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CROSS Safety Report

Failure of epoxy resin bonded anchors in concrete

Report ID: 429 Published: 1 April 2014 Region: CROSS-UK

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Overview

A reporter expresses concern about adhesive bonded anchors after anchors failed progressively on site during a lifting operation.

Key Learning Outcomes

For construction professionals and design engineers:

  • Connections can often be the weak link in structures and attention to detail is required

  • Manufacturers requirements and guidance for the installation and storage of fixings should be followed

  • Consider having representatives of the manufacturer attending site to train operatives on best practices. This can help to raise industry standards.

  • Where fixings are key components and part of the quality assurance procedure consider carrying out site testing to ensure their strength capacity

Full Report

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A reporter expresses concern about adhesive bonded anchors made by injecting epoxy resin into drilled holes to fix threaded bars at a pre-determined design depth. After curing the fixing is subjected to a pull-out test. The behaviour of the anchor under static tensile force depends on; concrete strength, steel strength, friction force between the concrete and the steel (adhesion of resin) and friction force between steel components of the anchor.

As a result, several failure modes are possible. In this case the contractor was undertaking the removal of a concrete guide wall and diaphragm wall as part of enabling works for the re-opening of a major roadway. To facilitate removal concrete panels had been cut into sections and all post-fixed anchors had been tested prior to lifting. During a lifting operation for a section of the guide-wall, the anchors failed progressively when the crane lifted the section 500mm off the ground. The anchors detached from the resin and came out of the holes (Figure 1).

During a lifting operation for a section of the guide-wall, the anchors failed progressively when the crane lifted the section 500mm off the ground

Image
Figure 1: lifting bracket with failed anchor

Contributory causes were:

  • Incorrect pull test load applied. Anchors tested at 35kN whereas the Work Package Plan required testing to 55kN.

  • The ambient temperature of the resin used dropped below the manufacturer's recommendations of 5-25oC. Temperature on the morning of incident was recorded as being 2oC.

  • The quality of the drilled holes was questionable as the manufacturer's installation best practice methodology was not followed

  • Threaded bars were not stored in a clean and dry environment so as to prevent contamination

  • To install the lifting plate, operatives were using scaffold tubes and hammers to straighten the threaded bars as they were not plumb due to free-hand percussion drilling

  • Sometime after the installation of the threaded bar core drilling was carried out near the anchor. This may have impeded the curing/setting of the resin.

 Action taken on site with immediate effect:

  • The number of operatives installing the anchors was limited and the only individuals allowed to carry out installation and testing were those appointed by site supervisor

  • Representatives from the manufacturer of the resin visited site and briefed the operatives on the best practice. Industry standards are now being exceeded.

  • A temperature controlled storage box was provided so that the resin was stored in line with manufacturer's recommendations

Going forward, says the reporter, for temporary and permanent works design requiring post fixed anchors, the resin manufacturer and the anchor manufacturer should be consulted prior to the installation to ensure that adequate controls are put in place.

Expert Panel Comments

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The Standing Committee on Structural Safety  (SCOSS) Alert Tension systems and post-drilled fixings - March 2014 has been published following a number of reports of similar type failures. Some of these, which also include hanger failures, are described and the serious consequences of progressive collapses are emphasised.

There are recommendations for the inspection of older systems and advice on the selection and installation of new anchors.  Resin grouted anchors need to be tested adequately and to a load above the design load and by a method that tests all failure modes. 

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