New Modular Precast Concrete Bridge System
Release date: 3rd March 2009 : www.concretecenter.com
The development of a new modular precast concrete bridge system offers an alternative to steel-composite construction for medium-span bridges believes Alan Bromage, head of civil engineering at The Concrete Centre.

The bridge system consists of precast concrete shell units that are precast off site. Once on site, the units are assembled, stressed together and in-situ concrete placed. It combines the best features of precast concrete and in-situ concrete into one single solution for bridges with a span of 15-50m.

The system’s development follows the successful and innovative use of segmental and incremental launching technology for the Taney Road Bridge and River Shannon Bridge projects in Ireland. The approach offers a number of benefits. Relatively light, 2.5m long precast shell units are transported to site for efficient assembly. The bridge units are precast concrete segmental shell units that are formed as a single rib and top slab which reduces the sophisticated mould technology that is usually associated with box forms. Permanent prestressing cables are then placed and covered by in-situ concrete to provide additional composite action and protection. The use of precast shell units allows the in-situ concrete to be poured in small bays. This eliminates most of the issues associated with early-age thermal and restraint effects. The internal prestressing of the deck from end-to-end allows an efficient use of materials whilst making the deck fully integral with bankseat abutments and fully monolithic with intermediate piers. This reduces future maintenance costs by eliminating all bearings and expansion joints.

The modular precast approach can be adapted to suit a wide range of bridge sites and project programme demands. Its flexibility is such that it can be constructed by the use of incremental launching, lifting using mobile cranes or transporters or by using a temporary gantry.

The system is also suitable for a wide range of highway bridge layouts. Varying span lengths, carriageway widths, horizontal and vertical curvatures and skew can be readily accommodated by the match-cast shell units. The modular precast system is adaptable to varying span requirements as the depth of the precast shells can be changed to allow different spans to be achieved. For typical spans between 25m and 35m, the section depth would vary between 1.5, and 2.0m. The minimum depth for shorter spans is 1.0m increasing to a maximum of 3.0m for longer spans of between 35m and 50m.

The system provides cost and programme savings compared with steel-composite. For example, it is estimated that using a span lift option for a modular concrete three-span highway bridge of 20m-25m-20m would save three weeks in construction time compared with a steel-composite alternative. Also, for a single 45m span highway overbridge, the span lift modular bridge option would be 10% less expensive than the steel composite option.

The use of modular precast concrete construction offers designers a well-engineered and cost competitive solution that will see concrete established as the preferred option for medium-to-long span bridges.

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