NETTCP Concrete Testing 5 — Questions and Answers
Question 1: A concrete cylinder fails at 3,800 psi at 28 days when the specified f'c is 4,000 psi. What is the immediate required action?
- Automatically reject the concrete placement
- Investigate per ACI 318 — core the structure or use other strength evidence (Correct answer)
- Re-test the same cylinders at 56 days
- Accept the result since it is within 10% of f'c
Correct answer: Investigate per ACI 318 — core the structure or use other strength evidence
ACI 318 requires investigation when cylinders fall below f'c, which may include coring the in-place concrete before rejection.
Question 2: During a concrete pour in cold weather, the minimum concrete temperature as delivered is typically required to be at least:
- 32°F (0°C)
- 40°F (4.4°C)
- 50°F (10°C) (Correct answer)
- 60°F (15.5°C)
Correct answer: 50°F (10°C)
ACI 306 requires fresh concrete temperature to be at least 50°F (10°C) as delivered to maintain adequate strength gain in cold weather.
Question 3: What does a Type C fracture pattern (cone and split) in a 4×8-inch cylinder typically indicate?
- Poor capping technique causing bending failure
- An acceptable fracture pattern within normal test expectations (Correct answer)
- Improper aggregate size relative to cylinder diameter
- Cylinder was tested before reaching 28-day strength
Correct answer: An acceptable fracture pattern within normal test expectations
A cone-and-split fracture (Type 3 per ASTM C39) is a normal, acceptable fracture pattern for cylinder compression tests.
Question 4: For a concrete air content test using the volumetric method (AASHTO T 196), which aggregate type requires this method instead of the pressure meter?
- Granite aggregate
- Limestone aggregate
- Lightweight aggregate (Correct answer)
- Crushed gravel aggregate
Correct answer: Lightweight aggregate
The volumetric method is required for lightweight aggregates and highly porous aggregates that absorb pressure, making the pressure method unreliable.
Question 5: When transporting fresh concrete field samples to the testing area, the maximum elapsed time between obtaining the sample and starting tests is:
- 5 minutes
- 15 minutes (Correct answer)
- 30 minutes
- 45 minutes
Correct answer: 15 minutes
AASHTO R 60 requires that tests begin within 15 minutes of obtaining the composite sample.
Question 6: In the NETTCP program, which test is used to evaluate the resistance of hardened concrete to chloride ion penetration?
- AASHTO T 97 (Flexural Strength)
- AASHTO T 259 (Ponding Test) or AASHTO T 277 (Rapid Chloride Permeability) (Correct answer)
- AASHTO T 121 (Unit Weight)
- AASHTO T 158 (Bleeding)
Correct answer: AASHTO T 259 (Ponding Test) or AASHTO T 277 (Rapid Chloride Permeability)
Chloride permeability is measured by the 90-day ponding test (T 259) or the rapid chloride permeability test (T 277/ASTM C1202).
Question 7: What is the primary reason concrete cylinders are stored in a moist room or lime-saturated water bath during standard curing?
- To prevent carbonation from reducing surface strength
- To maintain adequate moisture for continued cement hydration (Correct answer)
- To keep cylinder temperature above 90°F for faster strength gain
- To prevent shrinkage cracking of the cylinder ends
Correct answer: To maintain adequate moisture for continued cement hydration
Moist curing prevents moisture loss so that cement hydration can continue, which is essential for strength development.
NETTCP Concrete Testing 5
A concrete cylinder fails at 3,800 psi at 28 days when the specified f'c is 4,000 psi.
What is the immediate required action?