STRUCTURAL HEALTH MONITORING USING

NON DESTRUCTIVE TESTING

OF CONCRETE

A SEMINAR SUBMITTED IN PARTIAL FULFILLMENT OF THE

REQUIREMENTS FOR THE

DEGREE OF

Masters of Technology

In

Structural Engineering

By

PAWAN KUMAR AGGARWAL

Department of Civil Engineering

DAV Institute of Engineering & Technology

Jalandhar

Submitted to

Prof. Sanjeev Naval

H.O.D. (Civil Engineering Department)

DAV Institute of Engineering Technology

Jalandhar

CERTIFICATE

This is to certify that the seminar entitled,

“STRUCTURAL HEALTH MONITORING

USING NON DESTRUCTIVE TESTING OF CONCRETE” submitted by

Mr. PAWAN AGGARWAL

in fulfillment of the requirements for the award of

Master of Technology Degree

in Structural Engineering at the

DAV Institute of Engineering Technology

Jalandhar

is an authentic work carried out by him under my

Supervision and guidance.

Date:

Prof. Sanjeev Naval

Head Dept. of Civil Engineering

DAV Institute of Engineering Technology

Jalandhar

ACKNOWLEDGEMENT

I extend my deep sense of gratitude and indebtedness to our guide Prof. Sanjeev Naval,

(H.O.D.), Department Of Civil Engineering, DAV Institute of Engineering & Technology,

Jalandhar, for his kind attitude, invaluable guidance, keen interest, immense help, inspiration

and encouragement which helped me in carrying out my present work.

I am g r a t e f u l t o him for giving a lot of freedom, encouragement and guidance, and the

faculty members of Civil Engineering Department, DAV Institute of Engineering &

Technology, Jalandhar, for providing all kind of possible help throughout for the completion

of this seminar work..

I am also thankful to the Technical Laboratory Staff of DAV institute of Engineering &

Technology, Jalandhar for helping me during the experimental work . It is a great pleasure for

me to acknowledge and express my gratitude to my classmates and friends for their

understanding, unstinted support. Lastly, I thank all those who are involved directly or

indirectly in completion of the present seminar work.

PAWAN KUMAR AGGARWAL

ROLL NO: 81402105007

CIVIL ENGINEERING

DAV , JALANDAHAR

ABSTRACTTo keep a high level of structural safety, durability and performance of the infrastructure in

each country, an efficient system for early and regular structural assessment is urgently

required. The quality assurance during and after the construction of new structures

and after reconstruction processes and the characterisation of material properties and damage

as a function of time and environmental influences is more and more becoming a serious

concern.

Non-destructive testing (NDT) methods have a large potential to be part of such a system.

NDT methods in general are widely used in several industry branches. Aircrafts, nuclear

facilities, chemical plants, electronic devices and other safety critical installations are tested

regularly with fast and reliable testing technologies. A variety of advanced NDT methods are

available for metallic or composite materials.

In recent years, innovative NDT methods, which can be used for the assessment of existing

structures, have become available for concrete structures, but are still not established for

regular inspections. Therefore, the objective of this project is to study the applicability,

performance, availability, complexity and restrictions of NDT.

The purpose of establishing standard procedures for nondestructive testing (NDT) of concrete

structures is to qualify and quantify the material properties of in-situ concrete without

intrusively examining the material properties. There are many techniques that are currently

being research for the NDT of materials today. This chapter focuses on the NDT methods

relevant for the inspection and monitoring of concrete materials.

INTRODUCTION

Structures are assemblies of load carrying members capable of safely transferring the

superimposed loads to the foundations. Their main and most looked after property is the

strength of the material that they are made of. Concrete, as we all know, is an integral

material used for construction purposes. Thus, strength of concrete used, is required to be

‘known’ before starting with any kind of analysis. In the recent past, various methods and

techniques, called as Non-Destructive Evaluation (NDE) techniques, are being used for

Structural Health Monitoring (SHM).

The concept of nondestructive testing (NDT) is to obtain material properties of in place

specimens without the destruction of neither the specimen nor the structure from which it is

taken. However, one problem that has been prevalent within the concrete industry for years is

that the true properties of an in-place specimen have never been tested without leaving a

certain degree of damage on the structure. For most cast-in-place concrete structures,

construction specifications require that test cylinders be cast for 28-day strength

determination.

Usually, representative test specimens are cast from the same concrete mix as the larger

structural elements. Unfortunately, test specimens are not an exact representation of in-situ

concrete, and may be affected by variations in specimen type, size, and curing procedures.

Keeping in view the development phase in construction around us, it is appropriate time to

highlight the shortcomings of destructive tests. The quality of construction can only be

ensured, if the necessary in-situ tests are also conducted

The rebound hammer test is classified as a hardness test and 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 energy absorbed by the concrete is related to its strength. There is no unique

relation between hardness and strength of concrete but experimental data relationships can be

obtained from a given concrete. However, this relationship is dependent upon factors

affecting the concrete surface such as degree of saturation, carbonation, temperature,

surface preparation and location, and type of surface finish.

A correlation between rebound number and strength of concrete structure is established,

which can be used as well for strength estimation of concrete structures. The direct

determination of the strength of concrete implies that concrete specimens must be loaded to

failure. Therefore, the determination of concrete strength requires special specimens to be

taken, shipped, and tested at laboratories. This procedure may result in the actual strength of

concrete, but may cause trouble and delay in evaluating existing structures.

Because of that, special techniques have been developed in which attempts were made to

measure some concrete properties other than strength, and then relate them to strength,

durability, or any other property. Some of these properties are hardness, resistance to

penetration or projectiles, rebound number, resonance frequency, and ability to allow

ultrasonic pulses to propagate through concrete. Concrete electrical properties, its ability to

absorb, scatter, and transmit X-rays and gamma rays, its response to nuclear activation, and

its acoustic emission allow us to estimate its moisture content, density, thickness, and its

cement content. However, the term “nondestructive” is given to any test that does not damage

or affect the structural behavior of the elements and also leaves the structure in an acceptable

condition for the client.

The use of the ultrasonic pulse velocity tester is introduced as a tool to

monitor basic initial cracking of concrete structures and hence to introduce a threshold limit

for possible failure of the structures. Experiments using ultrasonic pulse velocity tester have

been carried out, under laboratory conditions, on various concrete specimens loaded in

compression up to failure.

LITERATURE SURVEYCompared with the development of nondestructive test (NDT) methods for steel structures,

the development of NDT methods for concrete has progressed at a slower pace, because

concrete is inherently more difficult to test than steel. Concrete is highly heterogeneous, it is

electrically nonconductive but usually contains significant amounts of steel reinforcement,

and it is often used in thick members. Thus, it has not been easy to transfer NDT technologies

developed for steel to the evaluation of concrete. In addition, there has been little interest

among those from the traditional NDT community (physicists, electrical engineers,

mechanical engineers) to develop test methods for concrete. Since the 1980s, however,

advances have occurred as a result of the microcomputer revolution and the development of

powerful signal-processing techniques. No standard definition exists for nondestructive test

as applied to concrete. For some people, it is any test that does not alter the concrete. For

others, it is a test that does not impair the function of a structure, in which case the drilling of

cores is considered to be a NDT test. For still others, it is a test that does less damage to the

structure than does drilling of cores. This presentation deals with methods that either do not

alter the concrete or that result in only superficial local damage. In this presentation various

methods are divided into two groups:

(1) those whose main purpose is to estimate in-place strength, and

(2) those whose main purpose is to evaluate conditions other than strength—that is, to

evaluate integrity.

It will be shown that the most reliable tests for strength are those that result in superficial

local damage, and this presentation prefers the term in-place tests for this group. The integrity

tests, on the other hand, are truly nondestructive.

The purpose of this presentation is to provide an introduction to commonly used NDT

methods for concrete. Emphasis is placed on the principles underlying the various methods so

as to understand their advantages and inherent limitations. Additional information on the

application of these methods is available in IS:1311;Part-I & II

2.1 Historical Background

Some of the first methods to evaluate the in-place strength of concrete were adaptations of

the Brinell hardness test for metals, which involves pushing a high-strength steel ball into the

test piece under a given force and measuring the area of the indentation. In the metals test, the

load is applied by a hydraulic loading system. Modifications were required to use this type of

test on a concrete structure. In 1934, Professor K. Gaede in Germany reported on the use of a

spring-driven impactor to supply the force to drive a steel ball into the concrete (Malhotra,

1976). A nonlinear, empirical relationship was obtained between cube compressive strength

and indentation diameter. In 1936, J.P. Williams in England reported on a spring-loaded,

pistol-shaped device in which a 4-mm ball was attached to a plunger (Malhotra, 1976). The

spring was compressed by turning a screw, a trigger released the compressed spring, and the

plunger was propelled toward the concrete. The diameter of the indentation produced by the

ball was measured with a magnifying glass and scale. In 1938, a landmark paper by D.G.

Skramtajev, of the Central Institute for Industrial Building Research in Moscow, summarized

14 different techniques for estimating the in-place strength of concrete, 10 of which were

developed in the Soviet Union (Skramtajev, 1938).

The various NDT methods for testing concrete are listed below –

A. For strength estimation of concrete

(i) Rebound hammer test

(ii) Ultrasonic Pulse Velocity Tester

(iii) Combined use of Ultrasonic Pulse Velocity tester and rebound hammer test

(iv) Pull off test

(v) Pull out test

(vi) Break off test

B. For assessment of corrosion condition of reinforcement and to determine reinforcement

diameter and cover

(i) Half-cell potentiometer

(ii) Resistively meter test

(iii) Test for carbonation of concrete

(iv) Test for chloride content of concrete

(v) Profometer

(vi) Micro covermeter

C. For detection of cracks/voids/ delamination etc.

(i) Infrared thermo graphic technique

(ii) Acoustic Emission techniques

(iii) Short Pulse Radar methods

(iv) Stress wave propagation methods

- pulse echo method

- impact echo method

- response method

2.2(a) Strength determination by NDE methods:

Strength determination of concrete is important because its elastic behavior & service

behavior can be predicted from its strength characteristics. The conventional NDE methods

typically measure certain properties of concrete from which an estimate of its strength and

other characteristics can be made. Hence, they do not directly give the absolute values of

strength.

2.2(b) Damage detection by NDE methods:

Global techniques: These techniques rely on global structural response for damage

identification. Their main drawback is that since they rely on global response, they are not

sensitive to localized damages. Thus, it is possible that some damages which may be present

at various locations remain un-noticed.

Local techniques: These techniques employ localized structural analysis, for damaged

detection

Their main drawback is that accessories like probes and fixtures are

required to be physically carried around the test structure for data recording. Thus, it no

longer remains autonomous application of the technique. These techniques are often applied

at few selected locations, by the instincts/experience of the engineer coupled with visual

inspection. Hence, randomness creeps into the resulting data.

NDE Methods in Practice

Visual inspection: The first stage in the evaluation of a concrete structure is to study the

condition of concrete, to note any defects in the concrete, to note the presence of cracking and

the cracking type (crack width, depth, spacing, density), the presence of rust marks on the

surface, the presence of voids and the presence of apparently poorly compacted areas etc.

Visual assessment determines whether or not to proceed with detailed investigation.

The Surface hardness method: This is based on the principle that the strength of concrete is

proportional to its surface hardness. The calibration chart is valid for a particular type of

cement, aggregates used, moisture content, and the age of the specimen.

The penetration technique: This is basically a hardness test, which provides a quick means

of determining the relative strength of the concrete. The results of the test are influenced by

surface smoothness of concrete and the type and hardness of the aggregate used. Again, the

calibration chart is valid for a particular type of cement, aggregates used, moisture content,

and age of the specimen. The test may cause damage to the specimen which needs to be

repaired.

The pull-out test: A pullout test involves casting the enlarged end of a steel rod after setting

of concrete, to be tested and then measuring the force required to pull it out. The test

measures the direct shear strength of concrete. This in turn is correlated with the compressive

strength; thus a measurement of the in-place compressive strength is made. The test may

cause damage to the specimen which needs to be repaired.

The rebound hammer test: The Schmidt rebound hammer is basically a surface hardness

test with little apparent theoretical relationship between the strength of concrete and the

rebound number of the hammer. Rebound hammers test the surface hardness of concrete,

which cannot be converted directly to compressive strength. The method basically measures

the modulus of elasticity of the near surface concrete. The principle is based on the

absorption of part of the stored elastic energy of the spring through plastic deformation of the

rock surface and the mechanical waves propagating through the stone while the remaining

elastic energy causes the actual rebound of the hammer. The distance travelled by the mass,

expressed as a percentage of the initial extension of the spring, is called the Rebound

number. There is a considerable amount of scatter in rebound numbers because of the

heterogeneous nature of near surface properties (principally due to near-surface aggregate

particles).

There are several factors other than concrete strength that influence rebound hammer test

results, including surface smoothness and finish, moisture content, coarse aggregate type, and

the presence of carbonation. Although rebound hammers can be used to estimate concrete

strength, the rebound numbers must be correlated with the compressive strength of molded

specimens or cores taken from the structure.

Ultra-sonic pulse velocity test: This test involves measuring the velocity of sound through

concrete for strength determination. Since, concrete is a multi-phase material, speed of sound

in concrete depends on the relative concentration of its constituent materials, degree of

compacting, moisture content, and the amount of discontinuities present. This technique is

applied for measurements of composition (e.g. monitor the mixing materials during

construction, to estimate the depth of damage caused by fire), strength estimation,

homogeneity, elastic modulus and age, & to check presence of defects, crack depth and

thickness measurement. Generally, high pulse velocity readings in concrete are indicative of

concrete of good quality. The drawback is that this test requires large and expensive

transducers. In addition, ultrasonic waves cannot be induced at right angles to the surface;

hence, they cannot detect transverse cracks.

Acoustic emission technique: This technique utilizes the elastic waves generated by plastic

deformations, moving dislocations, etc. for the analysis and detection of structural defects.

However, there can be multiple travel paths available from the source to the sensors. Also,

electrical interference or other mechanical noises hampers the quality of the emission signals.

Impact echo test: In this technique, a stress pulse is introduced at the surface of the structure,

and as the pulse propagates through the structure, it is reflected by cracks and dislocations.

Through the analysis of the reflected waves, the locations of the defects can be estimated.

The main drawback of this technique is that it is insensitive to small sized cracks.

3.

DETAIL METHODOLGY OF VARIOUS NDT TESTS OF

CONCRETE

3.1 REBOUND HAMMER TEST

The operation of rebound hammer is shown in the fig.1. When the plunger of rebound

hammer is pressed against the surface of concrete, a spring controlled mass with a constant

energy is made to hit concrete surface to rebound back. The extent of rebound, which is a

measure of surface hardness, is measured on a graduated scale. This measured value is

designated as Rebound Number (rebound index). A concrete with low strength and low

stiffness will absorb more energy to yield in a lower rebound value.

Fig: Operation of the rebound hammer

Fig: Rebound Hammer

IS 13311 Part-II – 1992 and BS 6089-81 and BS: 1881: Part-204 explains the standard

procedure for test and correlation between concrete cube crushing and strength rebound

number. The results are significantly affected by the following factors:

a) Mix characteristics

1. Cement type

2. Cement content

3. Coarse aggregate type

b) Angle of inclination of direction of hammer with reference to horizontal (fig2).

c) Member characteristics

1. Mass

2. Compaction

3. Surface type

4. Age, rate of hardening and curing type

5. Surface carbonation

6. Moisture condition

7. Stress state and temperature

Fig.2 – cube compressive strength is N/mm2 plotted against rebound number

Since each of these may affect the readings obtained, any attempts to compare or estimate

oncrete strength will be valid only if they are all standardised for the concrete under test and

for the calibration of specimens.

A) Strength Assessment

This test is conducted to assess the relative strength of concrete based on the hardness at or

near its exposed surface. Carrying of periodic calibration of rebound hammer using standard

anvil is desirable. However for new concrete construction, rebound hammer is calibrated on

concrete test cubes for a given source of constituent materials (i.e. cement, sand, stone

aggregates), this calibration data can be used with reasonable accuracy In arriving at

equivalent in-situ cube strength of relatively new concrete (not more than three months old

concrete. This calibration exercise may be carried out in a concrete lab by casting cubes of

designed mix and testing these under controlled condition with rebound hammer as well as

test to destruction in compression. Calibration graphs then can be drawn. Large number of

readings are desirable to reduce the affects of variability in readings due to various localized

as well as instrument factors. This method may give highly erroneous results for concrete

whose surface is exposed to atmosphere for longer periods say more than three months. This

is due to hardening of concrete surface due to carbonation, which may cause overestimation

as much as 50% for old structure. Hence strength assessment by rebound hammer test should

generally be restricted to relatively new structures only,

B) Survey of weak and delaminating concrete

as the test requires a flat surface and large number of readings to reduce variability, this test is

not generally suitable for use on spalled concrete surfaces of distressed structures. However,

comparison of rebound numbers which indicate the near surface hardness of the concrete will

help to identify relative surface weakness in cover concrete and also can be used to determine

the relative compressive strength of concrete. Locations possessing very low rebound

numbers will be identified as weak surface concrete and such locations will be identified for

further investigations like corrosion distress, fire damage and or any other reason including

original construction defects of concrete. This survey is to be carried our on each identified

member in a systematic way by dividing the member into well-defined grid points. The grid

matrix should have a spacing of approximately 300mm x 300mm. Table 1 gives guidelines

for qualitative interception of rebound hammer test results with reference to quality.

Table: 1 Quality of concrete from rebound values

Comparative Hardness

Average Rebound Quality of Concrete

>40 Very good

30 – 40 Good

20 – 30 Fair

<20 Poor and / or delaminated

0 Very poor and/or

delaminated

 

Procedure

i) Before commencement of a test, the rebound hammer should be tested against the test

anvil, to get reliable results, for which the manufacturer of the rebound hammer indicates the

range of readings on the anvil suitable for different types of rebound hammer.

ii) Apply light pressure on the plunger – it will release it from the locked position and allow it

to extend to the ready position for the test.

iii) Press the plunger against the surface of the concrete, keeping the instrument perpendicular

to the test surface. Apply a gradual increase in pressure until the hammer impacts. (Do not

touch the button while depressing the plunger. Press the button after impact, in case it is not

convenient to note the rebound reading in that position.)

iv) Take the average of about 15 readings.

3.2 ULTRA SOUND PULSE VELOCITY TEST

Ultrasonic scanning is a recognised non-destructive evaluation test to qualitatively asses the

homogeneity and integrity of concrete. With this technique, following can be assessed:

1. Qualitative assessment of strength of concrete, its gradation in different locations of

structural members and plotting the same.

2. Any discontinuity in cross section like cracks, cover concrete delamination etc.

3. Depth of surface cracks.

This test essentially consists of measuring travel time, T of ultrasonic pulse of 50 to 54 kHz,

produced by an electro-acoustical transducer, held in contact with one surface of the concrete

member under test and receiving the same by a similar transducer in contact with the surface

at the other end. With the path length L, (i.e. the distance between the two probes) and time

of travel T, the pulse velocity (V=L/T) is calculated (fig.2). Higher the elastic modulus,

density and integrity of the concrete, higher is the pulse velocity. The ultrasonic pulse

velocity depends on the density and elastic properties of the material being tested.

Fig.1: Ultrasonic Pulse Velocity Instrument

Though pulse velocity is related with crushing strength of concrete, yet no statistical

correlation can be applied.

The pulse velocity in concrete may be influenced by:

a) Path length

b) Lateral dimension of the specimen tested

c) Presence of reinforcement steel

d) Moisture content of the concrete

The influence of path length will be negligible provided it is not less than 100mm when

20mm size aggregate is used or less than 150mm for 40mm size aggregate. Pulse velocity

will not be influenced by the shape of the specimen, provided its least lateral dimension (i.e.

its dimension measured at right angles to the pulse path) is not less than the wavelength of the

pulse vibrations. For pulse of 50Hz frequency, this corresponds to a least lateral dimension of

about 80mm. the velocity of pulses in steel bar is generally higher than they are in concrete.

For this reason pulse velocity measurements made in the vicinity of reinforcing steel may be

high and not representative of the concrete. The influence of the reinforcement is generally

small if the bars runs in a direction at right angles to the pulse path and the quantity of steel is

small in relation to the path length. The moisture content of the concrete can have a small but

significant influence on the pulse velocity. In general, the velocity is increased with increased

moisture content, the influence being more marked for lower quality concrete.

Fig.1: Method of propagating and receiving pulses

Measurement of pulse velocities at points on a regular grid on the surface of a concrete

structure provides a reliable method of assessing the homogeneity of the concrete. The size of

the grid chosen will depend on the size of the structure and the amount of variability

encountered.

Table: 1 – General Guidelines for Concrete Quality based on UPV

PULSE VELOCITY CONCRETE QUALITY

>4.0 km/s Very good to excellent

3.5 – 4.0 km/s Good to very good, slight porosity may exist

3.0 – 3.5 km/s Satisfactory but loss of integrity is suspected

<3.0 km/s Poor and los of integrity exist.

 Table 1 shows the guidelines for qualitative assessment of concrete based on UPV test

results. To make a more realistic assessment of the condition of surface of a structural

member, the pulse velocity can be combined with rebound number. Table 2 shows the

guidelines for identification of corrosion prone locations by combining the results of pulse

velocity and rebound number.

Table:2 – Identification of Corrosion Prone Location based on Pulse Velocity and Hammer

Readings 

Sl. No. Test Results Interpretations

1 High UPV values, high

rebound number

Not corrosion prone

2 Medium range UPV values,

low rebound numbers

Surface delamination, low quality of surface

concrete, corrosion prone

3 Low UPV, high rebound

numbers

Not corrosion prone, however to be confirmed

by chemical tests, carbonation, pH

4 Low UPV, low rebound

numbers

Corrosion prone, requires chemical and

electrochemical tests.

 

Detection of Defects

When ultrasonic pulse travelling through concrete meets a concrete-air interface, there is a

negligible transmission of energy across this interface so that any air filled crack or void

lying directly between the transducers will obstruct the direct beam of ultrasonic when the

void has a projected area larger than the area of transducer faces. The first pulse to arrive at

the receiving transducer will have been directed around the periphery of the defect and the

time will be longer than in similar concrete with no defect.

Estimating the depth of cracks

An estimate of the depth of a crack visible at the surface can be obtained by the transit times

across the crack for two different arrangements of the transducers placed on the surface. One

suitable arrangement is one in which the transmitting and receiving transducers are placed on

opposite sides of the crack and distant from it. Two values of X are chosen, one being twice

that of the other, and the transmit times corresponding to these are measured. An equation

may be derived by assuming that the plane of the crack is perpendicular to the concrete

surface and that the concrete in the vicinity of the crack is of reasonably uniform quality. It is

important that the distance X be measured accurately and that very good coupling is

developed between the transducers and the concrete surface. The method is valid provided

the crack is not filled with water.

This test is done as per IS: 13311 (Part 1) – 1992.

Procedure for Ultrasonic Pulse Velocity

i) Preparing for use: Before switching on the ‘V’ meter, the transducers should be connected

to the sockets marked “TRAN” and ” REC”.

The ‘V’ meter may be operated with either:

a) the internal battery,

b) an external battery or

c) the A.C line.

ii) Set reference: A reference bar is provided to check the instrument zero. The pulse time

for the bar is engraved on it. Apply a smear of grease to the transducer faces before placing it

on the opposite ends of the bar. Adjust the ‘SET REF’ control until the reference bar transit

time is obtained on the instrument read-out.

iii) Range selection: For maximum accuracy, it is recommended that the 0.1 microsecond

range be selected for path length upto 400mm.

iv) Pulse velocity: Having determined the most suitable test points on the material to be

tested, make careful measurement of the path length ‘L’. Apply coolant to the surfaces of the

transducers and press it hard onto the surface of the material. Do not move the transducers

while a reading is being taken, as this can generate noise signals and errors in measurements.

Continue holding the transducers onto the surface of the material until a consistent reading

appears on the display, which is the time in microsecond for the ultrasonic pulse to travel the

distance ‘L’. The mean value of the display readings should be taken when the units digit

hunts between two values.

Pulse velocity=(Path length/Travel time)

v) Separation of transducer leads: It is advisable to prevent the two transducer leads from

coming into close contact with each other when the transit time measurements are being

taken. If this is not done, the receiver lead might pick-up unwanted signals from the

transmitter lead and this would result in an incorrect display of the transit time.

3.3 COVER METER

Cover meter survey

The necessity to provide adequate cover thickness to control corrosion needs no emphasis. A

cover thickness survey is useful to determine existing cover thickness in a specified location,

where a damage has been identified and elsewhere, for comparison on the same structure.

The cover thickness can be measured non-destructively using commercially known cover

meters. The cover meters are also used to identify the location and diameter of rebar:

COVERMASTER and PROFOMETER are commercially available instruments, which are

used to measure the cover thickness and rebar size. Table 1 shows how the cover reading are

to be interpreted for corrosion assessment.

Fig: Cover meter of Profometer

Table -1: Interpretation of Cover Thickness Survey

Sl. No. Test Results Interpretation

1 Required cover thickness and good quality

concrete

Relatively not corrosion

prone

2 Required cover thickness and bad quality

concrete cover

Corrosion prone

3 Very less cover thickness yet good quality

cover concrete

Corrosion prone

1. Half Cell Potential Survey

Corrosion being an electrochemical phenomenon, the electrode potential of steel wire with

reference to standard electrode undergoes changes depending on corrosion activity.

A schematic survey on well-defined grid points gives useful information on the presence or

probability of corrosion activity. The same grid points are used for other measurements,

namely, rebound hammer and UPV could be used for making the data more meaningful. The

common standard electrodes used are:

i. Copper – Copper sulphate electrode (CSE)

ii. Silver – Silver chloride electrode (SSE)

iii. Standard Calomel electrode (SCE)

The measurement consists of giving an electrical connection to the rebar and observing the

voltage difference between the bar and a reference electrode in contact with concrete surface.

(Fig. 1. (a)) Generally the voltage potential becomes more and more negative as the corrosion

becomes more and more active. However less negative potential values may also indicate the

presence of corrosion activity, if the pH values are less.

Fig-1 (a): Half Cell Potential Test

The general guidelines for identifying the probability of corrosion based on half cell potential

values are suggested as in ASTM C876 are given in Table 2.

Table-2: Corrosion Risk by Half Cell Potentiometer

In any case, the technique should never be used in isolation, but should be coupled with

measurements of chloride content of the concretes and its variation with depth and also the

cover to the steel and the depth of carbonation.

However, a systematic “potential mapping survey” is considered to be more useful for on-site

identification of the Corrosion State of rebars. This will facilitate setting out potential profile

or potential contour. A typical potential contour is shown in fig.1 (b) and (c). initially when

potential surveying was introduced as per ASTM C 876, each reading was interpreted in

isolation and the numerical value was directly correlated to the fegree of corrosion.

Subsequently, this approach was realised to be erroneous because non-corroded steel can

exhibit a wide range of potential values. It is now realised that potential values should be

assessed not in isolation but as a group and the inter relationship of the potentials within a

group should form the basis of interpretation. Analysis of potential contour will generally

consist of indenfying the locatins with accumulated potential lines indicating to the corroding

areas beneath.

(b) Shaded Mapping

(b) Contour Plot

Fig: 1 (b) and (c) Typical Half Cell Potential Contours

· Locating at a glance, the anodic areas identified by the gathering of isopotential lines having

more severe potential gradient.

· Ascertaining whether or not a structure is actively corroding.

It is necessary to realise certain important parameters (listed below) which influence the

measured potentials of the reinforcement.

1. The potentials of rebar measured on the surface of, or within concrete may not be a

true representation of the values at the surface of the steel

2. The physical i.e. moisture content and chemical state of concrete i.e. presence of

electrolyte ions can result in wide variation.

3. The ohmic drop due to electrical resistance of the concrete also can induce variations

4. With increased concrete cover, the potential values at the concrete surface over

actively corroding and passing slab become similar.

1. Resistivity Measurement

Resistivity Mapping:

The electrical rsistance of concrete plays an important role in determining the quality of

concre from the point of view ‘corrosion susceptibility potential’ at any specific location.

This parameter is expressed in terms of “Resistivity” in ohm-cm.

For general monitoritng, a resistivity check is important because long-term corrosion can be

anticipated in concrete structures where accurately measured values are below 10000 ohm-

cm. further if resistivity values fall below 5000 ohm-cm, corrosion must be anticipated at a

much earlier period (possibly within 5 years) in the life of a structure. Table 2 indicates the

general guidelines of resistivity values based on which areas having probable corrosion risk

can be identified in concrete structures.

Table-2: Corrosion risk from resistivity

Resistivity ohm-cm Corrosion Probability

Greater than 20,000 Negligible

10,000 – 20,000 Low

5,000 – 10000 High

Less than 5,000 Very high

The principle of resistivity testing in concrete is similar to that adopted in soil testing.

However when applied in concrete, a few drawbacks should be realised. The method

essentially consists of using a 4-probe technique in which a known current is applied between

two outer probes 100 mm apart and the voltage drop between the inner two elements at

50mm spacing is read off allowing for a direct evaluation of resistance, R. Using a

mathematical conversion factor, resistivity is calculated as per principle of four probe

resistivity testing illustrated in figure2.

Fig -2: Resistivity Meter (4 Probe System)

The following drawbacks are important to note while analysing and interpreting the

resistivity values:

The value obtained represents only the average evaluation over the depth regulated by

the chosen probe spacing and not that of concrete at steel interface.

The resistivity of concrete varies with varying moisture condition

The instrument should have adequate IR drop compensation for measurement.

Table-3: Corrosion Probability based on Resistivity and Potential Mapping

Measurement of corrosion rate:

In reinforced concrete structures, determination of actual rate at which the reinforcement is

corroding assumes larger importance since the laboratory results are not directly applicable to

field conditions. Another form of the polarization method has been developed and is known

as Linear Polarisation Resistance (LPR) method for the on-site study of corrosion rates of

steel in concrete. The fundamental principle of linear polarization is based on the

experimentally observed assumption that for a simple model corroding system, the

polarization curve for few milivolts around the corrosion potential obeys a quasi-linear

relationship. The slope of this curve is the so called polarization resistance

.

From this slope, the corrosion rate can be determined using stern-Geary equation

Where B is a constant which is a function of the Tafel slopes and ba, bc and determined from

the formula below:

The value of B usually lies between 13 and 52 mV depending on the passive and active

corroding system. For on-site measurements, the testing system consists of a potentiostat,

counter electrode, reference electrode and the reinforcement as working electrode. This

system is schematically illustrated in fig. 3.

Fig-3: Resistivity Testing for Concrete

A typical plot of linear polarization curve is shown in fig.4.It is necessary that for

measurements in concrete, the potentiostat should have electronic ohmic compensation (IR

drop) or otherwise, the valies to be obtained by calculation or separate experiments.

Fig-4: Linear Polarization Curve.

 Sl. No. Tests Description

1 Cover Meter/ Profo-meter (in-

situ Test)

Non-destructive method for measuring

- Thickness of cover concrete

- Reinforcement diameter

- Reinforcement spacing

2 Half Cell Method (in-situ

Test)

Non-destructive method for

measuring/plotting corrosion potential for

assessing probability of corrosion

3 Resistivity Measurement (in-

situ Test)

Non-destructive method for assessing

electrical resistivity of concrete.

4 Permeability

a) Water

b) Air

Assessment of in-situ permeability of

concrete due to water and air.

5 Initial surface absorption (Lab

Test)

An indicator of surface permeability

Half cell potential measurement method is used for finding out the status of corrosion

actively in the embedded steel.

3.4 WINDSOR PROBE TEST.

This technique offers a means of determining relative strengths of concrete in the same

structure or relative strength of different structures. Because of nature of equipments, it can

not and should not be expected to yield absolute values of strength. ASTM C-803 gives this

standard test method titled “Penetration Resistance of Hardened Concrete”.

Windsor Probe is penetration resistance measurement equipment, which consists of a gun

powder actuated driver, hardened alloy of probe, loaded cartridges, a depth gauge and other

accessories. In this technique a gunpowder actuated driver is used to fire a hardened alloy

probe into the concrete. During testing, it is the exposed length of probe which is measured

by a calibration depth gauge. But it is preferable to express the coefficient of variation in

terms of depth of penetration as the fundamental relation is between concrete strength and

penetration depth.

The probe shown in fig.1 has a diameter of 6.3mm, length of 73mm and conical point at the

tip. The rear of the probe is threaded and screwed into a probe-driving head, which is

12.6mm in diameter and fits snugly along with a rubber washer into the bore of the driver. As

the probe penetrates into the concrete, test results are actually not affected by local surface

conditions such as texture and moisture content. However damage in the form of cracking

may be cause to slender members. A minimum edge distance and member thickness of

150mm is required. It is important to leave 50mm distance from the reinforcement present in

the member since the presence of reinforcing bars within the zone of influence of penetrating

probe affects the penetration depth.

A pin penetration test device (PNR Tester) which requires less energy than the Windsor

Probe system is given in fig.2.

Fig.1: Windsor Probe

Fig.2: Penetration Tester

Being a low energy device, sensitivity is reduced at higher strengths. Hence it is not

recommended for testing concrete having strength above 28 N/sq.mm. in this a spring-loaded

device, having energy of about 1.3% of that of Windsor probe, us used to drive 3.56mm

diameter, a pointed hardened steel pin into the concrete. The penetration of pin creates a

small indentation (or hole) on the surface of concrete. The pin is removed from the hole, the

hole is cleaned with an air jet and the hole depth is measured with a suitable depth gauge.

Each time a new pin is required as the pin gets blunted after use.

The strength properties of both mortar and stone aggregate influence the penetration depth of

the probe in a concrete, which is contrastingly different than cube crushing strength, wherein

the mortar strength predominantly governs the strength. Thus the type of stone aggregate has

a strong effect on the relation of concrete strength versus depth of penetration as given in

fig.3.

Fig.3: Effect of aggregate type on relationship between concrete strength and depth of probe

penetration

For two samples of concrete with equal cube crushing strength, penetration depth would be

more in the sample with softer aggregate than the one with harder aggregates. Correlation of

the penetration resistance to compressive strength is based on calibration curves obtained

from laboratory test on specific concrete with particular type of aggregates. Aggregate

hardness is determined from standard samples provided along with the instrument. Aggregate

size in the mix also influence the scatter of individual probe readings. This technique offers a

means of determining relative strength of concrete in the same structure or relative strength of

different structures. Because of the nature of equipment it can not and should not be expected

to yield absolute values of strength. This test is not operator independent although verticality

of bolt relative to the surface is obviously important and safety device in the driver prevents,

if alignment is poor.

It is claimed an average coefficient of variation for a series of groups of three readings on

similar concrete of the order of 4% may be expected. It has been observed that ±20%

accuracy may be possible in strength determination of concrete. Fig.4 explains the

approximate shape of failure during the test.

Fig.4: Approximate shape of failure zone in probe penetration test

3.5 LOK (PULL OUT TEST)

The fundamental principle behind pull out testing with LOK-test and CAPO test is that the

test equipment designed to a specific geometry will produce results (pull-out forces) that

closely correlate to the compressive strength of concrete. This correlation is achieved by

measuring the force required to pull a steel disc or ring, embedded in fresh concrete, against a

circular counter pressure placed on the concrete surface concentric with the disc/ring.

For hardened concrete, an expandable steel ring is used instead. This ring expands to fit a

specially drilled hole and routed recessin the concrete. The first method, shown in figure 1

using the cast steel disc is called LOK test. The second method shown in fig.2 using

expandable ring is called CAPO test (i.e. Cut and Pull out Test). The diameter of both the

disc and ring is 25mm. the distance to the concrete surface is also 25mm. the inner diameter

of the counter-pressure is 55mm.

Fig.1: LOK Test Insert

Fig.2: CAPO test testing principle

The relationship between the pullout force Fu in kN and compressive strength Fc in MPa is

given in fig.3.

Fig.3: Typical Pull out Force Calibration Chart

By measuring the pull-out force of a cast-in disc or expanded ring, the compressive strength

of in-situ concrete can be determined from the relationship in fig.4 to a great degree of

confidence.

Fig.4: Pull off force compressive strength relationship

The pullout test produces a well defined in the concrete and measure a static strength

property of concrete. The equipment is simple to assemble and operate.

As the insert is pulled out, a roughly cone shaped fragment of the concrete is extracted. The

compressive strength which is considered as an indicator of quality is obtained from the

calibration curves, prepared based on laboratory and field tests conducted on concrete cubes

and pull out samples cast with various grades of concrete. In the test assembly, the

embedment depth and insert head diameter have to be equal with inner diameter of the

reaction ring could vary between 2 to 2.4 times the diameter of the insert head. The apex of

the conic frustum defined by the insert-head diameter and the inside diameter of the reaction

ring can vary between 54 degree and 70 degree. The compressive strength can be considered

as proportional to the ultimate pullout force. The reliability of the test is reported as good.

Since the embedment depths of the commercially available metallic inserts is of the order of

25 to 30mm, the test results cover a small portion of the near surface concrete located

adjacent to the fractured surface and below the reaction ring. Thus due to the inherent

heterogeneity of concrete, typical average within batch coefficient of variation of such

pullout tests has been found to be in the range of 7 to 10% which is better than that of

standard cube/cylinder compression test. It is superior to rebound hammer and Windsor probe

test because of greater depth of concrete volume tested. This is not affected by type of cement

and aggregate characteristics. However this test is not recommended for aggregates beyond

size of 38mm. the major limitation of this test is that it requires special care at the time of

placement of inserts to minimize air void below the disc besides a pre-planned usage.

Pull out tests are used to:

1. Determine in-situ compressive strength of the concrete

2. Ascertain the strength of concrete for carrying out post tensioning operations.

3. Determine the time of removal of forms and shores based on actual in-situ strength of

the structure.

4. Terminate curing based on in-situ strength of the structure.

Here item No.(i) only are of relevance for residual strength assessment of old and distressed

structures. It can be also used for testing repaired concrete sections. These tests are divided

under the following two categories:

a) Embedding an insert into the fresh concrete while casting and then pulling out with a jack

(LOK test – LOK meaning ‘punch’) after hardening after a specified period.

b) Insert fixed into a hole drilled into the hardened concrete and then pulling out with a jack

(CAPO test- cut and pull out test). This second method offers greater flexibility for

conducting in-situ tests on hardened concrete of existing structures and is explained above in

detail.

After the concrete has fractured by this test, the holes left in the surface are first cleaned of

the dust by a blower. It is then primed with epoxy glue and the hole is filled with a polymer-

modified mortar immediately thereafter and the surface is smoothened.

3.6 RADIOGRAPHY:

This technique enables to take a photograph showing details of the inside of a concrete

member. It is used to determine the location and size of reinforcement, to check for the

existence of voids and areas of poor compaction in concrete where other NDE tests methods

are not suitable, such as massive old concrete structural members when being assessed for

structural safety. It is also used for checking for voids in the grouting in prestressing ducts.

BS 1881: Part-205 gives detailed information / recommendation for radiographic inspection

of concrete.

This method uses sources of gamma rays for concrete upto about 500 mm thick and above

this thickness, the standard recommends the use of high energy X-rays.

Due to the inherent danger of using gamma rays and high energy X-rays, the use of this

method is confined to investigation which justify the cost of the special precautions which

have to be taken.

Impact – echo Test:

Most often non – destructive evaluation technique based on use of transient stress waves, has

been used for detecting flaws in many different types of structures including plate like

structures such a bridge decks, slabs and walls beams and columns, layered structures and

hollow cylindrical structures. To detect hidden damage and determine the extent of damage

inside a concrete cross section (delamination, honey-comb, cracks etc).

Dynamic Testing of Structures:

The dynamic characteristics of structures depend on its stiffness and mass. Frequency of a

structure gives a good indication of deteriorating or improvement of its stiffness. The

dynamic testing of a structure before and after repairs will give an idea of the deterioration of

structures and effectiveness of the repairs. The different techniques available for dynamic

testing of structures are following:

1. Frequency Shift: The method is base on the comparison of the natural frequency of

the damaged structure with the undamaged structure. In the absence of the knowledge

of the natural frequency of the undamaged structure an estimate has to be based on

theoretical computations. Care should be taken for the non-structural member on the

frequency. The frequency being a global property of a structure, this method can not

be used for locating the damage.

2. Mode – Shape Change: The change in mode shape is more sensitive to damage than

the frequency. The damage location using mode shape can be done without prior

knowlefge of the undamaged structure or the theoretical calculations. An alternative

to mode shape is to use derivatives of the mode shapes such as curvature.

3. Change in Dynamically Measured Flexibility: The flexibility matrix of a structure

can be synthesized using the frequency and mode shape in various modes. The

flexibility matrix of the damaged structures compared with that of undamaged

structure can indicate the damage. The same comparison before and after the repairs

will indicate the effectiveness of repair.

In addition to the above mentioned techniques a few techniques are available for

continuous /periodic health monitoring of a structure. The problem with dynamic testing is

the absence of the knowledge of the undamaged structure. However, the testing can be very

useful in assessing the effectiveness of repairs, as the testing can be done before and after the

repairs.

BENEFITS OF NDT OF CONCRETE

The conventional testing of control cubes can be at the best indicate the potential quality of

concrete assuming that the composition is same as that going into the structure. Two more

steps compaction & curing, can be different from cubes & the structure. So the use of NDT

can give a better understanding of the quality of concrete in its final position.

Another great advantage is possibility of repeating the NDT in case of any doubt. This is not

possible in case of destructive testing . So there is no scope for manipulation.

These methods are basically applicable to both metals as well as nonmetals. However in

general emphasis in the case of metals is on locating the local defects or discontinuities, in

case of concrete on the other hand , the emphasis is on estimating the gross properties or

quality of concrete . This is because the concrete is always full of features that can be called

as defects.

From the basic of structure of the material it can be expected that any change in one property

should affect all others properties as well .This is the main reason why indirect tests are

useful. In other words, it is desirable to determine the quality of the entire volume of concrete

in terms of the property required for the end use, strength in general. This is not just possible

using conventional destructive tests. With NDT, it is possible to cover the entire volume with

appropriate tests.

When estimating , say strength from any of the NDT methods, the question of accuracy is

always raised .With a proper correlation , it may be possible to get the results well, within

15% .In difficult situations also, it would still be possible to estimate the results within about

25% accuracy.

CONCLUSIONConsiderable engineering judgment is needed to properly evaluate a measurement.

Misinterpretation is possible when poor contact is made. For example, in some cases it may

not be possible to identify severely corroded reinforcing bar in poor quality concrete.

However, it is possible to identify poor quality concrete which could be the cause of

reinforcing bar problems. The poor quality concrete allows the ingress of moisture and

oxygen to the reinforcing bars, and hence corrosion occurs. Presently the system is limited to

penetration depths of 1 ft. Research is ongoing to develop a system that can penetrate to a

depth of 10 ft or more.

When variation in properties of concrete affect the test results, (especially in opposite

directions), the use of one method alone would not be sufficient to study and evaluate the

required property. Therefore, the use of more than one method yields more reliable results.

For example, the increase in moisture content of concrete increases the ultrasonic pulse

velocity but decreases the rebound number . Hence, using both methods together will reduce

the errors produced by using one method alone to evaluate concrete. Attempts have been

done to relate rebound number and ultrasonic pulse velocity to concrete strength.

Unfortunately, the equation requires previous knowledge of concrete constituents in order to

obtain reliable and predictable results.

The Schmidt hammer provides an inexpensive, simple and quick method of obtaining an

indication of concrete strength, but accuracy of ±15 to ±20 per cent is possible only for

specimens cast cured and tested under conditions for which calibration curves have been

established. The results are affected by factors such as smoothness of surface, size and shape

of specimen, moisture condition of the concrete, type of cement and coarse aggregate, and

extent of carbonation of surface.

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

be used on both existing structures and those under construction.

Usually, if large differences in pulse velocity are found within a structure for no apparent

reason, there is strong reason to presume that defective or deteriorated concrete is present.

Fairly good correlation can be obtained between cube compressive strength and pulse

velocity. These relations enable the strength of structural concrete to be predicted within ±20

per cent, provided the types of aggregate and mix proportions are constant.

In summary, ultrasonic pulse velocity tests have a great potential for concrete control,

particularly for establishing uniformity and detecting cracks or defects. Its use for predicting

strength is much more limited, owing to the large number of variables affecting the relation

between strength and pulse velocity.

The deviation between actual results and predicted results may be attributed to the fact that

samples from existing structures are cores and the crushing compressive cube strength was

obtained by using various corrections introduced in the specifications. Also, measurements

were not accurate and representative when compared to the cubes used to construct the plots.

The use of the combined methods produces results that lie close to the true values when

compared with other methods. The method can be extended to test existing structures by

taking direct measurements on concrete elements.

Unlike other work, the research ended with two simple chart that requires no previous

knowledge of the constituents of the tested concrete. The method presented is simple, quick,

reliable, and covers wide ranges of concrete strengths. The method can be easily applied to

concrete specimens as well as existing concrete structures. The final results were compared

with previous ones from literature and also with actual results obtained from samples

extracted from existing structures.

REFERENCES

1. IS-1311 (Part-1): 1992 Non-Destructive Testing of Concrete -methods of test, Part-I,

Ultrasonic Pulse Velocity.

2. IS 13311 (Part-2): 1992, Non-Destructive Testing of Concrete –methods of test, Part 2,

Rebound hammer.

3. RDSO Report No.BS-53: Guidelines on use of ultrasonic instruments for monitoring of

concrete structures.

4. Handbook on Non Destructive Testing of Concrete” (second edition) by V.M. Malhotra

and N.J. Carino

5. “Non destructive testing” by Louis Cartz.

6. “Concrete Technology” by M L Gambhir.

7. “Concrete Technology” by M S Shetty

8. Civil Engineering Construction Review, August 1998 Edition ( Non Destructive Testing of

Concrete ).

9. Concrete strength by combined nondestructive methods simply and reliably predicted

Hisham Y. Qasrawi , Civil Engineering Department, College of Engineering, Applied

Science University, Amman 11931, Jordan

10. Non-destructive testing of concrete material properties and concrete structures

Christiane Maierhofer Federal Institute for Materials Research and Testing (BAM), D-12205

Berlin, Germany .

11. The use of USPV to anticipate failure in concrete under compression Hisham Y. Qasrawi

and Iqbal A. Marie Civil Engineering Department, Faculty of Engineering, Hashemite

University, Zarqa 13115, Jordan .

RELATED BOOKS, JOURNALS AND ARTICLES

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ACI, Detroit, US (1976) monograph No. 9.

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International

Symposium held by RILEM, Belgium, E&FN SPON, UK (1991), pp. 377–386.

3. ASTM C 805-85, Test for Rebound Number of Hardened Concrete, ASTM,

USA (1993).

4. BS 1881: Part 202, 1986: Recommendations for Surface Hardness Tests

by the Rebound

Hammer, BSI, UK (1986).

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of Concrete

Practice, Part 2: Construction Practices and Inspection Pavements, ACI

228.1R-989, Detroit, MI (1994) 25 pp..

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Concrete, ACI SP-82, Detroit (1984), pp. 19–34.

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Proc Cem Conc 45 (1991), pp. 345–351.

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Strength of Hardened Concrete, FHWA Public Roads 30 (1958), pp. 45–50

Washington, DC, No. 2, June.

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Miscellaneous Paper No. 6-267, U.S. Army Engineer Waterways Experiment Station,

Vicksburg, MS (1958) June.

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(1988), pp. 137–148 ACI SP-112.

12. A. Neville. Properties of Concrete, Addison-Wesley Longman, UK (1995).

13. ASTM C 597-83 (Reapproved 1991), Test for Pulse Velocity Through

Concrete, ASTM, USA(1991).

14. BS 1881: Part 203: 1986: Measurement of Velocity of Ultrasonic Pulses

in Concrete, BSI,UK (1986).

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elasticity of concrete. J Am Concr Inst 51 (1955), pp. 461–469 January.

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strength concrete. Indian Concr J 34 4 (1960), pp. 139–141.

18. ACI 318-95, Building Code Requirements for Structural Concrete (ACI

318-95) and

Commentary-ACI 318R-95, ACI, USA (1995) 369 pp..

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433–444 Feb..

20. R. Jones and E. Gatfield. Testing Concrete by an Ultrasonic Pulse Technique,

DSIR Road

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methods for in-place strength determination. In: H.S. Lew, Editor, Nondestructive

Testing, ACISP-112, ACI, Detroit (1988), pp. 111–136.