- Email: [email protected]
Static and dynamic mechanical properties of concrete after high temperature exposure
Materials Science and Engineering A 544 (2012) 27–32
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Static and dynamic mechanical properties of concrete after high temperature exposure Zhiwu Li a,∗ , Jinyu Xu a,b , Erlei Bai a a b
The Engineering Institute, Air Force Engineering University, Xi’an, Shanxi 710038, China Department of Civil Engineering, Northwestern Polytechnical University, Xi’an, Shanxi 710072, China
a r t i c l e
i n f o
Article history: Received 20 June 2011 Received in revised form 18 February 2012 Accepted 18 February 2012 Available online 28 February 2012 Keywords: Concrete After high temperature exposure Mechanical property SHPB Strain rate effect Toughness
a b s t r a c t The aim of this paper is to investigate the static and dynamic mechanical properties of concrete before and after high temperature exposure. Quasi-static and impact loading experiments were carried out on concrete before and after exposure to the temperature of 200–800 ◦ C by using a servo-hydraulic testing machine and a 100-mm-diameter split Hopkinson bar (SHPB) apparatus, respectively. The results show that, mechanical properties of concrete after high temperature exposure change evidently, and the critical temperature for those changes acquiring dramatic character is 400 ◦ C. With the rise of temperature up to 400 ◦ C, compressive strength and critical strain change little compared with that of at room temperature. While with temperature increasing from 400 to 800 ◦ C, the decrease in strength and the increase in critical strain become obvious. Consequentially, specific energy absorption (SEA), which is the synthetic exhibition of strength and ductility, decreases much less than those observed for strength over 400 ◦ C. Dynamic compressive mechanical properties of concrete before and after high temperature exposure increase with the rise in strain rate rapidly, which exhibits strong strain rate dependency. Dynamic increase factor (DIF) increases approximately linearly with strain rate. In a word, concrete can be recognized as an excellent thermal-resistant and anti-impact construction material. © 2012 Elsevier B.V. All rights reserved.
1. Introduction As a kind of building and construction material, the properties of concrete under high temperature conditions have a direct bearing on the structural safety, so it is necessary to do researches. Since the beginning of 20th century, numerous studies have been carried out to investigate the properties of concrete at high temperature [1–3]. With the extensive application of high strength concrete (HSC) during the 80s of the last century, HSC was found to be prone to spalling at high temperature, thus spalling mechanism and methods of improving the thermal-resistant properties of HSC became research focus [4–6]. In recent years, researches on the thermal properties of other types of concrete are also put forth [7–9]. However, most of the present studies on the mechanical properties of concrete subjected to high temperature were carried out under static condition, with little focus on the dynamic properties. Results of many dynamic loading tests on not-heated concrete have shown that there were significant differences in the performances of concrete under the conditions between static and dynamic. A greater understanding of concrete behaviors under different conditions will improve confidence in its use. Otherwise, building collapses take place occasionally in conflagration, which is terribly
dangerous for rescue and fire fighter. The related researches show that high temperature, impact loading caused by chemistry explosion and construction member collapsing, and faster cooling by watering are the main reasons for collapses. Thus, for understanding the collapse mechanism and to prevent it, it is very important to study the anti-impact properties of concrete exposed to high temperature and cooled by water. The split Hopkinson bar (SHPB) apparatus has become popular for the studies on dynamic mechanical properties of material at the strain rate of 102 –104 s−1 [10,11]. As concrete is a composite material with complex constituents and original defects, the study of concrete-like materials needs large-diameter Hopkinson bar to ensure the representative of specimens. The aim of this paper was to investigate the static and dynamic mechanical behaviors of concrete subjected to high temperature and cooled down by watering. Tests were carried out using a servohydraulic testing machine and a 100-mm-diameter split Hopkinson bar (SHPB) apparatus.
2. Experimental details 2.1. Materials and specimen fabrication
∗ Corresponding author. Fax: +86 029 23053922. E-mail address: [email protected] (Z. Li). 0921-5093/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2012.02.058
Concrete was prepared using type 42.5R ordinary Portland cement, water, crushed limestone, sand, fly ash, silica fume and
28
Z. Li et al. / Materials Science and Engineering A 544 (2012) 27–32
Table 1 Mix proportion of concrete (kg/m3 ). Water
Cement
Fly ash
Silica fume
Sand
Gravel
Superplasticizer
180
375
125
25
690
1030
5
Table 2 Quasi-static compressive strength of cylinders at different temperature (MPa). Test no.
1 2 3 Average Maximum relative error (%) Percentage residual strength (%)
Room temperature
129.14 134.44 141.02 134.87 4.56 100
Temperature 200 ◦ C
400 ◦ C
600 ◦ C
800 ◦ C
118.69 117.20 120.81 118.90 1.61 88.16
129.08 128.36 134.91 130.78 3.16 96.97
96.88 87.56 105.67 96.70 9.45 71.70
55.55 52.33 45.10 50.99 11.55 37.81
200 ◦ C
400 ◦ C
600 ◦ C
800 ◦ C
50.32 54.23 49.56 51.40 5.51 85.03
57.89 58.21 54.32 56.81 4.38 93.98
32.41 34.98 29.38 32.26 8.93 53.37
15.32 17.28 19.53 17.38 12.37 28.75
Table 3 Quasi-static compressive strength of cubes at different temperature (MPa). Test no.
1 2 3 Average Maximum relative error (%) Percentage residual strength (%)
Room temperature
55.84 61.23 64.29 60.45 7.63 100
Temperature
superplasticizer. The cement used in the experiment complied with the requirements of Chinese standard (GB 175-2007), and the maximum gravel size was 20 mm. The mix proportion of concrete is listed in Table 1. After removal from the moulds, the mixtures were cured in a curing room (under a controlled condition of 20 ◦ C and 95% relative humidity) for 28 days. After curing, the long cylindrical concrete pastes were cut into cylinder specimens with 48 mm thickness and 98 mm diameter. 2.2. Heating and cooling To investigate the mechanical properties of concrete after high temperature exposure, 200 ◦ C, 400 ◦ C, 600 ◦ C and 800 ◦ C were selected as the heating target temperature according to our prior researches and other relevant papers [5,12–14]. The heating equipment was an electrically heated furnace, which had a capacity of 18 cylinders or 3 cubes. Specimens were placed in the furnace and the furnace was heated from room temperature to target temperature at a constant heating rate of 10 ◦ C/min. After 6 h at the target temperature, the heating was stopped, and the specimens were removed from the furnace and cooled down by watering for 30 min. Then they were transferred to laboratory. After 3 days, tests were carried on these specimens. 3. Results 3.1. Quasi-static compressive experiment For the cylindrical specimens are not the right size for static strength testing, tests were also carried out on cubic specimens (150 mm × 150 mm × 150 mm) that heated and cooled in the same way. A servo-hydraulic testing machine was adopted to determine the compressive strength of specimens at strain rate of 10−5 s−1 . The test results are listed in Tables 2 and 3. Fig. 1 presents the
Fig. 1. Quasi-static compressive strength.
relation between quasi-static compressive strength and temperature. As shown in Tables 2 and 3 and Fig. 1: (i) The maximum error of each compressive strength group is less than 13%, demonstrating that the experimental data are fairly reliable. (ii) The compressive strengths of cylinder are much higher than that of the cube at the same temperature, indicating that the size effect is obvious. (iii) The compressive strength of cubic specimens after high temperature exposure decreases in a similar manner to that of the cylinder. The quasi-static compressive strength decreases slightly at 200 ◦ C, and then increase closely to that of not-heated specimens at 400 ◦ C. The evident drop in the strength of specimens is observed at temperature between 400 ◦ C and 800 ◦ C. After exposure to 800 ◦ C, the residual compressive strength values of cylinder and cube are only 37.81% and 28.75%, respectively, of that at room temperature.
Z. Li et al. / Materials Science and Engineering A 544 (2012) 27–32
a
b
c
d
29
e
Fig. 2. (a) Stress–strain curve of concrete at room temperature, (b) at 200 ◦ C, (c) at 400 ◦ C, (d) at 600 ◦ C, and (e) at 800 ◦ C.
3.2. Impact compressive experiment In this study, strain rates ranging from 30 to 200 s−1 were obtained by means of changing the projectile’s impact velocity. To ensure the reliability of the test results, 3 specimens were impacted at each strain rate. Critical strain (strain at peak stress) was adopted to characterize the ductility of concrete. Specific energy absorption (SEA) [15,16], which is the energy absorption per unit volume of material, was adopted to characterize the
toughness of concrete, and it can be calculated as the following formula:
SEA =
AEc AS ls
T
[εi 2 (t) − εr 2 (t) − εt 2 (t)]dt
(1)
0
where c, E, A are respectively the bar’s elastic wave speed, Young’s modulus, and cross-sectional area, ls , As are length and crosssectional area of specimen; εi , εr , εt are, respectively, the incident
30
Z. Li et al. / Materials Science and Engineering A 544 (2012) 27–32
a
a
b
b
c
Fig. 3. (a) Dynamic compressive strength vs. projectile velocity, (b) critical strain vs. projectile velocity, and (c) specific energy absorption vs. projectile velocity.
pulse, reflect pulse and transmit pulse; T represents the complete failure moment of specimen. The experimental results of SHPB and the stress–strain curves of concrete at different temperature are shown in Table 4 and Fig. 2 separately. 4. Discussions 4.1. Dynamic compressive mechanical properties The relations between dynamic compressive mechanical properties and projectile velocity are presented in Fig. 3. As shown in Fig. 3(a), approximate change trends in compressive strength
c
Fig. 4. (a) Dynamic compressive strength vs. average strain rate, (b) critical strain vs. average strain rate, and (c) specific energy absorption vs. average strain rate.
of concrete under quasi-static and dynamic conditions are determined as the temperature increases. Dynamic compressive strength of concrete changes little below 400 ◦ C, but decreases rapidly above 400 ◦ C. As shown in Fig. 3(b), the critical strain keeps at ∼7‰ below 400 ◦ C, but increases greatly at 600 and 800 ◦ C. As shown in Fig. 3(c), the decrease in SEA at 600 ◦ C is obviously less than that observed for strength; and the SEA at 800 ◦ C is closely to that at 600 ◦ C, indicating that concrete still has good toughness under higher temperature condition. This is because toughness characterizes the deformation ability of material under loading, which is the synthetic exhibition of strength and ductility. In other words, SEA is concerned with both strength and critical strain.
Z. Li et al. / Materials Science and Engineering A 544 (2012) 27–32
31
Table 4 Results of SHPB test of concrete at different temperature. Temperature (◦ C)
Specimen no.
Projectile velocity (m/s)
Average strain rate (s−1 )
Critical strain (×10−3 )
Ultimate strain (×10−3 )
Dynamic compressive strength (MPa)
Specific energy absorption (KJ/m3 )
Room temperature Room temperature Room temperature Room temperature Room temperature 200 200 200 200 200 400 400 400 400 400 600 600 600 600 600 800 800 800 800 800
A1 B1 C1 D1 E1 A2 B2 C2 D2 E2 A3 B3 C3 D3 E3 A4 B4 C4 D4 E4 A5 B5 C5 D5 E5
5.29 6.4 7.53 8.29 9.99 4.85 6.63 7.05 7.66 8.4 6.76 7.28 8.08 10.6 12.85 5.43 6.11 7.46 8.37 9.34 4.21 4.92 7.64 8.14 9.4
40.95 60.79 86.42 95.01 125.70 33.70 80.41 95.19 107.10 117.43 74.25 80.47 96.23 139.16 194.12 76.05 81.71 110.44 125.53 141.90 57.52 68.74 129.62 140.78 172.25
6.30 6.45 7.14 7.21 8.60 6.16 6.53 6.62 7.02 7.35 6.53 6.93 7.25 7.81 8.52 7.96 9.61 10.61 10.20 11.93 12.24 14.54 17.28 17.89 24.12
12.60 19.99 21.51 26.09 31.10 6.55 9.02 11.91 14.16 15.03 10.47 12.81 14.20 18.42 24.43 14.40 17.10 18.78 20.11 21.86 16.19 20.85 23.14 32.11 35.76
76.60 84.08 95.82 107.57 130.75 67.53 80.61 85.78 88.09 94.09 90.00 93.30 94.62 101.16 105.56 33.64 47.51 58.46 67.80 84.62 12.28 20.02 31.77 37.64 50.98
555.89 827.55 1174.38 1344.52 1558.67 260.88 641.59 695.43 763.25 906.77 940.91 1002.51 1355.13 1931.04 2127.35 236.31 318.62 526.56 864.22 1068.34 98.41 178.74 444.69 724.00 1124.75
Though dynamic compressive strength of concrete drops evidently over 400 ◦ C, critical strain increases rapidly meanwhile, and with the consequent SEA less decrease. As mentioned above, 400 ◦ C is the critical temperature for dynamic mechanical properties changing seriously. 4.2. Strain rate effects Fig. 4 presents the relations among dynamic compressive strength, critical strain and SEA on strain rate of concrete. As shown in Fig. 4, the dynamic mechanical properties of concrete before and after high temperature exposure are prominently strain rate dependent. Dynamic compressive strength of concrete at different temperatures increases with the rise in strain rate evidently. With the rise in strain rate, critical strain increases slightly below 400 ◦ C, but increases rapidly over 400 ◦ C. Especially at 800 ◦ C, critical strain increases by 97.06% with the rise in strain rate from 57.52 to 172.25 s−1 . SEA at different temperature increases rapidly with the rise in strain rate.
Fig. 5. Dynamic increase factor vs. strain rate.
4.3. Comparison Affected by a series of factors, the present experimental results about the mechanical properties of concrete at high temperature under static condition are controversial, but the consensus is that the evident changes of mechanical properties take place at 400 ◦ C. The experimental results about static and dynamic mechanical properties obtained in this paper are similar to those relevant researches. Dynamic increase factor (DIF), which is the ratio of dynamic compressive strength of concrete to its corresponding quasi-static value, was applied to compare the effect of loading type on strength (Fig. 5). As shown in Fig. 5, DIF increases rapidly with the rise in strain rate, especially for the concrete after exposure to 600 and 800 ◦ C, demonstrating the strengthening effect of strain rate on concrete becomes more evidently over 400 ◦ C. The relations between DIF and strain rate can be described as Eqs. (2)–(6). Room temperature T = 200
◦
C
DIF = 0.01063ε˙ + 0.76771 (R = 0.951)
DIF = 0.00592ε˙ + 1.10531 (R = 0.984)
(2) (3)
T = 400
◦
C
DIF = 0.00217ε˙ + 1.45282 (R = 0.945)
(4)
T = 600
◦
C
DIF = 0.02083ε˙ − 0.42148 (R = 0.934)
(5)
T = 800
◦
C
DIF = 0.01744ε˙ − 0.22724 (R = 0.952)
(6)
where ε˙ is the strain rate, and R is the liner correlation coefficient. As mentioned above, after exposure to temperature up to 800 ◦ C, although strength of concrete decreases evidently caused by high-temperature attack and thermal attack due to faster cooling, ductility and toughness of concrete get improved significantly, especially under impact loading. That is to say, concrete can be recognized as an excellent thermal-resistant and anti-impact construction material. 5. Conclusions Based on this study, the following conclusions can be drawn:
32
Z. Li et al. / Materials Science and Engineering A 544 (2012) 27–32
(1) The mechanical properties of concrete after high temperature exposure change obviously, and 400 ◦ C is the critical temperature for changing. Below 400 ◦ C, compressive strength, critical strain and specific energy absorption of concrete change little. With the temperature increasing above 400 ◦ C, compressive strength decreases evidently, while critical strain increases greatly and specific energy absorption decreases less than strength, especially at 800 ◦ C. In other words, after exposure to the temperature above 400 ◦ C, strength drops remarkable, but ductility increases evidently with the consequent toughness less decrease. (2) Dynamic compressive strength and specific energy absorption of concrete before and after high temperature exposure increase with the rise in strain rate rapidly, exhibiting strong strain rate dependency. While critical strain does not show a strong strain rate sensibility until the temperature is above 400 ◦ C. (3) Dynamic increase factor increases rapidly with the rise in strain rate, especially for the concrete after exposure to 600 and 800 ◦ C, which indicates the strengthening effect of strain rate on concrete becomes more evidently over 400 ◦ C. (4) Above all, concrete can be recognized as an excellent thermalresistant and anti-impact construction material.
Acknowledgments This research program is supported by the National Natural Science Foundation of China (Grant No. 51078350) and Province Natural Science Foundation of Shanxi (Grant No. 2010JQ6011). References [1] H.L. Malhotra, Mag. Concr. Res. 23 (1956). [2] G.A. Khoury, B.N. Grainger, P.J.E. Sullivan, Mag. Concr. Res. 133 (1985) 195–215. [3] T.Q. Lü, G.F. Zhao, Z.S. Lin, Q.R. Yue, J. Build. Mater. 6 (2003) 135–141 (in Chinese). [4] K. Hertz, Institute of Building Design Report No. 166, Technical University of Denmark, 1984. [5] S.Y.N. Chan, G.F. Peng, J.K.W. Chan, Mater. Struct. 29 (1996) 616–619. [6] G.A. Khoury, C.E. Majorana, Mag. Concr. Res. 54 (2002) 77–101. [7] T. Harun, C. Ahme, Constr. Build. Mater. 22 (2008) 2269–2275. [8] F. Grondin, H. Dumontet, A.B. Hamida, H. Boussa, Cem. Concr. Res. 33 (2011) 424–435. [9] Y.A. Al-Salloum, H.M. Elsanadedy, A.A. Abadel, Constr. Build. Mater 25 (2011) 838–850. [10] Z. Han, Comput. Struct. 81 (2003) 1301–1310. [11] Q.M. Li, H. Meng, Int. J. Solids Struct. 40 (2003) 343–360. [12] G.A. Khoury, Eng. Med. 2 (2000) 429–447. [13] B. Georgali, P.E. Tsakiddis, Cem. Concr. Compos. 27 (2005) 255–259. [14] A.Y. Nassif, Fire Mater. 26 (2002) 103–109. [15] W.M. Li, J.Y. Xu, Mater. Sci. Eng. A 505 (2009) 178–186. [16] W.M. Li, J.Y. Xu, Mater. Sci. Eng. A 513 (2009) 145–153.
Contents lists available at SciVerse ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Static and dynamic mechanical properties of concrete after high temperature exposure Zhiwu Li a,∗ , Jinyu Xu a,b , Erlei Bai a a b
The Engineering Institute, Air Force Engineering University, Xi’an, Shanxi 710038, China Department of Civil Engineering, Northwestern Polytechnical University, Xi’an, Shanxi 710072, China
a r t i c l e
i n f o
Article history: Received 20 June 2011 Received in revised form 18 February 2012 Accepted 18 February 2012 Available online 28 February 2012 Keywords: Concrete After high temperature exposure Mechanical property SHPB Strain rate effect Toughness
a b s t r a c t The aim of this paper is to investigate the static and dynamic mechanical properties of concrete before and after high temperature exposure. Quasi-static and impact loading experiments were carried out on concrete before and after exposure to the temperature of 200–800 ◦ C by using a servo-hydraulic testing machine and a 100-mm-diameter split Hopkinson bar (SHPB) apparatus, respectively. The results show that, mechanical properties of concrete after high temperature exposure change evidently, and the critical temperature for those changes acquiring dramatic character is 400 ◦ C. With the rise of temperature up to 400 ◦ C, compressive strength and critical strain change little compared with that of at room temperature. While with temperature increasing from 400 to 800 ◦ C, the decrease in strength and the increase in critical strain become obvious. Consequentially, specific energy absorption (SEA), which is the synthetic exhibition of strength and ductility, decreases much less than those observed for strength over 400 ◦ C. Dynamic compressive mechanical properties of concrete before and after high temperature exposure increase with the rise in strain rate rapidly, which exhibits strong strain rate dependency. Dynamic increase factor (DIF) increases approximately linearly with strain rate. In a word, concrete can be recognized as an excellent thermal-resistant and anti-impact construction material. © 2012 Elsevier B.V. All rights reserved.
1. Introduction As a kind of building and construction material, the properties of concrete under high temperature conditions have a direct bearing on the structural safety, so it is necessary to do researches. Since the beginning of 20th century, numerous studies have been carried out to investigate the properties of concrete at high temperature [1–3]. With the extensive application of high strength concrete (HSC) during the 80s of the last century, HSC was found to be prone to spalling at high temperature, thus spalling mechanism and methods of improving the thermal-resistant properties of HSC became research focus [4–6]. In recent years, researches on the thermal properties of other types of concrete are also put forth [7–9]. However, most of the present studies on the mechanical properties of concrete subjected to high temperature were carried out under static condition, with little focus on the dynamic properties. Results of many dynamic loading tests on not-heated concrete have shown that there were significant differences in the performances of concrete under the conditions between static and dynamic. A greater understanding of concrete behaviors under different conditions will improve confidence in its use. Otherwise, building collapses take place occasionally in conflagration, which is terribly
dangerous for rescue and fire fighter. The related researches show that high temperature, impact loading caused by chemistry explosion and construction member collapsing, and faster cooling by watering are the main reasons for collapses. Thus, for understanding the collapse mechanism and to prevent it, it is very important to study the anti-impact properties of concrete exposed to high temperature and cooled by water. The split Hopkinson bar (SHPB) apparatus has become popular for the studies on dynamic mechanical properties of material at the strain rate of 102 –104 s−1 [10,11]. As concrete is a composite material with complex constituents and original defects, the study of concrete-like materials needs large-diameter Hopkinson bar to ensure the representative of specimens. The aim of this paper was to investigate the static and dynamic mechanical behaviors of concrete subjected to high temperature and cooled down by watering. Tests were carried out using a servohydraulic testing machine and a 100-mm-diameter split Hopkinson bar (SHPB) apparatus.
2. Experimental details 2.1. Materials and specimen fabrication
∗ Corresponding author. Fax: +86 029 23053922. E-mail address: [email protected] (Z. Li). 0921-5093/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2012.02.058
Concrete was prepared using type 42.5R ordinary Portland cement, water, crushed limestone, sand, fly ash, silica fume and
28
Z. Li et al. / Materials Science and Engineering A 544 (2012) 27–32
Table 1 Mix proportion of concrete (kg/m3 ). Water
Cement
Fly ash
Silica fume
Sand
Gravel
Superplasticizer
180
375
125
25
690
1030
5
Table 2 Quasi-static compressive strength of cylinders at different temperature (MPa). Test no.
1 2 3 Average Maximum relative error (%) Percentage residual strength (%)
Room temperature
129.14 134.44 141.02 134.87 4.56 100
Temperature 200 ◦ C
400 ◦ C
600 ◦ C
800 ◦ C
118.69 117.20 120.81 118.90 1.61 88.16
129.08 128.36 134.91 130.78 3.16 96.97
96.88 87.56 105.67 96.70 9.45 71.70
55.55 52.33 45.10 50.99 11.55 37.81
200 ◦ C
400 ◦ C
600 ◦ C
800 ◦ C
50.32 54.23 49.56 51.40 5.51 85.03
57.89 58.21 54.32 56.81 4.38 93.98
32.41 34.98 29.38 32.26 8.93 53.37
15.32 17.28 19.53 17.38 12.37 28.75
Table 3 Quasi-static compressive strength of cubes at different temperature (MPa). Test no.
1 2 3 Average Maximum relative error (%) Percentage residual strength (%)
Room temperature
55.84 61.23 64.29 60.45 7.63 100
Temperature
superplasticizer. The cement used in the experiment complied with the requirements of Chinese standard (GB 175-2007), and the maximum gravel size was 20 mm. The mix proportion of concrete is listed in Table 1. After removal from the moulds, the mixtures were cured in a curing room (under a controlled condition of 20 ◦ C and 95% relative humidity) for 28 days. After curing, the long cylindrical concrete pastes were cut into cylinder specimens with 48 mm thickness and 98 mm diameter. 2.2. Heating and cooling To investigate the mechanical properties of concrete after high temperature exposure, 200 ◦ C, 400 ◦ C, 600 ◦ C and 800 ◦ C were selected as the heating target temperature according to our prior researches and other relevant papers [5,12–14]. The heating equipment was an electrically heated furnace, which had a capacity of 18 cylinders or 3 cubes. Specimens were placed in the furnace and the furnace was heated from room temperature to target temperature at a constant heating rate of 10 ◦ C/min. After 6 h at the target temperature, the heating was stopped, and the specimens were removed from the furnace and cooled down by watering for 30 min. Then they were transferred to laboratory. After 3 days, tests were carried on these specimens. 3. Results 3.1. Quasi-static compressive experiment For the cylindrical specimens are not the right size for static strength testing, tests were also carried out on cubic specimens (150 mm × 150 mm × 150 mm) that heated and cooled in the same way. A servo-hydraulic testing machine was adopted to determine the compressive strength of specimens at strain rate of 10−5 s−1 . The test results are listed in Tables 2 and 3. Fig. 1 presents the
Fig. 1. Quasi-static compressive strength.
relation between quasi-static compressive strength and temperature. As shown in Tables 2 and 3 and Fig. 1: (i) The maximum error of each compressive strength group is less than 13%, demonstrating that the experimental data are fairly reliable. (ii) The compressive strengths of cylinder are much higher than that of the cube at the same temperature, indicating that the size effect is obvious. (iii) The compressive strength of cubic specimens after high temperature exposure decreases in a similar manner to that of the cylinder. The quasi-static compressive strength decreases slightly at 200 ◦ C, and then increase closely to that of not-heated specimens at 400 ◦ C. The evident drop in the strength of specimens is observed at temperature between 400 ◦ C and 800 ◦ C. After exposure to 800 ◦ C, the residual compressive strength values of cylinder and cube are only 37.81% and 28.75%, respectively, of that at room temperature.
Z. Li et al. / Materials Science and Engineering A 544 (2012) 27–32
a
b
c
d
29
e
Fig. 2. (a) Stress–strain curve of concrete at room temperature, (b) at 200 ◦ C, (c) at 400 ◦ C, (d) at 600 ◦ C, and (e) at 800 ◦ C.
3.2. Impact compressive experiment In this study, strain rates ranging from 30 to 200 s−1 were obtained by means of changing the projectile’s impact velocity. To ensure the reliability of the test results, 3 specimens were impacted at each strain rate. Critical strain (strain at peak stress) was adopted to characterize the ductility of concrete. Specific energy absorption (SEA) [15,16], which is the energy absorption per unit volume of material, was adopted to characterize the
toughness of concrete, and it can be calculated as the following formula:
SEA =
AEc AS ls
T
[εi 2 (t) − εr 2 (t) − εt 2 (t)]dt
(1)
0
where c, E, A are respectively the bar’s elastic wave speed, Young’s modulus, and cross-sectional area, ls , As are length and crosssectional area of specimen; εi , εr , εt are, respectively, the incident
30
Z. Li et al. / Materials Science and Engineering A 544 (2012) 27–32
a
a
b
b
c
Fig. 3. (a) Dynamic compressive strength vs. projectile velocity, (b) critical strain vs. projectile velocity, and (c) specific energy absorption vs. projectile velocity.
pulse, reflect pulse and transmit pulse; T represents the complete failure moment of specimen. The experimental results of SHPB and the stress–strain curves of concrete at different temperature are shown in Table 4 and Fig. 2 separately. 4. Discussions 4.1. Dynamic compressive mechanical properties The relations between dynamic compressive mechanical properties and projectile velocity are presented in Fig. 3. As shown in Fig. 3(a), approximate change trends in compressive strength
c
Fig. 4. (a) Dynamic compressive strength vs. average strain rate, (b) critical strain vs. average strain rate, and (c) specific energy absorption vs. average strain rate.
of concrete under quasi-static and dynamic conditions are determined as the temperature increases. Dynamic compressive strength of concrete changes little below 400 ◦ C, but decreases rapidly above 400 ◦ C. As shown in Fig. 3(b), the critical strain keeps at ∼7‰ below 400 ◦ C, but increases greatly at 600 and 800 ◦ C. As shown in Fig. 3(c), the decrease in SEA at 600 ◦ C is obviously less than that observed for strength; and the SEA at 800 ◦ C is closely to that at 600 ◦ C, indicating that concrete still has good toughness under higher temperature condition. This is because toughness characterizes the deformation ability of material under loading, which is the synthetic exhibition of strength and ductility. In other words, SEA is concerned with both strength and critical strain.
Z. Li et al. / Materials Science and Engineering A 544 (2012) 27–32
31
Table 4 Results of SHPB test of concrete at different temperature. Temperature (◦ C)
Specimen no.
Projectile velocity (m/s)
Average strain rate (s−1 )
Critical strain (×10−3 )
Ultimate strain (×10−3 )
Dynamic compressive strength (MPa)
Specific energy absorption (KJ/m3 )
Room temperature Room temperature Room temperature Room temperature Room temperature 200 200 200 200 200 400 400 400 400 400 600 600 600 600 600 800 800 800 800 800
A1 B1 C1 D1 E1 A2 B2 C2 D2 E2 A3 B3 C3 D3 E3 A4 B4 C4 D4 E4 A5 B5 C5 D5 E5
5.29 6.4 7.53 8.29 9.99 4.85 6.63 7.05 7.66 8.4 6.76 7.28 8.08 10.6 12.85 5.43 6.11 7.46 8.37 9.34 4.21 4.92 7.64 8.14 9.4
40.95 60.79 86.42 95.01 125.70 33.70 80.41 95.19 107.10 117.43 74.25 80.47 96.23 139.16 194.12 76.05 81.71 110.44 125.53 141.90 57.52 68.74 129.62 140.78 172.25
6.30 6.45 7.14 7.21 8.60 6.16 6.53 6.62 7.02 7.35 6.53 6.93 7.25 7.81 8.52 7.96 9.61 10.61 10.20 11.93 12.24 14.54 17.28 17.89 24.12
12.60 19.99 21.51 26.09 31.10 6.55 9.02 11.91 14.16 15.03 10.47 12.81 14.20 18.42 24.43 14.40 17.10 18.78 20.11 21.86 16.19 20.85 23.14 32.11 35.76
76.60 84.08 95.82 107.57 130.75 67.53 80.61 85.78 88.09 94.09 90.00 93.30 94.62 101.16 105.56 33.64 47.51 58.46 67.80 84.62 12.28 20.02 31.77 37.64 50.98
555.89 827.55 1174.38 1344.52 1558.67 260.88 641.59 695.43 763.25 906.77 940.91 1002.51 1355.13 1931.04 2127.35 236.31 318.62 526.56 864.22 1068.34 98.41 178.74 444.69 724.00 1124.75
Though dynamic compressive strength of concrete drops evidently over 400 ◦ C, critical strain increases rapidly meanwhile, and with the consequent SEA less decrease. As mentioned above, 400 ◦ C is the critical temperature for dynamic mechanical properties changing seriously. 4.2. Strain rate effects Fig. 4 presents the relations among dynamic compressive strength, critical strain and SEA on strain rate of concrete. As shown in Fig. 4, the dynamic mechanical properties of concrete before and after high temperature exposure are prominently strain rate dependent. Dynamic compressive strength of concrete at different temperatures increases with the rise in strain rate evidently. With the rise in strain rate, critical strain increases slightly below 400 ◦ C, but increases rapidly over 400 ◦ C. Especially at 800 ◦ C, critical strain increases by 97.06% with the rise in strain rate from 57.52 to 172.25 s−1 . SEA at different temperature increases rapidly with the rise in strain rate.
Fig. 5. Dynamic increase factor vs. strain rate.
4.3. Comparison Affected by a series of factors, the present experimental results about the mechanical properties of concrete at high temperature under static condition are controversial, but the consensus is that the evident changes of mechanical properties take place at 400 ◦ C. The experimental results about static and dynamic mechanical properties obtained in this paper are similar to those relevant researches. Dynamic increase factor (DIF), which is the ratio of dynamic compressive strength of concrete to its corresponding quasi-static value, was applied to compare the effect of loading type on strength (Fig. 5). As shown in Fig. 5, DIF increases rapidly with the rise in strain rate, especially for the concrete after exposure to 600 and 800 ◦ C, demonstrating the strengthening effect of strain rate on concrete becomes more evidently over 400 ◦ C. The relations between DIF and strain rate can be described as Eqs. (2)–(6). Room temperature T = 200
◦
C
DIF = 0.01063ε˙ + 0.76771 (R = 0.951)
DIF = 0.00592ε˙ + 1.10531 (R = 0.984)
(2) (3)
T = 400
◦
C
DIF = 0.00217ε˙ + 1.45282 (R = 0.945)
(4)
T = 600
◦
C
DIF = 0.02083ε˙ − 0.42148 (R = 0.934)
(5)
T = 800
◦
C
DIF = 0.01744ε˙ − 0.22724 (R = 0.952)
(6)
where ε˙ is the strain rate, and R is the liner correlation coefficient. As mentioned above, after exposure to temperature up to 800 ◦ C, although strength of concrete decreases evidently caused by high-temperature attack and thermal attack due to faster cooling, ductility and toughness of concrete get improved significantly, especially under impact loading. That is to say, concrete can be recognized as an excellent thermal-resistant and anti-impact construction material. 5. Conclusions Based on this study, the following conclusions can be drawn:
32
Z. Li et al. / Materials Science and Engineering A 544 (2012) 27–32
(1) The mechanical properties of concrete after high temperature exposure change obviously, and 400 ◦ C is the critical temperature for changing. Below 400 ◦ C, compressive strength, critical strain and specific energy absorption of concrete change little. With the temperature increasing above 400 ◦ C, compressive strength decreases evidently, while critical strain increases greatly and specific energy absorption decreases less than strength, especially at 800 ◦ C. In other words, after exposure to the temperature above 400 ◦ C, strength drops remarkable, but ductility increases evidently with the consequent toughness less decrease. (2) Dynamic compressive strength and specific energy absorption of concrete before and after high temperature exposure increase with the rise in strain rate rapidly, exhibiting strong strain rate dependency. While critical strain does not show a strong strain rate sensibility until the temperature is above 400 ◦ C. (3) Dynamic increase factor increases rapidly with the rise in strain rate, especially for the concrete after exposure to 600 and 800 ◦ C, which indicates the strengthening effect of strain rate on concrete becomes more evidently over 400 ◦ C. (4) Above all, concrete can be recognized as an excellent thermalresistant and anti-impact construction material.
Acknowledgments This research program is supported by the National Natural Science Foundation of China (Grant No. 51078350) and Province Natural Science Foundation of Shanxi (Grant No. 2010JQ6011). References [1] H.L. Malhotra, Mag. Concr. Res. 23 (1956). [2] G.A. Khoury, B.N. Grainger, P.J.E. Sullivan, Mag. Concr. Res. 133 (1985) 195–215. [3] T.Q. Lü, G.F. Zhao, Z.S. Lin, Q.R. Yue, J. Build. Mater. 6 (2003) 135–141 (in Chinese). [4] K. Hertz, Institute of Building Design Report No. 166, Technical University of Denmark, 1984. [5] S.Y.N. Chan, G.F. Peng, J.K.W. Chan, Mater. Struct. 29 (1996) 616–619. [6] G.A. Khoury, C.E. Majorana, Mag. Concr. Res. 54 (2002) 77–101. [7] T. Harun, C. Ahme, Constr. Build. Mater. 22 (2008) 2269–2275. [8] F. Grondin, H. Dumontet, A.B. Hamida, H. Boussa, Cem. Concr. Res. 33 (2011) 424–435. [9] Y.A. Al-Salloum, H.M. Elsanadedy, A.A. Abadel, Constr. Build. Mater 25 (2011) 838–850. [10] Z. Han, Comput. Struct. 81 (2003) 1301–1310. [11] Q.M. Li, H. Meng, Int. J. Solids Struct. 40 (2003) 343–360. [12] G.A. Khoury, Eng. Med. 2 (2000) 429–447. [13] B. Georgali, P.E. Tsakiddis, Cem. Concr. Compos. 27 (2005) 255–259. [14] A.Y. Nassif, Fire Mater. 26 (2002) 103–109. [15] W.M. Li, J.Y. Xu, Mater. Sci. Eng. A 505 (2009) 178–186. [16] W.M. Li, J.Y. Xu, Mater. Sci. Eng. A 513 (2009) 145–153.