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

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

Carbonation of filler type self-compacting concrete

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