IN fire situations, the loss of strength for rebars at elevated

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1 Effect of Heat on Mechanical Properties and Microstructure of Reinforcing Steel Bars made from Local Scrap J. O. Bangi, S. M. Maranga, S. P. Ng ang a and S. M. Mutuli Abstract Mechanical properties of reinforcing steel bars (rebars) degrade with elevated temperatures and this deterioration has to be properly accounted for to understand the behavior of rebars and composite structures in fire. In this study, the effect of heat on the mechanical properties and microstructures of rebars made from local scrap was studied. Rebar Samples of 10, 12, and 16 mm in diameter and 450 mm length were prepared using a power saw from samples purchased from the local steel factories and the spectrometry analysis was carried out. Seven specimens of the above types of rebars were heated in an electric furnace to temperatures ranging from 100 to 1000 o C for one hour and then cooled in air to room temperature. Thereafter, their residual mechanical properties (Yield Strength, Ultimate Tensile Strength, Percentage Elongation, and Young s Modulus) were determined using a 1MN Universal Testing Machine. Brinell hardness testing was performed using the Universal Hardness Tester and Metallographic analysis and grain size determination was studied by an Optical Microscope with an inbuilt camera to correlate mechanical properties to the microstructure. Results showed that normal mechanical properties and microstructure can be assumed after exposure to temperatures up to 500 o C for one hour except for ductility and Young s Modulus. For higher temperatures, the retained yield stress as a proportion of normal bar properties after exposure times of one hour was as follows: o C - 0.9, 900 o C - 0.7, 1000 o C Variation of the microstructure occurred above 500 o C, whereby the grain size reduced from 17.3 to 12 µm when the temperature increased from 500 to 1000 o C. The study provides precise information on mechanical properties and microstructure of rebars to steel producers, designers, building industry, and standardization bodies. The results may also be used to support other research projects aimed at studying the behavior of rebar steels structures in fire. Keywords Elevated temperatures; Fire; Mechanical properties; Microstructure; Reinforcing steel Nomenclature The following symbols are used in this paper MN Mega Newton R e Yield Strength (MPa) R m Ultimate Tensile Strength (MPa) R m/r e Ratio of Ultimate Tensile Strength / Yield Strength d n Grain Size WTC World Trade Centre J.O. Bangi, Department of Mechanical Engineering, JKUAT Corresponding author, ( bangij@kebs.org), Tel.:(+254) S.M. Maranga, Department of Mechanical Engineering, JKUAT ( smmaranga@yahoo.com) S.P. Ng ang a, Department of Mechanical Engineering, JKUAT ( spnganga@eng.jkuat.ac.ke) S. M. Mutuli, Department of Mechanical Engineering, University of Nairobi ( mutulis1@gmail.com) ISSN I. INTRODUCTION IN fire situations, the loss of strength for rebars at elevated temperatures may be significant and design requirements for fire are covered in Section 5 of AS3600 for rebars [1]. The yield strength of steel is reduced to about half at 550 o C. At 1000 o C, the yield strength is 10 percent or less [2]. Near-total depletion of strength occurs at approximately 1,204 o C [3]. Because of its high thermal conductivity, the temperature of unprotected internal steelwork normally will vary little from that of the fire [2]. Young s Modulus does not decrease with temperature as rapidly as does yield strength [2]. Cold-worked reinforced bars, when heated, lose their strength more rapidly than do hot-rolled high-yield bars and mild-steel bars. The differences in properties are even more important after heating. The original yield stress is almost completely recovered on cooling from a temperature of 500 to 600 o C for all bars but on cooling from 800 o C, it is reduced by 30 percent for cold-worked bars and by 5 percent for hotrolled bars [2]. The loss of strength for prestressing steels occurs at lower stressing temperatures than that for rebars [2]. Cold-drawn and heat-treated steels lose a part of their strength permanently when heated to temperatures in excess of about 300 o C and 400 o C, respectively [2]. Under fire conditions, the temperatures in the steel will increase, resulting in both thermal expansion of the member and transient deterioration of its mechanical properties. The magnitude of these effects depends upon several factors, including the composition of the steel, the sizes and shapes of the parts and whether it was protected or not and the duration and nature of the fire [4]. These are important considerations if heat has been applied to assist bending or if the rebars have been subjected to a fire. If the duration and the intensity of the fire are large enough, the load bearing resistance can fall to the level of the applied load resulting in the collapse of the structure [5]. Thus, load bearing of steel decreases when steel or composite structure is subjected to a fire action. However, the failure of the World Trade Centre on 11th September 2001 and, in particular, of building WTC7 alerted the engineering profession to the possibility of connection failure under fire conditions [6]. In addition, grain size has a measurable effect on most mechanical properties. For example, at room temperature, hardness, yield strength, tensile strength, fatigue strength and impact strength all increase with decreasing grain size [7]. Machinability is also affected; rough machining favors coarse grain size while finish machining favors fine grain size. The

2 effect of grain size is greatest on properties that are related to the early stages of deformation. Thus, for example, yield stress is more dependent on grain size than tensile strength [7]. Fine-grain steels do not harden quite as deeply and have less tendency to crack than coarse-grain steels of similar analysis. Also, fine-grain steels have greater fatigue resistance, and a fine grain size promotes a somewhat greater toughness and shock resistance. Cold working frequently alters grain size by promoting more rapid coarsening of the grains in critically stressed areas. The original grain size characteristics, however, can usually be restored by stress relieving. Coarsegrain steels have better creep and stress rupture properties because diffusion at high temperatures is impeded by sub grain low-angle boundaries present in coarse-grain steels [7]. Hence the response in mechanical properties of different structural steel grades at elevated temperatures should be well known in order to understand the behaviour of steel and composite structures when subjected to fire. To study thoroughly the behaviour of certain steel structures at elevated temperatures, one should use the material data of the used steel material obtained by testing. In this study, 10, 12, and 16 mm ribbed steel rebars were subjected to seven different temperatures to determine the high temperature behaviour of the rebars. This study was aimed at investigating the effect of heat on the mechanical properties and microstructure of rebars made from local scrap and heated to temperatures ranging from 100 o C to 1000 o C for one hour and then cooled in air to room temperature. II. MATERIALS AND METHODS The 10, 12 and 16 mm diameter and 12 m length rebar steel samples used for this study were from Athi-River Steel ltd and Apex steel ltd, Kenya. The chemical composition of the as received and elevated temperature samples of the 16 mm rebar was determined as given in Tables 1. Seven specimens each from the different diameter rebar steels were heated in an electric furnace for one hour and then cooled in air to room temperature. The furnace temperature was initially raised and when the desired level was attained, specimens were put into the heating chamber. To simulate temperatures likely to be experienced by the rebar steel during a fire, the rebar steels were heated to 100, 300, 500, 600, 900 and 1000 o C. and a computer connected to a data logger. Load-elongation data were recorded and converted into stress-strain graphs. The residual yield strength, Ultimate tensile strength, Young s modulus and Ductility (% elongation) were measured for the as-received materials and for the samples after a complete cycle of heating and air cooling. B. Grain size and Microstructure Examination The microstructure examination of the rebar steel in the as-received state and the heated and air cooled samples were examined using an optical microscopy as per the specifications in ASTM E3-01 [11] and ASTM E [12]. Each sample was carefully ground progressively on emery paper in decreasing coarseness. The grinding surfaces of the samples were polished using Al203 carried on a micro cloth. The crystalline structure of the specimens was made visible by etching using a solution containing 2% Nitric acid and 98% methylated spirit on the polished surfaces. Microscopic examination of the etched surface of various specimens was undertaken using a metallurgical microscope with an inbuilt camera through which the resulting microstructure of the samples were all photographically recorded with a magnification of 400. The average grain size measurement was done when ferrite grains, pearlite nodes, granular bainite and individual martensite are considered as separated grains. At the same time, the relative quantities of ferrite, pearlite, bainite, and martensite were assessed. III. EXPERIMENTAL RESULTS AND DISCUSSION A. Effect of Heat on Mechanical Properties The mechanical properties of the as-received and the heated samples for 10, 12, and 16 mm rebars are shown in Tables 2-4 respectively. The effects of the elevated temperature on the mechanical properties (Yield strength, Ultimate Tensile Strength, Percentage Elongation, Young s Modulus, and Brinell Hardness) of the air cooled samples are shown in Fig. 1-5 respectively. TABLE 1: Chemical Composition of 16 mm rebar (wt.%) A. Determination of Mechanical Properties Mechanical properties of the as-received, heated and air cooled samples were determined using standard methods. For hardness testing, oxide layers formed during heating were removed by stage-grinding and polishing. Average Brinell Hardness Number (BHN) readings were determined by taking three hardness readings at different positions on the samples, using a Brinell hardness tester as per ASTM E10 [8]. Tensile tests were carried out on samples according to ISO [9]. The tensile machine used was a Servo hydraulic universal testing machine of 1MN capacity. The tensile machine was calibrated as per ISO [10] before use. For each size of rebar steel and after each heating, two tensile tests were carried out. The values were recorded using the machine s own plotter ISSN

3 TABLE 2: Mechanical Properties of 10 mm rebar TABLE 4: Mechanical Properties of 16 mm rebar TABLE 3: Mechanical Properties of 12 mm rebar B. Yield Strength (R e ) Yield strength of 10, 12 and 16 mm rebars was affected by the elevated exposure temperatures. It can be seen in Fig. 1 that there is no significant variation in yield strength of rebars cooled by air up to 500 o C. Plain rebars have experienced the strain hardening already for this temperature. Composition is, however, not found to have a remarkable effect on the critical temperature above which the residual yield strength becomes lower than the standard allowed value of 460 MPa for high yield steel rebars [13]. This temperature lies between 500 and 600 o C for all rebars examined. According to Eurocode 3 [14], below 400 o C, there is no decrease in yield strength, but above this temperature a significant yield strength loss occurs. The yield strength losses for 10, 12 and 16 mm rebars were 25%, 12% and 38% for 900 o C exposure temperature, respectively when compared to that of the as received rebars. For further increase of temperature up to 1000 o C, yield strength decreased by 37%, 18% and 45%, respectively for rebar 10, 12, and 16 mm. Fig. 1: Yield strength of steel rebars against temperature. C. Tensile Strength (R m ) From Fig. 2, it is observed that the tensile strength remains high up to 500 o C for all types of rebars, after which it begins to drop. After 500 o C, the tensile strength left is lower than the tensile strength of the as received rebars. The tensile strength losses for the 10, 12 and 16 mm rebars were 19%, 14% and 26% for 900 o C exposure temperature, respectively when compared to that of the as received rebars. For the highest exposure temperature at 1000 o C, tensile strength decreases were 17%, 12% and 28%, respectively when compared to the as received rebars. However, it should be noted that there is a possibility of complete strength loss of rebars at high temperatures when a structure is subjected to a huge fire. Consequently, the remaining strength of the rebars in structures is influenced by the exposure time and type of fire and will depend on the rate of heat transfer through the concrete cover to steel reinforcement [15]. Fig. 2: Tensile strength of steel rebars against temperature. D. Elongation After the rebars cooled to room temperature, the retained ductility (elongation) after heating was measured as a ratio of the original room temperature elongation and the relationship is as shown in Fig. 3. The 10, 12 and 16 mm rebars showed similar elongation behaviour under elevated temperatures. The elongation decreased with increasing temperature until a minimum value was reached at 300 o C. At 600 o C, ductility increased and the exact effect depended on the diameter of rebar. The initial decrease in ductility was caused by strain aging and was most pronounced in the temperature range of 150 to 370 o C. The decrease in elongation for the rebars was determined from the ratio of elongation at elevated temperatures to that of the as received at normal room temperature (22 o C ) and for 10, 12 and 16 mm rebars, it was found to be 16%, 23% and 43% for 900 o C exposure temperature, respectively. For further increase of temperature to 1000 o C, elongation ratio decreases ISSN

4 were 36%, 30% and an increase of 45%, respectively. Another relevant expression of ductility involves a stress ratio between the ultimate tensile strength R m and the yield stress R e of the reinforcing bar (R m /R e ) [16]. The R m /R e ratio deduced from the stress-strain diagram could be used as an indirect means to express the extent of uniform elongation before fracture, i.e. the elongation up to the ultimate tensile strength. In all cases it is higher than the minimum allowed value of Fig. 5: Brinell hardness of steel rebars against temperature. Fig. 3: Elongation ratios of steel rebars against temperature. E. Young s Modulus The values of the Young s modulus of elasticity were calculated from the tensile strength and strain values and the graphs are presented in Fig.4. It can be seen that Young s modulus of elasticity decreased with an increase in the temperature above 400 o C. The relationship shown is nearly the same for all rebars steels. Youngs modulus does not decrease with temperature as rapidly as does yield strength [2]. m G. Effect of Heat on Microstructure The as received rebar shows a microstructure with a combination of ferrite (white) and pearlite (black) as shown in the micro graphs in Fig.6. The microstructures of the samples heated to 100, 500, 600, 900 and 1000 o C are shown Fig respectively. No significant microstructural changes occurred after heating the rebar up to 300 o C. However, a slight increase of hardness in this temperature range indicates that strain aging phenomena had occurred. Heating further up to 600 o C Fig.9 causes only recovery phenomena, as it is certified by a constant drop of hardness without remarkable microstructural changes, Its worth noting that, the degree of work hardening (approx. 2%) is not sufficient to cause recrystallization. At 900 o C, (Fig. 10) the deformed structure was fully homogenised and during the slow cooling from austenizing range to room temperature the final microstructure consisted of fine ferrite grains in which the pearlite was more uniformly distributed. The grain size of the rebars was comparatively small and the d n values for specimens was noted to decrease from 17.3 to 12 µm as the temperature increased from 500 to 1000 o C shown in Table 4. Fig. 4: Young s modulus of steel rebars against temperature. F. Brinell Hardness The results of Brinell hardness measurements are shown in Fig. 5. The precipitation phenomena occured at temperatures up to approximately 500 o C and are accompanied initially by a slight increase of hardness up to 300 o C, which thereafter becomes constant up to 500 o C. The hardness values of 12 mm rebar were slightly higher than the corresponding values of 10, and 16 mm rebars at temperatures from 100 up to 900 o C. The difference could presumably be attributed to their different chemical compositions though futher work on the rebar sizes may be necessary to verify this observation. ISSN Fig. 6: Optical micrographs of as received rebar (x400) Fig. 7: Optical micrographs of rebar heated to 100 o C (x400)

5 Fig. 8: Microstructure of rebar heated to 500 o C (x400) The effect of heating the rebars to temperatures from 500 o C to 1000 o C, was a corresponding decrease in grain size from 17.3 to 12 µm. The chemical Composition was found not to have a remarkable effect on the critical temperature above which the residual Yield strength becomes lower than the standard allowed value of 460 MPa. This temperature lies between 500 and 600 o C for all rebars examined. The load-carrying capacity of the rebars decreases with the increase of temperature. The residual capacity after cooling depends on the Yield strength of the rebar, temperature reached, and the degree of deformation. Fig. 9: Microstructure of rebar heated to 600 o C (x400) IV. CONCLUSIONS In this study, the effect of heat on mechanical properties and the microstructure of 10, 12 and 16 mm rebars made from local scrap were investigated after they were heated to 100, 300, 500, 600, 900 and 1000 o C for one hour and cooled normally in air to room temperature. From the results of this study, the following conclusions were made: The rebars sampled showed stable mechanical properties and microstructure after heating up to 500 o C, hence normal bar properties can be assumed after exposure to temperatures up to 500 o C for one hour. For higher temperatures, the retained yield Strength as a proportion of normal rebar properties was: o C - 0.9, 900 o C - 0.7, 1000 o C Yield strength, Tensile strength, Young s modulus, Brinell hardness, and Ductility of the rebars decreased as the heating temperature increased from 500 to 1000 o C. The weakening of these mechanical properties is attributed mainly to the extensive tempering occurring above this temperature. Fig. 10: Microstructure of rebar heated to 900 o C (x400) Fig. 11: Microstructure of rebar heated to 1000 o C (x400) ACKNOWLEDGMENT The authors wish to acknowledge Kenya Bureau of Standards and the University of Nairobi for providing the testing facilities for this work. Athi-River steel and Apex Steel ltd are also acknowledged for supplying rebars used for the experiments detailed in this paper. REFERENCES [1] AS3600, Australian Standard for Concrete Structures, Sydney Austria, 2009 [2] ASHI reporter, The Effects of Fire on Structural Systems, American Society of Home Inspectors, 2007 [3] American Institute of Steel Construction, Inc, Manual of Steel Construction, Facts for Steel Buildings-Fire, 2003, AISC, Chicago, IL, 2003 [4] Rassizadehghani, J., Raygan, Sh., and Askari, M, Comparison of the quenching capacities of hot salt and oil baths, Metal Sci. Heat Treat, vol. 48, pp. 5-6, 2006 [5] Topcu, I. B., Karakurt, C., Properties of Reinforced Concrete Steel Rebars Exposed to High Temperatures, Research Letters in Materials Science, vol. 2008, Article ID , 4 pages, 2008 [6] Wald, F., Da Silva, L. S., Moore, D. B., Lennond, T., Chladna, M., Santiagob, A.,Benes, M., Borgesf, M., Experimental behaviour of a steel structure under natural fire,, Fire Safety Journal, vol. 41, no. 7, pp , 2006 [7] Herring, D. H., Grain Size and Its Influence on Materials Properties, GrainSize.pdf, 2005 [8] ASTM E10-12 Standard Test Method for Brinell Hardness of Metallic Materials, Brinell,hardness, mechanical test, metals, Brinell hardness, Metallic hardness, Anual book of ASTM standards, ASTM International, West Conshohocken, PA, 2012 [9] ISO , Metallic materials-tensile testing-part 1: Method of test at room temperature, International Organization for Standardization, Geneva, Switzerland, 2009 [10] ISO , Metallic materials - Verification of static uniaxial testing machines - Part 1: Tension/compression testing machines -Verification and calibration of the force-measuring system, International Organization for Standardization, Geneva, Switzerland, 2004 [11] ASTM E3-11, Standard Guide for Preparation of Metallographic Specimens, Anual book of ASTM standards, ASTM International, West Conshohocken, PA,, 2011 [12] ASTM E 45, Test Methods for Determining the Inclusion Content of Steel, Anual book of ASTM standards, ASTM International, West Conshohocken, PA, 2005 [13] KS 573, High yield steel bars for the reinforcement of concrete- Specification, 2008 [14] CEN, Eurocode 3, pren :, Part 1.2: Structural Fire Design, Eurocode 3: Design of steel structures, Stage 49 draft, CEN, European Committee for Standardization, Brussels,2003 [15] Topcu, I. B. and Isikdag, B., The effect of cover thickness on rebars exposed to elevated temperatures, Construction and Building Materials. In press. [16] Nikolaou, J., Papadimitriou, G. D., Microstructures and mechanical properties after heating of reinforcing 500 MPa class weldable steels produced by various processes, Construction and Building Materials, vol. 18, pp , 2004 ISSN