NETTCP Concrete Testing 4 — Questions and Answers
Question 1: For a concrete pavement project, the maturity method is used to estimate strength. What two quantities are measured continuously?
- Air content and slump
- Temperature and time (Correct answer)
- Unit weight and temperature
- Compressive strength and time
Correct answer: Temperature and time
The maturity method (ASTM C1074) tracks temperature and time to calculate a maturity index that correlates to strength gain.
Question 2: What is the minimum number of test cylinders that must be made from a single composite sample to represent one strength test result?
- 1 cylinder
- 2 cylinders (Correct answer)
- 3 cylinders
- 4 cylinders
Correct answer: 2 cylinders
A strength test result is the average of at least 2 cylinders made from the same composite sample and tested at the same age.
Question 3: Which AASHTO standard governs the making and curing of concrete test specimens in the field?
- AASHTO T 119
- AASHTO T 121
- AASHTO T 23 (Correct answer)
- AASHTO T 152
Correct answer: AASHTO T 23
AASHTO T 23 is the standard method for making and curing concrete test specimens in the field.
Question 4: A concrete mix has a design strength (f'c) of 4,000 psi. What minimum average strength (f'cr) must be targeted to account for variability?
- 4,000 psi
- 4,200 psi
- 4,700 psi or higher depending on standard deviation (Correct answer)
- 5,000 psi always
Correct answer: 4,700 psi or higher depending on standard deviation
ACI 318 requires the required average strength f'cr to exceed f'c by an amount based on the standard deviation of past test data, typically 700–1,200 psi or more.
Question 5: When striking off the top of a concrete cylinder mold, what tool is most appropriate?
- A curved trowel
- A flat glass plate or steel plate (Correct answer)
- A wooden float
- A rubber mallet
Correct answer: A flat glass plate or steel plate
A flat glass or steel plate is used to strike off the cylinder top to achieve a smooth, level surface perpendicular to the axis.
Question 6: In a concrete flexural strength beam test (AASHTO T 97), the modulus of rupture is calculated differently based on where the fracture occurs. If the fracture is within the middle third, the formula used is:
- R = PL / bd² (Correct answer)
- R = 3Pa / bd²
- R = PL / bd² × correction factor
- R = Pa / bd²
Correct answer: R = PL / bd²
When fracture occurs within the middle third, the modulus of rupture R = PL / bd², using the full span length.
Question 7: What is the effect of high water content on the compressive strength of hardened concrete?
- Increases strength by improving hydration
- Reduces strength by increasing the w/cm ratio (Correct answer)
- Has no effect if cement content is also increased
- Increases strength up to a w/cm of 0.60
Correct answer: Reduces strength by increasing the w/cm ratio
Excess water increases the w/cm ratio, producing more porosity and reducing compressive strength.
NETTCP Concrete Testing 4
For a concrete pavement project, the maturity method is used to estimate strength.
What two quantities are measured continuously?