NON DESTRUCTIVE TESTING TECHNIQUES &

APPLICATIONS

by

B S RAO GROUP MANAGER

CENTRE FOR CONSTRUCTION DEVELOPMENT AND RESEARCH

NATIONAL COUNCIL FOR CEMENT AND BUILDING MATERIALS

DURABILITY OF CONCRETE

• A durable concrete is one that performs satisfactorily

in the working environment during its anticipated

exposure conditions during service.

• The materials and mix proportions specified and used

should be such as to maintain its integrity and, if

applicable, to protect embedded metal from

corrosion.

• One of the main characteristics influencing the

durability of concrete is its permeability to the ingress

of water, oxygen, carbon dioxide, chloride, sulphate

and other potentially deleterious substances.

FACTORS INFLUENCING DURABILITY

• The environment

• The cover to embedded steel

• The type and quality of constituent materials

• The cement content and water/cement ratio

of the concrete

• Workmanship, to obtain full compaction and

efficient curing.

• The shape and size of the member.

REQUIREMENTS FOR DURABILITY

• SHAPE AND SIZE OF MEMBER

• EXPOSURE CONDITIONS

• MINIMUM CONCRETE QUALITY

• EXPOSURE TO SULPHATE ATTACK

• CHLORIDE AND SULPHATE IN

CONCRETE

• ALKALI-AGGREGATE REACTION

ASSESSMENT OF DISTRESSED

CONCRETE STRUCTURES

VISUAL OBSERVATIONS

FIELD TESTS

INTERPRETAION

LABORATORY TESTS

TESTS STUDY OF RECORDS

AND

QUESTIONING

APPROACH

Further tests

ASSESSMENT OF CAUSE AND EXTENT OF DAMAGE

FORMULATION OF RECOMMENDATIONS FOR REPAIRS

TECHNIQUES ADOPTED FOR

ASSESSMENT OF DISTRESSED CONCRETE

STRUCTURES

• Rebound Hammer Test

• Ultrasonic Pulse Velocity Test

• Cover Measurement

• Core Extraction and Testing

• Carbonation Test

• Resistivity Test

• Half Cell Potential Test

• Chemical Analysis

REBOUND HAMMER TESTING

• In 1948, a Swiss engineer, Ernst Schmidt, developed a test

hammer for measuring the hardness of concrete by the rebound

principle.

• The Schmidt rebound hammer is principally a surface hardness

tester with little apparent theoretical relationship between the

strength of concrete and the rebound number of the hammer.

• However, within limits, empirical correlations have been

established between strength properties and the rebound

number.

• IS 13311-1992, part-II states, “As such, the estimation of

strength of concrete by rebound hammer method cannot be held

to be very accurate and probable accuracy of prediction of

concrete strength in a structure is ±25 percent.”

• The testing is conducted on smooth and uniform surface after

cleaning the surface with carborandum stone/grinding stone.

DIFFERENT TYPES OF REBOUND

HAMMER

Type of hammer Uses

L Type For testing Light weight

concrete

P Type For testing materials with low

hardness and strength such as

Plaster work and surfacing

N Type For testing concrete in ordinary

building and bridge

construction

M Type For testing the strength of

Mass Concrete

TYPICAL CORELATION FOR N TYPE

REBOUND HAMMER

FACTORS AFFECTING REBOUND

VALUE

• Type of Aggregates

• Degree of Compaction

• Age of Concrete

• Dryness/Wetness of The Surface

• Rigidity of The Member

• Surface Finish of Concrete- Moulded/ Troweled

• Maintenance of Rebound Hammer

• Inclination of The Rebound Hammer

• Type of Cement

• Carbonation

• Cover

APPLICATION OF REBOUND HAMMER

• Checking the uniformity of concrete quality

• Comparing a given concrete with a specified

requirement

• Approximate estimation of strength

• Abrasion resistance classification.

PHOTOGRAPHS SHOWING DIFFERENT

REBOUND HAMMER

N Type Rebound Hammer

M Type Rebound Hammer

ULTRASONIC PULSE VELOCITY

TESTING

• In this method, an ultrasonic pulse of longitudinal

vibrations is produced by an electro-acoustical transducer

which is held in contact with one surface of the concrete

member under test.

• The basic idea on which the pulse velocity method is

established is that the velocity of a pulse of compressional

waves through a medium depends on the elastic properties

and density of the medium

• Hence, comparatively higher pulse velocities are obtained

when the ‘quality’ of concrete in terms of density,

homogeneity and uniformity is good. In case of concrete of

poorer quality, lower velocities are obtained.

ULTRASONIC PULSE VELOCITY

TESTING

• Types of testing method

a) Direct transmission

b) Semi direct transmission

c) Surface transmission

• Direct transmission method is the best but it

requires access to two opposite sides of concrete

member

DIFFERENT MODES OF PROPOGATING

ULTRASONIC PULSES

FACTORS AFFECTING PULSE

VELOCITY

• Degree of coupling

• Presence of reinforcement

• Concrete temperature

• Moisture content

• Mix proportion

• Age of concrete

• Stress level in concrete

• Concrete strength can be predicted within + 20%

provided calibration curve is established

VELOCITY CRITERIA FOR CONCRETE

QUALITY GRADING

[As per IS: 13311 (Part I) – 1992]

[UPV by cross probing method]

Sl No. Pulse Velocity by Cross-

Probing (km/sec)

Concrete Quality Grading

1 Above 4.5 Excellent

2 3.5 to 4.5 Good

3 3.0 to 3.5 Medium

4 Below 3.0 Doubtful

APPLICATIONS OF PULSE VELOCITY

METHOD

• Main application for assessment of concrete

uniformity

• To establish areas of deteriorated concrete

• Detection of cracks

• Calculation of dynamic young’s modulus

PHOTOGRAPH SHOWING ULTRASONIC

PULSE VELOCITY TESTING

COVER METER

• Cover is a very important parameter dictating durability

of concrete

• Conventionally provision of cover is checked prior to

concreting. Post Construction Assessment of cover is

possible through cover meters.

• Ferro Scanning and Profometer instrument is used to

assess the cover of reinforcement. The instrument is based

on the magnetic technique and is calibrated for different

purposes.

• They are not very effective in heavily reinforced members

or members with spiral reinforcement

• Size of the reinforcement bar is required to be known for

accurate assessment of cover

APPLICATION OF COVER METER

• Identification of location of reinforcement bar

with the following applications

– Helps in avoiding drilling into the

reinforcement

– To avoid reinforcement in pulse velocity

measurements

• To improve quality control during construction

• Assessment of residual time till initiation of

corrosion

• Rehabilitation planning

PHOTOGRAPH SHOWING

FERROSCANNER

CONCRETE CORE TESTING

Concrete cores of 60-mm diameter are extracted

from different structural members identified, to

estimate equivalent cube compressive strength of the

structure. Equivalent cube strength does not

indicate 28 days standard cube strength rather it

represents the in-situ cube strength, and is compared

vis-à-vis strength used in design calculation with

safety of the structure under load in mind.

VARIOUS STEPS FOR

CORE TESTING • LOCATION AND NO. OF CORES

– The points from which cores are to be taken and the number of cores required shall be at the discretion of engineer-in-charge. In no case, however, fewer than three cores shall be taken.

( Clause 17.4.1 of IS 456-2000)

• DIAMETER OF CORE (Clause 4.3 of IS 1199)

– A core specimen for pavement thickness shall have a diameter of at least 10 cm.

– A core specimen for compressive strength shall have diameter at least 3 times MSA

– In no case Dia shall be less than 2 MSA

25

VARIOUS STEPS FOR

CORE TESTING

• LENGTH OF CORE (L/D RATIO)

– Length of specimen, when capped shall be nearly as practicable twice its diameter ( Clause 4.3.1 of IS 1199)

– A correction factor according to the height/dia of specimen after capping shall be obtained from the curve in Fig 1 of IS 516

• CUTTING AND CAPPING

– Cutting the ends with masonry saw.

– The specimen are capped with sulphur compound

• Pure Sulphur : 3 Parts

• Inert filler (fire clay) : 1 Part

CORRECTION FACTOR FOR HEIGHT/ DIAMETER

RATIO OF A CORE

HEIGHT

DIAMETER RATIO-

CO

RR

EC

TIO

N F

AC

TO

R

VARIOUS STEPS FOR

CORE TESTING

• TESTING THE CORES FOR COMPRESSIVE

STRENGTH

– The core shall be placed in water at a temperature of 240

to 300C for 48 hours before testing

– After testing, the measured compressive strength shall be

multiplied by correction factor for L/D ratio between 1

and 2.

– Equivalent cube strength of the concrete shall be

determined by multiplying the corrected cylinder strength

by5/4

VARIOUS STEPS FOR

CORE TESTING

• ACCEPTANCE CRITERIA (AS PER IS 456-2000)

Concrete in a member represented by a core test shall be considered acceptable if the average equivalent cube strength of the cores is equal to at least 85 percent of the cube strength of the grade of concrete specified for the corresponding age and no individual core has a strength less than 75 percent

PHOTOGRAPH SHOWING CORE

EXTRACTION

CARBONATION DEPTH

• Carbonation is chemical reaction between Ca

(OH)2 and CO2 of the atmosphere

• Carbonation destroys passive protection

provided by concrete to the reinforcement

• Carbonation proceeds from the surface into the

concrete

• When depth of carbonation equals concrete

cover reinforcement corrosion is imminent

CARBONATION DEPTH

• Method of Assessment

– Drill into concrete

– Spray phenolphthalein solution

– Uncarbonated concrete will show bright pink

stain

– Carbonated concrete will not change colour

– With the above visual indication depth of

carbonation can be easily measured

PHOTOGRAPH SHOWING MEASURED

CARBONATION DEPTH AT SITE

ELECTRICAL RESISTIVITY TESTING

• Concrete resistivity is geometrical independent

material property that indicates the ratio between the

applied voltage and resulting current in a unit cell.

• The resistivity of concrete impacts the current flow

between the cathodic and anodic regions of the

concrete.

• The higher the concrete resistivity, the lower the

current flowing between anodic and cathodic areas

will be, and therefore lower the corrosion rate.

• The proceq RESI Resistivity Meter i.e. a four-point

Wenner probe resistivity meter permits a rapid and

non-destructive measurement of the quality of

concrete with respect to its resistivity.

PHOTOGRAPH SHOWING RESISTIVITY

METER

HALF CELL POTENTIAL

MEASUREMENT

• This test method covers the estimation of electrical

Half Cell Potential of uncoated reinforcing steel, to

determine corrosion activity using reference electrode

copper; copper sulphate half-cell.

• It is not possible to expose all the reinforcements in the

structural element and observe the extent of corrosion.

So, this method has been very convenient to assess the

condition of the entire length of a member by exposing

a portion of the reinforcement at a suitable location,

which measures the half-cell potential on the entire

length, by placing the reference electrode on the wet

concrete surface.

HALF CELL POTENTIAL

MEASUREMENT

• The Half-Cell Potential measurement is based on

the principal of the corrosion, being an electro-

chemical process, induces certain voltage to the

reinforcement steel that is corroding.

• The wetting of the concrete is required to make

the portion between the concrete surface and the

reinforcing bar as electrolytes.

CRITERIA FOR DECIDING THE STATUS

OF CORROSION

[According to ASTM C – 876]

Phases of Corrosion

Activity

As measured By Copper

– Copper Sulphate Half

Cell

Initial phase-corrosion

activity not taking place

< - 200 mV

Transient phase-corrosion

activity uncertain

- 200 mV to –350 mV

Final Phase – corrosion

occurring positively

> - 350 mV

PHOTOGRAPH SHOWING HALF CELL

POTENTIAL MEASUREMENT

CHEMICAL ANALYSIS

• For analyzing Chloride content and pH of concrete,

concrete powder samples were extracted from 0-20mm, 20-

40mm & 40-60mm depths at identified locations and then

tested as per IS:14959(Part 2) -2001 (Determination of

water soluble and acid soluble Chlorides in Mortar and

Concrete – Method of Test).

• Corrosion of reinforcing steel due to chlorides in concrete

is one of the most common environmental attacks that lead

to deterioration of concrete structures. Whenever there is

chloride in concrete there is an increased risk of corrosion

of embedded metal. Chloride content is then expressed in

kg per cubic meter of concrete and compared with the

values of limits of chloride contents of concrete (Table 7 of

IS: 456–2000).

CHEMICAL ANALYSIS

• Sulphates (SO3) are present in most cements and in some

aggregates; excessive amounts of water-soluble sulphate

from these or other mix constituents can cause expansion

and disruption of concrete. To prevent it, IS: 456-2000

clause-8.2.5.3 states that the total water-soluble sulphate

content of the concrete mix, expressed as SO3, should not

exceed 4 percent by mass of the cement in the mix. The

sulphate content should be calculated as the total from the

various constituents of the mix.

• The pH value of the concrete should be above 11.5 to

maintain alkalinity of concrete surrounding the embedded

steel. A reduction in the pH value of concrete indicates loss

of passive layer around the reinforcement which protects

the steel from distress.

THANKS