•Notícia

In the field of civil engineering—and particularly in the construction of bridges—quality, beauty and strength are attributes traditionally associated works built in situ. In contrast, precasting brings to mind soulless structures of little architectural quality. This perception has endured for many years, probably because precasting was initially a relatively industrialised construction system that prioritised cost cutting and time saving. Moreover, the structures created were not expected to be of any aesthetic value.

The use of precast concrete elements in construction expanded rapidly between 1950 and 1970 in response to the urgent need to replace housing destroyed during the Second World War. The construction systems used were developed mainly in Germany, from where they spread rapidly, especially to countries in the East.

Since then, industrial and precast construction systems have undergone substantial changes, and we have now reached a point where the design possibilities are unlimited. Any element of a building or bridge can now be constructed in a factory and then slotted into place in the work.

Arch bridges, beam bridges, cable-stayed bridges, built using reinforced or prestressed concrete, of composite steel and concrete construction—more and more professionals are convinced that precasting offers versatile solutions that are technically interesting, formally attractive, and economically competitive.

But these are not the only advantages experts point to. Precasting also allows for parallel construction. While structural elements are being made in industrial facilities, preparation work can be done at the site where the bridge will be erected. As a result, time is saved and works can be executed within a shorter period.

Parallel construction also minimises the risk of cost overruns due to adverse weather conditions, particularly in the winter. “Precasting makes a project more independent of environmental conditions like the weather, flooding of rivers, and traffic,” says Antonio Marí of the UPC’s Department of Construction Engineering. Because it is an industrialised building solution, “quality control measures that would be impossible on-site can be applied. Materials are manufactured at the plant—whether it’s hot or cold out—with the right mix proportioning and tight control over conditions and methods. Thanks to this system, we can achieve a high level of quality and uniformity in finishes,” he adds. Finally, the system reduces occupational risks: work is done in a factory setting, and the process of assembling the structure on-site involves standardised, highly controlled procedures that are executed by a few specialised workers.

But precasting also has its drawbacks. The weak points in structures built using precast concrete elements are the connections between the different elements, and the joints between precast elements and the in-situ structure and bearings. Prestressing of concrete has provided a solution to this problem and driven the development of structures of this type. “Precast beams are now joined using prestressed cables or rods. As a result, a bridge made of separate pieces ends up being a continuous bridge, a monolithic structure, with all the advantages this implies in terms of strength and rigidity,” says Antonio Marí.

For many years, this project has been pursued by precasting companies rather than professional engineers. Enrique Mirambell, a specialist in metal structures with the Department of Construction Engineering, says that as a result there is a poor understanding of how to design structures with precast elements, and this has led to defects, usually linked to specific aspects of concrete production. “Structures made using precast elements evolve over time. You need to know how to support them so that, for example, thermal actions don’t end up causing support elements to break,” Mirambell explains. “So it’s essential to choose the right system for joining the pieces. These are factors that still haven’t been fully taken on board,” he concludes.

This is why the Spanish Code on Structural Concrete—regulations passed in 2008 that cover calculation methods and safety for concrete structures—for the first time includes a chapter on precast structures.

The transport and handling of large pieces can also lead to complications that need to be anticipated. These operations are carried out using computer-controlled thousand-ton cranes that can position 500-ton pieces at a height of up to 70 metres, within a radius of 30 metres (the length of the crane arm).

Access ways for trailers and heavy machinery are another factor that needs to be taken into account. At the design stage, the length and weight of precast elements must take into account such access constraints. Additional difficulties can arise at some sites that may be impossible to reach (those within the urban fabric, for instance), or where the type of equipment needed to handle precast elements cannot be used.

These factors make meticulous planning essential. Construction plans must be very detailed, because it is extremely costly to make corrections at the execution stage to tackle problems that were not anticipated at the outset of a project. Coordination between those involved in manufacturing and builders is also vital in order to ensure that precast elements are ready when they need to be incorporated into the structure under construction.