CROSS Safety Report
Continuously threaded reinforcing bar
Overview
This report highlights the difficulty of inspecting continuously threaded reinforcing bars to ensure that they have been adequately inserted into the cast-in ferrules.
When using such devices, it is very important to follow the manufacturer's advice and recommendations.
Key Learning Outcomes
For structural design engineers
- Specify inserts or couplers for which the design capacities are well established by testing that is relevant to the application
- Specify control measures that site inspectors and construction contractors should use to confirm compliance with their design intent
For site engineers and inspectors:
- When checking threaded bars installed into cast-in ferrules, be sure to check that they are fully wound into the insert
- Maintain records of all such inspections
For construction professionals and contractors:
- Follow the guidance provided by designers, product manufacturers, and suppliers
- Ensure that couplers and inserts are quality-assured and keep records of quality control of threaded depth of all coupling devices
- When installing continuously threaded bars into cast-in ferrules, these should not be tied to the slab reinforcement until after the pre-pour inspection has been completed by the engineer and/or site inspector
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When conducting pre-pour inspections of reinforced concrete structures, the reporter has found several examples where continuously threaded reinforcing bars have been tied into the slab reinforcement layer at the slab-to-wall joints. As a result, the reporter was unable to verify whether the threaded bars had been adequately wound into the cast-in ferrules within the wall. After the reporter had the tie-wire cut to check the adequacy, the threaded bars were found to be not fully wound into the ferrule on a number of occasions.
The reporter is concerned that there is a high risk of failure of the structure if threaded bars are not adequately wound into the cast-in ferrules as these bars provide direct shear support for the slab element.
Winding in threaded bars into ferrules can be labour-intensive. Reducing the time taken to undertake these construction activities may have been a contributing factor to the circumstances observed by the reporter at the time of inspection.
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Although this report specifically refers to the use of continuously threaded bars with cast-in ferrules, it could apply equally to the use of threaded bars with couplers and other splicing or joining devices and inserts.
When using such devices, reference should always be made to the manufacturer’s literature for the correct specification and installation of the particular product. For example, when using continuously threaded bars, such as Reid Bar, with couplers, the ReidBar design guide states: 'Tests show that to achieve the ultimate strength of the connection the thread engagement must be at least 80% of the maximum thread depth available in the fitting. Correct bar insertion is critical to the performance of the ReidBar™ system and it is recommended that good practice requires the user to mark the bar at half coupler length back from the inserted end so that a visual check is available.'
Thus, when selecting this type of connection, the design engineer should satisfy themselves as to the minimum thread length that has to be engaged to achieve the required performance, and the specification should require the bars to be clearly marked at the required distance back from the end of the bar. It would appear that this had not been done in the examples found by the reporter.
When determining the required performance of the connection, designers should take into account the particular location. For example, at floor slab-to-wall joints the bars are likely to be subject to a combination of shear and axial loads as they transfer bending moments to the walls.
As the reporter notes, winding in these bars is a labour-intensive operation and any grit or cement dust on the threads or inside the ferrule can make them difficult to install to the full depth. Keeping protective plugs in the ferrules until just before they are used and keeping the threads of the bars well protected will go a long way to eliminating this particular problem.
Threaded reinforcing bars
It is important to differentiate between continuously threaded bars which provide full capacity of the bar and those which rely upon a cut thread which reduces the bar capacity unless a proprietary system is used, such as Ancon reinforcing bar couplers. Again, it is very important to follow the manufacturer’s recommendations when using such items.
CROSS report 844 Defects in tapered thread reinforcement bars for coupling highlights the importance of good quality control procedures on site for the inspection of safety critical elements being delivered to ensure they meet the required standard. In this report reinforcing bars were observed to have visually different tapered threads cut into the bars after delivery to site and the reporter recommends the removal of couplers and visual inspection of threads for randomly sampled bars when delivered to site.
CROSS report 993 The use of cast-in ferrules as structural connections raises several issues with this type of connection and notes that when cast-in ferrules and similar threaded inserts are used to make structural connections using threaded reinforcing bar, the failure mode may be brittle. In this case, the design may not comply with relevant AS/NZS Standards, such as the ductility requirements in AS3600. Furthermore, AS 3850.1: Prefabricated concrete elements, makes it quite clear that only bolts should be fixed into such ferrules for fixing items such as lightweight architectural framing, temporary bracing, and the like.
National Precast Concrete Association Australia (NPCAA) has produced a very informative Tech Talk (Webinar): Code Compliant Reinforcing Connections (February 2022) on this subject.
As the Steel Reinforcement Institute of Australia (SRIA) notes in its newsletter bearing issue number 43/4 of December 2017 on “Mechanical Splices for Joining Reinforcement”: 'There are a number of proprietary systems available to mechanically splice reinforcing bars in Australia, including both coupler and coupling sleeve systems (both mechanically bolted and grouted), and specific design and detailing parameters for the various systems can be found via the links on our web site to our Associate Member manufacturers of these products. Detailers should consult the manufacturer’s websites and examine the details to satisfy themselves on the type and detail they should use and how to specify them. In some cases, mechanical splices are designed specifically for proprietary bar types, or to splice proprietary bars to ‘normal’ deformed reinforcing bars.'