PRESENTED BY S.SELVAPRAKASH

IMPACT STRENGTH

Impact strength: resistance to sudden shock or load.

Impact strength is of importance in driving concrete piles, in foundations for machines exerting impulsive loading, and also when accidental impact is possible, e.g. when handling precast concrete members.

There is no unique relation between impact strength and static compressive strength.

For this reason, impact strength has to be assessed, usually by the ability of a concrete specimen to withstand repeated blows and to absorb energy.

The number of blows which the concrete can withstand before reaching the 'no-rebound' condition indicates a definite state of damage.

Generally, for a given type of aggregate, the higher the compressive strength of the concrete the lower the energy absorbed per blow before cracking, but the greater the number of blows to reach 'no-rebound'.

Hence, the impact strength and the total energy absorbed by the concrete increase with its static compressive strength.

the relation between impact strength and compressive strength depends upon the type of coarse aggregate but the relation depends also on the storage condition of the concrete.

The impact strength of water-stored concrete is lower than when the concrete is dry.

Thus, the compressive strength without reference to storage conditions does not give an adequate indication of impact strength.

For the same compressive strength, impact strength is greater for concrete made with coarse aggregate of greater angularity and surface roughness,

a feature which suggests that impact strength of concrete is more closely related to its flexural strength than to the compressive strength .

Thus concrete made with a gravel coarse aggregate has a low impact strength owing to the weaker bond between mortar and coarse aggregate.

A smaller maximum size of aggregate significantly improves the impact strength.

NON DESTRUCTIVE TEST

In non destructive methods of testing , the specimen are not loaded to failure .

Schmidt hammer

This test is also known as the rebound hammer, impact hammer or sclero-meter test, and is a non-destructive method of testing concrete.

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.

REBOUND NUMBER:

The mass rebounds from the plunger (still in contact with the concrete surface), and

the distance travelled by the mass, expressed as a percentage of the initial extension of the spring, is called the rebound number; it is indicated by a rider moving along a graduated scale.

The rebound number is an arbitrary measure since it depends on the energy stored in the given spring and on the size of the mass.

AIM: To measure surface of the hardness concrete.

APPARATUS: Schmidt’s hammer test PROCEDURE: 1. press the plunger against the force

of the spring. 2. push the button the rider will held

in positions

3. Note down the reading. 4. The compressive strength of the

concrete can be interpolated from the chart.

5. This shows the relationship between compressive strength and rebound number.

CHART

Ultrasonic pulse velocity test The principle of this test is that the velocity of

sound in a solid material, V, is a function of the square root of the ratio of its modulus of elasticity, E, to its density, p

V = f [gE/ p]^1/2

where g is the acceleration due to gravity.

UPV APPARATUS

The apparatus generates a pulse of vibrations at an ultrasonic frequency

which are transmitted by an electro-acoustics at an ultrasonic frequency which are transmitted by an electro-acoustic transducer held in contact with the surface of the concrete under test.

After passing through the concrete, the vibrations are received and converted to an electrical signal by a second electro-acoustic transducer, the signal being fed through an amplifier to a cathode-ray oscilloscope.

The time taken by the pulse to travel through the concrete is measured by an electrical timing-unit with

an accuracy of ±0.1 microsecond and, knowing the length of path travelled through the concrete, the pulse velocity can be calculated.

KINDS OF WAVES

longitudinal waves, shear waves and surface waves. These three waves travel at different

speeds. The longitudinal or compression waves travel about twice as fast as the other two types. The shear or transverse waves are not

so fast, the surface waves are the slowest.

Techniques of Measuring Pulse Velocity

(a ) Direct transmission. (b) Indirect transmission. (c ) Surface transmission