• Cumunel, Gwendal
• Jeanjean, Yannick
• Hoang, Tien
• Forêt, Gilles
• Martin, Julien Castres Saint
• Aparicio, Jonas

Abstract

External prestressing is a construction technique widely used since the 1980s for large civil engineering structures. This technique is used to reduce the cross-sectional area of structures, and thus their perma- nent load, and more recently to reinforce structures. External prestressing tendons are composed of strands, each comprising seven wires with a very high elastic strength, laid in a HDPE sheath, anchored in reinforced concrete blocks and generally tensio- ned to 80 % of their tensile strength (excluding creep and shrinkage). For the protection of the cables, cement grout has been injected into the sheaths until the 2000s. However, this protection does not make it possible to preserve the cables in case of failure of the sheath (leakage, cracking, etc.) or injection default. The durability of the cable is therefore no longer guaranteed and stress-corrosion can occur. Because of the re-anchoring by friction in the grout of the wires and strands broken by corrosion, the stress of the damaged area of the cable increases until its breaking limit is reached. The fracture is then very sudden, releasing considerable energy that can endanger the structure and any operators present on site. Detecting the level of degradation of a cable before its fracture is therefore essential for the structure managers. Several cables 46,6m long between supports have been tested on the site of UGE Nantes and have been bringen to fracture by progressive cutting of the wires making up the strand. Firstly, three cables com- prising a single strand of seven wires have been tested. The cables have been tensioned to 0.70 Frg corresponding to the usual residual stresses observed in real structures after shrinkage and creep. Ex- citation devices have been used in order to measure the vibration response using accelerometers. As the wires break, variations in the natural frequencies are observed and used to characterize the condition of the cable.

Topics

• impedance spectroscopy
• corrosion
• strength
• cement
• tensile strength
• durability
• wire
• creep