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Ultrahochleistungsbeton (UHPC) zeichnet sich durch eine enorme hohe Druckfestigkeit von über 150MPa aus, während er gleichzeitig eine hohe Dichtheit besitzt. Im Schwerpunktprogramm SPP 1182 der DFG sind umfangreiche Untersuchungen zu verschiedensten Aspekten des UHPC durchgeführt worden, sodass die Voraussetzungen für eine breite Anwendung gegeben sind.
Ultra High Performance Concrete (UHPC) is characterized by high strength and high durability. This is achieved by an optimized grain size distribution, especially within fine grains, and addition of superplasticizer, which allow the reduction of the water/cement ratio in the cement paste and thereby the increase of the density of UHPC. Thermal treatment, i.e. curing at elevated temperature and pressure, contributes to a further increase of compressive strength. The aim of the presented study was to analyze the effect of thermal treatment at 90 ◦C and atmospheric pressure on UHPC samples. Varying factors were the age of the samples when heat treatment started (initial storage time), the duration of heat treatment and the type of heat treatment. It was applied in three ways: 1. treated without any protection, 2. sealed in plastic foil and 3. treated in hot water. Afterwards the samples were analyzed with respect to their mechanical properties and their phase composition. Furthermore, the weight (water absorption) of the samples was observed over 28 days and was correlated with the strength test results. The development of strength depends on the combination of initial storage time and the duration of heat treatment and is also influenced by the type of thermal treatment. The highest compressive strengths have been observed by implementing the hot water treatment. Thereby the weight of the samples increase due to additional absorbed water. This enables an increased hydration of cement clinker inducing a higher strength.
Ultra-high performance concrete (UHPC) is characterized by outstanding compressive strength of more than 150 MPa, double of normal concrete. Furthermore UHPC has a very dense structure which leads to an extremely low permeability for fluids resulting in with a very high durability.
Further structural improvement can be achieved with thermal treatment of UHPC. Recent studies have shown that an increase in compressive strength of more than 50% is possible. Nevertheless, the accurate conditions for an optimal thermal treatment are still not determined. A multitude of parameters can be varied: Temperature, pressure, water saturation, duration of the process steps.
Primary object of this study is the optimization of the conditions for thermal treatment focused to the treatment duration at a defined temperature and pressure (185°C / 1,1MPa). Therefore, the thermally treated UHPC samples are analysed by mean of mechanical properties and phase composition.
The development of phases and strength at higher temperature differs fundamentally from other cementitious systems (like normal concrete or autoclaved aerated concrete), although the chemical composition is similar. Because of the very dense structure and the low water/cement ratio the availability of water is low. This influences the hydration process. As a consequence tobermorite is absent in thermal treated UHPC in contrast to thermally treated normal concrete or predictions based on thermodynamic modelling.