An experimental investigation into bond behavior of prestressed CFRP to steel substrate

Nowadays, strengthening of existing steel structures using advanced carbon fiber reinforced polymer (CFRP) composites is becoming a common technique due to the unique advantages of the material such as light weight, high strength and fatigue endurance, as well as their relatively easy installation. Recently, the tendency to strengthen steel structures with prestressed CFRP plates is growing, especially in case of fatigue strengthening, where a reduction in acting stresses on the structures would be possible using prestressed reinforcements. However, the efficiency of using prestressed bonded CFRP reinforcements is severely affected by the undesirable premature debonding of CFRP from the steel substrate, before reaching the ultimate tensile capacity of the strengthening material. Consequently, it is of great importance to investigate the anchorage resistance and bond behavior of prestressed CFRP plates bonded to steel substrate. In the current study, a set of single lap-shear and prestress release tests was performed on CFRP-to-steel joints. All the tests were monitored using a 3D digital image correlation (DIC) system to understand the bond behavior of CFRP plates during lap-shear and prestress release tests. Moreover, the effect of accelerated curing on bond anchorage of the prestressed CFRP plates was investigated. Prestress release test results of the current study showed that the governing phenomenon in CFRP-to-steel joints is completely different than that in CFRP-to-concrete joints. The reason is that, despite in concrete, no tensile failure can occur in steel; consequently, relatively high prestressing forces can be transferred to the steel substrate before reaching debonding.

FRP-to-concrete debonding - global and local bond behaviour

Ardalan Hosseini

FRP (fiber reinforced polymer) strips are used to flexurally strengthen reinforced concrete (RC) beams. The global bond strength namely the maximum transferable bond stress due to the increase of FRP tensile stress in a cracked concrete element (subsequently named as global bond shear stress) is a relevant aspect of flexural strengthening design. The differences between the bond strength in an uncracked concrete (end anchorage) and in a cracked concrete element have been highlighted in the existing literature; however, despite a large number of research works on the simple lap-shear test (local bond shear stress), less effort has been dedicated to investigate and model debonding on a global view. The current version of Swiss code on externally bonded reinforcement (SIA 166-2004) limits the local and global bond strength by means of the shear stresses, τf,max=τl0 and τl,lim, which are both assumed to be only functions of the concrete tensile strength. Nevertheless, it has been demonstrated in the literature that the maximum interfacial shear stress between two adjacent flexural (or flexural-shear) cracks depends also on the stress level in the FRP (σo). In the current paper, the difference between the local and global bond behavior and its debonding processes is explained and discussed with analytical and numerical models. In analogy to the existing approach developed to study the bond behavior of prestressed carbon fiber reinforced polymer (CFRP) strips during force release, a fracture energy-based model is proposed in the current study to determine the maximum global bond developed in a so-called intermediate crack element (ICE). The proposed model is a function of the FRP tensile stress (σo) and assumes a constant shear stress law. It is here demonstrated that the proposed model provides similar results to the more complicated models available in the literature. The main findings are here discussed with the aim to evaluate the feasibility of this new model for a future enhancement of the Swiss code.

2018

Structural analysis of a composite bridge structure combining UHPFRC and prestressed concrete

Eugen Brühwiler, Cornelius Oesterlee, Hamid Sadouki

Reinforced concrete being the most applied construction material today performs very well in most applications but still lacks durability under severe environmental conditions. Especially existing structures built decades ago show degradation. Using Ultra-High Performance Fiber Reinforced Concrete (UHPFRC) to improve durability is a promising option seen the extraordinary performance of this material when applied in a composite section. Restrained shrinkage of the overlay plays an important role in this type of application. A conceptual bridge cross section combining UHPFRC and reinforced concrete has been numerically analysed. Two main parameters, tensile strength and strain hardening capacity, were varied. The analysis indicates the importance of strain hardening of UHPFRC and the influence of its tensile strength on the structural response under restrained shrinkage (deformation controlled loading) and traffic loads (force controlled loading). The results validate the concept of using a UHPFRC layer to improve the structural durability of concrete constructions.

Uni Kassel2008

Time-Dependent Response of Ultra High Performance Fibre Reinforced Concrete (UHPFRC) under Low to High Tensile Stresses

Agnieszka Ewa Switek

Ultra High Performance Fibre Reinforced Concretes (UHPFRC) are characterised by high mechanical performance, extremely low permeability and tensile strain hardening. For structural applications, in composite structures they are very well adapted for reinforcement and protection of existing concrete members. Cast in-situ, they undergo under restraint development of tensile eigenstresses at early age, which can lead in some specific cases of applications to localised macrocracking of a new layer and loss of protective function. On the other hand their tensile viscoelastic potential helps mitigate these stresses. The objective of this study was to investigate the tensile viscoelastic properties of UHPFRC at early ages and long-term in creep and relaxation tests. A large experimental campaign with tensile creep and relaxation tests was performed, with load levels ranging from 30 to 90% of the tensile strength, including creep and relaxation tests during the strain hardening phase, at two different loading ages of 3 and 7 days. Three main testing setups were used: TSTM (Temperature Stress Testing Machine), creep rigs and closed-loop servo-hydraulic testing machine with additional Acoustic Emission detection. The influence of early age evolution in mechanical response and underlying mechanisms governing this response were discussed. Linear viscoelastic models were used to analyse and to highlight on the basis of the experimental results the non-linear behavior depending on solicitation level and loading sequence. The non-linear threshold for viscoelastic response was estimated for the material loaded at 3 and 7 days. The early age mechanical response at 3 days was proportional to the load level in the instantaneous part for three stress levels tested 30, 60 and 90% of the tensile strength, and showed non-linear creep from 60% on. It was observed that for a very low stress level of 13% the creep deformation was decreasing. Relaxation tests with compensation of simultaneously monitored shrinkage deformation were performed, and showed a viscoelastic potential similar to the one observed for creep for the low load level of 30%. Incremental creep and relaxation tests were successfully performed for 3 days age specimens. For higher stress levels, the viscoelastic response from creep tests was more pronounced than in relaxation tests, which was highlighted by analysis with linear viscoelastic models. The creep response at 7 days showed that the threshold for non-linear viscoelasticity is close to 9 MPa (81% of the tensile strength) load level. The Acoustic Emission non-destructive testing method was used to get insight on the physical mechanisms involved in viscoelastic deformations. A strong correlation between AE number of events and the deformation was found, also the amplitude and energy of AE events was found to be of the same order during the instantaneous and delayed deformations. UHPFRC loaded at 7 days age in incremental test showed linear behavior until very high stress level corresponding to the hardening domain. The non-linear and tertiary creep was obtained during the last loading step, for 12 and 11 MPa respectively, which occurred in the hardening domain. The Acoustic Emission monitoring confirmed very close correlation between non-linear creep and AE cumulative number of events induced by microcracking. In order to precisely detect non-linear phenomena in incremental test, analysis with linear viscoelastic models was performed. Comparison between creep and relaxation test results showed that non-linear viscoelastic phenomena were more actively involved in creep tests. The outcomes of this research, bridging the material science and structural engineering, provide useful data to foster applications in new and existing structures, making the best use of the tensile viscoelastic potential of UHPFRC.