Open Access

Ein weiterer Fokus des 63. DAfStb-Forschungskolloquiums (Tagungsband: DOI 10.26272/opus4-61338) lag auf den Bauteil- und Bauwerksuntersuchungen. Insbesondere wurde hier ein Einblick in die Forschungsaktivitäten in den BAM-Themenfeldern „Infrastruktur“ und „Energie“ gegeben. Thematisch wird dabei der Bogen von der Dauerhaftigkeit von Betonfahrbahndecken über die Extrembeanspruchung von Bauteilen und Bauwerken mittels Brand und Impact bis zum Bauwerksmonitoring und der Zustandsanalyse von Bestandsbauwerken gespannt.

The cement industry is looking to reduce its overall CO2 footprint. Greener manufacturing can be achieved by the introduction of more clinker-reduced cements. Concrete exposed to fire tends to show explosive spalling caused by thermomechanical and thermohydraulic processes. It is therefore essential to determine how concretes with clinker-reduced cements behave under fire exposure, especially for concretes containing calcined clays, as these are expected to be the supplementary cementitious material of the future. In this paper, normal strength concretes with four different cements (CEM I, CEM II/A-LL, CEM III/A and CEM II/B-Q) were examined for their fire-induced spalling behaviour. In addition, a mix with PP fibres was investigated for each concrete. The experiments were conducted on ring-restrained cylindrical specimens exposed to the hydrocarbon fire curve. The results showed that the cement type influences spalling behaviour. Samples with CEM I spalled the least, followed by CEM II/A-LL and CEM III/A. Finally, samples with CEM II/B-Q showed the most severe damage. It was found that the spalling behaviour of different concretes correlates with the moisture content before exposure to fire, meaning that higher moisture content leads to higher spalling susceptibility. The use of 2 kg/m³ PP fibres completely inhibited spalling regardless of the cement type used and therefore remains a successful avoidance strategy.

Zur Verminderung der CO2-Emissionen der Bauindustrie gewinnen klinkereffiziente Zemente immer mehr an Bedeutung und werden vermehrt in Ingenieurbauwerken eingesetzt. Das brandinduzierte Abplatzverhalten von Betonen mit diesen Zementen ist allerdings nur bedingt bekannt und ist u.a. im Tunnelbau von hoher Bedeutung. Im Zuge dessen, wurden normalfeste Betone mit vier verschiedenen Zementen (CEM I, CEM II/A-LL, CEM III/A und CEM II/B-Q) auf ihr brandinduziertes Abplatzverhalten untersucht. Die Ergebnisse zeigen, dass Betone mit klinkereffizienten Zementen zu stärkeren Abplatzungen neigen.

The cement industry is responsible for high CO2 emissions, which occur during the production of Portland cement clinker. To reduce These emissions, blended cements become more and more popular. Therefore, it is expected that these cements find frequent application in infrastructure and housing construction in the future. The increased use leads to a higher probability of concrete buildings with blended cements being exposed to extreme conditions such as fire. Concrete under fire can experience heavy damage in the form of explosive spalling which is caused by thermohydraulic and thermomechanical mechanisms leading to the lowering of the cross section. Furthermore, the reinforcement may be subjected directly to the fire, which can lead to a rapid decline in its load-bearing capacity. The current state of the art showed that the cement type used can show great impact on the fire induced spalling behavior. However, large proportion of the reported spalling results took place under unsuitable testing conditions with insufficient evaluation. The smaller fraction of more profound spalling experiments showed inconsistences and contradictions, whereby no clear relation between cement type and spalling susceptibility could be drawn. Against this background in depth spalling experiments following the recommendations of RILEM TC 256-SPF of concrete with four different types of cement including Portland cement (CEM I), limestone cement (CEM II/A-LL), slag cement (CEM III/A) and Portland pozzolana cement (CEM II/B-Q) were carried out. Summarizing, CEM I concrete showed the lowest amount of spalling, followed by CEM II/A-LL and CEM III/A concrete and lastly CEM II/B-Q concrete. To understand the appearing differences in spalling susceptibility, the influence of the thermally induced moisture transport was studied by 1H-NMR relaxometry. From a thermohydraulic point of view, it was shown that initial differences of permeability and moisture content are responsible for the variations in spalling susceptibility. Furthermore, the appearance of the moisture clog was confirmed in every sample. To further analyze the contribution of the thermohydraulic mechanism, the phase composition of the cement paste was analyzed before and after high temperature exposure regarding the dehydration behavior. Less C-S-H and portlandite were observed in CEM III/A and CEM II/B-Q pastes. However, the C-S-H seemingly showed increased thermal stability. In addition, lower amounts of AFt and AFm phases were found in blended cement pastes corresponding to lower amounts of degradation and water released at lower temperatures. In conclusion, the increased spalling proneness in blended concretes is caused by higher initial moisture contents paired with lower initial permeabilities and less amounts of early dehydrating phases. It was also shown that the addition of PP-fibers remains a successful avoidance strategy regardless of the cement type used, by increasing permeability and thus the release of water vapor.

Blended cements are gaining increasing popularity due to their lower CO2-footprint in comparison to ordinary Portland cement (OPC). However, this growing use raises the potential risk of buildings made with blended cement concrete being exposed to fire, which can lead to heavy damages caused by explosive concrete spalling. It has already been shown that the cement type strongly influences the fire-induced concrete spalling and the thermally induced moisture transport, however, to understand the mechanisms behind these findings the thermal decomposition behavior of the cementitious matrix must be investigated more systematically. Therefore, the phase content of three blended cement pastes (CEM II/A-LL, CEM III/A and CEM II/B-Q) was studied in comparison with a Portland cement paste (CEM I) after temperature exposure to 20 ◦C, 105 ◦C, 300 ◦C and 500 ◦C. Clear differences in the initial phase composition and their dehydration behavior between the individual cement types were recognized. In conclusion, blended cements showed lower amounts of AFt and AFm phases and additionally lower amounts of portlandite and C-(A)-S-H were found in CEM III/A and CEM II/B-Q pastes. The results suggest that higher AFt and AFm contents in CEM I, which are associated with greater water release at relatively low temperatures may ultimately reduce the spalling risk. Furthermore, C-(A)-S-H in CEM III/A and CEM II/B-Q showed increased thermal stability and large amounts of non-hydrated phases were found in every blended cement paste. Both of those aspects might contribute to thermomechanical spalling and the overall increased spalling susceptibility observed in blended cement concrete.