Cube test of concrete Technology notes

              CUBE  TESTING 

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  this 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 their 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 
e)  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 day                 40%
7 day                 65%
14 day               90%
28 day               99%
                             By R k maurya 
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