Marc Quiertant

Fatigue-loading effect on RC beams strengthened with externally bonded FRP

External bonding of fiber reinforced polymers (FRP) on concrete beams is particularly attractive for the strengthening of civil engineering structures in order to increase their mechanical resistance. The composite material is generally bonded on the tensile part of the beam. In order to design these bonded reinforcements, an iterative computational method based on section equilibrium and material properties (concrete, steel, adhesive and composite) has been developed: this method can be extended to describe the fatigue behavior of RC beams. This paper focuses on the damage behavior of concrete structures subjected to fatigue loading. A specific modeling coupled with an experimental investigation on large-scale beams made it possible to compare the theoretical and experimental fatigue behaviors of RC beams with and without composite reinforcements Results showed that the beam deflection and the strain in each material could be calculated with a sufficient accuracy, so that the fatigue behavior of the FRP strengthened beams was correctly estimated by the model.

Characterization of FRP-to-concrete bonded interface: Description of the single lap shear test

ABSTRACT The single lap shear test is an experimental method available for the characterization of FRP-to-concrete bonded interfaces. The obtained experimental results are required for the determination of parameters used in the design of concrete structures reinforcement or repairing by bonding composite materials. The aim of this article is to precisely describe the test procedure and experimental scatter. The test procedure has since been adopted for several investigations revealing its suitability and robustness for many different reinforcement processes.

Analysis of the nonlinear creep behavior of concrete/FRP-bonded assemblies

This paper investigates the creep behavior of adhesively bonded concrete/fiber-reinforced polymer (FRP) joints, through experimental and modeling approaches. The first part proposes a methodology for predicting the long-term creep response of the bulk epoxy adhesive; such a procedure consists of (1) performing short-term tensile creep experiments at various temperatures and stress levels, (2) building the creep compliance master curves according to the time–temperature superposition principle in order to assess the long-term evolution for each stress level, and (3) developing a rheological model whose parameters are identified by fitting the previous master curves. In our case, it was found that master curves (and, consequently, parameters of the rheological model) are dependent on the applied stress level, highlighting the nonlinear creep behavior of the bulk epoxy adhesive. Therefore, evolution laws of the model parameters were established to account for this stress dependence. The second part focuses on the creep response of the concrete/FRP assembly in the framework of a double lap joint shear test configuration. Experiments showed that creep of the adhesive layer leads to a progressive evolution of the strain profile along the lap joint, after only one month of sustained load at 30% of the ultimate strength. Besides, a finite element approach involving the abovementioned rheological model was used to predict the nonlinear creep behavior of the bonded assembly. It confirmed that creep modifies the stress distribution along the lap joint, especially the stress value at the loaded end, and leads to a slight increase in the effective load transfer length. This result is of paramount interest since the transfer length is a key parameter in the design of FRP-bonded strengthening systems. Moreover, instantaneous and long-term calculated strain profiles were found in fair agreement with experimental data, validating the modeling approach.

Influence of accelerated corrosion on the reinforced cover concrete cracking behavior: experimental and numerical study

This article aims to present both experimental and numerical studies of the corrosion induced cracking pattern evolution of a reinforced concrete sample subjected to accelerated corrosion. The beam was not mechanically loaded. Rebars were intentiostatically corroded using a current density of 100μA/cm² of steel, in a chloride pond and for a 30 day period. Electrochemical tests and visual inspections were carried out in order to characterise the rebar state. Width evolution of the main longitudinal cover crack was measured thanks to ball-extensometer. A numerical modeling of the corroded RC beam has also been realised. The experimental and numerical results are in good agreement confirming the efficiency of the proposed modelling Cet article présente les principaux résultats d’une étude combinant approches expérimentale et numérique appliquées à un essai de corrosion accélérée sur un corps d’épreuve en béton armé. Ces résultats portent sur le suivi et la prédiction de l’évolution de la fissuration du béton sous l’effet de l’expansion des produits de corrosion. L’éprouvette n’a pas été chargée mécaniquement. Les armatures ont été corrodées en imposant un courant de densité de 100μA/cm² d’acier dans une solution saline pendant 30 jours. Des essais de caractérisation électrochimique ont été réalisés puis complétés par des inspections visuelles en vue de décrire l’état de dégradation de l’interface acier/béton. Les résultats de la simulation numérique conduisent à une évaluation satisfaisante de l’évolution de la fissure principale démontrant ainsi la pertinence de la modélisation proposée

MONITORING OF CONCRETE STRUCTURES USING OFDR TECHNIQUE

Structural health monitoring is a key factor in life cycle management of infrastructures. Truly distributed fiber optic sensors are able to provide relevant information on large structures, such as bridges, dikes, nuclear power plants or nuclear waste disposal facilities. The sensing chain includes an optoelectronic unit and a sensing cable made of one or more optical fibers. A new instrument based on Optical Frequency Domain Reflectometry (OFDR), enables to perform temperature and strain measurements with a centimeter scale spatial resolution over hundred of meters and with a level of precision equal to 1 μstrain and 0.1 °C. Several sensing cables are designed with different materials targeting to last for decades in a concrete aggressive environment and to ensure an optimal transfer of temperature and strain from the concrete matrix to the optical fiber. Tests were carried out by embedding various sensing cables into plain concrete specimens and representative‐scale reinforced concrete structural elemen...

Nondestructive Evaluation of FRP Strengthening Systems Bonded on RC Structures Using Pulsed Stimulated Infrared Thermography

In civil engineering, strengthening or retrofitting of reinforced concrete (RC) structures by externally bonded Fiber-Reinforced Polymer (FRP) systems is now a commonly accepted and widespread technique (Hollaway, 2010; Quiertant, 2011). However, the use of bonding techniques always implies following rigorous installation procedures (440.2R-08 Committee ACI, 2008; AFGC, 2011; FIB, 2001) and application personnel have to be trained in conformity with installation procedures to ensure both durability and long-term performances of FRP reinforcements. The presence of bonding defects can significantly affect the structural performance and durability of the strengthening systems. Defects have then to be detected, located and evaluated in order to estimate if injection or replacement is needed. In these conditions, conformance checking of the bonded overlays through in situ nondestructive evaluation (NDE) techniques is highly suitable. The quality-control program should involve a set of adequate inspections and tests.

Behavior of an innovative End-Anchored externally bonded CFRP Strengthening System under low cycle fatigue

Building and bridge columns are particularly vulnerable when earthquakes occur. Retrofit of deficient reinforced concrete columns with CFRP jackets and bonded CFRP plates is an efficient method to increase their strength and ductility and then to enhance their seismic resistance. However, issues related to anchorage of the plates can be a concern when strengthening flexural concrete members. This study presents specific end-anchors reinforcement systems which were designed and tested under monotonic and low cycle fatigue loading and compared with a reference system commercially available. A total of 16 specimens were tested up to failure, to check the performances of these anchors. It appears that anchors increase the ultimate capacity and ductility of bonded plates.

Qualification of a distributed optical fiber sensor bonded to the surface of a concrete structure: A methodology to obtain quantitative strain measurements

Distributed Optical Fiber Systems (DOFS) are an emerging and innovative technology that allows long-range and continuous strain/temperature monitoring with a high resolution. Sensing cables are either surface mounted or embedded into civil engineering structures to ensure long-term structural monitoring and early crack detection. However, strain profiles measured in the optical fiber (OF) may differ from actual strain in the structure, due to the shear transfer through the intermediate material layers between the OF and the host material (i.e., in the protective coating of the sensing cable and in the adhesive). Therefore, optical fiber sensors (OFS) need to be qualified to provide accurate quantitative strain measurements. This study presents a methodology for the qualification of a DOFS. This qualification is achieved through the calculation of the so-called Mechanical Transfer Function (MTF), which relates the strain profile in the OF to the actual strain profile in the structure. It is proposed to establish a numerical modeling of the system, in which the mechanical parameters are calibrated from experiments. A specific surface-mounted sensing cable connected to an Optical Frequency Reflectometry Domain (OFDR) interrogator is considered as case study. It was found that (i) tensile and pull-out tests can provide full information about materials and interfaces of the numerical modeling; (ii) the calibrated model made it possible to compute strain profiles along the OF and therefore to calculate the MTF of the system, (iii) which proved to be consistent with experimental data collected on a cracked concrete beam during a 4-point bending test. This paper is organized as follows: first, the technical background related to DOFS and interrogators is briefly recalled. Then, the MTF is defined and the abovementioned methodology is presented. In a second part, this methodology is applied to the specific cable. Finally, a confrontation with experimental evidences validates the proposed approach.

Durability of FRP to Concrete Bonded Interface Under Accelerated Ageing

Externally bonded carbon Fiber Reinforced Polymers (FRPs) are now commonly used for the strengthening and repair of Reinforced Concrete structures. However, if the effectiveness of this technique has been widely demonstrated, the durability of the adhesive bond at the concrete/composite interface is still a matter of investigation and remains a critical issue to be addressed in order to assess the long-term performance of FRP strengthening methods. The proposed paper aims at presenting the first results of an ongoing investigation on the time evolution of the concrete/composite adhesive bond strength. Such an evolution was studied by performing double lap shear tests, while changes in the mechanical properties of the polymer adhesive were investigated by means of tensile tests. Preliminary results show that shear tests are able to reveal evolutions of both the bond strength and the failure mode of concrete/composite assemblies subjected to various accelerated ageing conditions. The weakest part of the assembly, initially assigned to the concrete substrate (cohesive failure in concrete), is progressively transferred to the polymer joint (adhesive failure at the bonded interface).

Effects of Ageing on the Bond Properties of Carbon Fiber Reinforced Polymer/Concrete Adhesive Joints: Investigation Using a Modified Double Shear Test

Although externally bonded carbon fiber reinforced polymer (FRP) composites are now commonly used for the strengthening and repair of reinforced concrete structures, the durability of the adhesive bond at the concrete/composite interface is still a matter of investigation and remains a critical issue to be addressed in order to assess the long-term performance of FRP strengthening methods. The proposed paper aimed at presenting the results of an investigation on the time evolution of the adhesive bond strength of concrete/composite assemblies exposed to accelerated ageing. Such an evolution was studied by performing double lap shear tests at several intermediate periods of ageing up to 1 year, while changes in the mechanical properties of the weakest constitutive materials, namely polymer adhesive and concrete, were investigated by means of tensile and compressive tests, respectively. Considering that an increase in the transfer length of aged samples can counterbalance the ageing-induced degradation of the mechanical properties of the bulk adhesive and can finally lead to an unchanged shear capacity of the joint, a new geometry of double-shear test specimens was proposed. This geometry involved a smaller bonded length and consequently restricted the possible increase in transfer length during ageing. Moreover, their reduced size allows the storage of a large number of samples in standard climatic chambers. A specific anchorage device was designed, so that double lap shear tests could be carried out using a conventional universal testing machine. The obtained results showed that the shear tests based on this original setup are able to reveal the evolutions of both the bond strength and the failure mode of concrete/composite assemblies subjected to various accelerated ageing conditions. These ageing conditions were found to have only slight detrimental effects on the mechanical characteristics of samples, and finally a partial recovery was observed.