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Organisationseinheit der BAM
The existing cement and concrete standards are not capable of making full use of the current technology capacity due to strong focus on conventional concrete and thus they are not fit for the current and future challenges of construction industry. The paper highlights shortcomings with regard to the implementation of the existing standards. It can be seen that future-oriented standards are generally required to contribute to a lower-carbon footprint of the industry. These changes are significantly more relevant in sub-Saharan Africa, due to the rapidly increasing urbanisation challenge and the enormous potentials to develop lower-carbon technologies than elsewhere in the world.
This paper presents the analysis of crack propagation in strain-hardening cement-based composite (SHCC) under tensile and flexural load by using acoustic emission (AE). AE is a non-destructive technique to monitor the development of structural damage due to external forces. The main objective of this research was to characterise the cracking behaviour in SHCC in direct tensile and flexural tests by using AE. A better understanding of the development of microcracks in SHCC will lead to a better understanding of pseudo strain-hardening behaviour of SHCC and its general performance. ARAMIS optical deformation analysis was also used in direct tensile tests to observe crack propagation in SHCC materials. For the direct tensile tests, SHCC specimens were prepared with polyvinyl alcohol (PVA) fibre with three different volume percentages (1%, 1.85% and 2.5%). For the flexural test beam specimens, only a fibre dosage of 1.85% was applied. It was found that the application of AE in SHCC can be a good option to analyse the crack growth in the specimens under increasing load, the location of the cracks and most importantly the identification of matrix cracking and fibre rupture or slippage.
Polymers that help tailoring rheological properties during the casting process have become inevitable constituents for all kinds of high-performance concrete technologies. Due to lacking industries, these typically crude-oil based admixtures are not readily available in many parts of the world, which limits the implementation of more sustainable high-performance construction technologies in these regions. Alternative polymers, which often demand for less processing, can be derived from local plant-based resources. The paper provides experimental data of flow tests of cement pastes with polysaccharides from Triumfetta pendrata A. Rich, acacia gum and cassava without and in the presence of polycarboxylate ether superplasticizer. The flow tests are amended by observations of the zeta potentials and the hydrodynamic diameters in the presence of and without calcium ions in the dispersion medium. The results show that in the presence of and without calcium ions all polysaccharides provide negative zeta potentials, yet, they affect flowability and thixotropy in different ways. Cassava starch, acacia gum, and the gum of Triumfetta pendrata A. Rich qualified well for robustness improvement, strong stiffening, and additive manufacturing, respectively. The reason for the different effects can be found in their average sizes and size distribution. Due to the promising results, a flow chart for local value chains is derived on the example of yet unused cassava wastes, which can be converted in parallel.
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.
Sub-Saharan Africa is populated by more than half a billion people,
and rapid urbanization is creating challenges in the areas of housing,
roads, railways, power supply, dams and water pipelines —
aspects of infrastructure that are critical to the well being of the
population. This situation indicates that cement and concrete will
play a major role in the future construction technology in Africa, despite
the fact that it does not have a long-lasting tradition over most
of the continent. Although the continent is rich in natural resources
for the production of mineral binder systems, many sub-Saharan
African countries exhibit challenging boundary conditions, that have
to be well understood in order to use cement and concrete technologies
in a sustainable and reasonable way.
A thorough characterization of starting materials is the precondition for further research, especially for cement, which contains various phases and presents quite a complex material for fundamental scientific investigation. In the paper at hand, the characterization data of the reference cement CEM I 42.5 R used within the priority program 2005 of the German Research Foundation (DFG SPP 2005) are presented from the aspects of chemical and mineralogical compositions as well as physical and chemical properties. The data were collected based on tests conducted by nine research groups involved in this cooperative program. For all data received, the mean values and the corresponding errors were calculated. The results shall be used for the ongoing research within the priority program.
Two types of cements were selected as the reference cement in the priority program 2005 of the German Research Foundation (DFG SPP 2005). A thorough characterization of CEM I 42.5 R has been made in a recent publication. In this paper, the characterization data of the other reference cement CEM III/A 42.5 N are presented from the aspects of chemical and mineralogical compositions as well as physical and chemical properties. The characterization data of the slag, which is the second main constituent of this specific cement besides the clinker, are presented independently. For all data received, the mean values and the corresponding errors were calculated. The data shall be used for the ongoing research within the priority program. Also, researchers from outside this priority program can benefit from these data if the same materials are used.
A thorough characterization of base materials is the prereq- uisite for further research. In this paper, the characterization data of the reference materials (CEM I 42.5 R, limestone pow- der, calcined clay and a mixture of these three components) used in the second funding phase of the priority program 2005 of the German Research Foundation (DFG SPP 2005) are presented under the aspects of chemical and min- eralogical composition as well as physical and chemical properties. The data were collected based on tests performed by up to eleven research groups involved in this cooperative program.
The structural build-up of fresh cement paste is often considered as a purely thixotropic phenomenon in literature even though cementitious materials undergo a non-reversible hydration process that can have an influence on the structuration process. In the current paper a method is proposed to validate the impact of the non-reversible structural build-up. It is shown that fresh cement paste samples lose their structural gain almost completely due to thixotropy while the structural build-up due to hydration can be observed but occurs in a significantly lower order of magnitude over the course of the first hours of hydration. In addition, it is shown, that the chemical component of the structural build-up accelerates with the onset of the acceleration period of hydration, while its contribution in the entire structural build-up remains constant.
Viscosity-modifying agent (VMA) are used in several applications of concrete, such as underwater concrete, Self-Compacting Concrete (SCC) or Self-Levelling Underlayments (SLU) in order to improve the washout resistance and the stability. The study focuses on the modifications of cement pastes properties implied when a VMA (hydroxypropyl guars or HPG) and a superplasticizer are introduced together. For reaching this objective, two chemically different polycarboxylate-based superplasticizers (PCE) and two HPGs exhibiting different molar substitution ratios (MSHP), were studied. A method, combining total organic carbon and size exclusion chromatography, was developed in order to quantify the adsorption of the both admixtures. The adsorption of HPGs appears being significantly lowered by the presence of PCE, while only the adsorption of the less charged PCE is slightly affected by the HPG. As consequence, strong modifications of the rheological properties of cement pastes were noticed when HPG and PCE are combined. The desorption of HPG leads to higher yield stress and residual viscosity than with PCE alone.