Lesson - 3
Concrete Testing
Concrete Testing
There are two main tests to be done on
concrete:
The SLUMP test.
The SLUMP test shows the
WORKABILITY of concrete.
Workability measures how easy the
concrete is to place, handle and
compact.
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The COMPRESSION test.
The COMPRESSION test shows the
best possible strength concrete can
reach in perfect conditions.
The compression test measures
concrete strength in the hardened
state.
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Concrete Testing
Testing should always
be done carefully.
Wrong test results can
be costly and
unrepresentatively.
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Concrete Testing
SAMPLING
The first step is to take a test sample
from the large batch of concrete.
This should be done as soon as
discharge of the concrete commences.
The sample should be representative of
the concrete supplied.
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Concrete Testing
The sample is taken in one of two ways:
For purposes of accepting or rejecting
the load:
Sampling after 0.2 m3 of the load has
been poured.
For routine quality checks:
Sampling from three places in the load.
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THE SLUMP TEST
The slump test is done to make sure a
concrete mix is workable.
The measured slump must be within a
set range, or tolerance, from the target
slump.
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Concrete Testing
Tools
• Standard slump cone (100 mm top
diameter x 200 mm bottom diameter
x 300 mm high)
• Small scoop
• Bullet-nosed rod (600 mm long x 16
mm diameter)
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• Rule
• Slump plate (500 mm x 500 mm)
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Method
1. Clean the cone. Dampen with water
and place on the slump plate. The
slump plate should be clean, firm, level
and non-absorbent.
2. Collect a sample.
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3. Stand firmly on the footpieces and fill
1/3 the volume of the cone with the
sample. Compact the concrete by
'rodding' 25 times.
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Concrete Testing
Rodding: Rodding means to push a
steel rod in and out of the concrete to
compact it into the cylinder, or slump
cone. Always rod in a definite pattern,
working from outside into the middle.
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4. Now fill to 2/3 and again rod 25 times,
just into the top of the first layer.
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5. Fill to overflowing, rodding again this
time just into the top of the second
layer. Top up the cone till it overflows.
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6. Level off the surface with the steel
rod using a rolling action.
Clean any concrete
from around the base
and top of the cone,
push down on the
handles and step off
the footpieces.
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7. Carefully lift the cone straight up
making sure not to move the sample.
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8. Turn the cone upside down and place
the rod across the up-turned cone.
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9. Take several measurements and
report the average distance to the top
of the sample.
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10. If the sample fails by being outside
the tolerance (i.e. the slump is too high
or too low), another must be taken.
If this also fails
the remainder of
the batch should
be rejected.
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Concrete Testing
THE COMPRESSION TEST
The compression test shows the
compressive strength of hardened
concrete.
The testing is done in a laboratory off-
site.
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The only work done on-site is to make a
concrete cylinder for the compression
test.
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Concrete Testing
The strength is measured in Mega
Pascals (MPa) and is commonly
specified as a characteristic strength of
concrete measured at 28 days after
mixing.
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Concrete Testing
The compressive strength is a measure
of the concrete’s ability to resist loads
which tend to crush it.
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Concrete Testing
Compressive Strength vs. Age of
Concrete
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During the first week to 10 days of
curing it is important that the concrete
not be permitted to freeze or dry out.
In practical terms, about 90% of its
strength is gained in the first 28 days.
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Concrete compressive strength
depends upon many factors:
• quality and proportions of the
ingredients
• the curing environment.
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Stress vs. Strain Curves of Concrete
60
(MPa)
40
20
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Most structural concrete have f’c values
in the 20 to 35 MPa range.
High-rise buildings sometimes utilize
concrete of 80 or 100 MPa.
Concrete has no linear portion to its
stress-strain curve, therefore it is
difficult to measure the modulus of
elasticity.
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Concrete Testing
For concretes up to about 40 MPa, it
can be approximated as:
Ec = wc1.50.043(f’c)0.5
where:
wc is the unit weight (kg/m3), the value
is between 1440 and 2560 kg/m3,
f’c is the cylinder strength (MPa).
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The weight density of reinforced
concrete using normal aggregates is
about 2400 kg/m3.
If 80 kg/m3 of this is allowed for the
steel and wc is taken as 2320 kg/m3
then:
Ec = 4700(f’c)0.5
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Tools
• Cylinders (100 mm diameter x 200
mm high or 150 mm diameter x 300
mm high)
(The small cylinders are normally
used for most testing due to their
lighter weight)
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• Small scoop
• Bullet-nosed rod (600 mm x 16 mm)
• Steel float
• Steel plate
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Method
1. Clean the cylinder mold and coat the
inside lightly with form oil, then place
on a clean, level and firm surface, i.e.
the steel plate.
2. Collect a sample.
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3. Fill 1/2 the volume of the
mold with concrete then
compact by rodding 25
times. Cylinders may also
be compacted by vibrating
using a vibrating table.
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4. Fill the cone to
overflowing and rod 25
times into the top of
the first layer, then top
up the mold till
overflowing.
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5. Level off the top with
the steel float and
clean any concrete
from around the mold.
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6. Cap, clearly tag the
cylinder and put it in a
cool dry place to set for
at least 24 hours.
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7. After the mold is
removed the cylinder
is sent to the
laboratory where it is
cured and crushed to
test compressive
strength.
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