Post tensioned inspections

Post tensioned special inspections

Post tensioned special inspections (PTSI) were undertaken on numerous structures in the UK during the latter part of the 20th century as a result of a number of catastrophic failures in structures world wide.

Initial surveys by Nicholls Colton and others indicate post tensioning systems in the UK to be generally satisfactory in condition. Generally, some evidence of a lack of grout within the ducting was found and strands with end anchorages often showing signs of light corrosion, in a number of cases significant corrosion of strands and end anchors was identified resulting in loss of strength to the structural element (usually bridge deck) and significant costly and extensive remedial works including re-grouting of ducts and strengthening of the structure and in a few cases demolition and replacement of the bridge deck.   

Although a variety of post tensioning systems exist, commonly, post tensioning strands are enclosed within ducts (frequently metallic) and are protected by a cementitious grout being pumped into the duct once in-situ to prevent the strands from corrosion.  In some cases endoscopic examination of the duct and strands has identified incomplete or insufficient grouting, allowing ingress of moisture and chloride salts, penetrating the ducting, coming into contact with the strands and in turn promoting corrosion, leading to a reduction in the performance of a structure.

Whereas corrosion risk within normally reinforced structures can invariably be identified from visual inspection and specialist corrosion  testing techniques requiring little physical intrusion, with post tensioned bridges the elements to be inspected are frequently hidden, requiring more significant planning of site works and intrusive investigation once on site.

Nicholls Colton has been a leader in the development of post tensioned investigations procedures from the 1990’s through to the present day. Typically a project will involve a desk study of any construction details (drawings etc.) available to identify the theoretical position of ducts within a structure and to allow locations for intrusion to be chosen which minimise damage to a structure and disruption to users of a structure.  Once on site, the actual position of tendon ducts can be confirmed by techniques which may include cover/ferroscan/radar survey or drilling if non-destructive methods fail.  Mechanical means (coring, drilling) are then used to expose the ducting taking care not to damage it.  Once found the ducting is opened to reveal the grout/strands within.  At this stage an assessment of the volume of any void in the grouting can be made and an inspection can be made by endoscope including video analysis within the duct should a void in the grout be identified, estimating the extent of a void, degree of pitting and presence and level of corrosion products and loss of section to the strands.  Samples of the grout and any duct water present are taken for subsequent laboratory testing. 

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Anchorages for the post tensioning wires may also be inspected to identify factors such as water seepage, the integrity of capping mortar, evidence of poor construction etc. although more intricate methods of exposure may be required due to their location in a structure.  Details of exposed anchors/capping mortar are recorded and photographs taken; samples of capping mortar are taken for subsequent laboratory testing.

For structures falling under the responsibility of Highways England, requirements for undertaking a PTSI are detailed in the DMRB, Volume 3 Section 2 Part 5, BD54/15 Management of Post Tensioned Concrete Bridges. For all post tensioned structures it is recommended to follow a timely programme of investigation and testing to maintain the integrity and safety of the structure.