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Use of incinerator bottom ash in open-graded asphalt concrete
Construction and Building Materials 149 (2017) 497–506
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Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Use of incinerator bottom ash in open-graded asphalt concrete Huan-Lin Luo a, Shih-Huang Chen b, Deng-Fong Lin a,⇑, Xin-Rong Cai a a b
Department of Civil and Ecological Engineering, I-Shou University, No. 1, Sec. 1, Syuecheng Rd., Dashu District, Kaohsiung City 84001, Taiwan, ROC Department of Civil Engineering, National Central University, No. 300, Zhongda Rd., Zhongli District, Taoyuan City 32001, Taiwan, ROC
h i g h l i g h t s An effective and alternative way to recycle incinerator bottom ash (IBA). It is feasible to apply the IBA to open-graded asphalt concrete (OGAC). The IBA replacement improves the stability and indirect tensile strength of OGAC. Up to 80% of natural fine aggregates in OGAC could be replaced by IBA.
a r t i c l e
i n f o
Article history: Received 11 March 2016 Received in revised form 16 May 2017 Accepted 21 May 2017
Keywords: Incinerator bottom ash Open-graded mix Asphalt concrete
a b s t r a c t To observe the effects of the use of incinerator bottom ash (IBA) on open-graded asphalt concrete (OGAC), natural fine aggregates are partially replaced by IBA in OGAC. The results indicate that the use of IBA helps improve the adhesive force between asphalt and aggregates. The stability and indirect tensile strength of OGAC are effectively improved by the use of IBA as a replacement material. The natural fine aggregate content in the OGAC is 25%. The results indicate that 80% of natural fine aggregates could be replaced by IBA and demonstrate the feasibility of using IBA in OGAC in engineering applications. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction An incineration treatment is a method whereby high temperatures are applied to solid wastes. After incineration, the solid wastes turn into stable gases or ashes. During the incineration process, the wastes can be effectively incinerated, and hazardous materials can be destroyed. Moreover, heat produced during the incineration process is converted into electrical power. Hence, incineration is a suitable method for reducing, stabilizing, detoxifying, and recycling solid wastes. According to information released by the Environmental Protection Administration in Taiwan, the ashes produced from incinerators in Taiwan totaled 902,000 tons between January and October of 2011. Of this amount, bottom ash constitutes 719,000 tons, and 429,000 tons are sent to landfill sites [1]. If bottom ash continues to be sent to landfill sites, a large environmental loading will be placed on Taiwan, which is an island with little available land. Many applications of reusing incinerator bottom ash (IBA) have been observed worldwide. The main application is in construction materials, such as backfill, base layers, and surface layers of asphalt concrete, in pavement engineering [2]. ⇑ Corresponding author. E-mail address: [email protected] (D.-F. Lin). http://dx.doi.org/10.1016/j.conbuildmat.2017.05.164 0950-0618/Ó 2017 Elsevier Ltd. All rights reserved.
Tang et al. [3] investigated the heterogeneity and environmental properties of municipal solid waste incineration (MSWI) bottom ash from two waste plants. They found that their properties were stable and comparable to each other. Moreover, they noticed that the MSWI bottom ash exhibited a high fraction of fine particles (<125 lm), resulting in higher water absorption. When used as sand replacement, the bottom ash reduced the amount of water available for the reaction with cement in mortar. They suggested that the MSWI bottom ash fine particles, if applied as sand replacement, had a disadvantageous influence on cement hydration and strength development of the mortars. Because the MSWI bottom ash contained elemental aluminum, sulfate and harmful organics from a grate furnace (SF) and boiler and fly ash from a fluidized bed incinerator (BFA), Saikia et al. [4] pre-treated the ash samples used as the fine aggregate of sand to manufacture cement mortar. They found that the quality of the ash samples was improved for the application as a fine aggregate using a 0.25 M Na2CO3 solution to dissolve the Al and the sulfate-bearing minerals from the BFA. Furthermore, they noticed that the compressive strengths of the cement mortar specimens were considerably improved by replacing a portion of the sand with the ash samples and treating with heat and Na2CO3. Lynn et al. [5] suggested that the pre-treated municipal incinerated bottom ash (MIBA) had the potential for
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use as fine or coarse aggregate in the manufacturing of mortar, concrete, and blocks. They found that lightweight aggregate made by the pre-treated MIBA showed similar properties to Lytag, although with slightly reduced strength. Furthermore, when a portion of the sand was replaced by the fine MIBA in the manufacturing of foamed concrete, the requirements for high flowability and low strength were met using the MIBA mixes. Wu et al. [6] applied IBA to replace sandstone in the manufacturing of pervious concrete brick specimens by controlling different parameters in the various mix proportions, including water-to-cement (w/c) ratios and aggregate sizes. They found that using an IBA aggregate size of 4.76 mm and a 0.55 w/c ratio produced the maximum compressive strength for the brick specimens and was the most promising for future pavement applications. Moreover, by considering the effects of water permeability and the strength of IBA permeable bricks, they recommended that the IBA pervious concrete bricks could be applied to general bicycling paths, sidewalks, and landscaping but not for heavy traffic volume roads. IBA is a heterogeneous mixture, and its components are determined by the waste classification and affect its performance. The physical components of IBA include slag, iron, ceramics, glasses, non-combustible materials, and organic products resulting from incomplete combustion. In general, IBA is a lightweight porous polymer with a large particle size and irregular spherical shape. Its dry density is approximately 950 kg/m3, and its specific gravity is between 1.8 and 2.4, which is lighter than natural aggregates [7]. The main chemical compositions of IBA are SiO2, CaO, Fe2O3, and Al2O3, whose contents in the IBA are affected by the composition of the wastes. In Taiwan, IBA contains large amounts of SiO2 and Fe2O3 because of the characteristics of the collected wastes. CaO and lime are also observed in IBA. These two compositions may help improve the stripping of asphalt concrete [8]. The Federal Highway Administration (FHWA) suggests that after performing a magnetic separation to remove metallic and non-metallic materials, the IBA exhibits a good particle size distribution and can be further mixed with natural aggregates to produce asphalt concrete. When applying the IBA to the mix design of asphalt concrete, the characteristics of lightweight aggregates with smaller specific weights than those of natural aggregates must be considered. This suggests that less than 25% (weight %) of IBA should be used when the IBA is applied to the binder course or base layer, and less than 15% should be used when the IBA applied to the surface layer of asphalt concrete. The performances of pavement materials are assured if the above requirements are considered [9]. Aziz et al. [10] suggested that IBA is suitable for use in low-traffic-volume roads. However, when applied to asphalt concrete, the optimum asphalt binder was shown to be 10–20%, which increases the cost of construction. In this study, as suggested in the literature, the IBA is used in opengraded asphalt concrete (OGAC) to investigate the feasibility of its application to pavement engineering. 2. Materials and methods Modified type III asphalt was used in this study. The basic properties of the modified type III asphalt obtained for this study and the requirements set by the standards are shown in Table 1. The mixing temperature (170 ± 20 cSt) and compacting temperature (280 ± 30 cSt) were obtained using a linear regression on the
changes in asphalt viscosity at different temperatures. Moreover, after performing the basic tests on bottom ash and natural aggregates to understand the material properties, the natural fine aggregate was replaced by bottom ash at the following contents: 0, 20, 40, 60, 80, and 100%. The suggested aggregate mix designs are provided in Table 2. The optimal asphalt contents were obtained from an analysis of mix designs. The compaction method was based on the Standards of Provision 101.02798 (the general requirements of porous asphalt concrete) of the Highway Construction Technical Provisions set by the Directorate General of Highways [11]. The specimens were compacted 50 times on each side. Then, the mechanical properties of IBA asphalt concrete were evaluated through various tests, such as indirect tensile strength, static creep, dynamic creep, tensile strength ratio, and resilient modulus tests. 2.1. Tests of the material properties The properties of bottom ash and natural aggregates were obtained from sieve analyses, specific gravity tests, water absorption tests, and Los Angeles abrasion tests. Moreover, the toxicity characteristic leachate procedure (TCLP), scanning electron microscopy – energy-dispersive X-ray spectroscopy (SEM-EDS), and pH tests were performed on the bottom ash. Table 3 presents the tests, including their respective standards, performed on the bottom ash and natural fine and coarse aggregates. 2.2. Asphalt concrete mix design The main objective of the mix design for OGAC is to ensure that the asphalt concrete specimens have a sufficient asphalt content to increase the strength and avoid stripping of the OGAC. Moreover, the draindown of asphalt in the OGAC is prevented during the high-temperature transport of the asphalt mixture from plants to construction sites. According to Provision 089.02741 (the general requirements of asphalt concrete) of the Highway Construction Technical Provisions set by the Directorate General of Highways, Ministry of Transportation and Communications in Taiwan, the draindown for OGAC is required to be less than 0.3%, and the amount of abrasion must be less than 20% [12].
3. Results and discussion 3.1. Basic properties of aggregates Table 4 shows the results of the TCLP for the IBA and demonstrates that the results meet the requirements set by the Environmental Protection Agency in Taiwan. Table 5 shows the basic properties obtained for IBA and natural fine and coarse aggregates. The standard values of the Los Angeles abrasion, fracture (with 2 rupture surfaces), and flat and slenderness ratios are also provided in Table 5. The IBA constitutes the residuals obtained from thermal melting and subsequent performance of instant water cooling processes. Hence, numerous pores are produced on the surfaces of the IBA particles. As observed in Table 4, the water absorption of the IBA was higher than that of natural aggregates because the pore ratio of the IBA was larger than that of the natural aggregates. Moreover, because of the porous nature of the IBA, the specific weight of the IBA was small, thus resulting in the specific gravity of IBA being smaller than that of the natural aggregates. The pH of the IBA was higher than that of the natural fine aggregates because the IBA contained more alkali metals or alkalineearth metals than the natural fine aggregates. Fig. 1 shows the results of the sieve analyses obtained for the IBA and natural fine aggregates. Because the passing ratios of the IBA were smaller than those of the natural fine aggregates, the particles of the IBA were coarser than the particles of the natural fine aggregates. 3.2. Chemical composition analyses of the IBA and natural fine aggregates
Table 1 The basic properties of the modified type III asphalt. Test
Results
Criteria
Specific gravity Viscosity (60 °C, poise) Mix temperature (170 ± 20cSt, °C) Compact temperature (280 ± 30cSt, °C) Penetration (25 °C, 100 g, 5 s 0.1 mm)
1.039 9800 181–187 166–172 52.2
– >8000 170 ± 20 280 ± 30 >35
The chemical components of the natural fine aggregates and IBA obtained using 2.38 mm and 1.19 mm sieves and the bottom tray in the SEM-EDS analyses are shown in Table 6. The weight percentages of the main components of the IBA, such as Si, Al, and Ca, are similar to those of the natural fine aggregates. Among them, the
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H.-L. Luo et al. / Construction and Building Materials 149 (2017) 497–506 Table 2 The suggested aggregate mix designs. Amount of replacement (%)
0
20
40
60
80
100
Natural coarse aggregate (%) Natural fine aggregate (%) Incinerator bottom ash (%) Filler (%)
90 8 0 2
90 6.4 1.6 2
90 4.8 3.2 2
90 3.2 4.8 2
90 1.6 6.4 2
90 0 8 2
the IBA replacement content for the natural fine aggregates was also small. As observed in Fig. 2, the gradation curves for different IBA contents exhibited minimal variation.
Table 3 The tests and their respective standards performed on the bottom ash and natural fine and coarse aggregates [13–19]. Test
Coarse aggregate
Fine aggregate
Incinerator bottom ash
Sieve analysis
AASHTO T27
AASHTO T27
Specific gravity and water absorption Flat and elongated ratio
ASTM C127
AASHTO T27 ASTM C128 – – NIEA R208.04C –
– NIEA R208.04C
L.A. abrasion test pH value
ASTM D4791 ASTM C131 –
TCLP
–
3.3.2. Optimal asphalt content Based on the results obtained from a previous study, the optimal asphalt content of OGAC containing IBA is proportional to the IBA replacement content. Fig. 3 shows the relation between the optimal asphalt content and IBA replacement content for the OGAC mix design. In this study, the results of the mix design show that when the IBA replacement content was less than 60%, an approximately 0.1% increment in the optimal asphalt content was observed for the 12.5-mm OGAC containing IBA, and an approximately 0.3% increment was observed for the 12.5-mm OGAC having a 100% IBA replacement content. This implies that the use of IBA in the OGAC resulted in a minor increase in the cost of the asphalt.
ASTM C128 –
NIEA M103.02C
amounts of Ca in the IBA obtained using the 2.38 mm and 1.19 mm sieves and the bottom tray were larger than those in the natural fine aggregates. However, the Ca contents of the IBA were similar to those in stone powder and can help improve the properties of OGAC. As indicated in the reference, a metathesis reaction occurred between the Ca contained in the AC and the H, Na, Ka, and other cations present in the aggregate surface. Moreover, the Ca reacted with the naphthenic acids contained in the asphalt binder to produce calcium naphthenate, which facilitates the adsorption of the asphalt binder at the aggregate particles’ surface. The Na and K contents of the IBA were higher than those in the natural fine aggregates and helped produce calcium naphthenate. Because of the repulsive reaction between the negative charges on the surfaces of natural fine aggregates particles and the asphalt binder, the water easily penetrated the interface between the aggregate particles and asphalt binder. As a result, stripping of the asphalt concrete occurred. However, the positive charge of the Ca in the IBA strongly adsorbed the negative charges in the asphalt binder and thus reduced the stripping effects.
3.3.3. Draindown test Fig. 4 shows the relation between the draindown and IBA replacement content for the OGAC mix design. According to the 12.5-mm dense-graded asphalt concrete standard of Provision 089.02741 (the general requirements of asphalt concrete) of the Highway Construction Technical Provisions set by the Directorate General of Highways [12], the level of draindown must be less than 0.3%. As observed in the figure, the level of draindown decreased with increasing IBA replacement content. As stated above, the IBA contained many pores and exhibited a smaller specific gravity than the natural aggregates. Moreover, the weight method was considered to determine the quantity of natural fine aggregates replaced by the IBA substitution. Hence, when the IBA was applied to replace most of the natural fine aggregates, the optimal asphalt content increased with increasing IBA replacement; the draindown magnitude decreased with increasing IBA replacement. Furthermore, when the amount of the IBA replacement was fixed, the level of draindown increased with increasing asphalt content. Moreover, because the IBA surface was lipophilic, the IBA replacement helped to effectively adsorb the asphalt binder to reduce the level of draindown in the OGAC.
3.3. Mix design of asphalt concrete 3.3.1. Aggregate gradation mix design of asphalt concrete In this study, the aggregate gradation distributions must comply with the 12.5-mm dense-graded asphalt concrete standard of Provision 089.02741 (the general requirements of asphalt concrete) of the Highway Construction Technical Provisions set by the Directorate General of Highways, Ministry of Transportation and Communications in Taiwan [12]. Fig. 2 shows the aggregate gradation distribution curves (with volume ratio) for an OGAC mix design containing different amounts of natural fine aggregates replaced by IBA. The coarse and fine aggregate contents were approximately 75 and 25%, respectively. Because a small amount of natural fine aggregate was used in the open-graded mix design,
3.3.4. Cantabro test To obtain the required abrasion resistance and avoid stripping caused by the insufficient adhesive force among aggregate particles for the OGAC pavement, even when containing the lowest levels of asphalt, the draindown level and Cantabro abrasion resistance were considered to obtain the optimal asphalt content in the OGAC mix design. Fig. 5 shows the relation between the Cantabro abrasion and IBA replacement content for the OGAC mix design. As stated above, the IBA (containing relatively high percentages of
Table 4 The results of the TCLP for the IBA. Heavy metal
Hg
Cd
Se
Cr+6
Pb
Cr
As
Cu
Ba
Incinerator bottom ash Criteria
ND 0.2
<0.02 1.0
0.112 1.0
<0.1 2.5
0.152 5.0
<0.02 5.0
<0.1 5.0
1.45 15.0
0.732 100.0
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Table 5 The basic properties obtained for IBA and natural fine and coarse aggregates. Test
Coarse aggregate
Fine aggregate
Incinerator bottom ash
Criteria
Specific gravity pH value Water absorption (%) Los Angeles abrasion (%) Fracture (2 rupture surfaces), (%) Flat and elongated ratio Greater or equal 1:3 (%)
2.61 – 1.2 25.7 89.2 7.6
2.59 9.2 1.6 – – –
1.82 9.9 4.08 – – –
– – – Max35 Min60 Max10
Fig. 1. The results of the sieve analyses obtained for the IBA and natural fine aggregates.
CaO) particles’ surfaces were rough, thereby resulting in the absorption of additional asphalt on the coarse surface. The abrasion level increased with increasing IBA replacement content. However, the results obtained from the Cantabro abrasion tests for the OGAC containing different IBA replacement contents met the requirements set by the standard. 3.3.5. Density analysis Fig. 6 shows the relation between the density and IBA replacement content for the OGAC mix design. As observed in the figure, the density decreased with increasing IBA replacement. The largest density was at 0% IBA replacement, with a value of 2317 kg/m3; the smallest density was 2188 kg/m3 at 100% IBA replacement. The decrease in the density was approximately 20 kg/m3 for each 20% increase in the IBA replacement fraction. 3.3.6. Stability analysis According to the standards of Provision 101.02798 (the general requirements of porous asphalt concrete) of the Highway Con-
struction Technical Provisions set by the Directorate General of Highways [11], the stability value must be larger than 350 kgf to ensure the uniform quality of an AC mixture and the sufficient bearing capacity of AC specimens. Fig. 7 shows the relation between the stability value and IBA replacement content for the OGAC mix design. Each stability value was obtained from the optimal asphalt content at each IBA replacement content. Fig. 7 indicates that the stability value clearly increased with increasing IBA replacement content. The pore sizes between aggregate particles in the open-graded mix design were relatively large, and the aggregate particle structures formed in the specimens were not stable. Because the IBA particles were characterized by their relatively square-like shapes, the use of the IBA in the OGAC mix design helped to improve the aggregate particle structures formed in the specimens. Because the IBA contained high levels of CaO, IBA particles passing through the 0.074 mm sieve could replace stone powder as filler to increase the strength of the OGAC.
3.3.7. Flowability analysis Fig. 8 shows the relation between the flowability value and IBA replacement content for the OGAC mix design. Each flowability value was obtained from the optimal asphalt content at each IBA replacement content. The figure shows an R2 of 0.0311, thus implying no apparent correlation between the flowability value and IBA replacement content. Hence, the flowability of the OGAC was not affected by the natural fine aggregates being replaced by the IBA.
3.3.8. Void in the mineral aggregate The VMA can provide information that can be used to ensure that a suitable amount of asphalt is contained in the voids between aggregate particles for an asphalt concrete mixture. Fig. 9 shows the relation between the VMA and IBA replacement content in the OGAC mix design. The IBA possessed more pores and lower density than the natural aggregates. When the same mix designs and asphalt content were considered for OGAC with or without IBA, the weight of the aggregate decreased when the aggregate volume was held constant and when the IBA replacement content
Table 6 The chemical components of the natural fine aggregates and IBA obtained using 2.38 mm and 1.19 mm sieve sizes and the bottom tray in the SEM-EDS analyses. Element
Al Si Ca Mg P K Pt Na Fe Zr Cl S
Natural fine aggregate
Incinerator bottom ash
2.38 mm
1.19 mm
Bottom tray
2.38 mm
1.19 mm
Bottom tray
3.57 14.01 2.37 – 0.37 – 0.47 – – – – –
5.5 15.49 – – 0.08 0.68 – – – – – –
9.28 11.22 – 3.96 – – – – – – – –
2.48 13.84 8.1 1.8 1.98 0.57 0.86 8.58 – – – –
3.31 13.22 6.76 – – 0.89 – – 1.09 0.53 0.77 0.26
5.46 14.33 4.54 – – – – – – – – –
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Fig. 2. The aggregate gradation distribution curves for an OGAC mix design containing different amounts of natural fine aggregates replaced by IBA.
Fig. 3. The relation between the optimal asphalt content and IBA replacement content for the OGAC mix design.
Fig. 4. The relation between the draindown and IBA replacement content for the OGAC mix design.
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Fig. 5. The relation between the Cantabro abrasion and IBA replacement content for the OGAC mix design.
Fig. 6. The relation between the density and IBA replacement content for the OGAC mix design.
Fig. 7. The relation between the stability value and IBA replacement content for the OGAC mix design.
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Fig. 10. The relation between the asphalt film thickness and IBA replacement content for the OGAC mix design.
Fig. 8. The relation between the flowability value and IBA replacement content for the OGAC mix design.
the asphalt film thickness. Although the increase in the asphalt film thickness helped strengthen the resistance to the aging effect of the asphalt and prevented the invasion of water into the asphalt concrete pavement, this may increase the amount of ineffective additional asphalt content. As a result, this increase in additional asphalt content could reduce the quality of the pavement and increase the cost of the construction of the pavement. 3.4. Thermal imaging analyses
Fig. 9. The relation between the VMA and IBA replacement content in the OGAC mix design.
increased. Fig. 9 shows that the VMA increased with increasing IBA replacement content. 3.3.9. Asphalt film thickness The asphalt film thickness is one of the factors used to evaluate the resiliency of an asphalt concrete pavement. When the asphalt film thickness is insufficient, the adhesive force among aggregate particles will also be inadequate. As a result, the aggregate particles become easily stripped from the asphalt concrete pavement. The empirical equation derived by Hveen for the average asphalt film thickness wrapped around aggregate particles was applied in this study. Fig. 10 shows the relation between the asphalt film thickness and IBA replacement content for the OGAC mix design. Note that the asphalt thickness was calculated using the optimal asphalt content obtained from Fig. 3 for each IBA replacement. The increment in the asphalt film thickness was approximately 0.79 lm for the OGAC mix design with IBA contents from 0% to 100%. Moreover, the positive correlation between the asphalt film thickness and IBA replacement content is shown in Fig. 10. This implies that the rough IBA particle surface functioned similar to the addition of fibers to the asphalt concrete and can increase
To better understand the effects of the IBA replacement approach on the homogeneity of the OGAC, thermal imaging analyses were performed. To minimize possible errors, the thermal images were taken after the OGAC mixtures were paved. Furthermore, the images were obtained at the same angle and height. The results of the temperature distribution in the thermal images for OGAC containing with different levels of IBA replacement are shown in Fig. 11. The average OGAC temperatures for different levels of IBA replacement are shown in Table 7; the temperature distributions for the OGAC containing 0, 20, 40, 60, 80, and 100% IBA replacements exhibited the following ranges: 119–141, 121– 142, 114–140, 111–137, 107–133, and 106–128 °C, respectively. These results show that the temperature distribution variability of the OGAC changed slightly when the IBA replacement level was increased. Moreover, when the temperature of the mixing materials was fixed and the mixtures were mixed for the same duration, the average OGAC temperature decreased with increasing IBA replacement, suggesting that more attention should be given in engineering applications of IBA to OGAC. Hence, the homogeneity of the OGAC was slightly influenced by the IBA replacement. 3.5. Mechanical property test 3.5.1. Indirect tensile strength test Fig. 12 shows the relation between the indirect tensile strength and IBA replacement content for OGAC specimens containing IBA. The IBA particles were more similar to squares compared with natural fine aggregate particles. Hence, the interlocking ability among IBA particles was better than that of natural fine aggregate particles. Thus, the tensile strengths for the OGAC specimens containing IBA were increased. As shown in Fig. 12, the indirect tensile strength increased with increasing IBA replacement content. Moreover, because the IBA was lipophilic, the adhesive force between the IBA particles’ surfaces and the asphalt binder was increased. Therefore, the indirect tensile strengths of the OGAC specimens containing IBA were effectively improved.
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Fig. 11. The results of the temperature distribution in the thermal images for OGAC containing with (a) 0, (b) 20, (c) 40, (d) 60, (e) 80, and (f) 100% of IBA replacement.
Table 7 The average temperatures of OGAC containing with different level of the IBA replacement. Replacement content (%)
0
20
40
60
80
100
Average temperature (°C)
138.03
134.77
130.0
126.9
121.8
118.89
3.5.2. Tensile strength ratio (TSR) test Because of the effects of external physical conditions, such as moisture, temperature, traffic volume, and air pollution, on the asphalt concrete, the cohesive force of the asphalt and the adhesive force between aggregate particles and the asphalt binder were reduced, leading to the deterioration of the asphalt concrete structure. Fig. 13 shows the relation between the tensile strength ratio and IBA replacement content for the OGAC specimens containing IBA. All of the TSRs of the OGAC specimens were higher than the
standard value of 75%, as observed in the figure. Moreover, the TSRs for the OGAC specimens containing the IBA as a replacement were higher than that of control group (0% IBA replacement). This implies that the use of IBA in OGAC can help improve the resistance to moisture intrusion into the pavement. The lipophilic characteristic of the IBA helped to effectively adsorb the asphalt binder. Moreover, the hydrophobic nature of the IBA made it difficult for the IBA to combine with water. Therefore, the adhesive force between the aggregate particles and asphalt binder was improved,
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Fig. 12. The relation between the indirect tensile strength and IBA replacement content for OGAC specimens containing IBA.
Fig. 13. The relation between the tensile strength ratio and IBA replacement content for the OGAC specimens containing IBA.
and the moisture did not easily intrude into the voids generated between the aggregate particles and asphalt. The stripping of the pavement was reduced. To better understand the hydrophobicity of the applied materials, IBA and natural fine aggregates were ground into powders and passed through the size of 0.074 mm sieves. Afterward, mixtures were formed by mixing equal volumes of water and powder for
1 min, as shown in Fig. 14. The hydrophobicity of each material can be determined by the hydrophilicity. The hydrophobicity is a physical characteristic of a material and is related to the temporary bonding between hydrogen and water molecules. A stronger hydrophobicity leads to a weaker hydrophilicity; the molecules cluster together in water for a material with a high hydrophobicity. As shown in Fig. 14, the particles in the mixture containing the IBA powder were clustered together. Hence, the surface of the IBA exhibited a stronger hydrophobicity. Furthermore, hydrophobic materials can be characterized by lipophilic capability. As a result, the adhesion force between the asphalt binder and the aggregates was improved using the IBA aggregate replacement approach in the AC. It becomes harder for moisture to penetrate the pores between the asphalt binder and aggregates, leading to an improvement in the AC to stripping effect. However, the natural fine aggregate powder is instantly well mixed with water to form the mixture. Because the hydrophilicity of the natural fine aggregate powder is higher than that of the asphalt binder, the moisture can directly penetrate into the AC, leading to an easy separation of the asphalt binder film from the surface of the aggregates. Hence, the stripping effect is relatively easy to produce for AC containing natural fine aggregates. Moreover, water can easily penetrate into the angular gaps formed by the pores between the aggregates and asphalt binder. As a result, the asphalt binder film is relatively thin in the angular gaps. Because the IBA was more rectangular with smaller angular values as well as flat and elongated, there were fewer angular gaps in the AC containing IBA aggregate replacement in this study. Hence, the ability to produce thinner asphalt binder films is reduced, and the stripping effect caused by water penetration is decreased.
3.5.3. Static creep test Fig. 15 shows the relation between the static creep value and IBA replacement content for OGAC specimens containing IBA as a replacement. The static creep values obtained for OGAC containing IBA as a replacement were higher than that of the control group. For example, the static creep value for the 80% IBA replacement content was the highest. This result implies that the resistance to permanent deformation caused by static loading on the asphalt concrete pavement was improved when the natural fine aggregates were partially replaced by the IBA. However, the static creep decreased when the IBA replacement content was higher than 80%, thereby implying that the total replacement of the natural fine aggregates by the IBA for OGAC could not be suggested. The highest static creep value was observed at an 80% IBA replacement content, followed by 60, 40, 100, 20, and 0%.
Fig. 14. Hydrophobic properties of natural fine aggregates and IBA.
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4. Conclusions In this study, the results of analyses for the possible application of IBA to OGAC were evaluated. We provide the following conclusions:
Fig. 15. The relation between the static creep value and IBA replacement content for OGAC specimens containing IBA as a replacement.
3.5.4. Dynamic creep test Fig. 16 shows the relation between the dynamic creep value and IBA replacement content for the OGAC specimens containing IBA as a replacement. As stated above, the shape of the IBA particles was more similar to a square shape compared with the natural fine aggregate particles. The interlocking ability among IBA particles was better than that of natural fine aggregate particles. Hence, the space for aggregates to move around in OGAC specimens containing IBA as a replacement became relatively small. When the IBA replacement content was less than 80%, the dynamic creep value decreased with increasing IBA replacement content. This implies that OGAC pavement with an 80% IBA replacement content or less could effectively resist rutting deformation caused by dynamic loadings. However, when the IBA replacement content increased to 100%, the dynamic creep value increased and was larger than that of the 80% IBA replacement content. As stated in the previous section, this phenomenon can be explained as follows: when the IBA replacement content was higher than 80%, the asphalt film thickness increased. This thicker film thickness may lead to better lubrication among aggregate particles, and the deformation of OGAC pavement was observed under vehicle loadings. As a result, the dynamic creep value increased for the 100% IBA replacement content. These results of the dynamic creep test were similar to those obtained from the static creep tests. Therefore, the results of this study demonstrate that the IBA replacement content should be 80% or less to decrease the risks of degrading the engineering properties and durability of OGAC containing IBA as a replacement material.
Fig. 16. The relation between the dynamic creep value and IBA content for the OGAC specimens containing IBA as a replacement.
1. The IBA exhibited a smaller specific gravity and larger abrasion than did the natural fine aggregates. However, the IBA exhibited a rougher surface and had a higher CaO content than did the natural aggregates. The stripping of OGAC was effectively improved when fine aggregates were partially replaced by bottom ash. This result suggests the feasibility of applying IBA to OGAC. 2. Because the particle sizes of the IBA were larger than those of the fine aggregates, the amount of draindown for OGAC decreased with increasing IBA replacement content. However, the IBA particles had a rougher surface, thus resulting in the absorption of additional asphalt on the coarse surface. The abrasion level increased with increasing IBA replacement content. 3. Although the optimum asphalt binder content was slightly increased to 0.1% when the IBA replacement content was less than 60%, the properties of the OGAC containing IBA were largely improved. From the perspective of construction costs, the application of IBA to the OGAC is feasible. 4. Because the IBA contained more Ca than did the natural fine aggregates and was shown to be lipophilic, the adhesive force between the IBA particle surfaces and asphalt binder was increased. Hence, the indirect tensile strengths of OGAC containing IBA were effectively increased. 5. The test results obtained from the static or dynamic creep tests showed that the properties of OGAC containing IBA were better than those containing natural fine aggregates. These results suggest that the IBA replacement content should be less than 80% when applied to OGAC.
References [1] The Distribution of Refuse Incineration Plants, Statistical data, Environmental Protection Administration, Executive Yuan, Taiwan, R.O.C., Retrieved Jan 13 2015 from http://swims.epa.gov.tw/swims/swims_net/Page_en/Statistics_ List_en.aspx. [2] C.L. Wu, Feasibility study of Incinerator Bottom Ash regeneration as a Substitute Material for the Sub-base of Road Construction Master thesis, Tamkang University, Taiwan, 2011. [3] P. Tang, M.V.A. Florea, P. Spiesz, H.J.H. Brouwers, Characteristics and application potential of municipal solid waste incineration (MSWI) bottom ashes from two waste-to-energy plants, Constr. Build. Mater. 83 (2015) 77–94. [4] N. Saikia, G. Mertens, K. Van Balen, J. Elsen, T. Van Gerven, C. Vandecasteele, Pre-treatment of municipal solid waste incineration (MSWI) bottom ash for utilisation in cement mortar, Constr. Build. Mater. 96 (2015) 76–85. [5] C.J. Lynn, R.K. Dhir OBE, G.S. Ghataora, Municipal incinerated bottom ash characteristics and potential for use as aggregate in concrete, Constr. Build. Mater. 127 (2016) 504–517. [6] M.H. Wu, C.L. Lin, W.C. Huang, J.W. Chen, Characteristics of pervious concrete using incineration bottom ash in place of sandstone graded material, Constr. Build. Mater. 111 (2016) 618–624. [7] Taiwan Construction Research Institute and Sinotech Engineering Consultants Ltd, Report on Production Manufactured by Using Incinerator Bottom Ash, Executive Yuan, Taiwan, R.O.C, 2003. [8] C.C. Wiles, Municipal solid waste combustion ash: State-of-the- knowledge, J. Hazard. Mater. 47 (1996) 325–344. [9] L.S. Robin, The use of recycled materials in highway construction, The Free Library. Retrieved Jan 13 2015 from http://www.thefreelibrary.com/The+use +of+recycled+materials+in+highway+construction.-a016340226. [10] M.A. Aziz, S.D. Ramaswamy, Incinerator residue for roads, Geotech. Test. J. 15 (3) (1992) 300–304. [11] Directorate General of Highways, The general requirements of porous asphalt concrete, Highway Construction Technical Provisions 101.02798, Taiwan. [12] Directorate General of Highways, The general requirements of asphalt concrete, Highway Construction Technical Provision 089.02741, Taiwan. [13] American Association of State Highway and Transportation Officials (AASHTO), AASHTO T 27, Standard method of test for sieve analysis of fine and coarse aggregates, Revision: 2014 Edition, January 1, 2014.
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[14] ASTM International, ASTM C127–15, Standard test method for relative density (specific gravity) and absorption of coarse aggregate. [15] ASTM International, ASTM C128–15, Standard test method for relative density (specific gravity) and absorption of fine aggregate. [16] ASTM International, ASTM D4791–10, Standard test method for flat particles, elongated particles, or flat and elongated particles in coarse aggregate. [17] ASTM International, ASTM C131/C131M-14 Standard test method for resistance to degradation of small-size coarse aggregate by abrasion and impact in the Los Angeles machine.
[18] Environmental Analysis Laboratory, Environmental Protection Administration, Executive Yuan, Taiwan, ROC, NIEA R208.04C, Standard test method for measurement of power of hydrogen ions (pH value) in wastes by electrometric method. [19] Environmental Analysis Laboratory, Environmental Protection Administration, Executive Yuan, Taiwan, ROC, NIEA M103.02C, Standard test method for detection of heavy metals contents.
Contents lists available at ScienceDirect
Construction and Building Materials journal homepage: www.elsevier.com/locate/conbuildmat
Use of incinerator bottom ash in open-graded asphalt concrete Huan-Lin Luo a, Shih-Huang Chen b, Deng-Fong Lin a,⇑, Xin-Rong Cai a a b
Department of Civil and Ecological Engineering, I-Shou University, No. 1, Sec. 1, Syuecheng Rd., Dashu District, Kaohsiung City 84001, Taiwan, ROC Department of Civil Engineering, National Central University, No. 300, Zhongda Rd., Zhongli District, Taoyuan City 32001, Taiwan, ROC
h i g h l i g h t s An effective and alternative way to recycle incinerator bottom ash (IBA). It is feasible to apply the IBA to open-graded asphalt concrete (OGAC). The IBA replacement improves the stability and indirect tensile strength of OGAC. Up to 80% of natural fine aggregates in OGAC could be replaced by IBA.
a r t i c l e
i n f o
Article history: Received 11 March 2016 Received in revised form 16 May 2017 Accepted 21 May 2017
Keywords: Incinerator bottom ash Open-graded mix Asphalt concrete
a b s t r a c t To observe the effects of the use of incinerator bottom ash (IBA) on open-graded asphalt concrete (OGAC), natural fine aggregates are partially replaced by IBA in OGAC. The results indicate that the use of IBA helps improve the adhesive force between asphalt and aggregates. The stability and indirect tensile strength of OGAC are effectively improved by the use of IBA as a replacement material. The natural fine aggregate content in the OGAC is 25%. The results indicate that 80% of natural fine aggregates could be replaced by IBA and demonstrate the feasibility of using IBA in OGAC in engineering applications. Ó 2017 Elsevier Ltd. All rights reserved.
1. Introduction An incineration treatment is a method whereby high temperatures are applied to solid wastes. After incineration, the solid wastes turn into stable gases or ashes. During the incineration process, the wastes can be effectively incinerated, and hazardous materials can be destroyed. Moreover, heat produced during the incineration process is converted into electrical power. Hence, incineration is a suitable method for reducing, stabilizing, detoxifying, and recycling solid wastes. According to information released by the Environmental Protection Administration in Taiwan, the ashes produced from incinerators in Taiwan totaled 902,000 tons between January and October of 2011. Of this amount, bottom ash constitutes 719,000 tons, and 429,000 tons are sent to landfill sites [1]. If bottom ash continues to be sent to landfill sites, a large environmental loading will be placed on Taiwan, which is an island with little available land. Many applications of reusing incinerator bottom ash (IBA) have been observed worldwide. The main application is in construction materials, such as backfill, base layers, and surface layers of asphalt concrete, in pavement engineering [2]. ⇑ Corresponding author. E-mail address: [email protected] (D.-F. Lin). http://dx.doi.org/10.1016/j.conbuildmat.2017.05.164 0950-0618/Ó 2017 Elsevier Ltd. All rights reserved.
Tang et al. [3] investigated the heterogeneity and environmental properties of municipal solid waste incineration (MSWI) bottom ash from two waste plants. They found that their properties were stable and comparable to each other. Moreover, they noticed that the MSWI bottom ash exhibited a high fraction of fine particles (<125 lm), resulting in higher water absorption. When used as sand replacement, the bottom ash reduced the amount of water available for the reaction with cement in mortar. They suggested that the MSWI bottom ash fine particles, if applied as sand replacement, had a disadvantageous influence on cement hydration and strength development of the mortars. Because the MSWI bottom ash contained elemental aluminum, sulfate and harmful organics from a grate furnace (SF) and boiler and fly ash from a fluidized bed incinerator (BFA), Saikia et al. [4] pre-treated the ash samples used as the fine aggregate of sand to manufacture cement mortar. They found that the quality of the ash samples was improved for the application as a fine aggregate using a 0.25 M Na2CO3 solution to dissolve the Al and the sulfate-bearing minerals from the BFA. Furthermore, they noticed that the compressive strengths of the cement mortar specimens were considerably improved by replacing a portion of the sand with the ash samples and treating with heat and Na2CO3. Lynn et al. [5] suggested that the pre-treated municipal incinerated bottom ash (MIBA) had the potential for
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use as fine or coarse aggregate in the manufacturing of mortar, concrete, and blocks. They found that lightweight aggregate made by the pre-treated MIBA showed similar properties to Lytag, although with slightly reduced strength. Furthermore, when a portion of the sand was replaced by the fine MIBA in the manufacturing of foamed concrete, the requirements for high flowability and low strength were met using the MIBA mixes. Wu et al. [6] applied IBA to replace sandstone in the manufacturing of pervious concrete brick specimens by controlling different parameters in the various mix proportions, including water-to-cement (w/c) ratios and aggregate sizes. They found that using an IBA aggregate size of 4.76 mm and a 0.55 w/c ratio produced the maximum compressive strength for the brick specimens and was the most promising for future pavement applications. Moreover, by considering the effects of water permeability and the strength of IBA permeable bricks, they recommended that the IBA pervious concrete bricks could be applied to general bicycling paths, sidewalks, and landscaping but not for heavy traffic volume roads. IBA is a heterogeneous mixture, and its components are determined by the waste classification and affect its performance. The physical components of IBA include slag, iron, ceramics, glasses, non-combustible materials, and organic products resulting from incomplete combustion. In general, IBA is a lightweight porous polymer with a large particle size and irregular spherical shape. Its dry density is approximately 950 kg/m3, and its specific gravity is between 1.8 and 2.4, which is lighter than natural aggregates [7]. The main chemical compositions of IBA are SiO2, CaO, Fe2O3, and Al2O3, whose contents in the IBA are affected by the composition of the wastes. In Taiwan, IBA contains large amounts of SiO2 and Fe2O3 because of the characteristics of the collected wastes. CaO and lime are also observed in IBA. These two compositions may help improve the stripping of asphalt concrete [8]. The Federal Highway Administration (FHWA) suggests that after performing a magnetic separation to remove metallic and non-metallic materials, the IBA exhibits a good particle size distribution and can be further mixed with natural aggregates to produce asphalt concrete. When applying the IBA to the mix design of asphalt concrete, the characteristics of lightweight aggregates with smaller specific weights than those of natural aggregates must be considered. This suggests that less than 25% (weight %) of IBA should be used when the IBA is applied to the binder course or base layer, and less than 15% should be used when the IBA applied to the surface layer of asphalt concrete. The performances of pavement materials are assured if the above requirements are considered [9]. Aziz et al. [10] suggested that IBA is suitable for use in low-traffic-volume roads. However, when applied to asphalt concrete, the optimum asphalt binder was shown to be 10–20%, which increases the cost of construction. In this study, as suggested in the literature, the IBA is used in opengraded asphalt concrete (OGAC) to investigate the feasibility of its application to pavement engineering. 2. Materials and methods Modified type III asphalt was used in this study. The basic properties of the modified type III asphalt obtained for this study and the requirements set by the standards are shown in Table 1. The mixing temperature (170 ± 20 cSt) and compacting temperature (280 ± 30 cSt) were obtained using a linear regression on the
changes in asphalt viscosity at different temperatures. Moreover, after performing the basic tests on bottom ash and natural aggregates to understand the material properties, the natural fine aggregate was replaced by bottom ash at the following contents: 0, 20, 40, 60, 80, and 100%. The suggested aggregate mix designs are provided in Table 2. The optimal asphalt contents were obtained from an analysis of mix designs. The compaction method was based on the Standards of Provision 101.02798 (the general requirements of porous asphalt concrete) of the Highway Construction Technical Provisions set by the Directorate General of Highways [11]. The specimens were compacted 50 times on each side. Then, the mechanical properties of IBA asphalt concrete were evaluated through various tests, such as indirect tensile strength, static creep, dynamic creep, tensile strength ratio, and resilient modulus tests. 2.1. Tests of the material properties The properties of bottom ash and natural aggregates were obtained from sieve analyses, specific gravity tests, water absorption tests, and Los Angeles abrasion tests. Moreover, the toxicity characteristic leachate procedure (TCLP), scanning electron microscopy – energy-dispersive X-ray spectroscopy (SEM-EDS), and pH tests were performed on the bottom ash. Table 3 presents the tests, including their respective standards, performed on the bottom ash and natural fine and coarse aggregates. 2.2. Asphalt concrete mix design The main objective of the mix design for OGAC is to ensure that the asphalt concrete specimens have a sufficient asphalt content to increase the strength and avoid stripping of the OGAC. Moreover, the draindown of asphalt in the OGAC is prevented during the high-temperature transport of the asphalt mixture from plants to construction sites. According to Provision 089.02741 (the general requirements of asphalt concrete) of the Highway Construction Technical Provisions set by the Directorate General of Highways, Ministry of Transportation and Communications in Taiwan, the draindown for OGAC is required to be less than 0.3%, and the amount of abrasion must be less than 20% [12].
3. Results and discussion 3.1. Basic properties of aggregates Table 4 shows the results of the TCLP for the IBA and demonstrates that the results meet the requirements set by the Environmental Protection Agency in Taiwan. Table 5 shows the basic properties obtained for IBA and natural fine and coarse aggregates. The standard values of the Los Angeles abrasion, fracture (with 2 rupture surfaces), and flat and slenderness ratios are also provided in Table 5. The IBA constitutes the residuals obtained from thermal melting and subsequent performance of instant water cooling processes. Hence, numerous pores are produced on the surfaces of the IBA particles. As observed in Table 4, the water absorption of the IBA was higher than that of natural aggregates because the pore ratio of the IBA was larger than that of the natural aggregates. Moreover, because of the porous nature of the IBA, the specific weight of the IBA was small, thus resulting in the specific gravity of IBA being smaller than that of the natural aggregates. The pH of the IBA was higher than that of the natural fine aggregates because the IBA contained more alkali metals or alkalineearth metals than the natural fine aggregates. Fig. 1 shows the results of the sieve analyses obtained for the IBA and natural fine aggregates. Because the passing ratios of the IBA were smaller than those of the natural fine aggregates, the particles of the IBA were coarser than the particles of the natural fine aggregates. 3.2. Chemical composition analyses of the IBA and natural fine aggregates
Table 1 The basic properties of the modified type III asphalt. Test
Results
Criteria
Specific gravity Viscosity (60 °C, poise) Mix temperature (170 ± 20cSt, °C) Compact temperature (280 ± 30cSt, °C) Penetration (25 °C, 100 g, 5 s 0.1 mm)
1.039 9800 181–187 166–172 52.2
– >8000 170 ± 20 280 ± 30 >35
The chemical components of the natural fine aggregates and IBA obtained using 2.38 mm and 1.19 mm sieves and the bottom tray in the SEM-EDS analyses are shown in Table 6. The weight percentages of the main components of the IBA, such as Si, Al, and Ca, are similar to those of the natural fine aggregates. Among them, the
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0
20
40
60
80
100
Natural coarse aggregate (%) Natural fine aggregate (%) Incinerator bottom ash (%) Filler (%)
90 8 0 2
90 6.4 1.6 2
90 4.8 3.2 2
90 3.2 4.8 2
90 1.6 6.4 2
90 0 8 2
the IBA replacement content for the natural fine aggregates was also small. As observed in Fig. 2, the gradation curves for different IBA contents exhibited minimal variation.
Table 3 The tests and their respective standards performed on the bottom ash and natural fine and coarse aggregates [13–19]. Test
Coarse aggregate
Fine aggregate
Incinerator bottom ash
Sieve analysis
AASHTO T27
AASHTO T27
Specific gravity and water absorption Flat and elongated ratio
ASTM C127
AASHTO T27 ASTM C128 – – NIEA R208.04C –
– NIEA R208.04C
L.A. abrasion test pH value
ASTM D4791 ASTM C131 –
TCLP
–
3.3.2. Optimal asphalt content Based on the results obtained from a previous study, the optimal asphalt content of OGAC containing IBA is proportional to the IBA replacement content. Fig. 3 shows the relation between the optimal asphalt content and IBA replacement content for the OGAC mix design. In this study, the results of the mix design show that when the IBA replacement content was less than 60%, an approximately 0.1% increment in the optimal asphalt content was observed for the 12.5-mm OGAC containing IBA, and an approximately 0.3% increment was observed for the 12.5-mm OGAC having a 100% IBA replacement content. This implies that the use of IBA in the OGAC resulted in a minor increase in the cost of the asphalt.
ASTM C128 –
NIEA M103.02C
amounts of Ca in the IBA obtained using the 2.38 mm and 1.19 mm sieves and the bottom tray were larger than those in the natural fine aggregates. However, the Ca contents of the IBA were similar to those in stone powder and can help improve the properties of OGAC. As indicated in the reference, a metathesis reaction occurred between the Ca contained in the AC and the H, Na, Ka, and other cations present in the aggregate surface. Moreover, the Ca reacted with the naphthenic acids contained in the asphalt binder to produce calcium naphthenate, which facilitates the adsorption of the asphalt binder at the aggregate particles’ surface. The Na and K contents of the IBA were higher than those in the natural fine aggregates and helped produce calcium naphthenate. Because of the repulsive reaction between the negative charges on the surfaces of natural fine aggregates particles and the asphalt binder, the water easily penetrated the interface between the aggregate particles and asphalt binder. As a result, stripping of the asphalt concrete occurred. However, the positive charge of the Ca in the IBA strongly adsorbed the negative charges in the asphalt binder and thus reduced the stripping effects.
3.3.3. Draindown test Fig. 4 shows the relation between the draindown and IBA replacement content for the OGAC mix design. According to the 12.5-mm dense-graded asphalt concrete standard of Provision 089.02741 (the general requirements of asphalt concrete) of the Highway Construction Technical Provisions set by the Directorate General of Highways [12], the level of draindown must be less than 0.3%. As observed in the figure, the level of draindown decreased with increasing IBA replacement content. As stated above, the IBA contained many pores and exhibited a smaller specific gravity than the natural aggregates. Moreover, the weight method was considered to determine the quantity of natural fine aggregates replaced by the IBA substitution. Hence, when the IBA was applied to replace most of the natural fine aggregates, the optimal asphalt content increased with increasing IBA replacement; the draindown magnitude decreased with increasing IBA replacement. Furthermore, when the amount of the IBA replacement was fixed, the level of draindown increased with increasing asphalt content. Moreover, because the IBA surface was lipophilic, the IBA replacement helped to effectively adsorb the asphalt binder to reduce the level of draindown in the OGAC.
3.3. Mix design of asphalt concrete 3.3.1. Aggregate gradation mix design of asphalt concrete In this study, the aggregate gradation distributions must comply with the 12.5-mm dense-graded asphalt concrete standard of Provision 089.02741 (the general requirements of asphalt concrete) of the Highway Construction Technical Provisions set by the Directorate General of Highways, Ministry of Transportation and Communications in Taiwan [12]. Fig. 2 shows the aggregate gradation distribution curves (with volume ratio) for an OGAC mix design containing different amounts of natural fine aggregates replaced by IBA. The coarse and fine aggregate contents were approximately 75 and 25%, respectively. Because a small amount of natural fine aggregate was used in the open-graded mix design,
3.3.4. Cantabro test To obtain the required abrasion resistance and avoid stripping caused by the insufficient adhesive force among aggregate particles for the OGAC pavement, even when containing the lowest levels of asphalt, the draindown level and Cantabro abrasion resistance were considered to obtain the optimal asphalt content in the OGAC mix design. Fig. 5 shows the relation between the Cantabro abrasion and IBA replacement content for the OGAC mix design. As stated above, the IBA (containing relatively high percentages of
Table 4 The results of the TCLP for the IBA. Heavy metal
Hg
Cd
Se
Cr+6
Pb
Cr
As
Cu
Ba
Incinerator bottom ash Criteria
ND 0.2
<0.02 1.0
0.112 1.0
<0.1 2.5
0.152 5.0
<0.02 5.0
<0.1 5.0
1.45 15.0
0.732 100.0
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Table 5 The basic properties obtained for IBA and natural fine and coarse aggregates. Test
Coarse aggregate
Fine aggregate
Incinerator bottom ash
Criteria
Specific gravity pH value Water absorption (%) Los Angeles abrasion (%) Fracture (2 rupture surfaces), (%) Flat and elongated ratio Greater or equal 1:3 (%)
2.61 – 1.2 25.7 89.2 7.6
2.59 9.2 1.6 – – –
1.82 9.9 4.08 – – –
– – – Max35 Min60 Max10
Fig. 1. The results of the sieve analyses obtained for the IBA and natural fine aggregates.
CaO) particles’ surfaces were rough, thereby resulting in the absorption of additional asphalt on the coarse surface. The abrasion level increased with increasing IBA replacement content. However, the results obtained from the Cantabro abrasion tests for the OGAC containing different IBA replacement contents met the requirements set by the standard. 3.3.5. Density analysis Fig. 6 shows the relation between the density and IBA replacement content for the OGAC mix design. As observed in the figure, the density decreased with increasing IBA replacement. The largest density was at 0% IBA replacement, with a value of 2317 kg/m3; the smallest density was 2188 kg/m3 at 100% IBA replacement. The decrease in the density was approximately 20 kg/m3 for each 20% increase in the IBA replacement fraction. 3.3.6. Stability analysis According to the standards of Provision 101.02798 (the general requirements of porous asphalt concrete) of the Highway Con-
struction Technical Provisions set by the Directorate General of Highways [11], the stability value must be larger than 350 kgf to ensure the uniform quality of an AC mixture and the sufficient bearing capacity of AC specimens. Fig. 7 shows the relation between the stability value and IBA replacement content for the OGAC mix design. Each stability value was obtained from the optimal asphalt content at each IBA replacement content. Fig. 7 indicates that the stability value clearly increased with increasing IBA replacement content. The pore sizes between aggregate particles in the open-graded mix design were relatively large, and the aggregate particle structures formed in the specimens were not stable. Because the IBA particles were characterized by their relatively square-like shapes, the use of the IBA in the OGAC mix design helped to improve the aggregate particle structures formed in the specimens. Because the IBA contained high levels of CaO, IBA particles passing through the 0.074 mm sieve could replace stone powder as filler to increase the strength of the OGAC.
3.3.7. Flowability analysis Fig. 8 shows the relation between the flowability value and IBA replacement content for the OGAC mix design. Each flowability value was obtained from the optimal asphalt content at each IBA replacement content. The figure shows an R2 of 0.0311, thus implying no apparent correlation between the flowability value and IBA replacement content. Hence, the flowability of the OGAC was not affected by the natural fine aggregates being replaced by the IBA.
3.3.8. Void in the mineral aggregate The VMA can provide information that can be used to ensure that a suitable amount of asphalt is contained in the voids between aggregate particles for an asphalt concrete mixture. Fig. 9 shows the relation between the VMA and IBA replacement content in the OGAC mix design. The IBA possessed more pores and lower density than the natural aggregates. When the same mix designs and asphalt content were considered for OGAC with or without IBA, the weight of the aggregate decreased when the aggregate volume was held constant and when the IBA replacement content
Table 6 The chemical components of the natural fine aggregates and IBA obtained using 2.38 mm and 1.19 mm sieve sizes and the bottom tray in the SEM-EDS analyses. Element
Al Si Ca Mg P K Pt Na Fe Zr Cl S
Natural fine aggregate
Incinerator bottom ash
2.38 mm
1.19 mm
Bottom tray
2.38 mm
1.19 mm
Bottom tray
3.57 14.01 2.37 – 0.37 – 0.47 – – – – –
5.5 15.49 – – 0.08 0.68 – – – – – –
9.28 11.22 – 3.96 – – – – – – – –
2.48 13.84 8.1 1.8 1.98 0.57 0.86 8.58 – – – –
3.31 13.22 6.76 – – 0.89 – – 1.09 0.53 0.77 0.26
5.46 14.33 4.54 – – – – – – – – –
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Fig. 2. The aggregate gradation distribution curves for an OGAC mix design containing different amounts of natural fine aggregates replaced by IBA.
Fig. 3. The relation between the optimal asphalt content and IBA replacement content for the OGAC mix design.
Fig. 4. The relation between the draindown and IBA replacement content for the OGAC mix design.
501
Fig. 5. The relation between the Cantabro abrasion and IBA replacement content for the OGAC mix design.
Fig. 6. The relation between the density and IBA replacement content for the OGAC mix design.
Fig. 7. The relation between the stability value and IBA replacement content for the OGAC mix design.
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Fig. 10. The relation between the asphalt film thickness and IBA replacement content for the OGAC mix design.
Fig. 8. The relation between the flowability value and IBA replacement content for the OGAC mix design.
the asphalt film thickness. Although the increase in the asphalt film thickness helped strengthen the resistance to the aging effect of the asphalt and prevented the invasion of water into the asphalt concrete pavement, this may increase the amount of ineffective additional asphalt content. As a result, this increase in additional asphalt content could reduce the quality of the pavement and increase the cost of the construction of the pavement. 3.4. Thermal imaging analyses
Fig. 9. The relation between the VMA and IBA replacement content in the OGAC mix design.
increased. Fig. 9 shows that the VMA increased with increasing IBA replacement content. 3.3.9. Asphalt film thickness The asphalt film thickness is one of the factors used to evaluate the resiliency of an asphalt concrete pavement. When the asphalt film thickness is insufficient, the adhesive force among aggregate particles will also be inadequate. As a result, the aggregate particles become easily stripped from the asphalt concrete pavement. The empirical equation derived by Hveen for the average asphalt film thickness wrapped around aggregate particles was applied in this study. Fig. 10 shows the relation between the asphalt film thickness and IBA replacement content for the OGAC mix design. Note that the asphalt thickness was calculated using the optimal asphalt content obtained from Fig. 3 for each IBA replacement. The increment in the asphalt film thickness was approximately 0.79 lm for the OGAC mix design with IBA contents from 0% to 100%. Moreover, the positive correlation between the asphalt film thickness and IBA replacement content is shown in Fig. 10. This implies that the rough IBA particle surface functioned similar to the addition of fibers to the asphalt concrete and can increase
To better understand the effects of the IBA replacement approach on the homogeneity of the OGAC, thermal imaging analyses were performed. To minimize possible errors, the thermal images were taken after the OGAC mixtures were paved. Furthermore, the images were obtained at the same angle and height. The results of the temperature distribution in the thermal images for OGAC containing with different levels of IBA replacement are shown in Fig. 11. The average OGAC temperatures for different levels of IBA replacement are shown in Table 7; the temperature distributions for the OGAC containing 0, 20, 40, 60, 80, and 100% IBA replacements exhibited the following ranges: 119–141, 121– 142, 114–140, 111–137, 107–133, and 106–128 °C, respectively. These results show that the temperature distribution variability of the OGAC changed slightly when the IBA replacement level was increased. Moreover, when the temperature of the mixing materials was fixed and the mixtures were mixed for the same duration, the average OGAC temperature decreased with increasing IBA replacement, suggesting that more attention should be given in engineering applications of IBA to OGAC. Hence, the homogeneity of the OGAC was slightly influenced by the IBA replacement. 3.5. Mechanical property test 3.5.1. Indirect tensile strength test Fig. 12 shows the relation between the indirect tensile strength and IBA replacement content for OGAC specimens containing IBA. The IBA particles were more similar to squares compared with natural fine aggregate particles. Hence, the interlocking ability among IBA particles was better than that of natural fine aggregate particles. Thus, the tensile strengths for the OGAC specimens containing IBA were increased. As shown in Fig. 12, the indirect tensile strength increased with increasing IBA replacement content. Moreover, because the IBA was lipophilic, the adhesive force between the IBA particles’ surfaces and the asphalt binder was increased. Therefore, the indirect tensile strengths of the OGAC specimens containing IBA were effectively improved.
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Fig. 11. The results of the temperature distribution in the thermal images for OGAC containing with (a) 0, (b) 20, (c) 40, (d) 60, (e) 80, and (f) 100% of IBA replacement.
Table 7 The average temperatures of OGAC containing with different level of the IBA replacement. Replacement content (%)
0
20
40
60
80
100
Average temperature (°C)
138.03
134.77
130.0
126.9
121.8
118.89
3.5.2. Tensile strength ratio (TSR) test Because of the effects of external physical conditions, such as moisture, temperature, traffic volume, and air pollution, on the asphalt concrete, the cohesive force of the asphalt and the adhesive force between aggregate particles and the asphalt binder were reduced, leading to the deterioration of the asphalt concrete structure. Fig. 13 shows the relation between the tensile strength ratio and IBA replacement content for the OGAC specimens containing IBA. All of the TSRs of the OGAC specimens were higher than the
standard value of 75%, as observed in the figure. Moreover, the TSRs for the OGAC specimens containing the IBA as a replacement were higher than that of control group (0% IBA replacement). This implies that the use of IBA in OGAC can help improve the resistance to moisture intrusion into the pavement. The lipophilic characteristic of the IBA helped to effectively adsorb the asphalt binder. Moreover, the hydrophobic nature of the IBA made it difficult for the IBA to combine with water. Therefore, the adhesive force between the aggregate particles and asphalt binder was improved,
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Fig. 12. The relation between the indirect tensile strength and IBA replacement content for OGAC specimens containing IBA.
Fig. 13. The relation between the tensile strength ratio and IBA replacement content for the OGAC specimens containing IBA.
and the moisture did not easily intrude into the voids generated between the aggregate particles and asphalt. The stripping of the pavement was reduced. To better understand the hydrophobicity of the applied materials, IBA and natural fine aggregates were ground into powders and passed through the size of 0.074 mm sieves. Afterward, mixtures were formed by mixing equal volumes of water and powder for
1 min, as shown in Fig. 14. The hydrophobicity of each material can be determined by the hydrophilicity. The hydrophobicity is a physical characteristic of a material and is related to the temporary bonding between hydrogen and water molecules. A stronger hydrophobicity leads to a weaker hydrophilicity; the molecules cluster together in water for a material with a high hydrophobicity. As shown in Fig. 14, the particles in the mixture containing the IBA powder were clustered together. Hence, the surface of the IBA exhibited a stronger hydrophobicity. Furthermore, hydrophobic materials can be characterized by lipophilic capability. As a result, the adhesion force between the asphalt binder and the aggregates was improved using the IBA aggregate replacement approach in the AC. It becomes harder for moisture to penetrate the pores between the asphalt binder and aggregates, leading to an improvement in the AC to stripping effect. However, the natural fine aggregate powder is instantly well mixed with water to form the mixture. Because the hydrophilicity of the natural fine aggregate powder is higher than that of the asphalt binder, the moisture can directly penetrate into the AC, leading to an easy separation of the asphalt binder film from the surface of the aggregates. Hence, the stripping effect is relatively easy to produce for AC containing natural fine aggregates. Moreover, water can easily penetrate into the angular gaps formed by the pores between the aggregates and asphalt binder. As a result, the asphalt binder film is relatively thin in the angular gaps. Because the IBA was more rectangular with smaller angular values as well as flat and elongated, there were fewer angular gaps in the AC containing IBA aggregate replacement in this study. Hence, the ability to produce thinner asphalt binder films is reduced, and the stripping effect caused by water penetration is decreased.
3.5.3. Static creep test Fig. 15 shows the relation between the static creep value and IBA replacement content for OGAC specimens containing IBA as a replacement. The static creep values obtained for OGAC containing IBA as a replacement were higher than that of the control group. For example, the static creep value for the 80% IBA replacement content was the highest. This result implies that the resistance to permanent deformation caused by static loading on the asphalt concrete pavement was improved when the natural fine aggregates were partially replaced by the IBA. However, the static creep decreased when the IBA replacement content was higher than 80%, thereby implying that the total replacement of the natural fine aggregates by the IBA for OGAC could not be suggested. The highest static creep value was observed at an 80% IBA replacement content, followed by 60, 40, 100, 20, and 0%.
Fig. 14. Hydrophobic properties of natural fine aggregates and IBA.
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4. Conclusions In this study, the results of analyses for the possible application of IBA to OGAC were evaluated. We provide the following conclusions:
Fig. 15. The relation between the static creep value and IBA replacement content for OGAC specimens containing IBA as a replacement.
3.5.4. Dynamic creep test Fig. 16 shows the relation between the dynamic creep value and IBA replacement content for the OGAC specimens containing IBA as a replacement. As stated above, the shape of the IBA particles was more similar to a square shape compared with the natural fine aggregate particles. The interlocking ability among IBA particles was better than that of natural fine aggregate particles. Hence, the space for aggregates to move around in OGAC specimens containing IBA as a replacement became relatively small. When the IBA replacement content was less than 80%, the dynamic creep value decreased with increasing IBA replacement content. This implies that OGAC pavement with an 80% IBA replacement content or less could effectively resist rutting deformation caused by dynamic loadings. However, when the IBA replacement content increased to 100%, the dynamic creep value increased and was larger than that of the 80% IBA replacement content. As stated in the previous section, this phenomenon can be explained as follows: when the IBA replacement content was higher than 80%, the asphalt film thickness increased. This thicker film thickness may lead to better lubrication among aggregate particles, and the deformation of OGAC pavement was observed under vehicle loadings. As a result, the dynamic creep value increased for the 100% IBA replacement content. These results of the dynamic creep test were similar to those obtained from the static creep tests. Therefore, the results of this study demonstrate that the IBA replacement content should be 80% or less to decrease the risks of degrading the engineering properties and durability of OGAC containing IBA as a replacement material.
Fig. 16. The relation between the dynamic creep value and IBA content for the OGAC specimens containing IBA as a replacement.
1. The IBA exhibited a smaller specific gravity and larger abrasion than did the natural fine aggregates. However, the IBA exhibited a rougher surface and had a higher CaO content than did the natural aggregates. The stripping of OGAC was effectively improved when fine aggregates were partially replaced by bottom ash. This result suggests the feasibility of applying IBA to OGAC. 2. Because the particle sizes of the IBA were larger than those of the fine aggregates, the amount of draindown for OGAC decreased with increasing IBA replacement content. However, the IBA particles had a rougher surface, thus resulting in the absorption of additional asphalt on the coarse surface. The abrasion level increased with increasing IBA replacement content. 3. Although the optimum asphalt binder content was slightly increased to 0.1% when the IBA replacement content was less than 60%, the properties of the OGAC containing IBA were largely improved. From the perspective of construction costs, the application of IBA to the OGAC is feasible. 4. Because the IBA contained more Ca than did the natural fine aggregates and was shown to be lipophilic, the adhesive force between the IBA particle surfaces and asphalt binder was increased. Hence, the indirect tensile strengths of OGAC containing IBA were effectively increased. 5. The test results obtained from the static or dynamic creep tests showed that the properties of OGAC containing IBA were better than those containing natural fine aggregates. These results suggest that the IBA replacement content should be less than 80% when applied to OGAC.
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