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1、1-1 1 Surface Hardness Methods* 1.1Introduction 1-1 1.2Indentation Methods 1-2 Testing Pistol by Williams Spring Hammer by Frank Pendulum Hammer by Einbeck 1.3Rebound Method 1-3 Rebound Hammer by Schmidt 1.4Limitations. 1-7 Smoothness of Surface under Test Size, Shape, and Rigidity of Test Specimens
2、 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 1.5Rebound Number and Estimation of Compressive Strength. 1-10 1.6Rebound Number and Flexural Strength 1-11 1.7Rebound Number and Modulus
3、 of Elasticity. 1-12 1.8North American Survey on the Use of the Rebound Hammer 1-12 1.9Standardization of Surface Hardness Methods. 1-13 1.10Limitations and Usefulness 1-13 The chapter deals with surface hardness methods for nondestructive testing of concrete. These methods consist of the indentatio
4、n 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 conclude
5、d 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. 1.1Introduction The increase in the hardness of concrete with age and strength has led to the deve
6、lopment 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 an
7、d or depth of the resulting * Minister of Supply and Services Canada, 1989. V. Mohan Malhotra Department of Natural Resources Canada, Ottawa 1-2Handbook on Nondestructive Testing of Concrete: Second Edition indentation. The methods based on the rebound principle consist of measuring the rebound of a
8、 spring- driven hammer mass after its impact with concrete. 1.2Indentation 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
9、 little apparent theoretical relationship between the strength of concrete and its surface hardness. However, several researchers have published empirical correlations between the strength prop- erties of concrete and its surface hardness as measured by the indentation methods. The three known histo
10、rical methods based on the indentation principle are: Testing Pistol by Williams Spring Hammer by Frank Pendulum Hammer by Einbeck 1.2.1Testing 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 th
11、e ball is measured by a graduated magnifying lens or other means. The impression is usually quite sharp and well defi ned, 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
12、 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.
13、 1.2.2Spring 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 fi tted with balls having different diameters, and impact is achieved by placing the hammer against the surface under test and manipulating the
14、 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
15、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. 1.2.3Pendulum Hammer by Einbeck A line diagram of the pendulum hammer developed by Einbeck is given in Figure
16、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 t
17、he 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, l
18、ess versatile than the spring hammer by Frank. Surface Hardness Methods1-3 1.3Rebound Method 1.3.1Rebound Hammer by Schmidt In 1948 a Swiss engineer, Ernst Schmidt,79 developed a test hammer for measuring the hardness of concrete by the rebound principle. Results of his work were presented to the Sw
19、iss 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 appa
20、rent 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 r
21、ebound 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 fi eld. A schematic cutaway view of the rebound hammer is shown in Figure 1.3. The m
22、ain 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 ma
23、rked 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
24、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 1.1 Vertical elevation and plan of Einbeck pendulum hamme
25、r. (Adapted from Reference 6.) FIGURE 1.2 Schmidt rebound hammer. 55 cm 35 cm AB 1-4Handbook on Nondestructive Testing of Concrete: Second Edition 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
26、 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 dista
27、nce (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
28、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 man- ufacturer using standard cube specimens. However,
29、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 b
30、e 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 spec- ifi ed control age in the fi eld. 2. After capping, place the cylinders in a compression-
31、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
32、each cylinder. Avoid testing the same spot twice. For cubes, take fi ve 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. FIGURE 1.3 A c
33、utaway schematic view of the Schmidt rebound hammer. *In countries where a cube is the standard specimen, use 150-mm cube specimens. *Temperature 73.4 3F (23 1.7C) and 100% relative humidity. *Some erratic rebound readings may occur when a test is performed directly over an aggregate particle or an
34、air void. Accordingly, the outliers should be discarded and ASTM C 805 has a procedure for discarding these test results. Indicator Body Latch Hammer Spring Plunger A B CD Surface Hardness Methods1-5 6. Test the cylinders to failure in compression and plot the rebound numbers against the compressive
35、 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 c
36、urves 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. FIGURE 1.4 Relationship between 28-day compressive strength and rebound number for limestone aggregate con- crete obtained w
37、ith Type N-2 hammer. (Adapted from Reference 13.) 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.) 28-Day Compressive Strength, psi 5,500 5,000 4,500 4,000 3,500 3,000 2,500
38、 2,000 1,500 MPa 38 35 + 15% 31 28 24 21 17 14 10 152025303540 Rebound Number (hammer in horizontal position) Each rebound number is average of 10 hammer readings on each one of the 152 305-mm cylinders All cylinders were tested in SSD condition Hammer type : N-2 Hammer number: 3080 Coarse aggregate
39、: crushed limestone Fine aggregate: natural sand Total number of cylinders tested = 460 + 15% Cube Compressive Strength, psi 8000 7000 6000 5000 4000 3000 2000 1000 Mpa 50 40 30 20 10 50403020 Rebound Number Kolek Schmidt Greene Chefdeville 1-6Handbook on Nondestructive Testing of Concrete: Second E
40、dition 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 imp
41、act of the hammer mass produces a large compressive wave i and large refl ected stress wave r at the center of the plunger. The ratio r/i 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 wa
42、s found to be approximately proportional to the ratio of the two stresses, and was not signifi cantly 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
43、 stresses in the plunger showing the initial and refl ected 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 s
44、tress 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 r/i is shown in Figure 1.8. FIGURE 1.6 Schematic diagram of the equipment used for observing stress
45、 waves. (Adapted from Reference 12.) FIGURE 1.7 Oscilloscope trace of stress waves in the test plunger when testing concrete. (Adapted from Reference 12.) 0.74 MPa 0.74 MPa Schmidt Hammer Bridge Circuit DC Amplifier Active Gauge OscilloscopeTape Puncher Transient Memory Dummy Surface Hardness Method
46、s1-7 1.4Limitations 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. Smoothness of test surface 2. Size, shape, and rigidity o
47、f the specimens 3. Age of test specimens 4. Surface and internal moisture conditions of the concrete 5. Type of coarse aggregate 6. Type of cement 7. Type of mold 8. Carbonation of the concrete surface These limitations are discussed in the foregoing order. 1.4.1Smoothness of Surface under Test Surf
48、ace 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 w
49、ith 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 confi dence in estimated strength. 1.4.2Size, Shape, and Rigidity of Test Specimens If the concrete section or te