APC materials

Composite rehabilitation systems (CRS), i.e., structural hybrid systems involving advanced polymer composite (APC) materials (generally referred to as fibre-reinforced polymer, FRP), structural adhesives (SA) and conventional construction materials (CCM) (e.g., timber, concrete, masomy, steel, iron), constitute one such technology. 

The APC materials typically nsed for on-site rehabilitation of timber and concrete are composed of glass, carbon or aramid fibres and a polyester, epoxy or polynrethane polymeric matrix. Glass fibres are the most frequently used due to their moderate cost and good mechanical properties when compared to carbon fibres. They are used normally in the form of pultruded profiles or strips, fabrics (tissues) or mats. Carbon fibres are mainly used in the form of pultruded profiles of solid, open or hollow cross-sectional shapes. While in timber applications both thermoplastic and thermosetting matrix types are used, in concrete applications only the latter type is used. 

Structural rehabilitation of timber and concrete structures with composite systems can be generally accomplished in one of two ways (Karbhari and Seible, 2000) using wet lay-up or cured in-situ systems, by application of composite overlays, fabrics, sheets or fibre tows (Fig. 22.4) and using systems involving the bond of prefabricated APC materials, such as straight pultruded strips, and factory-made curved or shaped elements (Fig. 22.5). 

Pre-tensioning and post-tensioning systems using APC materials to rehabilitate concrete structures. 

Pre-stressed timber structural members with APC materials (this technique would allow for an increase of the economic efficiency of the reinforcement using APC materials in comparison to their use as a passive reinforcement only, because its final service stress may be substantially increased). 

The practical execution of the rehabilitation intervention includes the reception, storage, preparation and application of the SA and APC materials, as well as the curing of the adhesive materials. Site work should be carried out by well-informed, trained, experienced and certificated operatives, under the supervision of a qualified site manager to ensure eompliance with the specifications of the quality plan and to ensure a satisfaetory intervention programme (Petrie, 2006 Silva et al., 2011 Adams et al., 1997 Mays and Hutchinson, 1992). 

The condition of the materials that arrive at the work site should be assessed and recorded to guarantee that they ate in perfeet eondition so that if handled properly they will be able to produce rehabilitation work with adequate performance and durability. This verification also serves to confirm that the correct products have been delivered to meet project specifications and that they are under the expiry date. The storage of the materials is also very important and it should follow strictly the supplier or manufacturer indications present in the manufacturer s or supplier s Product Data Sheet. Generally, all adhesives and APC materials should be stored in a cool, dry place until they are used. While some materials are very tolerant to storage conditions, there are others that may have to be stored at low temperature or under special conditions. For instance, some adhesive systems are affected by light or moisture and others require periodic agitation to ensure that their components do not settle irreversibly (Petrie, 2006 Silva et a/., 2011 Adams et al., 1997 Mays and Hutchinson, 1992). 

This section focuses very briefly on the most relevant of the aforementioned factors, thus providing only a general understanding of the factors that influence the durability of bonded timber and concrete joints. The durability of APC materials has already been discussed in the previous chapters so is not dealt with here. The durability of the substrates, timber and concrete, is not also dealt with here the reader can obtain information on this topic from the relevant sub-section of Section 22.6. 

In general, low temperatures and freeze-thaw cycles can affect both the polymeric matrix of the APC materials and the adhesive. Freezing and thawing effects can be more severe due to moisture-initiated effects causing micro-crack development. Research results suggest that some degradation in the composite bonded joint due to freeze-thaw cycles can occur, particularly in the presence of humidity and sustained load (Ekenel and Myers, 2009). 

Development of APC materials with improved fire resistance. 

---