Cold-Rolled Ribbed Bar Mechanical Testing: Standards, Samples, Methods and Acceptance Criteria

Cold-rolled ribbed bar mechanical testing evaluates the tensile strength, elongation, bend performance and stress relaxation behavior of CRB reinforcement steel widely used in concrete structures. Testing performed in accordance with GB/T 13788, GB/T 228.1 and related ISO standards verifies that CRB550, CRB600H and prestressed CRB650 to CRB970 grades satisfy the strength and ductility requirements for safe structural application. This article explains sample preparation, testing items, test methods, equipment requirements and frequently asked questions.

Technical Overview

Cold-rolled ribbed bars (CRB bars) are reinforcement steels produced by cold-working hot-rolled low-carbon wire rod through profiled rolls, forming regularly spaced transverse ribs on the bar surface. The ribs improve mechanical anchorage with surrounding concrete, while the cold-working process substantially raises the strength of the steel. In China, the product and testing requirements are governed by GB/T 13788, which defines grades such as CRB550 and CRB600H for conventionally reinforced concrete, and CRB650, CRB800, CRB800H and CRB970 for prestressed concrete applications. Equivalent families of deformed cold-worked reinforcement are also covered internationally by standards such as EN 10080, ASTM A1064 and ASTM A496, and JIS G 3112.

Because cold working increases strength at the expense of ductility, mechanical testing is essential to confirm that every production lot achieves the specified balance of strength, elongation, bendability and, for prestressed grades, long-term stress relaxation resistance. Key purposes of mechanical testing include:

• Verifying tensile strength and proof strength against the specified minimum values for each grade.
• Confirming adequate ductility through elongation measurements such as A11.3, A100 and maximum-force elongation Agt.
• Demonstrating bend and reverse-bend performance to ensure the bars can be fabricated on site without cracking.
• Controlling stress relaxation behavior of prestressed grades to guarantee long-term prestress retention.
• Providing conformity evidence for factory production control, product certification and acceptance by accredited third-party laboratories.

Testing Samples

Reinforced Concrete Grades (CRB550, CRB600H)

CRB550 and CRB600H bars are typically supplied in coils or cut lengths with nominal diameters commonly ranging from 4 mm to 12 mm. Specimens are taken from production lots consisting of the same grade, nominal diameter, and manufacturing conditions. For tensile and bend testing, full-section specimens are used without machining, preserving the as-rolled ribs and cold-worked structure.

Prestressed Concrete Grades (CRB650, CRB800, CRB800H, CRB970)

Prestressed CRB grades, usually supplied in coil form, require additional sampling for stress relaxation testing. Relaxation specimens must be of sufficient free length for the loading frame and are tested in the as-delivered, de-coiled condition. For these high-strength grades, particular care is required during sampling to avoid introducing bends, kinks or surface damage that could affect test results.

Specimen Preparation

Coiled material must be straightened before testing. Straightening should be performed carefully, preferably by hand or with equipment that does not impose additional cold working, since excessive re-straightening can alter the strength and ductility being measured. Key preparation steps include:

• Cutting specimens to a length providing adequate gripping distance on each side of the gauge length.
• Measuring the nominal cross-sectional area, either from measured diameter or from the mass per meter as permitted by the product standard.
• Marking the gauge length: proportional gauge length 11.3√S0 for elongation A11.3, or a fixed 100 mm gauge length for A100 on prestressed grades.
• Identifying specimens with lot number, grade, diameter and sampling position for full traceability.

Testing Items

Mechanical testing of cold-rolled ribbed bars covers the following principal items, with typical acceptance values based on GB/T 13788:

• Tensile strength (Rm): the maximum force divided by the original cross-sectional area; for example, minimum values of 550 MPa for CRB550, 600 MPa for CRB600H, 650 MPa for CRB650, 800 MPa for CRB800 and CRB800H, and 970 MPa for CRB970.
• Proof strength (Rp0.2) or yield strength (Re): cold-worked bars generally exhibit no distinct yield plateau, so the 0.2% non-proportional extension proof strength is determined; CRB550 typically requires a minimum proof strength of about 500 MPa.
• Elongation after fracture: measured as A11.3 for reinforced concrete grades (commonly at least 8% for CRB550) or A100 for prestressed grades (commonly at least 4% for CRB650 and above).
• Maximum-force elongation (Agt): a measure of uniform ductility, particularly significant for high-ductility "H" grades, where values of 5% or more are typically specified.
• Strength ratio (Rm/Rp0.2): the ratio of tensile strength to proof strength provides an indication of the strength reserve of the bar.
• Bend test: bending the bar through a specified angle, commonly 180°, around a mandrel of specified diameter without visible cracking.
• Reverse bend (rebend) test: bending, then re-bending in the opposite direction, where required, to assess bendability after straining and aging.
• Stress relaxation: for prestressed grades, the loss of stress under constant strain, conventionally tested at an initial stress of 70% of the specified tensile strength, with 1000-hour relaxation commonly limited to 8% or less.
• Supplementary checks: mass per meter deviation and rib geometry may be evaluated alongside mechanical items as part of full conformity assessment.

Testing Methods

Tensile Testing

Tensile testing is carried out in accordance with GB/T 228.1, which is technically aligned with ISO 6892-1. A universal testing machine of verification class 1 or better, calibrated in accordance with ISO 7500-1 or GB/T 16825, applies axial loading to the full-section specimen. For determination of Rp0.2, an extensometer of class 1 or better per ISO 9513 or GB/T 12160 is attached within the gauge length, and loading is controlled within the stress-rate or strain-rate ranges specified by the standard, for example a stress rate in the elastic range typically between 6 MPa/s and 60 MPa/s. When Agt is required, the extensometer remains attached until maximum force so that the extension at maximum force can be recorded. After fracture, the pieces are carefully fitted together and the final gauge length measured to compute elongation after fracture. Valid results require fracture within the gauge length, away from the grips, unless otherwise permitted by the standard.

Bend and Reverse Bend Testing

Bend tests are performed per GB/T 232, aligned with ISO 7438, using a bending apparatus with interchangeable mandrels. The mandrel diameter is related to the nominal bar diameter and grade as specified in the product standard. The bar is bent through the required angle, typically 180°, and then examined for cracks along the outer bent surface. For reverse bend testing, the specimen is first bent through a specified angle, aged where required, and then bent back through the reverse angle; the absence of transverse cracks indicates acceptable bendability relevant to site fabrication such as bending of stirrups and mesh.

Stress Relaxation Testing

Relaxation testing of prestressed CRB grades is conducted per GB/T 10120, with related methods in the ISO 15630 series. The specimen is loaded to the specified initial stress, commonly 70% of the specified tensile strength, and held at constant strain while the decay of stress is recorded over time in a temperature-controlled environment, typically 20 ± 2 °C. The standard test duration is 1000 hours; the standard also provides procedures for determining relaxation from shorter-duration tests by extrapolation where permitted, which is useful for production control. Conformity is assessed by comparing the measured or extrapolated 1000-hour relaxation value with the specified limit.

Sampling and Lot Rules

Product standards define lot formation and sampling frequency. In practice, lots are constituted from bars of the same grade, nominal diameter and production conditions, with lot sizes commonly up to 60 tonnes. From each lot, specimens are drawn for tensile testing (including strength and elongation), bend testing and dimensional or mass checks, while relaxation testing is generally performed at defined periodic intervals or by agreement between manufacturer and purchaser. Retest and re-inspection rules, including doubling of specimens in case of a single failed result, follow the provisions of the product standard.

Applications

Mechanically tested cold-rolled ribbed bars are used across a wide range of construction and infrastructure applications, including:

• Welded reinforcement mesh for cast-in-place concrete slabs, walls and floor systems in residential and commercial buildings.
• Distribution and temperature reinforcement in precast concrete panels and elements.
• Prestressed concrete components such as hollow-core slabs, prestressed pipes, poles and piles using CRB650 and higher grades.
• Reinforcement for road pavements, airport runways, bridge decks and pavement overlays.
• Stirrups, ties and secondary reinforcement in structural members where small-diameter, high-strength bars offer fabrication efficiency.
• Tunnel lining segments, ground support mesh and mining reinforcement applications.

Reliable mechanical test data supports safe design, conformity certification, acceptance inspection and quality traceability throughout the construction supply chain, from steel mill to fabricator to jobsite.

FAQ

Why is proof strength Rp0.2 used instead of an upper yield point for CRB bars?

Cold-rolled ribbed bars are heavily cold-worked and usually do not exhibit a distinct yield plateau in the stress-strain curve. The 0.2% non-proportional extension proof strength (Rp0.2) is therefore determined as the conventional yield indicator, providing a reproducible measure of the stress at the onset of plastic deformation.

What is the difference between CRB550 and CRB650 bars?

CRB550 is a lower-strength, higher-ductility grade intended for conventionally reinforced concrete, with a minimum tensile strength of 550 MPa and elongation requirements such as A11.3 of at least 8%. CRB650 is a higher-strength grade with a minimum tensile strength of 650 MPa intended for prestressed concrete, where stress relaxation testing is additionally required.

What do the designations A11.3, A100 and Agt mean?

A11.3 is the elongation after fracture measured over a proportional gauge length of 11.3 times the square root of the original cross-sectional area. A100 is elongation over a fixed 100 mm gauge length. Agt is the percentage elongation at maximum force, which reflects uniform ductility before necking and is especially relevant for high-ductility "H" grades.

Can the 1000-hour stress relaxation test be shortened?

Yes, in a controlled way. Product and test standards permit estimation of the 1000-hour relaxation by extrapolation from shorter tests conducted under identical conditions, provided the extrapolation rules are followed. Full 1000-hour tests remain the reference method for certification and dispute resolution.

How should specimens be straightened before testing?

Coiled bars should be straightened with minimal additional cold working, preferably by hand or using a straightening method that does not rotate or re-roll the bar significantly, since extra deformation can change the strength and elongation results being measured.

Which standards govern mechanical testing of cold-rolled ribbed bars?

The product requirements are specified in GB/T 13788. Test methods are specified in GB/T 228.1 (tensile, aligned with ISO 6892-1), GB/T 232 (bend, aligned with ISO 7438) and GB/T 10120 (stress relaxation). Internationally comparable methods include the ISO 15630 series, EN 10080, ASTM A1064, ASTM A496 and JIS G 3112. Testing can be performed by the manufacturer's laboratory or by accredited third-party laboratories for independent conformity assessment.