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.