Elsevier

Geotextiles and Geomembranes

Volume 35, December 2012, Pages 14-24
Geotextiles and Geomembranes

Performance of geocell-reinforced recycled asphalt pavement (RAP) bases over weak subgrade under cyclic plate loading

https://doi.org/10.1016/j.geotexmem.2012.06.004Get rights and content

Abstract

Recycled Asphalt Pavement (RAP) is the most reused and recycled material in the United States. It has been included at percentage of 15–50% in new hot mix asphalt (HMA) concrete and used as a base course material up to 100% for pavement construction. Due to the existence of asphalt in RAP, RAP base courses may have increased or excessive permanent deformation under traffic loading. To minimize such deformation, use of geocell was proposed by authors to confine RAP. To verify the performance of geocell-reinforced RAP bases and the benefit of geocell reinforcement, an experimental study was conducted on geocell-reinforced RAP bases over a weak subgrade under cyclic plate loading. A large geotechnical test box was used for the cyclic plate loading tests. The subgrade was a mixture of sand and kaolin and compacted at the moisture content corresponding to a California Bearing Ratio (CBR) value of 2%. The fractionated RAP was compacted at the moisture content close to the optimum value. A total of four sections with three base thicknesses (0.15, 0.23, and 0.30 m) were prepared and tested, which included one 0.30 m thick unreinforced section and three geocell-reinforced sections. During the testing, surface deformations and vertical stresses at the interface of base and subgrade and strains in geocell walls were monitored. Test results show that the geocell-reinforced RAP bases had much smaller permanent deformations than the unreinforced RAP bases. The geocell-reinforced bases reduced the vertical stresses at the interface between base and subgrade as compared with the unreinforced base. The strain measurements demonstrated that the thicker geocell-reinforced RAP base behaved as a slab while the thinner base behaved as a tensioned membrane. The experimental results indicated that novel polymeric alloy (NPA) geocell reinforcement improved the life of 0.15, 0.23, and 0.30 m thick reinforced RAP base sections by factors of 6.4, 3.6, and 19.4 at a permanent deformation of 75 mm as compared with the 0.30 m thick unreinforced section at the same permanent deformation, respectively. Geocell reinforcement increased the minimum stress distribution angle by 2°, 3.5°, and 7° for the 0.15, 0.23, and 0.30 m thick reinforced RAP base sections as compared with the unreinforced section.

Introduction

Recycled Asphalt Pavement (RAP) is a removed and reprocessed pavement material containing asphalt binder and aggregates. RAP is obtained either by milling or by a full depth recovery method. Literature review shows that RAP has been mostly used with new asphalt binder to form hot-mix asphalt (HMA) concrete as a pavement layer. The percentage of RAP used in the HMA concrete typically ranges from 15% to 50%. RAP has also been used as a granular base material in paved and unpaved roadways, parking areas, bicycle paths, gravel road rehabilitation, shoulders, residential driveways, trench backfill, engineered fill, and culvert backfill (User Guidelines for Byproducts and Secondary Use Materials in Pavement Construction, 2008).

California, Colorado, Montana, and New Jersey DOTs have used RAP as a base course material (McGarrah, 2007). Past research showed that 100% RAP does not produce a product of base course quality as the CBR value and shear strength of RAP decrease with increasing percentage of RAP (McGarrah, 2007). It was reported that the CBR value for 100% RAP was 11%, but when the RAP percentage decreased to 80% after being mixed with virgin aggregate, the CBR value increased to 26% (Taha et al., 1999). However, Locander (2009) found that RAP had similar engineering pavement design properties as unbound aggregate. Canadian Strategic Highway Research Program (2000) concluded that the permanent deformation of RAP bases depends on magnitude, frequency, pressure, and speed of loading; temperature; aggregate gradation, shape and texture; binder type and amount; and construction variables such as compaction, quality control, and segregation. Bennert and Maher (2005) found that permanent deformation of the RAP-aggregate blend increased with the percentage of RAP using cyclic triaxial tests. It was also confirmed that the permanent deformation was a function of the shape of aggregate particles in RAP and depended on the gradation of the aggregates (Bennert and Maher, 2005). The authors believe that instead of blending RAP with virgin aggregate, geosynthetics may be used to reinforce a RAP base course to increase its strength and stiffness.

Geosynthetics have been widely used as construction materials for soil reinforcement in civil engineering projects such as slopes, retaining walls, roads, landfills, foundations, etc. since 1970s. Geosynthetic reinforcement has been one of the established techniques for subgrade improvement and base reinforcement for over 40 years (Giroud and Han, 2004a, b). Today, there are many types of geosynthetic products (e.g., geogrid, geotextile, geocell, geomembrane, etc.) available in the market. Each product is designed to solve a specific type of civil engineering problems. Geocell is a three-dimensional interconnected honeycomb type of geosynthetics used to confine unbound aggregates for base courses in roads since 1970s. Rajagopal et al. (1999) investigated the influence of geocell confinement on the strength and stiffness behavior of granular soil confined in single and multiple geocells and found that the apparent cohesive strength of granular soil increased due to geocell confinement. They also found that the induced apparent cohesive strength depended on the tensile modulus of geocell, however geocell confinement had no effect on frictional strength of granular soil. A comprehensive literature review by Yuu et al. (2008) indicated that theories and design methods for geocell were far behind its applications in the field up to that time, especially for roadway applications because the mechanisms of geocell reinforcement were not well understood and there was not enough research data.

Since then, Boushehrian et al. (2011), Han et al. (2008, 2011), Latha and Murthy (2007), Pokharel et al. (2010, 2011), Moghaddas Tafreshi and Dawson, 2010a, Moghaddas Tafreshi and Dawson, 2010b, 2012), Yang et al. (2012), and Zhang et al. (2009) have made significant efforts to improve the understanding of the mechanisms of geocell confinement and verify the performance of geocell-reinforced granular materials. A preliminary study done by Thakur et al. (2011) indicated the benefit of geocell in minimizing the creep deformation of RAP bases. Boushehrian et al. (2011) investigated the cyclic behavior of reinforced sand by conducting a series of laboratory tests, field tests, and numerical modeling using PLAXIS 3D Tunnel software and reported the benefit of the three-dimensional reinforced system (a grid-anchor reinforcement system) over the conventional geomesh system in reducing the settlements of foundations rested on sand bed. Latha and Murthy (2007) conducted triaxial tests to study the effect of planar, cellular, and discrete fiber reinforcements on strength improvement of geosynthetic-reinforced sand through regular triaxial compression tests. Cellular reinforcement was found to be more effective in improving the strength compared to planar and discrete reinforcements. Moghaddas Tafreshi and Dawson, 2010a, Moghaddas Tafreshi and Dawson, 2010b; 2012) showed the benefits of 3D geosynthetics (geocell or 3D reinforcement system made with geotextile) over planar geosynthetics (geotexile) in improving bearing capacity and reducing settlements of strip footings on sand by conducting a series of small-scale laboratory model tests on 3D geosynthetic-reinforced (geocell or 3D geotextile), geotextile-reinforced, and unreinforced sand. Zhang et al. (2009) developed a theoretical solution for the deformation of a geocell-reinforced soil layer as a beam on Winkler's foundation. Han et al. (2011), Pokharel et al. (2011), and Yang et al. (2012) reported the accelerated pavement testing of geocell-reinforced unpaved roads with different infill materials (sand, quarry waste, well-graded aggregate, and RAP). They demonstrated the benefits of geocell in reducing permanent deformations and increasing stress distribution angles; however, accelerated pavement testing is costly and the facility is not readily available for most research institutes. Large-scale box test results were used by Giroud and Han (2004a, b) to calibrate the design method for geosynthetic-reinforced unpaved roads. A preliminary study done by Thakur et al. (2012) investigated the benefit of geocell reinforcement on two 0.3 m thick RAP base sections (unreinforced and reinforced) over weak subgrade. This paper reports four large-scale box tests to evaluate the performance of geocell-reinforced RAP bases over weak subgrade under cyclic loading.

In this study, geocell was proposed to minimize the permanent deformations of RAP bases and improve their performance under cyclic loading through confinement. Four laboratory cyclic plate load tests were conducted in a large geotechnical test box at the University of Kansas to investigate the benefits of geocell on the reduction in the permanent deformations and the vertical stresses at the interface between base and subgrade as compared with an unreinforced base.

Section snippets

Geocell

The geocell, made of novel polymeric alloy (NPA), was manufactured and provided by PRS Mediterranean, Ltd. in Israel. It has three-dimensional honeycomb-interconnected cells as shown in Fig. 1. The geocell used in this study had two perforations of 100 mm2 area each on each pallet, 1.1-mm wall thickness, 100 and 150 mm cell heights, 19.1-MPa tensile strength, and 355-MPa elastic modulus at 2% strain. The tensile strength and elastic modulus were determined based on the tensile tests of geocell

Vane shear, DCP, and sand cone test results

Vane shear tests were performed just after the preparation of the subgrade for each test to confirm that the target CBR was achieved.

After each cyclic plate load test, two sand cone tests in accordance with ASTM D15556-07 were conducted to evaluate the density of the compacted RAP base. Vane shear, DCP, and sand cone test results for geocell-reinforced and unreinforced test sections are presented in Table 4.

The slightly higher subgrade CBR values were obtained during DCP tests because the DCP

Conclusions

This paper presents an experimental study to evaluate the performance of novel polymeric alloy (NPA) geocell-reinforced recycled asphalt pavement (RAP) bases over weak subgrade under cyclic loading. This study was conducted based on typical conditions in field for the construction of geocell-reinforced unpaved roads over weak subgrade. A nonwoven geotextile was placed between the subgrade and the geocell-reinforced RAP base. The thickness of the RAP cover over the geocell was 50–80 mm and the

Acknowledgments

This research was sponsored by the Mid-America Transportation Research Center. The geocell material used in this research was provided by PRS Mediterranean, Ltd. in Israel. RAP materials were supplied by R.D. Johnson Excavating, Co. Mr. Howard Jim Weaver, the laboratory manager, Mr. Kahle Loveless and Mr. Aj Rahman, undergraduate students, in the Department of Civil, Environmental, and Architectural Engineering (CEAE) at the University of Kansas (KU) provided great assistance during the

Nomenclature

α
the stress distribution angle in degree with respect to the vertical
CBR
California Bearing Ratio (%)
Cu
vane shear strength of subgrade (kPa)
h
the thickness of the base course (m)
P
the applied load (kN)
PI
Penetration Index (mm/blow)
pi
the distributed vertical stress at the center of the interface of base course and subgrade (kPa)
r
the radius of the tire contact area (m)

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