Hardened concrete

Department of Civil Engineering
MALLA REDDY INSTITUTE OF TECHNOLOGY AND SCIENCE
Presented By,
Pavankumar N.S.
Asst. Professor
Civil engineering Department
MRITS, Maisammaguda.
Concrete technology
UNIT-IV HARDENED CONCRETE
& TESTS OF HARDENED CONCRETE
Course Outcome: Understand the behavior of hardened concrete &
the durability requirements of concrete

MALLA REDDY INSTITUTE OF
TECHNOLOGY AND SCIENCE
stiffness of
Water/cement ratio and degree of compaction
Ratio of cement to aggregate
Grading, surface texture, shape, strength and
aggregate particles
Maximum size of aggregate.

Strength of concrete primarily depends upon the strength of
cement paste.
The strength of cement paste depends upon the dilution of
paste or in other words, the strength of paste increases with
cement content and decreases with air and water content.
In 1918; Abrams’ law states that “assuming full compaction, and
at a given age and normal temperature, strength of concrete
can be taken to be inversely proportional to the water/cement
ratio”
𝑆=
𝐴
𝐵𝑥
where x =water/cement ratio by volume and for 28 days
results the constants A and B are 96N/mm2 and 7 respectively.

Water/Cement Ratio:
– Typically:0.35 – 0.45
– Smaller w/cratio → stronger concrete

Since concrete is a brittle material, its porosity primarily
governs its strength. The compressive strength is found to be
severely decreasing with increase in the porosity.
The porosity of concrete which governs the strength of
concrete is affected by the gel/spaceratio in concrete.
The gel/space ratio is the ratio of the solid products of
hydration to the space availablefor these hydration products.
A higher gel/space ratio reduces the porosity and therefore
increases the strength of concrete.

The gel/space ratio, which
governs the porosity of
concrete affecting its strength,
is affected by the water/cement
ratio of concrete
A higher water/cement ratio
decreases the gel/spaceratio
increasing the porosity thereby
decreasing the strength of
concrete.

Power’s experiment showed that the strength of concrete
bears aspecific relationship with the gel/space ratio.
He found the relationship to be 240 x3, where x is the
gel/space ratio and 240 represents the intrinsic strength of the
gel in MPa for the type of cement and specimen used.
Calculation of gel/spaceratio for complete hydration
𝐺𝑒𝑙/ 𝑆𝑝𝑎𝑐𝑒𝑅𝑎𝑡𝑖𝑜=
𝑉𝑜𝑙𝑢𝑚𝑒𝑜𝑓𝑔𝑒𝑙
=
0.657𝐶
𝑆𝑝𝑎𝑐𝑒𝐴𝑣𝑎𝑖𝑙𝑎𝑏𝑙𝑒 0.319𝐶+ 𝑊𝑜
Calculation of gel/spaceratio for partial hydration
𝑆𝑝𝑎𝑐𝑒𝐴𝑣𝑎𝑖𝑙𝑎𝑏𝑙𝑒
𝐺𝑒𝑙/ 𝑆𝑝𝑎𝑐𝑒𝑅𝑎𝑡𝑖𝑜=
𝑉𝑜𝑙𝑢𝑚𝑒𝑜𝑓𝑔𝑒𝑙
=
0.657𝐶α
0.319𝐶α+ 𝑊𝑜

The aggregate/cement ratio, is only a secondary factor in the
strength of concrete but it has been found that, for a constant
water/cement ratio, aleaner mix leads to ahigherstrength.
Some water may be absorbed by the aggregate: a larger
amount of aggregate absorbs a greater quantity of water, the
effective water/cement ratio being thus reduced.
A higher aggregate content would lead to lower shrinkage and
lower bleeding, and therefore to less damage to the bond
between thee aggregate and the cement paste
As a result, in a leaner mix, the voids form a smaller fraction
off the total volume of concrete, and it is these voids that have
an adverse effecton strength

The larger the aggregate the lower is the total surface area and,
therefore, the lower is the requirement of water for the given
workability.
The use of larger size aggregate did not contribute to higher
strength as expected from the theoretical considerations due
to the following reasons.
The larger maximum size aggregate gives lower surface area for
developments of gel bonds which is responsible for the lower
strength of the concrete.
Secondly bigger aggregate size causes a more heterogeneity in
the concrete which will prevent the uniform distribution of
load when stressed.

When large size aggregate is used, due to internal bleeding, the
transition zone will become much weaker due to the
development of microcracks which result in lower
compressive strength.

With an increase in age, the degree of hydration generally
increases increasing the gel/space ratio so that strength
increases
Increase in the strength of concrete (at same w/c ratio) with
increase in early age(from 1 to 28 days) of concrete.

The relation between the flexural and compressive strengths
depends on the type of coarse aggregate because the
properties of aggregate, especially its shape and surface
texture, affect the ultimate strength in compression very much
less than the strength in tension or the cracking load in
compression.
In experimental concrete, entirely smooth coarse aggregate led
to a lower compressive strength, typically by 10 per cent, than
when roughened.
The influence of the type of coarse aggregate on the strength
of concrete varies in magnitude and depends on the
water/cement ratio of the mix.

 For water/cement ratios below 0.4,the use of crushed
aggregate has resulted in strengths up to 38 per cent
higher thanwhen gravelis used.
 With an increase in the
water/cement ratio to 0.5, the
influence
strength
of aggregate falls off, presumably because
the of the hydrated cement paste
itself becomesparamount and, at a water/cement ratio of 0.65, no difference
in the strengths of concretes made with crushed rock and
gravel has observed.

The rise in the curing temperature speeds up the chemical
reactions of hydration and thus affects beneficially the early
strength of concrete without any ill-effects on the later
strength.
Rapid initial hydration appears to form products of a poorer
physical structure, probably more porous, so that a proportion
of the pores will always remain unfilled.
The gel//space ratio rule that this will lead to a lower strength
compared with a less porous, though slowly hydrating, cement
paste in which a high gel//space ratio will eventually be
reached.

 At the end of the lecture, students should be
able to
“Understand the types of destructive testing
method and non destructive testing for
measuring hardened concrete properties
composite areas”

 Construction material is tested:
a) To ensure the QUALITY of the material
b) To minimize the maintenance cost
c) To spare or reduce the involved parties in
the construction from facing problem at
later stage
 There are 2 types of concrete test that is:
a) Destructive test
b) Non Destructive test

 Can be done for testing :
a) Compression strength concrete
- Cube test
b) Tensile strength of concrete
- Direct Tension Test
- Split-Cylinder Test
- Flexural Test
c) Flexural Strength

 Concrete cube testing is a primary quality
compliance check on the specified design
characteristic compressive strength of concrete
mix supplied to the site.
 Concrete cube is prepared by placing 3 layer of
concrete in the mould
 Each layer is compacted using rod for 35 times
 Then it is cured in a tank of water for 7, 14 and
28 days.
 On the 7th day, cube will be taken out for
compressive strength test.

Cube subjected to water curing
Cube must be oiled before placing
the concrete

Compression testDepartment of Civil Engineering

 Tensile strength of concrete should be high
enough to resist cracking from shrinkage
and temperature changes.
 It can be measured using the following test
a) Direct Tension Test
b) Split-Cylinder Test
c) Flexural Test
 Normally tensile strength is assessed using
flexural or split-cylinder test.

 SPLIT CYLINDER TEST (ASTM 496)
1. A cylinder specimen of minimum 2-in.
(50mm) dia, placed with it’s axis in a
horizontal plane.
2.Then it is subjected to a uniform load along
the length of the specimen.
LOADSide Elevation Of The Cylinder
Front Elevation

3.Logically, the load will split to 2 parts (P), so
the tensile strength can be calculated as
ft = 2P / Πld
l = length of the cylinder
d = diameter of the cylinder
4. The type and shape of coarse aggregate
particles also affect the tensile strength.
5. Split Cylinder Test to determine the tensile
strength of concrete by splitting cylinders of
the concrete in a compression testing machine.

Cylinder before testingDepartment of Civil Engineering

Split Cylinder TestDepartment of Civil Engineering

After Split Cylinder TestDepartment of Civil Engineering

FLEXURAL TEST
1. Most common method for measuring the
tensile strength of concrete
2. A concrete beam with span length equal to 3
times the beam depth ( the length of the beam
should be at least 2 in. (50mm) larger than the
span) is subjected to 3rd point loading (ASTM
C78-94).
3. This produces tensile stresses at the bottom of
the beam and compressive stresses at the top.Department of Civil Engineering

 Since concrete is weaker in tension than
compression, the specimen fails where it
breaks into 2 following the formation of a
nearly vertical crack called a flexural crack,
near the section of maximum moment.

Flexural TestDepartment of Civil Engineering

• Normally carried out:
1. Periodically to evaluate the performance of
building
2. To gather information on old building in
order to ascertain the methods of repair or
to demolish
3. To ascertain the strength of concrete if
cube tests failed.

 Rebound Hammer / Schmidt Hammer
 Ultrasonic Pulse Velocity
 Penetration Method
 Pull Out Test
 X-Rays
 Profometre

 Known as Schmidt hammer test
 Can be used to determine the in-place
compressive strength of concrete within a range
of 1500 – 8000 psi (10-55MPa)
 Useful in the assessment of uniformity of
concrete within a structure
 This is test can be used to establish whether the
rebound number has reached a value known to
correspond to the desired strength

 Measure the distance of rebound of a
spring-loaded plunger after it struck a
smooth concrete surface.
 A quick and simple mean of checking
concrete uniformity
 Results of the test can be affected by factors
such as smoothness of concrete surface,
size, shape, rigidity of speciment, age &
moisture condition, type of coarse aggregate
& the carbonation of the surface.

Schmidt HammerDepartment of Civil Engineering

Rebound hammer testDepartment of Civil Engineering

 It uses measurement of the speed of ultrasonic
pulses through the concrete through the
concrete to correlate concrete strength to
standard strength.
 Allows the determination of compressive
concrete strength and location of cracks.
 It will identify non homogenous condition in
the structure such as honeycomb, voids &
cracks.

 This test also can assist in determining sizes
of cracks
 Among the factors that could effect this test
are:
a) surface smoothness
b) travel path of the pulse
c) temperature effect on the pulse velocity
d) moisture content
e) presence of steel reinforcing bars
f) age of concrete

Ultrasonic Pulse Velocity testDepartment of Civil Engineering

 The Windsor probe is generally considered to
be the best means of testing penetration.
 It consists of a powder-actuated gun or driver
,hardened alloy probes, loaded cartridges, a
gauge for measuring penetration of probes &
other related equipment.
 A probe is driven into the concrete by means
of a precision powder charge.

 Depth of penetration provides an indication
of the compressive strength of the concrete.
 This apparatus provides a quick means of
determining the relative strength and quality
of concrete
 This test also useful in determining whether
formwork can be removed.
 Designed for in-place testing of compressive
strength and quality.

 Can be performed on slabs, floors, ceilings,
curved surfaces and pavements
 Cost of the test is higher than hammer test.
 This test is likely to be preferable to drilling
small-diameter cores.

 This is a test which measures, by means of a
special tension jack, the force required to
pull out a previously cast-in-metal insert
with an enlarged end.
 This test is superior to the rebound hammer
and to penetration resistance test because a
larger volume and greater depth of concrete
are involved in the pull out test.

 Base on magnetic field principle for
measurement of cover thickness, numbers of
reinforcement, location of reinforcement and
stirrups.
 Able to locate reinforcing bars and measures
concrete cover.

Test using profometreDepartment of Civil Engineering

 Radiography using gamma rays or high energy
X-rays to detect voids
 Radiometry to measure density
 Surface penetrating radar to detect voids,
cracks and delaminations

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