Protection of structures against corrosion enhances durability.
Three levels of protection for tendons
The protection of structures against corrosion is a critical challenge for project owners and becomes even more vital for post-tensioned structures and post-tensioning cables subjected to aggressive environments. VSL has developed solutions that improve durability and give owners greater peace of mind. Various solutions provide increasing levels of corrosion protection to suit the particular environment and structural details. An approach with three Protection Levels (PL) has been proposed by the International Federation of Structural Concrete (fib).
Protection Level 1 involves placing the tendon in a metal duct with a void filling material to provide a high degree of corrosion protection in cases where there is a fairly low exposure to an aggressive environment. The HPI® high – performance grout developed by VSL provides additional protection and has been specifically designed to improve the corrosion protection of anchorages and cables.
Protection Level 2 consists of PL 1 plus a corrosion-proof envelope over the full tendon length, creating a leak-tight barrier.
The VSL PT-Plus® plastic duct for post-tensioning cables fulfils this requirement and has become a standard in many countries. The duct creates a complete and leak-tight encapsulation of the tendon at the anchorage and over the free length of the cables to prevent any ingress of aggressive substances such as chlorides.
Protection Level 3 consists of PL 2 plus the capability to monitor and inspect the integrity of the tendon or tendon encapsulation at any time during its design life. VSL electrically isolated tendons (EIT) allow ongoing monitoring of the tendon encapsulation. A high electrical resistance between the prestressing steel inside the encapsulation and the reinforcing steel outside provides proof that there has been no breach. Any breach in the encapsulation – and therefore risk to the corrosion protection - results in an immediate and significant drop in electrical resistance. EIT tendon technology therefore allows early detection of any deficiencies, making maintenance and repair easier and cheaper to both plan and execute. It prevents major damage along with the associated risks and cost.
Protecting internal tendons with VSL PT-PLUS®
Encapsulating internal post-tensioned tendons with VSL PT-PLUS® plastic ducts, for example in segmental bridges, has several advantages over the use of traditional metal ducts:
- The VSL PT-PLUS® plastic ducts remain intact even under application of large strand pressures and tendon elongations during stressing.
- The VSL PT-PLUS® plastic ducts offer significantly higher fatigue strength and remain intact across large active cracks even at low temperatures.
- VSL PT-PLUS® plastic ducts reduce friction losses during stressing but provide sufficient bond for fully-bonded tendon design.
In addition, the VSL segmental duct coupler represents a major development in the provision of fully-encapsulated post-tensioning tendons in precast segmental construction. Monitoring can be used. Tendons are protected effectively in the critical sections across joints, evening very harsh climates or exposure conditions, including situations where de-icing salts are in use. The VSL segmental duct coupler has excellent performance and its compact design permits detailing of precast segmental structures identical to those using traditional construction methods with metal ducts. Fully-encapsulated tendons are expected to achieve a design life of 100 years or more, even in harsh environments.
VSL Void sensor
Prestressing steel is actively protected against corrosion when covered in cementitious grout. Corrosion of the steel inside the grout can only start once the alkaline environment created by the grout has been destroyed, for example by the presence of a sufficiently large amount of chlorides. Humidity and water alone cannot cause corrosion. The situation is however significantly different if the prestressing steel is not fully encased in cementitious grout, where voids in the grout leave the prestressing steel locally exposed and unprotected. Until recently it was quite difficult to detect such voids inside the duct of a tendon cast into a concrete structure. Once installed and grouted, it was also very difficult to confirm that the prestressing steel was well protected by the cementitious grout throughout the design life of the tendon, or that there was no corrosion. It was also difficult to prove, non-destructively, that chlorides were present in the grout, destroying its protective capability, and leading to corrosion of the prestressing steel.
VSL, together with a corrosion specialist, has now developed a sensor which allows confirmation that the tendon ducts have been completely filled with grout during the injection process. The system also allows monitoring to identify corrosion risk or the presence of corrosion inside the grouted tendon at any time during the tendon’s design life wherever sensors have been installed along the tendon’s length. The void and corrosion detection sensor for grouted tendons is a major step forward in the quality control and monitoring of posttensioning tendons. It can be used to confirm to owners that the grouting of the tendons has been done in full compliance with the specification, achieving complete filling of the tendon duct. It also confirms that the prestressing steel is not corroding or at risk of corroding at any time during the design life of the tendon, ensuring that the tendon strength has not been compromised by corrosion.
High strength for high protection
Ductal® is a material with unmatched strength and flexibility that has been developed jointly by Bouygues-TP and Lafarge and is marketed by VSL for specific uses. It generates a 20-fold increase compared with conventional concrete properties. Ductal® is a very dense material, and thus has much greater durability than conventional concrete, with virtually no maintenance needs. It comes in a wide palette of colours and can be used with precision in a range of potential shapes. Ductal® is almost impermeable and therefore provides excellent protection to other structural elements or materials beneath.