Abstract: Creep of concrete is a phenomenon that is not only significant in the long term, but also at young concrete. The paper deals with methods considering the tensile creep of concrete due to shrinkage. It was proved, that the tensile creep can be taken into account by simplified methods called Age-adjusted Effective Modulus Method with the ageing coefficient χ value falling between 0.4 and 0.45. Calculated values were experimentally verified by means of Ring-test.
Abstract: Design, preparation and testing of fibre-cement composites are a task of wide range of research workplaces and universities in our country and abroad. However, a question on homogeneity of all cement matrix components and mainly optimal dispersion of fibre reinforcement in a mixture has not been yet solved sufficiently. Within testing of properties of such designed composites there is usually a realized fact, that variability of these properties is distinctive mainly for the reason of uneven dispersion of commonly used fibres in a whole matrix volume. Elimination of this phenomenon could be achieved by means of a design of the optimal homogenization process of dry mixture components with fibre reinforcement. The aim of research works was to find suitable homogenization techniques, design of mixing process and optimal dosing of individual components. By means of these actions it is possible to achieve the best dispersion of selected fibre types, both metal and non-metal, in fine-grained cementitious matrixes, which is subsequently verified in hardened composites at first by non-destructive and then by destructive methods. Four different fibre-cement mixtures were chosen as representatives for commonly used reinforcement in fibre-cement composites and test specimens with a thickness of 40 mm were prepared using processes suitable for the specific fibre reinforcement. At the first stage non-destructive testing by means of ultrasound waves was carried out at first on a compact test slab with dimensions 500 x 500 x 40 mm and subsequently on individual test specimens with dimensions 250 x 40 x 40 mm, cut from the test slab according to a designed pattern. At the second stage destructive testing of test specimens was performed, mainly evaluation of flexural strength with 4-point bending and subsequently preparation of thin sections from the failure area for observation by means of polarizing microscopy. A purpose of all these research works is finding of correlation between testing by means of destructive and non-destructive methods.
Abstract: In cases when two concrete parts are cast against in different times are not connected by dowels, main contributors to the resistance are cohesion and friction. Shear resistance of the interface is highly dependent on surface treatment and its roughness. In this paper, besides the review of available methods of surface roughness determination, the optical profilometry will be introduced and described. Optical profilometry represents non-contact and non-destructive method for characterizing surface topography. Furthermore, results obtained by abovementioned method will be compared with Sand Patch Test, in order to determine its usability and limitations.
Abstract: This paper deals with determining the dependence of the value of the static modulus of elasticity of concrete in compression on the number of loading cycles. The deformation of specimens during multiple cyclic loading was measured in the elastic region of the stress-strain curve for concrete. The specimens were subjected to up to 1500 loading cycles. The main goal of the experiment was to ascertain whether the multiple cyclic loading causes significant changes in the static modulus of elasticity.
Abstract: This report deals with the description of development of waterproof steel fibre reinforced concrete (suitable for concrete lining of tunnel constructions in particular) in cooperation with the concrete supplier. With regard to newly released documents, two formulas of waterproof concrete supplemented with steel and polypropylene fibres were suggested. The formulas were designed according to different approaches (standards / Austrian Directive), and they differ only in various weight doses of the individual components. With regard to the general requirements for the tunnel lining (fire resistance tests, chemical stability, strength, impermeability and adhesion), strength and plastic properties, durability parameters (depth of penetration of water under pressure, depth of penetration of standing water, frost resistance, resistance of cement concrete surface to water and defrosting chemicals, depth of penetration of chloride) and the resistance to extreme thermal stress were determined in both the formulas. Standardized tests and non-standard testing have been completed.
Abstract: Hybrid cements represent a relatively new type of binders which combines some of the advantages of Ordinary Portland Cement and an application of mineral admixtures and alkali activation. Hybrid cements form then blends containing a low portion of OPC and a high proportion of mineral additions (such as blast furnace flag, fly ash, metakaoline, ...). This paper discusses the properties of mortars based on carbonate based activators. Mixtures composed from Ordinary Portland Cement, ground granulated blast furnace slag, fly ash and mechanically pre-activated fly ash were optimised with the target to achieve sufficient compressive strength. The influence of ratios between powder compounds, the dosage of activator and water to binder ratios are presented.
Abstract: The paper deals with the determination of the modulus of elasticity in tension for cementitious composites and comparing these values with the values of modulus in compression. It describes several methods, which are usually used for determination of modulus of elasticity of concrete and fibre reinforced concrete. In the experimental program modulus of elasticity in compression and tension of various types of concrete and fibre reinforced concrete were compared. The classic test with prismatic specimens was used for determination of the modulus in compression; a new arrangement of uniaxial tension test of cementitious composites was used for determination of the modulus of elasticity in tension.
Abstract: This article deals with the ways of mixing method for concrete with extremely low water-cement ratio (w/c) typical for ultra-high performance concretes. For this purpose, the mixture with w/c ratio of 0.2 and amount 6% superplasticizers of cement weight was used. In total 6 various mixing methods was tested, which differed in application of superplasticizers, mixing order or mixing length. Workability, electricity consumption during mixing and compressive and flexural strength after 28 days were examined. The performed experiments showed that workability can be increased by, for example, separate doses of superplasticizers and water or other mixing methods. Strength of concrete was identical for all produced mixtures, even though the total mixing time was shortened in two cases.
Abstract: The paper presents some results of experimental program focused on drying and shrinkage of large concrete specimens. Segments of walls with thicknesses 200, 400 and 800 mm and standard cylinders 150x300 mm were used as specimens. Each segment has embedded 4 vibrating wire strain gauges in axis plane for measurements of shrinkage strain and plastic tubes of various lengths for measurements of pore relative humidity in different depths. Relative humidity and temperature of ambient environment were not controlled, however they were recorded very closely. Measure shrinkage strains are compared with prediction based on shrinkage models. The most important predictive models are used for comparison: Model Code 2010, Eurokód 2, Model ACI 209-R92, Model B4 a Model B4s.