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Recently, there has been increased interest in constructing bridges that last longer, are less expensive, and take less time to construct. The idea is to generally increase the cost-effectiveness of bridges by increasing their durability (i.e., useful life) and by minimizing disruptions to the traveling public. There may be many ways to achieve more durable, less expensive and rapidly constructed structures, however, the most commonly discussed ideas currently include using some form of precast, segmental construction. This type of construction has the advantage that the individual components are manufactured off-site where increased quality is usually achieved. Further, because much of the work is completed away from the bridge site, user disruptions are minimized since the amount of labour intensive on-site work is reduced, leading to reduced onsite construction time.

Precast concrete has primarily been used for conventional beam and deck slab bridges and segmental bridges. Other applications of precast concrete elements used for the construction of bridge projects include; full and partial depth deck panels, retaining walls, arches, pier columns and pier caps. Precast concrete bridge elements are typically designed as mildly reinforced, pre-tensioned or post-tensioned.

Precast concrete bridge components offer a promising alternative to their Cast-In Place (CIP) concrete counterparts. Enormous benefits could arise from their use because precast concrete bridge components are typically fabricated off-site and then brought to the project site and quickly erected. Precast components also provide an opportunity to complete tasks in parallel. For example, the foundations can be cast on-site while the precast components are fabricated off-site. The use of precast components has the potential to minimize traffic disruptions, improve work zone safety, reduce environmental impacts, improve constructability, increase quality, and lower life-cycle costs. The use of precast concrete bridge elements can provide dramatic benefits for bridge owners, designers, contractors, and the traveling public.

PRECAST BRIDGE APPROACH SLABS

Approach slab is a structural concrete slab that spans from the back wall of the abutment (i.e. end of the bridge floor) to the beginning of the paving section. The purpose of the approach slab is to carry the dead and live loads over the backfill behind the abutments to avoid differential settlement that causes bumps at the bridge ends. Cast-in-place bridge approaches typically settle over time resulting in a “bump” in the pavement at the bridge ends which increases road roughness, driver discomfort, accident potential and vehicle wear and damage.

To significantly reduce the local settlement before the bridge, heavily reinforced precast concrete approach slabs are utilized as an alternate to CIP. The precast slabs reduce settling by acting as a land bridge spanning from the approach footing or bent to the abutment. In addition to decreasing settlement, precast also saves the installing contractor time and accelerates the schedule for bridge reconstructions, provides increased durability and enhanced user comfort. Approach slabs are available as partial depth which have exposed u-bars for bonding to a CIP wearing surface or as full depth where the precast is the final driving surface.

PRECAST DECK PANELS

Forming and pouring bridge decks on site are an expensive and time-consuming process, not to mention dangerous when over live commuter traffic.

The application of precast panels for a bridge deck allows the pieces to be cast at an off-site plant in a controlled environment. Precast decks panels are thinner than a typical CIP deck but have equal or better durability and structural capacity. This is attributed to the reduced rebar clearances allowed for precast and the use of high pressure concrete. Connection to the supporting structure below can be achieved by several different methods including weld plates or pockets in the precast that fit over studs and get grouted in to create a composite section. With any connection details, precast significantly decreases on-site construction time.

PRECAST PIER ELEMENTS

Difficulty in forming and pouring concrete high above the ground are two of the major disadvantages of traditional CIP pier elements which include foundations, columns and caps.

Precasting these components allows for the entire bridge pier to be set in a few days with minimal crane picks. This method cuts down on overall construction time and allows the contractor to move to other phases of the bridge’s construction that would have otherwise had to wait several weeks.

PRECAST BRIDGE ABUTMENTS, BACKWALLS & WINGWALLS

Bridge abutment and backwall components support the loadings of the bridge superstructure, support the bridge approaches, and retain the earth behind the beam ends. Wingwalls are adjacent to the abutment and act as small retaining walls to further support the embankment.

Typical on-site CIP construction for these components is a slow process. To decrease on-site construction time, the solution is to precast these items in an off-site plant. The pieces can then be trucked to the site and quickly erected and fastened using one, or a combination, of the innovative connection options available for precast.

SUPERSTRUCTURE SYSTEMS

These systems include both the deck and primary supporting members integrated in a modular manner.

A precast deck is poured integral with the supporting steel or concrete beams at an off-site, controlled precast plant. This allows the bridge system to be transported to the site already complete only requiring erection onto the substructure elements. These substructure elements may be existing abutments/piers or newly fabricated precast elements. The prefabricated system may also include the parapet or railing such that additional on-site construction isn’t required and the bridge may be operational sooner.

PRECAST MOMENT SLABS & COPINGS

Pouring these components on-site is a difficult task due to the complex forming and bracing required for the portion of the coping that overhangs the retaining wall. They are also labour intensive due to the heavy rebar cages required to withstand the vehicular impacts on the barrier that the moment slabs support.

Precast is the solution to these job-site problems. A precast coping section with exposed rebar eliminates the need to form over the side of the retaining wall and gives the contractor a ledge to setup forms on for their barrier. Rebar extends both vertically and horizontally to tie the CIP barrier and pavement/moment slab together. A precast moment slab system complete with a barrier and coping is also an option which eliminates the on-site forming altogether and significantly decreases on-site labour required.