Freeze Thaw Weathering and Degradation: The Effect on Road Pavements
by Piotr Mazurowski, on december 01, 2022
Freeze thaw weathering of asphalt surfacing has a major impact on pavement life, but the deeper influence of freeze thaw weakening of the subgrade soil can bring about more rapid pavement failures.
Each freezing event causes damage to the road pavement. In regions where air temperatures rise and fall around zero for some time during winter or the shoulder seasons before and after winter, there are multiple freezing and thawing events that cause a ratcheting accumulation of damage.
In this blog we explore the mechanisms and types of damage caused by freeze thaw cycles. We explain what is meant by freeze thaw weathering of asphalt and examine how freeze thaw effects on the subgrade causes major pavement damage. We also look at how climate change is likely to increase susceptibility to freeze thaw damage, and what part Tensar geogrids can play in increasing resiliency against the damage to pavements caused by freeze thaw cycles.
What is freeze thaw weathering?
Freeze thaw weathering occurs when water seeps into cracks in rocks or the pores between soil particles and freezes. The expansion of the water creates high pressures, which can widen the cracks or force apart the soil particles.
When we look at the forces involved in frost shattering, it's easy to understand how this degradation proccess can happen. As water crystalises during freezing, the increase in volume is slightly more than 9% and the expansion pressures generated if confined can exceed 220 MPa: more than enough to fracture rock.
What is a freeze thaw cycle?
When air temperatures drop below zero, the road surface and surrounding ground surface is cooled and eventually reaches zero - and at this point, water in the pavement will freeze. The longer the air temperature remains below zero, the deeper the zone of freezing (usually called the frost zone). Frost can then penetrate deep into the subgrade soil beneath.
As air temperatures rise above zero, the reverse effect occurs. Surface temperatures rise above zero, the ice thaws and the increased temperature slowly penetrates deeper into the pavement and soil beneath.
Taken together, the phases of decreasing and increasing temperatures form one freeze thaw cycle.
Where does freeze thaw cycling occur?
In near-polar regions, air temperatures remain below zero for long periods. There may be just one seasonal freeze thaw cycle per year. However, in Northern and Southern Temperate zones, there are extended periods when air temperatures rise and fall between zero for days or weeks at a time. In these regions there are multiple freeze thaw cycles per year.
What is the effect of climate change?
An effect of climate change is that some near polar regions in Canada, Northern Europe and Russia, that normally experienced one long frigid season are experiencing more temperate conditions with multiple freeze thaw cycles per year. Some regions that once had permafrost up to or close to ground level, now find that the permafrost has receded, resulting in subgrade soils and road foundations having to experience seasonal freeze thaw cycles.
How does freeze thaw weathering damage asphalt?
Micro effects – Freeze thaw cycles have been shown to cause damage to the binder, affecting its properties and causing a reduction in stiffness, compressive strength, and fatigue resistance of the asphalt concrete. The percentage of voids within the mineral aggregate increases under freeze thaw cycling, further reducing stability of the asphalt.
Mezzo Effects – The presence of water freezing within the asphalt causes a reduction in the bond between binder and aggregate particles, this loss of adhesion referred to as ‘stripping’, continues under repeated freeze thaw cycles.
Macro Effects – Water entering the asphalt through cracks expands during the freeze phase, bursting apart the asphalt and widening and lengthening cracks. This reduces structural performance of the asphalt layer and eventually leads to potholes forming. The potholes widen rapidly under the action of further freeze thaw cycles.
The impact of freeze thaw cycles on subgrade soils and road foundations: frost heave and thaw weakening
The adverse effects of freeze thaw on road pavements can be divided into two separate but related processes: frost heave and thaw weakening. Frost heave is the upward movement of the pavement resulting from the expansion as water in the pavement layers and subgrade freezes. Thaw weakening describes the weakening effect on the subgrade resulting from soil saturation as ice within the subgrade melts.
What is frost heave and how does it work?
The formation of ice crystals within the larger voids between soil particles extend to form continuous ice lenses. Ice lenses grow through capillary rise and they thicken in the direction of heat transfer until all water is frozen.
As water is frozen, a negative pore pressure develops in the pavement layers, which draws water upward from the lower unfrozen soil towards the frozen front: a phenomenon known as ‘cryosuction’. As the ice lenses thicken, the overlying soil and pavement layers ‘heave’ upward.
The longer sub-zero temperatures are maintained at the surface, the deeper the zone of frost penetration, the thicker the ice lenses and the greater the overall volume of frost heave.
Figure 1 – The formation of ice lenses leading to frost heave
What are the conditions for frost heave?
For frost heave to occur, three conditions need to be met:
- The presence of materials which are frost susceptible. Many agencies chose to classify materials as being frost susceptible if 10% or more passes a 0.075 mm sieve or 3 percent or more passes a 0.02 mm sieve
- Ground temperatures below zero degrees Celsius
- The presence of water.
What is the effect of frost heave?
The upward non-uniform soil heave lifts the pavement and induces bending stresses in the asphalt layer that can result in cracking. This bending effect can be aggravated by the weight of snow ploughed onto the road edges. Pavement damage caused by frost heave is a major problem in countries that experience frigid winter conditions, particularly on low volume roads with thinner asphalt layers.
What is thaw weakening and how does it work?
In a sudden spring thaw, as air temperatures rise above zero, the temperature within the pavement begins to rise, commencing from the surface. A temperature gradient develops through the pavement, extending into the frozen subgrade. As the ice melts in the upper zone of the subgrade, the soil immediately below is still frozen and impermeable, water is trapped in the thawed soil layer. The saturated subgrade layer is substantially weakened and its bearing capacity reduced. At this point, the pavement is highly susceptible to damage from traffic loading.
If there are several freeze thaw cycles that extend to the subgrade, the effect is to loosen the upper subgrade soil, reducing density and stiffness. This then leads to even greater weakening after the final thaw takes place.
The 5 stages of thaw weakening
Illustrated below are the 5 stages of thaw weakening. The temperature gradient for each stage is superimposed over the pavement structure. After stage 5 the subgrade is in a very weak condition and the pavement is susceptible to failure under traffic loading. This can result in pavement cracking and severe surface rutting.
Stage 1 - Frozen Pavement
The air temperature has remained below 0°C for an extended period. The subgrade is frozen down to the frost depth.
Stage 2 - First thawing
The air rises above 0°C. The pavement begins to warm from the surface downward. The upper subgrade thaws while the lower subgrade remains frozen down to the original frost depth.
Stage 3 - Thaw weakening
Water from the melted ice is trapped in the thawed subgrade above the frozen subgrade zone, with only slow lateral drainage possible. The thawed subgrade is saturated and weakened with reduced bearing capacity.
Stage 4 - Re-freezing
The air temperature drops again below zero. The pavement freezes down the the frost depth. The saturated upper subgrade re-freezes and expands - loosening the soil particles.
Stage 5 - Final thaw
After multiple freeze thaw cycles, the temperature rises for the 'final thaw'. The thawed and saturated upper subgrade is now fruther weakened by dilation during freeze cycles. The pavement is now highly susceptible to damage from traffic loading in this thaw weakened state.
How Tensar solutions can help
Tensar has developed solutions that have been proven to tackle the pavement problems associated with freeze thaw cycles.
Building in resilience – By incorporating Tensar InterAx geogrid above the subgrade surface to stabilise the subbase layer in new construction, the pavement stiffness is increased and the weakened subgrade protected during the spring thaw period.
Reducing the cost of reconstruction – where pavements have failed due to severe thaw weakening of the subgrade, Tensar InterAx geogrid can be incorporated into the reconstruction to avoid the need for deep excavation of weak subgrade soils.
Improving the life of asphalt overlays – Where pavement surfaces have cracked due to frost heave and an overlay is planned, Tensar Asphalt Interlayer products can be employed to increase the life of the overlay by reinforcing the overlay to delay further cracking.