Jaroslav Topič

Influence of Recycled Concrete Composition on its Elastic Stiffness

The presented study was focused on the influence of the micronized recycled a concrete sleeper on the mechanical properties of fine-grained concrete, specifically on dynamic modulus of elasticity. The tested material consisted of cement CEM I 42.5R (Radotín), crushed bricks fraction 2-5 mm, 0-4 mm sand and micronized recycled concrete fraction 0-0.125 mm. Individual material samples differed in the quantity of cement and finely ground recycled concrete (milled at Ltd. Lavaris, Libčice – Czech Republic). Cement was replaced by the recycled concrete in an amount of 30, 50 and 70 weight percent. Testing was performed on beams of dimensions 40 × 40 × 160 mm by using non-destructive testing – pulse method. The article compares the values obtained between 2-nd and 28-th day.

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.

Effect of Pva Modification on Properties of Cement Composites

Polymers are used for modification of the cement-based composites and others building materials since the thirties of 20th century. Based on the conclusions of recent studies, it is assumed that even water soluble polymers could be used as an admixture for such modification. Currently, there exist and are exploited several possibilities for polymer modification of mortars, wood-based products or bituminous asphalts. Various options differ in the way of modification, which can be basically applied to the entire volume or just a surface, but also in the form of the polymer used – either in the form of solution or fibers. The aim of our study was to investigate the influence of volume modification by the water soluble polymers, such as polyvinyl alcohol (PVA), on the properties of cement paste and find an optimum additive. It turned out that the addition of PVA solution into fresh cement paste results in an increase of porosity and therefore a stiffness and compressive strength reduction. On the other hand, the bending strength of PVA-rich specimens was significantly higher and their water absorption decreased, which may consequently result in enhanced frost resistance.

Mechanical Properties of Gypsum Matrix Reinforced with Recycled Fibers

This paper deals with using resonance method for determine a development of mechanical properties of gypsum matrix reinforced with steel fibers from recycled tires. Mechanical properties which were monitored are the dynamic Youngs modulus and dynamic shear modulus. Both properties were measured by a resonance method on samples 28 days old. The aim was to check the possibilities of using the recycled tires in construction as reinforcement to the gypsum binder. Many published papers deals with composite materials based on gypsum, but mainly with standard non-metallic reinforcement such as glass fibers, PP and PE fibers etc.

Cement Composite Reinforced with Synthetic Fibers: Comparison of Three-Point and Four-Point Bending Test Results

Presented article deals with the influence of PET fiber production on the bending strength of cement-based composite when incorporated into the fresh mortar, and comparison of results of 3-point and 4-point bending test. Cement paste samples were reinforced with 2 wt. % of primary or recycled PET fibers. The bending test was performed on prismatic samples with dimension of 40 × 40 × 160 mm. It was found that samples with recycled PET fibers, compared to primary ones, exhibit a decrease in bending strength. In the case of 4-point bending tests, the samples with recycled PET fibers exhibited higher bending strength than reference samples without any fibers. However, in the case of 3-point bending tests, the samples with recycled PET fibers had lower bending strength than the reference ones. The results suggest that recycled PET fibers could be used as an alternative to reinforce cement-based composites.

Non-Destructive Testing of Composite Gypsum Material Properties – Long Time Measurement

The paper presents results of long-time investigation of mechanical properties of composite gypsum material using nondestructive technique. The gypsum block was put to the façade of the building for four years. Then it was removed and cut to smaller specimens, which were investigated. The matrix of 6 × 4 specimens of dimensions 40 × 40 × 160 mm were cut from the block after its removal from the façade. After 6 and 9 years, the Young’s moduli and shear moduli of these specimens were determined from the transversal, longitudinal and torsional vibrations using the impulse excitation method.

MICROSTRUCTURE DESCRIPTION AND MICROMECHANICAL PROPERTIES OF SPRUCE WOOD

The knowledge about the microstructure and morphology of individual phases also allows an artificial modification of the material, or its processing, to produce engineered products with required properties. The purpose of our work was to investigate the distribution of elastic stiffness within the tissues of individual cells using the finest equipment and to observe the morphology of individual phases. A quasi-static nanoindentation was carried out on the cell walls of earlywood and latewood tracheids of spruce wood. The dynamic modulus mapping, also known as nanoDMA, was utilized to obtain the map of elastic moduli over the entire tracheid cross-section. In particular, it was found that the stiffness cells walls is approximately 10.5 GPa and 12.5 GPa in the case of earlywood and latewood tracheids, respectively. The difference between earlywood and latewood elastic stiffness is attributed to a different chemical composition and orientation of fibrils. The acquired data are indispensible for micromechanical modeling and design of engineered products with superior mechanical properties.

The Dependence of the Shrinkage of the Cement Composite with Fine Ground Recycled Concrete on its Microstructure

This paper is focused on the shrinkage and microstructure of composite cement based materials with different percentage of fine ground recycled concrete. Recycled concrete is in cement mixture as a partial replacement of the filler, which under certain circumstances also functions as a binder component. For the assessment of the microstructure were used images from the optical and scanning electron microscope. There were produced four mixtures of cement pastes containing 0, 33, 50 and 67 wt. % of fine ground recycled concrete. At the conclusion of the experiment it was shown that the mixture samples with increasing amounts of recycled concrete have greater shrinkage due to the increased water/cement ratio, which is for the individual mixtures different in the terms of the same workability of all mixtures.

Hydration Heat Evolution of the Cement Paste with Recycled Concrete: Influence of Grain Size Distribution of Recycled Concrete Powder

The paper deals with the influence of level of grinding of the recycled concrete. Level of grinding mostly influence the hydration heat evolution of cement paste. Measurement of hydration heat evolution is one of the ways to detect reactive properties of recycled concrete powder modified by high speed mill device. Basic characteristics that most influence the hydraulic properties of recycled concrete powder are grin size diagram and chemical composition of the original concrete. Non-hydrated inner part of cement grains could further react if it is exposed to water. In this experiment the recycled concrete from recycled railroad sleepers and structural layers of the D2 highway was used. Recycled concrete powder from highway was divided into 4 more variants which differ by grinding process during production. To detect difference in hydration process four mixtures of cement paste with different types of recycled concrete powder were made and further subjected to calorimetric measurement. The results were after compared with the hydration heat rate of reference cement paste. According to results the type of milling process had minor influence on hydraulic properties of recycled concrete powder compared with influence of origin of recycled concrete.

Influence of increasing amount of recycled concrete powder on mechanical properties of cement paste

This paper deals with using fine recycled concrete powder in cement composites as micro-filler and partial cement replacement. Binder properties of recycled concrete powder are given by exposed non-hydrated cement grains, which can hydrate again and in small amount replace cement or improve some mechanical properties. Concrete powder used in the experiments was obtained from old railway sleepers. Infrastructure offer more sources of old concrete and they can be recycled directly on building site and used again. Experimental part of this paper focuses on influence of increasing amount of concrete powder on mechanical properties of cement paste. Bulk density, shrinkage, dynamic Youngs modulus, compression and flexural strength are observed during research. This will help to determine limiting amount of concrete powder when decrease of mechanical properties outweighs the benefits of cement replacement. The shrinkage, dynamic Youngs modulus and flexural strength of samples with 20 to 30 wt. % of concrete powder are comparable with reference cement paste or even better. Negative effect of concrete powder mainly influenced the compression strength. Only a 10 % cement replacement reduced compression strength by about 25 % and further decrease was almost linear.