In the early 20th century construction techniques moved away from traditional load-bearing masonry towards masonry clad steel framed buildings. This marked the evolution of the modern high-rise building. Corrosion problems associated with these early steel framed buildings are inherent due to the nature of the early design and engineers and architects of the time failing to appreciate the destructive nature of corrosion. External masonry tightly notched around the steelwork with crudely in-filled cavities allows moisture to collect and come into contact with the steel, making the onset of corrosion inevitable. Moisture can penetrate through porous cladding materials, open or degraded mortar joints, cracked masonry, or faulty or degraded services such as cracked rainwater downpipes and gutters. Often the condition of cracking and spalling stonework is referred to as Regents Street Disease due to the number of buildings along London’s famous street that have suffered from this.
Treatment
There are two practical methods of treatment for corroded steelwork in buildings: cathodic protection or traditional treatment of the steelwork.
Cathodic Protection
This describes a general area of technology covering two forms of electrochemical corrosion prevention: sacrificial cathodic protection or impressed current cathodic protection (ICCP).
Sacrificial cathodic protection is the older method and consists of connecting a more corrosive metal like zinc or magnesium to the steel. The zinc or magnesium corrodes in preference to the steel and generates a protective current that prevents corrosion of the steel in contact with the sacrificial anode material. It is worth noting that sacrificial/galvanic protection has been shown to be unsuccessful on stone buildings.
ICCP involves the use of an inert non-corrosive anode material being placed in the structure or soil and the protective current being forced onto the steel structure through a power supply. It works on the principle of preventing corrosion by passing a current from an externally placed anode material, through a connecting electrolyte (facing and mortar) to the corroding steel work. As a result, electrochemical reactions are prevented at the steel surface through the prevention of ferrous ion formation on the steel surface and the removal of reactive negatively charged ions.
Benefits of ICCP
• It is a less invasive method than traditional repair work
• ICCP is generally more cost effective and is faster to implement
• The inert catalytic anodes are not consumed and have a high design life in comparison to sacrificial cathodic protection
• The cathodic protection current can be pushed through higher resistivity electrolytes (concrete, stone, mortar etc) by the higher drive voltages available from the power supply
Considerations with ICCP
ICCP requires highly specialised skills and only works effectively if the protective current passes consistently to the steel via a mortar, concrete or stone connection. Unfortunately, the quality and consistency of mortar in- fill in early steel framed buildings is highly variable. There is another risk with CP - Failure to ensure electrical continuity of all metallic elements in a building is an important issue as it can lead to the accelerated corrosion of any discontinuous metallic elements.
Other considerations are the costs associated with the ongoing monitoring. Although these are relatively modest, they are a cost none-the-less. There is also a need to slightly widen the joints. On balance widening joints is preferable to cutting out dozens of blocks to reach the rust, but it is still a negative impact. The external cladding is also a consideration; terracotta, faience and glazed bricks can act as an insulator and make it difficult to distribute protective currents.
Traditional Treatment
This involves the removal of damaged masonry followed by the removal of the corrosive material and then application of a suitable coating prior to reinstatement of the masonry. In cases where extensive corrosion problems are present and the masonry is displaced or damaged, traditional methods of repair are generally the only option. In instances where a mason needs to do lots of repair like this to address the problem, regardless of the long-term solution, cathodic protection is unlikely to be the right course of action. Generally speaking CP is best used as a “catch it early” solution as the cause can be addressed and much of the original fabric retained.
Traditional Repair Methods
Once the corroded area is revealed the following method may be used:
• Grit blasting the corroded steel
• Application of a corrosion inhibiting paint/coating system such as a zinc rich two part epoxy
• Where appropriate, the addition of DPC’s, particularly if a cavity cannot be achieved between the steel and external ashlar masonry
• Replacement of the masonry leaving a cavity between the steel frame and external cladding providing this is not visually evident
This method provides an adequate repair with a design-life expectancy dependent on the preparation of the steelwork and the choice of paint system.