Laboratory 5 Bend and Re-Bend Test of Steel Reinforcing Bar

Transcription

1 Department of Materials and Metallurgical Engineering Bangladesh University of Engineering Technology, Dhaka MME 222 Materials Testing Sessional 1.50 Credits Laboratory 5 Bend and Re-Bend Test of Steel Reinforcing Bar 1. Objective Reinforcing bars in concrete elements functions as the component carrying tensile stresses after concrete cracking. This is due to that fact that concrete exhibit very high compression strength combined with a very low tensile capacity. The tensile stressed are transferred to the bars by the bond between the reinforcement and the concrete. Bars are generally produced in strands 6 to 12 meter in length so that bending for transportation and handling purposes becomes necessary. At the site bars are straightened, and then sometimes re-bent for assembling purposes. This process will result in a loading and re-loading of steel. After completion of this experiment, students should be able to 1.1 conduct bend test and re-bend test of steel reinforcing bars, and 1.2 analyse the nature of the test data and conclude about the ductility of the tested steel bar. 2. Materials and Equipment 2.1 Bend samples 2.2 Universal testing machine 2.3 Heating furnace 3. Experimental Procedure 3.1 Guided-bend test of materials Select and install the mandrel to be used for application of the bending load Place the sample carefully on to the stage of the guided-bending fixture of a universal testing machine. Separate the two rounded supports by a clearance (C) equal to (D + 3d ± d/2), as shown in Fig. 1, where (d) is the specimen diameter and D is the mandrel diameter Bend the specimen by applying a force through the plunger or mandrel in contact with the specimen at the mid-length between supports (C/2) on the opposite side of the specimen from the end supports. Apply the bending force smoothly and without shock Continue bending until failure occurs, or until the bar is bended to an angle of 180. The angle of bend is measured while the specimen is under the bending force When a full bend is achieved, remove the load and detach the bar from the apparatus Repeat steps to to conduct bend tests of two other specimens of the same diameter. 1

2 3.2 Guided-re-bend test of materials Conduct guided-bend test through an angle of 90º round a mandrel with diameter not exceeding those specified in Table 11; Conduct artificial ageing treatment by placing the test specimen at a temperature of 100±10 ºC for a period of 1 hours (+15/-0) minutes Cool the specimens in still air to room temperature Bend back the test specimen towards its original shape (partially re-straightened) by a steadily applied force through at least 20º on the same bending machine as used above Remove the load and detach the bar from the apparatus Repeat steps to to conduct bend tests of two other specimens of the same diameter. 4. Results 4.1 In accordance to the specified Standard (e.g., ASTM Standard E 290), observe the bend and re-bend surface without any equipment (i.e., using naked eye) and inspected for cracks or chipping. 4.2 Give a general comment on the ductility of the tested steel bar. 4.3 Complete the Data Sheet, Table Discussion 5.1 Answer the following questions: (a) Discuss the conditions upon which a steel reinforcing bar is failed a bend or re-bend test. (b) Can you make comment on the tensile properties of the steel bar after the bend test? (c) Why do you need the ageing treatment of bended bar before re-bending? (d) What are the information needed to be specified in a typical bend/re-bend test of steel bar? 2

3 Table 3.1(a): Data Sheet for Bend Test Steel Bar, mm Mandrel, mm Bend Angle, Degrees Observation Bend Test Result Table 3.1(b): Data Sheet for Re-Bend Test Diameter of Steel Bar, mm Mandrel for Bend, mm Bend Angle, Degrees Ageing Condition Mandrel for Re-Bend, mm Re-Bend Angle, Degrees Observation Re-Bend Test Result Signature with Date of the Instructor/Course Tutor 3

4 6.0 Theoretical Background 6.1 Introduction The bend test consists of submitting a test piece of round, square, rectangular, or polygonal cross section to plastic deformation by bending, without changing the direction of loading, until a specified angle of bend is reached. This bend test is conducted for determining the ability of metallic materials to undergo plastic deformation in bending. Concrete reinforcing steel bars are generally produced in strands 6 to 12 meter in length so that bending for transportation and handling purposes becomes necessary. At the site bars are straightened, and then sometimes re-bent for assembling purposes. This process will result in a loading and re-loading of steel. Thus, bend and/or re-bend test is necessary to gain information about the ductility of the steel bar. The following ASTM Standard covers bend testing of bars primary for evaluation of their ductility: ASTM Standards E 290: Standard Test Methods for Bend Testing of Material for Ductility In the bend test procedure, ASTM E 290 Standard specifies four conditions of constraint on the bent portion of the specimen: 1. a guided-bend test using a mandrel or plunger of defined dimensions to force the mid-length of the specimen between two supports separated by a defined space, 2. a semi-guided-bend test in which the specimen is bent, while in contact with a mandrel, through a specified angle or to a specified inside radius (r) of curvature, measured while under the bending force, 3. a free-bend test in which the ends of the specimen are brought toward each other, but in which no transverse force is applied to the bend itself and there is no contact of the concave inside surface of the bend with other material, and 4. a bend and flatten test, in which a transverse force is applied to the bend such that the legs make contact with each other over the length of the specimen. Off these, the guided-bend test method is the more popular for laboratory test. After bending, the convex surface of the bend is examined with the unaided eye for evidence of a crack or other open defects or surface irregularities. Bend tests for ductility provide a simple way to evaluate the quality of materials by their ability to resist cracking or other surface irregularities during one continuous bend. While tests result will give a figure of the material s ductility, it does not reflect on the material strength or its influence to material strength. 6.2 The Guided Bend Testing Methods The guided-bend test using the Universal Testing Machine (UTM) is made by supporting the specimen on pins, rollers, or radiused flats near each end and applying a force through a pin, mandrel, or plunger midway between two supports, as shown schematically in Fig. 1(a) and 2(a), until the bar is bended to an angle of 180, Fig. 1(b) and 2(b). No force is applied directly to the outer face of the bend. The radii of the plunger and of the two supports shall be defined in the product specification as related to the diameter (d) of the specimen being tested. A clearance of three times the bar diameter with a tolerance of one half diameter shall be provided between the pins, plunger, and specimen in the initial bend fixture. The mandrel diameter for different grades of steel bars shall be as specified in Table 1.The surfaces of the supports and plunger shall be hardened to between 20 and 30 HRC. The supports can be fixed or free to rotate. A lubricant may be applied to the supports and plunger. The speed of bending is ordinarily not an important factor. The severity of the bend test is primarily a function of the angle of bend and inside diameter to which the specimen is bent, and of the cross-section of the specimen. These conditions are varied according to location and orientation of the test specimen and the chemical composition, tensile properties, hardness, type, and quality of the steel specified. 4

5 Load C = distance between lower supports = D + 3d ± 0.5d D = diameter of the end of the mandrel or plunger d = round specimen diameter D d Support c Support (a) Figure 1: (a) Schematic fixture for the guided-bend test, (b) Bar after testing (b) (a) (b) (c) Figure 2: (a) Bars prior to testing, (b) Bars bend at 180, (c) Observation after testing. 5

6 Table 1: Mandrel diameter for bend test. Nominal Size (mm) of Bar, d Mandrel Diameter (mm) for Different Grades of Steel Bars, D 40 Grade 60 Grade 500 W TMT 600 W TMT Up to and including 20 3d 4d 3d 5d Over 20 4d 5d 4d 6d 6.3 Cold Bending vs. Hot Bending When a bar is bent at room temperature, a mechanical torque is applied directly to the bar resulting in a condition where the outer fibers at the bend are in tension and the inner fibers undergo compression strain and stresses. This type of bending is distinguished as cold bending. The majority of bars are cold bend regardless as for transportation or assembling purposes. These cold bend bars will undergo strain aging and hardening. For hot bending, the bars at the position of bend are preheated to a temperature of 600 to 800 C using an oxygen- acetylene torch or other sources that can induce the same heat level. The heated bar is then bent manually in the same manner as for cold bending. Generally, only low strength non-deformed bars can be hot bent and the required temperature shall be 850 C. Hot bending of deformed bars is restricted. 6.4 Evaluation After bend test the convex surface of the bent specimen is examined for cracks or other open defects, using the unaided eye (without magnification). Cracks occurring in the corners or edge of the bent portion shall not be considered significant unless they exceed the size specified for corner cracks. If no crack size is specified, corner cracks not exceeding the nominal thickness of the specimen shall not be considered a failure. When the test is conducted as an acceptance criterion, the allowable crack size shall be specified by the code or specification requiring the test. Surface irregularities, loss of coating adherence, or other discontinuity developed by the bend test shall be evaluated according to the appropriate product specification. Surface disturbances, such as orange peel, that develop during a bend test, where there is no penetration of the surface, are not considered a crack failure. However, one should keep in mind that upon bending process steel reinforcement may fracture owing to the following reasons: (i) The ribs on steel bars serve as location of stress concentration which is a potential weak point for fracturing. (ii) Owing to their intrinsic high strength, large force is required during the bending process. (iii) The radius of bending is too tight. Temperature is also an important factor for controlling the risk of steel fracture. The risk of fracture is increased when there is a drop of temperature because steel has lower toughness at low temperatures. The bend test report normally contains the following information: 1. Specimen identification, 2. Size and type of specimen, 3. Type of test, 4. Radius used to form the bend, 5. Angle of bend, 6

7 6. Number and size of any visible cracks in the bend, and when the test is conducted for informational purposes, report the size and location of any and all cracks visible to the unaided eye, 7. Whether the specimen passed or failed to meet the requirements. 6.5 Re-bend Test The purpose of re-bend test is to measure the effect of strain ageing on steel. Strain ageing has embrittlement effect which takes place after cold deformation by diffusion of nitrogen in steel. Hence, there is limitation stated in some design codes to restrict the nitrogen content of steel to 0.012%. The test piece shall be bent through an angle of 90 using a mandrel of appropriate diameter, Table 2. The bent piece shall be aged by keeping in boiling water (l00 C) for 1 hours (+15/-0) minutes and then allowed to cool. The heating is necessary to reduce the extra residual stress generated during bending. Otherwise the sample will fail during re-bending. The piece shall then be bent back towards its original shape (partially re-straightened) to an angle of at least 20. The specimen shall be considered to have passed the test if there is no rupture or cracks visible under naked eye on the re-bend position. Table 2: Mandrel diameter for re-bend test. Nominal Size (mm) of Bar, d Mandrel Diameter (mm) for Different Grades of Steel Bars, D 40 Grade 60 Grade 500 W TMT 600 W TMT Up to and including 10 5d 5d 7d 7d Over 10 7d 7d 8d 7d 7