Precast Concrete Infrastructure Applications

David Fernández-Ordóñez

Fib Commission 6. Prefabrication

Fib Commission 6 on Prefabrication has been developing documents regarding the use of concrete prefabrications for many years. The intention to develop documents regarding prefabrication is to create and share the knowledge regarding precast concrete in all possible parts of the world.

Between those documents that have been developed by the Fib Commission of Prefabrication is a State of the Art report on Precast Bridges. This document also includes other relevant information regarding other types of precast elements that are normally used around the world in infrastructure construction.

Publications on the subject of bridge construction deal mostly with spectacular structures, both with respect to design matters - span length and total length - as with regard to execution. They are constructed on site, often using a technique of site-prefabrication of large and extremely heavy bridge parts. However, an important part of the bridge market consists of simple projects, where speed of construction, cost and minimum disturbance of existing traffic are important parameters. This is the domain of industrially precast bridges, although also relatively large bridges can be made by this technique. In the following, the term "precast bridges" is used for this type of bridges, made in permanent precasting factories.

The development and application of precast bridges is very different in the various countries: in some like Belgium, Canada, Italy, Spain, The Netherlands, United Kingdom, USA, etc. precast bridges are routinely used, in other countries they are hardly known or even not used at all. Also in other continents like for example China, precast bridges are frequently used, but information is missing.

The aim of the State of the Art report on Precast Bridges is to give a detailed overview of the existing solutions and applications world-wide and recent developments in the domain of precast bridges. They should enable the authorities and designers to form a realistic opinion about the possibilities and advantages of this technique, and get away from some still existing prejudices.

In the document, between other aspects, different kinds of bridges and complementary elements for infrastructure that are normally built around the world are described.

Earth retaining walls and Abutments

In some cases precast elements are used to form earth retaining walls and bridge abutments. The elements are full height, modular width, and usually have on the back side one or more webs from the top to the foundation. The resisting section is thus shaped as a T. The elements are placed on site side by side and are completed by the cast in-situ foundation and a top beam.

CROSS SECTION FRONT VIEW

Piers

Piers are generally cast on site, because of their large size and weight. However, there are good examples of precast piers. They can be made of a single shaft or with multiple elements depending of weight and design limitations.

Cast on site foundation

Culverts

There are different types of precast culverts. The most classical ones are the box culverts, which are often used for small underpasses and tunnels.

Another type concerns the arch culverts or vaults. The vaults are of curved or polygonal guideline structures of different types

Solid deck bridges

Small bridge decks can be constructed with precast units and a cast in-situ topping, acting together as a composite structure. They are used for decks of bridges, viaducts, culverts, tunnel decks etc. The type of solution leads to heavy but easy to erect structures. For this reason, it continues to be valid for short and in some cases even for medium span bridges.

The precast slabs are mostly prestressed and protruding bars at the top ensure a good connection with the structural topping. The longitudinal joint faces of the slabs are provided with a longitudinal slot to form a shear key. The edge of the bridge is normally finished with a profile cast on site together with the topping.

Girder bridges with T or I beams

Large girder bridges constitute the main solution for precast bridges built from the sixties on. The bridge deck is composed of several inverted T- or I-shaped beams positioned at a certain distance. The beams are connected by a transversal diaphragm beam at each support and sometimes also in the middle of the span, depending on the length. After erection of the beams and casting of the diaphragm beams, a deck slab is cast on site, mostly with concrete shuttering planks positioned on a notch at the top of the beams. The top of the beams has protruding reinforcement for the connection with the deck slab. The system is used both for simply supported and continuous bridge structures.  

 

 

 

 

 

Girder bridges with U beams

They are designed in a similar way as I girder bridges but with a U shaped beam that is also able to resist torsion.

Box-beams consist of U-shaped precast units, covered with precast deck slabs and completed by a cast in-situ structural topping.

The bridge deck is composed of a series of box beams placed side by side or at a small distance. After erection the site work is limited to the filling of the longitudinal joints and the transversal post-tensioning of the bridge.

Another variant type of box-beams for bridge decks comprises a large bottom flange. The system has been conceived for bridges with closed under deck. The cast in-situ top plate is made in the same way as for classical box-beams bridges.

Mono-box bridges

Curved beams bridges

Curved prestressed box beams have been developed by precasters to cope with the increasing demand for more aesthetic solutions. The torsional rigidity of box beams suits very well for bridges with a horizontal curvature.

The solution is now currently being applied in Spain, The Netherlands and UK. The radius varies from 200 m to as low as 100m.

Variable depth bridges

Precast mono-box bridges with curved soffit are similar to normal mono-box bridges, but the beams are cast in special moulds to achieve the curved shape. Pre-tensioning is normally not used and replaced by post-tensioning to cope with the curved moulds. Precast mono-box bridges with curved soffit are usually constructed with structural continuity and spans may reach more than 50 m.

Segmental Bridges

Precast segmental decks are regularly used in traditional cantilever construction of large span bridges. The segments comprise the full width of the bridge and the length of the units is related to their weight and to the means of transportation and lifting. Large segments are usually precast on site, but there are also good examples of large bridges with factory precast segments.

Arch Bridges

Bridges in dense populated areas are often designed with special attention to the aesthetical outlook and tradition.

Cable Stayed Bridges

It is possible to use stays to reach longer spans in the construction of precast bridges. Spans up to 400 metres can be achieved with precast decks in cable stayed bridges.

Precast decks can be designed for two planes of stays with a box girder under each plane of stays. It is also possible to design a deck with a single plane of stays with one or two box girders joined by a transverse beam at each anchorage of the stays.

Other aspects of design and production are dealt with in the document. For example there is a chapter on structural systems. There it is described from the simple support in isostatic bridges to the different kinds of structural continuity that can be achieved.

Some considerations are also given regarding Aesthetics in the design of bridges. This is an aspect that is more and more important. Sometimes decisions are made taking into account this aspect in the first part, even before economical aspects.

Besides design considerations on structural design and continuity, also some aspects regarding connections like the beam to transverse diaphragms, support, beam to slab connections and the connection of other elements like edge profiles.

Some indications on detailing are given for skew bridges, banking and edges design considerations.

Regarding design are also some considerations made, like different bibliography and code considerations, durability and seismic aspects.

Also some indications on the particularity of production are made, like the handling and storage, special kind of concrete, moulds, transport and erection and the final stages needed to be made on site.

Finally some considerations are made on quality assurance and research that has been made in this field.