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This paper introduces experimental study on mechanical behaviour of an ultra-high strength concrete (UHSC) at elevated temperatures and then a simple calculation method to predict the fire resistance of tubular column infilled with the UHSC. The cylinder compressive strength of the UHSC was 166 N/mm2 at room temperature. The compressive strength and modulus of elasticity of the UHSC were measured up to 800°C. Then the temperaturedependent mechanical properties were compared with those of normal/high strength concretes provided in Eurocode 2 and ANSI/AISC 360-10, and with those of concretes in literature. The comparisons showed that the compressive strength and elastic modulus of the UHSC were generally reduced less than those of normal/high strength concretes at the elevated temperatures. The temperature-dependent mechanical properties were proposed for evaluating fire resistance of steel tubular columns infilled with the UHSC. The UHSC investigated in this project was shown to markedly improve the fire resistance in a number of cases well documented in the literature concerning tubular columns filled with the normaland high-strength concretes.
Ờ Å ỊÙ× Ư Ờ Mechanical behaviour of ultra-high strength concrete at elevated temperatures and fire resistance of ultra-high strength concrete filled steel tubes Ming-Xiang Xiong, J.Y Richard Liew PII: DOI: Reference: S0264-1275(16)30655-4 doi: 10.1016/j.matdes.2016.05.050 JMADE 1798 To appear in: Received date: Revised date: Accepted date: February 2016 May 2016 13 May 2016 Please cite this article as: Ming-Xiang Xiong, J.Y Richard Liew, Mechanical behaviour of ultra-high strength concrete at elevated temperatures and fire resistance of ultra-high strength concrete filled steel tubes, (2016), doi: 10.1016/j.matdes.2016.05.050 This is a PDF file of an unedited manuscript that has been accepted for publication As a service to our customers we are providing this early version of the manuscript The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain ACCEPTED MANUSCRIPT Mechanical Bahviour of Ultra-High Strength Concrete at Elevated Temperatures and Fire Resistance of Ultra-High Strength Concrete Filled Steel Tubes School of Civil and Transportation Engineering, Guangdong University of Technology, 100 Waihuan Xi Road, GuangZhou, China 510006 b Department of Civil Engineering, National University of Singapore, 10 Kent Ridge Crescent, NU Singapore 117576 School of Civil Engineering, Nanjing Tech University, 30 Puzhu Road(S), Nanjing, China MA 211800 CE P TE D * Corresponding author, Email address: ceexm@nus.edu.sg AC c SC R a IP T Ming-Xiang Xiong a, b, *, J.Y Richard Liew b, c ACCEPTED MANUSCRIPT Abstract This paper introduces experimental study on mechanical behaviour of an ultra-high strength T concrete (UHSC) at elevated temperatures and then a simple calculation method to predict IP the fire resistance of tubular column infilled with the UHSC The cylinder compressive SC R strength of the UHSC was 166 N/mm2 at room temperature The compressive strength and modulus of elasticity of the UHSC were measured up to 800°C Then the temperature- NU dependent mechanical properties were compared with those of normal/high strength concretes provided in Eurocode and ANSI/AISC 360-10, and with those of concretes in MA literature The comparisons showed that the compressive strength and elastic modulus of the UHSC were generally reduced less than those of normal/high strength concretes at the D elevated temperatures The temperature-dependent mechanical properties were proposed for TE evaluating fire resistance of steel tubular columns infilled with the UHSC The UHSC CE P investigated in this project was shown to markedly improve the fire resistance in a number of cases well documented in the literature concerning tubular columns filled with the normal- AC and high-strength concretes Keywords: Ultra-High Strength Concrete, Elevated Temperatures, Mechanical Properties, Concrete Filled Steel Tubular Column, Simple Calculation Method, Fire Resistance ACCEPTED MANUSCRIPT Introduction High strength concrete (HSC) has been used in high-rise buildings and the other structures T because of its technical, architectural, and economical advantages over normal strength IP concrete (NSC) However, the need for sustainable constructions around the world, which SC R aims to further reduce the consumption of construction materials, requires higher-strength concretes to be introduced Nowadays, ultra-high strength concrete (UHSC) with NU compressive strength higher than 120MPa has been available with the development of concrete technology and the availability of variety of materials such as silica fume and high- MA range water-reducing admixtures However, the UHSC is mainly used in offshore and marine structures and for industrial floors, pavements and security barriers It has not been used in D building structures especially high-rise buildings This may be due to the fact that there are TE design concerns on its brittleness and fire resistance These concerns lead to the situations CE P that the current standards allow the use of concrete only up to Class C90/105 for concrete structures and Class C50/C60 for steel-concrete composite structures [1-4] AC To evaluate the fire resistance of structural members with the UHSC, the knowledge of the temperature-dependent mechanical properties, such as compressive strength and modulus of elasticity, is required In literature, the said properties of the NSC and HSC have been extensively studied where the compressive strength was found to be affected by the type of aggregate [5-7] Siliceous-aggregate concrete brought in greater strength losses than concrete with carbonate aggregate, whereas firebrick aggregate exhibited superior performance The strength was also affected by heating rate [8] Higher heating rate generally yielded lower strength and was more likely to induce spalling Furthermore, the loss of strength of HSC was larger than that of NSC [2; 9-13] The modulus of elasticity was generally governed by the type of aggregate and the water/cement ratio [14-16] The loss of modulus increased as the ACCEPTED MANUSCRIPT water/cement ratio increased According to the literature [7; 8; 17], the elastic modulus is less affected by the temperature in HSC compared with NSC The addition of fibers is deemed to T affect the mechanical properties of concrete Steel fibers generally increase both of the IP compressive strength and elastic modulus [18]; whereas polypropylene fibers decrease the SC R compressive strength but increase the elastic modulus [19] Overall, there is still little information in the available literature concerning the mechanical properties of UHSC at high NU temperatures Research efforts in this domain are, therefore, badly needed indeed Due to the brittleness, HSC is generally used in hollow steel tubes to form composite MA columns Concrete filled steel tubular (CFST) column integrates the respective advantages of steel and concrete materials thus exhibits many advantages over conventional steel or D reinforced concrete columns, such as high load bearing capacity, good ductility due to TE confinement effect, and convenience for fabrication and construction due to permanent CE P formwork from steel tubes [20] The CFST columns also have good fire resistance due to heat sink effect of the infilled concrete and prevention of spalling of the infilled concrete by the steel tube Researches on the fire resistance of CFST columns started from 1970s National AC Research Council of Canada (NRCC) is the pioneer in this area [21-24] Until now, the researches on the CFST columns with HSC have been carried on by Kodur [25], Lu et al [26-28] and Romero et al.[29] However, little information is found for studies on CFST columns with the UHSC of compressive strength higher than 120MPa A concept of CFST column with the UHSC was proposed for load-bearing system of the high-rise building constructions [30; 31] The compressive cylinder strength of the UHSC exceeded 160MPa This paper presents a study on the mechanical properties, such as the compressive strength and modulus of elasticity, of the UHSC under elevated temperatures The temperature dependent properties were obtained through standard compression tests ACCEPTED MANUSCRIPT With the tested mechanical properties, the fire resistance of CFST columns with the UHSC was evaluated when they were subject to standard ISO-834 fire, and compared with that of Basic Materials SC R IP T CFST columns with the NSC and HSC The basic materials to produce the UHSC were Ducorit® D4 and water Ducorit® D4 is one of the commercial Ducorit® products It is made from cementitious mineral powder, NU superplasticizer and fine bauxite aggregates with maximum sizes less than 4.75mm and 49% MA less than 0.6mm The mixing proportions for the UHSC are shown in Table Workability of the fresh UHSC was tested using the slump flow test in accordance with ASTM D C1611/C1611M-09b The slump flow spread was 735mm and the density was 2700 kg/m3 Standard Compression Tests at Elevated Temperatures CE P TE [32] 3.1 Test Specimens AC Spalling has been found for the HSC subject to high temperatures [33] The spalling is basically caused by thermal stresses due to a temperature gradient in concrete during heating, and by splitting force due to the release of vapor above 100oC It is believed that the present UHSC is more likely to spall under high temperatures With regard to this point, a series of trial tests have been done to investigate the spalling behavior of the UHSC [34] It was found that the plain UHSC specimens and the UHSC specimens with steel fibers (dosage up to 1.0% in volume) spalled around 490oC as shown in Figure and Figure 2, respectively The spalling was so severe that the cover plate of the casing was bent and the ceiling of the furnace was damaged However, the UHSC specimens with 0.1% polypropylene fibers did not spall at elevated temperature up to 800oC as shown in Figure The properties of steel ACCEPTED MANUSCRIPT and polypropylene fibers are shown in Table It is worth noting that the workability and flowability of the UHSC were not affected by the addition of polypropylene fibers as the T UHSC is most likely pumped into hollow tubes for CFST columns The dosage of IP polypropylene fibers was lower than that recommended by Eurocode where more than SC R 2kg/m3 (0.25% in terms of volume) of monofilament propylene fiber should be included in the HSC mixtures to prevent spalling [2] NU For the standard compression tests, cylinder specimens with a nominal diameter of 100mm and a height of 200mm were prepared The actual diameters and heights were measured MA before the test started The specimens were cured in lab air where the relative humidity was approximately 85% and the room temperature was around 30oC at daytime and 25oC at night D Owing to the fact that the moisture content in the UHSC is low, the effect of moisture on the TE mechanical properties is deemed to be insignificant [34] On the other hand, the moisture is CE P evaporated around 100oC, it may only have minor influence at 100oC but insignificant influences at higher temperatures Considering these, the unsealed specimens were used AC 3.2 Test Setup The compression tests were conducted by means of a servo-hydraulic testing machine with a maximum 300mm stroke displacement and capacity of 10000 kN The heat system was a split-tube furnace with a two-zone configuration and an optional side entry extensometer port The furnace is constructed with S304 stainless steel shell and alumina insulation material Heating elements are coils of Fe-Cr-Al alloy 0Cr27a17mo2 A type K thermocouple is mounted in the center of each heating zone The external dimensions (diameter x height) are 700 x 600mm and internal heating dimensions (diameter x height) are 350 x 400mm The furnace can heat up to a maximum temperature of 900oC Model 3548HI high temperature furnace extensometer was used to measure the relative deformation in gauge length of the ACCEPTED MANUSCRIPT specimen It is a strain gauged sensor and specified for a gauge length of 50mm and the maximum measurable strain is thus 20% The arms of the extensometer are alumina rods and T the rods were attached to the middle 1/4 height of the specimen which is the gauge length IP The test setup is shown in Figure Top and bottom cooling blocks were used to load the SC R specimen inside the furnace The cooling blocks were made from carbon steel which is not resistant to high temperature To bring down its temperature, channels were drilled inside the NU cooling blocks to allow for water circulating for the purpose of cooling The concrete specimen was protected by a steel casing in case where the crushing debris at failure would MA damage the furnace Diameter of 10mm holes were drilled on surface for heat propagation; and opening was cut at side for the pass of rods of the extensometer The compression force TE D was applied from the bottom of the loading frame by a hydraulic cylinder CE P 3.3 Test Method and Procedure In practice, different temperature–stress paths may appear in concrete and it is difficult to test for all of them Typically, two temperature-stress paths, unstressed and stressed, are AC considered to form the upper and lower bounds of the mechanical properties of concrete at elevated temperatures For the unstressed method, the specimen is loaded to fail with a constant temperature; whereas the specimen is heated to fail under a constant load level for the stressed method The unstressed method is mostly used due to its convenience to obtain stress-strain curves directly However, it is difficult to obtain the stress-strain curves in the stressed tests as the measured strain includes thermal strain and short-term creep strain [35] Supplementary tests are usually required to measure them independently The difference between the unstressed and stressed test methods is mainly that the stressed test could capture transient thermal strain For a CFST column subjected to a fire, ignoring the transient thermal strain could overestimate the buckling resistance of the CFST column, however the ACCEPTED MANUSCRIPT overestimation may not be much severe due to the existence of non-uniform temperature distribution through its cross-section [36] Nevertheless, the influence of transient thermal T strain should be considered for the fire resistance design of CFST columns In present study, IP the unstressed test method was adopted and the effect of transient thermal strain was SC R implicitly considered by a stiffness reduction factor given in Section 5.1 for the CFST columns containing the UHSC The validity of the said reduction factor has been established NU by test results For the unstressed tests conducted, a small compressive stress of approximately 0.05MPa was MA applied prior to testing in the direction of the specimen’s central axis in order to maintain the specimen at the center of loading machine Then the specimen was heated up to target D temperatures with a heating rate of 5oC/min In fact, the heating rate varies when a structural TE member is subjected to a realistic fire However, it would be rather difficult to conduct tests CE P for various heating rates With regard to this point, the heating rate herein is determined based on that of standard ISO-834 fire against which the structural members are generally designed The heating rate of the ISO-834 fire is shown in Figure At early minutes, the AC heating rate drops to 25oC/min, after 25 minutes, the heating rate is approximately 5oC/min Hence the heating rate of 5oC/min would be representative for most fire scenarios Especially when the UHSC is infilled in steel tubes, the heating rate would be further lower due to the heat sink effects of the steel tubes and the fire protection (if any) Thus if the heating rate of 5oC/min is used, the measured mechanical properties would be lower than they are in reality, which will turn out a more conservative but safer design In addition to ambient temperature which was approximately 30oC, the target temperature ranged from 100oC to 800oC at an increment of 100oC As the UHSC is denser and more impermeable than the NSC, a trial test was conducted to investigate the holding time of target ACCEPTED MANUSCRIPT temperature during which uniform temperature distributions can be achieved inside both the furnace and the UHSC specimens Figure shows the recorded temperatures for a T 100x200mm cylinder specimen heated up to 800oC in an electrical oven with a heating rate of IP 5oC /min It can be seen that the uniform temperature distribution can be achieved in hours SC R Hence, the holding time at target temperatures were taken as hours for all specimens After holding, the specimen was subjected to three load cycles between 0.05MPa and 15% or NU between 5% and 15% of the reference strength as shown in Figure [37] The holding time at 5% and 15% load levels was 60s Then the specimen was loaded to fail Displacement control MA was adopted during loading where the displacement rate was 0.4mm/min It should be mentioned that the full stress-strain curves were not recorded by the extensometer since the D sudden crush of the UHSC specimen would damage the extensometer The extensometer was TE removed when at least 40% of the compressive strength at target temperature was reached CE P The 40% compressive strength was measured to calculate the modulus of elasticity The compression continued after the extensometer was removed until the specimen was crushed The peak compression force was recorded by the loading machine In general, the peak AC compressive strength and the modulus of elasticity of the UHSC were obtained from the tests They are sufficient for the fire resistance design of CFST columns with the UHSC according to EN 1994-1-2 [4] Test Results 4.1 Compressive Strength Spalling was not observed during heating of all the UHSC specimens owing to the addition of 0.1% polypropylene fibers The compressive strength of UHSC at room temperature was 166MPa which was averaged from specimens specimens were used for the other target ACCEPTED MANUSCRIPT Table 5: Sensitivity study on section factor 139.7x8 28.6 SF02 219.1x8 18.3 SF03 273x8 14.7 SF04 355.6x8 11.2 SF05 406.4x8 9.8 SF06 508x8 7.9 SF07 610x8 6.6 fck (MPa) NSC UHSC Buckling length (m) Fire resistance time (minutes) NSC, UHSC, tu tu/tp Load level 22 24 1.091 26 38 1.462 30 76 2.533 0.35 42 111 2.643 55 150 2.727 99 255 2.576 167 401 2.401 T SF01 fy (MPa) IP Section factor (m-1) 355 35 SC R Sizes D x t (mm) 166 3.5 MA NU Circular Column Table 6: Sensitivity study on load level Sizes D x t (mm) NSC UHSC Buckling length (m) TE LL01 LL03 508x8 LL04 355 35 166 Fire resistance time (minutes) Load level NSC, UHSC, tu tu/tp 0.15 221 492 2.226 0.25 170 356 2.094 0.35 99 255 2.576 0.45 53 170 3.208 0.55 33 94 2.848 0.65 26 56 2.154 3.5 AC LL05 CE P LL02 LL06 fck (MPa) fy (MPa) D Circular Column Table 7: Sensitivity study on concrete contribution ratio Concrete contribution ratio NSC UHSC Circular Column Sizes Dxt (mm) CCR01 508x8 0.602 CCR02 508x12 CCR03 508x16 fy (MPa) 355 fck (MPa) NSC 35 UHSC Buckling length (m) Load level Fire resistance time (minutes) NSC, UHSC, tu tu/tp 0.853 99 255 2.576 0.496 0.802 72 218 3.028 0.418 0.754 67 193 2.881 166 3.5 0.35 CCR04 508x20 0.359 0.710 55 182 3.309 CCR05 508x25 0.303 0.659 51 146 2.863 CCR06 508x32 0.244 0.594 49 117 2.388 32 NU SC R IP T ACCEPTED MANUSCRIPT CE P TE D MA Figure 1: Spalled UHSC specimens without fibers AC Figure 2: Spalled UHSC specimens with steel fibers (a) Heated to 800oC (b) unheated Figure 3: Comparison between heated and unheated UHSC specimens 33 ACCEPTED MANUSCRIPT T Loading frame SC R IP Loading head with spherical hinge Furnace moving track NU Steel casing Furnace MA Cooling block AC CE P TE D Hydraulic cylinder Figure 4: Test setup 34 Concrete specimen Extensometer Water pipe ACCEPTED MANUSCRIPT 300 1200 1000 T 250 IP 200 150 100 NU 50 10 20 400 200 30 40 Time (minute) 50 60 MA 800 600 SC R Heating rate (oC/min) 1400 Heating rates based on time-temperature curve of ISO-834 fire Time-temperature curve of ISO-834 fire Temperature (oC) 350 TE D Figure 5: Time-temperature curve of ISO-834 fire and heating rates 700 600 500 AC Temperature (oC) 800 CE P 900 400 300 Temperature in oven 200 Temperature at point 100 0 50 100 150 200 250 Time (minutes) Figure 6: Temperatures in furnace and UHSC specimen 35 300 NU SC R IP T ACCEPTED MANUSCRIPT 60 At 800oC 35 30 TE 50 CE P 40 Stress (N/mm2) fck 30 20 10 0.0 Y = Ecm.X + A 25 20 15 10 At 800oC AC Stress (N/mm2) D MA Figure 7: Load cycles applied on the test specimens 2.0 4.0 6.0 Loading head movement (mm) (a) compressive strength fck 0.0000 0.0005 0.0010 0.0015 0.0020 0.0025 Strain measured by extensometer (b) modulus of elasticity Ecm Figure 8: Determination of compressive strength and modulus of elasticity 36 ACCEPTED MANUSCRIPT 1.0 IP T 0.8 0.6 SC R Reduction factor of strength 1.2 UHSC 0.4 NSC with siliceous aggregate-EN 1992-1-2 NSC with calcareous aggregate-EN 1992-1-2 0.2 NU NSC - AISC 360-10 0.0 100 200 300 400 500 o Temperature ( C) 600 700 800 MA D Figure 9: Comparison between strength reduction factors of UHSC and NSC as given in EN TE 1992-1-2 and AISC 360-10 CE P 1.0 0.8 0.6 AC Reduction factor of strength 1.2 UHSC 0.4 C55/67~C60/75-EN 1992-1-2 C70/80~C80/95-EN 1992-1-2 0.2 C90/105-EN 1992-1-2 0.0 100 200 300 400 500 Temperature (oC) 600 700 800 Figure 10: Comparison between strength reduction factors of UHSC and HSC as given in EN 1992-1-2 37 ACCEPTED MANUSCRIPT 0.6 IP T 0.8 SC R UHSC 79MPa (Chen et al,2004) 81.9MPa (Hammer,1995) 84.5MPa (Diederich et al,1988) 88MPa (Phan and Carino, 2002) 94.1MPa (Hammer,1995) 98.1MPa (Phan and Carino, 2002) 91.5MPa (Aslani and Samali, 2012) 91.8MPa (Diederich et al,1988) 102.2MPa (Hammer,1995) 106.6MPa (Diederich et al,1988) 117.4MPa (Hammer,1995) 0.4 NU 0.2 0.0 100 200 MA Reduction factor of compressive strength 1.0 300 400 500 600 Temperature (oC) 700 800 900 D Figure 11: Comparison between strength reduction factors of UHSC and HSC with results TE from previous researches CE P 1.0 0.8 AC Reduction factor of strength and residual strength 1.2 0.6 0.4 Strength 0.2 Residual Strength 0.0 100 200 300 400 500 600 700 800 Temperature (oC) Figure 12: Comparison between reduction factors of strength and residual strength 38 ACCEPTED MANUSCRIPT UHSC NSC with siliceous aggregate-EN 1992-1-2 NSC with calcareous aggregate-EN 1992-1-2 NSC - AISC 360-10 T 1.0 IP 0.8 SC R 0.6 0.4 0.2 0.0 100 200 300 400 500 600 700 800 MA NU Reduction factor of elastic modulus 1.2 Temperature (oC) D Figure 13: Comparison between elastic modulus reduction factors of UHSC and NSC as TE given in EN 1992-1-2 and AISC 360-10 CE P 1.0 0.8 0.6 UHSC 79MPa (Chen et al,2004) 81.9MPa (Hammer,1995) 84.5MPa (Diederich et al,1988) 88MPa (Phan and Carino, 2002) 91.5MPa (Aslani and Samali, 2012) 91.8MPa (Diederich et al,1988) 94.1MPa (Hammer,1995) 98.1MPa (Phan and Carino, 2002) AC Reduction factor of elastic modulus 1.2 0.4 0.2 0.0 100 200 300 400 500 600 700 800 Temperature (oC) Figure 14: Comparison between reduction factors of elastic modulus of UHSC and HSC as given in previous researches 39 ACCEPTED MANUSCRIPT 1.0 IP T 0.8 0.4 Elastic Modulus 0.2 Residual Elastic Modulus 0.0 100 200 SC R 0.6 NU Reduction factor of elastic modulus and residual elastic modulus 1.2 300 400 500 600 700 800 MA Temperature (oC) modulus Tnc+3 Tnc+2 Tm+1 Tm Tm-1 d Tnc+1 s n (1,1) element (2,2) m (2,1) AC CE P TE D Figure 15: Comparison between reduction factors of elastic modulus and residual elastic steel (3,3) air gap concrete (m,n) T2 T1 (nc+2,1) dc element concrete air gap steel (a) Circular CFST column (b) square CFST column Figure 16: Discretization on cross-section 40 TE D MA NU SC R IP T ACCEPTED MANUSCRIPT AC CE P Figure 17: Typical test setup for the standard fire test on CFST column Figure 18: Diagram for calculation of buckling length of pinned-fixed column 41 EQ 20 EQ EQ EQ EQ 1 1 EQ 40 60 80 Time (minute) 100 (a) LC-2-1 150 EQ EQ EQ EQ EQ EQ EQ EQ 1 1 EQ MA 100 50 10 20 30 40 D 600 EQ EQ 500 400 300 200 100 0 NU EQ 50 TE Time (minute) (c) LSH-2-1 EQ EQ EQ EQ 1 EQ EQ 10 20 EQ 30 40 50 Time (minute) EQ 10 20 30 Time (minute) 40 CE P Figure 19: Comparison between calculated and measured temperatures of UHSC 42 60 (b) LDC-2-1 (d) LDSH-2-1 AC Temperature (oC) EQ Temperature (oC) 200 EQ 700 EQ EQ 600 500 400 300 200 100 0 T EQ IP EQ SC R 350 300 250 200 150 100 50 Temperature (oC) Temperature (oC) ACCEPTED MANUSCRIPT 50 0.6 6000 0.5 P=2900kN 0.2 tp=90 0.0 0 20 40 60 80 Temperature (oC) 4000 2000 0 100 0.6 6000 0.5 P=3735kN 0.3 0.2 tp=33 0.0 0.9 0.8 8000 0.6 6000 0.5 P=3715kN 4000 2000 0.3 0.2 tp=44 0.0 40 D 20 30 Temperature (oC) (a) LSH-2-1 15 30 45 Temperature (oC) 60 (b) LDSH-2-1 CE P P=test axial load 60 10000 TE 10 0.0 30 45 Temperature (oC) NU 8000 Buckling resistance (kN) 0.8 Buckling reduction factor 10000 15 0.2 tp=53 12000 MA 0.9 2000 0.3 (b) LDC-2-1 12000 tp=predicted fire resistance time AC Figure 20: Comparison between calculated and measured fire resistance time 3.0 2.5 Improvement, tu/tp Buckling resistance (kN) (a) LC-2-1 4000 0.5 P=2428kN 2.0 1.5 1.0 0.5 10 15 Section factor 20 25 30 (m-1) Figure 21: Effect of section factor on improvement of fire resistance time 43 Buckling reduction factor 2000 0.3 0.6 SC R 4000 0.8 6000 Buckling reduction factor 0.8 0.9 T 8000 8000 IP 0.9 Buckling resistance (kN) 10000 Buckling reduction factor Buckling resistance (kN) ACCEPTED MANUSCRIPT ACCEPTED MANUSCRIPT IP T 3.0 2.5 SC R Improvement, tu/tp 3.5 2.0 NU 1.5 0.15 0.3 0.45 0.6 0.75 MA Load level Figure 22: Effect of load level on improvement of fire resistance time 2.5 2.0 D TE CE P 3.0 AC Improvement, tu/tp 3.5 1.5 0.2 0.3 0.4 0.5 0.6 0.7 Concrete contribution ratio Figure 23: Effect of concrete contribution ratio on improvement of fire resistance time 44 ACCEPTED MANUSCRIPT AC Concrete Material Test at Elevated Temperatures 2500 1.0 2000 0.8 1500 0.6 1000 0.4 500 0.2 0.0 Application in Composite Column for High-rise Building 45 20 40 60 Time (minute) 80 Design Method to Predict Fire Resistance Buckling reduction factor Buckling resistance (kN) CE P TE D MA N US CR IP T Graphical abstract ACCEPTED MANUSCRIPT Highlights T 1) A new ultra-high strength concrete achieved 166MPa; IP 2) Compressive strength and elastic modulus decreased slower than normal/high strength concretes at elevated temperatures; CR 3) Compressive strength and elastic modulus showed sharp reduction at 100 oC; AC CE P TE D MA N US 4) Simple calculation method effectively predicted fire resistance of ultra-high strength concrete infilled tubes; 46 ... MANUSCRIPT Mechanical Bahviour of Ultra-High Strength Concrete at Elevated Temperatures and Fire Resistance of Ultra-High Strength Concrete Filled Steel Tubes School of Civil and Transportation Engineering,... strength of UHSC at elevated temperatures were reduced less than those of HSCs in the literature The mechanical properties (compressive strength and modulus of elasticity) at elevated temperatures. .. literature concerning tubular columns filled with the normal- AC and high -strength concretes Keywords: Ultra-High Strength Concrete, Elevated Temperatures, Mechanical Properties, Concrete Filled Steel