Updated 29th March 2021
Across the globe 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 international standards for example BS EN 1504 in Europe and the UK and ACI CODE- 562-21 in the USA. The use of a suitably qualified contractor is critical as is diligent substrate preparation and the selection of a repair material compatible with the host concrete. Repairs will normally be carried out using pre-bagged materials, ensuring consistency and compatibility of components. Materials are available for either hand placed, flowable or spray application, 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. This is often referred to as the incipient anode effect, the ring anode effect’ or the halo effect. The good news is that this can be easily controlled by the installation of CPT PatchGuard™ galvanic anodes. Typically installed at 500mm centres into small drilled holes around the inside edge of the repair, each PatchGuard™ anode is individually connected, using an integral tie wire, to the exposed steel within the broken out area. Once connected to the steel reinforcement the PatchGuard™ anodes produce a galvanic current which renders the steel relatively cathodic within the zone of influence, preventing cracking and spalling at the repair boundary. PatchGuard™ anodes will slowly, but non-expansively, corrode in preference to the steel. This is why they are often referred to as ‘sacrificial anodes’. The PatchGuard™ system has been proven to effectively negate the so called ‘incipient anode’, or ‘ring anode’, effect and significantly extend patch repair life.
In currently sound but contaminated areas of concrete, CPT PatchGuard™ Connect anodes can be installed to prevent further spalling. The anodes are placed into drilled holes in a grid configuration, often at 500mm spacing. The key difference is that PatchGuard Connect™ anodes are wired together in series in strings of up to 20 anodes prior to connecting to an exposed section of steel. This approach minimises the need for multiple steel exposures. All wiring is recessed into saw cuts. PatchGuard Connect™ utilises the same purely galvanic technology as standard PatchGuard™ and enables maintenance free corrosion protection to be extended beyond existing patch repairs. The PatchGuard Connect™ system can be used to mitigate the effects of corrosion in moderately exposed conditions, greatly reducing future maintenance costs.
The PatchGuard™ and PatchGuard Connect™ systems will provide protection for between 10 to 20 years depending on the local environment.
If there is a limitation to galvanic anodes it is that the properties of the sacrificial alloy place a ceiling on the voltage available to power the system. In many cases a purely galvanic system is perfectly adequate and meets the needs of the client by providing long lasting, maintenance free protection. However, in aggressive environments where a long life is required, there are benefits to having access to a higher driving voltage, particularly at the outset, to halt corrosion and build up a passive layer around the steel. CPT have addressed this need by developing DuoGuard™, the world’s first hybrid corrosion protection system. DuoGuard™ anodes differ from PatchGuard Connect™ anodes in that they are powered by an external power source, generally for 7 days, before switching to purely galvanic mode. The DuoGuard™ system can be used to stop areas of active corrosion in aggressive environments before expensive and disruptive spalling occurs.
The design life of a DuoGuard™ system is typically 20 years plus.
For the last 15 years CPT corrosion control solutions have been installed worldwide. Our technologies are complimentary with each other and are often used side by side on the same structure. PatchGuard™ anodes are used around the boundary of concrete patch repairs. PatchGuard™ Connect and DuoGuard™ anodes are used to treat areas of significant corrosion risk, identified at survey stage, which will be vulnerable to future spalling if left unprotected.
From our UK base CPT manufacture cutting edge corrosion control technology to combat destructive corrosion around the world. For our domestic clients, we also undertake concrete testing works.
For expert advice, or to discuss your project or testing requirements, please call to talk personally to one of our friendly and professional team. Please do not hesitate to contact us at email@example.com or +44 (0)115 9724 238.
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