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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 .
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