Concrete Strength & Testing Methods Flashcards
7 cards from real ACI practice questions. Tap to flip, then mark Knew It or Still Learning — missed cards come back until you master them.
Read the first 7 Concrete Strength & Testing Methods flashcards as text
What is the primary purpose of the air content test (ASTM C231) on fresh concrete?
Answer: To verify adequate entrained air for freeze-thaw durability
Air content testing verifies that entrained air is within specification to protect concrete against freeze-thaw cycles.
A concrete cylinder is tested and fails by shearing along a diagonal plane at roughly 45°. This is considered a:
Answer: Normal and acceptable cone-and-shear failure pattern
A cone-and-shear failure (diagonal crack) is a normal, acceptable fracture pattern per ASTM C39.
Which factor does NOT directly influence the compressive strength of hardened concrete?
Answer: Color of the formwork
Formwork color has no effect on concrete compressive strength; the other factors directly influence the microstructure.
What is the Windsor probe test used to evaluate in concrete?
Answer: In-place compressive strength by measuring probe penetration resistance
The Windsor probe test fires a hardened steel probe into concrete and correlates penetration depth with in-place compressive strength.
What minimum number of standard cylinders should be made from each concrete sample per ACI and ASTM guidelines for a typical strength test?
Answer: 2
A minimum of two cylinders are made per sample so that the average of both is used for the strength test result, reducing variability.
What happens to concrete strength when ambient temperature during placement is excessively high (above 90°F)?
Answer: Early strength increases but long-term strength may decrease
High temperatures accelerate early hydration but can reduce 28-day and long-term strength due to incomplete or poorly distributed hydration products.
When testing concrete for flexural strength using the third-point loading method (ASTM C78), where does failure typically occur in a properly behaved specimen?
Answer: Within the middle third of the span
Third-point loading creates uniform moment in the middle third, so a valid test fracture occurs within that zone.