Abstract. The paper deals with information concerning properties and technology of a new generation cementitious composite i.e. Ultra-High Performance Concrete. High performance here means both high strength and high durability under the influence of environmental factors. This group of composites is mainly represented by Reactive Powder Concretes (RPC), which show both outstanding durability and mechanical properties. Characteristic features of RPC are mainly due to the very low water-cement ratio, which involves application of superplasticizer, significant reduction of aggregate grains size as well as hydrothermal treatment. In the first part of the paper selected properties of RPC are compared to ordinary concrete and to other groups of new generation concrete. Moreover, fundamental technological factors influencing properties of RPC are described as well. The second part deals with the RPC developed at Cracow University of Technology. The presented test results are mainly focused on the influence of steel fibres content on mechanical properties of reactive powder concrete and hydrothermal treatment on composites microstructure. The quantitative and qualitative evaluation of this relationship expand the knowledge of the UHPC technology. Finally, the third part presents the most significant and newest structures which have been erected with the use of RPC.

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This paper presents the results of comprehensive cement paste porosity and gas permeability tests. The tests conducted concerned ordinary Portland cement (OPC) cement pastes with varying water-cement ratios ranging from 0.3 to 0.6. The tests were conducted after the curing of cement paste for 90 days and two years under laboratory conditions. Open porosity was determined using three methods: helium pycnometry, mercury intrusion porosimetry, and water saturation. Permeability was determined using a modified RILEM-Cembureau method. The results obtained demonstrated that permeability does not change significantly over time despite the observed material shifts in open porosity characteristics caused both by further progress in hydration and by the carbonation process that occurs. The results of the tests conducted also permitted the quantitative determination of the impact of the water-cement ratio, age, and the progress of carbonation on open porosity measured using different methods and also on the gas permeability of the pastes.

Abstract. In this paper, an experimental study on the spalling behaviour and mechanical properties of Reactive Powder Concretes (RPCs) in high temperature are presented. The research program was established to evaluate the impact of low melting temperature polypropylene fibres PP on mechanical properties evolution with temperature but also to verify the effectiveness of their addition to prevent spalling. Three sets of RPC specimens were prepared for this study with different amount of PP fibres (no fibres, 1.0 kg/m 3 and 2.0 kg/m 3 ). The addition of PP fibres reduces the initial compressive strength of the RPC material by approx. 14% no significant influence on modulus of elasticity was observed. Addition of 1 kg/m 3 of PP fibres in RPC, seem not to give a sufficient protection against occurrence of spalling phenomenon. By adding 2 kg/m 3 of PP fibres the risk of spalling is significantly reduced.

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