Investigation on Characterization and Variability of Mechanical Properties of Reinforcing Steel Bars made from Scrap

Abstract

The quality requirements for concrete reinforcement have increased interest in optimizingthe mechanical properties of reinforcing bars used for the construction of alltypes of structures such as buildings, piers and hydraulic jibs. The variability of mechanicalproperties of reinforcing steel bars manufactured from scrap metals by localmanufacturers in Kenya have been investigated in this research. This was motivatedby the fact that it has been noticed that the use of the substandard reinforcing barsin construction industry could lead to collapse of the structures reinforced with thesebars in many developing countries.Therefore a complete understanding and knowledge of the extent of variability ofmechanical properties and the real behavior of these construction materials was ofprime importance for the proper behavior and integrity of the building structures.To address the above problem, a survey was carried out on a sample of manufacturersof reinforcing steel bars, construction companies and main distributors of steel barsin the country.Some bars were randomly selected from hardwares, rolling mills and other fromoutside the country and a few samples from construction sites. Laboratory tensiletests, chemical composition analysis, microstructure examination, micro hardnesstests and heat treatment were carried out on a set of the bars. The heat treatmentbehavior of reinforcing steel bars was investigated in this study and the results wereevaluated using the microvickers hardness tests.The results were compared with the existing set standards for specified class ofreinforcing steel bars and a statistical model on the variability of the mechanicalproperties of these bars was established and possible sources of the variation wasidentified. The yield strength of the bars sampled from the rolling mills compliedwith standards except in one case.The result show that the yield strength of 69% of reinforcing steel bars collectedfrom distributors failed in yield strength because the mean value for YS was belowthe BS 4449 standard value of 460 N/mm2. The possible cause of variability was theinconsistence in chemical composition. It was found that the twisted bars exhibitedhigher value of yield strength than self tempered ribbed bars. It was also foundthat the grain size have high influence on the tensile properties of the bars. It wasobserved that the yield strength of bars tested decreased with an increase of grainsize. Grain size greater than 28μm resulted in yield strength less than the standardvalue.