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CROSS Safety Report

Pre-stressing ducts

Report ID: 89 Published: 1 October 2008 Region: CROSS-UK

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Overview

A reporter has come across a situation where during grouting of the ducts in a slab, patches of concrete as big as 0.5m x 1m and 30mm thick fell on to the ground creating a safety hazard.

Key Learning Outcomes

For the construction team:

  • Post-tensioning and pre-stressing operations should be carried out by people who have the relevant training and are competent to do so

  • Quality control and competent supervision on site can help to ensure that the structure is built in accordance with the design

  • Regular toolbox talks with work crews are a good way of highlighting any risks associated with work activities

Full Report

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Pre-stressing in concrete has been used in bridges and heavy civil structures for many years and more recently in buildings. A reporter has come across a situation where during grouting of the ducts in a slab, patches of concrete as big as 0.5m x 1m and 30mm thick fell on to the ground creating a safety hazard (Figure 1). Investigation showed that the main reasons for falling patches of concrete were combinations of the following:

  1. During grout the pressure was not monitored at the delivery end and entry end into the duct and higher pressures than normal were used

  2. The grout was expanding very quickly - more than that which was specified

  3. The ambient temperature was too low during grouting

  4. Water found its way into the lowest points of the duct during rain and eventually froze resulting in partial ice expansion

Image
Figure 1: soffit of slab showing pre-stressing duct

Expert Panel Comments

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Grouting of pre-stressing ducts requires careful workmanship and supervision. Recent investigations have revealed the likelihood of a lack of properly grouted ducts in some buildings. One method of minimising the risks is to ensure that post tensioning installation is only carried out by properly trained teams, for example those certified for PT installation through the UK CARES certification Scheme.

It should be noted that point 4 above (water ingress and freezing) can occur in completed structures where grouting has not been properly executed. Guidance on procedures and techniques is given on the CARES website. Reference is made to the need for project specific reviews and advice is given for new structures.

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