In this technical article by the International Zinc Association Africa Desk (www.zinc.org.za), it is argued that zinc as used in galvanised light steel frame building (LSFB) construction elements, truly comes into its own in terms of offering long-lasting service.
LSFB is not the same as prefabricated or ‘kit’ buildings, as it is rather a building method. Another term is ‘off-site’ construction, due to the fact that a lot of the manufacturing takes place in factories, whereafter the components are transported and assembled on-site. LSFB consists of structured wall panels, trusses and floor joists that are assembled using cold-formed steel sections, which in turn are made from thin-gauge high-strength galvanized steel sheet.
Hot-dip galvanized substrate is produced on continuous zinc coating lines from either cold-rolled (thickness range from 0.27mm to <2.0mm) or hot-rolled (thickness 2.01mm to 3.0mm) steel substrate in coil form. It is produced to the requirements of EN 10142, EN 10147, EN 10143, ASTMA 924, SANS 4998 or SANS 3575, as well as Mittal Steel South Africa’s ISQ standards.
Rolled zinc products such as Rheinzink and Verozinc include sheet and strip, and can include alloying elements, depending on the requirements of the end product. This is a pure zinc sheet as opposed to galvanized sheet, where the zinc is applied to thin steel sheeting. Rolled zinc can be used in façades, cladding, roofing, gutters and drainpipes, as well as for ornamental applications and even as flashing. Rolled zinc products have a long service life and can be recycled and reused. Care must be taken when used at the coast under very corrosive conditions.
Zinc is used extensively to galvanize steel to prevent corrosion
Zinc generally corrodes at a low rate due to the protection offered by the zinc patina formed by the reaction between the metallic zinc surface, oxygen, rainwater and carbon dioxide. The compact, bonded and insoluble patina layer blocks exchange between the atmosphere and the zinc. Zinc is used extensively to galvanize steel to prevent corrosion. However, speak to an expert if you want to apply galvanized steel or roofing close to the sea, within 1km to 2km of the high-water mark.
The LSFB galvanized steel sections are joined together in the factory using rivets or self-tapping screws to form structural wall panels and roof trusses for on-site erection on foundations and floor slabs. Similar to timber frame construction, the wall frames are clad externally and internally on-site with a range of alternative cladding materials, with services such as electrical and plumbing and insulation material installed in the wall cavity itself.
LSFB offers a wide range of benefits
LSFB offers a wide range of benefits when compared with conventional building or other framing materials in terms of quality, cost, durability and speed. Reduced wastage, lower logistical costs and reduced time of construction could offer cost-savings of 20% or more compared with conventional building.
The typical life of a LSFB when using Z200 galvanized sheet will be 40 to 50 years due to the tenacity of the building envelope to keep out a corrosive atmosphere. In addition, LSFB complies with the rational design requirements of the National Building Regulations. This tried-and-tested building method has been deployed with great success in Australia, Europe and America for decades.
Each structure is signed off by a structural engineer and only quality certified materials are used. The frames are assembled under controlled factory conditions. In the event of poor foundation conditions, steel framed buildings can accommodate some movement in foundations without cracking.
Thermal insulation can be specified to be superior to conventional brick and mortar buildings, while professional finishes are readily achievable. Industry association SASFA (Southern African Light Steel Frame Building Association), acts as a quality watchdog that accredits system-competent frame erectors and builders.
Time-saving is estimated to be more than 30%
In terms of cost-efficiency, the time-saving is estimated to be more than 30% compared with conventional building. In addition, the steel frame dictates a high degree of accuracy and squareness of building dimensions. A steel framed wall clad with fibre cement or plaster board offers a mass saving of 90% compared with a double-skin brick wall. The low mass of the walls also means that lighter foundations are possible.
The LSFB building method lends itself to column foundations and suspended floors, reducing the cost of building on uneven sites. Other benefits are minimal wastage, energy-efficiency and flexibility as horizontal and vertical additions can be added easily in stages. There is also up to 4% extra floor space due to the reduced thickness of the external walls compared with double-skin or cavity brick walls. Services are installed in wall cavities without chasing of walls, which translates into earlier occupation of buildings for increased return on investment, as well as accurate cost budgets.
Using thin-gauge high-strength galvanized steel sheet is an important uptake and application of refined zinc in South Africa, according to Simon Norton from the IZA . While South Africa has 20% of the world’s zinc deposits, it no longer refines any zinc, following the closure of the Zincor refinery in Springs by mining company Exxaro at the end of 2011. Up to that point, the country produced 110 000 tons per year of refined zinc worth about R4.3 billion. From 2014 to 2019, South Africa imported R15 billion worth of refined zinc.
However, the uptake of refined zinc has declined rapidly from 86 000 t in 2015 to 47 000 tons in 2020. This represents a major challenge, and therefore the IZA’s short-term goal is to gradually boost zinc uptake to a new high of 60 000 tons per year, ultimately peaking at 90 000 tons per year in three years’ time.
Offers long-lasting corrosion protection
Zinc truly comes into its own in terms of offering long-lasting corrosion protection, an important consideration in South Africa’s coastal areas. As a barrier protection, zinc resists corrosion by isolating steel from the external environment. Zinc is also anodic to steel, meaning it corrodes sacrificially to protect the underlying material. In addition, a natural zinc patina develops as the coating weathers, slowing the overall corrosion rate, as well as creating a material with a distinctive appearance that is much in demand by architects and designers, for example.
Such a coating is applied by the process of either hot dip galvanizing or continuous galvanizing, the latter used mainly for steel sheet. The main mission of the IZA is to promote awareness of the corrosion protection capability of zinc among professionals such as designers and civil and consulting engineers.
For further information, contact independent Terry Smith, an independent galvanizing expert who is the go-to guy for questions on this technology. Contact him on 082 893 3911.