CREATED BY : RAJESH GOSWAMI

DESTRUCTIVE TESTNON-DESTRUCTIVE TESTREBOUND- HAMMER TESTFACTORS INFLUENCE THE TEST RESULTSULTRASONIC PULSE VELOCITY TESTPULL OUT TESTINGDRILLED HOLE METHODPENETRATION RESISTANCE TESTQUASSI BRITTLE NATURAE OF CONCRETE

In case of destructive tests, the concrete

specimens ( cubes, cylinders, beams, etc.) are

loaded till destruction in the laboratory and the

strength properties of concrete are determined

from the test results.

For example,

(1) compression test

(2) tensile strength test

• Splits cylinder test

• Modulus of rupture test

(3) Bond strength test

NON-DESTRUCTIVE TESTSIn case of non-destructive tests, the specimen are not loaded to failure.

Non-destructive testing is considered to be a powerful method of evaluating

existing concrete structures with regard to their strength and durability apart

from assessment and control of quality of hardened concrete . These tests are

also performed to determine then presence of internal cracks, micro cracks,

and progressive deterioration in the concrete.

These methods therefore, attempts to measure some other properties

of concrete from which an estimate of its strength ,durability and elastic

properties are obtained. Some such properties of concrete are:

• Hardness

• Resistance to penetration

• Rebound number

• Resonant frequency

• Ability to allow ultrasonic pulse velocity to propagate through it.

REBOUND-HAMMER TESTThe SCHMIDT rebound hammer , developed by a Swiss

engineer Ernst Schmidt ,in 1948 is one of the most frequently used method worldwide for non-destructive testing (NDT) of concrete structural element.

The test is based on the principle that the rebound of an elastic mass depends on the hardness of the surface against which the mass impinges. The hammer consists of a plunger connected with a spring driven metal mass. The plunger is held against at 90 to the smooth concrete surface , firmly supported and pressed . This will impact a fixed amount of energy . Upon release, by the metal mass or the amount of rebound is noted on a scale which gives an indication of the concrete strength . Larger is the rebound , higher is the strength of concrete .

FACTORS INFLUENCING THE TEST RESULTS

Although , rebound hammer saves effort , time and cost ,it has serious limitation .the results are significantly influenced by:

1. Mix characteristics like cement type, cement content

2. Smoothness of the surface under test

3. Type of coarse aggregate

4. Size shapes and rigidity of the specimen

5. Age of concrete

6. Moisture condition of the concrete

7. Carbonation of concrete surface

8. Type of mould

ULTRASONIC PULSE VELOCITY TESTThe ultrasonic pulse velocity method basically

involves the measurement of velocity electronic pluses passing through concrete from a transmitting transducer to a receiving transducer . The test is based on the principle that the velocity of sound in a solid material is a function of the square root of the ratio of its modulus of elasticity E to its density p . The density and elastic properties are in turn related to the quality and strength of the material . The pulse velocities range from about 3 to 5 km/s . The methods employ pulses in the frequency range of 15-175 KHz , generated and recorded by electronic circuits.

1. Transducer arrangement:

There are three ways of measuring pulse velocity through concrete

. They are:

I. Direct transmission

II. Semi-direct transmission

III. Indirect transmission

2. Factors affecting pulse velocity measurement:

I. Smoothness of contract surface

II. Path length

III. Moisture condition of concrete

IV. Temperature of concrete

V. Reinforcing steel

0

3. Application of the ultrasonic testing :

I. Can be used to detect the development of cracks in structures

such as dams , and to check deterioration due to frost or

chemical action.

II. Measurement of concrete uniformity.

III. Detection of ant development of voids or honeycombing in

concrete.

IV. Estimation of strength of concrete.

V. Measurement of the thickness of concrete roads or slabs.

VI. Measurement of elastic modulus.

VII. Studies on durability of concrete.

PULL – OUT TESTING As the names suggests , this method involves the

measurement of the force required to pull-out a specially

shaped steal rod or some similar device from a concrete

surface. There are two basic categories of pull-out , one

which involves an insert having to be cast into the

concrete and another where the insert is fixed into a hole

drilled into hardness concrete.

1.Cast in method (LOK-test) :

The main aim of this technique

is to measure the tensile force

required to pull-out a metal insert

which has been cast into the fresh

concrete . This test is now

commercially knows as LOK-test with

this system there are two main types

of insert , one which is attached

directly

to the formwork and another which is

fixed to a plastic buoyancy cup

which floats on the top surface of the

concrete.

2 .DRILLED HOLE METHOD

It is observed that one of the cast on method

was that the insert needed to be placed before or at

least during process . To overcome this problem

another type of pull-out test was development which

be completed on existing structure.

PENETRATION RESISTANCE TESTThe techniques of driving a steel nail into a concrete

surface to provide some from of fixing has been in

use for many years and from this it is well knows

that the depth of penetration achieved is greatly

influenced by the strength of the concrete . Based on

this approach an in-situ test technique was

development in the USA in the mid 1960’s which

involved an explosive charge being used to fire a

specially bolt into the concrete.

QUASSI-BRITTLE NATURE OF CONCRETE

Depending upon the extent of cracking , the

cracks can be classified as macro cracks , microcracks

and semi-microcracks. According ti the location of the

cracks , they are classified as bond cracks , mortar

cracks and aggregate cracks . The bond cracks are

formed at the interface of the aggregate and mortar ,

whereas the mortar cracks are formed through the

mortar and aggregate , respectively.

A knowledge of the microcracking of concrete

contributes considerably to the understanding of its

properties , such as its inelastic nature , the

descending portion of the stress-strain curve , the

strength under combined , repeated and sustained

loading etc. The stress –strain curve is related to the

internal cracking . The non-linearity of stress-strain

relation is due to propagation of bond and mortar

cracks.

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