The Compressive Strength of Cubic Concrete Specimens
Compressive strength of concrete: Out of many test applied to the concrete, this is the utmost important which gives an idea about all the characteristics of concrete. By this single test one judge that whether Concreting has been done properly or not.
Scope
The test method covers determination of compressive strength of cubic concrete specimens. It consists of applying a compressive axial load to molded cubes at a rate which is within a prescribed range until failure occurs. The compressive strength is calculated by dividing the maximum load attained during the test by the crosssectional area of the specimen.
Apparatus
➾ Weights and weighing device.
➾ Tools and containers for mixing.
➾ Tamper (square in cross section)
➾ Testing machine.
➾ Three cubes (150 mm side)
Procedure
1: Prepare a concrete mix as mentioned in (read this test) with the proportions suggested
Such as: 1: 2: 3 with w/c = 55% by mechanical mixer.
2: Prepare three testing cubes; make sure that they are clean and greased or oiled
thinly.
3: Metal molds should be sealed to their base plates to prevent loss of water.
4: Fill the cubes in three layers, tamping each layer with (35) strokes using a tamper,
square in cross-section with 2.54 cm side and 38.1 cm length, weighing 1.818 kg.
5: While filling the molds, occasionally stir and scrape together the concrete
remaining in the mixer to keep the materials from separating.
6: Fill the molds completely, smooth off the tops evenly, and clean up any concrete
outside the cubes.
7: Mark the specimens by a slip of paper on which is written the date and the
Specimen identification. Leave the specimens in the curing room for 24 hours.
8: After that open the molds and immerse the concrete cubes in a water basin for
7 days.
9: Before testing, ensure that all testing machine bearing surfaces are wiped clean.
10: Carefully center the cube on the lower platen and ensure that the load will be
applied to two opposite cast faces of the cube.
11: Without shock, apply and increase the load continuously at a nominal rate within
the range of ( 0.2 N/mm2.s to 0.4 N/mm2.s ) until no greater load can be
sustained. On manually controlled machines, as failure is approached, the
loading rate will decrease, at this stage operate the controls to maintain, as far as
possible, the specified loading rate. Record the maximum load applied to each
cube.
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