ACI Concrete Strength & Testing Methods 3

Question 1

The splitting tensile test (ASTM C496) applies load to a concrete cylinder in which direction?

Accepted Answer

Along the horizontal diameter (side-to-side)

Answer

ASTM C496 loads the cylinder diametrically (horizontally along its length), creating indirect tensile stress that splits the cylinder.

Question 2

Which curing method produces the highest 28-day compressive strength in concrete cylinders?

Accepted Answer

Moist curing at 73°F (23°C)

Answer

Moist curing at 73°F maintains the moisture needed for continued cement hydration, maximizing strength development.

Question 3

What is the purpose of the 7-day compressive strength test on a concrete cylinder?

Accepted Answer

It provides an early indication of whether 28-day strength will be achieved

Answer

The 7-day test gives an early estimate — typically 65–70% of 28-day strength — so corrective action can be taken if needed.

Question 4

When taking a concrete sample for cylinder fabrication from a truck, at what point in the discharge should the sample be taken?

Accepted Answer

After the first 10% and before the last 10% of the load is discharged

Answer

ASTM C172 requires sampling after discarding the first and last 10% of the load to obtain a representative sample.

Question 5

Cores drilled from hardened concrete are typically accepted if their compressive strength averages at least what percentage of the specified f'c?

Accepted Answer

85%

Answer

ACI 318 generally accepts drilled cores when the average of three cores is at least 85% of f'c and no single core is below 75% of f'c.

(ACI) American Concrete Institute Concrete Finisher Practice Test