Martin Labaj

Carbon Nanotubes in Cementitious Composites: Dispersion, Implementation, and Influence on Mechanical Characteristics

For effective utilization of nanoparticles in cementitious composite materials, their perfect dispersion is necessary. Appropriate quantity of ultrasonic energy (US energy), as well as usage of proper and compatible surfactant, is a prerequisite for carbon nanotubes’ (CNT) suitable deagglomeration. US energy was supplied by Bandelin Sonopuls HD 3200 ultrasonic homogenizer. Influence of US energy’s amount on CNT’s dispersion in aqueous medium was monitored in two steps: at first roughly by an optical microscope and in the next step precisely by the measurement of absorbance via UV/Vis spectrophotometry. After the appropriate amount of US energy was found, cement mortars with CNT were prepared and tested. Their physical and mechanical properties were examined and compared to the reference specimens.

Optimization of Concrete with High Volume of Fly Ash

The demand for concrete structures grows worldwide, which raises fears about sustainable development of Portland cement production. Its carbon footprint is relatively small compared to alternative building materials, but still it is not negligible. This argument together with lower cost and possibility of utilization of material, which would otherwise be disposed as waste, lead the research towards concrete with higher content of Portland cement replaced with fly ash. The experiment is divided into two parts: the first one determines influence of high volume fly ash replacement of Portland cement on behavior and properties of cement paste and mortar. The second part optimizes composition of concrete mix, in particular the granulometry of the cement-fly ash system in order to achieve maximal possible values of mechanical properties at high dosage of fly ash.

Comparison of Automated Concrete Shrinkage Measurement Methods

Volumetric changes are deformation properties of concrete that are caused by external or internal forces and can be reversible or irreversible. Relatively significant and irreversible volumetric changes occur during the hydration of clinker minerals during the first stages concrete setting and hardening. From the “real-world” point of view, the volumetric changes associated with drying are the most important – these take place when the leftover mixing water evaporates. The current legislation addresses the volumetric changes of concrete relatively briefly and the calculations of the assumed autogenous shrinkage according to Eurocode 2 are generally related only to the concrete’s strength class, not to the composition of the concrete mixture. The principle of determination of volumetric changes is classified by the standard ČSN 73 1320, which requires the measuring devices with an accuracy of at least 1∙10-5 of expected proportional deformation. In this experiment, the volumetric changes of the concrete will be monitored by two different measurement methods. Several different test mixtures will be prepared to monitor these changes continuously with an automatic measuring device. In one case, the measurement will take place from the point of fresh concrete and, in the case of the others, immediately after the acquisition of the handling strength, i.e. from the age of 1 day.

The Suitability of Fly Ash for Use in Concrete According to EN 450 Based on their Particle Size

The by-products of energy industry are nowadays often affected by new limits governing the production of harmful gases discharged into the air. These stricter and stricter criteria are often met by electricity producers by changing the combustion process in thermal power plants itself. Nowadays, the SNCR (selective non-catalytic reduction) application is quite common in the combustion process in order to help reduce the nitrogen oxide emission. This article deals with the primary measures of thermal power plants, which in particular consist of a modified treatment of raw materials (coal) entering the combustion process. These primary measures then often cause the formation of fly ash with unsuitable fineness for the use in concrete according to EN 450. The paper presents the comparison of the physico-mechanical parameters of several fly ashes with a different fineness values. The primary task is to assess the impact of non-suitable granulometry in terms of EN 450 on the other physico-mechanical parameters of fly ashes sampled within the same thermal power plant. Several fly ashes produced in the Czech Republic and surrounding countries were evaluated in this way.

Low-Strength Self-Compacting Concrete

Self-compacting concretes (SCC) are relatively modern building material that has great potential for using in a wide range of applications. Its origin and development is considered a major breakthrough in concrete technology, especially because of its ease of placement without the need to use external dynamic forces in the form of vibrations. This can significantly affect the resulting properties of concrete as well as working conditions on the building site.To maintain the fresh concrete’s rheological properties and, at the same time, achieve lower final strength, reduced amount of Portland cement needs to be proposed in mixture design. Then, to keep the number of fine particles at high level, it is necessary to use fine grained cement compatible additives which do not chemically participate on hydration process – at least not too much – and thus do not increase the resulting strength.This paper will address the verification of inert additives functionality for the production of lower-strength self-compacting concretes, namely in strength classes C16/20 and C25/30 according to ČSN EN 206. The inert admixture used in this experiment – stone dust from Zelesice quarry – has a relatively high water absorption. Therefore, the particularly crucial part was the fine-tuning of fresh SCC’s rheological properties. The results are clearly pointing to the possibility of lower-strength self-compacting concretes’ production and thus makes it possible to expand the usability portfolio of this type of modern construction material with regard to its lower production costs.

Nanoadditives - Future of High Performance Concrete

These days it is almost impossible to imagine the technology of high performance concrete without the use of any kind of additive. Whether it is a material capable of achieving high strength, excellent mobility of the fresh mix without losing cohesion or producing high quality architectural concrete surface, microadditives have their certain place for a long time now. Although the research in this field still has something to offer, it does not hurt to try to consider the future and imagine the path that will be taken in the production of high performance concrete of next generation. The article deals with the possibility of using nanoparticles in concrete technology. These materials can actively participate in the creation of very high-quality cement stone. In addition, due to the extreme reactivity of nanoparticles, these reactions can take place almost immediately after the onset of hydration and during its first hours. The experimental part of the paper assesses the impact of nanoparticles on selected properties of fresh cement paste and hardened cement mortar. In all cases, there was a positive effect and it has been demonstrated that nanoparticles may eventually create a new category of high performance concrete additives.

Foam Concrete Gravity Wedges as a Thermal Insulation of Flat Roofs

Proper selection of thermal insulation materials is an essential part of almost any construction project, whether it is a new construction, renovation or revitalization. There is a large number of materials available on the market and it is often very difficult to have a good grasp. Though some material has very similar features, they often cannot replace each other. The same applies to single-shell flat roofs. So far the most often used thermal insulation boards are from polystyrene and mineral wool. Their installation is accompanied by a large volume of waste. The experimental part deals with the general assessment of the behavior and properties of foam concrete. This allows simplification of technology of thermal insulation laying. Besides the construction speed up and the overall cost reduction the improvement in mechanical properties of the roof structure has been achieved.

Nanosilica Activated High Volume Fly Ash Concrete: Effects on Selected Properties

By volume, there is no other material used as much as concrete. Its mechanical properties, durability and favorable price makes concrete the perfect construction material. In last few decades, we are seeing a growing trend of partial Portland cement’s replacement with secondary raw materials, most commonly with fly ash. So-called high volume fly ash (HVFA) concretes usually contains over 50% of it. While HVFA concrete’s long-term properties and price are improved over the classical one, its early age properties are often affected negatively. Here, a highly reactive pozzolans enters the scene. Materials like microsilica and metakaolin are known to accelerate concrete’s strength development and improve early age characteristics. In this paper, nanosilica is used for this purpose. These SiO2 nanoparticles possesses a much higher surface area and thus reactivity. Three mixtures with 0, 40 a 60% portland cement’s replacement with fly ash were prepared and tested with and without addition of small amount of nanosilica. Effects on compressive strength, static and dynamic moduli of elasticity and resistivity against water pressure were observed. Results clearly demonstrates that even with dosage in the range of tenths of percent, nanosilica can significantly improve concrete’s properties.