Aspects of modelling plain and reinforced concrete at elevated temperatures
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Date
29/11/2012Author
Knox, Joanne J.
Metadata
Abstract
Extreme events such as the Mont Blanc Tunnel fire in 1999 (Bettelini et al. 2001)
or the Windsor Tower fire in 2005 (Calavera et al. 2005) have shown how
concrete failure at elevated temperatures can be hazardous to the safety of
members of the public. Generally, there is an absence of understanding of the
mechanical behaviour of both plain and reinforced concrete at elevated
temperatures, which is essential for computational modelling. Since fire is an
extreme event, a certain amount of damage within the structure would be seen
to be permissible within its performance objectives. This necessitates analysis in
the post-peak regime. As a material, concrete has a very low value of thermal
conductivity. This means that large thermal gradients often occur within
concrete, causing differential expansion of the material. This, coupled with the
change in mechanical properties at elevated temperatures, further complicates
analytical analysis procedures.
This study investigates issues associated with computational modelling of plain
and reinforced concrete at elevated temperatures and its residual behaviour
(behaviour when tested after the material has been heated, for example in a fire,
and then cooled). In order to achieve this, first the constitutive material
properties of both plain and reinforced concrete at ambient and elevated
temperatures were investigated. The study showed that mesh sensitivity and
localisation of strain softening occurs in plain concrete under both tensile and
compressive loading. Path dependency of the stress-strain behaviour of plain
concrete was also demonstrated, when it was subjected to loading and heating.
Tension stiffening was included in the reinforced concrete material model, to
represent the interaction between concrete and reinforcing steel. Complex
behaviours were seen for simple reinforced concrete benchmark tests, due to
changing material properties at elevated temperatures and differential thermal
expansion of steel and concrete. Non-linear load-displacement relationships were seen as a result of complex load-sharing between concrete and
reinforcement.
A hypothesis was proposed – that variation of temperatures during heating and
cooling of a specimen will cause damage, and hence material degradation, in
plain and reinforced concrete. On investigation, it was seen that damage due to
differential thermal expansion plays a small part in the reduction of elastic load-displacement
slope and peak strength seen in experimental data on residual
tests, indicating that other factors identified in previous research also affect the
residual behaviour of plain and reinforced concrete. Indeed, in reinforced
concrete, when tension stiffening was included, it was found that damage due to
differential thermal expansion and contraction had a negligible effect on the
residual response in the pre-peak regime.
The study also found that for a simply supported beam pure thermal expansion
caused a localised response, while pure thermal gradient gave distributed yield.
When both were present, in this study, distributed yield with no mesh
sensitivity was seen. Realistic heating of a restrained reinforced concrete plane
strain model caused compressive stresses accompanied by tensile longitudinal
total strains and tensile longitudinal plastic strains throughout the depth of the
slab, with the largest values occurring near to the model supports. Damage and
recovery variables were found to have no effect on the response of the model.
When a portal frame was exposed to heating, plastic strains were distributed
throughout the beam, with column rotation limiting downward thermal bowing
due to a uniformly distributed load or thermal gradient present. Application of
displacement loading causing plastic damage changed the behaviour of the
structure under heating – instead of symmetrical compressive plastic strains
being induced, areas of varying tensile and compressive strain were caused
within the beam.
Throughout, simple, easily reproducible simulations were used so that single
parameters could be altered and considered. This was important, so that the
important parameters to computational modelling could be identified. These can be used to guide experimental series to ensure that they are investigated, in
order to improve computational material models. Not all variations of
parameters were investigated in this study, but it is clear where further
repetition would be beneficial (e.g. in varying thermal expansion and thermal
gradient ratios in heating regimes). This study looks to address experimentalists
and people working in structural analysis, who would be interested in the
parameters investigated, as well as practitioners who may want to use these
results.