A Study on the Effect of Heat on Properties of Reinforcing Steel Bars made from Scrap Metal

Abstract

Fatigue performance of ribbed reinforcing bars (rebars) in concrete structures is of great interest to designers. This is because structures are becoming more slender, the traffic volume is increasing, the axel loads are larger, and the traffic speed limits are higher, the margin of reserve strength is progressively being reduced and the loading cycles are becoming more severe. Also the deterioration of mechanical properties of rebars at elevated temperature is of primary concern to the design and analysis of steel structures exposed to fire. However, data on fatigue performance and effect of heat on mechanical properties of rebars made from local metal scrap is unknown. Hence, a study was conducted to investigate fatigue strength and the effect of heat on mechanical properties of rebars made from local scrap metal against published standards. Fatigue was investigated using as-received 12 mm rebar specimens with 370 mm length. Axial load fatigue tests in air were conducted at room temperature using a stress ratio of 0.2 and 25 Hz at maximum stress amplitudes of 132, 136, 140, and 144 MPa until failure occurred using a MTS machine and thereafter S-N curves were plotted. Separately, experiments were conducted to establish the effect of heat on mechanical properties. Eighty four specimens were prepared from 10,12 and 16 mm rebars since these sizes are widely used in Kenya. Six specimens each (two from each diameter size) were heated in an electric furnace to temperatures ranging from 22 to 1000 o C for one hour. At the end of the curing processes, three heated specimens (one per diameter size) were cooled in air while the remaining three were quenched in water for 15 minutes. Thereafter, the changes in mechanical properties (Yield stress, Tensile Strength, Percentage Elongation and Modulus of Elasticity) were determined using a UTM. Brinell hardness testing was performed using a universal Hardness tester, while Charpy-V impact tests was investigated using a Wolpert impact tester. To correlate mechanical properties to microstructural characteristics, metallographic analysis and grain size determination was studied using an optical microscope. Curves of Yield stress, tensile strength, percentage elongation, Modulus of Elasticity and Brinell hardness versus temperature were plotted and compared with the results obtained from the as-received rebars. The results show that the 12 mm rebar had a fatigue life of 1.8 x l0 cycles at a stress amplitude of 132 MPa hence the rebar did not meet the requirements of the standards. Other results show that normal mechanical properties can be assumed after exposure to temperatures below 500 o C for one hour. Yield stress, tensile strength, Modulus of Elasticity, Brinell hardness and ductility of the rebars decreased with air cooling.However, with water quenching after heating from 500 to 1000 C, the Yield stress, tensile strength and Brinell hardness increased while ductility and Modulus of Elasticity decreased. Variation of the microstructure occurred as temperature increased from 22 to 1000 o C, whereby the grain size reduced from 18.9 to 13.7 µm and from 18.9 to 12.0 µm for air and water cooled specimens, respectively. Different rebars sizes and different steel mills showed varied mechanical properties. The 12 and 16 mm rebars exhibited superior impact toughness properties and chemical composition was found not to have a remarkable effect on fatigue and mechanical properties. The higher gripping pressure needed to prevent the rebars from slipping during fatigue testing caused some of the rebars to break in the grips but aluminium tubing was used to protect the gripped ends. The study provides precise information to the steel producers, designers, building industry and finally to the standardization bodies both at the national and international level. The results may also be used to support other research projects aimed at studying the behaviour of rebar steel structures exposed to extreme temperatures. o