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Over the last decades concrete has evolved from a simple mass construction material towards a sophisticated multi-component system. The design parameters for the mixture composition of concrete have significantly increased from strength based towards overall or specific performance based. As a result the variety of concrete has increased yielding a number of special concrete technologies such as Self-Compacting Concrete (SCC), High-Performance Concrete (HPC), Strain Hardening Cement Based Composites (SHCC), and many others. Due to their complex mixture compositions and a multitude of possible interactions between constituents, these concrete types are preferably composed of special components like well-defined powders and sophisticated chemical admixtures. This makes such concrete technology expensive and limits their application to regions with the required material supply chains. The paper puts focus on materials, which are less well studied in conjunction with high performance concrete, but which are available in many developing countries, and in particular sub-Saharan Africa. The paper shows how sec can be designed without polycarboxylate ether superplasticizer and well defined fillers, but with lignosulphonate, cassava starch and rice husk ash. The positive effect of well processed rice husk ashes is demonstrated. Furthermore results are presented of SHCC where typical components like polyvinyl alcohol fibres and fluy ash are replaced by sisal fibres and limestone filler, respectively. The results point out that high performance concrete applications do not have to be limited to a boundary framework with availability of well-defined raw material supply structures and sophisticated admixtures or fibres. Concepts are presented how innovative concrete technologies can be developed based on indigenous materials.
Polysaccharides are incorporated into cement based systems in order to modify the rheological properties. Typically cellulose ethers, sphingan gums or starch ethers are applied. Depending upon their chemistry, molecular architecture, and adsorption tendency, polysaccharides interact differently with the entire cementitious system. Some stabilising agents like diutan gum mainly affect the cementitious paste; other stabilising agents like starch tend to interact with the sand fraction and even with the coarse aggregates. Cellulose shows more divers performances. Often stabilising admixtures like polysaccharides are used, when sophisticated rheological properties are adjusted. Therefore, polysaccharides are often used in combination with superplasticisers. This can cause interactions, particularly when the stabilising agent shows a strong tendency to adsorb on particle surfaces. Adsorptive stabilising agents may reduce the amount of adsorbed superplasticisers, thus affecting both viscosity and yield stress, while non-adsorptive stabilising agents mainly affect the plastic viscosity independently of the superplasticiser. Due to the strong influence of superplasticisers on the yield stress, influences of the stabilising agent on the yield stress retreat into the background, so that their major effect is an increase of the viscosity. The paper provides a comprehensive overview of how different polysaccharide superplasticisers affect cementitious flowable systems and points out the challenges of the combined use of polysaccharides and superplasticisers. Based on rheometric experiments and Observations of the hydration process, time dependent effects on the workability as well as of the hydration of cement are presented.
A joint research project was accomplished by ibac and BAM with the aim to develop a numerical model
of cathodic protection (CP) of reinforced concrete with a special focus on CP of the rear reinforcement
layer. The model was intended to include the effects of chemical alterations within the concrete and the
steel concrete interface, which are induced by long-term application of CP. The investigations presented
in this paper focus on the migration of chloride ions due to small electric fields as applied during cathodic
protection of steel in reinforced concrete structures and its impact on polarisation characteristics. A
comparatively new method, laser induced breakdown spectroscopy (LIBS), was used to determine
chloride profiles on laboratory specimens in order to investigate one of several model parameters to
describe cathodic protection of the rear reinforcement of reinforced concrete structures. These
investigations are described elsewhere, [1].
The paper focuses on long term polarisation tests and their impact on the charge depending cathodic
polarisation behaviour of laboratory specimens.