Cold-rolled ribbed steel bar (CRB) tensile testing is the essential mechanical evaluation used to verify that this reinforcement meets the strength and ductility requirements of GB/T 13788, GB/T 228.1 and ISO 6892-1 before it is used in concrete structures. This guide explains the key measured parameters—yield strength (Rp0.2), tensile strength (Rm), elongation and the tensile-to-yield ratio—along with specimen preparation, testing procedures, applicable grades and typical applications. Whether you are a manufacturer, contractor or quality engineer, understanding CRB tensile testing helps ensure compliant, safe and durable reinforced concrete construction.
Technical Overview
Cold-rolled ribbed steel bars are produced by cold-rolling low-carbon steel wire rod at room temperature, forming two or three transverse ribs (or a helical rib pattern) on the bar surface. The cold-working process substantially increases strength, while the rib geometry significantly improves mechanical bonding with surrounding concrete. Because cold working raises strength at the cost of ductility, tensile testing is the decisive acceptance test for every production batch.
The product standard GB/T 13788 defines the common grades, including CRB550, CRB600H, CRB650, CRB680H, CRB800 and CRB800H. The "H" suffix designates high-ductility grades produced with a controlled tempering process. CRB550 and CRB600H are used as ordinary (non-prestressed) reinforcement, typically in diameters from 4 mm to 12 mm, while CRB650 and higher grades serve prestressed concrete applications.
The tensile test itself is performed in accordance with GB/T 228.1, which is technically aligned with ISO 6892-1 (Metallic Materials — Tensile Testing — Part 1: Method of Test at Room Temperature). In North America, cold-rolled deformed wire reinforcement is covered by ASTM A1064/A1064M, and in Europe the general framework of EN 10080 applies to weldable reinforcing steel. Testing is carried out by manufacturer quality-control laboratories and by accredited third-party laboratories for independent verification, factory acceptance and export certification.
Testing Samples
Specimen Form and Dimensions
Tensile specimens for cold-rolled ribbed bars must be tested in full cross-section. Machining or removal of the ribs is not permitted, because the cold-worked surface layer and the rib geometry carry a significant portion of the load and are part of the engineering cross-section. The specimen length must provide the required original gauge length plus sufficient material for the gripping zones; a total length of approximately 400 mm to 600 mm is typical depending on bar diameter.
Sampling Plan
- An inspection lot generally consists of bars of the same grade, same nominal diameter and same production conditions, with a maximum lot size of 60 tonnes.
- Samples are drawn at random from different coils within the lot, usually from the outer end of each coil selected.
- Samples must be straightened carefully by hand or with a gentle mechanical method; heating, hammering or any cold working that alters the mechanical properties is prohibited.
- Specimens must be free of visible surface defects, kinks or bending damage before testing.
Cross-Sectional Area and Gauge Length Preparation
Because the rib geometry prevents accurate direct measurement of diameter, the original cross-sectional area S0 is determined from the nominal mass per metre divided by the density of steel (7,850 kg/m³), in accordance with the product standard. The original gauge length L0 is then marked on the specimen using punched or scribed marks with closely spaced subdivisions to facilitate post-fracture measurement. Depending on the grade, the specified gauge length is typically 10d for ordinary grades, 5d for high-ductility grades, or 100 mm for high-strength prestressed grades.
Testing Items
The tensile test evaluates the following characteristic parameters:
- Proof strength, Rp0.2: the stress at 0.2% non-proportional extension, used as the yield strength because cold-rolled bars exhibit continuous yielding without a distinct yield plateau.
- Tensile strength, Rm: the maximum force divided by the original cross-sectional area.
- Tensile-to-yield ratio, Rm/Rp0.2: a ductility reserve indicator, typically required to be at least 1.03.
- Percentage elongation after fracture, A: measured on the specified gauge length after carefully butting the fractured ends.
- Total elongation at maximum force, Agt: a mandatory requirement for high-ductility "H" grades.
Representative acceptance criteria under GB/T 13788 include:
- CRB550: Rp0.2 ≥ 500 MPa, Rm ≥ 550 MPa, elongation ≥ 8.0%, Rm/Rp0.2 ≥ 1.03.
- CRB600H: Rp0.2 ≥ 540 MPa, Rm ≥ 600 MPa, elongation ≥ 8.0%, Agt ≥ 2.0%.
- CRB650: Rp0.2 ≥ 585 MPa, Rm ≥ 650 MPa, elongation on 100 mm gauge ≥ 4.0%.
- CRB680H: Rp0.2 ≥ 600 MPa, Rm ≥ 680 MPa, elongation ≥ 7.0%, Agt ≥ 2.5%.
The applicable edition of the product standard should always be consulted, as specific values may be revised.
Testing Methods
Equipment and Calibration
- A universal tensile testing machine verified to accuracy Class 1 or better in accordance with ISO 7500-1 or GB/T 16825.1.
- An extensometer of Class 1 or better per ISO 9513 or GB/T 12160, attached to the gauge length for determining Rp0.2 and, where required, Agt.
- Ambient test temperature between 10 °C and 35 °C unless otherwise specified.
Test Procedure
The specimen is mounted in the grips with careful axial alignment to eliminate bending stresses, and a small pre-load is applied to seat the specimen. Loading is controlled using Method A of GB/T 228.1 / ISO 6892-1: a stress rate in the elastic range (typically within 6 to 60 MPa/s) followed by closed-loop strain-rate control (typically 0.00025 s⁻¹ to 0.0025 s⁻¹) through proof-strength determination and the plastic range up to maximum force.
Rp0.2 is determined graphically or electronically as the stress corresponding to 0.2% non-proportional extension from the recorded force-extension curve. Loading continues to maximum force, where Rm is calculated as Fm divided by S0. For Agt, the extensometer remains attached until maximum force has passed. After fracture, the two ends are fitted together and the final gauge length Lu is measured; elongation after fracture is calculated as A = (Lu − L0)/L0 × 100%.
Data Evaluation and Retest Rules
Results are rounded in accordance with GB/T 8170, normally to the nearest 5 MPa, and compared against the specification minima for the declared grade. A test is invalid if fracture occurs outside the gauge length, immediately adjacent to the gauge marks, or within the grips; in such cases a retest on a replacement specimen from the same lot is performed. If any lot fails, double sampling and retesting are conducted as specified in the product standard before final rejection. The test report should state the grade and diameter, S0, L0, all measured values, the accuracy classes of the machine and extensometer, and the test temperature.
Applications
- Cast-in-place concrete structures: floor slabs, roof slabs, shear walls and staircases in residential and commercial buildings.
- Welded steel wire reinforcement mesh: CRB550 and CRB600H bars are the dominant raw material for prefabricated mesh panels used in slabs, pavement and tunnels.
- Prestressed concrete elements: CRB650, CRB680H and CRB800 grades are used in hollow-core slabs, precast panels, poles, pipes and other precast components.
- Municipal and infrastructure works: road pavements, drainage channels, culverts and small bridge components.
- Quality control and compliance: incoming material inspection by contractors and supervision engineers, production control by manufacturers, and independent verification by accredited third-party laboratories for project acceptance, dispute resolution and export documentation.
FAQ
Why is Rp0.2 used instead of a yield point for CRB bars? Cold rolling eliminates the distinct yield plateau of mild steel, producing a continuously rising stress–strain curve. The 0.2% proof strength is therefore the standardized measure of yield behavior for these bars.
Can specimens be machined to remove the ribs? No. The standards require full-section specimens because the ribs and cold-worked surface layer contribute to load capacity; machining would overstate the true performance of the as-delivered bar.
How is the cross-sectional area determined? Because direct measurement across ribs is unreliable, S0 is calculated from the nominal mass per metre divided by 7,850 kg/m³, as prescribed by the product standard.
What happens if the bar fractures outside the gauge length? The elongation result is invalid and a retest is performed on a new specimen from the same lot, since fracture position affects the measured ductility.
What is the difference between CRB550 and CRB600H? CRB600H is a high-ductility grade produced with a controlled tempering process; it offers higher strength than CRB550 and must additionally satisfy a stricter total elongation at maximum force (Agt), improving robustness in structural design.
Which standards govern the test? GB/T 13788 specifies the product and acceptance requirements, GB/T 228.1 and ISO 6892-1 specify the test method, while ASTM A1064/A1064M and EN 10080 provide comparable frameworks for deformed wire reinforcement in other markets.