Hans-Carsten Kühne

Segregation of Coarse Aggregates in Self-Compacting Concrete

A major requirement on self-compacting concrete (SCC) is the resistance to any kind of separation. In the presented studies the segregation behavior of aggregates was analyzed systematically. The rheology of selfcompacting mortars was dealt with at first. Following, the segregation of particles of different size, shape and density in various mortars was examined. The aim was to find an analytical relation to estimate the risk of sedimentation, using the characteristics of the particles and those of the mortars. The classification of the aggregates according to their potential segregation is rather simple. But the description of the segregation resistance of the mortar is much more difficult. It is not possible to evaluate a mortar solely based on the rheological properties. The mixture composition must always be considered as well. Tests on concrete samples were conducted additionally to determine the influence of the overall system of SCC on the sedimentation of the aggregates.

Effects of Superplasticizer and Viscosity-Modifying Agent on Fresh Concrete Performance of SCC at Varied Ambient Temperatures

Varying ambient temperatures in plants or on construction sites during casting of SCC can cause serious problems affecting the fresh concrete performances, such as rheological properties and setting time, with consequences for the hardened properties in the structure. By sensible choices of the components, the robustness of SCC mixtures against temperature variations can be improved. However, this aspect has not been the focus of intensive research yet. In this study the effects of varied ambient temperatures on the early performance of differently composed SCC mixtures are investigated, with emphasis on changes in rheology, early deformations, heat evolution and setting. Focus is placed on the study of the influence of the polycarboxylate ether molecule type as well as different viscositymodifying agents on the temperature robustness. Effects on the relevance for practical concrete applications are evaluated, which will provide a reliable framework of possible actions on the appropriate use of admixtures for SCC.

Outdoor performance tests of self-cooling concrete paving stones for the mitigation of urban heat island effect

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.

Computational Modeling of SCC Flow through Reinforced Sections

Computational modeling of fresh SCC flow is a comprehensive and time consuming task. The computational time is additionally increased when simulating casting of reinforced sections, where each single reinforcement bar has to be modeled. In order to deal with this issue and to decrease the computational time, an innovative approach of treating a reinforcement network as a porous medium is applied. This contribution presents the model for concrete flow through reinforced sections, based on Computational Fluid Dynamics (CFD), coupling a single-phase flow model for SCC and a continuum macroscopic model for porous medium. In the last part of this paper, numerical simulations are compared with experimental results obtained on model fluids.

Materials and technology solutions to tackle the challenges in daily concrete construction for housing and infrastructure in sub-Saharan Africa

Sub-Saharan Africas economic rise has caused a high demand for housing and infrastructure. This rapid growth has resulted in urgent challenges of enormous dimensions for urban and infrastructural planners and the entire construction sector. Considering the African supply chains and resources, cement and concrete are doubtless the most important materials that will support mastering the challenges and building the future in a sustainable way. Despite the promising perspectives, the social, economic and geographic boundary framework in most countries of sub-Saharan Africa exhibits a high number of peculiarities. Disadvantageous parameters during the concreting process, which negatively affect the quality and durability of concrete structures, are discussed with regard to implications for the safety and lifetime of structures. However, most sub-Saharan African countries feature a high potential for innovation and new, individual ways. Based on this consideration, non-standard concepts are discussed, namely how to bring about robust and well workable concrete reliably into practice, cost-efficiently and as ready-to-use pre-mixed dry mortar compound under consideration of local materials such as natural pozzolans, cassava starch and lignosulphonate. The development and performance of such a pre-mixed mortar compound is briefly demonstrated. These implemented in daily concrete practice could significantly contribute to improved durability of construction for the relevant sectors of housing and infrastructure.

Critical factors affecting the capacity of cylindrical grouted connections in offshore energy structures

Current trend suggests that global energy consumption will increase in the future. This growing energy demand and advancement of technology lead to explore all potential offshore fossil and non-fossil energy sources, necessitating erection of exploration and production structures, rigs, platforms and towers, which are susceptible to adverse environmental conditions along with their maintenances. Cylindrical grouted joints provide suitable connections between steel substructure and foundation in these offshore platforms and wind structures especially monopiles for ease of installation. However, these are composite connections with exterior sleeve, interior pile and infill grout. The capacity of these connections is affected by number of factors. The literature over last four decades by numerous researchers has shown the development of these connections with increasingly higher capacities and influences on these capacities due to various factors. This paper provides a comprehensive review on the factors affecting the connection capacity along with technical challenges for the future. Critical aspects and shortcomings of the current connection systems and potential solutions may be sought after for these issues are also discussed.