CROSS Safety Report
Defects in tapered thread reinforcement bars for coupling
This report is over 2 years old
A reporter highlights how B500 reinforcing bars were observed to have visually different tapered threads cut into the bars after delivered to site.
Key Learning Outcomes
For construction professionals:
Consider introducing a quality control procedure on site for the inspection of safety critical elements being delivered to ensure they meet the required standard
The reporter recommends the removal of couplers and visual inspection of threads for randomly sampled bars being delivered to site, at least for initial deliveries
Purchasing steel reinforcement bars and couplers from a supplier/fabricator with appropriate certification under the CARES (UK Certification Authority for Reinforcing Steels) scheme can ensure products are of the required quality
For suppliers and manufacturers of couplers:
The operation of machinery should be carried out by competent individuals who are trained and experienced
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A reporter says that B500 reinforcing bars of 12mm, 25mm and 32mm diameter were observed after delivery to site to have visually different tapered threads cut into the bars. The length of threaded section and depth and pitch of the threads varied between bars of the same diameter (Figure 1).
Production quality control
These bars were quality controlled during their production, including the threading of the bars, continues the reporter. The reinforcement manufacturer cuts the threads into the bars using equipment supplied by the coupler manufacturer. The manufacturer’s quality control process requires that 1 in 10 bars are checked using thread gauges.
Risk of pre-fitted bars in couplers
When these bars were delivered to site, most of the threaded bars were already fitted into the reinforcement couplers by the manufacturer. This is common, as the bars with couplers fitted are first cast into elements, with threaded bars later being installed into the couplers for construction of follow on elements. This means that it may not be possible to visually assess threaded bars before use.
Defects due to machine operator error
Representatives of both the reinforcement and coupler manufacturers inspected the bars on site. They confirmed that they were defective and were unlikely to provide the required tensile capacity when connected. A technician from the coupler manufacturer visited the reinforcement manufacturer and inspected the machinery used for cutting threads, and it was confirmed that the machine was working properly. The cause of the defects was determined to be machine operator error.
Four distinct defects identified
All threaded bars were returned to the manufacturer, fitted couplers were removed and 100% of the threaded bars were tested using thread gauges. This was witnessed by representatives from both the purchaser and reinforcement manufacturer. Over 25% of the couplers tested were found to have non-compliances with the coupler manufacturer specified thread requirements. Four distinct defects were identified:
Threads cut too short
Threads cut too long
Incorrect thread pitch
Non-contiguous threads cut into bars
The last defect left considerable surface area of bar unthreaded. All defective bars were replaced. The manufacturer's quality control process was amended to 100% testing of all manufactured bars using thread gauges before fitting of couplers and supply to site. To avoid recurrence, consideration should be given to specifying 100% thread gauge testing of bars. Removal of couplers and visual inspection of threads for randomly sampled bars is recommended by the reporter, at least for initial deliveries to site.
Removal of couplers and visual inspection of threads for randomly sampled bars is recommended by the reporter, at least for initial deliveries to site
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The photograph of the defective threads look like they may have been stripped or cross threaded. If this is a photo of the bar after the coupler has been removed, then it could be that the threads were cut correctly but became damaged when the coupler was attached. If that is the case, then 100% sampling of the threads before the couplers are attached might not address the problem - the threads on the bars would be OK but would become damaged once the coupler is attached and concealed. Maybe the coupler needs to have a slot in the side so the threads can be inspected in-situ?
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This is a valuable report highlighting what might be a very important safety issue. If the defective bars had been cast in, and not discovered, then safety would have been compromised. Alternatively, if the defects were discovered during or after construction, remedial costs and delays could have been huge. It is fortunate that vigilance on site detected the problem and that responsible action was taken to resolve the situation.
The construction industry relies on products meeting defined standards. The onus is on the supplier to operate quality systems to ensure such standards are met. However, as product quality often depends on workmanship, it is essential to have adequate supervision and inspection of work on site.
It is not known whether the reinforcement and mechanical couplers were specified as CARES approved in accordance with the UK National Structural Concrete Specification. Reinforcement and mechanical couplers provided in the UK would normally be certified by CARES (UK Certification Authority for Reinforcing Steels). This certification should cover the threading and attachment of the specified couplers prior to delivery to site.
It is not clear whether the reinforcement and mechanical couplers in the report were supplied by a supplier/fabricator with appropriate certification under the CARES scheme. Certification schemes rarely involve 100% sampling, and therefore rely on quality management systems and quality control procedures. Defects such as the one described in this report should be reported to the supplier/fabricator and, where the response is considered inadequate, to the certification body.
Certification schemes rarely involve 100% sampling, and therefore rely on quality management systems and quality control procedures
Continuous improvement processes
Certified suppliers should be operating continuous improvement processes which reports such as this would feed into to reduce future risk. Overall the benefits of independent third-party certification by an accredited body, of safety critical components, has been long acknowledged.
Indeed, the use of such schemes is a recommendation of the Hackitt report Building a safer future - Independent Review of Building Regulations and Fire Safety for safety critical products.
Similar coupler issues
In 2018, construction of a major railway project in Hong Kong was severely delayed due to allegations of steel reinforcing bars being improperly cut to imitate proper installation into couplers on parts of the project. The Interim Report of the Commission of Inquiry was published in February 2019 and shows the extent of problems that can be attributed to issues with couplers.