Minimización de defectos superficiales en barras de refuerzo de hormigón

Surface defects in the reinforcing bars of large diameter, were detected. Although these defects did not affect the mechanical properties of the bars, they generated internal claims. So, a thorough study of the casting and rolling process was carried out, including sampling, metallographic work, assessment of billet surface quality, rolling of sand blasted billets, rolling of billet with artificial defects, etc. Many samples of defective bars were taken, and polished specimens were observed in light microscope, before and after etching with several reagents, to characterize the defects and determine their origin. A survey of the billet's surface quality was made, studying in detail the different defects observed after sand blasting. The main problems observed were transverse corner cracks, and less frequent surface porosity (pinholes) and double skin or plating. After the assessment of their surface quality, the billets were rolled to verify the evolution of the defects during rolling. Also, billets with artificially generated defects were rolled, sampled and studied. It was possible to determine three different types of defects in the bars. Long but not frequent cracks in the bars were related to overlapping in the rolling mill. Shorter cracks associated to a shell, were thought to be generated by transverse corner cracks in the billets. Finally, related to accumulation of pin holes in the surface of the billets, short parallel cracks were observed. Following these results, actions were undertaken in the billet caster machine and the rolling mill. The lubrication system and the mould level were modified. Also the pass design for the rolling mill was optimized. These actions led the internal claims in rebar ∅ 36 mm to drop from 15.4 % in 1999, to 0.3 % in 2001.