Éva Lublóy

Improved fire resistance by using Portland-pozzolana or Portland-fly ash cements

Our study was directed to the analysis of the influence of various types of cements on the behaviour of concrete at high temperatures. In our experiments binary blended and ordinary Portland cements were involved: two Portland cements with different clinker compositions and Portland cements containing pozzolanic additives as replacement of clinkers. In the first part of the study we focused on the influence of cement types where cement paste specimens were investigated. Then, based on the results of cement paste specimens, concretes specimens were prepared with some selected types of cements. Most important observation of our experimental study was that the pozzolanic additives and their increased amount have a favourable effect on the heat resistance (fire resistance) properties of concrete.

Modifications of material properties due to elevated temperatures

Recent fire cases in tunnels (Mont blanc, 20 March 1999, Gotthardt, 24 October 2001) and in highrise buildings (World Trade Center, 11 September 2001) indicated again the importance of fire research. Fast development of construction technology requires new materials. Initiation and development of fire are strongly influenced by the choice of construction materials [8]. In addition to their mechanical properties, their behaviour in elevated temperature is also of high importance [3].

Behaviour of tyres in fire

The increasing numbers of used tyres constitute a serious threat to the natural environment. The progress made in recent years in the management of polymer wastes has meant that used tyres are starting to be perceived as a potential source of valuable raw materials. The objective of this research was to study the burning characteristics of various used tyres. Waste tyres of seven producers have been tested. In order to understand thermal properties, three different assessment methods were used to study the behaviour of the material: (1) determination of ash content, (2) flame propagation test and (3) thermal analysis. The ash content test can be used to analyse how many percentage of the tyre leaves in the form of smoke or gas. The flame propagation test gives information on the duration of the combustion and the degree of smoke generation. Results of thermal analysis (TG/DTG/DTA) show the degree and speed of the mass changes of the different tyre types, the enthalpy change and the temperature of the reactions during heating. By combustion tests, it was modelled how the tyres behave when they are burning in incineration plant. In terms of recycling, those tyres are better, which have low decomposition temperature and smaller residual mass. This also means that maximum combustion heat can be recovered. After burning, the samples showed the greatest difference in loss of mass; however, all are different in flammability, afterglow time and their thermal stability.

Fire Resistance of Concretes with Blended Cements

An extensive experimental study has been carried out to analyse the post-heating characteristics of concrete subjected to high temperatures up to 800 °C. Major parameters of our study were the type and amount of supplementary cementitious materials (slag, fly ash, trass, silica fume, metakaolin) in cements and the level of maximum temperature (50, 150, 300, 500 or 800 °C). Present study includes analyses of surface cracking and residual compressive strength.

Effect of thermal transformation and stability on the flammability of PAN precursors-based carbon fibres

During the development of the carbon fibres and oxidized fibres, the aim was to get a material with extraordinary mechanical properties. Nowadays, it becomes more and more important to make it also fire resistant. To achieve the fire resistance of the raw material PAN, a multistage technology process is needed in which its microstructure and flammability are changed. These special properties can be achieved by heat treatment, during which the microstructures of the fibres change at each step. The thermal transformation involves thermal stability. The thermal transformation can be followed by measuring the limited oxygen index (LOI), and compare it to traditional testing methods, such as infrared and Raman spectroscopy and elementary analysis. The results show that the oxygen index of the samples is a very sensitive and characteristic parameter of flammability. It seems a good correlation between the LOI and the structural parameters of the samples. The structural transformations are not linear but abrupt. Considering the temperature treatment ranges which are responsible for the increasing oxygen indices, three distinct areas can be determined. It has been confirmed by measurements that LOI can express the grade of stability. Based on spectroscopic measurements, it has also been confirmed that the oxidized fibres dramatically decompose when burning in enriched air with more than 50% oxygen content.

Improved Fire Resistance by Using Different Types of Cements

Composition and microstructure of hardened cement paste have important influences on the properties of concrete exposed to high temperatures. An extensive experimental study was carried out to analyse the post-heating characteristics of concretes subjected to temperatures up to 800 °C. Major parameters of our study were the content of supplementary materials (slag, fly ash, trass) of cement (0, 16 or 25 m%) and the value of maximum temperature. Our results indicated that (i) the number and size of surface cracks as well as compressive strength decreased by the increasing content of supplementary materials of cements due to elevated temperature; (ii) the most intensive surface cracking was observed by using Portland cement without addition of supplementary materials. The increasing content of the supplementary material of cement increased the relative post-heating compressive strength. Tendencies of surface cracking and reduction of compressive strength were in agreement, i.e. the more surface cracks, the more strength reduction.