Effect of Climate Change on Flexible Pavements

Dr. Abhishek Mittal, Principal Scientist, CSIR-CRRI New Delhi, discusses the effects of changes in temperature due to climate change on pavements with respect to their material, design, construction, and maintenance.

A long-term shift in temperature and weather patterns is referred to as climate change which may be triggered by variations in the solar cycle. The Inter-governmental Panel on Climate Change (IPCC), a UN body for assessing the science related to climate change, has accused human activities as the primary reason for climate change (IPCC, 2012).

The risks associated with climate change include more frequent and intense droughts, storms, heat waves, rising sea levels, melting glaciers, and warming oceans. These are the current risks; the future projections of climate change are even more scary. Table 1 summarises the global changes as a result of climate change. Such adverse climatic situations not only affect human beings but are severely detrimental to the transportation sector as a whole, and to roads in particular.

Table 1: List of main anticipated global climate-related changes (Dawson 2014)
Most Probable Scenario	Quantification (by 2010)
Increase in temperature	1.1 – 6.4 °C
Increase of sea level	18 – 60 cm
Ice reservoirs melting	Not quantified#
Increased frequency and intensity of extreme weather events	Not quantified#
Increase in precipitation	Annual net in Europe: 0-15 % Annual net in USA: -20 to +20%
Increase in radiative force	Anthropogenic: 0.6 – 6.4 W/m2
Increases in seasonally thawing layer thickness in high latitude soils	Not quantified#
# Indicates the effect is certain but the value has not been calculated reliably or varies too widely.

The three transportation sensitivities associated with climate change (Mills and Andrey 2002) are:

• Infrastructure (including roads): Planning of road network, design, construction, and maintenance
• Operations: Efficiency, mobility, safety, environment, and social externalities
• Demand: Include location, timing, mode(s), and sector.

Considerations for Road Materials
A road pavement is typically composed of various layers of different materials meeting desired specification requirements. For a flexible pavement, granular materials are commonly used in the lower layers with thick bituminous layers (one or more) as a wearing course. For low traffic applications, a thin bituminous surfacing (premix carpet or surface dressing) is provided. Stabilized bases and sub-bases are used for high strength pavements.

Pavements have to endure a wide variation of temperatures, both daily and seasonally, which affect the selection and specification of pavement materials. The most significant effect of temperature is on the bituminous layer, as bitumen is a temperature sensitive material. Bitumen ages when exposed to the environment; it loses volatiles, hardens, becomes brittle and then cracks.

Higher temperatures reduce the stiffness and strength of the asphalt layer, which leads to limiting the stress-strain response of the pavement and reduces the ability of a pavement structure to spread loads (AASHTO, 2009). Also, high temperatures reduce the ability of asphalt pavements to resist permanent deformation (or rutting). When the temperature ranges increase due to climate change, more thermal stresses are induced in the bituminous layers, resulting in enhanced thermal cracking of the pavement structure (Qiao 2020). In addition to this, aging of bituminous mixtures occurs at a faster rate due to the high temperatures, and pavements become more prone to cracking due to brittleness.

Overall, high temperatures reduce the life of the pavement and demands more frequent maintenance such as resealing / resurfacing.

Considerations for Pavement Design

For Indian conditions, pavements are designed for fatigue and rutting failure. The performance models for these two failures (IRC:37, 2018) are given below:

Fatigue Performance Model


Rutting Performance Model


Where,

N_f = fatigue life of bituminous layer in terms of cumulative repetitions of equivalent 80 kN standard axle load
N_r = subgrade rutting life in terms of cumulative repetitions of equivalent 80 kN standard axle load
ε_t = maximum horizontal tensile strain at the bottom of bituminous layer
ε_z = maximum vertical compressive strain at the top of the subgrade
M_R = resilient modulus of the bituminous layer (MPa)
V_be = percent volume of effective bitumen in the mix used in the bituminous layer
V_a = percent volume of air voids in the mix used in the bituminous layer
k_i = regression coefficients (i = 1 to 5)

The values of the regression coefficients for the fatigue and rutting performance models are given in Table 2.

Table 2 : Values of Coefficients for Fatigue and Rutting models (IRC:37 2018)
Coefficients		Design Traffic
		≥ 20 msa	< 20 msa
For Fatigue
	K1	1.6064 × 10-4	0.5161 × 10-4
	K2	3.89
	K3	0.854
For Rutting
	K4	4.1656 × 10-8	1.41 × 10-8
	K5	4.5337

The fatigue performance is affected by the resilient modulus and mix design properties of the bituminous mixes. The resilient modulus values of the bituminous mixes are given below in Table 3 (IRC:37-2018).

Table 3: Indicative Resilient modulus values of bituminous layers (IRC:37-2018)
Mix Type	Average Annual Pavement Temperature (° C)
	20	25	30	35	40
BC and DBM for VG10 Bitumen	2300	2000	1450	1000	800
BC and DBM for VG30 Bitumen	3500	3000	2500	2000	1250
BC and DBM for VG40 Bitumen	6000	5000	4000	3000	2000
BC with Modified bitumen	5700	3800	2400	1600	1300
BM with VG10 bitumen	500 MPa at 35 °C
BM with VG30 bitumen	700 MPa at 35 °C

As can be seen from Table 3, the modulus values of the bituminous mixes decrease with increase in temperature. For the purpose of pavement design, the modulus values corresponding to 35°C is selected which is close to Average Annual pavement temperature for climate conditions prevailing in the plains. For snow-bound regions, the modulus values at 20°C are considered for design purpose.

The most significant effect of climate change is manifesting in the form of high temperatures across the globe. Figure 1 presents the high temperatures recorded for different cities across the country. A closer look at the figure reveals that except for the Northern regions of J&K, Uttarakhand and Himachal Pradesh and the Southern part of the country, extremely high air temperatures were recorded throughout the country.

With the increase in temperatures due to climate change, the pavement design temperature may be needed to shift to a higher value. One would need to identify the design temperature which should be used for the purpose of design. When high temperatures are considered for design, the overall thickness of the pavement would increase due to reduced modulus values of bituminous mixes.

Considerations for Construction and Maintenance
Increased temperatures would demand thicker pavements and use of better quality materials which are able to resist the effects of high temperature. High temperatures coupled with heavy vehicle loads may require the wearing surfaces to utilise polymer modified bitumen, rather than conventional paving grade bitumen. On intersections and bridge deck slabs, mastic asphalt may be considered as a good alternative. It is a void-less mass consisting of an industrial grade bitumen (generally 85/25 bitumen is used), prepared in a mastic cooker (new variants of mastic cooker are non-polluting) and can be laid manually or with mechanical paver. The hard grade industrial binder is able to resist the high atmospheric temperature variations.

With increased temperatures, maintenance activities are expected to be more frequent, which would lead to an increased cost. During the extended periods of high temperatures due to hot, sunny conditions, it would be difficult to maintain the profile of the road during compaction as the mix remains workable for a considerably longer time (Willway et al., 2008).

Also, Hot Mix asphalt (HMA) mixes may maintain high temperatures even after the road is opened to traffic, thereby resulting in excessive rutting due to movement of loaded vehicles. Overall, this would lead to more expensive construction for the same traffic levels and increased maintenance cost for the pavements.

Associated Risks of Climate Change
High temperature is not the only risk associated with climate change. Other associated risks associated with climate change include more frequent and intense droughts, storms, heat waves, rising sea levels, melting glaciers, and warming oceans. These events will also affect the pavement performance in one way or the other. Rising sea levels would lead to a rise in the water tables or water levels, especially in the coastal areas. The unbound granular materials used in the lower layers of an asphalt pavement are more vulnerable to such moisture variations due to rising water levels. High moisture content results in the reduction of the strength of these layers. Increased precipitation may cause stripping and ravelling in the surface layers. Use of saline water during construction (i.e. due to drought) could result in the accumulation of salt in the pavement materials and may affect the bonding characteristics. The presence of salts could also exacerbate the shrink/swell behaviour in clay subgrade and fill materials, resulting in more cracking in pavements and requiring more frequent maintenance activities.

Closing Remarks
As climate changes have adverse impacts on pavement performance, suitable adaptations need to be done to mitigate these effects. Use of better-quality materials, proper drainage, and sub-surface drainage to maintain low saturation levels, better designs, construction and maintenance practices need to be developed and adopted to offset the adverse effects.

References

• Mills, B. and Andrey, J (2002), “Climate Change and Transportation: Potential Interactions and Impacts”, The Potential Impacts of Climate Change on Transportation, Workshop Proceedings, Brookings Institution, Washington, D.C., United States Department of Transportation.
• Willway T., Reeves S. and Baldachin L.(2008), “Maintaining Pavements in a Changing Climate”, Transport Research Laboratory, London, UK.
• IPCC (2012), “Managing the Risks of Extreme Events and Disasters to Advance Climate Change Adaptation”, Special Report of the Intergovernmental Panel on Climate Change, Cambridge University Press, New York, US.
• Dawson, A. (2014), “Anticipating and Responding to Pavement Performance as Climate Changes”, In K. Gopalakrishnan, W. J. Steyn, & J. Harvey (Eds.), Climate Change, Energy, Sustainability and Pavements (127-157). Springer. https://doi.org/10.1007/978-3-662-44719-2_4
• AASHTO (2009), “Mechanistic-Empirical Pavement Design Guide (MEPDG)”, American Association of State Highway and Transportation Officials, National Cooperative Highway Research Program: Washington, DC, USA, 2009.
• Qiao, Yaning, Dawson, Andrew R., Parry, Tony, Flintsch, Gerardo and Wang, Wenshun (2020), “Flexible Pavements and Climate Change : Comprehensive Review and Implications”, Sustainability,12(1057), MDPI.