Composites strengthen National Trust property in U.K.

An 18th Century manor in Oxfordshire, U.K, the Buscot House recently underwent structural rehabilitation, including the use of carbon fiber-reinforced polymer (CFRP) materials to strengthen an ornate concrete stairwell. The house hosts many visitors, and the weight on the stairwell was causing deflection on the landings. Pultruded Sika CarboDur CFRP plates from Sika (Baar, Switzerland) now strengthen the stone landings of the stairwell. The CarboDur CFRP structural strengthening system's plate material ranges in width from 50 mm to 150 mm (1.97 inches to 5.91 inches) at thicknesses of 1.2 mm and 1.4 mm (0.05 inch and 0.06 inch). Gifford & Partners (Woodlands, Southampton, U.K.), the engineering group for the project, selected a 50-mm wide by 1.2-mm thick (1.97-inch by 0.05-inch) plate material, which was shipped to the job site in a 250m/820-ft roll. Plates were cut to size on location.

The undersides of the landings were channeled out to permit subsurface installation of the 50-mm/1.97-inch wide plates, which were bonded in place with Sikadur 30, a special cold-cure, two-component epoxy structural adhesive formulated for effective bonding to different substrates. The adhesive holds the plates in place during cure with no additional support required. The adhesive is allowed to cure for three days at room temperature (20°C to 25°C or 68°F to 77°F). In situations where room temperature cannot be maintained — at 10C°/50°F, for instance, cure takes seven days — cure can be accelerated by using the CarboHeater, an electrical heating device that is attached to the plates using contacts that measure the temperature directly in the adhesive layer. The device keeps the temperature of the adhesive constant through the curing process. At 70°C/158°F, cure can be reduced to as few as three hours. According to Reto Clénin, Sika's corporate project product engineer — structural strengthening, cure at elevated temperatures also produces better mechanical properties and a higher glass transition temperature (Tg) in the adhesive.

The cured plates were coated with a layer of Sikadur 30 with sand filler to form a bonding bridge for Sika MonoTop-620 leveling mortar, thus leveling the repaired areas with the surrounding substrate. Once painted, the repaired area blends with the rest of the structure. The installation of the plates took about three days and the finishing steps brought the total project time to about one week. The repair strategy provided an optimum strengthening solution that had minimal visual impact on the stairwell.