- To determine the compressive strength of cement concrete cubes
- Batch mixer
- Vibrating table
- Cube moulds
- Tamping road 16mm diameter, 6m long and with bullet point at the lower end
- Standard compression testing machine
- Steel scale with an accuracy of 1mm
- Balance of capacity I Okgs and sensitivity 1 gram
- Test specimens shall be either from Laboratory mix or from mix produced by Batching plant.
Preparation of Mix in the Laboratory
- Collect representative samples of the materials for use in the particular concrete design by careful sampling.
- Bring all the materials to room temperature, preferably 27 ± 3°C before the mixing.
- Store the cement in a dry place, preferably in airtight metal containers.
- Samples of aggregates for each batch of concrete shall be of the desired grading and in air-dried condition.
- Weigh the portion of materials including water as required for specified mix design to an accuracy of 0.1% of the total weight of the batch.
- Each batch of concrete shall be of such a quantity as to give about 10% excess after casting the descried no of test specimens.
- When mixing is done by machine introduce all the mixing water into the drum before the solid materials.
- Load the drum with one half of the coarse aggregate, then with the fine aggregate, then with the cement and finally with the remaining coarse aggregates.
- The period of mixing shall not be less than 2 minutes after all the materials are placed in the drum and shall continue till the resulting concreting is uniform in appearance.
- After thoroughly mixing pour the concrete on a watertight non-absorbent platform and fill into the moulds before segregation occurs.
Casting of Specimens
- Fill the concrete into the mould in layers approximately 5cm deep.
- When compacting by hand use standard tamping rod and each layer is subjected to 35 strokes uniformly distributed.
- When compacting by means of table vibrator each layer shall be vibrated until the specified compaction is obtained.
- In both, the cases after the top layer has been compacted finish the surface of the concrete in level with the top of the mould using a trowel and cover with a glass or metal plate to prevent evaporation.
- Store the test specimens in a place free from vibration, in the moist air of at least 90 percent relative humidity and at a temperature of 27 ± 2°C for 24 ± 1/2 hours from the addition of water to the dry ingredients.
- After this period, mark the specimens and remove from the moulds and unless required for the test within 24 hours Curing
- Immediately submerge the cubes in a clean, freshwater or saturated lime solution and keep there until taken out just prior to testing.
- Renew the, water or solution in which the specimens are submerged for every seven days, and the temperature of the water is maintained with the specified limits.
- Conduct testing at recognized ages of the specimens, the most usual being 7 and 28 days.
- When it may be necessary to obtain the early strength, conduct tests at the ages of 24 ± 1/2 hours and 72 ± 2 hours.
- Calculate the ages from the addition of water to the dry ingredients.
- Test at least three specimens preferably from different batches at each selected age.
- Test the specimens stored in water immediately on removal from water and while they are still in wet condition.
- Wipe off surface water, grit and any projecting fins removed.
- Record the weight and dimensions of the specimens to the nearest 0.20mm.
- Wipe the bearing surface of the testing machine clean and dry.
- Place the cube specimens in the machine in such a manner that the load is applied to opposite sides of the cubes as cast, that is, not to the top and bottom.
- Carefully align the axis of specimen carefully with the center of the thrust of the spherically seated platen.
- Do not use any packing between the faces of the test specimen and the platens of the testing machine.
- Apply the load with out shock and increase continuously at a rate of approximately 140 kg/cm2/min until the resistance of the specimen of the increasing load breaks down and no further load can be sustained.
- The maximum load applied to the specimen shall then be recorded and the appearance of the concrete and any unusual features shall be noted.
Compressive strength = Load/Cross-sectional area N/mm2
- Report the individual and average of three values to the nearest second decimal.
- The individual variation of the specimens shall not be more than ± 15 percent of the average.
DETERMINATION OF COMPRESSWE STRENGTH
|Cube No||Weight in grams||Size of Cube||Area in mm2||Load in KN||Stress in N/mm2|
|Average Compressive Strength : Required Strength :|