Influence of Rock Heterogeneity on Fracture Pattern Formation

Imperial Users onl

Macro- and micro-modeling of crack propagation in encapsulation-based self-healing materials : application of XFEM and cohesive surface techniques

Encapsulation-based materials are produced introducing some small healing fluid-filled capsules in a matrix. These materials can self-heal when internal cracks intercept and break the capsules. If the healing agent is released, the crack can be sealed. However, this is not always the case. These capsules need to be designed with the adequate shape and material to be properly broken. This paper presents two application models based on the combination of eXtended Finite Element Method (XFEM) elements and Cohesive Surfaces technique (CS) to predict crack propagation. Two types of encapsulated systems are considered: a concrete beam in a three-point bending test, and a micro-scale model of a representative volume element of a polymer subjected to a uniaxial tensile test. Despite both systems rely on different capsule shapes and different constituent materials, the models predict a similar non-linear response of the overall material strength governed by the coupled effect of the interface strength and the capsule radii-to-thickness ratio. Furthermore, even if an inadequate material and geometry combination is used, it is found that the mere presence of capsules might achieve, under certain conditions, an interesting overall reinforcement effect. This effect is discussed in terms of clustering and volume fraction of capsules

Contributions to predicting contaminant leaching from secondary materials used in roads

Slags, coal ashes, and other secondary materials can be used in road construction. Both traditional and secondary materials used in roads may contain contaminants that may leach and pollute the groundwater. The goal of this research was to further the understanding of leaching and transport of contaminants from pavement materials. Towards this goal, a new probabilistic framework was introduced which provided a structured guidance for selecting the appropriate model, incorporating uncertainty, variability, and expert opinion, and interpreting results for decision making. In addition to the framework, specific contributions were made in pavement and embankment hydrology and reactive transport, Bayesian statistics, and aqueous geochemistry of leaching. Contributions on water movement and reactive transport in highways included probabilistic prediction of leaching in an embankment, and scenario analyses of leaching and transport in pavements using HYDRUS2D, a contaminant fate and transport model. Water flow in a Minnesota highway embankment was replicated by Bayesian calibration of hydrological parameters against water content data. Extent of leaching of Cd from a coal fly ash was estimated. Two dimensional simulations of various scenarios showed that salts in the base layer of pavements are depleted within the first year whereas the metals may never reach the groundwater if the pavement is built on adsorbing soils. Aqueous concentrations immediately above the groundwater estimated for intact and damaged pavements can be used for regulators to determine the acceptability of various recycled materials. Contributions in the aqueous geochemistry of leaching included a new modeling approach for leaching of anions and cations from complex matrices such as weathered steel slag. The novelty of the method was its simultaneous inclusion of sorption and solubility controls for multiple analytes. The developed model showed that leaching of SO4, Cr, As, Si, Ca, Mg, and V were controlled by corresponding soluble solids. Leaching of Pb was controlled by Pb(VO4)3 solubility at low pHs and by surface precipitation reactions at high pHs. Leaching of Cd and Zn were controlled by surface complexation and surface precipitation, respectively

Sustainability effects of including concrete cracking and healing in service life prediction for marine environments

With today’s focus on sustainable design, it is necessary to adequately predict and prolong service life of concrete in marine environments. By introducing self-healing properties, service life extension can be achieved. However, in prediction models, the required concrete mix specific input is usually not available. Moreover, little attention goes to the unavoidable presence of cracks. Finally, autonomous crack healing has almost never been taken into account. In this paper, the relevant model input was estimated from experimental chloride profiles. It enabled an adequate prediction of the chloride-induced steel depassivation period for cracked and uncracked 15% fly ash concrete (8–104 years, respectively). Comparison with self-healing by means of encapsulated polyurethane indicated a 48–76% self-healing efficiency. It could extend the corrosion initiation period to 36–68 years. Being much less subject to time-dependent repair, PU based self-healing concrete has a 77–88% lower environmental impact than traditional (cracked) concrete

Non-destructive evaluation of concrete using a capacitive imaging technique : preliminary modelling and experiments

This paper describes the application of capacitive imaging to the inspection of concrete. A two-dimensional finite-element method was employed to model the electric field distribution from capacitive imaging probe, and how it interacts with concrete samples. Physical experiments with prototype capacitive imaging probes were also carried out. The proof-of-concept results indicated that the capacitive imaging technique could be used to detect cracks on the surface of concrete samples, as well as sub-surface air voids and steel reinforcement bars

Issued as a Documentation Report on an Investigation of Field-Made Joints in Prestressed Reinforced Concrete Highway Girder Bridges, Project IHR-303, Phase 2

A prototype bridge girder was designed, built, and tested. The 250 ft long two-span girder was made of 3 precast segments about 88, 74, and 88 ft in length. The segments were supported on 3 final and 2 temporary supports. The joints were of cast-in-place concrete, as was the composite deck. After the site-cast concrete was cured, the structure was post-tensioned to establish continuity and the temporary supports were removed. The two longer segments were pretensioned to resist the girder and deck dead loads, while the shorter segment was reinforced with deformed bars for the same loads. The structure was subjected to a series of loadings, during which deflections, reactions, and concrete strains were measured. The loads approximated AASHTO HS-20 vehicles. The first 4 tests ,were to service loads, with total applied loads of 73.6 kips. The structure remained elastic and crack free during these tests. Two tests were to the design ultimate load, 198.7 kips. A load of 328.2 kips was applied in the final test without causing failure. The final loading was applied to produce maximum shear in one splice, and a shear failure, complicated by large flexural deformations, appeared to be developing when the test ended. The final test produced a maximum deflection of 10.8 in., and a residual of about 1.0 in. The joint details used in the prototype structure were adequate, and the presence of the, joint had no influence on the behavior of the structure until extremely large overloads were reached.State of Illinois Department of TransportationU.S. Department of Transportation. Federal Highway AdministrationProject IHR-30

New methodology for calculating damage variables evolution in Plastic Damage Model for RC structures

The behavior of reinforced concrete (RC) structures under severe demands, as strong ground motions, is highly complex; this is mainly due to joint operation of concrete and steel, with several coupled failure modes. Furthermore, given the increasing awareness and concern for the important seismic worldwide risk, new developments have arisen in earthquake engineering. Nonetheless, simplified numerical models are widely used (given their moderate computational cost), and many developments rely mainly on them. The authors have started a long-term research whose final objective is to provide, by using advanced numerical models, solid basis for these developments. Those models are based on continuum mechanics, and consider Plastic Damage Model to simulate concrete behavior. Within this context, this paper presents a new methodology to calculate damage variables evolution; the proposed approach is based in the Lubliner/Lee/Fenves formulation and provides closed-form expressions of the compressive and tensile damage variables in terms of the corresponding strains. This methodology does not require calibration with experimental results and incorporates a strategy to avoid mesh-sensitivity. A particular algorithm, suitable for implementation in Abaqus, is described. Mesh-insensitivity is validated in a simple tension example. Accuracy and reliability are verified by simulating a cyclic experiment on a plain concrete specimen. Two laboratory experiments consisting in pushing until failure two 2-D RC frames are simulated with the proposed approach to investigate its ability to reproduce actual monotonic behavior of RC structures; the obtained results are also compared with the aforementioned simplified models that are commonly employed in earthquake engineering.Postprint (published version