Non-Destructive Testing

Prepared by

Salahuddin

Khalil Ur Rehman

Non-Destructive Testing

Non- Destructive testing is the testing of material, used for the Analysis of internally embedded objects or flaws and defect inside it, without disturbing its suitability for the service

Non-destructive Testing (NDT)of concrete in Structures

In Concrete structures the NDT Purposes

locate objects embedded Condition and quality

in Concrete of concrete.

e.g Steel Reinforcement

location and development

of cracks and voids.

While the concrete Structure remains Undestroyed

Methods of NDT

Following are some important methods used for NDT:

. Infrared Thermography

. Ground Penetrating Radar (GPR)

. UltraSonic Pulse Velocity (UPV)

. Rebound Hammer Testing

. Radiography

INFRARED THERMOGRAPHY

The Thermographic images of Material (concrete) are taken that give an internal

view showing the voids and flaws.

Basic PrincipleThe presence of flaws within the concrete affects the heat conduction properties. Which

creates difference in surface temperature. That give an indication of defects.

INFRARED THERMOGRAPHY

> The presence and location of hidden defects (such as delaminations or voids) is indicated by the difference in surface temperature between sound and unsound concrete.

> The technique is considered suitable.

For Providing assessment of Flaws in concrete. i-e In Bridge Decks.

For Locating steel reinforcement,cableducts etc in Concrete.

Thermographic Process

Thermographic image

Drawbacks:

. Testing is not viable during periods of adverse weather (rain or wind).

. Testing can be adversely affected by several surface conditions such as:

Water

Wearing

Discolouration

Crack sealant

Strong wind, etc

GROUND PENETRATING RADAR (GPR)

Ground penetrating radar (GPR) provides subsurface reflection images of structures, By transmitting and recieving Ultra High Frequency Radio wave.

Operation:

> GPR operates by transmitting pulses of ultra high frequency radio waves (microwave electromagnetic energy) into the subsurface object through an antenna.

> The transmitting antenna is excited by repetitive high voltage pulses with repetition rates typically of the order of 10,000 Hertz.

> A part of the penetrating wave is reflected by interfaces or objects with dielectric properties different from that of the general medium.

> These reflections are detected by the receiving antenna and converted into electrical signals.

> The recorded reflections can be analysed in terms of shape, travel time and signal amplitude to provide information about the size, depth and properties in relation to the material object.

Contd……

Enables

Identification: reinforcing bars, prestressing cables, cable ducts, zones of varying moisture content and thickness of slabs, different concrete, asphalt-concrete overlay interface, plastic spacers.

Assessment of delaminations and large voids in concrete.

ULTRASONIC PULSE VELOCITY (UPV)

Ultrasonic Pulse velocity (UPV) testing is the Relative Strength assessment of concrete, that involves measuring the speed of travel of an ultrasonic pulse through concrete.

The speed of the pulse is influenced by the density and elastic modulus of the concrete.

Operation

• Low frequency ultrasonic pulses (about 30 kHz) are sent by the transmitting transducer and received by the second transducer with the time taken for the ultrasonic pulse to travel recorded by a timer.

• The pulse velocity is determined by dividing the path length between the two transducers by the pulse travel time.

• The pulse velocity can be affected by concrete mix composition, concrete density and compaction, moisture condition and age of concrete.

• The method is most effective when sound is transmitted directly between transducers placed on opposite faces.

• The method is less effective but possible when transducers are positioned at right angles over a corner or on the same surface.

Generally higher pulse velocity is associated with higher strength, and thus relative assessment is obtained.

REBOUND HAMMER TESTING

The rebound hammer (such as the Schmidt Hammer) is a simple device

that is used to measure the hardness and predict the strength of the concrete.

REBOUND HAMMERRebound hammer can be placed vertically, horizontally and inclined to the concrete

surface

Rebound Hammer Testing• The surface of the concrete is cleaned to remove laitance and shutter

board marks.

• A series of twelve readings are taken with hammer in a diamond pattern at the test point.

• The hammer is pressed against the surface, loading the sprung mass, and releasing this at the end of the stroke.

• There is a slide indicator on the outside of the unit that records the distance traveled during the rebound. This indication is known as the rebound number.

REBOUND HAMMER

The strength value can be obtain from Rebound No through a calibration chart given by the manufacturer

Factor Affecting the Results

Surface Smoothness

Concrete Age

Moisture Content

Aggregate, air void, Steel Reinforcement

Temperature

Surface Carbonation

RADIOGRAPHIC METHODS

Radiographic methods for concrete inspection include gamma-ray radiography,

X-ray radiography, and X-ray radioscopy .

A radiographic image is taken through a concrete component to reveal a pictureof the interior.

The Most commonly used is X-ray Radiograghy

Radiography

The concrete part is placed between the radiation source and a piece of film.

Some of Radiation will absorb by the material and some will pass through it,

Making an image of its internal Features.

It provides a fair assessment of flaws and defects in concrete.

Locating cables and cable ducts, reinforcing bars and cover thickness.

Advantage: The most important advantage of radiography over other NDT

techniques is that it is highly reliable and shows a “see-through” picture of the inspected area.

Disadvantage: The Rays are Hazardous for health and required Properly

trained operator.