CFRP Systems with Embedded FBG for Structural Monitoring and Retrofitting

The fields of activity in civil engineering are subjected to a constant change. Thereby maintenance, strengthening and mo nitoring of existing buildings have become more and more important. During th e last ten years an in creasing amount of Carbon Fiber Reinforced Polymer (CFRP) applications to rehabilitate damaged concrete or steel elements was observed. Thereby some important disadvantages of the brittle materials must be considered, for example the low ductility of the bond between CFRP and concrete and brittle failure of FRP. With embedded sensor systems it is possible to measure crack propag ation and strains. In this paper a sensor based CFRP system will be presented, that can be used for stre ngthening and measuring. The used optical fibers with Fiber Bragg Gratings (FBG) have a large number of advantages in op posite to electrical measur ing methods. Examples are small dimensions, low weight as well as high static and dynamic resolution of measured values. A Bragg Grating consists of a periodic sequence of artificial and equidistant refraction switches in the core of an optical fibe r. It can be produced over emblazing of an interference pattern of ultraviolet light. The core is surrounded by cladding. The main problem dur ing the investigations was th e fixing of the glass fiber and the small FBG at the desi gnated position. In this pape r the possibility of setting the glass fiber with embroidery at the rein forcing fiber material will be presented. The direct embroider of the optical fiber (and th e FBGS) clearly simpli fies the fixing. An embroidery machine, using computerized su pport, is able to fix the fiber optical system accurately fitting at the carbon fiber material. By using computer-controlled machines it is possible to ac hieve a very high degree of prefabrication as well as a high productiveness. The econo mic industrial fabrication of smart structures can be realized. Another possibility is the direct converting at th e building site by hand made lamination with an epoxy resign. _____________ Stefan Käseberg, HTWK Leipzig, Karl-Liebknecht-Str. 132, 04277 Leipzig 6th European Workshop on Structural Health Monitoring Fr.2. B.1 Licence: http://creativecommons.org/licenses/by-nd/3.0 1 M or e in fo a bo ut th is a rt ic le : ht tp :// w w w . n dt . n et /? id = 14 14 6 INTRODUCTION MO NITORING AND SM ART COMPOSITE STRUCTURES Monitoring in Civil Engineering The fields of activity in civil engineering are subjected to a constant change. Thereby maintenance, strengthening and mo nitoring of existing buildings have become more and more important. This tend s to result in smalle r investment for new buildings and significant increase for co st for maintenance and observation. These arrangements should start as early as possible and must be carefully maintained. In many cases this convention was not hold in the past. A lot of cases of damage at buildings at the age of 30 till 50 years underline th is fact. The reason s for damages are manifold and reach fr om faulty construction till unpred ictable natural phenomena [1]. Among other things natural facts have caused a stronger focus on building monitoring by publicity. One contribution fo r a safe structural monitori ng can be given by modern measurement techniques. They allow a blanket assessment of the actual situation additionally to visual controls. Thereby a clear distinction be tween temporary and permanent measurement should be made. For permanent measurement rugged measurement systems are needed. Electrical sy stems like strain gaug es are not the best alternative. Hence optical measurement systems move over to the foreground. Structural situation in Europe – building situation European-wide examinations prove the high importance of older existing buildings. Studies in big Euro pean countries like France or G eat Britain show that the fraction of apartments in older buildings (m ore than 20 years old) takes a portion of 75 percent of the whole floor space. It is ve ry important to prot ect these fundamental economic resources. One possibility of an effective an d sustainable maintenance can be smart composite struct res, which contain Fiber Reinforced Polymer and integrated fiber optical measurement systems. From FRP to Smart Composite Structures Fiber Reinforced Polymers (FRP) have got more and more important during the last decade. The fields of application ar e widespread and not only focused on civil engineering. In civil engine ering the main usage of FRP is the repair of concrete structures. But also for other building mate rials, for example wood constructions, FRP can be used. In the majority of cases carbon fibers are us ed as reinforcing material. Reasons are the superior technical properties of Carbon fibers compared to other highstrength fibers like glass fibe rs or aramid fibers [1]. Table 1 shows the properties of different fiber types. The hi gh modulus of elasticity and the great tensile strength of carbon can be seen clearly. The Carbon Fiber Reinforced Polymer (CFRP) arises when circa 70 vol.-% high-str ength carbon fibers are embe dd d into an epoxy matrix. The number of products of CF RP for reinforced concrete (RC) constructions is huge. Examples are at the surface bo nded CFRP laminates or sh eets as well as laminates which are placed in slots at the concrete surf ace. Furthermore it is possible to prestress CFRP laminates. Another favorable way is to combine these different types.