Corrosion

Related Resources:

The Importance of a Building Safety Case →

Glass Risk Assessment →

In a series of bitesize articles, Wintech’s Expert Witness Director, Nadha Dawood, draws upon her experience within the field and shares her insight into some of the challenges impacting the façade industry. In this instalment, we explore the issue of corrosion of Steel Framing Systems (SFS), examine its impact on façade construction projects and address the importance of proactive measures.

From my experience as an expert witness, where I have dealt with corrosion-related matters within the façade construction industry, this has the potential to not only compromise the structural integrity, but in doing so, also poses a substantial risk to public safety.

What is Corrosion?

Corrosion in façade construction poses a significant challenge, compromising the strength and stability of metal components and structures such as fixings, anchor bolts, SFS profiles and secondary steelwork. Inadequate preventative measures, including insufficient weathertightness protection, lack of separation between bimetallic components and improper use of membranes and seals can all contribute to corrosion-related issues.

Lack of Preventative Measures

Typically, I have found that a lack of preventative measures such as an inadequately specified galvanic zinc thickness suited to the environment, separation of bimetallic components, appropriate use of membranes and seals to address weathertightness and/or mitigating risk of condensation within the build-up, can all be contributing factors. The consequences of corrosion extend beyond surface deterioration as it can weaken load-bearing elements and compromise the stability of the entire supporting structure.

Furthermore, I have been witnessing an increase in corrosion to steel framed system (SFS) profiles. This is not just limited to surface corrosion or at cut edges but also around fixings that propagate to weaken the material considerably. Typically, the steel profiles within low corrosivity indoor environments are coated with a galvanic mass corresponding to 275 grams of zinc per square metre, which is approximately a minimum mean coating of 0.02mm thickness of zinc per face. This can provide a predicted design life of over 200 years if it is well protected. However, this has not always been the case.

Insulation backing wall within SFS

In a warm frame construction, SFS profiles are typically used to form the backing wall and the outer leaf of the construction is insulated. This helps to keep the SFS profiles above the dew point and minimises the risk of interstitial condensation. Whilst breather membranes were historically proposed to the outer face of the insulation to protect the insulation layer, they are now commonly installed between the SFS backing wall and insulation layer. This location offers greater protection of the breather membrane from damage from positive wind pressures, installation defects around bracketry and protection from the insulation layer, providing greater assurance of achieving weathertightness for the life of the building.

Insulation can also typically be placed within the SFS studs to meet the required thermal performance. If doing so, the external insulation should be thick enough so that internal condensation formation does not become problematic. This is often referred to as a hybrid construction.

Insulation within SFS

Furthermore, a vapour control layer installed between the internal lining and the SFS will assist in preventing moisture vapour migrating from the interior to the exterior, thus mitigating interstitial condensation. In addition to this, well installed and continuous waterproofing membranes will protect from any external moisture ingress.

In addition to poor design and workmanship of the cladding build-up, the other contributing factors I have observed are:

• Premature termination of ventilation ducts within cavity insulation which permanently keeps the insulation moist

• Water ingress at window surrounds that gets trapped between the concrete slab and SFS profiles leading to carbonation of concrete and subsequent corrosion of steel

• Over torquing of the fixings that damage the zinc coating leaving the base metal without sacrificial corrosion protection

Conclusions

Repairing, remediating or replacing corroded steel elements in fully functioning buildings is time consuming, labour intensive and cost prohibitive due to restricted access. To address these issues, it is crucial to raise awareness of effective corrosion management in the construction industry and incorporate necessary measures during the design and construction stages. By prioritising prevention and proactive maintenance, we can mitigate the risks associated with corrosion, ensure the longevity of structures and prioritise the well-being of building occupants.

Related Resources:

The Importance of a Building Safety Case →

Glass Risk Assessment →