# Assessment of Existing Bond Models for Externally Bonded SRP Composites
## 1 Introduction  
Strengthening of structures with externally bonded reinforcement is nowadays the most popular method of restoring or improving the bearing capacity. The most popular in the construction industry are organic fibers, especially glass and carbon. For this reason, most of the standards and design methods have been developed based on research on these types of composites. This is a barrier to popularizing alternative, often equally effective and cheaper solutions [1]. An example is ultra-high tensile steel (UHTS) and textile composites that use it. They were placed on the market in 2004. Most of the first applications of these composites, mainly in the USA and Italy, are related to the work of research teams developed in these two countries [2; 3; 4; 5]. Their research allowed for the dissemination of engineering knowledge necessary for the practical applications of steel-reinforced composites [6].  
A modern structural engineer expects clear guidelines, manuals, and design standards. Such international and local documents can be found for carbon fiber reinforced polymer (CFRP) composites. This paper aims to answer, at least partially, the question of whether they can be used in the design of steel-reinforced polymer (SRP) strengthening. The comparison of the mechanical properties showed many similarities. Due to the high carbon content and drawing down to diameters of 0.20-0.35 mm, the tensile strength of pure steel cord is around 3000 MPa. Additionally, the modulus of elasticity is comparable (around 180-210 GPa). The differences appear in the fabrication method. CFRP composites can be made as strips, by a pultrusion process, and then adhered to the substrates, or as textiles. SRP composites are always textiles that are fixed in a wet lay-up process. Fiber volume fraction in the pre-cured laminate is significantly greater; therefore, CFRP strips maintain the propertiesof the fibers. The strength and stiffness of the SRP related to the cross-section of the finished laminate can drop by more than three times [7], depending on the density of the cord and the amount of resin used. Both compared materials retain almost constant stiffness throughout the load range, although in the case of SRP composites, a short inelastic branch can be seen [8]; therefore, their fracture strain is about 50% greater [7].  
The question is, how described differences affect the behavior of strengthened specimens. The analysis provided by [PERSON] et al. [9] proved that in the case of flexural behavior, existing codes could be successfully used; however, in the case of SRP composites, the safety margin was significantly lower. It should be noted here that a structural element strengthened with an externally bonded composite usually exhibits forms of failure unknown for normal reinforced concrete. It could be a fracture of the fibers, or more commonly, debonding of the composite. [PERSON] et al. [8] showed clear differences in the bond strength and debonding slip. Proper modeling of debonding seems to be a key to the correct design of SRP strengthening. This manuscript assessed the existing strength anchorage models by testing them on a wide range of laboratory test results. The conclusions can be used in practice to select the safest and most accurate method of predicting the bond of SRP overlays.