Objective of the Project
Project aims on preparation and compressive strength testing of M25 grade concrete cubes of sizes 15 cm x 15cm x 15 cm and 10cm X 10 cm x 10 cm during 6th semester.
Procedure: Compressive Strength Test of Concrete Cubes
Cube test can be done using different sizes of molds, wich are: 15cm X 15cm X 15cm or 10cm X 10cm x 10cm (depends on aggregate size).
Commonly used molds are of size 15cm x 15cm x 15cm.
It should be made sure that Concrete is poured in the molds and tempered properly in such a way that no voids are left. Test performer must remove these molds after 24 and put them in water for curing. Top surface of these specimen should be even and smooth. This can be achieved by putting cement paste and spreading smoothly on whole area of specimen.
Specimens are tested after 7 days or 28 days of curing with the help of compression testing machine. Performer should apply load gradually at the rate of 140 kg/cm2 per minute till the specimens fails. Load at the failure divided by area of specimen gives the compressive strength of concrete.
Procedure for testing Compressive strength of Concrete Cubes is as follows:
Apparatus for Concrete Cube Test
Compression testing machine is used for this particular test.
Preparation of Concrete Cube Specimen
The proportion and material for making these test specimens are from the same concrete used in the field.
6 cubes of 15 cm size and of M15 mix or above
Mixing of Concrete for Cube Test
Mix the concrete either by hand or in a laboratory batch mixer.
(i) Examiner must mix the cement and fine aggregate on a water tight none-absorbent platform until the mixture is thoroughly blended and is of uniform color.
(ii) He/She should then add the coarse aggregate and mix with cement and fine aggregate until the coarse aggregate is uniformly distributed throughout the batch.
(iii) Add water and mix it until the concrete appears to be homogeneous and of the desired consistency.
Sampling of Cubes for Test
(i) Clean the molds and apply oil.
(ii) Fill the concrete in the molds in layers approximately 5cm thick
(iii) Compact each layer with not less than 35strokes per layer using a tamping rod (steel bar 16mm diameter and 60cm long, bullet pointed at lower end)
(iv) Level the top surface and smoothen it with a trowel
Curing of Cubes
Store test specimens in moist air for 24 hours and after this period the mark and remove the specimens from the molds and then keep them submerged in clear fresh water until taking out for test.
Precautions for Tests
The water for curing should be tested every 7 days and the temperature of water must be at 27+-2oC.
Procedure for Cube Test
(i) Remove the specimen from water after specified curing time and wipe out excess water from the surface.
(ii) Take the dimension of the specimen to the nearest 0.2m
(iii) Clean the bearing surface of the testing machine.
(iv) Specimen is placed in the machine in such a manner that the loading is applied to the opposite sides of the cube cast.
(v) Align the specimen centrally on the base plate of the machine.
(vi) Rotate the movable portion gently by hand so that it touches the top surface of the specimen.
(vii) Apply the load gradually without shock and continuously at the rate of 140 kg/cm2/minute till the specimen fails.
(viii) Record the maximum load and note any unusual features in the type of failure.
Examiner should Test a minimum of three specimens at each selected age. If strength of any specimen varies by more than 15 per cent of average strength, results of such specimen should be rejected. Average of three specimens gives the crushing strength of concrete. The strength requirements of concrete.
Calculations for Concrete Cube Tests for Compressive Strength
Size of the cube =15cm x15cm x15cm
Area of the specimen (calculated from the mean size of the specimen )=225 cm2
Characteristic compressive strength(f ck)at 7 days =
Expected maximum load =fck x area x f.s
28 day compressive strength…….. (similar calculation)
Maximum load applied =……….tones = ………….N
Compressive strength = (Load in N/ Area in mm2)=……………N/mm2
Reports of Cube Test
a) Identification mark
b) Date of test
c) Age of specimen
d) Curing conditions, including date of manufacture of specimen
f) Appearance of fractured faces of concrete and the type of fracture if they are unusual
Results of Concrete Cube Test
Average compressive strength of the concrete cube = ………….N/ mm2 (at 7 days)
Average compressive strength of the concrete cube =………. N/mm2 (at 28 days)
Compressive Strength of Concrete at Various Ages
The strength of concrete increases with age. Examiner can do comparison of strength after 28 days of casting.