Handbook of nondestructive testing of concrete:The Break-Off Test Method.pdf
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1、4-1 4 The Break-Off Test Method 4.1Introduction 4-1 4.2Theoretical Considerations. 4-2 4.3BO Test Equipment. 4-3 4.4Historical Background 4-4 4.5Test Procedure. 4-8 Inserting Sleeves in Fresh Concrete Preparation for Core Drilling from Hardened Concrete Conducting the BO Test The BO Tester Calibrati
2、on Procedure Developing a Correlation Curve 4.6Evaluation of Test Specimens. 4-13 4.7Applications. 4-14 4.8Advantages and Limitations. 4-15 4.9Standardization of the BO Method. 4-15 In-place concrete strength is not the same as cylinder concrete strength because the in-place concrete is placed, comp
3、acted, and cured in a different manner than the cylinder specimen concrete. Determination of accurate in-place strength is critical in form removal and prestress or post-tension force release operations. Fast construction techniques and construction failures emphasize the need for adopting methods f
4、or determining in-place concrete strength. Several such methods exist and a considerable amount of informa- tion is available. Out of many of these currently available nondestructive testing (NDT) methods, only the break-off (BO) and the pullout tests measure a direct strength parameter. The BO test
5、 consists of breaking off an in-place cylindrical concrete specimen at a failure plane parallel to the fi nished surface of the concrete element. The BO stress at failure can then be related to the compressive or fl exural strength of the concrete using a predetermined relationship that relates the
6、concrete strength to the BO strength for a particular source of concrete. The BO test was developed in Norway by Johansen in 1976, and it was then introduced in North America, initially by Malhotra in Canada and later by Naik in the United States. This chapter provides complete and detailed informat
7、ion regarding the theory behind the BO method, factors affecting this method, and the practical use of this method for laboratory and site investigations. Selected case histories and lab investigations are also included. 4.1Introduction For many years questions have been raised regarding concrete qu
8、ality assurance test methods based upon standard cylinder tests, which measure the potential strength of a concrete batch. In-place concrete strength is not the same as the cylinder concrete strength because the in-place concrete is placed, compacted, and cured in a different manner than the cylinde
9、r specimen concrete. Determination of accurate in-place strength is critical in form removal and prestress or post-tension force release operations. Tarun R. Naik University of WisconsinMilwaukee 4-2Handbook on Nondestructive Testing of Concrete: Second Edition Fast construction techniques and const
10、ruction failures emphasize the need for adopting methods for determining in-place concrete strength. Several such methods exist and a considerable amount of infor- mation is available.15 Out of many of these currently available NDT methods, only the BO and the pullout tests measure a direct strength
11、 parameter. The BO test consists of breaking off an in-place cylindrical concrete specimen at a failure plane parallel to the fi nished surface of the concrete element. The BO stress at failure can then be related to the compressive or fl exural strength of the concrete using a predetermined relatio
12、nship that relates the concrete strength to the BO strength for a particular source of concrete. The BO test was developed in Norway by Johansen in 1976.6 The BO test is still not very widely used in North America. The primary factor that limited the widespread use of this method was the lack of nec
13、essary technical data and experience in North America. Initial work at CANMET in the early 1980s had indicated a lack of reproducibility in results of this test method.* Several papers were published in Europe about the BO method. This chapter provides complete and detailed information regarding the
14、 theory behind the BO method, factors affecting this method, and the practical use of this method for laboratory and site investigations. Selected case histories and lab investigations are also included. 4.2Theoretical Considerations The BO method is based upon breaking off a cylindrical specimen of
15、 in-place concrete. The test specimen has a 55 mm (2.17 in.) diameter and a 70 mm (2.76 in.) height. The test specimen is created in the concrete by means of a disposable tubular plastic sleeve, which is cast into the fresh concrete and then removed at the planned time of testing, or by drilling the
16、 hardened concrete at the time of the BO test. Figures 4.1 and 4.2 show tubular plastic sleeves and a drill bit, respectively. Both the sleeve and the drill bit are capable of producing a 9.5 mm (3/8 in.) wide groove (counter bore) at the top of the test specimen (see Figure 4.3) for seating the loa
17、d cell (see Section 4.3). A force is applied through the load cell by means of a manual hydraulic pump. Figure 4.3 is a schematic of a BO concrete cylindrical specimen obtained by inserting a sleeve or drilling a core. The fi gure also shows location of the applied load at the top of the BO test spe
18、cimen. In essence, the load confi guration is the same as a cantilever beam with circular cross section, subjected to a concentrated load at its free end. The force required to break off a test specimen is measured by a mechanical manometer. The BO stress can then be calculated as: fBO = M/S where M
19、 = PBO h PBO = BO force at the top h = 65.3 mm S = (d)/32 d = 55 mm The BO method assumes that the ultimate fl exural strength of the concrete is reached at the extreme outside fi ber at the base of the BO test specimen. In this case, the circular cross-section area would restrict the ultimate fi be
20、r stress theoretically to a point, and a crack is initiated at this point. The exact location of the rupture is determined by the loading arrangement (see Figure 4.3) at a distance of 55 mm from the concrete surface. Away from the extreme outside fi ber at the base, the stresses successively change
21、in the direction of the neutral axis from tension to compression. The BO method is presently the only available test method for directly determining fl exural strength of in-place concrete; there is a linear relationship between the BO fl exural strength and modulus of rupture as determined by a bea
22、m test.712 In the above simplifi ed *Personal communication from V.M. Malhotra, 1984. The Break-Off Test Method4-3 formula, the manufacturer uses the elementary theory of strength of materials and does not apply the concept of deep beam analysis even though the diameter to length ratio is 1:1.3. Thi
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