Concrete technology ndt methods

SMCET/CIVIL/CONCRETE TECHNOLOGY/N.D.T METHOD/N.S.JADOUN/2016

STANI MEMORIAL COLLEGE OF ENGINEERING

& TECHNOLOGY

CIVIL ENGINEERING DEPARTMNET

CONCRETE TECHNOLOGY

TOPIC: - N.D.T method & Tools

Prepared By Approved By

Mr. N.S. Jadoun Mr.P.N. Mathur

Assistant Professor H.O.D. CIVIL


SUBJECT: - CONCRETE TECHNOLOGY

CLASS: - Second Year Civil Engineering

LIST OF TOPICS COVERD

Sr.No. NAME OF EXPERIMENT PAGE No.

FROM TO

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.


N.D.T: Introduction and their impo3rtance

1 Introduction

Non-destructive Testing is one part of the function of Quality Control and is complementary to other long established methods. 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:

� Radiography � 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 continues.


Rebound Hammer

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

results.

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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.”

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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

factors include:

1. Smoothness of the surface

2. Size and shape of the concrete sample


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

10. Carbonation

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 Granite 37 Limestone 32 Lightweight 31

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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.


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

hammer number.

9. Take 10 rebound hammer readings at each test area. All individual readings should be at least 25 mm (1

inch) apart.

10. Discard any reading that is over six units from the average and calculate the average of the remaining

readings.

11. If two units are over six units from the average, discard the entire set of reading and redo the test.

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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).


Ultra-sonic pulse velocity meter

INTRODUCTION

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 of geometry

• For an infinite, homogenous, isotropic elastic medium, the compression wave velocity is given as:


Where,

V=compression wave velocity (km/s)

Ed=dynamic modulus of elasticity (kN/mm2)

ρ=density (kg/m3) 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:


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 reliable results

• 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

smooth shutters

• 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 measurement 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 large. c. The indirect method is the least accurate because received signal is subject to errors due to scattering of pulse by discontinuities.


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 energy output).

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 μs.

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 faces.

• 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:


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:


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 proportions:

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 experimental data:

fc=AeBV

where , fc=equivalent cube strength e=base of natural logarithms V=pulse velocity and A and B are constants.

Figure 1. Effect of aggregate type (all concretes similar apart from

aggregate type) Figure 2. Comparison of lightweight and gravel aggregates


FACTORS AFFECTING ON RESULTS

There are following factors that affect the results:

I. Temperature II. Stress history III. Path length IV. Moisture conditions V. Reinforcement

I. Temperature

� 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 below:

Figure 4. Effect of temperature

The measured velocity should be corrected by multiplying with the factor obtained corresponding to the operating temperature

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 remains unaffected


� 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 pulse velocity

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 concrete)

� However, the strength of a dry concrete is found to be more than that of the same concrete in wet condition

� The effect of moisture condition on both pulse velocity and concrete strength is typically shown below:


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 control

= 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


ADVANTAGES AND LIMITATIONS

ADVANTAGES

� 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 site conditions

LIMITATIONS

� 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 affecting calibrations

� 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


Rebar & Cover meter

It is used to detect reinforcement bars and mesh, to measure their cover depth and estimate the bar diameter.

EQUIPMENT

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.


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

distinguished.

• 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.


Distinguish between balanced Situations


Half-cell potential meter

INTRODUCTION

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 reinforcing steel.

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

Advantages: –Inexpensive –Simple to perform –Whole structure quickly surveyed –Data analysis simple


Disadvantages: –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


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

without interpolation � 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

with.

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.


� 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 initiated.

� 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 above.

o Do not remove the sponges from the concrete surface until after the half-cell potential readings are taken.

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


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 following equation:

Where: 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

angle.

Figure 1. Contour mapping


Half-Cell Potential Test Method: Interpretation of Test Results

The half-cell potential values may be used to determine the probability of reinforcement corrosion using the criteria given in Table below:


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]


Probe Spacing

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 compromise.

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


Core sampling

Concrete technology ndt methods

Engineering

Transcript of Concrete technology ndt methods