A tool for non-destructive testing
The preliminary tests took place on a Cerema cable tensioning bench in Bordeaux. This bench is used to perform instrumented tests on cables, and in particular to test assessment and examination methods before they are deployed on engineering structures.
The tested prototype is a coil performing magnetic verification of cables. In partnership with the German company ROTEC, Cerema’s work consisted in transferring the technology developed for multi-layer single strand cables used mainly in ski lifts and suspension bridges to pre-stressed cables.
The design of pre-stressed cables is very different from that of single strand cables, and so the method had to be adapted, assessed and validated.
The coil is made up of permanent magnets (which create a magnetic field) and a magnetic field sensor (which records the signal). A code wheel associated with the recording is used to position the measurement on the cable. A system of castor wheels is used to move the equipment along the cable. The equipment is pulled using a rope in order to obtain measurements along the entire length of the cables.
When the cable is in good condition, the magnetic field remains stable at a constant amplitude. If there is a defect on the cable, this generates a local variation in the magnetic field. By detecting discontinuities in the magnetic field, this method makes it possible to identify the exact location on the cable where the section defects are situated (e.g. broken wires).
Several different sizes of coils can be used for cable diameters between 70 and 130 mm. Between 3 and 6 magnets form a circle around the cable.
The method deployed in real conditions
Cerema has implemented this method for the APPR, a motorway management company, on the pre-stressed cables of the Sylans motorway viaduct. The viaduct consists of two separate open web pre-stressed box sections, each 1266 m long. In all the operation took place over a period of one year, and Cerema personnel were present on site for one or two weeks every month.
During this operation, in addition to the magnetic examination of the cables, Cerema also provided the structure manager with:
-
its expertise in structures and materials,
-
advice to the contracting authority concerning asset management and specific assessment on polyethylene tubes (PEHD),
-
installation and execution of non conventional test protocols adapted to the new pathologies that were detected (ageing and characterisation of materials, products and processes),
-
assistance with access difficulties (securing of work areas, access for rope access technicians, etc.).
Cerema is strongly committed to guaranteeing the safety of engineering structures. It takes action regularly throughout France to carry out assessments of structures. These assessments include on-site investigations by visual inspections, examinations of materials, and measurements of real loads. These investigations are then used in theoretical approaches by means of modelling incorporating the observed conditions of the structure. These assessments are used to implement safety and repair work projects. Prior to and during the works phases, structures that require it may be monitored in order to carry out real-time tracking of changes in their condition.