[American Society of Civil Engineers Geo-Frontiers Congress 2011 - Dallas, Texas, United States (March 13-16, 2011)] Geo-Frontiers 2011 - EPS Geofoam Design Parameters for Pavement

EPS Geofoam Design Parameters for Pavement Structures

Xiaoodng Huang1, Ph.D., P.E. and Dawit Negussey2, Ph.D., P.E. 1Parsons, 301 Plainfield Road Suite 350, Syracuse NY 13212; PH (315) 552-9698; FAX (315) 451-9570; email: [email protected], formally graduate student at Syracuse University 2251 Link Hall, Syracuse University, Syracuse, NY 13244; PH (315) 443-3304; FAX (315) 443-4936; email: [email protected] ABSTRACT

Most of the current geofoam design methods adopt the working stress concept. To design pavement structures with geofoam as subgrade according to the AASHTO and other pavement design guides, the resilient modulus, California Bearing Ratio (CBR), and modulus of subgrade reaction of geofoam need to be determined. The standard test methods are not suitable for geofoam because they usually prescribe higher stress than can be applied on geofoam. This paper presents modified tests methods designed for geofoam to determine the three parameters. Modified tests showed geofoam has resilient modulus unacceptable for subgrade by normal standards. CBR values interpreted by a modified method yielded higher values than conventional methods but still less than acceptable soils. Modified plate load tests indicated low values of modulus of subgrade reaction for geofoam. The concrete slab-geofoam composite subgrade was evaluated by numerical modeling. By considering the composite action of the concrete slab and geofoam, higher resilient modulus and modulus of subgrade reaction values were obtained. Based on the modified tests and modeling, an approach for selecting appropriate geofoam design parameters is proposed.

INTRODUCTION

Among its many applications in geotechnical engineering, expanded polystyrene (EPS) geofoam has been successfully used in pavement structures to replace problematic soils for over three decades. Due to is super light weight compared with any geotechnical material, geofoam provides a substantial reduction of pavement weight, and thus offers a major solution to reduce the settlement of pavement structures in areas of poor soil conditions. The most commonly used density has been 20 kg/m3, which is named EPS19 (United States) or EPS20 (Europe and Japan). Exemplary applications of geofoam in pavement structures in the United States include the I-15 reconstruction project in Salt Lake City, Utah, Route 23A reconstruction in Greene County, New York, and Kaneohe Interchange construction in Oahu, Hawaii.

Most current geofoam design methods adopt the working stress concept, which limits the pressure applied on geofoam below its elastic limits to prevent

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excessive and plastic deformation (Negussey 2002). While the working stress concept is easy to understand, it does not align with most conventional pavement design methods. A summary of the required parameters for subbase/subgrade material in the major design guides in the United States is shown in Table 1.

Table 1 Design Parameters for Subbase/Subgrade

Pavement Design Guide Flexible Pavement Rigid Pavement AASHTO (1993) Resilient Modulus Modulus of Subgrade Reaction

FAA (1996) CBR Modulus of Subgrade Reaction AI (2005) Resilient Modulus Not available

PCA (1965) Not available Modulus of Subgrade Reaction Mechanistic-Empirical

(NCHRP 2004) Resilient Modulus Resilient Modulus

To design geofoam using the conventional pavement design methods, there

following three parameters need to be determined, resilient modulus, California bearing ratio (CBR), and modulus of subgrade reaction. In the sections that follow, this paper discusses test methods developed for geofoam, lab test results, and numerical simulation to determine the parameters.

CBR OF GEOFOAM

As specified in ASTM D1883 and AASHTO T193, the CBR test is a penetration test in which a standard piston of 3-inch2 (1935 mm2) is used to penetrate the soil at a standard rate of 0.05 inch (1.27 mm) per minute. The pressure at each 0.1 inch (2.54 mm) penetration up to 0.5 inch (12.7 mm) is recorded and its ratio to the bearing value of a standard crushed rock expressed in percent is termed as CBR. Some of the standard CBR procedures are not applicable to geofoam, namely: compaction, soaking and moisture content determination, seating load, and surcharge that exceeds the working stress of geofoam. Therefore, the following procedures were prepared based on AASHTO T193 for geofoam CBR tests:

1. Trim and weigh a 7-inch (178 mm) cubic geofoam sample to be tested, calculate the actual density.

2. Cut the cubic sample into cylinders using hotwire and a standard surcharge disc as guiding plate.

3. Trim the cylinder to the standard 4.5-inch (115 mm) tall sample for CBR mold.

4. Place the cylinder in the CBR mold and fill the annular space with uniform find sand of 1.3 mm average size. Tap the mold gently to pack the sand.

5. Place the mold on test frame. Perform the standard penetration test at 0.05 inch(1.27 mm) per minute to at least 0.5 inch (12.7 mm) total penetration. Record load and displacement every three seconds.

6. Plot pressure vs. displacement curve for each test and correct for seating error. 7. Calculate the CBR for geofoam in accordance with the standard test

procedure.


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0

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0 2 4 6

Pres

sure

(kPa

)

Displacement (mm)

12.8 kg/m^315.6 kg/m^319.3 kg/m^325.8 kg/m^334.7 kg/m^32.54mm5.08mm

Figure 1 Geofoam CBR Tests

Figure 1 presents the CBR test results of five geofoam densities performed in

the Geofoam Research Center (GRC) at Syracuse University. The CBR values for each density are summarized in Figure 2. It can be seen that the CBR values for all densities tested under AASHTO T193 are no more than 5.2, and for the most commonly used EPS20 the CBR is only 2.3. FAA pavement design method (FAA 1996) requires subgrade CBR to be greater than 3 or improvement is needed to increase the CBR. The low geofoam CBR values from tests render it not suitable for subgrade by the FAA design method. Observation during the CBR tests showed punching failure occurred under the piston at 0.1 inch (2.54 mm) penetration. A close examination of standard CBR tests on soil samples reveals that the 0.1 and 0.2 inch (2.54 and 5.08 mm) penetration depths are still within the linear portion of the pressure vs. displacement curves for most regular soils, whereas they are outside the linear portion on geofoam curves as shown in Figure 1.

0123456789

1011121314

10 15 20 25 30 35 40Density (kg/m3)

CB

R

GRC AASHTO T 193UK (Sanders,1996)FWD back calculation (Momoi and Kokusyo,1996)GRC Modified CBR at 2.54 mmGRC Modified CBR at 5.08 mm

Figure 2 Geofoam CBR Values


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By extrapolating the linear portion to 0.1 and 0.2 inch (2.54 and 5.08 mm) penetration depths and selecting the corresponding pressure, as shown in Figure 1, higher CBR values can be obtained, which is referred to as modified CBR in Figure 2. On average the modified CBR is approximately twice greater than the conventional CBR for all densities tested. Specifically, the modified CBR of the 19.3 kg/m3 sample is 4.6, which is acceptable to FAA design method without modification. Figure 2 also shows CBR test results by Sanders (1996) and back-calculated CBR from weight deflectometer (FWD) by Momoi and Kokusyo (1996). The back-calculated CBR agrees with the modified CBR at 0.2 inch (5.08 mm) penetration very well. It should be noted that the 0.1 and 0.2 inch (2.54 and 5.08 mm) of penetration are well beyond the elastic limit for both the conventional and modified CBR of all tested densities. In practice the modified CBR must be used in association with the stress limit that can be applied on geofoam.

RESILENT MODULUS OF GEOFOAM

As indicated in Table 1, resilient modulus of geofoam need to be determined to design flexible pavement with geofoam subgrade by the ASSHTO pavement design guide. Although the elastic modulus of geofoam has been extensively tested and researched, organized database of geofoam resilient modulus does not exist or has not been widely available. Duskov (1997) performed uniaxial cyclic loading tests on EPS20 samples with diameter of 100 mm and height of 200 mm. The dynamic moduli determined from these tests were between 6.9 to 8.3 MPa and much below the range acceptable for competent soil subgrades. Some resilient modulus and CBR for geofoam was reported in NCHRP Report 529 (Stark et al. 2004), which are higher than Duskov (1997) results, but detailed evidence as how the values were obtained was not provided. All these reported modulus values are below the range of competent soil subgrade; however, many highways constructed on geofoam continue to perform well. These highways were not designed assuming EPS19 or EPS20 to be equivalent to poor soil subgrade conditions. Much higher estimates of resilient modulus for EPS20 were suggested by back calculations from FWD field tests (Momoi and Kokusyo 1996). In the Netherlands, geofoam subgrades are designed according to conventional Dutch Pavement Design Method with special considerations to stress and strain for geofoam (Duskov, 2000).

Huang and Negussey (2007) proposed a modified geofoam resilient modulus test procedure based on AASHTO T 307 and performed tests on geofoam of various densities. The maximum axial stress specified in AASHTO T 307 for subgrade is 10 psi (69 kPa), which was established on findings from the AASHO Road Tests (HRB 1961-1962). According to ASTM D 6817, 10 psi (69 kPa) is greater than the linear limit stress at 1 percent strain for most geofoam grades. In practice, pavements on geofoam subgrades commonly feature mesh reinforced concrete slab for load distribution and protection. A numerical analysis showed the concrete slab is very effective in attenuating stress applied on geofoam subgrade under normal tire pressure (Huang 2006). The modified resilient modulus test procedure controls the maximum axial stress under the linear limit stress, and thus represents the stress condition of geofoam subgrade realistically. Figure 3 shows one set of test results for


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several densities. The linear limit stresses from 2-inch (50 mm) cube tests (ASTM D 6817) and 12-inch (600 mm) cube tests (Elragi et. al. 2000) are also presented for comparison. The geofoam resilient modulus generally decreases with the increase of axial stress, and is greater than the elastic modulus from 2-inch (50 mm) cube.

E 50mm-EPS15

E 600mm-EPS15E 50mm-EPS19

E 600mm-EPS19

E 50mm-EPS29

E 600mm-EPS29

0

5

10

15

20

0 10 20 30 40 50 60Axial stress (kPa)

Mr (

MPa

)

EPS15

EPS19

EPS29

Figure 3 Geofoam Resilient Modulus

Various correlations have been developed for soils to estimate resilient

modulus from CBR in the absence of lab testing. These correlations have been proved not suitable for geofoam (Huang 2006). Geofoam is a man-made material with quality control, so its properties are much less unpredictable than soils. Once the resilient modulus values are established for each density, it is not necessary to estimate the resilient modulus from CBR values.

MODULUS OF SUBGRADE REACTION OF GEOFOAM

Standard ASTM and AASHTO plate load tests are performed in the field by applying load increments in stages until plate settlement rates sufficiently diminish or to set load duration periods. The standard plate diameter is 30 inches (762 mm). The modulus of subgrade of reaction is commonly determined as the ratio of 10 psi (69 kPa) applied pressure and corresponding settlement. Similar as the standard resilient modulus test, this criterion is based on prior observation that the stress level developed at subgrades under heavy load tire pressures is approximately 10 psi (69 kPa) during the AASHO test road (HRB 1961-1962). Negussey and Huang (2006) performed a series of plate loading tests on geofoam to determine its modulus of subgrade reaction. Figure 4 shows comparison of stage loading and constant rate of loading test results for 6-inch (152 mm) diameter plate on EPS15 geofoam. The pressure-displacement curves confirm the initial segments of loading curves are relatively independent of the mode of loading (i.e., continuous vs. staged). This is consistent with expectation of the initial response to approach elastic behavior. However, the yield stress or linear limit is less than 10 psi (69 kPa). Similar to the rationale of the modified geofoam CBR test, the modulus of subgrade reaction based


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on 10 psi (69 kPa) pressure and the corresponding deformation would be less than at the same pressure and deformation at the linear extension of the initial linear segment. This latter approach was used to derive modulus of subgrade reaction for geofoam by plate loading tests.

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0 5 10 15 20

Stre

ss (k

Pa)

Displacement (mm)

Sample size: 430 mm (17 inch) cubeDensity: EPS15 (1pcf)Plate diameter:150 mm (6 inch)

10%strain/minstaged loading69 kPa (10psi)

Figure 4 Geofoam Plate Loading Tests

Constant strain rate plate loading tests by 30-inch (762 mm) diameter plate for

four different densities of geofoam are shown in Figure 5. The horizontal line at 69 kPa (10 psi) again represents the standard load level at which modulus of subgrade reaction is determined. Linear stress limits at 1 percent strain, as provided in ASTM D 6817, are also shown on each curve as the solid dot. With the exception of the 29 kg/m3 (2 pcf) density geofoam, the curves for the remaining three geofoam densities become non linear below the 69 kPa (10 psi) threshold. As aforementioned, the modulus of subgrade reaction values were determined by extrapolating the initial linear segments.

50kPa

25kPa

40kPa50kPa

75kPa

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0 2 4 6 8 10 12Displacement (mm)

Stre

ss (k

Pa)

15 kg/m3 (1pcf)19 kg/m3 (1.25pcf)22 kg/m3 (1.5pcf)29 kg/m3 (2pcf)69 kPa (10psi)

Figure 5 30-inch (762 mm) Plate Load Test on Geofoam


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The test results are summarized in Figure 6. The geofoam modulus of subgrade reaction decreases with diameter in a similar trend like previously indicated for soils (Brebner and Wright 1953). The trend shown for soils is an average curve from an early study of plate loadings on silt loam (A4 type) compacted subgrades at the Arlington Experiment Farm (Teller, et al. 1935, 1936, 1943). Much of the variability at a geofoam density and plate diameter was associated with sample source. The solid line curves associated with the geofoam test data are from Equation 2 in the next section for rigid plates on semi-infinite half space and for maximum and minimum moduli. For the soils and geofoam of different densities, the trend lines indicate plate size effects tend to diminish beyond 30-inch (762 mm) diameter. Regardless of the geofoam grade, the modulus of subgrade reaction derived from tests with the standard plate size (30-inch diameter) were less than 100 pci (27.7 MN/m3) and presumably equivalent to inferior subgrade type for airport and highway pavement construction.

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k (M

N/m

3 )

Plate diameter (mm)

k (p

ci)

Plate diameter (in)

EPS15 (1 pcf) test results

EPS29 (2 pcf) test results

A4 Soil-Arlington Experiment Farm

E 50mm

E 600mm

E 600mm

E 50mm

Theoretical-Rigid plate-EPS29 (2 pcf)

Theoretical-Rigid plate-EPS15 (1 pcf)

Figure 6 Modulus of Subgrade Reaction of Geofoam and Soil

NUMERICAL ANALYSIS

As expected, direct testing on geofoam indicate the three design parameters of geofoam, namely CBR, resilient modulus, and modulus of subgrade reaction, are too low to be used alone as subbase or subgrade by any conventional pavement design method, no matter how the test methods are modified and results are interpreted. In practice, a concrete slab or a layer of coarse material is always placed on top of geofoam to attenuate the stress applied on geofoam, and this has been the focal point of the stress limit design method of geofoam. In addition to lab testing, Huang (2006) performed numerical analysis to estimate the resilient modulus and modulus of subgrade reaction for geofoam and concrete slab composite. Figure 7 shows the composite modulus of subgrade reaction using upper and lower bonds of elastic modulus, along with the modulus of subgrade reaction from geofoam tests and the A4 soil at Arlington experiment Farm (Teller, et al. 1935, 1936, 1943) for comparison. It


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can be seen that the concrete slab and EPS20 geofoam composite using the high bond elastic modulus has modulus of subgrade reaction equivalent to the A4 soil.

A4 soil

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10 15 20 25 30

Com

posi

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(MN

/m3 )

Com

posi

te k

(pci

)

Density (kg/m3)

composite k- 10 cm concrete slab

Tested average k from source A (without slab)

Composite k-E from 50mm cube (Anasthas 2001)

Composite k-E from 600mm cube (Elragi 2000)

Figure 7 Equivalent modulus of subgrade reaction of geofoam and concrete

composite Based on a linear elastic model with flexible loading area, AASHTO

suggested the following relationship between modulus and subgrade reaction (k) and resilient modulus (Mr) (AASHTO, 1993):

4.19rM

k = (1)

in which k is in pci and Mr is in psi. Composite concrete slab-geofoam

resilent modulus calculated using numerical analysis results and Equation 1 is referred to as estimated resilent modulus herein.

Theoretically the displacement of rigid circular plate on surface of semi-infinite elastic body can be expressed as:

EqRy

2)1( 2μπ −

= (2)

For flexible plate:

centerat )1(2 2

max EqRy μ−

= (3)

Where: y= displacement of loaded area under rigid plate; ymax= maximum displacement of loaded area under flexible plate; q= average stress applied through plate; R= plate radius; μ = Poisson’s ratio;


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E= elastic modulus of semi-infinite elastic body. Composite resilient modulus calculated using numerical analysis results and Equation 2 is referred to as back-calculated concrete slab-geofoam composite resilient modulus herein.

Figure 8 presents the estimated composite resilient modulus (using Equation 1) and back-calculated resilient modulus (using Equation 2). For EPS20 the composite resilient modulus ranges from 25 to 49 MPa (3626 to 7107 psi), which is competent for conventional design methods.

Composite Modulus- 0.1m concrete slab

0

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10 15 20 25 30Density (kg/m3)

Estim

ated

com

posi

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r (M

Pa)

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Bac

kcal

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(MPa

)

E from 50mm cube (Anasthas 2001)E from 600mm cube (Elragi 2000)Backcalculation-E from 50mm cube (Anasthas 2001)Backcalculation-E from 600mm cube (Elragi 2000)

Figure 8 Estimated and Back-calculated Composite Moduli

SUMMARY

The presence of thin concrete slab can spread the load to a much greater area and reduce the stress on geofoam below its elastic limit, without sacrificing the geofoam’s advantage of light weight. Treating the concrete slab and geofoam as one composite subgrade makes it suitable for design by conventional pavement design methods. Numerical analyses indicate the composite resilient modulus and modulus of subgrade reaction are comparable to competent soils. Composite geofoam subgrade with various concrete slab thickness can be developed in the same manner as shown in Figure 8. However, only very limited field data is available to validate the numerical analysis results. FWD tests can test and back-calculate the composite modulus to fill the data gap between numerical analysis and design. With sufficient amount of field data, the numerical model can be calibrated and a database can be established for selecting proper design parameters for various geofoam density and slab thickness. Other variables, such as the geofoam thickness and effect of underlying material, can also be accounted for as part of the FWD test process. As the pavement design methods advancing to the mechanistic-empirical approach, resilient modulus will be the single input parameter for subgrade, and this will further rationalize the modeling and field testing effort.


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REFERENCES AI, (2005). SW-1 Asphalt Pavement Thickness Design Software for Highways,

Airports, Heavy Wheel Loads and Other Applications User’s Guide. Asphalt Institute, Lexington, KY.

Anasthas, N. (2001). Young’s Modulus by Bending Test and Other Proteries of EPS Geofoam Related to Geotechnical Applications, Master’s Thesis, Syracuse University, Syracuse, New York.

ASTM D 6817-07 (2007). “Standard Specification for Rigid Cellular Polystyrene Geofoam”, Annual Book of ASTM Standards, Philadelphia, PA.

Brebner, A, and Wright. W. (1953). “An Experimental Investigation to Determine the Variation in the Subgrade Modulus of a Sand Loaded by Plates of Different Breadths”, Geotechnique, Vol. 3.

Duskov, M. (1997). EPS as a Light-Weight Sub-base Material in Pavement Structures. Ph. D. Thesis, Delft University of Technology, Delft, the Netherlands, 1997.

Duskov, M. (2000). “Dutch Design Manual for Lightweight Pavements with Expanded Polystyrene.” TRR, No.1736:103-109.

Elragi, A.F., Negussey, D., and Kyanka, G. (2000). “Sample Size Effects on the Behavior of EPS Geofoam.” Soft Ground Technology Conference, ASCE Geotechnical Special Publication 112, the Netherlands.

FAA, (1995). “Airport Pavement Design and Evaluation.” AC150/5320-6D, FAA. HRB, (1961-1962). “The AASHO Road Test.” HRB Special Report 61 (7 reports).

National Research Council (U.S.). HRB; AASHO. Washington, D.C. Huang, X. (2005). Evaluation of EPS Geofoam as Subgrade Material in Pavement

Structures. Ph.D. thesis, Syracuse University, Syracuse, New York. Huang, X., and Negussey, D. (2007). “Resilient Modulus for EPS Geofoam.” Proc.

Geo-Denver 2007 International Conference (ASCE Geotechnical Special Publication No. 169), Denver, Co.

Momoi, T., and Kokusyo. T. (1996). “Evaluation of Bearing Properties of EPS Subgrade.” Proc. Intl. symposium on EPS construction method.

Negussey, D. (2002). “Design parameters for EPS geofoam.” Keynote Lecture – IW-LGM 2002 International Workshop on Lightweight Geo-Materials, March 26-27, Tokyo, Japan.

Negussey, D., and Huang, X. (2006). “Modulus of Subgrade Reaction for EPS Geofoam.” Proc.Geo Shanghai 2006 Intl. Conference (ASCE Geotechnical Special Publication No. 169:165-172), Shanghai, China.

NCHRP, (2004). Guide for Mechanistic-empirical Design of New and Rehabilitated Pavement Structures Final Report. Transportation Research Board, National Research Council, March 2004.

PCA, (1966). Thickness Design for Concrete Pavements. Potland Cement Association.

Teller, L.W, et al, (1935, 1936, 1943). “The Structural Design of Concrete Pavements”, Public Roads, Vol. 16, No. 8,9,10, Vol.23, No.8.


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Sanders, R.L, and Seedhouse, R.L. (1994). “The Use of Polystyrene for Embankment Construction.” Transport and Road Research Laboratory Contractor Report 356, HMSO, United Kingdom.

Stark, T., Horvath, J.S., and Leshchinsky, D. (2004). “Guideline and Recommended Standard for Geofoam Applications in Highway Embankments.” NCHRP Report 529, TRB, National Research Council, Washington, D.C.


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[American Society of Civil Engineers Geo-Frontiers Congress 2011 - Dallas, Texas, United States (March 13-16, 2011)] Geo-Frontiers 2011 - EPS Geofoam Design Parameters for Pavement