Deformed reinforcing bars (rebar) are essential for concrete structural integrity, providing the tensile strength and ductility that concrete inherently lacks. Ensuring the quality of rebar procurement is directly linked to life safety. Key performance indicators are the Yield Strength (Re) and Percentage Elongation, which govern the steel’s load-bearing capacity and its ability to absorb energy before catastrophic failure.
Critical Mechanical Properties and Standards
1. Yield Strength (Re)
Yield strength is the maximum stress a rebar can withstand before permanent deformation occurs. In structural engineering, this value is the basis for all load calculations.
- Standard Grades: Common standards include ASTM A615 (Grade 60 equivalent to 420 MPa minimum yield) and higher strength options like Grade 75 (520 MPa). European standard B500B specifies 500 MPa minimum yield strength.
- Testing Protocol: Yield strength is determined via a uniaxial tensile test (e.g., ASTM A370 or ISO 6892-1). The measured yield strength must meet or exceed the specified minimum value of the purchased grade, although excessive overstrength can reduce ductility.
- B2B Insight: Procurement specifications must clearly denote the required grade and the specific testing standard the supplier must adhere to, as slight variations exist between national standards.
2. Percentage Elongation (Ductility)
Elongation is the steel’s ability to stretch before fracturing, measured as a percentage of the original gauge length. Ductility is paramount for seismic zones, as it allows the structure to yield and absorb energy during an earthquake without instantaneous collapse.
- Total Elongation at Fracture (A): Measures the overall stretch. For typical seismic-grade rebar (such as ASTM A706), minimum elongation requirements are high (e.g., 14% for smaller sizes) to ensure adequate performance under dynamic loads.
- Uniform Elongation (Agt): Measures the percentage elongation at maximum force (tensile strength). This is a precise indicator of the material’s plasticity before necking begins. Higher minimum Agt values are mandatory in strict seismic design codes.
- The Yield-to-Tensile Ratio: Many codes limit the ratio of Yield Strength to Tensile Strength (Re/Rm). A ratio too close to 1.0 indicates low strain-hardening capacity and poor ductility, which is undesirable for safety-critical structures.
Quality Verification and Procurement Due Diligence
Procurement departments must establish rigorous inbound QC procedures for rebar due to its critical role:
- Mandatory MTRs (Material Test Reports): Suppliers must furnish EN 10204 Type 3.1 MTRs for every shipment, verifying that the actual Re and elongation results meet the project specification’s minimum values.
- Verification of Rib Configuration: Beyond strength, the geometry of the deformations (ribs) must comply with standards to ensure adequate bonding with the concrete. Non-compliant spacing or height compromises the integrity of the reinforced section.
- Chemical Composition Control: For critical welding on site, the Carbon Equivalent Value (CEV) must be reported and kept below maximum limits (e.g., 0.55% for Grade 60) to ensure predictable weldability without specialized procedures.
