Segments, a prefabricated solution

New transport (motorways and high-speed rail) infrastructure require the construction of large number of viaducts of large and medium lights, and more and more the construction of these viaducts determines the term of the work or is a significant percentage of the budget. Becomes therefore necessary the development of systems for the implementation of viaducts to address the construction of cheaply and quickly, without giving up quality.

In the construction of the boards of the viaducts can opt for two possibilities: the industrialization of boards built in situ or prefabrication. Here we are going to deal with this second solution.

The constructive system of prefabricated segments consists of the Division of the Board obtained elements to cut it by planes perpendicular to its axis, in which each element has the complete cross section of the Board. Each of these elements is a keystone. Usually prefabrican in the vicinity of the work (usually weigh between 50 and 100 tons each) and mounted with the help of specialized shoring.

The main peculiarity of this constructive method is that joints between segments not makes connection of passive armor, so that the continuity of the Board is only achieved by the prestressed, achieving a single montage of elements that allows a great speed of mountingthat it leads to some characteristic of the system constraints.

Joints between segments can be dry or with a resin epoxy. In dry joints transmission of efforts is directly through the contact between the concrete of the two adjacent elements. On the boards with resin epoxy, before mounting extends over one of the two surfaces that are to come into contact, a layer of resin; to assemble the segments is applied between the two a provisional prestressed which distributes the resin in the entire surface, expelling the excess. The resin is not colaborante to resist efforts to court, only to be used for improving contact between the elements and provide the Board with waterproof.

In both types of Board to get an adequate transmission of efforts in the walls between segments, it is necessary that there is no difference of geometry between the faces of two adjacent elements, in the manufacture of each Keystone used as formwork of a face the keystone that will be in contact with herWhat is known as "method of conjugate Keystone".

There are two methods for the manufacture of segments by the method of conjugate Keystone: long line and short line manufacturing. In the absence of any element between the segments the geometry of the Board resulting in mounting is the result of surface mounting the segments each other, forcing these must have a variable geometry in order to build dashboards of arbitrary geometry. The geometry of the Board to build is determined by the layout of the Board more deformations of the same during the Assembly.

Long line method in the implementation of a bed with the geometry of the bottom of the Board to be has manufactured, which is hormigona all of the Board keystone to Keystone with the help of a formwork that moves along the same.

In the manufacture in short line, the Keystone is manufactured in a cell of prefabrication in which the formwork is formed by the adjacent Keystone (Keystone conjugate) on the side in contact with it, but a fixed formwork or mask for face of the Board opposite, but a table adjustable to the formwork lower, more formwork folding wings, which are adapted to the other forms, but a retractable internal formwork that is inserted through the mask.

In the manufacture in short line geometry of each Keystone gets properly positioning the keystone conjugate with respect to the mask and adapting the rest of the formwork geometry delimited by both.

In summary, the viaducts of prefabricated segments provide a solution to the problems currently in the construction of viaducts, in response to the constraints of speed of execution, economy, quality of construction and independence of the land. The manufacture and Assembly of repetitive elements allow the industrialization of construction through the use of standardized manufacturing processes, the implementation of systems of quality control, getting the advantages of prefabrication in a fixed installation and installation of the elements with specialized machinerywhich allows to get returns far greater than those obtained through on-site construction.

For viaducts of lights mean the vain prefabricated dowels mounted system is used to no avail. Because this range of lights, there are other methods of industrialized or prefabricated construction that they compete with him and require a smaller initial investment and less specialization, their use is usually limited to viaducts of several kilometers in length in which the best yields are obtained. Panels of prefabricated segments mounted by advance cantilever, being the only method of industrialized for the construction of boards of concrete of more than 70 meters lights are used to viaducts of large lights. The speed of construction obtained by this system is several times exceeding the construction spot, achieving significant savings in addition.

In the case of viaducts of highway with average height of batteries exceeding 50 metres only for both roads Board typology provides the most competitive solution. The development of this constructive system is limited by the high cost of the equipment for manufacture and Assembly that makes need to be a large volume of work for the amortization and the need for the control of the manufacture and Assembly technology to address the construction with the appropriate guarantees of success.