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CROSS Safety Report

Temporary movement joint in slabs not installed correctly

Report ID: 998 Published: 29 March 2021 Region: CROSS-UK


Overview

Dowels in temporary movement joints for a slab were not installed correctly, impacting the structural behaviour.

Key Learning Outcomes

For civil and structural design engineers:

  • If possible, attend site and inspect the installation of safety critical components such shear connectors to ensure they are installed as per the design intent

  • If you are unable to attend site, ask the contractor for site photos of the installation of these components

  • Where there are time dependant activities, these should be clearly conveyed to the contractor and included in the design risk assessment

  • If shear connectors are required to transfer lateral loads through slabs to provide in-plane stability, then raise awareness of this to the contractor and stress the importance of their installation 

For the construction team:

  • Where specialist systems such as shear connectors are used, it is beneficial to have a close working relationship with the supplier from the earliest opportunity to ensure installation requirements are understood

  • If the work crew are not experienced in the installation of these components, toolbox talks and workshops, are a good way of training the crew and conveying important information to them

  • Consider introducing a quality management procedure for the inspection of safety critical components to ensure they are installed in accordance with the construction drawings and manufacturer’s specification

  • It is good practice to keep detailed site records including photos of the installation of critical structural elements such as shear connectors

Full Report

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A concrete framed building completed within the past few years has post-tensioned roof slabs with temporary movement joints. A reporter found that one of these had opened up recently causing the roof finishes to fail and allowing water ingress. The type of shear connector used allows movement in the temporary condition but is to be locked once the majority of early shrinkage has completed.

In the temporary condition, the dowels only allow shear to be transferred across the joint, however when locked, the detail should not allow for any movement so that axial actions can then be transferred across the joint. In this case, as the slabs are moving, it is clear that the dowels are not functioning as intended. The reporter believes this has the potential to undermine the structural engineer’s intention to provide in-plane stability by transferring lateral loads through the roof.

Investigations showed that the dowels had not been locked as per the manufacturers’ instructions - the locking pins were not installed and grouting of the installation wells not properly carried out. The contractor does not have records to show how these were installed nor inspection sheets that should have been produced during the locking of the dowels. There has been, according to the reporter, a trend of issues with temporary and permanent movement joints when shear dowels are used.

Investigations showed that the dowels had not been locked as per the manufacturers’ instructions

Expert Panel Comments

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Expert Panels comment on the reports we receive. They use their experience to help you understand what can be learned from the reports. If you would like to know more, please visit the CROSS-UK Expert Panels page.

This is another example of the common pattern of what has been built not being in accord with design intent. Referring back to Report 987, the locking of the dowels must fall into a category of ‘critical areas of construction’ and should have been highlighted in the Designer’s Risk Assessment as an activity that needed to be verified. The report does not point out that there must have been time lag to allow the majority of early shrinkage to take place and someone would have had to define what that time period should be. Given a time lag, a clear risk is that the activity will be overlooked if not highlighted in the risk assessment.

Access to accurate information about a building is very important when incidents like this occur. It should be seen as essential that contractors keep full records of their work, and that they are open and accessible to those who may need them in the best interests of the safety of the structure. Owners should have this information to hand so that timely investigations and repairs can be made, without having to refer back to contractors’ records.

It should be seen as essential that contractors keep full records of their work, and that they are open and accessible to those who may need them in the best interests of the safety of the structure.

This type of joint is normally placed at the points of contraflexure as the anchorages either side of the joint are then at mid depth. Under normal loading vertical shear is the main action to carry, obviously if the slab acts as a diaphragm lateral shear is also important.

As the joint is required to work in the un-grouted case it is unlikely that the construction error would lead to an immediate safety concern. However, a larger than expected crack can lead to failure of finishes as noted in the report and potentially later durability issues.

The industry needs to invest more in adequate supervision and the completion of specified inspection and testing in order to improve quality control. This report is also another good example of when record photos of activity completion could have been helpful.

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