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Eingeladener Vortrag
- nein (60)
Grouting is a universal repair and strengthening technique, which is constantly used for structural remediation of concrete components, trenches, mine subsidence, dam joints, restoration of masonry structures, and geological stabilizations. Having an extremely small particle size of only few microns, ultrafine cements are ideal for grouting applications due to their superior permeability and compressive strength properties of the hardened cement paste compared to that of the less-expensive, but coarser ordinary Portland cements. Supplementary cementitious materials (SCMs) are often used to replace ultrafine cement in order to modify certain properties and to reduce costs. The aim of this experimental study is to investigate the effect of three supplementary materials: microsilica (MS), fly ash (FA), and metakaolin (MK) on the workability, and mechanical properties of an ultrafine cement based grout with a constant water-binder ratio and constant superplasticizer content. Maximum percentages of replacement with ultrafine cement were 6% by volume of cement for MS and 16% for FA, and MK. In general, results suggest that the workability is improved by addition of FA, whereas is reduced, when modified with MS and MK. The compressive strength of grout after cement replacement remains comparable to that of pure cement grout. However, there is a tendency of the MS to positively affect the compressive strength opposite to FA, whereas flexural strength is positively affected by FA. Based on the results, it is evident that grouts with Hägerman cone flow more than 500 mm and compressive strength of more than 90 MPa after 28 days can be produced.
In den vergangenen Jahrzehnten hat sich die Betonbauweise beträchtlich von einer Massenanwendung hin zu einer innovativeren, zukunfts- und leistungsorientierten Werkstofftechnologie weiterentwickelt. Heutzutage besteht Beton nicht mehr länger nur aus Zement, Wasser und Gesteinskörnungen, sondern kann unter Anwendung von Zusatzstoffen und chemischen Zusatzmitteln erheblich verbessert werden. Gleichzeitig beschränken die Betonspezifikationen sich nicht mehr auf den Elastizitätsmodul und Druckfestigkeit, sondern Beton kann ebenfalls zur Erfüllung zahlreicher Anwendungskriterien wie Dauerhaftigkeit, Verarbeitbarkeit, Ästhetik, Brandverhalten und vieler anderer mehr beitragen. Gleichzeitig ist jedoch die Zusammensetzung von Beton wegen der Vielzahl an interagierenden Mischungskomponenten komplizierter geworden. Das Verständnis der Auswirkungen der veränderten Mischungskomponenten ist eine Herausforderung, aber auch der Schlüssel zu einer innovativen Betontechnologie.
Portland cement, as we know it today, has its origin in Great Britain approximately 170 years ago. Since then, concrete technology has spread out to Europe, the United States, and Japan, where it became a key component for rapid industrial development. Europe, the Unites States and many Asian countries today have developed a high level of technology regarding concrete construction.
However, each of them has a unique history and as a result, different “concrete philosophy” depending upon the social, environmental and financial boundary conditions, as well as their evolution throughout the years and local construction traditions. As a result, the word concrete may refer to rather different materials in America, Europe, and Asia.
Apart from South Africa, most sub-Saharan African countries cannot look back on a similarly long cement and concrete history. Cement and concrete are rather new materials and not yet well established. This gives African engineers the unique opportunity to learn from past mistakes and to develop a concrete technology, which refers to the best available practice. However, in many sub-Saharan African countries, standards and regulations are adopted (preferably from Europe or the US) without consideration of the historical background of these standards. Although this practice helps saving resources for the implementation, it does not necessarily yield the best result in the African environment, and also from an economic point of view it might come back disadvantageously due to unnecessary overdesigning.
By comparing the differing states-of-the-art in North America, Europe, and Asia, this paper emphasizes, how regional conditions determine the practice of concrete technology in the sub-Saharan area. It is therefore important for Africa to develop a unique African concrete technology, which is perfectly fitted to the specific local conditions, even if it may vary distinctively from the established practice elsewhere. The paper concludes that African nations should effort into adapting existing principles that have proved to function well rather than adopting existing standards.
Why Africa can spearhead innovative and sustainable cement and concrete technologies globally
(2016)
The perception of concrete in the society as well as in the politics is rather negative. This becomes obvious in the fact that the phrase “concrete jungle” has become synonym for hapless living with no perspectives. In politics and research funding, it is also not easy to create a broader audience, since concrete is falsely considered as old-fashioned material that is sufficiently understood today and does not need further considerations, particularly compared to allegedly newer materials. However, particularly since the last two decades the technology has completely changed. Binders of today are no more the same binders as used before, and concrete mixture compositions of today diverge quite significantly from compositions in the past. There is little understanding world-wide about that. This causes that potentials the concrete technology bears are wasted.
In the broadly found opinion that concrete is old-fashioned and ugly, it is ignored that architectural sins are not inherent to the material, which actually is extremely versatile and CO2-friendly compared to all other construction materials available. It is also ignored that 98% of the outer Earth’s crust are made of the elements cement and concrete are made from, and therefore it will be an illusion to believe that the complementary 2% can create materials to develop regions and infrastructures in less developed areas in the world.
For betterment in Africa the infrastructural development should have highest priority, since poor connections between settlements are responsible for enormous Price increases [2], and urban traffic congestion is responsible for an incredible loss of productivity. It is not unrealistic to assume that earners that are dependent on a car get stuck in traffic about 3-4 hours per day in cities like Lagos, Nairobi or Dar es Salaam. However, the traffic congestions do not only affect the car owners negatively but the living of the entire urban population every day.
Besides infrastructure, housing should be the other priority, since a large part of the African population does not live in adequate condition. This is a societal problem, since unequal Distribution of wealth is a major driving force for instability in societies. The latter has a global impact, since 8 many phenomena that can be observed all over the world such as political radicalism, xenophobia, terrorism, and migration can often be linked to instable societies.
However, the importance of infrastructure has an even wider range. Most African countries go through a change process recently. In order to strengthen very positive perspectives, the focus in politics and research funding is put on issues such as agriculture, energy, and health, which are without doubt extremely important issues. However, it is typically overlooked that all enhancements in these areas can only become effective, when an infrastructure is created to support the implementation of better concepts. Mobility is the key to a prosperous future, and mobility can only be granted by infrastructural construction activities. Hence, compared to many other regions in the world, cement and concrete technologies have a significantly higher relevance in Africa.
Viscosity modifying agents: Key components of advanced cement-based materials with adapted rheology
(2022)
Viscosity modifying agents (VMAs) are essential ingredients for the production of flowable cement-based materials. This paper presents an overview of commonly used VMAs and attempts to shed some light on the underlying physics at the origin of their mechanisms of action. The main molecular parameters of VMA controlling the rheological properties of the cement pore solution are highlighted. As the mechanisms of action of VMAs in cement-based materials are closely related to their affinity with the surface of cement particles, the adsorption of the main VMA types is discussed. The effect of VMAs on flow properties and stability of cement-based materials is presented for VMAs added without any superplasticizer, and then in systems incorporating both VMAs and superplasticizers. Finally, the effect of VMAs in enhancing concrete properties to secure adequate performance of different construction applications, and perspectives for future developments of novel cement-based materials made with VMAs are showcased.
Fünf verschiedene Messverfahren wurden zur Partikelgrößenanalyse von nanodispersen ZrO₂-Pulver verglichen. Mit der Laserstreulichtanalyse, der dynamischen Lichtstreuung (heterodyne DLS und homodyne DLS-PCS), der Sedimentationsanalyse im Zentrifugalfeld und der Ultraschallspektrometrie wurden wässrige ZrO₂-Suspensionen mit verschiedenen Feststoffkonzentrationen hinsichtlich ihrer Partikelgrößenverteilung analysiert. Als Referenz diente die REM-Analyse zur Ermittlung der Primärpartikelgröße (ca. 40 nm). Mit den hier vorgestellten Messverfahren konnten in den entsprechenden Suspensionen nur Sekundärpartikel im Bereich von 105 nm bis 224 nm detektiert werden, die somit auf das Vorhandensein von Aggregaten bzw. harten Agglomeraten hinweisen.
Verbesserung der Verarbeitungseigenschaften fließfähiger Betone bei veränderlichen Temperaturen
(2010)
The built environment significantly impacts the health of individuals and populations in various ways. The health and durability of the built environment are intertwined with availability ofnaturally occurring and man-made resources and their supply chains. Therefore, resource con servation is a key to ensure sustainability of built environments. Many industrial wastes can be turned into valuable resources for reuse in construction of the built environment. For instance, biowaste (woody/ leafy biomass and animal waste) have been used to make construction adhesives (Fini et al., 2011), some urban wastes have been used in road construction (Poulikakos et al., 2017; Schmidt et al., 2021), end of life plastics and polymers have made their way to roadway construction, bio-oils and algae harvested from wastewater treatment plants has been used to make antiaging for outdoor building elements to mitigate UV aging (Kabir et al., 2021); sulfur has been used as an extender in asphalt and recycled mineral powders such as silica and alumina have been used to increase strength and durability against acidic compounds (Fini et al., 2019). These are just a few examples of recycling venues with beneficial uses in the built environment.
The construction and operation of the built environment and the traffic that it attracts or facilitates significantly contribute to the emis sion of greenhouse gasses (GHG) and cause air pollution. The direct and indirect impacts of GHG and air pollutants on the environment and so cieties have been weil established. The built environment can be used as a powerful platform not only for recycling and resource conservation but also to remove near-ground gaseous contaminants. This can be done via tailored design and engineering of adsorptive construction materials via recycling of waste materials. For instance, advanced sorbent systems can be made for removing C02, H2S, and formaldehyde from air. This re quires tailored sorbent design, topology optimization, and catalytic conversion of collected gaseous compounds to name a few.
This special issue covers innovative materials, methods, and man agement practices which aim to simultaneously address durability ofthe built environment, air quality, resource conservation, and supply chain resilience. Such innovative materials, methods, and management prac tices will transform the built environment into not only an active contributor to no waste, no pollution for healthy environment, but also a medium that converts the waste and pollution into beneficial products for use in the built environment, thereby promoting resource conser vation. Followings are examples of topics that the special issue is interested in:
• Venuses to advance resource conservation specifically via novel ap proaches in the built environment.
• Innovative construction materials for passive or active adsorption of harmful gaseous emissions to conserve air, energy, and water.
• Advanced materials, methods, and management practices to pro mote well-being of the built environment.
• Smart buildings to promote resource conservation.
• Research convergence in the built environment for zero waste, zero pollution economy.
Time-resolved in situ investigation of Portland cement hydration influenced by chemical admixtures
(2016)
Numerous admixtures are used in the building practice to customize the properties of the cement paste during application. The influences of admixtures on the course of cement hydration and formation of hydrate phases have to be considered. Polycarboxylate ether (PCE) based polymeric superplasticizers (SPs) are known to retard the setting of the cement paste. The extent of the retardation differs depending on the molecular structure of the SP. Additionally, the presence of a stabilizing agent (SA) in the cement paste has a retarding side effect on the setting. The initial cement hydration processes and the detailed mechanisms of the retardation influenced by PCEs, as well as their interactions with particular SAs, are insufficiently understood. Up to now, only the results of phenomenological studies were taken into account to describe this retardation process. A detailed structure analysis monitoring the change of the phase composition during the hydration was never applied. Both SP and SA affect the adsorption of the sulphate ions on the clinker particles, causing changes in the formation of ettringite during the initial hydration, and are therefore a crucial part of the setting process itself. Here, the initial hydration of cement influenced by the interaction of SP and SA was monitored in situ by synchrotron X-ray diffraction. The high time resolution of the measurements allowed a continuous detection of the hydrates formed. The hydration was followed from the starting point of water addition and for couple of hours afterwards. The hydration of the levitated cement pellets containing starch as SA was initialized by adding aqueous solutions of different commercial SPs. Changes in the ettringite formation were detected in comparison to the reference hydration of pure cement.
Synopsis: The paper presents the results of a study on the influence of agitation on the structural build-up of fresh cement pastes using a penetration test. It first presents results about the influence of the penetrating shape’s geometry on the influence on the cement paste specimen. Then, results are shown for the load-deflection curves depending upon time and agitation before testing. Based on the observation of the force required to penetrate the sample, conclusions on the structural build-up can be made. The observations were made over the course of time with samples that were left at rest and partly agitated before testing at different time steps. The setup allows to identify the contribution of the chemical reaction to the structural build-up process. The presence of superplasticizer obviously reduces the load that cement paste can withstand, while the loss of thixotropy is found notably lower than that of neat cement pates
Polysaccharides provide high potential to be used as rheology modifying admixtures in mineral binder systems for the construction industry such as concrete or mortar. Since superplasticizers have become state of technology, today, concrete is more and more adjusted to flowable consistencies. This often goes along with the risk of segregation, which can be effectively avoided by adding stabilising agents supplementary to superplasticizers. Stabilising agents are typically based on polysaccharides such as cellulose, sphingan gum, or starch. Starch clearly distinguishes in its effect on rheology from other polysaccharides, mainly due to the strong influence of amylopectin on the dispersion and stabilisation of particles. Based on rheometric investigations on cementitious and limestone based dispersions with different volumetric water to solid ratios, the mode of operation of modified potato starch is explained in comparison to a sphingan gum. It is shown that the stabilising effect of starch in a coarsely dispersed system is mainly depending upon the water to solid ratio and that above a certain particle volume threshold starch mainly affects the dynamic yield stress of dispersions, while plastic viscosity is affected only to a minor degree. Sphingans operate more independent of the particle volume in a coarsely dispersed system and show significantly higher effect on the plastic viscosity than on the yield stress. In systems incorporating superplasticizers, influences of both stabilising agents on yield stress retreat into the background, while both observed polysaccharides maintain their effect on the plastic viscosity.
The SPIN project
(2011)
Globally, cement and concrete experts are at the cutting-edge to sustainable, green, healthy but nonetheless high-performance concrete. In the present age, concrete is not yet well established in Africa, which öfters the unique opportunity to build up a cement and concrete market based on the highest available state of technology. As this industry needs high level expertise, a central issue in implementation of skilled technology is cross-linking research institutions and laboratories. It should not be neglected that concrete is a product with comparably low transport ranges, which means that an improved concrete market will mainly support the local economy without exceeding financial drains to the international market, thus fostering the fight against poverty, which is an urgent need in most African countries.
The SPIN project highlights recent developments in the field of cement and concrete research with impact on the local and global economy. Challenges, future developments and opportunities for the African construction industry are in the focus.
The SPIN project is funded by the European Commission (EC) and supported by the African, Caribbean and Pacific (ACP) Group of States under the project body of the ACP Science and Technology Programme. SPIN is acronym for “Spearhead network for Innovative, Clean and Safe Cement and Concrete Technologies”. The project aims to cross-link experts with industry and policy making bodies, aiming to establish sustainable cement and concrete construction in Africa.
NMR is becoming increasingly popular for the investigation of building materials as it is a non-invasive technology that does not require any sample preparation nor causes damage to the material. Depending on the specific application it can offer insights into properties like porosity and spatial saturation degree as well as pore structure. Moreover it enables the determination of moisture transport properties and the (re-)distribution of internal moisture into different reservoirs or chemical phases upon damage and curing. However, as yet most investigations were carried out using devices originally either designed for geophysical applications or the analysis of rather homogeneous small scale (< 10 mL) samples. This paper describes the capabilities of an NMR tomograph, which has been specifically optimized for the investigation of larger, heterogeneous building material samples (diameters of up to 72 mm, length of up to 700 mm) with a high flexibility due to interchangeable coils allowing for a high SNR and short echo times (50 - 80 m s).
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.
Starches and its derivatives are known to exhibit viscosity modifying characteristics. In an ongoing work, the influence of com and cassava starches on some properties of concrete such as compressive strength, heat of hydration and creep are examined. Various percentages (0.0, 0.5, 1.0, 1.5 and 2 %) of starches by weight of cement were added to concrete mixes prepared in the laboratory. Preliminary results of compressive strengths showed that both starches have some positive impact (e.g. there was 5.3 % increase in strength due to a 1 % addition of com starch by weight of cement in comparison to the control while cassava starch of the same percentage gave 4.9 % increase in strength) at certain percentages of starch addition to concrete at 28 days. The creep and hydration results shows the starch additions compares well and in some instance performs better.
Abstract: Increasing demand for sustainable, resilient, and low-carbon construction materials has highlighted the potential of Compacted Mineral Mixtures (CMMs), which are formulated from various soil types (sand, silt, clay) and recycled mineral waste. This paper presents a comprehensive inter- and transdisciplinary research concept that aims to industrialise and scale up the adoption of CMM-based construction materials and methods, thereby accelerating the construction industry’s systemic transition towards carbon neutrality. By drawing upon the latest advances in soil mechanics, rheology, and automation, we propose the development of a robust material properties database to inform the design and application of CMM-based materials, taking into account their complex, time-dependent behaviour. Advanced soil mechanical tests would be utilised to ensure optimal performance under various loading and ageing conditions. This research has also recognised the importance of context-specific strategies for CMM adoption. We have explored the implications and limitations of implementing the proposed framework in developing countries, particularly where resources may be constrained. We aim to shed light on socio-economic and regulatory aspects that could influence the adoption of these sustainable construction methods. The proposed concept explores how the automated production of CMM-based wall elements can become a fast, competitive, emission-free, and recyclable alternative to traditional masonry and concrete construction techniques.
We advocate for the integration of open-source digital platform technologies to enhance data accessibility, processing, and knowledge acquisition; to boost confidence in CMM-based technologies; and to catalyse their widespread adoption. We believe that the transformative potential of this research necessitates a blend of basic and applied investigation using a comprehensive, holistic, and transfer-oriented methodology. Thus, this paper serves to highlight the viability and multiple benefits of CMMs in construction, emphasising their pivotal role in advancing sustainable development and resilience in the built environment.
Synopsis: Lately, there has been rising attention to superplasticizers (SP) based on polyphosphate esters. However, the influence of the molecular structure of the polyphosphate polymers on time-dependent properties such as structural build-up has not been examined yet intensively. To investigate this effect, three comb polyphosphate superplasticizers with different charge densities were synthesised by free radical polymerisation. Our findings indicate that SP with the lowest and medium charge densities extend the induction period more strongly than the SP with the highest charge density. The reduction of the structural build-up rate is linearly dependent on the dosage and concentration of the functional group of polyphosphate SP in the cementitious system. This study proposes a mathematical equation expressing the relationship between the structural build-up rate during the induction period and the molecular structure of the polyphosphate SP.
Superplasticizers (SPs) have been employed in concrete technology for decades to improve the workability of concrete in its fresh state. The addition of SPs in cement-based systems affects the early properties. Although the interaction of the cement particles with various SPs has been extensively researched, there still exists limited research on the interaction of SPs with supplementary cementitious materials such as rice husk ash (RHA). This paper investigates the rheological properties and early hydration kinetics of RHA-blended systems with three types of SPs, a polycarboxylate ether (PCE) and two lignosulphonates (LS-acc and LS-ret). In rheological properties, the addition of SP causes an initial improvement of workability as the yield stress is significantly reduced. The pastes with PCE and LS-acc show a slight increase of yield stress over time whereas pastes with LS-ret tend to lower the yield stress slightly over time, further improving the workability. Without SP, pastes with RHA show a lower yield stress but an increase in plastic viscosity as cement is further replaced with RHA. The addition of the LS SPs is observed to lower the plastic viscosity but remains constant with further replacement of cement with RHA. This indicates that LS SPs further adsorbs on RHA particles and hydration products produced causing dispersion of the particles
within the system. In early hydration kinetics, pastes with PCE retard hydration and the degree of retardation is further increased with LS SPs. In the presence of RHA, the retardation of LS SP systems is significantly reduced. The pastes with PCE show more ettringite in the SEM micrographs, but is observed to be shorter needles. This indicates an initial good workability for PCE. However, C-S-H and CH were observed to be low in quantity, whereby the pastes with LS show more nucleation sites for C-S-H and CH. The ettringite needles in the LS systems were similar in quantity and more elongated in some cases but not abundant as in the PCE systems.
The African-European SPIN Network - Linking Experts to Foster Innovation in Cement and Concrete
(2012)
Most factors acting on concrete rheology work at an extremely small-scale level. Influencing factors in the millimetre or centimetre area are essentially restricted to sand and aggregates. The latter, however, make up 50 to 70% of the total volume of most concretes – a fact often ignored in research on controlling concrete processing properties.
Whereas suitably chosen concrete admixtures and additives can influence rheology in a very targeted manner, sand and aggregates are less suitable for controlling rheology but nonetheless contribute to the rheology of the Overall system. The actions of sand and aggregate can impose themselves upon the actions of admixtures and additives
and, in unfavourable circumstances, even render them redundant. For this reason, any results concerning the processability of binding agent systems can only be transferred to concrete with great care. It is important to better understand the action of sand and aggregates in order to be able to harmonise them in such a way that they complement the action of superplasticisers positively, instead of
working against them. Savings on costs can also be made by this targeted fine-tuning.
Repair is an indispensable part of the maintenance of structures over their lifetimes. Structural grouting is a widely used remediation technique for concrete components, trenches, mine subsidence, dam joints, restoration of masonry structures, and geological stabilizations. A structural grout system should be injectable in narrow spaces and hence include ingredients with finer particles. Ultrafine cements are ideal for these type of demanding grouts due to their superior properties compared to that of the less expensive, but coarser ordinary Portland cement (OPC). Supplementary cementitious materials (SCMs) are often used to replace OPC clinker based binder in order to modify certain properties and to reduce costs. The most commonly used SCMs are fly ash (FA), and ground granulated blast furnace slag (GGBS). For various special applications microsilica (MS), and metakaolin (MK) are also used. Identifying the optimum replacement contents of OPC by SCMs are a challenge during the design of such grouts. The aim of this experimental study is to investigate the effect of the selected SCMs (FA, MS and MK) on the slump flow, time of efflux, viscosity, shrinkage, and compressive and flexural strength of ultrafine cement based grouts with constant water-binder ratio and superplasticizer content. The test program was formulated using Box-Behnken design principles. Maximum percentages of replacement with ultrafine cement was 6% by volume of cement for MS and 16% for FA, and MK. The results suggest that most investigated grouts have the potential to be used for structural applications. The appropriate quadratic models are then formulated through statistical tools and presented as response surfaces. The trends indicate that fly ash improves the rheological properties, whereas microsilica and metakaolin positively affect shrinkage and mechanical properties to some extent. Based on the influence of SCMs and priorities among the properties, Decision Matrix Analysis (DMA) is carried out to select the most suitable ones among the SCMs. The analysis suggests that microsilica and fly ash are more suitable as SCMs than metakaolin without affecting the properties.
Structural build-up describes the stability and early-age strength development of fresh mortar used in 3D printing. lt is influenced by several factors, i.e. the composition of the print able material, the printing regime, and the ambient conditions. The existing modelling approaches for structural build-up usually define the model parameters for a specific material composition with out considering the influence of the ambient conditions. The goal of this contribution is to explicitly include the temperature dependency in the modelling approach. Temperature changes have signifi cant impact on the structural build-up process: an increase of the temperature leads to a faster dissol ution of cement phases and accelerates hydration. The proposed extended model includes temperature dependency using the Arrhenius theory. The new model parameters are successfully calibrated based on Viskomat measurement data using Bayesian inference. Furthermore, a higher impact of the temperature in the re-flocculation as in the structuration stage is observed.
Supplementary Cementitious Materials an Admixtures - Part I: Supplementary Cementitious Materials
(2014)
Superplasticizer and Shrinkage Reducing Admixture Dosages for Microfine Cement in Grout Systems
(2019)
Grouts have numerous applications including crack repair as maintenance in construction industries. Microfine cements are intensively used for high strength mortar and grout products. They are ideal for injection grouting in structural repair. Such grouts should have suitable rheological properties to be injectable, especially those used in repair and rehabilitation. The use of superplasticizers (SP) in these products is thus becoming increasingly crucial to achieve favorable workability and viscosity properties. A difficulty in such grouts is the plastic shrinkage due to finer particles used. It is thus necessary to determine optimum SP and shrinkage reducing admixture (SRA) dosages for a microfine cement based grout. In this study, a saturation dosage was decided from two Polycarboxylate ether (PCE) based SPs in relation to neat cement using slump flow and rheological parameters. A range of grout mixtures was formulated containing micro silica (MS) and fly ash (FA), and tested for suitable rheological and mechanical parameters. Based on the results, a grout mixture with MS and FA was selected to determine optimum SRA content. According to the results, a SP dosage of 3% by weight of neat cement is sufficient to achieve saturation. The grout material including MS and FA can produce comparable properties to neat cement grout. MS is found to improve compressive strength within the range considered, whereas a higher FA content provides favourable rheological properties. Finally, a SRA dosage of 4%, which could reduce the shrinkage by about 43% after 28d days, is determined for the grout system.
Nowadays, superplasticizers (SPs) are widely used to increase fluidity and reduce water content in concrete; thus, allowing better workability for final applications. The present study will focus on the hydration effect using comb shape polycarboxylates (PCEs), which are known to allow a very low water/cement ratio (w/c of 0.20) or less.Optical methods have been successfully used for the analysis and monitoring of the interactions between a broad variety of nanoscale and molecular systems like nanoparticles of various chemical composition and different types of organic ligands or biomolecules.This encouraged us to assess the potential of these methods, and particularly fluorescence, for the investigation of the interactions that occur at the interface between hydrate surfaces of cement particles and PCE at a very early stage of concrete formation and to differentiate between the impact of PCE’s molecular structures on such interactions.
Steuerung der Mischung
(2015)
Worldwide, cement and concrete experts are at the cutting-edge to sustainable, green, healthy but nonetheless high-Performance concrete. The current relatively low development of the cement and concrete industry in Africa offers the unique opportunity to start directly on the best achievable and sustainable Level, if only Expertise is sufficiently available. It should not be neglected that concrete is a product with comparably low Transport ranges, which means that an improved concrete market will mainly Support the local economy without exceeding financial Drains to the international market, thus fostering the fight against poverty, which is an urgent Need in most African countries.
Globally, cement and concrete experts are at the cutting-edge to sustainable, green, healthy but nonetheless high-performance concrete. Today concrete is not yet well established in Africa, which offers the unique opportunity to build up a cement and concrete market based on the highest available state of technology. As this industry needs high level expertise, a central issue in implementation of skilled technology is crosslinking research institutions and laboratories. It should not be neglected that concrete is a product with low transport ranges. This means that an improved concrete market mainly supports the local economy without exceeding financial drains to the international market. Thus it fosters the fight against poverty, which is an urgent need in most African countries. The project aims to cross-link experts with industry and policy making bodies, aiming to establish sustainable cement and concrete construction in Africa.
Im Hinblick auf die Konformitätsbewertung der Druckfestigkeit von Beton wird es in der für 2014 geplanten Fassung der EN 206 signifikante Änderungen geben. Neben den bisher bekannten Verfahren werden auch sogenannte Kontrollkartensysteme als Konformitätsnachweis zugelassen. Dabei können beliebige Kontrollkartensysteme verwendet werden, sofern nachgewiesen werden kann, dass der maximale Durchschlupf (Average Outgoing Quality Limit, AOQL) 5 % beträgt. Im informativen Normenanhang werden zwei grundlegende Verfahren, das sogenannte Shewhart-Verfahren sowie das CUSUM-Verfahren (Cumulative Sum) beschrieben, welche beide die Forderung an das AOQL erfüllen.
Kontrollkartensysteme sind eigentlich Werkzeuge zur Prozesssteuerung. Sie können effektiv dabei helfen, die Produktion technisch, ökonomisch und ökologisch zu optimieren. Allerdings sind Kontrollkarten deutlich komplexer in der Anwendung als die bisher normativ erfassten Methoden. Der vorliegende Beitrag stellt die neuen Verfahren und deren Besonderheiten anhand von Beispielen vor. Darüber hinaus werden der Zusatznutzen bei sachgerechter Anwendung im Vergleich und Vor- und Nachteile zu den bisherigen Verfahren dargestellt und diskutiert.--------------------------------------------------------------------------------------
With regard to the conformity assessment of the compressive strength of concrete, there will be significant changes in the EN 206, which is scheduled for publication in 2014. So called control chart systems will be approved supplementary to the existing methods. Any control chart can be used as long as the system can grant an average outgoing quality limit (AOQL) of 5 %. The informative appendix of the standard presents two fundamental control chart methods, the so called Shewhart method and the CUSUM method (cumulative sum), both of which fulfill the required AOQL.
Control card systems are actually tools for process control. They can effectively help to optimise production processes technically, economically, and ecologically. However, control charts are much more complex to use than the methods known from the recently applied standard. This paper presents these new methods and their characteristics using examples. Furthermore, the additional benefit of these systems is illustrated and advantages and disadvantages are discussed in comparison to existing methods.
SCM potential in Africa
(2012)
Innovative admixture technology has significantly widened up the ränge of possibilities of concrete engineers. For many decades the water to cement ratio (w/c) was the major influencing factor for the performance of concrete. Due to the need to adjust a consistency, which still allowed reasonable workability, the w/c was typically significantly higher than technologically reasonable. Rheology modifying admixtures Support adjusting the concrete consistency largely independent of the w/c. It was only after the invention of the first superplasticizers that modern concrete technology significantly evolved in terms of flowability, strength, and durability, and only due to the steady evolution of the technology modern innovations, such as Self-Compacting Concrete, Ultra-High-Performance Concrete, or Engineered Cementitious Composites were made possible. Today’s superplasticizers are extremely versatile and can be adjusted to individual technological specifications. However, the other side of the coin of versatility is that cementitious Systems incorporating superplasticizers have become more sensitive against environmental influences, such as the environmental temperature, which may cause unwanted effects or demand for supplementary admixture use such as stabilizing admixtures. Hence, concrete mixture composition with admixtures demands for a high level of expertise and offen there is lack of awareness about the mode of Operation of rheology modifying admixtures among concrete technologists. The paper gives a comprehensive overview about rheology modifying admixtures such as superplasticizers or stabilizing agents, and how they can be used depending upon the application in the most favourable way. Based on experiences with the sub-Saharan African concreting boundary conditions, which exhibit many challenges in terms of environmental boundary conditions and construction site logistics, conclusions are finally drawn, how admixtures can be used in the most beneficial way to improve the concrete casting Situation.
Innovative admixture technology has significantly widened up the ränge of possibilities of concrete engineers. For many decades the water to cement ratio (w/c) was the major influencing factor for the performance of concrete. Due to the need to adjust a consistency, which still allowed reasonable workability, the w/c was typically significantly higher than technologically reasonable. Rheology modifying admixtures Support adjusting the concrete consistency largely independent of the w/c. It was only after the invention of the first superplasticizers that modern concrete technology significantly evolved in terms of flowability, strength, and durability, and only due to the steady evolution of the technology modern innovations, such as Self-Compacting Concrete, Ultra-High-Performance Concrete, or Engineered Cementitious Composites were made possible. Today’s superplasticizers are extremely versatile and can be adjusted to individual technologicalspecifications. However, the other side of the coin of versatility is that cementitious Systems incorporating superplasticizers have become more sensitive against environmental influences, such as the environmental temperature, which may cause unwanted effects or demand for supplementary admixture use such as stabilizing admixtures. Hence, concrete mixture composition with admixtures demands for a high level of expertise and offen there is lack of awareness about the mode of Operation of rheology modifying admixtures among concrete technologists. The paper gives a comprehensive overview about rheology modifying admixtures such as superplasticizers or stabilizing agents, and how they can be used depending upon the application in the most favourable way. Based on experiences with the sub-Saharan African concreting boundary conditions, which exhibit many challenges in terms of environmental boundary conditions and construction site logistics, conclusions are finally drawn, how admixtures can be used in the most beneficial way to improve the concrete casting Situation.
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.
Rheological Optimisation for Flowable Mixture Compositions Specified for African Boundary Conditions
(2013)
Rheological optimisation for flowable mixture compositions specified for African boundary conditions
(2013)
To date concrete technology is not yet well established in sub-Saharan Africa but considering the construction technological challenges of the region, without doubt, concrete will play a major role in future. The social, economic, and geographic boundary conditions distinguish greatly from those that can be found in many other regions of the world, where concrete is considerably well established. Düring the last decades, concrete technology underwent a rapid evolution process, but in most countries, where concrete is already well established, innovations are difficult to be brought into practice, Africa, currently has the opportunity and potential, of directly establishing the best available practice in concrete technology.
The conclusion is drawn that self-compacting concrete (SCC) or highly flowable concrete is a reasonable solution for African construction sites. Since on most African construction sites the disadvantageous concreting environment is the most crucial factor for the concrete quality, the outstanding workability properties outweigh any disadvantages of SCC.
Based on this consideration, concepts are developed, how to bring about robust SCC cost-efficiently and reliably into practice as pre-mixed dry compound under consideration of local materials such as rice husk ashes, bagasse ashes, natural pozzolans, cassava starch and lignosulphonate.
Admixtures are important constituents to enhance the performance of concrete. They allow for more efficient use of binders which can mitigate negative environmental impacts from producing cement-based materials. Commonly used rheology modifying agents like polycarboxylate ethers or cellulose ethers are synthetic or semi-synthetic, respectively. This requires additional energy consumption for their production and global supply chains particularly for many developing regions, which will be large consumers of concrete in the future. However, many locally available bio-based polysaccharides could be effectively used instead. These polymers are often overlooked by engineers and scientists due to their limited distribution and inherent complexity, yet they represent an underleveraged source of precursors for admixtures. This study investigates the action mechanisms of some bio-based rheology modifying agents, i.e., acacia gum and miscanthus gum, and provides a comparison to a conventionally modified starch. The results show that the mechanism of action of these polymers is closely related to the composition of the mixture, mixing regime, and the composition of the aqueous phase. Depending on the agent, either plasticizing or stabilizing effects on rheology can be revealed.
The cement and concrete market in East, Central and Southern Africa is highly fragmented. The concrete industry in this area consists of multiple parties, including producers and suppliers of construction materials, formal and informal contractors, engineers and architects, unions of trades persons and workmen, governmental bodies and formal institutions of research and education. All these institutions mostly do not interact adequately, which makes building with cementitious materials susceptible to damage and failures. Completely opposed to the situation in Europe or North America, cement in Africa is often unaffordable, while manpower is cheap, which results in a questionable economisation of cement. Typically, there is not sufficient awareness of methods to sensibly reducing the cement content in concrete or replace Ordinary Portland Cement by adequate alternative materials. Research activities in this field of technology are often missing completely. Only few countries in the area, such as South Africa, are exempted from these issues.
This paper presents the SPIN project, which is a joint project of a consortium of 8 African and 3 European partners within the ACP Science and Technology Programme. The project is funded by the EC and ACP Secretariat is the project body. The main objective of the current project is to strengthen the cement and concrete industry in the East and Central African regions. The project shall generate reasonable solution strategies to implement clean, safe and sustainable cement and concrete technology on the African continent, including general and specific guidelines for sensible application. Furthermore it shall be the kick-off for future projects, research activities and the world-wide expansion of a European-African network.
The paper addresses special problems the cement and concrete market in Eastern, Central and Southern African countries has to face. Several options are presented in detail, which shall help overcoming the current situation. Customized solutions for the African market include rational methods for reducing the amount of cement used and the replacement of Ordinary Portland Cement with cheaper alternatives. The use of recycled concrete through a new and economically effective method, as well as the opportunity of using locally available resources is also discussed.
Today, concrete engineers can vary consistencies between very stiff and self-compacting. The possibility to tailor rheological properties of concrete eventually opened up the path to multiple new technologies, where design criteria are no longer limited to the Young’s modulus and the compressive strength, but often comprise additional added value. It can therefore be concluded that the capability to control the rheology of concrete systems can be considered as the catalyst for concrete innovations such as polymer modified cementitious composites (PCC) self-compacting concrete (SCC), high-performance concrete (HPC), ultra-high performance concrete (UHPC) or strain hardening cement based composites (SHCC). In the same way rheology modifying admixtures will become key parameters in mastering the challenges of the next decades such as:
- Additive manufacturing/3D-printing
- Overcoming pumping height limitations
- Casting at extreme temperatures
- Tailored rheology at delivery
In many regions in the Southern hemisphere, and particularly in Africa, the effective use of chemical admixtures for concrete could significantly contribute to solve problems induced by the challenging climatic conditions, but particularly in Africa it is difficult to use them, due to lacking local supply and supply infrastructure. For Africa, concrete admixtures are largely shipped or transported from Europe, the Arabian Peninsula, or Asia. The long distance transportation of chemicals is not very environmentally friendly and the economic consequences are dramatic: the agents are difficult to purchase, expensive, and there is not a large variety of products available in the market.
However, bio-based chemicals have been used in construction for ages effectively. Due to the enormous relevance of rheology modifying admixtures it is worthwhile to recollect that nature provides an enormous variety of chemicals that can be used readily or with low processing. Today, the use of petrol based polycarboxylate ether superplasticizers has become quite common in concrete technology due to their versatility, but their uncomplicated and cost efficient availability is limited in most countries in Africa. However, alternatives are available, which can be found in many regions.
Plant extracts have been used to modify the properties of concrete for long time. These plants are typically regionally abundantly available, cheap, and they are environmental friendly. They do not require special caring for seedlings, weeding or manuring to grow since they grow wildly.
The talk will present an overview of various options for rheology modifying admixtures, that can be found in nature, and that may become a real alternative, once their interaction within the complex cementitious system is well understood.
Today, concrete engineers can vary consistencies between very stiff and self-compacting. At the same time engineers can opt for a vast variety of binders. The possibility to use optimised mineral binders and to tailor rheological properties of concrete eventually opened up the path to multiple new technologies, where design criteria are no longer limited to the Young’s modulus and the compressive strength, but often comprise additional added value. It can therefore be concluded that the capability to control the rheology and the interactions particles can be considered as the catalyst for concrete innovations such as polymer modified cementitious composites (PCC) self-compacting concrete (SCC), high-performance concrete (HPC), ultra-high performance concrete (UHPC) or strain hardening cement based composites (SHCC). In the same way rheology modifying additions, SCMs and admixtures will become key parameters in mastering the challenges of the next decades such as:
- Additive manufacturing/3D-printing
- Overcoming pumping height limitations
- Casting at extreme temperatures
- Tailored rheology at delivery
In many regions in the Southern hemisphere the effective use of SCMs, additions and chemical admixtures for concrete could significantly contribute to solve problems induced by the challenging climatic conditions, but particularly in many countries with challenging climatic conditions, it is difficult to use them, due to lacking local supply and supply infrastructure, and often the awareness of the value of local mineral resources is missing. However, the long distance transportation of mineral resources and chemicals is not very environmentally friendly and the economic consequences are dramatic.
However, bio-based constituents and chemicals have been used in construction for ages effectively. Due to the enormous relevance of binders, fillers and rheology modifying admixtures it is worthwhile to recollect that nature provides an enormous variety of products that can be used readily or with low processing. Today, the use of petrol based polycarboxylate ether superplasticizers has become quite common in concrete technology due to their versatility, but their uncomplicated and cost efficient availability is limited in many countries in the world. However, alternatives are available, which can be found locally. In addition many agricultural wastes today are dumped, although they could be converted to reactive ashes easily.
Plant extracts have been used to modify the properties of concrete for long time. These plants are typically regionally abundantly available, cheap, and they are environmental friendly. They do not require special caring for seedlings, weeding or manuring to grow since they grow wildly.
The talk will present an overview of various options for binders, fillers and rheology modifying admixtures, that can be found in nature, and that may become a real alternative, once their interaction within the complex cementitious system is well understood.
Potentials for sustainable cement and concrete technologies - Comparison between Africa and Europe
(2017)
Cement and concrete technologies in Africa provide vast potentials for sustainable future technologies as well as enormous challenges. The main focus of the symposium is to address existing challenges, possible solutions, and future potentials with all participants. This symposium also aims at offering a platform for networking with relevant players in science and industry.
Potentials for sustainable cement and concrete technologies - Comparison between Africa and Europe
(2017)
The fundamental knowledge about cement and concrete has made enormous progress over the last decades, and today it would be possible to find optimised sustainable concrete solutions tailored for every given boundary framework and raw material supply. However, this knowledge barely finds implementation into practice despite the urgent global need to minimise carbon emissions and energy consumption. A major reason is that most concrete developments were historically made in the northern hemisphere, where today over-regulations and stagnating market perspectives slow down innovation drive towards higher sustainability. In most African countries, however, sustainable building is simply an urgent real-life problem. The demand for building is enormous, Standard solutions are not an option, and the pool of innovative local raw materials and concrete concepts is enormous. The paper provides a comprehensive comparison between the boundary frameworks of Europe and Africa, and it is explained why local African solutions shall be given priority over imported solutions. Examples of local African concrete solutions are given, and ideas for a rapid implementation are developed. Most of the potentially useful materials such as agricultural ashes, natural and calcined pozzolans, polysaccharides, etc. have not yet been subject to intensive research to date. Therefore, it is not unlikely to assume that with an open mind for non-Standard solutions, combined with creativity and particularly knowledge and awareness, the next generation of innovative and sustainable concretes will be developed and applied on the African continent. Therefore, the conclusion is that particularly the African continent provides the best starting position to develop better and more sustainable concrete solutions than anywhere else in the world. Hence, Africa can become a global pioneer in green cement and concrete technology with impact to the entire world.
Over the course of the last decades, concrete construction has significantly changed from mass application towards sophisticated more innovative, future oriented, and performance based materials technology. Today concrete no longer consists of cement, water, and aggregates, but can be enhanced significantly by using supplementary additions and chemical admixtures. Advancements in admixtures eventually facilitated the development of new concrete types such as self-compacting concrete (SCC) and high performance concrete (HPC) by uncoupling the workability from the water-cement ratio (w/c).
While for traditional concrete types the major mixture parameter for the workability and strength were the w/c and the grading of the aggregates, for SCC and HPC the performance parameters are more complicated and the performance depends highly on the powder components. This makes the system more sophisticated and in parallel more prone to scatter during the production. The complexity and difficulty to predict the effects of changes in the process and mixture makes it very difficult to identify the right potentials for savings.
While today, engineers can choose from a wide range of rheology modifying admixtures, in some parts of the world, these are difficult to access, due to their complex processing. However, alternatives can be bio‐based polymers such as polysaccharides from various sources. These are easily accessible all over the world, do not demand for complicated processing, and typically they are more sustainable than many established materials, which are crude oil‐based.
The paper presents the effects of acacia gum, cassava starch and the gum of triumfetta pendrata A. Rich on the rheological performance of cementitious systems. It is shown that acacia gum can be as efficient as polycarboxylate based superplasticisers, cassava starch can reduce the yield stress slightly with little effect on the plastic viscosity, and the gum of triumfet ta pendrata A. Rich increases the thixotropy of cement pastes with plasticizing polymers significantly.
Während für die Bauwirtschaft in den meisten Ländern der nördlichen Halbkugel die Instandhaltung bestehender Strukturen und Infrastruktur im Vordergrund steht, geht es in vielen Ländern der Südhalbkugel in der Hauptsache um die Schaffung von Bauwerken und Infrastruktur zur Verwirklichung angemessener Lebensbedingungen, was wiederum das schnelle Wirtschaftswachstum weiter befeuert.
Beton ist als Massenbaustoff, trotz landläufig anderer Bewertung, aufgrund seines im Vergleich zu anderen Baustoffen günstigen CO2-Footprints und der globalen Verfügbarkeit hierbei aktuell ohne Alternative. Allerdings ist Beton nach Wasser das am zweithäufigsten genutzte Produkt auf Erden [1] und die globale Nachfrage und Produktion steigt dramatisch. Dies hat Konsequenzen für das globale Klima, denn gerade die als Bindemittel eingesetzten Portlandzemente verursachen bei ihrer Herstellung selbst unter modernsten Produktionsbedingungen große Mengen an CO2.
Deshalb müssen für zukünftige nachhaltigere Technologien neuartige Betone entwickelt werden, die bei gleicher oder verbesserter Leistungsfähigkeit, einen geringeren Bedarf an Portlandzement aufweisen.
Um den CO2-Ausstoß, der mit der Betonherstellung verbunden ist zu minimieren, müssen Anteile an Portlandzement im Bindemittel reduziert und durch nachhaltigere Ersatzstoffe ausgetauscht werden. Darüber hinaus muss das eingesetzte Bindemittel im Beton so effizient wie möglich ausgenutzt werden. Das heißt, ein Bauteil oder Bauwerk sollte gerade so viel Bindemittel enthalten, wie für die Tragfähigkeit und Dauerhaftigkeit erforderlich ist. Hierfür erscheint der Einsatz von leistungsfähigen bauchemischen Produkten unumgänglich. Diese können zum einen die veränderten Verarbeitungseigenschaften, die Zementersatzstoffe mit sich bringen, ausgleichen und gleichzeitig helfen, den Gesamtwassergehalt zu minimieren, so dass das Bindemittel seine maximale Leistungsfähigkeit erzielt. Fließmittel sind aktuell die wichtigsten Zusatzmittel für Beton. Sie reduzieren den Wasserbedarf und ermöglichen, Zement im Beton effizienter auszunutzen. Darüber hinaus sind Stabilisierer wichtig zur Erhöhung der Robustheit, insbesondere im Hinblick auf die bautechnischen Herausforderungen der Zukunft – vom Pumpen über weite Distanzen, dem Spritzen, dem Betonieren bei extremen Klimabedingungen bis hin zum 3D-Druck.
Supplementary cementitious materials (SCM) can reduce the total amount of Portland cement clinker in concrete production. Rice husk ashes (RHA) can be converted from an agricultural by-product to a high-performance concrete constituent due to a high amount of reactive silica with pozzolanic properties if they are burnt under controlled conditions. The way and duration of combustion, the cooling process as well as the temperature have an effect on the silica form and thus, the chemical and physical performance of the RHA. Various studies on the best combustion technique have been published to investigate the ideal combustion techniques. Yet, the process mostly took place under laboratory conditions. Investigating the difference between the performance of RHA produced in a rural environment and laboratory conditions is useful for the assessment and future enhancement of RHA production, and its application both as building material, for example in rural areas where it is sourced in large quantities, and as additive for high performance concrete. Thus, the paper presents a comparison between RHA produced under rudimentary conditions in a self-made furnace in the rural Bagamoyo, Tanzania and under controlled laboratory conditions at the Technical University of Munich, Germany, with different combustion methods and temperatures. In a second step, RHA was ground to reach particle size distributions comparable to cement. In a third step, cement pastes were prepared with 10%, 20% and 40% of cement replacement, and compared to the performance of plain and fly ash blended cement pastes. The results show that controlled burning conditions around 650 °C lead to high reactivity of silica and, therefore, to good performance as SCM. However, also the RHA burnt under less controlled conditions in the field provided reasonably good properties, if the process took place with proper burning parameters and adequate grinding. The knowledge can be implemented in the field to improve the final RHA performance as SCM in concrete.
Rice husk as (RHA) is an eco-friendly material, which can be used as a supplementary cementitious material (SCM) in cement and concrete. Due to the high water demand for the material, superplasticizers (SPs) are essential to improve the performance. However, the interaction between the SPs and RHA systems is limited. This paper investigates the interaction of the binders with three SPs, i.e. two polycarboxylate ethers (PCEs) and one lignosulphonate (LS). The investigations are performed on blended systems of mortar containing various percentages of RHA and limestone powder (LSP). LSP is used in this research to improve the workability of the mortar. The results from the zeta potential (ZP) shows that the SPs are extremely dependent on the pH of the suspension. At higher pH values such as in a cementitious system, the ZP becomes less negative indicating that the ions in the suspension interact with the carboxyl groups in the backbone of the polymers thus reducing the surface charges of the SP. The mini-slump flow shows that the workability of the blended mortar systems is significantly improved with the addition of SPs. LS systems with increasing RHA is observed to have similar workability as the control mix over time. The compressive strength test results show increased strength for all mortar specimens with added RHA and LSP at later ages.
Today's African concrete construction environment differs greatly from that in Europe, Japan or North America. Europe can look back on a concrete tradition of more than 150 years. Standardising documents and guidelines have existed for over 100 years, and formed the foundation of modern Standards adopted and applied in many regions of the world. Technological developments in cement production are also strongly influenced by environmental conditions of the Northern hemisphere. In contrast, with the exception of South Africa, most countries of sub-Saharan Africa started cement production in the 1950s to 1960s, followed by a period of political and economic upheaval. As a result, there have not been serious approaches towards customised Solutions for concrete construction for conditions specific to sub-Saharan Africa; in most countries overseas Standards are applied, as a matter of course.
The African environment for the construction with concrete differs greatly from the Situation that can be found in Europe, Japan or North America. Europe can look back upon a concrete tradition of more than 150 years. Standardising documents and guidelines exist since more than 100 years, which formed the foundation of modern Standards that are adopted and applied and in many regions of the world. The technological developments in cement production are also strongly influenced by environmental conditions of the Northern hemisphere. In contrast, with the exception of South Africa, in most countries of sub-Saharan Africa, cement production started in the 1950s to 1960s, followed by a period of political and economical disturbances in many countries. As a result, there have not been serious approaches towards customised Solutions for the construction with concrete for the specific boundary conditions of sub-Saharan Africa. In most countries Standards from overseas are applied. Today, most African economies are growing at a rapid pace and construction activities in infrastructure and housing are urgently required to support and keep up the economic verve. Concrete technology will become of major interest in this context, since no other material can be produced similarly efficiently based on raw materials, which can be found largely only locally. In order to produce concrete with high efficiency, the local boundary framework has to be taken into account. Sub-Saharan Africa exhibits special peculiarities that are not covered by the recently applied technological guidelines and Standards. These need to be addressed to in local codes of practice in order to maximise the benefit of concrete technology.
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.
Concrete of today has only little in common with the traditional concrete used a few decades ago. It has become a high performance material, which can be adjusted for high performance applications and according to ultimate user specifications. The reason for the rapid evolvement was the increasing awareness about how the rheology of concrete can be improved without negatively affecting the mechanical properties of concrete by chemical admixtures.
Rising temperatures worldwide pose an increasing challenge for safe and healthy living conditions. Particularly inner cities have been affected by these environmental changes because of the materials used to build houses, streets and infrastructure. The most common building material is concrete. It shows a specific heat capacity, while the heat conductivity for Standard concrete is low. Thus, the use of concrete generates a high capacity of heat storage. In addition, extensive soil sealing also contributes to the temperature rise of inner city areas compared to
their surroundings. To mitigate this so-called urban heat island effect, a self-cooling concrete paver was developed. This paver is able to store water. The evaporation of the water at elevated temperatures provides a cooling effect. This paper focuses on determination of this new paver’s capability to cool the surface and the surrounding. The new paver’s cooling Qualities were analysed in a series of laboratory tests. To prove the results outside of laboratory conditions, two fields (12 m × 8 m) with self-cooling and reference pavers were installed in Spain.
This paper presents and discusses the results of the tests. Correlation between reduced surface temperature of the self-cooling concrete pavers and the air temperature is examined.
There is a wide range of research worldwide on supplementary cementitious materials (SCMs) such as fly ash and slag for substituting pure cement. Such materials are suitable to be considered in a cementitious system with ordinary Portland cement (OPC) due to their high pozzolanic properties. In addition, majority of the SCMs are said to significantly improve concrete properties especially in terms of increased strength and durability. Unfortunately, the production of such SCMs is not entirely eco-friendly and also limited to certain parts of the world, hence one has to look at alternative options.
The issue of availability of resources is a strong concept that is ever-increasing, and the use of more eco-friendly SCMs in a cementitious system is furthermore attractive. Hence this paper addresses the use of eco-friendly SCMs in concrete such as rice husk ash (RHA). Despite the fact that extensive research has been done on this material, its application in a cementitious system to obtain sufficient concrete properties is still rather limited. In a country like Tanzania, high strength concrete construction is applicable in special construction cases but certainly not a high priority. Majority of construction is still undertaken using normal strength concrete. In a ternary cementitious system consisting of OPC, RHA and other readily available resources such as limestone filler (LSF), normal strength concrete can still be produced having good performance and suitable for regular on-site construction. This paper explains a possible application of obtaining sufficient concrete properties from the available resources.
Selbstverdichtender Beton verhält sich unter Temperatureinfluss anders als Normalbeton, da die Rheologie neben der fortschreitenden Hydratation zusätzlich durch die von der Zeit und dem Hydratationsfortschritt abhängige Adsorption von Fließmitteln beeinflusst wird. Anhand rheometrischer Betonversuche an SVB unterschiedlicher Entwurfskonzepte mit variierter anionischer Ladungsdichte im Fließmittel wird verdeutlicht, dass mehlkornreiche SVB bei niedrigen Temperaturen sehr robust sind, während bei hohen Temperaturen mehlkornärmere Entwürfe zu bevorzugen sind. Darüber hinaus wird gezeigt, wie sich unterschiedliche Fließmittelmodifikationen in bestimmten Temperaturbereichen verhalten. Anhand des Wasser-Feststoff-Verhältnisses und des Adsorptionsverhaltens von Fließmitteln werden die maßgeblichen Prozesse erläutert und Möglichkeiten für die Entwicklung robuster Mischungen für individuelle Temperaturbereiche aufgeführt.--------------------------------------------------------------------------------------------------------------------------------------------------------------
Regarding the temperature dependent performance, self-compacting concrete (SCC) distinguishes from normal concrete, since its rheology does not only depend upon the hydration itself but supplementary upon the adsorption of superplasticizers, which is affected by the time and the hydration progress. Based on rheometric concrete investigations with different SCC mixture compositions and varied anionic charge densities of the superplasticizers, it is shown that SCC, which is rich in powder components, shows robust performance at low temperatures, while compositions with lower powder contents are favourable at high temperatures. Furthermore, the performance of different superplasticizer modifications at different temperature ranges is demonstrated. The relevant processes are explained by means of the water to powder ratio as well as the adsorption behaviour of superplasticizers, and options for the development of robust mixture compositions for individual temperature ranges are itemised.
The paper addresses numerical modelling of fresh self-compacting concrete. In the previous studies, a numerical tool for casting prediction is built based on the assumption that fresh concrete behaves as a yield-stress fluid and treating zones with rebars as porous media. The present contribution discusses the determination of material parameters, which are required as an input for numerical simulations of casting processes.
The study introduces the porous medium model for the simulation of concrete flow through highly-reinforced sections. It shows that numerical simulations can predict concrete behavior during casting and help to avoid expensive mistakes.
The possibility to tailor the rheology of concrete eventually opened up the path to new technologies, where design criteria are no longer limited to the compressive strength. Thus, it can be concluded that the capability to control the rheology of concrete can be considered as catalyst for many of today’s concrete innovations. In the same way rheology modifying admixtures will be key to mastering the challenges of the next decades. In many regions of the Southern hemisphere, the effective use of chemical admixtures would significantly contribute to solve problems induced by the challenging climate, but particularly in Africa there is often a lack of local supply and supply infrastructure. In Africa, concrete admixtures are largely shipped or transported from outside the continent. The long distance transportation of chemicals is not very environmentally friendly and the economic consequences are dramatic: the agents are difficult to purchase, expensive, and there is no large variety of products available in the market. Due to the enormous relevance of rheology modifying admixtures, it is worthwhile to recollect that nature provides an enormous variety of chemicals that can be used readily or with low processing and without enormous transportation distances, since these plants are typically regionally abundantly available, cheap, and they are environmental friendly. The paper presents an overview of various options for rheology modifying admixtures, that can be found in Africa, and that may become a real alternative, once their interaction within the complex cementitious system is well understood.
Selbstverdichtender Beton (SVB) findet kaum Anwendung als Transportbeton. Ein wesentlicher Grund hierfür ist, dass die Außentemperaturen auf der Baustelle nicht gesteuert werden können. Während des Betonierens können allerdings Schwankungen in der Umgebungstemperatur die rheologischen Eigenschaften aber auch das Erstarrungsverhalten signifikant beeinflussen, wodurch in der Folge auch Veränderungen im Festbeton auftreten können. Durch geeignete Wahl der Komponenten kann die Robustheit von SVB gegenüber Einflüssen aus den Umgebungsbedingungen deutlich verbessert werden.
In der vorgestellten Arbeit werden anhand von Untersuchungen am Betonrheometer, am Setzfließmaß und am Erstarren nach Vicat die Einflüsse von veränderlichen Umgebungstemperaturen auf die frühen Eigenschaften von SVB dargestellt. Der Fokus liegt auf Effekten, die durch die Modifikation von Polycarboxylatether-Fließmitteln und durch Stabilisierer eingetragen werden. Darüber hinaus wird der Einfluss des Mischungsentwurfes gezeigt. Die Ergebnisse werden mit Relevanz für praktische Betonanwendungen analysiert und es werden die wichtigen Einflussgrößen zur Verbesserung der Robustheit von SVB gegenüber Temperaturschwankungen herausgearbeitet.
The large scale application of self-cooling concrete pavers is a future-oriented approach to reduce the urban heat island effect in inner-city areas. Making use of evaporation enthalpy, these new pavements counterbalance the absorption of solar radiation and the subsequent transfer of heat to the surrounding environment. The concrete pavers consist of a permeable top-layer and a storage layer. As the different requirements of these layers demand different concrete mixtures they are developed and optimised for their respective functions. This paper presents some results of the concrete mixture design and Solutions that combine a sufficient compressive strength as well as good water transportation properties for the face concrete layer.
This study focuses on concrete flow in presence of obstacles and develops a mathematical model and a computational approach for SCC flow through reinforced formworks. In order to decrease high computational times needed to simulate castings through reinforced elements, an innovative approach to model the reinforced sections as porous media is proposed here. In the previous work, this numerical model is proved able to simulate the free-surface flow of non-Newtonian fluids through the reinforcement networks. In the present study, the applicability of the model on the concrete flow will finally be proved. The large-scale form-filling experiments with SCC will be conducted and the experiments will be simulated using the proposed numerical model. The numerical model will then be validated through the comparison of the experimental results and the results of the numerical simulations.