Integrated Life-Cycle Assessment

Integrated Life-Cycle Assessment and Life-Cycle CostAnalysis Model for Concrete Bridge Deck Applications

Alissa Kendall1; Gregory A. Keoleian2; and Gloria E. Helfand3

Abstract: An integrated life-cycle assessment and life-cycle cost analysis model was developed and applied to enhance the sustainabilityof concrete bridge infrastructure. The objective of this model is to compare alternative bridge deck designs from a sustainabilityperspective that accounts for total life-cycle costs including agency, user, and environmental costs. A conventional concrete bridge deckand an alternative engineered cementitious composite link slab design are examined. Despite higher initial costs and greater material-related environmental impacts on a per mass basis, the link slab design results in lower life-cycle costs and reduced environmental impactswhen evaluated over the entire life cycle. Traffic delay caused by construction comprises 91% of total costs for both designs. Costs to thefunding agency comprise less than 3% of total costs, and environmental costs are less than 0.5%. These results show life-cycle modelingis an important decision-making tool since initial costs and agency costs are not illustrative of total life-cycle costs. Additionally,accounting for construction-related traffic delay is vital to assessing the total economic cost and environmental impact of infrastructuredesign decisions.

DOI: 10.1061/�ASCE�1076-0342�2008�14:3�214�

CE Database subject headings: Life cycles; Environmental issues; Bridge design; Fiber reinforced materials; Concrete pavements;Traffic delay; Construction costs.

Introduction

Concrete pavements and structures such as bridges are fundamen-tal components of our transportation network, and thus also fun-damental to economic vitality and personal mobility. Yet, theAmerican Society of Civil Engineers estimates that the poor con-ditions of United States roads cost users $117.2 billion in addedoperating costs and time lost in traffic delay annually �ASCE2005�. Poor roadway conditions persist despite economic and ma-terial investment in highways and roads of approximately $64.6billion and 260 million metric tons of concrete annually in theUnited States �FHWA 2002; Kelly 1998�. The most recent high-way bill signed into law, the Safe, Accountable, Flexible, EfficientTransportation Equity Act: A Legacy for Users �SAFETEA-LU�,authorizes $25.2 billion for interstate maintenance alone throughthe year 2009, and $21.6 billion for preventive maintenance andimprovements on highway bridges through 2009 �FHWA 2005�.

1Assistant Professor, Dept. of Civil and Environmental Engineering,Univ. of California, 1 Shields Ave., Davis, CA 95616; formerly, DoctoralStudent, Center for Sustainable Systems, School of Natural Resourcesand Environment, Univ. of Michigan, 440 Church St., Dana Building,Ann Arbor, MI 48109.

2Associate Professor, Center for Sustainable Systems, School ofNatural Resources and Environment, Univ. of Michigan, 440 Church St.,Dana Building, Ann Arbor, MI 48109 �corresponding author�. E-mail:[email protected]

3Associate Professor, School of Natural Resources and Environment,Univ. of Michigan, 440 Church St., Dana Building, Ann Arbor,MI 48109.

Note. Discussion open until February 1, 2009. Separate discussionsmust be submitted for individual papers. The manuscript for this paperwas submitted for review and possible publication on June 22, 2006;approved on October 9, 2006. This paper is part of the Journal of Infra-structure Systems, Vol. 14, No. 3, September 1, 2008. ©ASCE, ISSN
1076-0342/2008/3-214–222/$25.00.
214 / JOURNAL OF INFRASTRUCTURE SYSTEMS © ASCE / SEPTEMBER

Downloaded 19 Jan 2009 to 141.211.206.116. Redistribution subject to

The magnitude of investment demonstrated by these allocationsunderscores the need to approach road building and repair from anew perspective—long term and preventive, rather than shortterm and corrective. Life-cycle assessment �LCA� and life-cyclecost analysis �LCCA� methodologies provide the means for thiskind of evaluation.

An integrated LCA and LCCA model �LCA-LCCA� was de-veloped to provide a holistic assessment of the economic costs ofconcrete infrastructure applications, in this case a highway bridgedeck. LCA is a framework designed to evaluate the environmentalperformance of a product or process throughout its life cycle,including raw material acquisition, production, use, final disposalor recycling, and the transportation needed between these phases�ISO 1997�. Often, LCA elucidates unseen environmental and so-cial burdens incurred over a product or system’s lifetime. Byquantifying environmental and social burdens, the LCA modelprovides the data necessary for comprehensive LCCA. The LCAmodel was developed prior to integration with the LCCA model.Its methods and results are described in Keoleian et al. �2005�.

Like the LCA model, the LCCA model analyzes the costs as-sociated with all phases of an infrastructure application through-out its life cycle. LCCAs vary in scope and depth, accounting fordifferent kinds of costs and benefits. For example, LCCA mayaccount only for agency costs, which are the costs incurred by thefunding agency; it may account for user costs in addition toagency costs, such as costs incurred by motorists who are delayedor detoured by construction related traffic; and LCCA, morerarely, may include environmental costs, such as the pollutiondamage costs associated with construction processes.

According to a study conducted on behalf of the New JerseyDepartment of Transportation �DOT�, only 12.5% of state DOTsapply any sort of LCCA on bridges �Ozbay et al. 2003�. However,LCCA could prove extremely useful in bridge applications be-
cause bridges require significant capital investment but also con-
2008

ASCE license or copyright; see http://pubs.asce.org/copyright

siderable investment for maintenance and rehabilitation duringthe bridge’s service life.

Other studies have developed life-cycle costing methods forroads, highways, and highway infrastructure. Ehlen identifiedlife-cycle costs for polymer-reinforced concrete bridges that in-clude agency and user costs �driver delay, vehicle operating, andvehicle accident costs� and third party costs �Ehlen 1999�. Ehlenclassified third party costs as the upstream environmental costsassociated with construction materials �pollution from mining,processing, and transportation� and the downstream environmen-tal costs related to construction activities such as runoff. WhileEhlen noted the increasing importance of third party costs, theywere not quantified and environmental impacts from constructionrelated traffic delay were not identified either.

In addition to LCCA studies like Ehlen’s, there has been aconsiderable effort to model the expected maintenance costs ofcivil infrastructure. Frangopol and others have done extensivework to optimize bridge maintenance costs using reliability-basedoptimization methods to predict and reduce life-cycle costs forbridges. These analyses examine and optimize agency costs, andgenerally neglect user, third party, and environmental consider-ations �Enright and Frangopol 1999; Estes and Frangopol 2001;van Noortwijk and Frangopol 2004�. Despite neglecting no-nagency costs, these studies develop valuable methods for pre-dicting and optimizing maintenance and rehabilitation schedules.Maintenance and rehabilitation schedules are extremely importantin LCCAs since they determine the number and timing of con-struction events that drive life-cycle cost results.

There is also existing software for applying LCCA to bridges,namely BLCCA, created by the National Cooperative HighwayResearch Program and funded by the Transportation ResearchBoard, and BridgeLCC created by the Office of Applied Econom-ics, Building and Fire Research Laboratory at the National Insti-tute of Standards and Technology �Ehlen 2003; Hawk 2002�.These software applications were reviewed in a 2004 FederalHighway Administration �FHWA� report �FHWA 2004a; PalisadeCorporation 2000�. Both BLCCA and BridgeLCC provide userand agency cost calculations. In addition, BridgeLCC allows thesoftware user to enter third party cost parameters, and BLCCAincludes vulnerability costs, which are the expected costs associ-ated with extreme events, such as earthquakes or explosions, thatare unlikely but possibly disastrous. Both software programs haveuncertainty modeling capabilities. Some limitations and weak-nesses noted by the FHWA include a complex user interface andlimited uncertainty models for BLCCA, and for BridgeLCC, alack of details in the cost output and the opinion that BridgeLCCdoes not reflect “realistic bridge-related experience.”

Other models have also been developed for LCCA of pave-ments, rather than bridges, such as RealCost, developed by theFHWA Office of Asset Management. This program accounts foruser and agency costs and allows for both deterministic andprobabilistic output �FHWA 2004b�. None of these software toolswere specifically designed for bridge deck decision making, butall are useful points of reference to understand previous measuresthat computerized and automated LCCA for road and highwayinfrastructure.

The objective of this research was to create a model that washighly tailorable, where new materials, new designs, and multiplebridge deck sites could be modeled. Input variables include com-ponent materials, material durability, and the bridge deck repairand rehabilitation schedule. In addition, traffic parameters, fueleconomy improvements, and roadway and construction work
zone characteristics can be manipulated and are integrated with
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the traffic model. The emissions model and fuel economy modelcan be updated to replicate the climate characteristics and re-gional fleet characteristics for the location of the bridge deckunder evaluation. Moreover, the integrated LCA-LCCA modeldeveloped here is unique in its capacity to calculate life-cycleuser, environmental, and agency costs, including the upstreamburdens of material and fuel production. This ability to quantifyupstream burdens and environmental and user costs is an addi-tional capability not available in other models.

Model Description

LCA-LCCA model development began with the LCA model.Within the LCA model, a life-cycle inventory �LCI� is performed.A LCI evaluates the inputs and outputs of the system under con-sideration by examining each phase of the system’s life cycle. Thelife-cycle phases include material production, consisting of theacquisition and processing of raw materials; distribution, whichaccounts for transport of materials and equipment to and from theconstruction site; construction and rehabilitation of the bridgedeck, including all construction processes and construction re-lated congestion effects; use of the bridge deck, which modelsvehicular travel over the bridge during its service life; and finallyend of life, which assesses demolition of the bridge deck, trans-portation of the material to a landfill or recycling facility, andprocessing of the materials. LCI datasets for each input are re-quired. A LCI dataset provides the life-cycle information for agiven material or energy source. For example, the LCI dataset forcement provides the total primary energy needed to produce a unitof cement. Total primary energy includes the energy required forextraction, refining, and production of the energy source. Thedataset also quantifies nonfuel material inputs, such as the mass oflimestone required, and all the inputs and outputs required toextract and process these raw materials. The sources for thesedatasets may be found in Keoleian et al. �2005�.

The LCA modeling required integration of three additionalmodels: a vehicle emissions model, MOBILE6 developed by theU.S. Environmental Protection Agency �EPA�; a constructionequipment model, NONROAD, also developed by the EPA; and atraffic flow model developed at the University of Kentucky �Ken-tucky Transportation Center 2002; U.S. EPA 2000, 2002�.

The LCCA model, whose methods were first developed byChandler, but enhanced in this model, was integrated with theLCA model �Chandler 2004�. The LCCA model uses some of thesame modeling parameters and user inputs to the LCA model,such as the bridge deck system specifications, material specifica-tions, traffic flow rate, and the construction timeline. The LCCAmodel also uses some of the results of the LCA model to calculateenvironmental and user costs. The cost model requires inputs forpollution damage costs, the value of lost time to personal andcommercial vehicles delayed in traffic, costs of agency construc-tion activities, and discount rates for environmental, user, andagency costs. Fig. 1 shows the integrated model framework.

System Definition

The LCA-LCCA model was applied to a highway overpass bridgedeck, and two alternative designs that could be used to constructit. The first design uses conventional mechanical steel expansionjoints, and the second design applies engineered cementitious
composite �ECC� link slabs in place of conventional expansion
F INFRASTRUCTURE SYSTEMS © ASCE / SEPTEMBER 2008 / 215


joints. ECC is an advanced cement-based material that has theadvantages of concrete such as great compressive strength, butalso is capable of ductile behavior like a metal. ECC is consideredmore durable than conventional concrete and the link slab designis expected to eliminate key failure modes associated with theconventional expansion joint design. Thus, the ECC link slab de-sign is expected to increase the life of the bridge deck and reducethe number of repairs necessary over the bridge deck life �Li et al.2003; Li 2003�.

The bridge examined in this application was modeled after anoverpass in southeast Michigan at the intersection of two high-ways, M-14 and US-23. During nonconstruction periods thebridge has two lanes with one-way average annual daily trafficflow �AADT� of 35,000 vehicles with no annual growth rate. Thestructure is 0.16 km �0.1 mi� long and contains eight expansionjoints in the case of the conventional design, and eight link slabsin the ECC link slab design. Fig. 2 depicts the two bridge deckjoint designs evaluated in this study. The bridge substructure isassumed to be 30 years old and is undergoing its first deck re-placement at the beginning of this analysis. The time horizon forthe analysis is 60 years. At the end of 60 years the bridge sub-structure will be 90 years old and is expected to need replace-ment.

The study evaluates three repair processes performed on thedeck; bridge deck replacement, shallow overlay, and pothole fill-ing. The repair and maintenance schedules, shown in Fig. 3,reflect the assumption that only these three repair and reconstruc-tion activities take place for both systems. This timeline is a sim-plification of all possible repair processes that could beundertaken, and excludes some maintenance activities. The con-ventional deck design requires another deck replacement after 30years, but the ECC link slab deck design is expected to last twiceas long, meeting the bridge’s remaining 60 years of life.

The timeline shown in Fig. 3 illustrates a doubling of bridgedeck life, 20 rather than 15 year intervals for deck overlays, and adoubling in time between pothole patching events for the ECCsystem compared to the conventional system. A doubling of decklife for the ECC system significantly influences modeling results.However, as noted before, the link slab design eliminates keyfailure modes initiated at the expansion joint in the conventionaldesign. A key failure mode associated with the expansion joint iscaused by degradation of the joint’s seal, a strip of rubber sand-wiched between the two sides of the steel expansion joint. Whenthe seal deteriorates, water and salts creep into the joint and ini-

Fig. 1. Integrated LCA

tiate corrosion on the reinforcing bar, which eventually causes



cracking and potholes on the surface of the deck. By eliminatingthis failure mode, the link slab design extends the life of thebridge deck and deck surface.

Research supports this doubling of bridge deck life when theECC link slab system is treated as a jointless bridge. Jointlessbridges are designed with no expansion joints or similar mecha-nisms such as link slabs, and like the link slab design, eliminatethe expansion joint failure mode. A study in New York concludedthat jointless bridges remained in acceptable conditions abouttwice as long as comparable jointed bridges �Yanev and Chen1993�.

Key Cost Parameters

Agency Costs

Agency costs were provided by a Michigan construction companythat requested to remain anonymous. Table 1 shows the costbreakdown for the three construction activities modeled. Thesecosts reflect material cost assumptions of $130.80/m3 �$100 /yd3�for conventional concrete and $329.99/m3 �$250 /yd3� for ECC.This ECC cost is the expected cost once the material is morewidely used and mass produced; the cost of this material uponintroduction would be higher.

Environmental Costs

In this study pollution damage costs were used to characterizeenvironmental costs. These costs are calculated for key pollutants

model flow diagram

Fig. 2. Bridge deck with ECC link slab and conventional mechanicalsteel expansion joint

-LCC

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including six of the seven criteria pollutants specified by the EPA,and three primary greenhouse gases �GHGs�. The criteria pollut-ant costs were derived from Banzhaf and colleagues, except forthe cost of volatile organic compounds �VOCs� which was pro-vided by Matthews and Lave �Banzhaf et al. 1996; Matthews andLave 2000�. The GHG costs were adapted from Tol’s value of $60per metric ton �t� of carbon �Tol 1999�.

The Banzhaf study estimated damage costs from Midwesterncoal-fired electric power plants. While the regional aspects of thisstudy are well suited to this application, it represents cost esti-mates from point sources, not mobile sources, and a significantportion of the criteria pollutants generated over the bridge decklife cycle comes from mobile vehicle sources. In general vehicleemissions emitted at ground level rather than from an elevatedstack result in increased exposure for humans and thus greaterdamage to health, so the costs estimated by Banzhaf can be con-sidered conservative for this LCCA application �Lai and Thatcher2000�. Pollutant damage costs from Banzhaf et al. 1996 are basedon “morbidity health values, mortality risks, and �willingness topay� to avoid mortality risks” �Banzhaf et al. 1996�.

The VOC damage cost, taken from Matthews and Lave, isbased on a mean value from five studies on social damage costsfor VOCs. Matthews and Lave emphasize the most importanteffects of pollution are human health effects, but do not specifythe basis for VOC damage cost estimates of each source used inthe average.

Tol’s marginal cost of carbon is based on results from theclimate framework for uncertainty, negotiation and distribution�FUND� model. FUND integrates social factors such as popula-tion distribution, technology, and economics with climate model-ing �The Research Unit for Sustainability and Global ChangeUniversität Hamburg 1999�. The marginal cost of carbon as aGHG is developed based on a cost benefit and cost effectiveness

Table 1. Agency Cost Breakdown for ECC Bridge Deck System Rehabi

Total cost

Construction event Deck type �$�

Deck replacement ECC link slab 431,186

Conventional 385,963

Resurfacing ECC link slab 100,521

Conventional 185,102

Patching ECC link slab 654

Conventional 654

Fig. 3. Bridge dec

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analysis of different possible climate and emissions scenarios.Costs resulting from climate change, such as the economic im-pacts of sea level rise and the migration of affected populations,as well as human health impacts such as increased exposure totropical diseases like malaria, and heat and cold stress, are evalu-ated and weighed against the costs of emissions reductions �Tolet al. 2003�.

The marginal cost of carbon must be converted to apply to theGHGs evaluated in this study, which include carbon dioxide�CO2�, methane �CH4�, and nitrous oxide �N2O�. Each of thesegases has a different global warming potential reflecting the de-gree to which they trap heat in the earth’s atmosphere, with thebaseline defined by the effects of CO2. The cost per metric tonof carbon can be converted to cost per metric ton CO2�$ / tCO2�by multiplying by a factor of 44/12, the ratio of the molecularweights of CO2 and carbon. CH4, and N2O have global warmingpotentials of 23 and 296, respectively, so these factors thenmultiply the cost of CO2 for their cost per metric ton �Joos andMcFarland 2001�.

Table 2 shows the pollution damage cost estimates used in theLCCA model for each pollutant. In all cases, pollutant damagecosts were adjusted to 2003 United States dollars.

Pollution damage costs are difficult to calculate, and all have asignificant amount of uncertainty associated with them. Thosepollutants that contribute to global warming pose perhaps evenmore difficulties for cost estimation. In order to address the un-certainty in pollution damage costs, Monte Carlo simulation wasperformed on pollution costs.

User Costs

User costs consist of three types; traffic delay, increased risk oftraffic crashes, and increased vehicle operating costs due to con-

Material cost Labor cost Equipment cost

�$� �$� �$�

97,038 229,439 51,427

89,557 192,448 47,409

15,615 58,556 16,591

31,625 117,413 22,007

92 471 92

92 471 92

bilitation timelines

liation

k reha



struction. User delay, or time spent in construction related traffic,dominates user costs. Delay costs for passenger automobiles,single unit trucks, and combination trucks are $13.61, $21.78, and$26.21, respectively. These costs are based on estimates devel-oped in a Federal Highway Administration study �Walls andSmith 1998�. The costs are in 1996 dollars and are brought for-ward using the consumer price index to 2003 dollars. User delayis calculated based on the time spent in construction related con-gestion above what would be spent traveling the equivalent dis-tance in normal traffic flow conditions.

Increased traffic crashes related to construction work zonesand increased distance traveled when detours are used to avoidconstruction zones also contribute to user costs. Data for workzone accidents in the state of Michigan were used to calculate theincreased cost for users in construction work zones due to ahigher risk of fatality and injury compared to roadway use whenno construction zone is in place �Michigan Department of StatePolice 1994–2001; Michigan Department of Transportation2002�. The result is an estimated $0.08 per vehicle kilometertraveled �VKT� �$0.13 per vehicle mile traveled �VMT�� inthe construction zone and a $0.06 increased cost per additionalVKT �$0.09 increased cost per additional VMT� traveled when adetour is taken. These calculations assume that the relative riskof crashes is proportional to the distance traveled through awork zone versus distance traveled on typical roadway with noconstruction.

The last element of user cost is based on increased vehicleoperating costs when a construction work zone is in place. Ve-hicles delayed in construction related traffic have higher fuel con-sumption and thus higher fuel costs compared to normal flowconditions. If vehicles avoid congestion by detouring, they travelfurther than those that stay on the highway and also have in-creased fuel consumption. Increased fuel consumption in thework zone is estimated based on the difference in fleet averagefuel economy between highway fuel economy and city fueleconomy for passenger vehicles and heavy duty trucks �Bradley2000; Hellman and Heavenrich 2003�. Fuel costs are based on a10 year average �1993–2003� of retail fuel costs, approximately$0.40/L for gasoline and $0.35/L for diesel fuel in constant 2003dollars �Davis and Diegel 2004�. Fuel costs comprise less than2% of total user costs so fuel price volatility is unlikely to affectresults.

Discount Rate

The discount rate used in this model is based on values recom-

Table 2. Air Pollution Damages Costs by Impacted Region

Average cost

�2003 US$/t�

Pollutant name Urban Urban fringe Rural Global

Particulate matter 6,144 2,750 800 —

Nitrogen oxides 156 65 19 —

Sulfur dioxides 170 88 21 —

Carbon monoxide 2 1 0 —

Lead 3,955 2,059 480 —

VOC 1,960 1,960 1,960 —

Carbon dioxide — — — 21

Nitrous oxide — — — 7,112

Methane — — — 384

mended by the United States Office of Management and Budget



�OMB� and is estimated at a real discount rate of 4% for user andagency costs �Office of Management and Budget 2005�. Somegoods, typically including environmental goods, may be dis-counted at a different rate than private market transactions due toa concern that society is underinvesting in these goods �Gramlich1990�. Pollution damage costs here are subject to a variation ofexponential discounting, a sliding discount rate that accounts forthe immediate, near, and medium future. This scale was devel-oped by Weitzman, who conducted a survey of over 2,000 leadingeconomists and created the following method: for the immediatefuture, Years 1–5, a 4% discount rate is used; for the near future,Years 6–25, a 3% discount rate is used; and for the mediumfuture, Years 26–75, a 2% discount rate is used �Weitzman 2001�.The reason for a sliding scale is that, given the significant uncer-tainty in environmental impacts and their costs, appropriate ratesof return on capital many years into the future are unknown�Weitzman 1998�.

The selection of a discount rate can be a topic of controversyin LCCA, since there are no hard and fast rules regardingdiscount rate selection. In the system evaluated here, sincecosts are incurred over a 60-year service life, the selectionof a discount rate significantly influences results. While the OMBdiscount rates are considered to be a reasonable assumptionof discount rates for public infrastructure investment, opinionsvary on what discount rate should be applied. Because of thisuncertainty, sensitivity analysis was performed on discount rateselection.

Results

Life-Cycle Cost Results

The results for life-cycle costs, shown in Table 3, demonstratethat overall the ECC link slab system has a cost advantage overthe conventional system in all categories assessed. These costs arebased on the 60-year service schedule for construction eventsshown in Fig. 3.

The repair and rehabilitation timeline drives the results shownabove. Despite higher initial costs for the ECC system, shown inTable 1 for bridge deck replacement costs, the more frequentrepair and replacement rates for the conventional system meanthat over time the conventional system accumulates costs thatexceed ECC system costs.

User costs overwhelmingly dominate total life-cycle costs,which are made up of agency, user, and environmental costs. En-vironmental costs are notably small compared with agency anduser costs. Of these user costs, time lost to vehicles delayed inconstruction related traffic account for 94% of all user costs and91% of total life-cycle costs in both cases. Essentially, the mag-nitude of the cost results is driven by parameters for traffic and

Table 3. Life-Cycle Costs for Bridge Deck System Designs

Conventionalsystem

ECCsystem

ECCadvantagea

�$� �$� �%�

Agency cost 640,000 450,000 29

User cost 21,000,000 18,000,000 14

Environ. costs 100,000 80,000 21

Total costs 22,000,000 19,000,000 15aCalculated from prerounded costs.

traffic modeling. For example, on a road with lower traffic vol-

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umes but similar capacity, the agency and environmental costs areexpected to have more influence on results. Backup at a workzone is caused by traffic volumes exceeding the construction zonecapacity. Thus, if traffic volume is lower, construction eventswould be less likely to cause congestion and user costs woulddrop significantly. In all cases, user costs refer only to trafficimpacts as they differ from nonconstruction traffic conditions.Because of the dominance of traffic related costs on results, sen-sitivity analysis was run on AADT, a key parameter in determin-ing traffic congestion.

Sensitivity to AADT Variations

Fig. 4 shows the total life-cycle cost results, as well as the cost ofvehicle delay, as one-way AADT varies from 25,000 to 35,000�one-way traffic� with no annual growth rate. The figure shows adramatic increase in total life-cycle costs, driven by the cost ofvehicle delay, between AADT values of 30,000 and 35,000. Thissuggests that user costs may vary greatly from one project to thenext, and collecting accurate AADT data and accurately assessingroad capacity can be particularly important on stretches of roadwhere capacity loss during construction causes excessive delays,as in the 35,000 AADT case. DOTs may want to spend more timeand effort on managing traffic by providing the public with infor-mation far in advance and providing sufficient opportunities formotorists to detour or plan trips differently if delays are immi-nent. Reducing the number and duration of repair and rehabilita-tion events would also be important for reducing traffic delayimpacts.

Sensitivity to Discount Rate Selection

As mentioned previously, selecting a discount rate in LCCA canbe a polemic issue. Moreover, the results of an LCCA, especiallyfor long-lived systems such as public infrastructure, can varygreatly based on the selection of a discount rate. Because bridgemanagement is funded by government agencies, using the OMB’sestimates for the discount rate is a reasonable method of approxi-mation. However, even the OMB’s estimates have varied greatlyover the last 2 decades. For this reason, the LCCA was run atdiscount rates from 0 to 10%. While the ECC bridge deck systemhas a cost advantage in the base case scenario, and all lowerdiscount rate values, the two systems become equal in total life-cycle costs at about 7.2%, and at higher discount rates the con-

Fig. 4. Life-cycle and vehicle delay costs with increasing AADT

ventional system gains a cost advantage over the ECC system.

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The better cost performance of the conventional system at higherdiscount rates can be explained by the construction schedule.For the initial construction process, the ECC system performspoorly compared with the conventional system. However, as timegoes on, the conventional system requires more frequent repairactivities. The cost of these future repair activities are greatlyaffected by the discount rate. For example the agency cost of aconventional bridge deck replacement, scheduled to take place inYear 30, is $385,963. When discounted at 4% the present value ofthe cost of the bridge replacement today is approximately$124,000, when discounted at 7.2% the cost is $51,000, and whendiscounted at 10% the present value falls to $24,000. So, despitethe more frequent repairs needed in the conventional system, asthe discount rate grows these repairs become less significant andtotal costs are dominated by the costs incurred in the first year. Inthis sensitivity analysis environmental costs are discounted in thesame manner �no sliding scale of discount rate� and at the samerate as all other costs.

Fig. 5 shows the agency, user, and environmental life-cyclecosts. The conventional system initially costs 59% more than theECC system at a 0% discount rate, but with a 10% discount ratethe conventional system costs less than the ECC system by 9%.The break even discount rate for total life-cycle costs, where bothsystems cost approximately $16,500,000, is 7.2%. The ECC sys-

Fig. 5. Life-cycle costs at nominal discount rates from 0 to 10%

Fig. 6. Total life-cycle pollution damage costs



tem environmental cost advantage at a constant 4% discount rateis only 10%, notably different from the 21% cost advantageshown in Table 3 where a sliding scale for discounting is applied.Despite the change in environmental cost advantage, the effect ofthe discount rate change on ECC system total life-cycle cost ad-vantage is less than 1%.

Pollution Damage Costs

Pollution damage costs are small compared to the agency anduser cost results. However, only air pollution damage costs wereincluded in these results, while the damage costs of water pollu-tion and runoff were not.

Fig. 6 shows the environmental damage costs for the twobridge deck systems, broken down by the different air pollutantsanalyzed in this LCCA.

CO2 and VOCs dominate the total pollution damage costs. Forthe conventional system, CO2 accounts for 76% of total emissionscost. Seventy nine percent of the CO2 emissions result from fossilfuel combustion in the traffic phase, about 20% from the materialsphase, and only 1% from all other phases combined. The contri-bution of GHGs other than CO2 amount to less than 1% of totalenvironmental costs and only slightly more than 1% of total GHGcosts, despite their higher global warming potentials. The ECCsystem has similar results. CO2 accounts for 69% of all emissionscost, where 83% of the emissions result from the traffic phase,16% from the materials phase, and only 1% from all other life-cycle phases. For VOCs, the costs are even more dependent ontraffic phase results; nearly 100% in the conventional case and99% in the ECC.

Monte Carlo Simulation Applied to Pollution DamageCosts

LCCA, especially of long lived systems, is subject to inherentuncertainty. Much of this uncertainty is simply because of diffi-culty in predicting exactly what events and conditions will exist

Table 4. Probability Distribution Functions for Pollution Damage Costs�2003 United States $/t�

Air pollutant name Probability distribution function

Carbon dioxide Lognormal: mean=26, SD=76

Methane Lognormal: mean=600, SD=1,700

Nitrous oxide Lognormal: mean=7,900, SD=22,000

Carbon monoxide Uniform: range=0.09−1.51

Lead Uniform: range=1,865−2,253

Nitrogen oxides Uniform: range=38−91

Particulate matter �10 � Uniform: range=2,243−3,258

Sulfer oxides Uniform: range=51−125

Volatile organic compounds Uniform: range=210−5,767

Table 5. Monte Carlo Simulation Results

Bridge design

GHG cost ECC link slab

Conventional

Environ. cost ECC link slab

Conventional



in the future. While most parameters in this model are subject toa degree of uncertainty, the uncertainty for damage costs of pol-lutants, especially GHGs, is high. This is primarily a result of thecomplexity of the science and economics behind climate changeissues. For example, which types of costs are examined, how farinto the future the cost of pollution is assessed, and which climatemodels are used all affect the magnitude of damage costs. Due tothe uncertainty in damage cost estimates, Monte Carlo simulationwas used to develop a range of costs for pollution damage overthe service life of the bridge.

Monte Carlo simulation requires that each variable, in thiscase pollution damage costs, be defined by a probability distribu-tion. Table 4 shows the probability distributions used in theMonte Carlo simulation for all of the assessed pollutants.

For criteria pollutant costs, a uniform distribution defined bythe range in costs provided in their source articles was used. In allcases, damage costs for urban fringe areas were used when pos-sible. In Banzhaf et al. �1996� the damage cost ranges representthe 90% confidence interval created in that study’s Monte Carlosimulation. Despite large ranges, the damage costs are quite con-servative compared to estimates for mobile source emissions. Forexample, in a study by Delucchi, the estimated lower bound forPM10 costs is $9,750/t, still higher than the upper bound in thecosts reported by Banzhaf et al. 1996 �Delucchi 1998�. Conse-quently, the environmental cost estimated in this analysis is likelya conservative estimate.

GHG costs for this study were calculated using Tol’s 1999value of 60 $/tC. Tol did not include a range of uncertaintyfor this estimate. In a 2005 study, Tol performed a review ofprevious estimates for the marginal cost of CO2, and providedcentral estimates from each source, along with other factors suchas author weights, whether the study was subjected to peer re-view, the time horizon of the study, etc. �Tol 2005�. For thisanalysis, the central estimates from all peer reviewed sourceswere taken, unweighted, and a best fit curve created within Crys-tal Ball, the software used to perform the Monte Carlo simulation�Decisioneering 2004�.

Results from the Monte Carlo simulation are based on a trialof 2,000 runs, and costs were discounted using Weitzman’s slid-ing scale of discount rates. Table 5 shows a summary of resultsfrom the simulation for the ECC and conventional system. Notsurprisingly, results for total environmental costs were highly cor-related with the cost of greenhouse gas emissions.

Within the 90% confidence interval for total environmentalcosts, there is a significant range of potential costs. GHGs, andCO2 in particular, drive this observed variance in total environ-mental costs. For the conventional system, GHGs contribute 86%of the variance, and 77% for the ECC system.

These results show that compared with user costs, environ-mental costs are small, even when uncertainty is taken into ac-

n Median 90% certainty

�$� �$�

00 22,000 1,700–290,000

00 31,000 2,300–400,000

00 65,000 2,300–290,000

00 71,000 3,400–400,000

Mea

�$�

68,0

94,0

100,0

130,0

2008


count. However, at the upper limit of the 90% certainty intervalenvironmental costs are equal to about half that of the totalagency costs.

Conclusion

This paper describes the development of an integrated LCA-LCCA model and its application to a concrete bridge deck.Two potential bridge decks were evaluated: a conventional me-chanical steel expansion joint design and an ECC link slab de-sign. Model results show reduced costs for the ECC linkslab design over the 60-year service life while the real discountrate is less than 7.2%. The superior results for the ECC lifecycle costs are driven by the repair and rehabilitation schedule,which is based on an approximate doubling of time betweenrepair events over the conventional system. User costs domi-nate other costs, and comprise more than 90% of total lifecycle costs for both systems. Sensitivity analysis of AADT andtraffic related costs show that at lower traffic volumes, the domi-nance of user cost drops to a magnitude where agency and envi-ronmental costs play a significant role in the results. Byaccounting for user costs in this LCCA, a stronger argument forthe ECC system can be made. While agency costs are also lowerfor the ECC system, the savings in avoided user costs are muchgreater in magnitude and augment the cost advantage of the ECCsystem.

Because of significant uncertainty in damage costs, an uncer-tainty analysis using Monte Carlo simulation was performed. Re-sults show that total environmental costs are highly correlatedwith the cost of carbon, and that the range of uncertainty is great.At the upper end of the 90% certainty range, the environmentalcosts approach about half those of agency costs, which is signifi-cant given that conservative damage cost estimates were used,and only air pollutants, and not water effluents, were modeled.

This study highlights the importance of evaluating costs ofroad infrastructure over time. A short-term perspective would leadto selection of the conventional bridge deck despite the ECC linkslab system’s life-cycle cost advantage. These results also dem-onstrate the importance of including user costs in road infrastruc-ture accounting. While the importance of these factors will varyin other applications, this study demonstrates that a LCCA includ-ing both time dimensions and costs external to the funding agencycan indicate different net benefits for a project than a conventionalcost analysis.

Future model development will include enhanced uncertaintymodeling, sensitivity analysis to the construction timeline, andincorporation of additional construction event types. In addition,the integrated LCC-LCCA model is being applied to other roadinfrastructure applications including pavement overlays.

Acknowledgments

This research was funded through a NSF MUSES BiocomplexityProgram Grant �Grant Nos. CMS-0223971 and CMS-0329416�.Materials Use: Science, Engineering, and Society �MUSES� sup-ports projects that study the reduction of adverse human impacton the total interactive system of resource use, the design andsynthesis of new materials with environmentally benign impactson biocomplex systems, as well as the maximization of efficient
use of materials throughout their life cycles.
JOURNAL O


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Integrated Life-Cycle Assessment