Cost Energy Ash in Place Stabilized

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Road Materials and Pavement Design, 2013

Vol. 14, No. 3, 537–550, http://dx.doi.org/10.1080/14680629.2013.779302

Cost, energy, and greenhouse gas analysis of fly ash stabilised cold in-placerecycled asphalt pavement

Xiaojun Lia, Haifang Wena*, Tuncer B. Edilb, Renjuan Sunc and Timothy M. VanRekena

a Department of Civil & Environmental Engineering, Washington State University, 405 Spokane Street,

Pullman, WA 99164, USA; b Department of Civil & Environmental Engineering, University of Wisconsin- Madison, 1415 Engineering Drive, Madison, WI 53706, USA; cSchool of Civil Engineering, Shandong University, 17922 Jingshi Road, Jinan 250061, People’s Republic of China

The purpose of this study is to evaluate the performance, costs, energy, and greenhouse gasemission of cementitious high-carbon fly ash (CHCFA)-stabilised recycled pavement materials(RPMs) as base course. Three test road cells were built in Minnesota. These cells have the same

pavement structure except different base courses: conventional crushed aggregates, untreated RPM, and CHCFA-stabilised RPM. Results of laboratory and field tests were used in the

Mechanistic-empirical pavement design guide (MEPDG) to predict the pavement performance.MEPDG performance prediction indicated that Cell 79 containing CHCFA-stabilised RPM had the longest service life in the three cells. The life-cycle analysis indicated that the usage of the CHCFA-stabilised RPM as the base of the flexible pavement can significantly reduce thelife-cycle cost, energy consumption, and greenhouse gas emission.

Keywords: performance; cost; energy; greenhouse; stabilisation; base

1. Introduction

Due to the increasingly stringent environmental policy stipulated by the Environmental Protection

Agency and/or local authority, the power-generation industry must take measures to reduce the

emission of nitrogen oxides (NOx), sulphur oxides (SOx), and mercury (Hg). Low-NOx burners

reduce emissions by changing the combustion characteristic of coal boilers, but they increase

the amount of residual unburned carbon in fly ash. Additionally, activated carbon is injected to

reduce Hg emission, which also increases the carbon level in fly ash. Increased carbon levels

in fly ash make air-entrained concrete production more difficult (Ramme & Tharaniyil, 2000).

High-carbon fly ash can reduce the durability and workability of concrete significantly (Bhatty,

Gajda, & Miller, 2003). Even though some measures exist to beneficiate high-carbon fly ash, such

as carbon/ash separation and reburning ash as a fuel for coal boilers, there are disadvantages to

these technologies. The carbon/ash separation process consumes energy and ash reburning needs

a burner capable of producing marketable fly ash. Therefore, direct utilisation of cementitious

high-carbon fly ash (CHCFA) without treatment is still the best possible scenario to consume

high-carbon fly ash.

Simultaneously, quarrying virgin aggregate for highway construction also results in environ-

mental problems and energy consumption. An alternative to quarrying virgin materials is in-place

recycling of asphalt pavement. This recycling process is relatively inexpensive and contributes

towards sustainable pavement rehabilitation (Koch & Ksaibati, 2010). Existing hot-mix asphalt

*Corresponding author. Email: [email protected]



538 X. Li et al.

(HMA) layer is pulverised and blended with some of or the entire base course to form a broadly

graded material referred to as recycled pavement material (RPM) (Li, Benson, Edil, Hatipoglu,

& Tastan, 2008). There is an increasing trend towards recycling existing asphalt pavement and

using as base course for the new pavement (Sullivan, 1996). However, there are concerns on the

load-carrying capacity and deformability of a base layer made of RPM (Koch & Ksaibati, 2010;

Senior, Szoke, & Rogers, 1994). Mechanical or chemical stabilisation of RPM are often needed to improve the engineering properties. There is a potential to use CHCFA as the admixture to

stabilise the RPM in pavement recycling.

It will have great environmental and cost benefits if CHCFA can be used to stabilise RPM in

pavement construction. However, before CHCFA-stabilised RPM base layer can be promoted

in the industry with confidence, a comprehensive knowledge about its material properties and

performance, a quantification of its life-cycle cost and environmental benefits, such as the energy

consumption and greenhouse gas emissions, is imperative. In order to comparatively evaluate the

performance and effectiveness of CHCFA-stabilised RPM base course material in a real highway

construction, three test cells at MnROAD test facility were built, with the same asphalt surface

layer, subbase, and subgrade, but three different base courses: conventional crushed aggregates,untreated RPM, and CHCFA-stabilised RPM materials.

2. Construction

In this study, three flexible pavement test cells with different base materials were constructed at the

MnROAD test facility to evaluate the effectiveness of RPM with and without fly ash addition and

to compare them with traditional crushed aggregate base course which is a Minnesota Department

of Transportation (MnDOT) Class 6 granite aggregate (Clyne & Palek, 2008). The MnROAD test

facility is a two-lane pavement test track located about 64 km (40 miles) northwest of Minneapolis.

A 12.7 cm (5 in.)-thick clay subbase was placed and compacted over existing subgrade soils inCells 77, 78, and 79. The RPM base course, Class 6 aggregate base course, and CHCFA-stabilised

RPM base course were installed in Cell 77, Cell 78, and Cell 79, respectively. For the CHCFA-

stabilised RPM base course, the contractor added 14% of fly ash based on the dry weight of the

RPM. Two lifts of 5.08 cm (2 in.) of bituminous concrete-wearing course were placed about a

month after the placement of subgrade and base course materials in Cells 77, 78, and 79. When the

placement of the wearing course was initiated in Cells 77 and 78, the base material appeared to be

unstable under the weight of the paving machine and other construction traffic. It was discovered

that the subgrade material in Cells 77 and 78 had become very wet possibly due to the excess

precipitation following the placement of the base course materials. The subgrade material in Cell

79 was protected from excessive seepage of rain water, probably due to the presence of the flyash-stabilised RPM. Therefore, the base course material from Cells 77 and 78 was removed.

The subgrade clay material was disked to dry it out and then compacted to the required density.

Following the compaction of subgrade material, the RPM and Class 6 aggregate base courses

were placed again in Cell 77 and Cell 78, respectively. Cell 79 was completed in September 2007

and Cells 77 and 78 were completed in October 2007.

3. Laboratory tests and results

Observing the field performance of the pavement and materials will require many years of service.

Therefore, the performance of pavements was predicted by the Mechanistic-empirical pavement design guide (MEPDG) in which various laboratory testing results were needed. The test samples

were taken in the construction field



Road Materials and Pavement Design 539

3.1. Asphalt concrete material

3.1.1. Binder

The Elvaloy/PPA binder used in this research meets PG 64-34 requirements. The dynamic shear

reometer (DSR) test results of the binder after rolling thin film oven aging are shown in Table 1

and input to the MEPDG programme.

3.1.2. Hot mixed asphalt

Two different sets of loose HMA mixtures, Cells 77, 78, and 79, were sampled at the MnROAD

facility. The sampled HMA was used for the dynamic modulus and the creep compliance tests.

3.1.2.1 Dynamic modulus of HMA. Dynamic modulus is needed in the MEPDG for the pave-

ment analysis and determination of stress/strain which will be used in the performance modelling.

The dynamic modulus was tested at temperatures of −10◦

C, 4◦

C, and 21◦

C. The dynamic modu-lus at temperature of 54.4◦C was predicted based on the time-temperature superposition principle,

as shown in Figure 1. These results were input to the MEPDG programme.

3.1.2.2 Creep compliance of HMA. The creep compliance was tested at −36◦C, −24◦C, and

−12◦C and was shifted to the MEPDG-required temperature of −20◦C, −10◦C, and 0◦C based

on the time-temperature superposition principle, as shown in Figure 2. The creep compliance was

input to the MEPDG to characterise the thermal cracking.

Table 1. DSR test results of the Elvaloy/PPA binder.

Temperature (◦C) G∗/Sin δ (Pa) Phase angle (δ) G∗ (Pa)

64 3900 69.0 3640.96258 7700 66.2 7045.18652 15,000 63.7 13447.29

Figure 1 The measured and predicted dynamic moduli used in MEPDG modelling



540 X. Li et al.

Figure 2. The measured and predicted creep compliance used in MEPDG modelling.

3.2. Base course materials

During construction, the base course materials were sampled and stored for laboratory testing.

The CHCFA-stabilised RPM specimens were fabricated in the laboratory. The moisture-density

relationship of RPM, CHCFA-stabilised RPM, and Class 6 were obtained in accordance with

ASTM D 1557. Resilient modulus ( M r) was determined in accordance with the National Coop-

erative Highway Research Programme 1-28A test protocol (Witczak, 1997). The field density

and moisture contents of base materials on site were measured using a nuclear density gauge.

The field densities and moisture contents were used to fabricate laboratory specimens for

testing.

3.2.1. Fly ash

Fly ash obtained from Unit 8 of the Riverside Power Plant in Minneapolis, MN (operated by Xcel

Energy) was used to stabilise the RPM. This fly ash has a calcium oxide (CaO) content of 22.37%

and a carbon content of 16.35% which is a CHCFA. A fly ash application rate of 14% by weight

of dry mix was used to stabilise RPM as base course.

Figure 3 The gradation of RPM and Class 6 aggregate




Table 2. Parameters in the resilient modulus model.

Base type k 1 k 2 k 3 k 6 k 7

Clay 2380.4971 0.8451 −5.4203 −9.4677 1.0317Class 6 376.4932 1.4858 −0.7325 −67.4420 1.0000

RPM 764.7781 1.2642 −1.0341 −55.4167 1.0000RPM + CHCFA 7D 31,194.3790 0.1545 −0.4353 −8.9163 1.0301RPM + CHCFA 28D 50,966.4110 0.1424 −0.6308 0.0000 1.0000

Figure 4. Resilient modulus test results of field-sampled materials.

3.2.2. RPM and Class 6 aggregate

3.2.2.1 Gradation. The RPM was produced by pulverising the in-situ asphalt pavement at

MnROAD. The RPM consisted of 50% of recycled asphalt pavement and 50% of existing crushed

aggregate base course. The Class 6 aggregate is a granite base course material used by MnDOT.

The gradations of the RPM and Class 6 are shown in Figure 3.

3.2.2.2 Resilient modulus. The resilient modulus model in the MEPDG is shown by Equation

(1) (Witczak, 2004). The parameters for different base materials were resolved by fitting the test

data with the model and are shown in Table 2. The resilient moduli reported in Figure 4 was

predicted by the resolved model at the peak cyclic stress of 103 kPa and confining stress of 45 kPafor aggregate base, and peak cyclic stress of 41 kPa and confining stress of 14 kPa for subgrade

(Witczak, 2004). Figure 4 indicates that the CHCFA-stabilised RPM has a much higher resilient

modulus at 28 days curing age than that at 7 days of curing age, and has much higher resilient

modulus than untreated RPM and Class 6 aggregate.

M r = k 1 ∗ P a

σ b − 3k 6

P a

k 2

∗

ιoct

P a+ k 7

k 3

, (1)

where M r = resilient modulus, psi; k 1, k 2, k 3, k 6, and k 7 = regression constants (obtained by

fitting resilient modulus test data to the equation); σ b = bulk stress = σ 1 + σ 2 + σ 3; σ 1, σ 2, and σ 3 = the major, intermediate, and minor principal stress, respectively; P a = normalising stress

(atmospheric pressure); τ = octahedral shear stress



542 X. Li et al.

Figure 5. The average FWD back-calculated base moduli by ‘MODULUS’.

4. Field tests and results

Falling weight deflectometer (FWD) tests were conducted directly on the base courses during the

construction. Prior to the placement of HMA, the base course had one month of curing. After the

placement of HMA surface, FWD tests were conducted on HMA to back-calculate the modulus

of base materials.

FWD tests were conducted using a 40 kN (9000 lbf) load level which represents the load level

exerted by a dual-wheel load. The modulus back-calculation of the FWD test on the base coursewas based on the equations developed by George, Bajracharya, and Stubstad (2004). For the

modulus back-calculation of the FWD test on HMA surface, the Texas Transportation Institute

(2001) software MODULUS was used.

Figure 5 summarises the averaged back-calculated moduli at different testing dates.

The CHCFA-stabilised RPM had higher moduli than unstabilised RPM, followed by Class 6.

The back-calculated moduli of the base courses from FWD tests were extremely high in the Win-

ter season when the base course materials were frozen (as indicated by the temperature gauges

placed in the base course). The moduli of base courses were lower in Spring than the moduli

in other seasons, indicating the weakening of materials by the Spring thaw. The back-calculated

moduli of base materials from the FWD tests on HMA surface were higher than those tested

directly on the base course during construction. One unexpected result is that the back-calculated

moduli of the CHCFA-stabilised RPM did not show significant increase with the increase of cur-

ing age. This might be due to the micro-shrinkage crack within the CHCFA-stabilised RPM layer

which compensated for the strength increase. However, this assumption needs further study. It

should be noted that although the back-calculated moduli of CHCFA-stabilised RPM are higher

than the other two base materials, the difference is not as significant as the difference between the

laboratory-tested resilient moduli, as shown in Figure 4.

5. MEPDG performance prediction and results No distresses have been found three years after the construction. The observation of the field

performance of pavement and materials may need many years of service Therefore the service




Figure 6. Truck configuration (MnDOT).

Table 3. The traffic configuration (MnDOT).

The traffic configuration Weight ( N )

Total weight 353,634Steering axle 53,379Front axle tractor tandem 75,175 149,015Back axle tractor tandem 73,840

Front axle trailer tandem 69,392 151,240Back axle trailer tandem 81,847

lives for the life-cycle analysis needs to be predicted. The MEPDG programme, version 1.1

(Applied Research Associates, Inc., 2009), was used to predict the service lives because it is

partially based on mechanistic theory.

5.1. MEPDG input

5.1.1. Traffic

The traffic applied on Cells 77, 78, and 79 is an 18-wheel, 5-axle tractor/trailer with the loading

configurations of gross vehicle weight of 355.9 kN (80 kips), as shown in Figure 6 and Table 3

(Minnesota Department of Transportation, 2009). The truck runs on these three cells during a

normal 8-hour working time with an average of 48 laps per day. The real traffic information was

input into the MEPDG software to predict future performance.

5.1.2. Climate

The Cells 77, 78, and 79 are located about 64 km (40 miles) north-east of Minneapolis, MN.The hourly climatic data of the nearest station were exported from the climatic database provided

in the MEPDG programme



544 X. Li et al.

Table 4. Summary of service life analysis.

Time to failure (month)

Distress Cell 77 (RPM) Cell 78 (Class 6) Cell 79 (RPM + CHCFA)

AC surface down cracking 132 90 >300AC bottom-up cracking >300 >300 >300AC thermal fracture 144 144 144Total rutting >300 >300 >300IRI 270 270 282Service life (Year) 11 7.5 23.5Rehabilitation times (within

service life of Cell 79)1.14 2.13 0

5.2. Predicted performance

The performance of Cells 77, 78, and 79 were predicted by MEPDG software with same traf-fic, climate, HMA surface layer, and subgrade properties. Only the properties of the base-layer

materials were different as shown in Figure 4. A reliability of 90% was used in the modelling.

The performance modelling results are summarised in Table 4. It can be seen that the time to

thermal cracking failure is same for Cells 77, 78, and 79, which is 144 months. This is because the

same HMA material was used in the three cells. However, the purpose of this study is to compare

the performance of base materials. Therefore, the modelling results of other distresses such as

top-down cracking, bottom-up cracking, rutting, and IRI which are related to the materials of base

course were used to determine the service lives.

6. Life-cycle cost analysis and results

Life-cycle cost analysis (LCCA) is an effective way to assess the cost of the construction operation

over the service life (Mearig, Coffee, & Morgan, 1999). The RealCost software of version 2.1

which was developed by Federal Highway Administration (2004) was used to perform the cost

analysis.

Based on the MEPDG performance prediction, the service life of Cell 79 with CHCFA-

stabilised RPM is 23.5 years, which is about twice the service life of Cell 77 with RPM base

(11 years, Table 4), and about three times the service life of Cell 78 with Class 6 aggregate base

(7.5 years, Table 4). The typical rehabilitation of surface milling and HMA overlay was assumed

at the end of each service life. It is assumed that the overlay life is essentially same as that of the original asphalt pavement life (Wisconsin Department of Transportation, 2011). Therefore,

for Cell 77, the rehabilitation would be carried out at 11 and 22 years. At the end of 23.5 years,

the pavement would be still there and had a remaining service life of 9.5 years. For Cell 78,

the rehabilitation would be carried out at 7.5, 15, and 22.5 years. At the end of 23.5 years, the

pavement would be still there and had a remaining service life of 6.5 years. The salvage value

was considered in the LCCA. No maintenance cost was considered in this study. A 3% inflation

rate was assumed in the analysis.

For the convenience of comparison, the procedure of the rehabilitationwas assumed to consist of

milling of the entire roadway surface, the application of tack coat to the milled surface, placement

of HMA overlay, and the compaction of the HMA overlay. The depth of the milling and thethickness of the HMA overlay were assumed to be 7.62 cm (3 in.) (Washington State Department

of Transportation 2005) for both Cell 77 with untreated RPM base course and Cell 78 with Class




$0

$10,000

$20,000

$30,000

$40,000

$50,000

$60,000

$70,000

$80,000

$90,000

Cell 77 (RPM) Cell 78 (Class 6) Cell 79(RPM+FA)

N P V

o f L i f e C y

c l e C o s t s ( $ )

Base Materials of Cells

Life Cycle Cost ($)

HMA Overlay

Milling

HMA Surface

Re-work Due toWeather

OriginalConstruction

$0

$20,000

$40,000

$60,000

$80,000

$1,00,000

N P V

C o s t ( $ )

Items

Life Cycle Cost ($)

Cell 77 (RPM)

Cell 78 (Class 6)

Cell 79

(RPM+CHCFA)

Figure 7. Comparison of life-cycle costs.

6 aggregate base course. There will be no rehabilitation within the 23.5 years of life-cycle analysis

period for Cell 79 with the CHCFA-stabilised RPM base. The cost analysis results were reported

as net present value.

The comparison of the life-cycle costs are shown in Figure 7 which indicates that, from the

life-cycle point of view, Cell 79 with the CHCFA-stabilised RPM base has the lowest cost and

Cell 78 with the Class 6 aggregate base has the highest cost. The ratio of the life-cycle cost of

Cells 77, 78, and 79 is 1.57:2.34:1. Even when the cost of the reconstruction of the base due to

the rain was not considered, the ranking of Cells 77, 78, and 79 remains the same.

7. Life-cycle energy and greenhouse gas analysis and results

The method of determining the life-cycle energy consumption and greenhouse gas emission is

relatively straightforward. First, the construction material quantities were determined for original

construction and projected rehabilitation. Second, the energy consumption and the greenhouse

gas emission per unit construction material was determined as reported by others (Halstead, 1981;

Meil, 2006). Finally, the energy consumption and the greenhouse gas emission were obtained by

multiplying the unit value by the quantities of construction materials.The assumption of the rehabilitation procedure was the same as that in the LCCA. The same

procedure as calculating the salvage value in the LCCA was used in the energy and greenhouse gas



546 X. Li et al.

Table 5. Energy consumption and greenhouse gas emission of Cells 77, 78, and 79.

Total CO2 TotalItems Energy (MJ) energy (MJ) (Mg) = GWP CO2 (kg)

RPM (Cell 77) 1,273,149

(with ForceAccount)1,195,537(withoutForceAccount)

27,144 (with

ForceAccount)27,139(withoutForceAccount)

Initial constructionBase

Materials production 24,425 2.84

Materials transportation 0 0.00

Processes (equipment) 3313 0.25

Force Account (due to rain) 77,612 5.00

HMA surface 623,547 14,149.87

Rehabilitation

Milling (twice-salvage) 12,821 1059.10

HMA overlay (twice-salvage) 531,432 11,926.71

Class 6 Agg. (Cell 78) 1,823,287(with ForceAccount)1,745,675(withoutForceAccount)

38,542 (withForceAccount)38,537(withoutForceAccount)

Initial construction

Base


Materials transportation 13,147 0.98


Force Account (due to rain) 77,612 5.00

HMA surface 623,547 14,149.87

Rehabilitation

Milling (thrice-salvage) 24,069 1988.28

HMA overlay (thrice-salvage) 997,674 22,390.41

RPM + FA (Cell 79) 661,728 (withand withoutForceAccount)

14,154 (with and without ForceAccount)

Initial construction

Base


Materials transportation 9141 0.68


Force Account (due to rain) 0 0.00

HMA surface 623,547 14,149.87

Rehabilitation

No 0 0.00

analysis. PaLate programme was used to determine life cycle energy consumption and greenhouse

gas emission (Horvath, Pacca, Masanet, & Canapa, 2003).

7.1. Energy analysis

The energy involved in highway development consists of construction energy, transport energy,

processing energy, and calorific energy (Halstead, 1981). The calculation results of the life-cycle

energy consumption are shown in Table 5. As indicated in the introduction of construction,

for Cells 77 and 78, the subgrade and base course were reconstructed due to the rain water.The energy consumed during the reconstruction due to precipitation is indicated as ‘Force

Account’ in Table 5 The ratio of the life cycle energy consumption between Cells 77 78 and




Figure 8. Comparison of life-cycle energy consumption (considering reconstruction due to rain).

79 are 1.92:2.76:1 and 1.81:2.64:1, respectively, with and without considering the reconstruction

due to rain. Figure 8 shows the comparison of the life-cycle energy consumption of Cells 77,

78, and 79, considering the reconstruction due to rain. Cell 79, with the CHCFA-stabilised RPM

base, has lower life-cycle energy consumption than Cell 77 with RPM base, followed by Cell 78

with Class 6 base. This conclusion would remain the same if the reconstruction due to rain was

not considered.

7.2. Greenhouse gas emissions analysis

The greenhouse gasses CO2, CH4, N2O, etc. were converted to a measure of direct global warm-ing potential (GWP) using the well-accepted CO2 equivalence method as developed by the

International Panel on Climate Change The calculation results are shown in Table 5 Because



548 X. Li et al.

Figure 9. Comparison of life-cycle greenhouse gas emission (considering reconstruction due to rain).

the CO2 emission during the subgrade and base reconstruction is very small, the ratio of the life-cycle CO2 emission between Cells 77, 78, and 79 is the same as 1.92:2.72:1, with and without

considering the reconstruction. The life-cycle CO2 emission is plotted on a log scale in Figure 9.

It can be seen that Cell 79 has the lowest CO2 emission, followed by Cells 77 and 78.

8. Conclusions and recommendations

This paper studied the use of CHCFA-stabilised RPM as a base material and compared it to

unbound RPM and crushed aggregates, in terms of pavement lives and life-cycle analysis of costs,

energy consumption, and greenhouse gas emission. The conclusions can be drawn as follows:

(1) The back-calculated moduli of the base courses from FWD tests were extremely higher

during the Winter seasons when the base course materials were frozen The moduli of base




courses were lower in Spring than the moduli in other seasons, indicating the weakening

of materials by Spring thaw.

(2) The back-calculated moduli of CHCFA-stabilised RPM in the field does not show

significant increase with the increase of curing age.

(3) The CHCFA-stabilised RPM has much higher resilient modulus at 28 days curing age

than that at 7 days of curing age, and has much higher resilient modulus than untreated RPM and Class 6 aggregate.

(4) Based on the MEPDG performance prediction, the service life of Cell 79 with CHCFA-

stabilised RPM is 23.5 years, which is about twice the service life of Cell 77 with RPM

base (11 years), and about three times the service life of Cell 78 with Class 6 aggregate

base (7.5 years).

(5) The life-cycle analysis indicates that the usage of the CHCFA-stabilised RPM as the base

of the flexible pavement can significantly reduce the life-cycle cost, energy consumption,

and greenhouse gas emission compared with the untreated RPM and Class 6 aggregate

base.

Follow-on performance monitoring of Cells 77, 78, and 79 is needed to validate the MEPDG

performance prediction.

Acknowledgements

The authors want to express their appreciation to the US Department of Energy for funding this study and Minnesota Department of Transportation for their cooperation during the construction and field studies and Mathy Technology & Engineering Services, Inc. for performing the binder test. Thanks are given to theAdvanced Transportation Research and Engineering Laboratory at the University of Illinois at Urbana-Champaign for the laboratory testing of the properties of the HMA.

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Cost Energy Ash in Place Stabilized