ORN THE ONLY SOURCE FOR PROFESSIONAL ICI ROOFING CONTRACTORS IN ONTARIO ONTARIO ROOFING NEWS – ISSUE 2 2026 13 panels are moulded and cut into the desired shape, the mixture is heated and cured in high-pres- sure steamed autoclaves. The product was first devel- oped in Sweden in the 1930s, and although the use of RAAC is more common in Europe, the only RAAC sold to the Ontario market goes by the brand name of “Siporex,” which was manufac- tured by Domtar at their Montre- al plant between 1955 and 1972. Other RAAC products include Durox, Celcon, Hebel, Ytong, Aircrete and Thermalite, some of which are now being imported into parts of Canada. Siporex panels used conventional steel re- inforcement, coated with cement latex coating, to provide tensile and compressive strength to the panels. The panels also included transverse reinforcement, but these were mainly to facilitate pro- duction of the panels rather than to provide additional strengthen- ing. The product became popular in Canada due to its good insu- lating properties, which allowed for insulation to be omitted from roof assemblies, acoustical prop- erties and lightweight nature, al- lowing for ease of construction, and was mainly used in schools, hospitals, commercial and light industrial buildings.10 Although it is called concrete, RAAC differs from conventional concrete in that it has different materials and physical proper- ties than conventional concrete. The aluminum powder reacts with the water and lime to cre- ate hydrogen gas, which create uniform air bubbles throughout the concrete. These air bubbles give the concrete the appearance of looking like a sponge and sig- nificantly reduces the weight of RAAC to approximately 30 per cent of conventional concrete.6 These air bubbles also reduce the strength of RAAC, as well as its ability to bond to steel reinforce- ment. RAAC does not use coarse aggregate, which also contributes to its reduced strength.2 RAAC Durability Concerns Prior to the 2018 collapse, a string of RAAC roof panel fail- ures occurred in the U.K. in the 1980s, affecting panels initially installed in the 1960s.2 This tem- porarily halted the production of RAAC products in the U.K. and led to significant research being conducted in the early 1990s. Al- though there have been no known RAAC failures in Ontario, out of an abundance of caution, schools and other public building owners are being tasked with implement- The porous nature of RAAC makes it highly susceptible to moisture ingress and carbonation. ing a management strategy to ad- dress the safety concerns posed by RAAC. The primary durability con- cerns associated with RAAC con- struction are listed below: • RAAC has an effective ser- vice life of 30 years.7 Given that RAAC production in Ontario was stopped in 1972, all RAAC panels cur- rently in place in Ontario are well beyond the end of their expected service life and are due for replacement. • The porous nature of RAAC makes it highly sus- ceptible to moisture ingress and carbonation. This vul- nerability leads to moisture- related degradation and concrete carbonation effects such as shrinkage, sulfate attack and leaching of ce- ment hydrates.5 As such, all RAAC requires exterior weather protection. • The porosity of RAAC also compromises the al- kaline protective layer that normally shields steel reinforcement within con- crete, making it prone to corrosion from rainwater penetration and carbon- ation. To mitigate this is- sue, steel reinforcement in RAAC is typically coated with bituminous or cement latex coatings. However, assessments have revealed corrosion even in areas where these coatings appear visually intact, rais- ing concerns about their long-term effectiveness.2 • Corrosion of the steel re- inforcement can occur without apparent cracks or spalling occurring on the exterior of the RAAC due to its high porosity and low compressive strength.7 This hidden corrosion contrib- utes to the unforeseen sud- den failures that can occur with RAAC panels. • The low modulus of elas- ticity of RAAC causes it to undergo permanent creep deflection when subjected to loading. This permanent deflection increases the like- lihood of water ponding on roofs, which further increas- es loading and likelihood of moisture infiltration. This makes RAAC roof panels particularly susceptible to failure; although, RAAC used in wall systems are also susceptible to degradation via the same mechanisms.
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