Jaroslav Válek

Selected Properties of Cementitous Composites with Portland Cements and Blended Portland Cements in Extreme Conditions

The paper is focused on research of physico-mechanical properties of concretes with Portland-limestone cement, Blastfurnace cement and Portland-composite cement in comparisom with concrete with Portland Cement CEM I. Following physico-mechanical properties of concretes exposed to extreme conditions were tested: compressive strength, flexural strength, tensille splitting strength, velocity of propagation of ultrasonic pulse, dynamic elasticity modulus and density of hardened concrete. Following environments were used in tests: sulphates, magnesic ions, nitrates, gaseous CO2, high temperatures.

Mixing of Different Particle-Size Fractions of Fly Ash to Increase its Activity in Cement Composites

Fly ash residue from coal burning process in power plants have been used as an active additive in concrete since middle of nineteenth century. Nowadays, we are trying to maximize its reactivity in cement composites by several methods. One of the methods is mechanical activation by mixing several size fractions to reach ideal particle size distribution. Ideal particle size distribution of fine components in cement composites helps to reach maximal compactness and thus increases strength and durability. In this paper we present results of compressive and tensile strength of cement-fly ash mortars with mixing 3 size fractions origins in 3 electrostatic precipitators from power plant Tušimice. Fly ashes from 1st precipitator, 2nd precipitator and 3rd precipitator were mixed with ration 10:4:1, 5:4:1 a 0:4:1.

Development of Concretes with Resistance to High Temperatures in the Czech Republic and Surrounding European Countries

The resistance of concrete constructions to high temperatures at present is a much monitored issue for many scientific teams and experts in the stated area. This fact is mainly caused by fatal consequences originating in the case when concrete constructions are loaded by the effect of fire and consequent loss of their load-bearing capacity, for the population and the environment of our planet, in which we live in. The development of society goes hand in hand with the development of new building materials and as a consequence the requirements for building constructions increase which bring about extraordinary strict regulations in the area of fire safety. So, many high, non-traditional or specific constructions originate, e.g. nuclear power plants due to permanently higher demand for transport linkage and many tunnels have originated between European countries as a result. Unfortunately, in this relation the threat of terrorist attacks increases and unexpected natural disasters which also threaten the stability of the mentioned constructions. The objective of the article is to familiarize readers with the results of research concerning the improvement of the resistance of the concrete to high temperatures originated during fire instances.

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.

Failure of Concrete at High Temperatures - The Influence of the Type of Cement

Experimental work verifies behavior of concrete with various types of cement and aggregate when exposed to high temperatures. Concrete samples differed in the aggregate used (two modes - basalt aggregates, lightweight aggregates), combined with four types of cement (CEM I 42.5 R – Mokrá Czech Republic, CEM II/B-M (S-LL) 32.5 R – Mokrá Czech Republic, CEM II/B-S 32.5 R – Cemex Czech Republic, CEM III/B 32.5 N-SV – Cemex Czech Republic). Properties were determined before and after thermal loading. The aim of the presented experiments was to verify behavior of different types of cement in concrete at high temperatures and the effect of various types of cement for subsequent damage concrete samples after exposure to high temperatures.

Testing of Action of Direct Flame on Concrete

The paper states results of experimental exposition of concrete test specimens to direct flame. Concrete test specimens made from various mixtures differing in the type of aggregate, binder, dispersed reinforcement, and technological procedure were subjected to thermal load. Physicomechanical and other properties of all test specimens were tested before exposition to open flame: density, compressive strength, flexural strength, moisture content, and surface appearance. The specimens were visually observed during exposition to open flame and changes were recorded. Exposed surface was photographically documented before thermal load and at 10-minute intervals. Development of temperature of the specimens was documented with a thermocamera. After exposition to thermal load and cooling down, concrete specimens were visually observed, network of cracks was photographically documented, and maximal depth of spalled area was measured.

Alkali-Activated Matrix Based on Metakaoline with Lightweight Aggregate

The paper focuses on design and verification of mix-design of lightweight concrete with alkali-activated matrix with lightweight aggregate Liapor. Alkali-activated matrix was designed on the basis of ground metakaoline and micronized limestone. Mixes of concrete with alkali-activated matrix and lightweight porous aggregate Liapor of size fraction 8 mm were designed and their rheological and physico-mechanical properties were tested. To reduce shrinkage and to increase resistance to high temperatures, dispersed polypropylene fiber reinforcement was used and testing specimens made from this concrete with alkali-activated matrix were exposed to thermal load and tested.

Functional Vibro Pressed Pavement with Ecological Benefits

The paper describes results of functional tests of photo catalytic materials directly in matrix of prefabricated vibro pressed pavement blocks. Resulting surfaces of photo catalytic concrete products were tested with respect to conversion of NOx and surface was analyzed with Scanning Electron Microscope (SEM), so called element maps were elaborated by means of Energy Dispersive Spectroscopy (EDS).