PSI - Issue 13

Libor Topolář et al. / Procedia Structural Integrity 13 (2018) 1177 – 1182 Author name / Structural Integrity Procedia 00 (2018) 000 – 000

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burning objects. Since concrete does not burn, it does not produce smoke, gases or toxic fumes in case of a fire. Unlike some plastics or metals, there are no hot parts falling off concrete that may subsequently cause another fire. Concrete does not increase the fire load, however, in case of a fire it loses its mechanical properties such as strength, load-bearing capacity etc. Collepardi (2010) and Bodnárová et al. (2013). Suitable modification of concrete compositions and selection of input materials can increase the resistance of concrete to elevated temperatures. The presented article discusses mixtures that have different aggregate grain sizes but the same amount and type of the used cement. The temperature affecting a concrete structure plays an important part in the use of concrete for different engineering structures (nuclear reactors, blast furnaces, cooling towers and high-rise buildings). Particularly in case of an accident or terrorist attack, the load-bearing constructions can be subjected to elevated temperatures. Temperature changes of concrete cause both physical and chemical changes in the cement matrix as well as in the aggregates. The analysis of the damage is therefore even more complicated due to the fact that concrete is composed of different components (Pazdera et al. (2017)). Table 1 presents the expected changes of each concrete component (cement matrix, water and gravel aggregate) depending on the changes of the temperature.

Table 1. The changes of thermally degraded concrete (Hager (2013)). Temperature ( o C) Expected changes 20 – 200

slow capillary water loss and reduction in cohesive forces as water expands

80 – 150 150 – 170

ettringite dehydration and C-S-H gel dehydration

gypsum decomposition (CaSO 4 · 2H 2 O) and physically bound water loss break up of some siliceous aggregates (flint) and critical temperature of water

~ 350

460 – 540

Portlandite decomposition, thus Ca (OH) 2 → CaO + H 2 O quartz phase change β − α in aggregates and sands second phase of the C-S- H decomposition, formation of β -C 2 S

~ 570

600 – 800

~ 840

dolomite decomposition

930 – 960

calcite decomposition CaCO 3 → CaO + CO 2 , carbon dioxide release and start of ceramic binding which replaces hydraulic bonds

1050

basalt melting

> 1200

total decomposition of concrete, melting

The acoustic emissions method (AE) was used to evaluate the degree of damage to concrete samples exposed to elevated temperatures. The AE method belongs among the most advanced methods for material engineering and fatigue applications in Kreidl (2006). The source of AE can be of various phenomena depending on the type of the material. In concrete, they can cause microscopic and macroscopic damage as well as tearing off or shifting of the reinforcement. In case of composites, the AE originates in matrix cracking, delamination, separation of the matrix from the fibres, fibre tearing, and fibre dislocation (Grosse (2008)). Most acoustic emission sources are associated with damage. Detection and monitoring of these emissions is commonly used to predict material failure. The AE method is considered useful in fatigue testing and during destructions. The advantage of AE is that it is rather a global than local method, which means that the technique focuses more on the overall structure than on a small local area. As a result, monitoring can be done in a short time and is not demanding in Ativitavas (2002). However, the disadvantage of this technology is that the AE depends on the applied load. This means that some distortions may not generate detectable acoustic emission under certain types and levels of load. The AE method is successfully used today for monitoring metals, composite materials, rocks, and other materials. Unlike most other non-destructive testing methods, the AE method monitors only active defects occurring inside the observed structure. These defects can only form when the observed structure is loaded. Passive defects or structure shapes do not have any significant effect on the localization of AE. Acoustic emission occurs in an AE source when energy is released by stimulation of the internal or external stress (Grosse (2008) and Nair (2010)).