How to Prepare Cube Samples from Fresh Concrete?

To prepare concrete cube samples properly, take a representative sample of the fresh concrete, remix it, fill clean and lightly oiled moulds, compact each layer without segregation, finish and identify every specimen, then protect and cure the cubes under the conditions required by the governing standard.

What a Concrete Cube Sample Represents

A cube result is meaningful only when the concrete in the mould represents the concrete placed in the structure. The sample should therefore be taken from the required point of discharge or placement and linked to the correct batch, truck, grade, pour location, casting time, and test age.

The 7-day strength gives an early indication. The 28-day result is commonly used for acceptance, but one cube should not be treated as the final decision. The project specification and acceptance code define how individual specimens, sample means, and groups of test results are judged.

Equipment for Preparing Concrete Cubes

The following equipment is normally needed:

• personal protective equipment for wet concrete, including suitable gloves and eye protection;
• sampling scoop, containers, and a clean non-absorbent remixing tray;
• 150 mm cube moulds, or another size specifically permitted by the governing standard;
• spanner, wire brush, cleaning rags, and a suitable mould-release agent;
• compacting bar or approved vibration equipment;
• steel float or trowel and a soft-faced mallet;
• moist cover or plastic sheet for initial protection;
• permanent marker, specimen labels, and sampling/cube-making report; and
• controlled curing tank and a safe means of transport.

Concrete Cube Preparation Procedure

Sampling, Remixing, and Filling

1. Prepare the moulds. First, remove old concrete, check the faces and joints, assemble the correct mould parts, tighten all bolts, and apply only a thin layer of release agent. Place each mould on a rigid, level surface away from vibration.
2. Take a representative sample. Next, obtain the sample according to IS 1199 Part 1 and the project inspection plan. Do not make acceptance cubes from a convenient scoop taken only from the first part of a truck discharge.
3. Remix the sample. Then, combine and remix the concrete on the non-absorbent tray until it is uniform. Cast the cubes as soon as practicable after sampling.
4. Fill the moulds. After remixing, for the common 150 mm cube and hand compaction, place the concrete in layers not more than 50 mm thick. Move the scoop around the mould to distribute the concrete evenly.
5. Compact each layer. In addition, apply the specified number of uniformly distributed strokes. For the traditional 150 mm Indian cube procedure, this is at least 35 strokes for each of three layers, or 105 strokes per cube. Let the rod penetrate the underlying layer without striking the mould bottom. Tap the mould sides with a soft-faced mallet to close rod holes and release large air bubbles.

Finishing, Identification, and Curing

6. Use vibration only when specified. However, stop when full compaction is achieved. Excess vibration can cause segregation and laitance. Self-compacting concrete is normally filled and levelled without rodding or mechanical vibration unless the approved procedure says otherwise.
7. Finish the surface. Next, remove surplus concrete and level the top with a steel float or trowel. Clean the outside and top edges of the mould.
8. Identify and record. Then, mark the mould immediately. The specimen ID should remain linked to the represented concrete from casting through testing.
9. Protect the fresh cubes. After that, cover the moulds to prevent evaporation and store them where they will not be shocked, moved, heated, chilled, or vibrated. Under IS 1199 Part 5, initial storage is at 27 ± 3°C.
10. Demould and cure. Finally, leave the specimens in the mould for at least 16 hours and not more than 3 days; about 24 hours is normal site practice. After careful demoulding, place them immediately in clean water at 27 ± 2°C until the required test age. Protect the cubes from drying during transport and handover.

How Many Cubes and Samples Are Required?

Do not assume that every job requires exactly six cubes. The Indian Railways procedure based on IS 516 (Part 1/Sec 1):2021 states that at least three specimens are tested at each selected age. Therefore, testing at both 7 and 28 days commonly means six cubes, but the project may require additional early-age, reserve, durability, or field-cured specimens.

The following schedule is the familiar minimum sampling frequency from IS 456 practice. Use it only when it is the governing project rule; stricter contract or owner requirements take priority.

Samples should be distributed over the concreting period and taken at random. The responsible parties can agree to a higher frequency. A lower frequency should never be assumed without written authority from the governing specification.

Example of a Traceable Cube ID

A short mark on the cube can refer to a complete record. For example, T2-C35-S04-28D may identify tower 2, concrete grade C35, sample 04, and a 28-day specimen. The accompanying report should contain the full details.

Common Cube-Making Errors

Before Sending Cubes to the Laboratory

• Confirm every cube ID is legible on two cast faces and matches the report.
• Record unusual sampling, water addition, delay, damage, temperature, or curing interruption.
• Keep water-cured specimens wet and protected from damage during transfer.
• Send the required test ages and project acceptance information with the specimens.
• Review the separate compressive strength test of concrete for the laboratory testing stage.

References

• Bureau of Indian Standards – current concrete standards family listing.
• Indian Railways Institute of Civil Engineering – Compressive Strength of Concrete, based on IS 516 (Part 1/Sec 1):2021.
• BSI – BS EN 12390-2:2019 current release and scope.
• Mineral Products Association/BRMCA – Practical Guide for Site Personnel.
• The Concrete Society – Making Cubes on Site, updated 2025.