Crack healing under sustained load in concrete: An experimental/numerical study

The need of sustainable resilient structures and infrastructures push towards the use of cementitious materials able to heal micro-cracks and defects. For real structural application under service loading the time-dependent behavior is of the utmost importance, especially in presence of cracks which can lead to a nonlinear creep behavior that might cause the structural failure. Now the new challenge is to study and quantify the effect of crack-healing on the nonlinear creep behavior. This study aims at the following goals: 1) to characterize with experimental investigations the effect of the healing in tests in which the specimens, along the exposure time and under controlled environmental conditions, are under sustained load, the expected service load, determined as a fraction of the pre-cracking load; 2) develop a comprehensive numerical framework for the interpretation and simulation of the experimentally observed results. To this purpose an experimental investigation is currently ongoing at Politecnico di Milano with reference to an Ultra High-Performance Concrete developed in the framework of the H2020 ReSHEALience project for exposure to extremely aggressive environments. The numerical framework is based on the recent developments of the multiphysics lattice particle model

From slender columns to branching structures

The first part of this paper reviews current approaches and methods used for modelling of structural concrete columns. Several methods are presented and their benefits and drawbacks are discussed. In the second part, a transition between traditional shapes and organic forms is described together with an example of a real structure. An insight for design and assessment of members with organic geometrical forms is then given, including a proposal for an inner bio-inspired filing of the structural member itself

AIFIT : user orientated identification for infrastructure : field test

In general damages of reinforced concrete result by cracks and thus lead to loss of stiffness within the location of this defect. These cracks are difficult to detect by visual inspections, because it is time consuming to localize and evaluate cracks and limited to accessible parts of the structure. Aim of the research project AIFIT is the development of robust identification methods suitable for practical use. In this paper an overview of the steps of the AIFIT-project are given and particular focuses the item on the field test at a three span reinforced concrete bridge of Austrian Railways

Comprehensive database for concrete creep and shrinkage : analysis and recommendations for testing and recording

The first large worldwide database of creep and shrinkage tests was assembled at Northwestern University (NU) in 1978. It was expanded as the RILEM database in 1992 and further in 2008. A major expansion, completely restructured and verified, named the NU Database, is now presented. The number of the test curves of creep and drying shrinkage is more than doubled and over 400 test curves of autogenous shrinkage are added. The database covers longer measurement periods and encompasses the effects of admixtures in modern concrete mixtures. The database contains roughly 1400 creep and 1800 shrinkage curves, of which approximately 800 creep and 1050 shrinkage curves contain admixtures. Their analysis shows significant influence of admixtures on the creep and shrinkage behavior The mixture proportions, testing conditions, and specimen geometries are documented in greater detail, and information on the admixture contents and aggregate types is included. The new database makes it possible to calibrate and verify improved creep and shrinkage prediction models. Additionally, the statistics of the mixture parameters, strength distributions, and scatter of the compliance curves have been extracted for applications in reliability engineering and probabilistic performance assessment. Data analysis brings to light various recommendations for testing and recording, and suggests corrections of various oversights distorting the reported data. These recommendations would make future test data more useful, consistent, complete, and reliable. The NU database is now available for free download at www.civil.northwestern.edu/people/bazant/ as well as at www.baunat.boku.ac.at/creep.html

Integrale Brücken und derer Zuverlässigkeitsaspekte unter Einbeziehung von Monitoring

Bei Integrale Brücken handelt es sich um zumeist in Form von Rahmentragwerken ausgeführte lager- und fugenlose Brücken, welche sich durch hochgradige statische Unbestimmtheit auszeichnen. Obwohl dieser Bauwerkstyp in letzter Zeit auf ständig wachsendes Interesse seitens der Bauwerkseigner stößt, welche mit signifikanten Einsparungen in der Bauwerkserhaltung aufgrund des Fehlens der stark gefährdeten Brückenausrüstung (Lager und Fahrbahnübergänge) rechnen, ist deren Akzeptanz unter Planern und Bauherrn nach wie vor eingeschränkt. Gründe hierfür sind die zahlreichen Unsicherheiten im Entwurf, die durch die Entwicklung der Materialfestigkeit mit der Zeit, Kriech- und Schwindvorgänge im Beton sowie geeignete Modellierungsmöglichkeiten begründet sind. Aus diesem Grund werden gegenwärtig in mehreren Forschungsprojekten durch Monitoring des Strukturverhaltens von neugebauten integralen Brücken, deren Interaktion mit dem Untergrund und numerischen Untersuchungen die notwendigen Grundlagen für eine österreichische Richtlinie zur Bemessung Integraler Brücken und die Verifikation beziehungsweise Verbesserung der Entwurfskriterien erarbeitet. Integral abutment and joint-less bridges are structures without bearings and expansion joints. These types of structures do gain much popularity among bridge owners, since they expect reduced costs in maintenance due to the missing of significantly endangered details like bearings and joints. Nevertheless, the acceptance by designers is not that high since there are a lot of uncertainties with regard to material strength development,creep, shrinkage and availability of suitable numerical models. These topics are currently being addressed by several research projects with the aim to work out an Austrian guideline for the design of jointless bridges and to verify or improve current design criteria. Work includes monitoring of the actual structural response of newly built jointless bridges, their interaction with the underground and numerical simulations

Inspection of bridges in Austria : practice and outlook

During the last decades many structures-buildings and bridges-have been erected, many of which have already reached their initial design life demanding high efforts in inspection and maintenance. Due its geographical characteristics the Austrian railroad and street network consists of a high number of bridges, further complicating the task at hand. In course of a research project called AIFIT demands by operators and engineers were listed and future possibilities for more efficient inspection in accordance with current codes were derived

Nonlinear design and monitoring aspects of jointless structures

Jointless bridges are characterized by integral abutments and their lack of bearings and expansion joints. Recently this construction type has gained much popularity among bridge owners, since reduced costs in maintenance and rehabilitation are to be expected caused by the lack of significantly endangered details like joints and bearings. However, due to uncertainties regarding numerical modeling, material strength development and interaction with the soil the acceptance by designers is not that high. Currently those topics are being addressed by a number of research projects aiming at (a) the verification of current design criteria for jointless bridges, (b) the improvement of those, and (c) the derivation of an Austrian guideline regarding the design of this construction type. By monitoring the real structural response using suitable sensor-systems and performing advanced finite-element calculations to study the influence of creep, shrinkage and loading, current design concepts are being evaluated. Finally based on those models the most relevant design criteria can be optimized even taking into account the non-linearities in material laws and geometrical properties. This paper presents a suitable concept for non-linear design which allows for the activation of reserves in bearing capacity and thus more efficient design. Modeling uncertainties as well as sensitivities between material properties, loading and design parameters are covered by the second paper “Probabilistic Design Aspects of Jointless Structures associated with Monitoring Information”

Shear test equipment for testing various polymeric materials by using standardized multipurpose specimens with minor adaptions

In this research, a novel shear test equipment was developed and finally used in a validity check with neat and reinforced polymers with a subsequent exemplary case study to investigate the short- and long-term shear behavior of two commercially available polymer-based injection mortar systems. A shear testing device was designed and implemented to be used on a universal testing machine allowing shear tests on standardized multipurpose specimens with minor adaptions. Therefore, the design of an existing standardized clamping system for composite materials was adapted and optimized for V-notched specimens of materials with lower stiffness. Simultaneously, it has the flexibility to test specimens with other dimensions. In the exemplary case study, an epoxy resin-based injection mortar system (EP) and .a vinyl ester resin-based injection mortar system (VE) were investigated. Due to the sensitivity of the injection mortar systems to curing state and moisture content, three reference states were defined. Reference state I represents an upper bound material condition with a defined curing state (cured) and moisture content (dry) allowing for long-term tests under stable material conditions. To consider the effect of curing state and moisture content on the material performance, reference state II and reference state III were defined as lower bound material conditions. While monotonic short-term tests were conducted for all three reference states at 23 degrees C, static long-term tests were performed with the reference state I material at different temperatures

Long-term shear creep behavior of polymer-based injection mortar systems

To assess the lifetime of resin-based injection mortar systems, a fundamental knowledge of their long-term behavior is necessary. In this research, the long-term shear creep modulus of two commercially available polymer-based injection mortar systems was investigated. Besides an epoxy resin based injection mortar system (EP), a vinyl ester resin based injection mortar system (VE) was utilized. Therefore, an accelerated characterization method providing shear creep modulus data considering various material states was implemented. Due to the sensitivity of the injection mortar systems to curing state and moisture content, the shear creep master curve was generated by testing under stable material conditions. Hence, upper bound material conditions with a defined curing state (cured) and moisture content (dry) were introduced. For this reference state I, shear creep curves were determined at different temperatures. Based on the time-temperature shift methodology, for both injection mortar systems shear creep master curves were generated for up to 50 years. To allow for an evaluation of the influence of curing state and moisture content, reference state II and reference state III were defined as lower bound material conditions. Due to the long-term instability of these reference states, short-term tests were utilized to evaluate reduction factors enabling a shift of the shear creep master curve of reference state I. Together, these results provide an evident effect of curing state and moisture content. For both injection mortar systems, the effect of curing state was rather small and the influence of moisture content was more pronounced

Mesoscale Modeling of Concrete Time-Dependent Behavior

Creep and shrinkage of concrete are time-dependent deformations that influence primarily the serviceability, and in some cases also the safety, of reinforced concrete structures with and without prestressing. Shrinkage is mainly driven by both self-desiccation and moisture drying if exposed to lower relative humidity environments. In addition, and in combination to that, the large and widely unrecoverable creep deformations of concrete can cause significant modifications of action effects in structures in terms of internal stress distributions, excessive deflections and loss of prestressing forces, and produce large cracks. All these effects affect the serviceability and the durability of structures and may impact on their structural safety as well.Many models were formulated to explain and simulate the time-dependent behavior of concrete, among others. Also, several methods have been presented in the literature to simplify the calculation of creep strain for structural calculations, such as the effective modulus method, rate of creep method, the ageing coefficient method (AAEM method), and the approach based on the aging linear viscoelastic theory. More refined and advanced approaches for detailed numerical analyses of the structural effects of creep and shrinkage of concrete in complex, heterogeneous and sequentially built structures have also been developed in recent times. However, time-dependent behavior of concrete must be contextualized in a wider comprehensive framework since it is a result of interplay between multiple chemical, physical, and mechanical processes that are functions of the material composition and its curing as well as the surrounding environmental and loading conditions. The nature and scales at which all these aforementioned processes take place represent a challenge for the numerical modeling. Concrete is a heterogeneous material made of two components having very large differences: a cementitious matrix and aggregates. The aggregates are typically much stiff, less porous and their time-dependent deformations are orders of magnitude lower than those of the cementitious matrix. Staying at the mesoscale, these two phases represent the main heterogeneity of concrete since at this scale the contribution of the matrix/aggregate interface, called the Interfacial Transitional Zone (ITZ), can be lumped together with the matrix and distinguish them from the aggregate to represent the main heterogeneity of concrete. By differentiating aggregate from the matrix, mesoscale interaction at that level can be directly captured; for example, when concrete is loaded in compression, the meso-structure experiences a well-known splitting mechanism of the aggregates. Mesoscale models are capable of resolving the stresses and strains at such level and can differentiate between tensile and compressive deformations, while macroscopic models have to average it. This distinction becomes very important during damage and creep/shrinkage interaction or when internal self-equilibrated stresses are the only source of loading like in non-uniform drying or free expansion under ASR progression in which cases the macroscopic stresses are equal to zero. Hence, macroscopic continuous models have to explicitly account for these lower-scale phenomena in their constitutive laws. In the literature there are many meso-scale approaches based on continuum finite element (FE) models and discrete models, such as the classical particle discrete element methods (DEM), the lattice methods, a comprehensive approach that combines both of them called Lattice Discrete Particle Model, the Rigid-Body-Spring Networks (RBSN), and interface element models with constitutive laws based on non-linear fracture mechanics. Only through physically based constitutive approaches the problem of establishing reliable prediction models can be overcome, but this still requires a calibration on an extensive database. In this paper, a recent mesoscale approach capable of representing remarkably well the concrete time-dependent behavior will be first reviewed. This mesoscale approach consists of the combination between the mesoscale discrete model termed Lattice Discrete Particle Model (LDPM) that is a comprehensive concrete model. It represents the internal structure (heterogeneity) of the material using an assemblage of coarse aggregates that interact at discrete interfaces. The model has been successfully used in modeling concrete samples and reinforced concrete structures under various static and dynamic loading conditions. The LDPM was recently coupled with a hygro-thermo-chemical (HTC) model resulting in a multi- physics framework that later was extended to account for coupled creep, shrinkage and ASR deformations. In this framework, creep and shrinkage deformations are modeled based on a discrete version of the Microprestress-Solidification theory. Finally, different experimental data sets available in the literature are here used to show the capabilities and the unique features of the proposed computational framework. Since the computational framework consists of several components, it requires an objective solid calibration strategy of the numerous parameters. In the numerical applications the calibration of each model component based on suitable test data is first presented. Then, the validation is performed using the experimental data that were not employed for the calibration. The examples considered in the manuscript deal with the creep and shrinkage under varying hygro-thermal conditions, the aging effects on strength, the tertiary creep and its application to time to failure analysis, the deterioration effect of the Alkali-Silica-Reaction (ASR) coupled with creep and shrinkage