Cold-rolled ribbed steel bar testing verifies the mechanical properties, chemical composition, dimensional accuracy, and surface quality of CRB reinforcement before it is used in concrete structures. Testing is performed in accordance with recognized standards such as GB/T 13788, ISO 6892-1, ISO 15630, ASTM A1064, and EN 10080 to confirm that each batch meets the strength and ductility requirements for its intended grade. This article outlines the sample types, test items, test methods, typical applications, and frequently asked questions related to cold-rolled ribbed steel bar evaluation by manufacturers and accredited third-party laboratories.
Technical Overview
Cold-rolled ribbed steel bars, commonly designated as CRB bars, are concrete reinforcement products manufactured by passing hot-rolled plain wire rod through profiled rolls at ambient temperature. The cold-rolling process reduces the cross-section and simultaneously forms longitudinal and transverse ribs on the bar surface, which significantly improves mechanical anchorage and bond performance with surrounding concrete. Because the cold-working operation strain-hardens the steel, the finished bars achieve substantially higher yield and tensile strength than the original feedstock, although at the cost of reduced ductility, which is why systematic testing is essential.
In the Chinese national standard GB/T 13788, cold-rolled ribbed bars are classified into grades identified by the letters CRB followed by a number indicating the minimum tensile strength in megapascals. Common grades include CRB550, CRB600H, CRB650, CRB680H, CRB800, CRB800H, and CRB970, where the suffix H designates high-ductility variants with enhanced elongation requirements. Grades such as CRB550, CRB600H, and CRB680H are intended for ordinary reinforced concrete, while higher-strength grades such as CRB650, CRB800, and CRB970 are used in prestressed concrete members. Nominal diameters typically range from 4 mm to 12 mm.
Comparable international frameworks include ISO 6934-2 for cold-worked prestressing steel wire, ASTM A1064/A1064M for plain and deformed steel wire reinforcement, EN 10080 for weldable reinforcing steel, and JIS G 3112 for deformed bars. Supporting test-method standards include GB/T 228.1 and ISO 6892-1 for tensile testing, GB/T 232 and ISO 7438 for bend testing, GB/T 238 for reverse bend testing of wire, GB/T 10120 for stress relaxation, and GB/T 4336 for optical emission spectrometry. Testing may be carried out by in-house quality departments, government supervision stations, or accredited third-party laboratories.
Testing Samples
Coiled Bars (Bar-in-Coil)
The most common delivery form is wound coils. Specimens are cut from the outer end of selected coils after the production line has stabilized. A test batch is normally defined as bars of the same grade, same nominal diameter, and same manufacturing process, commonly limited to approximately 60 tonnes. From each batch, at least one tensile specimen and one bend specimen are prepared, with additional samples taken when the batch is large or when results are marginal.
Straightened and Cut Bars
When bars are delivered as straight lengths, typically 6 m to 12 m long, specimens are selected at random from different bundles within the batch. Care is taken to avoid the cropped ends, where straightening or shearing may have altered the mechanical properties, and the sampling positions are documented in the test report.
Welded Wire Fabric
For bars fabricated into welded mesh panels, additional specimens are cut from finished panels so that both the wire properties and the integrity of the welded intersections can be evaluated. Longitudinal and transverse wires are tested separately for tensile strength, and cross-wire specimens are prepared for weld shear strength assessment in accordance with ISO 15630-2.
Wire Rod Feedstock
To control quality at the source, the hot-rolled wire rod used as raw material is sampled for chemical composition and surface condition before cold rolling. Feedstock verification helps prevent inherited defects such as seams, inclusions, or excessive carbon content that could compromise the finished bar.
Testing Items
A complete evaluation program for cold-rolled ribbed bars covers the following categories:
- Chemical composition: carbon, silicon, manganese, phosphorus, sulfur, and residual elements such as chromium, nickel, and copper, with phosphorus and sulfur commonly limited to 0.040% to 0.045% depending on the specification.
- Tensile properties: proof strength (Rp0.2 for ordinary grades, Rp1.0 for prestressed grades), tensile strength Rm, percentage elongation after fracture (A11.3 or A100), total elongation at maximum force Agt, and the strength-to-proof ratio.
- Bend and reverse bend performance: the ability to withstand a 180-degree bend around a mandrel of specified diameter, and for certain grades a reverse bend sequence, without transverse cracking.
- Dimensional and geometric characteristics: nominal diameter, rib height, rib spacing, rib flank angle, layout of longitudinal and transverse ribs, and calculated relative rib area, which governs bond capacity.
- Mass per unit length: deviation between actual and theoretical mass per meter, commonly required to remain within about plus or minus 4%.
- Surface quality: absence of cracks, seams, folds, scabs, damaging corrosion, and other visible defects over the full length.
- Stress relaxation: for prestressed grades, the loss of stress over time under sustained tensile load, assessed at an initial stress commonly equal to 70% of the specified tensile strength.
- Weld intersection strength: shear strength of welded joints when the bars are supplied as mesh.
- Marking and documentation: verification of grade, diameter, and manufacturer identification on the product and consistency with the mill test certificate.
Testing Methods
Tensile Testing
Tensile tests are performed on universal testing machines in accordance with GB/T 228.1 or ISO 6892-1, using a gauge length of 11.3 times the square root of the original cross-sectional area or a fixed 100 mm gauge length as appropriate. For ordinary grades, the 0.2% non-proportional proof strength Rp0.2 is recorded; for prestressed grades, the 1.0% proof strength Rp1.0 applies. An extensometer is used to capture Agt, the total elongation at maximum force, which reflects uniform ductility. As an example, GB/T 13788 requires CRB550 bars to achieve a minimum proof strength of 500 MPa, a tensile strength of 550 MPa, and an elongation A11.3 of at least 8%, while high-ductility CRB600H bars must meet at least 540 MPa, 600 MPa, and 14% respectively.
Bend and Reverse Bend Testing
Bend tests follow GB/T 232 or ISO 7438: the specimen is bent 180 degrees around a mandrel whose diameter is a specified multiple of the nominal bar diameter, commonly between three and six times depending on the grade. After bending, the outer surface of the tension zone is examined at magnification; no transverse cracks are permitted. For wire-type products and certain grades, a reverse bend test per GB/T 238 is applied, in which the bar is bent through a defined angle and then bent back, verifying ductility after cold deformation such as that experienced during fabrication and placing.
Chemical Composition Analysis
Composition is determined using spark discharge optical emission spectrometry in accordance with GB/T 4336, supplemented where necessary by combustion infrared analysis for carbon and sulfur. Product analysis results are compared with ladle analysis and against the acceptance limits of the applicable standard. Composition verification is particularly important for confirming weldability and for tracing the feedstock quality of the cold-rolled product.
Dimensional and Rib Geometry Measurement
Diameter is measured with calipers or micrometers in two perpendicular directions and averaged over multiple sections. Rib height is measured on a defined number of ribs across the sample, rib spacing is averaged over at least ten intervals, and flank angles are determined using optical projectors or laser profilometry. From these data the relative rib area is calculated, a parameter that directly controls bond strength between the bar and concrete and must satisfy minimum values assigned to each nominal diameter.
Stress Relaxation Testing
For prestressed grades, long-duration relaxation tests are conducted in accordance with GB/T 10120 or ISO 15630-3. The specimen is loaded to the specified initial stress, typically 70% of the specified tensile strength, and held at constant initial strain while the decay of force is monitored. The relaxation value after 1,000 hours must not exceed the limit stipulated in the product standard. Because full-duration tests are time-consuming, some producers use validated accelerated procedures with established correlation factors for routine control, reserving full-scale tests for type approval and periodic verification.
Visual and Surface Inspection
Every coil or bundle is inspected visually for surface defects, uniform rib formation, and legibility of markings. Suspect areas are examined more closely, and where localized imperfections are found, their depth is checked against the permissible limits in the product standard. The inspection also confirms that packaging and labeling correspond to the delivery documents and the mill test certificate.
Applications
Cold-rolled ribbed bars occupy an important niche in reinforced and prestressed concrete construction, particularly where small-diameter, high-strength reinforcement is required at high volume. Their main applications include:
- Floor slabs, roof slabs, and walls in cast-in-place reinforced concrete structures, where CRB550 and CRB600H bars serve as distribution and main reinforcement.
- Welded wire fabric used in highway pavements, airport runways, tunnel linings, industrial floors, and precast concrete panels.
- Prestressed concrete elements such as hollow-core slabs, spun concrete pipes, transmission poles, and piles, which use CRB650, CRB800, CRB800H, or CRB970 grades.
- Masonry reinforcement, plaster mesh, and light structural elements where fine-diameter ribbed wire improves crack control.
- Substitution for small-diameter hot-rolled deformed bars in non-seismic structural members, delivering material savings due to the higher strength-to-weight ratio of cold-worked steel.
In every application, the suitability of the bar depends on verified compliance with grade-specific strength, ductility, bond, and relaxation requirements, which is why inspection certificates from accredited third-party laboratories are routinely required for project acceptance.
FAQ
What is the difference between ordinary and prestressed CRB grades?
Ordinary grades such as CRB550, CRB600H, and CRB680H are used as conventional reinforcement and are evaluated using the 0.2% proof strength Rp0.2. Prestressed grades such as CRB650, CRB800, and CRB970 are intended for prestressed members, are evaluated using the 1.0% proof strength Rp1.0, and must additionally satisfy stress relaxation requirements.
How is a test batch defined and how many specimens are taken?
A batch consists of bars of the same grade, nominal diameter, and production process, commonly limited to around 60 tonnes. At least one tensile specimen and one bend specimen are normally taken per batch, with extra samples drawn from additional coils when required by the specification or by the size of the delivery.
Is stress relaxation testing required for every batch?
No. Relaxation testing is a type test and periodic verification requirement for prestressed grades rather than a routine per-batch test. Manufacturers typically perform it during qualification of a new composition or process and at defined intervals thereafter, and the results are documented in the product certification.
Can cold-rolled ribbed bars replace hot-rolled deformed bars?
They can substitute small-diameter hot-rolled bars in many non-seismic applications because of their higher strength and good bond performance. However, their lower ductility compared with hot-rolled bars means that design codes restrict their use in seismic-critical zones and highly ductile members, so compliance with the governing structural design standard must always be confirmed.
Why is rib geometry tested and not just strength?
Rib height, spacing, and the calculated relative rib area control the mechanical bond between steel and concrete. A bar with adequate strength but underdeveloped ribs could fail structurally through bond slippage, so geometric verification is an integral part of the test program.
How long does a complete test program take?
Tensile, bend, dimensional, and chemical tests are usually completed within a few working days. Stress relaxation testing requires substantially longer because the standard evaluation period is 1,000 hours, so projects using prestressed grades should schedule type testing well in advance of production approval.