In this article, Ian Godson outlines the growing risk of corrosion caused deterioration to prestressed concrete elements and summaries the investigation, repair and long term durability options for prestressed structures
The use of Prestressed Concrete for construction of bridge, wharf and other civil structures expanded rapidly from the 1960’s onwards, due mainly to the increased spans and economy that it provided. The prestressed elements have also proved to be very durable, due mainly to the high quality of construction including good concrete cover, excellent quality concrete and high quality (commonly steam) curing. Another major durability factor is that cracking is limited due to the prestress compression, minimizing access of chlorides and carbonation to the reinforcement and prestressing strands.
However, prestressed elements are in no way immune to corrosion especially that initiated by chlorides. Further, the amount of chloride known to initiate corrosion in prestressed strands is around 50% of that of conventional reinforcement, with concern for active pitting corrosion if chloride levels at the steel depth exceed 0.2% Cl/wt cmt (~0.03%Cl/wt concrete). Once strand corrosion has commenced, the corrosion is commonly to multiple wires in the strand, and with only minimal pitting required to rupture the stressed strand, serious structural ramifications follow.
The investigation of prestressed elements is significantly more complicated than for conventionally reinforced structures. Firstly, the strands are commonly not electrically continuous, complicating the common testing using half cell potential and corrosion rate equipment. Accordingly, the electrical continuity must be evaluated at the start of the investigation, with access to the strands usually completed by drilling at the ends of the beams where the structural requirement of the strands is low. Access to deep strands may be very difficult especially in cases of draped strands. The use of a ground penetrating radar (GPR) is required to confirm the location of the strands.
Having confirmed the electrical continuity, or most likely the lack of it, electrical connections are completed to multiple strands, allowing half cell potential and corrosion rate analysis to be completed. Chloride sampling is targeted to likely corroding locations with drill sampling at depth preferred to coring unless a thorough GPR survey is completed. Delamination or spalling of the concrete will almost certainly indicate failure of the strand(s), unless the conventional ligature reinforcement is responsible. Exposure of suspect corroding reinforcement is highly recommended to confirm the state of the strand, with hydro-demolition favoured over jackhammer concrete removal to avoid damage to the brittle, fragile strands.
Prevention is the best defence for prestressed elements, with early application of silanes or alternative breathable coatings to reduce chloride ingress, preferably applied well before critical chloride levels approach the strands. But if it is too late for prevention and active strand corrosion is occurring, the repair options are very limited. Damaged strand cannot be repaired and is commonly cut out, with supplementary strengthening methods including carbon fibre utilized to compensate for the lost strength.
Conventional Patch Repairs cannot be used due to the in-built compression forces in the concrete and the likelihood of damage to the brittle, prestressed strands. The main remaining alternative repairs are limited to forms of cathodic protection as discussed below.
Prior to the installation of any form of CP, the lack of electrical continuity commonly found in prestressed elements must be rectified before any of the CP options can be utilized. In some cases, the ends of the beams are accessible allowing easy access to the strands to establish electrical continuity. More typically, the ends of the beams are not accessible, requiring identification by GPR and exposure of all strands from the side of the beams. This strand exposure is undertaken via hydro-demolition within the first 1m of the beam end, in the low bending moment of the element, allowing connection to each strand by appropriate cable connections. Once the electrical continuity is confirmed, the CP system can be installed on the element.
Forms of cathodic protection for prestressed elements include ICCP (Impressed Current CP), Hybrid CP and galvanic systems but there are significant challenges for all these options. Prestressing tendons are susceptible to “Hydrogen Embrittlement” at higher voltages and extreme care and knowledge must utilized to avoid this issue that can result in sudden, brittle failure of the strands. The Australasian CP code limits the voltage of the strand to >-900mV (AgAgCl 0.5M at instant off). Infracor adopts a more conservative figure of >-800mV in its specifications and is an ongoing requirement throughout the life of the applied CP systems.
Impressed Current CP (ICCP) usually utilizes Titanium MMO discrete anodes installed in drilled holes into the beam or alternatively ribbon anodes installed in slots cut into the beam surface. A conservative design is required with small zone sizes and close spacing of anodes to minimize the driving voltage to protect the strands. Increased numbers of monitoring reference electrodes should be installed to ensure the accurate recording of the potential at representative areas of the structure. The control system should preferably be remotely accessible and automatically record the potential of the reference electrodes to ensure the >-800mV criteria is not exceeded. With the use of a remote monitoring system with in-built remote alarms is ideal as the potential must be recorded and controlled for the entire life of the CP system.
Hybrid CP utilizes zinc anodes installed in drilled holes into the concrete element, connected by small titanium wires, and is normally operated in impressed current mode from temporary power supplies at ~9V for 10 days. The power supplies are then removed and the zinc anodes then connected to the steel to provide the long term galvanic protection, with the zinc anodes providing ~0.5V ongoing protection. In the case of prestressed steel, the normally applied voltage must be reduced to ~1.5V to ensure the >-800mV limit is not exceeded in the impressed current stage, with the duration of the impressed current increased to approximately 8-10 weeks. The instant off potential at the implanted reference electrodes must be carefully monitored during the impressed current phase, but once this is completed and the system converted to galvanic mode, there is no ongoing risk of Hydrogen embrittlement for the low voltage of the Hybrid system.
In summary, early investigation of prestressed elements and the prevention of chloride ingress through coatings is the best method of extending the life of the elements. When faced with serious levels of chloride ingress resulting in active corrosion of the strands, cathodic protection methods offer the best alternative remediation options. However, the challenges of CP to prestressed elements, including electrical continuity and hydrogen embrittlement, require a high level of design, installation and monitoring.
Ian Godson, Managing Director, Infracorr Consulting PL