Concrete spalling effects a wide variety of structures including concrete framed buildings, multi-storey car parks, bridges, jetties, tanks and bunds. Spalling can have wide ranging consequences in terms of health and safety, structural integrity and asset value. Unfortunately a sticking plaster approach is often taken to repairing spalling structures, with the underlying causes left untreated. So, what exactly is spalling? What are its effects and how can structures be successfully repaired and further corrosion prevented?
Spalling is a term used to describe areas of concrete which have cracked and delaminated from the substrate. There are a number of reasons why spalling occurs including freeze thaw cycling, the expansive effects of Alkali Silica Reaction or exposure to fire. However, the most common cause of spalling is the corrosion of embedded steel reinforcement bars or steel sections. Corroding steel can expand up to ten times its original volume, exerting stress on the surrounding concrete.
When steel is cast into concrete, the naturally high alkalinity helps to protect the embedded steel from corrosion. However, the protection afforded by high alkalinity can be compromised by the ingress of acidic atmospheric gases, a process normally referred to as carbonation, or by the presence of salts in the concrete, typically from marine environments or from the use of de-icing salts. Combine these contaminants with oxygen and water and we have the key ingredients for corrosion. The reason some concrete structures exhibit widespread spalling whilst other appear to be in a sound condition is down to a combination of age, maintenance, concrete quality, the depth of concrete cover and local environmental conditions.
Firstly, spalling is not attractive and makes a structure look uncared for. More seriously, if the structure is in an area accessed by the public, or by workers, then spalling can be hazardous in terms of falling debris or trip hazards. If left unchecked then spalling will tend to accelerate and spread so that, eventually, the structure could become unstable. Diligent maintenance is therefore of great importance and legislation places a duty of care on owners to ensure that structures are not hazardous to users.
There are also serious commercial considerations. If a structure is left to deteriorate then the value of the asset will diminish and, furthermore, maintenance costs will tend to increase as corrosion and spalling becomes more widespread. For some structures, such as car parks, customers will stay away if the environment is unattractive and potentially hazardous.
Concrete repairs should be carried out in accordance with BS EN 1504, the European and British Standard for the repair and protection of reinforced concrete. The use of a suitably qualified contractor is critical as is diligent substrate preparation and the selection of a repair material which is compatible with the host concrete. Typically concrete repairs will be carried out using pre-bagged materials, thus ensuring consistency and compatibility of components. Materials are available for either hand placed, flowable or spray application, normally determined by the size and geometry of the application.
However, despite best intentions, concrete patch repairs often fail prematurely due to on-going corrosion around the periphery of the repairs. The good news is that this can be easily controlled by the installation of CPT PatchGuard™ sacrificial anodes, typically at 500mmm centres, into small drilled holes around the edge of the repairs. When connected to the steel reinforcement PatchGuard™ anodes produce a galvanic current which renders the steel relatively cathodic within the zone of influence. The PatchGuard™ anodes will thus slowly, but non-expansively, corrode in preference to the steel. PatchGuard™ anodes will typically last 15 to 20 years and have been proven to effectively negate the so called ‘incipient anode’ effect and significantly extend patch repair life.
To extend corrosion protection beyond existing patch repairs and prevent further spalling in currently sound but contaminated areas of concrete, CPT DuoGuard™ anodes can be installed into small drilled holes in a grid configuration, typically at 400mm spacing. DuoGuard™ anodes differ from PatchGuard™ in that they are initially powered by an external power source for seven days prior to switching to self-contained galvanic mode. DuoGuard™ anodes can be used to stop active areas of corrosion before expensive and disruptive spalling occurs.
For more information please visit the DuoGuard™ and PatchGuard™ pages at www.cp-tech.co.uk. CPT undertakes concrete testing works as well as manufacturing cutting edge corrosion control technology. For expert advice, or to discuss your testing requirements, please call to talk personally to one of our friendly and professional team. Please do not hesitate to contact us at firstname.lastname@example.org or +44 (0)115 9724 238.
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