Nondestructive Evaluation Methods for Quality Acceptance of Hardened Concrete in Structures
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NBSIR 80-2163 Nondestructive Evaluation Methods for Quality Acceptance of Hardened Concrete in Structures James R. Clifton Erik D. Anderson Center for Building Technology National Bureau of Standards U.S. Department of Commerce Washington, DC 20234 January 1981 °repared for - (• f - _ J.S. Army Construction Engineering 1UU Research Laboratory Champaign, Illinois , U56 30-2163 1981 National Bureau of Standards Library, E-01 Admin. Bldg. FEB 2 7 1981 NBSIR 80-2163 not clcc- - Quod NONDESTRUCTIVE EVALUATION • METHODS FOR QUALITY ACCEPTANCE no. 86’ 20,3 OF HARDENED CONCRETE IN &ZI STRUCTURES James R. Clifton Erik D. Anderson Center for Building Technology National Bureau of Standards U.S. Department of Commerce Washington, DC 20234 January 1981 Prepared for U.S. Army Construction Engineering Research Laboratory Champaign, Illinois U.S. DEPARTMENT OF COMMERCE, Philip M. Klutznick, Secretary Jordan J. Baruch, Assistant Secretary for Productivity. Technology, and Innovation NATIONAL BUREAU OF STANDARDS. Ernest Ambler, Director tr>r v ?vj 1 UriiUH l !Q . "SnI A 10- J ,yv*<lt, isei vs i3i ABSTRACT Nondestructive evaluation methods which can be used in quality acceptance programs for hardened concrete have been critically reviewed and are described in this report. Methods have been identified which provide information on the strength, uniformity, thickness, air content, stiffness, finish, density of concrete as well as the location and condition of steel reinforcement. Both commonly used methods and potentially useful test methods are covered. In addition, the feasibility of combining two or more test methods for improving the prediction of the strength or quality of concrete is explored. Key Words: Concrete; construction; nondestructive evaluation; quality assurance; steel reinforcing bars; test methods. iii TABLE OF CONTENTS Page 1. INTRODUCTION 1 2. SURVEY OF NONDESTRUCTIVE EVALUATION (NDE) METHODS FOR CONCRETE 2 3. COMMONLY USED NONDESTRUCTIVE EVALUATION METHODS 7 3.1 Probe Penetration Method 7 3.1.1 Probe Equipment and Use 7 3.1.2 Applications 9 3.1.3 Advantages and Limitations 12 3.2 Rebound Method 13 3.2.1 Description of Method 13 3.2.2 Applications 15 3.2.3 Advantages and Limitations 16 3.3 Cast-In-Place Pullout 17 3.3.1 Test Method 17 3.3.2 Reliability of Method 19 3.3.3 Advantages and Limitations 19 3.4 Ultrasonic Pulse Velocity Method 21 3.4.1 Principle of Method 21 3.4.2 Assessment of the Uniformity and Quality of Concrete ... 23 3.5 Gamma Radiography 27 3.5.1 Principle and Applications 27 3.5.2 Limitations 28 3.6 Neutron Moisture Meters 29 3.6.1 Principle and Applications 29 3.6.2 Limitations 29 3.7 Cover Meters 30 3.7.1 Principle and Applications 30 3.7.2 Limitations 33 3.8 Electric Potential Measurements 33 3.8.1 Principle 33 3.8.2 Limitations 35 iv 4. TABLE OF CONTENTS (CONTINUED) Page APPLICATION OF OTHER NDE METHODS FOR CONCRETE 35 4.1 Ultrasonic Pulse Echo 36 4.2 Microwaves 36 4.3 Infrared Thermography 37 4.4 Maturity Concept .... 38 4.5 Acoustic Emission ... 40 5. COMBINATIONS OF NONDESTRUCTIVE EVALUATION METHODS ,.. 40 f 5.1 Combination of Ultrasonic Pulse Velocity and Rebound Hammer Methods 41 5.2 Other Possible NDE Combinations , 42 6. SUMMARY . 44 r 7. REFERENCES 45 v ’ •, • * - * u c NONDESTRUCTIVE EVALUATION METHODS FOR QUALITY ACCEPTANCE OF HARDENED CONCRETE 1 . INTRODUCTION Quality-^ and uniformity of hardened concrete in a newly constructed building depends on a series of processes including the selection of concrete ingredients, design of the concrete mix, batching of and mixing the concrete ingredients, placing and consolidating plastic concrete, and curing operations. Specification codes, standard guidelines, and standard test methods form the basis for decision making in these processes. For example, concrete ingredients can be specified by citing the appropriate American Society for Testing and Materials (ASTM) standards. In addition, standard test methods atre available for checking the characteristic^ of plastic concrete prior to placement. However, the quality and uniformity of hardened concrete also are affected by construction practices, workmanship, and curing conditions. It is the role of concrete inspectors to determine if the building contractor is adhering to acceptable practices and to the design specifications. Often, an inspector only can form subjective conclusions which are highly dependent on his level of training and experience. The present testing and inspection approach to quality control of concrete does not always adequately ensure that the hardened concrete in the structure is of the desired quality or uniformity. An effective quality control program also should include evaluation of hardened concrete and reinforcement at the time of acceptance of the building. -/ In this paper "quality of concrete" refers to the levels of the chemical and physical properties of concrete needed to meet the designed perfor- mance and service life. 1 Quality control testing of hardened concrete, i.e., quality assurance testing, is gaining recognition as being an important phase of quality acceptance programs. Because of the importance of such programs, an international symposium was held in 1979 [1] which addressed the "Quality Control of Concrete Structures." The nondestructive evaluation (NDE) approach is becoming the foundation for quality assurance testing of hardened concrete. NDE methods which can be used in quality acceptance programs are critically reviewed and described in this report. Quality acceptance programs based on NDE methods have been reported to be cost effective [2] and reliable [2, 3]. The on-job time for concrete inspectors can be reduced. In addition, poor quality or nonuniform concrete can be readily detected and the contractor required to replace it before a building is accepted. This can increase the service life of the building and also reduce maintenance costs. Another application of NDE is in selecting sites for coring. Cores are often taken if hardened concrete appears to be substantially below the specified quality or strength. Taking and testing numerous cores from a concrete structure is undesirable for several reasons including the time consumed, the expense, and the damage to the structure. NDE can be used to identify those areas that should be cored, thereby reducing the number of cores needed to quantitatively evaluate the concrete. 2. SURVEY OF NONDESTRUCTIVE EVALUATION METHODS FOR CONCRETE The selection of NDE methods for characterizing concrete should be based on considerations of its important functional properties. In quality acceptance testing of hardened concrete, properties and factors of concrete which might be determined include strength, uniformity, thickness, air content, stiffness, 2 finish, density, and location and condition of steel reinforcment . NDE methods which can be used to obtain information on these factors are given in table 1. The Recommended Test Methods in table 1 are commonly used and their limitations are understood, while the reliability and feasibility of Possible Test Methods are still being assessed. Operation, principles, and applications of commonly used NDE methods are outlined in table 2. The test equipment for all the methods is sufficiently portable so that the concrete can be tested in the field. In addition, results can normally be obtained and evaluated within one or two days. With the exception of gamma radiography and the neutron moisture meter, the equip- ment can be operated by most concrete testers or inspectors. These radiation techniques only can be used by personnel licensed by the Nuclear Regulatory Commission. The methods outlined in table 2 are described in more detail in section 3. The ASTM has recognized the need for standards and specifications covering nondestructive evaluation methods for a variety of materials and applications. ASTM standards for NDE methods for concrete and for taking test cores from concrete are listed in table 3. The Possible Test Methods listed in table 1 may become recognized methods after their reliabilities and feasibilities have been demonstrated. Princi- ples and potential applications of these methods are discussed in section 4. The feasibility of combining two or more test methods for predicting the strength or quality of concrete is being explored with some success. This subject is covered in section 5. 3 TABLE 1. NDE METHODS FOR QUALITY ASSURANCE TESTING OF HARDENED CONCRETE CONCRETE PROPERTIES RECOMMENDED TEST METHODS POSSIBLE TEST METHODS Strength Windsor Probe Schmidt Rebound Hammer Cast-In-Place Pullouts Maturity Concept Cores General Quality Windsor Probe and Uniformity Schmidt Rebound Hammer Ultrasonic Pulse Ultrasonic Pulse Echo Gamma Radiography Cores Microwaves Thickness Covermeter Microwaves (Radar) Gamma Radiography Cores Ultrasonic Pulse Air Content Cores Neutron Density Gage Dead Weight Loading Moduli of with Acoustic elasticity Emission Surface Visual Microwaves Properties Schmidt Rebound Hammer Ultrasonic Pulse Echo Cores Density Gamma Radiography Neutron Density Gage Cores Rebar Size and Covermeter Microwaves (Radar) Location Gamma Radiography Ultrasonic Pulse Echo Corrosion State of Visual Reinforcing Steel Electrical Potential Measurement 4 . .. TABLE 2. SURVEY OF COMMONLY USED METHODS FOR CONCRETE INSPECTION Method Principle Main Applications Equipment Cost User Expertise Advantages Llaltatlons 1. Windsor Probe Probe fired Into concrete Estimations of $900 Low, can be Equipment is simple Slightly damages and depth of penetration compressive plus operated