Prevention of expansion due to alkali silica reaction in concrete containing reactive aggregates from Mactaquac

Mactaquac Generating Station was constructed in the mid 1960's and is located in the province of New Brunswick in Eastern Canada. Approximately 10 years after construction, the concrete structures, which consist of an intake structure, a powerhouse and two spillways, started to exhibit distress, which was subsequently attributed to alkali-silica reaction (ASR). Since 1985, various remedial measures have been undertaken to mitigate the effects of concrete expansion. However, it is envisioned that at some time in the future it will no longer be economically feasible to continue remediation and replacement of the concrete structures will have to be considered. Currently, it is projected that reconstruction will commence in the year 2025. The reactive rock in the existing concrete structures is a greywacke-type material, which was produced by crushing the rock excavated during construction. Due to the lack of any suitable locally-available non-reactive aggregate, consideration is being given to using the same source of material for reconstruction. With the luxury of lead-time, a number of different strategies to prevent deleterious expansion from occurring with this reactive aggregate are under evaluation. The most obvious choice is the use of low-calcium fly ash currently produced at a thermal generating station in New Brunswick. However, due to the possibility that this generating station won't be using coal as a fuel source in 20 years time, other strategies are also being evaluated, including the use of other supplementary cementing materials and low-alkali cement. In addition, the use of reclaimed fly ash from various landfill sites in the province is being explored. The various strategies are being screened using accelerated tests and then further evaluated by expansion testing of concrete prisms in the laboratory and larger instrumented blocks stored both on an outdoor exposure site at the University of New Brunswick and in the inspection gallery of the existing intake structure at Mactaquac. It is planned by the end of 2007 to cast massive monolithic specimens at the proposed construction site using the most promising strategies selected from the laboratory program.;This study found that mineralogical and chemical composition analysis and accelerated mortar bar testing indicated that aggregate retrieved from the Springhill quarry would act as an acceptable surrogate for testing to the Mactaquac aggregate. Concrete prisms cast with both the Springhill and Mactaquac aggregates responded similarly to fly ash replacement levels, with an increase in effectiveness with increased replacement. Fly ash replacement levels tested with the Mactaquac aggregate were unable to control the expansion to an acceptable level at 1-year in the concrete prism test, warranting further investigation. The composition of the fly ash (i.e. the CaO and Na2O e content) has a major impact on the ability for fly ash to control expansion caused by ASR. It was also found that as the level of fly ash replacement increases, the compressive strength and resistivity of the concrete decreases. Ternary blends of cementitious materials are very effective in reducing expansion caused by ASR. Results also indicate that limiting the total equivalent alkalis of the cement may be an effective method of controlling expansion, but long-term results are required to be certain.