Iveta Nováková

High Strength Concrete with Enhanced Properties by Addition of Chopped Basalt Fibres

Use of high strength concrete (HSC) becoming inevitable in case of modern structures, which require application of advanced materials. High strength of concrete can be reached by higher dose of cement or addition of various kinds of fibres as a dispersed reinforcement. In practice are most commonly used steel, glass and polypropylene fibres, but basalt fibres are still under investigation. Basalt is highly resistance to aggressive chemicals and stable in high temperatures in comparison to other materials used for dispersed reinforcement. This study focuses on use of chopped basalt fibres in purpose to enhance properties of harden concrete. Six mixes with various dose of cement and chopped basalt fibres were designed for evaluation of properties of fresh and hardened concrete. As it was expected, chopped basalt fibres have negative influence on workability of fresh concrete but in comparison to other types of fibres the decrease is not that dramatic. Test results indicate increase of compressive strength after 28 days and even higher gain of strength after 90 days in case of mixes with addition of chopped basalt fibres. Reduction of strength due to exposure to 400 and 800 °C is also evaluated and in case of 400 °C temperature exposure the results are very promising.

Quality Evaluation System for Description of Recycled Concrete Aggregates and Verification of its Functionality

Article presents quality evaluation system for description of recycled concrete aggregates (RCA), verification of RCA properties and subsequent application in to concrete mixtures as a partial or total replacement of natural aggregates (NA). Modernization and rehabilitation of constructions is accompanied by creation of demolition waste from old buildings and structures. The necessity of recycling is unavoidable, because volume of construction and demolition waste (C&DW) is increasing and the landfills are reaching their maximum capacity. Nowadays, there are numerous research teams focused on analysis of characteristics and application of RCA into new concrete as a replacement of NA. Test samples of RCA have always different source, grain composition and other physical and mechanical properties, which are variously described in each different paper. Up to now, there is no any uniform quality evaluation system for description of recycled concrete aggregates, which can easily describe their source and assumed properties. Our aim is to set up rules for description of RCA and simplify the evaluation of properties of various RCA. Qualification system will be applied on three different samples of RCA and verified by the selected properties tests. Tested samples of RCA will be than used as a replacement of natural aggregates in concrete mixtures. The replacement amount was set up on 20%, 40% and 100% according to the reviewed literature, to have a comparable replacement amount and valuable results for discussion. The results of RCA testing and testing of concrete with partial and total replacement of NA showed that the evaluation system is working properly. It can be concluded, that accuracy of the quality evaluation system for description of recycled concrete aggregates is sufficient, but more tests on RCA should be done to prove all connections in between description of RCA and their properties.

Characteristics of recycled concrete aggregates from precast slab block buildings

Precast slab block buildings (PSBB) typically and frequently occur in Central and Eastern Europe, as well as elsewhere in the world. Some of these buildings are currently used beyond their service life capacity. The utilization of recycled materials from these buildings with regard to applying the principles of sustainable construction and using recycled materials will probably be significant in the following years. Documentation from the manufacturing processes of prefabricated blocks for precast slab block buildings is not available, and also it is difficult to declare technological discipline during the construction of these buildings. Therefore, properties of recycled concrete aggregates (RCA) produced from construction and demolition waste (C&DW) of precast slab block buildings build between 1950s to 1990s are not sufficiently known. The demolition of these buildings is very rare today, but it can be assumed an increase in demolitions of these buildings in the future. The use of RCA in new concrete requires verification/testing of the geometrical and physical properties of RCA according to the EN 12 620+A1 standard. The aim of the contribution is to present a case study of the demolition of slab block building with emphasis on RCA usage. The paper presents the results of the tests according to European standards for determining selected geometrical and physical properties of the RCA. The paper describes and evaluates tests such as determination of particle size distribution - Sieve Analysis, content of fine particles, determination of density and water absorption. The results of the properties testing of RCA are compared with the properties of natural aggregate. The general boundary conditions of RCA particular tests are presented.

Possibilities of Recycled Concrete Aggregate in Cement-Based Flowscreed

This paper presents some test-results of using fine recycled concrete as a partial replacement of natural aggregates in cement-based flowscreed. Special attention is given to describing and studying rheological characteristics and mechanical properties of cement-based flowscreed. Experimental results indicate that it is possible to prepare flowscreed with partial replacement of natural aggregates by recycled aggregates and obtain a quality that meets the requirements. This study was partially connected to the research-project RecyFlowscreed [1].