Ludger Lohaus

A key requirement onto the concrete properties is that during the placement process the fresh concrete needs to completely fill the formwork while staying homogeneous. Whereas substantial research onto the flow and sedimentation behavior of fresh concrete has been presented regarding highly flowable self-compacting concretes, research regarding these properties on ordinary, vibrated concretes has been missing. In the presented research thus the sedimentation behavior of ordinary concrete as well as model systems was investigated. Here it could be shown, that even concretes which are seen as stable show significant changes in their homogeneity due to vibration compaction and that these local variations in aggregate content and particle size distribution have pronounced influence on the local rheological properties of the mix. The results could be closly linked to the nanoscopic particle inventory. Further, a novel measurement technique for the rheological properties of fresh concrete was developed which relies on the inverse analysis of the flow behavior of the concrete during the discharge process of a mixing truck. Using so-called Computer Vision Methods, video images of the concrete flow process can be used to derive rheological parameters such as the yield stress of the plastic viscosity.

The expected long-term deformations of concrete structures are calculated using creep models, derived from experiments performed with constant mechanical loads. However, in the majority of real structures, such as bridges, constant creep loads are superimposed with cyclic loads of substantial magnitude. Additionally, such structures are subject to changes in environmental conditions (temperature and humidity). Deformation measurements of existing bridges have shown significant underestimations by established creep models, which might be traced back to the superimposition of cyclic loads and different moisture contents. Therefore, the developments of strains, viscoplastic strains and modulus of elasticity under creep and cyclic loading of a normal strength concrete have been comparatively investigated for two different pore moisture contents (approx. 100 and 75%). The results show that viscous strains due to cyclic loading are significantly higher than those due to creep loading at the mean stress level of cyclic loading. Furthermore, the strains are higher for the higher moisture content. The differences in the development of the modulus of elasticity and viscoplastic strains of both load types give clear indication for load type dependent microstructural deformation mechanisms. The results obtained concerning the influence of the load type and the moisture content need to be considered for the improvement of existing models.

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The influence of the compressive strength of concrete on fatigue resistance has not been investigated thoroughly and contradictory results can be found in the literature. To date, the focus of concrete fatigue research has been on the determination of the numbers of cycles to failure. Concerning the fatigue behaviour of high-strength concrete (HPC) and, especially, ultra-high-strength concrete (UHPC), which is described by damage indicators such as strain and stiffness development, little knowledge is available, as well as with respect to the underlying damage mechanisms. This lack of knowledge has led to uncertainties concerning the treatment of high-strength and ultra-high-strength concretes in the fatigue design rules. This paper aims to decrease the lack of knowledge concerning the fatigue behaviour of concrete compositions characterised by a very high strength. Within the priority programme SPP 2020, one HPC and one UHPC subjected to monotonically increasing and cyclic loading were investigated comparatively in terms of their numbers of cycles to failure, as well as the damage indicators strain and stiffness. The results show that the UHPC reaches a higher stiffness and a higher ultimate strain and strength than the HPC. The fatigue investigations reveal that the UHPC can resist a higher number of cycles to failure than the HPC and the damage indicators show an improved fatigue behaviour of the UHPC compared to the HPC.

The influence of a wet environment on the fatigue behaviour of high-strength concrete has become more important in recent years with the expansion of offshore wind energy systems. According to the few investigations documented in the literature, the fatigue resistance of specimens submerged in water is significantly lower compared to that of specimens in dry conditions. However, it is still not clear how the wet environment and the moisture content in concrete influence its fatigue behaviour and which damage mechanisms are involved in the deterioration process. Here the results of a joint project are reported, in which the impact of moisture content in concrete on fatigue deterioration are investigated experimentally and numerically. Aside from the number of cycles to failure, the development of stiffness and acoustic emission (AE) hits are analysed as damage inductors and discussed along with results of microstructural investigations to provide insights into the degradation mechanisms. Subsequently, an efficient numeric modelling approach to water-induced fatigue damage is presented. The results of the fatigue tests show an accelerated degradation behaviour with increasing moisture content of the concrete. Further, it was found that the AE hits of specimens submerged in water occur exclusively close to the minimum stress level in contrast to specimens subjected to dry conditions, which means that additional damage mechanisms are acting with increasing moisture content in the concrete.

Lime-sand bricks of different ages were investigated using IR-spectroscopy, thermogravimetry, and X-ray diffraction/scattering. After subtraction of the dominant quartz contribution (80%), the IR spectra show the absorption peaks of the hydrothermally formed binder phases. The spectra also show the alteration of the binder during ageing under atmospheric conditions by the influence of CO₂ forming carbonate and a condensed SiO₂-gel (secondary gel). The alteration could also be proven in X-ray pattern, obtaining a separation between crystalline CSH and amorphous contributions in the freshly produced lime-sand brick, too. Here, the formation of CSH_amorph could be understood as a precursor state (primary gel) to the crystallization of CSH phases. X-ray patterns of aged bodies of alkali-silicate solution activated slags (AAS), CEM-I/CEM-III pastes, and CEM-I concrete indicate that in all cases a similar amorphous CSH-type phase (CSH_amorph) was formed, which is responsible for the hardening properties as the glue. The main X-ray peak of CSH_amorph obtained using CuK_α-radiation with a usual diffractometer is observed between 24° and 35° 2 Theta with maximum at about 29° 2 Theta, whereas it appears much more broadly distributed between 15° and 35° 2 Theta with maximum between 26° and 28° 2 Theta for a geopolymer body prepared using the reaction of alkali-silicate solution and metakaolin (AAMK). This is due to the network formed by siloxo and sialate units in the case of AAMK, given that any crystallization can be ruled out. The origin of increasing mechanical strength during the ageing of AAS mortars must be due to further crosslinking of the preformed siloxo chains. Thermal treatment up to 800 °C leads to a complete loss of any mechanical strength of the CEM pastes due to the destruction of crystalline CSH-phases, whereas geopolymer bodies maintain their strength. Implications for next generation concrete include that cement clinker could be completely replaced by using a using alkali silcate solution technology for gel formation.

The impact of low-pressure treatment on the crystal structure, morphology, and chemical composition of ettringite, due to their major importance with respect to processability (i.a., drying conditions) and to the analysis of ettringite-containing samples, is examined utilizing X-ray diffraction, thermogravimetric analysis, Raman spectroscopy, and environmental scanning electron microscopy. Synthetic ettringite was treated for various durations (5 min up to 72 h) and at two different levels of low-pressure (4.0 mbar and 60 µbar). Evaluation showed a correlation between the procedural parameters (time and pressure), the chemical composition, and the morphology of ettringite. The experiments reveal that, when exposed to 4 mbar pressure, nearly no changes occur in the ettringite’s morphology, whereas the crystals undergo swelling and slight deformations at very low pressures (60 µbar and 35.3 nbar), which is attributed to the loss of bound water and the partial transformation from ettringite to quicklime, anhydrite, and calcium aluminate. Furthermore, the strongly dehydrated ettringite shows the same morphology.