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Study of the performance of steel fiber reinforced concrete to water and salt freezing condition
Materials and Design 44 (2013) 267–273
Contents lists available at SciVerse ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Study of the performance of steel fiber reinforced concrete to water and salt freezing condition Ditao Niu, Lei Jiang ⇑, Min Bai, Yuanyao Miao Xi’an University of Architecture and Technology, Xi’an, Shaanxi 710055, PR China
a r t i c l e
i n f o
Article history: Received 26 April 2012 Accepted 29 July 2012 Available online 13 August 2012 Keywords: B. Steel fiber A. Concrete E. Frost resistance
a b s t r a c t Properties of plain concrete and steel fiber reinforced concrete (SFRC) (with volume fraction of 0.5%, 1%, 1.5% and 2%) subjected to freeze–thaw cycles in water and in the 3.5% NaCl solution were investigated in this paper. Through the experiment, surface damage, weight loss and splitting tensile strength loss of SFRC were measured after different numbers of freeze–thaw circulations. The microstructure and the pore structure of SFRC were analyzed on the basis of scanning electron microscope (SEM) and mercury intrusion experiment. The test results show that the use of steel fiber could improve the pore structure and decelerate the damage of concrete during freeze–thaw cycles. However, the ability of steel fiber to reduce surface scaling of concrete is limited subjected to freeze–thaw cycles in the NaCl solution. Furthermore, the weight loss and the splitting tensile strength loss of concrete tested in the NaCl solution were larger than those in water. It is also shown that the steel fiber content has the great influence on the frost-resisting property of SFRC. When a relatively steel fiber content is introduced (1.5 vol.%), the deterioration process of concrete subjected to the frost damage is considerably reduced. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction In cold regions, the deterioration of concrete resulting from freezing–thaw damage has great impact on the durability and service life of concrete structures [1,2]. The decrease of concrete structural durability caused by freeze–thaw cycles ultimately leads to the formation and development of cracks in the concrete. Incorporation of steel fibers is the effective way to improve crack resistance behavior and consequently the ductility and fracture toughness of concrete, it can be explain that the steel fibers are able to transfer emerging loads by bridging the cracks [3–5]. For most structural and non-structural purposes, steel fiber is the most commonly used of all the fibers [6]. SFRC is a multiphase composite material which added disordered distribution of short steel fibers in plain concrete. With the development of economy and technology, SFRC has been applied widely in the engineering construction fields step by step. Numerous works for evaluating mechanical properties of SFRC have been reported, it now has been well accepted that incorporation of steel fiber can significantly improve the mechanical behaviors of concrete. Düzgün et al. [7] investigated the effect of steel fibers on the mechanical properties of pumice aggregate concrete, ⇑ Corresponding author. Address: College of Civil Engineering, Xi’an University of Architecture and Technology, No. 13 Yanta Road, Xi’an, Shaanxi 710055, PR China. Tel.: +86 136 0912 6920. E-mail addresses: [email protected], [email protected] (L. Jiang). 0261-3069/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2012.07.074
declaring that the compressive strength, splitting tensile strength and flexural strength of concretes increased up to 21.1%, 61.2% and 120.2%, respectively. Song and Hwang [8] studied the mechanical properties of HSC, indicating that the compressive strength reached a maximum at 1.5 vol.% fraction, being a 15.3% improvement over the HSC. The splitting tensile strength improved with increasing the volume fraction, achieving 98.3% at 2 vol.% fraction. Amr and Dieb [9] investigated the compressive strength and splitting tensile strength of UHSC, indicating that improvement increases as the fiber volume fraction increases. According to Marar et al. [10], the compressive strength of HSFRC improved with the increase in fiber volume. And Daniel and Loukili [11] declared, the compressive strength of HSFRC held 15% advantage over the HSC. Incorporation of fibers into concrete is not only an effective way to improve concrete mechanical behaviors, but also durability. Pigeon et al. [12,13] obtained that the use of steel and particularly carbon microfibers improves the frost and deicer salt scaling resistance of mortars. Cantin and Pigeon [14] indicated that steel fibers have no significant influence on the deicer-scaling resistance of concrete. Yang and Zhu [15] reported that the deicer-scaling resistance of concrete is reduced by the addition of steel fibers at the same air content, especially for the air-entrained concrete. According to Sun et al. [16], steel fiber could retard the performance deterioration of the concrete and improve the resistance against multidamaging under severe conditions. From the literatures, it is obvious that most research has focused on the mechanical properties and little information available
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Table 1 Mix proportions of concrete (kg/m3). Mix code
Cement
Sand
Coarse aggregate
Water
Steel fiber
Water reducer (%)
B1 B2 B3 B4 B5
367 367 367 367 367
765 735 733 718 702
1146 1102 1100 1066 1053
165 165 165 165 165
0 39 78 117 156
0.6 0.6 0.6 0.6 0.6
Table 2 Mechanical properties of concrete mixtures at 28 days. Concrete
Cube compressive strength (MPa)
Splitting tensile strength (MPa)
Flexural strength (MPa)
B1 B2 B3 B4 B5
37.9 38.3 39.5 40.7 35.2
6.1 8.1 9.2 11.0 7.2
5.03 5.34 6.53 7.16 9.77
concerning the freeze and thaw durability of SFRC. Furthermore, microstructural features including the pore structure and the microcrack characteristics of SFRC under the action of freeze–thaw needs more investigate. In this paper, the volume fraction of steel fiber and number of freeze–thaw circulation were taken as variable parameter. Basic experimental research that the performance of SFRC to water and salt freezing condition was conducted on the basis of the macroscopic and microscopic test. Furthermore, the mechanism of SFRC such as reinforcement, damage and cracking resistance under the action of freeze–thaw was analyzed.
Fig. 3. Weight loss in concrete subjected to freeze–thaw cycles. (a) In water and (b) in the NaCl solution.
2. Experimental details 2.1. Materials and mix proportions
Fig. 1. Strength increased rate of SFRC.
A Chinese standard Portland cement produced by Cement Factory of TongChuan was adopted, river sand with fineness modulus of 2.69, and coarse aggregate of crushed basalt stone with diameter of 5–16 mm were used in the test. A naphthalene-type superplasticizer was used, and the dosage was adjusted to keep the slump of fresh mixed plain concrete in the range of 50–120 mm. A steel fiber with a rectangular cross section, an aspect ratio of 60, and a length of 30 mm was adopted to prepare SFRC. In this experiment, the water-binder ratio (W/B) was 0.45. A series of plain concrete (B1) and SFRC with additions of 0.5%, 1%, 1.5% and 2% steel fiber were prepared (denoted as B1, B2, B3, B4, B5 respectively). The proportions and basic characteristics of the various concrete mixes are given in Table 1.
Fig. 2. Surface damage of B3 subjected to freeze–thaw cycles in the NaCl solution.
D. Niu et al. / Materials and Design 44 (2013) 267–273
where fcc is the compressive strength (MPa), F is the maximum load (N), A is the area of the cube loading face (mm2).
2.2. Specimens preparation and curing conditions The components of SFRC mixture were batched by weight, the steel fiber were premixed with cement, sand and coarse aggregate before adding the water and the admixtures for 1 min, then, the entire amount of mixing water with the dissolved superplasticizer was added and mixed for 3 min, finally, SFRC mixture was mixed for an additional 2 min. The concrete specimens were cast in steel moulds and compacted on a vibration table. All samples were cured at 20 °C in molds covered by a polyethylene film to prevent moisture loss according to GB/T 50081-2002 [17], the specimens were demolded after 24 h of casting and cured in a condition of 20 ± 3 °C and 95% relative humidity until the age of testing. 2.3. Test methods 2.3.1. Mechanical properties The mechanical properties measurements were carried out according to CECS13:89 [18]. The specimens were cured in a humidity room for 28 days. For each mixture, 6 specimens of 100 mm cubes were used to determined the compressive and splitting tensile strength (three cubes for compressive strength test and three for splitting tensile strength test), 3 specimens of 100 mm 100 mm 400 mm prisms were used to determined the flexural strength. All experiments were performed on three specimen replicates. The average values are used in the discussion of the test results. The following Eqs. (1)–(3) were used to calculate the compressive strength, splitting tensile strength and flexural strength.
fcc ¼ F=A
269
ð1Þ
Fig. 4. Effect of steel fiber content on weight loss. (a) In water and (b) in the NaCl solution.
fts ¼ 2F=pA ¼ 0:637F=A
ð2Þ
where fts is the splitting tensile strength (MPa), F is the maximum load (N), A is the area of the cube splitting face (mm2). 2
ff ¼ Fl=bh
ð3Þ
where ff is the flexural strength (MPa), F is the maximum load (N), l is the distance between the supporting rollers (mm), b is the width of the cross-section (mm), h is the height of the cross-section (mm). 2.3.2. Freezing and thawing cycle tests The specimens for the freeze–thaw cycles tests in gap water or in the NaCl solution were cured for 24 days, and then were immersed in gap water or in a 3.5 mass% sodium chloride solution, respectively, for 4 days. At the age of 28 days, testing of concrete exposed to freeze–thaw cycles in gap water or in the NaCl solution were carried out in accordance with CECS13: 89 [18] and GBT50082-2009 [19] separately. In this paper, the concentration of 3.5% NaCl solution was used [20,21]. For each mixture, 6 specimens of 100 mm 100 mm 400 mm prisms and 45 specimens of 100 mm cubes were used for the study of weight loss and splitting tensile strength at every 25 times of freeze–thaw circulations. The temperature of concrete samples was controlled by a Pt sensor embedded in the center of a concrete sample. The temperature of the sample center ranged from 17.0 ± 1 to 6.0 ± 1 °C. According to the test procedure, the deterioration of the specimens was investigated by determining the weight loss, the
Fig. 5. Splitting tensile strength loss in concrete subjected to freeze–thaw cycles. (a) In water and (b) in the NaCl solution.
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thaw cycles (kg), Wn is the average weight of concrete specimens at every 25 freeze–thaw cycles in water or in the 3.5% NaCl solution (kg). All experiments were performed on three specimen replicates. The average values are used in the discussion of the test results. 2.3.3. Microcosmic-test The microstructural features of the concrete were analyzed on the basis of scanning electron microscope (SEM) and mercury intrusion experiment. This investigation helps to understand the mechanical properties and durability performance of the SFRC. 3. Test results and discussion 3.1. Mechanical properties test results Table 2 and Fig. 1 gives the test results about compressive strength, splitting tensile strength, and flexural strength of five kinds of concrete in 28 days-curing cycles. By increasing the fiber content from 0% to 1.5%, the 28 day cube compressive strength, the splitting tensile strength and the flexural strength increased from 1.06% to 7.39%, 32.79% to 80.33% and 6.16% to 42.35% respectively, compared with B1. Steel fiber significantly improves the mechanical properties of SFRC compared with ordinary concrete. However the effect of steel fiber improve the splitting tensile strength and the flexural strength is better than the compressive strength. These results are in agreement with those reported in literature [22,23], which show that adding steel fibers to concrete composition increases the splitting tensile strength 1.5–2 times and lead a slight increase in the compressive strength values (up
Fig. 6. Effect of steel fiber content on splitting tensile strength loss. (a) In water and (b) in the NaCl solution.
Table 3 Pore structure parameter of SFRC. Total porosity (%)
Total pore volume (ml/g)
Total pore area (m2/g)
Critical pore size (nm)
Average pore size (nm)
Most probable pore diameter (nm)
B1 B2 B3 B4 B5
15.22 14.17 12.69 10.33 14.60
0.0785 0.0664 0.0620 0.0561 0.0681
23.63 19.39 17.04 13.52 18.21
40.31 20.24 15.73 12.47 34.98
52.14 25.85 18.01 13.80 40.45
59.37 30.57 21.60 15.23 46.82
Fig. 7. Pore size distribution histogram.
Table 4 Pore size distribution of SFRC. Code
B1 B2 B3 B4 B5
Pore size volume fraction (%) d < 20 nm
20 nm 6 d < 50 nm
50 nm 6 d < 200 nm
d P 200 nm
13.14 43.24 52.39 80.01 21.39
43.70 40.35 30.29 12.41 44.40
38.45 14.02 10.32 3.09 29.35
4.30 2.15 4.51 2.62 3.74
specimen fails if its weight loss exceeds 5%, which was calculated as follows.
DW n ¼ ½ðW 0 W n Þ=W 0 100
ð4Þ
Log differential intrusion (ml/g)
Code
0.4 B1 B2
0.3
B3 B4 B5
0.2
0.1
0
1
10
100
Pore size diameter (nm) where DWn is the loss of specimens at every 25 freeze–thaw cycles (%), W0 is the average weight of concrete specimens before freeze–
Fig. 8. Pore size distribution differential curve of SFRC.
1000
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Fig. 9. Microstructure observation of B1 subjected to freeze–thaw cycles in the NaCl solution: (a) 0 cycles, (b) 50 cycles, (c) 100 cycles.
Fig. 10. Microstructure observation of B4 subjected to freeze–thaw cycles in the NaCl solution: (a) 0 cycles, (b) 50 cycles, (c) 100 cycles.
to 10–25%). With a 2 vol.% fraction of steel fiber, the distribution of steel fiber in concrete is not uniform and a reunion phenomenon occurred, which weaken the interfacial bond strength instead of reinforcing it.
more, the coarse aggregate exposed in some severely scaled specimens and even peeled off as freeze–thaw cycles increases. Similar studies reported in literature [14,15], which reported that the ability of steel fiber to reduce surface scaling of concrete is limited when subjected to freeze–thaw cycles in the NaCl solution.
3.2. Surface damage 3.3. Weight loss As can be seen from the Fig. 2, surface scaling of SFRC is damaged seriously due to freeze–thaw cycles in the NaCl solution. In the first several freeze–thaw cycles, surface layer of mortar is destroyed, and the surface of concrete becomes uneven. Further-
As can be seen from the Fig. 3a, the weight loss rate of SFRC is lower than plain concrete when subjected to the action of freezing–thawing. For B4, the weight loss is 2.28% after 300 freez-
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weight loss of SFRC subjected to freeze–thaw cycles. The weight loss rate can be inhibit effectively when the steel fiber content is 1.5%, but the weight loss of concrete increased with 2% steel fiber content. The weight loss of concrete specimens is caused by the scaling of concrete surface. Scaling due to freezing and thawing cycles is a progressive phenomenon which slowly destroys successive surface layers of concrete. The randomly scattered steel fiber has no effect on the disintegrated slurries in surface layers and could not prevent the scaling off the concrete surface. 3.4. Splitting tensile strength loss As shown in Fig. 5a, the splitting tensile strength decreased by 40% after 150 cycles and 250 cycles respectively for B1 and B4, but B1 was completely destroyed after 200 cycles. Fig. 5b shows that the NaCl solution accelerate the splitting tensile strength loss rate of SFRC. At 100 freeze–thaw cycles, the splitting tensile strength loss of B1 and B4 are 34% and 22% in the NaCl solution, and 23%, 9% in water. It can be concluded that the cracking resistance of SFRC has been weakened and the interior damage has been greatly accelerated when subjected to freeze–thaw cycles in the NaCl solution. Fig. 6 shows that the splitting tensile strength loss improved with increasing the volume fraction, reached a best performance at 1.5%, and the damage rate of SFRC under the action of freeze–thaw cycles has been reduced significantly. But the splitting tensile strength loss of concrete increased when the steel fiber content is 2%. 3.5. Microstructural features
Fig. 11. Microstructure observation of fiber–cement matrix interfaces of SFRC subjected to freeze–thaw cycles in the NaCl solution: (a) Ca(OH)2 crystals, (b) ettringite crystals and (c) microcracks in fiber–cement interfaces.
ing and thawing cycles, which is almost half of that of B1 after 225 cycles. Fig. 3b shows that the weight loss rate of SFRC in the NaCl solution is much greater than that in water. After 100 freeze–thaw cycles, the weight loss of B1 and B4 are 2.1%, 1.3% in water and 4.2%, 2.5% in the NaCl solution. The results are in agreement with those reported in literature [16,21], which show that the weight loss in the NaCl solution is twice as large as those in water, and steel fiber has a little effect on the weight loss of concrete when subjected to freeze–thaw cycles in the NaCl solution. Fig. 4 shows that the steel fiber volume fraction has the great influence on the
3.5.1. Pore structure As can be seen from Table 3, the pore structure has been improved when adding the amount of steel fibers to the concrete. Compared with B1, the total porosity, total pore volume and total pore area of B4 are reduced by 32.13%, 28.54% and 42.78% respectively. Furthermore, the average pore size and most probable pore diameter of SFRC also decrease. The results are in a good agreement with what were suggested by Pigeon et al. [12,14]; however, it disagree with the results reported by Yang and Zhu [15]. The pore structure parameters of SFRC increase when the content reaches to 2%, thus making the air-voids in concrete become coarser. It is an important cause for the reduction of the mechanical property and the frost resistance of B5. Table 4 and Fig. 7 show that, harmless hole increases and harmful hole decreases as the volume fraction of steel fiber ranging from 0% to 1.5%. The volume percentages of the hole aperture (d < 20 nm and 20 nm 6 d < 50 nm) increase greatly, especially in the case of (d < 20 nm), where the hole aperture increases obviously. The volume percentage of the hole aperture (50 nm 6 d < 200 nm and d > 200 nm) decrease obviously. As shown in Fig. 8, the peak value of the mercury injection curvilinear, which means the most probable pore diameter, moves to the direction of the small pore when the steel fiber content increases from 0% to 1.5%. However, the most probable pore diameter enlarges gradually as the content increases from 1.5% to 2%. Experiment results revealed that with a appropriate amount of steel fibers (0–1.5 vol.%), the pore structure of concrete can be greatly improved, therefore enhancing the mechanical property as well as the frost resistance of concrete. 3.5.2. SEM test results It is clearly seen from the Figs. 9 and 10 that the cement hydration products form with each other in a continuous phase with no microcracks before freeze–thaw cycles. After 50 times of freeze– thaw cycles, the microcracks can be clearly observed from the micrographs, but the cracks in B1 not only appear to be much wider than B4 but also interconnect with each other. Besides, the
D. Niu et al. / Materials and Design 44 (2013) 267–273
density of microcracks increases in B1 is more obvious than B4 after 100 cycles. It is known that concrete has to bear hydraulic pressure and osmotic pressure when subjected to freeze–thaw cycles [24,25]. When the tensile stress generated by the two kinds of pressure exceeds the tensile strength of concrete, crack may occur. However the high elastic modulus and strength of steel fiber enhances the cracking resistance of concrete [26]. The addition of steel fiber reduces the rate of crack propagation and improve the frost-resisting property of concrete. There exists the sheet structure of Ca(OH)2 (Fig. 11a) and the cluster structure of ettringite crystals (AFt) (Fig. 11b) in the interfacial transition zone of SFRC. They mainly exist in pores or on the aggregate surface in concrete [27]. This shows that the fiber–cement matrix interface has a high porosity and porous network structure. Microcracks generated in the fiber-cement matrix interface appear to be much wider (Fig. 11c). Therefore, the interface is the sensitive area that microcracks occur and propagate when subject to freeze–thaw cycles. It is also reported that the fiber–cement matrix interface is the weak link of SFRC [28]. The excessive quantity of steel fiber increase the fiber–matrix interface in concrete, which weaken the interfacial bond strength as well as frost-resisting property of concrete. As a result, B5 has a low frost-resist quality compared with other SFRC with different contents in the freezing and thawing process. 4. Conclusion When the plain concrete was subjected to the freeze–thaw cycles, the splitting tensile strength dropped sharply and the weight losses increased considerably, severe scaling occurring on concrete surfaces. Steel fiber could reduce the rate of crack propagation and retard the performance deterioration of the concrete. Furthermore, adding the appropriate amount of steel fibers (0–1.5 vol.%), the pore structure of concrete can be greatly improved, therefore enhancing the mechanical property as well as the frost resistance of concrete. However, the ability of steel fiber to reduce surface scaling of concrete is limited and the deterioration in the concrete is obviously accelerated when subjected to freeze–thaw cycles in the NaCl solution. Besides, the numbers of freeze–thaw cycles at failure in the NaCl solution were significantly lower than those in water. The steel fiber volume fraction has the great influence on the frost-resisting property of SFRC. When a relatively steel fiber content is introduced (1.5 vol.%), the deterioration process of concrete subjected to the frost damage is considerably reduced. However, the frost-resisting property of concrete is significantly decreased with 2% steel fiber content. Acknowledgements This project was supported by the National Excellent Young Scientists Fund (Chinese, No. 50725824) and supported by
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Program for Changjiang Scholars and Innovative Research Team in University. References [1] Kim YJ, Yoon DK. Identifying critical sources of bridge deterioration in cold regions through the constructed bridges in North Dakota. J Bridge Eng 2010;15(5):542–52. [2] Ahani RM, Nokken MR. Salt scaling resistance – the effect of curing and presaturation. Constr Build Mater 2012;26(1):558–64. [3] Xu BW, Shi HS. Correlations among mechanical properties of steel fiber reinforced concrete. Constr Build Mater 2009;23(12):3468–74. [4] Hannant DJ. Fibre-reinforced concrete. Adv Concr Technol Set 2003;4:1–17. [5] Mindess S, Boyd AJ. High performance fibre reinforced concrete. Adv Build Technol 2002;I:873–80. [6] Mehta PK, Monteiro PJM. Concrete; microstructure, properties, and materials. 3rd ed. New York: McGraw-Hill; 2006. [7] Düzgün OA, Gül R, Aydin AC. Effect of steel fibers on the mechanical properties of natural lightweight aggregate concrete. Mater Lett 2005;59(27):3357–63. [8] Song PS, Hwang S. Mechanical properties of high-strength steel fiberreinforced concrete. Constr Build Mater 2004;18(9):669–73. [9] Amr S, Dieb El. Mechanical, durability and microstructural characteristics of ultra-high-strength self-compacting concrete incorporating steel fibers. Mater Des 2009;30(10):4286–92. [10] Marar K, Eren Ö, Çelik T. Relationship between impact energy and compression toughness energy of high-strength fiber-reinforced concrete. Mater Lett 2001;47(4–5):297–304. [11] Daniel L, Loukili A. Behavior of high-strength fiber-reinforced concrete beams under cyclic loading. ACI Struct J 2002;99(3):248–56. [12] Pigeon M, Pleau R, Azzabi M, Banthia N. Durability of microfiber-reinforced mortars. Cem Concr Res 1996;26(4):601–9. [13] Pigeon M, Azzabi M, Pleau R. Can microfibers prevent frost damage? Cem Concr Res 1996;26(8):1163–70. [14] Cantin R, Pigeon M. Deicer scaling resistance of steel-fiber reinforced concrete. Cem Concr Res 1996;26(11):1639–48. [15] Yang QB, Zhu BR. Effect of steel fiber on the deicer-scaling resistance of concrete. Cem Concr Res 2005;35(12):2360–3. [16] Sun W, Mu R, Luo X, Miao CW. Effect of chloride salt, freeze–thaw cycling and externally applied load on the performance of the concrete. Cem Concr Res 2002;32(12):1859–64. [17] GB/T 50081–2002. Standard for test method of mechanical properties on ordinary concrete. Chinese standard Institution; 2002. [18] CECS13:89. Test methods used for steel fiber reinforced concrete. Chinese standard Institution; 1989. [19] GBT50082-2009. Standard for test method of long-term performance and durability of ordinary concrete; 2009. [20] John JV, George WS. A review of salt scaling: I. Phenomenology. Cem Concr Res 2007;37:1007–21. [21] Mu R, Miao CW, Luo X, Sun W. Interaction between loading, freeze–thaw cycles, and chloride salt attack of concrete with and without steel fiber reinforcement. Cem Concr Res 2002;32(7):1061–6. [22] Dvorkin L, Nwaubani S, Dvorkin O. Construction materials. Nova Science Publishers; 2010. 409. [23] Day KW. Concrete mix design, quality control, and specifications. 3rd ed. New York: Taylor & Francis; 2006. 214. [24] Powers TC. The air requirement of frost-resistance concrete. Proc Highway Res Board 1949;29:184–202. [25] Powers TC. A working hypothesis for further studies of frost resistance of concrete. J Am Concr Inst 1945;16(4):245–71. [26] Holschemacher K, Mueller T, Ribakov Y. Effect of steel fibres on mechanical properties of high-strength concrete. Mater Des 2010;31(5):2604–15. [27] Salah U, Dulaijan Al. Sulfate resistance of plain and blended cements exposed to magnesium sulfate solutions. Constr Build Mater 2007;21(8):1792–820. [28] Butler M, Mechtcherine V, Hempel S. Experimental investigations on the durability of fibre–matrix interfaces in textile-reinforced concrete. Cem Concr Compos 2009;31(4):221–31.
Contents lists available at SciVerse ScienceDirect
Materials and Design journal homepage: www.elsevier.com/locate/matdes
Study of the performance of steel fiber reinforced concrete to water and salt freezing condition Ditao Niu, Lei Jiang ⇑, Min Bai, Yuanyao Miao Xi’an University of Architecture and Technology, Xi’an, Shaanxi 710055, PR China
a r t i c l e
i n f o
Article history: Received 26 April 2012 Accepted 29 July 2012 Available online 13 August 2012 Keywords: B. Steel fiber A. Concrete E. Frost resistance
a b s t r a c t Properties of plain concrete and steel fiber reinforced concrete (SFRC) (with volume fraction of 0.5%, 1%, 1.5% and 2%) subjected to freeze–thaw cycles in water and in the 3.5% NaCl solution were investigated in this paper. Through the experiment, surface damage, weight loss and splitting tensile strength loss of SFRC were measured after different numbers of freeze–thaw circulations. The microstructure and the pore structure of SFRC were analyzed on the basis of scanning electron microscope (SEM) and mercury intrusion experiment. The test results show that the use of steel fiber could improve the pore structure and decelerate the damage of concrete during freeze–thaw cycles. However, the ability of steel fiber to reduce surface scaling of concrete is limited subjected to freeze–thaw cycles in the NaCl solution. Furthermore, the weight loss and the splitting tensile strength loss of concrete tested in the NaCl solution were larger than those in water. It is also shown that the steel fiber content has the great influence on the frost-resisting property of SFRC. When a relatively steel fiber content is introduced (1.5 vol.%), the deterioration process of concrete subjected to the frost damage is considerably reduced. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction In cold regions, the deterioration of concrete resulting from freezing–thaw damage has great impact on the durability and service life of concrete structures [1,2]. The decrease of concrete structural durability caused by freeze–thaw cycles ultimately leads to the formation and development of cracks in the concrete. Incorporation of steel fibers is the effective way to improve crack resistance behavior and consequently the ductility and fracture toughness of concrete, it can be explain that the steel fibers are able to transfer emerging loads by bridging the cracks [3–5]. For most structural and non-structural purposes, steel fiber is the most commonly used of all the fibers [6]. SFRC is a multiphase composite material which added disordered distribution of short steel fibers in plain concrete. With the development of economy and technology, SFRC has been applied widely in the engineering construction fields step by step. Numerous works for evaluating mechanical properties of SFRC have been reported, it now has been well accepted that incorporation of steel fiber can significantly improve the mechanical behaviors of concrete. Düzgün et al. [7] investigated the effect of steel fibers on the mechanical properties of pumice aggregate concrete, ⇑ Corresponding author. Address: College of Civil Engineering, Xi’an University of Architecture and Technology, No. 13 Yanta Road, Xi’an, Shaanxi 710055, PR China. Tel.: +86 136 0912 6920. E-mail addresses: [email protected], [email protected] (L. Jiang). 0261-3069/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.matdes.2012.07.074
declaring that the compressive strength, splitting tensile strength and flexural strength of concretes increased up to 21.1%, 61.2% and 120.2%, respectively. Song and Hwang [8] studied the mechanical properties of HSC, indicating that the compressive strength reached a maximum at 1.5 vol.% fraction, being a 15.3% improvement over the HSC. The splitting tensile strength improved with increasing the volume fraction, achieving 98.3% at 2 vol.% fraction. Amr and Dieb [9] investigated the compressive strength and splitting tensile strength of UHSC, indicating that improvement increases as the fiber volume fraction increases. According to Marar et al. [10], the compressive strength of HSFRC improved with the increase in fiber volume. And Daniel and Loukili [11] declared, the compressive strength of HSFRC held 15% advantage over the HSC. Incorporation of fibers into concrete is not only an effective way to improve concrete mechanical behaviors, but also durability. Pigeon et al. [12,13] obtained that the use of steel and particularly carbon microfibers improves the frost and deicer salt scaling resistance of mortars. Cantin and Pigeon [14] indicated that steel fibers have no significant influence on the deicer-scaling resistance of concrete. Yang and Zhu [15] reported that the deicer-scaling resistance of concrete is reduced by the addition of steel fibers at the same air content, especially for the air-entrained concrete. According to Sun et al. [16], steel fiber could retard the performance deterioration of the concrete and improve the resistance against multidamaging under severe conditions. From the literatures, it is obvious that most research has focused on the mechanical properties and little information available
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D. Niu et al. / Materials and Design 44 (2013) 267–273
Table 1 Mix proportions of concrete (kg/m3). Mix code
Cement
Sand
Coarse aggregate
Water
Steel fiber
Water reducer (%)
B1 B2 B3 B4 B5
367 367 367 367 367
765 735 733 718 702
1146 1102 1100 1066 1053
165 165 165 165 165
0 39 78 117 156
0.6 0.6 0.6 0.6 0.6
Table 2 Mechanical properties of concrete mixtures at 28 days. Concrete
Cube compressive strength (MPa)
Splitting tensile strength (MPa)
Flexural strength (MPa)
B1 B2 B3 B4 B5
37.9 38.3 39.5 40.7 35.2
6.1 8.1 9.2 11.0 7.2
5.03 5.34 6.53 7.16 9.77
concerning the freeze and thaw durability of SFRC. Furthermore, microstructural features including the pore structure and the microcrack characteristics of SFRC under the action of freeze–thaw needs more investigate. In this paper, the volume fraction of steel fiber and number of freeze–thaw circulation were taken as variable parameter. Basic experimental research that the performance of SFRC to water and salt freezing condition was conducted on the basis of the macroscopic and microscopic test. Furthermore, the mechanism of SFRC such as reinforcement, damage and cracking resistance under the action of freeze–thaw was analyzed.
Fig. 3. Weight loss in concrete subjected to freeze–thaw cycles. (a) In water and (b) in the NaCl solution.
2. Experimental details 2.1. Materials and mix proportions
Fig. 1. Strength increased rate of SFRC.
A Chinese standard Portland cement produced by Cement Factory of TongChuan was adopted, river sand with fineness modulus of 2.69, and coarse aggregate of crushed basalt stone with diameter of 5–16 mm were used in the test. A naphthalene-type superplasticizer was used, and the dosage was adjusted to keep the slump of fresh mixed plain concrete in the range of 50–120 mm. A steel fiber with a rectangular cross section, an aspect ratio of 60, and a length of 30 mm was adopted to prepare SFRC. In this experiment, the water-binder ratio (W/B) was 0.45. A series of plain concrete (B1) and SFRC with additions of 0.5%, 1%, 1.5% and 2% steel fiber were prepared (denoted as B1, B2, B3, B4, B5 respectively). The proportions and basic characteristics of the various concrete mixes are given in Table 1.
Fig. 2. Surface damage of B3 subjected to freeze–thaw cycles in the NaCl solution.
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where fcc is the compressive strength (MPa), F is the maximum load (N), A is the area of the cube loading face (mm2).
2.2. Specimens preparation and curing conditions The components of SFRC mixture were batched by weight, the steel fiber were premixed with cement, sand and coarse aggregate before adding the water and the admixtures for 1 min, then, the entire amount of mixing water with the dissolved superplasticizer was added and mixed for 3 min, finally, SFRC mixture was mixed for an additional 2 min. The concrete specimens were cast in steel moulds and compacted on a vibration table. All samples were cured at 20 °C in molds covered by a polyethylene film to prevent moisture loss according to GB/T 50081-2002 [17], the specimens were demolded after 24 h of casting and cured in a condition of 20 ± 3 °C and 95% relative humidity until the age of testing. 2.3. Test methods 2.3.1. Mechanical properties The mechanical properties measurements were carried out according to CECS13:89 [18]. The specimens were cured in a humidity room for 28 days. For each mixture, 6 specimens of 100 mm cubes were used to determined the compressive and splitting tensile strength (three cubes for compressive strength test and three for splitting tensile strength test), 3 specimens of 100 mm 100 mm 400 mm prisms were used to determined the flexural strength. All experiments were performed on three specimen replicates. The average values are used in the discussion of the test results. The following Eqs. (1)–(3) were used to calculate the compressive strength, splitting tensile strength and flexural strength.
fcc ¼ F=A
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ð1Þ
Fig. 4. Effect of steel fiber content on weight loss. (a) In water and (b) in the NaCl solution.
fts ¼ 2F=pA ¼ 0:637F=A
ð2Þ
where fts is the splitting tensile strength (MPa), F is the maximum load (N), A is the area of the cube splitting face (mm2). 2
ff ¼ Fl=bh
ð3Þ
where ff is the flexural strength (MPa), F is the maximum load (N), l is the distance between the supporting rollers (mm), b is the width of the cross-section (mm), h is the height of the cross-section (mm). 2.3.2. Freezing and thawing cycle tests The specimens for the freeze–thaw cycles tests in gap water or in the NaCl solution were cured for 24 days, and then were immersed in gap water or in a 3.5 mass% sodium chloride solution, respectively, for 4 days. At the age of 28 days, testing of concrete exposed to freeze–thaw cycles in gap water or in the NaCl solution were carried out in accordance with CECS13: 89 [18] and GBT50082-2009 [19] separately. In this paper, the concentration of 3.5% NaCl solution was used [20,21]. For each mixture, 6 specimens of 100 mm 100 mm 400 mm prisms and 45 specimens of 100 mm cubes were used for the study of weight loss and splitting tensile strength at every 25 times of freeze–thaw circulations. The temperature of concrete samples was controlled by a Pt sensor embedded in the center of a concrete sample. The temperature of the sample center ranged from 17.0 ± 1 to 6.0 ± 1 °C. According to the test procedure, the deterioration of the specimens was investigated by determining the weight loss, the
Fig. 5. Splitting tensile strength loss in concrete subjected to freeze–thaw cycles. (a) In water and (b) in the NaCl solution.
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thaw cycles (kg), Wn is the average weight of concrete specimens at every 25 freeze–thaw cycles in water or in the 3.5% NaCl solution (kg). All experiments were performed on three specimen replicates. The average values are used in the discussion of the test results. 2.3.3. Microcosmic-test The microstructural features of the concrete were analyzed on the basis of scanning electron microscope (SEM) and mercury intrusion experiment. This investigation helps to understand the mechanical properties and durability performance of the SFRC. 3. Test results and discussion 3.1. Mechanical properties test results Table 2 and Fig. 1 gives the test results about compressive strength, splitting tensile strength, and flexural strength of five kinds of concrete in 28 days-curing cycles. By increasing the fiber content from 0% to 1.5%, the 28 day cube compressive strength, the splitting tensile strength and the flexural strength increased from 1.06% to 7.39%, 32.79% to 80.33% and 6.16% to 42.35% respectively, compared with B1. Steel fiber significantly improves the mechanical properties of SFRC compared with ordinary concrete. However the effect of steel fiber improve the splitting tensile strength and the flexural strength is better than the compressive strength. These results are in agreement with those reported in literature [22,23], which show that adding steel fibers to concrete composition increases the splitting tensile strength 1.5–2 times and lead a slight increase in the compressive strength values (up
Fig. 6. Effect of steel fiber content on splitting tensile strength loss. (a) In water and (b) in the NaCl solution.
Table 3 Pore structure parameter of SFRC. Total porosity (%)
Total pore volume (ml/g)
Total pore area (m2/g)
Critical pore size (nm)
Average pore size (nm)
Most probable pore diameter (nm)
B1 B2 B3 B4 B5
15.22 14.17 12.69 10.33 14.60
0.0785 0.0664 0.0620 0.0561 0.0681
23.63 19.39 17.04 13.52 18.21
40.31 20.24 15.73 12.47 34.98
52.14 25.85 18.01 13.80 40.45
59.37 30.57 21.60 15.23 46.82
Fig. 7. Pore size distribution histogram.
Table 4 Pore size distribution of SFRC. Code
B1 B2 B3 B4 B5
Pore size volume fraction (%) d < 20 nm
20 nm 6 d < 50 nm
50 nm 6 d < 200 nm
d P 200 nm
13.14 43.24 52.39 80.01 21.39
43.70 40.35 30.29 12.41 44.40
38.45 14.02 10.32 3.09 29.35
4.30 2.15 4.51 2.62 3.74
specimen fails if its weight loss exceeds 5%, which was calculated as follows.
DW n ¼ ½ðW 0 W n Þ=W 0 100
ð4Þ
Log differential intrusion (ml/g)
Code
0.4 B1 B2
0.3
B3 B4 B5
0.2
0.1
0
1
10
100
Pore size diameter (nm) where DWn is the loss of specimens at every 25 freeze–thaw cycles (%), W0 is the average weight of concrete specimens before freeze–
Fig. 8. Pore size distribution differential curve of SFRC.
1000
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Fig. 9. Microstructure observation of B1 subjected to freeze–thaw cycles in the NaCl solution: (a) 0 cycles, (b) 50 cycles, (c) 100 cycles.
Fig. 10. Microstructure observation of B4 subjected to freeze–thaw cycles in the NaCl solution: (a) 0 cycles, (b) 50 cycles, (c) 100 cycles.
to 10–25%). With a 2 vol.% fraction of steel fiber, the distribution of steel fiber in concrete is not uniform and a reunion phenomenon occurred, which weaken the interfacial bond strength instead of reinforcing it.
more, the coarse aggregate exposed in some severely scaled specimens and even peeled off as freeze–thaw cycles increases. Similar studies reported in literature [14,15], which reported that the ability of steel fiber to reduce surface scaling of concrete is limited when subjected to freeze–thaw cycles in the NaCl solution.
3.2. Surface damage 3.3. Weight loss As can be seen from the Fig. 2, surface scaling of SFRC is damaged seriously due to freeze–thaw cycles in the NaCl solution. In the first several freeze–thaw cycles, surface layer of mortar is destroyed, and the surface of concrete becomes uneven. Further-
As can be seen from the Fig. 3a, the weight loss rate of SFRC is lower than plain concrete when subjected to the action of freezing–thawing. For B4, the weight loss is 2.28% after 300 freez-
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weight loss of SFRC subjected to freeze–thaw cycles. The weight loss rate can be inhibit effectively when the steel fiber content is 1.5%, but the weight loss of concrete increased with 2% steel fiber content. The weight loss of concrete specimens is caused by the scaling of concrete surface. Scaling due to freezing and thawing cycles is a progressive phenomenon which slowly destroys successive surface layers of concrete. The randomly scattered steel fiber has no effect on the disintegrated slurries in surface layers and could not prevent the scaling off the concrete surface. 3.4. Splitting tensile strength loss As shown in Fig. 5a, the splitting tensile strength decreased by 40% after 150 cycles and 250 cycles respectively for B1 and B4, but B1 was completely destroyed after 200 cycles. Fig. 5b shows that the NaCl solution accelerate the splitting tensile strength loss rate of SFRC. At 100 freeze–thaw cycles, the splitting tensile strength loss of B1 and B4 are 34% and 22% in the NaCl solution, and 23%, 9% in water. It can be concluded that the cracking resistance of SFRC has been weakened and the interior damage has been greatly accelerated when subjected to freeze–thaw cycles in the NaCl solution. Fig. 6 shows that the splitting tensile strength loss improved with increasing the volume fraction, reached a best performance at 1.5%, and the damage rate of SFRC under the action of freeze–thaw cycles has been reduced significantly. But the splitting tensile strength loss of concrete increased when the steel fiber content is 2%. 3.5. Microstructural features
Fig. 11. Microstructure observation of fiber–cement matrix interfaces of SFRC subjected to freeze–thaw cycles in the NaCl solution: (a) Ca(OH)2 crystals, (b) ettringite crystals and (c) microcracks in fiber–cement interfaces.
ing and thawing cycles, which is almost half of that of B1 after 225 cycles. Fig. 3b shows that the weight loss rate of SFRC in the NaCl solution is much greater than that in water. After 100 freeze–thaw cycles, the weight loss of B1 and B4 are 2.1%, 1.3% in water and 4.2%, 2.5% in the NaCl solution. The results are in agreement with those reported in literature [16,21], which show that the weight loss in the NaCl solution is twice as large as those in water, and steel fiber has a little effect on the weight loss of concrete when subjected to freeze–thaw cycles in the NaCl solution. Fig. 4 shows that the steel fiber volume fraction has the great influence on the
3.5.1. Pore structure As can be seen from Table 3, the pore structure has been improved when adding the amount of steel fibers to the concrete. Compared with B1, the total porosity, total pore volume and total pore area of B4 are reduced by 32.13%, 28.54% and 42.78% respectively. Furthermore, the average pore size and most probable pore diameter of SFRC also decrease. The results are in a good agreement with what were suggested by Pigeon et al. [12,14]; however, it disagree with the results reported by Yang and Zhu [15]. The pore structure parameters of SFRC increase when the content reaches to 2%, thus making the air-voids in concrete become coarser. It is an important cause for the reduction of the mechanical property and the frost resistance of B5. Table 4 and Fig. 7 show that, harmless hole increases and harmful hole decreases as the volume fraction of steel fiber ranging from 0% to 1.5%. The volume percentages of the hole aperture (d < 20 nm and 20 nm 6 d < 50 nm) increase greatly, especially in the case of (d < 20 nm), where the hole aperture increases obviously. The volume percentage of the hole aperture (50 nm 6 d < 200 nm and d > 200 nm) decrease obviously. As shown in Fig. 8, the peak value of the mercury injection curvilinear, which means the most probable pore diameter, moves to the direction of the small pore when the steel fiber content increases from 0% to 1.5%. However, the most probable pore diameter enlarges gradually as the content increases from 1.5% to 2%. Experiment results revealed that with a appropriate amount of steel fibers (0–1.5 vol.%), the pore structure of concrete can be greatly improved, therefore enhancing the mechanical property as well as the frost resistance of concrete. 3.5.2. SEM test results It is clearly seen from the Figs. 9 and 10 that the cement hydration products form with each other in a continuous phase with no microcracks before freeze–thaw cycles. After 50 times of freeze– thaw cycles, the microcracks can be clearly observed from the micrographs, but the cracks in B1 not only appear to be much wider than B4 but also interconnect with each other. Besides, the
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density of microcracks increases in B1 is more obvious than B4 after 100 cycles. It is known that concrete has to bear hydraulic pressure and osmotic pressure when subjected to freeze–thaw cycles [24,25]. When the tensile stress generated by the two kinds of pressure exceeds the tensile strength of concrete, crack may occur. However the high elastic modulus and strength of steel fiber enhances the cracking resistance of concrete [26]. The addition of steel fiber reduces the rate of crack propagation and improve the frost-resisting property of concrete. There exists the sheet structure of Ca(OH)2 (Fig. 11a) and the cluster structure of ettringite crystals (AFt) (Fig. 11b) in the interfacial transition zone of SFRC. They mainly exist in pores or on the aggregate surface in concrete [27]. This shows that the fiber–cement matrix interface has a high porosity and porous network structure. Microcracks generated in the fiber-cement matrix interface appear to be much wider (Fig. 11c). Therefore, the interface is the sensitive area that microcracks occur and propagate when subject to freeze–thaw cycles. It is also reported that the fiber–cement matrix interface is the weak link of SFRC [28]. The excessive quantity of steel fiber increase the fiber–matrix interface in concrete, which weaken the interfacial bond strength as well as frost-resisting property of concrete. As a result, B5 has a low frost-resist quality compared with other SFRC with different contents in the freezing and thawing process. 4. Conclusion When the plain concrete was subjected to the freeze–thaw cycles, the splitting tensile strength dropped sharply and the weight losses increased considerably, severe scaling occurring on concrete surfaces. Steel fiber could reduce the rate of crack propagation and retard the performance deterioration of the concrete. Furthermore, adding the appropriate amount of steel fibers (0–1.5 vol.%), the pore structure of concrete can be greatly improved, therefore enhancing the mechanical property as well as the frost resistance of concrete. However, the ability of steel fiber to reduce surface scaling of concrete is limited and the deterioration in the concrete is obviously accelerated when subjected to freeze–thaw cycles in the NaCl solution. Besides, the numbers of freeze–thaw cycles at failure in the NaCl solution were significantly lower than those in water. The steel fiber volume fraction has the great influence on the frost-resisting property of SFRC. When a relatively steel fiber content is introduced (1.5 vol.%), the deterioration process of concrete subjected to the frost damage is considerably reduced. However, the frost-resisting property of concrete is significantly decreased with 2% steel fiber content. Acknowledgements This project was supported by the National Excellent Young Scientists Fund (Chinese, No. 50725824) and supported by
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