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- High strength concrete (2)
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High-strength concrete (HSC) will experience thermal microcracking, explosive spalling, and undesirable chemical changes when exposed to high temperatures, such as during fire, engulfment by lava flow, or nuclear meltdown. Knowledge of the resultant changes in mechanical, physical, and chemical properties is paramount for hazard mitigation. We present a multidisciplinary study on the influence of thermal-stressing on HSC. Our study shows that thermal microcracking in HSC initiates at 180 °C, is more prevalent during cooling, and exhibits the Kaiser 'temperature-memory' effect. We show that residual compressive strength, indirect tensile strength, ultrasonic wave velocities, and Youngs modulus and Poissons ratio decrease, whilst porosity and permeability increase with increasing temperature. We discuss these data in terms of the chemical changes during thermal-stressing, provided by thermo-gravimetric analysis, differential scanning calorimetry, and X-ray diffraction, and from optical microscopic analysis of thermally-stressed samples. We provide implications for thermally-damaged HSC structures and a new method for non-destructive monitoring.
This paper presents the results of an
experimental study on the transient strain of high
strength concrete (HSC) under heating up to 750 °C
and the impact of polypropylene (PP) fibers. Concerning
this topic only few results are available in the
literature and systematic investigations are missing.
However, basic knowledge is necessary for the
understanding of the internal damage processes in
the material as well as for heated structures. The
transient strain during heating can be separated in two
basic components: the free thermal strain and the
mechanical strain. They were experimentally determined
exemplarily for one HSC. For the determination
of the mechanisms of transient strain and particularly
the influence of PP fibers different techniques were
applied. In this context the monitoring of the microcracking
was done for the first time with acoustic
emission analysis in combination with ultrasonic
measurements. This new approach helps fundamentally
to explain the impact of PP fibers on free thermal
strain and mechanical strain during heating up.
Furthermore weight loss measurements were carried
out to characterize the moisture transport. It was
shown that the PP fibers cause an acceleration of the
moisture transport in the temperature range from 200
to 250 °C which leads to drying shrinkage in opposite
direction to the free thermal strain. Hence this paper is
a contribution to the general understanding of the
impact of PP fibers in HSC at high temperatures and
emphasizes the important influence of PP fibers on the
thermal and mechanical induced strain of HSC.
Ausgehend von einer kritischen Analyse der Materialkennwerte für normalfesten (NFB) und hochfesten Beton (HFB) im Eurocode 2 wurden in grundlegenden Untersuchungen an der BAM exemplarisch für einen HFB die thermomechanischen Materialkennwerte bestimmt, um auf der Basis eines geeigneten Materialmodells das Verhalten von Stahlbetonstützen aus HFB unter Brandbeanspruchung zu simulieren. Die Validierung des Modells erfolgte mittels großmaßstäblicher Brandversuche an belasteten Stahlbetonstützen aus dem gleichen HFB wie in den Materialuntersuchungen. Es konnte gezeigt werden, dass die Festigkeit als charakteristischer Parameter des Materialmodells zwingend auf der Basis instationärer Versuche und nicht aus stationären Versuchen wie bei NFB üblich bestimmt werden muss. Mit den in vorgeschlagenen Materialbeziehungen für HFB wird das Tragverhalten der untersuchten Stützen dagegen deutlich überschätzt, was sich in den numerischen Simulationen insbesondere durch zu geringe Verformungen äußert.------------------------------------------------------------------------------------------------------
Development and Validation of an Advanced Calculation Model for High Strength Concrete Columns Subjected to Fire Loading
Following a critical analysis of the material parameters of normal strength (NSC) and high strength concrete (HSC) presented in Eurocode 2 the thermo-mechanical material parameters of one representative HSC were identified. Using these parameters and based on an appropriate material model the behavior of HSC columns was simulated. The validation of the model was realized through real scale fire resistance tests on loaded columns of the same type of HSC as in the material identification tests. It was found that the strength as a characteristic parameter of the material model has to be identified on the basis of transient creep tests and not of stationary tests, respectively as realized usually for NSC. It could be shown that the structural behavior of the tested columns is overestimated if the numerical simulations are based on the material properties of HSC as proposed in. This feature is expressed especially through insufficient displacements.