Concrete Strength & Testing Methods Flashcards

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

Answer: Along the horizontal diameter (side-to-side)

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

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

Answer: Moist curing at 73°F (23°C)

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

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

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

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

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

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

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

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

Answer: 85%

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.

What ASTM standard governs the making and curing of concrete test specimens in the field?

Answer: ASTM C31

ASTM C31 covers the standard practice for making and curing concrete test specimens in the field.

How does increasing the water-to-cementitious material (w/cm) ratio affect concrete compressive strength?

Answer: Decreases strength due to higher porosity

A higher w/cm ratio creates more capillary voids after excess water evaporates, directly reducing compressive strength.