the life cycle of a road. Furthermore, it is concluded that better understanding of the binder would lead to better optimized pavement design, hence reducing the.
Using Life Cycle Assessment to Optimize Pavement Crack-Mitigation Ali Azhar Butt, Denis Jelagin, Björn Birgisson, and Niki Kringos Division of Highway and Railway Engineering, Transport Science Department, KTH Royal Institute of Technology, Sweden
Abstract. Cracking is very common in areas having large variations in the daily temperatures and can cause large discomfort to the users. To improve the binder properties against cracking and rutting, researchers have studied for many years the behaviour of different binder additives such as polymers. It is quite complex, however, to decide on the benefits of a more expensive solution without looking at the long term performance. Life cycle assessment (LCA) studies can help to develop this long term perspective, linking performance to minimizing the overall energy consumption, use of resources and emissions. To demonstrate this, LCA of an unmodified and polymer modified asphalt pavement using a newly developed open LCA framework has been performed. It is shown how polymer modification for improved performance affects the energy consumption and emissions during the life cycle of a road. Furthermore, it is concluded that better understanding of the binder would lead to better optimized pavement design, hence reducing the energy consumption and emissions. A limit in terms of energy and emissions for the production of the polymer was also found which could help the polymer producers to improve their manufacturing processes, making them efficient enough to be beneficial from a pavement life cycle point of view.
1 Introduction Problems like low temperature cracking and fatigue cracking have always been an issue in cold regions like the European Nordic countries [1]. Cracking is in fact very common in areas having large variations in the daily temperatures and can cause large discomfort to the users due to uncomfortable rides and disturbances caused by frequent maintenance periods. It also increases the cost to the society, as often higher taxes will have to be paid to overcome increased number of maintenance actions. Improvement of crack-mitigation in asphalt pavements could therefore have a significant contribution to the society at large. The rheological properties of bitumen have an important effect on the cracking of the asphalt mixtures, since they provide the glue of the aggregate skeleton [2]. To improve the binder properties against cracking and rutting, researchers have studied for many years the behaviour of different binder additives such as
A. Scarpas et al. (Eds.), 7th RILEM International Conference on Cracking in Pavements, pp. 299–306. © RILEM 2012
300
A.A. Butt et al.
polymers [3-5]. The benefit of using polymers to modify the binder properties is well established but to quantify the long term benefit, an investigation of the effect of this modification over the entire life time of the pavement should be made. Life Cycle Assessment (LCA) tools can therefore be utilized. Due to the depletion of resources and concerns of climatic change, LCA for different products, systems and activities have increased in popularity among researchers for the past years. LCA studies can help to determine and minimize the energy consumption, use of resources and emissions to the environment by giving a better understanding of the systems. LCAs can also purpose different alternatives for different phases of a life cycle of the system. Unfortunately, LCA has not yet been adopted by the industry or the road authorities as part of the procurement and material selection procedure. This could partly be explained due to the lack of a technical tool that accurately represents all the aspects of the pavement sector and is able to make close predictions of the in-time pavement response. For this reason, a new technical LCA framework is being developed. This paper is giving an example of the application width of such a tool for the case of prevention of low-temperature cracking in asphalt pavements.
1.1 Low Temperature Properties of Asphalt Concrete To improve the quality of our roads and prevent pavement distresses such as cracking and rutting, certain measures can be taken. For example, improved road design, optimal use of materials or improving mixtures properties as a whole. Polymers like Styrene-Butadiene-Styrene (SBS) and natural rubbers are often used in the pavement industry to enhance the properties of the asphalt mixtures against premature damage. Polymers have the ability to create a secondary network or a balance system in the bitumen by either molecular interactions or react chemically with the bitumen [6]. Several studies have concluded that adding small amount of polymer (3-6% depending on what type of polymer is used) usually results in dispersed polymer particles in the continuous bitumen matrix and improves the properties of the binder against rutting and cracking [3-5, 7-9]. Due to heavy loads on the pavements and inefficient maintenance operations, roads sometimes deteriorate much quicker than expected. This directly leads to increased energy usage, higher cost and more emissions to the environment. It is therefore in favour of all stakeholders to optimize the efficiency of the maintenance operation over the lifetime of the pavement as much as possible. To achieve this, different case studies and possibilities are to be studied based on different design alternatives. Hence, an approach is required which could help in decision support during the lifetime of the pavement.
1.2 Development of an Open LCA Framework LCA is a versatile tool to investigate the environmental aspect of a product, a service, a process or an activity by identifying and quantifying related input and
Using Life Cycle Assessment to Optimize Pavement Crack-Mitigation
301
output flows utilized by the system and its delivered functional output in a life cycle perspective [10]. Ideally, it includes processes from the cradle to the grave of a product. In the case of asphalt pavements, the cradle can be the extraction of materials and the grave can be the burial of the asphalt pavement in the sub-grade. Use of resources and environmental loads can be reduced by studying the effects and the impacts on the environment during the different phases of a road’s lifetime. A new open LCA framework for asphalt pavements was recently developed by Butt et al. [11] that considers energy consumption and emissions produced during the lifetime of the pavement. The LCA framework is fed the output from pavement design tools which are then processed to quantify energy, raw materials and emissions during different phases of a road’s life time. The functional unit was defined as the construction, maintenance and end of life of 1 km asphalt pavement per lane for a nominal design life. Certain system boundaries have to be assumed while developing the LCA framework. The study was focused on the project level, therefore it was assumed that the road location was known and the use of the land for some other purpose was not considered. Furthermore, the thickness of the asphalt layer was assumed to be constant along the length of the road per functional unit. Fuel and electric energies were accumulated separately for different processes in the lifetime of a road. This assumption was necessary because electricity being a secondary energy source could only be added to the fuel energy if the electricity production energy and efficiency are known. The raw materials considered for the framework are bitumen, aggregate and additives like waxes and polymers.
1.3 Pavement Design (Mechanistic Calibrated MC Model) A calibrated mechanistic design tool used in this study has recently been evaluated for Swedish conditions [12]. The analysis and design framework presented by Gullberg et al. [12] is an extension of the earlier work by Birgisson et al. [13], in which a framework for a pavement design against fracture based on the principles of viscoelastic fracture mechanics has been reported. One key observation regarding this approach is that each mix is evaluated based on its dissipated creep strain energy limit (DCSElim), which is a measure of how much damage mixture can tolerate before a non-healable macro-crack forms. In a design procedure the DCSElim acts thus as a threshold between healable micro-cracks and non-healable macro-cracks. This is a threshold that has proven to be fundamental and independent of mode of loading [14]. In Romeo et al. [9], SuperPave indirect tension (IDT) tests were performed on unmodified and polymer modified asphalt mixtures. In this study it was found that the polymer modification results in a higher damage tolerance of the asphalt mixture, i.e. higher DCSElim. The impact of the DCSElim increase on the design thickness is presently investigated with the design framework reported in Gullberg et al. [12]. All other material properties are assumed not to be affected by polymer modification.
302
A.A. Butt et al.
1.4 Research Aims In this paper, LCA of an unmodified and polymer modified asphalt pavement is performed using the newly developed open LCA framework. The effect of a polymer modification for crack resistance on the energy consumption and emissions during the life cycle of a road is investigated in this paper. The polymer production and transportation energy is also estimated in order to determine the benefit of polymer modification of asphalt pavements in terms of environmental costs.
2 LCA Case-Study The design of the pavement section is based on the work by Almqvist [15]. The asphalt pavement thickness design is done for a lifetime of 20 years using a mechanistic calibrated pavement design model [12]. The pavement consists of a 50 mm thick wearing course above a structural course. The thickness of the structural course changes for different cases depending on the design. The base layer is 178 mm thick whereas the sub-base is 1.0 m lying on top of the bedrock. The design is done for a mean temperature of 5 °C (corresponds to Swedish climate zone 3) assuming the design equivalent single axle load (ESALs) to be 10e6. The following three cases are analysed using the LCA framework: Simulations are performed with unmodified asphalt, SBS polymer modification and unknown polymer modification of asphalt which results in 0%, 50% and 100% increase of the DCSElim, respectively. SBS polymer enhances the properties of the asphalt against rutting and cracking [8-9]. For the case 2, 3.5% SBS polymer modified asphalt has been considered [9]. With the addition of 3.5% SBS to the unmodified asphalt, IDT tests have shown that the DCSElim changes from 3.57 to 5.34 kJ/m3. Hence an increase of almost 50% is achieved. For case 3, it is assumed that the unknown polymer is 3.5% by weight of the blend and provides 100% increase in the DCSElim. The thicknesses of asphalt layers used for the LCA are as shown in Table 1. For the analyses, the total asphalt pavement thickness has been considered containing 5.2% binder content. The construction, and bitumen and aggregates storage sites are considered to be 25, 75 and 35 km from the asphalt plant, respectively. The emissions from electricity and diesel production are as inventoried by Stripple [16]. Energy consumption data for the asphalt production was acquired from Skanska, a large Swedish contractor. It is also assumed that an increase of 17% in fuel consumption is required for polymer modification of the asphalt mixture. The functional unit (FU) defined for the study is construction of 1 km of asphalt pavement for a nominal design life. Lane width is selected to be 4 m wide.
Using Life Cycle Assessment to Optimize Pavement Crack-Mitigation
303
Table 1. Asphalt pavement layer thicknesses for different cases
Cases
Description
1
Unmodified asphalt Unmodified asphalt with 3.5% SBS Unmodified asphalt with 3.5% unknown polymer
2 3
Increase in DCSElim (%)
Wearing Course Thickness (mm)
Structural Course Thickness (mm)
0
50
100
Total asphalt pavement Thickness (mm) 150
50
50
69
119
100
50
36
86
The comparison between Case 1 and Case 2, 3 will give insight into the added benefits in terms of reduced energy and greenhouse gas (GHG) emissions when polymer is added to the asphalt against crack resistance. Based on the results of the previous studies mentioned in the above, it was found that a small percentage of polymers not only provide resistance against cracking but also allows for the reduction of the asphalt layer thickness. This decrease in thickness itself will save energy and reduce emissions in a road’s life cycle, but polymers production and transportation should also be considered in this number.
2.1 Results The results of the LCA analysis are summarized in Table 2 and Table 3. Parameters a, b, c are the unknown energy values (in GJ) for the SBS whereas d, e, f are energy values (in GJ) for the unknown polymer which are associated with the electric, fuel and transportation energies, respectively. Parameters g, h, i and j are CO2-eq values (in tonnes) for the polymer production and transportation. For Case 2, SBS polymer modification of asphalt led to an increase of 50% in the DCSElim which resulted in a decrease of the structural course by 31%, assuming the same service life of the pavement. For the calculation of Case 3 it was assumed that 3.5% of an unknown polymer is added in asphalt which would increase the DCSElim to 100% which lead to a decrease of 64% w.r.t. case 1 and a further decrease of almost 50% w.r.t. case 2. From Table 2 can be seen that the total used energy therefore reduces from 830 GJ (Case 1) to 700 GJ (Case 2) to 508 GJ (Case 3). From Table 3 can be seen that the total CO2-eq reduces from 55 (tonnes) to 47 to 34, respectively. These values, however, still do not include the energy spent and emissions created when including polymers into the process. For this reason, in the following the thresholds will be determined for these.
304
A.A. Butt et al. Table 2. LCA results from the case study
Table 3. Resulting emissions for different case studies CASE STUDY 1 Emissions to air (tonnes) CO2
N2O
CASE STUDY 3
CH4
CO2
N2O
CH4
CO2
N2O
CH4 1.46E-06
2.64E-06
9.92
6.08E-06
2.02E-06
7.17
4.39E-06
-
-
-
g'
g''
g'''
i'
i''
i'''
Aggregate production
1.94
4.93E-05
5.21E-06
1.54
3.91E-05
4.13E-06
1.11
2.82E-05
2.99E-06
Asphalt production
27.72 5.79E-04
2.45E-05
25.53 5.31E-04
2.17E-05
18.45 3.84E-04
1.57E-05
Paving
0.31
6.18E-06
1.93E-07
0.31
6.18E-06
1.93E-07
0.31
6.18E-06
1.93E-07
Compacting
0.18
3.64E-06
1.14E-07
0.18
3.64E-06
1.14E-07
0.18
3.64E-06
1.14E-07
Transportation
12.04 2.44E-04
7.62E-06
9.53
1.93E-04
6.03E-06
6.89
1.39E-04
4.36E-06
-
h'
h''
h'''
j'
j''
Bitumen production Polymer production
Polymer transportation Σ CO2-eq
12.95 7.94E-06
CASE STUDY 2
-
-
55.14 8.90E-04 55.41
4.03E-05
47.00 7.79E-04
3.42E-05
47.23 + g + h
34.10 5.66E-04
j''' 2.48E-05
34.27 + i + j
2.2 Polymer Production and Transportation The polymers production and transportation energies are not included in Case 2 and 3, which should be considered to make an objective judgement of the long term effect of the modification. For this reason, in the following the thresholds of
Using Life Cycle Assessment to Optimize Pavement Crack-Mitigation
305
the energy and emission limits are determined for the polymer production and transportation based on the study’s cases results (Table 4). Table 4. Beneficial bitumen modification boundaries w.r.t. energy and emissions allocation
Energy spent on polymer ETE Electricity used/FU Fuel consumption/FU Transportation Energy/FU
(GJ/FU) Case 1 Vs Case 2 Case 1 Vs Case 3
