Now showing 1 - 10 of 14
  • Publication
    The performance of Irish aggregates in the 'ultra-accelerated test' for alkali-aggregate reactivity with particular reference to chert content
    Irish aggregates have a satisfactory history of service regarding alkali-aggregate reaction despite containing significant amounts of chert and use for a period in conjunction with cements of high alkali level. An investigation was conducted of the performance in the draft RILEM ‘ultra-accelerated mortar-bar test’ of 23 individual aggregates and 14 combinations found in Irish practice. An X-ray diffraction study was also carried out. Definitive expansion limits have yet to be finalised but a value of 0.15% after 14 days immersion has been suggested as the innocuous/deleterious threshold and 0.25% expansion is suggested as the lower limit for reactive aggregates. Despite their satisfactory history of use, almost half of the aggregates tested individually and one third of the combinations exceeded the innocuous/deleterious threshold value. However, the lower limit for reactive aggregates was reached by only four individual aggregates and by one of the combinations. The X-ray diffraction study revealed the presence of well-crystallised quartz, confirming the likelihood that the aggregates would not be reactive. The highest expansions were recorded in samples from argillaceous limestones and those containing small amounts of greywacke and chalcedonic chert. As expected, there was no correlation between chert content and degree of expansion. It is concluded that most Irish cherts are unreactive; that the draft mortar-bar test returns pessimistic results and is therefore a suitable screening test but could not be used universally for definitive assessment unless national expansion limits could be agreed in the place of use; and that X-ray diffractometry may be used in conjunction with the rapid screening test to assist early judgement on likely reactivity.
  • Publication
    Assessment of ultrasonic signals to determine the early age properties of concretes incorporating secondary cementitious materials
    Secondary cementitious materials (SCMs) such as ground granulated blast-furnace slag (GGBS) are used in increasing quantities in concrete practice internationally. While these materials offer benefits such as reduced CO2 and a more dense microstructure, they also have drawbacks in terms of slower initial gain of strength. There are significant financial implications associated with this, as it can lead to delays in the construction process. Key to overcoming this challenge is the development of a methodology to assess the early-age stiffness development in concretes manufactured using GGBS. This paper presents the results of a study into the application of ultrasonic sensors to assess the early age concrete stiffness. A novel wavelet-based approach is used to overcome the difficulties associated with wave reflections and classical wave theory is used to determine the concrete small-strain stiffness based on P and S wave velocities. It was found that the results are largely in agreement with those obtained using standard strength testing, suggesting potential practical applications of this method.
  • Publication
    Effective use of corrosion inhibitors in highway structures
    The corrosion of European concrete highway structures leads to traffic disruption, significant expenditure on remedial works and ultimately threatens to impact on European competitiveness. A potentially more efficient component in maintenance strategies is the use of surface applied corrosion inhibitors, which may delay the onset of corrosion or retard the corrosion rate of steel in concrete. This paper presents a proposed framework of guidelines for the effective use of corrosion inhibitors based on a study conducted as part of the EU Fifth Framework SAMARIS project. The proposed guidelines call for an initial desk study to assess the potential use of inhibitor and an assessment of risk control to the specifiers satisfaction. If necessary (for risk assessment and control) a preview trial is recommended, based on defined performance criteria from which a proposed rehabilitation strategy is finalised and again considered against the risk assessment. If resources permit, performance monitoring post repair is recommended as part of a pro-active maintenance strategy, since such an approach may represent one of the most effective uses of corrosion inhibitors in service life management.
  • Publication
    Significance of the concentration of chloride in the repair of concrete highway structures using surface applied corrosion inhibitors
    Advances in surface-applied corrosion inhibitors suggest that they have the potential to prevent or significantly retard corrosion of steel in reinforced concrete structures. It is thought that the effectiveness of the inhibitor depends on both the chloride concentration at the steel reinforcement and the inhibitor concentration. This paper presents the preliminary findings of a laboratory study into this assumption. Concrete specimens were ponded with chloride solutions to initiate corrosion. Inhibitor was applied to one face and the influence on corrosion activity was monitored by linear polarisation resistance measurement. The preliminary results of this continuing study showed that the surface-applied inhibitors could reduce the corrosion rate and this reduction depends on the chloride concentration. The practical implication is that there exists a chloride concentration range within which inhibitor use is most effective.
  • Publication
    Chloride diffusion coeficient determination for specifications
    The chloride diffusion resistance of reinforced concrete can have a major influence on durability. A move to performance-based specifications is therefore to be encouraged in the case of highway structures, where the client expects a long trouble-free service life. The introduction to practice of service life modelling and performance-based specifications requires a parallel development in user-friendly test methods. This paper reports on experience with three techniques used to determine apparent chloride diffusion coefficients: long-term immersion tests with chloride analysis by Volhard titration, tests of medium duration with chloride analysis by an internal-calibration potentiometric method and short-term Nordtest rapid migration tests. Concrete compositions tested included Portland cement, pfa and ggbs mixes. It was found that medium and long-term tests were consistent, but care is needed with early age tests.
  • Publication
    Performance of high alkali slag concretes in the context of alkali-silica reaction
    The depletion of natural resources, the requirement for sustainable development and environmental restrictions, such as those associated with the Kyoto Agreement, makes the re-use of waste materials increasingly important as we enter the new millennium. The use of one such material in concrete, ground granulated blast furnace slag is well-accepted in many parts of the world. However much research and experience of use is based on slags with low alkali levels and low chloride contents. Environmental and economic constraints may force the increased use of slags with higher alkali levels and chloride contents in future works. The alkali level parameter raises concern in respect of alkali-silica reaction (ASR) where susceptible aggregates are used. The performance of high alkali slags in the context of ASR was assessed by a modified version of the concrete prism expansion test, X-Ray diffraction and scanning electron microscopy. Variables in the test programme were binder combination, slag alkali level, aggregate combination, and storage temperature. Previous research by the authors had identified Irish argillaceous limestone aggregate as potentially classifiable as ASR-vulnerable, despite a low silica content and a satisfactory service record. The performance, in the context of ASR, of argillaceous limestone slag concretes was specifically studied. The study found that inclusion of slag in concrete can be beneficial in inhibiting ASR irrespective of the alkali content of the slag. The particular case of argillaceous limestone slag concrete is commented on.
  • Publication
    Biochemical attack on concrete in wastewater applications : a state of the art review
    The costs associated with the provision and maintenance of drinking water and wastewater infrastructure represents a significant financial demand worldwide. Maintenance costs are disproportionately high, indicating a lack of adequate durability. There remains a lack of consensus on degradation mechanisms, the performance of various cement types, the role of bacteria in the corrosion process associated with wastewater applications and testing methodologies. This paper presents a review of the literature, outlining the various research approaches undertaken in an effort to address this problem. The findings of these varying approaches are compared, and the different strategies employed are compiled and discussed. It is proposed that a key step in advancing the understanding of the associated deterioration mechanism is a combined approach that considers the interaction between biological and chemical processes. If this can be achieved then steps can be taken to establishing a performance-based approach for specifying concrete in these harsh service conditions.
  • Publication
    Reactivity assessment of aggregates : the role of chert crystallinity
    A systematic assessment scheme for determining the alkali reactivity of aggregates has been developed for international use. Expansion tests were conducted on Irish aggregates. Aggregates were subjected to a sequence of tests for alkali reactivity: petrographic examination and mortar bar and concrete prism expansion testing. No cases of damaging alkali-silica reaction have been identified to date in Ireland despite the presence of chert in significant commercial sources. Chert-bearing Irish aggregates performed poorly in expansion tests despite an exemplary service record. Chert crystallinity was examined through domain size and quartz crystallinity index. The crystallinity study helped explain the apparently anomalous behavior of the aggregates in tests. A preliminary study of the effect of alkali loading provided confirming evidence. It is suggested that enhanced crystallinity raises the alkali threshold value for reaction to one intermediate between that experienced in practice and in tests.
  • Publication
    Numerical Model for Quantifying Degree of Hydration in Concrete Mixes with Reduced CO2 Footprint
    The widespread application of innovative cementitious combinations in concrete raises the need for more comprehensive investigation of the resulting concrete properties. Early age behaviour is a major factor to be addressed, and tools are required for quantifying the hydration state of concrete members, particularly at early-ages. Numerical models can potentially be used in mass concrete construction to predict and prevent possible thermal crack formation. They also provide an indirect means for characterizing development of the hydration reaction in concrete. The latter can then be utilised in modelling and predicting secondary concrete properties, such as diffusion coefficient. This is gaining increasing importance as we harness the ability to develop innovative combinations. The cement industry is estimated to be responsible for about 7% of the carbon dioxide generated globally. As such, reducing the amount of CO2 emitted during cement production is a key issue if the construction industry is to fully participate in sustainable development. Under the terms of the Kyoto Protocol Emissions Trading Scheme it is also potentially profitable for cement companies to reduce their CO2 emissions. By using blended cement instead of ordinary Portland cement, it is possible to lower the share of clinker in cement, resulting in reduced CO2 and energy emissions. In Ireland, CEM II now accounts for over 80% of the Irish cement production portfolio. GGBS is a by-product of steel industry and a common replacement for cement. When compared to Portland cement it has a reduced CO2 footprint and concretes containing GGBS are less prone to deterioration due to aggressive chemical attacks. Its use has the potential to produce more durable concrete with increased service life, lower maintenance costs and a lower carbon footprint, increasing the sustainability of concrete construction. The aim of the current study is to use numerical models to quantify the development of heat of hydration when mixtures of CEM II and GGBS are utilised. Experiments were conducted where the temperature profiles in 4 different mixes of concrete (CEM II with 0%, 30%, 50% and 70% GGBS) are recorded. This was achieved by casting 6 identical concrete samples from each mix, with thermocouples embedded to record the internal temperature of the mix at regular time steps. Temperature changes of the mix are then used to quantify the heat evolved, based on the principles of heat transfer. To account for the combined effect of time and temperature on hydration development, activation energy of the mix is used, along with the equivalent age maturity method. Total heat of hydration is determined based on the composition and amount of cementitious materials. It has long been accepted that the liberated heat of hydration, divided by the total available heat of hydration is a good measure of the degree of hydration. The experimental data describing hydration development with equivalent age are then used to calibrate the exponential formulation presenting the S-shaped hydration curve. Values of β, τ, and αu (the hydration parameters) are obtained for each mix, from the results of multivariate non-linear regression analysis. Comments on the use of this method in quantifying concrete hydration are then made.
  • Publication
    A study of the influence of slag alkali level on the alkali-silica reactivity of slag concrete
    Ground granulated blast furnace slag (ggbs), can reduce the alkali load in concrete, despite its relatively high alkali content. Most research has been devoted to the efficacy of slag with an alkali content of less than 1.0% and this is reflected in guidance documents. A comparative assessment was made of the effect, if any, of the alkali level of ggbs on potential alkali-silica reactivity. Expansion tests were performed on a matrix of concrete mixes using Irish normal Portland cement, two slags of differing alkali content, three aggregates and alkali loads of 5 and 6 kg Na2Oeq./m(3). A replacement level of 50% was used throughout. No significant difference in behaviour was apparent, irrespective of aggregate type or alkali load, indicating that the alkali level of the slag is not a contributory factor at the 50% replacement level.
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