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International audience

International audience

Formwork removal by means of polarization offers an alternative solution to compensate for some of the disadvantages and risks of using mineral-based oil due to its petrochemical origin. This technique consists of applying an electrical current between the steel reinforcing bars and the metal formwork wall in order to obtain a thin water film at the concrete/formwork interface and allow for formwork removal. This paper focuses on the facing quality of the reinforced concrete and formwork surface at various temperatures for both conventional and self-compacting concretes. A series of experimental tests have been conducted in the laboratory to determine the optimal polarization parameters (voltage value, time and duration of its application). On-site tests were then performed to observe the scaling effect. Results indicate that the proposed polarization technique can indeed provide an effective means to separate the concrete from its formwork without requiring demoulding oil. Concrete mixtures implemented both in the laboratory and on-site display high-quality facing without the presence of any attachment points. A chromatographic analysis of the concrete pore solution and water transported by electro-osmosis shows that ion concentrations remain unaffected. This polarization approach can thus be used as an alternative to avoid the use of oils of a chemical or petrochemical origin.

In recent years, many studies have focused on the mix-design and characterization of cementitious composites reinforced with plant fibres. It appears that in the fresh state, the morphology and the hydrophilic nature of these fibres significantly affect the rheology of the material. In the hardened state, the mechanical performances of the composites are lower than that of composites reinforced with standard fibres (glass, synthetic, etc.). These poor mechanical performances are mainly due to weak neo-formed fibre/matrix interface. Indeed, the high-water absorption capacity causes swelling/shrinkage of the fibre within the matrix, which subsequently generates an unbounded interface. In order to address these issues, numerous studies have been carried out over the past few years on compatibilisation treatments applied to plant fibres. In most cases, these are chemical treatments to leach the fibre, which often have a significant environmental impact. This work examines the influence of a coating treatment with linseed oil applied to flax fibres on the properties of cementitious composites. The first part of this study deals with the optimisation of this treatment. The second part focuses on the effect of this treatment on the properties of the composite in the fresh and hardened states. The results showed a noticeable improvement in the workability of the treated fibre composites. The linseed oil coating treatment can also significantly improve the flexural strength of the composite, depending on the mix-design. Flax fibres and linseed oil can also slightly enhance the thermal insulation properties of the material.

International audience; The friction of concrete on the interior skin of formworks takes place during the pouring of concrete into the molds. The present work investigates the friction of a granular material (i.e. fresh concrete) against metallic and polymeric surfaces. Interfacial behavior between different formulated concretes and formwork skins is characterized using a plane-plane tribometer dedicated to concrete tribometer. The formwork surface is measured before and after testing to quantify the wear issue. A Coulomb friction law is observed within the range of tested normal pressures. Tribological tests reveal that friction mechanisms depend on the interface properties. Two underlying mechanisms are hypothesized to explain the wearing of the formwork skin: a granulate-formwork solid-solid friction and a capillary-dominated friction.

Alfa plants are endemic to the Maghreb, where they cover great areas and are of prime socio-economic interest. This study investigated use of the alfa plant in a fibre-reinforced cement composite for facade panels. Alfa fibres were ground in a knife mill, sieved through a 2 mm sieve and added to the mixture in different addition ratios (1, 2 and 3 vol%). The results showed that the incorporation of ground alfa fibres did not significantly influence the fresh properties of the mortar. Compared with a control mortar, there was a decrease of 4% in the workability of the mortar reinforced with 2 vol% of alfa fibres. An enhancement in flexural strength was observed for all three addition ratios, with the evolution of flexural strength linearly related to the increase of the fibre addition ratio. However, the compressive strength of the composites decreased with the addition of fibres, mainly due to the high porosity inside the matrix. This was confirmed by a capillary test, which showed that a high addition ratio led to high water absorption by capillary motion.

In the aim of enhancing the reuse potential of poplar sawdust stemming from the transformation of wood into particleboard for woodworking purposes, wood-reinforced concrete mix designs are optimised to obtain composites in compliance with French standard NF EN 771-3/CN. Two criteria are assessed herein for structural elements, namely dimensional tolerance and mechanical strength (7-day levels above 6 MPa). Wood aggregates are introduced by means of a volumetric substitution for sand. Easily demoldable concrete blocks have been produced by means of vibrocompaction. A high vibration frequency is coupled with a compaction process in order to generate semi-dry cylindrical concrete blocks (100 mm in diameter, 200 mm high). The addition of wood into the cementitious composite exerts a noticeable influence on its workability, as well as on its thermophysical and mechanical behaviour. Wood alters how the cementitious composite is hydrated and has an impact on its mechanical strength in compression, which drops considerably with an increase in sawdust content. A 50% rate of substituting poplar sawdust for sand was chosen for testing purposes. This concrete composition actually yields mechanical strength values similar to that of the reference concrete used to manufacture masonry blocks.

An experimental investigation was undertaken to evaluate the potential use of poplar by-products in cement mortars. Two by-products from poplar processing were studied: sawdust (from sawing wood) and milled fibres (from wood waste obtained during cutting). Poplar fibres and sawdust were incorporated into mortars as sand volume substitution with rates ranging from 0 to 100%. Before introduction into the mortar, poplar aggregates were characterized (morphology and water absorption). It appeared that these wood aggregates absorb very quickly a large amount of water (up to 200%). The introduction of these poplar by-products has significant effects on the composite properties in both fresh and hardened states. It was noted that the workability of the mixtures with the poplar wood varies with the substitution rate. The inhibition of cement setting with poplar sawdust is characterized by maturity tests and affects the mechanical properties of mortars. In addition, increasing the poplar aggregates content causes a continuous increase of the material porosity, which leads to a decrease in density. Besides, it appears that the incorporation of poplar aggregates into mortars also decrease their thermal conductivities, which would help to reduce building heat losses. A significant decrease in the mechanical properties of poplar-based mortars was also noted, related to the rate of fibres or sawdust introduced into mixtures. The hydration disturbances, the high internal porosity of the wood particles and their low density can explain these lower mechanical strengths. However, despite these disturbances, the valorisation of poplar wood by-products remains possible in cementitious materials for the manufacture of concrete masonry blocks.

In this study, a nonlinear numerical analysis is carried out to predict the creep response of reinforced concrete (RC) beams externally strengthened using carbon fiber-reinforced polymer (CFRP) and glass fiber-reinforced polymer (GFRP) laminates. The beams are tested under static four-point bending; the level of sustained service load is equal to 59% of the ultimate capacities of the un-strengthened beams. The Burger rheological model is used to evaluate the flexural creep response (compressive and tensile creep) and then to validate the findings with experimental results. It is shown that the Burger model effectively predicts the creep of CFRP-bonded RC. Furthermore, parametric studies are performed to investigate the effects of: three fiber orientations of CFRP composites (0°, 45° and 90°), the CFRP thickness, and the type of FRP (CFRP and GFRP) on the flexural creep of FRP-externally retrofitted RC beams based on the Burger rheological model. According to the numerical results, a major reduction in creep strain is obtained with the 45° orientation of CFRP. An increase in CFRP-thickness significantly lowers the creep strain. Moreover, the CFRP laminates prove to be more effective than GFRP in improving the creep response.

This study deals with an experimental and numerical investigation focused on the total shrinkage of concrete made with recycled coarse aggregates (RCA) and subjected to various isothermal temperatures. The objective of the experimental work is to assess the influence of the external temperatures (20, 35 and 50 °C) and the percentage of RCA replacements on the total shrinkage of recycled aggregates concrete. Natural coarse aggregates (NCA) are substituted by RCA at different weight levels (0%, 25%, 50%, 75% and 100%). The obtained results relate that the mechanical properties are adversely affected by the RCA replacement ratio. In addition, the total shrinkage strain rises with increase in the replacement ratio and the curing temperature. On the other hand, finite element analysis (FEA) using ANSYSⒸ software is used to predict the shrinkage of concrete regarding the thermal and mechanical properties. The shrinkage is estimated by means of the maturity approach and the two-phase serial model. The results founded from the FEA illustrate a good accuracy with the experimental ones.

This study deals with experimental investigations of beam performances in a marine environment. Two kinds of concrete beams, unreinforced and reinforced with carbon plates and carbon rods, are being tested. The first one is stored in a laboratory, the other is exposed to a marine environment located in the north of France. After 12 months, all beams are tested via a four-point bending test in a laboratory. Results obtained have shown that beams stored in marine environment have a better behaviour than those stored in laboratory. It should be noted that no damage has occurred on these beams. However, we observe a significant increase of load of about 32% to 48% causing the first crack observed on the beams stored in marine environment compared to those stored in the laboratory. This means that beams in situ offer increased stiffness and a slight gain of failure loads. This may be due to the development of living organisms (in a marine environment) which acted as additional adhesive and sealing, providing a protection of concrete structures against damage.

This article presents an experimental study aiming to provide reliable experimental data for the prediction of the shear strength of reinforced concrete beams without stirrups. The shear strength results obtained from the proposed model as well as those from the design codes (ACI 318, Eurocode 2 and BS 8110) are compared with the database containing 700 beams made of both high strength concrete and normal strength concrete. Furthermore, the experimental results were used to assess the contribution of each shear mechanism obtained from Chen’s, Cavagnis’s and Fernández Ruiz’s models. The contribution of these various shear-transfer actions is also quantified experimentally, using a digital image correlation. As a result, the measured contributions of dowel action, aggregate interlock and compression zone to the total shear resistance were estimated as 45% to 50%, 20% to 35% and 17% to 31%, respectively, for high strength concrete beams. On the contrary, the average test-to-predicted contribution of the shear-transfer action ratio determined by the Chen formula is 1.15 for short beams, whereas the Fernández Ruiz and the Cavagnis models yielded average ratios of 1.04 and 1.52, respectively, for slender beams. The proposed formulae give a rational prediction for either both short and slender beams, and yield accurate and consistent results compared to the other models used in this study, with a lowest average value of the test-to-predicted at 1.08 and that of the coefficient of variation at 23.06%, particularly for short beams. However, ACI 318 is the only code that does not take into account the size effect, leading to a severe underestimation of the shear strength for short beams.

Diaphragm walls are used either to support excavation walls on site or prevent water move through the ground. The observations recorded following excavation and planning of diaphragm walls demonstr...

In this study, a numerical investigation is conducted to simulate the total shrinkage strain of High-Strength Concrete (HSC) and Steel Fiber Reinforced High-Strength concrete (SFRHSC) by means of the transient thermal-stress analysis. The ANSYS finite element software has been used in order to evaluate the shrinkage by taking into account both the thermal and mechanical properties of the concrete. These properties are calculated using the maturity concept and the two-phase serial model. The experimental work was carried out to assess the influence of the external temperatures, steel fibers (SF) and their volume fraction on the total shrinkage strain of the SFRHSC which has been exposed to isothermal temperature of 20, 35 and 50 °C. Two dosages of 0.5% and 1% for the SF with aspect ratio of 55 have been considered. The main results obtained from the FE analysis show a good agreement with the founded experimental results under different thermal conditions. According to the obtained numerical results, an increase of the dosage of fibers will reduce the total shrinkage strain. Additionally, the curing temperature raises significantly the evolution of the total shrinkage strain.

The present study is focused on experimental investigations and numerical analyses of compressive and tensile total creep at long-term of high-strength concrete (HSC) beams. Reinforced concrete (RC) beams with and without steel fibres were investigated for their long-term creep behaviour. These beams are under sustained uniformly distributed load (SUDL) in bending containing steel fibres (SF) with two aspect ratios (55 and 80) and two steel fibre dosages (0.5% and 1%). The objective of this work is to evaluate the influence of steel fibres, their dosage and their aspect ratio on compressive and tensile total creep. Experimental results show that the long-term total creep is influenced by the volume fractions of steel fibres. Moreover, steel fibres decrease the tensile total creep more than the compressive one. A non-linear finite element method (FEM) is performed to simulate the long-term total creep strain rate included thanks to the time hardening model called generalized Garofalo creep which is based on Bailey-Norton law called power law creep. It was found a good agreement between the experimental and numerical results.

Wall roughness plays a crucial role in granular medium - rough wall interface friction. In this study, an experimental device has been designed to study the influence of boundary conditions, more specifically wall roughness, on the behavior of sheared granular medium. The study is based on use of an analog model, and consists of simulating roughness by means of notches and grains in the medium by monodisperse beads and on use of a numerical model based on the discrete element method. The test protocol entails displacing at fixed speed notched rods under confined granular medium. Movement of the beads layer near the rods as well as friction of the beads against the rods are both studied herein. Results indicate that the parameter controlling friction at the granular medium - rough wall interface is primarily the depth of beads embedment in surface asperities. The objective of the associated numerical modeling is to supplement the experimental results.

Architects nowadays demand impeccable aesthetic qualities of demoulded rough concretes. New mix designs are increasingly complex, and a concrete mix characterization often limits the workability attributes, in the aim of providing an assessment of concrete behavior at the time of its placement. The aesthetics of concrete facings however also depend on the actual mix design, location and release agents used. This study focuses on analyzing the influence of self compacting concrete (SCC) and the choice of release agent on the evolution of frictional stresses at the concrete/formwork interface. A follow-up study has been conducted on the aesthetic quality of facings between an SCC and a concrete with additives, including mix designs with both mineral and vegetable oil. An analysis of shading color was also carried out over three periods, namely: 1 day, 2 days, and 7 days.

International audience; Les travaux présentés ici rentrent dans le cadre du projet de bétons de fondations profondes financé par la FNTP. Ils s'intéressent aux problèmes de mise en œuvre rencontrés sur les chantiers de pieux forés à la tarière creuse, parmi lesquels la difficulté d'introduction de la cage d'armatures après le bétonnage du pieu. Ils se proposent de valider un protocole d'essai de laboratoire permettant de caractériser l'enfoncement d'une cage d'armatures en fonction de la charge qui lui est appliquée et du temps de repos du béton. Pour ce faire, des essais expérimentaux ont été réalisés sur une formulation courante de béton de pieu pour étudier l'effet du temps de repos sur les propriétés rhéologiques du béton et la capacité des armatures à y pénétrer. Les résultats obtenus ont permis de déterminer, pour le dispositif proposé, les paramètres d'essai à prendre en compte.

International audience; To ensure an easy demoulding of the concrete, the release agent must form a homogeneous film on the formwork to resist the casting concrete. The adherence of oil with the support was studied via the solid/liquid energy adhesion. This energy was calculated using the Zisman equation involving the angle of drop and the surface tension. Various formulations were studied, in order to establish correlations between their composition and the interfacial properties. Formwork release tests were performed to assess cladding aesthetics; it was possible to define a minimal level of adhesion energy to meet the facings quality after concrete demoulding.

International audience; The release agents protect the internal surfaces of formwork against corrosion and also ensure easy removal of the concrete elements. On the other hand, the superplasticizers are used to enhance the fluidity of the fresh concrete without adding water, thanks to their surfactant properties. Their contact with release agents can cause interactions near the formwork surfaces during the concrete pouring. The aim of this study is to identify the effect of a polycarboxylic ether superplasticizer on the tribological behavior of fresh concrete at the concrete/oil/formwork interface. Friction tests were performed using a plane/plane tribometer on three concretes with 30% of paste at different dosages of superplasticizer. Moreover, the purpose of the physicochemical study was to clarify the nature of the interactions between concrete pore solutions and release agents. Results showed that the superplasticizer dosage did not have the same effect on the friction according to the oil nature involved at interface. Thus, a description of the processes occurring at the concrete/oil/formwork interface is proposed. When the superplasticizer generates a deflocculating action of the cement particles, it also acts on the stabilization of the soap-oil micellae present at the interface.

International audience; Improving the knowledge of rheological and tribological characteristics of fresh concrete is important to contribute to the progress of construction sites and the final quality of the work. The objective of this study is to identify the effect of a superplasticizer based on polycarboxylic ether on the tribological behavior of fresh concrete at the concrete/formwork and concrete/oil/formwork interfaces. Friction tests on fresh concrete were carried out using a plan/plan tribometer. In order to study the behavior of the superplasticizer close to the formwork, three concretes with 30% of paste and different dosage of superplasticizer were formulated. The results show that the increase of the dosage of superplasticizer reduces the friction stress. The properties of the superplasticizer generate a deflocculating action of concrete grains and lead to a stabilisation of the soap-oil micellae present in the vicinity of the formwork. Thus, the efficiency of superplasticizer depends on the quantity of fines, on the quantity of soap formed and so, on the release agent formulation.

This study concerns the prediction of lateral pressure exerted by self-compacting concrete (SCC) on formwork during the casting process. This estimation is obtained using Janssen's model as used in the statics of ensiled granular material. The model has been adapted to SCC. The friction coefficient between the concrete and the wall occurring in the model is measured with a new type of tribometer. The case of SCC placed in 12 m high formwork by three companies using two different techniques is presented. Comparison of the predicted results with on site measurements shows good agreement.