sustainability Article
Investigation on the Effect of Recycled Asphalt Shingle (RAS) in Portland Cement Mortar Jinwoo An 1 , Boo Hyun Nam 1, * and Heejung Youn 2, * 1 2
*
Department of Civil, Environmental, and Construction Engineering, University of Central Florida, 12800 Pegasus Drive, Orlando, FL 32816, USA;
[email protected] Department of Civil Engineering, School of Urban and Civil Engineering, Hongik University, 94, Wausan-ro, Mapo-gu, Seoul 04066, Korea Correspondence:
[email protected] (B.H.N.);
[email protected] (H.Y.); Tel.: +1-(407)-823-1361 (B.H.N.); +82-(2)-320-7691 (H.Y.)
Academic Editor: Muge Mukaddes Darwish Received: 4 March 2016; Accepted: 14 April 2016; Published: 19 April 2016
Abstract: Tear-off roofing shingle, referred to as Reclaimed asphalt shingle (RAS), is the byproduct of construction demolition and it is a major solid waste stream in the U.S. Reuse of this byproduct in road construction sector can contribute to the success of materials sustainability as well as landfill conservation. Ground RAS has similar particle distribution as sand and its major component includes aggregate granules, fibers, and asphalt. To promote the beneficial utilization of RAS, this study evaluates the effect of RAS in cement mortar when used as replacement of sand. In addition, the study investigates how cellulose fibers from RAS behave under high alkaline environment during cement hydration process, which may significantly affect mortar’s strength performance. The laboratory study includes measurements of physical, mechanical, and durability behaviors of cement mortar containing RAS replacing sand up to 30%. It was found that the optimum mixture proportions are 5% and 10% for compressive strength and toughness, respectively. Keywords: cement mortar; reclaimed asphalt shingle (RAS); cellulose fiber; toughness; crack propagation
1. Introduction US Environmental Protection Agency (EPA) reported that approximately 250 million tons of wastes were generated in the US during 2010, but only 34% (85 million tons) was either recycled or composed [1]. Roofing shingle is one of construction and demolition (C&D) debris and it is a major solid waste stream in the US. According to the USEPA, total shingle waste generated in the U.S. is approximately 11 million tons per year [2]. Beneficial utilization of byproduct and waste materials in road construction have been made and it contributes to the success of sustainability [3–14]. It consists of roughly 90% representing post-consumer scrap (or tear-off shingles) and 10% comprising post-manufacture scrap [15]. In addition, shingle waste generates up to 8% of the total building-related waste stream and more than 10% of construction and demolition debris [16]; Most of them are being disposed by landfilling, and the estimated cost is up to $60 per ton [17]. Therefore, beneficial reuse of shingle waste can lead to not only land conservation but also cost saving in waste disposal and construction materials. Tear-off roofing shingle, referred as Recycled asphalt shingle (RAS) hereafter, is a complex material. In general the RAS is composed of 40% to 70% of aggregate granules, 20% to 40% of asphalt, and 1% to 25% of fibrous base materials (which can be either cellulose or glass fibers). Figure 1 shows the processing procedure of tear-off roofing shingles from C&D debris to ground RAS that has similar size of sand.
Sustainability 2016, 8, 384; doi:10.3390/su8040384
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Figure 1. Procedure of Recycled asphalt shingle (RAS) processing.
There have been several attempts that beneficially reuse RAS in Hot-Mix Asphalt (HMA), and some of them have shown improvements to mechanical properties of HMA [18–22]. According to Nam et al., the usage of RAS is a good solution to save the virgin binder because shingles contain about 30%–40% of bitumen [20]. Another good reason to use shingle in HMA claimed by Sengoz et al. is that the usage of shingle makes easier compaction of the HMA because the HMA with shingle contains more filler (around 30% of the mass of shingle) [18]; in addition, he also mentioned the lower cracking due to the fibers in the shingle . One more important finding about RAS is that according to Yang et al. the HMA is stiffer at high temperatures than the regular HMA [22]. On the other hand, there has been almost no study on the usage of RAS (consisting of aggregate granules, asphalt, and fiber) in cement-based materials. Instead, there were several studies that evaluates the use of reclaimed asphalt pavement (RAP) in cement-based materials [23–25] and also fibrous base materials in cement-based materials (such as cement paste, mortar and concrete) [26–32]. Regarding RAP, to Mang et al. found several advantages on the usage of RAS in terms of the increment of failure strain, toughness, poison’s ratio, drying shrinkage and coefficient of thermal expansion [23]. Huang et al. concludes that the toughness of Portland cement concrete can be increased by adding RAP as replacement of aggregate [33,34]. On the other hand, however, Abdel-Mohti et al. mentions that compressive strength and flexural strength decrease as the content of RAP increases. The addition of RAP in concrete may reduce the load bearing capacity [24]. Erdem et al. also reports that concrete containing RAP aggregate may not be feasible for structural application [25]. Several researchers report that cellulose fiber provides adequate strength, toughness and the capacity of bonding to cement-based materials because the fibers function as bridge between the cement matrix cracks and transfer the stresses [26–28]. Regarding fibers, fibers might be responsible for the reduction of plastic cracking shrinkage and for some increments on mechanical properties of the cementitious mechanical properties [26]. However, the usage of cellulose fiber in cement-based composites is still limited by durability issue of cellulose fibers. The main problems of cellulose fiber are volume variation due to high water absorption capacity and degradation of cellulose fiber under high alkaline environment [29–32]. This paper investigates the beneficial utilization of RAS in cement mortar when used as replacement of fine aggregate in the mixture. The beneficial and side effects of RAS in mortar were evaluated. For the specimen preparation, the fine aggregate was replaced by RAS with different percentages of 0%, 5%, 10%, 15%, 20%, 25%, and 30%; thus, optimum proportioning (best RAS replacement ratio) was determined for mechanical performance. 2. Materials and Testing Methods 2.1. Materials Physical Properties The cement used in this study was Type I ordinary Portland cement (ASTM C150) with specific gravity of 3.15. The fine aggregate used was sand passing the sieve no. 4 (4.75 mm) with a fineness modulus of 2.36. The specific gravity and water absorption are 2.66 and 2%, respectively. Figure 2a,b shows the pictures of ground RAS and cellulose fibers extracted from RAS, respectively. Figure 2c,d shows those aggregate and fibers in larger magnifications. As seen in Figure 2, RAS has shown irregular
2.1. Materials Physical Properties The cement used in this study was Type I ordinary Portland cement (ASTM C150) with specific gravity of 3.15. The fine aggregate used was sand passing the sieve no. 4 (4.75 mm) with a fineness modulus of 2.36. The specific gravity and water absorption are 2.66 and 2%, respectively. Figure Sustainability 2016, 8, 384 3 of 16 2a,b shows the pictures of ground RAS and cellulose fibers extracted from RAS, respectively. Figure 2c,d shows those aggregate and fibers in larger magnifications. As seen in Figure 2, RAS has shown shape, rough surface texture and highly angular particles [34]. Inparticles general the RAS composed 40% irregular shape, rough surface texture and highly angular [34]. In is general the by RAS is to 70% of aggregate granules, 20% to 40% of asphalt, and 1% to 25% of fibrous base materials (which composed by 40% to 70% of aggregate granules, 20% to 40% of asphalt, and 1% to 25% of fibrous can be either cellulose or glass fibers) [18,19]. base materials (which can be either cellulose or glass fibers) [18,19].
(a)
(b)
(c)
(d)
Figure 2.2. Pictures Pictures ground cellulose fiber RAS with different magnifications: (a) Figure of of ground RASRAS and and cellulose fiber in RASin with different magnifications: (a) Ground Ground RAS; (b) Cellulose fibers in RAS; (c) Aggregate and asphalt binder in RAS; (d) Cellulose RAS; (b) Cellulose fibers in RAS; (c) Aggregate and asphalt binder in RAS; (d) Cellulose fibers in RAS. fibers in RAS.
The basic physical properties of RAS along with cement and fine aggregates are summarized in The basic physical properties of RAS along with cement and fine aggregates are summarized Table 1. Krivit reported that RAS has a low specific gravity [35]. The low specific gravity is due to the in Table 1. Krivit reported that RAS has a low specific gravity [35]. The low specific gravity is due to presence of asphalt binder and cellulose fibers, which ranges from 18% to 40% of RAS by weight [35]. the presence of asphalt binder and cellulose fibers, which ranges from 18% to 40% of RAS by weight The specific gravity of RAS used in this study equals to 2.17, which is lower than normal sand. RAS [35]. The specific gravity of RAS used in this study equals to 2.17, which is lower than normal sand. has high absorption capacity compared to general fine and coarse aggregates due to the presence of RAS has high absorption capacity compared to general fine and coarse aggregates due to the cellulose fibers and woods that absorbs more water. The water absorption of RAS was found 7% which presence of cellulose fibers and woods that absorbs more water. The water absorption of RAS was is 3 times higher than sand. found 7% which is 3 times higher than sand.
Table 1. Physical properties of Portl and cement, fine aggregate and RAS. Measured Property
Cement
Fine Aggregate (Sand)
RAS
Reference
Fineness modulus Specific gravity (OD) Absorption capacity (%)
N/A 3.15 N/A
2.36 2.66 2.0
2.25 2.17 7.0
ASTM C 33 ASTM C 127 and C128 ASTM C 127 and C128
Table 1. Physical properties of Portl and cement, fine aggregate and RAS.
Measured Property Cement Fineness modulus N/A Sustainability 2016, 8, 384 Specific gravity (OD) 3.15 Absorption capacity (%) N/A
Fine Aggregate (Sand) 2.36 2.66 2.0
RAS 2.25 2.17 7.0
Reference ASTM C 33 4 of 16 ASTM C 127 and C128 ASTM C 127 and C128
In this ground RAS waswas used as partial replacement of fineof aggregate becausebecause its particle In this study, study, ground RAS used as partial replacement fine aggregate its distribution is similar to the required gradation of sand in Portland cement concrete (PCC). Figure 3 particle distribution is similar to the required gradation of sand in Portland cement concrete (PCC). shows the particle distribution of sand and ground RAS as received. Figure 3 shows the particle distribution of sand and ground RAS as received.
ASTM High
120
ASTM Low
100 Percent (%) finer
80 60 40 20 0 0.05
0.5 Particle size (log scale)
5
Figure 3. Gradation curves for fine aggregate (required by ASTM C33 [36]) and ground RAS used in Figure 3. Gradation curves for fine aggregate (required by ASTM C33 [36]) and ground RAS used in this study. this study.
2.2. Mixture Proportioning 2.2. Mixture Proportioning A water‐to‐cement (w/c) ratio of 0.5 was used for all mortar samples. Each mixture was A water-to-cement (w/c) ratio of 0.5 was used for all mortar samples. Each mixture was prepared prepared in 5‐liter (0.005‐m³) batches. RAS was used to replace fine aggregate (sand) in the range in 5-liter (0.005-m³) batches. RAS was used to replace fine aggregate (sand) in the range from 5 wt% from 5 wt% to 30 wt% (with 5 wt% incremental) so that optimum replacement ratio for mortar’s to 30 wt% (with 5 wt% incremental) so that optimum replacement ratio for mortar’s strength and strength and durability are evaluated. Detailed information of mixture proportioning are presented durability are evaluated. Detailed information of mixture proportioning are presented in Table 2. in Table 2. Table 2. Mixture proportions of mortar samples per cubic meter. Table 2. Mixture proportions of mortar samples per cubic meter. Sample Water Cement Fine Aggregate RAS RAS Slump Fine Aggregate w/c Slump (cm) 3) Cement (kg/m 3) Sample ID w/c Water (kg/m ID (kg/m3 ) (kg/m3 ) (kg/m3 ) (kg/m3 ) 3 3 C C R5 R5 R10 R10 R15 R20R15 R25R20 R30R25
R30
0.5
0.5
180 180 180 180 180 180 180 180 180
180 180 180 180 180
360 360 360 360 360
360 360 360 360 360 360 360 360 360
(kg/m ) 760 722 684 646 608 570 532 570 532
760 722 684 646 608
(kg/m ) ‐ 38 76 114 190 152 228 190 228 38 76 114 152
(cm) 18 18 16 16 15 15 13 11 13 11 10 810 8
Fine aggregate and RAS used in this study meets the grading requirement of ASTM C33. Figure 3 Fine and RAS used in study meets the grading requirement of C33 ASTM C33. shows theaggregate particle size distributions of this fine aggregate with the corresponding ASTM grading Figure 3 shows the particle size distributions of fine aggregate with the corresponding ASTM C33 requirements and also the gradation of ground RAS used in this study. The aggregate granules used in grading requirements and particles also the coated gradation ground RAS ground used in coal this study. The aggregate RAS is likely crushed rock with of ceramic oxides, slag, backsurfacer sand, granules used in RAS is likely crushed rock particles coated with ceramic oxides, ground coal slag, or mineral filler [34]. Based on the sieve analysis, RAS has slightly more fine aggregates than typical backsurfacer or mineral fine aggregatesand, (see Figure 3). filler [34]. Based on the sieve analysis, RAS has slightly more fine aggregates than typical fine aggregate (see Figure 3). 2.3. Properties of Fresh Mortar Several testing methods were employed to investigate properties of early cement hydration when RAS was used in replacement of fine aggregate. The selected tests include measurements of heat of hydration (ASTM C186 [37]), setting time (ASTM C807 [38]) and Porosity (ASTM C1754 [39]). A
2.3. Properties of Fresh Mortar Several testing methods were employed to investigate properties of early cement hydration when RAS was used in replacement of fine aggregate. The selected tests include measurements of Sustainability 2016, 8, 384 5 of 16 heat of hydration (ASTM C186 [37]), setting time (ASTM C807 [38]) and Porosity (ASTM C1754 [39]). A thermometer with four‐channel data logger provided by Omega was used to measure heat of hydration. with Vicat consistency apparatus of Humboldt Mfg. was Co. used was to used to measure the initial thermometer four-channel data logger provided by Omega measure heat of hydration. setting time of mortar mixture. All results were compared with a control sample that includes 0% Vicat consistency apparatus of Humboldt Mfg. Co. was used to measure the initial setting time of RAS (mortar with only Portland cement and sand). mortar mixture. All results were compared with a control sample that includes 0% RAS (mortar with only Portland cement and sand). 2.4. Properties of Harden Mortar 2.4. Properties of Harden Mortar Mechanical behaviors of RAS‐combined mortar samples were investigated by compressive strength (ASTM C109 [40]), of flexural strength (ASTM [41]), and investigated indirect toughness tests. The Mechanical behaviors RAS-combined mortar C348 samples were by compressive sample size for the compressive strength test was 5 cm × 5 cm × 5 cm (cubic) and the sample size for strength (ASTM C109 [40]), flexural strength (ASTM C348 [41]), and indirect toughness tests. The the flexural strength test was 5 cm × 10 cm × 2.5 cm. After these mechanical tests, visual survey was sample size for the compressive strength test was 5 cm ˆ 5 cm ˆ 5 cm (cubic) and the sample size conducted on fractured surface investigate of strength change to RAS in for the flexural strength test was 5to cm ˆ 10 cm ˆany 2.5 mechanism cm. After these mechanical tests,due visual survey cement matrix. on As fractured a means of durability investigation, porosity measurement (ASTM due C1754 [39]) was conducted surface to investigate any mechanism of strength change to RAS was conducted. in cement matrix. As a means of durability investigation, porosity measurement (ASTM C1754 [39]) was conducted. 2.5. Degradation Check of RAS at High pH 2.5. Degradation Check of RAS at High pH RAS contains significant amount of cellulose fibers (approximately 7 wt% by weight). This RAS contains significant amountof of cellulose wt% by weight). This study study investigated the behavior cellulose fibers fiber (approximately contained in 7RAS under high alkaline investigated the behavior of cellulose fiber contained in RAS under high alkaline environment where environment where cement hydration process produces high pH (in the range of 12 to 13). High pH cement hydration process is produces highto pH (in the2 range of 12 towhich 13). High pH during during cement hydration mainly due Ca(OH) (portlandite), is byproduct of cement cement hydration is mainly due to Ca(OH)2 (portlandite), which is byproduct of cement hydration. 500 µm of hydration. 500 μm of distilled water and 20 grams of pure sodium hydroxide (NaOH) were used to distilled water and 20 grams of pure sodium hydroxide (NaOH) were used to make sodium hydroxide make sodium hydroxide solution with a pH of 12.5. 50 grams of cellulose fibers extracted from RAS solution with a pH of 50 grams of cellulose fibers extracted RAS wereAfter submerged in sodium were submerged in 12.5. sodium hydroxide (NaOH) solution from for 7 days. 7 days of the hydroxide (NaOH) solution forwere 7 days. After with 7 days thesieve submergence, cellulose fiberswith weredistilled filtered submergence, cellulose fibers filtered a of fine (75 μm sieve), washed with fine air sieve (75 to µmremove sieve),the washed with distilled water and air driedmicroscope to removewas the sodium water a and dried sodium hydroxide. A high‐resolution used to hydroxide. A high-resolution microscope was used to investigate the condition of cellulose fibers after investigate the condition of cellulose fibers after the degradation under high pH. Figure 4 shows a the degradation under high pH. Figure 4 shows a schematic illustration of the laboratory testing to schematic illustration of the laboratory testing to investigate the behavior of cellulose fibers in high investigate the behavior of cellulose fibers in high pH condition. pH condition.
Figure 4. Test setup for investigating the behavior of cellulose fibers in high pH condition. Figure 4. Test setup for investigating the behavior of cellulose fibers in high pH condition.
3. Results and Discussion 3. Results and Discussion 3.1. Heat of Hydration 3.1. Heat of Hydration The tested onon thethe mortar samples thatthat contain 10%,10%, 20%,20%, and 30% RAS, The heat heat of of hydration hydration was was tested mortar samples contain and of 30% of and the results compared with the control sample. Allsample. results shown in Figure 5 illustrate that5 RAS, and the were results were compared with the control All results shown in Figure the heat of hydration decreases as the replacement percentage of RAS increases in the mix. The control illustrate that the heat of hydration decreases as the replacement percentage of RAS increases in the sample exhibits regular behavior of cement hydration with two humps of which the second one is about the temperature around 29.5 ˝ C. On the other hand, the RAS-combined mixtures (all three cases) show a clear decrement in the heat of hydration. One of main causes for the reduction of hydration
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mix. The control sample exhibits regular behavior of cement hydration with two humps of which 6 of 16 the second one is about the temperature around 29.5 °C. On the other hand, the RAS‐combined mixtures (all three cases) show a clear decrement in the heat of hydration. One of main causes for the can reduction of hydration heat can be related to the of higher water absorption capacity of RAS heat be related to the higher water absorption capacity RAS compared to sand. RAS is composed compared to sand. RAS composed several materials fiber and pieces wood of several materials such is cellulose fiberof and pieces of woodsuch thatcellulose can largely absorb water. of Table 1 that can largely absorb water. Table 1 shows that the absorption capacity of RAS is 3 times higher shows that the absorption capacity of RAS is 3 times higher than fine aggregate which is sand. Effective than fine aggregate which is sand. Effective water in the cement matrix decreases due to the high water in the cement matrix decreases due to the high absorption; thus, cement grains in the mix may absorption; thus, cement grains in the mix may be hindered to fully react with water during early be hindered to fully react with water during early hydration process. In addition, Table 2 shows hydration process. In increased, addition, Table shows that as This the RAS content might increased, slump have that as the RAS content slump2 have decreased. phenomenon be due to higher decreased. capacity This phenomenon due to higher absorption capacity of fully cellulose fibers the and absorption of cellulosemight fibersbe and pieces of wood in RAS. In order to investigate pieces of wood in RAS. In order to fully investigate the effect of RAS as is, the study does not effect of RAS as is, the study does not include pre-processing on RAS such as removal of deleterious include pre‐processing on RAS such as removal of deleterious materials or leads water materials or water correction. Another observation is that the replacement of RAS to correction. the retard Another is that replacement RAS leads to the retard cement hydration. of cement observation hydration. The peakthe temperature hasof been delayed gradually byof increasing the amountThe of peak temperature has been delayed gradually by increasing the amount of RAS replacement. RAS replacement.
Temperature (C)
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32 31 30 29 28 27 26 25 24 23 22
Mortar (Control) Mortar (10% RAS) Mortar (20% RAS) Mortar (30% RAS)
0
2
4
6
8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 Time (hour)
Figure 5. Heat of hydration with RAS (10%, 20% and 30% of RAS). Figure 5. Heat of hydration with RAS (10%, 20% and 30% of RAS).
3.2. Setting Time 3.2. Setting Time The setting setting time time of of mortar mortar was was analyzed analyzed for for all all mixtures mixtures (0%, (0%, 10%, 10%, 20%, 20%, and and 30% 30% of of RAS RAS The replacement). The results of setting time are summarized in Table 3. ASTM C807 recommends to replacement). The results of setting time are summarized in Table 3. ASTM C807 recommends to measure only the initial setting, thus the final setting was not recorded. measure only the initial setting, thus the final setting was not recorded. Table 3. Setting time of mortar samples containing RAS. Table 3. Setting time of mortar samples containing RAS.
Sample Time (mins) Time (mins) Control 210 Control 210 10% RAS 200 10% RAS 200 20% RAS 200 20% RAS 200 30% RAS 195 30% RAS 195 Sample
As the replacement percentage of RAS increases from 0% to 30%, the setting time decreases As the replacement percentage of RAS increases from 0% to 30%, the setting time decreases from from 210 min to 195 min. It can be explained by two reasons. The first reason is that fibers of RAS in 210 to 195 min. It canresist be explained by twoof reasons. Theapparatus first reason isforming that fibers of RASof infiber the the min mortar mixture may to the needle the setting by a matrix mortar mixture may resist to the needle of the setting apparatus by forming a matrix of fiber bridges. bridges. The second reason might be the un‐hydrated cement grains in the mortar mixture due to The second reason might be the un-hydrated cement grains in the mortar mixture due to the high the high absorption capacity of RAS. Fibers in RAS are hydrophilic materials and can absorb more absorption of RAS. Fibers actual in RAScement are hydrophilic and can absorb more water than water than capacity sand. Consequently, grains to materials be hydrated become less and the initial sand. Consequently, actual cement grains to be hydrated become less and the initial cement hydration (representing the initial setting time) occurs faster when the replacement ratio of RAS is increased.
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Sustainability 2016, 8, 384 7 of 16 cement hydration (representing the initial setting time) occurs faster when the replacement ratio of
RAS is increased. 3.3. Compressive Strength Measurement 3.3. Compressive Strength Measurement The results results of of compressive compressive strength strength test testare areshown shownin inFigure Figure6. 6.Overall, Overall, the usage RAS The the usage of of RAS in in cement mortar causes reduction in compressive strength. As the percentage of RAS in the mix cement mortar causes reduction in compressive strength. As the percentage of RAS in the mix increases the strength decreases. Interestingly, the 5%the replacement exhibits the maximum increases the compressive compressive strength decreases. Interestingly, 5% replacement exhibits the compressive strength, which is slightly higher than the control. The control and 5% replacement maximum compressive strength, which is slightly higher than the control. The control and 5% shows the compressive of 23.62 and 23.65 MPa,and respectively. However, this increment might replacement shows the strength compressive strength of 23.62 23.65 MPa, respectively. However, this possibly be caused by experimental errors such as mixing, vibrating and pouring time. After the increment might possibly be caused by experimental errors such as mixing, vibrating and pouring 5% replacement, the compressive strength proportionally decreases with increasing the RAS content, time. After the 5% replacement, the compressive strength proportionally decreases with increasing exhibiting 17.80 MPa for the 30% replacement. The mechanism to control mortar strength includes the RAS content, exhibiting 17.80 MPa for the 30% replacement. The mechanism to control mortar asphalt film around aggregate, degradation of cellulose fiber, filler effect, and any combination of these. strength includes asphalt film around aggregate, degradation of cellulose fiber, filler effect, and any RAS used in this studyRAS has finer than has sandfiner (see particles Figure 3),than thussand a slight effect combination of these. used particles in this study (see filler Figure 3), occurs, thus a resulting in a slight increase of the strength from 0% to 5% RAS. However, the effect of asphalt film and slight filler effect occurs, resulting in a slight increase of the strength from 0% to 5% RAS. However, cellulose fiber become dominant and the strength decreases with increasing the RAS content. More the effect of asphalt film and cellulose fiber become dominant and the strength decreases with discussionsthe on the effects of asphalt and cellulose fiber degradation at high environment are increasing RAS content. More film discussions on the effects of asphalt film pH and cellulose fiber presented later sections. degradation at high pH environment are presented later sections.
Compressive strength (MPa)
30
7 days
25
28 days
20 15 10 5 0 0
5
10
15
20
25
30
Percentage of replaced RAS (%)
Percentage of strength increment (%)
(a)
35% 30% 25% 20% 15% 10% 5% 0%
y = 0.024x + 0.1246 R² = 0.8982
0
5 10 15 20 25 Percentage of replaced RAS (%)
30
(b) Figure 6. Results of compressive strength test: (a) compressive strength of mortars containing RAS Figure 6. Results of compressive strength test: (a) compressive strength of mortars containing RAS and and (b) percentage of strength increment from 7 days to 28 days. (b) percentage of strength increment from 7 days to 28 days.
Another observation is that the difference in compressive strength between 7 and 28 days. The Another observation that the difference in compressive strength between ratio 7 andincreases. 28 days. difference between 7 and is28‐day strengths increases as the RAS replacement The difference between 7 and 28-day strengths increases as the RAS replacement ratio increases.
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Considering the strength increment is one of indicators of cement hydration process, this trend Considering the strength increment is one of indicators of cement hydration process, this trend indicates that higher RAS replacement cause larger amount of cement hydration after 7 days. Figure indicates that RAS replacement largerfrom amount of cement afteris 7the days. Figure 6a 6a shows the higher percentage of strength cause increment 7 days to 28 hydration days, which ratio of the shows the percentage of strength increment from 7 days to 28 days, which is the ratio of the strength strength difference between 7 and 28 days to the 7‐day strength. Asphalt binder in RAS may retard difference betweenin 7 and 28stage daysbut to the strength. Asphalt binder in RAS retard cement cement hydration early the 7-day hydration continues after 7 days. This may later‐stage cement hydration in early stage but the hydration continues after 7 days. This later-stage cement hydration hydration combined with the filler effect obviously causes the strength increment after 7 days. combined with the filler effect obviously causes the strength increment after 7 days. 3.4. Porosity Measurement 3.4. Porosity Measurement The results of porosity testing are shown in Figure 7. Since Portland cement hydrates and The results of porosity testing are shown in Figure 7. Since Portland cement hydrates and consumes unbound water in the mixture over time, the voids due to unbound water in the mixture consumes unbound water in the mixture over time, the voids due to unbound water in the mixture is being reduced in process of time. Figure 7 shows that all the results of porosity measurements at is being reduced in process of time. Figure 7 shows that all the results of porosity measurements 28 days are lower than those measured at 7 days. Interestingly, the control and 5% replacement at 28 days are lower than those measured at 7 days. Interestingly, the control and 5% replacement shows relatively high porosity values at 7 days and significant reduction in porosity at 28 days. shows relatively high porosity values at 7 days and significant reduction in porosity at 28 days. Other Other mortar samples including RAS more than 5% shows smaller reduction in porosity as cement mortar samples including RAS more than 5% shows smaller reduction in porosity as cement hydration hydration proceeds from 7 days to 28 days. The 30% replacement exhibits almost same porosity proceeds from 7 days to 28 days. The 30% replacement exhibits almost same porosity between 7 days between 7 days and 28 days. This can be explained by that the mortar including RAS less than 5% and 28 days. This can be explained by that the mortar including RAS less than 5% produces significant produces significant amount of cement hydration but higher amount of RAS has limited level of amount of cement hydration but higher amount of RAS has limited level of cement hydration. Another cement hydration. Another observation is that the mortar containing 5% RAS at 28 days has the observation is that the mortar containing 5% RAS at 28 days has the lowest porosity, which is 29.6%. It lowest porosity, which is 29.6%. It is an important note that the mortar containing 5% RAS is an important note that the mortar containing 5% RAS produces the highest compressive strength. produces the highest compressive strength. As discussed in Section 3.3, the RAS used in this study As discussed in Section 3.3, the RAS used in this study contains finer particles than sand. As a result, a contains finer particles than sand. As a result, a filler effect likely occurred and led to lower porosity filler effect likely occurred and led to lower porosity at 5% RAS at 28 days, resulting in the highest at 5% RAS at 28 days, resulting in the highest compressive strength at 5% RAS (see Figure 6a). compressive strength at 5% RAS (see Figure 6a).
7days 28 days
35
Porosity (%)
33 31 29 27 0
5 10 15 20 25 30 Percentage of replaced RAS (%)
Figure 7. Porosity of mortars containing RAS. Figure 7. Porosity of mortars containing RAS.
3.5. Flexural Strength 3.5. Flexural Strength Figure the results of flexural strength test. The strength reduction in flexural Figure 88 presents presents the results of flexural strength test. trend The of trend of strength reduction in strength similar to results compressive test. This reduction bereduction explainedcan by the flexural is strength is the similar to of the results of strength compressive strength test. can This be aggregate coated by asphalt film and material degradation of cellulose fibers. In general, the bonding explained by the aggregate coated by asphalt film and material degradation of cellulose fibers. In between film and cementasphalt paste matrix is weaker than thematrix bonding regular general, asphalt the bonding between film and cement paste is between weaker than the aggregate bonding between regular aggregate and cement paste matrix [11]. Cellulose fibers are also being degraded and cement paste matrix [11]. Cellulose fibers are also being degraded under high pH environment. under high pH environment. More details of the degradation mechanism are discussed Section 3.6. More details of the degradation mechanism are discussed Section 3.6. Although the overall pattern of Although the overall pattern flexural in decline, shows the mortar with 10% RAS flexural strength is in decline, theof mortar withstrength 10% RASis replacement higher flexural strength replacement shows higher flexural than the RAS replacement. than the mortar incorporating 5% RASstrength replacement. Onemortar possibleincorporating explanation of5% this increment might One explanation increment might there be the fiber‐cement bonding in the mortar. be thepossible fiber-cement bondingof in this the mortar. Although might be a degradation of cellulose fibers Although might be ofa cement degradation of some cellulose fibers due function to high pH of cement due to highthere pH condition mixture, of fibers might as a condition bridge in the mortar mixture, In some fibers might as a bridge in RAS the mortar samples. In athis sense, it is samples. thisof sense, it is very function recommendable to use with glass fibers as replacement ofvery fine recommendable to use RAS with glass fibers as a replacement of fine aggregates. Since glass fiber
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Sustainability 2016, 8, 384 9 of 16 Sustainability 2016, 8, 384 9 of 15 has more resistibility to high pH condition than cellulose fiber, mortar with RAS containing glass
Flexural strength Flexural strength (MPa)(MPa)
fiber may increase the flexural strength. has more resistibility to high pH condition than cellulose fiber, mortar with RAS containing glass aggregates. Since glass fiber has more resistibility to high pH condition than cellulose fiber, mortar fiber may increase the flexural strength. with RAS containing glass 6.0 fiber may increase the flexural strength. 7 days
5.0 6.0
28 days 7 days
4.0 5.0
28 days
3.0 4.0 2.0 3.0 1.0 2.0 0.0 1.0
0
5
25
30
0
Percentage of replaced RAS (%) 5 10 15 20 25
30
0.0
10
15
20
Figure 8. Flexural strength test for the mortar specimens containing RAS. Percentage of replaced RAS (%)
Figure 8. Flexural strength test for the mortar specimens containing RAS. 3.6. Effect of RAS in Cement Mortar Figure 8. Flexural strength test for the mortar specimens containing RAS.
This section presents discussions on how RAS reacts with cement and behaves in the cement 3.6. Effect of RAS in Cement Mortar 3.6. Effect of RAS in Cement Mortar matrix. Important mechanisms involve effects of asphalt binder and cellulose fiber on cement This section presents discussions on how RAS reacts with cement and behaves in the cement mortar. This section presents discussions on how RAS reacts with cement and behaves in the cement matrix. Important mechanisms mechanisms involve effects of asphalt binder and cellulose on mortar. cement matrix. Important involve effects of asphalt binder and cellulose fiber onfiber cement mortar. 3.6.1. Influence of the Asphalt Binder 3.6.1. Influence of the Asphalt Binder The bonding between asphalt and cement paste matrix is generally weaker than the bonding 3.6.1. Influence of the Asphalt Binder The bonding between asphalt and cement paste matrix is generally weaker than the bonding between regular sand and cement paste matrix [11]. Thus, crack may propagate around the asphalt The bonding between asphalt and cement paste matrix is generally weaker than the bonding between regular sand and cement paste in matrix [11].9b) Thus, maythe propagate around the asphalt film film surrounding aggregates (shown Figure not crack through aggregate particle (shown in between regular sand and cement paste matrix [11]. Thus, crack may propagate around the asphalt surrounding aggregates (shown in Figure 9b) not through the aggregate particle (shown in Figure 9a). Figure 9a). film surrounding aggregates (shown in Figure 9b) not through the aggregate particle (shown in Figure 9a).
(a) (a)
(b) Figure 9. Crack propagation in Mortar with sand (a) and RAS (b) [33]. (b) with sand (a) and RAS (b) [33]. Figure 9. Crack propagation in Mortar Figure 9. Crack propagation in Mortar with sand (a) and RAS (b) [33].
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This mechanism will decrease the overall compressive strength of mortar since the crack resistance This mechanism will decrease the overall compressive strength of mortar since the crack by the bonding between harden paste and aggregate may not be fully developed. Figure 10 supports resistance by the bonding between harden paste and aggregate may not be fully developed. Figure the10 supports the phenomenon that a crack propagates around the asphalt layers. Figure 10d clearly phenomenon that a crack propagates around the asphalt layers. Figure 10d clearly shows asphalt film in the fractured surface (meaning no split of aggregate) while Figure 10c shows split aggregates in shows asphalt film in the fractured surface (meaning no split of aggregate) while Figure 10c shows thesplit aggregates in the fracture section of the control mortar sample. fracture section of the control mortar sample.
(a)
(b)
(c)
(d)
Figure 10. Photographs of mortar sections: (a) Fracture section (Control); (b) Fracture section (R15); Figure 10. Photographs of mortar sections: (a) Fracture section (Control); (b) Fracture section (R15); (c) Split aggregates (Control); (d) Asphalt layer in mortar (R15). (c) Split aggregates (Control); (d) Asphalt layer in mortar (R15).
The asphalt film layers, an interface between harden cement paste and RAS aggregate, might The asphalt film layers, an interface between harden cement paste and RAS aggregate, might impede the crack propagation. While the crack detours around the side of the RAS aggregate due to impede the crack propagation. While the crack detours around the side of the RAS aggregate due the asphalt layer, the fracture energy to mobilize crack initiation and propagation might increase. In to the asphalt layer, the fracture energy to mobilize crack initiation and propagation might increase. order to check this mechanism, the toughness of mortar samples was measured. In this study, the In order to check this mechanism, the toughness of mortar samples was measured. In this study, the area of force–displacement curve of flexural test at 28 days was used as an indirect toughness index. Figure 11 shows the results of of indirect toughness flexural testing at 28 days. It was area of force–displacement curve flexural test at 28index days from was used as an indirect toughness index. found that the mortar samples containing 5% and 10% RAS have larger areas compared to that of Figure 11 shows the results of indirect toughness index from flexural testing at 28 days. It was found thatthe control sample, which indicates that the asphalt film layer requires larger energy to fracture the the mortar samples containing 5% and 10% RAS have larger areas compared to that of the control specimen and also more behavior. all mortar containing shows sample, which indicates thatductile the asphalt filmAlthough layer requires largersamples energy to fracture RAS the specimen lower flexural strength than the control, the mortar samples containing 10% RAS exhibited the and also more ductile behavior. Although all mortar samples containing RAS shows lower flexural highest “toughness”. strength than the control, the mortar samples containing 10% RAS exhibited the highest “toughness”.
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Indirect toughness index (N · mm) Indirect toughness index (N · mm)
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2500
2500 2000 2000 1500 1500 1000 1000 500 500
0 0
0 0
5 5
10 15 20 (%) 10 RAS15content 20 RAS content (%)
25 25
30 30
Figure 11. Indirect toughness index of mortar samples containing RAS. Figure 11. Indirect toughness index of mortar samples containing RAS. Figure 11. Indirect toughness index of mortar samples containing RAS.
3.6.2. Influence of the Cellulose Fiber 3.6.2. Influence of the Cellulose Fiber 3.6.2. Influence of the Cellulose Fiber The effect of cellulose fiber in cementitious material can be either increase or decrease of The effect of cellulose fiber in cementitious material can fibers be either increase or decrease of compressive strength. Figure 12 shows the existence of RAS’s in cement matrix. The main The effect of cellulose fiber in cementitious material can be either increase or decrease of compressive strength. Figure 12 shows existence of RAS’sgravity fibers inof cement matrix. The main cause cause of that decreased strength is the that low specific the fibers causes a lack of compressive strength. Figure 12 shows the existence of RAS’s fibers in cement matrix. The main of that decreased strength is that low specific gravity of the fibers causes a lack of homogeneity in homogeneity in the mixture [42]. On the other hand, there might be a positive effect of cellulous cause of that decreased strength is that low specific gravity of the fibers causes a lack of the mixture [42]. On other mixture. hand, there might a positive effect cellulous replacement in fiber replacement in the cement When the becellulous fiber is of mixed well fiber in the cement paste homogeneity in the mixture [42]. On the other hand, there might be a positive effect of cellulous cement mixture. When the cellulous fiber is mixed well in the cement paste matrix, those fibers can matrix, those fibers can form the fiber‐cement matrix bonding; hence, the fiber network can increase fiber replacement in cement mixture. When the cellulous fiber is mixed well in the cement paste form the fiber-cement matrix bonding; hence, the fiber network can increase the compressive strength the compressive strength of mortar. As shown in Figure 6, the mortar sample containing 5% RAS matrix, those fibers can form the fiber‐cement matrix bonding; hence, the fiber network can increase of mortar. As shown in Figure 6, the mortar sample containing 5% RAS exhibited a slight higher exhibited a slight higher compressive strength than the control, the positive effect of fiber is quite the compressive strength of mortar. As shown in Figure 6, the mortar sample containing 5% RAS compressive strengthof than theat control, the positive effect ofcan fiberbe is one quiteof small. Degradation of fiberlow at small. Degradation fiber high alkaline condition mechanism to explain exhibited a slight higher compressive strength than the control, the positive effect of fiber is quite high alkaline condition can be one of mechanism to explain low positive effect of cellulose fibers. positive effect of cellulose fibers. small. Degradation of fiber at high alkaline condition can be one of mechanism to explain low positive effect of cellulose fibers.
(a)
(b)
Figure 12. 12. Photographs Photographs of mortar mortar section: section: (a) (a) Fracture Fracture section section (R15) (R15) and and (b) Fracture Fracture section section with with (a) of (b) (b) Figure cellulose fiber (R15). cellulose fiber (R15). of mortar section: (a) Fracture section (R15) and (b) Fracture section with Figure 12. Photographs cellulose fiber (R15).
Cement hydration creates high pH condition (pH 12–13) in the cement mixture due to Cement hydration creates high pH condition (pH 12–13) in the cement mixture due to byproducts byproducts of hydroxyl ions and calcium hydroxide (Ca(OH)2). This high pH condition may cause creates hydroxide high pH (Ca(OH) condition (pH high 12–13) in the cement mixture due to ofCement hydroxylhydration ions and calcium pH condition may cause the degradation 2 ). This the degradation of cellulose fibers in RAS when fibers are incorporated in the cement mixture. byproducts of hydroxyl ions and calcium hydroxide (Ca(OH) 2). This high pH condition may cause of cellulose fibers in RAS when fibers are incorporated in the cement mixture. Figure 13a shows a Figure 13a shows a diagrammatic sketch of cellulose fiberʹs alkaline degradation process [43]. To the diagrammatic degradation of cellulose fibers in RAS when fibers are incorporated cement mixture. sketch of cellulose fiber's alkaline degradation process [43].in Tothe verify the degradation verify the degradation of cellulose fiber in high pH condition, high alkaline solution was prepared Figure 13a shows a in diagrammatic sketch of cellulose fiberʹs alkaline [43]. To of cellulose fiber high pH condition, high alkaline solution was degradation prepared by process dissolving sodium by dissolving sodium hydroxide (NaOH) in distill water. Manually collected cellulose fibers (50 g) verify the degradation of cellulose fiber in high pH condition, high alkaline solution was prepared hydroxide (NaOH) in distill water. Manually collected cellulose fibers (50 g) from RAS were submerged from RAS were submerged in high alkaline solution (measured pH was 12.5) for 7 days. Figures by dissolving sodium hydroxide (NaOH) in distill water. Manually collected cellulose fibers (50 g) in high alkaline solution (measured pH was 12.5) for 7 days. Figure 13b,c shows the comparison of from RAS were submerged in high alkaline solution (measured pH was 12.5) for 7 days. Figures
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cellulose fibers in RAS before and after the chemical process. The same amount of cellulose fibers was 13b and 13c shows the comparison of cellulose fibers in RAS before and after the chemical process. tested and same magnification used to tested check the of fibers. Although it to is very hard The same amount of cellulose was fibers was and conditions same magnification was used check the to quantify of thefibers. reduction of length all fibers, the length the of the fiber after chemical processthe is conditions Although it is of very hard to quantify reduction of the length of all fibers, generally reduced. The reduction of fibers can be seen and compared with the same fibers and same length of the fiber after the chemical process is generally reduced. The reduction of fibers can be magnification in Figure 13. seen and compared with the same fibers and same magnification in Figure 13.
(a)
(b)
(c)
Figure 13. Illustration of degradation process of cellulose fiber in high alkaline: (a) and photos of Figure 13. Illustration of degradation process of cellulose fiber in high alkaline: (a) and photos of cellulose fibers before; (b) and after; (c) exposed to high alkaline solution for 3 days. cellulose fibers before; (b) and after; (c) exposed to high alkaline solution for 3 days.
Several researchers studied fiber mixed cementitious composites such as mortar and concrete Several researchers studied fiber mixed cementitious composites such as mortar and concrete contain either cellulous, glass fibers or both in the past. The results of their studies and this study contain either cellulous, glass fibers or both in the past. The results of their studies and this study were compared in Table 4 to show the differences and similarities. Table 4 shows the comparison of were compared in Table 4 to show the differences and similarities. Table 4 shows the comparison of compressive and flexural strengths of this study and other studies. compressive and flexural strengths of this study and other studies. Table 4. Comparison of compressive and flexural strengths of this and other studies [44–46]. Table 4. Comparison of compressive and flexural strengths of this and other studies [44–46]. Mortar Mortar (Current (Current Study) Study) C C R5
R5
R10
R10 R15 R20
R15
R25
R20 R30 R25 R30
Mortar Compressive Compressive Strength Strength 23.64 23.64 (100%) (100%) 23.65 23.65 (100%)
Flexural Flexural Strength Strength 5.1 5.1 (100%)
(100%) 4.4 21.8(100%) (92.2%) 4.6(86.3%) (90.2%) 21.8 4.6 21.34 4.1 (80.4%) (90.3%) (92.2%) (90.2%) 4.1 19.521.34 (82.5%) 3.9 (76.5%) (90.3%) (80.4%) 18.25 3.6 (70.6%) (77.2%) 19.5 3.9 17.9 (75.7%) 3.6 (70.6%) (82.5%) (76.5%) 3.6 18.25 (77.2%) (70.6%) 17.9 3.6 (75.7%) (70.6%) 4.4 (86.3%)
[45] Mortar [45] (Hemp (Hemp Fiber) Fiber)
Control Control Hemp Hemp fiber 1% Hemp fiber 1% fiber 2% Hemp Hemp fiber 4% fiber 2% Hemp Hemp fiber10%
fiber 4% Hemp fiber10%
Mortar Compressive Flexural Compressive Flexural Mortar [46] [46] Compressive Strength Flexural Strength Compressive Flexural Strength Strength Strength Strength Strength (Glass (Glass Strength (MPa) (MPa) Fiber) (MPa) (MPa) Fiber) 32.4 9.1 70.2 6.57 32.4 (100%) 9.1 (100%) Control 6.57 (100%) Control 70.2 (100%) (100%) (100%) (100%) (100%) 7.78 Glass 32.3 (99.7%) 8.2 (90.1%) 57.58 (82%) 32.3 8.2 Glass 57.58 7.78 fiber 3% (118.4%) Glass 50.46 8.37 (99.7%) 5.6 (61.5%) (90.1%) fiber 3% (82%) (118.4%) 27.2 (83.9%) fiber 5% (71.9%) (127.4%) 27.2 5.6 Glass 50.46 Basalt 57.15 8.728.37 25.1 (77.5%) 5.6 (61.5%) 3% (81.4%) (132.7%) (83.9%) (61.5%) Fiber fiber 5% (71.9%) (127.4%) Basalt 52.10 8.538.72 57.15 25.1 5.6 Basalt 16.1 (49.7%) 5.9 (64.8%) Fiber 5% (74.2%) (129.8%) Fiber 3% (81.4%) (132.7%) (77.5%) (61.5%) 16.1 5.9 Basalt 52.10 8.53 (49.7%) (64.8%) Fiber 5% (74.2%) (129.8%)
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‐
‐
‐
‐
‐
‐
‐
‐
‐
‐
‐
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As shown in Table 4, compressive and flexural strengths of mortars which contain hemp fiber (one of cellulose fibers) were gradually decreased due to the degradation of cellulose fiber in high pH condition as we discussed [45]. However, flexural strengths of mortars which contain either glass or basalt fibers were increased up to 132.7% compared to the control mortar sample. This result can be explained by the bridging effect of glass and basalt fibers. Glass and basalt fibers have higher resistance to alkali attack than cellulose fiber so that these fibers could function as bridges. Which means that RAS contains either glass or basalt fiber may increase the flexural strength of mortar. On the other hand, compressive strengths of mortar which contain glass and basalt fiber had been deceased [46]. Greater amount of glass and basalt fibers may cause to increase the viscosity of fresh cement matrix. In addition, this higher viscosity may allow the entrapment of residual air bubble while material mixing [47]. 4. Conclusions The present work investigated the effect of RAS when used as replacement of fine aggregate in cement mortar samples. Laboratory testing procedure includes measurements of physical, mechanical, and durability properties/behaviors of RAS-mixed mortar specimens. In addition, the visual survey on the fractured surface of mortar was conducted to evaluate how RAS behaves in the cement matrix. Lastly, the degradation of cellulose fibers at high pH (due to byproduct of cement hydration) was investigated. The following conclusions have been drawn from this study. ‚
‚
‚
‚
The particle size distributions of ground RAS meets ASTM C33 grading requirements of fine aggregate for Portland cement concrete (PCC); thus, ground RAS as received could be used as replacement of sand in mortar and PCC. Overall the usage of RAS in mortar causes reduction in compressive and flexural strengths, which can be explained by the mechanism that high absorption capacity of RAS may cause reduced effective water to participate in cement hydration. It is concluded that an optimum mixture proportion for compressive strength of mortar is 5% and that for toughness is 10% RAS replacement. The asphalt film layer by RAS likely impedes the crack propagation so that the mortar samples containing RAS shows more ductile behavior than the control under flexural loading; however, the improvement is slight. The alkaline degradation of cellulose fibers due to high pH condition in cement matrix can cause the disconnection of discrete cellulose fiber-cement matrix. Thus, bridging effect of cellulose fibers in the cement matrix may not be significant. However, reusing RAS containing glass fibers in cementitious materials can result in positive influence because glass fiber has higher resistance to alkali attack than cellulose fiber.
Acknowledgments: This work was supported by National Research Foundation of Korea (NRF) funded by Ministry of Science, ICT & Future Planning (NRF-2013R1A1A1011983). Author Contributions: Jinwoo An conducted the laboratory experiments; Heejung Youn and Boo Hyun Nam designed the experiments and analyzed the data. Conflicts of Interest: The authors declare no conflict of interest.
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