Handbook of nondestructive testing of concrete：The Break-Off Test Method.pdf

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 Calibration 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, compacted, 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 for 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 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 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 information 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 quality 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 cylinder 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 construction 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 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 relationship 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 necessary 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 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 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 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 load 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 specimen. 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 = 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 ber 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 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 beam 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. This point has been further explored.1315 4.3BO Test Equipment The BO tester (see Figure 4.4), consists of a load cell, a manometer, and a manual hydraulic pump capable of breaking a cylindrical concrete specimen having the specifi ed dimensions given in Section 4.2. The load cell has two measuring ranges: low range setting for low strength concrete up to approximately 20 MPa (3000 psi) and high range setting for higher strength concrete up to about 60 MPa (9000 psi) (see Figure 4.5). A tubular plastic sleeve, with internal diameter of 55 mm (2.17 in.) and geometry shown in Figure 4.1, is used for forming cylindrical specimen in fresh concrete. A sleeve remover (see Figure 4.6) is used for removing the plastic sleeve from the hardened concrete. A diamond tipped drilling bit is used for drilling cores for the BO test in hardened concrete (see Figure 4.2). The bit is capable of producing a cylindrical core, along with a reamed ring (counter bore) in the hardened concrete at the top with dimensions similar to that produced by using a plastic sleeve. The manufacturer also provides a calibrator for calibration and adjustment of the BO tester (see Figure 4.7). The procedure of calibrating a BO tester is discussed later. FIGURE 4.1 Tubular plastic sleeves for inserting in fresh concrete for the BO test. FIGURE 4.2 Core drill bit for drilling a core for BO testing of existing concrete element. 4-4Handbook on Nondestructive Testing of Concrete: Second Edition 4.4Historical Background The BO method is a relatively new NDT. The fi rst paper was published by Johansen in 19766 and the research work was done at the Norwegian Technical University (NTH). In 1977 researchers at the NTH and the Research Institute for Cement and the test is fast to perform, requiring only one exposed surface. The BO test does not need to be planned in advance of placing the concrete because drilled BO test specimens can be obtained. The test is reproducible to an acceptable degree of accuracy and correlates well with the compressive strength of concrete. Two limitations for the BO test equipment are worth noting: (1) the maximum aggregate size; and (2) the minimum member thickness for which it can be used. The maximum aggregate size is 19 mm (3/4 in.) and the minimum member thickness is 100 mm (4 in.). However, the principle of the method can be applied to accommodate larger aggregate sizes or smaller members. The major disadvantage of the BO test is that the damage to the concrete member must be repaired if the member is going to be visible. However, this test is nondestructive since the tested member need not be discarded. 4.9Standardization of the BO Method In the 1980s, the BO method was standardized in England,3 Norway,19 and Sweden.20 In 1990, ASTM adopted the method as Test Method C 1150.21 The method, however, was not used frequently in the fi eld. In 2002, ASTM discontinued the test method because “the break-off test is not being used in North America, and there is little use in other parts of the world. Therefore, without feedback based on fi eld experience, it is diffi cult to recommend meaningful revisions to this test method.”22 While the BO method is not used widely, there may be applications where it is a useful technique to assess in-place strength, and thus discussion of the method has been retained in this handbook. References 1. Malhotra, V., Testing hardened concrete: nondestructive methods, ACI Monog., No. 9, 1976. 2. Jones, R., A review of nondestructive testing of concrete, Proc. Symp. Nondestructive Testing of Concrete and Timber, Institute of Civil Engineers, London, June 1969, 1. 3. British Standard, B.S. 1881, Part 201, 1986, 17. 4. Methods of Mechanical Nondestructive Determination of Probable Compressive Strength of Con- crete, Bulgarian National Standard 381665. 5. Vossitch, P., Outline of the various possibilities of nondestructive mechanical tests on concrete, Proc. Int. Symp. Nondestructive Testing of Materials and Structures, Vol. 2, Paris, 1959, 30. 6. Johansen, R., A New Method for Determination of In-Place Concrete Strength of Form Removal, 1st Eur. Colloq. on Construction Quality Control, Madrid, Spain, March 1976. 7. Byfors, J., Plain Concrete at Early Ages, Swedish Cement and Concrete Research Institute, Rep. No. Facks-10044, Stockholm, 1980. 8. Dahl-Jorgensen, E., In-situ Strength of Concrete, Laboratory and Field Tests, Cement and Concrete Research Institute, the Norwegian Institute of Technology, Rep. No. STF 65A, Trondheim, Norway, June 1982. 9. Johansen, R., In-situ Strength of Concrete, the Break-off Method, American Concrete Institute, Farmington Hills, MI, 1979, 45. 4-16Handbook on Nondestructive Testing of Concrete: Second Edition 10. Nishikawa, A., A Nondestructive Testing Procedure for In-Place Evaluation of Flexural Strength of Concrete, Rep. No. JHRP 83-10, Joint Highway Research Project, School of Civil Engineering, Purdue University, West Lafayette, IN, 1983. 11. Smith, L., Evaluation of the Scancem Break-Off Tester, Ministry of Works and Development, Central Laboratories, Rep. No. 86/6, New Zealand, 1986. 12. Dahl-Jorgensen, E. and Johansen, R., General and Specialized Use of the Break-Off Concrete Strength Test Method, ACI, SP 82-15, 1984, 294. 13. Hassaballah, A., Evaluation of In-Place Crushed Aggregates Concrete by the Break-Off Method, M.S. thesis, Department of Civil Engineering and Mechanics, University of WisconsinMilwaukee, December 1987. 14. Salamech, Z., Evaluation of In-Place Rounded Aggregates Concrete by the Break-Off Method, M.S. thesis, Department of Civil Engineering and Mechanics, University of WisconsinMilwaukee, December 1987. 15. Naik, T.R., Salameh, Z., and Hassaballah, A., Evaluation of In-Place Strength of Concrete by the Break-Off Method, Department of Civil Engineering and Mechanics, University of Wisconsin Milwaukee, March 1988. 16. Dahl-Jorgensen, E., Break-Off and Pull-Out Methods for Testing Epoxy-Concrete Bonding Strength, Project No. 160382, The Foundation of Scientifi c and Industrial Research of the Norwe- gian Institute of Technology, Trondheim, Norway, September 1982. 17. Carlsson, M., Eeg, I., and Jahren, P., Field Experience in the Use of Break-Off Tester, ACI, SP 82- 14, 1984. 18. Barker, M. and Ramirez, J., Determination of Concrete Strengths Using Break-Off Tester, School of Civil Engineering, Purdue University, West Lafayette, IN, 1987. 19. Nordtest Method NTBUILD 212, Edition 2, Approved 1984-05. 20. Swedish Standard, SS 13723l9, Approved 1983-06. 21. ASTM C1150, Standard Test Method for the Break-Off Number of Concrete, Annual Book of ASTM Standards, Vol. 04.02, 1997. 22. ASTM C1150, Standard Test Method for the Break-Off Number of Concrete, Annual Book of ASTM Standards, Vol. 04.02, 640, 2002.