Lenka Bodnárová

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.

Contribution to the issue of evaluating the compressive strength of concrete exposed to high temperatures using the Schmidt rebound hammer

Concrete properties change as a result of ambient conditions. The internal microstructure changes under high temperatures. As a consequence, this change affects both physical and mechanical properties of concrete and therefore the parameters determined from nondestructive testing. It was determined that for concrete exposed to temperatures in the range of 200 to 400°C the rebound number measured by the rebound hammer is approximately 9% higher after heat load than for wet concrete placed in a standard environment. Temperatures from 600 to 800°C cause a drop in the rebound number, which correlates with a decrease in the concrete’s compressive strength. A strength dependency was established for the N-type Schmidt rebound hammer expressing the relationship between the rebound number and the compressive strength of concrete exposed to heat load, which is characterized by a high correlation between variables (correlation coefficient is 0.98). To estimate the compressive strength of concrete in a structure which has been exposed by fire, can be used both the calculated strength relationship, both the calibration curves for N-type Schmidt rebound hammer calculated by manufacturer, eventually the basic curve from CSN EN 13791. If the calibration equation for N-type Schmidt rebound hammer or basic curve of CSN EN 13791 is taken, the calculated strength for the rebound numbers should be lowered using the corrective coefficient, the value of which is 0.54, respectively 0.65.

Evaluation of Degradation of Concrete Exposed to High Temperature by Means of Ultrasonic Pulse Method

Ultrasonic pulse method is a non-destructive testing method used for testing materials. For concrete, it is used mostly for determination of dynamic elasticity modulus, compressive strength, homogeneity, to determine depth of cracks or as a supportive method for testing frost resistance. Applicability of using ultrasonic pulse method for evaluation of degradation of concrete exposed to high temperature was proved. This method is unambiguously utilizable for rationalization of experimental work focused on optimization of composition of concrete resistant to high temperatures. Ultrasonic pulse method can be also used for mapping the degree of degradation of concrete elements and structures, which can be measured by means of direct sounding. Appropriateness of the use of ultrasonic pulse method for evaluation of degradation of concrete exposed to high temperature was proved. This method is unambiguously applicable for rationalization of experimental work focused on optimization of composition of concrete resistant to high temperatures. Ultrasonic pulse method can be also used for mapping degree of degradation of concrete elements and structures, which can be measured by means of direct sounding.

Experimental Verification of Behavior of Glass and Carbon Fibers in Alkali Environment

The paper discusses durability of glass and carbon fibers which are exposed to alkali environment of concrete. Mainly glass fibers are affected by the high alkali environment with pH of concrete exceeding 12.0. The degradation of fibers is reflected by decreasing of its tension strength, while the rate of decreasing is depended on exposition time and temperature. The paper briefly presents results of a large experimental investigation which should specify an influence of alkali ions concentration in a solution on reduction of the mechanical properties of fibers. Reference samples were immersed in a bath with distilled water and the others were stored at ambient air only.

Study of Effect of Various Types of Cement on Properties of Cement Pastes

This article studies the effect of various types of cement on rheological properties of cement pastes. Rheological properties are significantly related to workability of concrete and to requirements of mixing water amount and thus affect the development of strength, durability of concrete and resistance of concrete to extreme conditions and to high temperature. This article describes the influence of cement type, type and dose of superplasticising admixtures on development of torque in time. Torque was monitored by the rotational viscometer Viskomat NT. Theoretical part of the article briefly describes the phenomenons that can accompany mixing and processing of cement pastes. Experimental part is focused on monitoring the rheological behaviour of cement pastes of different formulas by monitoring the evolution of torque in time.

Laboratory Experiments on Effects of Water Jet on Heat-Affected Concretes

The paper is dealing with the effects of flat high-speed water jet on concretes affected by high temperature. Research should help to correct use of water jet technology in repair works on concrete structures especially after wildfires in tunnels, underground garages, etc., which are exposed to enormous thermal stress.

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.

Comparison of the actual costs during removal of concrete layer by high-speed water jets

This paper describes the process of possible evaluation of costs of using high speed water jet technology for concrete removal methods. High speed water jet technology is a progressive technology of removing damaged concrete used in civil engineering since the 80s of 20-super-th century. It has been changing and developing since that time. But there is little information in literature devoted to the economic evaluation of this technology. Detailed economic analysis is still missing. This paper aims to compare comprehensively in economic terms the costs of removing concrete using the technology of both continuous oscillating and pulsating oscillating water jets. The research was realized in cooperation with research institutions and industrial companies and was supported by state budget of the Czech Republic and from the European Union. The scheme of cooperation of the University, research institutions, industrial companies and government follows the Triple helix model.

Testing of the High-Velocity Water Jetting on Concrete Samples Inside the Overpressure Vessel

The concrete samples with various erosion states were disintegrated inside the overpressure vessel using high-velocity water jet. Their erosion states were prepared by application of several laboratory techniques simulating the concrete aging under the natural conditions due to the real applications in practice. The influence of the erosion state on the disintegration rate was tested and the surface topography was studied both prior the application of water jet and post it. The water jet was applied in the overpressure vessel used for simulation of pressures equivalent to the submersion to several depths under the water level. Usual experimental scale responsive to the national practical application range was from the depth of submersion close to the water level up to the depth about 100 meters with the 20 meters step. Nevertheless, the data were obtained for overpressures equivalent to the submersion levels up to 140 meters enlarging so the range of results useful for regression analysis of the physical trend. Several samples were tested even in overpressures simulating depth of submersion equal to 160 meters and more. Few samples of special decorative concretes were also studied and special techniques of their working up were prepared and tested. All results are discussed regarding their application in practice and further development of special routings.Copyright

Erosion Test with High-speed Water Jet Applied on Surface of Concrete Treated with Solution of Modified Lithium Silicates

The paper focuses on the erosion resistance test based on surface treatment by a high-speed water jet applied on concrete. A reference concrete sample and concrete samples with three types of solution of modified lithium silicates applied on the concrete surface were tested under various erosion conditions. Two different jet impact angles (45° and 90°) were investigated. A significant difference in the erosion rate in the surface layers was observed comparing the untreated concrete and the concrete treated by various solutions of modified lithium silicates, but only in the initial phase of erosion.