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Safe masonry prestressing

Letters

The use of deflected strands in precast prestressed beams has always seemed a little risky to me. It is difficult to envisage a method that does not require a penetration of the concrete to achieve the deflection. Your first reports of the Lowes speedway bridge collapse (NCE 25 May) seemed to place the blame squarely on this detail.

Since the collapse of the Ynys y Gwas bridge (which involved precast elements and prestessing) UK engineers have been acutely aware of the need to protect prestressing wires.

The Concrete Society's TR47 report advises engineers of the need to adequately protect all prestressed concrete structures and in particular their grouting.

Your latest report (NCE 1 June) suggests that the grout used to fill the penetration was poor. A classic case of a poor detail and poor materials/ workmanship leading to failure.

The current version of TR47 is limited to bonded post tensioned bridges. However, the basic philosophy of multi layer protection is applicable to all types of prestressed concrete structures. Updated and expanded guidance will be given in a revised TR47 due to be published soon.

David Collings (F) purplle@hotmail.com

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