Handbook of nondestructive testing of concrete:Combined Method.pdf
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1、9-1 0-8493-1485-2/04/$0.00+$1.50 2004 by CRC Press LLC 9 Combined Methods 9.1Introduction 9-1 9.2Historical Development 9-2 9.3Combined Ultrasonic Pulse Velocity and Hardness Measurement Techniques. 9-3 Theoretical Considerations Description of Test Methods Development of Test Methods for Practical
2、Applications Limitations and Advantages of Combined Pulse Velocity and Rebound Number Technique Application of Combined Test Methods 9.4Conclusions . 9-10 This chapter describes the theoretical and empirical based concepts as well as the history of development of combined nondestructive test methods
3、 for hardened concrete. Of a number of purely nondestructive tests, the rebound (Schmidt) hammer and the ultrasonic pulse velocity combinations are the most commonly used. In the majority of cases, the need for in situ concrete strength evaluation arises as a result of suspect quality of concrete. B
4、y developing a prior correlation for a range of concrete grades and types, having only the source of coarse aggregate and a broad age group in common, it is possible to obtain good indication of the in situ strength of concrete, expressed as the value of a test result of a standard laboratory compre
5、ssive specimen. The quality of concrete, using combined nondestructive methods, is evaluated through the measure- ments and correlation of the surface hardness, density, elastic constants and the predicted compressive strength. Use of combined methods is generally justifi able only if a reliable cor
6、relation for a particular type of concrete is developed prior to the evaluation of the subject quality concrete. The benefi t of the small additional reliability of a combined test vs. a single nondestructive test should be assessed against the additional time, cost, and complexity of combined techn
7、iques. 9.1Introduction Although the need for nondestructive in situ testing of concrete has long been realized, it is seldom used in its own right for quality control and compliance purposes. In fact, the practice is to bring in a destructive (e.g., cores), a semi-destructive (e.g., pullout or break
8、-off tests), or a nondestructive in situ test at the postmortem stage, either following noncompliance of a standard specimen test result or on observing signs of deterioration and distress in a structure. Occasionally, nondestructive in situ tests are used to evaluate the “quality” of existing struc
9、tural members for the purpose of subsequent modifi cations to that structure. The “quality” of concrete in practice is still commonly described in terms of its uniaxial compressive strength, evaluated statistically from the results of standard laboratory specimens, cast, compacted, cured, and tested
10、 under strictly prescribed conditions. There is, of course, a very good reason for maintaining this system. In those Aleksander Samarin Consultant 9-2Handbook on Nondestructive Testing of Concrete: Second Edition countries where ready-mixed concrete can be purchased from a premixed concrete manufact
11、urer, this method remains the only fair and reasonable way of evaluating the potential quality, i.e., strength of concrete as supplied to a building site. However, with the present emphasis on the design of reinforced concrete structures according to the limit state concepts, there seems to be much
12、greater need for better defi nition of the relationship between concrete quality and variability in actual structures and in standard, laboratory-cured test specimens of the same concrete. A combination of these in situ tests, if properly used, can improve some of these correlations. The extent to w
13、hich the correlation can be improved should be balanced, however, by the additional time, cost, and resources required to use combined methods. 9.2Historical Development In the preceding chapters several nondestructive and semi-destructive (or partially destructive) test methods have been described.
14、 These methods were developed to evaluate the strength, or strength related properties of concrete. In order to predict the strength of in situ concrete more accurately, a number of investigators have tried to apply more than one nondestructive method at the same time. Some of the pioneering work in
15、 this fi eld, carried out in the 1950s and 1960s, was reported by Kesler and Higuchi,1 Skramtaev and Leshchinsky,2 Wiebenga,3 Facaoaru,4 and McLeod.5 Combined methods, as used in this chapter, refers to techniques in which one test improves the reliability and precision of the other in evaluating a
16、property of concrete, e.g., strength or elastic modulus. When an additional test is used to provide new, but supporting information (e.g., location of reinforcing steel, using a covermeter), the tests are not considered promising by their investigators. Dynamic modulus of elasticity and damping cons
17、tant, as determined from resonance tests by Kesler and Higuchi,1 were reported to correlate well with the compressive strength of concrete, regardless of its mix proportions, age, or moisture content. The accuracy of prediction was considered to be within 5%. Laboratory techniques were used, as the
18、method was unsuitable for in situ measurements. Wiebenga3 used ultrasonic pulse velocity in place of dynamic modulus of elasticity and the damping constant as well as pulse attenuation in his laboratory tests. In each case, the use of either damping constant or pulse attenuation improved the accurac
19、y of the predicted compressive strength. Galan6 also used the combination of ultrasonic pulse velocity and the damping constant to estimate the in situ strength of concrete. The damping constant was determined by calibrating experimental curves of an oscillogram with the corresponding damped reverbe
20、rated impulses. According to Galan, good correlation between the strength of concrete and the two acoustic characteristics, i.e., pulse velocity and damping constant, can be established. The pulse velocity expresses the elastic properties and the damping constant represents the inelastic behavior of
21、 concrete. In the majority of cases, the differences between the estimated strength values and the values obtained by destructive testing was of the order of 5%, i.e., similar to the accuracy of the laboratory tests by Kesler and Higuchi. Most of the recent research work using the above technique ha
22、s been conducted in the eastern European countries. The portable ultrasonic pulse velocity units currently used in Western countries have digital displays and are not equipped with oscilloscopes. Thus the application of pulse attenuation techniques combined with the ultrasonic pulse velocity would r
23、equire additional equipment. It would also require highly skilled technical personnel, which can render this test method cost ineffective. By far, the most popular combination, however, is the method based on the measurement of ultrasonic pulse velocity in conjunction with hardness measurements. Thi
24、s was confi rmed by a number of surveys, such as Jones and Facaoaru7 and Malhotra and Carette.8 Historically, most of the credit for the develop- ment of this combined method should be attributed to Skramtaev and Leshchinsky,2 Wiebenga,3 MacLeod5 and particularly to Facaoaru,4,7,9 and for the promot
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