Karim Benzarti

Characterisation of cement pastes by inverse gas chromatography.

Two cement pastes, commonly used in concrete formulations, were characterised by IGC at 35-80 degrees C before and after coating with an epoxy resin and a hardener. The cements are mixtures of hydrates in various proportions, such as calcium silicate hydrate (CaO-SiO2-H2O) and calcium hydroxide Ca(OH)2. Apolar and polar probes were used to determine the dispersive and acid-base characteristics of the cement pastes. These materials have high surface energy as judged from the dispersive contribution to the surface free energy (gamma(s)d) values lying in the 50-70 mJ/m2 range at 60-80 degrees C. Examination of the specific interactions permitted to show that the cement pastes are strongly amphoteric species with a substantial predominant Lewis basicity that is in line with the basic pH of their aqueous suspensions. Following coating with an epoxy resin (DGEBA) and a hardener (triethylene tetramine), the surface energy of the cements decreases substantially with the mass loading of the organic material. The surface thermodynamic properties were also correlated with the surface chemical composition as determined by X-ray photoelectron spectroscopy.

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.

Interactions of fully formulated epoxy with model cement hydrates

The surface energy of cement paste components (calcium silicate hydrate [C-S-H], ettringite and portlandite) before and after treatment with an organic coating has been characterised by X-ray photoelectron spectroscopy (XPS) and inverse gas chromatography at 35 °C using n-alkanes, 1-alkene, chloroform, tetrahydrofuran, diethyl ether and CCl4 molecular probes. Complementary investigations on the interfacial chemistry were also conducted by Fourier transform infrared spectroscopy and Differential scanning calorimetry analyses. Changes in the dispersion contribution to the surface energy ( ) and acid–base interaction energies were found to be significantly reduced by the organic coating. The XPS allowed the surface chemistry changes induced by the organic coating to be monitored. In particular, the C1s spectra were peak fitted in order to deduce the contribution of the organic coating materials to the total carbon content on the surface. This study showed not only the existence of hydrate-hardener donor–acceptor adduct formation but also presence of interactions between the hydroxide groups OH of the epoxy and portlandite.

Role of interfacial chemistry on the rheology and thermo-mechanical properties of clay-polymer nanocomposites for building applications

This study is directed towards investigating the role of the surface treatment of clay particles on the rheological and thermomechanical behaviour of clay-epoxy blends. Nanocomposites were prepared by mixing small amounts (5–10 mass %) of commercial organoclays or raw clays with an epoxy system commonly used in civil engineering. Rheological characterisations in the liquid state revealed a pronounced thixotropic character of the organoclay-based systems, which all exhibited a shear-thinning behaviour above a critical stress threshold (yield stress), depending on both the intensity of interfacial interactions and the degree of filler dispersion. On the other hand, systems based on raw clay particles behaved like Newtonian fluids, in the same way as the unreinforced polymer matrix. Complementary dynamic mechanical analyses (DMA) performed on the cured cross-linked nanocomposites also showed significant changes in the viscoelastic behaviour of the epoxy matrix due to the introduction of organoclays, whereas only minor variations were observed following the introduction of raw fillers. These results were consistent with nanoscale morphological characterisations performed by conventional X-ray diffraction (XRD) on the various hybrid systems. In this context, rheology and DMA appear as attractive alternative methods for assessing the filler dispersion at a macroscopic (and possibly more relevant) scale. This research is of practical interest for civil engineers, since clay reinforced-epoxies could in the future be used as coating materials with enhanced barrier performances, in order to protect infrastructures against environmental ageing or corrosion.

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.

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.

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).

Durability of bonded assemblies: A predictive theory coupling bulk and interfacial damage mechanisms

ABSTRACT An advanced model coupling bulk and interfacial damages is proposed in order to predict the durability of adhesively bonded joints. The underlying theory, based on the principle of virtual power, is briefly presented in the first part of the paper. The second part is devoted to the validation the cited theory. The model is first implemented to describe the damage behavior of bonded assemblies subjected to either homogeneous tension or shear loading conditions. In each case, the theoretical simulations are compared to experimental data obtained in the same configuration. Such comparisons yield objective indications on the validity of the model.