Steel Rebar Tensile Testing: Standards, Equipment, and Key Parameters

Why Test Rebar?

Reinforcing steel bars (rebar) are the primary tensile element in reinforced concrete structures. Failure to meet minimum yield strength, tensile strength, or elongation can result in brittle structural failure under seismic or overload conditions. Tensile testing of rebar is mandatory under virtually all structural codes.

Key Parameters Measured

• Yield Strength (Re or Fy): The stress at which permanent deformation begins. For Grade 420 rebar: minimum 420 MPa.
• Tensile Strength (Rm or Fu): Maximum stress before fracture. ASTM A615 Grade 420 requires minimum 620 MPa.
• Elongation (A%): Percentage increase in gauge length at fracture. Indicates ductility.
• Yield Ratio (Re/Rm): Required by seismic codes to be ≤ 0.85–0.90, ensuring ductile overstrength.

Standards Comparison

Testing Machine Requirements

• Capacity: For common rebar diameters (Ø8–Ø32 mm), a 600 kN electromechanical or servo-hydraulic universal testing machine is typical. Heavy bars (Ø40+) may require 1000 kN.
• Grips: Wedge-action rebar grips that accommodate the deformed bar surface without slipping. Self-aligning grips prevent bending loads.
• Extensometer: A clip-on extensometer (gauge length typically 5d or 10d) measures yield point and elongation accurately. Required for ISO 6892-1 Method A (strain rate controlled).
• Control: ISO 6892-1 specifies two methods — Method A (strain rate control, more accurate) and Method B (stress rate control, simpler but less reproducible at yield).