Mechanical Bahviour of ultra-high strength concrete at elevated temperatures and fire resistance of ultra-high strength concrete filled steel tubes

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.

and fire resistance of ultra-high strength concrete filled steel tubes

Ming-Xiang Xiong, J.Y Richard Liew

PII: S0264-1275(16)30655-4

DOI: doi: 10.1016/j.matdes.2016.05.050

Reference: JMADE 1798

To appear in:

Received date: 2 February 2016

Revised date: 2 May 2016

Accepted date: 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

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Mechanical Bahviour of Ultra-High Strength Concrete at Elevated Temperatures and

Fire Resistance of Ultra-High Strength Concrete Filled Steel Tubes

Ming-Xiang Xiong a, b, *, J.Y Richard Liew b, c

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Abstract

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

temperature-dependent 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 normal-

and high-strength concretes

Keywords:

Ultra-High Strength Concrete, Elevated Temperatures, Mechanical Properties, Concrete

Filled Steel Tubular Column, Simple Calculation Method, Fire Resistance

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1 Introduction

High strength concrete (HSC) has been used in high-rise buildings and the other structures

because of its technical, architectural, and economical advantages over normal strength

concrete (NSC) However, the need for sustainable constructions around the world, which

aims to further reduce the consumption of construction materials, requires higher-strength

concretes to be introduced Nowadays, ultra-high strength concrete (UHSC) with

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-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

building structures especially high-rise buildings This may be due to the fact that there are

design concerns on its brittleness and fire resistance These concerns lead to the situations

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]

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

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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

affect the mechanical properties of concrete Steel fibers generally increase both of the

compressive strength and elastic modulus [18]; whereas polypropylene fibers decrease the

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

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

columns Concrete filled steel tubular (CFST) column integrates the respective advantages of

steel and concrete materials thus exhibits many advantages over conventional steel or

reinforced concrete columns, such as high load bearing capacity, good ductility due to

confinement effect, and convenience for fabrication and construction due to permanent

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

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

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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

CFST columns with the NSC and HSC

2 Basic Materials

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,

superplasticizer and fine bauxite aggregates with maximum sizes less than 4.75mm and 49%

less than 0.6mm The mixing proportions for the UHSC are shown in Table 1 Workability of

the fresh UHSC was tested using the slump flow test in accordance with ASTM

C1611/C1611M-09b The slump flow spread was 735mm and the density was 2700 kg/m3

[32]

3 Standard Compression Tests at Elevated Temperatures

3.1 Test Specimens

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 1 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 3 The properties of steel

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and polypropylene fibers are shown in Table 2 It is worth noting that the workability and

flowability of the UHSC were not affected by the addition of polypropylene fibers as the

UHSC is most likely pumped into hollow tubes for CFST columns The dosage of

polypropylene fibers was lower than that recommended by Eurocode 2 where more than

2kg/m3 (0.25% in terms of volume) of monofilament propylene fiber should be included in

the HSC mixtures to prevent spalling [2]

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

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

Owing to the fact that the moisture content in the UHSC is low, the effect of moisture on the

mechanical properties is deemed to be insignificant [34] On the other hand, the moisture is

evaporated around 100oC, it may only have minor influence at 100oC but insignificant

influences at higher temperatures Considering these, the unsealed specimens were used

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

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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

the rods were attached to the middle 1/4 height of the specimen which is the gauge length

The test setup is shown in Figure 4 Top and bottom cooling blocks were used to load the

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

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

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

was applied from the bottom of the loading frame by a hydraulic cylinder

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

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

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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

strain should be considered for the fire resistance design of CFST columns In present study,

the unstressed test method was adopted and the effect of transient thermal strain was

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

by test results

For the unstressed tests conducted, a small compressive stress of approximately 0.05MPa was

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

temperatures with a heating rate of 5oC/min In fact, the heating rate varies when a structural

member is subjected to a realistic fire However, it would be rather difficult to conduct tests

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 5 At early 5 minutes, the

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

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temperature during which uniform temperature distributions can be achieved inside both the

furnace and the UHSC specimens Figure 6 shows the recorded temperatures for a

100x200mm cylinder specimen heated up to 800oC in an electrical oven with a heating rate of

5oC /min It can be seen that the uniform temperature distribution can be achieved in 4 hours

Hence, the holding time at target temperatures were taken as 4 hours for all specimens

After holding, the specimen was subjected to three load cycles between 0.05MPa and 15% or

between 5% and 15% of the reference strength as shown in Figure 7 [37] The holding time at

5% and 15% load levels was 60s Then the specimen was loaded to fail Displacement control

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

sudden crush of the UHSC specimen would damage the extensometer The extensometer was

removed when at least 40% of the compressive strength at target temperature was reached

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

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]

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 6 specimens 3 specimens were used for the other target

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temperatures The compressive strength was taken as the peak stress on the curve of loading

head movement versus compressive stress as shown in Figure 8(a) The test values are shown

in Table 3 The reduction factor in this paper is defined as the ratio of strength or elastic

modulus at target temperature divided by their counterpart at room temperature The

reduction factors of strength are shown in Figure 9 It can be seen that the strength was

sharply reduced at 100oC, and then it was partly recovered up to 300oC It is believed that the

chemical composition of the cement paste were not noticeably changed around 100oC Hence,

the sharp reduction of strength at 100oC could be either due to the built-up internal pressure

by the evaporation of free water, or the expansion of water between the C-S-H layers causing

a decrease in the surface forces For the recovery of strength up to 300°C, it might be

attributed to the general stiffening of the cement gel by shrinkage, in other words, the

increase of surface forces (Van der Walls forces) between the gel particles due to the removal

of water [38;39] The temperature at which water is removed and the strength begins to

recover depends on the porosity of the concrete [40] Beyond 300oC, the strength decreased

as the temperature increased The decrease of strength was attributed to the decomposition of

hydration products such as C-S-H and Ca(OH)2, the deterioration of aggregates, and the

cracks due to thermal incompatibility between the aggregate and the cement paste which led

to stress concentration At 800oC, the strength was about 30% of that at room temperature

The comparisons between the strength reduction factors of UHSC and NSC given in EN

1992-1-2 [2] and AISC 360-10 [41] are also shown in Figure 9 The NSC is implicitly

defined as compressive cylinder strength less than 55MPa in EN 1992-1-2, and not greater

than 55MPa in AISC 360-10 The reduction factors are applicable to both NSCs with

siliceous aggregates and calcareous aggregates in AISC 360-10 It was supposed that the

strength of UHSC would reduce greater than that of NSC However, it can be seen that,

beyond 300°C, the reduction factors of UHSC were similar with those of NSC with

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calcareous aggregates, but higher than those of NSC with siliceous aggregates The reason

was due to the effect of aggregate type as discussed in Section 1 Generally, the aggregates

occupy 65% to 75% of the concrete volume The effect of aggregate mainly depends on the

thermal stability or integrity of aggregate at high temperatures [14] Conventional calcareous

or siliceous aggregates are thermally stable up to 300°C~350°C Bauxite aggregates in UHSC

are more stable due to high melting point, and thus produced significant improvements in

heat resistance of the UHSC The bauxite aggregates have been used for refractory concretes

to achieve super fire performance [42; 43] The comparison between the strength reduction

factors of UHSC and HSC in EN 1992-1-2 are shown in Figure 10 The reduction factors are

not provided for HSC in AISC 360-10 It is clear that the strength of UHSC was reduced less

than that of HSC due to the effect of aggregate type The comparisons indicated that, for stub

CFST columns with the UHSC governed by compressive resistance, they would withstand

longer time when exposed to fire than the CFST columns with the NSC and HSC

The strength reduction factors of UHSC are compared with those of HSC in the literature as

shown in Figure 11 [7; 8; 19; 40; 44] It can be seen that the sharp deterioration at 100°C and

the recovery of strength between 100°C~300°C were also captured in previous researches In

general, the 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 and their counterparts measured after heating and cooling are also compared in

this paper In tests for residual properties, the heating rate was 5 °C/min, the dosage of

polypropylene fibers was 0.1% by volume, and the specimens were naturally cooled down in

lab air The reduction factors of strength and residual strength are shown in Figure 12 [34] It

can be seen that the residual strength was reduced more than the strength when the

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temperature was higher than 300oC This might be attributed to two facts One fact is that the

residual strength was affected by further internal micro-cracking induced by differential

thermal strain during cooling down Another fact is that the calcium oxide (CaO) from the

decomposition of hydration products Ca(OH)2 absorbed water after cooling down from high

temperatures Then, it expanded and induced more cracks inside the concrete As a result, the

residual strength was lower [15]

4.2 Modulus of Elasticity

The elastic modulus was generally defined as the secant modulus between the stress equal to

40% of peak stress and the stress corresponding to strain of 5x10-5 in accordance with ASTM

C469-02 [45] For some stress-strain curves with the existence of turbulence, the slope of the

regressed linear equation for a straight portion was taken as the modulus of elasticity as

shown in Figure 8(b) The elastic modulus of UHSC at room temperature was 61GPa as

shown in Table 3 The reduction factors of elastic modulus at elevated temperatures are

shown in Figure 13 Similar to the compressive strength, the sharp reduction and the recovery

were also observed for the elastic modulus due to the built-up internal pressure by the

evaporation of free water Figure 13 also gives the comparisons between the modulus

reduction factors of UHSC and NSC as given in EN 1992-1-2 and AISC 360-10 The

modulus of elasticity of UHSC was less affected than that of NSC

The reduced elastic modulus of UHSC are also compared with those of HSC in the literature

as shown in Figure 14 [7; 8; 19; 40; 44] The sharp reduction and recovery were also

observed in some researches Overall, the elastic modulus of UHSC was reduced less than

most of the HSCs in the literature due to the effect of aggregate The practical implication is

that, for slender CFST columns with the UHSC governed by buckling resistance, they would

withstand longer time when exposed to fire than the CFST columns with the NSC and HSC

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A comparison of the reduction factors for the elastic modulus and the residual elastic

modulus are shown in Figure 15 [34] For the residual elastic modulus, there was no unusual

deterioration and recovery at the temperature range of 100°C~200°C The elastic modulus

was reduced slightly less than the residual elastic modulus at temperatures between

200°C~500°C, but more beyond 500°C It could be concluded that the modulus of elasticity

at and after elevated temperatures are comparable

5 Simple Calculation Method for Predicting Fire Resistance of Ultra-High Strength Concrete Filled Steel Tubes

5.1 Fire Resistance Calculation

Similar to the calculation of buckling resistance of a CFST column under axial load at room

temperature [3], the buckling resistance of CFST columns with the UHSC exposed to fire can

be determined in accordance with the following simple calculation method (SCM):

a,j a,θ c,k c,θ fi,Rd

the external fire load as the fire exposure time goes To determine the buckling resistance,

cross section of the CFST column should be discretized into elements where Aa,j and Ac,k are

the areas of steel and concrete elements, and fa,θ and fc,θ are the yield strength of steel element

and the compressive strength of concrete element at temperature θ The is the buckling

reduction coefficient which depends on the buckling curve “c” according to EN 1994-1-2 [4]

γM,fi,a and γM,fi,c are partial safety factors for steel and concrete which are taken as 1.0 at fire situation [4] The determination of involves the buckling length of the column in fire lθ,

the effective flexural stiffness (EI)fi,eff, the Euler buckling resistance Nfi,cr under fire, and the

fi cr

EI N

As mentioned in Section 3.3, the transient thermal strains of the UHSC were not measured in

the unstressed tests, which would overestimate the flexural stiffness of column The

overestimation should be less for CFST columns with the UHSC as the transient thermal

strain of the UHSC is generally smaller than that of NSC [46] To determine the effective

flexural stiffness properly, reduction factors φa,θ and φc,θ are taken into account respectively

for the steel and concrete as shown in Eq.(2) φc,θ is recommended as 0.8 for concrete [4], and

φa,θ can be taken as 1.0 for steel [47] Ia,j and Ic,k are th e second moment of areas of the steel

and concrete elements about neutral axis; whereas the Ea,θ and Ec,θ are the elastic modulus of

the steel and concrete elements at temperature θ Basically for NSC/HSC and carbon steel,

the temperature dependent mechanical properties can be referred to EN 1992-1-2 [2] and EN

1993-1-2 [48], respectively For high tensile strength steel and the present UHSC, they can be

taken respectively from Ref [49] and Section 4 of this paper

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It is worth noting that the increase in strength of concrete caused by confinement is not taken

into account in Eq.(1) for the present fire resistance design This has been found in researches

by Ibanez et al [50], and Wang and Young [51] The ignorance of confinement is mainly due

to three facts Firstly, the CFST column with the UHSC fails at small deformation and has not

developed significant confinement [30] Secondly for temperature lower than 250oC, the steel’s thermal expansion is larger than that of concrete, there is actually no contact between the steel tube and the concrete, thus no confinement occurs At higher temperatures, there

may be contact but the steel has been softened Thirdly, the magnitude of confinement is

related to the ratio fy/fck according to Eurocode 4 [3] For the CFST column directly exposed

to fire, the steel temperature arises faster than that of concrete, as a result, the steel loses its

yield strength fy much faster than the concrete losing its compressive strength fck The ratio

fy/fck decreases rapidly, and the steel is not capable to provide sufficient confinement to the

concrete

5.2 Heat Transfer Analysis

Temperature distribution on the cross section of the CFST column should be determined for

each time step for the determination of fire resistance To the authors’ best knowledge, there

is no method available for calculating temperatures of a CFST column in worldwide design

codes Herein the modified finite difference method (FDM) is adopted The modifications are

based on the FDM used by Lie et al [52] and Kodur et al [53] to determine temperature

profiles of the circular and square CFST columns under fire The main modifications are

inclusion of heat convection, development of thermal resistance between steel and concrete

interface, and introduction of square mesh network for square columns To determine the

temperature distribution, the cross sections should be discretized as shown in Figure 16 Then

the elemental temperature, represented by the nodal temperature at center of an element, can

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be solved on the basis of energy conservation where the heat flowing into one element from

adjacent elements should be equal to the energy consumed by the temperature increase of the

element considered For instance at node (1,1) in Figure 16(b), the temperature is calculated

The left-hand size of Eq.(7) represents the heat flowing into node (1,1) at ith time step, the

right-hand side of Eq.(7) stands for the energy consumed by the increase of nodal

temperature The Tf is the fire temperature and t is the fire exposure time The Δx, Δy are

element sizes Δt is the time step The λ, ρ, c are the temperature-dependent thermal

conductivity, density and specific heat, respectively The thermal properties of NSC and HSC

at elevated temperatures can be referred to EN 1992-1-2 [2], whereas their counterparts in EN

1993-1-2 [48] can be used for steel For the UHSC, little information on its thermal properties

is available in the literature As the UHSC is less porous than the NSC and HSC, it should

have higher thermal conductivity and less moisture content Considering this, the thermal

properties of HSC can be used for the UHSC, except that the upper limit of the thermal

conductivity and the specific heat with a moisture content of 0% in EN 1992-1-2 may be used

For NSC exposed to fire, the time-temperature curve usually shows a plateau at 100oC due to

the evaporation of water However for the UHSC shown in Figure 6, there is no such plateau

With regard to this, its moisture content is assumed to be 0%, which is validated by test

results in Figure 19

In Eq.(7), the h is the sum of coefficients of the heat convection he and the thermal radiation

h r The convection coefficient he can be taken as 25 W/mK for exposure to standard fire of

ISO-834 [54] The thermal radiation coefficient hr should be calculated as follow:

Φ is the configuration factor which can be taken as 1.0 by ignoring the position and shadow

effects [55] εm is the steel surface emissivity where it may be taken as 0.7 for unprotected

columns [48] εf is the emissivity of the fire and taken as 1.0 [55] σ is the Stephan Boltzmann

constant equal to 5.67x10-8 W/m2K4 The finite difference equations for nodal temperatures at

the steel-concrete interface can be derived similar to Eq.(7), except that the thermal contact

resistance at the interface should be used to replace the coefficient h The thermal contact

resistance is considered due to the air gap existing at the steel-concrete interface Basically, it

can be taken as 100 W/mK according to Ref [56]

5.3 Buckling Length of CFST Column in Standard Fire Test

As mentioned in Section 5.1, the column buckling length is needed to determine the buckling

reduction coefficient χfi For the CFST columns in frames subject to a realistic fire, it can be

easily determined according to EN 1994-1-2 [4] However it is difficult for the CFST

columns in standard fire tests This is because only the mid-height of the column is exposed

to fire and the other parts are unexposed A typical setup of the standard fire test on a CFST

column with a pinned-pinned boundary condition and subject to axial compression is shown

in Figure 17 The column is partly exposed to fire with two ends outside the furnace The

non-uniform temperature distribution yields differences in flexural stiffness along the column

length, thus the buckling length would be different from that of column exposed to a uniform

temperature In general, the buckling length of a CFST column in standard fire test can be

calculated by solving a fourth-order differential equation of its lateral displacement For

instance for a pinned-fixed column shown in Figure 18, it is divided into three segments

according to the exposed/unexposed parts, the fourth-order differential equation of lateral

displacement for each segment is then given as:

Substituting the boundary and compatibility conditions shown in Figure 18 into the general

solutions of , , , and their derivatives will yield the solution of buckling length

5.4 Validations for Simple Calculation Method

Standard fire test data on CFST columns subject to axial compression from Lie and Chabot

[23] Romero et al [29], and the authors of this paper [57] were used to establish validity of

the proposed simple calculation method There were totally 29 tests carried out by Lie and

Chabot [23] All columns were fixed at both ends The concrete strength varied from

23.8MPa to 58.3MPa All column length was 3810mm with the length exposed to fire being

3200mm For the tests by Romero et al.[29], there were 5 columns used for the validations

The concrete strength varied from 28.55MPa to 71.14MPa The column length was 3180mm

and the exposed length to fire was 3000mm All columns were fixed at one end and pinned at

the other end except for one column with both ends pinned 4 unprotected CFST columns

infilled with the UHSC by Xiong et al [57] were used for the validation The column length

was 3810mm with 3000mm exposed to fire For all the CFST columns used for validations

hereinafter, the fire resistance time was predicted based on the actual furnace temperatures

which were designed to follow the time-temperature curve of standard ISO-834 fire Details

of the CFST columns are summarized in Table 4

The heat transfer analysis in Section 5.2, the determination of buckling length in Section 5.3,

and the calculation of buckling resistance under fire in Section 5.1 were conducted by using

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MATLAB The modified FDM is deemed to be qualified for the heat transfer analysis on

CFST columns under fire as the predicted temperatures for the CFST columns with the

UHSC, given in Figure 19, show reasonable agreements with the test temperatures The

moisture content could be taken as 0% as there was no clear plateau on the time-temperature

curves of the UHSC infilled in steel tubes

Table 4 gives the tested and predicted fire resistance time Two cases were considered where

the NSC/HSC in Refs.[23; 29] was replaced by the UHSC having fck=166MPa in Case 1; and

the UHSC in Ref.[57] was replaced by a NSC with fck=40MPa in Case 2 The mean

prediction/test ratio is 1.017 and 0.889, respectively for Case 1 and Case 2 The mean value

for CFSTs with NSC/HSC is much close to unity, whereas the mean value for CFSTs with

UHSC shows conservative predictions The conservativeness is approximately 11%, mainly

due to the over-conservativeness from column LSH-2-1 Overall, reasonable predictions by

the proposed simple calculation method are generally observed

5.5 Discussions

The proposed simple calculation method is powerful as it can provide the buckling reduction coefficient and the bucking resistance Nfi,Rd, changing with the fire exposure time This is difficult for conventional finite element software, such as ABAQUS, ANSYS, etc., as a full

package of heat transfer analysis and coupled thermal-mechanical analysis is needed for each

time step This is tedious for many time steps involved for a rather long fire exposure time

The buckling reduction coefficient and the buckling resistance for the CFST columns with

the UHSC are shown in Figure 20 It shows that the buckling resistance is rapidly reduced at

early stage of fire exposure, then decreases smoothly at later stage The fire resistance time is

determined when the buckling resistance approaches the applied test load Regarding the

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buckling reduction coefficient, it decreases as the fire exposure time continues The reduction

factor is less than 1.0 at the column failure, indicating a global buckling failure

The predictions for the CFST columns with the replaced concrete are shown in Table 4 The

replacement was based on the same load level which is defined as the ratio between the

applied axial load over the buckling resistance at room temperature calculated according to

EN 1994-1-1 [3] For Case 1 with the NSC/HSC replaced by the UHSC, the ratio between the

two predictions tu/tp varies from 1.083 to 2.674 with a mean value of 1.523 For Case 2 with

the UHSC replaced by the NSC, the said ratio is in the range of 0.682 ~ 0.849 with an

average of 0.767 The comparisons show significant improvements on fire resistance when

the NSC/HSC with conventional siliceous or calcareous aggregates are replaced by the novel

UHSC with the bauxite aggregates

It is well known that the fire resistance time of a CFST column increases with the decrease of

section factor and load level, and the increase of concrete contribution ratio It is also worthy

to know if the improvement, represented by the ratio tu/tp, follows the same trend The section

factor is defined as the ratio between the exposed area and the volume of the CFST column It

is generally used to measure the rate of temperature increase in a column The higher the

section factor, the faster the section heats up For a CFST column with a uniform

cross-sectional profile within its length, the section factor can be calculated as the ratio between the

perimeter and the area of the cross-section The CFST columns for sensitivity study on the

section factor is shown in Table 5 British hot finished steel tubes are used Figure 21 shows

the relationship between the improvement and the section factor It can be seen that the

improvement is slightly more at early increase of the section factor, but sharply less for the

further increase Nevertheless, there is an improvement (i.e tu/tp >1.0) as long as the NSC is

replaced by the UHSC

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The effect of load level is shown in Table 6 and Figure 22 High load levels are not used as

the maximum value of the load level is 0.74 according to EN 1994-1-2 [4] The improvement

is generally getting more with the increase of load level, this is contrary to the trend of fire

resistance time After its maximum is achieved, the improvement is getting less with the

increase of load level

Table 7 and Figure 23 shows the effect of concrete contribution ratio which is defined as:

c c

,

ck

pl Rd

A f N

  (19)

where Ac is the cross-sectional area of concrete, Npl,Rd is the plastic resistance to compression

according to EN 1994-1-1 [3] The concrete contribution ratio stands for the contribution of

concrete to the resistance of section The higher the ratio is, the larger the influence should be

due to the replacement of concrete Herein the variation of concrete contribution ratio is made

by the change of steel tube thickness Figure 23 shows that the improvement is more with the

increase of concrete contribution ratio, but generally less for the further increase

Overall, the improvement is not monotonically changed with the increase of section factor,

load level, and the concrete contribution ratio This reflects a counterbalance between the

benefit from the use of UHSC and the said parameters to affect the fire resistance time

6 Conclusions

An experimental investigation on the mechanical properties of UHSC at elevated

temperatures is presented in this article The mechanical properties included cylinder

compressive strength and modulus of elasticity The experimental results were compared with

those of concretes given in design codes and in the literature The fire resistance of CFST

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composite columns with the UHSC was evaluated based on a proposed simple calculation

method The following conclusions can be drawn

(1) Spalling of the UHSC was prevented during heating to 800oC due to the addition of 0.1%

polypropylene fibers

(2) Sharp deterioration of strength of the UHSC was observed at 100oC and then it was

partly recovered up to 300oC The deterioration and recovery of strength were induced by

the evaporation of free water and the resulted shrinkage The deterioration around 100°C

and recovery of strength up to 300°C were also observed for HSC from previous

researches

(3) Strengths of the UHSC at elevated temperatures were reduced less than those of NSC and

HSC as introduced in Eurocode 2 and AISC 360, and the HSC reported in the literature

This can be explained through the types of aggregates

(4) Deterioration and recovery of the elastic modulus of the UHSC were observed at the

temperature range of 100oC~200oC, similar to HSC from previous researches The elastic

modulus of the UHSC were reduced less than that of NSC in Eurocode 2 and AISC 360

(5) Compressive strength at elevated temperature was generally greater than the residual

strength at the same target temperature; whereas the elastic modulus was comparable

with the residual elastic modulus

(6) Fire resistance of CFST columns with the UHSC were improved compared with the

NSC/HSC infilled in hollow steel tubes By studying on 38 CFST columns from previous

researches subject to standard fire tests, the fire resistance time was averagely prolonged

by 30% ~ 50% when the NSC/HSC was replaced by the present UHSC