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In Germany the damages on concrete motorways have increased during the past 15 years. This led often to a reduction of service life from 30 to 10 years. In spite of comprehensive research the reasons for these damages have not been clarified yet. But it is known that in this context the ASR plays a decisive role. Against this background ASR performance tests have been developed. However, these tests do not take into account the effects of mechanical loading due to cyclic traffic and climatic impact. Therefore in Germany the ASR research group 1498 was founded. The research group consists of three experimentally- and two multi-scale modelling oriented subprojects, which are working closely together. Selected findings of BAM’s experimental subproject SP4 are content of this contribution. The aim was to investigate the impact of a cyclic mechanical pre-damage on moisture transport. The pre-damaging was carried out on large-format beams using a cyclic four point bending test with a total number of 5 million load cycles. Thereby the damage process was monitored by using different NDT-techniques. After the mechanical pre-damaging, smallsized specimens were taken from loaded as well as unloaded areas of the large format beams. Absorption tests were carried out on these specimens using water and NaCl solution. Thereby moisture distribution was measured with Nuclear Magnetic Resonance (NMR) and Time Domain Reflectometry (TDR). Additionally, moisture suction was quantified integrally by means of gravimetric measurements. The analysis of the first test data shows that the mechanical loading influences the moisture transport.
Thermal treatment improves the excellent properties of UHPC. Recent studies have shown that an increase in compressive strength of more than 30 % is possible. However, the accurate conditions of thermal treatment for a maximal strength are considered as yet undetermined. A multitude of parameters can be varied: temperature, pressure, water saturation, and duration of the process steps. These parameters influence the phase development and in consequence the macroscopic properties of UHPC. The primary objective of the presented study was the optimisation of the conditions for thermal treatment, concerning compressive strength. It focuses on pre-storage time and duration of the treatment at defined temperatures and pressures (90 °C and 185 °C/1.1MPa).
As expected, experimental results showed a fundamental change of phase composition in hydrothermally treated UHPC in comparison to standard cured UHPC: Ettringite decomposes at higher temperatures and is absent after thermal treatment; the amount of portlandite and clinker phases decreases. The change of phase composition is accompanied by increased compressive strength.
Experimental studies of calcium-silicate-systems at hydrothermal conditions predict the formation of the crystalline C-S-H phase tobermorite. In fact, this is a typical phase occurring in other hydrothermally treated calcium-silicate-systems like Autoclaved Aerated Concrete (AAC). Commonly, high strength is attributed to the presence of tobermorite; however, in the presented study tobermorite was not detected in the hydrothermally treated UHPC. Therefore, tobermorite cannot be responsible for the increased strength of hydrothermally treated UHPC.
In conclusion the development of phases and strength of UHPC at hydrothermal conditions differs fundamentally from AAC and the experimental studies with water saturation. Results of these systems cannot be transferred to UHPC. In thermally treated UHPC, the hydration of clinker is enhanced and the puzzolanic reaction is intensified. Hence, more C-S-H is formed that fills pores and cracks, leading to a denser structure and finally to higher strength.