Indian Geotech J DOI 10.1007/s40098-015-0168-0
ORIGINAL PAPER
Novel Use of Geosynthetic Reinforced Chip Seal in Asphalt Pavements Rajib B. Mallick1 • G. L. Sivakumar Babu2
Received: 28 July 2015 / Accepted: 8 October 2015 Ó Indian Geotechnical Society 2015
Abstract High temperature causes a number of problems including rutting and premature aging/cracking in asphalt pavements. The high temperature results primarily from high absorptivity and low conductivity of the asphalt binder and the hot mix asphalt (HMA). This paper presents a novel approach of utilizing geosynthetic reinforced chip seal (GRCS) with high reflectivity (reflectivity greater than 0.2) aggregates to reduce the amount of heat that is absorbed at the surface and the heat that is conducted to the lower layers. Results of small scale and full scale tests as well as finite element analyses are presented. The results show that GRCS is highly effective in reducing pavement temperatures at different depths, specifically, at a depth of 12.5 mm below the surface, which corresponds with the depth of maximum temperature for most asphalt pavements. The GRCS section used in the full scale test shows a reduction of 8 °C, compared to a conventional HMA section. Mechanistic-empirical analysis shows that for such a reduction, an extension of pavement life by 8 years is achievable. It is recommended that GRCS section be constructed along with conventional HMA section, and trafficked, for evaluation of life cycle cost benefits.
& Rajib B. Mallick
[email protected] G. L. Sivakumar Babu
[email protected] 1
Civil and Environmental Engineering, Worcester Polytechnic Institute (WPI), 100 Institute Road, Worcester, MA 01609, USA
2
Department of Civil Engineering, Indian Institute of Science, Bengaluru 560012, India
Keywords Geosynthetic reinforced chip seal Asphalt Temperature Rutting
Introduction Geosynthetic materials have been used extensively in pavements primarily as reinforcing and/or separator layers. The main properties of the geosynthetic materials that are typically utilized in such applications include tensile strength, opening size and flexibility. Chip seals are utilized in pavements primarily as preservation (or maintenance) treatments, in which aggregates are partially embedded in a layer of asphalt binder or emulsion. The layer acts as a friction enhancing non-structural layer, and a functional surface layer for traffic. A combination of geosynthetic layer and a chip seal layer, although not rare, is not widely used. While this product, commonly known as geosynthetic reinforced chip seal (GRCS) [1], can serve the same purpose as a geosynthetic layer plus a chip seal, this paper presents a novel use, in which its thermal properties are utilized to lower temperature, and hence reduce the potential of temperature related distresses in pavements. Temperature Related Distresses in Asphalt Pavements and Proposed Solution High thermal absorptivity (0.85–0.93) and low thermal conductivity (0.76–1.4 W/mK) [2] of asphalt mixes lead to higher temperatures in asphalt surfaces during warm weather [3]. As Fig. 1 shows, an increase in the temperature leads to faster aging (primarily through oxidation) of the asphalt binder and associated cracking, and an increased risk of accelerated rutting. These
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Indian Geotech J Fig. 1 Temperature related distresses in asphalt pavements
Chip seal with high reflectivity aggregates (suggested, r=> 0.25)
Nonwoven geotextile layer Tack coat Existing HMA
Fig. 3 Schematic of GRCS
Fig. 2 Heat flow in asphalt pavements
phenomena could be slowed down with a reduction in pavement temperature; an insulation of the asphalt surface from the oxygen-rich environment can also reduce maintenance costs and the long term use of aggregate and asphalt for maintenance chip seals. The different mechanisms of heat transfer in pavements are shown in Fig. 2. The pavement absorbs part of the solar radiation and conducts that energy as heat through the layers while the convective action of the air helps in cooling the surface. The pavement surface also emits thermal radiation back to the environment, when the temperature of the air drops below that of the surface. The high temperature causes a reduction in the stiffness of the asphalt mix, and makes it susceptible to rutting. Therefore, if the absorption of the surface is lowered and the conductivity of the layers is reduced, then less heat will penetrate through the depth of the pavement resulting in a lower pavement temperature.
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The absorption at the surface can be reduced by using a surface layer with greater reflectivity, and the conductivity could be reduced with the help of a layer of a material that has very low conductivity—lower than conventional HMA or chip seals. Therefore, a composite pavement system incorporating an insulation layer and a high reflectivity surface layer is proposed for lowering of temperature in asphalt pavements. The insulating layer will also effectively protect the asphalt surface of the pavement from air, and hence significantly reduce the oxidation exposure and hence the aging of the asphalt binder and the resultant stiffening of the asphalt mix. Geosynthetic reinforced chip seal (GRCS) with high reflectivity aggregates (for the chip seal) can be utilized to serve this purpose (Fig. 3). A surface with partially exposed aggregate with a reflectivity of 0.24 (for example) would absorb 76 % of the incident solar radiation, compared to 92 % absorption for a new conventional HMA surface (reflectivity or albedo, r = 0.08). The geosynthetic layer used in a pavement is generally saturated with asphalt binder during placement. The polypropylene geosynthetic layer and the asphalt binder have very low thermal conductivities (0.16 W/mK, [3, 4]) compared to HMA. The asphalt binder saturated geosynthetic layer along with the chip seal, when placed on top of a new HMA pavement or
Indian Geotech J Fig. 4 Close-up of HMA sample with geosynthetic layer and chip seal aggregates, and test set-up
a HMA overlay, will act as a barrier between the HMA surface and the environment. This will protect the HMA from sunshine, UV rays and moisture, which are responsible for oxidative aging of the asphalt binder [5, 6]. Note that currently chip seals are widely used as part of pavement preservation strategies. Typically, a chip seal consists of an application of asphalt binder or rapid-setting emulsion followed with an application of aggregate layer. If multiple layers are used (such as double or triple seal coats), finer gradations are used in each successive layer. Precoated aggregates could also be used, and one-size aggregates are often used. Compaction of the aggregate layer is required by a steel-wheeled or rubber-tired (preferred) roller.
Test Results and Analyses Results of Laboratory and Small Scale Testing In order to investigate the concept of using GRCS for lowering of pavement temperature, first small scale experiments were carried out. Samples of HMA with and without geosynthetic reinforced chip seals with light colored aggregates (Fig. 4) were used. Massachusetts Highway Department 12.5 mm nominal maximum aggregate size (NMAS) gradation with 6 % PG 64-28 asphalt binder was used. The ambient conditions during the tests were as follows: air temperature: 19.7–37.84 °C; solar radiation: 41–887 kW/m2, and wind speed: 0.2–12.7 km/h. The HMA samples were prepared at optimum asphalt content, using a Superpave gyratory compactor. After compaction thermocouples were inserted into holes that were drilled at
different depths of the samples. The thermocouples were connected to a data acquisition system that collected data at regular intervals of time. The samples were subjected to solar radiation outdoors, and temperatures at different depths were collected over a time period of 12 h during the daytime, using thermocouples. From the results (Fig. 5) it is evident that the temperatures are significantly reduced at different depths. To separate the effects of the light colored aggregates in the chip seal and the geosynthetic layer, similar experiments were carried out with a geosynthetic layer with and without aggregates, as well as a control sample (air temperature: 24–29 °C, solar radiation: 74–1007 kW/m2, wind speed: 2–9 km/h). Figure 6 shows that both the geosynthetic layer and the light colored aggregates are separately contributing towards the reduction in temperature. Experiments with one (2 mm) and three (6 mm) geosynthetic layers also show the positive effect of increasing the layer thickness (Fig. 6; air temperature: 23–28 °C, solar radiation: 166–930 W/m2, wind speed: 3–12.5 km/h). Note that the light-colored aggregate that was used in the laboratory and small scale testing was also used in the field study. No measurements of thermal absorptivity or conductivity were conducted for these aggregates; however, thermal reflectivity was measured with the use of a dual-pyranometer technique, and was found to be 0.25. Results from Field Testing Next, full scale testing was carried out to evaluate the temperature reduction potential of GRCS. A 4 m by 4 m fully weather-data-instrumented test section of GRCS was constructed over an existing HMA layer at the University
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Fig. 6 Results of experiments showing effect of chip seal and geosynthetic layer and thickness of geosynthetic layers
Fig. 7 Test section in UC Davis pavement testing facility
Fig. 5 Plot of temperature versus time data from experiment
of California Pavement Center (UCPRC) test facilities (7) in Davis, California in September 2013 (Fig. 7). The details of the section are provided in Mallick et al. [7]. For
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the GRCS a locally available PG 64-16 binder was first applied over the existing HMA surface at a rate of 1.3 l per square meter (residual), and it was followed by the placement and rolling of a non-woven geotextile layer. A rubber-coated 90-kg hand roller was first used for rolling, and then a side-walk roller was utilized for compaction, after the application of sand. A polymer modified cationic rapid set emulsion at a rate of 2.7 l per square meter (residual) was applied then, followed by the spreading of the chip seal aggregates (an alluvial gravel from central Massachusetts meeting South Dakota Type 1 chip seal gradation requirements with reflectivity of 0.24) at a rate of 20 kg/sq m. The gradation of the chip seal aggregates was
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as follows: 100, 55, 8, 3 and 1 % passing the 12.5, 9.5, 4.75, 2.36 and 0.075 mm sieve, respectively. The hand held roller and a pickup truck were used to roll the aggregates. The section was instrumented with a weather station and thermocouples, to obtain wind speed, solar radiation, air temperature and temperatures at different depths of the pavement, as well as rainfall and humidity. Note that the temperature data from the existing HMA section had already been acquired in the preceding year. Tables 1 and 2 show the weather data and the temperature data from the GRCS and the HMA sections, respectively. The weather conditions for the 2 years in which the data were collected from the two sections were very close, and it can be seen that the maximum temperature at all depths in the GRCS are significantly lower than that of the HMA. Of particular importance is the reduction in temperature by about 8 °C at a depth of 12.5 mm from the surface of the HMA. The temperature at this depth is generally considered in the analysis of rutting potential of asphalt pavements [2]. Figure 8 shows that the benefit of reduction in temperature (at the same point, compared to a HMA) increases with an increase in air temperature. That is, at higher air temperature when the potential of rutting would be higher because of higher HMA temperatures (in a conventional
pavement) the reduction of temperature due to the application of GRCS will be higher. So for example, when the air temperature is 10 °C, there is 0 °C difference between the GRCS and the HMA pavement at a depth of 12.5 mm below the surface; however, when the air temperature rises to 40 °C, there is a 10 °C difference between the HMA and the GRCS pavement, at the same depth. Note that here four points have been used as they include the range of high temperatures that were observed in the study for the used location. The air temperature was used so that the utility of the GRCS with high albedo aggregates could be considered for regions with specific air temperatures (which are generally easily available, in comparison to pavement surface temperature).
Table 1 Weather data for GRCS and (conventional) HMA sections
Extension of Pavement Life
Maximum solar radiation (W/m2)
Maximum wind speed (m/s)
HMA 32.2
1195
12.3
GRCS 32.5
1143
6.8
HMA 33.7
1292
11.6
GRCS 35.2
1420
8.6
June HMA 40.6
1074
7.8
GRCS 40.0
1085
7.1
HMA 41.0
1021
6.2
GRCS 38.6
1261
5.9
Section Maximum air temperature (°C) April
May
July
Fig. 8 Plot of air temperature versus reduction in temperature due to the use of GRCS
The MEPDG/MEPDS [2] software, along with its weather database, was used to predict rutting damage of conventional and insulated pavement systems, for different maximum pavement temperatures. Four cities were selected to consider a range of maximum pavement temperatures, from 70 to 52 °C. These are, in decreasing average temperatures: Houston, TX, Raleigh–Durham, NC, Chicago, IL, and Portland, ME. A pavement located in Houston was simulated, using the climatic information for the above four cities, to determine the rutting damage over the years, and the years to failure, for the range of temperatures from 70 to 52 °C. Figure 9 shows that for the same traffic loading and the same materials, the life of the pavement can be extended by 5 years for a 5 °C drop in maximum
Table 2 Maximum and mean temperatures in July Section
Temperature (°C) Surface
12.5 mm below HMA surface
37.5 mm below the HMA surface
62.5 mm below the HMA surface
Max.
Mean
Max.
Mean
Max.
Mean
Max.
Mean
HMA
71.4
25.8
59.5
22.9
60.8
24.7
59.5
25.3
GRCS
58.5
33.0
51.9
33.9
50.8
34.6
49.3
35.1
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be reduced significantly with the use of GRCS with high albedo aggregates.
Practical Considerations
Fig. 9 Plot of service life versus maximum pavement temperature
temperature. The drop in temperature is more effective in extending the life of the pavement at higher temperatures. Based on this analysis, it can be said that for the results obtained from field testing (presented in the preceding section), an extension of life of 8 years is achievable.
Results of Theoretical Analysis and Modeling The GRCS test section at the UC Davis test facility was constructed adjacent to an instrumented Open Graded Friction Curse (OGFC) section [8]. Finite element (FE) models of the GRCS and the OGFC sections were developed [9] and utilized with appropriate material properties to predict the temperatures for a specific 24-h period, which was selected on the basis of relatively high temperature. The material and layer details are shown in Fig. 10. Details of similar FE modeling have been presented elsewhere [10]. The solar radiation data obtained from the field instrument were utilized as irradiation in the FE model. Figure 10 compares the results of the simulation with the field data at 12.5 mm below the pavement surface. The simulated and the actual temperatures compare very well, confirming the validity of FE model, and demonstrating the effectiveness of a GRCS layer in lowering pavement temperature. Impact on Aging-Related Cracking Potential High temperature related oxidation and aging in HMA leads to an increased potential of cracking. The use of GRCS can act as a barrier to protect the HMA from air, moisture, UV radiation and sunshine, which are mainly responsible for aging [5, 11, 12]. An analysis of the impact of GRCS was conducted using the Global Aging Model (GAM) and System (GAS) [13, 14]. In this analysis, the time required by the asphalt binder in the HMA of two sections—a conventional and a GRCS to reach critical stiffness (that triggers cracking, [15, 16]) were determined. Details of this analysis are presented in [17]. Figure 11 shows the results of the analysis—a significant drop in the aging related increase in stiffness with GRCS, compared to a conventional section. Hence, the aging related cracking potential could be expected to
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High reflectivity aggregates (r [ 0.2) may not be locally available everywhere. In such a case the application of this concept should be reviewed in conjunction with the added cost of transporting such aggregates from elsewhere. In lieu of light color aggregates, broken/crushed pieces of recycled concrete (that meet the specific gradation of chip seal) could be considered for this application although the concept needs to be validated first. Regarding the choice of geosynthetic material, nonwoven fabrics are recommended as they can elongate and resist rupturing, and hence can be successfully recycled. During construction the application rate of binder must be monitored closely, as insufficiently saturated nonwoven paving fabric can cause problems during recycling. Loss of chips can also occur if the binder is not applied at the specified rate under the paving fabric. Finally, if the loss of chip seal under traffic is a concern, double seals can also be applied for added protection against surface distress caused by moisture damage. The high reflectivity aggregates need to be applied only on the top seal. The use of GRCS with high reflectivity aggregates involves some special considerations: laying of a geosynthetic layer, paving of chip seal, and utilization of an aggregate or reflective coating with reflectivity of preferably 0.24 or higher. However, it may be worth the extra effort as the modeling and testing indicate a potential of significant enhancement of the pavement life. This will also help in reduction of the number of maintenance cycles needed to keep the pavements in serviceable conditions. Using a protective covering layer in the form of a GRCS with the reflective aggregates or coating is intended to provide a relatively simple and yet effective way to reduce the effect of the environment to solve a long-standing problem in asphalt pavements. There is also the added benefit of prevention of moisture ingress and associated problems (stripping) in HMA layers. The use of GRCS does not require the development of new materials—materials and processes do exist; what is needed is effective guidance from qualified personnel during the construction of the GRCS.
Conclusions and Recommendations The following conclusions and recommendations can be made.
Indian Geotech J Fig. 10 Data and results from finite element (FE) analysis
1,500 900
Thermal conductivity (W/mC) 1.73 0.16
Heat capacity (J/kgC) 1,250 1,800
Not relevant Not relevant
2,269 2,250
1.24 2.5
763 1,250
0.08 Not relevant
2,269 2,250
1.24 2.5
763 1,250
Section Material
Reflectivity
Density (kg/m3)
GRCS
0.24 Not relevant
Temperature, C
OGFC
Chip seal, 6.25 mm Geosynthetic saturated with asphalt, 2mm HMA, 100 mm Aggregate, 150 mm (insulated underneath) HMA, 100 mm Aggregate, 300 mm (insulated underneath)
50 45 40 35 30 25 20 15 10 5 0 0
5
10
15 Time, hour
20
25
30
Field data, OGFC, 12.5 mm below surface Field data, GRCS, 12.5 mm below original surface FE Simulation, OGFC, 12.5 mm below surface FE Simulation, GRCS, 12.5 mm below surface
1.
2.
3.
4. Fig. 11 Plots of asphalt viscosity versus time for conventional HMA and GRCS sections
A significant reduction of maximum pavement temperatures at different depths is possible with the use of GRCS; A reduction in the maximum pavement temperature should lead to a significant enhancement in the life of the pavement; Theoretical calculations show a significant potential of the GRCS to reduce the aging related increase in stiffness of asphalt binder in HMA pavements; Side by side conventional HMA and GRCS section should be set up and trafficked to evaluate comparative performance, and benefits in terms of life cycle cost;
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5.
The utility of GRCS in lowering temperature gradient along the depth of concrete pavements (and hence reducing stress) should be investigated.
Acknowledgments The authors gratefully acknowledge the help of Mr. Don Pellegrino and Ryan Worsman of WPI, Dr. John Harvey, Dr. Hui Li and Mr. David Jones of UC Davis and Mr. Steve Thaxton of Propex Operating Company, LLC.
8.
9. 10.
11.
References 1. Myers R (2012) Geosynthetic reinforced chip seals. California Pavement Preservation Center, CP2 Center News, March 2. National Cooperative Highway Research Program (NCHRP) (2004) Guide for mechanistic–empirical design. Transportation Research Board, Washington, DC, Design Inputs 3. Solaimanian M, Kennedy TW (1993) Predicting maximum pavement surface temperature using maximum air temperature and hourly solar radiation. Transportation Research Record, No. 1417, Transportation Research Board, National Research Council, Washington, DC 4. The engineering toolbox (2013) www.EngineeringToolBox.com. Accessed 1 Feb 2013 5. Read J, Whiteoak J (2003) The shell bitumen handbook, 5th edn. Thomas Telford, London 6. Cheolmin B, Underwood BS, Kim YR (2012) Effects of oxidative aging on asphalt mixture properties. Transportation Research Record 2296, Transportation Research Board, National Research Council, Washington, DC 7. Mallick RB, Worsman RK, Li H, Harvey J, Bhowmick S (2014) Effective reduction of asphalt pavement temperatures. In:
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Presented and published in the proceedings of the 12th ISAP conference on asphalt pavements, Rayleigh, NC, USA Li H (2012) Evaluation of cool pavement strategies for heat island mitigation, UC Davis, 50 Institute of Transportation Studies. December COMSOL Multiphysics, version 4.3 (software) (2012) http://www.comsol.com Mallick RB, Sakulich A, Chen B-L, Bhowmick S (2013) Insulating pavements to extend service life. In: RILEM symposium on multi-scale modeling and characterization of infrastructure materials, Stockholm, Sweden, June 10–12, 2013 Airey GD, Choi YK, Collop AC, Moore AJ, Elliott RC (2005) Combined laboratory ageing/moisture sensitivity assessment of high modulus base asphalt mixtures. J Assoc Asph Paving Technol 74:307–346 Lerfald BO (2000) Ageing and degradation of asphalt pavements on low volume roads. Thesis submitted to the Norwegian University of Science and Technology. February Houston WN, Mirza MW, Zapata CE, Raghavendra S (2005) Environmental effects in pavement mix and structural design systems. NCHRP, Transportation Research Board, National Research Council, Washington, DC Mirza MW, Witczak MW (1995) Development of a global aging system for short and long term aging of asphalt cements. J Assoc Asph Paving Technol 64:393–431 Hubbard P, Gollomb H (1937) The hardening of asphalt with relation to development of cracks in asphalt pavements. J Assoc Asph Paving Technol 9 Kandhal PS (1997) Low temperature ductility in relation to pavement performance, ASTM, STP 628 Mallick RB, Li H, Harvey J, Myers R, Veeraragavan A, Reck N (2015) Pavement life-extending potential of geosynthetic-reinforced chip seal with high-reflectivity aggregates. Transp Res Rec J Transp Res Board 2474:19–29