Cracking in asphalt materials

This chapter provides a comprehensive review of both laboratory characterization and modelling of bulk material fracture in asphalt mixtures. For the purpose of organization, this chapter is divided into a section on laboratory tests and a section on models. The laboratory characterization section is further subdivided on the basis of predominant loading conditions (monotonic vs. cyclic). The section on constitutive models is subdivided into two sections, the first one containing fracture mechanics based models for crack initiation and propagation that do not include material degradation due to cyclic loading conditions. The second section discusses phenomenological models that have been developed for crack growth through the use of dissipated energy and damage accumulation concepts. These latter models have the capability to simulate degradation of material capacity upon exceeding a threshold number of loading cycles.Peer ReviewedPostprint (author's final draft

3D numerical modelling of twisting cracks under bending and torsion of skew notched beams

The testing of mode III and mixed mode failure is every so often encountered in the dedicated literature of mechanical characterization of brittle and quasi-brittle materials. In this work, the application of the mixed strain displacement e-ue-u finite element formulation to three examples involving skew notched beams is presented. The use of this FE technology is effective in problems involving localization of strains in softening materials. The objectives of the paper are: (i) to test the mixed formulation in mode III and mixed mode failure and (ii) to present an enhancement in terms of computational time given by the kinematic compatibility between irreducible displacement-based and the mixed strain-displacement elements. Three tests of skew-notched beams are presented: firstly, a three point bending test of a PolyMethyl MethaAcrylate beam; secondly, a torsion test of a plain concrete prismatic beam with square base; finally, a torsion test of a cylindrical beam made of plain concrete as well. To describe the mechanical behavior of the material in the inelastic range, Rankine and Drucker-Prager failure criteria are used in both plasticity and isotropic continuum damage formats. The proposed mixed formulation is capable of yielding results close to the experimental ones in terms of fracture surface, peak load and global loss of carrying capability. In addition, the symmetric secant formulation and the compatibility condition between the standard irreducible method and the strain-displacement one is exploited, resulting in a significant speedup of the computational procedure.Peer ReviewedPostprint (author's final draft

INTEGRATED COMPUTATIONAL AND EXPERIMENTAL EVALUATION OF ELECTROMAGNETIC ENERGY-INDUCED SELF-HEALING PERFORMANCE OF ASPHALT COMPOSITES

The objective of this doctoral research is to investigate the electromagnetic energy induced self-healing effect of modified asphalt mixture material by developing computational and experimental characterization tools. More than 90% of the pavements in United States are constructed by asphalt mixture. The durability of pavement decreased by distresses has significant impact on maintenance costs. The asphalt mixture has self-healing capability because the asphalt could flow and fill the microcracks if enough external energy can be transmitted to the asphalt mixture system. However, the self-healing capability of asphalt is limited based on the climatic condition and traffic volume. Therefore, it is necessary that a new method named electromagnetic-induced healing needs to be used to accelerate the self-healing process of the asphalt mixture. In this research, different materials were added into asphalt to produce the modified asphalt binder samples and modified asphalt mixture samples, including steel wool, carbon fiber, graphite flake and exfoliated graphite nanoplatelets (xGNP). Some relative asphalt binder tests were conducted to evaluate the performance of modified asphalt, including rotational viscosity, light absorbance, aging and thermal conductivity. Some other tests were employed to evaluate the performance of the asphalt mixture, including disk-shaped compact tension test, dynamic modulus test, and rutting test. Three EM-induced healing approaches were utilized to investigate the induction healing effect of the asphalt mixture material, including the longwave radiation, visible/near-infrared light and microwave healing, respectively. In addition, a multi-phase triangle-shaped finite element bilinear cohesive zone model (CZM) was developed to simulate the fracture behavior of the original and strength recovered asphalt mixture samples during the cyclic fracture-induction healing tests. The digital image correlation (DIC) method was used to analyze the crack displacement variation of the fracture samples. The relative strain ratio was incorporated to determine the recovered fracture energy for the simulation model. The experimental results indicated that three added materials all could increase the healing effect of the asphalt mixture samples. The favorable numerical results compared with the experimental results indicated that the finite element bilinear CZM with defined crack path can be used to predict the recovered fracture strength after fracture-induction healing cycles

Active thermography for the investigation of corrosion in steel surfaces

The present work aims at developing an experimental methodology for the analysis of corrosion phenomena of steel surfaces by means of Active Thermography (AT), in reflexion configuration (RC). The peculiarity of this AT approach consists in exciting by means of a laser source the sound surface of the specimens and acquiring the thermal signal on the same surface, instead of the corroded one: the thermal signal is then composed by the reflection of the thermal wave reflected by the corroded surface. This procedure aims at investigating internal corroded surfaces like in vessels, piping, carters etc. Thermal tests were performed in Step Heating and Lock-In conditions, by varying excitation parameters (power, time, number of pulse, ….) to improve the experimental set up. Surface thermal profiles were acquired by an IR thermocamera and means of salt spray testing; at set time intervals the specimens were investigated by means of AT. Each duration corresponded to a surface damage entity and to a variation in the thermal response. Thermal responses of corroded specimens were related to the corresponding corrosion level, referring to a reference specimen without corrosion. The entity of corrosion was also verified by a metallographic optical microscope to measure the thickness variation of the specimens

Fatigue reliability and post-fracture residual capacity of a two-girder steel bridge

2016 Fall.Includes bibliographical references.Due to the immense and always increasing traffic volume, bridges are permanently subjected to repetitive loadings. These high numbers of cyclic loads can cause the initiation of fatigue cracks. If these flaws remain undetected they may become through-thickness cracks and further propagate, if left unrepaired, until they eventually lead to fracture of the entire member. The criticality of a full member fracture is not well defined nor agreed upon. Previous failure cases have demonstrated the ability of two-girder steel bridges to withstand full girder fracture of one of the two girders without structural collapse. Other cases, however, have shown the criticality of a complete girder failure on complete system collapse. Due to uncertainties in bridge redundancy and the ability to develop alternative load path, the American Association of State Highway and Transportation Officials (AASHTO) attempts to prevent fracture or collapse by classifying bridges with respect to their redundancy into fracture critical bridges (FCB) and decreasing their inspection periods. However, this leads to higher construction and maintenance costs for the owners of FCBs. Clearly, the level of uncertainty in bridge performance when one of its two girders suffer complete fracture should be represented in a probabilistic manner to evaluate the probability of fatigue crack growth and system collapse. To that end, thesis uses probabilistic analysis to assess the crack propagation behavior in a girder of a two-girder steel bridge by conducting finite element Monte Carlo simulations. The simulations account for the scatter in the load and the resistance by treating those uncertainties as random variables with predefined statistical distributions. Additionally, the post fracture redundancy is evaluated by comparing the resulting equivalent plastic strain to the failure strain of steel. The results show that the bridge provides sufficient redundancy to redistribute the load after full depth fracture a FC member. Furthermore, the results of the probabilistic analyses provide a basis for choosing the inspection intervals for FCBs

Three-dimensional cohesive fracture modeling of non-planar crack growth using adaptive FE technique

AbstractIn this paper, the three-dimensional adaptive finite element modeling is presented for cohesive fracture analysis of non-planer crack growth. The technique is performed based on the Zienkiewicz–Zhu error estimator by employing the modified superconvergent patch recovery procedure for the stress recovery. The Espinosa–Zavattieri bilinear constitutive equation is used to describe the cohesive tractions and displacement jumps. The 3D cohesive fracture element is employed to simulate the crack growth in a non-planar curved pattern. The crack growth criterion is proposed in terms of the principal stress and its direction. Finally, several numerical examples are analyzed to demonstrate the validity and capability of proposed computational algorithm. The predicted crack growth simulation and corresponding load-displacement curves are compared with the experimental and other numerical results reported in literature

Efficiency of different techniques in flexural strengthening of RC beams under monotonic and fatigue loading

In the context of flexural strengthening of concrete structures, fiber reinforced polymers (FRP) have been used mostly by two main techniques: Externally Bonded Reinforcement (EBR) and Near-Surface Mounted (NSM). Both strengthening techniques are applied on the cover concrete, which is normally the weakest region of the element to be strengthened. Consequently, the most common problem is the premature failure of the strengthening system that occurs more frequently in the EBR one. In an attempt of overcoming this weakness, another technique has been proposed, called MF-EBR – Mechanically Fastened and Externally Bonded Reinforcement, which uses multi-directional carbon fiber laminates, simultaneously glued and anchored to concrete. To compare the efficiency of NSM, EBR and MF-EBR techniques, four-point bending tests with RC beams were carried out under monotonic and cyclic loading. In this work the tests are described in detail and the obtained results are discussed. Additionally, to assess the performance of a FEM-based computer program for the prediction of the behaviour of RC beams strengthening according to these techniques, the beams submitted to monotonic loading were numerically simulated.FCT; Hilti Portugal - Productos e Serviços Lda.; S&P Clever Reinforcement Ibérica Lda.; SECIL; TSwaterjet, Lda