koya university lab report

1. Koya University
Faculty of Engineering
Civil Department - 2nd Stage
Concrete Technology
Test no: 9
Compressive Strength of Concrete Cubs
Student Name: Muhammad Saleem Asaad
Group: B
Expr Date: 27-3-2015
Submission Date: 15-5-2015

2. Introduction
All concrete design is based on a specific strength of concrete.
This may vary from project to project, but is usually in the
range from 15 MPa to 50 MPa. This procedure describes the
method for determining the compressive strength of concrete
cubes. The concrete strength is normally tested at an age of 7
and 28 days.

3. Apparatus:
1. Mold of cast iron or plastic or steel with (150 x 150 x 150
mm).
2. Scoop.
3. Steel compacting rod weighing 1.8 kg, 380 mm long and
having a ramming face
25 mm (1 in.) square.
4. Trowel.
5. Sampling tray.
6. Compression testing machine.
7. Balance with min. 10 kg capacity and accuracy of 1g.

4. Test Procedure
Take a sample of fresh concrete (the sample shall be thoroughly mixed just
before it is molded). For each sample of fresh concrete, there shall be made two
test cubes specimens.
 Place the molds on a rigid horizontal surface. Lubricate the internal face of
molds thoroughly (avoid excess lubricant).
 Fill the molds with concrete in three equal layer and compact each layer with
compacting rod using 36 evenly distributed strokes.
 With the sharp edge of a trowel remove the excess concrete from the surface
of the mold and smoothen the surface without pressing.
 Following the above procedure, prepare 6 samples (3 samples for 7 days test &
3 samples for 28 days test).
 Cover the test cubes (in the molds) with an impervious sheet and store them in
a place free from vibration (avoid moving samples before 24 hrs.). The storing
place preferably shall have a relative humidity of min. 90% and temperature of
about 25⁰C.
 After initial curing period of 24 hours, make each cube so that it can be clearly
identified, and remove it carefully form the mold.
 Place the cubes immediately after removing the molds in a temperature
controlled water bath having 25⁰C temperature.
 After 7 days and 28 days, take three cubes for each mentioned age and weigh
each sample in surface dry condition.
 Check the nominal dimensions of each sample by measuring.
 Determine the density of each specimen.
 Carefully center the cube on the lower plate of compression machine and
ensure that the load will be applied to two opposite cast face of the cube.
 Apply and increase the load continuously at a nominal rate within the range 12
MPa/min. to 24 MPa/min. until no greater load can be sustained. Record the
maximum load applied to the cube.
 Note any unusual feature in the type of failure. (Figure 9.1 & 9.2)
 Repeat the above procedure for the rest of samples.

5. Calculation
Calculate the cross-sectional area of the cube face from the checked nominal
dimensions or from measure dimension. Calculate the compressive strength of
each cube by dividing the maximum load by the cross-sectional area. Express
the result to the nearest 0.5 MPa (N/mm2).
An average of three values should be taken as the representative of the batch,
provided the individual variation is not more than ±15% of the average.
Otherwise repeat tests should be done. This is reported as the compressive
Strength of the test sample.
Figure 9.1: Satisfactory failures

7. Calculation
Cube size b=100mm h=100mm w=100mm
Area of cube = b h =100*100=10000mm2
Sample І (1.5 Kg water)
1- 𝜎 =
P
𝐴
=
600.1∗103
10000
=60 N/mm2(Mpa)
2- 𝜎 =
P
𝐴
=
566.6∗103
10000
=56.66 N/mm2(Mpa)
3- 𝜎 =
P
𝐴
=
616.8∗103
10000
=61.68 N/mm2(Mpa)
Average =
60+56.66+61.68
3
∗ 0.9 = 53.508 Mpa
Sample П (1.5Kg water + 40g super plasticizer)
1- 𝜎 =
P
𝐴
=
627∗103
10000
=62.70 N/mm2(Mpa)
2- 𝜎 =
P
𝐴
=
645∗103
10000
=64.50 N/mm2(Mpa)

8. 3- 𝜎 =
P
𝐴
=
555.6∗103
10000
=55.56 N/mm2(Mpa)
Average =
62.70+64.50+55.56
3
∗ 0.9 = 54.828Mpa
Sample Ш (2 Kg water)
1- 𝜎 =
P
𝐴
=
449.9∗103
10000
=44.99 N/mm2(Mpa)
2- 𝜎 =
P
𝐴
=
479.9∗103
10000
=47.99 N/mm2(Mpa)
3- 𝜎 =
P
𝐴
=
478.3∗103
10000
=47.83 N/mm2(Mpa)
Average =
44.99+47.99+47.83
3
∗ 0.9 = 42.243 Mpa

9. Discussion
Compressive strength test is a main and very useful test for all
concrete design that base on a specific strength of a concrete, this test
do on a cube shape concrete in British standard with a 100×100×100
mm at an age 7-28 day. Compressive strength of a sample have
opposite relation with amount of water when water is increase
compressive strength decrease so when we need a high slump
instead of increase amount of water is better use a some amount of
super plasticizer and concrete will be more workable and higher
compressive strength than without super plasticizer , in our test we
make three sample first with 1.5kg water and second with same
amount of water and add a 40g super plasticizer and third with 2 kg
water in the result second sample have a higher compressive strength
and more workability and the first one is better than second one
because of a less amount of water , during the test may be have same
errors for example not put sample in water or test the sample before
or after permissible age or may be Compression testing machine is
read error in conclusion we learn that with super plasticizer
compressive strength is higher and we know compressive strength is
useful for design the structure load of a building.