• Hansen, Søren Gustenhoff

Abstract

Alkali-silica reaction (ASR) is a deterioration mechanism that can occur in concrete<br/>structures. It is a chemical reaction between alkalis, silica minerals in the reactive aggregates and water. The reaction causes severe cracking of the concrete, which results in<br/>significant reductions of the strength parameters. This material degradation has raised<br/>serious concerns regarding the safety of ASR-damaged structures; particularly structures,<br/>which may be sensitive to shear failure. The Danish Road Directorate has estimated that<br/>more than 600 Danish road bridges have the potential to develop ASR in the future. The<br/>majority of these bridges has been constructed as slabs without shear reinforcement, i.e.<br/>structures where the shear capacity relies entirely on the strength of the concrete. Unfortunately, there exists no satisfactory method to assess the residual shear capacity of<br/>ASR-damaged slabs without shear reinforcement - in spite of nearly 80 years of research<br/>on ASR.<br/>The aim of this PhD project is therefore to develop an approach that can be used to<br/>determine the shear capacity of ASR-damaged slabs without shear reinforcement. The<br/>approach includes a shear model as well as recommendations and descriptions of how<br/>the relevant strength parameters should be determined by simple tests on samples taken<br/>from the structure. The works that have been undertaken to develop this approach are<br/>as follows.<br/>In the first part of the project, a literature study on how ASR affects the parameters<br/>that are important for the shear capacity is conducted. One of the main findings here<br/>is that ASR affects slabs differently than other types of structures, e.g. the way that<br/>the ASR-induced cracks are orientated. The majority of the existing ASR research on<br/>material characteristics and/or residual capacity of reinforced members is therefore not<br/>directly applicable for this PhD project. Based on the findings as well as shortcomings<br/>in the existing literature, a number of research questions that need answers in order to<br/>develop a shear model for ASR-damaged slabs are formulated.<br/>In the second part of the project, answers to the formulated research questions are found<br/>by means of a thorough experimental investigation, where the effects of ASR on the<br/>material properties as well as on the structural response are studied. The investigation<br/>includes a large shear testing campaign with specimens cut out from two ASR-damaged<br/>bridges. The material properties are investigated by means of standard test methods<br/>and Digital Image Correlation (DIC). By a critical examination of the results and an<br/>optical investigation of the underlying mechanisms, recommendations of testing methods<br/>to obtain the anisotropic residual compressive- and tensile strength are formulated.<br/>In the last part of the project, a model to determine the shear capacity of ASR-damaged<br/>slabs without shear reinforcement is established. The model is based on the upper bound<br/>theorem of plasticity theory, where the specific solutions are derived with inspiration<br/>from the failure mechanisms observed in shear tests with the ASR-damaged slab bridge<br/>specimens. The calculated shear capacity correlates well with test results, both for simply<br/>supported members and for continuous members.<br/>Based on the model, some recommendations are given for how practical assessment of<br/>members subjected to arbitrary loading can be carried out.

Topics

• impedance spectroscopy
• mineral
• theory
• reactive
• crack
• strength
• anisotropic
• shear test
• plasticity
• tensile strength