HANDBOOK ON NONDESTRUCTIVE TESTING OF CONCRETE.pdf

H A N D B O O K

O N

NONDESTRUCTIVE TESTING OF CONCRETE SECOND EDITION

V.M.

Edited by MALHOTRA and N.J.

CARINO

CRC PR E S S Boca Raton London New York Washington, D.C.

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Foreword

ASTM International is pleased to join CRC Press to co-publish Handbook on Nondestructive Testing of Concrete, Second Edition. ASTM distributes many publications including standards, adjuncts, journals, special technical publications, manuals, data series, monographs, and reference radiographs. ASTM is proud to include this book in its publications series. ASTM International has 130 technical committees including Committee C09 on Concrete and Concrete Aggregates. This committee was formed in 1914 and currently has over 700 members. The committee has jurisdiction of 152 standards. It meets in June and December. ASTM Committee C09 on Concrete and Concrete Aggregate’s scope is (1) the assembling and study of data pertaining to the properties of hydraulic-cement concrete and its constituent materials, including the study of the effect of characteristics of materials and mixtures on the properties of concrete; and (2) the development of standards for concrete and for the constituent materials of concrete (except cement), as well as for certain related materials, such as materials used in curing. The scope of the Committee does not include the field of design and construction of concrete structures except insofar as references need to be made to construction methods in special case of concrete as “overthe-counter” material. For additional information on ASTM International, 100 Barr Harbor Drive, West Conshohocken, PA 19428, visit www.astm.org, e-mail: [email protected], phone: +1.610.832-9500.

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Preface

In the inspection of metals, nondestructive testing is an accepted practice. For example, radiographic and ultrasonic techniques are routinely used to identify anomalies in steel pipelines, and there are recognized national and international standards on their use. In the inspection of concrete, however, the use of nondestructive testing is relatively new. The slow development of nondestructive testing techniques for concrete is because, unlike steel, concrete is a highly nonhomogeneous composite material with varying composition and different raw materials. Apart from precast concrete units that, like steel products, are fabricated at a plant, most concrete is produced in relatively small ready-mixed concrete plants and delivered to the construction site. The placing, consolidation, and curing of concrete takes place in the field using labor that is relatively unskilled. The resulting product is, by its very nature and construction method, highly variable and does not lend itself to testing by traditional nondestructive methods as easily as steel products. Despite the above drawbacks, there has been progress in the development of nondestructive methods for testing concrete, and several methods have been standardized by the American Society for Testing and Materials (ASTM), the Canadian Standards Association (CSA), the International Standards Organization (ISO), the British Standards Institute (BSI), and others. The direct determination of mechanical and other properties requires that concrete specimens taken from the structure be tested destructively; therefore, nondestructive methods cannot yield absolute values of these properties. Methods have been developed to measure other properties of concrete from which estimates of mechanical properties or other characteristics related to performance can be inferred. Broadly speaking, there are two classes of nondestructive test methods for concrete. The first class consists of those methods that are used to estimate strength. The surface hardness, penetration resistance, pullout, break-off, pull-off, and maturity techniques belong to this category. Some of these methods are not truly nondestructive because they cause some surface damage, which is, however, minor compared with that produced by drilling a core. The second class includes those methods that measure other characteristics of concrete such as moisture content, density, thickness, resistivity, and permeability. Also included in the second class are such methods as stress wave propagation, ground probing radar, and infrared thermography techniques, which are used to locate delaminations, voids, and cracks in concrete. In addition, there are methods to provide information on steel reinforcement such as bar location, bar size, and whether the bars are corroding. This second edition provides comprehensive treatment of nondestructive test methods that are used to evaluate concrete structures. The opening chapter deals with surface hardness test methods, followed by chapters on penetration resistance, pullout, break-off, maturity, pull-off, permeation, resonant frequency, and pulse velocity techniques. These chapters are followed by a chapter on the combined methods, in which more than one technique are used to estimate strength of concrete. The remaining chapters deal with magnetic, electrical, radioactive, nuclear, radar, stress wave propagation, infrared thermography, and acoustic emission techniques.

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This handbook is written primarily for practicing engineers engaged in quality control or investigations of hardened concrete. The chapter authors are recognized specialists in the subject areas, and are, or have been, active participants in technical committees on nondestructive testing of concrete. Each chapter discusses the basic principles of the methods and provides practical information for their use. In-depth mathematical treatment and derivations have been kept to a minimum. Those interested in more detailed information about the development of these methods are referred to the original papers cited at the end of each chapter. Some of the test methods described in this handbook are based on fairly simple principles and are easy to carry out, whereas others are based on complex principles and require sophisticated equipment and trained personnel to perform the tests. Regardless of which test is used, it is strongly recommended that interpretation of test results be performed by persons who are thoroughly familiar with the principles and limitations of the method. Interpretation should not be delegated to unqualified field technicians. It is hoped that this second edition of the handbook will meet the growing needs of practicing engineers and technologists in the area of nondestructive testing of concrete. Graduate students in concrete technology should also find this handbook useful as a comprehensive state-of-the-art document, and as a source of reference material on the subject. V. Mohan Malhotra Nicholas J. Carino

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The Editors

V. Mohan Malhotra, P.Eng., DDL (Hon), D.Eng. (Hon), internationally known researcher, author, and speaker, is Scientist Emeritus, CANMET, Department of Natural Resources, Ottawa, Canada. Dr. Malhotra received his B.Sc. degree in 1951 from Delhi University, India and his B.E. (Civil) in 1957 from the University of Western Australia, Perth. In 1984, he was awarded the honorary degree of Doctor of Laws (DDL) by the University of Dundee, Dundee, Scotland, and in 1999 he was awarded the honorary degree of Doctor of Engineering by the University de Nuevo Leon, Monterrey, Mexico. Dr. Malhotra has been actively engaged in research in all aspects of concrete technology, including nondestructive testing, for the past 35 years. From 1975 to 1990 he was Chair of ASTM subcommittee C09 02.05 on Nondestructive Testing of Concrete. Dr. Malhotra is an Honorary Member and a Charter Fellow of the American Concrete Institute, Fellow of the American Society for Testing and Materials (ASTM), a Fellow of the Canadian Society for Civil Engineering, a Fellow of the Engineering Institute of Canada, an Honorary Member of the Concrete Society, U.K., and an Honorary Fellow of the Institute of Concrete Technology, U.K. He has received numerous awards and honors from the American Concrete Institute (ACI) and ASTM. Dr. Malhotra is on the editorial board of several international journals on concrete technology. He has published more than 150 technical papers on concrete technology including nondestructive testing. He is the author or co-author of several books, including Condensed Silica Fume, published by CRC Press in 1988. He has edited numerous special publications for the American Concrete Institute and CANMET. His book, High Performance, High Volume, Fly Ash Concrete, was published in 2002. Dr. Malhotra has organized and chaired numerous international conferences on concrete technology throughout the world. In 1984, he organized and chaired the CANMET/ACI International Conference on In-situ/Non-destructive Testing of Concrete in Ottawa, Canada. Nicholas J. Carino, Ph.D., is a research structural engineer in the Materials and Construction Research Division of the National Institute of Standards and Technology (formerly National Bureau of Standards) in Gaithersburg, Maryland. Dr. Carino received his undergraduate and graduate education at Cornell University (B.S., 1969; M.S., 1971; and Ph.D., 1974). Upon receiving his Ph.D., Dr. Carino accepted a teaching position at The University of Texas at Austin, where he received several awards for teaching excellence. In 1979, Dr. Carino accepted a research position at the National Bureau of Standards. His research has dealt with methods for in-place testing of concrete for strength, with nondestructive methods for flaw detection in concrete, and with testing high-strength concrete. His work on the maturity method gained national and international recognition for which he received a U.S. Department of Commerce Bronze Medal in 1983. In 1996 he was co-recipient of the ACI Wason Medal for Materials Research for a paper dealing with the effects of cementitious materials on maturity functions. He was co-developer

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with Dr. Mary Sansalone of the impact-echo method, and in 1996 they received the ACI Wason Medal for their paper describing the application of the impact-echo method for delamination detection. In addition to research, Dr. Carino has been involved in structural investigations. In 1987, he was project leader in the structural assessment of the new office building of the U.S. Embassy in Moscow. He participated in the investigation of the 1981 condominium collapse in Cocoa Beach, Florida; the 1982 highway ramp failure in East Chicago, Indiana; and conducted damage assessments after the 1989 Loma Prieta earthquake and the 1994 Northridge earthquake. Dr. Carino is a Fellow of the ACI, a Fellow of the ASTM, and a member of the American Society of Civil Engineers. He serves as Chair of ASTM Committee C09 on Concrete and Concrete Aggregates. He has held numerous other technical committee leadership positions including Chair of the ACI Committee on Nondestructive Testing for Concrete and the ASTM Subcommittee on Nondestructive and In-Place Testing. He also served on the ACI Board of Direction and the ACI Technical Activities Committee.

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Contributors

P.A. Muhammed Basheer

Kenneth R. Lauer

Tarun R. Naik

Professor and Chair of Structural Materials School of Civil Engineering The Queen’s University of Belfast Northern Ireland United Kingdom

Professor Emeritus Department of Civil Engineering and Geological Sciences University of Notre Dame Notre Dame, Indiana

Director, Center for By-Products Utilization Professor, Civil Engineering College of Engineering and Applied Science < The University of Wisconsin< Milwaukee Milwaukee, Wisconsin

Georges G. Carette1 CANMET Department of Natural Resources Canada Ottawa, Ontario Canada

Nicholas J. Carino Research Structural Engineer Building and Fire Research Laboratory National Institute of Standards and Technology Gaithersburg, Maryland

Gerardo G. Clemeña Principal Research Scientist Virginia Transportation Research Council Charlottesville, Virginia

Graeme D. Henderson Dr. Doran and Partners Belfast Northern Ireland United Kingdom 1

Deceased

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Adrian E. Long Professor School of Civil Engineering The Queen’s University of Belfast Northern Ireland United Kingdom

V. Mohan Malhotra Scientist Emeritus CANMET Department of Natural Resources Canada Ottawa, Ontario Canada

Sidney Mindess Professor, Civil Engineering University of British Columbia Vancouver, British Columbia Canada

Terry M. Mitchell Materials Research Engineer Turner-Fairbank Highway Research Center Federal Highway Administration U.S. Department of Transportation McLean, Virginia

John S. Popovics Assistant Professor Department of Civil and Environmental Engineering University of Illinois at Urbana-Champaign Urbana, Illinois

Aleksander Samarin Consultant Rhodes, New South Wales Australia

Vasanthy Sivasundaram Research Engineer CANMET Department of Natural Resources Canada Ottawa, Ontario Canada

Gary J. Weil President EnTech Engineering, Inc. St. Louis, Missouri

Contents

1

Surface Hardness Methods V. Mohan Malhotra 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 1.10

2

Penetration Resistance Methods V. Mohan Malhotra, Georges G. Carette 2.1 2.2 2.3 2.4 2.5 2.6

3

Introduction Probe Penetration Test System Evaluation of the Probe Penetration Test Pin Penetration Test Standardization of Penetration Resistance Techniques Limitations and Usefulness of Penetration Resistance Methods

Pullout Test 3.1 3.2 3.3 3.4 3.5 3.6

4

Introduction Indentation Methods Rebound Method Limitations Rebound Number and Estimation of Compressive Strength Rebound Number and Flexural Strength Rebound Number and Modulus of Elasticity North American Survey on the Use of the Rebound Hammer Standardization of Surface Hardness Methods Limitations and Usefulness

Nicholas J. Carino Introduction Historical Background Failure Mechanism Statistical Characteristics Applications Concluding Remarks

The Break-Off Test Method Tarun R. Naik 4.1 4.2 4.3 4.4 4.5 4.6 4.7

Introduction Theoretical Considerations BO Test Equipment Historical Background Test Procedure Evaluation of Test Specimens Applications

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4.8 4.9

5

The Maturity Method Nicholas J. Carino 5.1 5.2 5.3 5.4 5.5 5.6

6

Adrian E. Long Introduction In Situ Strength Assessment Measuring Concrete Permeation Properties Description of Test Methods Concluding Remarks

Resonant Frequency Methods V. Mohan Malhotra, Vasanthy Sivasundaram 7.1 7.2 7.3 7.4 7.5 7.6 7.7 7.8 7.9 7.10 7.11 7.12 7.13

8

Introduction Historical Background Theoretical Basis Application of Maturity Method Standard Practice Conclusion

Pull-Off Test and Permeation Tests Graeme D. Henderson, P.A. Muhammed Basheer, 6.1 6.2 6.3 6.4 6.5

7

Advantages and Limitations Standardization of the B.O. Method

Introduction Resonant Frequency Method Other Methods of Resonant Frequent Testing Factors Affecting Resonant Frequency and Dynamic Modulus of Elasticity Resonant Frequency and Durability of Concrete Reproducibility of Test Results Correlation between Dynamic Modulus of Elasticity and Strength Properties of Concrete Comparison of Moduli of Elasticity Determined from Longitudinal and Transverse Frequencies Comparison of Dynamic and Static Moduli of Elasticity Specialized Applications of Resonance Tests Damping Properties of Concrete Standardization of Resonant Frequency Methods Limitations and Usefulness of Resonant Frequency Methods

The Ultrasonic Pulse Velocity Method Tarun R. Naik, V. Mohan Malhotra, 8.1 8.2 8.3 8.4 8.5 8.6 8.7 8.8

John S. Popovics Historical Background Theory of Wave Propagation Pulse Velocity Test Instrument The Pulse Velocity Method Factors Affecting Pulse Velocity Standardization of the Pulse Velocity Method Applications Advantages and Limitations

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9

Combined Methods Aleksander Samarin 9.1 9.2 9.3 9.4

10

Introduction Historical Development Combined Ultrasonic Pulse Velocity and Hardness Measurement Techniques Conclusions

Magnetic/Electrical Methods Kenneth R. Lauer 10.1 Introduction 10.2 Magnetic Methods 10.3 Electrical Methods

11

Methods to Evaluate Corrosion of Reinforcement Nicholas J. Carino 11.1 11.2 11.3 11.4 11.5 11.6 11.7

12

Radioactive/Nuclear Methods Terry M. Mitchell 12.1 12.2 12.3 12.4 12.5

13

Introduction Principle of Short-Pulse Radar Instrumentation Applications Standardization of Short-Pulse Radar Methods Conclusions

Stress Wave Propagation Methods Nicholas J. Carino 14.1 14.2 14.3 14.4

15

Introduction General Principles Radiometry Radiography Neutron-Gamma Techniques

Short-Pulse Radar Methods Gerardo G. Clemeña 13.1 13.2 13.3 13.4 13.5 13.6

14

Introduction Principles of Corrosion Corrosion of Steel in Concrete Half-Cell Potential Method Concrete Resistivity Polarization Resistance Summary

Introduction Basic Principles Test Methods Summary

Infrared Thermographic Techniques Gary J. Weil 15.1 Introduction 15.2 Historical Background 15.3 Theoretical Considerations

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15.4 15.5 15.6 15.7 15.8

16

Testing Equipment Testing Procedures Case Histories Advantages and Limitations Summary

Acoustic Emission Methods Sidney Mindess 16.1 16.2 16.3 16.4 16.5 16.6 16.7 16.8 16.9

Introduction Historical Background Theoretical Considerations Evaluation of Acoustic Emission Signals Instrumentation and Test Procedures Parameters Affecting Acoustic Emissions from Concrete Laboratory Studies of Acoustic Emission Field Studies of Acoustic Emission Conclusions

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1 Surface Hardness Methods* 1.1 1.2

Introduction Indentation Methods Testing Pistol by Williams · Spring Hammer by Frank · Pendulum Hammer by Einbeck

1.3

Rebound Method

1.4

Limitations

Rebound Hammer by Schmidt Smoothness of Surface under Test · Size, Shape, and Rigidity of Test Specimens · Age of Test Specimen · Surface and Internal Moisture Condition of the Concrete · Type of Coarse Aggregate · Type of Cement · Type of Mold · Carbonation of Concrete Surface

1.5

V. Mohan Malhotra Department of Natural Resources Canada, Ottawa

Rebound Number and Estimation of Compressive Strength 1.6 Rebound Number and Flexural Strength 1.7 Rebound Number and Modulus of Elasticity 1.8 North American Survey on the Use of the Rebound Hammer 1.9 Standardization of Surface Hardness Methods 1.10 Limitations and Usefulness

The chapter deals with surface hardness methods for nondestructive testing of concrete. These methods consist of the indentation type and those based on the rebound principle. The rebound method is described in detail, and a procedure is given for the preparation of correlation curves between compressive strength and rebound number. The advantages and limitations of the surface hardness methods are discussed. it is concluded that these methods must be regarded as substitutes for standard compression tests, but as a means for determining the uniformity of concrete in a structure and comparing one concrete against another.

1.1 Introduction The increase in the hardness of concrete with age and strength has led to the development of test methods to measure this property. These methods consist of the indentation type and those based on the rebound principle. The indentation methods consist principally of impacting the surface of concrete by means of a given mass having a given kinetic energy and measuring the width and or depth of the resulting

*

©Minister of Supply and Services Canada, 1989.

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indentation. The methods based on the rebound principle consist of measuring the rebound of a springdriven hammer mass after its impact with concrete.

1.2 Indentation Methods According to Jones,1 the indentation methods originated in Germany in 1934 and were incorporated in the German standards in 1935.2,3 The use of these methods has also been reported in the U.K.4 and the USSR.5 There is little apparent theoretical relationship between the strength of concrete and its surface hardness. However, several researchers have published empirical correlations between the strength properties of concrete and its surface hardness as measured by the indentation methods. The three known historical methods based on the indentation principle are: • Testing Pistol by Williams • Spring Hammer by Frank • Pendulum Hammer by Einbeck

1.2.1 Testing Pistol by Williams In 1936 Williams4 reported the development of a testing pistol that uses a ball as an indenter. The diameter of the impression made by the ball is measured by a graduated magnifying lens or other means. The impression is usually quite sharp and well defined, particularly with concrete of medium and high strength. The depth of indentation is only about 1.5 mm for concrete with compressive strengths as low as 7 MPa. The utility of the method according to Williams4 depends on the approximate relationship found to exist between the compressive strength of concrete and the resistance of its surface to indentation during impact. Skramtaev and Leshchinszy5 have also reported the use of a pistol in the testing of concrete in the USSR.

1.2.2 Spring Hammer by Frank The device developed by Frank consists of a spring-controlled mechanism housed in a tubular frame. The tip of the hammer can be fitted with balls having different diameters, and impact is achieved by placing the hammer against the surface under test and manipulating the spring mechanism. Generally about 20 impact readings are taken at short distances from one another and the mean of the results is considered as one test value. The diameter and/or depth of the indentation is measured, and this, in turn, is correlated with the compressive strength of concrete.6 The spring mechanism can be adjusted to provide an energy of 50 kg/cm or of 12.5 kg/cm so that the indentation on the concrete surface is within 0.3 to 0.7 times the diameter of the steel ball.

1.2.3 Pendulum Hammer by Einbeck A line diagram of the pendulum hammer developed by Einbeck is given in Figure 1.1.6 The hammer consists of horizontal leg, at the end of which is pivoted an arm with a pendulum head with a mass of 2.26 kg. The indentation is made by holding the horizontal leg against the concrete surface under test and allowing the pendulum head to strike the concrete. The height of fall of the pendulum head can be varied from full impact (180˚) to half impact (90˚). The diameter and depth of indentation are measured, and these are correlated with the compressive strength of concrete. The biggest drawback to this hammer is that it can be used only on vertical surfaces and is, therefore, less versatile than the spring hammer by Frank.

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55 cm

35

cm

A FIGURE 1.1

B

Vertical elevation and plan of Einbeck pendulum hammer. (Adapted from Reference 6.)

1.3 Rebound Method 1.3.1 Rebound Hammer by Schmidt In 1948 a Swiss engineer, Ernst Schmidt,7–9 developed a test hammer for measuring the hardness of concrete by the rebound principle. Results of his work were presented to the Swiss Federal Materials Testing and Experimental Institute of Zurich, where the hammer was constructed and extensively tested. About 50,000 Schmidt rebound hammers had been sold by 1986 on a worldwide basis. Principle — The Schmidt rebound hammer is principally a surface hardness tester with little apparent theoretical relationship between the strength of concrete and the rebound number of the hammer. However, within limits, empirical correlations have been established between strength properties and the rebound number. Further, Kolek10 has attempted to establish a correlation between the hammer rebound number and the hardness as measured by the Brinell method. Description — The Schmidt rebound hammer is shown in Figure 1.2. The hammer weighs about 1.8 kg and is suitable for use both in a laboratory and in the field. A schematic cutaway view of the rebound hammer is shown in Figure 1.3. The main components include the outer body, the plunger, the hammer mass, and the main spring. Other features include a latching mechanism that locks the hammer mass to the plunger rod and a sliding rider to measure the rebound of the hammer mass. The rebound distance is measured on an arbitrary scale marked from 10 to 100. The rebound distance is recorded as a “rebound number” corresponding to the position of the rider on the scale. Method of Testing — To prepare the instrument for a test, release the plunger from its locked position by pushing the plunger against the concrete and slowly moving the body away from the concrete. This causes the plunger to extend from the body and the latch engages the hammer mass to the plunger rod (Figure 1.3A). Hold the plunger perpendicular to the concrete surface and slowly push the body toward

FIGURE 1.2

Schmidt rebound hammer.

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A Body

B C

D

Latch Indicator Hammer

Spring

Plunger

FIGURE 1.3

A cutaway schematic view of the Schmidt rebound hammer.

the test object. As the body is pushed, the main spring connecting the hammer mass to the body is stretched (Figure 1.3B). When the body is pushed to the limit, the latch is automatically released, and the energy stored in the spring propels the hammer mass toward the plunger tip (Figure 1.3C). The mass impacts the shoulder of the plunger rod and rebounds. During rebound, the slide indicator travels with the hammer mass and records the rebound distance (Figure 1.3D). A button on the side of the body is pushed to lock the plunger in the retracted position, and the rebound number is read from the scale. The test can be conducted horizontally, vertically upward or downward, or at any intermediate angle. Due to different effects of gravity on the rebound as the test angle is changed, the rebound number will be different for the same concrete and will require separate calibration or correction charts. Correlation Procedure — Each hammer is furnished with correlation curves developed by the manufacturer using standard cube specimens. However, the use of these curves is not recommended because material and testing conditions may not be similar to those in effect when the calibration of the instrument was performed. A typical correlation procedure is given below. 1. Prepare a number of 150 = 300-mm cylinders* covering the strength range to be encountered on the job site. Use the same cement and aggregates that are to be used on the job. Cure the cylinders under standard moist-curing room conditions,** keeping the curing period the same as the specified control age in the field. 2. After capping, place the cylinders in a compression-testing machine under an initial load of approximately 15% of the ultimate load to restrain the specimen. Ensure that cylinders are in a saturated surface-dry condition. 3. Make 15 hammer rebound readings, 5 on each of 3 vertical lines 120˚ apart, against the side surface in the middle two thirds of each cylinder. Avoid testing the same spot twice. For cubes, take five readings on each of the four molded faces without testing the same spot twice. 4. Average the readings and call this the rebound number for the cylinder under test.*** 5. Repeat this procedure for all the cylinders. *

In countries where a cube is the standard specimen, use 150-mm cube specimens. Temperature 73.4 ± 3˚F (23 ± 1.7˚C) and 100% relative humidity. ***Some erratic rebound readings may occur when a test is performed directly over an aggregate particle or an air void. Accordingly, the outliers should be discarded and ASTM C 805 has a procedure for discarding these test results. **

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FIGURE 1.4 Relationship between 28-day compressive strength and rebound number for limestone aggregate concrete obtained with Type N-2 hammer. (Adapted from Reference 13.)

6. Test the cylinders to failure in compression and plot the rebound numbers against the compressive strengths on a graph. 7. Fit a curve or a line by the method of least squares. A typical curve established by Zoldners11 for limestone aggregate concrete is shown in Figure 1.4. This curve was based on tests performed at 28 days using different concrete mixtures. Figure 1.5 shows four calibration curves obtained by research workers in four different countries.10 It is important to note that some of the curves deviate considerably from the curve supplied with the hammer. 8000

6000

50

40 Schmidt

5000 30

Mpa

Cube Compressive Strength, psi

Kolek 7000

4000 3000

20 Greene

2000 1000

Chefdeville 20

30 40 Rebound Number

10 50

FIGURE 1.5 Correlation curves obtained by different investigators with a Schmidt rebound hammer Type N-2. Curve by Greene was obtained with Type N. (Adapted from Reference 5.)

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To gain a basic understanding of the complex phenomena involved in the rebound test, Akashi and Amasaki12 have studied the stress waves in the plunger of a rebound hammer at the time of impact. Using a specially designed plunger instrumented with strain gauges, the authors showed that the impact of the hammer mass produces a large compressive wave mi and large reflected stress wave mr at the center of the plunger. The ratio mr/mi of the amplitudes of these waves and the time T between their appearance was found to depend upon the surface hardness of hardened concrete. The rebound number was found to be approximately proportional to the ratio of the two stresses, and was not significantly affected by the moisture condition of the concrete. A schematic diagram of the equipment used for observing stress waves is shown in Figure 1.6, and Figure 1.7 is an oscilloscope trace of the impact stresses in the plunger showing the initial and reflected waves. From their research, the authors concluded that to correctly measure the rebound number of hardened concrete, the Schmidt hammer should be calibrated by testing a material with a constant hardness and measuring the resulting impact stress waves. Thus, by measuring the impact waves in the plunger, the surface hardness of concrete can be measured with a higher accuracy. A typical relationship between the rebound number R and stress mr/mi is shown in Figure 1.8.

0.74 MPa

Schmidt Hammer

Bridge Circuit

DC Amplifier

Dummy

0.74 MPa

Active Gauge

Oscilloscope

Tape Puncher

Transient Memory

FIGURE 1.6

Schematic diagram of the equipment used for observing stress waves. (Adapted from Reference 12.)

FIGURE 1.7 12.)

Oscilloscope trace of stress waves in the test plunger when testing concrete. (Adapted from Reference

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Stress Ratio σr /σi

1.0 0.9 0.8 Curing Dry Wet

0.7 0.6 10

FIGURE 1.8

20 30 40 Rebound Number R

Stress ratio vs. rebound number. (Adapted from Reference 12.)

1.4 Limitations Although the rebound hammer provides a quick, inexpensive means of checking the uniformity of concrete, it has serious limitations and these must be recognized. The results of the Schmidt rebound hammer are affected by: 1. 2. 3. 4. 5. 6. 7. 8.

Smoothness of test surface Size, shape, and rigidity of the specimens Age of test specimens Surface and internal moisture conditions of the concrete Type of coarse aggregate Type of cement Type of mold Carbonation of the concrete surface

These limitations are discussed in the foregoing order.

1.4.1 Smoothness of Surface under Test Surface texture has an important effect on the accuracy of the test results. When a test is performed on a rough textured surface, the plunger tip causes excessive crushing and a reduced rebound number is measured. More accurate results can be obtained by grinding a rough surface to uniform smoothness with a carborundum stone. it has been shown by Kolek10 and Greene13 that trowelled surfaces or surfaces made against metal forms yield rebound numbers 5 to 25% higher than surfaces made against wooden forms. This implies that if such surfaces are to be used, a special correlation curve or correction chart must be developed. Further, trowelled surfaces will give a higher scatter of individual results and, therefore, a lower confidence in estimated strength.

1.4.2 Size, Shape, and Rigidity of Test Specimens If the concrete section or test specimen is small, such as a thin beam, wall, 152-mm cube, or 150 = 300mm cylinder, any movement under the impact will lower the rebound readings. In such cases the member has to be rigidly held or backed up by a heavy mass. It has been shown by Mitchell and Hoagland14 that the restraining load for test specimens at which the rebound number remains constant appears to vary with the individual specimen. However, the effective restraining load for consistent results appears to be about 15% of the ultimate strength of 152 = 305-mm cylinders (Figure 1.9). Zoldners,11 Greene,13 and Grieb15 have indicated effective stresses of 1,

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