Field Experience and Lessons Learned in Bending High-Strength Steel Reinforcement
High-strength steel reinforcement in new concrete structures is becoming more commonplace in practice. This reinforcement offers equivalent strength with less cross sectional area, which helps mitigate concrete consolidation concerns in highly congested regions. ACI 318-14 (2014) presently limits its applicability, as research is ongoing. High-strength reinforcement use requires verification that design and structural behavior assumptions embedded in the code equations remain valid for the higher steel grades. One important attribute of using high-strength reinforcement is its ability to be fabricated and bent into the shapes necessary for placement in a structure. The material must have enough ductility to be “cold-bent” without bar damage. ACI minimum bend diameters must be verified for a range of bend angles and pin diameters, and process changes may be required, as high-strength steel requires greater forces to bend. To achieve higher strengths, steel usually sacrifices ductility, and potentially fracture toughness, given the alloying elements and production process used to produce the reinforcement. This paper provides a case study regarding use of high-strength reinforcement in practice. A massive, reinforced concrete foundation element was designed with Grade 80 (552 MPa) reinforcement. These high-strength bars were bent into many different configurations. During cold weather construction, several #11 (~#35M) bars fractured at bends during normal bar placement. The author’s investigation identified the material properties, chemistry, bending procedures, and fabrication temperatures all contributed to the fracture cause.This paper analyzes the investigation into the fracture cause, including review and observation of the bar bending processes; conducting metallurgical examinations and bar mechanical testing; review of industry standards and literature; discusses the quality control/quality assurance testing protocol for the refabrication; and presents general conclusions.
Forensic Engineering 2018