Iva Rozsypalová

Thermal Analysis of Concrete from Panels Subjected to Fire Experiments

This paper presents the results of the differential thermal analysis of cement matrix samples taken from concrete panels with nominal dimensions of 2300 × 1300 × 150 mm after one-sided exposure to high temperatures. The panels were subjected to maximum nominal temperature loads of 550, 600, 800 and 1000 °C. Concrete was also taken from a reference panel (without temperature loading) and investigated. Five samples with a nominal thickness of 20 mm were taken for thermal analysis. They were cut from the central part of the panels using a diamond blade saw. The thermal analysis covered the effects of temperature load on the concrete to a depth of approximately 100 mm from the heated surface of the panel.

Detailed Determination of Mechanical Fracture Parameters of Concrete after Fire Experiments

The aim of this paper is to describe the procedure of determining the mechanical fracture parameters of selected concrete specimens taken from panels after the fire experiments. The records (in form load vs displacement diagrams) of three-point bending fracture tests of these specimens with initial stress concentrators was first advanced corrected and subsequently evaluated using the Effective Crack Model and the work-of-fracture method. The increasing temperatures during the fire experiments ranging between 550 to 1000 °C led to a decrease of modulus of elasticity and fracture toughness values and to the increase of fracture energy value. The 2D laser profile scanner was used to estimate the degree of complexity of fracture surfaces; its statistical dependence on the mechanical fracture parameters proved to be moderate – the absolute value of the correlation coefficient was about 0.5°[–].

A pilot study of methods for measuring the residual properties of concrete exposed to elevated temperatures

This pilot study and literature review was performed with the purpose of preparing for subsequent extensive research focused on the design of effective diagnostic methods for determining the current state of concrete structures damaged by high temperatures during exposure to fire. During the pilot study, specimens were prepared from ordinary concrete with dense aggregate, and were subsequently loaded by temperatures of (200, 400, 600, 800, 1000 and 1200) °C. In addition, one set of the specimens was kept as reference specimens without thermal loading. After the thermal loading of the six sets of specimens had cooled, selected non-destructive tests were performed; i.e., the Silver Schmidt hammer rebound test, the ultrasonic pulse velocity test, and the impact-echo method. Furthermore, the compressive strength of the specimens was determined destructively to provide reference data. The experiments indicated the fact that the commonly used non-destructive methods are not suitable for estimating the compressive strength of concrete exposed to temperatures higher than 600 °C. This indicates that it is necessary to derive new calibration relationships for such conditions. It was also found that an apparent increase occurred in the hardness and modulus of elasticity for the tested concrete when it was exposed to temperatures greater than 1000 °C.

Monitoring of the Setting and early Hardening with Ultrasonic Waves

The paper deals with the experimental investigation aimed at the continual monitoring of the process of setting and early hardening in cement pastes and mortars using the measurement equipment Vikasonic. The measurement principle consists in measuring the time of ultrasonic pulses transit through a test specimen placed between two ultrasonic transducers. This innovative method of measurement could in the future suitably complement or, in some cases, completely substitute the measurement of setting in cement composites using the Vicat apparatus. The pilot measurement results performed on the cement pastes and mortars are presented in the paper.

Non-Destructive Schmidt Rebound Hammer Evaluation of the Degradation of Concrete Exposed to Elevated Temperatures

Concrete suffers significant changes in its internal structure due to fire exposition. Chemical and physical transformations cause further changes in its properties [1]. The research focuses on non-destructive Schmidt rebound hammer testing of experimental concrete panels exposed to high temperature. Five concrete panels with the dimensions of 2300 × 1300 × 150 mm were made from concrete of ordinary fire-resistance. According to the standard temperature curve in EN 1991-1-2 [2], the specimens were heated up to the target temperature (550, 600, 800 and 1000 °C) and afterwards, the given thermal load was maintained for 60 more minutes. The equation of the standard temperature curve represents a common fire by which a building structure may be affected in rare cases. The rebound hammer SilverSchmidt L and Original Schmidt N was used for determining the rebound number. Rebound number area charts of all experimental panels were created in order to identify most damaged areas of the concrete due to the effects of high temperatures. Differences between values of destructive and non-destructive compressive strength were determined.

Evaluation of Permeability Tests of Surface Layer of Concrete of Various Composition

Concrete is one of the most common building materials and its durability has been observed with increased attention. It can be said that concrete durability is closely connected with permeability and, generally speaking, the quality of its surface layer. Evaluating the quality of the surface layer of concrete is a rather difficult issue which can be addressed in different ways. The authors of this paper focus on three internationally used methods – TPT, GWT and ISAT. The paper sums up the results of the experiment whose goal was to investigate the influence of concrete composition on the outputs of the above-mentioned methods. For the purposes of the experiment, specimens were made using 9 mixtures which differed only in the amount of cement and plasticiser, i.e. water/cement ratio. The experiment was designed and evaluated using the statistical methodology DOE (Design of Experiment). Next, the paper discusses a new view of statistical evaluation of test results of the methods described above.