Fire behaviour of HPLWC hollow core slabs: full scale furnace tests and numerical modelling Annibale Luigi MATERAZZI Professor University of Perugia Perugia, ITALY Marco BRECCOLOTTI Assistant Researcher University of Perugia Perugia, ITALY Summary The preliminary results of full scale furnace tests on HPLWC prestressed hollow core slabs are presented. The results supplied useful information to tune the available procedures for the evaluation of the thermal field under fire and of the load bearing capacity. The fire endurance of the tested specimens was reduced by the phenomenon of the explosive spalling, which confirmed to be a drawback of this innovative material. Keywords: High performance light weight concrete, hollow core PC slabs, fire resistance. 1. Introduction High performance light weight concrete (HPLWC) combines three properties that are favourable for civil engineering structures: the lightness (its unit weight is approximately 19 kn/m 3 instead of 25 kn/m 3 of normal weight concrete), the relatively high strength (f ck around 6 N/mm 2 ) and the superior durability. The aforementioned characteristics make this material very suitable for use in prestressed concrete hollow core slabs where, generally, the elements self weight represent a big amount of the total load. Hollow core slabs, made of standard characteristics concrete, are commonly used in the precast concrete industry to build large floors for industrial facilities or commercial buildings where a high fire resistance is commonly needed. While in the literature is reported the favourable fire performance of the normal concrete units [1], the fulfilment of this requirement is considered by several experts as a serious problem for the application of HPLWC, because it is well known that this material is potential exposed to the phenomenon of the explosive spalling, which may seriously affect the heat propagation and the fire endurance. The assessment of fire endurance can be carried out, according to the current standards and regulations (among which we remember [2], [3] and [4]), by means of and numerical methods. These latter are, generally, easier to carry out and cheaper than the other but require the knowledge of the thermal and mechanical properties of concrete and steel, data not always available in a complete form in literature. As a first step to overcome this problem, a research programme has been started at the University of Perugia. The programme of the planned investigations includes the execution of full scale furnace tests and the development of numerical model for the analytical assessment. 2. Furnace tests 2.1 General programme of the research A series of full scale furnace tests has been planned at the University of Perugia to investigate the mechanism of heat propagation in hollow core slabs made with this innovative material and to compare the numerical and the load bearing capacity. The programme includes two furnace tests. During each test two slabs will be exposed to the ISO 834 thermal programme, for a total of four HPLWC hollow core slabs. In every experiment a slab will be loaded during the fire with 6% of the service load, while the other one will be left unloaded, to study its residual strength after cooling. At the present time only one furnace test has been carried out.
Slurry 41 kg/m 3 Fig. 2: Four point bending test scheme. 2.2 Characteristic of the hollow core slabs The hollow core slabs, built by extrusion, are 4.3 m long and their cross section is 1.2 x.2 m (Fig. 1). The slabs have been made with C48/58 N/mm 2 concrete having a unit weight of 1.9 kn/m 3. The prestressing reinforcement is made up of seven 3/8 strands. The mix design of the concrete used for the slabs is listed in the Table 1. The specimens were built on June 29 22 and were stored in the factory under the same curing conditions used for the everyday production until September 23 24 when the first furnace test was carried on. Fig. 1: Cross section of the hollow core slabs. Table 1 - Mix design of the HPLW concrete. Material Quantity Cement 38 kg/m 3 Water 1 kg/m 3 T8 expanded clay 578 kg/m 3 Fine sand 526 kg/m 3 Sand 279 kg/m 3 2.3 Experimental setup During the furnace test one of the slabs has been loaded following a four point bending scheme (Fig. 2). The value of the vertical load has been calculated to cause a bending moment at the midspan section equal to 6% of the corresponding value in the service condition. The vertical deflection of the loaded slab has been monitored for the whole duration of the test. Both slabs have been instrumented with thermocouples placed at different locations and depth (Fig. 3). The thermocouples have been installed inside the hardened concrete by drilling holes and using insulating material to close them. It was chosen to use insulating material instead of cement mortar to avoid possible measurement inaccuracy due the evaporation of the residual moisture of the sealing material. 2.4 Experimental results A first test on two slabs has been already completed and the corresponding results are reported in the present communication. In Fig. 4 and Fig. 5 are depicted the temperature time histories of the furnace together with the theoretical ISO 834 temperature history and the data recorded by the thermocouples placed in the slabs. In some cases irregularities occurred in the temperature history (see sensors no. 18 and no. 21 in Fig. 5), to be ascribed probably to the effect of the spalling, which occurred during the test. The brittle collapse in shear of the loaded slab occurred after 76 minutes. It was anticipated at 4 minutes by the formation of a pass-trough hole which allowed the hot gases to escape and was closed on the spot with insulating material to continue the test until failure.
Fig. 3: Temperature sensors location and depth. 125 1 ISO 834 Temp. Furnace Temp. Sensor n. 1 Sensor n. 2 Sensor n. 3 Sensor n. 4 Sensor n. 5 Sensor n. 7 Sensor n. 9 Sensor n. 11 Sensor n. 12 Sensor n. 13 125 1 ISO 834 Temp. Furnace Temp. Sensor n. 15 Sensor n. 16 Sensor n. 17 Sensor n. 18 Sensor n. 19 Sensor n. 21 Sensor n. 23 Sensor n. 25 Sensor n. 26 Sensor n. 27 Temperature ( C) 75 5 Temperature ( C) 75 5 25 25 15 3 45 6 75 Time (min) Fig. 4: Temperature histories for slab n. 1. 15 3 45 6 75 Time (min) Fig. 5: Temperature histories for slab n. 2. 3. Numerical simulation of the furnace tests The results were used to tune a finite element numerical model of the heat propagation across the slabs and to refine the analytical evaluation of the fire endurance. The heat propagation inside the slabs as well as the degradation of the load bearing capacity of the hollow core slabs exposed to fire have been simulated with numerical analysis. As the latest version of the Eurocode 2 [3] provides no data on the HPLW concrete, the relevant thermal properties have been taken from Eurocode 4 [4] even if this document directly addresses LWC and not HPLWC. The corresponding mechanical properties at high temperature have been taken from the previous release of the Eurocode 2 [5], where the mechanical properties for normal strength LWC are provided. The temperature-dependent mechanical properties used in the numerical analysis are depicted in Fig. 6 and Fig. 7.
6 σ (N/mm 2 ) 4 2 C 2 σ(n/mm 2 ) 15 2 C 1 C 2 C 3 C 2 C3 C 4 C 1 C 5 C 6 C 7 C 1 4 C 2 8 C 5 5 C 9 C 1 C..1.2.3.4 Fig. 6: Stress-strain curves for concrete. ε 6 C..5.1.15 ε Fig. 7: Stress-strain curves for prestressing steel. 3.1 Thermal analysis The thermal problem has been analyzed using the finite element software FIRES-T3 [6], considering uncoupled, as usually accepted, the thermal and the mechanical problems. The following cases have been considered, which differ one from the other for the boundary conditions. Case no. 1 the lower surface is exposed to the ISO 834 standard fire; the upper surface is in contact with air at 2 C constant temperature; the internal hollow core surfaces are in contact with air at 2 C constant temperature. Case no. 2 the lower surface is exposed to the ISO 834 standard fire; the upper surface is in contact with air at 2 C constant temperature; the internal hollow core surfaces are in contact with air at 2 C constant temperature. Case no. 3 the lower surface is exposed to the ISO 834 standard fire; the upper surface is in contact with air at 1 C constant temperature; the internal hollow core surfaces are in contact with air at 2 C constant temperature. A two-dimension analysis has been carried out assuming that the temperature distribution does not vary from section to section in the slabs. Temperature fields for the case no. 3 and for 3, 6 and 9 minutes exposure to the ISO 834 fire are shown, as an example, in Fig. 8, Fig. 9 and Fig. 1. Fig. 8: 3 min 1 C 2 C. Fig. 9: 6 min 1 C 2 C. Fig. 1: 9 min 1 C 2 C. 3.2 Mechanical analysis 3.2.1 Ultimate moment The degradation of the ultimate moment due to the increasing temperature has been evaluated at the ultimate limit state, following the provision of the Eurocodes. Since the material properties are known, the corresponding partial safety factors were set to unity. For each time step (every 5 minutes of fire exposure) the moment curvature relationship has been evaluated, taking into account the variation of the material strength due to the high temperature. The maximum value of
the bending moment for each temperature distribution has been assumed as the ultimate moment for the corresponding time. The results of the numerical analysis are depicted in Fig. 12. It can be noted that the bending load capacities of the slab for the case no. 2 and no. 3 are practically the same, while a bigger value of the ultimate moment have been obtained for the case no. 1. This has to be ascribed to the fact that the degradation of the concrete in compression is very small (the upper surface of the slab doesn t undergo significant temperature variation) and that the flexural bearing capacity of the slab depends mainly on the degradation of the prestressing steel. 3.2.2 Ultimate shear The shear resistance of the hollow core slabs was calculated according to Eurocode 2 [7]. I b V w,fi ( ) 2 Rd,c, fi = fctd +α1σ cp, fi fctd (1) S with bw,fi = kc,t i bw,i (2) where: Ι is the moment of inertia; S is the first moment of area above and about the centroidal axis; α I = l x /l pt2 1. for prestressed tendons; l x is the distance of section considered from the starting point of the transmission length; l pt2 is the upper bound value of the transmission length of the prestressing element; σ cp is the concrete compressive stress at the centroidal axis due to prestressing; b w,i is the width of the cross-section at level i; k c,t is the tensile strength reduction factor for concrete at temperature t. The ultimate shear was evaluated at 5 different levels to take into account the different temperature and the different web thickness (Fig. 11). Fig. 11: Levels for ultimate shear strength evaluation. 4. Conclusions In the present paper the preliminary results of full scale furnace tests on PC hollow core slabs made with HPLW concrete have been reported. The comparison between the and the numerical temperature data exhibited a good agreement especially for the boundary condition assumed in the case no. 3. Anyway some discrepancies were noticed. They are probably related to the initial inertia of the whole furnace to warm up and to the phenomenon of the explosive spalling which is a known possible drawback of this innovative material, drawback that actually occurred during the test. The spalling locally modified the mass distribution and the concrete cover depth of the prestressed strands. For the same reasons the numerical simulation of the load bearing capacity showed some differences from the results of the test. While the collapse of the loaded slab, which happened after 76 minutes, was identified as a shear failure, the numerical simulations foresee an ultimate shear resistance well above the value. Moreover the numerical analyses set the failure in bending at roughly 95 minutes. The investigations confirmed the role of the spalling in reducing the fire endurance of HPLWC slabs and suggested to improve the mix design to mitigate the problem.
Ultimate Moment (knm) 11 1 9 8 7 6 5 4 num. ext. temp. 2-2 C num. ext. temp. 2-2 C num. ext. temp. 1-2 C Ultimate shear (kn) 6 5 4 3 2 level 1 level 2 level 3 level 4 level 5 3 2 1 1 2 4 6 8 1 12 14 16 18 Fig. 12: Ultimate moment. 2 4 6 8 1 12 14 16 18 Fig. 13: Ultimate shear 2 C - 2 C. Ultimate shear (kn) 6 5 4 3 2 level 1 level 2 level 3 level 4 level 5 Ultimate shear (kn) 6 5 4 3 2 level 1 level 2 level 3 level 4 level 5 1 1 2 4 6 8 1 12 14 16 18 Fig. 14: Ultimate shear 2 C - 2 C. 5. Acknowledgments 2 4 6 8 1 12 14 16 18 Fig. 15: Ultimate shear 1 C - 2 C. The Authors gratefully acknowledge the contribution of the Firm Generale Prefabbricati S.p.A of Perugia (Italy) which manufactured the slabs used for the tests. References [1] VAN ACKER A., "Shear resistance of prestressed hollow core floors exposed to fire", Structural Concrete, Vol. 4, No. 2, 23, pp. 65-74. [2] Eurocode 1: Action on structures - Part 1-2: General actions - Action on structures exposed to fire, EN 1991-1-2, 22. [3] Eurocode 2: Design of concrete structures - Part 1-2: General rules - Structural fire design, pren 1992-1-2, 24. [4] Eurocode 4: Design of composite steel and concrete structures - Part 1-2: General rules - Structural fire design, pren 1994-1-2, 23. [5] Eurocode 2: Design of concrete structures - Part 1.2: General rules - Structural fire design, pren 1992-1-2 (1 st draft), 2. [6] IDING R., BRESLER B., NIZAMUDDIN Z., NIST, Fires - T3 A Computer program for the fire response of structures. [7] Eurocode 2: Design of concrete structures - Part 1-1: General rules and rules for buildings, pren 1992-1-1, 23.