HOW TO MAKE TEST CUBES FROM FRESH CONCRETE?

IS-456 sets the acceptance criteria for quality concrete. In all cases, the 28-day compressive strength will be the criterion for acceptance or rejection of concrete. 7 days of compressive strength can be done to get a relatively quick idea about the quality of concrete.

Device

The following tools are required to make a concrete cube.

  1. sample tray;
  2. mold for making test cubes;
  3. spanner;
  4. Scoop;
  5. steel float or trowel;
  6. compacting bar;
  7. cleaning rags;
  8. a bucket or barrow for transporting samples;
  9. treatment tank
  10. permanent marker.

lb001

number of cubes

6 cubes of 150 x 150 x 150 mm size will be cast, 3 for a 7-day trial and 3 for a 28-day trial. A sample contains 3 cubic samples and their average compressive strength represents the test result of that sample. The individual variation of a set of 3 cubes should not exceed ±15% of the average. If higher, the test result of the sample is invalid.

concrete sample

The concrete sample for the test sample shall be taken at the time of discharge in the mixer or in the case of ready mixed concrete from the transport vehicle. Such samples must be obtained by repeatedly passing a scoop or pail through the discharge stream of concrete. The samples thus obtained shall be mixed with a shovel on a non-absorbent basis until it becomes uniform in appearance.


lb005


The number of samples to be taken depends on the quantity of concrete. As per IS-456, the following table shall be followed while taking the sample.

Volume of concrete in work (m3, number of samples
1-5 1
6-15 2
16-30 3
31-50 4
51 and up 4 plus one additional sample for every additional 50 square meters3 or part of it.

Note:- Frequency of sampling may be agreed internally by the supplier and the buyer.

casting of cubes

Cube mold plates must be removed, thoroughly cleaned and assembled and all bolts fully tight. Then a thin layer of oil should be applied to all the faces of the mould. It is important that the faces of the sides of the cube should be parallel.

lb002

lb003

After taking concrete samples and mixing them, the cubes will be made as described below as soon as possible.

hand casting and compaction

The concrete sample shall be filled in cube moulds in 3 layers, each layer approximately 5 cm deep. In placing each scoopful of concrete, the scoop will be moved around the top edge of the mold to ensure symmetrical distribution of the concrete within the mold as the concrete slides through it. Each layer will be compressed either by hand or by vibration. Each layer of concrete loaded into the mold should be compacted by a tamping bar by at least 35 strokes. The strokes will be penetrating the underlying layer and the layer below will be rotated to its full depth. Where voids are left by the tamping bar, the sides of the mold should be tapped to close the voids.

lb007

lb008

Treatment

Casted Cubes to 22. shall be stored under the shed in a place free from vibration at a temperature of0C to 330c Covered with a wet straw or sack for 24 hours.

marking

They should be clearly marked immediately after the initial curing of the cubes. This can be done by writing a description of the cube in ink on a small piece of paper and placing it on top of the concrete until it is demolished.

lb011

At the end of 24 hours the cube will be removed from the molds and 24 . will be immersed in clear water at a temperature of0c to 300By 7 or 28 days of age after the C test. The cubes shall be tested in saturated and surface dry condition.

For an accurate representation of the actual strength of the concrete in the structure, additional cubes shall be poured, stored and curdled according to the same conditions of that structure, and tested to the required age.

pay attentionWhen the mold is not in use, apply a thin layer of oil on the inner surface of the mold and keep it in a safe place.

Er. Mukesh Kumar

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Er. Mukesh Kumar is Editor in Chief and Co-Funder at ProCivilEngineer.com Civil Engineering Website. Mukesh Kumar is a Bachelor in Civil Engineering From MIT. He has work experience in Highway Construction, Bridge Construction, Railway Steel Girder work, Under box culvert construction, Retaining wall construction. He was a lecturer in a Engineering college for more than 6 years.