Bellingham Marine, headquartered in Newport Beach, Calif., recently installed its first fully GFRP-reinforced Unifloat® concrete dock system in a New Zealand marina. The docks, which feature GFRP thru-rods, rebar and beams, are lighter, stronger, easier to maintain and more environmentally friendly than steel and timber-reinforced ones. With an expected lifespan of 100 years or more, they also offer extreme durability.
“Most floating docks use treated timber for the walers and steel bolts to hold them onto the floats, which are reinforced with steel rebar,” says Pete Renshaw, business development director at Pultron Composites Ltd. in Auckland, New Zealand. “All of these are subject to deterioration over time and limit the lifespan of the structure.”
The dock system is the culmination of a decade-long collaboration between Bellingham Marine and Pultron Composites to replace timber and steel dock reinforcements with composites. The companies first tackled the thru-rods, which attach floating concrete sections to walers (structural beams mounted to the deck). To develop the thru-rods, which were introduced in 2015, Pultron relied on its experience producing GFRP rock bolts for tunnels and mine reinforcement and its work in the sports and recreation industry.
“We knew that the tie rods would be subjected to constant wave movement affecting the dock system, so we used some of our know-how from [GFRP] trampoline rods that we developed, which have high flexural fatigue resistance,” says Renshaw.
While the GFRP thru-rods have twice the tensile strength of steel, they have a low tensile modulus. This allows them to stretch and contract and maintain tension.
“This has proven to be a big advantage as it saves a lot of maintenance,” says Renshaw. “As the timber dries, it can shrink enough for the steel bolts to become loose, but the extra stretch in the GFRP bolts accommodates this shrinkage so they do not come loose. Much less retightening is now required.”
Pultron Composites pultrudes 20-millimeter-diameter thru-rods using corrosion-resistant ECR glass rovings and a proprietary resin system in lengths up to 12 meters. After fabrication, a purpose-built CNC grinding machine turns the smooth rods into threaded bolts.
“Rather than cutting the thread, as is normally done for steel bolts, our threads are ground into the surface using profiled diamond grinding wheels,” says Renshaw. The nuts are injection-molded from glass-filled nylon by Lane Plastics, another Auckland company.
After developing thru-rods, Bellingham began efforts to replace the steel rebar in concrete floats with Mateenbar™, a GFRP rebar developed by Pultron and manufactured by Mateenbar Ltd. Although Mateenbar had been used in more than a thousand infrastructure projects, including roads, bridges, tunnels, seawalls and aluminum smelters, concrete floats were new territory. Consequently, Bellingham trialed them in marinas in Australia and Cyprus before introducing them commercially in 2018.