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By: Pasindu Weerasinghe, The University of Melbourne, Australia
Priyan Mendis, The University of Melbourne, Australia
Maurice Guerrieri, Victoria University, Australia
Kate Nguyen, RMIT University, Australia
The use of concrete flat slabs in multi-storey buildings is increasing, and the current fire design guidelines for flat slabs are based on research carried out a few decades ago. In this work, the fire resistance of flat slabs was investigated at a large scale (3.78m x 4.75m) under structural loading and exposed to ISO 834 fire conditions. The test was also extended to investigate the behaviour during the cooling phase, as it is critical with restrained support conditions. Results show that the duration of fire resistance is significantly higher than that of similar tests with no lateral restraint. Heat propagation and deformation recovery during the cooling phase were also measured. Improved fire resistance duration suggests that the punching shear resistance is enhanced by restrained support conditions.
*This is a short version of the journal article titled ‘Large-scale experiment on the behaviour of concrete flat slabs subjected to standard fire’ published in Journal of Building Engineering [1].
Existing Design Practice for Fire Design of Concrete Flat Slabs
Current code-based approach to structural fire design of concrete flat slabs specifies a minimum thickness and a minimum cover to withstand a particular fire duration [2,3]. Thicknesses specified for flat slabs are considerably higher than thicknesses specified for conventional slabs with beams as flat slabs tend to due to punching shear near the slab-column connection (see Figure 1). However, the design guidelines are based on a limited number of flat slab fire tests [4-7] where most of the specimens are small-scale isolated specimens with no lateral restrains, except for one series of test that considered restrained support conditions on small-scale specimens [8].
Figure 1. Typical punching shear failure of a concrete flat slab
Experimental Investigation on Fire Resistance of Concrete Flat Slabs
The authors tested a large-scale flat slab specimen (3.78m x 4.75m x 0.18m) under standard ISO 834 fire [9] with structural loading. Details of the specimen are shown in Figure 2 – 4. The supporting frame provided lateral restraints in order to more closely represent the continuous action of flat slabs in actual buildings.
Figure 2. Slab specimen with the supporting frame before placing on the furnace
Figure 3. Test set-up
Figure 4. TCs, String pots and hydraulic actuator fixed to the specimen while it is on top of the furnace
Test Results and Discussion
The behaviour of the slab
The tension side of the slab specimen was exposed to ISO 834 standard fire, as shown in Figure 5.With the downward deformation due to heat, the pressure of the hydraulic actuator was dropping as the test started. In order to maintain the same load, continuous pumping was done while monitoring the load level. It was managed to keep the load level in the range of 225kN-275kN for 120 mins, with only a small drop, as shown in Figure 6. However, the length of the piston reached the limit at 120 mins, and therefore the load could not be maintained beyond this point
Figure 5. Temperature inside the furnace
Figure 6. Applied load on the slab
Temperature Variation
Figure 7 shows the temperature recordings obtained from TCs fixed within the depth, as shown in Figure 4. It can be seen from the graphs that the exposed surface has reached a maximum temperature of 1000 o C at 3.5hrs, but the temperature of the unexposed surface is only 55oC. It shows the good insulation characteristic of concrete, and importantly the unexposed surface temperature is well below the 180 oC limit specified by AS1530.4 [10] as the failure criteria for insulation. Furthermore, the measured temperatures are in close agreement with the temperature profiles for slabs given in EC2 part 1-2 Annex A [3].
Deflection
After heating commenced, downward deflection started to increase considerably, although the applied load was kept constant (see Figure 8). This implies that the deflection towards the fire was due to the thermal strain as a result of elevated temperature. Deflection at the centre was higher, and it decreases as it goes away from the centre as expected. Initial load was maintained until 2 hrs and the maximum deflection at that time was 85mm which is much less than the deflection limit (L2/400d) for structural adequacy failure criteria specified by AS 1530.4 [10]. Therefore, it can be confidently stated that the slab has a fire-resistance period of 2hrs.
Figure 7. Temperature variation of the slab during heating and cooling down. (a)exposed surface – heating, (b)exposed surface-cooling, (c)At 45mm-heating, (d) At 45mm-cooling, (e)At 90mm-heating, (f)At 90mm-cooling, (g)At 135mm-heating, (h)At 135mm-cooling, (i)Unexposed surface-heating, (j)Unexposed surface-cooling
Figure 8. Deflection along the short span during (a)heating and (b)cooling
Fire Resistance
Fire resistance is defined as the time until the slab reaches one of the failure criteria for structural adequacy, integrity and insulation. The maximum deflection of 95mm reached during the heating phase is well below the L2/ 400d failure criteria for structural adequacy. There were no visible cracks during that period. Although the slab was cured only for 28 days, there was no spalling during the complete test. Therefore, it is evident that the slab has passed the structural adequacy and integrity criteria during the heating phase.
In contrast to the guidelines specified by the design codes which requires to have a 200mm thick slab with 35mm axis distance to have a 2hr FRL, the 180mm thick slab used in the experiment with 35mm axis distance has survived more than 2hrs of standard fire exposure. Due to the lateral restraint against expansion, membrane action is allowed to develop during fire, and this could have influenced the enhancement in fire performance in this particular case. It closely represents the actual conditions in a building as the flat slab is continuous and adjacent slab panels could provide lateral restraint against heating; hence, membrane actions will be developed.
References
[1] P. Weerasinghe, K. Nguyen, P. Mendis, M. Guerrieri, Large-scale experiment on the behaviour of concrete flat slabs subjected to standard fire, Journal of Building Engineering 30 (2020): 101255. doi:https://doi.org/10.1016/j.jobe.2020.101255
[2] Standards Australia, AS 3600 - Concrete structures, (2009).
[3] European committee for standardization, Eurocode 2: Design of concrete structures - Part 1-2: General rules -Structural fire design, (2004).
[4] K. Kordina, Flat slabs under fire–Redistribution of the internal forces and punching tests, Inst. Für Baustoffe, Massivbau Und Brand. Tech. Univ. Braunschweig, Ger. CEN/TC. 250 (1993) 1–2.
[5] E. Annerel, L. Lu, L. Taerwe, Punching shear tests on flat concrete slabs exposed to fire, Fire Saf. J. 57 (2013) 83–95. doi:10.1016/j.firesaf.2012.10.013.
[6] J.-S. Liao, F.-P. Cheng, C.-C. Chen, Fire resistance of concrete slabs in punching shear, J. Struct. Eng. 140 (2013) 4013025. doi:https://doi.org/10.1061/(ASCE)ST.1943-541X.0000809.
[7] M. Ghoreishi, A. Bagchi, M.A. Sultan, Punching shear behavior of concrete flat slabs in elevated temperature and fire, Adv. Struct. Eng. 18 (2015) 659–674. doi:https://doi.org/10.1260/1369-4332.18.5.659
[8] H.K.M. Smith, T.J. Stratford, L.A. Bisby, Punching shear of restrained reinforced concrete slabs under fire conditions, in: 8th Int. Conf. Struct. Fire, 2014: pp. 11–13.
[9] International Organization for Standardization, ISO 834-1 : Fire-resistance tests - Elements of building construction - Part 1: General requirements, (1999).
[10] Standards Australia, AS 1530.4: Methods for fire tests on building materials, components and structures–Part 4: Fire-resistance test of elements of construction, (2014).