2. SMCET/CIVIL/CONCRETE TECHNOLOGY/N.D.T METHOD/N.S.JADOUN/2016 Page 2
SUBJECT: - CONCRETE TECHNOLOGY
CLASS: - Second Year Civil Engineering
LIST OF TOPICS COVERD
Sr.No. NAME OF EXPERIMENT PAGE No.
1. NDT: Introduction and their importance 3 3
2. Application & use of Rebound Hammer 4 6
3. Ultra-sonic pulse velocity meter 7 16
4. Rebar & Cover meter 17 19
5. half-cell potential meter 20 25
6. corrosion resistivity meter 26 27
7. core sampling 28
Time Allotted for each topic season = 20 min.
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N.D.T: Introduction and their impo3rtance
Non-destructive Testing is one part of the function of Quality Control and is complementary to other long
By definition non-destructive testing is the testing of materials, for surface or internal flaws or metallurgical
condition, without interfering in any way with the integrity of the material or its suitability for service.
There are various varieties of methods available to inspect the offshore structures or any structural system
without causing any physical harm neither to the material nor to the member.
The technique can be applied on a sampling basis for individual investigation or may be used for 100%
checking of material in a production quality control system.
Also an assurance that the supposedly good is good. The technique uses a variety of principles; there is no
single method around which a black box may be built to satisfy all requirements in all circumstances.
What follows is a brief description of the methods most commonly used in industry, together with details of
typical applications, functions and advantages. The methods covered are:
Magnetic Particle Crack Detection
Dye Penetrant Testing
Ultrasonic Flaw Detection
Eddy Current and Electro-magnetic Testing
However, these are by no means the total of the principles available to the N.D.T. Engineer. Electrical
potential drop, sonics, infra-red, acoustic emission and spectrography, to name but a few, have been used to
provide information that the above techniques have been unable to yield, and development across the board
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The rebound hammer is one of the most popular non-destructive testing methods used to investigate
concrete. Its popularity is due to its relatively low cost and simple operating procedures. The rebound hammer
is also one of the easiest pieces of equipment to misuse; thus, many people do not trust the rebound test
The rebound hammer measures the surface hardness of the concrete. This is accomplished by placing the
rebound hammer plunger against the concrete surface and releasing a spring loaded weight. The amount the
plunger rebounds or bounces back is measured. This rebound number is shown on a scale and will be between
10 and 100. The Impact Hammer is another name for Schmidt Hammer.
The surface of concrete gets harder as concrete gains strength; thus, we have a method of estimating the
strength of concrete. A low rebound number will indicate that the surface of the concrete is soft and the
concrete is weak. A high rebound number will indicate that the concrete is hard and strong. Unfortunately,
there is no theoretical relationship between surface hardness and the strength of concrete.
“Nondestructive tests of the concrete in place, such as by probe penetration, impact hammer, ultrasonic
pulse velocity, or pullout may be useful in determining whether or not a portion of the structure actually
contains low-strength concrete. Such tests are of value primarily for comparisons within the same job rather
than as quantitative measures of strength.”
Fa a Aff u a uFa a Aff u a uFa a Aff u a uFa a Aff u a u
Since the rebound hammer measures the surface hardness of the concrete, it is important to understand all the
items that might affect surface conditions of the concrete and thus, the rebound hammer numbers. These
1. Smoothness of the surface
2. Size and shape of the concrete sample
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3. The rigidity of the test area
4. Age of the concrete
5. Surface moisture
6. Internal moisture (moisture gradient)
7. Coarse aggregates
8. Type of cement
9. Forms used
11. Location of the reinforcement
12. Frozen concrete
For these reasons, the user of the rebound hammer must follow exact procedures and use engineering
judgment. To illustrate this, the following chart shows how the effects of the coarse aggregates in concrete of
the same strength can have on the rebound hammer.
CONCRETES OF SAME STRENGTH
Aggregates Rebound Hammer
River Rock 40
a a u f u aa a u f u aa a u f u aa a u f u a
Standard Test for Rebound Numbers of Hardened Concrete, provides some standard procedures so that the
user can have consistency when using the rebound hammer. Some of these standard procedures are:
1. Do not test frozen concrete.
2. The test area must be at least 150 mm (6 inches) in diameter and fixed rigidly within the structure.
3. The surface to be tested must be flat with no loose mortar.
4. The surface to be tested must be free from water.
5. If the layer of carbonated concrete is thick, it should be removed before testing.
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6. The hammer must be held in the same direction — horizontal, upward, downward and it should always be
at a right angle to the surface being tested.
7. Do not test over reinforcement with a cover of less than 20 mm (3/4 inch).
8. If estimating concrete strength takes at least two cores from six locations that have different rebound
9. Take 10 rebound hammer readings at each test area. All individual readings should be at least 25 mm (1
10. Discard any reading that is over six units from the average and calculate the average of the remaining
11. If two units are over six units from the average, discard the entire set of reading and redo the test.
U u a a uU u a a uU u a a uU u a a u
A a ! aA a ! aA a ! aA a ! a
One of the ways to use the rebound hammer is to locate those areas that may need additional investigation. In
this procedure the round hammer is used at several locations to identify those areas that have a lower rebound
number. Since the structure would have the same mixture, curing history, moisture content, etc., the rebound
hammer can identify those areas that appear to have the weakest concrete (lowest rebound hammer number).
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Ultra-sonic pulse velocity meter
UPV measurement through concrete was initiated in the USA in the mid-1940s and later adopted everywhere
as NDT on concrete
• UPV methods basically consists of transmitting the mechanically generated pulses (in the frequency ranges
of 20-150/s) through concrete with the help of electro-acoustic transducers and measuring the velocity of the
longitudinal waves generated by the applied pulses
• UPV is correlated to many desirable information pertaining to concrete, such as:
– Elastic modulus, strength, and uniformity of concrete
– Layer thickness, cracking, honeycombing, and deterioration of concrete
If the method is properly used by an experienced operator, a considerable amount of information about the
interior of a concrete member can be obtained. However, since the range of pulse velocities relating to
practical concrete qualities is relatively small (3.5–4.8 km/s), great care is necessary, especially for site usage.
Furthermore, since it is the elastic properties of the concrete which affect pulse velocity, it is often necessary
to consider in detail the relationship between elastic modulus and strength when interpreting results.
Theory of pulse propagation through concrete
• Following three types of waves are generated by an impulse applied to a mass:
I. Surface waves having an elliptical particle displacement and slowest.
II. Shear or transverse waves with particle displacement at right angles to the direction of travel
and faster than the surface waves.
III. Longitudinal or compressive waves with particle displacement in the direction of travel and
fastest providing more useful information
• Electro-acoustical transducers used for UPV measurements on concrete produce longitudinal waves
which, as mentioned above, are fastest and provide more useful information
• UPV depends primarily upon the elastic properties of the material and found to be almost independent
• For an infinite, homogenous, isotropic elastic medium, the compression wave velocity is given as:
8. SMCET/CIVIL/CONCRETE TECHNOLOGY/N.D.T METHOD/N.S.JADOUN/2016 Page 8
V=compression wave velocity (km/s)
Ed=dynamic modulus of elasticity (kN/mm2)
v=dynamic Poisson’s ratio.
Pulse velocity equipment and use
• The UPV equipment is used for the following purposes:
I. Generating a pulse mechanically.
II. Transmitting the generated pulse through concrete.
III. Receiving and amplifying the pulse. Measuring and displaying the transit time
• The circuitry of a typical UPV testing equipment is shown below:
Commercially available UPV equipment’s are shown below:
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UPV TEST PROCEDURE:
There are following test procedure should be determined are as follows:-
I. Coupling of transducers
II. Arrangement of transducers
III. Selection of transducers
IV. Equipment calibration
V. Velocity determination
I. Coupling of transducers
• A good acoustic coupling between the concrete surface and the face of the transducers is essential for
• Coupling is provided by a medium such as petroleum jelly, liquid soap or grease
• Air pockets must be eliminated, and it is important that only a thin separating layer exists-any surplus
must be squeezed out
• A light medium such as petroleum jelly or liquid soap is found to be the best for smooth surfaces
• A thicker medium such as grease is recommended for rough surfaces which have not been cast using
• In case of very rough or uneven surfaces, grinding or preparation with plaster of Paris or quick-setting
mortar may be necessary before coupling
II. Arrangement of transducers
Following are three basic ways in which the transducers may be arranged:
I. Transducers coupled on opposite faces (direct transmission).
II. Transducers coupled on adjacent faces (semi-direct transmission).
III. Transducers coupled on same faces (indirect transmission).
The above mentioned arrangements of transducers are shown below:
a. The direct method is the most reliable from the point of
view of transit time measurement as well as path length
b. The semi-direct method is less reliable than the direct
method and should only be used if the angle between the
transducers is not too great, and if the path length is not too
c. The indirect method is the least accurate because
received signal is subject to errors due to scattering of pulse by
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III. Selection of transducers
Selection of the transducers for UPV test mainly depend on the following:
• Whether point contact is needed or not, as in case of rough or curved surface, the exponential probe
transducer is suitable.
• The required transducer frequency, which is related to the dimensions of the member under test, for
example, for 10 m path length a transducer should have a frequency of 54 kHz and the transducer
should have a frequency of 82 kHz for a path length of 3 m (higher frequency required for lower
IV. Equipment calibration
• Before use, the time delay adjustment must be made by setting the zero reading for the equipment. For
this, the equipment should regularly be checked during and at the end of each period of use.
• The time delay adjustment is carried out with the help of a calibrated steel reference bar which has a
transit time of around 25 μs.
• It is recommended that the accuracy of transit time measurement of the equipment should also be
checked by measurement of a second reference specimen, preferably with a transit time of around 100
V. Velocity determination
• Determination of pulse velocity requires measurement of the transit time using the UPV equipment
with an accuracy of ± 0.1 μs and measurement of path length with an accuracy of ± 1%
• The transit time readings are repeated by complete removal and reapplication of transducers to obtain
a minimum value for the transit time, which is taken as final reading
• Once the transit time and the path length are measured, the pulse velocity is determined by dividing
the path length by the transit time, as follows:
V = path length/transit time
• In case of direct transmission, the path length is just the thickness of the member under test. In case of
semi-direct transmission, the path length is taken as distance between center to center of transducer
• In case of indirect transmission, the pulse velocity is determined by recording the transit times by
placing the receiver at different distances from the fixed position of the transmitter and then obtaining
the mean pulse velocity as inverse of slope of a best fit line plotted using spacing versus transit time
data, as follows:
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V = 1/ slope of the best-fit line
USE OF UPV TEST RESULTS
Use of UPV test results we find some properties of concrete are as follows
I. Dynamic elastic modulus
II. Compressive strength
I. Dynamic elastic modulus
The calibration chart between pulse velocity and
dynamic elastic modulus shown below (developed
by conducting resonance and UPV tests on prisms)
may be used to determine the dynamic elastic
modulus of concrete:
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II. Compressive strength
• Coarse aggregate type, shape, size, quantity; sand type; cement type; w/c ratio; and maturity of
concrete are the important factors which affects the correlation between pulse velocity and strength
• Therefore separate strength calibration charts are needed for accurate interpretation of the test results
for strength, considering the effect of each of the above factors
• Following are few typical strength calibration charts taking the effect of aggregate types and
Due to the fact that the precise relationship between
pulse velocity and strength is affected by many
variables, a calibration model in the following form
should be fitted by least squares techniques using the
fc=equivalent cube strength
e=base of natural logarithms
and A and B are constants.
Figure 1. Effect of aggregate type (all concretes similar apart from
Figure 2. Comparison of lightweight and gravel aggregates
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FACTORS AFFECTING ON RESULTS
There are following factors that affect the results:
II. Stress history
III. Path length
IV. Moisture conditions
Normal operating temperature (i.e., around 20 0C) does not significantly influence the pulse velocities
However, the peak temperatures (above 20 0C and below 0 0C) affect the pulse velocity, as shown
Figure 4. Effect of temperature
The measured velocity should be corrected by multiplying with the factor obtained corresponding to the
II. Stress history
Any type of stress (compressive or tensile or flexural or prestress in prestressed concrete members)
with a low magnitude does not affects the pulse velocity
It is reported that the pulse velocity in laboratory cubes stressed up to 50% of its crushing strength
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No correction is required for measured velocity through concrete members stressed less than or up to
one-third of cube strength
Care should be taken for overstressed members and in case if tensile stresses have caused cracking
The internal micro cracks affect both path length and width resulting into reduction in the measured
III. Path length
Unless the path length is excessively small, pulse velocity is not affected by it
The effect of path length on pulse velocity for a concrete with a maximum aggregate size of 20 mm is
typically shown below:
For no effect of path length, it is recommended to select a
minimum path length of 100 mm in case of concrete with
aggregate having max. Size of 20 mm and a minimum path
length of 150 mm for concrete with aggregate having max.
Size of 40 mm.
A reduction of 5% in the measured velocity is typically observed for a path length increase from
approximately 3 m to 6 m.
The pulse velocity is also affected if the path length is too long because of attenuation of the higher
frequency pulse components.
IV. Moisture conditions
Pulse velocity through a wet concrete is found to be up to 5% higher than that through the same
concrete in dry condition (effect of moisture is less for high strength concrete than the low-strength
However, the strength of a dry concrete is found to be more than that of the same concrete in wet
The effect of moisture condition on both pulse velocity and concrete strength is typically shown
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Tomsett (1980) has proposed following calibration model for determining actual strength of in-situ concrete
tested in any moisture condition:
k is a constant reflecting compaction
= 0.015 for normal concrete
= 0.025 for poorly compacted concrete
APPLICATION OF UTV TEST RESULTS
Monitoring strength development or deterioration in laboratory specimens subjected to varying curing
conditions or to aggressive environment.
Measurement of in-situ concrete uniformity.
Detection of cracking and honeycombing in in-situ concrete.
Measurement of crack depth.
Strength estimation of in-situ concrete
Assessment of in-situ concrete deterioration
Measurement of layer thickness in in-situ concrete
Measurement of elastic modulus of in-situ concrete
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ADVANTAGES AND LIMITATIONS
UPV test is truly non-destructive and can be performed both in lab as well as in-situ
UPV measurement has been found to be a valuable and reliable method of examining interior of a
body of concrete
Modern UPV test equipment is robust, reasonably cheap and easy to operate, and reliable even under
Operators must be well trained and aware of the factors affecting the readings
It is essential that the test results are properly evaluated and interpreted by experienced engineers who
are familiar with the technique.
The UPV method only gives an estimate of the extent of cracking within concrete, however, the use
for detection of flaws within the concrete is not reliable when the concrete is wet
The UPV test is least reliable for estimation of strength of concrete because of the many factors
Application of the UPV test for determining depth of fire damage is limited to only the portions which
are free from cracking due to very high temperature
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Rebar & Cover meter
It is used to detect reinforcement bars and mesh, to measure their cover depth and estimate the bar diameter.
PRINCIPAL & MEASURMENT OF REBAR (RENFORCEMENT)
It is uses the pulse-induction method. Coils in the probe are periodically charged by current pulses and thus
generate a magnetic field. On the surface of any electrically conductive material which is in the magnetic field
eddy currents are produced. They induce a magnetic field in opposite direction. The resulting change in
voltage can be utilized for the measurement.
Rebar that are closer to the probe or of larger size produce a stronger magnetic field. The strongest signal also
results, when the center line of the probe is parallel to a bar.
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When moving the probe across the concrete, the measured signal gets stronger and weaker. The max. Signal
Value signals the rebar.
For one single bar, e.g. the Test Block, the situation is easy. The Signal Value starts from 0 and has a peak
above the bar. For an arrangement of several parallel bars the characteristics of the signal can be as shown
above. If the spacing of the bars is closer the curve gets rather straight or there is just one peak in the middle
of the bars.
This means the bars cannot be detected individually anymore. For a clear identification of bars a sufficient
decrease of Signal Value is required.
MEASURMENT OF CONCRETE COVER DEPTH
• The signal value is converted to a cover value in [mm].
• The accuracy of reber meter is 95% 5%.
• It also define the spacing between the bars
• The spacing between the bars determines the maximum depth at which bars of a specific diameter can be
• e.g. In order to distinguish a 10mm diameter bar at a depth of 100mm, the bar spacing has to be at least
125mm when measuring on the large range.
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Half-cell potential meter
Corrosion potential mapping is carried out on concrete structures non-destructively for identifying the spots of
rebar’s undergoing corrosion.
The corrosion status is related to the measured corrosion potential value
The contours obtained by plotting the corrosion potential values are useful in delineating corroding portions of
the structure from non-corroding portions.
Potential mapping does not give information regarding corrosion rate.
Following are the methods used for corrosion potential mapping:
Half-cell or open-circuit potential test method (frequently used)
Double-probe corrosion potential test method (rarely used)
Significance and Uses
This test method is suitable for in-situ evaluation and for use in research and development work.
This test method is applicable to members regardless of their size or the depth of concrete cover over the
This test method may be used at any time during the life of a concrete member.
The results obtained by the use of this test method shall not be considered as a means for estimating the
structural properties of the steel or of the reinforced concrete member.
The potential measurements should be interpreted by engineers or technical specialists experienced in the
fields of concrete materials and corrosion testing.
Half-Cell Potential Test Method: Advantages and Limitations
–Simple to perform
–Whole structure quickly surveyed
–Data analysis simple
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–Limited information for potentials between –200 and –350 mV CSE
–No information on corrosion rate
–Difficult to perform when contaminants present on or in concrete
Half-Cell Potential Test Method Circuit & Reference electrode
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Electrical contact solution
In order to standardize the potential drop through the concrete portion of the circuit, an electrical
contact solution shall be used to wet the electrical junction device
One such solution is composed of a mixture of 95 mL of wetting agent (commercially available
wetting agent) or a liquid household detergent thoroughly mixed with 5 gal (19 L) of potable water
Under working temperatures of less than about 50°F (10°C), approximately 15 % by volume of either
isopropyl or denatured alcohol must be added to prevent clouding of the electrical contact solution,
since clouding may inhibit penetration of water into the concrete to be tested.
Voltmeter and electric lead wire
The voltmeter shall have the capacity of being battery operated and have ±3 % end-of-scale accuracy
at the voltage ranges in use
The input impedance shall be no less than 10 MΩ when operated at a full scale of 100 mV
The divisions on the scale used shall be such that a potential difference of 20 mV or less can be read
The electrical lead wire shall be of such dimension that its electrical resistance for the length used will
not disturb the electrical circuit by more than 0.1 mV
This has been accomplished by using no more than a total of 150 m. The wire shall be suitably coated
Half-Cell Potential Test Method: Testing Procedure
Spacing between measurements
The spacing between the test points should be properly selected depending on the type of member
being investigated and the intended end use of the measurements.
On very large structures, e.g. bridge decks, the test should start with initial spacing of about 1 m and
then sections should be resurveyed with 300 mm spacing where the potential difference between
adjacent readings exceeded 100 mV.
A spacing of about 300 mm is gradually becoming a more universally accepted initial spacing,
reducing to 100 mm over the high-gradient sections.
With present techniques it appears that spacing’s of less than 100 mm are unlikely to greatly influence
the effectiveness of the survey.
Electrical connection to the steel
Make a direct electrical connection to the rein-forcing steel by means of a compression-type ground
clamp, or by brazing or welding a protruding rod.
To ensure a low electrical resistance connection, scrape the bar or brush the wire before connecting to
the reinforcing steel.
Electrically connect the reinforcing steel to the positive terminal of the voltmeter.
Electrical continuity of steel components with the reinforcing steel can be established by measuring
the resistance between widely separated steel components on the deck.
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Where duplicate test measurements are continued over a long period of time, identical connection
points should be used each time for a given measurement.
Electrical connection to the half-cell
Electrically connect one end of the lead wire to the half-cell and the other end of this same lead wire to
the negative (ground) terminal of the voltmeter.
Pre-wetting of the concrete surface
Under certain conditions, the concrete surface or an overlaying material, or both, must be pre-wetted
by either of the two methods (A and B), using the same solution used for making contact of reference
electrode with concrete surface, to decrease the electrical resistance of the circuit.
A test to determine the need for pre-wetting may be made as follows:
–Place the half-cell on the concrete surface and de not move.
–Observe the voltmeter for one of the following conditions:
(a) The measured value of the half-cell potential does not change or fluctuate with time.
(b) The measured value of the half-cell potential changes or fluctuates with time.
Method A for Pre-Wetting Concrete Surfaces–
o This method is used for those conditions where a minimal amount of pre-wetting is required to
obtain condition (a) as described above.
o Pre-wetting by this method consists of spraying or otherwise wetting either the entire concrete
surface or only the points of measurement.
o No free surface water should remain between grid points when potential measurements are
Method B for Pre-Wetting Concrete Surfaces
o In this method, sponges saturated with the solution are placed on the concrete surface at
locations marked for measurements.
o Leave the sponges in place for the period of time necessary to obtain condition (a) described
o Do not remove the sponges from the concrete surface until after the half-cell potential readings
Recording of half-cell potential values
Record the electrical half-cell potentials to the nearest 10 mV.
Report all half-cell potential values in volts or mV and correct for temperature if the half-cell
temperature is outside the range of 72 ±10°F (22.2 ±5.5°C).
The temperature coefficient for the correction is 0.5 mV more negative/°F for the temperature range
from 32 to 120°F (0 to 49°C).
Test measurements may be presented by one or both of the following two ways:
o an equipotential contour map, provides a graphical delineation of areas in the member where
corrosion activity may be occurring
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o a cumulative frequency diagram
Equipotential contour map
On a suitably scaled plan view of the concrete member, equipotential contours with a maximum interval of
100 mV may be plotted as shown in the figure below:
Cumulative frequency distribution
To determine the distribution of the measured half-cell potentials for the concrete member, make a
plot of the data on normal probability paper in the following manner:
o Arrange and consecutively number all half-cell potentials by ranking from least negative
potential to greatest negative potential.
o Determine the plotting position of each numbered half-cell potential in accordance with the
fx= plotting position of total observations for the observed value, %
r = rank of individual half-cell potential, and
Σn = total number of observations.
Draw two horizontal parallel lines intersecting the -0.20 and -0.35 V values on the ordinate,
respectively, across the chart.
After plotting the half-cell potentials, draw a line of best fit through the value.
Figure 2. Cumulative frequency diagram
Note: If a break in the straight line is observed, the line
of best fit shall be two straight lines that intersect at an
Figure 1. Contour mapping
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Corrosion resistivity meter
Corrosion resistivity measurement provides extremely useful information about the state of a concrete
structure. Not only has it been proven to be directly linked to the corrosion and the corrosion rate, recent
studies have shown that there is a direct correlation between resistivity and chloride diffusion rate. The
versatility of the method can be seen in these example applications:
• Estimation of the likelihood of corrosion
• Indication of corrosion rate
• Correlation to chloride permeability
• On site assessment of curing efficiency
• Determination of zonal requirements for catholic protection systems
• Identification of wet and dry areas in a concrete structure
• Indication of variations in the water/cement ratios within a concrete structure
• Identification of areas within a structure most susceptible to chloride penetration
• Correlation to water permeability of rock
The measurement principle
Operating on the principle of the Wenner probe, it is designed to measure the electrical resistivity of concrete
or rock. A current is applied to the two outer probes, and the potential difference is measured between the two
inner probes. The current is carried by ions in the pore liquid. The calculated resistivity depends on the
spacing of the probes.
Resistivity ρ= 2πaV/l [kΩcm]
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A wider probe spacing provides a more consistent reading when measuring on an inhomogeneous material
like concrete. However, if the spacing is too wide, there is more danger of the measurement being affected by
the reinforcement steel. The industry standard 50 mm probe spacing has long been seen as a good
The 38mm (1.5”) model is designed specifically to comply with the Indian standard for “Surface Resistivity
Indication of Concrete’s Ability to Resist Chloride Ion Penetration”
The Concrete Resistivity (SR) test is a much quicker and easier test for estimating concrete permeability. It is
a proven and mature test method which can replace the more laborious rapid chloride permeability test.
Instrument picture with meter or cable