Il ponte in compositi (o meglio costruito grazie ai compositi)

[Rand]

Interessante soluzione per diminuire i costi di costruzione e aumentare la resistenza agli elementi dei ponti in cemento armato:

“We were looking for ways to use … composites … to reduce the time it takes to erect formwork or, better yet, get rid of it — and rebar — altogether,” recalls Center director Dr. Habib Dagher of the program’s beginnings. “Plus, we wanted to find a way to protect concrete from the environment.”

Reinventing the concrete bridge
The university assembled a team to pursue these goals in the context of an arched structure (considered to be the most efficient for bridges) that could be produced at the work site. Among the large number of design options considered and then discarded were rigid foam-backed forms and composite overmolding, using “tools” like bent thermoplastic tubes.

The concept that showed the most promise for formwork was an array of fabric sleeves that could be inflated and then made rigid by applying and curing a thermoset resin. In theory, it would serve as a stay-in-place concrete form, an exoskeleton that eliminates the need for rebar and a waterproof layer to protect the concrete from environmental deterioration, but it proved to be very challenging in the realization. Years were devoted to fiber architecture alone — finding the right types and orientations of fibers to handle the hoop stress (to contain the wet concrete), biaxial shearing forces, and longitudinal stresses. Other questions concerned the most efficient methods for producing the sleeves and introducing concrete into them, as well as how to ensure they would be watertight and that fiber buckling on the inner curve when the sleeves were formed into arches wouldn’t sacrifice mechanical properties.

In final form, the sleeve fabric is a complex and proprietary blend of glass, carbon and other unnamed fibers. The custom-designed sleeves are cut to length and width, joined in a proprietary manner to form a tube and mounted on custom-built fixtures. Next, the tube ends are sealed, the sleeves are inflated with air, and then they are impregnated with resin. Curing occurs at room temperature and normal atmospheric pressure. According to Dagher, tube length, diameter, wall thickness and the number of arches can be varied to accommodate the needs of any given application. Longer, larger-diameter arches placed closer together can carry higher loads.

UMaine also had to custom-formulate a durable, ductile resin that would adhere well to concrete and develop a proprietary self-consolidating Portland cement-based concrete that expands slightly to ensure a continuous bond between it and the composite shell.

Accelerated fatigue testing subjected the concrete-filled arches to the equivalent of 50 years of truck traffic over a busy interstate bridge. Afterward, the arches retained their full load-bearing capacity — residual strength was equivalent to initial strength. Testing also revealed that, compared to reinforced concrete, the arches are extremely ductile, a function of confinement of the concrete and elimination of rebar. (If the bridge exceeds its design stresses and the rebar is put into compression, it can buckle outward, fracturing the concrete.) The arches effectively form three plastic hinges (constrained at both ends and at the top) and maintain peak strength throughout fatigue testing. Adverse environmental testing has shown that the arches are effective at the temperatures at which conventional concrete bridges are used, that is, from -30ºF to 150ºF (-34ºC to 66ºC).

The next day, composite spandrel walls were erected along the sides of the cured arch structure to contain the granular, sand-filled backfill that was placed and compacted on top of the arch structure. Next, the bridge was paved with asphalt and conventional bridge rails were set in place before the road was reopened to traffic. The total installation time was about three days.

Now confident that its design will minimize a bridge’s life cycle cost and double the bridge’s life span, UMaine has issued a commercial license to newly launched Advanced Infrastructure Technologies (Orono, Maine, see the article entitiled "UMaine commercializes two hybrid composite/concrete bridge technologies" under "Editor's Picks," at right), which will sell and install bridges using the university’s inflatable-arches technology and design software. Bridges that will use the arches technology are already planned for 2009 in Maine and other states.