Article
Mechanical Properties and Eco-Efficiency of Steel Fiber Reinforced Alkali-Activated Slag Concrete Sun-Woo Kim 1 , Seok-Joon Jang 2 , Dae-Hyun Kang 2 , Kyung-Lim Ahn 2 and Hyun-Do Yun 2, * Received: 1 October 2015 ; Accepted: 27 October 2015 ; Published: 30 October 2015 Academic Editor: Jorge de Brito 1 2
*
Department of Construction Engineering Education, Chungnam National University, Daejeon 34134, Korea;
[email protected] (S.-W.K.) Department of Architectural Engineering, Chungnam National University, Daejeon 34134, Korea;
[email protected] (S.-J.J.);
[email protected] (D.-H.K);
[email protected] (K.-L.A) Correspondence:
[email protected]; Tel.: +82-42-821-6281; Fax: +82-42-823-9467
Abstract: Conventional concrete production that uses ordinary Portland cement (OPC) as a binder seems unsustainable due to its high energy consumption, natural resource exhaustion and huge carbon dioxide (CO2 ) emissions. To transform the conventional process of concrete production to a more sustainable process, the replacement of high energy-consumptive PC with new binders such as fly ash and alkali-activated slag (AAS) from available industrial by-products has been recognized as an alternative. This paper investigates the effect of curing conditions and steel fiber inclusion on the compressive and flexural performance of AAS concrete with a specified compressive strength of 40 MPa to evaluate the feasibility of AAS concrete as an alternative to normal concrete for CO2 emission reduction in the concrete industry. Their performances are compared with reference concrete produced using OPC. The eco-efficiency of AAS use for concrete production was also evaluated by binder intensity and CO2 intensity based on the test results and literature data. Test results show that it is possible to produce AAS concrete with compressive and flexural performances comparable to conventional concrete. Wet-curing and steel fiber inclusion improve the mechanical performance of AAS concrete. Also, the utilization of AAS as a sustainable binder can lead to significant CO2 emissions reduction and resources and energy conservation in the concrete industry. Keywords: alkali-activated slag (AAS); mechanical performance; eco-efficiency; ordinary Portland cement (OPC); sustainable binder
1. Introduction Concrete is the most widely used construction material and uses a great amount of cement. Furthermore, anthropogenic CO2 emissions due to the production of cement is rapidly increasing with the increase of urban development. In order to address this, alkali-activated slag (AAS) has been considered as an alternative binder to cement, as well as a way of reusing available industrial by-products. Damineli et al. [1] proposed two indicators which allow measuring the eco-efficiency of cement use, binder intensity and CO2 intensity. Both indicators were tested using two sets of data from literature, one Brazilian and the other from 28 different countries. Yang et al. [2] derived the relationship between the CO2 and binder intensities of different concretes from a regression analysis of a comprehensive database in Korea. However, their applicability was hindered because the AAS concretes obtained with sodium silicate activators exhibit higher drying shrinkage rates than in ordinary Portland cement (OPC). At present, fiber inclusion is commonly accepted as the way to improve shrinkage behavior of AAS concrete [3,4]. In addition to above, many researches have been conducted to improve or enhance the performance of AAS concrete by incorporation of fibers [5–7]. The primary purposes of the fiber inclusion are to control cracking and to increase Materials 2015, 8, 7309–7321; doi:10.3390/ma8115383
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Materials 2015, 8, 7309–7321
the fracture toughness of the cement matrix by a bridging action that is controlled by debonding, sliding and pulling-out the reinforcing fibers during both micro and macro-cracking of the matrix. Bernal et al. [5] concluded that alkali-activated slag concrete reinforced with steel fibers shows three times higher flexural toughness than Portland cement concretes at early ages of curing. Aydin and Baradan [6] reported that alkali-activated slag/silica fume mortars present significantly higher mechanical performance than OPC based mortar at the same fiber dosage due to the higher bonding properties between fiber and alkali-activated slag/silica fume mortar interfacial zone compared to OPC mortar. In relation to the use of fibers and development of sustainable materials, the structural behaviors of timber beams have also been studied [8–10]. The aim of this study is to improve the mechanical performance of Ca(OH)2 -based AAS concrete by incorporation of steel fibers because it has been found from other previous research results that microfiber such as polyethylene (PE) or polyvinyl alcohol (PVA) is useful for improving tensile performance of a cementitious composite without coarse aggregate. In this study, concrete was mixed with crushed granite, and therefore steel fibers that have high tensile performance and larger diameter compared to the microfiber were used for mixing the concrete. The effects of incorporation of steel fibers on the compressive and flexural performance of AAS concretes with the main variables being steel fiber volume fraction and curing condition are investigated in this paper. The test results are compared to the OPC concrete specimens, with a view to identifying their performances and potential applications as construction materials. In this study, an alternative fiber-reinforced concrete to OPC concrete with a specified compressive strength value of 40 MPa was produced by using 30 mm length hooked-end steel fibers. The utilization of AAS in concrete will be helpful in reducing environmental problems and greenhouse gas emissions associated with the Portland cement production, and in conserving existing natural resources. 2. Experimental Section 2.1. Materials and Specimen Preparation Ground granulated blast-furnace slag (GGBS) and Type I OPC were used as binders for AAS concrete and ordinary concrete, respectively. The chemical compositions of the GGBS and OPC used in this study are given in Table 1. The specific surfaces for the OPC and the GGBS were 325 and 430 m2 /kg, respectively. The GGBS has a 21.2 µm maximum particle size and an 8.5 µm average particle size. To produce the AAS concrete, AAS binder was produced by the activation of GGBS with calcium hydroxide as the primary activator. Sodium silicate (Na2 SiO3 ) and sodium carbonate (Na2 CO3 ) were used as auxiliary activators. The selection of primary and auxiliary activators was based on studies previously conducted in Korea [2,11,12]. From the chemical composition of GGBS presented in the table, the basicity coefficient (Kb ) and hydration modulus (HM) of GGBS were calculated to be 0.92 and 1.68, respectively. Locally available river sand (maximum particle size of 5 mm) and crushed granite (maximum particle size of 20 mm) were used as fine and coarse aggregates, respectively. The results of sieve analysis for fine and coarse aggregates met the continuous standard curves specified in KS F 2526 [13]. As listed in Table 2, OPC and AAS concretes were prepared with a water-to-binder ratio of 0.55 and a sand-to-coarse aggregate ratio of 0.45. Hooked-end steel fibers shown in Figure 1 were made from mild carbon steel with a tensile strength of 1100 MPa. The fibers were 30 mm long and 0.5 mm in diameter, giving an aspect ratio of 60. The percentage of the steel fiber added ranged from 0%–2.0% by weight of the binder as seen in Table 3. To produce OPC and AAS concretes, the binder and the aggregate were initially dry-mixed for a minute. After the dry-mixing, water including a superplasticizer was added and the time for mixing was planned to be long enough to prevent any segregation in concrete. The required quantities of steel fibers were then added separately in small amounts to avoid fiber balling. The freshly mixed steel fiber-reinforced concrete was poured in two layers into cylindrical (Φ100 mm ˆ 200 mm) and
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Materials 2015, 8, page–page prismatic (100 mm ˆ 100 mm ˆ 400 mm) steel moulds for compressive and flexural tests, respectively. For each mix, nine cylinders (three cylinders at each age) and three prisms were cast in steel moulds tests, respectively. For each mix, nine cylinders (three cylinders at each age) and three prisms were ˝ and 95% relative humidity for 24 h until demoulding. After and kept a mist roomand at 23 cast in in steel moulds kept C in a mist room at 23 °C and 95% relative humidity for 24 h until demoulding, specimens for wet-curing were preserved in water at 23 ˝ C and the other specimens demoulding. After demoulding, specimens for wet‐curing were preserved in water at 23 °C and the for dry-curing were placed in air at 23 ˘ 5 ˝ C and 50 ˘ 5% relative humidity until 1 day before other specimens for dry‐curing were placed in air at 23 ± 5 °C and 50 ± 5% relative humidity until 1 day before testing. For all mixes, 225 and 75 specimens were made and tested for compressive and testing. For all mixes, 225 and 75 specimens were made and tested for compressive and flexural flexural properties, respectively. properties, respectively.
Table 1. Chemical composition (% by mass) of GGBS and OPC. Table 1. Chemical composition (% by mass) of GGBS and OPC.
Component
GGBS
OPC 20.9 OPC Silicon Aluminium oxide (Al dioxide (SiO2 ) 2O3) 34.7 13.8 5.39 20.9 AluminiumCalcium oxide (CaO) oxide (Al2 O3 ) 13.8 40.1 64.7 5.39 Calcium oxide (CaO) 40.1 64.7 Iron oxide (Fe2O3) 0.11 2.38 2.38 Iron oxide (Fe2 O3 ) 0.11 Magnesium oxide (MgO) 4.38 1.51 1.51 Magnesium oxide (MgO) 4.38 2) 0.74 0.74 1.33 1.33 TitaniumTitanium dioxide (TiO dioxide (TiO2 ) Sodium oxide (Na2 O) 0.20 Sodium oxide (Na 2O) 0.20 0.27 0.27 Potassium oxide (K2 O) 0.48 Potassium oxide (K2O) 0.48 0.22 0.22 Sulfur trioxide (SO3 ) 4.83 1.65 Sulfur trioxide (SO 3) 4.83 1.65 5.80 Loss on ignition (LOI) 2.70 Loss on ignition (LOI) 2.70 5.80 2.52 Basicity coefficient (Kb ) 0.92 HydrationBasicity coefficient (K modulus (HM) 1.68 b) 0.92 2.52 3.43 Hydration modulus (HM) 3.43 + Al2 O3 )/SiO2 . Notes: Kb = (CaO + MgO)/(SiO2 + Al2 O3 ); HM =1.68 (CaO + MgO Component Silicon dioxide (SiO2)
GGBS 34.7
Notes: Kb = (CaO + MgO)/(SiO2 + Al2O3); HM = (CaO + MgO + Al2O3)/SiO2.
Figure 1. Shape of a hooked‐end steel fiber.
Figure 1. Shape of a hooked-end steel fiber. Table 2. Mixture proportions of OPC and AAS concretes.
Mix
3) Table 2. Mixture proportionsUnit Weight (kg/m of OPC and AAS concretes. Mix w/b S/a W C AAS S G 3) Unit Weight (kg/m OPC 0.55 0.45 205 373 ‐ 756 924 w/b S/a AAS 0.55 0.45W 205 ‐ C 373 AAS 756 924 S
G
OPC 0.55 0.45 205 373 756 924 Notes: w/b is water‐to‐binder ratio; S/a is sand‐to‐aggregate ratio; W is water; C is cement; S is sand; AAS 0.55 0.45 205 373 756 924 and G is coarse aggregate. Notes: w/b is water-to-binder ratio; S/a is sand-to-aggregate ratio; W is water; C is cement; S is sand; and G is coarse aggregate. Table 3. Variables for mechanical tests.
Test
Specimen Vf (%) Curing Method 0.0, for 0.5,mechanical 1.0, 1.5, 2.0 tests. Wet-curing TableOPC 3. Variables Compression AAS-dry 0.0, 0.5, 1.0, 1.5, 2.0 Dry-curing AAS-wet 0.0, 0.5, 1.0, 1.5, 2.0 Wet-curing V f (%) Test Specimen Curing Method OPC 0.0, 0.5, 1.0, 1.5, 2.0 Wet-curing Flexure OPC 0.0, 0.5, Wet-curing AAS-wet 0.0, 0.5, 1.0, 1.0, 1.5,1.5, 2.02.0 Wet-curing
Compression Flexure
AAS-dry 0.0, 0.5, 1.0, 1.5, 2.0 Notes: Vf is steel fiber volume fraction. AAS-wet 0.0, 0.5, 1.0, 1.5, 2.0 OPC 0.0, 0.5, 1.0, 1.5, 2.0 AAS-wet 0.0, 0.5, 1.0, 1.5, 2.0
3
Notes: V f is steel fiber volume fraction.
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2.2. Test Methods Materials 2015, 8, page–page
The cross-sectional area of each cylinder for compressive test was calculated using an average of three 2.2. Test Methods diameter measurements taken in two intersecting directions at the mid-height of the specimen. At 3, 7, and 28 days after concrete casting, compressive strength tests of the cylindrical specimens The cross‐sectional area of each cylinder for compressive test was calculated using an average were of performed. The cylinders were taken testedin intwo compression as directions per ASTMat Cthe 39 mid‐height [14] until failure. three diameter measurements intersecting of the The flexural strength tests were conducted at 28 days in accordance with ASTM C 78 [15]. specimen. At 3, 7, and 28 days after concrete casting, compressive strength tests of the cylindrical specimens were performed. The cylinders were tested in compression as per ASTM C 39 [14] until
3. Results and Discussion failure. The flexural strength tests were conducted at 28 days in accordance with ASTM C 78 [15]. 3.1. Compressive Performance 3. Results and Discussion Compressive strength versus strain curves of OPC, AAS-dry, and AAS-wet specimens at 28 days 3.1. Compressive Performance with different fiber contents are presented in Figure 2. The average curves were drawn from the three Compressive strength versus in strain curves of OPC, was AAS‐dry, and AAS‐wet specimens at 28 of test results for each mix. The error the measurement calculated as the standard deviation days with different fiber contents are presented in Figure 2. The average curves were drawn from the compressive strength from three samples. The maximum standard deviations in the compressive the three test results for each mix. The error in the measurement was calculated as the standard strength were 1.56, 2.45, and 3.05 MPa at 3, 7, and 28 days, respectively. As seen in Figure 2a, the deviation of the compressive strength from three samples. The maximum standard deviations in the descending curvestrength of OPC-0.0 has 3.05 almost due brittle failure right afterin peak compressive were specimen 1.56, 2.45, and MPa vanished at 3, 7, and 28 to days, respectively. As seen stress,Figure whereas the other OPC specimens with fiber and all of the AAS specimens have various 2a, the descending curve of OPC‐0.0 specimen has almost vanished due to brittle failure descending curves as seen in Figure 2a,b. As the steel fiber volume fraction increases, the curve right after peak stress, whereas the other OPC specimens with fiber and all of the AAS specimens after have various descending curves as seen in Figure 2a,b. As the steel fiber volume fraction increases, peak stress descended slowly. It can be inferred that the compressive failure mode of concrete the curve after peak stress descended slowly. It can be inferred that the compressive failure mode of changed from a brittle to a more ductile failure due to the steel fiber inclusion. The effect of steel fibers concrete changed from a brittle to a more ductile failure due to the steel fiber inclusion. The effect of on the compressive strength of AAS concrete is more noticeable in case of higher fiber inclusion [5]. steel fibers on the compressive strength of AAS concrete is more noticeable in case of higher fiber Karunanithi and Anandan (2014) reported that the increase of steel fiber inclusion improved the inclusion [5]. Karunanithi and Anandan (2014) reported that the increase of steel fiber inclusion compressive strength of AAS concrete [16]. As shown in Figure 2b,c, AAS-wet specimens exhibit improved the compressive strength of AAS concrete [16]. As shown in Figure 2b,c, AAS‐wet morespecimens stable post behavior than AAS-dry specimens. It reflects that water-curing enhances the bond exhibit more stable post behavior than AAS‐dry specimens. It reflects that water‐curing properties in the zone in between fibers and matrix phase binders enhances the interfacial bond properties the interfacial zone the between fibers and for the AAS matrix phase compared for AAS to dry-curing. binders compared to dry‐curing.
Figure 2. Stress-strain curves of test specimens at 28 days. (a) OPC-wet; (b) AAS-dry; (c) AAS-wet. 4
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The effects of fiber inclusion on the compressive strengths with age are presented in Figure 3. Figure 2. Stress‐strain curves of test specimens at 28 days. (a) OPC‐wet; (b) AAS‐dry; (c) AAS‐wet. Until 7 days, OPC-0.5 specimen shows a little less strength than OPC-0.0. At 28 days, however, OPC-0.5 specimens show higher compressive strength value than OPC-0.0. For OPC specimens The effects of fiber inclusion on the compressive strengths with age are presented in Figure 3. with Until 7 days, OPC‐0.5 specimen shows a little less strength than OPC‐0.0. At 28 days, however, OPC‐0.5 over 1.0% of fiber volume fraction, there is no noticeable increase in the compressive strength specimens show higher compressive strength value than OPC‐0.0. For OPC specimens with over 1.0% compared to OPC-0.0 specimen. It is thought that the steel fibers were not effectively dispersed in of fiber volume fraction, there is no noticeable increase values in the of compressive strength have compared to concrete when mixing. However, compressive strength AAS specimens significantly OPC‐0.0 specimen. It is thought that the steel fibers were not effectively dispersed in concrete when increased by fiber inclusion. At 28 days, the addition of steel fibers in AAS concrete has a significant mixing. However, compressive strength values of AAS specimens have significantly increased by effect on the enhancement of the compressive strength by 1.9%–17.8%. In addition to the fiber fiber inclusion. At 28 days, the addition of steel fibers in AAS concrete has a significant effect on the inclusion, the compressive strength development of concrete is affected by curing methods; it is clear enhancement of the compressive strength by 1.9%–17.8%. In addition to the fiber inclusion, the that water-curing is moredevelopment efficient for of theconcrete strength of concrete than as shown compressive strength is development affected by curing methods; it is dry-curing clear that water‐ in Figure 3b,c. curing is more efficient for the strength development of concrete than dry‐curing as shown in Figure 3b,c.
Figure 3. Compressive strength development with age (a) 3 days; (b) 7 days; (c) 28 days. Figure 3. Compressive strength development with age (a) 3 days; (b) 7 days; (c) 28 days.
Figure 4 shows the effect of fiber inclusion on the strain at maximum stress of concrete at each
Figure 4 shows the effect of fiber inclusion on the strain at maximum stress of concrete at each age. As shown in the figure, the strain value has a tendency to decrease with the age of concrete as age. As shown in the figure, has a except tendency tospecimens decrease at with the age of concrete as expected. From the results the for strain strain value at each age, OPC 7 days, all specimens show a similar trend in the strain increase ratio, so it can be inferred that the effect of binder type on expected. From the results for strain at each age, except OPC specimens at 7 days, all specimens show the strain of concrete is negligible. curing method, that the increase ratio the strain a similar trendcapacity in the strain increase ratio, so For it can be inferred the effect of on binder type of on the AAS concrete is similar between dry‐ and wet‐curing. For strain values at 7 days, regardless of strain capacity of concrete is negligible. For curing method, the increase ratio on the strain of AAS curing methods, the AAS‐1.5 specimen showed a noticeably higher strain than OPC. The concrete is similar between dry- and wet-curing. For strain values at 7 days, regardless of curing compressive test results are summarized in Table 4. methods, the AAS-1.5 specimen showed a noticeably higher strain than OPC. The compressive test results are summarized in Table 4. 5
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Figure 4. Variation of compressive strain at maximum stress with age. (a) 3 days; (b) 7 days; (c) 28 days. Figure 4. Variation of compressive strain at maximum stress with age. (a) 3 days; (b) 7 days; (c) 28 days. Table 4. Compressive test results.
Compressive Strength (MPa) Strain at Max. Compressive Strength (%) Table 4. Compressive test results. Specimens 3 days 7 days 28 days 3 days 7 days 28 days wet dry wet dry wet wet dry wet dry wet Compressive Strength (MPa) Strain at Max. Compressive Strength (%) OPC‐0.0 21.7 ‐ 29.2 ‐ 36.3 0.212 ‐ 0.211 ‐ 0.214 Specimens 3 Days 7 Days 28 Days 3 Days 7 Days 28 Days OPC‐0.5 Wet 20.2 Dry ‐ Wet 27.8 Dry ‐ 42.2Wet 0.205 Wet ‐ Dry0.186 Wet ‐ Dry 0.225 Wet OPC‐1.0 14.6 ‐ 25.5 ‐ 34.0 0.241 ‐ 0.247 ‐ 0.231 OPC-0.0 21.7 29.2 36.3 0.212 0.211 0.214 OPC‐1.5 16.4 ‐ 24.5 ‐ 35.9 0.486 ‐ 0.239 ‐ 0.311 OPC-0.5 20.2 27.8 42.2 0.205 0.186 0.225 OPC‐2.0 14.6 18.2 - ‐ 25.5 24.2 ‐ - 36.434.0 1.167 OPC-1.0 0.241 ‐ - 1.4130.247 ‐ -0.457 0.231 AAS‐0.0 16.4 14.3 - 21.8 24.5 22.0 31.8 0.236 -0.207 0.311 OPC-1.5 - 34.635.9 0.290 0.486 0.195 - 0.2090.239 AAS‐0.5 15.5 24.8 23.0 35.9 39.3 0.245 0.238 0.227 0.225 OPC-2.0 18.2 24.2 36.4 1.167 1.413 -0.238 0.457 AAS‐1.0 15.8 23.4 25.5 34.6 34.7 0.285 0.246 0.272 0.228 0.219 0.207 AAS-0.0 14.3 21.8 22.0 31.8 34.6 0.290 0.195 0.209 0.236 AAS-0.5 35.9 42.339.3 0.692 0.245 0.5610.2380.6970.227 AAS‐1.5 15.5 17.5 24.827.8 23.0 28.5 38.1 0.333 0.225 0.334 0.238 AAS-1.0 34.6 41.434.7 1.020 0.285 0.7190.2460.8830.272 AAS‐2.0 15.8 23.5 23.431.8 25.5 34.6 40.1 0.480 0.228 0.447 0.219 AAS-1.5 17.5 27.8 28.5 38.1 42.3 0.692 0.561 0.697 0.333 0.334 AAS-2.0 23.5 31.8 1.020 0.719 0.883 0.480by fiber 0.447 The compressive strength 34.6 values 40.1 of AAS 41.4 concrete were significantly increased inclusion whereas OPC concrete shows no noticeable change in the compressive strength. It can be noted that lateral confinement by steel fiber concrete is improved due to the smaller particle by size of GGBS The compressive strength values of AAS were significantly increased fiber inclusion compared to OPC. Furthermore, the increase ratio in compressive strength for AAS concrete with whereas OPC concrete shows no noticeable change in the compressive strength. It can be noted that wet‐curing is higher than that with dry‐curing. This indicates that wet‐curing enhances the bonding lateral confinement by steel fiber is improved due to the smaller particle size of GGBS compared to properties between fiber and mortar phase for AAS based binders. Figure 5 shows SEM OPC.micrographs of the steel fiber–matrix transition zone. From the micrographs in the figure, it is clear Furthermore, the increase ratio in compressive strength for AAS concrete with wet-curing is higher than thatAAS withbinder dry-curing. This indicates that wet-curing thecoarse. bonding that using turns the surface of the steel fiber from enhances smooth to The properties SEM between fiber and mortar phase for AAS based binders. Figure 5 shows SEM micrographs of the steel micrographs also confirmed that the bond property in the interfacial zone between steel fibers and
fiber–matrix transition zone. From the micrographs in the figure, it is clear that using AAS binder turns the surface of the steel fiber from smooth 6to coarse. The SEM micrographs also confirmed that the bond property in the interfacial zone between steel fibers and matrix phase for AAS binders is better than that for OPC based binders. These enhanced bond characteristics of alkali activated 7314
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matrix phase AAS binders in is other better research than that results for OPC based Shi binders. These bond that binders have alsofor been reported [17,18]. and Xie [17]enhanced also reported characteristics of alkali activated binders have also been reported in other research results [17,18]. ´ matrix phase for dense AAS binders is better than that zone for OPC based These enhanced bond can the formation of the and uniform transition in the Na2binders. SiO3 activated slag mortars Shi and Xie [17] also reported that the formation of the dense and uniform transition zone in the characteristics of alkali activated binders have also been reported in other research results [17,18]. be attributed to several factors such as the water reducing function of Na2 SiO3 and the high initial Na 2SiO3− activated slag mortars can be attributed to several factors such as the water reducing Shi and Xie [17] also reported that the formation of the dense and uniform transition zone in the concentration of [SiO ]4´ in the pore solution. function of Na 2SiO43 and the high initial concentration of [SiO4]4− in the pore solution. − Na2SiO3 activated slag mortars can be attributed to several factors such as the water reducing function of Na2SiO3 and the high initial concentration of [SiO4]4− in the pore solution.
(a)
(b)
(c)
(a) (b) (c) Figure 5. SEM micrographs of steel fiber–matrix transition zone. (a) OPC‐wet; (b) AAS‐dry; (c) AAS‐wet. Figure 5. SEM micrographs of steel fiber–matrix transition zone. (a) OPC-wet; (b) AAS-dry; (c) AAS-wet. Figure 5. SEM micrographs of steel fiber–matrix transition zone. (a) OPC‐wet; (b) AAS‐dry; (c) AAS‐wet.
3.2. Flexural Performance
3.2. Flexural Performance
3.2. Flexural Performance A flexural strength‐deflection curve itself may be quite sensitive in distinguishing amongst A flexural strength-deflection curve be quite quitesensitive sensitive distinguishing amongst different fiber inclusions, and the curve roughly indicates differences in the toughness performance A flexural strength‐deflection curve itself itself may may be in in distinguishing amongst of concrete with different fiber volume fractions. Figure 6 shows flexural strength‐deflection curves different fiber inclusions, and the curve roughly indicates differences in the toughness performance different fiber inclusions, and the curve roughly indicates differences in the toughness performance for all specimens. All the are fractions. the average values for three specimens. As expected, the of concrete with different fibercurves volume Figure 6 shows flexural strength-deflection curves of concrete with different fiber volume fractions. Figure 6 shows flexural strength‐deflection curves ultimate strength is higher for concrete with more fiber volume fractions, due to bridging action. for allfor specimens. All the are the average values for three specimens. As expected, the ultimate all specimens. All curves the curves are the average values for three specimens. As expected, the However, there was no significant effect of binder type or fiber inclusion on to the first cracking ultimate strength is higher for concrete with more fiber volume fractions, due bridging action. strength is higher for concrete with more fiber volume fractions, due to bridging action. However, strengths. there was no significant effect of binder type or fiber inclusion on the first cracking thereHowever, was no significant effect of binder type or fiber inclusion on the first cracking strengths. The The ultimate strength was noticeably improved when the fiber volume fraction was above 1.0%. strengths. ultimate strength was noticeably improved when the fiber volume fraction was above 1.0%. In In particular, AAS‐1.5 and AAS‐2.0 specimens showed very similar flexural as The ultimate AAS‐1.0, strength was noticeably improved when the fiber volume fraction was behavior above 1.0%. particular, AAS-1.0, AAS-1.5 and AAS-2.0 specimens showed very similar flexural behavior as shown shown in Figure 6a, whereas the ultimate strengths of AAS‐0.0 and AAS‐0.5 specimens were lower In particular, AAS‐1.0, AAS‐1.5 and AAS‐2.0 specimens showed very similar flexural behavior as in Figure 6a, whereas the ultimate strengths of AAS-0.0 and AAS-0.5 specimens were lower than than those of OPC‐0.0 and OPC‐0.5 specimens due to lower compressive strengths. In Table 5, first shown in Figure 6a, whereas the ultimate strengths of AAS‐0.0 and AAS‐0.5 specimens were lower thosecrack strength (f of OPC-0.0 and OPC-0.5 specimens fdue to lower compressive strengths. In Table 5, firstf) crack f), first crack deflection (δ ), ultimate strength (f f), and deflection at the peak load (δ than those of OPC‐0.0 and OPC‐0.5 specimens due to lower compressive strengths. In Table 5, first are given. As shown in flexural responses in Figure 6, the descending portion of the curve is steeper strength (ff ), first crack deflection (δf ), ultimate strength (ff ), and deflection at the peak load (δf) f ) are f), first crack deflection (δ f), ultimate strength (f f), and deflection at the peak load (δ crack strength (f when the ultimate strength is higher. Therefore, it can be inferred that the residual strength beyond given. As shown in flexural responses in Figure 6, the descending portion of the curve is steeper are given. As shown in flexural responses in Figure 6, the descending portion of the curve is steeper when the ultimate strength is higher. Therefore, it can be inferred that the residual strength beyond whenthe peak load decreases faster so that the post‐crack toughness changes. the ultimate strength is higher. Therefore, it can be inferred that the residual strength beyond the peak load decreases faster so that the post‐crack toughness changes. the peak load decreases faster so that the post-crack toughness changes.
Figure 6. Flexural strength‐deflection curves of test specimens at 28 days. (a) OPC specimens; (b) AAS specimens. Figure 6. Flexural strength‐deflection curves of test specimens at 28 days. (a) OPC specimens;
Figure 6. Flexural strength-deflection curves of test specimens at 28 days. (a) OPC specimens; (b) AAS specimens. (b) AAS specimens. 7
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Table 5. Strengths and deflections at both first crack and peak load. Table 5. Strengths and deflections at both first crack and peak load.
Specimen OPC-0.0 OPC-0.5 OPC-1.0 OPC-1.5 OPC-2.0 AAS-0.0 AAS-0.5 AAS-1.0 AAS-1.5 AAS-2.0
First Crack First crack ff (MPa) δf (mm) Specimen ff (MPa) δf (mm) 6.329 0.049 OPC‐0.0 6.329 0.049 6.577 0.047 OPC‐0.5 6.577 0.047 5.755 0.042 OPC‐1.0 5.755 0.042 5.064 0.047 OPC‐1.5 5.064 0.055 0.047 6.646 OPC‐2.0 6.646 0.050 0.055 5.501 4.674 AAS‐0.0 5.501 0.044 0.050 5.249 AAS‐0.5 4.674 0.029 0.044 5.910 AAS‐1.0 5.249 0.019 0.029 4.180 AAS‐1.5 5.910 0.047 0.019 AAS‐2.0 4.180 0.047
Peak Load Peak load fu (MPa) δu (mm) fu (MPa) δu (mm) 6.329 0.049 6.329 0.049 8.197 1.303 8.197 10.5571.303 0.531 10.557 10.8550.531 1.035 10.855 13.1301.035 2.221 13.130 5.5012.221 0.050 1.145 5.501 6.6380.050 0.705 6.638 11.6151.145 1.427 11.615 11.7140.705 0.931 11.714 13.2621.427
13.262
0.931
ASTM C1018 [19] is the most common method used for evaluating the flexural toughness of FRC ASTM C1018 [19] is the most common method used for evaluating the flexural toughness of (fiber reinforced concrete). In this study, the ASTM flexural toughness indices and the ASTM residual FRC (fiber reinforced concrete). In this study, the ASTM flexural toughness indices and the ASTM strength factors are adopted for calculating the post-crack toughness of test specimens. The ASTM residual strength factors are adopted for calculating the post‐crack toughness of test specimens. specifies the flexural toughness indices as I5 (3δ), I10 (5.5δ), I20 10(10.5δ), (15.5δ),30and I50 (25.5δ). For The ASTM specifies the flexural toughness indices as I 5 (3δ), I (5.5δ), II2030 (10.5δ), I (15.5δ), and I 50 the residual strength factors, R5,10 , R10,20 , R20,30 and R30,50 are also specified in the ASTM. In this paper, (25.5δ). For the residual strength factors, R 5,10, R10,20, R 20,30 and R30,50 are also specified in the ASTM. for the flexural toughness indices and residual strength factors, I100 (50δ), I200 (100δ),200R In this paper, for the flexural toughness indices and residual strength factors, I 100 (50δ), I (100δ), 50,100 , and R50,100 R100,200 were added consider overall flexural response including the deflection at peak load of test , and R 100,200to were added to consider overall flexural response including the deflection at peak load of For test specimens. For the R50,100 and the R100,200 added, factors reduced as per the specimens. the R50,100 and R100,200 added, factors werethe reduced aswere per the following equations: following equations:
R50,100 “ = 2 × (I 2 ˆ pI100 100 R50,100 − I´50I) 50 q
(1)
(1)
R100,200 “= 2 × (I 2 ˆ pI200 R100,200 − I´100I) 100 q 200
(2)
(2)
Figure 7 shows the effect of binder on ASTM the ASTM toughness indices all specimens. Figure 7 shows the effect of binder typetype on the toughness indices for for all specimens. All are All are average values for three specimens. It can be seen that the ASTM toughness indices are average values for three specimens. It can be seen that the ASTM toughness indices are particularly particularly sensitive in distinguishing amongst different fiber volume fractions, is more sensitive in distinguishing amongst different fiber volume fractions, which is morewhich obvious when the obvious when the fiber volume fraction is higher than 1.0%. AAS specimens exhibit similar or fiber volume fraction is higher than 1.0%. AAS specimens exhibit similar or higher toughness index higher toughness index than OPC specimens, regardless of fiber volume fraction. In particular, for than OPC specimens, regardless of fiber volume fraction. In particular, for R50,100 and R100,200 which R50,100 and R100,200 which are toughness indices added in this paper, the difference of indices between are toughness added inwas this widened. paper, theThis difference of indices between and OPC specimens AAS and indices OPC specimens means that the AAS binder AAS is more efficient for was widened. This means that the AAS binder is more efficient for bonding in the interfacial zone bonding in the interfacial zone between steel fibers and the matrix phase than OPC binder during between steel fibers and the matrix phase than OPC binder during post-cracking behavior. post‐cracking behavior.
Figure 7. Comparison of toughness indices. Figure 7. Comparison of toughness indices.
Figure 8 shows the effect of binder type on the ASTM residual strength factors for all specimens. 8 As shown in the figure, AAS binder could be better for residual strength of FRC than OPC binder. It
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Figure 8 shows the effect of binder type on the ASTM residual strength factors for all specimens. As shown in the figure, AAS binder could be better for residual strength of FRC than can bebinder. thoughtIt that distribution the tension zone in would be enhanced to the OPC can the be internal thought stress that the internal in stress distribution the tension zone due would be formation of the dense and uniform transition zone in the matrix as seen in Figure 5. enhanced due to the formation of the dense and uniform transition zone in the matrix as seen in Figure 5.
Figure 8. Comparison of residual strength factors. Figure 8. Comparison of residual strength factors.
3.3. Eco‐Efficiency of Alkali‐Activated Slag 3.3. Eco-Efficiency of Alkali-Activated Slag Korea LCI Database Information Network [20] and Japanese database [21] were referred to in Korea LCI Database Information Network [20] and Japanese database [21] were referred to in order to evaluate evaluate the the CO CO22 emissions of concrete. concrete. Tables show CO CO22 evaluation OPC order to emissions of Tables 6 6 and and 77 show evaluation of of the the OPC and the AAS concrete, respectively. As listed in the tables, the CO 2 emission of the AAS concrete is and the AAS concrete, respectively. As listed in the tables, the CO2 emission of the AAS concrete 15%~24% of of that of of the lower CO CO22 is 15%~24% that theOPC OPCconcrete concretebecause becausethe theAAS AASbased based binder binder has has about about 97% 97% lower emission than the OPC based binder. The CO 2 emissions of concretes considering fiber inclusion are emission than the OPC based binder. The CO2 emissions of concretes considering fiber inclusion are compared in Table 8. compared in Table 8. To compare environmental efficiency, a binder intensity (bi) and CO2 intensity (ci) were proposed To compare environmental efficiency, a binder intensity yields (bi) and CO2 intensity (ci) were by Damineli BL et al. [1]. It was reported that the binder intensity the efficiency of using clinker proposed by Damineli BL et al. [1]. It was reported that the binder intensity yields the efficiency of and other hard to find clinker substitutes. The CO2 intensity permits an estimation of the mix design’s using clinker and other hard to find clinker substitutes. The CO 2 intensity permits an estimation of global warming potential. The indices are calculated as: the mix design’s global warming potential. The indices are calculated as: bܾ bi (3) ܾ݅“ൌ p (3) cܿ ciܿ݅“ൌ p
(4) (4)
where b is the total consumption of binder materials (kg/m (kg/m3) emitted to ) emitted to where b is the total consumption of binder materials (kg/m33),), c is the total CO c is the total CO22 (kg/m produce, and p is the compressive strength (MPa) at 28 days. produce, and p is the compressive strength (MPa) at 28 days. Table 6. CO Table 6. CO22 evaluation of OPC concrete. evaluation of OPC concrete.
Material and Production Material and Production Item A BB A∙B A A¨ B Item 3 3 -kg/kg kg/m3 kg/m3 COCO CO 2‐kg/kg CO 2 ‐kg/m 2 2 -kg/m OPC 373 373 0.944 352.1 352.1 OPC 0.944 Sand 0.0026 2.0 Sand 756 756 0.0026 2.0 Coarse 924 0.0075 6.9 Coarse 924 0.0075 ´4 6.9 Water 205 0.0 1.96 ˆ 10 Water 205 1.96 × 10−4 0.0 Admixture 0.1492 0.25 0.0 Admixture 2258 0.1492 0.25 0.0 Conc. production 0.008 18.1 2258 0.008 18.1 Sum Conc. production – – 379.2 Wet-curing – 38.5 – –– Sum 379.2 3 Total * = 417.7 CO -kg/m 2 – – 38.5 Wet‐curing Note: * CO2 emission of steel fiber (1.6 CO is not included. 3 2 -kg/kg) Total * = 417.7 CO 2‐kg/m
Note: * CO2 emission of steel fiber (1.6 CO2‐kg/kg) is not included.
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Table 7. CO2 evaluation of AAS concrete. Materials 2015, 8, page–page
Material and Production A¨ B CO2 -kg/kg CO2 -kg/m3 Material and production GGBS 373 9.9 Item A B0.0265 A∙B Sand 756 0.0026 2.0 3 3 kg/m CO2‐kg/kg CO2‐kg/m Coarse 924 0.0075 6.9 GGBS 373 0.0265 Water 205 0.0 1.96 ˆ 10´4 9.9 Sand 0.1492 756 0.0026 2.0 Admixture 0.25 0.0 924 0.0075 6.9 Conc. production Coarse 2258 0.008 18.1 Water – 205 1.96 × 10–−4 0.0 Sum 36.9 Dry-curing 0.0 Admixture – 0.1492 0.25 – 0.0 Wet-curing Conc. production – 38.5 2258 0.008 – 18.1 3 Total * = 36.9 CO –2 -kg/m for – AAS-dry;36.9 Sum 3 for AAS-wet 75.4 CO -kg/m 2 – – 0.0 Dry‐curing Note: * Wet‐curing CO2 emission of steel– fiber (1.6 CO– included. 38.5 2 -kg/kg) is not Total * = 36.9 CO2‐kg/m3 for AAS‐dry; 75.4 CO2‐kg/m3 for AAS‐wet Table 8. Final CO2 emission of concrete with fiber inclusion. Item
A 2 evaluation of AAS concrete. B Table 7. CO kg/m3
Note: * CO2 emission of steel fiber (1.6 CO2‐kg/kg) is not included.
Mix OPC AAS AAS
CO22 emission of concrete with fiber inclusion. Emission with Fiber Volume Fractions (CO2 -kg/kg) Curing Table 8. Final CO 0.0% 0.5% 1.0% 1.5% 2.0% Method Curing CO2 Emission with Fiber Volume Fractions (CO2‐kg/kg) 417.7 420.6 423.6 426.6 429.6 Mix wet 0.0% 0.5% 1.0% 42.9 1.5% 45.9 2.0% method dry 36.9 39.9 48.9 OPC wetwet 417.7 420.6 423.6 81.4 426.6 84.4 429.6 75.4 78.4 87.4 AAS dry 36.9 39.9 42.9 45.9 48.9 AAS wet 75.4 78.4 81.4 84.4 87.4
The binder intensity with variables and the relationship between the binder intensity and compressive are presented in Figure seen in Figure 9a,the it binder is clearintensity that theand binder The strength binder intensity with variables and 9. the As relationship between intensity can be reduced by wet-curing fiber case 9a, of concrete lessbinder than 1.0% compressive strength are presented in and Figure 9. inclusion. As seen in In Figure it is clear with that the of fiber volume fraction, OPC specimens have lower binder intensity compared to AAS specimens. intensity can be reduced by wet‐curing and fiber inclusion. In case of concrete with less than 1.0% of fiber volume specimens have lower binder intensity ofcompared to AAS specimens. However, as fiberfraction, volume OPC fraction increases, the binder intensities AAS specimens are lower than However, as fiber volume fraction increases, the binder intensities of AAS specimens are lower than those of OPC specimens. It can be inferred that AAS binder can enhance performance, especially those of OPC specimens. It can be inferred that the AAS binder can enhance performance, the compressive strength of concrete, and reduce total amount of binder necessary especially to achieve the the compressive strength of concrete, and reduce the total amount of binder necessary to achieve performance required. The best-fit curves in previous research [2] are adopted to compare the binder the performance required. The best‐fit curves in previous research [2] are adopted to compare the intensity in this study and are presented in Figure 9b. As shown in the figure, the binder intensity binder intensity in this study and are presented in Figure 9b. As shown in the figure, the binder calculated in this study is below the best-fit curve of OPC (Yang et al., 2013). AAS concrete has lower intensity calculated in this study is below the best‐fit curve of OPC (Yang et al., 2013). AAS concrete binder intensity compared to compared OPC concrete even though itthough was reported that Ca(OH) AA 2 -based has lower binder intensity to OPC concrete even it was reported that Ca(OH) 2‐ GGBSbased AA GGBS concrete requires a greater amount of binder in order to obtain the same compressive concrete requires a greater amount of binder in order to obtain the same compressive strength as OPC concrete [2]. strength as OPC concrete [2].
Figure 9. Comparison of binder intensity (bi). (a) fiber volume fraction; (b) compressive strength.
Figure 9. Comparison of binder intensity (bi). (a) fiber volume fraction; (b) compressive strength. 10
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Figure thethe COCO withwith variables and the relationship betweenbetween the CO2 intensity 2 intensity Figure 10 10 shows shows 2 intensity variables and the relationship the CO2 and compressive strength. As estimated in Table 8, the CO intensity of AAS concrete is about intensity and compressive strength. As estimated in Table 8, 2the CO2 intensity of AAS concrete is 20%–25% of that of of OPC concrete. In view of curing method, shown in wet-curing about 20%–25% that of OPC concrete. In the view of the curing asmethod, as Figure shown 10a, in Figure 10a, increases theincreases CO2 intensity by 40%–50% by compared to compared dry-curing to because electric equipment is wet‐curing the CO 2 intensity 40%–50% dry‐curing because electric needed to maintain the water at a constant temperature for curing. The best-fit curves in previous equipment is needed to maintain the water at a constant temperature for curing. The best‐fit curves research [2] are adopted to compare the CO2 intensity in2 intensity in this study and are presented in this study and are presented in Figure 10b. in previous research [2] are adopted to compare the CO As shown in the figure, it is indicated that the estimated CO for OPC is around for the OPC best-fit 2 intensity Figure 10b. As shown in the figure, it is indicated that the estimated CO2 intensity is curve. However, there is little difference between the curves because Yang et al. [2] considered CO2 around the best‐fit curve. However, there is little difference between the curves because Yang et al. [2] emitted in production and transport as c. In this study, CO2 emitted for transport was omitted because considered CO 2 emitted in production and transport as c. In this study, CO 2 emitted for transport the CO emission is influenced by the transportation distance between the plant and construction 2 was omitted because the CO2 emission is influenced by the transportation distance between the sites. In this study, CO2sites. emitted in production (cradle to gate) was considered to gate) calculate plant and construction In this study, CO2only emitted in production only (cradle to was CO intensity (ci), and the best-fit curves for AAS-wet and -dry lie under the curve proposed by 2 considered to calculate CO2 intensity (ci), and the best‐fit curves for AAS‐wet and ‐dry lie under the Yang et al. [2]. curve proposed by Yang et al. [2].
Figure 10. Comparison of CO2 intensity (ci). (a) Fiber volume fraction; (b) compressive strength. Figure 10. Comparison of CO2 intensity (ci). (a) Fiber volume fraction; (b) compressive strength.
4. Conclusions 4. Conclusions In this study, AAS was used as an alternative binder to OPC in order to reduce environmental In this study, AAS was used as an alternative binder to OPC in order to reduce environmental impact of concrete. To estimate eco‐efficiency of AAS based binder, compressive tests were impact of concrete. To estimate eco-efficiency of AAS based binder, compressive tests were conducted conducted and the effects of fiber inclusion and wet‐curing on the two indicators (bi and ci) were and the effects of fiber inclusion and wet-curing on the two indicators (bi and ci) were evaluated. The evaluated. following observations and conclusions can be made and drawn on the basis of the compressive test The following observations and conclusions can be made and drawn on the basis of the results and intensity estimation in this study. compressive test results and intensity estimation in this study. 1. AAS based binders may provide similar compressive performance with a lower fiber content 1. AAS based binders may provide similar compressive performance with a lower fiber due to better dispersion and enhanced bond characteristics in the steel fiber–matrix content due fiber to better fiber dispersion and enhanced bond characteristics in the steel fiber– transition zone. Furthermore, the wet-curing method is very helpful for the performance matrix transition zone. Furthermore, the wet‐curing method is very helpful for the enhancement of AAS with steel fibers. performance enhancement of AAS with steel fibers. 2.2.It It can bebe inferred can inferred that that AAS AAS binder binder can can enhance enhance performance, performance, especially especially the the compressive compressive strength of concrete, and can reduce the total amount of binder necessary strength of concrete, and can reduce the total amount of binder necessary to to achieve achieve the the performance required. performance required. 3.3.Based on on the results of theof ASTM toughness index and residual strength factor, it isfactor, concluded Based the results the ASTM toughness index and residual strength it is that AAS binder is more efficient for bonding in the interfacial zone between steel fibers and concluded that AAS binder is more efficient for bonding in the interfacial zone between matrix than matrix OPC binder post-cracking behavior. This is because theThis internal stress steel phase fibers and phase in than OPC binder in post‐cracking behavior. is because distribution in the tension zone would be enhanced due to the formation of the dense and the internal stress distribution in the tension zone would be enhanced due to the formation uniform transition zone in the matrix. of the dense and uniform transition zone in the matrix.
4. The significant effects of fiber inclusion and wet‐curing on the eco‐efficiency of AAS were confirmed on the basis of the binder intensity and CO2 intensity. It indicates that more economic and eco‐efficient fiber reinforced composites with AAS can be produced. In the 7319 11
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4. The significant effects of fiber inclusion and wet-curing on the eco-efficiency of AAS were confirmed on the basis of the binder intensity and CO2 intensity. It indicates that more economic and eco-efficient fiber reinforced composites with AAS can be produced. In the future, additional studies on the mechanical properties of AAS concrete under fatigue and creep are needed. Acknowledgments: The work was financially supported by the General Researcher Support Program (No. NRF-2013R1A1A2066034) through a National Research Foundation (NRF) grant funded by the Korea Ministry of Education (MOE). Author Contributions: Sun-Woo Kim is a main writer of the manuscript and advised the experiments. Seok-Joon Jang, Dae-Hyun Kang, and Kyung-Lim Ahn performed the experiments and assisted the research. Hyun-Do Yun is a consultant to the research and supervised the entire research work. All authors were involved in the test results analysis and in the finalizing of the manuscript. Conflicts of Interest: The authors declare no conflict of interest. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18.
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