Post-tensioned (PT) construction, with particular focus on bridges has been a common design preference in the UK since the 1940’s. Post-tensioned bridges were widely used across the world and in the 50’s and 60’s such bridges typically comprised 19 individual strands which were spirally wound together, rather than being individually stressed, to increase the pre-compression force into the concrete. In more recent times the technology of gripping the strands improved and 7-wire strands are more common-place.  Post tensioning is now widely used throughout the construction industry such as buildings, multi story car parks and bridges due to reduction in element thickness and larger spans which can be achieved.

On an internally post tensioned bridge, various numbers of these 7-wire strands are typically fed through profiled steel/plastic ducts and stressed at the ‘live’ end. The strand material is a high yield steel which is particularly susceptible to corrosion in the right conditions. To protect the strands and aid the force transfer to the concrete, a cementitious fluid grout is pumped through the ducts housing the strands. This process is not always carried out correctly leaving sections of ducts voided and subsequently unprotected from potential corrosion.   

Across the UK large amounts of de-icing salts are placed on our roads during the winter months. On many structures, defects such as failed waterproofing systems, failed bridge joints and non-functional drainage systems can allow chlorides to penetrate into the concrete. This in turn reaches embedded metallic elements, triggering the corrosion process. If the grouting process has not been carried out correctly then voids could be present and the tensioned strands more susceptible to corrosion. The concerning factor linked to this defect is that during routine inspections this hidden critical defect will likely not be identified. 

Empirical can provide the following services to determine the condition of the post tensioned system and assist consultants with DMRB CS 465 specifying Post Tensioned Special Inspections (PTSI): 

• Excavations to expose the ends of longitudinal and transverse anchors to determine their condition. 

• Intrusive openings to expose tendon ducts at critical points confirming the presence of grout and current condition of strands. 

• Ground Penetrating Radar surveys to locate post tensioned ducts/anchors prior to intrusive investigations and to confirm profiles.  See GPR – Structures  

• Ultra-sonic surveys using tomography to identify potential voids within the ducts.  See Ultra-Sonic Testing of Concrete  

• Extraction of samples to determine material quality. See Materials Sampling and Testing  

• Concrete condition surveys including half-cell potential, resistivity and cover meter surveys. 

• Exposure of PT ducts and tendons by experienced operatives using either diamond drilling or local breaking out of cover concrete. 

• Reinstatement of materials including road surfacing, waterproofing and concrete after intrusive investigations. 

• Supply of interpretive reports 

Empirical provide professionally experienced engineers to manage and supervise post tensioned special inspections.