Concrete Strength & Testing Methods Flashcards

According to ASTM C39, how many days after casting is the standard compressive strength test for concrete cylinders performed?

Answer: 28 days

ASTM C39 specifies 28 days as the standard age for compressive strength testing of concrete cylinders.

What is the standard size of a concrete cylinder used for compressive strength testing per ASTM C39?

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.

The Schmidt hammer (rebound hammer) test is used to estimate which concrete property?

Answer: Surface hardness related to compressive strength

The Schmidt hammer measures surface rebound to estimate surface hardness, which correlates with compressive strength.

When capping concrete cylinders for testing, what material is typically used to create a flat bearing surface?

Answer: Sulfur mortar or neoprene pad

Sulfur mortar caps or neoprene pads are used to provide uniform, plane bearing surfaces on concrete test cylinders.

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?

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.

What does the term 'modulus of rupture' describe in concrete testing?

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.

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?

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.