NETTCP Concrete Testing 2 — Questions and Answers
Question 1: According to AASHTO T 23, how long must freshly molded concrete cylinders remain undisturbed in their molds after casting?
- 8 hours minimum
- 16 to 32 hours (Correct answer)
- 24 hours maximum
- 48 to 72 hours
Correct answer: 16 to 32 hours
AASHTO T 23 requires cylinders to remain undisturbed in molds for 16 to 32 hours before initial curing.
Question 2: When performing the unit weight test on fresh concrete per AASHTO T 121, what is the correct procedure after filling the measure?
- Weigh immediately without striking off
- Strike off the top and weigh the measure (Correct answer)
- Allow to set 5 minutes then weigh
- Vibrate the measure then weigh
Correct answer: Strike off the top and weigh the measure
After filling and consolidating, the top surface must be struck off level before weighing the measure.
Question 3: Which condition would cause a concrete compressive strength test result to be considered invalid?
- Cylinder tested at 28 days
- Cylinder end is not within 0.5 degrees of perpendicular (Correct answer)
- Cylinder diameter is 4 inches
- Fracture pattern is a well-formed cone
Correct answer: Cylinder end is not within 0.5 degrees of perpendicular
If cylinder ends are not within 0.5 degrees of perpendicular, the result may be unreliable and end preparation is required.
Question 4: What does the term 'w/cm ratio' refer to in concrete mix design?
- Weight of coarse to medium aggregate
- Water to cementitious material ratio by weight (Correct answer)
- Width to compressive modulus ratio
- Water content to concrete mass ratio
Correct answer: Water to cementitious material ratio by weight
The w/cm ratio is the ratio of the mass of water to the mass of all cementitious materials in the mix.
Question 5: During the modified Proctor compaction test of soils under a concrete slab, how many layers are compacted?
- 3 layers
- 4 layers
- 5 layers (Correct answer)
- 6 layers
Correct answer: 5 layers
The modified Proctor test (AASHTO T 180) uses 5 layers compacted with 25 blows each.
Question 6: What is the maximum aggregate size typically allowed in a standard 4-inch diameter concrete test cylinder?
- 3/4 inch
- 1 inch
- 1-1/3 inches (Correct answer)
- 1-1/2 inches
Correct answer: 1-1/3 inches
The maximum aggregate size should not exceed one-third of the cylinder diameter; for a 4-inch cylinder that is 1-1/3 inches.
Question 7: In a bleeding test for fresh concrete (AASHTO T 158), water that accumulates on the surface is collected at what intervals?
- Every 2 minutes for the first 40 minutes
- Every 10 minutes for 60 minutes then every 30 minutes (Correct answer)
- Every 5 minutes until bleeding stops
- Every 15 minutes for 2 hours
Correct answer: Every 10 minutes for 60 minutes then every 30 minutes
AASHTO T 158 requires collecting bleed water every 10 minutes for the first 60 minutes, then every 30 minutes until bleeding stops.
NETTCP Concrete Testing 2
According to AASHTO T 23, how long must freshly molded concrete cylinders remain undisturbed in their molds after casting?