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Evaluation of the mechanical properties of 200 MPa ultra-high-strength concrete at elevated temperatures and residual strength of column
Construction and Building Materials 86 (2015) 159–168
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Evaluation of the mechanical properties of 200 MPa ultra-high-strength concrete at elevated temperatures and residual strength of column Gyeongcheol Choe a, Gyuyong Kim a,⇑, Nenad Gucunski b, Seonghun Lee c a
Department of Architectural Engineering, Chungnam National University, 99 Daehak-ro, Yuseong-gu, Daejeon 305-764, Republic of Korea Department of Civil and Environmental Engineering, Rutgers University, Piscataway, NJ 08855-0909, USA c Construction Technology Center, Samsung C&T Corporation, Seocho-Gu, Seoul, Republic of Korea b
h i g h l i g h t s Mechanical properties of USHC subjected to elevated temperature were evaluated. Fire resistance performance of RC column USHC was investigated. Prediction process for residual load of column subjected to fire has been proposed.
a r t i c l e
i n f o
Article history: Received 28 October 2014 Received in revised form 9 March 2015 Accepted 11 March 2015 Available online 10 April 2015 Keywords: Fire resistance Ultra high-strength concrete Reinforced concrete column Mechanical properties Residual strength
a b s t r a c t In this study, material properties of ultra-high-strength concrete (UHSC) and residual load of reinforced concrete column have been evaluated at elevated temperature. On the basis of the results of experiments, prediction process for residual load of column member subjected to elevated temperature have been proposed. For evaluation on mechanical properties at elevated temperature of UHSC, 100 200 mm cylinder specimen of 100, 150, 200 MPa was heated to 100, 200, 300, 500 and 700 °C by 1 °C/min. After that, the ratio of compressive strength at room temperature to compressive strength at elevated temperature was measured. Also, comparison evaluation between experimental results of this paper and previously presented properties of compressive strength properties of normal strength concrete (NSC) and high strength concrete was carried out. Reinforced concrete column member was made of 200 MPa concrete and was heated by ISO-834 standard heating curve for 3 h. Internal force degradation and fire resistance performance of column member was evaluated by measuring internal temperature and axial deformation. Also, after heating, residual axial load capacity was evaluated through natural cooling for 24 h. Prediction process of residual internal force of UHSC column was proposed by deriving model for internal force degradation with experimental results of residual mechanical properties and spalling properties of UHSC subjected to elevated temperature. Based on the experimental results, it was observed that internal force degradation of UHSC by elevated temperature occurred more than that of NSC. In the test for fire resistance performance of column, 26% of section was lost due to spalling, however, it was observed that residual internal force was 52% of design axial load. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Driven by the increasing demand for high-rise buildings, over 200 m high buildings are being built around the world. The development of high-strength concrete (HSC) with larger permissible stress and smaller cross-sections is one of the key technological elements for building such ultrahigh-rise structures [1]. The American Concrete Institute (ACI) specifies a compressive strength
⇑ Corresponding author. E-mail address: [email protected] (G. Kim). http://dx.doi.org/10.1016/j.conbuildmat.2015.03.074 0950-0618/Ó 2015 Elsevier Ltd. All rights reserved.
of 40 MPa as the minimum strength requirement for HSC, which should outperform normal-strength concrete in terms of compressive strength and stiffness [18]. Recently, high-rise buildings have been constructed using concrete with a compressive strength of 100 MPa. Furthermore, there are reports that applications of 200 MPa concrete was used. Therefore, it is expected that ultra-high-strength concrete is about to be on commercialization in construction industry [2,3]. On the other hand, despite numerous advantages of HSC in terms of strength, stiffness, and durability, it can exhibit undesirable behaviors in high-temperature environments, such as spalling
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and reduced strength due to the formation of dense internal structures [4–6]. To address this issue, researchers such as Abrams [7], Malhotra [8], and Shchneider [9] have carried out studies on the strength deterioration of concrete exposed to high temperatures since the 1950s, analyzing the association between heating temperature and concrete strength deterioration. Based on the results of these studies, predictions and recommendations have been established in codes that are widely used. However, these studies deal mostly with normal-strength concrete with compressive strengths of less than 40 MPa. Phan reported that HSC of more than 40 MPa suffered a strength loss of 20–30% more in the temperature range of 100– 400 °C as compared to normal-strength concrete [10–13], and they presented material models capable of coping with the HSC strength deterioration after evaluating the mechanical properties of 100 MPa HSC at elevated temperatures [14]. These material models are used as final test materials for the resistance prediction of structural members exposed to high temperatures [15]. Dotreppe calculated the temperature distribution area of a concrete column using SFAIR, a finite element analysis (FEA) program. He suggested a calculation method to design reinforced concrete (RC) columns under fire conditions, thereby taking into account the properties of concrete at elevated temperatures as specified in Eurocode [16]; the calculated values proved to agree closely with the experimental data (n = 83) [17]. The aforementioned studies on the properties of concrete at elevated temperatures or calculations of the fire resistance of concrete members are based on research results of concrete with compressive strengths of less than 100 MPa; thus, there is a lack of research on the properties of concrete at elevated temperatures or the fireproof performance of concrete members with compressive strengths of 100 MPa or above. Unlike ordinary concrete, UHSC has a dense internal structure and hence tends towards condensation of water vapor pressure and is thus susceptible to spalling. The UHSC column thus becomes prone to abrupt deterioration due to spalling and subsequent cross-sectional loss of the concrete as well as to degradation of the mechanical properties at elevated temperatures. Mechanical performance and spalling behavior of UHSC at elevated temperatures were evaluated. Also, the deterioration and residual strength of an RC column fabricated using UHSC were evaluated. Thereupon, the concomitant effects of the materialrelated mechanical properties under high-temperature conditions and the fire resistance performance of structural members were investigated in order to determine how to reflect the safety performance index in fire resistance design.
of the ultimate residual load after leaving the specimen to cool for 24 h. Finally, on the basis of the mechanical properties at elevated temperatures and the date obtained from the fire resistance test, ultimate load of an RC column exposed to high temperatures was calculated using the residual load calculation method suggested by Dotreppe. 3. Evaluation of properties of UHSC at elevated temperatures 3.1. Test program and mixture proportions Experimental plan for evaluation on material properties of concrete subjected to elevated temperature is presented in Table 1. UHSC specimens with compressive strengths of 100, 150, and 200 MPa were produced with dimensions of /100 200 mm. Evaluation of material properties was divided into spalling property and compressive strength degradation. In spalling property test, nylon and polypropylene fiber were used in order to verify the spalling prevention effect by organic fiber mixing. Fiber combination conditions were no fiber condition, single fiber condition, and composite fiber condition. In single fiber condition, nylon and polypropylene fibers accounted for 0.15 vol%, 0.25 vol% respectively of the total volume of concrete. In composite fiber condition, the total volume percent of concrete was 0.15 vol% with 0.075 vol% of nylon fiber and 0.075 vol% of polypropylene fiber. Heating was carried out up to 900 °C with ISO-834 standard heating curve. After heating, damage from spalling of specimens subjected to heating was identified by conducting visual inspection and weight loss rate evaluation. In this study, consistent weight loss rate was observed in the concrete specimen that spalling did not occur. Therefore, weight loss rate was evaluated in order to use it as the index for quantitative assessment of the extent of spalling of UHSC. Also spalling levels were rated on a scale of 1 to 4 referring to Lee [30]. Spalling levels were defined as grade 1 with 0–25%, grade 2 with 25–50, grade 3 with 50–75, and grade 4 with 75–100 by comparing to before and after experiment of weight loss rate. The values for each grade was calculated by Eq (1).
W¼
W1 W2 100 W1
ð1Þ
where W: weight loss (%) W1: weight of specimen before heating (g) W2: weight of specimen after heating (g)
2. Test process The flow of this study is shown in Fig. 1. Experiments for material properties of concrete and fire resistance of column member have been conducted separately. First, in order to determine the high-temperature properties of UHSC, the spalling properties and degradation of compressive strength of UHSC specimens were evaluated. Spalling properties of UHSC was evaluated by visual inspection and weight loss rate in accordance with the ISO-834 standard heating curve. The compressive strength of the specimens at elevated temperatures with respect to that at ambient temperature was evaluated after heating the specimens to 100, 200, 300, 500, and 700 °C at a rate of 1 °C/min. Next, the fire resistance test for 3 h was performed in accordance with the ISO-834 standard heating curve on a RC column (500 500 600 mm, 200 MPa UHSC). Also, the heating temperature-dependent internal temperature increase, spalling property, and axial deformation were evaluated followed by an evaluation
Also, concrete specimen without fiber was used for compressive strength degradation test, heated to 100, 200, 300, 500 and 700 °C at the rate of 1 °C/min. in order to make the inside and outside temperature of specimen equal and simultaneous. The test for determining the compressive strength property at elevated temperatures was performed under two loading conditions: unstressed, in which the compressive strength was assessed after heating the specimen up to the target temperature without preloading in order to determine the effect of baseline stress, and stressed, in which the specimen was preloaded at ambient temperature to 25% of its ambient temperature compressive strength and heated to the target temperature prior to evaluating the compressive strength. Table 2 outlines the mixture proportions and the properties of the concrete. UHSC spcimens producted by mixing ASTM Type I Portland cement, crushed granite (coarse aggregate), river sand (fine aggregate), silica fume, fly ash, blast furnace slag, and gypsum. The specimens were moist-cured for 7 days and dry-cured
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Fig. 1. Research flow.
Table 1 Test program for evaluation of material properties of USHC. fck (MPa)
Material property
Fiber (added contents (vol%))1
Heating method
Exposure temperature (°C)
Pre-load condition
100
Spalling
Non fiber Nylon (0.15, 0.25) Polypropylene (0.15, 0.25) Composite fiber (0.15) –
ISO-834 heating curve
900
Unstressed
1 °C/min
100, 200, 300, 500, 700
Stressed2 unstressed
150 200 1 2
Compressive strength
Composite fiber (0.15): nylon (0.075 vol%) + polypropylene (0.075 vol%). 25% of compressive strength at room temperature.
3.2. Experimental results and discussion Table 2 Mixture proportions and properties of UHSC. Property
100 MPa
150 MPa
200 MPa
Cement content-type 1 (kg/m3) Silica fume-dry weight (kg) Slag (kg/m3) Fly ash (kg/m3) Gysum (kg/m3) Fine aggregate (kg/m3) Coarse aggregate (kg/m3) (maximum size: 13 mm) Water (kg/m3) Water:Cement ratio Water:Cementitious ratio
525 75 0 150 0 644 848
652 124 207 0 52 448 870
660 240 240 0 60 389 736
150 0.286 0.200
150 0.230 0.145
150 0.227 0.125
Fresh concrete Slump-flow (mm) Air concrete (%)
735 1.9
740 1.9
785 2.1
Hardened concrete Compressive strength (MPa) 28 days 56 days 300 days
75 86 101
125 135 149
173 182 205
for 300 days under constant temperature (20 °C) and humidity (60%) conditions. Fig. 2 shows the test device for the evaluation of the mechanical properties of the concrete test specimens at elevated temperatures. It is a 2000 kN universal test machine (UTM) with an electric heating device attached to it, capable of performing heating and loading simultaneously.
3.2.1. Spalling properties of UHSC The weight loss rate from spalling and evaporation of the UHSC specimens in accordance with the ISO-834 standard heating curve are illustrated in Fig. 3. All of the specimens without fiber exhibited explosive spalling and could not maintain their forms by the end of the heating process. Therefore, since obtained concrete fragments after experiment were pieces by spalling, weight loss rate was evaluated as 100%. No spalling appeared in the 100 and 150 MPa concrete specimens containing more than 0.15 vol% of PP and NY fibers, and they exhibited a weight loss of about 6% because of the water evaporation inside the specimens. However, the 200 MPa concrete specimen containing 0.15 vol% of PP and NY fibers suffered surface spalling, thereby exhibiting 20% and 16% weight loss, respectively. In contrast, none of the specimens with 0.25% fiber content experienced spalling. According to the reported results of a previous study, cement reinforced with two or more types of fiber is better protected from spalling than cement reinforced with a single fiber [19]. This result is consistent with the results of this study, in that no spalling occurred in the specimen containing 0.15 vol% of the PP and NY blend. 3.2.2. Compressive strength of UHSC at elevated temperatures Fig. 4 shows the results of the strength evaluation test of the UHSC specimens at elevated temperatures. Under the unstressed condition, strength deterioration of 10%, 30%, and 40% occurred in the 100, 150, and 200 MPa test specimens, respectively, at
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Fig. 2. Heating machine (left) and method of strain measurement (right).
Fig. 3. Weight loss according to spalling and evaporation.
100 °C. At 200 °C, however, compressive strength increased to an extent similar to that tested at ambient temperature. Once this temperature was reached, the compressive strength tended to decrease as the heating temperature increased. Remarkably, the 200 MPa specimens suffered spalling at a heating temperature of 280 °C under the unstressed condition; therefore, the compressive strength at 300 °C and higher could not be evaluated. These results, which are consistent with those reported in studies by Diederichs [22], Hammer [21], and Furumura [20], support the assumption that HSC and UHSC can suffer explosive spalling at low heating temperature conditions as well. The stressed test exhibited tendencies similar to those of the unstressed test, whereby the residual rates of compressive strength were observed to be slightly higher, more markedly in the temperature ranges exceeding 500 °C under loading. Unlike
in the unstressed test, the 200 MPa specimens did not show spalling; thus, compressive strength could be evaluated at all temperature conditions. The absence of spalling in this study is considered to be attributable to the 25% preloading. In this study, as demonstrated in Fig. 4, except for the strength deterioration below 200 °C, the 100 and 150 MPa concrete showed tendencies similar to those of Eurocode 2, and the 200 MPa concrete showed properties similar to those suggested by the National Institute of Standards and Technology (NIST) for HSC at elevated temperature. Consequently, on the basis of the existing codes and the results of this study, code for the 200 MPa UHSC property model at elevated temperatures is present as the following Relative compressive strength of ultrahigh-strength concrete (k(T), fc,t/fc,20 °C)
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1.4
column fire resistance performance was assessed in accordance with these standards [25–29].
1.2
Relative Strength(fc,t/fc,20ഒ)
163
4.2. Fire resistance of UHSC column
1.0
100MPa 150MPa 200MPa 100MPa 150MPa 200MPa
Unstressed Test 0.8 Stressed Test
0.6 0.4 0.2 0.0 0
200
400 600 Temperture(ഒ)
800
1000
1200
Fig. 4. Comparison of test data with codes.
T 6 20 C;
kðTÞ ¼ 1
20 C < T 6 100 C; 100 C < T 6 200 C;
kðTÞ ¼ 1:0375 0:0019t kðTÞ ¼ 0:85
200 C < T 6 700 C;
kðTÞ ¼ 1:1 0:0013t
700 C < T 6 900 C;
kðTÞ ¼ 0:62 0:0006t
700 C < T 6 1200 C; kðTÞ ¼ 0:32 0:0003t T > 1200 C
kðTÞ ¼ 0
4. Fire resistance performance of reinforced concrete column 4.1. Test method for assessing the fire resistance of RC column Table 3 and Fig. 5 provide an overview of the test RC column used in this study. The same concrete with a compressive strength of 200 MPa as was used for the fabrication of the high-temperature property test specimen was used. In order to prevent spalling, 0.1, 0.15, and 0.3 vol% of PP, NY, and steel fibers were add, respectively. For heating and loading, a combined device consisting of a 3000 kN compression testing machine and a gas fuel heating apparatus were used. The design load was set at 48,000 kN. A test load of 12,000 kN force, which was 25% of the design load, was applied to the test specimen prior to heating. The fire resistance test was performed by heating the specimen for 3 h in accordance with the ISO-834 standard heating curve (Fig. 6). As the test setup for the evaluation of the fire resistance performance of an RC column, three temperature sensors were installed on the vertical reinforcing bar (rebar) and six additional temperature sensors were installed inside the column at different distances from the surface to assess the increase in internal temperature. ISO, ASTM, KS, BS, and JIS specify allowable temperatures or axial deformation of the rebar as the evaluation criteria for column fire resistance tests, as outlined in Tables 4 and 5. The
Table 3 Fabrication of RC column. Column Size (mm)
Reinforcing bar
Design load (kN)
Concrete strength (MPa)
Test load (kN)
Fire duration (min.)
500 500 3600
D29500 MPa
48,000
207
12,000
180
4.2.1. Heating characteristics Fig. 7 shows the temperature increases inside the concrete column during the heating process in accordance with the ISO-834 standard heating curve. The rising temperature on the heated concrete surface was transferred to the inside of the column. The closer surface column subjected to direct fire, the higher temperature was measured. The rate of temperature rise tended to increase sharply after 30 min of heating as a result of the reduction of crosssection size due to spalling. In order to determine the change in the rate of temperature increase induced by the damage to the concrete cover, a heat transfer analysis was performed using the Midas FEA program for finite element analysis assuming the non-spalling scenario. Although the spalling did not have significant impact on the temperature increase of the concrete inside the rebar, the parts closer to the surface area showed a significant increase in temperature, presumably due to the damage to the concrete cover. The temperatures measured at sensor positions 4, 6, and 7 installed on the rebar were taken as reference temperatures inside the column for the evaluation of fire resistance performance as compared to the standards for the maximum allowable temperature as set out in ISO, ASTM, KS, and JIS. The measured values from the fire resistance test after 3 h of heating did not exceed the maximum and average allowable temperatures of 649 °C and 538 °C, respectively; thus, the test column was verified to have a fire resistance performance of 3 h. On the other hand, the temperature sensor 7 of the test column showed temperatures of more than 500 °C, the allowable temperature as specified by JIS, in 2.3 h, and its fire resistance performance was thus evaluated to be 2.3 h. 4.2.2. Spalling property Although no spalling occurred in the specimens fabricated with the 0.25 vol% multi-fiber blend. The test column exhibited a 26% decrease in cross section by the end of the 3 h heating test. All four sides suffered intense spalling, with the first spalling occurring within 30 min of heating and last for 90 min. The surface concrete directly exposed to heating underwent sequential spalling with gradual peeling, but unlike in the case of small specimens, no explosive spalling from the center was observed. The average and maximum spalling depths were measured to be 36.85 mm and 98 mm, respectively, from the corner. The cross-section shape of RC column subjected to spalling becomes irregular by loss of area in concrete. However, it was considered that regular shape of cross-section was more efficient than irregular shape of cross-section in heat transfer analysis. Therefore, the square cross-section whose size was reduced by average spalling depth on the size of the initial cross-section after 30 min of heating was used in heat transfer analysis of spalling occurred RC column. The comparison between the results of the heat transfer analysis and the actual temperature increase as shown in Fig. 8. The comparison reveals substantial similarity between the internal temperatures of the spalling-induced deformed cross section and those of the idealized normal cross section. It is considered useful to idealize the cross section of the RC column exposed to heating by removing the average spalling depth of the RC column. 4.2.3. Axial deformation and residual load A preload of 12,000 kN was applied to the test column prior to heating, whereby the pure deformation of the test column was measured at 5.12 mm. Fig. 9 illustrates the axial deformation of
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Fig. 5. Time–temperature curve used in the column fire tests.
Fig. 10 shows the residual strength of the column after 24 h of post-test cooling. Since the pre-heating baseline ultimate strength could not be assessed due to the limitations of the test equipment, the load–deformation curve during preloading was plotted. The gradient of the load–deformation curve indicated the rigidity deterioration of the RC column under elevated temperatures, and after 3 h of heating, the RC column showed 25,000 kN residual strength, which is 52% of the design axial load.
5. Calculation of residual load of UHSC column 5.1. Determination of cross section size
Fig. 6. Time–temperature curve used in the column fire tests.
the column for the fire resistance test. This represents the deformation evaluation of expansion between the heating start point (0 deformation) and 1.2 h of heating. The figure shows a slight tendency towards shrinkage between 1.2 and 2 h of heating and constant shrinkage down to baseline value after 2 h. The maximum axial shrinkage of the test column during the 3 h heating test was 6.55 mm, far smaller than the allowable axial shrinkage of 36 mm and shrinkage rate of 10.8 mm/min calculated with the calculation methods specified by KS and ISO. It also satisfied the load-bearing ability as specified by ASTM and BS; thus, its fire resistance performance against axial shrinking deformation for 3 h was verified.
Table 4 Limit temperature of steel bar in the concrete column for determining the fire resistance time during elevated temperature. Standard
ISO, ASTM, KS
Limit temp.
Max. temp. (649 °C)
JIS Avg. temp. (538 °C)
500 °C
An RC column exposed to elevated temperatures is prone to strength deterioration because its surface is in direct contact with high temperatures. In the case of spalling, its cross section shrinks, losing the homogeneous strength of the initial pre-heating state. It is therefore of crucial importance to determine the post-fire effective cross section for the calculation of the residual strength of the RC column exposed to elevated temperatures. Concrete spalling is known to arise from internal vapor pressure and thermal stress, and it has been reported that the higher the compressive strength of the concrete, the denser its internal pore structure, thus making it more vulnerable to spalling. Thus, the risk for spalling is reflected in the fire-resistance design of a member. Eurocode [16], for example, addresses this problem by subtracting 30 mm from the cover thickness, and in Dotreppe’s research report [17], a coefficient of 0.85 is applied after 30 min, the usual time point for explosive spalling. In Kodur’s study [24], the internal temperature area below 500 °C was calculated as the effective area
Table 5 Limit axial deformation and velocity deformation for determining the fire resistance time during elevated temperature. Standard
ISO, KS
Limit axial deform. & velocity
h/ 100
ASTM, BS 3 h/1000 per min
Bearing load during the heating
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ཱཱིིུ ྲྀ
ཱུ
Non spalling, 500 500mm
spalling
Heating Temperature
Furnace Temperature
ུ Aver. rebar
ཱུ ྲྀ
ཱི ི ཱ
Fig. 7. Temperature in RC column.
under the assumption that the initial pre-heating level of strength is maintained in this temperature area. Since strength varies according to concrete temperature and the spalling amount varies depending on the compressive strength, the cross section reflecting spalling, and the high-temperature mechanical properties were determined as a function of the effective cross section. In Mitsui’s study [23], the spalling depth was calculated using Eq. (2). Substituting the test conditions of this study, into this equation, it was 32 mm, which proved to be similar to the actual spalling depth. Consequently, the spalling depth was calculated using Eq. (2) and defined the post-fire cross section as the difference between the baseline cross section and the spalling depth. This definition is expressed in Eq. (3).
dc ds ¼ a ðrB 70Þ0621 V f ð232 0:883rB Þ 16:2
ð2Þ
where ds: depth of explosive spalling (mm) dc: cover depth (mm) rB: compressive strength of concrete (MPa, rB P 70 MPa) Vf: added ratio of PP fiber (vol%) a: reduction coefficient by adding steel fiber (more than 0.5 vol%: 0.17, no fiber: 1.00)
A0c ¼ ðd 2ds Þ ðb 2ds Þ
ð3Þ
5.2. Determination of strength In Dotreppe’s research report, the heating time-dependent temperature distribution of the member’s cross section was quantified using SAFIR. The heating time-dependent strength deterioration patterns of concrete and rebar, in accordance with the ISO-834 standard heating curve, were quantified using the concrete strength deterioration coefficient set out by Eurocode. Based on the resulting values, the relationship between the cross section size and its effect on the residual strength of the member was determined. Furthermore, the strength deterioration coefficient of the member’s concrete part (b1(t)) and that of the rebar part (b2(t)), which has the cover thickness as a variable, were calculated. However, to reflect the deviation of 200 MPa UHSC from Eurocode, the strength deterioration coefficient of the member’s concrete part b1(t) was corrected using the model proposed in this study, as expressed by Eq. (4). Fig. 11 shows the b1(t) function according to Eurocode and to this study.
1 b1 ðtÞ ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi A00:23 1 þ ð0:36A0c tÞ c
ð4Þ
where b1(t): strength deterioration coefficient of the member’s concrete part t: heating time(h).
where Ac0 : effective cross section according to explosive spalling (mm) d: width of cross section (mm) b: height of cross section (mm)
Additionally, Dotreppe arranged the patterns of b2(t) by Curve a in Fig. 12. This curve is characterized by points P1 (t1, 1) and P2 (t2, 0.1). Because the impact of rebar on the compressive strength is insignificant in comparison to that of the concrete, one can use
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Fig. 8. Comparison of temperature with analysis.
4
30,000 After heating test (Residual property) 25,000
-4
Load(kN)
Axial deformation(mm)
0
-8
20,000 Before heating test (Initial property)
15,000
6.55mm 10,000
-12 5,000
-16 0
Limit deformation : 36mm Limit deformation rate : 10.8mm/min.
0
-20 0.0
0.5
1.0
1.5 2.0 Heating time(h)
2.5
3.0
3.5
3
6 9 12 Axial deformation(mm)
15
18
Fig. 10. Residual strength of RC column.
Fig. 9. Axial deformation during heating.
a more simplified representation for b2(t) (Curve b), which is on the safe side. In this way, b2(t) is only a function of t2, which itself is a function of the concrete cover depth c.
In this study, in consideration of the definition of an efficient cross section reflecting spalling, Dotreppe’s b2(t) function was corrected, as expressed by Eq. (5). Fig. 13 shows b2(t) that reflects spalling and b2(t) that does not reflect spalling.
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1. 1.2
1.2 Dotreppe(Eurocode) This study
1.0
1. 1.0 0. 0.8 β2(t)
β1(t)
0.8 0.6
D igneed ccoveer dept Desi d th(c)
0. 0.6 0. 0.4
0.4 0. 0.2
0.2 0.0 0. 0.0
0.0 0
1
2
3
4
5
M ified Modi d cooverr deepthh ( ds) (c-d
0.5
1. 1.0
1.5
2.0
2 2.5
3.0
3. 3.5
4.0 .0
H eating ing tim time( e(h) Heat
Heating time(h)
Fig. 13. b2(t) with cover depth.
Fig. 11. Variation of factor b1(t) as a function of time.
b2 ðtÞ ¼ 1 0:9t >0 t2 ð5Þ
t 2 ¼ 0:046 ðc ds Þ þ 0:11 where
b2(t): strength deterioration coefficient of the member’s steel part c: concrete cover depth(mm) 5.3. Calculation of residual load of RC column after fire The residual strength of the RC column exposed to fire can be calculated on the same basis as the ambient temperature-stressed design. The only difference is the necessity to substitute the ambient temperature variables with those reflecting the mechanical properties at elevated temperatures because of the fire-induced reduction of the efficient cross-section size and the strength deterioration of fire-damaged rebar and concrete. Consequently,
Nr ¼ b1 ðtÞA0c f c þ b2 ðtÞAs f y
ð6Þ
Where Nr: residual load of RC column (kN) As: steel area (mm2) fc: compressive strength of concrete (MPa) fy: yielding strength of steel (MPa)
Fig. 14. Residual load of RC column with heating time.
The residual strength of an RC column exposed to fire was presented with Eq. (6). The heating time-dependent residual strength of an RC column was calculated by Eq. (6) as shown in Fig. 14. With these, residual load of RC column subjected to fire for 3 h was calculated as 17,568 kN. Therefore, it was verified that the residual load of RC column could be predicted by the Eq (5) with the 70% of accuracy. 6. Conclusion
β2(t) 1 a
b 0.1 t1
t2
Fig. 12. Schematic representation of b2(t).
t
(1) In this study, the rate of compressive strength degradation of UHSC subjected to elevated temperature was higher than that of NSC which has been presented by Eurocode. Therefore, prediction formula for compressive strength of UHSC in high temperature has been proposed considering proposed formula and results of this study. (2) Although the 200 MPa RC column exhibited 26% spalling, it maintained strength enough to bear 25% of the design load for up to 3 h and retained 52% of its design axial load, as verified by the calculation of residual axial load after cooling. A 3 h fire resistance performance was verified by the axial deformation amount and load-bearing strength, but the strength deterioration amount is considered to be too high from the perspective of structural safety performance.
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(3) The ultimate strength of the UHSC column calculated on the basis of the idealization of the section corrected for cross section damage induced by high-temperature properties and spalling after 3 h of heating was verified to account for 70% of the actually measured values. This significant discrepancy of 30% between the calculated and measured strengths arose presumably from the difference in heat capacities due to the size effect of the actual size of the member and may be ascribed to the strength of the tested member exceeding the strength deterioration amount calculated by the prediction equation.
Acknowledgement This research was financially supported by the Ministry of Education, Science Technology (MEST) and National Research Foundation of South Korea (NRF) through the Human Resource Training Project for Regional Innovation and the National Research Foundation of South Korea (NRF) grant funded by the South Korea government (MSIP) (No. 2010-0014723). References [1] Kimura Hideki, Ishikawa Yuji, Kambayashi Atsushi, Takatsu Hiroto. Seismic behavior of 200 MPa ultra high strength steel fiber reinforced concrete columns under varying axial load. J Adv Concr Technol 2007;5(2):193–200. [2] Yamamoto K, Nakajima T, Watanabe S, Shimizu Y. High-rise building operations using ultra strength concrete with a design strength of 200 MPa. Concr J 2011;49(8):37–42. [3] Koizumi Shinichi, Sugamata Takumi, Masuda Yochihiro. The temperature dependency of the reaction of silica fume on strength development of 200 N/ mm2-class high strength concrete. J Struct Constr Eng (Trans AIJ) 2011;76(659):15–22. [4] Kalifa P, Menneteau FD, Quenard D. Spalling and pore pressure in HPC at high temperatures. Cem Concr Res 2000;30:1915–27. [5] Abe T, Furumura F, Tomatsuri K, Kuroha K, Kokubo I. Mechanical properties of high strength concrete at high temperatures. J Struct Constr Eng Trans AIJ 1999;515:163–8 (in Japanese). [6] Japanese architectural standard specification JASS 5 reinforced concrete work. Maruzen: Architectural Institute of Japan; 2003 (in Japanese). [7] Abrams MS. Compressive strength of concrete at temperatures to 1600 °F. American Concrete Institute (ACI) SP 25, Temperature and Concrete, Detroit, Michigan; 1971. [8] Malhotra HL. The effect of temperature on the compressive strength of concrete. Magazine of Concrete Research, v. 8 (22) (London, 1956). p. 85–94. [9] Schneider U. Concrete at high temperatures – a general review. Fire Saf J Neth 1988:55–68. [10] Phan LT, Carino NJ. Review of mechanical properties of HSC at elevated temperature. J Mater Civ Eng Am Soc Civ Eng 1998;11(1). [11] Phan LT, Carino NJ. Mechanical properties of high strength concrete at elevated temperatures, NISTIR 6726. Building and Fire Research Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland, March; 2001.
[12] Phan LT, Carino NJ. Effects of test conditions and mixture proportions on behavior of highstrength concrete exposed to high temperatures. ACI Mater J Am Conc Inst 2002;99(1):54–66 (January–February). [13] Phan LT, Lawson JR, Davis FR. Effects of elevated temperature exposure on heating characteristics, spalling, and residual properties of high performance concrete. RILEM Mater Struct J 34(236) (March 2001) 83–91. (February, 1998) 58–64. [14] Phan LT, High-strength concrete at high temperature––an overview. In: Konig G, Dehn F, Faust T, editors. Utilization of high strength/high performance concrete, 6th international symposium. Proceedings, vol 1. June 2002, Leipzig, Germany; 2002. p. 501–518. [15] Kuroiwa S, Jinnai H, Kobayashi Y, Kawabata I, Nishikawa Y, Kimura Y, et al. Application of high-strength concrete with fire resistance improved by polypropylene fibers. AIJ J Technol 2002;16:17–22 (in Japanese). [16] Comité Européen de Normalisation, ENV 1992-1-2: Eurocode 2: Design of Concrete Structures. Part 1–2: Structural Fire Design, CEN/TC 250/SC 2; 1993. [17] Dotreppe J-C, Franssen J-M, Vanderzeypen Y. Calculation method for design of reinforced concrete columns under fire conditions. ACI Struct J 1999;96(1):9– 18. ACI Committee 363. State-of-the art report on high strength concrete (ACI 363R–92). Farmington Hills, MI: American Concrete Institute; 1992. [18] Heo YS, Sanhayan JG, Han CG, Han MC. Synergistic effect of combined fibers for spalling protection of concrete in fire. Cem Con Res 2010;40:1547–54. [19] Furumura F, Abe T, Shinohara Y. Mechanical properties of high strength concrete at high temperatures. Proceedings, fourth weimar workshop on high strength concrete: material properties and design. Hochschule für Architektur und Bauwesen (HAB), Weimar, Germany October 4th and 5th; 1995, p. 237– 254. [20] Hammer TA. High-strength concrete phase 3, compressive strength and emodulus at elevated temperatures, SP6 fire resistance, Report 6.1, SINTEF Structures and Concrete, STF70 A95023; February 1995. [21] Diederichs U, Jumppanen UM, Penttala V. Behavior of high strength concrete at high temperatures, Report #92, Helsinki University of Technology; 1989. [22] Diederichs U, Jumppanen U-M, Schneider U. High temperature properties and spalling behavior of high strength concrete. Proceedings, fourth weimar workshop on high performance concrete: material properties and design, Hochschule für Architektur und Bauwesen. Weimar, Germany, October 4–5; 1995, p. 219–236. [23] Kenro Mitsui, Toshio Yonezawa, Masaro Kojima, Hirozo Mihashi. Effect of incorporating organic and steel fiber on fire resistance of 80 to 200 N/mm2 high strength concrete columns. J Structand Constr Eng (Trans AIJ) 2010;75(648):461–468. [24] Kodur VKR, Raut NK, Mao XY, Khaliq W. Simplified approach for evaluating residual strength of fire-exposed reinforced concrete columns. Mater struct 2013; 6(12):2059–2075. [25] ISO 834. Fire resistance tests-elements of building construction. Geneva, Switzerland: International Standards Organization; 1999. [26] KS F 2257-1. Methods of fire-resistant testing for structural element-general requirements. Korean Standards Association, Korean Industrial Standards; 1999. [27] ASTM E119. Standard test methods for fire tests of building construction and materials. West Conshohocken, Pennsylvania: American Society for Testing and Materials; 2002. [28] BSI, British Standard BS476. Part 20: Method for determination of the fire resistance of elements of construction. London: British Standards Institution; 1987. [29] JIS A 1304. Method of fire resistance test for structural parts of buildings; 1994. [30] Lee G, Han D, Han MC, Han CG, Son HJ. Combining polypropylene and nylon fibers to optimize fiber addition for spalling protection of high-strength concrete. Constr Build Mater 2012;34:313–20.
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Evaluation of the mechanical properties of 200 MPa ultra-high-strength concrete at elevated temperatures and residual strength of column Gyeongcheol Choe a, Gyuyong Kim a,⇑, Nenad Gucunski b, Seonghun Lee c a
Department of Architectural Engineering, Chungnam National University, 99 Daehak-ro, Yuseong-gu, Daejeon 305-764, Republic of Korea Department of Civil and Environmental Engineering, Rutgers University, Piscataway, NJ 08855-0909, USA c Construction Technology Center, Samsung C&T Corporation, Seocho-Gu, Seoul, Republic of Korea b
h i g h l i g h t s Mechanical properties of USHC subjected to elevated temperature were evaluated. Fire resistance performance of RC column USHC was investigated. Prediction process for residual load of column subjected to fire has been proposed.
a r t i c l e
i n f o
Article history: Received 28 October 2014 Received in revised form 9 March 2015 Accepted 11 March 2015 Available online 10 April 2015 Keywords: Fire resistance Ultra high-strength concrete Reinforced concrete column Mechanical properties Residual strength
a b s t r a c t In this study, material properties of ultra-high-strength concrete (UHSC) and residual load of reinforced concrete column have been evaluated at elevated temperature. On the basis of the results of experiments, prediction process for residual load of column member subjected to elevated temperature have been proposed. For evaluation on mechanical properties at elevated temperature of UHSC, 100 200 mm cylinder specimen of 100, 150, 200 MPa was heated to 100, 200, 300, 500 and 700 °C by 1 °C/min. After that, the ratio of compressive strength at room temperature to compressive strength at elevated temperature was measured. Also, comparison evaluation between experimental results of this paper and previously presented properties of compressive strength properties of normal strength concrete (NSC) and high strength concrete was carried out. Reinforced concrete column member was made of 200 MPa concrete and was heated by ISO-834 standard heating curve for 3 h. Internal force degradation and fire resistance performance of column member was evaluated by measuring internal temperature and axial deformation. Also, after heating, residual axial load capacity was evaluated through natural cooling for 24 h. Prediction process of residual internal force of UHSC column was proposed by deriving model for internal force degradation with experimental results of residual mechanical properties and spalling properties of UHSC subjected to elevated temperature. Based on the experimental results, it was observed that internal force degradation of UHSC by elevated temperature occurred more than that of NSC. In the test for fire resistance performance of column, 26% of section was lost due to spalling, however, it was observed that residual internal force was 52% of design axial load. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Driven by the increasing demand for high-rise buildings, over 200 m high buildings are being built around the world. The development of high-strength concrete (HSC) with larger permissible stress and smaller cross-sections is one of the key technological elements for building such ultrahigh-rise structures [1]. The American Concrete Institute (ACI) specifies a compressive strength
⇑ Corresponding author. E-mail address: [email protected] (G. Kim). http://dx.doi.org/10.1016/j.conbuildmat.2015.03.074 0950-0618/Ó 2015 Elsevier Ltd. All rights reserved.
of 40 MPa as the minimum strength requirement for HSC, which should outperform normal-strength concrete in terms of compressive strength and stiffness [18]. Recently, high-rise buildings have been constructed using concrete with a compressive strength of 100 MPa. Furthermore, there are reports that applications of 200 MPa concrete was used. Therefore, it is expected that ultra-high-strength concrete is about to be on commercialization in construction industry [2,3]. On the other hand, despite numerous advantages of HSC in terms of strength, stiffness, and durability, it can exhibit undesirable behaviors in high-temperature environments, such as spalling
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and reduced strength due to the formation of dense internal structures [4–6]. To address this issue, researchers such as Abrams [7], Malhotra [8], and Shchneider [9] have carried out studies on the strength deterioration of concrete exposed to high temperatures since the 1950s, analyzing the association between heating temperature and concrete strength deterioration. Based on the results of these studies, predictions and recommendations have been established in codes that are widely used. However, these studies deal mostly with normal-strength concrete with compressive strengths of less than 40 MPa. Phan reported that HSC of more than 40 MPa suffered a strength loss of 20–30% more in the temperature range of 100– 400 °C as compared to normal-strength concrete [10–13], and they presented material models capable of coping with the HSC strength deterioration after evaluating the mechanical properties of 100 MPa HSC at elevated temperatures [14]. These material models are used as final test materials for the resistance prediction of structural members exposed to high temperatures [15]. Dotreppe calculated the temperature distribution area of a concrete column using SFAIR, a finite element analysis (FEA) program. He suggested a calculation method to design reinforced concrete (RC) columns under fire conditions, thereby taking into account the properties of concrete at elevated temperatures as specified in Eurocode [16]; the calculated values proved to agree closely with the experimental data (n = 83) [17]. The aforementioned studies on the properties of concrete at elevated temperatures or calculations of the fire resistance of concrete members are based on research results of concrete with compressive strengths of less than 100 MPa; thus, there is a lack of research on the properties of concrete at elevated temperatures or the fireproof performance of concrete members with compressive strengths of 100 MPa or above. Unlike ordinary concrete, UHSC has a dense internal structure and hence tends towards condensation of water vapor pressure and is thus susceptible to spalling. The UHSC column thus becomes prone to abrupt deterioration due to spalling and subsequent cross-sectional loss of the concrete as well as to degradation of the mechanical properties at elevated temperatures. Mechanical performance and spalling behavior of UHSC at elevated temperatures were evaluated. Also, the deterioration and residual strength of an RC column fabricated using UHSC were evaluated. Thereupon, the concomitant effects of the materialrelated mechanical properties under high-temperature conditions and the fire resistance performance of structural members were investigated in order to determine how to reflect the safety performance index in fire resistance design.
of the ultimate residual load after leaving the specimen to cool for 24 h. Finally, on the basis of the mechanical properties at elevated temperatures and the date obtained from the fire resistance test, ultimate load of an RC column exposed to high temperatures was calculated using the residual load calculation method suggested by Dotreppe. 3. Evaluation of properties of UHSC at elevated temperatures 3.1. Test program and mixture proportions Experimental plan for evaluation on material properties of concrete subjected to elevated temperature is presented in Table 1. UHSC specimens with compressive strengths of 100, 150, and 200 MPa were produced with dimensions of /100 200 mm. Evaluation of material properties was divided into spalling property and compressive strength degradation. In spalling property test, nylon and polypropylene fiber were used in order to verify the spalling prevention effect by organic fiber mixing. Fiber combination conditions were no fiber condition, single fiber condition, and composite fiber condition. In single fiber condition, nylon and polypropylene fibers accounted for 0.15 vol%, 0.25 vol% respectively of the total volume of concrete. In composite fiber condition, the total volume percent of concrete was 0.15 vol% with 0.075 vol% of nylon fiber and 0.075 vol% of polypropylene fiber. Heating was carried out up to 900 °C with ISO-834 standard heating curve. After heating, damage from spalling of specimens subjected to heating was identified by conducting visual inspection and weight loss rate evaluation. In this study, consistent weight loss rate was observed in the concrete specimen that spalling did not occur. Therefore, weight loss rate was evaluated in order to use it as the index for quantitative assessment of the extent of spalling of UHSC. Also spalling levels were rated on a scale of 1 to 4 referring to Lee [30]. Spalling levels were defined as grade 1 with 0–25%, grade 2 with 25–50, grade 3 with 50–75, and grade 4 with 75–100 by comparing to before and after experiment of weight loss rate. The values for each grade was calculated by Eq (1).
W¼
W1 W2 100 W1
ð1Þ
where W: weight loss (%) W1: weight of specimen before heating (g) W2: weight of specimen after heating (g)
2. Test process The flow of this study is shown in Fig. 1. Experiments for material properties of concrete and fire resistance of column member have been conducted separately. First, in order to determine the high-temperature properties of UHSC, the spalling properties and degradation of compressive strength of UHSC specimens were evaluated. Spalling properties of UHSC was evaluated by visual inspection and weight loss rate in accordance with the ISO-834 standard heating curve. The compressive strength of the specimens at elevated temperatures with respect to that at ambient temperature was evaluated after heating the specimens to 100, 200, 300, 500, and 700 °C at a rate of 1 °C/min. Next, the fire resistance test for 3 h was performed in accordance with the ISO-834 standard heating curve on a RC column (500 500 600 mm, 200 MPa UHSC). Also, the heating temperature-dependent internal temperature increase, spalling property, and axial deformation were evaluated followed by an evaluation
Also, concrete specimen without fiber was used for compressive strength degradation test, heated to 100, 200, 300, 500 and 700 °C at the rate of 1 °C/min. in order to make the inside and outside temperature of specimen equal and simultaneous. The test for determining the compressive strength property at elevated temperatures was performed under two loading conditions: unstressed, in which the compressive strength was assessed after heating the specimen up to the target temperature without preloading in order to determine the effect of baseline stress, and stressed, in which the specimen was preloaded at ambient temperature to 25% of its ambient temperature compressive strength and heated to the target temperature prior to evaluating the compressive strength. Table 2 outlines the mixture proportions and the properties of the concrete. UHSC spcimens producted by mixing ASTM Type I Portland cement, crushed granite (coarse aggregate), river sand (fine aggregate), silica fume, fly ash, blast furnace slag, and gypsum. The specimens were moist-cured for 7 days and dry-cured
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Fig. 1. Research flow.
Table 1 Test program for evaluation of material properties of USHC. fck (MPa)
Material property
Fiber (added contents (vol%))1
Heating method
Exposure temperature (°C)
Pre-load condition
100
Spalling
Non fiber Nylon (0.15, 0.25) Polypropylene (0.15, 0.25) Composite fiber (0.15) –
ISO-834 heating curve
900
Unstressed
1 °C/min
100, 200, 300, 500, 700
Stressed2 unstressed
150 200 1 2
Compressive strength
Composite fiber (0.15): nylon (0.075 vol%) + polypropylene (0.075 vol%). 25% of compressive strength at room temperature.
3.2. Experimental results and discussion Table 2 Mixture proportions and properties of UHSC. Property
100 MPa
150 MPa
200 MPa
Cement content-type 1 (kg/m3) Silica fume-dry weight (kg) Slag (kg/m3) Fly ash (kg/m3) Gysum (kg/m3) Fine aggregate (kg/m3) Coarse aggregate (kg/m3) (maximum size: 13 mm) Water (kg/m3) Water:Cement ratio Water:Cementitious ratio
525 75 0 150 0 644 848
652 124 207 0 52 448 870
660 240 240 0 60 389 736
150 0.286 0.200
150 0.230 0.145
150 0.227 0.125
Fresh concrete Slump-flow (mm) Air concrete (%)
735 1.9
740 1.9
785 2.1
Hardened concrete Compressive strength (MPa) 28 days 56 days 300 days
75 86 101
125 135 149
173 182 205
for 300 days under constant temperature (20 °C) and humidity (60%) conditions. Fig. 2 shows the test device for the evaluation of the mechanical properties of the concrete test specimens at elevated temperatures. It is a 2000 kN universal test machine (UTM) with an electric heating device attached to it, capable of performing heating and loading simultaneously.
3.2.1. Spalling properties of UHSC The weight loss rate from spalling and evaporation of the UHSC specimens in accordance with the ISO-834 standard heating curve are illustrated in Fig. 3. All of the specimens without fiber exhibited explosive spalling and could not maintain their forms by the end of the heating process. Therefore, since obtained concrete fragments after experiment were pieces by spalling, weight loss rate was evaluated as 100%. No spalling appeared in the 100 and 150 MPa concrete specimens containing more than 0.15 vol% of PP and NY fibers, and they exhibited a weight loss of about 6% because of the water evaporation inside the specimens. However, the 200 MPa concrete specimen containing 0.15 vol% of PP and NY fibers suffered surface spalling, thereby exhibiting 20% and 16% weight loss, respectively. In contrast, none of the specimens with 0.25% fiber content experienced spalling. According to the reported results of a previous study, cement reinforced with two or more types of fiber is better protected from spalling than cement reinforced with a single fiber [19]. This result is consistent with the results of this study, in that no spalling occurred in the specimen containing 0.15 vol% of the PP and NY blend. 3.2.2. Compressive strength of UHSC at elevated temperatures Fig. 4 shows the results of the strength evaluation test of the UHSC specimens at elevated temperatures. Under the unstressed condition, strength deterioration of 10%, 30%, and 40% occurred in the 100, 150, and 200 MPa test specimens, respectively, at
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Fig. 2. Heating machine (left) and method of strain measurement (right).
Fig. 3. Weight loss according to spalling and evaporation.
100 °C. At 200 °C, however, compressive strength increased to an extent similar to that tested at ambient temperature. Once this temperature was reached, the compressive strength tended to decrease as the heating temperature increased. Remarkably, the 200 MPa specimens suffered spalling at a heating temperature of 280 °C under the unstressed condition; therefore, the compressive strength at 300 °C and higher could not be evaluated. These results, which are consistent with those reported in studies by Diederichs [22], Hammer [21], and Furumura [20], support the assumption that HSC and UHSC can suffer explosive spalling at low heating temperature conditions as well. The stressed test exhibited tendencies similar to those of the unstressed test, whereby the residual rates of compressive strength were observed to be slightly higher, more markedly in the temperature ranges exceeding 500 °C under loading. Unlike
in the unstressed test, the 200 MPa specimens did not show spalling; thus, compressive strength could be evaluated at all temperature conditions. The absence of spalling in this study is considered to be attributable to the 25% preloading. In this study, as demonstrated in Fig. 4, except for the strength deterioration below 200 °C, the 100 and 150 MPa concrete showed tendencies similar to those of Eurocode 2, and the 200 MPa concrete showed properties similar to those suggested by the National Institute of Standards and Technology (NIST) for HSC at elevated temperature. Consequently, on the basis of the existing codes and the results of this study, code for the 200 MPa UHSC property model at elevated temperatures is present as the following Relative compressive strength of ultrahigh-strength concrete (k(T), fc,t/fc,20 °C)
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1.4
column fire resistance performance was assessed in accordance with these standards [25–29].
1.2
Relative Strength(fc,t/fc,20ഒ)
163
4.2. Fire resistance of UHSC column
1.0
100MPa 150MPa 200MPa 100MPa 150MPa 200MPa
Unstressed Test 0.8 Stressed Test
0.6 0.4 0.2 0.0 0
200
400 600 Temperture(ഒ)
800
1000
1200
Fig. 4. Comparison of test data with codes.
T 6 20 C;
kðTÞ ¼ 1
20 C < T 6 100 C; 100 C < T 6 200 C;
kðTÞ ¼ 1:0375 0:0019t kðTÞ ¼ 0:85
200 C < T 6 700 C;
kðTÞ ¼ 1:1 0:0013t
700 C < T 6 900 C;
kðTÞ ¼ 0:62 0:0006t
700 C < T 6 1200 C; kðTÞ ¼ 0:32 0:0003t T > 1200 C
kðTÞ ¼ 0
4. Fire resistance performance of reinforced concrete column 4.1. Test method for assessing the fire resistance of RC column Table 3 and Fig. 5 provide an overview of the test RC column used in this study. The same concrete with a compressive strength of 200 MPa as was used for the fabrication of the high-temperature property test specimen was used. In order to prevent spalling, 0.1, 0.15, and 0.3 vol% of PP, NY, and steel fibers were add, respectively. For heating and loading, a combined device consisting of a 3000 kN compression testing machine and a gas fuel heating apparatus were used. The design load was set at 48,000 kN. A test load of 12,000 kN force, which was 25% of the design load, was applied to the test specimen prior to heating. The fire resistance test was performed by heating the specimen for 3 h in accordance with the ISO-834 standard heating curve (Fig. 6). As the test setup for the evaluation of the fire resistance performance of an RC column, three temperature sensors were installed on the vertical reinforcing bar (rebar) and six additional temperature sensors were installed inside the column at different distances from the surface to assess the increase in internal temperature. ISO, ASTM, KS, BS, and JIS specify allowable temperatures or axial deformation of the rebar as the evaluation criteria for column fire resistance tests, as outlined in Tables 4 and 5. The
Table 3 Fabrication of RC column. Column Size (mm)
Reinforcing bar
Design load (kN)
Concrete strength (MPa)
Test load (kN)
Fire duration (min.)
500 500 3600
D29500 MPa
48,000
207
12,000
180
4.2.1. Heating characteristics Fig. 7 shows the temperature increases inside the concrete column during the heating process in accordance with the ISO-834 standard heating curve. The rising temperature on the heated concrete surface was transferred to the inside of the column. The closer surface column subjected to direct fire, the higher temperature was measured. The rate of temperature rise tended to increase sharply after 30 min of heating as a result of the reduction of crosssection size due to spalling. In order to determine the change in the rate of temperature increase induced by the damage to the concrete cover, a heat transfer analysis was performed using the Midas FEA program for finite element analysis assuming the non-spalling scenario. Although the spalling did not have significant impact on the temperature increase of the concrete inside the rebar, the parts closer to the surface area showed a significant increase in temperature, presumably due to the damage to the concrete cover. The temperatures measured at sensor positions 4, 6, and 7 installed on the rebar were taken as reference temperatures inside the column for the evaluation of fire resistance performance as compared to the standards for the maximum allowable temperature as set out in ISO, ASTM, KS, and JIS. The measured values from the fire resistance test after 3 h of heating did not exceed the maximum and average allowable temperatures of 649 °C and 538 °C, respectively; thus, the test column was verified to have a fire resistance performance of 3 h. On the other hand, the temperature sensor 7 of the test column showed temperatures of more than 500 °C, the allowable temperature as specified by JIS, in 2.3 h, and its fire resistance performance was thus evaluated to be 2.3 h. 4.2.2. Spalling property Although no spalling occurred in the specimens fabricated with the 0.25 vol% multi-fiber blend. The test column exhibited a 26% decrease in cross section by the end of the 3 h heating test. All four sides suffered intense spalling, with the first spalling occurring within 30 min of heating and last for 90 min. The surface concrete directly exposed to heating underwent sequential spalling with gradual peeling, but unlike in the case of small specimens, no explosive spalling from the center was observed. The average and maximum spalling depths were measured to be 36.85 mm and 98 mm, respectively, from the corner. The cross-section shape of RC column subjected to spalling becomes irregular by loss of area in concrete. However, it was considered that regular shape of cross-section was more efficient than irregular shape of cross-section in heat transfer analysis. Therefore, the square cross-section whose size was reduced by average spalling depth on the size of the initial cross-section after 30 min of heating was used in heat transfer analysis of spalling occurred RC column. The comparison between the results of the heat transfer analysis and the actual temperature increase as shown in Fig. 8. The comparison reveals substantial similarity between the internal temperatures of the spalling-induced deformed cross section and those of the idealized normal cross section. It is considered useful to idealize the cross section of the RC column exposed to heating by removing the average spalling depth of the RC column. 4.2.3. Axial deformation and residual load A preload of 12,000 kN was applied to the test column prior to heating, whereby the pure deformation of the test column was measured at 5.12 mm. Fig. 9 illustrates the axial deformation of
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Fig. 5. Time–temperature curve used in the column fire tests.
Fig. 10 shows the residual strength of the column after 24 h of post-test cooling. Since the pre-heating baseline ultimate strength could not be assessed due to the limitations of the test equipment, the load–deformation curve during preloading was plotted. The gradient of the load–deformation curve indicated the rigidity deterioration of the RC column under elevated temperatures, and after 3 h of heating, the RC column showed 25,000 kN residual strength, which is 52% of the design axial load.
5. Calculation of residual load of UHSC column 5.1. Determination of cross section size
Fig. 6. Time–temperature curve used in the column fire tests.
the column for the fire resistance test. This represents the deformation evaluation of expansion between the heating start point (0 deformation) and 1.2 h of heating. The figure shows a slight tendency towards shrinkage between 1.2 and 2 h of heating and constant shrinkage down to baseline value after 2 h. The maximum axial shrinkage of the test column during the 3 h heating test was 6.55 mm, far smaller than the allowable axial shrinkage of 36 mm and shrinkage rate of 10.8 mm/min calculated with the calculation methods specified by KS and ISO. It also satisfied the load-bearing ability as specified by ASTM and BS; thus, its fire resistance performance against axial shrinking deformation for 3 h was verified.
Table 4 Limit temperature of steel bar in the concrete column for determining the fire resistance time during elevated temperature. Standard
ISO, ASTM, KS
Limit temp.
Max. temp. (649 °C)
JIS Avg. temp. (538 °C)
500 °C
An RC column exposed to elevated temperatures is prone to strength deterioration because its surface is in direct contact with high temperatures. In the case of spalling, its cross section shrinks, losing the homogeneous strength of the initial pre-heating state. It is therefore of crucial importance to determine the post-fire effective cross section for the calculation of the residual strength of the RC column exposed to elevated temperatures. Concrete spalling is known to arise from internal vapor pressure and thermal stress, and it has been reported that the higher the compressive strength of the concrete, the denser its internal pore structure, thus making it more vulnerable to spalling. Thus, the risk for spalling is reflected in the fire-resistance design of a member. Eurocode [16], for example, addresses this problem by subtracting 30 mm from the cover thickness, and in Dotreppe’s research report [17], a coefficient of 0.85 is applied after 30 min, the usual time point for explosive spalling. In Kodur’s study [24], the internal temperature area below 500 °C was calculated as the effective area
Table 5 Limit axial deformation and velocity deformation for determining the fire resistance time during elevated temperature. Standard
ISO, KS
Limit axial deform. & velocity
h/ 100
ASTM, BS 3 h/1000 per min
Bearing load during the heating
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ཱཱིིུ ྲྀ
ཱུ
Non spalling, 500 500mm
spalling
Heating Temperature
Furnace Temperature
ུ Aver. rebar
ཱུ ྲྀ
ཱི ི ཱ
Fig. 7. Temperature in RC column.
under the assumption that the initial pre-heating level of strength is maintained in this temperature area. Since strength varies according to concrete temperature and the spalling amount varies depending on the compressive strength, the cross section reflecting spalling, and the high-temperature mechanical properties were determined as a function of the effective cross section. In Mitsui’s study [23], the spalling depth was calculated using Eq. (2). Substituting the test conditions of this study, into this equation, it was 32 mm, which proved to be similar to the actual spalling depth. Consequently, the spalling depth was calculated using Eq. (2) and defined the post-fire cross section as the difference between the baseline cross section and the spalling depth. This definition is expressed in Eq. (3).
dc ds ¼ a ðrB 70Þ0621 V f ð232 0:883rB Þ 16:2
ð2Þ
where ds: depth of explosive spalling (mm) dc: cover depth (mm) rB: compressive strength of concrete (MPa, rB P 70 MPa) Vf: added ratio of PP fiber (vol%) a: reduction coefficient by adding steel fiber (more than 0.5 vol%: 0.17, no fiber: 1.00)
A0c ¼ ðd 2ds Þ ðb 2ds Þ
ð3Þ
5.2. Determination of strength In Dotreppe’s research report, the heating time-dependent temperature distribution of the member’s cross section was quantified using SAFIR. The heating time-dependent strength deterioration patterns of concrete and rebar, in accordance with the ISO-834 standard heating curve, were quantified using the concrete strength deterioration coefficient set out by Eurocode. Based on the resulting values, the relationship between the cross section size and its effect on the residual strength of the member was determined. Furthermore, the strength deterioration coefficient of the member’s concrete part (b1(t)) and that of the rebar part (b2(t)), which has the cover thickness as a variable, were calculated. However, to reflect the deviation of 200 MPa UHSC from Eurocode, the strength deterioration coefficient of the member’s concrete part b1(t) was corrected using the model proposed in this study, as expressed by Eq. (4). Fig. 11 shows the b1(t) function according to Eurocode and to this study.
1 b1 ðtÞ ¼ qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi A00:23 1 þ ð0:36A0c tÞ c
ð4Þ
where b1(t): strength deterioration coefficient of the member’s concrete part t: heating time(h).
where Ac0 : effective cross section according to explosive spalling (mm) d: width of cross section (mm) b: height of cross section (mm)
Additionally, Dotreppe arranged the patterns of b2(t) by Curve a in Fig. 12. This curve is characterized by points P1 (t1, 1) and P2 (t2, 0.1). Because the impact of rebar on the compressive strength is insignificant in comparison to that of the concrete, one can use
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Fig. 8. Comparison of temperature with analysis.
4
30,000 After heating test (Residual property) 25,000
-4
Load(kN)
Axial deformation(mm)
0
-8
20,000 Before heating test (Initial property)
15,000
6.55mm 10,000
-12 5,000
-16 0
Limit deformation : 36mm Limit deformation rate : 10.8mm/min.
0
-20 0.0
0.5
1.0
1.5 2.0 Heating time(h)
2.5
3.0
3.5
3
6 9 12 Axial deformation(mm)
15
18
Fig. 10. Residual strength of RC column.
Fig. 9. Axial deformation during heating.
a more simplified representation for b2(t) (Curve b), which is on the safe side. In this way, b2(t) is only a function of t2, which itself is a function of the concrete cover depth c.
In this study, in consideration of the definition of an efficient cross section reflecting spalling, Dotreppe’s b2(t) function was corrected, as expressed by Eq. (5). Fig. 13 shows b2(t) that reflects spalling and b2(t) that does not reflect spalling.
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1. 1.2
1.2 Dotreppe(Eurocode) This study
1.0
1. 1.0 0. 0.8 β2(t)
β1(t)
0.8 0.6
D igneed ccoveer dept Desi d th(c)
0. 0.6 0. 0.4
0.4 0. 0.2
0.2 0.0 0. 0.0
0.0 0
1
2
3
4
5
M ified Modi d cooverr deepthh ( ds) (c-d
0.5
1. 1.0
1.5
2.0
2 2.5
3.0
3. 3.5
4.0 .0
H eating ing tim time( e(h) Heat
Heating time(h)
Fig. 13. b2(t) with cover depth.
Fig. 11. Variation of factor b1(t) as a function of time.
b2 ðtÞ ¼ 1 0:9t >0 t2 ð5Þ
t 2 ¼ 0:046 ðc ds Þ þ 0:11 where
b2(t): strength deterioration coefficient of the member’s steel part c: concrete cover depth(mm) 5.3. Calculation of residual load of RC column after fire The residual strength of the RC column exposed to fire can be calculated on the same basis as the ambient temperature-stressed design. The only difference is the necessity to substitute the ambient temperature variables with those reflecting the mechanical properties at elevated temperatures because of the fire-induced reduction of the efficient cross-section size and the strength deterioration of fire-damaged rebar and concrete. Consequently,
Nr ¼ b1 ðtÞA0c f c þ b2 ðtÞAs f y
ð6Þ
Where Nr: residual load of RC column (kN) As: steel area (mm2) fc: compressive strength of concrete (MPa) fy: yielding strength of steel (MPa)
Fig. 14. Residual load of RC column with heating time.
The residual strength of an RC column exposed to fire was presented with Eq. (6). The heating time-dependent residual strength of an RC column was calculated by Eq. (6) as shown in Fig. 14. With these, residual load of RC column subjected to fire for 3 h was calculated as 17,568 kN. Therefore, it was verified that the residual load of RC column could be predicted by the Eq (5) with the 70% of accuracy. 6. Conclusion
β2(t) 1 a
b 0.1 t1
t2
Fig. 12. Schematic representation of b2(t).
t
(1) In this study, the rate of compressive strength degradation of UHSC subjected to elevated temperature was higher than that of NSC which has been presented by Eurocode. Therefore, prediction formula for compressive strength of UHSC in high temperature has been proposed considering proposed formula and results of this study. (2) Although the 200 MPa RC column exhibited 26% spalling, it maintained strength enough to bear 25% of the design load for up to 3 h and retained 52% of its design axial load, as verified by the calculation of residual axial load after cooling. A 3 h fire resistance performance was verified by the axial deformation amount and load-bearing strength, but the strength deterioration amount is considered to be too high from the perspective of structural safety performance.
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(3) The ultimate strength of the UHSC column calculated on the basis of the idealization of the section corrected for cross section damage induced by high-temperature properties and spalling after 3 h of heating was verified to account for 70% of the actually measured values. This significant discrepancy of 30% between the calculated and measured strengths arose presumably from the difference in heat capacities due to the size effect of the actual size of the member and may be ascribed to the strength of the tested member exceeding the strength deterioration amount calculated by the prediction equation.
Acknowledgement This research was financially supported by the Ministry of Education, Science Technology (MEST) and National Research Foundation of South Korea (NRF) through the Human Resource Training Project for Regional Innovation and the National Research Foundation of South Korea (NRF) grant funded by the South Korea government (MSIP) (No. 2010-0014723). References [1] Kimura Hideki, Ishikawa Yuji, Kambayashi Atsushi, Takatsu Hiroto. Seismic behavior of 200 MPa ultra high strength steel fiber reinforced concrete columns under varying axial load. J Adv Concr Technol 2007;5(2):193–200. [2] Yamamoto K, Nakajima T, Watanabe S, Shimizu Y. High-rise building operations using ultra strength concrete with a design strength of 200 MPa. Concr J 2011;49(8):37–42. [3] Koizumi Shinichi, Sugamata Takumi, Masuda Yochihiro. The temperature dependency of the reaction of silica fume on strength development of 200 N/ mm2-class high strength concrete. J Struct Constr Eng (Trans AIJ) 2011;76(659):15–22. [4] Kalifa P, Menneteau FD, Quenard D. Spalling and pore pressure in HPC at high temperatures. Cem Concr Res 2000;30:1915–27. [5] Abe T, Furumura F, Tomatsuri K, Kuroha K, Kokubo I. Mechanical properties of high strength concrete at high temperatures. J Struct Constr Eng Trans AIJ 1999;515:163–8 (in Japanese). [6] Japanese architectural standard specification JASS 5 reinforced concrete work. Maruzen: Architectural Institute of Japan; 2003 (in Japanese). [7] Abrams MS. Compressive strength of concrete at temperatures to 1600 °F. American Concrete Institute (ACI) SP 25, Temperature and Concrete, Detroit, Michigan; 1971. [8] Malhotra HL. The effect of temperature on the compressive strength of concrete. Magazine of Concrete Research, v. 8 (22) (London, 1956). p. 85–94. [9] Schneider U. Concrete at high temperatures – a general review. Fire Saf J Neth 1988:55–68. [10] Phan LT, Carino NJ. Review of mechanical properties of HSC at elevated temperature. J Mater Civ Eng Am Soc Civ Eng 1998;11(1). [11] Phan LT, Carino NJ. Mechanical properties of high strength concrete at elevated temperatures, NISTIR 6726. Building and Fire Research Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland, March; 2001.
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