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Baustoffe im Betonbau
(2019)
In dem Kap. „Baustoffe im Betonbau“ werden zunächst die Baustoffe vorgestellt, die für die Herstellung von Stahlbeton- und Spannbetonbauwerken erforderlich sind (im Wesentlichen Beton, Betonstahl und Spannstahl). Herstellungsbedingte, typische Eigenschaftsprofile der Baustoffe werden aufgezeigt und ein Bezug zu entsprechenden Anwendungsgebieten hergestellt.
Engineering Modelling
(2019)
This chapter presents the most commonly used approach to analyse the
thermo-mechanical behaviour of concrete structures subjected to high temperatures as in the case of fire loading. Prescriptions of the Eurocode are detailed for the thermal as well as the mechanical analysis. Finally, recommendations from two national (Austrian and German) guidelines give some improvements for Underground infrastructure.
Advanced Modelling
(2019)
Properties of interstitial liquid phase in cement paste, including the species and concentrations of polymers and ion etc., play an important role for the rheological properties of cementitious materials. In order to better understand their effect, an inert model substance, spherical silica beads (SBs) with defined surface and granulometry were used in the presence of electrolytes (CaCl2) and/or different polymers, including polycarboxylate superplasticizer (PCE) and polyethylene glycol (PEG). It was found the presence of Ca2+ greatly increases the viscosity and yield stress of silica beads paste (SBP), which is proportional to the [Ca2+]. For the effect of PCE, the addition of PCE is beneficial to the flowability of SBP, but a high dosage of PCE leads to a reversal effect. Furthermore, the yield stress firstly increases and then decreases with increasing [Ca2+] under the same dosage of PCE. The addition of PEG always increases the yield stress of SBP, regardless of the ion concentration and the presence or not of PCE.
The flowability of cement paste is of great importance in today’s construction industry and is influenced by additives such as superplasticizers (SP). One type of SPs are polycarboxylate ether type SPs. These additives electrostatically bind with the negatively charged carboxylic groups at the backbone to the positively charged clinker phases.
To model positively charged clinker phases with adsorbed SP, silicon Wafers are pre-coated with cationic polyethylenimine (PEI) and SP is adsorbed onto the coated surface (Si/PEI/SP). Two different polycarboxylate ether type (PCE) SP are compared – one for ready-mix concrete and one for precast concrete. In this preliminary study the interaction forces between Si/PEI/SP surface and a silica microsphere (colloidal probe) are investigated under mild physico-chemical conditions (pH *6, ion concentration <10−5 M) using Colloidal Probe Atomic Force Microscopy (CP-AFM).
The interaction force between the model surfaces is attractive for low concentration of SP. The force changes from attractive to repulsive by increasing amount of SP. The force upon approach reveals a biexponential behavior. The exponential decay at large and short surface separations are attributed to electrostatic and steric interactions, respectively. The steric forces of the SP for ready-mix concrete show a steeper onset than the SP for precast concrete.
The quantification of these interaction forces will be compared to rheological measurements of similar systems. Furthermore, the parameters will be changed to better approach the conditions in real systems, i.e. higher pH and ionic strength. This helps to understand how the forces on the nanoscale influence the macroscopic rheology.
Special applications like pumping, spraying or printing of concrete require the precise adjustment of very specific rheological properties at different time steps during the casting process. Superplasticizers such as polycarboxylate ethers (PCE) can be used to obtain the required flowability, which, possibly in combination with additional rheology modifying admixtures, generate the required specified consistency. However, after the application, the concrete should change the rheological properties immediately in order to avoid deformations at rest. Therefore, the use of accelerators can be effective. Accelerators influence the hydration of cementitious materials, and thus the rheological properties over the course of time and the setting.
In this paper, the influence of different accelerators on the rheology and early hydration of cement paste as well as the interaction of accelerator and PCE are presented. Methods like rheometry, needle penetration tests and practical Tests like spread flow were applied. The used accelerators showed accelerating behavior on the cement pastes without and in the presence of PCE. At the same time an influence on the rheology could be observed. This effect was less in the mixes with PCE, especially at the highest water/cement ratio (w/c).
Methods have been presented for detailed studies of railway vibration and for the fast prediction of train-induced ground vibration. The ground vibration is generated by static or dynamic loads. The main purpose of this contribution was to show the influence of inhomogeneous soils on the different vibration components.
Layered soils, namely a soft layer on a stiffer half-space, yield a quite specific transmission behavior. The low-frequency and sometimes also high-frequency cut-off of the transfer function of the soil is demonstrated in theory and by experiments at many sites of which the soil model is approximated from dispersion and transfer function measurements. The layer frequency divides the frequency range in a low-frequency range, where the stiff half-space rules the low amplitudes, and a high amplitude high-frequency range which is mainly determined by the softer top layer. A thick soft layer yields a very low layer frequency, so that the higher soft soil amplitudes have a wider range down to low frequencies. A thin layer yields a high layer frequency, so that the high frequencies above this layer frequency are dominant. The higher the contrast between the stiff half-space and the soft layer is, the stronger the increase between the half-space and layer amplitudes, the more characteristic are the spectra of the soil transfer function. The range of measured soils has been from vS1 down to 125 m/s, vS2 up to 1000 m/s and the layer frequencies are within 10 Hz < f0 < 75 Hz. Moreover, during this measuring campaign in Switzerland, all 11 sites showed clearly the layer-on-half-space behaviour. The transfer functions of inhomogeneous soils have been used to predict the ground vibration due to dynamic axle loads which is usually thought to be the most important component.
The passage of static loads, in the contrary, results in very small vibration amplitudes for low train speeds, which can only be found at near distances and at low frequencies. They attenuate very rapidly with distance and lose very rapidly the higher frequency content. The passage of static axle loads can be included in the prediction of railway vibration just for completeness.
Special attention should be given to the case if the train runs with the Rayleigh-wave speed of the soil (Rayleigh train). The Rayleigh-train effect is strongest for a homogeneous half-space: At the near-field of the track the amplitudes are raised strongly compared to normal trains, and in addition, little attenuation with distance is observed. In case of a layered soil, the low-frequency cut-off reduces the frequency range and the amplitudes of the homogeneous quasi-static ground vibrations. Therefore, the Rayleigh-train effects are clearly reduced by a layered soil and they disappear if the layer frequency (for example for a thin layer) is higher than the frequency band of the axle impulse. The Rayleigh-train effect could completely disappear in a randomly inhomogeneous soil, but this has not been analysed so far.
The axle impulses from static loads can have an additional, quite different effect. They can be scattered by a randomly inhomogeneous soil so that a part (the scattered part) of the axle impulse can reach further distances from the track. This can establish a certain mid-frequency component of the ground vibration which becomes dominant in the far-field, and this important component exists for all train speeds. Experimental results from BAM and international measurements show the importance of the corresponding frequency range.
The mitigation of train induced ground vibration by elastic and stiff track elements has been analysed threefold. The vehicle-track interaction yields the reduction at high frequencies above the vehicle-track resonance. This is the standard effect. The filtering of trackbed errors by the bending stiffness of the track yields a certain mid-frequency effect. An even stronger mid-frequency effect is predicted for the mitigation of the scattered axle impulses by the bending stiffness and elastic elements of the track.
Elastomers are usually reinforced by large amount of fillers like carbon black (CB) or silica in order to improve various mechanical properties, such as Young’s modulus, hardness, tear resistance, abrasion resistance, and gas barrier properties. In recent years, such improvements were also obtained by using nanoparticles at significantly lower filler loadings. Graphene is a twodimensional (2D) sheet of a thickness in the atomic scale, composed of a honeycomb structure of sp2 carbon atoms. Besides significant mechanical reinforcement, graphene harbors the potential to be used as a multifunctional filler, as it can also increase the conductivity and weathering stability of elastomer matrices. Ultraviolet (UV) irradiation and oxidative agents can lead to the degradation of elastomers due to a multistep photooxidative process, including the formation of radicals. Carbon-based fillers have an influence on these reactions, as they can absorb UV radiation and act as radical scavengers.
This chapter summarizes the results of our larger project on multilayer graphene (MLG)/elastomer nanocomposites, previously published, which present a comprehensive case study of MLG as a multifunctional nanofiller in elastomer/graphene nanocomposites. Different elastomeric matrices are compared in order to demonstrate the outstanding impact of MLG as a general benefit. The dependency of this effect on concentration is discussed in detail. Taking into account the key role of dispersion, different mixing procedures are compared, evaluating a facile implementation of graphene nanocomposites into conventional rubber processing. Finally, the most probable commercial uses of MLG nanofillers in combination with conventional CB are studied. The nanocomposites were prepared in the kg scale in order to obtain enough specimens to investigate various properties of the uncured and vulcanized rubbers at the highest quality level, including rheology, curing, morphology, several mechanical properties, abrasion, conductivity, gas permeation, burning behavior, and weathering stability. The structure property relationships are asserted and questioned, for example, by investigating the radical scavenging ability or aspect ratio of the MLG. This chapter illustrates the state of the art of graphene/rubber nanocomposites targeted for commercial mass applications.
This chapter report the most recent experimental results on mechanical behaviour at high temperature of high-performance concretes. After a short introduction, subsection 5.2 describes the main testing methods that were used in the analysed studies.
Sub-section 5.3 collects and compares; the temperature-dependency of the compressive strength and modulus of elastici1ty of many experimental studies. The influence of parameters such as the initial compressive strength, the type of aggregate, the presence of additions, the W/C ratio, the moishrre content and the way the mechanical test was performed is analysed. Sub-section 5.4 presents the experimental results obtained under a constant temperature, i.e. creep tests at high temperature.
Sub-section 5.5 presents experimental results obtained under increasing temperahrre.
These results allow assessing the free thennal strain of concrete (when no mechanical load is applied) and the so-called "transient the1mal strain". Finally, subsection 5.6 collects and analyses the few results conceming the temperature-dependency of the tensile strength of high-perfo1mance concretes.
Although concrete itself is not a combustible material, concrete mixtures with high density, such has high-performance concretes (HPCs), are susceptible to significant damage during fires due to explosive spalling. Past research has shown that the inclusion of polymer fibres in high density concrete can significantly mitigate this fire damage. The exact mechanisms causing this increased spalling resistance are not yet fully under-stood, but it is thought that the fibres facilitate moisture transport during fire exposure, which in turn contributes to relief of internal stresses in the spalling-susceptible region. In this study, X-ray Computed Tomography (CT) was applied to observe the interaction between polymer fibres and cracking during thermal exposure. For this purpose, two concrete samples containing different polymer fibre types were subjected to incremental application of a defined thermal exposure. CT images were acquired before and after each thermal exposure and powerful image processing tools were used to segment the various material components. This enabled a detailed analysis of crack formation and propagation as well as the visualization and quantification of polymer fibre characteristics within the concrete. The results demonstrated that the orientation of both fibres and cracks in polymer-fibre reinforced concrete tend to be anisotropic. The results also indicated that crack geometry characteristics may be correlated with fibre orientation, with cracks tending to run parallel to fibre beds. Clear quantitative relationships were also observed between heating and increasing cracking levels, expressed in terms of both crack surface area and crack volume.