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African cement inffastructure is quite complex. Apart from Northern Africa and South Africa in particular, cement plants are scarce resulting in highly unstable cement pricing. Clinker and cement are imported from overseas, e.g. from Portugal, Turkey, Pakistan, Indonesia, and China. Imports are typically determined by the lowest price, and as a result the countries of origin of products vaiy regularly yielding large scatter of properties. Quality control and a good quality infrastructure are thus of utmost importance for the safety of the popuiace, an issue, which is actually often neglected. With funding of the German Metrology Institute (PTB) and Support of the SPIN project, a proficiency testing scheme for cement testing according to EN 196 was set up for African laboratories. Proficiency testing schemes, also called round robins. are interlaboratory performance comparisons allowing participants to evaluate themselves against pre-established criteria. Thev are a powerful tool to help laboratories improve their performance as well as demonstrate their competences to accreditation bodies or Customers. 26 laboratories from 20 nations, 18 of which from Africa, participated. The BAM Federal Institute for Materials Research and Testing acted as coordinator and provider of the scheme. The aim of the round robin was to interpret the submitted data further beyond the pure statistic analyses. The data provided a positive picture of the performance of the participants in general, but it also exhibited a number of technical fields that need improvement. The paper provides the general results of the scheme and analyses identified strengths and weak points based on the submitted and non submitted data as well as on discrepancies from the EN 196 procedures during measurements. The application of EN Standards for material testing is critically discussed and since quality infrastructure is also always an issue between industrial and political stakeholders, suggestions for the mitigation of the identified shared problems are given.
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.
Die Charakterisierung von Oberflächen mittels optischer Technologien wird dank des ständigen Fortschritts bei der Entwicklung von Sensoren und Lichtquellen immer attraktiver. Die kommerziell erhältlichen Systeme werden dabei immer leistungsstarker, empfindlicher und genauer. Mit diesen Systemen ist es möglich, viele hundert Messpunkte innerhalb weniger Sekunden zu erfassen und auszuwerten. Dieser Fortschritt führt auch im Baubereich zu einer ständigen Weiterentwicklung und Verbesserung von Messmethoden und Systemen. So werden bereits standardisierte und angewandte Messprinzipien weiterentwickelt, den neuen Möglichkeiten angepasst und deren Anwendung auf neue Bereiche ausgeweitet. An der BAM ist es gelungen ein Messsystem zu entwickeln, mit dem Rautiefen von Bauteiloberflächen in Anlehnung an bereits standardisierte Verfahren bestimmt werden können. Mit dem Mer vorgestellten System können sowohl horizontale als auch vertikale, gewölbte und geneigte Oberflächen untersucht werden. Dank der Implementierung geeigneter Algorithmen entsprechen die vom System ausgegebenen Ergebnisse denen der seit Jahrzehnten mit den volumetrischen Messverfahren erzielten Ergebnisse. Vor allem bei der Instandsetzung von Bauwerken bringt das an der BAM entwickelte System erhebliche Vorteile, da es zukünftig auch möglich sein wird Wände und Decken genauer zu untersuchen.
African cement infrastructure is quite complex. Apart from Northern Africa and South Africa in particular, cement plants are scarce resulting in highly unstable cement pricing. Clinker and cement are imported from overseas, e.g. from Portugal, Turkey, Pakistan, Indonesia, and China. Imports are typically determined by the lowest price, and as a result the countries of origin of products vary regularly yielding large scatter of properties. Quality control and a good quality infrastructure are thus of utmost importance for the safety of the populace, an issue, which is actually often neglected. With funding of the German Metrology Institute (PTB) and support of the SPIN project, a proficiency testing scheme for cement testing according to EN 196 was set up for African laboratories. Proficiency testing schemes, also called round robins, are inter-laboratory performance comparisons allowing participants to evaluate themselves against pre-established criteria. They are a powerful tool to help laboratories improve their performance as well as demonstrate their competences to accreditation bodies or customers. 26 laboratories from 20 nations, 18 of which from Africa, participated. The BAM Federal Institute for Materials Research and Testing acted as coordinator and provider of the scheme. The aim of the round robin was to interpret the submitted data further beyond the pure statistic analyses. The data provided a positive picture of the performance of the participants in general, but it also exhibited a number of technical fields that need improvement. The paper provides the general results of the scheme and analyses identified strengths and weak points based on the submitted and non submitted data as well as on discrepancies from the EN 196 procedures during measurements. The application of EN standards for material testing is critically discussed and since quality infrastructure is also always an issue between industrial and political stakeholders, suggestions for the mitigation of the identified shared problems are given.
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.
Worldwide an increasing migration from rural to urban regions can be observed. Hence cities are growing and as a result the building density and the land sealing rise. Concrete as commonly used building material in urban structures provides a high heat storage capacity. Therefore the microclimate in cities has become warmer than in the surrounding areas. This phenomenon is called Urban Heat Island Effect. To mitigate this situation a large scale application of self-cooling concrete pavers is an approach to reduce the urban heat island effect. Making use of evaporation enthalpy, this new type of pavements counterbalances the absorption of solar radiation and the subsequent transfer of heat to the surrounding environment. The typical double-layer structure of concrete paving stones can be maintained. The mass concrete acts as a water storage layer and is covered by a permeable face concrete. As the different requirements of these layers demand different concrete mixtures, they are developed and optimised for their respective functions. This paper presents some suitable no-slump concrete mixtures that combine a sufficient compressive strength as well as good water transportation properties for the above mentioned approach.
Todays superplasticizers for self-compacting concrete and high Performance cementitious materials are very versatile. Typically superplasticizers are composed of a polycarboxylic backbone equipped with polyethylene oxide graft chains of variable length and grafting degree. The mode of operation of these admixtures is based on the steric repulsion of the particles upon adsorption. The adsorption is strongly depending upon the charge density of a superplasticizer and the time dependent Adsorption processes control the retention of the flow performance – an important issue particularly for ready-mix and construction site concrete. However, the properties of polycarboxylic superplasticizers typically remain a black box for users. The paper suggests a simple and rapid test method, which can be conducted without sophisticated equipment, to qualitatively distinguish between two types of superplasticizers based on the adsorption mechanism. Interpreting the results in the right way provides a powerful tool to choose the right admixture for individual time dependent flow specifications.
A silica residue from waste treatment of chlorosilane production was used together with solid sodium aluminate to test its applicability for the production of one-part geopolymers. The blend was activated with water and cured at 70 °C. The degree of reaction and strength were determined after 1, 3, and 7 days. The reaction products were analyzed by XRD and SEM/EDX. Until the third day of curing the degree of reaction of the residue reached 51 % and the strength was 8.9 MPa. The reaction product was identified as geopolymer containing zeolite A. Thus, the results confirmed that the residue may be used in the production of geopolymers. However, after 3 days of curing no further progress of reaction was observed and the strength slightly decreased, which was attributed to changes in the structure of the geopolymeric gel. It was further observed that even harsh vacuum drying left some water (presumably zeolitic water and surface hydroxyl groups) in the geopolymer.
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.
Weltweit besteht einen hoher, stetig wachsender Bedarf an Natursteininstandsetzung im Bereich des denkmalgeschützten Kulturguts. Das Ausbessem beschädigter Elemente mit sogenannten Steinersatzmassen ist dabei eine etablierte Technik. Hierfür werden Mörtel unterschiedlichster Zusammensetzung verwendet. Die Schwierigkeit für den Restaurator liegt in der Wahl, bzw. der Verfügbarkeit des optimalen Produkts, um im individuellen Fall sowohl den mechanischen und bauphysikalischen Anforderungen als auch den gestalterischen und denkmalpflegerischen Belangen gerecht zu werden. Anhand der im Beitrag an Fallbeispielen erläuterten häufig auftretenden Probleme und der daraus abgeleiteten Anforderungen an Natursteininstandsetzung, lässt sich die Komplexität dieser Aufgabe erkennen. An der BAM wird derzeit im Rahmen eines Forschungsprojekts ein modulares Steinersatzsystem entwickelt. Ziel der Entwicklung ist es dem Restaurator die größtmögliche Freiheit in der Anpassung des Mörtels an die Originalsubstanz zu geben. Erste Ergebnisse aus Parameterstudien zur Bindemittel- und Sandvariation sowie zu Polymermodifikationen werden vorgestellt.