International Journal of

Environmental Research and Public Health Article

The Optimum Production Method for Quality Improvement of Recycled Aggregates Using Sulfuric Acid and the Abrasion Method Haseog Kim 1 , Sangki Park 1, * and Hayong Kim 2 1 2

*

Building and Urban Research Institute, Korea Institute of Civil Engineering and Building Technology, 283, Goyang-daero, Ilsanseo-gu, Goyang-si 10223, Korea; [email protected] Department of Civil & Environmental Engineering, Kongju National University, 1223-24, Cheonan-daero, Seobuk-gu, Cheonan-si 31080, Korea; [email protected] Correspondence: [email protected]; Tel.: +82-31-910-0724

Academic Editors: Rao Bhamidiammarri and Kiran Tota-Maharaj Received: 13 June 2016; Accepted: 26 July 2016; Published: 29 July 2016

Abstract: There has been increased deconstruction and demolition of reinforced concrete structures due to the aging of the structures and redevelopment of urban areas resulting in the generation of massive amounts of construction. The production volume of waste concrete is projected to increase rapidly over 100 million tons by 2020. However, due to the high cement paste content, recycled aggregates have low density and high absorption ratio. They are mostly used for land reclamation purposes with low added value instead of multiple approaches. This study was performed to determine an effective method to remove cement paste from recycled aggregates by using the abrasion and substituting the process water with acidic water. The aim of this study is to analyze the quality of the recycled fine aggregates produced by a complex method and investigate the optimum manufacturing conditions for recycled fine aggregates based on the design of experiment. The experimental parameters considered were water ratio, coarse aggregate ratio, and abrasion time and, as a result of the experiment, data concerning the properties of recycled sand were obtained. It was found that high-quality recycled fine aggregates can be obtained with 8.57 min of abrasion-crusher time and a recycled coarse aggregate ratio of over 1.5. Keywords: recycled fine aggregates; response surface methodology; design of experiment; orthogonal arrays; abrasion-crusher time; acid treatment

1. Introduction There has been increased deconstruction and demolition of reinforced concrete structures due to the aging of the structures and redevelopment of urban areas, i.e., urban renewal, reindustrialization, and large-scale housing reconstruction plans, which result in the generation of massive amounts of construction waste that cause damage to the urban and residential environments [1,2]. There are more than 300 construction waste treatment companies in Korea with booming businesses that are producing recycled aggregates. However, the recycled aggregates manufactured by these companies are not being used from multiple approaches, but are instead used in low value-added fields, such as land reclamation. This is thought to be due to the low quality of the recycled aggregates. The volume of construction wastes generated has risen dramatically, exceeding 68 million tons in 2012, and it is expected to rise above 100 million tons by 2020 [3]. Accordingly, the production volume of waste concrete, which accounts for the highest proportion of construction wastes, increased to 42 million tons in 2012, and it is projected to increase to over 100 million tons by 2020 [4]. This has made it necessary to develop technologies for the recycling of waste concrete and establish the necessary measures for their practical use. Although there have been research and development efforts to derive a wide range Int. J. Environ. Res. Public Health 2016, 13, 769; doi:10.3390/ijerph13080769

www.mdpi.com/journal/ijerph

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of technologies and methods to produce high-quality recycled aggregates, the recycled aggregates that areInt. manufactured using the current technology are incomparable natural aggregates in terms J. Environ. Res. Public Health 2016, 13, 769 2 of 14 of quality, and this has resulted in limitations of their use. Thus, considering the aspect of effective technologies and methods recycled aggregates, the recycled that utilization of resources, there istoaproduce need tohigh-quality promote the high-value-added use of aggregates recycled aggregates. arethis, manufactured using the current technology are incomparable aggregates in terms of Based on there is high value in research performed to enhancenatural the quality of recycled aggregates. quality, and this has resulted in limitations of their use. Thus, considering the aspect of effective The reason behind the low quality of recycled fine aggregates is that the cement paste attached to utilization of resources, there is a need to promote the high-value-added use of recycled aggregates. the aggregate surface has isa high negative on performed the absorption ratiothe and specific gravityaggregates. [5]. Accordingly, Based on this, there valueimpact in research to enhance quality of recycled there have been numerous with the to effectively cement paste from The reason behindstudies the low conducted quality of recycled fine aim aggregates is that theremove cement paste attached to the aggregate has amethods negative of impact the absorption and specific gravity aggregate surface. Mostsurface common suchon removal are the ratio mechanical method in [5]. which the there have been numerous studies conducted with the to effectively cement cement Accordingly, paste is removed by physically crushing and grinding theaim waste concreteremove and the innovative from aggregate surface. Most common methods of such removal are the mechanical method in methodpaste using heat or acid. Although a crushing process, for example, can lead to a significant which the cement paste is removed by physically crushing and grinding the waste concrete and the reduction of the cement paste content, there are issues of increased costs and an overall reduction in innovative method using heat or acid. Although a crushing process, for example, can lead to a the production volume of of coarse aggregates as it involves crushing of relatively aggregates significant reduction the cement paste content, there arethe issues of increased costs andweak an overall into finereduction aggregates In order to of resolve these issues, acid may used of asrelatively a meansweak to remove in the[6–8]. production volume coarse aggregates as it involves thebe crushing into fine aggregates [6–8]. In order resolve these issues, acida may be used as a means to It has only theaggregates cement paste, which is alkaline and cantobe removed through neutralization reaction. remove only the cement paste, which is alkaline and can be removed through a neutralization been reported that the use of sulfuric and hydrochloric acids results in a high rate of cement paste reaction. It has been reported that the use of sulfuric and hydrochloric acids results in a high rate of removal [9–11]. Additionally, using a rotary mixing system in addition to the acid treatment can cement paste removal [9–11]. Additionally, using a rotary mixing system in addition to the acid enhancetreatment the reactivity between the cement paste the paste acidic substance, thereby facilitating the can enhance the reactivity between theand cement and the acidic substance, thereby removalfacilitating of the cement paste and alumino-silicate gelalumino-silicate [12]. Thus, ingel this[12]. study, in in addition to executing the removal of the cement paste and Thus, this study, in addition to acid treatment, the abrasion method combination with a was chemical reaction acid treatment, theexecuting abrasion method in combination with ainchemical reaction applied as shown was applied as shown in 1. The of abrasion be largely divided in Figure 1. The mechanisms of Figure abrasion can mechanisms be largely divided intocan compression, impact,into shearing, compression, impact, shearing, and friction. However, it is highly rare for only one of these abrasion and friction. However, it is highly rare for only one of these abrasion mechanisms to occur; instead, mechanisms to occur; instead, generally speaking, two or more types of abrasion mechanisms occur generally speaking, two or more types of abrasion mechanisms occur in combination [5,7,13]. In order in combination [5,7,13]. In order to improve the efficiency of abrasion, various forms of abrasion to improve the efficiency of abrasion, forms of abrasion media be combined in order to media may be combined in order to various apply higher energy to the ground itemmay and enhance the grinding apply higher energy to the ground item and enhance the grinding efficiency [14,15]. efficiency [14,15].

Figure 1. Principle of abrasion crusher.

Figure 1. Principle of abrasion crusher. Accordingly, this study was conducted with the aim to mitigate the issues arising from the crushing andthis grinding used in the with conventional recycled aggregate practices Accordingly, studymethods was conducted the aim to mitigate theproduction issues arising from the and to empirically and statistically review and analyze the optimum conditions in which crushing and grinding methods used in the conventional recycled aggregate productionrecycled practices and fine aggregates could be produced after neutralizing strong alkaline water. In other words, the to empirically and statistically review and analyze the optimum conditions in which recycled fine amount of washing water, which exerts an impact during abrasion, the amount of coarse aggregates aggregates could be produced after neutralizing strongasalkaline water. In parameters, other words, to be ground, and the abrasion time were chosen the experimental andthe theamount of washing water, which exerts an impact during abrasion, the amount of coarse aggregates effectiveness of the recycled fine aggregate and water ratio, the recycled fine aggregate, and coarse to be ratio, and time the abrasion time were under an orthogonal and design. the ground,aggregate and the abrasion were chosen as thereviewed experimental parameters, the Then, effectiveness of optimum conditions for the production of recycled fine aggregates derived using the the recycled fine abrasion aggregate and water ratio, the recycled fine aggregate, andwere coarse aggregate ratio, and response surface methodology (RSM). During the optimization procedure, unit production amount was the abrasion time were reviewed under an orthogonal design. Then, the optimum abrasion conditions expressed as a function of coarse aggregate ratio and the abrasion time and was taken as the objective for the production of recycled fine aggregates were derived using the response surface methodology function. Nelder-Mead sequential simplex algorithm, i.e., fminsearch function in MATLAB (R2015b, the (RSM). During the optimization procedure, production expressed as a function of Mathworks Inc., Natick, MA, USA, 2015), wasunit applied to solve theamount problem was with various constraints. coarse aggregate ratio and the abrasion time and was taken as the objective function. Nelder-Mead sequential simplex algorithm, i.e., fminsearch function in MATLAB (R2015b, the Mathworks Inc., Natick, MA, USA, 2015), was applied to solve the problem with various constraints.

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2. Experimental Design and Method 2.1. Experiment Design In general, the orthogonal design method [16], proposed by Genichi Taguchi, is an experimental design method that can reduce a number of experiments by means of sacrificing the information on the parameters effecting on the test results. Therefore, the orthogonal design method has been adopted to estimate variable interactions among water ratio, coarse aggregate ratio, and abrasion time [17]. In this study, a three-level design system, mainly used in cases where factors are measured values, was used as represented in Equation (1), which is commonly used when the parameters are indiscrete values, was used [18]: 3m´1 L3m “ 3p 2 q (1) where L is an orthogonal array; m is an integer number of experiment factor; 3m is a size of the experiment; and (3m ´ 1)/2 is the number of columns in the orthogonal array. Additionally, RSM employed in order to derive the optimum abrasion method for the production of recycled fine aggregates is a statistical analysis method focusing on the response surface on which changes occur when multiple explanatory variables pξ1, ξ2, ξ3, . . . . . . ξn ) exert an impact on a certain variable η in a complex manner [7]. RSM used in this study employed the secondary regression model, and the T-surface is expressed as shown in Equation (2): η “ β0 `

ÿ

(2)

βij χi χ j

where η is an dependent variable; χ is an independent variable; and β is an constant. As for the experimental design and level of this study, three factors, i.e., the fine aggregate to water ratio, the fine aggregate to coarse aggregate ratio, and abrasion time, were selected as the experimental parameters, as shown in Table 1 and 27 experimental batches were arranged in nine experimental levels, as shown in Table 2, using an orthogonal design based on the experimental design. It should be commented that the water ratio is the ratio of the volume of water to the volume of total aggregates. When the washing water ratio is less than 0.7, it is impossible to conduct experiments due to flocculation. If the washing water ratio exceeds 1.3, the grinding efficiency is decreased. Based on these reasons, two values, i.e., 0.7 and 1.3, are selected as a lower and an upper boundary, respectively. The coarse aggregate ratio is the ratio of the weight of coarse aggregates to the weight of fine aggregates. If the coarse aggregate ratio is under 0.5, then the grinding efficiency is decreased. On the contrary, if it is over 1.5, fine aggregates are ground with a size of 1.2 mm or less due to the excessive crushing action. Therefore, a lower and an upper boundary for the coarse aggregate ratio is 0.5 and 1.5, respectively. In addition, based on the results of applying the orthogonal design, RSM was used to derive the optimum abrasion conditions. The coarse aggregates to be ground in this experiment were broken pebbles that were bigger than 100 mm, and they were to replace the fine aggregates in terms of weight. The amount of washing water to be used was determined for the total volume of fine and coarse aggregates. The abrasion time was divided into three levels: 5 min, 10 min, and 15 min. This was because field application of this experimental method would decrease the production rate for long-term processing and reduce the cost effectiveness. Table 1. Experimental plan using the design of experiment. Levels

Factors A B C 1

Water ratio 1 (%) Coarse aggregate ratio 2 (%) Abrasion time (min)

Test Items

0

1

2

0.7 0.5 5

1.0 1.0 10

1.3 1.5 15

Density Absorption ratio Percentage of solid volume

Volume ratio of water to total aggregate; 2 Weight ratio of coarse aggregate to fine aggregate.

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Table 2. The level of experiment. A

B

C

Water Ratio (%)

Coarse Aggregate Ratio (%)

Abrasion Time (min)

A0 B0 C0 A1 B0 C2 A2 B0 C1

0.7 1.0 1.3

0.5 0.5 0.5

5 15 10

A0 = 0.7 A1 = 1.0 A2 = 1.3

A0 B1 C1 A1 B1 C0 A2 B1 C2

0.7 1.0 1.3

1.0 1.0 1.0

10 5 15

B0 = 0.5 B1 = 1.0 B2 = 1.5

A0 B2 C2 A1 B2 C1 A2 B2 C0

0.7 1.0 1.3

1.5 1.5 1.5

15 10 5

C0 = 5 C1 = 10 C2 = 15

ID

1

Levels 1

0, 1, and 2 represent the experimental level, respectively.

2.2. Experimental Methods The recycled fine aggregates, having a size of 5 mm or less, used in this experiment were obtained from Green and Environments Co., Cheonan, Korea and their physical properties are shown in Table 3. Additionally, the washing water used in this experiment was water from the common waterworks, which is typically used by recycled aggregate manufacturers. Table 3. The physical properties of recycled fine aggregate. Density (kg/m3 )

Absorption Ratio (%)

Fineness Modulus

Solid Content in Aggregate (%)

Unit Weight (kg/m3 )

2270

6.56

3.40

59.6

1443

It should be noted that the fineness modulus (FM) is an empirical factor obtained by adding the total percentages of a sample of the aggregate retained on each of a specified series of sieves, and dividing the sum by 100. Additionally, the unit weight is the weight per unit volume of a material. It depends on the value of the gravitational acceleration. The density of a substance is its mass per unit volume [19–21]. As shown in Figure 2, recycled fine and coarse aggregates were fed first into the system according to the weight ratio, and the washing water, made by diluting sulfuric acid, was fed into the system afterwards according to the volume ratio. Then, the experiment was conducted by varying the abrasion time. Furthermore, the tests on the density, absorption ratio and solid volume percentage of the recycled fine aggregates generated were conducted in accordance with KS F 2504 [22], which is similar to [23], and KS 2505 [24], similar to ASTM C29 [25], respectively. 5 of 14 Int.ASTM J. Environ.C128 Res. Public Health 2016, 13,F 769 Recycled Fine Aggregate + Coarse Aggregate (0.5, 1.0, 1.5 Weight ratio)

Washing water (0.7, 1.0, 1.3 Volume ratio)

Abrasion crusher (5, 10, 15 mins)

Test Physical properties Figure 2. Experiment process of the manufacture of high-quality of recycled fine aggregate.

Figure 2. Experiment process of the manufacture of high-quality of recycled fine aggregate. 2.3. Measurement Method The testing on the quality of the recycled aggregates was conducted according to the items listed in the quality standards for recycled aggregates (limited to recycled fine aggregates used in concrete manufacture), and of the physical properties specified in the quality standards, the most important quality characteristics of recycled fine aggregates were reviewed and a statistical analysis was

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2.3. Measurement Method The testing on the quality of the recycled aggregates was conducted according to the items listed in the quality standards for recycled aggregates (limited to recycled fine aggregates used in concrete manufacture), and of the physical properties specified in the quality standards, the most important quality characteristics of recycled fine aggregates were reviewed and a statistical analysis was performed on the results. Table 4 shows the items that were measured in this study [22,24,26]. Table 4. Testing items and their measurement methods. Item

Experimental Method

Standard of Recycled Fine Aggregate

Density (kg/m3 ) Absorption ratio (%) Solid content in aggregate (%)

KS F 2503 (ASTM C128) KS F 2504 (ASTM C128) KS F 2505 (ASTM C29)

2.2 over 5.0 under 53 over

3. Results and Discussion 3.1. Analysis of the Results of the Experiment Performed According to an Orthogonal Design Table 5 shows the results for the density, absorption ratio, and solid volume percentage according to the abrasion conditions that were obtained according to an orthogonal design. Table 5. Test results. Density (kg/m3 ) Oven-Dry

Saturated Surface-Dry

Absorption Ratio (%)

ID

Solid Content in Aggregate (%)

Base

2270

2420

6.56

59.6

A0 B0 C0 A1 B0 C2 A2 B0 C1

2410 2480 2440

2500 2540 2520

3.43 2.46 3.20

60.7 65.4 64.6

A0 B1 C1 A1 B1 C0 A2 B1 C2

2460 2410 2500

2540 2500 2550

3.03 3.95 2.02

64.6 62.3 65.3

A0 B2 C2 A1 B2 C1 A2 B2 C0

2530 2480 2430

2570 2550 2500

1.65 2.90 3.24

65.7 64.7 62.7

3.1.1. Oven-Dry Density IBM SPSS Statistics (V19.0, IBM Corp., Armonk, NY, USA, 2015) [27], the commercial statistics software package, is selected to perform the F-test in this study. Table 6 shows the results of the F-test performed on each experimental parameter after performing ANOVA on the density results. The results of the ANOVA show that the F values of A (water ratio), B (coarse aggregate ratio), and C (abrasion time) were 3.0, 31.0, and 169.0, respectively. As for the experimental values, the degree of freedom was 2 and the error value was 2. Thus, the critical values, F0.01 , F0.05 , and F0.10 , were found to be 99.0, 19.0, and 9.0, at a confidence limit of 99% (level of significance, α = 0.01), 95% (level of significance, α = 0.05), and 90% (level of significance, α = 0.10). Based on this, C (abrasion time) was found to satisfy the 99% (level of significance, α = 0.01) level, which means that it exerted the most significant impact on the quality of recycled aggregates. In addition, B (coarse aggregate ratio) satisfied the 95% (level of significance, α = 0.05) level, meaning that it had an impact on the quality. However, A (water ratio) was found to be insignificant even at a confidence limit of 90% (level of significance, α = 0.10), meaning that it did not exert any significant impact on quality.

found to satisfy the 99% (level of significance, α = 0.01) level, which means that it exerted the most significant impact on the quality of recycled aggregates. In addition, B (coarse aggregate ratio) satisfied the 95% (level of significance, α = 0.05) level, meaning that it had an impact on the quality. However, A (water ratio) was found to be insignificant even at a confidence limit of 90% (level of Int. J. Environ. Res. Health 2016, 13,that 769 it did not exert any significant impact on quality. 6 of 14 significance, α =Public 0.10), meaning Table 6. Variance analysis for density of recycled fine aggregate. Table 6. Variance analysis for density of recycled fine aggregate. ID A B C

ID Factors S1 Factors water S1 A Washing 0.0002 B Washing Coarse water aggregate 0.0002 0.0021 Coarse aggregate 0.0021 C Abrasion Abrasion time 0.0113 time 0.0113 Error 0.0001 Error 0.0001 Total 0.0136 Total 0.0136

1

2

Ø2 Ø2 2 22 222 22 88

V3 V3 0.0001 0.0001 0.0010 0.0010 0.0056 0.0056 0.0000 0.0000

3

F0 4 Evaluation 4 3.00F0 -Evaluation 3.00 31.00 ** 31.00 ** 169.00 *** *** 169.00 4

sumof of squares; squares; 2Ø:Ø: degree of freedom; V: mean the sum of squares; F0 : F-statistics *** accepted 3 V: of 4 F0:value; S:S:sum degree of freedom; mean of the sum of squares; F-statistics value; at the 0.01 significance level; ** accepted at the 0.05 significance level; - not accepted. *** accepted at the 0.01 significance level; ** accepted at the 0.05 significance level; - not accepted.

1

Figure 33 shows shows the the density density values values estimated estimated using using the the test test results. results. The The mean mean density density varied varied Figure between 2.24 2.24 and and 2.51. 2.51. As Asshown shownin inthe thefigure, figure,coarse coarseaggregate aggregateratio ratio and abrasion time were found between and abrasion time were found to to be positively correlated with density, and of particular note, abrasion time was found to be a major be positively correlated with density, and of particular note, abrasion time was found to be a major factor contributing contributing to to an an increase increase in in density. density. On On the the other other hand, hand, the the water water ratio, ratio, which which was was found found to to factor have little to no effect based on the F-test results, was shown to reduce the density, albeit slightly, if have little to no effect based on the F-test results, was shown to reduce the density, albeit slightly, if it it was decreased. was decreased. 2500

2500

2480

2480

2460

2460

2440

2440 2420

2420 0.5

1

4

1.5

(a)

6

8

10

12

14

16

Abrasion time (min)

Coarse aggregate ratio (%)

(b)

(c)

Figure 3. 3. Assumed Assumed density recycled fine fine aggregate aggregate by by variance variance analysis: analysis: (a) (a) water water ratio; ratio; (b) (b) coarse coarse Figure density of of recycled aggregate ratio; and (c) abrasion crusher time. aggregate ratio; and (c) abrasion crusher time.

3.1.2. Absorption Ratio 3.1.2. Absorption Ratio Table 7 shows the results of the F-test performed on each experimental parameter after Table 7 shows the results of the F-test performed on each experimental parameter after performing performing ANOVA on the absorption ratio results. The F values of A (water ratio), B (coarse ANOVA on the absorption ratio results. The F values of A (water ratio), B (coarse aggregate ratio), aggregate ratio), and C (abrasion time) were found to be 6.50, 9.00, and 89.2, respectively. Since C and C (abrasion time) were found to be 6.50, 9.00, and 89.2, respectively. Since C (abrasion time) was (abrasion time) was larger than 19.0, the critical value at a confidence limit of 95% (level of larger than 19.0, the critical value at a confidence limit of 95% (level of significance, α = 0.05), it was significance, α = 0.05), it was determined that this parameter had the most significant impact on the determined that this parameter had the most significant impact on the post-abrasion absorption ratio. post-abrasion absorption ratio. Additionally, B (coarse aggregate ratio) was found to be a significant Additionally, B (coarse aggregate ratio) was found to be a significant factor at a confidence limit of 90% factor at a confidence limit of 90% (level of significance, α = 0.10). In the case of water ratio, it was (level of significance, α = 0.10). In the case of water ratio, it was smaller than 9.0, the critical value at a confidence limit of 90% (level of significance, α = 0.10) and, thus, it was deemed to have little impact on the post-abrasion absorption ratio. Table 7. Variance analysis for absorption ratio of recycled fine aggregate. ID

Factors

S

Ø

V

F0

Evaluation

A B C

Washing water Coarse aggregate Abrasion time Error Total

0.2539 0.3514 3.4867 0.0391 4.1310

2 2 2 2 8

0.1269 0.1757 1.7433 0.0195

6.4952 8.9886 89.1990

* ***

*** accepted at the 0.01 significance level; * accepted at the 0.10 significance level; - not accepted.

A B C

Washing water Coarse aggregate Abrasion time Error Int. J. Environ. Res. Public Health 2016, 13, 769 Total

0.2539 0.3514 3.4867 0.0391 4.1310

2 2 2 2 8

0.1269 0.1757 1.7433 0.0195

6.4952 8.9886 89.1990

* *** 7 of 14

*** accepted at the 0.01 significance level; * accepted at the 0.10 significance level; - not accepted.

Figure Figure 44 shows shows the the changes changes in in the the values values estimated estimated for for the the population population mean mean for for each each level level of of the the experimental respect to to thethe absorption ratio. As shown in the the absorption ratio experimentalparameters parameterswith with respect absorption ratio. As shown infigure, the figure, the absorption decreased significantly with with increased abrasion time. TheThe absorption ratio also ratio decreased significantly increased abrasion time. absorption ratio alsodecreased decreasedwith with an an increased but not asas substantially as the absorption ratio. On the hand, the increasedcoarse coarseaggregate aggregateratio, ratio, but not substantially as the absorption ratio. On other the other hand, water ratioratio showed a point of inflection at 1.0, at making it difficult to identify a consistent trend. trend. the water showed a point of inflection 1.0, making it difficult to identify a consistent 4 3.5 3 2.5 2 1.5 0.6

0.8

1

1.2

1.4

Water ratio (%)

(a)

(b)

(c)

Figure Figure 4. 4. Assumed Assumedabsorption absorptionratio ratioof ofrecycled recycledfine fine aggregate aggregate by by variance variance analysis: analysis:(a) (a)water waterratio; ratio; (b) coarse aggregate ratio; and (c) abrasion crusher time. (b) coarse aggregate ratio; and (c) abrasion crusher time.

3.1.3. Solid Volume Percentage 3.1.3. Solid Volume Percentage Table 8 shows the results of the F-test performed on each experimental parameter after Table 8 shows the results of the F-test performed on each experimental parameter after performing performing ANOVA on the solid volume percentage results. The F values of A (water ratio), B (coarse ANOVA on the solid volume percentage results. The F values of A (water ratio), B (coarse aggregate aggregate ratio), and C (abrasion time) were found to be 0.60, 1.16, and 24.7, respectively. The sum of ratio), and C (abrasion time) were found to be 0.60, 1.16, and 24.7, respectively. The sum of squares of squares of A (water ratio) was smaller than the error term and, thus, A (water ratio) was included in A (water ratio) was smaller than the error term and, thus, A (water ratio) was included in the error the error for pooling. for pooling. Table 9 shows the pooling results. C (abrasion time) was higher than 18.0, the critical value at a confidence limit of 99%8.(level of significance, α solid = 0.01), while (coarsefine aggregate ratio) was smaller Table Variance analysis for the volume of B recycled aggregate. than 4.32, the critical value at a confidence limit of 90% (level of significance, α = 0.10). Thus, C ID was found Factors S Ø V even forF0the solidEvaluation (abrasion time) as the most significant parameter volume percentage, whereas A (water ratio) and Bwater (coarse aggregate found to0.5984 be insignificant. A Washing 0.5067 ratio) 2 were 0.2533 B C

Coarse aggregate 0.9800 2 0.4900 1.1575 Abrasion time 20.8867 2 10.4433 24.6693 *** Table 8. Variance analysis for the solid volume of recycled fine aggregate. Error 0.8467 2 0.4233 Total 23.2200 8

ID Factors S Ø V F0 Evaluation *** accepted at the 0.01 significance level; - not accepted. A Washing water 0.5067 2 0.2533 0.5984 B Coarse aggregate 0.9800 2 0.4900 1.1575 Table 9 shows pooling time results. C (abrasion was higher than 18.0,***the critical value at a C the Abrasion 20.8867 2 time) 10.4433 24.6693 confidence limit of 99%Error (level of significance, while B (coarse aggregate ratio) was smaller 0.8467α = 20.01),0.4233 than 4.32, the critical value limit8of 90% (level of significance, α = 0.10). Thus, C Total at a confidence 23.2200 (abrasion time) was found as the most significant parameter even for the solid volume percentage, *** accepted at the 0.01 significance level; - not accepted. whereas A (water ratio) and B (coarse aggregate ratio) were found to be insignificant. Table 9. Variance analysis for the solid volume of recycled fine aggregate after pooling. ID

Factors

S

Ø

V

F0

Evaluation

B C

Coarse aggregate Abrasion time Error Total

0.9800 20.887 1.3534

2 2 4 8

0.4900 10.443 0.3384

1.45 30.9

***

*** accepted at the 0.01 significance level; - not accepted.

Figure 5 shows the changes in the values estimated for the population mean for each level of the experimental parameters with respect to the solid volume percentage. Similar to the experimental

Error Total

1.3534

4 8

0.3384

*** accepted at the 0.01 significance level; - not accepted.

Figure 5 shows the changes in the values estimated for the population mean for each level of8 of the Int. J. Environ. Res. Public Health 2016, 13, 769 14 experimental parameters with respect to the solid volume percentage. Similar to the experimental results for density and absorption ratio, an increase in the abrasion time substantially increased the results for density and absorption ratio,was an increase in dramatic the abrasion time substantially the solid volume percentage. The increase especially between 5 and 10 minincreased of abrasion solid volume percentage. The increase was especially dramatic between 5 and 10 min of abrasion time. time. On the other hand, an increase in the water ratio or the coarse aggregate ratio caused only a On theincrease other hand, increase in the water ratio the coarse aggregate ratio only crusher a slight slight in theansolid volume percentage. Anorincrease of processing time caused in abrasion increaseininan theincrease solid volume percentage. An increase of processing time in abrasion crusher results results of friction between the recycled fine aggregates. Therefore, it leads to in an increase of friction between the recycled fine aggregates. Therefore, it leads to accelerating the accelerating the removal of cement paste from the recycled fine aggregates, enhancing the grain size removal of cement paste from theactions recycled fine aggregates, enhancing grain sizewhich of aggregates, of aggregates, etc. Finally, these improve the solid contents in the aggregates, is shownetc. in Finally,5c. these actions improve the solid contents in aggregates, which is shown in Figure 5c. Figure 70 68 66 64 62 60 58 4

6

8

10

12

14

16

Abrasion time (min)

(a)

(b)

(c)

Figure ofof recycled fine aggregate by by variance analysis: (a) water ratio;ratio; (b) Figure 5.5. Assumed Assumedsolid solidvolume volume recycled fine aggregate variance analysis: (a) water coarse aggregate ratio; and (c) abrasion crusher time. (b) coarse aggregate ratio; and (c) abrasion crusher time.

3.2. and Optimization 3.2.Response ResponseSurface SurfaceMethodology Methodology and Optimization RSM was applied with the aim to RSM was applied with the aim to determine determine the the abrasion abrasion conditions conditions that that would would induce induce optimum optimum performance based on the results derived by applying an orthogonal array design. The target performance performance based on the results derived by applying an orthogonal array design. The target levels were over 2460were oven-dry density and lessdensity than 3.0and absorption It should beratio. notedItthat the performance levels over 2460 oven-dry less thanratio. 3.0 absorption should recycled fine aggregate, which meets the quality standard, has been limited to use in concrete be noted that the recycled fine aggregate, which meets the quality standard, has been limited to use manufacture. To overcomeToit,overcome the quality standard the natural aggregate, which iswhich over in concrete manufacture. it, the quality for standard for thefine natural fine aggregate, 2450 density and less than 3.0 absorption ratio, has been considered as the quality standard for the recycled is over 2450 density and less than 3.0 absorption ratio, has been considered as the quality standard fine aggregate in this The solid volume percentage was omitted as it was shown to be excellent in for the recycled finestudy. aggregate in this study. The solid volume percentage was omitted as it was all experimental values at over 60%. Moreover, as analyzed above, A (water ratio) was excluded as it was shown to be excellent in all experimental values at over 60%. Moreover, as analyzed above, A (water determined to have low significance, and RSM was applied only for B (coarse aggregate ratio) and C ratio) was excluded as it was determined to have low significance, and RSM was applied only for B (abrasion time). The results were then compiled to derive the optimum abrasion conditions. (coarse aggregate ratio) and C (abrasion time). The results were then compiled to derive the optimum abrasion conditions. 3.2.1. Density 3.2.1.Relationship Density between two factors, i.e., the coarse aggregate ratio and the abrasion time, and the density of recycled fine aggregate was derived using Equationratio (3) and expressed as:and the Relationship between two factors, i.e., the by coarse aggregate andcan thebe abrasion time, density of recycled = fine aggregate was×derived by using can × be expressed as: 2.3833 − 0.0333 + 0.0057 × Equation + 0.02 ×(3) and + 0.003 ×

(3)

2 where YD is the Ydensity of recycled fine aggregate, x1 is the coarse aggregate ratio, and x2 is the (3) D “ 2.3833 ´ 0.0333 ˆ x1 ` 0.0057 ˆ x2 ` 0.02 ˆ x1 ` 0.003 ˆ x1 ˆ x2 abrasion time. where YD is the density of recycled fine aggregate, x1 is the coarse aggregate ratio, and x2 is the abrasion time. Table 10 shows the results of applying RSM with respect to density in cases where there were changes in B (coarse aggregate ratio) and C (abrasion time). R2, the coefficient of determination for the regression equation, with respect to density, was 0.99, and its significance within the significance level of 99% was recognized.

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Table 10 shows the results of applying RSM with respect to density in cases where there were changes in B (coarse aggregate ratio) and C (abrasion time). R2, the coefficient of determination for Int. Environ. Res.equation, Public Healthwith 2016,respect 13, 769 to density, was 0.99, and its significance within the significance 9 of 14 theJ.regression level of 99% was recognized. Table 10. Variance analysis for the density of recycled fine aggregate. Table 10. Variance analysis for the density of recycled fine aggregate. Factors Factors

S

S ØØ Regression 0.0136 5 5 Regression 0.0136 Error Error 0.0000 0.0000 3 3 Total Total 0.0136 0.0136 8 8

Evaluation VV F0 F0 Evaluation 0.0027 *** *** 0.0027 195.2400 195.2400 0.0000 0.0000

accepted at the 0.01 significance level. ******accepted at the 0.01 significance level.

Figure 6 shows shows the the results results of of applying applying RSM in in relation relation to to the the oven-dry oven-dry density density of recycled recycled fine fine aggregates that were were obtained obtainedfrom fromBB(coarse (coarseaggregate aggregateratio) ratio)and andCC(abrasion (abrasiontime). time).Line Line Figure 6 TT inin Figure 6 is is a line that satisfies the target performance level for the oven-dry density, which was 2.46. In the a line that satisfies the target performance level for the oven-dry density, which was 2.46. In the case case of recycled aggregates, increased density results qualityimprovement improvementand, and,thus, thus, the the upper of recycled fine fine aggregates, increased density results inin quality part of Line T satisfies the target performance level. The target performance level was satisfied at an abrasion time of over 11 min for a coarse aggregate ratio of 1.0, over 8 min for a coarse aggregate ratio of 1.5, a coarse aggregate ratio of 2.0. Increased abrasion time means increased input 1.5, and andover over6 6min minforfor a coarse aggregate ratio of 2.0. Increased abrasion time means increased energy, and this improves the quality of recycled aggregates. Increasing the coarse aggregate ratio, on input energy, and this improves the quality of recycled aggregates. Increasing the coarse aggregate the other hand, results a relative decreasedecrease in the target of recycled fine aggregates as the ratio, on the other hand,inresults in a relative in theamount target amount of recycled fine aggregates capacity of the abrasion device isdevice fixed. is Therefore, these variables be should considered to determine as the capacity of the abrasion fixed. Therefore, these should variables be considered to the optimum coarse aggregate ratio and abrasion time. determine the optimum coarse aggregate ratio and abrasion time. 20

2.56

2.52

2.62 2.58

Increase of Density

2.48

15

2.6

2.54 2.5

10

T

2.44

2.46

Decrease of Density

5

0

2.4

2.42 2.38

0

0.5

1

1.5

2

Coarse Aggregate Ratio (%) Figure 6. 6. Analysis aggregate drive drive from from coarse coarse Figure Analysis response response surface surface for for the the density density of of recycled recycled fine fine aggregate aggregate ratio and abrasion time. aggregate ratio and abrasion time.

3.2.2. Absorption Ratio 3.2.2. Absorption Ratio Again, the relationship between two factors, i.e., the coarse aggregate ratio and the abrasion Again, the relationship between two factors, i.e., the coarse aggregate ratio and the abrasion time, time, and the absorption ratio of recycled fine aggregate was derived by using Equation (4) and can and the absorption ratio of recycled fine aggregate was derived by using Equation (4) and can be be expressed as: expressed as: (4) = 2.7244 + 1.68 × + 0.1137 × − 0.7467 × − 0.062 × × − 0.0101 × YAR “ 2.7244 ` 1.68 ˆ x1 ` 0.1137 ˆ x2 ´ 0.7467 ˆ x12 ´ 0.062 ˆ x1 ˆ x2 ´ 0.0101 ˆ x22 (4) where YAR is the absorption ratio of recycled fine aggregate, x1 is the coarse aggregate ratio, and x2 is the abrasion where YAR is time. the absorption ratio of recycled fine aggregate, x1 is the coarse aggregate ratio, and x2 is Table 11 shows the results of applying RSM with respect to the absorption ratio in cases where the abrasion time. thereTable were11changes in results B (coarse aggregate ratio) C (abrasion time). R2, theincoefficient of shows the of applying RSM withand respect to the absorption ratio cases where determination for the equation with to absorption 0.95, and its there were changes in regression B (coarse aggregate ratio) respect and C (abrasion time).ratio, R2, was the coefficient of significance within the significance level of 90% was recognized. determination for the regression equation with respect to absorption ratio, was 0.95, and its significance within the significance level of 90% was recognized.

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Table 11. Variance analysis for absorption ratio of recycled fine aggregate. Table 11. Variance analysis for absorption ratio of recycled fine aggregate. Factors Factors

S

S ØØ Regression 3.9341 5 5 Regression 3.9341 Error Error 0.1969 0.1969 3 3 Total Total 4.1310 4.1310 8 8

Evaluation VV F0 F0 Evaluation 0.7868 ** 0.7868 11.9896 11.9896 ** 0.0656 0.0656

** accepted at the 0.05 significance level.

** accepted at the 0.05 significance level.

Figure 77shows results of the analysis of the optimum abrasion abrasion range for range the absorption Figure showsthe the results of contour the contour analysis of the optimum for the ratio according B (coarsetoaggregate and C (abrasion Thetime). resultsThe show that,show similar to absorption ratioto according B (coarse ratio) aggregate ratio) and Ctime). (abrasion results that, density, an abrasion time of over 15 min and fine aggregate and coarse aggregate ratio of over 1 resulted similar to density, an abrasion time of over 15 min and fine aggregate and coarse aggregate ratio of in a performance exceeding level the target level proposed the proposed quality standards for recycled fine over 1 resulted in level a performance exceeding the targetin level in the quality standards aggregates, which is up to the standard in comparison to natural fine aggregates. for recycled fine aggregates, which is up to the standard in comparison to natural fine aggregates. 1

20

1.5

15

2.5

-1 -0.5

0

1

0.5

Decrease of Absorption ratio 2

10

T 3

5

Increase of Absorption ratio

3

0

0

0.5

1

3.5

1.5

2

Coarse Aggregate Ratio (%) Figure 7. Analysis response surface for the absorption ratio of recycled fine aggregate derived from Figure 7. Analysis response surface for the absorption ratio of recycled fine aggregate derived from the the coarse aggregate ratio and abrasion time. coarse aggregate ratio and abrasion time.

3.2.3. Derivation Optimum Abrasion Conditions 3.2.3. Derivation Optimum Abrasion Conditions An optimization technique is applied to seek the optimum abrasion conditions. Typically, an An optimization technique is applied to seek the optimum abrasion conditions. Typically, an optimization problem with constraints can be expressed as: optimization problem with constraints can be expressed as: Maximize f(x) (5) Maximize f(x) (5) ( )≤0 : = 1,2, … , # Subjected to (6) ( )ď=00 : : =i “+1,1, gℎi pxq 2, . .+. ,2, q… Subjected to (6) h j pxq “ inequality 0 : j “ qconstraints, ` 1, q ` 2, . q. .is m the number of inequality where f(x) is an objective function, g(x) are the constraints, h(x) the equality constraints, providesconstraints, the numberqofisequality constraints [28]. where f(x) is anare objective function, g(x) areand them–q inequality the number of inequality It is difficult to determine the global maximum or minimum solution if the objective function constraints, h(x) are the equality constraints, and m–q provides the number of equality constraints [28]. does It not onetoordetermine more constraints. Therefore, a penalized objective function is adopted and is is have difficult the global maximum or minimum solution if the objective function expressed as follows [28]: does not have one or more constraints. Therefore, a penalized objective function is adopted and is expressed as follows [28]: m ( ) = ( ) +ÿ ( ) f p pxq “ f pxq ` Ci pPi qβ

(7) (7)

i “1

δ ( ) : = 1,2, … , δi gi pxq : i “ 1, 2, . . . , q ˇ ℎ ( ˇ) : = ˇh j pxqˇ : j “ q `+1,1, q `+ 2, 2, . .… .m

(8) (8)

1 : if − ℎ constraint is violated (i. e. , ( ) > 0) 0 : otherwise ( i. e. , g (x) ≤ 0 )

(9)

# ) = pPi q(β “

δ =

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# δi “

1 0

: :

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if i ´ th constraint is violated pi.e., gi pxq ą 0q otherwise pi.e., gi pxq ď 0q

(9)

where fP (x) is a penalized objective function, f(x) is the (unpenalized) objective function, Ci is a value imposed for violation of the ith constraint with values equal to a relatively large number, β is a user-defined exponent, with values of 1 or 2 typically used, δi is the Kronecker delta function, and constraints 1 through q are inequality constraints. One can see that the penalty will only be activated when the constraint is violated, while constraints q + 1 through m are equality constraints that will activate the penalty if there are any non-zero values [28,29]. In this study, the productivity per day, i.e., the unit productivity, is introduced as the objective function and is expressed as a function of abrasion time and coarse aggregate ratio. It should be noted that the actual equation for the unit productivity is very complicated and must consider various factors, such as human labor, material cost, delivery fee, machine operations time, etc. Rather, it is simply assumed like a unit time to product, which is 8 h per day divided by the abrasion time, multiplying the amount of coarse aggregates, and is expressed as: Qday “

480 x1 ˆ x2 2

(10)

where Qday is an expression for the unit productivity, x1 is coarse aggregate ratio, and x2 is abrasion time. Design variables, i.e., x1 is coarse aggregate time and x2 is abrasion time, are selected and have a range from 0.5% to 1.5% and from 5 min to 15 min, respectively. To determine optimal conditions, six constraints are selected: (1) lower/upper limits for coarse aggregate ratio and abrasion time; (2) lower/upper limits for density; and (3) lower/upper limits for absorption time. Density and absorption time are derived and are expressed by means of design variables in Tables 10 and 11, respectively. Assumptions for density and absorption time are introduced for the optimization problem in this study, i.e., the aforementioned target performance levels are considered as the limitations for density and absorption time. In other words, density should be greater than the target performance level, i.e., 2.46. Absorption time should be less than the target performance level, i.e., 3.0, and should be greater than zero. Thus, constraints are expressed as: x1 Limits: x g1 pxq “ 0.5 ď x1 “ 1 ´ 1 ď 0 (11) 0.5 1.5 g2 pxq “ x1 ď 1.5 “ 1 ´ ď0 (12) x1 x2 Limits: x2 ď0 5 15 g4 pxq “ x2 ď 15 “ 1 ´ ď0 x2

g3 pxq “ 0.5 ď x2 “ 1 ´

(13) (14)

Density Limits: g5 pxq “ 2.46 ď YD “ 1 ´

YD ď0 2.46

(15)

Absorption Limits: g6 pxq “ YAR ď 3 “ 1 ´

3 ď0 YAR

(16)

where x1 is the coarse aggregate ratio, x2 is the abrasion time, YD is the density, and YAR is the absorption time. Optimization has been performed using MATLAB [30] and Figure 8 shows the optimum mixing conditions satisfying the required performance level that was determined based on Figures 6 and 7, showing the results of analyzing the density and the absorption ratio. The results of the contour

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analysis showed that aggregates manufactured through a process had an oven-dry density level of Int. J. Environ. Res. Public Health 2016, 13, 769 12 of 14 over 2200 kg/m3 and absorption ratio of under 5%, which are the standard quality levels prescribed in in the the quality quality standards standards for for recycled recycled aggregates, aggregates, and and satisfied satisfied the the standards standards for for natural natural aggregates aggregates 3 and an absorption ratio of under 3%. Thus, proposed in KS, which are a density of over 2460 kg/m 3 proposed in KS, which are a density of over 2460 kg/m and an absorption ratio of under 3%. Thus, considering thethe manufacture of recycled fine fine aggregates, it is deemed that it would considering the theproductivity productivityofof manufacture of recycled aggregates, it is deemed that it be economical to feed relatively large amounts of coarse aggregates to reduce the production time. would be economical to feed relatively large amounts of coarse aggregates to reduce the production In addition, the optimum abrasion conditions for improved productivity and cost effectiveness would time. In addition, the optimum abrasion conditions for improved productivity and cost effectiveness be an abrasion time oftime 8.57ofmin, min is theisexact value, andand a coarse aggregate ratio of would be an abrasion 8.57i.e., min,8.5695 i.e., 8.5695 min the exact value, a coarse aggregate ratio over 1.5. In that case, the unit productivity was maximized and has a value of 42.01. of over 1.5. In that case, the unit productivity was maximized and has a value of 42.01. 1

20

1 2.52 1.5

0

2.56

-1 2.62 -0.5

2.6

2.58 0.5 2.48 2.5

15

2.54 2.5

10

2

2.44 2.46 3

5

2.4 3

0

0

0.5

Optimum Pts (1.5, 8.5695) 1

3.5

2.42

2.38

1.5

2

Coarse Aggregate Ratio (%) Figure quality of of recycled recycled fine fine aggregate. aggregate. Figure 8. 8. The The optimum optimum condition condition for for improved improved quality

4. Conclusions 4. Conclusions Typically, the recycled aggregates have a limited usage due to its low quality comparing with Typically, the recycled aggregates have a limited usage due to its low quality comparing with natural aggregates. In this study, experimental testing has been conducted considering three factors, natural aggregates. In this study, experimental testing has been conducted considering three factors, i.e., the fine aggregate to water ratio, the fine aggregate to coarse aggregate ratio, and the abrasion i.e., the fine aggregate to water ratio, the fine aggregate to coarse aggregate ratio, and the abrasion time, to improve the quality of the recycled aggregates. To do this, experimental plans have been time, to improve the quality of the recycled aggregates. To do this, experimental plans have been arranged in a three-level system using the orthogonal design method. Overall, 27 experimental plans arranged in a three-level system using the orthogonal design method. Overall, 27 experimental plans have been arranged in nine experimental levels. The abrasion conditions for the production of have been arranged in nine experimental levels. The abrasion conditions for the production of recycled recycled fine aggregates have been derived using the response surface methods and its optimum fine aggregates have been derived using the response surface methods and its optimum conditions conditions to maximize the production have been found using the Nelder-Mead sequential simplex to maximize the production have been found using the Nelder-Mead sequential simplex algorithm algorithm with various constraints. During the procedure, the unit production amount has been with various constraints. During the procedure, the unit production amount has been selected as the selected as the objective function. objective function. The following conclusions were made based on the results of this study conducted on the The following conclusions were made based on the results of this study conducted on the manufacturing method for recycled fine aggregates that satisfy the quality standards. Of the abrasion manufacturing method for recycled fine aggregates that satisfy the quality standards. Of the abrasion conditions, such as water ratio, coarse aggregate ratio, and abrasion time, abrasion time was found conditions, such as water ratio, coarse aggregate ratio, and abrasion time, abrasion time was found to to have the most significant impact on the density change, while the water ratio had no significant have the most significant impact on the density change, while the water ratio had no significant impact. impact. The results of reviewing the changes in the quality of recycled fine aggregates caused by The results of reviewing the changes in the quality of recycled fine aggregates caused by varying the varying the water ratio, coarse aggregate ratio, and abrasion time showed that, similar to the density water ratio, coarse aggregate ratio, and abrasion time showed that, similar to the density experiment, experiment, abrasion time was found to have the most significant impact on the changes in the abrasion time was found to have the most significant impact on the changes in the absorption ratio, absorption ratio, and the trend in the changes was similar to that of the changes observed in density. and the trend in the changes was similar to that of the changes observed in density. The results of the The results of the abrasion experiment performed to determine the optimum conditions for the abrasion experiment performed to determine the optimum conditions for the manufacture of recycled manufacture of recycled fine aggregates that satisfy the quality standards showed that an abrasion fine aggregates that satisfy the quality standards showed that an abrasion time of 8.57 min and coarse time of 8.57 min and coarse aggregate ratio of over 1.5 have been the optimum conditions for producing aggregate ratio of over 1.5 have been the optimum conditions for producing aggregates with the target aggregates with the target oven-dry density of 2460 kg/m3 and absorption ratio of under 3%. 3 oven-dry density of 2460 kg/m and absorption ratio of under 3%. Therefore, it is advantageous to use pulverized materials to ensure the quality of the production efficiency and aggregate when the optimal abrasion conditions for producing the high-quality of the recycled aggregates are applied. Additionally, it gives more benefits in productivity and economics to use the recycled aggregates from deconstruction and demolition of reinforced concrete structures and will be helpful to save the limited amount of the natural aggregates and environmental protection. Finally,

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Therefore, it is advantageous to use pulverized materials to ensure the quality of the production efficiency and aggregate when the optimal abrasion conditions for producing the high-quality of the recycled aggregates are applied. Additionally, it gives more benefits in productivity and economics to use the recycled aggregates from deconstruction and demolition of reinforced concrete structures and will be helpful to save the limited amount of the natural aggregates and environmental protection. Finally, it can be expected that a further study related to apply the recycled aggregates, having the improved quality, to the mortar and concrete in order to evaluate its material properties and stability. Acknowledgments: The research reported in this paper was supported by the Korea Science and Engineering Foundation (KOSEF) grant funded by the Korean Ministry of Education, Science and Technology (MEST) (No.2014R1A1A2008855). Author Contributions: Haseog Kim and Hayong Kim conceived and designed the experiments; Haseog Kim performed the experiments; Haseog Kim and Sangki Park analyzed the data; Sangki Park contributed materials/analysis tools; Haseog Kim and Sangki Park wrote the paper. Conflicts of Interest: The authors declare no conflict of interest.

Abbreviations The following abbreviations are used in this manuscript: S Ø V0 F0 RSM

Sum of squares Degree of freedom Mean of the sum of squares F-statistics value Response surface methodology

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7. 8. 9. 10. 11. 12. 13. 14.

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