Q2. What are the future works mentioned in the paper "Unbonded post tensioned concrete in fire: a review of data from furnace tests and real fires" ?
Because full-scale fire tests on actual or model UPT buildings, however badly needed, are unlikely to occur in the foreseeable future, research is currently restricted largely to using computational analysis tools [ 52, 53 ] to study their response to fire. A detailed experimental and computational examination of the potential consequences of localized heating on UPT tendons is therefore needed with a view to eventually developing the ability to defensibly model real UPT buildings in real fires.
Q3. What is the reason for the lack of shearcritical designs?
• Shear-critical designs – Because UPT slabs allow shallow, flat floor-plates over large spans they are often shearcritical under ambient design loads [12, 31].
Q4. What is the reason why the UPT slabs are less likely to spall?
The fact that higher concrete strengths are used in modern UPT elements also results in less reserve cross-sectional capacity being available should fire-induced spalling occur.
Q5. What is the risk of a slab breaking during fire?
Given that tendon rupture during fire, before the prescribed fire rating is achieved, is a credible concern in UPT structures, slab pre-compression may be lost during fire.
Q6. How long did the beams experience spalling?
The beams experienced spalling, typically to a depth not greater than 25 mm but in some areas up to 64 mm deep, beginning at 14 minutes of fire and continuing for about one hour.
Q7. What is the common cause of tendons to rupture?
Localized heating of tendons at these cracks (even in the absence of cover spalling) could induce localized heating and premature tendon rupture during fire.
Q8. What are the potential concerns associated with UPT elements during fire?
the structural optimization and efficiency of UPT elements generate potential concerns associated with their performance during fire.
Q9. How many tests were observed with a premature tendon rupture?
Premature tendon rupture during heating was observed in at least 9 of the 27 tests (33%), and was particularly evident in tests with multiple spans and localized heating.
Q10. How many slabs were not cast integrally with precast planks?
The remaining six slabs (not cast integrally with precast planks) had minimum clear covers to the prestressed reinforcement, at the middle of the central span, varying between 20 mm and 40 mm.
Q11. What is the key conclusion stated in the PCA report on these tests?
A key conclusion stated in the PCA report on these tests [8], which is reiterated in a later summary paper by Gustaferro [42] and which appears to have been embraced in the years since these tests, is that beams with UPT reinforcement have about the same fire endurance as their counterparts with bonded prestressed reinforcement.
Q12. What is the importance of the initial load ratio in a standard furnace test?
This is of considerable interest, since the initial load ratio is universally assumed to be of central importance to the structural fire resistance of a flexural assembly in a standard furnace test – to the extent that larger fire endurances are assigned to assemblies with lower load ratios for some types of construction [42, 45].
Q13. How many mm of clear concrete cover was required to the UPT cables?
The minimum clear concrete cover to the UPT cables, each of which consisted of five 6.3 mm diameter cold drawn, high tension stress-relieved wires, each having a minimum guaranteed ultimate tensile strength of 1655 MPa, was 25 mm in the larger spanning direction.
Q14. What was the effect of compression membrane action on the cardington concrete frame fire test?
The beneficial effects of compression membrane action in preventing collapse of two-way reinforced concrete flat plate slabs in fire, even when the majority of the bottom steel reinforcement is lost due to heating and/or cover spalling, was clearly shown during the concrete frame fire test performed at Cardington in the 1990s [32].
Q15. What is the reason why a slab may experience greater deflections during fire?
A UPT slab may therefore experience proportionally greater deflections during fire, due to thermal bowing and smaller lateral restraint forces, than would occur for a non-prestressed slab.
Q16. What is the likelihood of cover spalling?
In combination with the use of expanded shale aggregates, the preconditioning makes the likelihood of cover spalling very low for this slab.
Q17. How did the bonded PT slabs perform?
recent furnace testing by Bailey and Ellobody [3] (described below) showed that bonded PT slabs were capable of achieving their designed target fire resistance whereas otherwise identical UPT slabs had fire resistances that were lower than expected.
Q18. What are the only tests to consider the possible influence of important parameters noted previously?
These are also the only available tests to rationally consider the possible influence of important parameters noted previously including: cover spalling, load ratio, and the presence and amount of “secondary” bonded mild steel reinforcement.
Q19. What is the difference between pre-conditioned and non-prestressed slabs?
Such slabs have less reserve shear capacity than their non-prestressed counterparts, making them more susceptible to shear failure during fire.
Q20. Why is the span-to-depth ratio used in available furnace tests unrealistic?
It is also worth noting that the span-to-depth ratios used in available furnace tests on UPT members have generally been unrealistically small due to limits in available furnace sizes.
Q21. How was the relative humidity in the slab at the time of testing?
The relative humidity (RH) in the slab at the time of testing was 62% at a depth of 38 mm (≈1.9% moisture by mass) and the concrete compressive strength was 41 MPa.