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Compressive Strength of Concrete
Among the many tests applied to concrete, this is the most important
one which gives an idea about all the properties of concrete and its
compressive strength. Through this single test, one judges whether
.the concrete casting has been done correctly or not
The compressive strength of concrete depends on many factors such
as water-cement ratio, cement strength, quality of concrete
.materials, quality control during concrete production, etc
Compressive strength test is performed on cube or cylinder
For cube testing, two types of specimens are used either cubes of
size 15 cm x 15 cm x 15 cm or 10 cm x 10 cm x 10 cm depending on
the size of aggregate. For most of the works, cube moulds of size 15
.cm x 15 cm x 15 cm are commonly used
It is properly stirred and tempered so that there are no voids. After
24 hours these moudls are removed and the test specimens are
placed in water for curing. The top surface of this specimen should
be level and smooth. This is done by placing cement paste and
.spreading smoothly over the entire area of the specimen
These specimens are tested by compression testing machine after 7
days of curing or 28 days of curing. The load should be applied
gradually at a rate of 140 kg/cm2 per minute until the specimens fail.
The load at failure divided by the area of the specimen gives the
.compressive strength of concrete
:Procedures
Preparing the tools used-)1
.Pressure testing machine-
Steel compaction rod with a diameter of 1.6 cm and a length of -
. 60 cm
- .Cylindrical or cubic molds
- Mixers .
Preparing concrete samples2-) :
At this stage, samples are prepared and taken from the concrete
mixture that is used on site, where three samples are taken, either
cylinders or cubes. It is worth noting that three samples must be
.taken at each specific age for testing (1, 3, 7, 28) days
:Mixing concrete -)3
Either it is mechanical mixing using laboratory mixers or manual
mixing according to stages in which cement and fine gravel sand are
added and mixed well, then coarse gravel is added and mixed until
the gravel is distributed in a homogeneous and uniform manner, and
then water is added until a good and homogeneous concrete
.appears
:Pouring concrete into the molds-)4
The molds are cleaned and greased from the inside with an oily -
.substance that helps prevent concrete from sticking to them
The concrete is poured in three layers, and each layer is compacted -
35 times using a compacting rod, and its upper surface is leveled, the
.mold is covered, and placed without disturbance
Treating concrete samples-)5
At this stage, the sample is kept in the molds in humid air for 24
hours a day, and then we take the samples out of the molds and
.immerse them in fresh water for the specified period of treatment
:Conducting the concrete compressive strength test-)6
First, the sample is removed from the fresh water after the end of -#
.the treatment period, and its surface is wiped of water
.Measure the dimensions of the sample-#
The sample is placed in its appropriate place in the testing machine -#
between the supports so that the load is applied vertically to the
.casting face
Place the moving part of the upper machine in contact with the -#
.upper surface of the sample
The loads are applied gradually until we reach the maximum load -#
.that causes the sample to fail
Finally, the maximum load is recorded-#
The concrete durability strength is recorded at different ages as a
.percentage of the resistance at day 28
At the age of one day, the resistance is 16 percent of the strength of
.28 days
At the age of three days, the compressive resistance is 40 percent of
.the strength of 28 days
At the age of seven days, the resistance is 65 percent of the strength
.of 28 days
At the age of 14 days, the resistance is 90 percent of the strength of
.28 days
.At the age of 28 days, the resistance is 100 percent -
