Concrete tests

 

 CONCRETE TESTS

 

SLUMP TEST

 

                  Concrete slump test is to determine the workability or consistency of concrete mix. It is most simple workability test for concrete, involves low cost and provides immediate results.

Procedure:

•      Take a mould and clean the internal surface of the mould and apply oil

•      Place the mould on a smooth horizontal non-porous base plate

•      Fill the mould with the prepared concrete mix in 4 layers

•      Tamp each layer with 25 strokes of the rounded end of the tamping rod

•      Remove the excess concrete and level the surface with a trowel 

•      Clean away the mortar or water leaked out between the mould and base plate

•      Raise the mould from the concrete immediately and slowly in vertical direction

•      Measure the slump as the difference between the height of the mould and that of height point of the specimen being tested

v    Slump between 25mm – 50mm means that the concrete mix has low workability

v    Slump between 50mm – 100mm indicates medium workability

v    Slump  greater than 100mm indicates higher workability   HOW TO PREVENT LOW SLUMP:

ª Time

•            Eliminate every possible delay

•            Workable concrete must be established by trial ª Temperature

•            Cooling concrete materials such as aggregate and water (ice, liquid nitrogen)

•            Slow rate of setting time by retarder or supplementary cement material

ª Aggregate

•    Sprinkled to make aggregate wet and pre-saturate. So that they will not take water from concrete after mixing or during pumping

•    Highly absorption poor aggregate preferably should not be used

 CUBE TEST

              Concrete cube test gives the compressive strength of concrete. It can be defined as the characteristic strength of 150mm size concrete cube tested at 28 days. Compressive strength depends on many factors such as water cement ratio, cement strength, quality of concrete material and quality control during production, of concrete etc.  

Procedure:

•      First take 150mm*150mm*150mm*150mm mould

•      Then pour concrete into the mould with three layers

•      Each layer shall be compacted with 35 blows

Figure7.6 – Cube crushing machine

using tamping rod. Diameter of tamping rod is 16mm. It has 600mm length

•      After compacted top layer, surface is made in flush using trowel

•      Cover the mould with damp hessian cloth immediately 

•      Keep it under 24 hours 22 ̊C - 32 ̊ C temperature

•      Label the cubes to identify

•      After 24 hours, strip the mould and store the cubes in 24  ̊C  - 30  C water for curing till taken out just testing

•      Cured cubes are tested on a compression testing machine after 3, 7 or 28 days

Compressive strength of concrete = maximum compressive load

                                                            Cross sectional area of cube

 

 

Grade of concrete	Minimum compressive strength at 7 days             N/mm2	Compressive strength at 28 days             N/mm2
M15	10	15
M20	13.5	20
M25	17	25
M30	20	30
M35	23.5	35
M40	27	40
M45	30	45

                                  Table 7.1 – Compressive strength for concrete grades

Causes for low compressive strength:

•      Improper concrete mix design

•      Improper concrete cube sample preparation

•      Improper curing condition

•      Calculation errors

•      Errors in crushing machine

 

 REBOUND HAMMER TEST

 

                       

Rebound hammer test is done to determine the compressive strength of concrete. If cube test is fail after 28 days, Engineer can instruct to do hammer test for the concrete structure. In our site rebound hammer test was done for some pile caps.                      The rebound of an elastic mass depends on the hardness of the surface against which its mass strikes. When the plunger of the rebound hammer is pressed against the surface of the concrete, the sprig controlled mass rebounds and the extent of such a rebound depends upon the surface hardness of the concrete. The rebound value is red from a graduated scale and the compressive strength can be read directly from the graph provided on the body of

the hammer.                                                                    

Figure 7.10 – Rebound hammer test

 

 

Precautions for rebound hammer test:

 

•      The surface should be smooth, clean and dry.

•      The loose surface should be rubbed off with a grinding wheel before testing.

•      The point of impact should be at 20mm way from the edge or sharp discontinuity.

•      The test should not be conducted on the rough surface resulting from incomplete compaction of concrete, loss of grout, spoiled or tooled surface.  

 

 

Average rebound number	Quality of concrete
>40	Very good
30-40	Good
20-30	Fair
<20	Poor or delaminated

  

Table 7.2 – Concrete quality for average rebound number