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Sustainable roadway construction using recycled aggregates with geosynthetics
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Sustainable roadway construction using recycled aggregates with geosynthetics Jie Han a,∗ , Jitendra K. Thakur b,1 a b
Civil, Environmental, and Architectural Engineering Department, The University of Kansas, 1530W, 15th Street, Lawrence, KS 66045-7609, United States Terracon Consultants, Inc., 1211 W. Florida Avenue, Midland, Texas 79701, United States
a r t i c l e Keywords: Geosynthetic Recycled aggregate Stabilization
i n f o
a b s t r a c t Concrete, asphalt pavements, and ballast are removed during the re-construction of existing roads and have been increasingly recycled as aggregates for the construction of roadways. Due to existence of asphalt, cement, and fines, mechanical properties of recycled aggregates may not be sufficient for load support. They may also have long-term durability problems. Geosynthetics have been used to improve mechanical properties and long-term durability of recycled aggregates. This paper reviews recent research work on the use of geosynthetics to stabilize recycled aggregates in roadway construction and summarizes the main findings on permanent deformation, creep deformation, degradation, stress distribution, and/or crack propagation. © 2014 Published by Elsevier B.V.
1. Introduction According to the Federal Highway Administration (FHWA, 2004), two billion tons of aggregates are quarried annually in the United States and the quantity of quarried aggregates will reach 2.5 billion tons by 2020, which force construction industries to consider new sources of aggregates. Roadways (highways and railways) that have reached the end of their service lives are frequently rehabilitated by removing the existing roadway surfaces and replacing the removed portion with new construction materials. A large amount of recycled aggregates are created every year during the rehabilitation and reconstruction of existing roadways. Currently, great emphasis is placed on sustainable construction and infrastructure with green technologies because the demand for sustainable and environmental-friendly roads is increasing daily. More technologies for sustainable roadway construction are needed. One way to construct sustainable roads is through the use of recycled aggregates. Recycled Asphalt Pavement (RAP), Recycled Concrete Aggregate (RCA), and Recycled Ballast (RB) are the three types of recycled aggregates as shown in Fig. 1 and are discussed in this paper. According to the Recycled Material Resource Center (RMRC, 2008), RAP is a removed and reprocessed pavement material from deteriorated asphalt pavements containing asphalt binder (3–7%) and aggregates (97–93%) by weight. The use of RAP has
∗ Corresponding author. Tel.: +1 7858643714; fax: +1 7858645631. E-mail addresses: [email protected] (J. Han), [email protected] (J.K. Thakur). 1 Tel.: +432 684 9600.
been in practice since 1930s. The U.S. FHWA estimated that 100.1 million tons of asphalt pavement materials are milled off each year during resurfacing and widening of road projects, of which 80.3 million tons are reclaimed and reused for roadbeds, shoulders, and embankments (Missouri Asphalt Pavement Association, 2010). RCA is a removed and reprocessed construction material from demolished concrete structures, such as high-rise buildings, bridges, highways, railways, etc. containing cement and natural aggregates. The natural aggregates contain 60–75% of the total volume of RCA (RMRC, 2008). Ballast is a free-draining granular material composed of medium to coarse gravel-sized aggregates (10–60 mm in diameter) with a small percentage of cobble-sized particles, commonly used as a load-bearing material in railway tracks (Indraratna, Khabbaz, Salim, & Christie, 2006). The good quality of ballast consists of angular particles with rough surface and minimum hairline cracks and should have high specific gravity, shear strength, toughness and hardness, and enough resistance to weathering (Indraratna et al., 2006). Railway ballast degrades and deteriorates progressively under repeated cyclic loading. Degraded ballast is usually replaced by fresh ballast during routine track maintenance. The railway track constructed using recycled ballast (RB) shows excessive settlement and lateral deformation, which affect the performance of railroads. The use of recycled aggregates can reduce the cost of construction materials, reduce the amount of waste to be land-filled, reduce the transportation and energy costs to import virgin aggregates, and conserve natural resources by requiring less virgin aggregates in road construction projects. Several agencies are seriously considering the economic and environmental benefits of using recycled aggregates in roadways and facing challenges to maintain
2210-6707/$ – see front matter © 2014 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.scs.2013.11.011
Please cite this article in press as: Han, J., & Thakur, J.K. Sustainable roadway construction using recycled aggregates with geosynthetics. Sustainable Cities and Society (2014), http://dx.doi.org/10.1016/j.scs.2013.11.011
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Fig. 1. Types of recycled aggregates.
high-quality road infrastructure. The mechanical properties (mainly strength and stiffness) and long term durability (breakage and abrasion) of recycled aggregates may not be sufficient for load support due to the existence of asphalt and cement, or loss of angularity of ballast. In the past, most of the research studies on the improvement of RAP and RCA quality focused on blending them with virgin aggregates or stabilizing them using chemical additives. However, the blending of RAP and RCA with virgin aggregate still consumes natural resources and the chemical stabilization is not always environmental friendly. The use of 100% recycled aggregate with geosynthetic is a sustainable solution. Liu, Scarpas, Blaauwendraad, and Genske (1998) were few early researchers to explore such a possibility. Recent research work done by Indraratna, Salim, and Christie (2002), Indraratna et al. (2006), Han et al. (2011), Thakur, Han, Pokharel, and Parsons (2012), Thakur, Han, and Parsons (2013), and others has further evaluated the behavior and performance of geosynthetic-reinforced recycled aggregates. This paper reviews these studies on the use of geosynthetics to stabilize recycled aggregates (RAP, RCA, and RB) in roadway construction and summarizes the main research findings. 2. Reinforcement mechanism of geosynthetic Geosynthetics manufactured from polymeric materials have been widely used as construction materials to solve many civil engineering problems since 1970s. Geosynthetics are used to improve the performance of unpaved and paved roads for over 40 years (Giroud & Han, 2004). The use of geotextile, geogrid, and geocell with recycled aggregates are discussed in this paper. Geotextile and geogrid are planar geosynthetics whereas geocell is a threedimensional honeycomb type of geosynthetic. Geogrid and geocell improve the performance of aggregate layers by providing lateral confinement whereas geotextile improves the performance of aggregate layers by providing a tensioned membrane effect. Different types of geosynthetics used in roadway construction are shown in Fig. 2. The most efficient and convenient location of geosynthetic in roadway construction is at the interface of subgrade and granular base course (Das & Shin, 1998). Geosynthetic installed at this location provides full or partial separation, lateral confinement of granular base materials, a tensioned membrane or beam effect when a road deforms extensively. The tensioned membrane or beam effect is referred to as the tension developed in the curved geosynthetic-reinforced base to resist the vertical load (Rajagopal, Krishnaswamy, & Madhavi Latha, 1999). The tensioned membrane effect mechanism is shown in Fig. 3a. Nonwoven geotextile provides separation, filtration, and drainage whereas woven geotextile provides separation and reinforcement. Geogrid and geocell provide reinforcement to aggregate base and subgrade by providing lateral confinement due to their tensile strength and stiffness. Uniaxial, biaxial, and triaxial geogrids are three types of
geogrids available in the market. Uniaxial geogrid provides tensile resistance in only one direction, biaxial geogrid provides tensile resistance in two directions, and triaxial geogrid can provide nearuniform tensile resistance (Dong, Han, & Bai, 2010; Qian, 2009) when it is subjected to tension in different directions. Qian (2009) reported that the confinement of granular base aggregates was obtained through the interlocking between geogrid apertures and aggregate particles as shown in Fig. 3b. Webster (1992) reported that the degree of interlocking depended on geogrid aperture size and aggregate particle size and the effectiveness of interlocking depended on the in-plane stiffness, rib strength, and junction strength of the geogrid. Thakur et al. (2012) reported that the geocell-reinforced bases had improved bending resistance. The beam effect of geocell-reinforced bases is demonstrated in Fig. 3c.
3. Geosynthetic-reinforced recycled asphalt pavement (RAP) Geotextile, geogrid, and geocell have been used to stabilize RAP bases. This section discusses the effects of geosynthetic reinforcement on the permanent deformation, resilient deformation, creep deformation, and stress distribution of RAP bases. Foye (2011) presented the work of a design-build contractor who used a geosynthetic stabilization technique for reconstruction of 19,500 m2 asphalt parking lot on a site with very weak subgrade (CBR ranging from 1 to 3%). The remedial design parking lot section consisted of very weak subgrade soil overlaid by 200 mm thick geocomposite (a 271 g/m2 needle-punched nonwoven geotextile – geogrid)-stabilized blended RAP aggregate base, 64 mm thick dense-graded asphalt course, and 25 mm thick asphalt wearing course. The geocomposite was placed at the interface of subgrade and granular base course to provide separation and reinforcement. It was found that the geocomposite-stabilized parking lot section showed little rutting or deflection under proof rolling and the use of the geocomposite reduced the cost of construction from about $890,000 (estimated for the original cut and replacement specification) to about $200,000. In addition, the geocomposite stabilization technique saved time, resources, and energy as compared with the traditional cut and replacement technique. Han et al. (2011) conducted moving wheel tests on five geocellreinforced and two unreinforced RAP bases over weak subgrade (target CBR = 3%) to evaluate the effect of geocell reinforcement on rut depth and stress distribution angle at a certain number of passes of the wheel load. Two types of recycled asphalt materials, named RAP and FRAP (fractioned RAP or RAP with finer gradation) were used in this study. The following base sections were prepared and tested: (1) 300 mm thick unreinforced RAP. (2) 150 mm thick geocell-reinforoced RAP with a 20 mm thick RAP cover.
Please cite this article in press as: Han, J., & Thakur, J.K. Sustainable roadway construction using recycled aggregates with geosynthetics. Sustainable Cities and Society (2014), http://dx.doi.org/10.1016/j.scs.2013.11.011
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Fig. 2. Types of geosynthetics. (a) Non-woven geotextile and geocell. (b) Triaxial geogrid. (c) Biaxial geogrid. (d) Uniaxial geogrid. (e) Woven geotextile.
(3) 100 mm thick geocell-reinforced RAP with a 70 mm thick RAP cover. (4) double layered geocell-reinforced RAP with a 30 mm thick RAP cover above a 100 mm thick bottom geocell layer and a 70 mm thick RAP cover above a 100 mm thick top geocell layer. (5) 250 mm thick unreinforced FRAP. (6) 100 mm thick geocell-reinforced FRAP over a 100 mm thick unreinforced FRAP base course with a 50 mm thick FRAP cover. (7) 75 mm thick geocell-reinforced FRAP over a 100 mm thick unreinforced FRAP base course with a 75 mm thick FRAP cover. They found that the geocell improved the life of unpaved sections by a factor of 1.3 using one layer of 75 mm high geocell and 1.8 using one layer of 100 mm high geocell at a rut depth of 75 mm. They concluded that the geocell reinforcement reduced the rut depth and vertical stresses transferred to the subgrade by distributing the load over a wider area at the same number of passes. For a demonstration purpose, the vertical stresses at the interface of subgrade and FRAP base versus the number of passes are shown in Fig. 4. The measured vertical stresses at the interface of subgrade and base were much lower than the tire pressure of 552 kPa applied
on the road surface for each section. The vertical stress increased or remained constant with the number of passes for unreinforced sections but decreased with the number of passes for reinforced sections. They attributed this phenomenon to the beam or slab effect of the geocell-reinforced bases. Thakur et al. (2012) conducted large-scale cyclic plate loading tests on one unreinforced RAP base (300 mm thick) and three geocell-reinforced RAP bases (150, 230, and 300 mm thick) over weak subgrade (target CBR = 2%) to evaluate the performance of unreinforced and geocell-reinforced RAP bases over weak subgrade. The permanent deformation, the resilient deformation, the vertical stress at the interface of subgrade and base, and the strains in the geocell wall were measured during the cyclic plate loading tests. Fig. 5 shows the permanent deformations of the unreinforced and reinforced bases over weak subgrade at the center of the loading plate versus the number of loading cycles. The permanent deformation increased with the number of loading cycles. The rate of increase in the permanent deformation decreased with the number of loading cycles. On the weak subgrade, the geocellreinforced RAP bases (150, 230, and 300 mm thick) improved the performance (i.e., the number of cycles) by a factor of 6.4, 3.6, and 19.4 as compared with the 300 mm thick unreinforced RAP base,
Please cite this article in press as: Han, J., & Thakur, J.K. Sustainable roadway construction using recycled aggregates with geosynthetics. Sustainable Cities and Society (2014), http://dx.doi.org/10.1016/j.scs.2013.11.011
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Fig. 3. Different reinforcement mechanism of geosynthetics. (a) Tensioned membrane effect mechanism (from Maxwell, Kim, Edil, & Benson, 2005). (b) Aggregate interlocking between geogrid aperture and aggregate particle (from Tensar International). (c) Beam effect of geocell.
respectively, at 75 mm permanent deformation. The 230 mm thick geocell-reinforced base had a lower improvement factor than the 150 mm thick geocell-reinforced base because of the lower CBR values of the base and subgrade in the 230 mm thick base as compared with those in the 150 mm thick base. They concluded that geocell-reinforced RAP bases provided a sustainable solution for roadway construction technology by improving the performance of RAP bases. In addition to reducing the permanent deformation of bases, geocell reinforcement reduced the vertical stresses transferred to the subgrade and increased the percentage of resilient deformation of RAP bases.
Thakur et al. (2013) conducted plate loading tests to investigate the vertical stress-displacement responses of the following RAP specimens:
(1) unreinforced sample (unreinforced RAP sample extruded from a Proctor compaction mold) (2) unreinforced base (a RAP base prepared in a test box without geocell) (3) single geocell-reinforced base (a RAP base prepared by placing RAP into the single geocell pocket and the test box)
Please cite this article in press as: Han, J., & Thakur, J.K. Sustainable roadway construction using recycled aggregates with geosynthetics. Sustainable Cities and Society (2014), http://dx.doi.org/10.1016/j.scs.2013.11.011
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(4) multi geocell-reinforced base (a RAP base prepared by placing RAP into the multiple geocell pockets and the test box). The applied vertical stress versus displacement curves are shown in Fig. 6. They found that the unreinforced RAP sample failed at 172 kPa while other sections did not fail up to a vertical stress of 586 kPa and showed a linear vertical stress-displacement response. The stress-displacement responses were analyzed in terms of a modulus improvement factor. The modulus improvement factor is the ratio of the slope of the initial portion of the vertical stress versus displacement curve for the geocell-reinforced base to that of the unreinforced base. The test results showed that the moduli of the single geocell-reinforced and the multi geocell-reinforced bases were increased by 1.2 and 1.6 times compared to the unreinforced base, respectively. Thakur et al. (2013) also conducted static plate loading tests in a test box and a compaction mold at a room temperature of about 25 ◦ C to investigate the effects of confinement, stress, and cover material on creep deformations of unreinforced and geocell-reinforced RAP bases. The plate loading tests were conducted at vertical stresses of 276 and 552 kPa under five confining conditions including single and multiple geocell-reinforced RAP bases. They concluded that geocell confinement reduced the initial deformation and the rate of creep of the RAP bases, RAP bases crept more at a higher vertical stress and at a lower degree of confinement, and the well-graded aggregate cover significantly reduced the creep of geocell-reinforced RAP bases as compared with the RAP cover. Fig. 7 shows the creep deformations of the geocell-reinforced RAP bases at the vertical stress of 276 kPa as compared with the unreinforced RAP bases. It is shown that geocell
Fig. 6. Vertical stress-displacement curves for unreinforced and geocell-reinforced RAP bases (data from Thakur et al., 2013, ©ASCE).
confinement significantly reduced the creep deformation of the RAP bases and multiple geocell-reinforced base crept least followed by the single geocell-reinforced base and the unreinforced base. Han, Acharya, Thankur, and Parsons (2012) conducted largescale cyclic plate loading tests on asphalt pavements with geocell-reinforced RAP bases over moderate subgrade (target CBR = 5%) and concluded that geocell reduced the permanent deformations of HMA layer, RAP base, and subgrade, reduced the vertical stress at the interface of base and subgrade, and increased the elasticity of the RAP base. Bortz, Hossain, Halami, and Gisi (2012) conducted moving wheel tests on eight asphalt pavement sections, in which two unreinforced sections had well-graded crushed limestone aggregate (AB-3) bases and six geocell-reinforced sections had AB-3, quarry waste (QW), and RAP bases. The details of these bases are provided below: (1) (2) (3) (4) (5) (6) (7) (8)
300 mm thick unreinforced AB-3 75 mm thick geocell-reinforced QW with 25 mm thick cover 75 mm thick geocell-reinforced RAP with 25 mm thick cover 75 mm thick geocell-reinforced AB-3 with 25 mm thick cover 200 mm thick unreinforced AB-3 150 mm thick geocell-reinforced QW with 50 mm thick cover 150 mm thick geocell-reinforced RAP with 50 mm thick cover 150 mm thick geocell-reinforced AB-3 with 50 mm thick cover.
All the cover materials were the same as the infill materials. The subgrade was AASHTO A-7-6 clay and was prepared to obtain target CBR values of 6% and 12%. The bases (1)–(4) were prepared over the subgrade with a CBR of 6% and were paved with a 50 mm thick
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Time (minute) Fig. 7. Creep behavior at vertical stress of 276 kPa (data from Thakur et al., 2013, ©ASCE).
Please cite this article in press as: Han, J., & Thakur, J.K. Sustainable roadway construction using recycled aggregates with geosynthetics. Sustainable Cities and Society (2014), http://dx.doi.org/10.1016/j.scs.2013.11.011
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4. Gesosynthetic-reinforced recycled concrete aggregate (RCA) Geotextile and geogrid have been used to stabilize RCA bases. This section discusses the effects of geosynthetic reinforcement on the permanent deformation and crack propagation of RCA bases. Liu et al. (1998) performed finite element analysis on asphalt concrete pavements with geogrid reinforced recycled concrete aggregate and natural aggregate bases to evaluate the performance of the geogrid-reinforced RCA base as compared with the recycled masonry aggregate (RMA) base and the crushed natural aggregate (CNA) base. Influence of the material characteristics of the recycled aggregate and the geogrid reinforcement on the development and rate of propagation of reflective cracking in the top layer of the pavement was selected as the criterion for the comparison purpose. They used stress intensity factor (K) distributions for both unreinforced and reinforced pavements to evaluate the performance in terms of the energy available at a crack tip for additional crack propagation. Fig. 9 shows that geogrid reinforcement reduced the K factor for different base materials thus improved the pavement performance. They concluded that geogrid reduced the rate of crack propagation into the top layer of the pavement, improved the load spreading in the base layer, and enhanced the pavement life. The geogrid-reinforced RCA base performed better than the
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Fig. 9. K factor distribution versus crack length (redrawn and modified from Liu et al., 1998).
unreinforced CNA base followed by the unreinforced RCA base, the reinforced RMA, and the unreinforced RMA. Donovan (2011) conducted dynaflect tests on five pavement test sections (150 m long each) constructed at city of Edmonton, Canada to investigate the possibility of using RCA in roadway construction. The pavement structure of each section consisted of cement stabilized subgrade (CSS) overlaid by granular aggregate base (GAB), asphalt concrete base (ACB), and asphalt concrete overlay (ACO). The city had added the second lift of ACO after two years of original construction in 2004. The total thickness of asphalt layer in each section was same (50 mm thick ACB + 60 mm thick first ACO + 50 mm thick second ACO = 160 mm) with a variation in type and thickness of the aggregate base course. Sections 1 and 2 used natural aggregate and crushed natural aggregate, respectively, whereas Sections 3–5 used recycled crushed aggregate as the granular base course. Recycled crushed concrete consisted of 60% RCA, 25% RAP, and 15% other materials such as cement treated granular base aggregate, brick and other recyclable materials. The thicknesses of GAB for Sections 1–4 were 325 mm while that for Section 5 was 150 mm. Each section had 150 mm thick CSS except Section 3 which had 325 mm thick CSS. Geotextile and geogrid were installed on top of subgrade in Sections 3 and 5, respectively. All other remaining sections were unreinforced. The deflection test results are shown in Fig. 10. The recycled crushed aggregate sections performed better than the natural aggregate and crushed natural aggregate sections. The deformation of each section increased with time. The deformations measured in 2005 were less than those in 2004 because of the placement of the second lift of ACO in 2004. 0.05 0.045
Deformation (mm)
HMA layer, whereas the bases (5) to (8) were prepared over the subgrade with a CBR of 12% and were paved with a 100 mm thick HMA layer. All eight sections were tested to evaluate the effects of base, cover and HMA thicknesses and geocell reinforcement on rut depth and stress distribution at the interface of subgrade and base at a certain number of passes of the wheel load. They concluded that a minimum cover thickness of 50 mm over geocell and a minimum HMA thickness of 100 mm over a base were necessary for better performance of pavements. The geocell-reinforced waste materials (RAP and QW) performed as well as the geocell-reinforced AB-3. The pavement sections constructed over the firm subgrade (CBR = 12%) performed better than those over the moderate subgrade (CBR = 6%) as shown in Fig. 8. In this figure, RAP (moderate) and RAP (firm) stand for asphalt pavements with geocell-reinforced RAP bases over moderate and firm subgrades, respectively. The same representation holds for AB-3 and QW. Fig. 8 does not show the rut behavior at 100,000 and 500,000 passes for the pavement sections constructed over the moderate subgrade because these sections reached the failure criterion of 12.5 mm rut depth before 100,000 passes.
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Year Fig. 10. Deformation behavior of pavement sections (redrawn and modified from Donovan, 2011).
Please cite this article in press as: Han, J., & Thakur, J.K. Sustainable roadway construction using recycled aggregates with geosynthetics. Sustainable Cities and Society (2014), http://dx.doi.org/10.1016/j.scs.2013.11.011
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5. Gesosynthetic-reinforced recycled ballast (RB) Railroad ballast provides a support for railroad tracks and distributes loads to weak subgrade. Geotextile and geogrid have been used to stabilize fresh and recycled ballasts. This section discusses the effects of geosynthetic reinforcement on the deformation and degradation of RB. Indraratna et al. (2002) conducted large-scale laboratory cyclic load tests in a cubical triaxial chamber, which simulated the field load and boundary conditions on unstabilized, geogrid-stabilized, and geogrid-geotextile composite stabilized-fresh and recycled ballasts. Indraratna and Salim (2003), Indraratna, Shahin, and Salim (2005), and Indraratna et al. (2006) conducted cyclic laboratory triaxial tests in a prismoidal chamber, which simulated field loading conditions on unstabilized and geotextile-stabilized fresh and recycled ballasts. The test sections consisted of 50 mm thick
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Section 4 performed best followed by Sections 5, 3, 1 and 2 in long term as shown in Fig. 10. They concluded from this study: (1) the geotextile had a positive effect when the subgrade soil was very weak; (2) the geogrid improved the life of pavement section and reduced the thickness of granular base by 50% as compared with a similar unreinforced base to provide the same performance; (3) the recycled crushed aggregate was successfully used as the granular base course. Gongora and Palmira (2012) conducted laboratory cyclic plate loading tests on unreinforced and biaxial geogrid-reinforced unpaved road test sections prepared inside a steel tank (750 mm in diameter and 530 mm high) to investigate the performance of unreinforced and geogrid-reinforced unpaved roads on weak subgrade (CBR ≈ 4%). The cyclic load was applied with a peak force of 17.6 kN at a wave frequency of 1 Hz. Each test section consisted of 300 mm thick subgrade overlaid by a 230 mm thick granular aggregate base. Fresh aggregate (FA) and recycled concrete aggregate (RCA) were used as granular aggregate base materials. RCA was coarser in size than FA. The geogrid was installed at the interface of subgrade and base course in case of the reinforced sections. The deformations at different numbers of cycles were measured using linear variable differential transducers (LVDT). The deformation behavior of unreinforced and geogrid reinforced sections is shown in Fig. 11. The geogrid-reinforced sections had much smaller permanent deformations than the unreinforced sections. The geogrid-reinforced RCA performed best, followed by the reinforced FA, RCA, and FA as shown in Fig. 11. The test results indicated that the geogrid improved the life of reinforced RCA and FA sections by factors of 33.5 and 15 at a permanent deformation of 25 mm as compared with the corresponding unreinforced sections, respectively.
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Fig. 12. Ballast settlement versus number of loading cycles. (a) Redrawn and modified from Indraratna et al. (2002). (b) Redrawn and modified from Indraratna and Salim (2003).
subgrade, a 100 mm thick subbase layer (i.e. a capping layer of gravel and sand), 300 mm thick load bearing ballast, and a 150 mm thick crib ballast layer of fresh or recycled ballast. They found that the geosynthetics (woven geotextile, geogrid, and geogridgeotextile composite) at the interface of ballast and the capping layer reduced the amount and rate of vertical and lateral deformations. The wet RB had more deformations than the dry RB. They developed a semi-logarithmic equation (S = A + B logN) to predict the settlement of RB, where A and B are empirical constants depending on the initial compaction, type of ballast, type of reinforcement, magnitude of cyclic loading, and degree of saturation; N is the number of load cycles; and S is the tie (also called sleeper) settlement. Fig. 12 shows the effects of geosynthetic reinforcement and saturation on the deformation behavior of fresh and recycled ballasts in two different studies. Indraratna et al. (2005) reported that the optimum location of the geosynthetic was 200 mm beneath the tie to improve the railway track performance; however, it was easy to place the geosynthetic at the ballast-capping interface. Indraratna et al. (2005) also investigated the effects of geosynthetic reinforcement and saturation on degradation behavior of fresh and recycled ballasts under cyclic loading by sieving each ballast sample before and after the test, and recording the change in percentage retained on each sieve size (Wk ). They calculated a breakage index (Bg ) for each specimen using the method proposed by Marsal (1967). Breakage index (Bg ) is the sum of the positive values of Wk . Higher Bg indicates higher potential to degradation. The degradation behavior of unreinforced and geosynthetic-reinforced ballasts is shown in Fig. 13. The unreinforced RB had 97 and 95% more breakage as compared with the unreinforced FB in dry and wet conditions, respectively. Each specimen had slightly higher degradation in the wet condition than that in the dry condition. Geogrid-reinforced RB, geotextile-reinforced RB, and geocomposite-reinforced RB
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Parsons, Jowkar, and Han (2012) conducted full-scale laboratory cyclic loading tests on one unreinforced and one triaxial geogrid-reinforced recycled ballast railroad sections to investigate the deformation and degradation behavior of unreinforced and geogrid-reinforced RB. A full-scale trapezoidal railroad section consisted of a 600 mm thick recycled ballast layer over a 600 mm thick fat clay subgrade. The top and bottom widths of the railroad cross-section were 2700 mm and 7500 mm, respectively. The subgrade and ballast layers were prepared at 2:1 slope with 2700 mm top width. The ties were embedded in the ballast to a depth of 175 mm. The railroad track was placed over ties. The geogrid was installed 175 mm below the tie in case of the reinforced section. They concluded that geogrid reduced the vertical settlement of
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improved the performance by 41, 48, and 50% as compared with unreinforced RB, respectively. The geosynthetic-reinforced RB performed as good as or even better than the unreinforced FB in terms of breakage consideration. Indraratna, Nimbalkar, Christie, Rujikiatkamojorn, and Jayan (2010) conducted full-scale moving wheel load tests on test track sections. Four test sections (two unreinforced and two geocomposite-reinforced, 15 m each in length) were prepared and tested. The geocomposite consisted of biaxial geogrid and nonwoven geotextile. The track had an overall bed thickness of 450 mm with a 300 mm thick ballast layer and a 150 mm thick capping layer. The particles smaller than 9.5 mm were removed from the recycled ballast. The capping materials consisted of a mixture of sand and gravel. The particle shapes of the fresh ballast (FB), the recycled ballast (RB), and the capping materials were highly angular, semi-angular, and angular to rounded, respectively. The geocomposite-reinforced FB performed best, followed by the geocomposite-reinforced RB, RB, and FB. The RB performed better than the FB because of the difference in gradation of ballast. The RB was moderately graded whereas the FB was relatively uniformly graded. The geocomposite improved the performance of FB significantly as compared with the RB. The geocomposite installed at the interface of ballast and capping layer improved the performance of the reinforced track sections by reducing the vertical and lateral strains in the ballast layer as compared with that of the unreinforced sections as shown in Fig. 14. Fatahi and Khabbaz (2011) conducted finite element modeling using PLAXIS to investigate the effect of shoulder ballast width, ballast type, and geosynthetic on the deformation of a track due to train load. The track section consisted of 3000 mm thick subgrade overlaid by 150 mm thick sub-ballast, 300 mm thick ballast, 15 mm thick tie, and 100 mm thick rail. The ballasts used were fresh ballast (FB), recycled ballast (RB), and blended ballast (BB). The blended ballast consisted of 50% FB and 50% RB (FB and RB, each of 150 mm thick). The widths of shoulders were varied from 0 to 700 mm to investigate the shoulder width effect. The geogrid was installed at the interface of subgrade and sub-ballast for the reinforced RB section whereas for the reinforced BB sections, the geogrid was installed at different locations to investigate the location effect. They concluded that track settlement was reduced by increasing the shoulder ballast width; the reinforced BB section performed best, followed by the reinforced RB, unreinforced FB, and unreinforced BB sections; and the effective location of geogrid was at the interface of subgrade and sub-ballast. Fig. 15 shows the maximum settlements of the track in different sections. The test results for unreinforced FB and unreinforced RB are not shown in Fig. 15 as the analysis was not conducted for these sections.
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the reinforced recycled ballast between the ties and geogrid by 37–65% as compared with that of the unreinforced test section and increased the resistance against breaking under cyclic loading. 6. Conclusions This paper reviews the research work done in the past on the use of geosynthetics to stabilize recycled aggregates including recycled asphalt pavement (RAP), recycled concrete aggregate (RCA), and recycled ballast (RB). The following conclusions can be made: (1) 100% RAP and 100% RCA have been used with geosynthetic reinforcement as base course materials for sustainable roadway construction. (2) 100% RB has been used with geosynthetic reinforcement (woven geotextile, geogrid, and geogrid and non-woven geotextile composite) as a load bearing layer in railway track construction. (3) Geocell improved the performance of RAP bases by reducing the permanent and creep deformations and vertical stresses transferred to the subgrade, increasing the percentage of resilient deformation and the modulus of the RAP bases. (4) Geogrid improved the performance of RCA bases in flexible pavements by reducing the rate of crack propagation into the top layer of the pavement and spreading the load in the base layer in a wider area. (5) Geogrid improved the life of reinforced RCA and FA base sections by factors of 33.5 and 15 at a permanent deformation of 25 mm as compared with the corresponding unreinforced sections, respectively. (6) Geosynthetics (geotextile, geogrid, and geotextile–geogrid composite) improved the degradation resistance of the reinforced RB by 41 to 50% as compared with the unreinforced RB. The geosynthetic-reinforced RB performed as good as or even better than the unreinforced FB in terms of breakage consideration. (7) The performance of ballast depended on its gradation. The geocomposite (geotextile–geogrid) reduced the vertical and lateral deformations of the uniform graded ballast more significantly as compared with the well-graded ballast. (8) Geosynthetics (woven geotextile, geogrid, and nonwoven geotextile–geogrid composite) reduced the vertical and lateral deformations and enhanced the long-term durability of recycled ballast. References Bortz, B. S., Hossain, M., Halami, I., & Gisi, A. (2012). Low-volume paved road improvement with geocell reinforcement. In Transportation Research Board, annual meeting (online publication). Das, B. M., & Shin, E. C. (1998). Strip foundation on geogrid-reinforced clay: Behavior under cyclic loading. Geotextiles and Geomembranes, 13(10), 657–666. Dong, Y. L., Han, J., & Bai, X. H. (2010, 20–24 February). A numerical study on stress–strain responses of biaxial geogrids under tension at different directions. In D. O. Fratta, A. J. Puppala, & B. Muhunthan (Eds.), Advances in analysis, modeling and design, Geotechnical special publication no. 199 (pp. 2551–2560). West Palm Beach, FL: GeoFlorida.
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Donovan, H. (2011). Recycled aggregate and geosynthetic study – City of Edmonton. In Annual conference of the transportation association of Canada Edmonton, Alberta, 19p. Fatahi, B., & Khabbaz, H. (2011). Enhancement of ballasted rail track performance using geosynthetics. In GeoHunan international conference (pp. 222–230). Federal Highway Administration (FHWA). (2004). Transportation applications of recycled concrete aggregates. FHWA State of the Practice National Review. Foye, K. C. (2011). Use of reclaimed asphalt pavement in conjunction with ground improvement: A case history. Advances in Civil Engineering, 2011. Hindawi Publishing Corporation, 7p. Giroud, J. P., & Han, J. (2004). Design method for geogrid-reinforced unpaved roads. I. development of design method. Journal of Geotechnical and Geoenvironmental Engineering, 130(8), 775–786. Gongora, I. A. G., & Palmira, E. M. (2012). Influence of fill and geogrid characteristics on the performance of unpaved roads on weak subgrades. Geosynthetics International, 19(2), 191–199. Han, J., Acharya, B., Thankur, J. K., & Parsons, R. L. (2012). Onsite use of recycled asphalt pavement materials with geocells to reconstruct pavements damaged by heavy trucks. Report no. 25-1121-0001-462. Mid-America Transportation Center., 121p. Han, J., Pokharel, S. K., Yang, X., Manandhar, C., Leshchinsky, D., Halahmi, I., et al. (2011). Performance of geocell-reinforced RAP bases over weak subgrade under full-scale moving wheel loads. ASCE Journal of Materials in Civil Engineering, 23(11), 1525–1534. Indraratna, B., Khabbaz, H., Salim, W., & Christie, D. (2006). Geotechnical properties of ballast and the role of geosynthetics in rail track stabilization. Journal of Ground Improvement, 10(3), 91–102. Indraratna, B., Nimbalkar, S., Christie, D., Rujikiatkamojorn, C., & Jayan, V. (2010). Field assessment of the performance of a ballasted rail track with and without geosynthetics. Journal of Geotechnical and Geoenvironmental Engineering, 136(7), 907–917. Indraratna, B., & Salim, W. (2003). Deformtion and degradation mechanism of recycled ballast stabilized with geosynthetics. Soils and Foundations, 43(4), 35–46. Indraratna, B., Salim, W., & Christie, D. H. (2002). Improvement of recycled ballast using geosynthetics. Rail International, 33(4), 20–29. Indraratna, B., Shahin, M. A., & Salim, W. (2005). Use of geosynthetics for stabilizing recycled ballast in railway track substructures. In Proceedings of NAGS2005/GRI 19 cooperative conference (pp. 1–15). Liu, X., Scarpas, A., Blaauwendraad, J., & Genske, D. D. (1998). Geogrid reinforcing of recycled aggregate materials for road construction: Finite element investigation. Transportation Research Record, 1611, 78–85. Marsal, R. J. (1967). Large scale testing of rockfill materials. Journal of Soil Mechanics and Foundation Engineering, 93(SM2), 27–43. Maxwell, S., Kim, W., Edil, T. B., & Benson, C. H. (2005). Effectiveness of geosynthetics in stabilizing soft subgrades. Report to the Wisconsin Department of Transportation. Missouri Asphalt Pavement Association. (2010). Recycling of asphalt pavement. http://www.moasphalt.org/facts/environmental/recycling.htm Retrieved 10.12.10 Parsons, R. L., Jowkar, M., & Han, J. (2012). Performance of geogrid reinforced ballast under dynamic loading. Report no. 25-1121-0001-363. Mid-America Transportation Center., 119 pp. Qian, Y. (2009). Experimental study on triangular aperture geogrid-reinforced bases over weak subgrade under cyclic loading (MS thesis). CEAE Department, The University of Kansas. Rajagopal, K., Krishnaswamy, N. R., & Madhavi Latha, G. (1999). Behaviour of sand confined with single and multiple geocells. Geotextiles and Geomembranes, 17(3), 171–184. Recycled Material Research Center (RMRC). (2008). User guideline for byproducts and secondary use materials in pavement construction. http://www.recycledmaterials. org/tools/uguidelines/rcc4.asp Retrieved 24.05.12 Thakur, J. K., Han, J., & Parsons, R. L. (2013). Creep behavior of geocell-reinforced recycled asphalt pavement (RAP) bases. ASCE Journal of Materials in Civil Engineering, 25(10), 1533–1543. Thakur, J. K., Han, J., Pokharel, S. K., & Parsons, R. L. (2012). Performance of geocellreinforced recycled asphalt pavement (RAP) bases over weak subgrade under cyclic plate loading. Geotextiles and Geomembranes, 35, 14–24. Webster, S. L. (1992). Geogrid reinforced base courses for flexible pavements for light aircraft: test section construction, behavior under traffic, laboratory tests, and design criteria, final report, DOT/FAA/RD-92/25. U.S. Department of Transportation and Federal Aviation Administration, 91 p.
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Contents lists available at ScienceDirect
Sustainable Cities and Society journal homepage: www.elsevier.com/locate/scs
Sustainable roadway construction using recycled aggregates with geosynthetics Jie Han a,∗ , Jitendra K. Thakur b,1 a b
Civil, Environmental, and Architectural Engineering Department, The University of Kansas, 1530W, 15th Street, Lawrence, KS 66045-7609, United States Terracon Consultants, Inc., 1211 W. Florida Avenue, Midland, Texas 79701, United States
a r t i c l e Keywords: Geosynthetic Recycled aggregate Stabilization
i n f o
a b s t r a c t Concrete, asphalt pavements, and ballast are removed during the re-construction of existing roads and have been increasingly recycled as aggregates for the construction of roadways. Due to existence of asphalt, cement, and fines, mechanical properties of recycled aggregates may not be sufficient for load support. They may also have long-term durability problems. Geosynthetics have been used to improve mechanical properties and long-term durability of recycled aggregates. This paper reviews recent research work on the use of geosynthetics to stabilize recycled aggregates in roadway construction and summarizes the main findings on permanent deformation, creep deformation, degradation, stress distribution, and/or crack propagation. © 2014 Published by Elsevier B.V.
1. Introduction According to the Federal Highway Administration (FHWA, 2004), two billion tons of aggregates are quarried annually in the United States and the quantity of quarried aggregates will reach 2.5 billion tons by 2020, which force construction industries to consider new sources of aggregates. Roadways (highways and railways) that have reached the end of their service lives are frequently rehabilitated by removing the existing roadway surfaces and replacing the removed portion with new construction materials. A large amount of recycled aggregates are created every year during the rehabilitation and reconstruction of existing roadways. Currently, great emphasis is placed on sustainable construction and infrastructure with green technologies because the demand for sustainable and environmental-friendly roads is increasing daily. More technologies for sustainable roadway construction are needed. One way to construct sustainable roads is through the use of recycled aggregates. Recycled Asphalt Pavement (RAP), Recycled Concrete Aggregate (RCA), and Recycled Ballast (RB) are the three types of recycled aggregates as shown in Fig. 1 and are discussed in this paper. According to the Recycled Material Resource Center (RMRC, 2008), RAP is a removed and reprocessed pavement material from deteriorated asphalt pavements containing asphalt binder (3–7%) and aggregates (97–93%) by weight. The use of RAP has
∗ Corresponding author. Tel.: +1 7858643714; fax: +1 7858645631. E-mail addresses: [email protected] (J. Han), [email protected] (J.K. Thakur). 1 Tel.: +432 684 9600.
been in practice since 1930s. The U.S. FHWA estimated that 100.1 million tons of asphalt pavement materials are milled off each year during resurfacing and widening of road projects, of which 80.3 million tons are reclaimed and reused for roadbeds, shoulders, and embankments (Missouri Asphalt Pavement Association, 2010). RCA is a removed and reprocessed construction material from demolished concrete structures, such as high-rise buildings, bridges, highways, railways, etc. containing cement and natural aggregates. The natural aggregates contain 60–75% of the total volume of RCA (RMRC, 2008). Ballast is a free-draining granular material composed of medium to coarse gravel-sized aggregates (10–60 mm in diameter) with a small percentage of cobble-sized particles, commonly used as a load-bearing material in railway tracks (Indraratna, Khabbaz, Salim, & Christie, 2006). The good quality of ballast consists of angular particles with rough surface and minimum hairline cracks and should have high specific gravity, shear strength, toughness and hardness, and enough resistance to weathering (Indraratna et al., 2006). Railway ballast degrades and deteriorates progressively under repeated cyclic loading. Degraded ballast is usually replaced by fresh ballast during routine track maintenance. The railway track constructed using recycled ballast (RB) shows excessive settlement and lateral deformation, which affect the performance of railroads. The use of recycled aggregates can reduce the cost of construction materials, reduce the amount of waste to be land-filled, reduce the transportation and energy costs to import virgin aggregates, and conserve natural resources by requiring less virgin aggregates in road construction projects. Several agencies are seriously considering the economic and environmental benefits of using recycled aggregates in roadways and facing challenges to maintain
2210-6707/$ – see front matter © 2014 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.scs.2013.11.011
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Fig. 1. Types of recycled aggregates.
high-quality road infrastructure. The mechanical properties (mainly strength and stiffness) and long term durability (breakage and abrasion) of recycled aggregates may not be sufficient for load support due to the existence of asphalt and cement, or loss of angularity of ballast. In the past, most of the research studies on the improvement of RAP and RCA quality focused on blending them with virgin aggregates or stabilizing them using chemical additives. However, the blending of RAP and RCA with virgin aggregate still consumes natural resources and the chemical stabilization is not always environmental friendly. The use of 100% recycled aggregate with geosynthetic is a sustainable solution. Liu, Scarpas, Blaauwendraad, and Genske (1998) were few early researchers to explore such a possibility. Recent research work done by Indraratna, Salim, and Christie (2002), Indraratna et al. (2006), Han et al. (2011), Thakur, Han, Pokharel, and Parsons (2012), Thakur, Han, and Parsons (2013), and others has further evaluated the behavior and performance of geosynthetic-reinforced recycled aggregates. This paper reviews these studies on the use of geosynthetics to stabilize recycled aggregates (RAP, RCA, and RB) in roadway construction and summarizes the main research findings. 2. Reinforcement mechanism of geosynthetic Geosynthetics manufactured from polymeric materials have been widely used as construction materials to solve many civil engineering problems since 1970s. Geosynthetics are used to improve the performance of unpaved and paved roads for over 40 years (Giroud & Han, 2004). The use of geotextile, geogrid, and geocell with recycled aggregates are discussed in this paper. Geotextile and geogrid are planar geosynthetics whereas geocell is a threedimensional honeycomb type of geosynthetic. Geogrid and geocell improve the performance of aggregate layers by providing lateral confinement whereas geotextile improves the performance of aggregate layers by providing a tensioned membrane effect. Different types of geosynthetics used in roadway construction are shown in Fig. 2. The most efficient and convenient location of geosynthetic in roadway construction is at the interface of subgrade and granular base course (Das & Shin, 1998). Geosynthetic installed at this location provides full or partial separation, lateral confinement of granular base materials, a tensioned membrane or beam effect when a road deforms extensively. The tensioned membrane or beam effect is referred to as the tension developed in the curved geosynthetic-reinforced base to resist the vertical load (Rajagopal, Krishnaswamy, & Madhavi Latha, 1999). The tensioned membrane effect mechanism is shown in Fig. 3a. Nonwoven geotextile provides separation, filtration, and drainage whereas woven geotextile provides separation and reinforcement. Geogrid and geocell provide reinforcement to aggregate base and subgrade by providing lateral confinement due to their tensile strength and stiffness. Uniaxial, biaxial, and triaxial geogrids are three types of
geogrids available in the market. Uniaxial geogrid provides tensile resistance in only one direction, biaxial geogrid provides tensile resistance in two directions, and triaxial geogrid can provide nearuniform tensile resistance (Dong, Han, & Bai, 2010; Qian, 2009) when it is subjected to tension in different directions. Qian (2009) reported that the confinement of granular base aggregates was obtained through the interlocking between geogrid apertures and aggregate particles as shown in Fig. 3b. Webster (1992) reported that the degree of interlocking depended on geogrid aperture size and aggregate particle size and the effectiveness of interlocking depended on the in-plane stiffness, rib strength, and junction strength of the geogrid. Thakur et al. (2012) reported that the geocell-reinforced bases had improved bending resistance. The beam effect of geocell-reinforced bases is demonstrated in Fig. 3c.
3. Geosynthetic-reinforced recycled asphalt pavement (RAP) Geotextile, geogrid, and geocell have been used to stabilize RAP bases. This section discusses the effects of geosynthetic reinforcement on the permanent deformation, resilient deformation, creep deformation, and stress distribution of RAP bases. Foye (2011) presented the work of a design-build contractor who used a geosynthetic stabilization technique for reconstruction of 19,500 m2 asphalt parking lot on a site with very weak subgrade (CBR ranging from 1 to 3%). The remedial design parking lot section consisted of very weak subgrade soil overlaid by 200 mm thick geocomposite (a 271 g/m2 needle-punched nonwoven geotextile – geogrid)-stabilized blended RAP aggregate base, 64 mm thick dense-graded asphalt course, and 25 mm thick asphalt wearing course. The geocomposite was placed at the interface of subgrade and granular base course to provide separation and reinforcement. It was found that the geocomposite-stabilized parking lot section showed little rutting or deflection under proof rolling and the use of the geocomposite reduced the cost of construction from about $890,000 (estimated for the original cut and replacement specification) to about $200,000. In addition, the geocomposite stabilization technique saved time, resources, and energy as compared with the traditional cut and replacement technique. Han et al. (2011) conducted moving wheel tests on five geocellreinforced and two unreinforced RAP bases over weak subgrade (target CBR = 3%) to evaluate the effect of geocell reinforcement on rut depth and stress distribution angle at a certain number of passes of the wheel load. Two types of recycled asphalt materials, named RAP and FRAP (fractioned RAP or RAP with finer gradation) were used in this study. The following base sections were prepared and tested: (1) 300 mm thick unreinforced RAP. (2) 150 mm thick geocell-reinforoced RAP with a 20 mm thick RAP cover.
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Fig. 2. Types of geosynthetics. (a) Non-woven geotextile and geocell. (b) Triaxial geogrid. (c) Biaxial geogrid. (d) Uniaxial geogrid. (e) Woven geotextile.
(3) 100 mm thick geocell-reinforced RAP with a 70 mm thick RAP cover. (4) double layered geocell-reinforced RAP with a 30 mm thick RAP cover above a 100 mm thick bottom geocell layer and a 70 mm thick RAP cover above a 100 mm thick top geocell layer. (5) 250 mm thick unreinforced FRAP. (6) 100 mm thick geocell-reinforced FRAP over a 100 mm thick unreinforced FRAP base course with a 50 mm thick FRAP cover. (7) 75 mm thick geocell-reinforced FRAP over a 100 mm thick unreinforced FRAP base course with a 75 mm thick FRAP cover. They found that the geocell improved the life of unpaved sections by a factor of 1.3 using one layer of 75 mm high geocell and 1.8 using one layer of 100 mm high geocell at a rut depth of 75 mm. They concluded that the geocell reinforcement reduced the rut depth and vertical stresses transferred to the subgrade by distributing the load over a wider area at the same number of passes. For a demonstration purpose, the vertical stresses at the interface of subgrade and FRAP base versus the number of passes are shown in Fig. 4. The measured vertical stresses at the interface of subgrade and base were much lower than the tire pressure of 552 kPa applied
on the road surface for each section. The vertical stress increased or remained constant with the number of passes for unreinforced sections but decreased with the number of passes for reinforced sections. They attributed this phenomenon to the beam or slab effect of the geocell-reinforced bases. Thakur et al. (2012) conducted large-scale cyclic plate loading tests on one unreinforced RAP base (300 mm thick) and three geocell-reinforced RAP bases (150, 230, and 300 mm thick) over weak subgrade (target CBR = 2%) to evaluate the performance of unreinforced and geocell-reinforced RAP bases over weak subgrade. The permanent deformation, the resilient deformation, the vertical stress at the interface of subgrade and base, and the strains in the geocell wall were measured during the cyclic plate loading tests. Fig. 5 shows the permanent deformations of the unreinforced and reinforced bases over weak subgrade at the center of the loading plate versus the number of loading cycles. The permanent deformation increased with the number of loading cycles. The rate of increase in the permanent deformation decreased with the number of loading cycles. On the weak subgrade, the geocellreinforced RAP bases (150, 230, and 300 mm thick) improved the performance (i.e., the number of cycles) by a factor of 6.4, 3.6, and 19.4 as compared with the 300 mm thick unreinforced RAP base,
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Fig. 3. Different reinforcement mechanism of geosynthetics. (a) Tensioned membrane effect mechanism (from Maxwell, Kim, Edil, & Benson, 2005). (b) Aggregate interlocking between geogrid aperture and aggregate particle (from Tensar International). (c) Beam effect of geocell.
respectively, at 75 mm permanent deformation. The 230 mm thick geocell-reinforced base had a lower improvement factor than the 150 mm thick geocell-reinforced base because of the lower CBR values of the base and subgrade in the 230 mm thick base as compared with those in the 150 mm thick base. They concluded that geocell-reinforced RAP bases provided a sustainable solution for roadway construction technology by improving the performance of RAP bases. In addition to reducing the permanent deformation of bases, geocell reinforcement reduced the vertical stresses transferred to the subgrade and increased the percentage of resilient deformation of RAP bases.
Thakur et al. (2013) conducted plate loading tests to investigate the vertical stress-displacement responses of the following RAP specimens:
(1) unreinforced sample (unreinforced RAP sample extruded from a Proctor compaction mold) (2) unreinforced base (a RAP base prepared in a test box without geocell) (3) single geocell-reinforced base (a RAP base prepared by placing RAP into the single geocell pocket and the test box)
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Number of passes Fig. 4. Vertical stress at the interface of base and subgrade versus number of passes (data from Han et al., 2011, ©ASCE).
(4) multi geocell-reinforced base (a RAP base prepared by placing RAP into the multiple geocell pockets and the test box). The applied vertical stress versus displacement curves are shown in Fig. 6. They found that the unreinforced RAP sample failed at 172 kPa while other sections did not fail up to a vertical stress of 586 kPa and showed a linear vertical stress-displacement response. The stress-displacement responses were analyzed in terms of a modulus improvement factor. The modulus improvement factor is the ratio of the slope of the initial portion of the vertical stress versus displacement curve for the geocell-reinforced base to that of the unreinforced base. The test results showed that the moduli of the single geocell-reinforced and the multi geocell-reinforced bases were increased by 1.2 and 1.6 times compared to the unreinforced base, respectively. Thakur et al. (2013) also conducted static plate loading tests in a test box and a compaction mold at a room temperature of about 25 ◦ C to investigate the effects of confinement, stress, and cover material on creep deformations of unreinforced and geocell-reinforced RAP bases. The plate loading tests were conducted at vertical stresses of 276 and 552 kPa under five confining conditions including single and multiple geocell-reinforced RAP bases. They concluded that geocell confinement reduced the initial deformation and the rate of creep of the RAP bases, RAP bases crept more at a higher vertical stress and at a lower degree of confinement, and the well-graded aggregate cover significantly reduced the creep of geocell-reinforced RAP bases as compared with the RAP cover. Fig. 7 shows the creep deformations of the geocell-reinforced RAP bases at the vertical stress of 276 kPa as compared with the unreinforced RAP bases. It is shown that geocell
Fig. 6. Vertical stress-displacement curves for unreinforced and geocell-reinforced RAP bases (data from Thakur et al., 2013, ©ASCE).
confinement significantly reduced the creep deformation of the RAP bases and multiple geocell-reinforced base crept least followed by the single geocell-reinforced base and the unreinforced base. Han, Acharya, Thankur, and Parsons (2012) conducted largescale cyclic plate loading tests on asphalt pavements with geocell-reinforced RAP bases over moderate subgrade (target CBR = 5%) and concluded that geocell reduced the permanent deformations of HMA layer, RAP base, and subgrade, reduced the vertical stress at the interface of base and subgrade, and increased the elasticity of the RAP base. Bortz, Hossain, Halami, and Gisi (2012) conducted moving wheel tests on eight asphalt pavement sections, in which two unreinforced sections had well-graded crushed limestone aggregate (AB-3) bases and six geocell-reinforced sections had AB-3, quarry waste (QW), and RAP bases. The details of these bases are provided below: (1) (2) (3) (4) (5) (6) (7) (8)
300 mm thick unreinforced AB-3 75 mm thick geocell-reinforced QW with 25 mm thick cover 75 mm thick geocell-reinforced RAP with 25 mm thick cover 75 mm thick geocell-reinforced AB-3 with 25 mm thick cover 200 mm thick unreinforced AB-3 150 mm thick geocell-reinforced QW with 50 mm thick cover 150 mm thick geocell-reinforced RAP with 50 mm thick cover 150 mm thick geocell-reinforced AB-3 with 50 mm thick cover.
All the cover materials were the same as the infill materials. The subgrade was AASHTO A-7-6 clay and was prepared to obtain target CBR values of 6% and 12%. The bases (1)–(4) were prepared over the subgrade with a CBR of 6% and were paved with a 50 mm thick
Unreinforced base
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60 50
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25
Multi geocell-confined base
20 15 10 5
10 0 0
20
40
60
80
100
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Number of loading cycles Fig. 5. Permanent deformations at the center versus the number of loading cycles for RAP bases over weak subgrade (Thakur et al., 2012).
0 1
500
2000
4000
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Time (minute) Fig. 7. Creep behavior at vertical stress of 276 kPa (data from Thakur et al., 2013, ©ASCE).
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4. Gesosynthetic-reinforced recycled concrete aggregate (RCA) Geotextile and geogrid have been used to stabilize RCA bases. This section discusses the effects of geosynthetic reinforcement on the permanent deformation and crack propagation of RCA bases. Liu et al. (1998) performed finite element analysis on asphalt concrete pavements with geogrid reinforced recycled concrete aggregate and natural aggregate bases to evaluate the performance of the geogrid-reinforced RCA base as compared with the recycled masonry aggregate (RMA) base and the crushed natural aggregate (CNA) base. Influence of the material characteristics of the recycled aggregate and the geogrid reinforcement on the development and rate of propagation of reflective cracking in the top layer of the pavement was selected as the criterion for the comparison purpose. They used stress intensity factor (K) distributions for both unreinforced and reinforced pavements to evaluate the performance in terms of the energy available at a crack tip for additional crack propagation. Fig. 9 shows that geogrid reinforcement reduced the K factor for different base materials thus improved the pavement performance. They concluded that geogrid reduced the rate of crack propagation into the top layer of the pavement, improved the load spreading in the base layer, and enhanced the pavement life. The geogrid-reinforced RCA base performed better than the
150
200
Fig. 9. K factor distribution versus crack length (redrawn and modified from Liu et al., 1998).
unreinforced CNA base followed by the unreinforced RCA base, the reinforced RMA, and the unreinforced RMA. Donovan (2011) conducted dynaflect tests on five pavement test sections (150 m long each) constructed at city of Edmonton, Canada to investigate the possibility of using RCA in roadway construction. The pavement structure of each section consisted of cement stabilized subgrade (CSS) overlaid by granular aggregate base (GAB), asphalt concrete base (ACB), and asphalt concrete overlay (ACO). The city had added the second lift of ACO after two years of original construction in 2004. The total thickness of asphalt layer in each section was same (50 mm thick ACB + 60 mm thick first ACO + 50 mm thick second ACO = 160 mm) with a variation in type and thickness of the aggregate base course. Sections 1 and 2 used natural aggregate and crushed natural aggregate, respectively, whereas Sections 3–5 used recycled crushed aggregate as the granular base course. Recycled crushed concrete consisted of 60% RCA, 25% RAP, and 15% other materials such as cement treated granular base aggregate, brick and other recyclable materials. The thicknesses of GAB for Sections 1–4 were 325 mm while that for Section 5 was 150 mm. Each section had 150 mm thick CSS except Section 3 which had 325 mm thick CSS. Geotextile and geogrid were installed on top of subgrade in Sections 3 and 5, respectively. All other remaining sections were unreinforced. The deflection test results are shown in Fig. 10. The recycled crushed aggregate sections performed better than the natural aggregate and crushed natural aggregate sections. The deformation of each section increased with time. The deformations measured in 2005 were less than those in 2004 because of the placement of the second lift of ACO in 2004. 0.05 0.045
Deformation (mm)
HMA layer, whereas the bases (5) to (8) were prepared over the subgrade with a CBR of 12% and were paved with a 100 mm thick HMA layer. All eight sections were tested to evaluate the effects of base, cover and HMA thicknesses and geocell reinforcement on rut depth and stress distribution at the interface of subgrade and base at a certain number of passes of the wheel load. They concluded that a minimum cover thickness of 50 mm over geocell and a minimum HMA thickness of 100 mm over a base were necessary for better performance of pavements. The geocell-reinforced waste materials (RAP and QW) performed as well as the geocell-reinforced AB-3. The pavement sections constructed over the firm subgrade (CBR = 12%) performed better than those over the moderate subgrade (CBR = 6%) as shown in Fig. 8. In this figure, RAP (moderate) and RAP (firm) stand for asphalt pavements with geocell-reinforced RAP bases over moderate and firm subgrades, respectively. The same representation holds for AB-3 and QW. Fig. 8 does not show the rut behavior at 100,000 and 500,000 passes for the pavement sections constructed over the moderate subgrade because these sections reached the failure criterion of 12.5 mm rut depth before 100,000 passes.
100
Crack length (mm)
Number of passes Fig. 8. Rut depth at wheel path versus number of passes of geocell-reinforced sections (redrawn and modified from Bortz et al., 2012).
50
Section 1 Section 3 Section 5
Section 2 Section 4
0.04 0.035 0.03 0.025 0.02 2001 2002 2003 2004 2005 2006 2007 2008 2009 2010 2011
Year Fig. 10. Deformation behavior of pavement sections (redrawn and modified from Donovan, 2011).
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Unreinforced fresh ballast (dry) Unreinforced recycled ballast (dry) Geogrid-reinforced recycled ballast (dry) Geogrid-reinforced recycled ballast (wet) Geogrid-geotextile-reinforced recycled ballast (dry) Geogrid-geotextile-reinforced recycled ballast (wet)
5 10 15 20
0 0
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20000
30000
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50000
25
60000
(a)
Number of cycles
Number of loading cycles
Fig. 11. Permanent deformation versus number of loading cycles (redrawn and modified from Gongora & Palmira, 2012).
5. Gesosynthetic-reinforced recycled ballast (RB) Railroad ballast provides a support for railroad tracks and distributes loads to weak subgrade. Geotextile and geogrid have been used to stabilize fresh and recycled ballasts. This section discusses the effects of geosynthetic reinforcement on the deformation and degradation of RB. Indraratna et al. (2002) conducted large-scale laboratory cyclic load tests in a cubical triaxial chamber, which simulated the field load and boundary conditions on unstabilized, geogrid-stabilized, and geogrid-geotextile composite stabilized-fresh and recycled ballasts. Indraratna and Salim (2003), Indraratna, Shahin, and Salim (2005), and Indraratna et al. (2006) conducted cyclic laboratory triaxial tests in a prismoidal chamber, which simulated field loading conditions on unstabilized and geotextile-stabilized fresh and recycled ballasts. The test sections consisted of 50 mm thick
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0
Settlement (mm)
Section 4 performed best followed by Sections 5, 3, 1 and 2 in long term as shown in Fig. 10. They concluded from this study: (1) the geotextile had a positive effect when the subgrade soil was very weak; (2) the geogrid improved the life of pavement section and reduced the thickness of granular base by 50% as compared with a similar unreinforced base to provide the same performance; (3) the recycled crushed aggregate was successfully used as the granular base course. Gongora and Palmira (2012) conducted laboratory cyclic plate loading tests on unreinforced and biaxial geogrid-reinforced unpaved road test sections prepared inside a steel tank (750 mm in diameter and 530 mm high) to investigate the performance of unreinforced and geogrid-reinforced unpaved roads on weak subgrade (CBR ≈ 4%). The cyclic load was applied with a peak force of 17.6 kN at a wave frequency of 1 Hz. Each test section consisted of 300 mm thick subgrade overlaid by a 230 mm thick granular aggregate base. Fresh aggregate (FA) and recycled concrete aggregate (RCA) were used as granular aggregate base materials. RCA was coarser in size than FA. The geogrid was installed at the interface of subgrade and base course in case of the reinforced sections. The deformations at different numbers of cycles were measured using linear variable differential transducers (LVDT). The deformation behavior of unreinforced and geogrid reinforced sections is shown in Fig. 11. The geogrid-reinforced sections had much smaller permanent deformations than the unreinforced sections. The geogrid-reinforced RCA performed best, followed by the reinforced FA, RCA, and FA as shown in Fig. 11. The test results indicated that the geogrid improved the life of reinforced RCA and FA sections by factors of 33.5 and 15 at a permanent deformation of 25 mm as compared with the corresponding unreinforced sections, respectively.
0
5
Unreinforced fresh ballast (dry) Unreinforced recycled ballast (dry) Unreinforced recycled ballast (wet) Geotextile-reinforced recycled ballast (dry) Geotextile-reinforced recycled ballast (wet)
10 15 20 25
(b)
Fig. 12. Ballast settlement versus number of loading cycles. (a) Redrawn and modified from Indraratna et al. (2002). (b) Redrawn and modified from Indraratna and Salim (2003).
subgrade, a 100 mm thick subbase layer (i.e. a capping layer of gravel and sand), 300 mm thick load bearing ballast, and a 150 mm thick crib ballast layer of fresh or recycled ballast. They found that the geosynthetics (woven geotextile, geogrid, and geogridgeotextile composite) at the interface of ballast and the capping layer reduced the amount and rate of vertical and lateral deformations. The wet RB had more deformations than the dry RB. They developed a semi-logarithmic equation (S = A + B logN) to predict the settlement of RB, where A and B are empirical constants depending on the initial compaction, type of ballast, type of reinforcement, magnitude of cyclic loading, and degree of saturation; N is the number of load cycles; and S is the tie (also called sleeper) settlement. Fig. 12 shows the effects of geosynthetic reinforcement and saturation on the deformation behavior of fresh and recycled ballasts in two different studies. Indraratna et al. (2005) reported that the optimum location of the geosynthetic was 200 mm beneath the tie to improve the railway track performance; however, it was easy to place the geosynthetic at the ballast-capping interface. Indraratna et al. (2005) also investigated the effects of geosynthetic reinforcement and saturation on degradation behavior of fresh and recycled ballasts under cyclic loading by sieving each ballast sample before and after the test, and recording the change in percentage retained on each sieve size (Wk ). They calculated a breakage index (Bg ) for each specimen using the method proposed by Marsal (1967). Breakage index (Bg ) is the sum of the positive values of Wk . Higher Bg indicates higher potential to degradation. The degradation behavior of unreinforced and geosynthetic-reinforced ballasts is shown in Fig. 13. The unreinforced RB had 97 and 95% more breakage as compared with the unreinforced FB in dry and wet conditions, respectively. Each specimen had slightly higher degradation in the wet condition than that in the dry condition. Geogrid-reinforced RB, geotextile-reinforced RB, and geocomposite-reinforced RB
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(a)
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(b)
Fig. 14. Deformation behavior of unreinforced and geocomposite-reinforced ballast (redrawn and modified from Indraratna et al., 2010). (a) Vertical. (b) Lateral.
Parsons, Jowkar, and Han (2012) conducted full-scale laboratory cyclic loading tests on one unreinforced and one triaxial geogrid-reinforced recycled ballast railroad sections to investigate the deformation and degradation behavior of unreinforced and geogrid-reinforced RB. A full-scale trapezoidal railroad section consisted of a 600 mm thick recycled ballast layer over a 600 mm thick fat clay subgrade. The top and bottom widths of the railroad cross-section were 2700 mm and 7500 mm, respectively. The subgrade and ballast layers were prepared at 2:1 slope with 2700 mm top width. The ties were embedded in the ballast to a depth of 175 mm. The railroad track was placed over ties. The geogrid was installed 175 mm below the tie in case of the reinforced section. They concluded that geogrid reduced the vertical settlement of
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Maximum settlement (mm)
improved the performance by 41, 48, and 50% as compared with unreinforced RB, respectively. The geosynthetic-reinforced RB performed as good as or even better than the unreinforced FB in terms of breakage consideration. Indraratna, Nimbalkar, Christie, Rujikiatkamojorn, and Jayan (2010) conducted full-scale moving wheel load tests on test track sections. Four test sections (two unreinforced and two geocomposite-reinforced, 15 m each in length) were prepared and tested. The geocomposite consisted of biaxial geogrid and nonwoven geotextile. The track had an overall bed thickness of 450 mm with a 300 mm thick ballast layer and a 150 mm thick capping layer. The particles smaller than 9.5 mm were removed from the recycled ballast. The capping materials consisted of a mixture of sand and gravel. The particle shapes of the fresh ballast (FB), the recycled ballast (RB), and the capping materials were highly angular, semi-angular, and angular to rounded, respectively. The geocomposite-reinforced FB performed best, followed by the geocomposite-reinforced RB, RB, and FB. The RB performed better than the FB because of the difference in gradation of ballast. The RB was moderately graded whereas the FB was relatively uniformly graded. The geocomposite improved the performance of FB significantly as compared with the RB. The geocomposite installed at the interface of ballast and capping layer improved the performance of the reinforced track sections by reducing the vertical and lateral strains in the ballast layer as compared with that of the unreinforced sections as shown in Fig. 14. Fatahi and Khabbaz (2011) conducted finite element modeling using PLAXIS to investigate the effect of shoulder ballast width, ballast type, and geosynthetic on the deformation of a track due to train load. The track section consisted of 3000 mm thick subgrade overlaid by 150 mm thick sub-ballast, 300 mm thick ballast, 15 mm thick tie, and 100 mm thick rail. The ballasts used were fresh ballast (FB), recycled ballast (RB), and blended ballast (BB). The blended ballast consisted of 50% FB and 50% RB (FB and RB, each of 150 mm thick). The widths of shoulders were varied from 0 to 700 mm to investigate the shoulder width effect. The geogrid was installed at the interface of subgrade and sub-ballast for the reinforced RB section whereas for the reinforced BB sections, the geogrid was installed at different locations to investigate the location effect. They concluded that track settlement was reduced by increasing the shoulder ballast width; the reinforced BB section performed best, followed by the reinforced RB, unreinforced FB, and unreinforced BB sections; and the effective location of geogrid was at the interface of subgrade and sub-ballast. Fig. 15 shows the maximum settlements of the track in different sections. The test results for unreinforced FB and unreinforced RB are not shown in Fig. 15 as the analysis was not conducted for these sections.
500000
10
Wet
Fig. 13. Breakage indices of unreinforced and geosynthetic-reinforced ballast (redrawn and modified from Indraratna et al., 2005).
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Unreinforced FB Unreinforced RB Geocomposite reinforced FB Geocomposite reinforced RB
4
18
0
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Unreinforced Geogrid at Subgrade-sub ballast interface Geogrid at Sub ballast-ballast interface Geogrid at fresh ballast-recycled ballast interface
56 54 52 50 48 46
Fresh ballast
Recycled ballast
Blended ballast
Fig. 15. Settlement behavior of unreinforced and geogrid reinforced ballasts (redrawn and modified from Fatahi & Khabbaz, 2011).
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the reinforced recycled ballast between the ties and geogrid by 37–65% as compared with that of the unreinforced test section and increased the resistance against breaking under cyclic loading. 6. Conclusions This paper reviews the research work done in the past on the use of geosynthetics to stabilize recycled aggregates including recycled asphalt pavement (RAP), recycled concrete aggregate (RCA), and recycled ballast (RB). The following conclusions can be made: (1) 100% RAP and 100% RCA have been used with geosynthetic reinforcement as base course materials for sustainable roadway construction. (2) 100% RB has been used with geosynthetic reinforcement (woven geotextile, geogrid, and geogrid and non-woven geotextile composite) as a load bearing layer in railway track construction. (3) Geocell improved the performance of RAP bases by reducing the permanent and creep deformations and vertical stresses transferred to the subgrade, increasing the percentage of resilient deformation and the modulus of the RAP bases. (4) Geogrid improved the performance of RCA bases in flexible pavements by reducing the rate of crack propagation into the top layer of the pavement and spreading the load in the base layer in a wider area. (5) Geogrid improved the life of reinforced RCA and FA base sections by factors of 33.5 and 15 at a permanent deformation of 25 mm as compared with the corresponding unreinforced sections, respectively. (6) Geosynthetics (geotextile, geogrid, and geotextile–geogrid composite) improved the degradation resistance of the reinforced RB by 41 to 50% as compared with the unreinforced RB. The geosynthetic-reinforced RB performed as good as or even better than the unreinforced FB in terms of breakage consideration. (7) The performance of ballast depended on its gradation. The geocomposite (geotextile–geogrid) reduced the vertical and lateral deformations of the uniform graded ballast more significantly as compared with the well-graded ballast. (8) Geosynthetics (woven geotextile, geogrid, and nonwoven geotextile–geogrid composite) reduced the vertical and lateral deformations and enhanced the long-term durability of recycled ballast. References Bortz, B. S., Hossain, M., Halami, I., & Gisi, A. (2012). Low-volume paved road improvement with geocell reinforcement. In Transportation Research Board, annual meeting (online publication). Das, B. M., & Shin, E. C. (1998). Strip foundation on geogrid-reinforced clay: Behavior under cyclic loading. Geotextiles and Geomembranes, 13(10), 657–666. Dong, Y. L., Han, J., & Bai, X. H. (2010, 20–24 February). A numerical study on stress–strain responses of biaxial geogrids under tension at different directions. In D. O. Fratta, A. J. Puppala, & B. Muhunthan (Eds.), Advances in analysis, modeling and design, Geotechnical special publication no. 199 (pp. 2551–2560). West Palm Beach, FL: GeoFlorida.
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Donovan, H. (2011). Recycled aggregate and geosynthetic study – City of Edmonton. In Annual conference of the transportation association of Canada Edmonton, Alberta, 19p. Fatahi, B., & Khabbaz, H. (2011). Enhancement of ballasted rail track performance using geosynthetics. In GeoHunan international conference (pp. 222–230). Federal Highway Administration (FHWA). (2004). Transportation applications of recycled concrete aggregates. FHWA State of the Practice National Review. Foye, K. C. (2011). Use of reclaimed asphalt pavement in conjunction with ground improvement: A case history. Advances in Civil Engineering, 2011. Hindawi Publishing Corporation, 7p. Giroud, J. P., & Han, J. (2004). Design method for geogrid-reinforced unpaved roads. I. development of design method. Journal of Geotechnical and Geoenvironmental Engineering, 130(8), 775–786. Gongora, I. A. G., & Palmira, E. M. (2012). Influence of fill and geogrid characteristics on the performance of unpaved roads on weak subgrades. Geosynthetics International, 19(2), 191–199. Han, J., Acharya, B., Thankur, J. K., & Parsons, R. L. (2012). Onsite use of recycled asphalt pavement materials with geocells to reconstruct pavements damaged by heavy trucks. Report no. 25-1121-0001-462. Mid-America Transportation Center., 121p. Han, J., Pokharel, S. K., Yang, X., Manandhar, C., Leshchinsky, D., Halahmi, I., et al. (2011). Performance of geocell-reinforced RAP bases over weak subgrade under full-scale moving wheel loads. ASCE Journal of Materials in Civil Engineering, 23(11), 1525–1534. Indraratna, B., Khabbaz, H., Salim, W., & Christie, D. (2006). Geotechnical properties of ballast and the role of geosynthetics in rail track stabilization. Journal of Ground Improvement, 10(3), 91–102. Indraratna, B., Nimbalkar, S., Christie, D., Rujikiatkamojorn, C., & Jayan, V. (2010). Field assessment of the performance of a ballasted rail track with and without geosynthetics. Journal of Geotechnical and Geoenvironmental Engineering, 136(7), 907–917. Indraratna, B., & Salim, W. (2003). Deformtion and degradation mechanism of recycled ballast stabilized with geosynthetics. Soils and Foundations, 43(4), 35–46. Indraratna, B., Salim, W., & Christie, D. H. (2002). Improvement of recycled ballast using geosynthetics. Rail International, 33(4), 20–29. Indraratna, B., Shahin, M. A., & Salim, W. (2005). Use of geosynthetics for stabilizing recycled ballast in railway track substructures. In Proceedings of NAGS2005/GRI 19 cooperative conference (pp. 1–15). Liu, X., Scarpas, A., Blaauwendraad, J., & Genske, D. D. (1998). Geogrid reinforcing of recycled aggregate materials for road construction: Finite element investigation. Transportation Research Record, 1611, 78–85. Marsal, R. J. (1967). Large scale testing of rockfill materials. Journal of Soil Mechanics and Foundation Engineering, 93(SM2), 27–43. Maxwell, S., Kim, W., Edil, T. B., & Benson, C. H. (2005). Effectiveness of geosynthetics in stabilizing soft subgrades. Report to the Wisconsin Department of Transportation. Missouri Asphalt Pavement Association. (2010). Recycling of asphalt pavement. http://www.moasphalt.org/facts/environmental/recycling.htm Retrieved 10.12.10 Parsons, R. L., Jowkar, M., & Han, J. (2012). Performance of geogrid reinforced ballast under dynamic loading. Report no. 25-1121-0001-363. Mid-America Transportation Center., 119 pp. Qian, Y. (2009). Experimental study on triangular aperture geogrid-reinforced bases over weak subgrade under cyclic loading (MS thesis). CEAE Department, The University of Kansas. Rajagopal, K., Krishnaswamy, N. R., & Madhavi Latha, G. (1999). Behaviour of sand confined with single and multiple geocells. Geotextiles and Geomembranes, 17(3), 171–184. Recycled Material Research Center (RMRC). (2008). User guideline for byproducts and secondary use materials in pavement construction. http://www.recycledmaterials. org/tools/uguidelines/rcc4.asp Retrieved 24.05.12 Thakur, J. K., Han, J., & Parsons, R. L. (2013). Creep behavior of geocell-reinforced recycled asphalt pavement (RAP) bases. ASCE Journal of Materials in Civil Engineering, 25(10), 1533–1543. Thakur, J. K., Han, J., Pokharel, S. K., & Parsons, R. L. (2012). Performance of geocellreinforced recycled asphalt pavement (RAP) bases over weak subgrade under cyclic plate loading. Geotextiles and Geomembranes, 35, 14–24. Webster, S. L. (1992). Geogrid reinforced base courses for flexible pavements for light aircraft: test section construction, behavior under traffic, laboratory tests, and design criteria, final report, DOT/FAA/RD-92/25. U.S. Department of Transportation and Federal Aviation Administration, 91 p.
Please cite this article in press as: Han, J., & Thakur, J.K. Sustainable roadway construction using recycled aggregates with geosynthetics. Sustainable Cities and Society (2014), http://dx.doi.org/10.1016/j.scs.2013.11.011