COMPRESSIVE STRENGTH OF CONCRETE CUBES


concrete cubesconcrete cube mould

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. For cube test two types of specimens either cubes of 15 cm X 15 cm X 15 cm or 10cm X 10 cm x 10 cm depending upon the size of aggregate are used. For most of the works cubical moulds of size 15 cm x 15cm x 15 cm are commonly used.

This concrete is poured in the mould and tempered properly so as not to have any voids. After 24 hours these moulds are removed and test specimens are put in water for curing. The top surface of these specimen should be made even and smooth. This is done by putting cement paste and spreading smoothly on whole area of specimen.

These specimens are tested by compression testing machine after 7 days curing or 28 days curing. Load should be applied 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.

Following are the procedure for Compressive strength test of Concrete Cubes

APPARATUS

Compression testing machine

PREPARATION OF CUBE SPECIMENS

The proportion and material for making these test specimens are from the same concrete used in the field.

SPECIMEN

6 cubes of 15 cm size Mix. M15 or above

MIXING

Mix the concrete either by hand or in a laboratory batch mixer

HAND MIXING

(i)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)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

(i) Clean the mounds 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

The test specimens are stored in moist air for 24hours and after this period the specimens are marked and removed from the molds and kept submerged in clear fresh water until taken out prior to test.

PRECAUTIONS

The water for curing should be tested every 7days and the temperature of water must be at 27+-2oC.

PROCEDURE

(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) Place the specimen in the machine in such a manner that the load shall be 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 140kg/cm2/minute till the specimen fails

(VIII) Record the maximum load and note any unusual features in the type of failure.

NOTE

Minimum three specimens should be tested 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 there specimens gives the crushing strength of concrete. The strength requirements of concrete.

 

CALCULATIONS

Size of the cube =15cm x15cm x15cm

Area of the specimen (calculated from the mean size of the specimen )=225cm2

Characteristic compressive strength(f ck)at 7 days =

Expected maximum load =fck x area x f.s

Range to be selected is …………………..

Similar calculation should be done for 28 day compressive strength

Maximum load applied =……….tones = ………….N

Compressive strength = (Load in N/ Area in mm2)=……………N/mm2

=……………………….N/mm2

REPORT

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

RESULT

Average compressive strength of the concrete cube = ………….N/ mm2 (at 7 days)

Average compressive strength of the concrete cube =………. N/mm2 (at 28 days)

Percentage strength of concrete at various ages:

The strength of concrete increases with age. Table shows the strength of concrete at different ages in comparison with the strength at 28 days after casting.

Age

Strength per cent

1 day

16%

3 days

40%

7 days

65%

14 days

90%

28 days

99%

Compressive strength of different grades of concrete at 7 and 28 days

Grade of Concrete

Minimum compressive strength N/mm2 at 7 days

Specified characteristic compressive strength (N/mm2) at 28 days

M15

10

15

M20

13.5

20

M25

17

25

M30

20

30

M35

23.5

35

M40

27

40

M45

30

45


191 COMMENTS

      • i have 2 cm height x 1.5 cm Length x 1.5 cm wide. Cubic sample. I stand it with height 2 cm. then loading the test and i toke 1.5 tons max Load. how to calculate the Compressive strength because its not same in size. ???? please help me. thankz

          • dear sir,
            how about the height of sample, I have 3 sample with same mixture, same Area perpendicullar, but different height and maximum load resulted so different. offcourse I think the Compressive Strength are different for 3 samples. can you explain it sir? About height correction.

        • dear sir,
          how about the height of sample, I have 3 sample with same mixture, same Area perpendicullar, but different height and maximum load resulted so different. offcourse I think the Compressive Strength are different for 3 samples. can you explain it sir? About height correction

  1. Please i wish to know which elements should core and hammer test should carried being it,foundation,pedestal,roof or base foundation(lean concrete) or columms in accordance with BS standard or Specifications

    Thank you

  2. Concrete slab (12 cm thick) of surface area of about 600 sq meter is to be cast with M20 concrete. Compressive strength is to be tested after 7 & 28 days. How many cube samples should be taken ?

  3. Dear SIR,
                       will you please specify about the standard which says that 50 % is ok for concrete in 3 days compressive strength or any information on compressive strength of poly propylene fibre mixed concrete.in 3 days compressive strength . Regards Sooraj.

  4. please, i need a mathematical relationship between the compressive strength of concrete with respect to the days of curing.i’ll appreciate if i get it early,cos i need it for my project calculation.THANKS

  5. SIR,FOR M-20 GRADE CONCRETE THE STRENGTH IS M-20 @ 28 DAYS, BUT IF @28DAYS THE STRENGTH ACHIVED IS LESS THAN 20 N/MM2 , THEN WOULD THE WHOLE CONCRETING IS REJECTED? , DOES THERE IS ANY CRITERIA FOR THIS PROBLEM

  6. While assessing compressive strength of concrete by testing 150mmX150mm cube, shall we take stress at the point of failure noticed (in the form of crack) or allow the stressing till needle of dial gauge moves? Please indicate the relevant code (IS code, if available, would be preferable).

    • While assessing compressive strength of concrete by testing 150mmX150mm cube, shall we take stress at the point of failure noticed (in the form of crack)?.-Ans:-..The Crack point is the point of failure/breaking point..

  7. Dear Sir, 
    I would like to know, for compressive strength measurement of cubes in which cement partially replaced with composites, can we use 10 cm mould? Is there a practical advantage of using 15 cm over 10 cm ?

    • when you test a number of standard cube and a number of standard cylinderical specimen of concrete, and draw the normal distribution curve and then calculate the characteristic strength from cube and from cylinder, you will find the following
      Fck(cube) = 1.25 Fck (cylinder)
      => Fck of cylinder = 0.8 Fck (cube)

      Fck being characteristic strength.
      This relation has been obtained from number of tests done on different samples.

    • cube size is 150x150x150, because it has been set as standard size, the value of compressive strength of which has been standardized at 28 days.We can also use 200mm cube, but you have to convert the compressive strength of 200mm cube to the equivalent of 150mm cube to know the actual strength of the concrete.

    • There can be many problems like thorough mixing of concrete, compaction, cement content, water content, size of aggregates etc. These are the main things to consider while mixing concrete. Also, transporting and placing can be a problem, as segregation can take place during transporting and placing. If you take care of your mixing, cement content, aggregate size proportion, water content, compaction etc, you will get good strength concrete.

    • Rate of loading is kept uniform, preferably 140kg/cm2/min. If you calculate the same load on 15cm cube per second, it will be around 5.25kN/sec. The load on structure in actual case is not sudden, and impact strength and compressive strength differs, so we take some uniform loading on cubes, such that cube is tested compression but not impact. So, uniform load of 140kg/cm2/min is used which means 5.25kN/sec, i.e. a M20 concrete will fail in 5 to 6 seconds.

  8. @zwa ryuny:These faces must be thinly coated with mould oil to prevent leakage during filling, and a similar oil film should be provided between the contact surfaces of the bottom of the mould and the base. The inside of the mould must also be oiled to prevent the concrete from sticking to it. The two sections must be bolted firmly together, and the moulds held down firmly on the base plates.

  9. it was really help full to know the strength of concrete at different ages
    n it was always confusion about testing of specimen that it should be tested after 7 , 14 … days of curing nt on de same day

  10. if the rate of loading is increased the strength achieved is more n If you apply the rate of loading still less u will get more strength which is not feasible hence the rate of loading shall b kept at constant rate as mentioned for further information u can refer properties of concrete by neveli

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