Effect of Fiber Orientation and Specimen Thickness on the Tensile Response of Strain Hardening UHPFRC Mixes with Reduced Embodied Energy

Ultra-high performance fiber reinforced concretes (UHPFRC) have demonstrated their potential to contain the explosion of maintenance costs (Economy and Environment) for civil engineering structures, due to their extremely low permeability associated with the outstanding mechanical properties. Substitution of embodied-energy (EE)-costly components of UHPFRC such as clinker and steel fibers, is the next step towards sustainability, to make it even more efficient and more environment-friendly. In this study, a strain hardening UHPFRC mix with two main modifications has been developed in which (1) 75% of steel fibers have been replaced by ultra-high molecular weight polyethylene (UHMWPE, henceforth referred to as PE) fibers and (2) 50% volume of cement type CEM I have been replaced with limestone filler. The effect of the fiber orientation and the specimen thickness on the mechanical properties of such mixes have been investigated. The mechanical properties have been investigated using direct tensile test, and 4-point bending test. Finally, the dramatic effect of fiber orientation on the ultimate strength and deformability has been demonstrated. Moreover, the results confirm that the specimen thickness affects the deformation capacity of the specimens. Finally, improvements in terms of reduction of EE of the proposed mixes, are highlighted.

Effect of fiber orientation and specimen thickness on the tensile response of strain hardening UHPFRC mixes with reduced Embodied Energy

Emmanuel Denarié, Amir Hajiesmaeili

Ultra-High Performance Fiber Reinforced Concretes (UHPFRC) have demonstrated their potential to contain the explosion of maintenance costs (Economy and Environment) for civil engineering structures, due to their extremely low permeability associated with the outstanding mechanical properties. Substitution of Embodied- Energy (EE)-costly components of UHPFRC such as clinker and steel fibers, is the next step towards sustainability, to make it even more efficient and more environment- friendly. In this study, a strain hardening UHPFRC mix with two main modifications has been developed in which (1) 75% of steel fibers have been replaced by ultra-high molecular weight polyethylene (UHMWPE, henceforth referred to as PE) fibers and (2) 50% volume of cement type CEM I have been replaced with limestone filler. The effect of the fiber orientation and the specimen thickness on the mechanical properties of such mixes have been investigated. The mechanical properties have been investigated using direct tensile test, and 4-point bending test. Finally, the dramatic effect of fiber orientation on the ultimate strength and deformability has been demonstrated. Moreover, the results confirm that the specimen thickness affects the deformation capacity of the specimens. Finally, improvements in terms of reduction of EE of the proposed mixes are highlighted.

Springer Netherlands2017

Mixed-Mode Static and Fatigue Failure Criteria for Adhesively-Bonded FRP Joints

Moslem Shahverdi

Fiber-reinforced polymer (FRP) composites are increasingly used in many load-bearing structures. Joints are the most critical structural elements as they often become the weak links in engineering structures. Bonding and bolting are two of the main joining techniques used for composite elements but, although bolted and bonded joints have their advantages and disadvantages, adhesively-bonded joints are often preferred. This is thanks to their better performance in terms of cost-effective structures with uniformly distributed stresses without the need for cutting fastener holes that result in high stress concentrations in the FRP adherends at the edges of the holes. In addition, due to the absence of stress concentrations, adhesively-bonded joints exhibit better fatigue behavior. However, the efficient and reliable use of adhesively-bonded joints requires a thorough understanding of their failure mechanisms under mixed-mode quasi-static and fatigue loading conditions. In real structural applications, failure of adhesively-bonded joints occurs due to crack initiation and propagation under varying mixed-mode ratios. Since it is difficult to describe this progressive fracture in a structural joint, fracture joints with constant mode-mixity ratios are used to determine the strain energy release rates for crack initiation and propagation. Subsequently, mixed-mode failure criteria based on the determined strain energy release rates can be established and used to model/predict the fatigue and fracture behaviors of structural joints. The main objectives of this thesis were to understand the fracture and fatigue behaviors of adhesively-bonded FRP fracture joints under Mode I, Mode II, and mixed-Mode I/II loading conditions and, consequently, develop mixed-mode static and fatigue failure criteria applicable to structural joints. To fulfill these objectives a combined experimental-analytical-numerical study was undertaken. Double cantilever beam (DCB-Mode I), end-load split (ELS-Mode II), and mixed-mode bending (MMB-Mode I/II) specimens were examined under quasi-static and constant amplitude fatigue loading. Analytical approaches were used to determine the strain energy release rates under quasi-static and fatigue loading, whereas phenomenological models were developed to characterize the fatigue and fracture behaviors of the joints. A new approach designated the “extended global method” was established and applied for the analysis of the quasi-static and fatigue experimental data and the fracture mode partitioning in the mixed-mode experiments. In parallel, non-linear finite element models were developed to simulate the quasi-static fracture behavior and investigate the effects of both the existing asymmetry and the observed fiber bridging on the fracture behavior of the examined joints. The bridging zone was modeled by zero-thickness cohesive elements. An exponential traction-separation description of the cohesive zone model was shown to appropriately model the fiber bridging and calculate its contribution to the fracture energy under quasi-static loading. The FE models were successfully employed for the separation of the fracture parameters, i.e. strain energy released at the crack tip (Gtip) and the amount of energy released along the crack-bridging zone (Gbr), in the quasi-static mixed-mode failure criterion for crack propagation. In addition to the studies on the quasi-static fracture behavior of the joints, a new phenomenological fatigue crack growth formulation for the modeling and prediction of Rratio effects on the Mode I fatigue behavior of adhesively-bonded joints was also derived. The model was used for prediction of the fatigue behavior of the examined joints under different R-ratios. The results proved that the model could accurately simulate the fatigue behavior of the examined joints and was also capable of predicting behavior exhibited under unseen loading conditions, i.e. the R-ratios used for estimating the model parameters were different from those used for validating its prediction capability. The experimental results and numerical analyses combined with the phenomenological models were used to establish mixed-mode static and fatigue failure criteria for crack initiation and crack propagation for the adhesively-bonded joints. The derived mixed-mode failure criteria can be used for simulating/predicting the crack initiation and progressive crack propagation in structural joints comprising the same adhesive and adherends.

EPFL2013

Structural performance of permanent post-tensioned CFRP ground anchors with strap ends

Haifeng Fan

Permanent post-tensioned ground anchors are widely used for the stabilization of different structures in civil engineering. Carbon fiber-reinforced polymer (CFRP) tendons have been proposed to replace steel tendons in ground anchors to solve the durability problem. The objective of this research is to develop permanent post-tensioned CFRP ground anchors with strap ends to achieve a load-bearing capacity of at least 2500 kN. The CFRP tendon comprises a multi-strap anchor head on the ground side, in which the CFRP strap end is prefabricated in a lightweight high-strength grout cylinder confined with CFRP rings. The confinement rings deviate the spreading forces occurring in the strap region into the cylinderâs axial direction. The grout cylinder is stepwise axially loaded in compression. The uniaxial compressive stress vs strain curves, including the softening branch, of four cement- and resin-based grout materials were obtained in compression experiments. The concrete compression model developed by Sargin was successfully applied to also describe the compressive grout behavior and subsequently used for finite element (FE) analyses of the CFRP ground anchors. In a first stage, CFRP ground anchor heads with one-strap ends on the ground side were developed and investigated in pull-out experiments simulating anchor applications in rock and soil. In the rock application, the anchor can be used without additional confinement, while in the case of soil, an additional CFRP confinement ring is needed to prevent premature grout failure in the strap region. In a second stage, CFRP ground anchors with two-strap ends on the ground side were developed and investigated in pull-out experiments simulating anchor applications in different rock types. The anchors reached an average load-bearing capacity of 1384 kN with final failure occurring in the CFRP straps. In a third stage, based on the pull-out experiments on the one- and two-strap anchors and the newly conducted FE analyses on the one-strap anchors, the load-transfer mechanism in multi-strap anchors was investigated. An empirical model was developed for deriving the load-transfer diagram along the embedded straps for multi-strap anchors and subsequently applied to a new three-strap anchor with a targeted capacity of 2500 kN. Furthermore, since strap rupture occurred in the one- and two-strap anchors, the tensile behavior of non-laminated and laminated straps, both applicable for strap anchors, was investigated using experimental, numerical and analytical methods. An empirical model for the non-laminated straps and an analytical model for the laminated straps were developed to predict the strap capacity; in the latter, the strap anisotropy and friction at the strap-pin interfaces were also taken into account. The findings of this research therefore demonstrate that conventional steel anchors can be replaced by high-capacity CFRP strap anchors. This will lead to cost saving since the anchor can be easily handled and will no longer need to be monitored and replaced.