24th Concrete Days 2017

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Authors: Munenobu Murasaka, Motoyuki Suzuki
Abstract: In this paper, made a PC beam specimen using ASR reactivity aggregate made in Chinese Zhejiang Ministry and made them do an accelerated curing and the influence by which swell of ASR reactivity aggregate gives it to a PC beam specimen was evaluate. Moreover the PC beam specimen which degraded by swell was classified every degradation grade, a loading test was put into effect and the load carrying capacity was inspected. As a result, I got the following conclusion. ①The thing which classifies the degradation grade into 3 stages (incubation period, acceleration period and degradation period) from the accelerated curing results of the test piece performed in advance. In the incubation period, it was confirmed that the compressive strength and the modulus of static elastic tend to increase more than before accelerated curing. ②Expansion of PC beam specimen was inflated from the upper edge part with less influence of prestress, Moreover as the expansion stabilized, the tendency of the lower edge portion which is highly affected by the prestress to expansion could be confirmed from the strain behavior by monitoring.In addition, with expansion of the lower edge part the prestress amount of the PC steel material placed inside the PC beam specimen (The lower edge position) is increasing and as the prestress amount stabilizes the tendency of expansion of the lower edge part is stabilized can also be confirmed. ③When accelerated curing was further promoted, the expansion of the upper edge and lower edge turned to a decreasing trend, and it was confirmed that stabilize tendency at about 25 to 35% of the maximum expansion. ④The P-δ curve up to fracture was measured for PC beam specimen before accelerated curing and ② to ③. As a result, it was not possible to confirm a large difference at the P-δ curve before accelerated curing and ②, but a significant decrease in load bearing capacity was confirmed at ④.
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Authors: Jan Nováček, Miloš Zich
Abstract: The paper deals with strengthening structures against punching shear with focus on strengthening of existing concrete foundation slabs. These can be slabs of basements of residential, administrative and other buildings. Also, foundations slabs placed on piles can be considered. This is very specific case of strengthening where only one surface of the structure is accessible. Strengthening is often designed using massive new bearing elements, which are space-demanding. Attention is paid to the development of technology that minimizes the resulting constraints on the surrounding area and is sufficiently effective. Inspiration is strengthening of ceiling flat slabs with additional shear reinforcement. Compared to the ceiling slabs, a larger load must be bear to the foundation structures and therefore the shear reinforcements demand larger dimensions. Mechanical expanded anchors are used for anchoring on the inaccessible side. A significant contribution of mechanical anchoring is demonstrated in the tests results which make possible more efficient design of additional punching reinforcement.
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Authors: Pavel Jursík, Jan L. Vítek, David Čítek
Abstract: Nowadays the extraordinary mechanical properties of UHPC can be used for real structures. The exceptionally high bond of reinforcement and UHPC together with significant shortening of lap length are the key features for the simplification of joining of precast elements. The properties of UHPC can be very well used also for providing of subsequent shear connection of composite structures. This paper is focused on analysis of performance of a combined joint where connection of precast elements as well as subsequent composite action between steel and concrete parts of steel-concrete composite structure are carried out in one compact detail. Instead of the traditional headed shear studs there were used local perforated puzzle sheets. More resilient connection of shear sheets to the top flange and a high resistance of UHPC allow for reduction of the extent of shear connectors. In experimental measurements carried out on six beams the behavior of shear connection of conventional monolithic concrete slab with continuous perforated shear sheet and shear connection of precast slab with the joint made of UHPC with local puzzle sheets were compared. During the experiment it has been proven that the performance of the beams with both variants of shear connection is very similar even with the significant reduction of the extent of shear connectors. Numerical analysis which describes the stressing in this detail confirms such behavior.
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Authors: Adam Svoboda, Ladislav Klusáček
Abstract: When strengthening existing reinforced concrete structures, we can find many successful applications of post-tensioning. The method of substituted cable ducts is a very convenient especially in the case of strengthening existing reinforced concrete bridges whose load capacity can be increased up to 300 %. Strengthening of structures by post-tensioning is based on active influence of internal forces by prestressing force and radial forces that occur in places of bending the unbonded prestressing tendons. Thus, the forces which acts directly on structure in the anchorage areas and in the locations where the prestressing tendons change their trajectory (in saddles) are considered as the effects of prestressing. Saddles are one of the most important structural details of the additional prestressing system. For the purpose of closer examination of the action of monostrand in steel saddles with different radii of curvature, the methodology of experiment was developed and the concrete test panel in which holes for sensors and substituted cable ducts for monostrand are prepared was created. The concrete test panel enables change of the radius of curvature and was designed for analysis of monostrands in saddles with radii of 600, 1000 and 1500 mm. The aim of short-term experiments with monostrands is to determine suitable application of post-tensioning of existing structures with prestressing tendons in HDPE sheaths in saddles with small radii of curvature.
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Authors: Vladimír Suchánek, Tomáš Bednarz, Tomáš Svojanovský
Abstract: This paper deals with an evaluation of long-term experimental work carried out in cooperation with concrete suppliers and a specific company (named Sobriety). The key part of this work is focused on the experimental determination of secant modulus of elasticity and Poisson's Ratio of special concrete (self-compacting concrete, steel fibre reinforced concrete, polymer fibre reinforced concrete, specific high-strength concrete). Two different real-time approaches were used to detect modulus of elasticity. In parallel, both approaches - the (European) standard approach and the DIC (2D DIC; 3D DIC with varying lengths in the vertical direction) - were applied.
154
Authors: Radek Vašátko, Jan L. Vítek
Abstract: Long-term deformations of the concrete are important for determination of behavior of concrete structures. For prediction of the long-term deformations rheological models are used. The models are usually derived from measurements on laboratory specimens. There are only few measurements performed on real structures. The objective of the realized experimental program is to compare predicted values of strains and strains measured on a real structure. Vibrating wire strain gauges have been installed in two columns in underground floors of eight-storey office building. Target of this paper is evaluation of measurement from the initial period (7 to 8 months). Measured values have been compared with results of first calculations and with values measured on laboratory specimens placed at the construction site.
160
Authors: Michal Ženíšek, Tomáš Vlach, Jakub Řepka, Tereza Pavlů
Abstract: The mixture of ultra-high performance concrete is often supplemented by steel fibres which can significantly improve the fracture energy of the concrete. The incorporation of steel fibres results in especially a significant increase of tensile strength and minimize the risk of brittle failure. However, these positive properties can be fully realised only if the fibres are uniformly distributed. Otherwise, that leads to different quality of the concrete and to deterioration of strength characteristics. The subject of this article is the segregation of steel fibres which is one of the frequent problems of poor distribution of fibres. The degree of segregation has been investigated depending on amount of fibre dosage, size of maximum aggregate size and degree of consistency of fresh concrete. Two types of steel fibres with circular cross-section of a diameter of 0.4 mm but with different lengths (12.5 and 25 mm) were used for experiments. The segregation was evaluated by the drop of the fibres on the cutting surface of specimens. The results showed that segregation was the most affected by the consistency of fresh concrete. The amount of fibre dosage as well as the size of maximum aggregate size did not significantly affect segregation.
166
Authors: Michaela Kopálová, Vladimír Příbramský
Abstract: UHPC (ultra-high-performance concrete) is one of the essential elements in the development and research of new modern concrete structures. It is a material showing all the positive qualities in terms of the design efficiency and construction process. In this paper, we analyze the behavior of prestressed UHPC I-beam with lightened web that is loaded by four-point bending test. Two variants of I-beam were used for the analysis. The first variant is beam with a continuous thin web and the second variant is beam with lightened web, which is analogous to castellated steel I-profiles, commonly used types of steel beams with larger span. Although these are relatively small specimens, light beam analysis can be extrapolated for large span structures, especially bridges. In case of structures of larger magnitude with precast members of UHPC, a certain amount of discontinuous action cannot be avoided, which is the case of using a lightened beam with openings across the entire height of the web. Application of prestressed tendons ensures sufficient resistance for bending action; members are designed so failure occurs by shear response.
172
Authors: Vladimír Křístek, Jaroslav Průša, Jan L. Vítek
Abstract: According to the common design methods of calculation of the stress state induced by torsion of massive prismatic concrete structural elements, the structural system is reduced to a simple cage consisting of ties and struts. This model has, however, a number of principal shortcomings, the major of them is the fact that all of simultaneously acting effects like axial forces, bending moments and shear forces are not taken into account – the compressive axial forces increase very significantly the torque capacity of structural members, while due to action of tensile forces, bending moments and shear forces the torque capacity is reduced. These phenomena, applying non-linear approaches, are analysed and assessed.
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Authors: Barbora Svobodova, Hana Šimonová, Zbyněk Keršner
Abstract: The structural concrete can be also characterized via parameters obtained by evaluation of fracture tests. Therefore, this paper presents software CheF developed in Java programming language, designed for evaluation of three-point bending fracture tests of core based concrete specimens with chevron type notch. The records of fracture tests in form load vs. displacement and load vs. crack mouth opening displacement diagrams are analysed by software CheF to obtain values of selected mechanical fracture parameters: modulus of elasticity E, fracture toughness KIc, and fracture energy GF, determined based on the linear elastic fracture mechanics approach and work-of-fracture method.
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