SCIENCE CHINA Technological Sciences • RESEARCH PAPER •
November 2012 Vol.55 No.11: 3102–3108 doi: 10.1007/s11431-012-5027-y
Effect of ground granulated blast-furnace slag (GGBFS) and silica fume (SF) on chloride migration through concrete subjected to repeated loading ZHANG WuMan1* & BA HengJing2 1
School of Transportation Science and Engineering, Beihang University, Beijing 100191, China; 2 School of Civil and Engineering, Harbin Institute of Technology, Harbin 150006, China Received March 12, 2012; accepted August 2, 2012; published online September 28, 2012
The effect of ground granulated blast-furnace slag (GGBFS) and silica fume (SF) on the chloride migration through concrete subjected to repeated loading was examined. Portland cement was replaced by 20%, 30%, 40% GGBFS and 5%, 10% SF, respectively. Five times repeated loadings were applied to specimens, the maximum loadings were 40% and 80% of the axial cylinder compressive strength ( fc ), respectively. Chloride migration through concretes was evaluated using the rapid chloride migration test and the chloride concentration in the anode chamber was measured. The results indicate that the transport number of chloride through concrete containing 20% and 30% GGBFS replacements and 5% and 10% SF replacements is lower than that of the control concrete, but 40% GGBFS replacement increases the transport number of chloride. Five loadings at 40% fc or 80% fc increase the transport number of chloride for all mixes investigated in this study. 5% SF replacement has a very close effect on the chloride permeability of concrete with 20% GGBFS when concrete is subjected to 40% fc or 80% fc . concrete, ground granulated blast-furnace slag (GGBFS), silica fume (SF), chloride migration, repeated loading Citation:
Zhang W M, Ba H J. Effect of ground granulated blast-furnace slag (GGBFS) and silica fume (SF) on chloride migration through concrete subjected to repeated loading. Sci China Tech Sci, 2012, 55: 31023108, doi: 10.1007/s11431-012-5027-y
1 Introduction Chloride ion penetration into concrete and the resulting deterioration (cracking and spalling due to the corrosion of reinforcement) is a major concern to the durability and service life of a reinforced concrete structure [1–4]. Chloride-induced corrosion of reinforcement has been identified as one of the most predominant degradation mechanisms in reinforced concrete structures [5]. It has been found that corrosion can begin with a chloride ion content of only 593.3 to 949.3 g m3 at the level of the steel in the concrete
[6]. Chloride attack is considered as one of the most important factors affecting the service life of concrete structures [7]. Ground granulated blast-furnace slag (GGBFS) and silica fume (SF) have been used for many years as a partial replacement of Portland cement to improve the mechanical and durability properties of concrete [8–15]. Ozyildirim and Halstead [16] concluded that concrete containing slag had strength similar to the controls but had lower chloride permeability. Concrete containing slag exhibited higher resistance to chloride ion penetration than concrete without slag, as reported by Fukute et al. [17], Geiseler et al. [18], Detwiler et al. [19], Yeau and Kim [20]. Vejmelková et al. [21] showed that a lower GGBFS re-
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placement as environmentally more friendly and still very valuable alternative binder either affected positively or at least did not worsen the substantial properties of the hardened concrete mix. Gesoğlu and Güneyisi [22] showed that SF may be considered as a remedy to enhance the chloride penetration resistance of the rubberized concretes. Shekarchi et al. [15] reported that partial cement replacement with up to 7.5% silica fume reduced the diffusion coefficient, whereas for higher replacement rates the diffusion coefficient did not decrease significantly. Toutanji et al. [23] also observed the similar result. Bentz [24] concluded that addition of 10% SF may reduce the diffusivity of concrete to chloride ions by more than 15 times. Saito and Ishimori [25] found that the application of static loading up to 90% of the ultimate strength had a little effect on the chloride permeability. However, a repeated loading at the maximum stress levels of 60% or more caused the chloride permeability to increase significantly. Samaha and Hover [26] reported that the chloride permeability of concrete was generally not affected after one cycle of the uniaxial compressive loading to below 75% of the maximum load capacity of the concrete. However, for higher load levels, chloride permeability could be enhanced by as much as 20%. Cao et al. [27] concluded that a moderate compressive load reduced the coefficient of chloride penetration, while it was enhanced by higher mechanical loads as microcracks were developed further. Concrete structures usually are subjected to different environmental conditions and external loadings at the same time [28–31]. Service load may cause microcracks in concrete. Determination of whether service load has a significant impact on transport properties could have an impact on design and construction procedures in regard to limiting service load stresses [26]. This information could also be useful in predicting changes in transport properties over the service life of the reinforced concrete, especially for rein-
forced concrete in chloride environment. The objective of this study is to evaluate the effect of GGBFS and SF on chloride migration through concrete subjected to repeated loading.
2 Experimental program 2.1
C
Mix proportions of concrete with GGBFS and SF SF Cement GGBFS Fine aggregate (kg m3) (kg m3) (kg m3) (kg m3) 450 – –
GS20
360
90
GS30
315
135
GS40
270
180
– – –
SF5
427.5
–
22.5
Materials
For making concrete specimens, ordinary Portland cement (OPC 42.5), GGBFS and SF complying with Chinese standard GB/T 18046-2008 were used. Their chemical and physical properties were given in Table 1. River sand with fineness modulus of 2.82 and apparent density of 2610 kg m3 was prepared. Saturated surface crushed diabase with maximum size of 5 mm and apparent density of 2690 kg m3 was used as the coarse aggregate. M-100 naphthalene based superplasticizer was used to obtain the desired workability. 2.2
Mix proportions and samples preparation
The concrete mixes were produced with a binder content of 450 kg m3 and a sand to aggregate ratio of 0.40. The water to binder ratio was maintained at 0.35 for all mixes. Portland cement was replaced by 20%, 30%, 40% GGBFS and 5%, 10% SF, respectively. The superplasticizer dosage was adjusted to maintain a slump value of 200 mm± 20 mm for all the investigated mixtures, and the superplasticizer used in its powder form was systematically mixed with the solid materials in preparing mixtures investigated in this paper. The mix proportions of the concrete were shown in Table 2. Up to about 10% of the mixing water was placed in the drum before the solid materials were added. Water then was added uniformly with the solid materials; after mixing, a number of cylindrical specimens were cast for each mix.
Table 1 Chemical compositions and physical properties of cement, GGBFS and SF Fe2O3 MgO SO3 Al2O3 CaO SiO2 (%) (%) (%) (%) (%) (%) OPC 62.28 21.08 5.47 3.96 1.73 2.63 GGBFS 42.49 38.61 6.72 0.40 6.71 0.80 SF 0.2 93.7 0.3 0.8 0.2 0.5 a) R2O=Na2O+K2O. Table 2
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R2Oa) (%) 0.80 0.70 0.3
Coarse aggregate (kg m3)
Loss (%) 1.61 2.53 3.7
Density (g cm3) 3.17 2.86 2.26
Superplasticizer (kg m3) 2.7
Specific area surface (m2 kg1) 335 501 15000
Water (kg m3)
W/B
157.5
0.35
2.6 737
1105.5
2.5 3.0 3.2
SF10 405 45 3.3 – Note: W/B is the ratio of water to binder; C is the control concrete; GS20, GS30 and GS40 are the concrete mixed with 20%, 30% and 40% GGBFS, respectively; SF5 and SF10 are the concrete mixed with 5% and 10% SF, respectively.
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The diameter of the cylinders is 95 mm and the height is 300 mm. A vibrating table was used to ensure good compaction. The specimens’ surface was then smoothed, and plastic film was used to cover concrete. After demoulding (24 h after casting), the specimens were cured at a constant temperature 20oC for 28 d in an alkaline solution to ensure the saturation of specimens and to avoid leaching phenomena [32]. Three cylinders for each mix were used to determine the compressive strength at 28 d. Another two cylinders were subjected to five-cycles repeated load by a 200 kN full automatic testing machine (WHY series). Five repeated cycles were used in this paper because that they were the maximum number of setting cycles of the WHY testing machine. Although there is a difference between five repeated cycles and the actual condition, the results obtained in this study can still be referenced for the related research. Kermani [33] found that the water permeability was related to the applied stress and identified a threshold stress approximately 40% of the ultimate strength. Choinska et al. [34] observed a significant increase in gas permeability only beyond 80% of the ultimate stress. Hearn [35] obtained a significantly higher threshold than others –- at 80% of peak stress–for the water permeability. It had been revealed that bond cracks began to increase in length, width, and number at about 30%–50% of the ultimate strength. Mortar cracks began to increase noticeably and to form continuous crack patterns at about 70%–90% of the ultimate strength, when concrete was subjected to an increasing compressive load [25]. Uchikawa et al. [36] found that the crack revealed generally at 60% of the fracture stress, and it grew rapidly, exceeding at 80% of the fracture stress. The growth of the cracks in the hardened body was more conspicuous in repeated loading than in single loading. Therefore, 40% f c and 80% f c were selected as the maximum repeated loadings in this study. A rich layer of cement paste at the top and a rich layer of aggregate at the bottom of concrete specimens will occur during the vibration, which is caused by the settlement of solid particles and the simultaneous upward migration of water [37]. In addition, both ends of the specimen are not in a uniform uniaxial compression because of the constraints of the testing machine up and down platens to concrete specimens. Therefore, concrete disks cut off from both ends of the specimen will have a wall effect on the test results. In this paper, three concrete disks (95 mm in diameter and 10 mm in thickness) are cut off from the middle part of the cylindrical specimens with a diamond blade saw for chloride migration tests, which does not introduce any excessive cracks [38]. 2.3
Setup of chloride migration test
In this study, an equipment of chloride migration test was used, as shown in Figure 1.
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Figure 1 Equipment of chloride migration test. 1, Upstream chamber; 2, downstream chamber; 3, sample; 4, electrode; 5, pick devices.
The upstream chamber contained 1.5 L of 0.5 mol L1 NaCl and 0.3 mol L1 NaOH. The upstream chamber was kept large to prevent the build-up of hydroxyl ions and the depletion of chloride ions. The experiment created a near constant concentration of migrating anions at concentrations somewhat representative of real cement pore solution. The downstream chamber contained 0.6 L of 0.3 mol L1 NaOH dissolved with distilled water, which was to prevent chlorine gas occurrence due to the decrease of pH [39]. The volume of the downstream chamber was kept small to allow early detection of chloride break-through from the sample, but still large enough to avoid significant electrolyte concentration changes. The graphite electrodes were selected to reduce the risk of oxidation and corrosion [40]. A 12 V voltage was controlled between two sides of the sample during testing, which did not make the solutions temperature high [41]. A PClS-10 chlorine meter with a measuring range of 5.0×105 mol L1–1.0 mol L1 was used to measure the chloride concentration. For the chloride migration test, three specimens (95 mm in diameter and 10 mm in thickness) were used and the lateral surface of the samples was covered with epoxy resin to ensure a unidirectional transfer. The mean values of chloride concentration were reported.
3 Experimental results and discussion 3.1
Strength of concrete
The compressive strength values of concrete investigated in this study are summarized in Table 3. The cylinder compressive strength of the 20% and 30% GGBFS mixes was greater than that of the control concrete Table 3 Compressive strength values of concrete cured for 28 d Compressive strength (MPa) Mean value (MPa) 1 2 3 C 45.1 44.6 42.9 44.2 GS20 47.6 48.4 50.1 48.7 GS30 52.1 49.3 48.6 50.0 GS40 25.8 26.5 26.9 26.4 SF5 53.3 54.7 53.9 54.0 SF10 57.1 56.7 61.9 58.6
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at 28 d, whereas 40% GGBFS replacement resulted in a 40.3% strength reduction. For 20% and 30% GGBFS replacements, the unreacted slag particles in the paste may act as micro-aggregates with higher modulus of elasticity, increasing the resistance to crack propagation. Also, cracking around the slag particles required more energy to be dissipated before failure [42, 43]. However, the micro-aggregate effect of the unreacted slag particles was insufficient to compensate for the lower concentration of cement hydration [44] when the replacement of cement by GGBFS was 40%. Therefore, the strength of concrete mixed with 40% GGBFS was significantly lower than that of the control concrete. In addition, for concrete with high replacement levels of slag, curing time will play a critical role in realizing the full potential of concrete in terms of strength and especially durability characteristics [11]. It can be seen that the strength of concrete at 28 d increased by 22.2% and 32.6% at average for 5% and 10% SF replacements, respectively. The main mechanisms involved in improvement of strength by SF were (1) strength enhancement by pore size refinement and matrix densification, (2) strength enhancement by reduction in content of Ca (OH)2, and (3) strength enhancement by cement pasteaggregate interfacial refinement seems to be responsible for the strength development capability of SF [45, 46]. 3.2
Deformation of concrete
During the repeated loading test, a continuous stress-strain curve can be automatically given. The maximum deformation of a specimen can be obtained when the applied load is equal to 40% f c or 80% f c . The residual deformation of a specimen can be obtained when five loading cycles finish and the load is unloaded to zero. In this study, the maximum deformation and the residual deformation are defined as shown in eqs. (1) and (2).
Dmax ( L0 Lmin ) 10 6 L0 ,
(1)
Dr ( L0 Lunloading ) 10 6 L0 ,
(2)
where Dmax is the maximum deformation, Dr is the residual deformation, L0 is the initial length of a specimen, Lmin is the minimum length of a specimen subjected to cycle loading, Lunloading is the length of a specimen after the repeated test. Figure 2 shows the deformations of concrete subjected to the repeated loading. Compared with the control concrete, the maximum deformations of concrete at 28 d decreased by 9.7% and 17.3%, and the residual deformations by 24% and 36% on average for 20% and 30% GGBFS replacements, respectively, when the repeated loading was 40% f c . However, the maximum and residual deformations of concrete subjected to 40% f c increased by 66.4% and 240%, respectively, for a 40% GGBFS replacement. There was a
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similar trend for GGBFS concrete when the repeated loading was raised to 80% f c . The maximum deformations of concrete at 28 d decreased by 6.9% and 11.1%, and the residual deformations by 4% and 12% at average for 5% and 10% SF replacements when the repeated loading was 40% f c . The deformations of SF concrete had a significant increase when the repeated loading was raised to 80% f c . Compared with the control concrete, the maximum deformations of concrete at 28 d decreased by 6.8% and 23.5%, and the residual deformations by 27.8% and 42.6% at average for 5% and 10% SF replacements when the repeated loading was 80% f c . This result is due to the combination of the filling action and the pozzolanic reaction [47]. Detwiler and Mehta [48] reported that at an age of 7 days, the influence of SF on the compressive strength of concrete may be attributed mainly to physical effects. By the age of 28 days, both physical and chemical effects became significant. 3.3
Chloride migration test
The results of chloride migration tests of concrete are shown in Figure 3. The concentration of chloride migration through concrete showed an approximate linear relationship and increased with the testing time after a different stage. This indicated that the mass flux in all the regions perpendicular to migration direction was constant. The concentration of chloride migration through concrete slightly decreased when the GGBFS replacement was 20% or 30%. However, an obvious increase was observed for 40% GGBFS replacement. For concrete mixed with SF, the chloride concentration migration through concrete was lower than that of the control concrete, and decreased with the increment of SF replacement. In addition, 5% SF replacement had a similar effect on the chloride permeability of concrete to 20% or 30% GGBFS.
Figure 2 80% fc .
Deformations of concrete subjected to (a) 40% fc and (b)
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Figure 3
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Chloride migration test of concrete.
Figure 4 shows the chloride migration tests of concrete subjected to 40% f c . The chloride concentration migration through these concretes subjected to 40% f c was greater than that of the corresponding nonloaded concretes. A further increment of chloride concentration migration through these concretes was found when the repeated loading was raised from 40% to 80% f c , as shown in Figure 5. The effect of GGBFS and SF on chloride migration through concrete subjected to 40% and 80% f c was similar to that of the nonloaded concretes. 5% SF replacement had a very close effect on the chloride permeability of concrete as 20% GGBFS had when concrete was subjected to 40% f c or 80% f c . 3.4
Discussion
The steel rebar in concrete is susceptible to corrosion when the concentration of chloride at the steel surface exceeds a critical value [4]. Therefore, the amount of chloride migration through concrete cover is an important factor for predicting the service life of concrete structures exposed to chloride in the environment.
Figure 4
Chloride migration test of concrete subjected to 40% fc .
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Figure 5
Chloride migration test of concrete subjected to 80% fc .
The transport number of chloride migration through concrete tested for 168 h is shown in Figure 6. It can be noted that GGBFS and SF had a significant effect on the transport number of chloride migration through the concretes. Compared with the control concrete, the transport number of chloride migration through concretes cured for 28 d decreased by 43.3% and 47.9% for 20%, 30% GGBFS replacements, respectively. These decreases may have occurred because the pozzolanic reaction of GGBFS in concrete at an early age may form more C-S-H gel and 3CaO·Al2O3, which can reduce the pore sizes and cumulative pore volume [19, 44, 49–51]. In addition, C-S-H gel can bind more chloride ions and block the diffusing path, reducing the permeability of the concrete. The number of total ions of Ca2+, Al3+, AlOH2+, and Si4+ in GGBFS concrete is greater than that in pure Portland cement concrete, and the ion concentration of GGBFS concrete is higher than that of the control concrete. Ions with a lower diffusing ability may restrict the movement of chloride ions [52–54]. However, the transport number of chloride increased by 26.6% for 40% GGBFS replacement compared with the control concrete. This is possibly because that the calcium hydroxide provided by cement is not sufficient for full pozzolanic reaction when the slag replacement is at a high level [44, 55]. The transport number of chloride migration through concretes cured for 28 d decreased by 47.1% and 69.2% for 5% and 10% SF replacements, respectively. SF increased the diffusion resistance of concrete in several ways. As a mineral admixture with extreme fineness and high pozzolanic reactivity, SF improves the diffusion resistance of concrete as well as strength by refining the pore structure of interfacial transition zone (ITZ) of concrete [47] and by producing a greater solid volume of C-S-H gel [56], and also reducing the porosity for fixed degree of cement hydration [57, 58]. Bentz et al. [59] observed that the pozzolanic gel produced from SF had about 25 times less diffusivity than the gel produced from normal cement hydration. This may be due
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transport number of chloride. 3) Five loadings at 40% f c or 80% f c increased the transport number of chloride for all the mixes investigated in this study. 5% SF replacement had a very close effect on the chloride permeability of concrete with 20% GGBFS when concrete was subjected to 40% f c or 80% f c . This research was supported by the National Natural Science Foundation of China (Grant No.50808045), Natural Science Foundation of Beijing (Grant No.2112024), and Fok Ying Tung Education Foundation (Grant No.132016).
Figure 6
Transport number of chloride migration through concrete.
to pore size refinement and matrix densification, reduction in content of Ca(OH)2 and cement paste-aggregate interfacial refinement [60]. In addition, during the hydration process the transition interfacial zone is gradually densified due to pozzolanic reaction between SF and calcium hydroxide [61]. Compared with nonloaded specimens, repeated loading increased the transport number of chloride for all mixes. For example, the transport number of chloride increased by 34.2% and 85.0% when the control concrete was subjected to 40% and 80% f c , respectively. The length, width, and number of micro-cracks in concrete subjected to 40% f c will increase. Mortar cracks will increase noticeably and form continuous crack patterns when concrete is subjected to 80% f c [25]. Thus, 40% f c and 80% f c repeated loadings usually increase the permeability of concrete with or without mineral admixtures. Therefore, the replacement percentages of mineral admixtures and the repeated loading have significant effects on chloride migration through concretes. More research is needed on the effects of changes in exposure conditions or curing temperatures on concretes with mineral admixtures so that the results can better be related to actual practical conditions.
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4 Conclusions From the results obtained in this work the following conclusions can be drawn: 1) 20% and 30% GGBFS replacements had a significantly positive effect on the cylinder compressive strength but 40% GGBFS replacement reduced the strength at the age of 28 d. 2) The transport number of chloride through concrete containing 20% and 30% GGBFS replacements and 5% and 10% SF replacements was lower than that of the control concrete, but 40% GGBFS replacement increased the
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