Lightweight and slender timber-concrete composite floors made of CLT-HPC and CLT-UHPC with ductile notch connectors

Cross-Laminated Timber (CLT) structures have been emerging worldwide for residential floors in multi-storey buildings thanks to their lightness, fast construction and low ecological footprint. This work aims at fostering this application, which is often limited by vibrational and deflection limits, by investigating composite slab floors made of CLT and High-Performance Concrete (HPC) slab as well as CLT and Ultra High-Performance Fiber Reinforced Concrete (UHPFRC). Firstly, the composite floors CLT-HPC and CLT-UHPFRC with a span of 8 m were designed by considering a multicriteria analysis. To assure a certain structural ductility, previously developed ductile notch connectors were employed. As an economic choice, no shear reinforcement in the concrete slab was employed. Then, fullscale composite beams were fabricated in order to verify the predicted flexural behaviour and natural frequency. A numerical analysis was carried out to verify the connectors could effectively yield before the timber collapse. The comparison between the numerical simulation and the slip measurements confirmed that about 50% of the notch connections fully yielded and underwent inelastic deformation which favors the structural ductility. In the case of the CLT-HPC floor, a reduction of the notch contact surface due to the use of plastic sheet waterproofing as well as shear cracks developing in the concrete close to the notch corner both reduced the expected structural stiffness. Finally, the CLT-UHPFRC floor is endowed with outstanding values of slenderness ratio (~35) and lightness (~2 kPa), while eliminating the use of shear reinforcement and sheet waterproofing

Will Blended-Cement Systems with Similar Chloride Penetration Potentials Resist Similarly to Corrosion?

The capacity of concrete to prevent chloride ions penetration represents a key durability factor for steelreinforced structures exposed to de-icing salts and/or marine environments. Although blended-cement systems are commonly characterized with accelerated chloride penetration tests developed for Portlandcement concrete (e.g. the ASTM C1202 method), their microstructures and pore solutions are very different. This study aims to illustrate how the full potential of blended-cement systems to resist chloride ingress (and thus, chloride-induced corrosion) may not be justly disclosed by these accelerated tests using electrical current, even when tested after three months of curing. A Portland-cement-only mortar and five binary blended-cement mortars containing typical dosages of fly ash, slag, metakaolin, glass powder or rice husk ash were characterized using the ASTM C1202 chloride penetration test, bulk resistivity measurements and pore solution resistivity measurements. The results showed similar very low chloride penetration potential after three months for the investigated systems (except for the slag system which showed a low potential). The microstructure was densified with time particularly for systems with fly ash or glass powder, as shown by comparing bulk resistivity measurements after three months and one year of curing. However, measurements of the pore solution resistivity suggested a reinterpretation of the observed trends and the glass powder showed unique features for long-term resistance to chloride-induced corrosion. Finally, this work illustrates the importance of understanding the effects of supplementary cementitious materials on both the microstructure and the pore solution, while motivating further work on complementary aspects such as chloride migration coefficients, chloride binding, porosity distribution, or interfacial transition zone

Uncertainty Propagation of a Multiscale Poromechanics-Hydration Model for Poroelastic Properties of Cement Paste at Early-Age

International audienceThe durability of concrete materials with regard to shrinkage and cracking phenomena depends on the evolution of the poroelastic properties of cement paste. The ability of engineers to control the uncertainty of the percolation threshold and the evolution of the elastic modulus, the Biot-Willis parameter and the skeleton Biot modulus is a keypoint of design practices to minimize the vulnerability of concrete structures at early-age. This article presents the uncertainty propagation and the sensitivity analysis of a multiscale poromechanics-hydration numerical model for cement pastes of water-to-cement ratio between 0.35 and 0.70. The model provides poroelastic properties required to model the behavior of partially saturated aging cement pastes (\emph{e.g.} autogenous shrinkage) and it predicts the percolation threshold and the undrained elastic modulus in good agreement with experimental data. The development of a stochastic metamodel using polynomial chaos expansion allows to propagate the uncertainty characteristic of the kinetic parameters of hydration, the quantitative cement phase composition, the elastic moduli of elementary material phases and the morphological parameters of the microstructure. The propagation does not magnify the uncertainty of the single poroelastic properties although, their correlation may amplify the variability of the estimates obtained from poroelastic state equations of cement paste. In order to reduce the uncertainty of the percolation threshold and of the poroelastic properties at early-age, it is recommanded to improve the accuracy of the apparent activation energy of calcium aluminate and, later on, of the elastic modulus of low density calcium-sillicate-hydrate

Optical fiber sensors implementation for monitoring the early-age behavior of full-scale timber-concrete composite slabs

The study of the early-age behavior of Timber-Concrete Composite (TCC) structures is of great interest as it provides valuable information for manufacturing specification development, quality control, and optimization of the formwork design. In this study, the results of the continuous monitoring of the short-term behavior of TCC slabs using Brillouin Distributed Optical Fiber Sensors (DOFS) are reported. Two TCC slabs with 8.5 m of length were monitored. The composite elements are constituted of Cross-Laminated Timber (CLT) connected to a High-Performance Concrete (HPC) slab. During a monitoring period of about 30 days, the early-age temperature/strain variation in the fresh concrete and in the CLT slab was measured in great details by DOFS. From the presented results, the significant influence of the curing conditions on the early-age shrinkage was highlighted. It was also observed that creep and the daily hygrometric variations of environment affect considerably the composite action between the timber and the concrete. In addition, it was experimentally demonstrated that such mechanisms generate considerable structural changes in the composite elements even before their entry into service

A modified accelerated chloride migration tests for UHPC and UHPFRC with PVA and steel fibers

Accelerated migration tests which are commonly used to measure chloride diffusion in ordinary cement-based materials cannot be directly applied to composite with very low permeability, such as Ultra High-Performance Fiber Reinforced concretes (UHPFRC). In order to assess the UHPFRC enhancement on the structural durability, there is a critical need to accurately assess the permeability level of the material to chloride ions. The objective of this work is to adapt an existing set-up of accelerated chloride migration test in order to (i) better characterize the resistance of chloride ion penetration in UHPFRC; and (ii) to compare the resistance of chloride ion penetration between UHPC and UHPFRC. The material characterization, the set-up modifications of the existing accelerated migration test, the results are presented. In conclusion, the modification of the test-set-up allowed to accurately measure chloride transport of very low permeability UHPFRC and to shed light on the effect of the fiber reinforcement

A Coupled Nanoindentation/SEM-EDS Study on Low Water/Cement Ratio Portland Cement Paste: Evidence for C–S–H/Ca(OH)2 Nanocomposites

A low water/cement ratio (w/c=0.20) hydrated Portland cement paste was analyzed by grid-indentation coupled with ex situ scanning electron microscope-energy-dispersive X-ray spectra (SEM-EDS) analysis at each indentation point. Because finite element and Monte-Carlo simulations showed that the microvolumes probed by each method are of comparable size (approximately 2 μm), the mechanical information provided by nanoindentation was directly comparable to the chemical information provided by SEM-EDS. This coupled approach provided the opportunity to determine whether the local indentation response was a result of a single- or a multiphase response—the latter being shown predominant in the highly concentrated w/c=0.20 hydrated cement paste. Results indicate that, in the selected microvolumes where C–S–H and nanoscale Ca(OH)2 (CH) are present, increasing fractions of CH increase the local indentation modulus (and hardness), yielding values above those reported for high-density (HD) C–S–H. Micromechanical analyses show that C–S–H and CH are associated, not merely as a simple biphase mixture, but as an intimate nanocomposite where nanoscale CH reinforces C–S–H by partially filling the latter's gel pores. The paper discusses the mechanism of forming the C–S–H/CH nanocomposite, as well as the impact of nanocomposites on various macroscopic properties of concrete (e.g., shrinkage, expansion). On a general level, this study illustrates how a coupled nanoindentation/X-ray microanalysis/micromechanics approach can provide otherwise inaccessible information on the nanomechanical properties of highly heterogeneous composites with intermixing at length scales smaller than the stress field in a nanoindentation experiment

Non-local numerical treatment of non-linear behavior by means of Helmholtz equation, with variable coefficients. Application to reinforced concrete structures.

Numerous work has been done with the aim of modeling the cracking of reinforced concrete (RC) structures. Among the recent methods proposed in the literature, the combination of reinforcement-concrete equilibrium combined with the linear behavior of the interface leads to a Helmholtz equation which takes account of the slip between the homogenized reinforcements and the concrete in presence of localized cracks [1][2]. In the case of large cracks openings, it is necessary to consider the non-linear behaviors of material and interfaces, such as the plasticity of reinforcements or the damage of the matrix-reinforcement interface. These phenomena induce variations of the coefficients in the Helmholtz equation, which leads to two levels of iterative procedures: one at a global level considering equilibrium of homogenized RC, and another one at a non-local level taking account of equilibrium between reinforcement and concrete. The implementation of a convergence criterion is then needed at each level. The goal of this paper is to describe the developments implemented in the Finite Element code Cast3m to perform non-local Helmholtz type calculations with non-constant coefficients. This method, using an acceleration method [3] is illustrated by the cases of reinforced concrete tie and beam, with homogenized reinforcements. References : [1] A. Sellier and A. Millard, “A homogenized formulation to account for sliding of non-meshed reinforcements during the cracking of brittle matrix composites: Application to reinforced concrete,” Eng. Fract. Mech., vol. 213, pp. 182–196, May 2019, doi: 10.1016/j.engfracmech.2019.04.008. [2] A. Sellier and A. Millard, “Traitement numérique non local de phénomènes physiques par l’équation d’Helmholtz : les effets d’échelle et le glissement renfort-matrice,” in Club Cast3M 2018, Paris, 2018, vol. 1, no. 1, pp. 12–18. Available: http://www-cast3m.cea.fr/html/ClubCast3m/club2018/Presentation_Sellier.pdf. [3] A. C. Aitken, “On the iterative solution of a system of linear equations.,” Proc. Roy. Sot. Edinburgh, pp. 52–60, 1950

Structural elements made with highly flowable UHPFRC: Correlating computational fluid dynamics (CFD) predictions and non-destructive survey of fiber dispersion with failure modes

Structural design with highly flowable Fibre Reinforced Concrete has to duly take into account the preferential alignment of fibers, which can be governed through the rheological properties of the fluid mixture and the casting process and by the geometry of the structure. The possibility of predicting the fiber alignment, by tailoring the casting process, and of non-destructively monitoring it, can foster more efficient structural applications and design approaches. Focusing on UHPFRC slabs with pre-arranged casting defects, the flow-induced alignment of the fibers has been predicted by means of a suitable CFD modelling approach and hence monitored via a non-destructive method based on magnetic inductance properties of the fiber reinforced composite. The comparison between the assessed data on the fiber orientation and the crack patterns as visualized by image analysis supports the effectiveness of casting flow modelling and non-destructive fiber dispersion monitoring in supporting the structural design of elements made with highly flowable fiber reinforced cementitious composites