24th Concrete Days 2017

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Authors: Ondřej Januš, František Girgle, Vojtěch Kostiha, Petr Štěpánek
Abstract: This article deals with the bond of internal composite (FRP) reinforcement with different surface treatments. Bars with additional sand-coating exhibit different behavior due to different transmission of forces in relation to bars with ribbed surface. To compare the bond behavior of these types of reinforcement, an experimental program consisting of several sets of pull-out tests was designed. This test configuration was chosen primarily for simplicity, comparability with previously published results, and especially easy modification for assessment the effect of concrete cover thickness. The paper presents the results confirming the direct influence of the surface treatment and also the position of the reinforcement on the resulting bond behavior.
Authors: Jakub Řepka, Tomáš Vlach, Lenka Laiblová, Tereza Pavlů, Michal Ženíšek, Petr Hájek
Abstract: This paper presents the simplified method of efficiency evaluation of surface protection products for concrete elements. This evaluation method was designed to consist of as few testing procedures as possible and to use as few specimens as well. Several testing processes most of which were based on the European technical standards were used to compare five commercially available products. Those comparisons served to assess whether the chosen method has sufficient informative value and does not require a disproportionate amount of time and money. The main examined properties were impact on color and reflectivity of the original surface, durability of the coating in extreme weather conditions, wettability of the surface and amount of labor needed for its maintenance. All of these experiments were optimized so that they could be performed on one type of specimen.
Authors: Petr Misák, Tomáš Vymazal, Dalibor Kocáb, Barbara Kucharczyková
Abstract: The aim of this paper is to summarize the results of 11 experiments which focused on assessing the precision of results of fresh concrete testing. The observed characteristics are slump test, flow table test, density and air content. More than twenty participants (testing laboratories) took part in these experiments. The test results were evaluated with statistical methods used for evaluations of interlaboratory comparisons. The article deals especially with the values of repeatability and reproducibility, which provide further understanding of the interval within which it is possible to expect with the given probability the test results measured in one or more laboratories.
Authors: Milan Holý, Lukáš Vráblík
Abstract: This paper deals with the connection of timber beams and precast concrete slabs. The connection of timber and concrete has many advantages associated with the efficient use of both materials, not only in terms of their stress. Timber is a natural renewable material. It can be achieved some savings of volume of the concrete by its application and thereby also reducing of the environmental burden. By the combining of the timber and ultra-high performance concrete (UHPC), it can be designed very subtle, bearable, aesthetic and durable structures. The conventional timber-concrete composite structures are most often realized by joining of the timber beams and the cast in-situ reinforced concrete slabs. However, the cast in-situ slab is not very suitable for UHPC application and it has some structural disadvantages, in particular the need to protect the timber beams against moisture penetration from the fresh concrete mix, the need for formwork, etc. The prefabrication eliminates some disadvantages of the cast in-situ design, increases the quality of the structure and speeds up the construction process. In the case of the timber-concrete composite structures, the prefabrication has a positive impact on the reduction of the concrete shrinkage projections as the development of deflections and the redistribution of internal forces between the connected parts of the cross-section. Some special coupling elements must be used for connection in the case of precast slabs. This paper summarizes the research findings in the field of development of special coupling elements for composite timber-precast concrete structures. The development of the new coupling elements for pedestrian and cyclist timber-UHPC composite footbridges is presented.
Authors: Dalibor Kocáb, Vlastimil Bílek Jr., Libor Topolář, Petr Daněk, Barbara Kucharczyková, Petr Pőssl
Abstract: This article deals with an experimental determination of the static modulus of elasticity in compression on fine-grained composites based on alkali-activated slag. This experiment included an alkali-activated composite without a shrinkage-reducing admixture and the same composite with a shrinkage-reducing admixture. The test specimens were subjected to testing of the dynamic modulus of elasticity using the ultrasonic pulse velocity test and the resonance method as well as of the static modulus of elasticity in compression. The static modulus of elasticity test was accompanied by the measurement of the acoustic activity of the material using the acoustic emission method, whose advantages is the possibility to detect early formation and propagation of cracks in the internal structure of the material. The output of the described experiment is a detailed evaluation of the differences in the behaviour of the tested alkali-activated composites based on the observed values of the modulus of elasticity and the recorded acoustic activity of the material during loading.
Authors: Bohdan Nešpor, Martin Nejedlík
Abstract: The paper presents the development results of "smart" pressure-sensitive fibre-cement compositematerials as well as thin-film coating sensors designed to detect disruptions of the base materialstructure. Basic material characteristics of fine-grained and coarse-grained cement matrices wereacquired during the research. The benefits and influence of conductive inorganic components,metallic components in the form of iron fillings and steel wires, and last but not least carbon-basednon-metallic fibrous reinforcement, have been verified. Thin-film epoxy resin coatings were enrichedwith amorphous carbon black, multi-walled carbon nanotubes (MWCNT) and natural micronizedgraphite. The article closely describes the measurement of electrical and electro-mechanical(piezoresistive) properties of conductive fibre-cement composites and thin-film organic coatings in anon-loaded state, during static loading and especially during dynamic ballistic and shock tests on thefall tower. Specific electrical characteristics and the course of change in electric conductivity wasexpressed as electrical resistivity (the real component of the impedance).The performed experiments confirmed excellent electrical conductivity of dense steel-fibrereinforced composites and graphite-doped hybrid fibre reinforced concrete. The coatings showedsignificant and permanent changes in impedance in the order of tens of ohms. The newly developedfibre composites and coating layers change the impedance during destructive and non-destructivedynamic loading tests. The impedance changed not only during failure of the matrix, but also in thecase of indirect impact. Moreover, carbon reinforced concrete with incorporated graphite showedsome piezoresistive properties. These detection materials were intended to be part of a ballistic-resistant monitoring system.
Authors: Tomáš Augustín, Ľudovít Fillo, Jaroslav Halvonik, Marián Marčiš
Abstract: Flat slabs are commonly used structures in contemporary architecture. Although their common use there is still problem in design of these structures. The openings adjacent to a column are often used for plumbing and such a position of the openings increases shear stresses in the flat slab near the column. This paper deals with experimental work focused on the punching shear resistance of the flat slab specimens with openings adjacent to column compared to the flat slab specimens without openings. The opening influence is determined experimentally and by using models for the assessment of punching resistance from relevant standards and codes. The material properties of concrete and reinforcing steel were obtained from the laboratory tests.
Authors: Hana Šimonová, Iva Rozsypalová, Pavla Rovnaníková, Petr Daněk, Zbyněk Keršner
Abstract: This paper presents the results of the differential thermal analysis of cement matrix samples taken from concrete panels with nominal dimensions of 2300 × 1300 × 150 mm after one-sided exposure to high temperatures. The panels were subjected to maximum nominal temperature loads of 550, 600, 800 and 1000 °C. Concrete was also taken from a reference panel (without temperature loading) and investigated. Five samples with a nominal thickness of 20 mm were taken for thermal analysis. They were cut from the central part of the panels using a diamond blade saw. The thermal analysis covered the effects of temperature load on the concrete to a depth of approximately 100 mm from the heated surface of the panel.
Authors: Ondrej Keseli, Juraj Bilčík, Ivan Hollý
Abstract: More restrictive requirements of new codes, increasing of imposed loads connected with usage change of the existing structures, errors in design process or during execution can lead to necessity of the strengthening of the existing buildings. In paper, the new and modern strengthening system of flat slabs against punching shear using screw anchors is presented. Screw anchors are installed from lower face of the slab into vertically drilled boreholes. After the installation, the screw anchors operate as punching shear reinforcement. The advantages of the system are significant resistance and deformation capacity increment of the flat slab as well as easy and fast installation with no need to drill through the whole slab thickness and access the upper face of the slab. System is suitable for strengthening of office buildings, since removal of the flooring is not needed or in parking garages, where the water resistance is not disturbed by the system. Moreover, paper deals with special problems that are connected with design of the presented strengthening system e.g. considering of load (deformation) level of the slab at the moment of strengthening, performance of the screw anchors in cracked and non-cracked concrete and anchorage conditions improvement of the screw anchors by application of glue or mortar into the borehole prior the screw installation.

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