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BÜRGE, T. A. High performance chloride-free hardening accelerator. In: INTERNATIONAL CONFERENCE ON HIGH- PERFORMANCE CONCRETE, AND PERFORMANCE AND QUALITY OF CONCRTETE STRUCTURES, 2., 1999, Gramado. Anais… Gramado, 1999
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Abstract

Concretes for the use in construction may be produced from portland cements with the appropriate admixtures if high early strength is required in the time range of 8 to 24 hours. To attain significant cost savings in construction works, early strength has to be attained within 4 - 8 hours after placing the concrete. A variety of substances that accelerate the setting and hardening of concrete are already known. The most conventional are e.g. nitrate, formate, thiocyanate, nitrite, mono-, di- and triethanolamine, highly alkaline agents like alkali hydroxide, alkali carbonate, alkali silicate, alkali aluminate as well as alkaline earth metal chlorides. From a concrete technology point of view, highly alkaline setting-accelerators reduce the final strength and increase the shrinkage. Accelerators containing chlorides are generally not desired on a building site, because they can lead to corrosion of the steel reinforcement. Furthermore, it is known, that chlorides reduce substantially the chemical resistance. Nitrate has been known for a long time for being an "anti-freeze", i.e. it causes an acceleration of setting at low temperatures. However at room temperature the effect of nitrate is insignificant. With an unique, patented combination of 5 components, an accelerator could be produced, which increases strength at early and later ages without affecting the slump loss behaviour. The combinations do not only accelerate at low temperature, but also at room temperature, as well as at warm climate. To be suitable for the use in construction, concrete properties should conform to the standard concreting equipment and practice as well as to the requirements of civil engineers. Properties and practice of the new hardening accelerator and its influence on concrete are reviewed. A case study shows the suitability of the hardening accelerator for the construction of a 300 m vertical ventilation shaft of a roadway tunnel using slip form technique. With the application of the new hardening accelerator construction time could be reduced by 57%.
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