Bond strength of reinforcing bars with different encasement qualities : guidelines for the development length of reinforcing bars in shotcrete

Depuis plusieurs années, le béton projeté a été utilisé pour la stabilisation des talus et le renforcement des structures dégradées. À ce jour, il est également utilisé pour construire des éléments complets tels que des murs de refend, colonnes, coquilles minces, revêtements de tunnels et poutres. Cependant, depuis quelques années, les ingénieurs en structures ont soulevé une préoccupation particulière concernant la qualité d'enrobage des barres d'armature. En effet, des imperfections peuvent être créées spécifiquement derrière les barres d'armature si le béton n'est pas projeté correctement ce qui pourrait provoquer la rupture prématurée des éléments structuraux. Essentiellement, peu de recherche a été faite à ce sujet et les recommandations courantes reposent sur des fondements entièrement empiriques servant uniquement à évaluer la qualité de l'enrobage de barres d'armature des carottes des panneaux de caractérisation. Cette étude a pour but d'augmenter les connaissances scientifiques concernant la réduction de l'adhérence entre les barres d'armature et le béton causé par ces imperfections afin d'inclure ce phénomène dans les guides destinés à l'inspection et à la conception des structures en béton projeté. Pour contrebalancer la perte d'adhérence, le cas échéant, des facteurs de modification pour l'équation de la longueur de développement des barres d'armature en traction est proposée. À cette fin, l'étude présente une phase expérimentale, une phase de modélisation et finalement une phase analytique. La phase expérimentale inclut des éprouvettes de type « pull-out » faites en béton projeté et en béton coulé ayant des vides artificiels (pour recréer les imperfections parfois observées en béton projeté). Des éprouvettes de type « beam-end » coulées avec des vides artificiels ont également été testées. La phase de modélisation inclut seulement des éprouvettes de type « beam-end » et les principaux résultats étudiés comportent la charge maximale et le type de rupture des éprouvettes, lesquels se sont montrés influencés principalement par la longueur transversale des vides (périmètre non-adhéré) et le recouvrement du béton. Les résultats ont permis d'établir des périmètres non-adhérés limites pour lesquels une rupture par déchaussement pourrait survenir. Cependant, puisqu'une certaine perte d'adhérence a été observée même avant les limites établis, la phase analytique a permis de proposer des facteurs de modification pouvant être utilisés avec l'équation de la longueur de développement. Ainsi, des recommandations ont été développées pour permettre aux ingénieurs de prendre des décisions concernant l'intégrité des structures pendant les inspections ou d'inclure le béton projeté lors de la conception de ces structures si des vides sont relevées ou susceptibles d'être créés derrière les barres d'armature.For many years, shotcrete (sprayed concrete) has been used for slope stabilization and the reinforcement of degraded structures. Nowadays, it's also used to build full-depth structural reinforced concrete elements such as shear walls, columns, thin shells, tunnel linings and girders. However, concerns regarding the encapsulation quality of the reinforcing bars have been raised by structural engineers. Indeed, imperfections could be created specifically behind the reinforcing bars if concrete is inappropriately sprayed which could cause the premature failure of structural elements. Essentially, very little research has been completed on the subject and the current guidelines rely completely on empirical evidence which serves only to evaluate the encasement quality of reinforcing bars from cores taken from pre-construction panels. This study aims to increase the scientific understanding regarding the bond stress reduction between reinforcing bars and concrete caused by the presence of such imperfections in order to include this phenomenon in the current inspection and design guidelines for shotcrete structures. To counteract the bond stress loss, if any, modification factors to be used in conjunction with the development length equation of reinforcing bars in tension is proposed. To do so, the study includes an experimental, a modeling and lastly an analytical phase. The experimental phase includes sprayed as well as cast in-place with artificial voids (to recreate the imperfections observed when shotcrete is incorrectly applied) “pull-out” specimens. Cast in-place “beam-end” specimens with artificial voids were also studied. The modeling phase only includes “beam-end” specimens and the main studied results were the ultimate load and the mode of failure of the specimens which were found to be mainly influenced by the transversal length of the voids (or un-bonded perimeters) and the concrete cover. The results allowed to establish un-bonded perimeters limits beyond which a possible reinforcing bar pull-out failure could occur. However, since a certain bar stress loss was still observed even below the limits established, the analytical phase served to propose modification factors to be used in conjunction with the development length equation. Thus, important guidelines have been created for structural engineers allowing them make decisions regarding the integrity of shotcrete structures during the inspection phase or to take into account shotcrete during the design phase of structures if imperfections are observed or are susceptible to be created behind the reinforcing bars

Encapsulation quality of reinforcement: Impact on bond strength and structural design considerations

Proper reinforcement encapsulation is a concern among structural engineers who must work with limited specific guidelines for shotcrete. Indeed, imperfections behind reinforcing bars (or any other obstacles) are often reported with the excessive use of set-accelerating admixtures or with unskilled nozzlemen. To address such concerns, past research has mainly focused on optimal mixture consistencies and best nozzle handling techniques to obtain perfect encapsulation [1, 2]. Please Click Additional Files below to see the full abstract

Experimental proof of the reciprocal relation between spin Peltier and spin Seebeck effects in a bulk YIG/Pt bilayer

We verify for the first time the reciprocal relation between the spin Peltier and spin Seebeck effects in a bulk YIG/Pt bilayer. Both experiments are performed on the same YIG/Pt device by a setup able to accurately determine heat currents and to separate the spin Peltier heat from the Joule heat background. The sample-specific value for the characteristics of both effects measured on the present YIG/Pt bilayer is (6.2 \pm 0.4)\times 10^{-3} \,\, \mbox{KA^{-1}}. In the paper we also discuss the relation of both effects with the intrinsic and extrinsic parameters of YIG and Pt and we envisage possible strategies to optimize spin Peltier refrigeration.Comment: 11 pages, 3 figure

Non-equilibrium thermodynamics of the spin Seebeck and spin Peltier effects

We study the problem of magnetization and heat currents and their associated thermodynamic forces in a magnetic system by focusing on the magnetization transport in ferromagnetic insulators like YIG. The resulting theory is applied to the longitudinal spin Seebeck and the spin Peltier effects. By focusing on the specific geometry with one YIG layer and one Pt layer, we obtain the optimal conditions for generating large magnetization currents into Pt or large temperature effects in YIG. The theoretical predictions are compared with experiments from the literature permitting to derive the values of the thermomagnetic coefficients of YIG: the magnetization diffusion length $l_M \sim 0.4 \, \mu$m and the absolute thermomagnetic power coefficient $\epsilon_M \sim 10^{-2}$ TK$^{-1}$.Comment: accepted for publication on Physical Review

Non-equilibrium thermodynamics of the longitudinal spin Seebeck effect

In this paper we employ non equilibrium thermodynamics of fluxes and forces to describe magnetization and heat transport. By the theory we are able to identify the thermodynamic driving force of the magnetization current as the gradient of the effective field $\nabla H^*$. This definition permits to define the spin Seebeck coefficient $\epsilon_M$ which relates $\nabla H^*$ and the temperature gradient $\nabla T$. By applying the theory to the geometry of the longitudinal spin Seebeck effect we are able to obtain the optimal conditions for generating large magnetization currents. Furthermore, by using the results of recent experiments, we obtain an order of magnitude for the value of $\epsilon_{M} \sim 10^{-2}$ TK$^{-1}$ for yttrium iron garnet (Y$_3$Fe$_5$O$_{12}$).Comment: accepted for publication on Physics Procedi

Longitudinal spin Seebeck coefficient: heat flux vs. temperature difference method

The determination of the longitudinal spin Seebeck effect (LSSE) coefficient is currently plagued by a large uncertainty due to the poor reproducibility of the experimental conditions used in its measurement. In this work we present a detailed analysis of two different methods used for the determination of the LSSE coefficient. We have performed LSSE experiments in different laboratories, by using different setups and employing both the temperature difference method and the heat flux method. We found that the lack of reproducibility can be mainly attributed to the thermal contact resistance between the sample and the thermal baths which generate the temperature gradient. Due to the variation of the thermal resistance, we found that the scaling of the LSSE voltage to the heat flux through the sample rather than to the temperature difference across the sample greatly reduces the uncertainty. The characteristics of a single YIG/Pt LSSE device obtained with two different setups was $(1.143\pm0.007)\cdot 10^{-7}$ Vm/W and $(1.101\pm0.015)\cdot 10^{-7}$ Vm/W with the heat flux method and $(2.313\pm0.017)\cdot 10^{-7}$ V/K and $(4.956\pm0.005)\cdot 10^{-7}$ V/K with the temperature difference method. This shows that systematic errors can be considerably reduced with the heat flux method.Comment: PDFLaTeX, 10 pages, 6 figure

Domain-wall motion in random potential and hysteresis modeling

Two different approaches to hysteresis modeling are compared using a common ground based on energy relations, defined in terms of dissipated and stored energy. Using the Preisach model and assuming that magnetization is mainly due to domain-wall motion, one can derive the expression of magnetization along a major loop typical of the Jiles–Atherton model and then extend its validity to cases where mean-field effects and reversible contributions are present

Non-equilibrium Thermodynamics of the Longitudinal Spin Seebeck Effect

In this paper we employ non equilibrium thermodynamics of fluxes and forces to describe magnetization and heat transport. By the theory we are able to identify the thermodynamic driving force of the magnetization current as the gradient of the effective field ▿H∗. This definition permits to define the spin Seebeck coefficient ϵM which relates ▿H∗ and the temperature gradient ▿T. By applying the theory to the geometry of the longitudinal spin Seebeck effect we are able to obtain the optimal conditions for generating large magnetization currents. Furthermore, by using the results of recent experiments, we obtain an order of magnitude for the value of ϵM ∼ 10-2 TK-1 for yttrium iron garnet (Y3Fe5O12)

Local spin Seebeck imaging with scanning thermal probe

In this work we present the results of an experiment to locally resolve the spin Seebeck effect in a high-quality Pt/YIG sample. We achieve this by employing a locally heated scanning thermal probe to generate a highly local non-equilibrium spin current. To support our experimental results, we also present a model based on the non-equilibrium thermodynamic approach which is in a good agreement with experimental findings. To further corroborate our results, we index the locally resolved spin Seebeck effect with that of the local magnetisation texture by MFM and correlate corresponding regions. We hypothesise that this technique allows imaging of magnetisation textures within the magnon diffusion length and hence characterisation of spin caloritronic materials at the nanoscale