Concrete Testing & Quality

"Trust but verify" is the philosophy of concrete quality control. Even the best mix design can be compromised by variations in materials, batching errors, delivery delays, improper placement, or inadequate curing. That is why concrete is one of the most heavily tested construction materials. This lesson covers the field and laboratory tests used to verify that concrete meets its design requirements.

Fresh concrete tests are performed at the jobsite before the concrete is placed. They verify that the concrete delivered is consistent with the approved mix design.

Slump Test (ASTM C143)

The slump test is the most common field test for concrete workability.

Procedure:

1. Moisten the slump cone (a 12-inch tall truncated cone, 4" top opening, 8" bottom opening).
2. Fill the cone in three equal layers, rodding each layer 25 times with a 5/8" diameter, 24" long tamping rod.
3. Strike off the top surface flush with the top of the cone.
4. Lift the cone straight up in 5-7 seconds.
5. Measure the difference between the top of the cone and the highest point of the slumped concrete. This measurement is the slump.

Interpretation:

• Specified slump is stated on the mix design (e.g., "4-inch slump, ±1 inch").
• Higher slump = more fluid concrete. Lower slump = stiffer concrete.
• If slump is out of specification, the concrete may be rejected. Do not add water to correct low slump — request a plasticizer from the batch plant.

Air Content Test (ASTM C231 - Pressure Method)

Air content is critical for freeze-thaw durability. The pressure method is the most common field test.

Procedure:

1. Fill a Type B pressure meter bowl with concrete in three layers, rodding each layer.
2. Strike off the top and attach the pressure lid.
3. Close air valves, pump air to the initial pressure, then open the valve between the upper and lower chambers.
4. Read the air content directly from the gauge dial.

Typical specifications: 4-8% entrained air for concrete exposed to freeze-thaw (varies with aggregate size; smaller aggregate requires higher air content).

Temperature Test (ASTM C1064)

Concrete temperature affects setting time, strength development, and cracking risk.

Procedure: Insert a thermometer at least 3 inches into fresh concrete and wait for the reading to stabilize.

Specifications: Most specifications require concrete temperature between 50°F and 90°F at the time of placement. Hot concrete (>90°F) sets too fast and is prone to cracking. Cold concrete (<50°F) sets too slowly and is vulnerable to freezing.

Unit Weight (Density) Test (ASTM C138)

The unit weight test verifies that the concrete has the expected density, which can indicate the correct proportioning of ingredients.

Procedure: Fill a known-volume container with concrete, consolidate, strike off, and weigh. Calculate density: weight ÷ volume.

Normal-weight concrete: 140-150 lb/ft³ (typically about 145 lb/ft³). Lightweight concrete: 90-120 lb/ft³ (uses lightweight aggregate).

Compressive Strength Test (ASTM C39)

The compressive strength test is the primary quality control test for hardened concrete. It verifies that the concrete has achieved its specified design strength (f'c).

Specimen preparation (ASTM C31 — Field-Made Cylinders):

1. Standard cylinder size: 6" diameter × 12" height (or 4" × 8" for high-strength concrete).
2. Fill in three layers (6"×12") or two layers (4"×8"), rodding each layer 25 times.
3. Strike off the top and cap or seal the cylinder.
4. Store cylinders on-site for 24 hours in an insulated container (initial curing).
5. Transport to the testing laboratory.
6. Moist-cure in a lime-saturated water bath or fog room.

Testing schedule:

• 7-day break: Two cylinders tested at 7 days. Results give an early indication of strength development. Concrete typically reaches 65-75% of its 28-day strength at 7 days.
• 28-day break: Two cylinders tested at 28 days. This is the official strength test. Results must meet the specified f'c.

Acceptance criteria (per ACI 318):

1. No individual strength test (average of two cylinders) shall be more than 500 psi below f'c.
2. The running average of any three consecutive tests shall equal or exceed f'c.

If test results fail to meet these criteria, the concrete may be:

• Investigated further (cores drilled from the structure)
• Subjected to load testing
• Required to be removed and replaced (worst case)

Core Testing (ASTM C42)

When cylinder tests indicate potentially low strength, cores may be drilled from the actual structure to assess in-place concrete strength.

Procedure:

1. Diamond-core drill is used to extract cylindrical samples (typically 4" diameter) from the structure.
2. Cores are trimmed, capped or ground flat, and tested in compression.
3. Core strengths are typically 85-90% of standard cylinder strengths due to the drilling process and differences in curing.

Acceptance criteria (ACI 318): Concrete is considered structurally adequate if the average of three cores is at least 85% of f'c and no individual core is less than 75% of f'c.

Non-destructive methods assess concrete without removing samples:

Rebound Hammer (Schmidt Hammer - ASTM C805)

A spring-loaded hammer strikes the concrete surface, and the rebound distance is measured. Harder, stronger concrete produces a higher rebound number. The rebound hammer provides a quick estimate of concrete strength but is not highly accurate — it is best used for comparing relative strengths across a structure, not for determining absolute strength.

Ultrasonic Pulse Velocity (ASTM C597)

Ultrasonic pulses are transmitted through the concrete and the travel time is measured. Faster travel time indicates denser, stronger concrete. Slower travel time may indicate voids, cracks, or deterioration.

Ground-Penetrating Radar (GPR)

GPR uses electromagnetic waves to "see" into concrete. It can locate:

• Rebar position and spacing
• Post-tension cables
• Voids and delaminations
• Conduits and embedded objects

GPR is essential before cutting, drilling, or coring existing concrete to avoid hitting reinforcement or embedded utilities.

A concrete test report (also called a "break report" or "cylinder break report") contains:

• Project information: Project name, location, contractor, supplier.
• Mix information: Mix design number, specified f'c, slump, air content, w/c ratio.
• Delivery information: Truck number, batch time, delivery time, quantity.
• Field test results: Slump, air content, temperature, unit weight at the time of sampling.
• Cylinder identification: Specimen numbers, date cast, curing conditions.
• Break results: Date tested, age at test (7-day, 28-day), compressive strength (psi), type of fracture.
• Pass/fail determination: Does the strength meet the specified f'c?