Non Destructive Tests

What is Non Destructive Testing?

Non Destructive testing are non-invasive techniques to determine the integrity of a material, component or structure or quantitatively measure some characteristic of an object”.

DEFNITION:

These tests can be categorized in two parts:

1. Tests used to determine the strength of concrete2. Tests used to determine the other characteristics of concrete like voids, cracks etc.

Why we are using N.D.T?

It’s an assessment without doing harm, destroying the test. Plays crucial role in ensuring cost effective operation, safety and reliability of plant. Cost saving technique in quality assurance process.

APPLICATION OF NDT

quality control of pre-cast units or construction in situ confirming or negating doubt concerning the workmanship involved in batching,

mixing, placing, compacting or curing of concrete location and determination of the extent of cracks, voids, honeycombing and similar

defects within a concrete structure determining the position, quantity or condition of reinforcement Confirming or locating suspected deterioration of concrete resulting from such factors

as overloading, fatigue, external or internal chemical attack or change, fire, explosion, environmental effect.

The most common tests which are usually conducted are:

1- Schmidt (Rebound) Hammer Test

Schmidt hammer test is done to find out the compressive strength of concrete by using Schmidt hammer.

PRINCIPLE

The elastic mass depends on the hardness of the surface. It is related to the compressive strength of the concrete. The rebound value is designated as the rebound number or rebound index. The results are significantly affected by :

A) Mix characteristics.B) Angle of inclination of direction of hammer.C) Member characteristics.

Uses – OBJECTIVES

Assessing the likely compressive strength of concrete . Assessing the uniformity of concrete Assessing the quality of concrete in relation to standard requirements. Assessing the quality of one element of concrete in relation to another.

ADVANTAGES

Assessing the likely compressive strength of concrete. Assessing the quality of concrete in relation to standard requirements.

APPLICATIONS

for testing normal weight concrete 2.25 for light weight concrete or small and impact sensitive part of concrete For testing mass concrete i.e. In roads, airfield pavements and hydraulic structure.

2- Ultrasonic Pulse Velocity Test

It measures the time of travel of an ultrasonic pulse passing through the concrete. The apparatus for ultrasonic pulse velocity measurement consists of the following:

(a) Electrical pulse generator(b) Transducer – one pair(c) Amplifier(d) Electronic timing device

How it works?

Pulse velocity tests measure the transit time of an ultrasonic pulse between two transducers, a transmitter and a receiver. The velocity of the pulse can be determined from the transit time and the distance between the transmitter and the receiver.

There are two methods of receiving the ultrasound waveform, reflection & attenuation.

OBJECTIVES

To assess the uniformity and homogeneity of concrete. To assess the quality of concrete in relation of standard requirement. Detection of presence of voids, cracks & imperfection of concrete. Measurement of changes occurring with time in the properties of concrete. To compare quality of two different concrete element.

ADVANTAGES

Low cost Equipments used are easy to handle. No health hazards. High penetrating power. High sensitivity. Greater accuracy. Some capability in estimating the size, shape, nature of the flaws. Portability.

DISADVANTAGES

Manual operation requires careful attention by experienced technicians Difficulty in inspecting the parts which are irregular. Requirement of the complaints. Test objects should be water resistant.

APPLICATIONS.

Monitoring strength development or deterioration in laboratory specimens subjected to varying curing conditions or to aggressive environment.

Measurement of in-situ concrete uniformity. Detection of cracking & honeycombing in in-situ concrete. Measurement of crack depth. Strength estimation of in-situ concrete. Assessment of in-situ concrete deterioration. Measurement of layer thickness in in-situ concrete. Measurement of elastic modulus of in-situ concrete.

APPLICATIONS & LIMITATIONS.

The pulse velocity method is an ideal tool for establishing whether concrete is uniform.

Applied to both existing structures and those under construction. High pulse velocity readings are generally indicative of good quality

concrete.

3- Resonance Frequency test

Principle

The principle used in the E-Meter Mk II™ is based upon the determination of the fundamental resonant frequency of vibration of a specimen generated by an impact and sensed by an accelerometer. The frequency spectrum is computed and displayed by the meter.

Features & Benefits

Conforms to ASTM C-215 and C-666.

The only method of calculating the following material parameters non destructively:

o Young's Modulus of Elasticity,

o Modulus of Rigidity,

o Poisson’s Ratio,

o Damping Constant.

Available for specimen sizes up to 6 inches (150mm) cross section dimension and from 1.75 inches (45mm) to 28 inches (700mm) in length.

Automatic identification of the resonance frequency. Large easy to view display for data analysis of time domain and frequency spectrum signals.

Data can be stored and uploaded to a PC for further analysis and inclusion in reports.

Fast and easy to use system.

Applications

Freeze Thaw Analysis

Young's Modulus Determination

Damping

Coefficient Analysis

Applications in mass-production

Resonant Inspection is very suited to inspecting mass produced parts, and is easily able to detect "outliers" or components that differ from the normal production.

4- Rebar Detector Test

PRINCIPLE

The instrument is based upon measurement of change of an electromagnetic field caused by steel embedded in the concrete.

OBJECTIVES, ADVANTAGES

To determine

1. Diameter of reinforcement 2. Location of a rebar. 3. Cover

FEATURES.

Eddy current design for greater accuracy. Single sensor for all depth ranges. Daylight visible display Locates up to 8",(200mm) Economical

Factor affecting test methods

Cover measurement may be affected by neighboring bars parallel to the bras being measured.

• Magnetic effects from the aggregate and matrix of the concrete. • Variation in the properties of steel. • Cross sectional shape of bars. • Roughness of surface.

APPLICATIONS.

Rebar Locating Detection and Orientation Measurement of Concrete Cover Concrete Drilling and Sawing Repair and Rehabilitation Location of Pre-stressed Cables Contractors.5- Impact-echo test

Impact-echo is a method for nondestructive testing of concrete & masonry structures that is based on the use of impact-generated stress (sound) waves that propagate through concrete & masonry & are reflected by internal flaws & external surfaces. Impact-echo.

How Impact-Echo Works

A short-duration mechanical impact, produced by tapping a small steel sphere against a concrete or masonry surface, produces low-frequency stress waves (up to about 80 kHz) that propagate into the structure and are reflected by flaws &/or external surfaces. The wavelengths of these stress waves are typically between 50mm and 2000mm -- longer than the scale of natural inhomogeneous regions in concrete (aggregate, air bubbles, micro-cracks, etc.).

APPLICATIONS. Impact Echo testing has many applications, including:

Quality control programs, such as measuring pavement thickness or assessing pile integrity.

Routine maintenance evaluations to detect cracks, voids, or delaminations in concrete slabs.

Delineate areas of damage & corrosion in walls, canals & other concrete structures. Assess quality of bonding and condition of tunnel liners.

SIGNIFICANCE & USE.

This test method may be used as a substitute for, or in conjunction with, coring to determine the thickness of slabs, pavements, decks, walls, or other plate structures. There is a certain

level of systematic error in the calculated thickness due to the discrete nature of the digital records that are used. The absolute systematic error depends on the plate thickness, the sampling interval, and the sampling period.

6- Pull out Test

This test measure the strength of specimen by means of special tension jacks that are usually used to be inserted in test specimen

A pullout test consists of casting a specially-shaped

steel insert with an enlarged end into fresh concrete.

This steel insert is then pulled-out from the concrete

and the force required for pullout is measured using

a dynamometer.

A bearing ring is used to confine failure to a well-defined shape.

As the steel insert is pulled out, a cone of concrete is

also removed, thereby damaging the concrete

surface (which must be repaired after the test).