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Organisationseinheit der BAM
Paper des Monats
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Eingeladener Vortrag
- nein (3)
A sustainable approach for the conservation of the Timur Shah Mausoleum in Kabul, Afghanistan
(2005)
This paper presents the results of an
experimental study on the transient strain of high
strength concrete (HSC) under heating up to 750 °C
and the impact of polypropylene (PP) fibers. Concerning
this topic only few results are available in the
literature and systematic investigations are missing.
However, basic knowledge is necessary for the
understanding of the internal damage processes in
the material as well as for heated structures. The
transient strain during heating can be separated in two
basic components: the free thermal strain and the
mechanical strain. They were experimentally determined
exemplarily for one HSC. For the determination
of the mechanisms of transient strain and particularly
the influence of PP fibers different techniques were
applied. In this context the monitoring of the microcracking
was done for the first time with acoustic
emission analysis in combination with ultrasonic
measurements. This new approach helps fundamentally
to explain the impact of PP fibers on free thermal
strain and mechanical strain during heating up.
Furthermore weight loss measurements were carried
out to characterize the moisture transport. It was
shown that the PP fibers cause an acceleration of the
moisture transport in the temperature range from 200
to 250 °C which leads to drying shrinkage in opposite
direction to the free thermal strain. Hence this paper is
a contribution to the general understanding of the
impact of PP fibers in HSC at high temperatures and
emphasizes the important influence of PP fibers on the
thermal and mechanical induced strain of HSC.
Sustainable evaluation - concept to assess the applicability of secondary aggregates in concrete
(2012)
Self-compacting concrete (SCC) differs substantially from normal concrete as regards the quantity of superplasticiser dosage. The functionality of superplasticisers, based on the polycarboxylate ether (PCE) superplasticiser customary with SCC, is nowadays very well understood at construction material research level. However, knowledge concerning pertinent correlations does not always permeate practice to an equal extent. PCEs are extremely polymorphic as opposed to earlier superplasticiser groups, which still possess a considerable proportion of the superplasticiser market. The geometry of polymers can be adjusted individually to performance characteristics required for certain uses. Yet, in selecting a superplasticiser for its specified rheological properties, a very strong influence can equally be exerted simultaneously on the subsequent hydration process. The charge density of the PCE employed also plays a key role in connection with both rheology and early hydration.
Since its invention in the early 1990’s, self-consolidating concrete has never become well established in the ready-mix sectors worldwide. The reason for this can be mainly found in the fact that the sophisticated compositions are sensitive against changing environments. This lack of robustness can be attributed to the interaction between cement hydration reaction and high range water reducing agent (HRWRA). Understanding the relevant mechanisms that control the initial flow performance as well as the flow retention helps optimizing SCC mixtures that perform either steadily in one specific environment or that perform largely similarly at steadily changing environments.
This paper depicts how HRWRAs interact with clinker and hydration phases and discusses the important role of the charge density of a polycarboxylic HRWRA in the way the rheology is affected. Based on rheometric results and observations of the Vicat setting times, it is shown that increasing charge densities of the HRWRA and decreasing water to powder ratios (w/p) reduce the flow retention and have lesser retarding effect on the setting. Based on the discussion, optimization procedures for the mixture composition and the HRWRA modification are suggested to achieve optimized performance for varying environmental situations or highest robustness for specific conditions.