It is well known that steel corrodes and that the resulting rust occupies ten times the volume of the original steel. When the steel is embedded in concrete, whether as steel beams or reinforcing bars, this expansive reaction cracks the concrete and forces the cover concrete to spall away. This lets in more moisture, accelerating the corrosion.
One might think that stopping this process is simply a matter of chipping back the loose concrete, cleaning the steel back to bright metal and reapplying new concrete. Whilst this is a perfectly acceptable method for the direct area of repair, it does nothing to improve the situation in adjacent parts of the structure and may even make matters worse. To understand why, we need to look at the basic corrosion mechanism with steel embedded in concrete, which is an electro-chemical process.
It takes an enormous amount of energy to convert iron ore into steel and, left to its own devices, the steel readily converts back to iron ore (rust). The alkaline environment that the freshly poured concrete affords arrests this decay. However, atmospheric carbon (principally from CO2 gas) and chlorides in de-icing salts or sea spray neutralise the alkalinity through carbonation and chloride attack respectively. When the alkalinity drops at the level of the embedded steel, corrosion can start. This may take decades to set in but many of our structures are old enough that they are now in trouble and need intervention to prevent degradation.
At the point where the alkalinity is lowest, an anode is set up in the steel. Next to this, a cathode is established and electrons flow from the anode to the cathode. The corrosion happens at the anode.
After a concrete patch repair has been carried out, that section is once again highly alkaline.
However, adjacent to the repair is old concrete that might be barely alkaline at all. A new anode therefore establishes itself in the old concrete, leading to new corrosion adjacent to the repair. Due to this phenomenon, which is known as the incipient anode effect, in as little as three years after the repair the member can be in just as sorry a state as before.
The most effective remedy is to apply a cathodic protection (CP) system. This imposes a low electrical current by artificial means that stops the electro- chemical reaction just described. Either the current can be provided from an external power source or from a metal which is less noble than steel, ie a metal that will corrode in preference to the steel. Such metals are usually zinc or mixed metal oxides on a titanium matrix.
Two projects that illustrate the successful implementation of cathodic protection are Tilbury Dock and Nettlehill.
The A1089 Tilbury Dock Road crosses over the Fenchurch Street to Southend railway just north of Tilbury Town Station on a nine-span viaduct. It mostly comprises of a steel composite deck supported on reinforced concrete piers, crossheads and abutments, and it was these support structures that needed repair and protection.
The project brief was to carry out concrete repairs and install an impressed current cathodic protection system to these supports. Specialist contractor Freyssinet was brought in to:
- Remove defective and delaminated concrete using hand-held breakers for small areas or hydro-demolition for larger areas with all repairs having saw cut edges;
- Replace the concrete using hand-trowelled polymer modified repair concrete, shuttered and flowable concrete or sprayed concrete;
- Design and install an impressed current cathodic protection system, noting the need for the system to take account of lateral post- tensioning in the pier base beam.
The CP system was designed and commissioned by Freyssinet’s in house cathodic protection design company, Corrosion Control Services Limited. It was installed on three crossheads, two abutments and three piers. The solution was designed as two independent systems, one either side of the bridge, each linked into its own stainless steel enclosure which contained all the controllers and data links.Installation entailed grit blasting the concrete surface where CP was to be applied and fixing titanium mesh anodes against the roughened surface. Various electrical connections were made between the original reinforcement cage, the control panel and the new mesh. Reference electrodes were installed at specified locations (these check that the system is working effectively), then 25mm of concrete overlay was sprayed over the cables and mesh as permanent protection.
A second example of this type of problem is Nettlehill Railway Bridge which carries the A899 over the Edinburgh to Bathgate line immediately south of junction 3 on the M8 and close to Livingston. The crossing comprises three separate two- span structures with precast concrete beam decks.
These three structures carry the two carriageways plus a sliproad and were suffering from corroded reinforcement in their blade piers and abutments. Earlier patch repairs were starting to blow again and there were many additional areas of rusting and spalling.
Defective concrete had to be removed by hydro-demolition due to the extensive areas involved. The average depth of repair was 120mm in order to give good penetration for the replacement concrete which was sprayed back in behind the reinforcing bars. Propping of the precast beams that formed the deck was necessary before works could commence on repairs immediately below their supports.
The decision had been taken to apply an impressed current CP system to the bridge abutments and central blade piers. To accomplish this, two types of anode system were used. An MMO (mixed metal oxide) coated titanium mesh anode was applied directly to the atmospherically exposed faces of the structures and an MMO coated ribbon canister groundbed system was buried adjacent to the abutment wingwalls to provide protection to those elements below ground level.
Both of these projects were completed with minimal disruption to rail services. The currents now flowing through the reinforcing cages of both structures may well be tiny but the effect is powerful. Corrosion will be kept at bay and the fine blank canvas that the new sprayed concrete provides can be enjoyed by graffiti artists for years.
Report by Kevin Bennett, senior technical director, Freyssinet Ltd.