Compressive Strength of Concrete and Cube Test
Compressive strength is the
capacity of material or structure to resist or withstand under compression. The
Compressive strength of a material is determined by the ability of the material
to resist failure in the form cracks and fissure.
In this test, the push force
applied on the both faces of concrete specimen and the maximum compression that
concrete bears without failure is noted.
Concrete testing helps us to
majorly focus on the Compressive strength of concrete because it helps us to
quantify the ability of concrete to resists Compressive stresses among
structures where-as other stresses such as axial stresses and tensile stresses are
catered by reinforcement and other means.
In
technical point of view,
Compressive Strength of
concrete is defined as the Characteristic strength of 150mm size concrete cubes
@28 days.
Factors affecting compressive strength of
concrete:-
Coarse aggregate:-
Concrete is made homogenous
by combining aggregates, cement, sand, water and various other admixtures. But
even with proper mixing, there may arise some microcracks due to differences in
thermal and mechanical properties of coarse aggregates and cement matrix, which
leads to failure of concrete.
Concrete technologists came
up with theoretical concepts regarding size of aggregates, which as the size of
aggregate being the major contributor of compressive strength. So if the size
of aggregate is increased, then it would lead increased compressive strength.
This theory was later
discarded, as experiments proved that greater size of aggregates showed
increased strength in initial phases but reduced exponentially.
The sole reason for this
strength drop was due to the reduced surface area for bond strength between
cement matrix and aggregates and weaker transition zone.
Air-entrainment:-
Air entrainment in concrete
was one of the concepts developed by cold countries in order to prevent damages
due to freezing and thawing. Later on, as experimentation’s proved
multidimensional benefits of air entrainment along with improved the
workability of concrete at lower water/cement ratio.
As the achievement of the
desired workability at lower water content helped one to achieve concrete with
the greater compressive strength which in turn, leads to light concrete with
greater compressive strength.
Water/Cement ratio:-
We are all very aware of how
excess water can be harmful to the strength of concrete. Cement being the major
binding material in concrete needs water for hydration process, but that is
only limited to about (0.20 to 0.25) % of cement content. The excess water
turns out to be beneficial in contributing to workability and finishing of
concrete.
The very aspect where excess
water is considered harmful because as the water in the concrete matrix dries,
it leaves large interstitial spaces among aggregate and cement grains. This
interstitial space becomes primary cracks during compressive strength testing
of concrete.
Why do we test concrete for 7 days, 14 days
& 28 days?:
Concrete gains maximum
strength at 28days. Since in construction sector great amount of capital is at
stake, so instead of checking strength at 28 days we can check strength in
terms of concrete strength psi at 7 and 14 days to predict the target strength
of construction work.
From the below table it is
clear that, Concrete gains 16 % of its strength within 24 hrs, whereas concrete
gains 65% of the target strength by the time of 7 days of its casting.
Till 14 days concrete shows
90% of the target strength and thereafter the gain in strength slows down and
it takes 28days to achieve 99% of strength.
We cant judge the strength
of concrete until it becomes stable. And we also won’t wait for 28 days to
judge the concrete whether it suitable for construction or not to keep it
balanced, concrete is tested at various intervals.
|
Age in Days
|
Percentage of Strength
|
|
1
Days
|
16%
|
|
3
Days
|
40%
|
|
7
Days
|
65%
|
|
14
Days
|
90%
|
|
21
Days
|
94%
|
|
28
Days
|
99%
|
The maximum strength gain
spike is observed till 14 days, so we test concrete at an intervals of 7 days,
10 days & 14days and If concrete fails to show results of 90% of its
overall strength @14 days then that batching is rejected.
Compressive Strength of Different Grades of Concrete at 7, 14, 21
and 28 Days:
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Compressive strength testing of concrete:-
The test is carried out using 150mm concrete cubes on a Universal testing machine or compressive testing machine.
The test is carried out using 150mm concrete cubes on a Universal testing machine or compressive testing machine.
Apparatus
As
per IS: 516-1959 Compressive testing machine (2000Kn),15cm×15cm×15cm steel cube
molds or Cylinder having Dia 15cm and length 30cm are used.
The
test includes following steps:-
Preparing of material for Cube
test:
All
the material must be brought and stored to an approximate temperature of 27 ±3
degree Celsius. Cement must be uniformly mixed with a trowel in order there
exist no lumps.
Mixing of concrete:
Machine mixing: The
ingredient must not be rotated for more than 2 minutes and the following
pattern must be followed
1>Calculated water,2>50% coarse aggregates,3>fine aggregates,4>cement,5>50% coarse aggregates.
Hand mixing: The process
must be done on the rectangular pan until a homogenous mix is obtained.
Dry mixing of fine
aggregates and cement>addition of coarse aggregate with the even distribution>addition of calculated water
in batch till consistency is achieved.
Casting of specimen
The casting molds
are chosen to be made of cast iron and must be rubbed with grease on inner side
for easy removal of cubes. The specimen must be cast in 3 layers (5cm each) and
properly compacted in order that honeycombing formation
does not take place.
Compaction
In compacting through
tamping bar, 35 strokes must be done in all parts of a cube for proper compacting.
This tamping bar has the dimension of diameter 16mm and length of 0.6m.
Age of test
The cube test for
Compressive strength can be done on 1,3, 7, 14 and 28 days. In some cases, the
strength of greater ages is required which is performed from 13 to 52 weeks.
Number of specimens
It is mandatory to have at
least 3 specimens for testing from different batches. The mean of compressive
strength achieved by this specimen is used to determine actual strength of the
batch.
Procedure for Compressive strength of concrete or Cube test
:-
1.
Place the prepared
concrete mix in the steel cube mould for casting.
2.
Once it sets, After 24
hours remove the concrete cube from the mould.
3.
Keep the test
specimens submerged underwater for stipulated time.
4.
As mentioned the specimen
must be kept in water for 7 or 14 or 28 days and for every 7 days
the water is changed.
5.
Ensure that concrete
specimen must be well dried before placing it on the UTM.
6.
Weight of samples is
noted in order to proceed with testing and it must not be less than 8.1Kg.
7.
Testing specimens are
placed in the space between bearing surfaces.
8.
Care must be taken to
prevent the existence of any loose material or grit on the metal plates of
machine or specimen block.
9.
The concrete cubes are
placed on bearing plate and aligned properly with the center of thrust in the
testing machine plates.
10.
The loading must be
applied axially on specimen without any shock and increased
at the rate of 140kg/sq cm/min. till the specimen collapse.
at the rate of 140kg/sq cm/min. till the specimen collapse.
11.
Due to the constant
application of load, the specimen starts
cracking at a point & final breakdown of the specimen must be noted.
Calculations
Compressive strength of
Concrete Formula:
The Compressive strength of specimen
can be calculated by dividing maximum load carried by the specimen by
cross-sectional area of the specimen cubes.
The
surface area of specimen: = 150 x 150 = 22500mm² = 225cm²
Assume,
The Max compression load is 450KN
1KN
= 1000N ; 450Kn = 450×100 = 450000N
The
same calculation is done for the specimen at different ages as stated below:
Important Note: As per IS: 516-1959
Minimum three specimens should be tested at each selected age (that means three
specimens at 7 days, three specimens @ 14 days & 28 days) If strength of
any specimen varies by more than 15% of average strength, such specimen
should be rejected.
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Results of cube test
Average
Compressive strength at 7 days = _____N/mm²
The
average Compressive strength at 28 days = _____N/mm²
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