Luca Sorelli

Steel Fiber Concrete Slabs on Ground: A Structural Matter

Steel fiber-reinforced concrete (SFRC) is used in industrial pavements, roads, parking areas, and airport runways as an alternative to conventional reinforcement with reinforcing bars or welded mesh. This article reports on a study of the structural behavior of slabs on ground made of SFRC. Several full-scale slabs reinforced with different volume fractions of steel fibers having different geometries were tested under a point load in the slab center. A hybrid combination of short and long fibers was also tested to optimize structural behavior. The authors found that steel fibers significantly enhance the bearing capacity and the ductility of slabs on ground. The nonlinear behavior of these SFRC structures is well captured by performing nonlinear fracture mechanics analyses where the constitutive relations of cracked concrete under tension were experimentally determined. The authors conclude by proposing a simplified analytical equation for predicting the minimum necessary thickness of SFRC slabs on ground.

A closer look at the light-induced changes in the mechanical properties of azobenzene-containing polymers by statistical nanoindentation

The mechanical properties of azobenzene-containing polymer films are statistically measured by instrumented nanoindentation experiments in the dark and under illumination in the absorption band of the azobenzene molecules. The material is obtained from a commercial PMMA compound grafted with Disperse Red 1 (DR1) azobenzene derivative. In the dark, DR1 molecules remain in the stable trans isomer state while, under illumination, they undergo photoisomerisation cycling and form a photo-stationary equilibrium between cis and trans isomers. This material is known to exhibit light-induced deformation phenomena related to the photoisomerization cycling of the DR1 units. Statistical loading/unloading tests performed in the tens of μN load range reveal a significant change in the mechanical properties of the film under light excitation. The material hardness and irreversible viscosity are seen to decrease, while the creep coefficient value increases, indicating a significant reinforcement of the viscoplastic response of the film under illumination. Moreover, creep experiments performed at a constant load show striking dissipative effects when light is turned on and also, surprisingly, when light is turned off. These features are supposedly related to the transient changes in the balance between the cis and trans isomer populations.

Innovative Design of Ultra High-Performance Fiber Reinforced Concrete Ribbed Slab: Experimental Validation and Preliminary Detailed Analyses

A new generation of cementitious composites, ultra high performance fiber reinforced concrete (UHPFRC), represents an important breakthrough for addressing civil engineering challenges. The most significant feature of UHPFRC is the nearly elasto-plastic ductile behavior in tension, which allows safe exploitation of the tensile and shear capacity in structural elements, while also potentially benefits the dynamic behavior of concrete structures. Where traditional steel elements have shown fatigue resistance problems at the connections in orthotropic slab bridge decks, an attractive application of UHPFRC has been developed within MIKTI coordinated R&D French national project. It consists in a thin 2D-ribbed slab, pre-stressed transversally, made of 2.5 m-long segments connected by post-tension, further connected to conventional longitudinal steel beams which take advantage of the slab lightness. A major critical aspect of the project consists in the safe accounting for local with respect to global bending, even under repeated local fatigue loading. Moreover, safety barriers have to be anchored at the edges of such a thin structure. The capacity of the deck to withstand the load representative of a truck shock, without being damaged before the fuse connecting system of the barrier yields, appears as highly critical also. The detailed design of this innovative structure has been carried out applying French interim recommendations for UHPFRC. However, detailed verifications of the local bending (corresponding to a wheel load directly applied and concentrated over the center of one “honeycomb” delimited by the transverse and longitudinal ribs) and of the behavior of the transverse joint under representative bending loads, require refined Finite Element analyses. Both general design and detailed analyses are being compared to scale one experiments.

Creep Behavior of Cement Paste, Mortar, and Concrete: The Role of Relative Humidity and Interface Porosity

Delayed deformations in concrete under sustained loading may depend on complex chemo-hydro-mechanical phenomena. Generally, the creep deformation of a cement paste is distinguished in the short-term and long-term. Simply speaking, the former can be associated to the diffusion of gel water in pores and cracks and it is mainly volumetric (i.e., volumetric deformation), reversible, and bounded. The latter can be associated to the sliding of the sheets of Calcium-Silicate-Hydrate (C-S-H), which is mainly deviatoric (i.e., shape deformation), irreversible and with an unbounded asymptotic deformation. From an experimental point of view, recent studies have shown that microindentation is a powerful technique to quickly characterize basic creep thanks to the reduced characteristic time of a sample size of few micrometers. The objective of this project is twofold: (1) investigating the effect of relative humidity on the creep behavior by microindentation; (2) characterize the viscous behavior of the interface transition zones (ITZ) around a rigid inclusion. In particular, the project investigated two different relative humidity, 33% and 85%, by considering both creep and relaxation microindentation tests. The presented results show the effect of the moisture content on the creep behavior of cement paste both at short and long terms. In addition, the results showed that the ITZ zone around an aggregate showed higher creep than the bulk cement paste. Those results provide new insights to understand the role of water and interface porosity on the creep mechanisms of concrete.

Micro-Chemo-Mechanical Characterization of a Limestone-Calcinated-Clay Cement Paste by Statistical Nanoindentation and Quantitative SEM-EDS

To address the sustainability concerns associated with Portland cement clinker production, the ternary blend of limestone, calcined clay and cement (which was named LC3) has recently been demonstrated to be an efficient solution. This work aims to contribute to the development of this promising material by applying latest techniques to characterize the chemo-mechanical properties of the complex heterogeneous microstructure of an LC3 paste by combining statistical nanoindentation and quantitative SEM-EDS techniques (SNI-QEDS). The results showed that the mechanical properties of LC3 come from a complex microstructure assemblage of C-A-S-H, Al-rich hydrates and anhydrous grains. Thus, the LC3 microstructure is composed by large anhydrous grains (clinker and calcined clay) embedded in a cementitious paste made of hydrates incorporating finely graded grains of anhydrous calcined clay, limestone and quartz. Moreover, the partial reaction of the calcined clay, limestone and Portlandite formed C-A-S-H and other Al-rich hydrates (likely including carboaluminates). Notably, the latter exhibited higher mechanical properties than those of C-A-S-H. Finally, the present work provides new knowledge for better understanding the complex LC3 microstructure towards advanced modelling and mix design optimization.

NLFM for FRC Pavement Analysis and Design

Non Linear Fracture Mechanics (NLFM) represents a powerful tool for analyzing Fiber Reinforced Concrete (FRC) slabs on grade; in fact, because of the stress redistribution, possible in these statically indeterminate structures, the bearing capacity can remarkably increase after reaching the ultimate strain up the formation of a collapse mechanism. This evidences that slab behavior is highly influenced by the crack propagation and underlies the importance of a fiber reinforcement. In fact, concrete roughness is significantly increased by steel fibers that may substitute (partially or totally) conventional reinforcement. Fibers also increase the crack resistance of concrete pavement. The use of a NFLM method for modeling concrete slab on grade with different types of reinforcement is presented in this paper. The validation of the methods was firstly obtained by simulating experiments on full-scale slabs subjected to different loading conditions. The numerical analyses proved to be in a good agreement with the experimental response for the load-displacement behavior, for the ultimate load and for the final crack pattern. Finally, some deign considerations are introduced by comparing the numerical response or slabs characterized by the same geometry and loading conditions and a different reinforcement arrangement. In particular, the behavior of concrete slabs reinforced by steel fibers only, by steel rebars only or by steel rebars and fibers together is presented.

Crack Growth Resistance of Thin Mortar Layers with Hybrid Fiber Reinforcement

Hybrid fiber reinforcement of cement composites is rapidly emerging as an innovative and promising way of improving mechanical performance and durability of cement-based materials. Fracture behavior of medium, high and very high strength mortars reinforced with hybrid fibers was experimentally studied in this paper by using contoured double cantilever beam specimens. Different combinations of small steel fibers and fibrillated polypropylene micro-fibers are investigated. These composites are very suitable for thin sheet products such as roofing sheets, tiles, curtain walls, cladding panels, permanent forms, etc. The objective this paper was to study the influence of matrix strength, fiber type and fiber combinations on the fracture toughness of the resulting fiber reinforced mortars. The results indicate that some combinations of fibers and matrix strengths exhibit a higher resistance to crack growth and evidence the contribution of polypropylene fibers to mortar toughness.

Full thermo-mechanical coupling using eXtended finite element method in quasi-transient crack propagation

This work aims to present a complete full coupling eXtended finite element formulation of the thermo-mechanical problem of cracked bodies. The basic concept of the extended finite element method is discussed in the context of mechanical and thermal discontinuities. Benchmarks are presented to validate at the same time the implementation of stress intensity factors and numerical mechanical and thermal responses. A quasi-transient crack propagation model, subjected to transient thermal load combined with a quasi-static crack growth was presented and implemented into a home-made object-oriented code. The developed eXtended finite element tool for modeling two-dimensional thermo-mechanical problem involving multiple cracks and defects are confirmed through selected examples by estimating the stress intensity factors with remarkable accuracy and robustness.

Identification of the Stress Intensity Factor of Carbon Cathode by Digital Image Correlation

Crack propagation in carbon cathode used in the aluminium industry has been investigated through flexural tests on notched specimens. The main parameters of interest were the geometrical evolution of the crack and the stress intensity factor at the tip ends. The latter, in the case of interfacial fracture in two-dimensional geometries, can be related to normal (mode I) and shear (mode II) stresses. In this work, a new methodology has been applied which optically measures the crack tip displacement field by Digital Image Correlation. The stress intensity factors derived from the experimental data are consistent with results available in the literature. Furthermore, the preliminary results showed that characterizing the mode I (opening) only is somehow challenging due to the heterogeneity of such carbonaceous materials.

Duality between Creep and Relaxation of a Cement Paste at Different Levels of Relative Humidity: Characterization by Microindentation and Analytical Modeling

Recent studies have showed that microindentation techniques allow assessing the logarithmic creep rate of a cement paste in good correlation with the long-term creep rates measured by compressive tests at macroscopic scale. After having applied microindentation techniques to characterize the effect of relative humidity (RH) on both the creep and relaxation behavior of a cement paste, the objective of this work is to analyze the duality between creep and relaxation curves by means of the analytical models which are currently employed in open literature. First, large grids of creep and relaxation microindentation tests were carried out on a cement paste sample in hygral equilibrium at different levels of RH. Thus, the results were modeled by a viscoelastic model by considering different creep functions (logarithmic and power-law) and corrective terms for initial plasticity under loading. The presented results provide new insights to understand the duality between creep and relaxation rates of a cement paste measured at micrometer scale, especially considering the possible plastic effect.