To achieve thinner and longer floor slabs, rapid construction, and tight control of inservice
deflections, modern concrete structures increasingly use high-strength, posttensioned
prestressing steel as reinforcement. The resulting structures are called posttensioned
(PT) concrete. Post-tensioned concrete slabs are widely believed to benefit
from ‘inherent fire endurance.’ This belief is based largely on results from a series of
standard fire tests performed on simply-supported specimens some five decades ago.
Such tests are of debatable credibility; they do not capture the true structural
behaviour of real buildings in real fires, nor do they reflect modern PT concrete
construction materials or optimization methods. This thesis seeks to develop a more
complete understanding of the structural and thermal response of modern
prestressing steel and PT concrete slabs, particularly those with unbonded
prestressing steel conditions, to high temperature, in an effort to steer current practice
and future research towards the development of defensible, performance-based, safe
fire designs.
An exhaustive literature review of previous experimentation and real case
studies of fire exposed PT concrete structures is presented to address whether current
code guidance is adequate. Both bonded and unbonded prestressing steel
configurations are considered, and research needs are identified. For unbonded
prestressing steel in a localised fire, the review shows that the interaction between
thermal relaxation and plastic deformation could result in tendon failure and loss of
tensile reinforcement to the concrete, earlier than predicted by available design
guidance. Since prestressing steel runs continuously in unbonded PT slabs, local
damage to prestressing steel will affect the integrity of adjacent bays in a building. In
the event that no bonded steel reinforcement is provided (as permitted by some
design codes) a PT slab could lose tensile reinforcement across multiple bays; even
those remote from fire. Using existing literature and design guidance, preliminary
simplified modelling is presented to illustrate the stress-temperature-time interactions
for stressed, unbonded prestressing steel under localised heating. This exercise
showed that the observed behaviour cannot be rationally described by the existing
design guidance. The high temperature mechanical properties of modern prestressing steel are
subsequently considered in detail, both experimentally and analytically. Tests are
presented on prestressing steel specimens under constant axial stress at high
temperature using a high resolution digital image correlation (DIC) technique to
accurately measure deformations. A novel, accurate analytical model of the stresstemperature-
time dependent deformation of prestressing steel is developed and
validated for both transient and steady-state conditions. Modern prestressing steel
behaviour is then compared to its historical prestressing steel counterparts, showing
significant differences at high temperature.
Attention then turns to other structural actions of a real PT concrete structure
(e.g. thermal bowing, restraint, concrete stiffness loss, continuity, spalling, slab
splitting etc.) all of which also play inter-related roles influencing a PT slab’s
response in fire. A series of three non-standard structural fire experiments on heavily
instrumented, continuous, restrained PT concrete slabs under representative sustained
service loads were conducted in an effort to better understand the response of PT
concrete structures to localised heating. To the author’s knowledge this is the first
time a continuous PT slab which includes axial, vertical and rotational restraint has
been studied at high temperature, particularly under localised heating. The structural
response of all three tests indicates a complex deflection trend in heating and in
cooling which differs considerably from the response of a simply supported slab in a
standard fire test. Deflection trends in the continuous slab tests were due to a
combination of thermal expansion and plastic damage. The test data will enable
future efforts to validate computational models which account for the requisite
complexities.
Overall, the research presented herein shows that some of the design
guidance for modern PT concrete slabs is inadequate and should be updated. The
high temperature deformation of prestressing steel under localised heating, as would
be expected in a real fire, should be considered, since uniform heating of simplysupported
elements is both unrealistic and unconservative with respect to tensile
rupture of prestressing steel tendons. The most obvious impact of this finding would
be to increase the minimum concrete covers required for unbonded PT construction,
and to require adequate amounts of bonded steel reinforcement to allow load shedding to the bonded steel at high temperature in the event that the prestressing
steel fails or is severely damaged by fire
