Products - Bridge Components - Lafarge Precast

By: Lafarge Precast  09-12-2011
Keywords: Prestressed Concrete

Lafarge has extensive experience in bridge design and construction and completed numerous projects with many different cross sections. They range in size from short and long span pedestrian bridges to multi-span and spliced girder bridges.

Proven economy there were no prestressed concrete bridges in North America prior to 1950. Thousands of prestressed bridges have now been built in the past 50 years and many more are under construction in all parts of Canada. They range in size from short span bridges to some of the largest bridge projects in the world. The design of prestressed concrete bridges is covered by CSA Standard CAN/CSA-S6-00 Canadian Highway Bridge Design Code specifications.

Precast prestressed concrete bridges have gained wide acceptance because of:

  1. Proven economic factors:
    • low initial and long-term cost
    • minimum maintenance
    • fast easy construction
    • minimum traffic interruption
  2. Sound engineering reasons:
    • simple design
    • minimum depth-span ratio
    • assured plant quality
    • durability
  3. Desirable aesthetics – precast prestressed bridges can be designed to be very attractive. Bridge designers are often surprised to learn that precast prestressed bridges are usually lower in first cost than other types of bridges. Coupled with savings in maintenance, precast bridges offer maximum economy.

Up until the mid 1960’s, transportation equipment and available cranes limited the length of precast pretensioned girders to around 34 m. Some innovative designers began to look for ways to use the economy and high quality of plant produced precast girders for longer span bridges. Canadian engineers led the way in constructing long span prestressed precast girder bridges using spliced beams. Precast girder segments of manageable weight and length are transported to the site.

Girder segments are either spliced and posttensioned on the ground and launched or the girder segments are erected on temporary supports in their final position and post-tensioned together. Normally, precast girders can be fabricated and transported in lengths of 40 to 50 m and weights of up to 75 to 90 tonnes.

The spliced girder method of construction has extended the practical use of precast beams to span lengths of 75 m or more by joining and post-tensioning girder segments at the site.

  • Economy
    Fewer piers result in lower overall cost, especially where soil conditions are problematic.
  • Safety
    For overpasses, fewer piers result in longer sight distances and more spacious horizontal clearances. There is less likelihood of vehicle collisions with supporting columns.
  • Navigation
    Across waterways, fewer piers allow improved navigation, better movement of ice and debris and minimal disruption to the natural environment.
  • Deck Joints
    Fewer joints result in a smoother driving surface and less maintenance.
  • Aesthetics
    Long span bridges are more attractive.

Reinforced splice
Precast girders are cast with splicing reinforcement projecting from the ends. The beams are positioned end-to-end on a temporary support, usually near the dead load inflection point, and concrete is cast-in-place at the splice. The girder segments are usually pretensioned to resist shipping and handling forces.

Cast-in-place post-tensioned splice
Precast girders are placed on falsework or temporary end supports, usually locate near the dead load inflection points. The joint is poured and continuous post-tensioning is applied. Mechanical keys are often used. Sinusoidal keys work well because they transfer shear more uniformly.

Stitched splice
This splice is a compromise between reinforced and post-tensioned splices. The ends of pretensioned segments are clamped together by short cables or threaded bars.

Drop-in splice
This splice is used when the erection of a temporary support is not feasible (e.g. over river crossings or traffic lanes). The splice may be designed as a hinge or post-tensioning may be applied locally to induce continuity.

Structural steel splice
Steel plates are cast in the ends of girder segments to overlap at the matching ends of precast units. The plates are bolted together temporarily while free standing without support. The joints are later welded together and encased in concrete.

Epoxy-filled post-tensioned splice
Girders are aligned end to end, either in their final position or on the ground. The gap is filled with epoxy gel or grout and later the post-tensioning force is applied. A compressible gasket often protects the post-tensioning duct splice area. Match casting, while not essential, allows precision placement and expedites the work.

Bridge Decks Bridge decks often wear out well before the supporting beams. Some provinces have evidence that concrete bridges are more rigid than steel bridges and this results in superior deck performance (less cracking and longer life).

Lafarge precast concrete composite bridge deck panels are 75-100 mm thick slabs that span between the top flanges of concrete or steel beams. The panels provide a working platform for deck reinforcement placement and a stay-in-place form for the cast-in-place concrete overlay. Prestressing strands in the panels are perpendicular to the longitudinal axis of the beams and provide all of the positive reinforcement required for the span of the deck between beams. The panels are shimmed to the correct height and become composite with the cast in place overlay to resist superimposed dead and live loads.

Full depth precast concrete bridge deck panels are used to replace worn or corroded decks on bridges where traffic must be maintained during the construction. Prestressing strands in the panels are perpendicular to the longitudinal axis of the beams and can be in two layers to provide all of the positive reinforcement required for the span of the deck between beams. The panels are shimmed to the correct height. Shear studs on the beams and longitudinal post-tensioning are usually used to tie the deck panels together. Consult your local Lafarge location for our standard panel sizes and reinforcing layouts.

Lafarge precast prestressed concrete products are inspected and quality controlled at each of its plants. Each operation in the manufacturing process provides an opportunity for inspection and control. During fabrication, prestressed beams are proof tested at release of prestress and subjected to some of the highest stresses they will ever encounter in service. Lafarge manufactures certified products per the CSA Program for Architectural and Structural Precast Concrete in accordance with CSA Standard A23.4 “Precast Concrete - Materials and Construction”. Lafarge bridge components are certified by the CPCI Certification Program. See:

Keywords: Prestressed Concrete

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