requires the mix design target strength (f'cr) to exceed f'c by a margin based on standard deviation to ensure adequate strength.
Question 6: What does the term 'modulus of rupture' describe in concrete testing?
- Compressive strength of a cylinder
- Flexural tensile strength of a beam (Correct answer)
- Splitting tensile strength of a cylinder
- Shear strength of a core
Correct answer: Flexural tensile strength of a beam
Modulus of rupture (MOR) is the flexural tensile strength determined by loading a concrete beam to failure per ASTM C78.
Question 7: If a set of concrete test cylinders fails to meet the specified strength, what is typically the FIRST step a finisher or supervisor should take?
- Immediately remove and replace all concrete placed that day
- Notify the engineer and investigate the cause before taking further action (Correct answer)
- Add water to the remaining mix to improve workability
- Ignore the results if the concrete looks acceptable
Correct answer: Notify the engineer and investigate the cause before taking further action
The engineer of record must be notified and the cause investigated before any remediation, removal, or acceptance decision is made.
ACI Concrete Strength & Testing Methods 2
According to ASTM C39, how many days after casting is the standard compressive strength test for concrete cylinders performed?