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CROSS Safety Report

Concern about rebending starter bars

Report ID: 1164 Published: 24 May 2023 Region: CROSS-UK


Overview

This report concerns the bending and rebending of starter bars on site during the construction of a multi storey tower. The reporter considered that such a practice could compromise the structure under extreme load conditions.

Key Learning Outcomes

For builders and contractors:

  • Rebending of bars on site is poor practice and the design engineer should be consulted prior to rebending any bars
  • Concrete Society publication Straightening and rebending reinforcement on site provides guidance where site changes are sanctioned by the designer
  • Reinforcement should be installed as per the detailed drawings. If constraints arise with placing reinforcement, the design engineer should be consulted prior to any alterations being made
  • Unauthorised bending or rebending of reinforcement on site may invalidate the design

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A reporter writes with their concerns regarding starter bars for a multi storey tower under construction. The original contractor had been unable to continue when the building was partly constructed and a new contractor took over. The reporter's concerns related to the interface between earlier and new construction work.

A jump form self-climbing system for cast in-situ concrete walls was in use and the reporter noted that many jacks had been installed on top of previously cast columns and walls. A number of starter bars had been bent or rebent on site to suit the positions of the jacks. It was also clear to the reporter that some starter bars had been bent down at the tops of walls but were not rebent, to stand vertically, in preparation for the next lift.

The reporter says that bending and rebending starter bars is prohibited in codes of practice. Worse still in this case, they say, starter bars had been bent to suit the operation of the jump form. This meant that in some instances, there might not be continuity between starters and the reinforcement in the next lift, which could prejudice the structure under extreme load conditions.

Expert Panel Comments

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As the reporter suggests, the deliberate rebending of bars is poor practice. Sometimes it occurs on site due to damage, often from plant, rather than deliberately as in this case. Bending of reinforcement on site should not occur without the permission of the designer.  It should only be carried out using the appropriate equipment to ensure that the required minimum bend radii is achieved, to avoid damage to the bar metal and cast concrete. The rebending of reinforcement is even more problematic and should only be carried out on reinforcement certified for the process. Uncontrolled bending and rebending can lead to bar failure, which should be obvious, and it can also lead to less obvious damage, including a loss of ductility, which may invalidate the design assumptions.

Uncontrolled bending and rebending can lead to bar failure ... and ... loss of ductility, which may invalidate the design assumptions

The design rules for lapping reinforcement assume the bars are in close proximity. However, if due to bending this is not the case normally there are additional requirements including extending the lap length, which would clearly be problematic for a bar that is already cast in. Designers should give very close consideration to this issue. Alternative means of providing anchorage, rather than using non-designed bent bars, could be considered.

If it is known that the construction process has the potential to damage some of the reinforcement, then one solution is to over-provide a limited amount of reinforcement so there is some tolerance for damaged bars.

In summary, generally, any unauthorised site changes should be discouraged as they potentially undermine the design. If changes are required, they should receive design sanction and be made under an agreed procedure. The Concrete Society publication Straightening and rebending reinforcement on site provides guidance on this.

CROSS-UK report 894 - Checking reinforcement before concrete pours, published in 2020, considers some of the typical reinforcement issues which those inspecting reinforcement on-site may come across.

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