Cold-Rolled Ribbed Steel Bar Tensile Testing: Standards, Methods, and Acceptance Requirements

Cold-rolled ribbed steel bar (CRB) tensile testing is the core mechanical evaluation used to verify the proof strength, tensile strength, and elongation of CRB550, CRB600H, CRB650, CRB680H, and CRB800 bars before they are placed in reinforced or prestressed concrete structures. This guide explains the applicable GB/T, ISO, and ASTM standards, specimen preparation, cross-sectional area determination, loading procedures, and acceptance criteria for cold-rolled ribbed steel bar tensile testing performed by accredited third-party laboratories.

Technical Overview

Cold-rolled ribbed steel bars are manufactured by cold-working hot-rolled wire rod through rolling or pressing operations that form transverse or combined transverse and longitudinal ribs on the bar surface. The ribs provide a mechanical bond with surrounding concrete, while the cold-working process increases strength through strain hardening. High-ductility (H) grades receive an additional heat treatment that restores part of the elongation capacity, making them suitable for more demanding structural applications.

The product standard for these bars is GB/T 13788, which defines grade designations, dimensions, and mechanical property requirements. In the grade designation, CRB stands for Cold Rolled Ribbed bar, the number indicates the minimum tensile strength in megapascals (MPa), and the suffix H denotes a high-ductility grade with elevated elongation and maximum-force total elongation requirements.

Because cold-rolled ribbed bars generally exhibit continuous yielding without a pronounced yield plateau, the tensile test focuses on the 0.2% proof strength (Rp0.2), tensile strength (Rm), elongation after fracture (A), and, for high-ductility grades, the maximum-force total elongation (Agt). The test method itself is specified in GB/T 228.1, which is technically aligned with ISO 6892-1. For international projects, comparable requirements can be found in ASTM A1064/A1064M and ASTM A496/A496M for deformed and welded-wire reinforcement.

Testing Samples

Coiled Bars

CRB650, CRB680H, CRB800, and CRB800H bars are typically delivered in coils, particularly when they are drawn as relatively small diameters for prestressed concrete elements such as hollow-core slabs. Test specimens are cut from the coil and carefully straightened. Straightening must be performed by hand or with suitable apparatus in a manner that does not alter the mechanical properties of the bar; heating, machining, or aggressive mechanical straightening is not permitted.

Straight-Length Bars

CRB550 and CRB600H bars for general reinforced concrete use are commonly supplied in straight lengths. Specimens are cut directly from the delivered bars at locations selected according to GB/T 2975 and the sampling provisions of GB/T 13788, ensuring the sample is representative of the lot.

Specimens from Welded Wire Fabric

Where cold-rolled ribbed bars are supplied as welded steel fabric or mesh, individual wires are cut so that the tensile specimen consists of the bar itself. When a weld junction falls within the gauge length, the requirements of the applicable product and fabrication standard govern whether the specimen is tested with or without the cross weld.

Testing Items

A full mechanical evaluation of cold-rolled ribbed bars under GB/T 13788 typically includes the following items:

• Proof strength (Rp0.2): the stress at 0.2% non-proportional plastic extension, used as the yield characteristic because cold-worked bars show continuous yielding.
• Tensile strength (Rm): the maximum force divided by the original cross-sectional area.
• Elongation after fracture (A): measured on a proportional gauge length (commonly 11.3·√S0) for lower grades, or on a fixed 100 mm gauge length for the thinner prestressing grades.
• Maximum-force total elongation (Agt): required for high-ductility H grades to demonstrate uniform deformation capacity.
• Tensile-to-proof strength ratio (Rm/Rp0.2): evaluated where design specifications require a minimum strength ratio for ductility and robustness.
• Complementary mechanical tests: bend and rebend testing per GB/T 232, and 1000-hour stress relaxation testing for prestressed grades, when specified.

Representative minimum requirements from GB/T 13788 include: CRB550 with Rp0.2 of at least 500 MPa, Rm of at least 550 MPa, and elongation of at least 8%; CRB650 with Rp0.2 of at least 585 MPa and Rm of at least 650 MPa; and CRB800 with Rp0.2 of at least 720 MPa and Rm of at least 800 MPa. The H grades (CRB600H, CRB680H, CRB800H) carry corresponding minimum strength values together with stricter elongation and Agt limits. These values are indicative, and the authoritative limits are always those stated in the current edition of the product standard.

Testing Methods

Lot Formation and Sampling

A test lot consists of bars of the same grade, same nominal diameter, and same production process, with lot weights typically limited to about 60 tonnes. From each lot, tensile specimens (and, for CRB550, bend specimens) are selected randomly and symmetrically from different coils or bundles so that the sample represents the entire lot.

Specimen Preparation and Cross-Sectional Area Determination

Tensile specimens are tested in full section without machining. Because the ribbed geometry makes direct measurement of the cross-section unreliable, the original cross-sectional area (S0) is determined by the mass-per-length method: a straightened specimen of measured length, commonly at least 500 mm, is weighed accurately, and S0 is calculated as mass divided by length and by the steel density of 7850 kg/m³. The gauge length is then marked on the specimen, and the free length between grips must exceed the gauge length by a sufficient margin, commonly at least 50 mm.

Equipment Requirements

Testing is performed on a universal testing machine verified to Class 1 or better in accordance with ISO 7500-1 or GB/T 16825.1. Proof strength and Agt determination require an extensometer of Class 1 or better per ISO 9513 or GB/T 12160, attached within the gauge length and, for Agt, monitored up to the point of maximum force. The test is normally conducted at ambient temperatures between 10 °C and 35 °C.

Loading and Speed Control

During the elastic range up to the specified proof strength, the stress rate is controlled within the range of 6 to 60 MPa/s as required by GB/T 228.1 for steels with an elastic modulus of at least 150 GPa. After the proof strength has been determined, loading continues at a constant strain rate or an equivalent constant crosshead separation rate within the limits of the standard until fracture. Grip pressure and alignment are adjusted to prevent slippage and premature failure at the jaws.

Result Calculation, Rounding, and Retest Rules

Proof strength, tensile strength, elongation, and Agt are calculated and rounded in accordance with GB/T 228.1 and the rounding rules of GB/T 8170, with strength values commonly reported to the nearest 1 or 5 MPa. If any single specimen fails to meet a requirement, re-inspection follows the rules of the product standard: normally, double the original number of specimens is taken from the same lot and retested for the failed item only, and the lot is accepted only if all retest specimens conform. Failure by brittle fracture at a grip, or evidence of improper specimen preparation, is grounds for a replacement test.

Applications

Tensile testing of cold-rolled ribbed bars supports quality control and acceptance across a wide range of construction uses:

• Cast-in-place reinforced concrete floor slabs, walls, and stairs in residential and commercial buildings.
• Prefabricated components such as precast slabs, pipes, culverts, manholes, and small hollow sections.
• Welded steel fabric and reinforcement mesh for road pavements, airport runways, tunnel linings, and flooring systems.
• Prestressed concrete elements, including hollow-core slabs, poles, piles, and sleepers, using CRB650, CRB680H, and CRB800 grades.
• Masonry reinforcement, mesh for shotcrete and slope stabilization, and general secondary structural elements.

Application guidance for design and construction is provided in the Chinese technical specification for concrete structures with cold-rolled ribbed bars (JGJ 95), while international projects frequently reference ASTM A1064/A1064M or equivalent EN standards for welded and deformed reinforcement products.

FAQ

Why is Rp0.2 used instead of upper and lower yield strength?

Cold rolling strain-hardens the steel, so the stress-strain curve rises continuously without a distinct yield plateau. The 0.2% proof strength, determined with an extensometer, is therefore the standard yield characteristic for these bars.

How is the cross-sectional area of a ribbed bar determined?

Because the ribbed profile prevents accurate direct measurement, the area is calculated from the mass of a measured straight length divided by the steel density of 7850 kg/m³, as permitted by GB/T 228.1 for specimens of uniform full section.

How many specimens are tested per lot?

Under typical GB/T 13788 practice, a lot is limited to about 60 tonnes of the same grade and diameter, and tensile specimens are taken from each lot; bend specimens are added for CRB550. Project specifications or the client may require higher sampling frequencies.

Can specimens be straightened after being cut from a coil?

Yes, but only by methods that do not change the mechanical properties. Careful hand straightening or purpose-designed apparatus is acceptable, while heating or any treatment that alters the cold-worked structure is prohibited.

What happens if one specimen fails the requirement?

The re-inspection rules of the product standard apply: normally twice the number of specimens is drawn from the same lot and retested for the failed item, and acceptance requires all retest results to conform.

Which standards govern the test method?

GB/T 13788 specifies the product requirements, GB/T 228.1 (aligned with ISO 6892-1) specifies the tensile test method, and GB/T 2975 governs sample selection and preparation. Comparable international product requirements appear in ASTM A1064/A1064M and ASTM A496/A496M.