PSI - Issue 23

Lucyna Domagała / Procedia Structural Integrity 23 (2019) 342 – 347 Author name / Structural Integrity Procedia 00 (2019) 000 – 000

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partly melted. Such a performance of concretes LAC1 and LAC2 is connected with low melting temperatures for polypropylene (ca 160 o C) and polystyrene (ca 240 o C). However, these concretes revealed only minor changes in their surface appearance. LAC2 showed a net of surface cracks only after heating at 400 o C, while in LAC1 cracks did not develop at all. Meanwhile, in the case of expanded glass concretes LAC3 and LAC4, which revealed no visible interior changes, surface cracks were observed at lower temperatures (300 o C and 200 o C). Fig. 4 and Fig. 5 present external surfaces and interior fractures of specimens heated at the temperature of 400 o C.

LAC2

LAC3

LAC4

LAC1

Fig. 4. The specimens’ surface (4 0 mm · 40 mm) of different types of insulating-structural concretes after heating at 400 o C.

LAC1

LAC2

LAC3

LAC4

Fig. 5. The specimens’ fracture (40 mm · 40 mm ) of different types of insulating-structural concretes after heating at 400 o C.

The decrease of compressive strength of concretes subject to the temperature of 400 o C ranged from 27 % to 54 % and was ca two times lower for composites made of expanded glass than for those with perlite and polystyrene aggregates. Such a high strength decrease in the case of LAC1 and LAC2 was caused by melting of polystyrene granulate. It should be noted that there is almost no difference in strength of specimens of LAC1 and LAC2 heated at 200 o C and 300 o C. It was probably caused due to melting of polypropylene fibres and forming a net of tubules enabling to lower the inner pressure of vapors. However, glass fibres seemed to have no impact on resistance of LAC3 and LAC4 to high temperature up to 400 o C. It should be stated that all concretes revealed the strength decrease even at as low temperature as 100 o C. Usually the strength of normal-weight concretes as well as of structural lightweight concretes subject to heating at 100 o C does not decrease. Moreover, when these types of concretes are subject to heating at moist condition at 100 ÷ 200 o C they may reveal even a certain increase in strength resulting from evaporation of water. Meanwhile, tested concretes showed the strength decrease up to 5 % and up to 20 %, respectively for expanded glass concretes (LAC3 and LAC4) and perlite and polystyrene concretes (LAC1 and LAC2). In the case of concretes containing polystyrene aggregate this relatively high strength loss was caused by deformation of the granulate. Polystyrene usually begin to soften already at temperature of 80 o C ÷ 100 o C. Additionally, the phenomenon of strength decrease at such a low temperature may be explained by a scale effect. Structural concretes, usually containing coarse aggregate and used for larger elements, are tested on much bigger specimens with a side of at least 100 mm. In the case of insulating structural concretes applied to thin-walled elements specimens thickness is usually as low as 40 mm or even lower. Such smaller specimens heat up and dry up faster and there is no specimen core protected by external concrete layer. In result, the negative effect of cement paste dehydration can be observed already at the temperature of 100 o C. The scale effect also makes it impossible to compare the achieved results of high temperature resistance to results given in literature for other types of concrete.