About Segmental Construction
As its name implies, a segmental bridge is a bridge built in short sections (called segments), i.e. one piece at a time, as opposed to traditional methods that build a bridge in very large sections. The bridge is made of concrete that is either cast-in-place (constructed fully in its final location) or precast concrete (built at another location and then transported to their final location for placement in the full structure).
These bridges are very economical for long spans (over 100 meters), especially when access to the construction site is restricted. They are also chosen for their aesthetic appeal.
– The first segmental concrete bridge, built in 1950, was cast-in-place across the Lahn River in Balduinstein, Germany.
– The first precast segmental concrete bridge, built in 1962, crossed the Seine River in France.
– The first U.S. precast segmental concrete bridge, built in 1973, in Corpus Christi, Texas.
– The first U.S. cast-in-place segmental bridge, built in 1974, was built near San Diego, California.
– The first U.S. precast segmental concrete arch bridge is the Natchez Trace Parkway Bridge, completed in 1993.
From these beginnings, both precast and cast-in-place segmental concrete bridge construction have grown to become a major component of the bridge construction industry in nearly all parts of the world.
– The sequence of construction is similar to traditional concrete bridge building, i.e. build the support towers (columns), build the temporary falsework, build the deck, and perform the finishing work. The principle differences are as follows:
– The support towers may be built segmentally. Often this is accomplished using “slip-form” construction, where the falsework moves (slips) upward following sequential concrete “pours.” The falsework uses the newly constructed concrete as the basis for moving upward.
– After the towers are built, a superstructure is built at top of the towers. This superstructure serves as the “launching” point for building the deck. (The deck is often built in both directions away from the tower, simultaneously.)
– The deck is now constructed sequentially, beginning at the tower, one section at a time.
– In cast-in-place bridges, the falsework is connected to the previously installed concrete and allowed to cantilever freely. Next, the permanent reinforcing steel and supports are installed. Finally, the concrete is placed and cured, freeing the falsework to be moved.
– In pre-cast bridges, the concrete segment is constructed on the ground, and then transported and hoisted into place. As the new segment is suspended in place by the crane, workers install steel reinforcing that attaches the new segment to preceding segments. Each segment of the bridge designed to accept connections from both preceding and succeeding segments.
– The process in step 3 is repeated until the span is completed.
A lot of concrete bridges, railway overbridges, flyovers and viaducts were constructed in recent years and many more are under construction. On the evidence of durable concrete structures it was earlier considered sufficient to prevent visible water leakages through concrete roofs, water reservoirs, tunnels, etc. In contrast, it is seen in the case of concrete structures, built with today’s cement and rebars, that the mere arresting of visible water leakages is not sufficient for making such structures reasonably durable. As in the case of steel structures, all surfaces of today’s concrete structures, which may be exposed to the atmosphere, more particularly structures, which might be intermittently exposed to water, need be given surface protection, i.e. made waterproof.
Waterproofing of bridges and viaducts, and large infrastructure elements in general, is crucial to their operational performance, safety and long service life. All man-made structures, especially bridge and viaduct decks, are under constant physical and chemical attack from rainwater, freeze-thaw cycles and traffic, which results in wear induced by loads, vibrations and mechanical strain. These negative effects are further compounded by road salting during the winter months. Systems designed to protect structures of this kind are therefore, receiving more attention, particularly waterproofing membranes. Indeed, harmonised standards have even been issued at a European level specifically concerning bridge and viaduct waterproofing: notably, reference standard EN 14695, as described below. In detail, waterproofing is installed between the substrate and the road surfacing produced with a hot mix asphalt (binder course), which is applied at a high temperature directly on top of the actual waterproofing membrane.
The waterproofing of reinforced concrete slabs for road decks (bridges or viaducts) was already being done with bituminous products in the late 70s, building up the various layers directly on site. For over twenty years now, the waterproofing practice of installing modified bitumen and polyester nonwoven reinforcement on site has been replaced or supplemented with the installation of prefabricated polymer distilled bitumen membranes. Standard EN 14695-2010 issued by the European Committee for Standardisation has been in force since January 2010, certifying membranes that can be applied as a waterproofing layer and overlaid directly with the binder course. The standard specifies the characteristics and performance of reinforced bituminous membranes for waterproofing bridge and viaduct decks and all other concrete surfaces trafficked by vehicles where the waterproofing system is bonded to the concrete deck and overlaid with a binder course or asphalt. In addition to the usual tests to determine thickness, cold flexibility, ultimate tensile strength, etc., the specific new EN 14695-2010 standard also entails testing specifically designed for this particular type of application. Membranes are also tested following application to verify: adhesion to the concrete substrate and to the binder course (pull-off test); resistance to heat and dimensional stability of the carrier; and bond strength of the bituminous compound.
Waterproofing Installation Instructions
Concrete substrates must be even, smooth, sound, clean and dry in order for the waterproofing system to be applied correctly. The deck must be given a suitable gradient to encourage rainwater to run off correctly, as specified by the designer. In some cases, specific preparation work may be required to bring substrates up to scratch
Application of Adhesion-promoting Primers
To stop dusting and boost the bitumen membranes’ bond strength, once substrates have been suitably prepared, they must be coated by roller, block brush or airless spray equipment with a bituminous or epoxy primer to promote adhesion. All surfaces to be treated must be clean, perfectly dry, free of loose parts and oily or greasy residues. The waterproofing membranes can only be laid once the primer has dried completely and, in any case, no earlier than 24 hours after priming.
Installing Waterproofing Membranes
Polymer bitumen membranes are applied using a heat-welding process whereby they are torched onto the substrate by heating the underside correctly with a specific propane torch. The membranes must be laid with a suitable overlap. Side laps must be at least 100 mm, while end laps must be at least 150 mm. The rolls must be laid in a staggered pattern so as to avoid four sheets overlapping at any one point (end laps).
Laying the Bituminous Binder
Once the waterproofing system has been installed, the binder course is hot applied, to the thickness specified by the designer, directly on top of the membrane. This course must be applied taking extra care not to damage the waterproofing system. Installation is completed with any detail work (vertical turn-ups; connection of membranes to drains and structural joints, if any), ensuring that the structure is fully waterproofed.