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, generally showing post tensioning systems to be in a satisfactory condition, although some evidence of a lack of grout was found, strands often showing signs of light corrosion. 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.  In some cases though this grouting has been found to be incomplete, allowing moisture and chloride salts to penetrate 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 or testing 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 a wide range of experience in carrying out post tensioned special inspections from the 1990’s to 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 are identified from the desk study and confirmed by techniques which may include cover/ferroscan 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 by testing, or if appropriate, an inspection can be made by endoscope to estimate the extent of a void.  Samples of the grout and any duct water present can be taken for subsequent laboratory testing.

During all stages of the above, records are taken of the condition of materials encountered (overlying concrete, ducting, grout, strands) with photographic records taken where appropriate.

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 taken for subsequent laboratory testing if required.

Findings of Post tensioned special inspections are presented in typed format supplemented by CAD drawings and photographic evidence where applicable.

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.