Descripción de los derechos vinculados a los valores y procedimiento para

Despite their great potential, fiber-reinforced polymer profiles present several drawbacks when compared to their steel counterparts: a relatively low stiffness (especially for glass FRP), which can lead to design constraints due to instability or large deformations, an inherent brittle behavior, and a partially developed connection technology. In addition, the lack of authoritative codes as well as the current high initial costs of these advanced materials prevent a widespread use of composite profiles in civil engineering applications. To overcome some of these issues, researchers have proposed the introduction of new hybrid elements that combine the advantages of FRP profiles with those of conventional materials in order to obtain superior structural members.

Most of the hybrid members designed up to date have been built by combining fiber-reinforced polymer shapes with concrete, given the lower cost and higher structural efficiency of the resulting constructive solution. Concrete is also preferred because it can provide confinement, increase flexural stability, strength and stiffness. Besides, the added weight from the concrete part may be beneficial in the sense that the system will have better damping, as light structures are normally predisposed to unacceptable vibrations. Because composite materials can be tailored in function of expected needs, the

FRP-concrete combined solution allows engineers to take advantage of the preeminent properties of each component and to optimize the use of both materials.

After a thorough analysis of the available and possible applications of advanced polymer composites in civil infrastructure, it has been suggested that hybrid FRP-concrete members represent the future in this field [7], as they possess great in-service properties and mechanical characteristics. Moreover, these novel elements can be employed in a wide variety of situations, and based on their promising results, extensive investigations have been undergone in North America [22], Europe and Asia [23].

In a recent review of the present and future utilization of FRP composites in construction [2], the author recommended that the following three criteria should be met for a successful implementation of hybrid systems in new structures:

 Cost effectiveness in terms of the most advantageous combination of whole-life cost and of high quality and performance.

 The composite material should be used ideally in areas subjected to tension.  The fire resistance should not be critical.

Regarding FRP-concrete beams, the large majority of the proposed designs rely on pultruded FRP sections connected to concrete slabs. The main role of the composite profile is to carry the tension and shear in the member, while the concrete top serves to resist compression and to stabilize the flexural behavior. Most of the designs favor glass FRP (GFRP) pultruded profiles due to their lower production costs, whereas the top slabs are generally made from normal strength reinforced concrete. The FRP profiles and the concrete layers can be connected using a bonded joint, mechanical joint, or combined joint, as will be detailed further on.

The major advantages of the FRP-concrete beams over conventional reinforced concrete beams are:  Higher strength-to-weight ratio

 Extended service life and reduced maintenance  Resistance to aggressive external factors  Lower transportation and installation costs  Reduced formwork

Compared to single pultruded FRP profiles, the hybrid beams possess the following benefits:  Enhanced strength and stiffness

 Better resistance to instability phenomena and impact loading  Improved vibrational characteristics

 Elevated structural redundancy and ductility

Some of the notable disadvantages of hybrid FRP-concrete beams at the present time are related to:  Interface/connection problems

 Little available data and experience

 High initial costs and environmental concerns (i.e., recycling of FRPs)

Initial applications of hybrid FRP-concrete beams in civil infrastructure commenced in the 1980s with a few experimental projects and grew substantially along the years, as the technology evolved and the price of the advanced polymer composites decreased. More recently, there has been a tremendous attention provided to the use of hybrid solutions and to broadening their application range. At the current moment, FRP-concrete beams may be employed in designing bridge superstructures, building floors, industrial platforms, and offshore structures. Extensive reviews of pedestrian and vehicular bridges utilizing hybrid solutions may be found in ref. [24–27].

To exemplify a few practical cases, a joint project developed in 2003 in Spain led to the completion of three highway overpass bridges with hybrid superstructures [28]. One of the bridges, spanning a four- lane highway and designed to carry 60 ton traffic, has a total length of 46 m and four continuous spans, as illustrated in Figure 2.11(a).

Figure 2.11: Examples of infrastructure projects incorporating hybrid FRP-concrete beams: (a) the Cantábrico highway

overpass bridge; (b) the M-111 highway overpass bridge (both images courtesy of ACCIONA Infraestructuras); (c) the marine pier of the Downeast Institute for Applied Marine Research and Education (courtesy of Downeast Institute).

The superstructure of the first bridge is made of three carbon fiber sandwich beams with a polymer foam that support the roadway deck which is formed from reinforced concrete and asphalt. Installation of the beams and concreting of the deck took place in under two days due to the reduced weight of the beams at 1 kN/m. The other two bridges were built identical, each made up of three simply supported spans with a 20.40 m wide box-girder deck, as seen in Figure 2.11(b). The former design was improved by combining GFRP with CFRP layers to reduce the costs of the composite girders, and by using a different manufacturing and assembly method. The overall objective of the research project was the development of a new high performance and cost-effective construction concept for bridges based on the application of rapid-renewal and long-life service infrastructures [29].

As a last case study, the composite marine pier depicted in Figure 2.11(c) was commissioned to replace an old wooden pier that was damaged due to the harsh environmental conditions present at the location. The project demanded the structure to necessitate a minimal maintenance and to support important supply loads. Thus, an innovative solution was applied, where 10 m long hybrid FRP-concrete beams were mounted on top of composite piles, over thee spans.