TEST ON HARDENED CONCRETE

ADVANCED CONCRETE TECHNOLOGY
10CV81
UNIT-8
TEST ON HARDENED CONCRETE
By
Jaswanth J
Jayanth V Gowda
Nandeesh S

INTRODUCTION
• Testing of hardened concrete plays an important role
in controlling and confirming the quality of cement
concrete works.
• The test methods should be simple, direct and
convenient to apply.
• The main purpose of testing hardened concrete is to
confirm that the concrete used at site has developed
the required strength. As the hardening of the
concrete takes time, one will not come to know, the
actual strength for some time. This is an inherent
disadvantage in conventional test.

• But if the strength of concrete is to be known at an
early period, accelerated strength test can be carried
out to predict 28 days strength. But mostly when
correct materials are used and careful steps are taken
at every stage of the work, concrete normally give the
required strength.

Capping specimens and their effects:
• Capping is applicable to cylindrical specimen. The
ends of all cylindrical specimens that are not plane
within 0.05mm are capped.
• Caps are made as thin as practicable and care should
be taken so that flaw or fracture does not take place,
when the specimen is tested.
• Capping can be done on completion of casting or a
few hours prior to testing of specimen.

Capping is required to be carried out according to the
following methods:
1. Neat cement:
• The test cylinders are capped with a thin layer of stiff,
neat Portland cement paste after the concrete has set in
the moulds.
• Capping is done after 4 hours of casting so that concrete
in the cylinder undergoes plastic shrinkage and subsides
fully.
2. Sulphur:
• Just prior to testing, the cylindrical specimens are capped
with a sulphur mixture consisting of 1 part of sulphur to 2
or 3 parts of inert filler, such as fire-clay.
• The specimens are securely held in a special jig so that
the caps formed have a true plane surface.

3. Hard plaster:
• Just prior to testing, specimens are capped with hard
plaster having a compressive strength of at least 42
Mpa cm in an hour.
• The caps are formed by means of a glass plate not
less than 13 mm in thickness, having a minimum
surface dimension at least 25 mm larger than the
diameter of the mould.

Effect of the Height/Diameter ratio on Strength:
• Normally, height of the cylinder “h” is made twice the
diameter “d”, but sometimes, particularly, when the core is cut
from the road pavements or airfield pavements or foundations
concrete, it is not always possible to keep the height/diameter
ratio of 2:1.
• The diameter of the core depends upon the cutting tool, and
the height of the core will depend upon the thickness of the
concrete member.
• If the cut length of the core is too long, it can be trimmed to
h/d ratio of 2 before testing.
• But if the length is too short, it is necessary to estimate the
strength of the same concrete, as if it had been determined on a
specimen with h/d ratio equal to 2.

• Fig 1 below shows the correction factor for height/diameter
ratio of a core(IS 516:1959).
Fig 1: Influence of h/d ratio on apparent strength of the cylinder

• Murdock and Kesler found that correlation factor is not
constant, instead it depends on the strength level of concrete.
• High strength concrete is less affected than the low strength
concrete.
• Fig 2 shows the influence of h/d ratio on the strength of
cylinder for different strength levels.
Fig 2: Correction Factor for h/d ratio of core

Rate of loading:
• The specimen is placed in the machine in such a manner that
the load is applied to the uppermost surface as cast in the
mould, along two lines spaced 20.0 or 13.3 cm apart.
• The load is applied without shock and increasing continuously
at rate such that extreme fibre stress increases at approximately
0.7 kg/sq cm/min that is, at a rate of loading of 400kg/min for
the 15.0 cm specimens and at a rate of 180 kg/min for the 10.0
cm specimens.
• The load is increased until the specimen fails, and the
maximum load applied to the specimen during the test is
recorded.

• The flexural strength of the specimen is expressed as
the modulus of rupture ‘fb’, which is given by:
fb=(P*l)/(b*d*d)
where b=specimen width measured (cm),
d= depth of the specimen at the point of failure(cm),
l=length of the span on which specimen is
supported(cm),
and P=maximum load applied to the specimen(kg).

• Compression test is the most common test
conducted on hardened concrete, partly
because it is an easy test to perform, and partly
because most of the desirable characteristic
properties of concrete are qualitatively related
to its compressive strength.
• The compression test is carried out on
specimens cubical or cylindrical in shape.
Prism is also sometimes used, but it is not
common in our country.

• The cube specimen is of the size 15 x 15 x 15 cm
and 10 x 10 x10 cm
• Metal moulds, preferably steel or cast iron, thick
enough to prevent distortion

Failure Pattern of Cube & Cylinder –
Two Pyramid Failure - Ideal

Failure of Compression Specimen
• Due to compression load, the cube or cylinder
undergoes lateral expansion owing to the
Poisson’s ratio effect.
• With friction acting i.e. , under normal
conditions of test, the elements within the
specimen is subjected to a shearing stress as
well as compression

TENSILE STRENGTH
• Tensile strength is one of the basic and important
properties of concrete. A knowledge of its value is
required for the design of concrete structural elements.
• Its value is also used in the design of prestressed concrete
structures, liquid retaining structures, roadways and
runway slabs.
• Direct tensile strength of concrete is difficult to
determine; recourse is often taken to the determination
of flexural strength or the splitting tensile strength and
computing the direct tensile.

What is split tensile strength test?
A method of determining the tensile
strength of concrete using a cylinder which
splits across the vertical diameter. It is an
indirect method of testing tensile strength of
concrete.

Why we are going for split tensile test?
• In direct tensile strength test it is impossible to
apply true axial load. There will be always
some eccentricity present.
• Another problem is that stresses induced due to
grips. Due to grips there is a tendency for
specimen to break at its ends.

MOULDS
Cylinders
• The cylindrical mould shall be of 150mm diameter and 300mm
height. Similarly the mould and base plate shall be coated with a
thin film of mould oil before use, in order to prevent adhesion of
the concrete.

•The load shall be applied without shock and
increased continuously at a nominal rate within
the range 1.2 N/(mm2/min) to 2.4 N/
(mm2/min).
•Record the maximum applied load indicated
by the testing machine at failure. Note the type
of failure and appearance of fracture.

Computations: Calculate the splitting tensile
strength of the specimen as follows:
T = 2P
πLd
Where:
T : splitting tensile strength, kPa
P : maximum applied load indicated by testing
machine, kN
L : Length, m
d : diameter, m

Result :
• It is found that the splitting test is closer to the true
tensile strength of concrete it gives about 5 to 12%
higher value than the direct tensile strength test.

Advantage of using this method:
• Same type and same specimen can also be used
for compression test.
• It is simple to perform and it gives uniform
results than the other tension tests like ring
tension test and double punch test.

• Flexure
• The state of being flexed (i.e. being bent)
• Flexural strength
• It is also known as modulus of rupture, bend strength, or
fracture strength, a mechanical parameter for brittle material, is
defined as a material's ability to resist deformation under load.
• The flexural strength represents the highest stress
experienced within the material at its moment of rupture.
• When an object formed of a single material, like a wooden
beam or a steel rod, is bent, it experiences a range of stresses
across its depth.

• Most materials fail under tensile stress before they
fail under compressive stress, so the maximum tensile
stress value that can be sustained before the beam or
rod fails is its flexural strength
• The stress will be at its maximum compressive
stress value

• The standard size of the specimens are 15 x 15
x 70 cm. Alternatively, if the largest nominal
size of the aggregate does not exceed 20 mm,
specimens 10 x 10 x 50 cm may be used.

• Procedure
Test specimens are stored in water at a
temperature of 24° to 30°C for 48 hours
before testing. They are tested immediately
on removal from the water whilst they are still
in a wet condition. The dimensions of each
specimen should be noted before testing. No
preparation of the surfaces is required.

The flexural strength of the specimen is expressed as the
modulus of rupture fb
fb = Pl/bd2
When ‘ a ’ is greater than 20.0 cm for 15.0 cm specimen
or greater than 13.3 cm for a 10.0 cm specimen

1) If ‘ a ’ is less than 20.0 cm but greater than 17.0 cm for 15.0
specimen
2) If ‘ a ’ less than 13.3 cm but greater than 11.0 cm for a 10.0
cm specimen
3) If ‘ a ’ less than 17 cm and 11 cm for 15 & 10 cm specimen
resp

Calculation of the flexural strain
,
Calculation of flexural modulus
,
D= maximum deflection of the center of the beam, (mm)
m = The gradient (i.e., slope) of the initial straight-line portion of
the load deflection curve,(P/D), (N/mm)

Non Destructive
Testing in
Concrete

NDT-Need & Importance.
• To test concrete structure after concrete has hardened.
• To test structure without damaging.
• To determine whether the structure is suitable for design
use.
• It can be applied on both old and new structure.
• Cost effective.
Test available ranges from
• SDT(semi destructive test)-causes negligible repairable
damages on surface.
• NDT(non destructive test)-do not cause any damage to
structure.

Where to use NDT
• Determining parameters- density, elastic
modulus, strength, surface adsorption.
• Location of Cracks/Joints/Honeycombing
• Determining position of reinforcement
• Quality control of Construction , in situ
• Confirming Workmanship

METHODS
• VISUAL TESTING
• SCHMIDTS REBOUND HAMMER TEST-surface hardness.
• ULTRASONIC PULSE VELOCITY TEST-compressive strength.
• PERMEABILITY TEST-flow of water.
• HALF CELL ELECTRIC POTENCIAL METHOD-corrosion
potential.
• COVERMETER TESTING-dia and distance of bars from surface.
• RADIO GRAPHIC TESTING-detects voids.
• CARBONATION DEPTH MEASUREMENT-detects corrosion.
• IMPACT ECO-TESTING.
• GROUND PENETRATION RADAR TESTING.
• INFRARED THERMOLOGY.

Qualification/Certification
• A person / Organization should have
Certification From - ISO – 9712
• IS 1311 -Non Destructive Testing
• IS 13311 (PART 1) : 1992-Ultrasonic Pulse
Velocity
• IS 13311 (PART 2) : 1992-Rebound Hammer
Test

1 . REBOUND HAMMER
TEST
• This is used for measuring
surface hardness of existing
concrete mass which in turn is
correlated with the grade of
concrete.
• calibration curves are available
to relate the rebound number
with the grade of concrete for the
hammer held either horizontal or
vertical (down or up) for both
dry and wet condition of surface.
• Result depends upon– Type and
nature of aggregate used, Surface
and internal moisture condition,
presence of void, Smoothness of
surface

• It can be best used to compare strength of one concrete against
another but usually not reliable in determining absolute strength.
• Moreover, each hammer varies considerably in performance and
require individual calibration
1. Original Schmidt Hammer-Impact direction perpendicular to
the surface, Used for concrete/mortar testing,900 g.
2. Silver Schmidt Hammer-Independent of impact
direction,Suitable for testing a wide variety of concrete, mortar
and rock,600 g.
• Quick, simple and in expensive method.

Components of Hammer and Procedure
• Schmidt’s rebound hammer
consist of a spring
controlled mass that slides
on a plunger within a
tubular housing.
• When plunger is pressed
against the surface of the
concrete ,the spring
controlled mass rebounds .
• The extent of such rebound
depends upon the surface
hardness of concrete.

Limitation
• Accuracy is 25% only.
• Results effects by-angle of test, surface smoothness, mix proportions.
• Only suitable for closed structured concrete.
• Rebound hammer have to be calibrated frequently.
• Test results depends on-
1. moisture condition of surface-wet surface produce lower rebound
upto 20%.
2. surface carbonation-increases rebound number(carbonated layer
should be removed
before testing)
3. location of plunger-if plunger is placed on aggregate, high rebound
no. is obtained
(hence values are taken at several places)

2 . Pulse Velocity Method
• This is used for measuring the time of travel of pulse of
vibrations in ultrasonic ranges, passing through the concrete to
judge qualitatively, how good or bad the concrete is.
• It can be operated in direct, semi-direct or indirect, i.e. surface
mode
• The result depends upon-
Heterogeneity of concrete within a short length
Presence of reinforcing steel or other impurities in concrete
• Used to judge uniformity of concrete and to establish acceptance
criteria, correlation with strength is possible

• It can be operated in direct, semi-direct or indirect,
i.e. surface mode
Direct transmission Semi-direct transmission Indirect or surface
transmission
• Long waves travel twice fast as other, surface waves are slowest
• The time of travel b/w initial onset and reception of the pulse is
measured electronically.
• The path length b/w transducer divided by time of travel gives avg
velocity.

Factors affecting
1. Smoothness of contact surface under test
2. Influence of path length on pulse velocity
3. Temperature of concrete
4. Moisture condition of concrete
5. Presence of reinforcing steel

Applications
• Determine quality of concrete.
> 4500 m/s à Excellent
3500-4500 m/s à Good
3000-3500 m/s à Doubtful
2000-3000 m/s à Poor
<2000 m/s à Very poor
• Determine the setting characteristic.
• Durability study-freeze thaw, acid attack..
• Detects cracks
• Measures detoration of concrete due to fire exposure.

TEST ON HARDENED CONCRETE

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