articulo asfalto

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Resources, Conservation and Recycling 47 (2006) 356374

Air emissions from pavement layers composedof varying rates of reclaimed asphalt

Agnes Jullien a,, Pierre Moneron a,Gaetana Quaranta b, David Gaillard c

a Division TGCE-LCPC, Route de Bouaye BP4129, 44341 Bouguenais Cedex, Franceb CGS/CNRS, 1 rue Blessig, 67084 Strasbourg Cedex, France

c SETRA, 46 Avenue Aristide-Briand, BP 100, 92223 Bagneux, France

Received 18 December 2003; accepted 30 September 2005

Available online 18 April 2006

Abstract

An experimental study of road building and recycling of used pavement has been conducted within

the framework of a Life Cycle Analysis. Four equivalent asphalt concretes made with different recy-

cling rates have been investigated during road construction. Airborne emissions, pollutant release over

time and odor production related to asphalt laying have all been determined and compared among thevarious recycling rates. All of the results (VOC, PAH and odors), expressed in terms of concentra-

tions and fluxes, exhibit quite monotonic variations with respect to the recycling rate. These results,

however, did not reveal the same trend as a function of the recycling rate (increases or decreases),

dependingon theselected target parameter (VOC or PAH). Indicators have been proposed fora discus-

sion of results that take into account: (i) raw material emissions with respect to the reference defined

in the case of pavement without reclaimed asphalt; and (ii) emissions in each case from all material

production sources.

2006 Published by Elsevier B.V.

Keywords: Gas emissions; Asphalt; Road pavement; Recycling; Environmental inventory

Corresponding author. Tel.: +33 2 40 84 59 38; fax: +33 2 40 84 59 92.

E-mail address: [email protected] (A. Jullien).

0921-3449/$ see front matter 2006 Published by Elsevier B.V.

doi:10.1016/j.resconrec.2005.09.004
mailto:[email protected]://dx.doi.org/10.1016/j.resconrec.2005.09.004http://dx.doi.org/10.1016/j.resconrec.2005.09.004mailto:[email protected]


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A. Jullien et al. / Resources, Conservation and Recycling 47 (2006) 356374 357

1. Introduction

1.1. General background

Throughout history, the earths resources have been exploited without considering their

eventual limitations. At present, one of the key challenges is to concentrate on saving natural

resources for future generations while bringing industrial activities into a more stable long-

term balance between environmental preservation and costs. As the result of increased

awareness of environmental concerns and in order to satisfy the 2002 French legislation

regarding landfill sites, recycling is bound to be further developed.

According to the World Health Organization (WHO, 1999), clean air is now recognized

as a basic requirement for human health and well-being. Moreover, some countries have

been continuously working since the 1992 Rio Conference to reduce the airborne emis-

sions produced by engineering processes. The recent protocol adopted in Kyoto, Japan in

December 1997, within the United Nations Framework Convention on Climate Change,

concluded the process that began in Berlin in 1995. According to the protocol, the indus-

trialized nations agree to reduce their emissions of greenhouse gases by 5%, on average,

in relation to the levels emitted in 1990 over the period 20082012. As a consequence,

several chapters of the French Environmental Code (Ordinance No. 2000-914, 2000) are

now devoted to atmospheric pollution, with pollutants being monitored at a local (munici-

pal) scale by specialized agencies. The measurements carried out by associations pertain to

ambient air pollutants, for which rules have been set either in France or within the European

Union, with respect to maximum concentration levels. Included among these are CO2, CO,

NOx, dust (e.g. PM10) and PAH. Such pollutants are characterized every 15 min through

analyzers in conjunction with meteorological data.

1.2. Recycling in road works

For now, the trend in road construction and maintenance in France is to consider not only

the economic and technical factors, but environmental factors as well. Increased mobility

and transport has led to expending major efforts on road construction and maintenance

under conditions of ever-increasing traffic. Specific problem areas are associated with: (i)

savings of natural raw material and recycling or reuse of road-building materials; (ii) design

and processing for minimum waste production; (iii) avoidance of pollutant release into air,

water and soil; and (iv) preservation of the human environment. A few details about these

problems are given below:

(i) Natural aggregates as well as bitumen are widely consumed and therefore consti-

tute natural resources to be preserved. Natural aggregate consumption within Frances

civil engineering sector amounts to 200 million tons per year on roads alone out of a

total production of 400 million tons (Michel, 1997). As for road infrastructure, only

non-hazardous waste is allowed for recycling or reuse (Decree No., 2002-540, 2002).

Research on the mechanical properties of asphalt pavement and reclaimed asphalt

pavement has been conducted for a long time since the 1980s, due to the importance

ascribed to such properties. A considerable body of literature has been devoted to this


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358 A. Jullien et al. / Resources, Conservation and Recycling 47 (2006) 356374

subject, e.g. Bonnot (1992) and Bonnotet al. (1993) and Moneron (1993). International

guidelines (US EPA) also provide technical elements for review within this topic area.

In short, technical problems have been extensively investigated over the years. How-ever, by 2002, the quantity of recycled aggregates had been estimated at less than 10%

in the road sector, despite Frances 1992 Law (1992) stipulating no more inert waste

disposal by July 2002, which served to incite the future practice of recycling.

(ii) Asphalt pavement is much more widely used in France than concrete pavement, i.e.

93% and 7%, respectively of the countrys annual road network volumes. Reclaimed

asphalt pavement as a secondary raw material supply is therefore of great interest and

should become, in the near future, a valuable alternative in road construction and a

source of great natural resource savings.

(iii) Use of road waste will thus prove to be of major benefit, if environmental studies are

able to demonstrate its global advantages in comparison with new raw materials. Such

advantages could then be investigated through either global or local impact assessments

of various industrial technical solutions, by employing the best currently available

techniques according to the European Asphalt Pavement Association (EAPA, 1990).

In subsequent road projects, focusing on airborne emissions associated with the use of

asphalt containing various RAP rates would seem to be a sound approach.

(iv) Despite the considerable amount of work performed in this area, it can still be noted

that the data available on road projects are partially focused on the fields of bitumen

fume, mean exposure values and epidemiological studies. Bitumen fume particulates,

volatile hydrocarbons, PAH compound particulates and PAH compound volatiles have

been mainly characterized by laboratory tests and in some cases in situ experiments

(Riala et al., 1996; Tang et al., 1999). The differences between modified bitumen and

classical bitumen for asphalt production are now well known (CONCAWE, 1992). For

certain situations, mean exposure values for road workers have been set by nationalregulations (WHO, 1999), given that asphalt fumes are known to cause irritation to

the mucous membranes of eyes and the respiratory tract (Tang et al., 1999). Lastly,epidemiological studies have been carried out on road workers and then summarized

(CONCAWE, 1992); no clear conclusions regarding bitumen effects have yet to be

reached, however, due to the numerous parameters involved throughout the working

lives of road construction crew members.

1.3. Presentation of the study

As explained above and based on current knowledge, asphalt recycling has been studied

herein at an industrial scale. This study concentrates on airborne emissions during theasphalt-laying operation. The asphalt was producedat a hot-mix plant with varying recycling

rates ranging from 0% (new raw materials) to 10%, 20% and 30% of RAP. Industrial hot-

mix plants must petition for regional authorizations in France (MATE, 1998). Airborne

emissions during asphalt production are thus supposed to match threshold values, yet no

public data is available in this area; the existence of such data wouldcontributeto thepractice

of recycling as regards hot-mix plant-generated airborne emissions, with the exception of

solid particles. On the other hand, certain data are available on bituminous mixture use

patterns and optimal paver operating conditions (ISAP, 1990; EAPA, 1990; CONCAWE,


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1992; AIPCR, 1995). Moreover, environmental concerns at the road pavement scale have

recently been investigated through global approaches such as life cycle analyses ( Stripple,

2001). No in situ measurements, however, have been identified within the framework ofairborne emissions caused by road waste and reclaimed asphalt pavement.

The objectives behind this study have been to collect data relative to the asphalt-laying

process in order to discriminate, by means of a pertinent and precise gas and odor analysis,

asphalt made using four recyclingratesranging from 0% to 30%. For thesake of comparison,

asphalt production was performed at the same hot-mix plant with a parallel-flow dryer-drum

mixer. After laying, measurements were carried out above the pavement in accordance with

a specific methodology using a gas-flux sensor developed specifically for such analyses.

The methods, materials and methodology for this laying process analysis are presented in

Section 2. Emissions and odor results are provided in Section 3 as a function of recycling

rate and then discussed in Section 4 from the standpoint of data validation and gas emission

inventory with respect to the data available in the literature.

2. Methods, materials and methodology

2.1. LCA requirements for data collection

Since 1980, the Life Cycle Assessment methodology has been applied in several indus-

trial sectors (SETAC, 1993). Its use has been expanding within the building industry as

regards of raw materials (AFNOR, 2002), but yet remains only narrowly employed in

the area of road construction work, especially in France. LCA consists of several phases

described in ISO 14040 standards series (AFNOR, 2000). Here, LCA for data collection

was done as follows.(i) Goal and scope definition: this phase provides a description of the product system in

terms of system boundaries and functional unit. The whole studied system focused on

the maintenance of an area of pavement that can be analyzed by considering several

subsystems (old pavement disassembly, new pavement construction and pavement

utilization). The present study pertains only to part of one of the subsystems (new

pavement construction), narrowed to the asphalt-laying step. The functional unit for

the whole road study is indeed an asphalt road section (surface 3.8 m 150 m and

thickness 0.07 m) using a given recycling rate. However, the results for asphalt laying

were expressed per m2 to ease interpretation.

(ii) Life Cycle Inventory represents the step of LCA methodology for identifying and

evaluating resource and energy consumption along with emissions, e.g. emissions intothe air, water and soil at all stages in a product life cycle. In this study, the impact

factors considered herein will be volatile organic compounds (VOC) and polycyclic

aromatic hydrocarbons (PAH), which are expected to be released immediately upon

asphalt laying. Pollutant fluxes were calculated in accordance with the functional unit

and expressed per m2 as explained above.

(iii) Life Cycle Impact Assessment (LCIA) consists of classifying, characterizing and

assessing the impacts generated by this set of impact factors. In this paper, only LCIA

restricted to olfactory impacts has been undertaken.


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Since the ISO 14040 standard series (AFNOR, 2002) do not yield advice about the

methodology to apply in conducting the required measurements, this line of inquiry con-

stituted an important part of the work performed and presented herein.

2.2. Primary and secondary raw materials

The studied materials were correlated with the maintenance of a binding course for a

heavily traveled road, located close to the French city of Romorantin and submitted to a T1+

traffic rate corresponding to 500750 annual average daily trips (AADT). The total road

lengthfor theexperimentwas 600 m, divided into four test sections,each featuringa different

recycling rate. The old pavement was partially recycled under normal operating conditions.

Pavement removal was performed in order to produce a rather homogeneous raw material

for this in situ recycling experiment. The old 3-cm thick upper layer was then removed

separately from the binding layer during the milling operation (Fig. 1a). Furthermore, only

the old 4-cm thick binding layer was milled at a suitable speed to be directly recycled as

reclaimed asphalt pavement. Lastly, four test road sections, each requiring 100 asphalt tons

including both new asphalt (with a 0% recycling rate being set as the reference case) and

reclaimed asphalt pavements, were rebuilt, hence just half of total road width. It is important

to note in all cases that the hot-mix plant produced the same material, i.e. an asphalt concrete

called BBSG 0/10 (according to French standards). This BBSG material contained 5.5% of

35/50 bitumen, while the aggregate was either an entirely new or a mixture of new aggregate

and reclaimed asphalt pavement aggregate.

Before the disassembly phase, some tests were run to determine optimal milling speed

(9 m/min) for the aggregates to be recycled directly without any additional crushing and

screening processes. The new pavement made of a 7-cm thick binding layer and covered

with a 3-cm thick top layer could then be rebuilt ( Fig. 1b).It should also be pointed out that even if such disassembly conditions allowed for use of a

maximum RAP rate (equal to 57%), only in a range 030% have been successively selected

for this study to avoid technical operating problems, linked to the hot-mix-plant technol-

ogy (parallel-flow dryer-drum mixer). Rates higher than 30% (up to 100%) can indeed be

produced at an industrial scale, but using other hot-mix-plant technologies (Kandhal and

Mallick, 1997). Prior to rebuilding the binding course into a 7-cm thick layer, a careful

analysis of the old bitumen was performed to both characterize its actual set of properties

and choose the appropriate new bitumen for operations.

Before asphalt production, a number of mechanical laboratory characterizations were

undertaken as well, to further guarantee the long-term geomechanical performance of the

new asphalt products. Part of these tests focused on the old bitumen and its index of pen-etrability, which led to classifying bitumen according to its sensitivity to temperature. As

the algebraic value of this index rises, the bitumen becomes less likely to show sensitiv-

ity. Although initially graded at 50/70, the old bitumen exhibited a 35/50 grade, thereby

indicative of a classical aging process. In considering the properties of the old bitumen

samples, no additional binder, outside of the classical 35/50 grade, was used for asphalt

production thus simplifying the analysis of airborne emissions. Table 1 provides the ratios

of new bitumen in asphalt composition for each road-recycling rate as well as for the new

material.


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Fig. 1. Old and new pavement: (a) old pavement milling for secondary raw material production; (b) principle of

new binding course construction.


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Table 1

Mass ratios of old and new 35/50 additional bitumen

Recycling rate/ratioin mass of RAP (%)

Old bitumenfrom RAP (%)

Additional bitumen forasphalt production (%)

Total bitumen innew asphalt (%)

0 0 5.5 5.5

10 0.53 4.97 5.5

20 1.05 4.45 5.5

30 1.57 3.92 5.5

2.3. Asphalt production

It is well known that gas emissions depend upon both constituent material contents

and the initial state (Blomberg et al., 1999), as well as upon the target industrial process

(CITEPA, 2002). In some instances, meteorological phenomena such as rain or sunshinemay influence material processing, especially the initial water content of raw materials. In

the field of asphalt production, some data are available, but without precise link between

the rate of RAP and the hot-mix-plant process. It has thus seemed important to validate

material production prior to undertake a precise study on the emissions from processed new

asphalt samples.

Natural aggregates, milled aggregates (RAP) and bitumen were successively processed

on the same day in order to obtain four equivalent asphalt products, which were prepared

so as to exhibit the same geotechnical properties regardless of recycling rate. One differ-

ence, however, between the new asphalt and RAP processing must be noted; some of the

details on asphalt hot-mix-plant processing in both cases have been given in Fig. 2. Raw

material input, i.e. new aggregates, bitumen and RAP, were introduced in different zones

of the plant cylinder along the cylinder axis. In addition, a link tying the initial state ofbitumen, aggregates and milled RAP with emissions released first from the asphalt pro-

cessing (hot-mix plant) and then, after pavement laying was to be expected. Therefore,

Fig. 2. Schematic view of the drum-dryer mixer recycling.


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before beginning asphalt production, the initial aggregate water content was controlled

mainly due to the fact that water had been used during milling in order to reduce teeth

wear and avoid an additional milling temperature rise. This setup yielded the followingfindings: 1.9% for the reference asphalt (0% RAP), and 2%, 2.1% and 2.2%, respectively

for the increasing recycling rates (10%, 20% and 30% RAP, respectively). These gravimet-

ric water content values were found to be close enough to one another to exert negligible

effect on:

material processing, by means of a required increase in aggregate temperature inside the

cylinder (mean temperature of around 165 C) to produce suitable asphalt;

layer emissions,which arenot independentof eitherthe asphalt producedor themaximum

temperature rise during processing.

2.4. Asphalt sampling for characterization of the asphalt-laying subsystem

Asphalt is usually stored for a period of time at the hot-mix plant before truck loading

and transport to the site. A time lapse, which corresponds to the storage period prior to

truck loading and which cannot be easily controlled at the plant location, is thus typically

encountered before final pavement laying. For all tests conducted, asphalt storage within a

hot-mix storage silo lasted less than 10 min. This procedure served to avoid, to the greatest

extent possible, excessive initial gas emissions during preliminary storage, thereby reducing

potential scattering of the entire data set. It was also decided that measurements would be

taken at the hot-mix plant instead of the road site for purposes of the emissions analysis.

The road site was located 57 km from the hot-mix plant and 1 h was needed for asphalt

transport. Moreover, at the road site, any gas emissions analysis should have been performedimmediately upon laying and before rolling, whereas the BBSG 0/10 asphalt concrete

had to be rolled immediately upon laying in order to attain desired asphalt density. Gas

sampling above the pavement over a fixed area during a sufficiently long time period did not

therefore seem plausible. Under such road-building conditions, neither gas sampling control

at the beginning nor follow-up gas concentration appeared to be possible. The methodology

chosen then consisted of performing tests using small-scale asphalt plates, with dimensions

on the order of 1 m, just 20 m from the hot-mix plant at a sheltered location so as to avoid

any climate-related influence (sun, rain).

In order to study a homogeneous material, the asphalt was first sampled during the

1 h production period required for each asphalt plate, as shown in Fig. 3. Sampling was

conducted after 25 min of asphalt production (50 tonnes of output); a 100-kg mass wasraised to produce each of the given plates. This methodology was then repeated for each of

the recycling rates. A 650 cm 1 m70 mm mold, which required a welded beam frame,

was prepared as part of the procedure. In order to avoid undesirable odors, metallic beams

were used and the asphalt sample was placed on a concrete slab. Immediately after sampling,

the mold was carefully filled with three successive asphalt layers, each of which had been

leveled. The asphalt was then compacted (by means of two cross passes) from a height of

8 cm down to 7 cm thus corresponding with the desired final thickness. The entire procedure

for both asphalt sampling and plate compaction lasted approximately 10 min.


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Fig. 3. Preparation of an asphalt plate at the hot-mix-plant site.

2.5. Experimental protocol for gas sampling and emissions analysis

The pollutants expected to be released immediately after asphalt laying were mainly

volatile organic compounds and polycyclic aromatic hydrocarbons. The PAH were inves-

tigated as part of this study by virtue of the Environmental Protection Agency priority

list, which classifies PAH according to US EPA toxicity levels. A flux chamber technique,

advised by Rognon and Pourtier (2001) for diffuse odor characterization, was developed.

Such a technique is indeed capable of leading to a determination of both pollutant maxi-

mum concentration and pollutant fluxes, as well as identifying the maximum emission level

reached after asphalt laying. The specific equipment and measurement procedures applied

to asphalt plates are described below.

Gas chamber geometry called for a thin half-cylindrical shell made of steel, designed to

be easily removed and to allow for confined measurements close to the asphalt pavementsurface. No additionalodor should have been provided by the chambers constituent material

when submitted to an increase in temperature of about 100 C due to the hot asphalt.

Furthermore, quick and easy connections to gas sampling systems also had to be included.

The chamber outlet was connected to both a flame ionization detector and a thermocouple to

yield real-time total VOC and temperature quantification as shown in Fig. 4. An additional

10-l filter wasalso used for detailedVOC andPAH analysis by means of gaschromatography

after sampling (AFNOR, 1991). Lastly, an 80-l inert bag allowed for gas sampling before

Fig. 4. Installation of the flux chamber on the asphalt plate.


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odor characterization. Under such conditions, the results from asphalt laying were obtained

for all recycling rates using the same equipment for purposes of comparison.

From preliminary testing experiments, it was then decided to study the recycling effectson emissions with the flux chamber under confined conditions over asphalt plates and for

a period greater than 30 min (with the control parameter being VOC variations over time).

The parameter that had to be determined for odor analysis was the dilution factor (denotedK) at the yield olfactory perception, which is also known as odor concentration. The

odor concentration obtained using this technique is thus the characteristic of the diffuse

source and must be investigated by experts. As regards odor quantification, however, the

French 1998 Decree (MATE, 1998) indicates a number of standards, such as NF X43-

101 (AFNOR, 1986) and X43-104 (AFNOR, 1995), which define a specific measurement

method. Since no absolute unity for odor analysis has been derived, the dilution rate used

in the analysis was allowed to vary. The gas collected was successively diluted until odor

perception had dissipated entirely. Moreover, only the dilution factor corresponding to

odor perception with a probability of 0.5 (denoted K50) has been considered herein. This

K50 factor was obtained once the gas had been collected inside a removable bag by six

experts. Data were collected using a dynamic apparatus for olfactory measurements, in

accordance with French standard NF X43-101 (AFNOR, 1986). In following the above

standards, gas samples were diluted at given known values and each expert analyzed the

gas odor with a mask connected to the apparatus. K50 was then measured by each expert

so that the result for each asphalt recycling rate ultimately corresponded to a mean K50value calculated from all the results provided by experts and expressed in terms of odorant

units (OU).

3. Results

3.1. Material processing validation

In order to avoid any confusion between hazardous fluctuations in hot-mix-plant pro-

cesses and process changes due to the addition of recycled aggregates, some preliminary

controls were undertaken for the purpose of study validation. The industrial process was

first validated during the experiment by means of quick bitumen analysis and gyratory com-

pactor tests; gas consumption at the hot-mix plant was examined as well. Quality control

tests on bitumen content of the produced asphalt plates were also subsequently analyzed. All

of these results proved that the four materials (i.e. 0%, 10%, 20% and 30% recycling rates)

were, as expected, exactly the same with respect to both bitumen content and geotechnicalproperties, as illustrated in Fig. 5. These controls have thus allowed for emission compar-

isons among all result sets.

3.2. Pollutant and odor emissions versus recycling rate

Only the total VOC and individual PAH were targeted in the measurements of pollutant

emissions from asphalt laying. The VOC have been plotted versus time in Fig. 6 over

a 1-h period, in accordance with the chosen functional unit. Both the transient regime


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Fig. 5. Control of hot asphalt composition (content of bitumen and filler).

and mean VOC values could be highlighted. As shown in Fig. 6, VOC emissions reach a

maximum after about 12 min for all recycling rates and exhibit a reproducible curve for

all rates as well. The maximum concentration values for an increasing recycling rate were

35 mgC/Nm3, 58 mgC/Nm3, 56 mgC/Nm3 and 66 mgC/Nm3, respectively; moreover, they

displayed no difference between 10%and 20% RAP, yeta strong difference between 0% and

30%. Fig. 7 provides an overview of the detected PAH, i.e. benzo(a)anthracene, chrysene

and benzo(b)fluoranthene. These PAH were composed of four aromatic cycles. As shown

in Fig. 7, as recycling rate rises, the total emitted PAH fluxes increase with a strong effectfrom chrysene at 30% RAP. Together with this individual PAH analysis and in the aim of

improving the presentation of trends versus recycling rate, the total PAH values have also

Fig. 6. Evolution of the content of total VOC with time during the experiment.


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Fig. 7. PAH concentration during the experiment.

been expressed. Both VOC and PAH were thus normalized with respect to the reference

asphalt (0% RAP), according to the following formula:

ratio = flux (recycling rate)flux (reference)

100 (1)

The normalized VOC and PAH values are plotted in Fig. 8a and b versus recycling rate.

The relative increase in such emissions with recycling rate revealed by this ratio (Eq. (1))

proves to be very sizable.

In contrast, odor concentration and fluxes have been plotted on Fig. 9 with varying

recycling rates. A decrease in odor concentration with recycling rate can be observed,

although odor variation trends do not seem to decrease linearly with increasing RAP. In

any event, a significant drop in odor units within the range 030% RAP is apparent. Flux

decrease is thus much higher than concentration decrease. If odors are indeed linked to the

volatile part of bitumen, it would seem normal to observe a higher flux decrease than RAP

increase because of the percentage variation in new bitumen.

3.3. Inventory of gas and odor emissions

Table 2 lists the total VOC fluxes within the (1-h) road functional unit along with indi-

vidual and total PAH fluxes and odor inventory. The order of magnitude of VOC emissions

is around 20 gC/m2, while that of PAH ranges between 0.11 g/m2and 0.62g/m2, with

marked differences between recycling rates.


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Fig. 8. Normalized results vs. recycling rate: (a) VOC; (b) PAH.

The final inventory of pollutant emissions and nuisance obtained is given in Table 2

per functional unit versus recycling rate. The trends derived indicate that VOC and PAH

fluxes increase with recycling rate, and odors decrease with an increasing recycling rate.

The global trend (whether increase or decrease) observed with respect to recycling rate thus

depends on the target emission.

Fig. 9. Odors concentration and fluxes vs. recycling rate.


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Table 2

Inventory for the road per m2 (sampling during 1 h)

Production (%) Total VOC (mgC/m2

) for 1 h Total PAH (g/m2

) for 1 h Odors (UO/m2

) for 1 hNo recycling 0 14.3 0.11 154700

Rate of recycling 10 22.6 0.27 55700



4. Discussion

For all of the measurements performed regarding the asphalt-laying process, significant

differences between recycling rates have been highlighted. Fluxes have been plotted either

in absolute value terms with respect to zero or in relative values normalized to consider the

0% RAP as the reference. None of the results, however, have made it easy to discriminateamong the recycling solutions. In what follows, results will first be discussed within the

context of other results from the literature in the area of airborne emissions. The influence

of recycling rate on this study will then be analyzed and a normalized indicator proposed.

4.1. Asphalt emissions

In this study, measurements have focused on both pollutants emitted during asphalt

laying (inventory) and their associated odors (impacts). The values obtained are discussed

below using data from the literature.

A pollutant inventory of this kind has already been investigated in the case of bitumen

production. Bitumen life cycle inventory in terms of airborne emissions for a very frequentlyused bitumen grade, identified as 50/70 according to the EN standard, has been assessed

(Blomberg et al., 1999). Results, however, do not lendassistance in derivingdirect comments

on asphalt laying because bitumen industrialprocessingimplies veryhigh temperature levels

around 1000 C while asphalt is produced at the hot-mix plant at 165 C.

Furthermore, only a few results are available on asphalt emissions within a road-building

framework. de Groot et al. (2001) measured PAC values near a hot-mix plant at various

sites, yet did not perform any direct measurements over asphalt pavement. Only pollutant

concentrations are therefore given (in ng/m3), whereas, no other results such as pollutant

fluxes, which prove helpful in drawing comparisons with this study, were obtained by the

authors. In another study, Stripple (2001) analyzed asphalt emissions; the asphalt studied

therein, however, was made with just new raw materials. In addition, Stripple did notprovideany data for VOC and PAH compounds, neither forasphalt production nor forasphalt laying;

hence, comparisons with this study cannot be readily performed.

Although pollutants correlated with raw materials such as bitumen and aggregates have

been given in these studies in terms of tons of produced resources (Blomberg et al., 1999;

Stripple, 2001), no emphasis seems to have been placed on either working on RAP or testing

exposure to RAP emissions over significant working periods.

The lack of data on RAP emissions has led this research effort to investigate the order of

magnitude for VOC and PAH with respect to both general knowledge on air quality and the


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particular pollutants involved. Air quality and therefore air emissions have been analyzed

over periods spanning up to several dozens of years by the CITEPA organization in France

(CITEPA, 2002). Among greenhouse gases, CO2 and CH4 are produced in the millions oftons and kilotons, respectively while road transport and industry exhibit contributions of

around 26% and 21% of total CO2; industry alone produces 27% of the total CH4. As for

persistent organic pollutants such as PAH, road transport contributes, according to CITEPA

(2002), 6% in terms of Mg.The very strongcontrast in magnitudebetween massesof emitted

gases like CO2 and PAH into the surrounding air may be easily noticed. Such differences

in magnitude and then in flux per functional unit have also been observed in this study for

asphalt laying, with VOC being about two orders of magnitude higher than PAH regardless

of the recycling rate.

As far as odors are concerned, it is a well-known fact that only an expert nose can

generate a characterization, since no equipment can be used successfully for such measure-

ments (Benoit and Pannier, 1982; Rognon and Pourtier, 1999). Inhalation of foul-smelling

products may have undesirable effects on human well-being, although the sense of smell is

not considered a major sense. Hence, the overlap between foul smell, hazard and toxicity

may be confusing. Furthermore, as opposed to taste, which is based on four fundamental

characteristics (salty, sweet, acidic, bitter), it proves impossible to define a basic list of

odors that help discriminate the four studied recycling rates. Odor intensity (see Table 2,

results in odorant units) has been expressed and quantified in this study; it has served to

indicate significant differences among the four solutions, thereby allowing for a classifi-

cation of recycling rates. The trend observed (decrease in odor with increasing recycling

rate) is indeed directly linked to the mass of new additional bitumen ( Table 1). Taking into

account the measurement methods stipulated in NF X43-101 (AFNOR, 1986), it is now

known that no relationship exists between the K50 factor and odor-generated nuisances. No

other odor characteristic, such as quality (pleasant, acceptable, unpleasant, unbearable), hasbeen analyzed, however, to better characterize the impact on workers.

Unlike pollutant release, which led to conducting a flux inventory associated with asphalt

laying, odor measurements did not ultimately allow for impact determination.

4.2. Effects of recycling rate

Although the emitted PAH values were small (see Table 2), a significant increase in

source emission has been measured between 0% RAP and 10% RAP, which indicates the

presence of an effect induced by recycling. As regards VOC, the relative increase would

seem to be lower than that of PAH, despite a higher total mass of emitted pollutants, yet an

effect due to recycling remains. This same significant effect can be noted for odors, whichdecreased by a factor of more than three from 0% RAP to 10% RAP. In addition, the trend

exhibited with respect to the reference seemed to show that a full decrease in odor (down

to zero) was nearly reached at a rate of just about 40% RAP (see Fig. 9). Nonetheless, such

a result could hardly be obtained since for a 40% RAP rate, a modified binder would need

to be used to maintain comparable mechanical properties.

The above analysis has only concerned airborne emissions due to asphalt laying and was

performed in order to discriminate between different recycling rates; in terms of mechanical

properties, however, the asphalt produced was all the same. A more global approach using


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Table 3

Comparisons of emitted VOC for different productions

Recycling rate/ratioin mass of RAP (%)

Total VOC(mgC/m2)

Total VOC forbitumen

(mg/m2)

Total VOC foraggregates

(mg/m2)

Total VOC forRAP (mg/m2)

Indicator

0 14.3 1828 148 0 0.72

10 22.6 1656 133 132 1.18

20 21.9 1481 118 263 1.18

30 24.5 1305 103 395 1.35

fluxes was then undertaken using another indicator, which had been determined as the

ratio of the asphalt-laying flux to the sum of fluxes released for each asphalt component

produced, i.e.:

indicator =laying flux

bitumen flux + aggregate flux + RAP flux 100 (2)

Calculations were performed with Eq. (2) by considering the existing data in the lit-

erature on both raw material production (Stripple, 2001) and pollutant values for milled

aggregate production. Complete data were available only for VOC; the pertinent calculated

VOC values are given in Table 3 and Fig. 10. This indicator does not change the previous

classification of results over the range of recycling rates.

Recyclingbituminous materialsup to a 30%recycling rate mayproduce differentimpacts

depending on the targets. With respect to asphalt laying, local impacts would be expected,

in particular on workers health. The experimental conditions for source characterization in

this study were severe as a result of being performed under confined conditions. Maximum

concentration values would thus have to be considered as much higher than actual ones

Fig. 10. VOC indicator.


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under unconfined conditions at a height of 160 cm instead of 15 cm. Nevertheless, results

indicate an unexpected increase in pollutants (both VOC and PAH) with an increase in

recycling rate, corresponding to a decrease in additional bitumen binder. According to thecurrent state of knowledge, aggregating PAH and VOC would require further investigation

within the framework of a broader study. It may then prove more worthwhile to calculate

this type of indicator value (Eq. (2)) in including the asphalt production process into the

analysis as well, since asphalt-laying emissions are correlated with production emissions

(hot-mix plant).

5. Conclusion

Sustainable development objectives within the framework of road building and main-

tenance signifies the use of a properly validated database associated with a powerful

environmental assessment tool, e.g. LCA. The recycling of asphalt pavement may of coursesignificantly reduce natural aggregate use, yet still requires further examination not only

during the early stages of resource production, but also at the time of the pavement operating

stage. This study has been aimed at identifying asphalt-laying gas emissions; such emis-

sions comprise the pollutants and odors obtained from road-building activities that need

to be measured at the same time, even though LCA is able to separate sources (pollutant

emissions) from impacts (nuisances caused by odors). Special measurement procedures

have been developed and applied in order to discriminate between three recycling rates

(10%, 20% and 30%), for which significant differences have been highlighted. Despite the

chemical analysis techniques that had to be employed, gas sampling over asphalt plates was

investigated first. Preliminary tests were performed prior to in situ experiments in order to

determine gas-sampling conditions. These tests revealed that sampling should be conductedimmediately after asphalt laying under confined conditions.

In this study, the RAP mechanical characteristics of the old constitutive binder allowed

to use the same new classical binder for all recycling rates. Furthermore, a smaller bitumen

mass ratio was applied for asphalt production with an increasing recycling rate, i.e.: (i)

5.5% for new raw materials; (ii) 4.97% for a 10% recycling rate; (iii) 4.45% for a 20%

rate; and (iv) 3.92% for a 30% rate; this rate schedule leads to supposing that a decrease in

gas emissions should be observed over the proportion investigated. Depending on the gas

component to analyze, either real-time concentrations were monitored or gas was stored

inside a specific filter. The former method was applied for total continuous VOC. As for

PAH and VOC collected during asphalt laying, a complete identification was performed.

These results have shown that some gas emissions increase with recycling rate while

odors decrease;moreover, significant differencesin orders of magnitudehave beenobserved.They also demonstrate that a more globalanalysis is necessary and mayinclude, for instance,

using global indicators to incorporate both production and asphalt-laying effects.

Acknowledgments

This study was financially supported by the French Ministere de lEquipement that the

authors would like to thank, as well as C. Lachet, M. Schemid and all participants of the Parc


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Routier de Blois, Ministere de lEquipement, for road maintenance realisation. P. Morcel

and E. Brayard from SODAE are also thanked for helpful technical discussions.

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