Vacuum mixing technology to improve the mechanical properties of ultra-high performance concrete

Ultra-high performance concrete is an important evolution in concrete technology, enabled by the combination of a good particle packing density, a suitable mixing procedure and compatible binders and admixtures. In the last decades a lot of research has been performed to explore the boundaries of this new type of concrete. Mixers equipped with a vacuum pump able to lower the mixing pressure from 1,013 to 50 mbar are an interesting way to improve the performance by lowering the air content. Profound research is necessary, because little is known about this technique of air content reduction. The influence of a reduced air content on the mechanical properties of ultra-high performance concrete is tested at The Magnel Laboratory for Concrete Research. This paper reports the results of the compressive strength, the splitting and bending tensile strength and the modulus of elasticity. All the mechanical properties after 28 days curing are improved by reducing the air content in the ultra-high performance concrete. An increase in compressive strength between 7 and 22 % is measured. The bending tensile strength increases maximum with 17 % and the splitting tensile strength gains 3-22 % in performance. Furthermore, the modulus of elasticity improves with 3-8 %. In conclusion, the air content can be controlled and a higher performance can be achieved by vacuum mixing technology. Finally, it is shown that the vacuum technology is not as effective in a 75 l capacity vacuum mixer as it is for a smaller vacuum mixer with a capacity of 5 l

Thixotropic effects during large-scale concrete pump tests on site

During recent years, the fundamentals of pumping fresh concrete have been intensively studied worldwide. New insights have been gained concerning the important role of the lubrication layer near the surface pipe. The influence of rheological properties of the fresh concrete, like yield stress, viscosity, and shear thickening behavior, is now well understood, and can be fairly accurately modelled. One major challenge is found in the potential effect of thixotropy while pumping fresh concrete, especially in case the pumping operation is temporarily paused and resumed after a while. Due to thixotropic effects, restarting the pumping operation can be a very challenging task. This paper reports on a large-scale concrete pump test on a construction site, specifically focusing on the risk of thixotropy. Fresh concrete has been pumped in a horizontal closed-loop pumping pipe with a total length of 600 meter. In steady-state pumping conditions, the pumping operation was very successful, and in agreement with the expectations. However, the thixotropic effects, occurring during a short stop of the pumping operations, showed to provoke major problems while trying to resume pumping. The lessons-learnt helped to define a successful pumping procedure for this major construction site

Damage to Concrete Structures

Serious degradation mechanisms can severely reduce the service life of concrete structures: steel reinforcement can corrode, cement matrix can be attacked, and even aggregates can show detrimental processes. Therefore, it is important to understand how damage can occur to concrete structures and to appreciate the timing of the actions leading to damage. Damage to Concrete Structures summarizes the state-of-the-art information on the degradation of concrete structures, and gives a clear and comprehensive overview of what can go wrong. Offering a logical flow, the chapters are ordered according to the chronological timing of the actions leading to concrete damage. The author explains the different actions or mechanisms in a fundamental manner, without too many physical or chemical details, to provide greater clarity and readability. The book describes the different causes of damage to concrete, including inappropriate design, errors during execution, mechanisms occurring during hardening of concrete, and actions or degradation mechanisms during service life (hardened concrete). The degradation mechanisms are illustrated with numerous real-world examples and many drawings and photographs taken of actual structures. Written as a textbook for students as well as a reference for professionals, this easy-to-comprehend book gives readers a deeper understanding of the damage that can occur to concrete during the construction process and service

Smart casting of concrete structures by active rheology and stiffening control (ERC advanced grant project)

Concrete production processes do not take fiill advantage of the rheological potential of ffesh cementitious materials, and are still largely labour-driven and sensitive to the human factor. The recently started ERC Advanced Grant project 'SmartCast' proposes a new concrete casting concept to transform the concrete industry into a higlily automated technological industry. Currently, the rheological properties of the concrete are defined by mix design and mixing procedure without any further active adjustment during casting. The goal of the 'SmartCast' project is the active control of concrete rheology during casting, and the active triggering of early stiffening of the concrete as soon as it is put in place. The ground-breaking idea to achieve this goal, is to develop concrete with actively controllable rheology by adding admixtures responsive to extemally activated electromagnetic frequencies. Inter-disciplinary insights are important to achieve these goals, including inputs from concrete technology, polymer Science, electrochemistry, rheology and computational fluid dynamics. In the short term, achieving the active control of the pumping slip layer will have an immediate impact on concrete industry, as this can be applied on pump trucks without interfering with the clements to be cast. In the longer term, making possible concrete casting with active control of flow and stiffening will be a totally new paradigm for concrete industry. Moving from ‘passively’ relying on evolving properties of ffesh concrete, to ‘actively’ controlling rheology and stiffening will revolutionize concrete industry and bring quality levels to higher standards. The developed active rheology control will also provide a fundamental basis for the development of future-proof 3D printing techniques in concrete industry. For society, it will mean more reliable construction, with less damage cases and less failures, while better preserving the environment (reduced carbon footprint, reduced noise and vibration levels, reduced exposure of technicians to safety and health risks)

The comparison between sulfate salt weathering of portland cement paste and calcium sulfoaluminate cement paste

In this paper, the damage performances of sulfate salt weathering of Portland cement paste and calcium sulfoaluminate (CSA) cement paste were compared according to authors' previous studies. It was found that the evaporation zone of speciments partially immersed in 10% Na2SO4 solution were both severely deteriorated for Portland cement and CSA cement. However, the differences were more significant: (1) the CSA cement paste were damaged just after 7 days exposure compared to the 5 months exposure of Portland cement paste under the same exposure condition of RH 60% and 20°C; (2) the cement paste specimen was split into several pieces along the shrinkage cracks, and the damaged CSA cement paste consisted of a detachment of successive paste layers; (3) gypsum and ettringite were identified in the Portland cement paste and attributed to the paste failure mechanism, however sodium sulfate crystals were clearly observed in the detached paste layers. According to the comparison the so-called sulfate weathering of Portland cement concrete was discussed

Self-desiccation and self-desiccation shrinkage of silica fume-cement pastes

Self-desiccation is one common phenomenon of high-performance cementitious materials, which are characterized by low water/binder (w/b) ratio and high mineral admixture incorporation. As a consequence, large magnitude of self-desiccation shrinkage, a key factor which influences the cracking behavior of concrete, develops rapidly in the cement matrix due to the internal relative humidity (RH) decrease and capillary pressure induced by self-desiccation. The objective of this study is to evaluate the behavior of self-desiccation and self-desiccation shrinkage in silica fume (SF) blended cement pasts with low w/b ratio of 0.25. The self-desiccation process was revealed by the measurement of internal RH of the sealed cement pastes with conventional method of hygrometer. The shrinkage of the sealed cement pastes was measured by the corrugated tube method, permitting measurements to start at early age. Experimental results revealed that SF blending leads to a higher internal RH, indicating slower self-desiccation process, compared with pure cement paste. Consequently, less self-desiccation shrinkage was observed in SF blended cement pastes than that in pure cement paste