Residual Compressive Behavior of Self-Compacting Concrete after High Temperature Exposure-Influence of Binder Materials.

This paper presents an experimental investigation of the compressive behavior of high-strength self-compacting concrete exposed to temperatures up to 600 °C. Ten different concrete compositions were tested, in which part of the cement (by weight) was replaced by three different mineral additives (5-15% metakaolin, 20-40% fly ash and 5-15% limestone). The stress-strain curves, compressive strength, modulus of elasticity and strain at peak stress were evaluated from uniaxial compression tests. Scanning electron microscope micrographs were also taken to evaluate the damage caused by the high temperatures. A sharp decrease in mechanical properties and an increase in peak strain were observed already after 200 °C for all mixes tested. The different mineral additives used in this study affected the variations of residual compressive strength by 24% and peak strain by 38%, while the variations of residual modulus elasticity were 14%. Comparing the obtained results with the recommendations for compressive strength given in regulatory code EN 1992-1-2 for high strength concrete, it can be concluded that the strength loss observed in EN 1992-1-2 at temperatures up to 400 °C is too conservative. The Popovics model for the relationship between stress and strain provided a good approximation for the experimentally determined stress-strain curves at different temperatures.

Influence of Wood Fly Ash on Concrete Properties through Filling Effect Mechanism.

This paper presents the results of an experimental study aimed at determining the influence of wood fly ash (WFA) from three Croatian power plants on the properties of concrete. First, the chemical and physical properties of WFA's were determined. It was found that these properties are highly influenced by combustion technology, the type and parts of wood used as fuel, and the local operating conditions. Subsequently, workability, heat of hydration, stiffness development, 28-day compressive strength, apparent porosity, and capillary absorption were determined on concrete mixes prepared with WFA as cement replacement from 5-45% by weight. Cement replacement up to 15% with the finest WFA accelerated hydration, stiffness development, and increased compressive strength of concrete up to 18%, while replacement with coarser WFA's led to a decrease in compressive strength of up to 5% and had more gradual heat liberation. The dominant effect that could explain these findings is attributed to the filler and filling effect mechanisms. At the same time replacement content of up to 45% had very little effect on capillary absorption and could give concrete with sufficiently high compressive strength to be suitable for construction purposes.

On-line identification and reconstruction of finite automata with generalized recurrent neural networks.

In this paper finite automata are treated as general discrete dynamical systems from the viewpoint of systems theory. The unconditional on-line identification of an unknown finite automaton is the problem considered. A generalized architecture of recurrent neural networks with a corresponding on-line learning scheme is proposed as a solution to the problem. An on-line rule-extraction algorithm is further introduced. The architecture presented, the on-line learning scheme and the on-line rule-extraction method are tested on different, strongly connected automata, ranging from a very simple example with two states only to a more interesting and complex one with 64 states; the results of both training and extraction processes are very promising.