Josef Fládr

Replacement of Cement with Finely Ground Recycled Concrete: Influence on Mechanical Properties

The waste production from construction sites become very serious problem. Recycling is the best option for disposal of such waste, and the proper sorting and knowledge of the recycled concrete history allows its further use in the construction. The current studies are mostly focused on utilization of recycled concrete in the form of aggregate. The presented work is focused on the utilization of Finely Ground Recycled Concrete (FGRC) used as a filler and partial substitution for binder. Recycled concrete was ground from concrete railway sleepers in the Lavaris Company (Czech Republic). Through the testing of mechanical properties, we demonstrate the influence of FGRC’s amount in cement paste on mechanical properties of the composite. To clearly show the relationship between the amount of FGRC and the composite properties, samples with 33, 67 and 100 wt. % of cement replaced by FGRC were tested. The composite with 33 wt. % of FGRC attained the compressive and flexural strength comparable with reference cement paste without any FGRC additions. The results indicate that the partial substitution of cement by FGRC could lead to a cost reduction of cement composites with minimal impact on their mechanical properties.

Experimental Verification of the Behavior of Protective Steel Liner of Concrete Containment

Containment is the last barrier protecting the environment from contamination in case of serious accident in nuclear power plant. Steel liner is often used to guarantee leaktightness of concrete containment and therefore the design of the liner must be done with utmost care. Several methods are used in practice to anchor the liner plate to concrete containment wall, differing in layout and type of anchorage elements used. The anchorage system must ensure full connection of the liner to the wall, liner buckling and tearing have to be prevented. The objective of the research was to determine the behavior of typical details of steel liner of concrete containment subjected to axial and shear loads. Two types of experiments were proposed. The first one was aimed at defining the response of two different types of liner anchorage system – L-profiles and headed studs – to the axial loads imposed on the liner plate. Behavior of specimens with and without initial imperfection of the steel plate was also compared. The goal of the second experiment was to determine the shear load-bearing capacity of the anchorage and to verify that liner tearing will not occur before the failure of the anchorage – this one of the main conditions of the safe design. The experimental program was successfully executed and the results are presented in the paper.

Utilization of Surface-Modified Polymer and Glass Micro-Fibers as Reinforcement in Cement Composites

The presented work focuses on plasma modifications of polymer and glass micro-fibers (having 32 and 14 μm in the diameter, respectively) used as randomly distributed and oriented reinforcement of concrete composites. Fiber surfaces were modified by means of the low-pressure coupled cold oxygen plasma in order to attain a strong adhesion with the cement matrix. From the perspective of micro scale, an impact of modifications on both the physical and the chemical surface changes of treated fibers was examined using: (i) a wettability measurements – an evaluation of an interphase interaction between demineralized water and fibers and (ii) the SEM microscopy – an assessment of a surface morphology. From the perspective of macro scale, the interaction between the two materials was examined by destructive four-point bending tests of the cement paste containing both the reference and treated fibers (specimens having dimensions equal to 40×40×160 mm, water to cement ration 0.4) were done. It was shown that the wettability of modified fibers was increased by approx. 10 % and 70 % in the case of glass and polymer fibers, respectively. The SEM morphology analysis revealed fine roughening of treated fibers, if compared to the reference ones. The mechanical testing pointed out on a toughness increase in the post-cracking response of loaded specimens.

Mechanical properties and durability of crumb rubber concrete

This paper is focused on concrete with admixture of rubber powder, generally called crumb rubber concrete (CRC). The inspiration was found in Arizona, where one of the first CRCs has been created. However, Arizona has completely different climates than Central Europe. Could we use the crumb rubber concrete on construction applications in the Central European climate too? The paper evaluates the influence of the rubber powder on material characteristics and durability of CRC. CRCs with various contents of fine and coarse crumb powder were compared. The tested parameters were slump, air content, permeability, resistance of concrete to water with deicing chemicals, compressive and splitting tensile strength. The tests showed that workability, compressive strength and permeability decreased as the amount of rubber increased, but the air content increased as the rubber content increased. Photos of air voids in cement matrix from electron microscope were captured. The results of laboratory tests showed that admixture of rubber powder in concrete could have a positive impact on durability of concrete and concurrently contribute to sustainable development. Considering the lower compressive strength, CRC is recommended for use in applications where the high strength of concrete is not required.

Analysis of laboratory compaction methods of roller compacted concrete

Roller-Compacted Concrete (RCC) is an ordinary concrete poured and compacted with machines typically used for laying of asphalt road layers. One of the problems connected with this technology is preparation of representative samples in the laboratory. The aim of this work was to analyse two methods of preparation of RCC laboratory samples with bulk density as the comparative parameter. The first method used dynamic compaction by pneumatic hammer. The second method of compaction had a static character. The specimens were loaded by precisely defined force in laboratory loading machine to create the same conditions as during static rolling (in the Czech Republic, only static rolling is commonly used). Bulk densities obtained by the two compaction methods were compared with core drills extracted from real RCC structure. The results have shown that the samples produced by pneumatic hammer tend to overestimate the bulk density of the material. For both compaction methods, immediate bearing index test was performed to verify the quality of compaction. A fundamental difference between static and dynamic compaction was identified. In static compaction, initial resistance to penetration of the mandrel was higher, after exceeding certain limit the resistance was constant. This means that the samples were well compacted just on the surface. A specimen made by pneumatic hammer actively resisted throughout the test, the whole volume was uniformly compacted.

PLASMA TREATMENT IMPACT ON PHYSICAL AND CHEMICAL PROPERTIES OF POLYMERIC FIBERS

Presented work focuses on chemical and physical properties of plasma modified polymeric macro-fibers. Polyethylene terephthalate (PET) and polypropylene (PP) fibers having approx. 300 μm in diameter were modified using cold oxygen plasma in order to achieve their surface changes needed for durable bond and adhesion with cement matrixes. A duration of plasma modification differed between 5 to 480 seconds, where an effect of the treatment was examined. Fiber surfaces chemical changes were researched via wettability measurement with demineralized water (the measurement was repeated immediately and after 1, 7 and 30 days to find out the changes stability). Physical changes were studied by means of weight balance (determination of weight loss) and tensile strength tests. It was found that wettability was enhanced significantly – up to two times, while mechanical properties of treated fibers decreased only slightly.

Distribution of Stresses in Masonry Pillar with Fully Filled and Unfilled Perpend Joints

The paper summarizes the results of numerical analysis conducted with the aim to compare the distribution of stresses in masonry pillars constructed using different bricklaying techniques. The analysis was carried out in reaction to the discussion of members of Czech standardization committee TNK 37 – Masonry structures. Currently, most of masonry load-bearing structures in the Czech Republic are made from clay blocks without mortar in perpend joints. The analysis seeks the answer to the question whether it is possible, in case of the eccentrically loaded masonry pillars with unfilled perpend joints, to consider the value of design compressive strength calculated using the same approach as for pillars with filled perpend joints for the check of vertical load resistance. Supplementary comparison of the behavior of the pillars with filled and unfilled perpend joints loaded by lateral load in the plane of the pillar (corresponding to short shear walls) was also conducted. 2D FEM model created in ATENA Science software was exploited for the analysis. The results confirmed that the approaches contained in ČSN EN 1996-1-1 [1] are basically applicable for pillars with unfilled perpend joints.

Influence of Finely Ground Recycled Concrete on Microstructure of Cement-Based Composite Material

This article deals with using recycled concrete in cement-based composite materials. The recent studies were mostly focused on utilization of recycled concrete in the form of an aggregate filler. In this study we are investigating the possibility of using finely ground recycled concrete as microfiller or partial substitution for binder. In particular, we focus on changes in microstructure of the cement paste which differed in amount of finely ground recycled concrete (FGRC). We used four mixtures of cement pastes containing 0, 33, 50 and 67 wt. % of FGRC, respectively. For the examination of their microstructure, phase distinction and determination of FGRC influence, the images obtained using optical and electron microscope were used. The first results indicate that cement paste with 33 wt. % of FGRC has similar mechanical properties as reference cement paste. Partial replacement of cement by finely ground recycled concrete leads to a cost reduction of cement-based composites and also can reduce environmental impacts of construction waste disposal and cement production.