Has global warming eliminated the need to consider frost heave in cold climates? I think not. After several warm winters, last winter showed that long cold spells could return to damage roadbeds, shallow foundations and slabs on grade. Aggregate Base Courses provide the primary pavement layer designed to prevent heaving and displacement due to prolonged freezing and frost penetration into subgrade soils. Frost heave in soils and in improperly designed aggregate base layers can be a major destructive force in deteriorating pavements and structures. It is essential that any subgrade soil or subbase with silt-size fines be protected from prolonged freezing temperatures when a source of water is present. The best protection is to use coarse ABC layers not susceptible to frost or foundations that extend below the frost line.

As freezing temperatures penetrate the ground and the aggregate structure under pavements and slabs, moisture is frozen. Saturated pore structures are more susceptible to damage as water freezes and expands. Frost heaving occurs when additional water can be drawn into the pores between soil and aggregate particles by capillary action. The most susceptible are silt-sized materials forming a continuous path in soil or aggregate layers. They contain pores that are fine enough to hold water under tension and to draw sufficient additional water into the mass. ABC products that have no more than 3% silt sizes (less than 0.020 mm as specified in ASTM D 2940) have a much coarser pore structure — one that cannot pull in water by capillary attraction and can drain by gravity. Clay soils can hold large quantities of water but are not susceptible to frost heave. Because of their very fine pore structure, they have low permeability and cannot move enough water to cause ice lenses to grow.

In addition to the upward displacement caused by heaving during long frozen periods, the principal damage to highways occur during spring breakup — when thawing starts from the top down. Thawing first weakens the layer just under the pavement as ice lenses melt and the structure above collapses. As thawing continues (with the layer of frozen material below), excess water has nowhere to go and further reduces support for the pavement. This is when heavy truck traffic can do the most damage to pavement.

Using a quality ABC with limited silt fines at the depth of the frost line will cut off the supply of capillary water to the layers above — whatever their type or design. It is also important to maintain good drainage from the pavement cross section and the base. In cold regions that only have susceptible fine-grained soils or aggregates, explore options for using stabilization. By blending a cementing material to stabilize a soil or aggregate base, the particles are held together with more strength. The pore structure also is modified to reduce permeability and the capacity to move water to freezing sites. Test methods are available to evaluate the capillary suction of aggregate gradings and the frost resistance of stabilized materials.

Frost heave can occur in soils and bases under concrete and asphalt pavements, under driveways, and under interior slabs on grade in unheated structures. When designing to prevent frost heave, it is important to estimate the depth of frost penetration during exceptionally cold winters. Consider the elevation of the area and that snow will be removed from a pavement, so the freezing depth may be more than for nearby snow-covered terrain. Northern Arizona, for example, is not immune to frost heave. Aggregate producers should be aware of the frost heaving potential for different climates and soils so they can recommend the correct depth and type of ABC.

Richard Meininger, PE has been active in developing aggregate standards for decades. He is past vice president of engineering for both the former National Aggregates Association and the National Ready Mixed Concrete Association.

Soil and Aggregate Particle Sizes

• Gravel and Crushed Stone Coarse Aggregate — No. 4 (4.75 mm) to 3 in. (75 mm)
• Sand and Manufactured Fine Aggregate — No. 200 (0.075 mm) to No. 4 (Atterberg Defines Fine Sand Down to a Smaller Size — 0.020 mm)
• Silt (Most Susceptible to Frost Heave) — 0.002 mm to 0.020 mm
• Clay (Clay Minerals Vary in Size) — Generally Below 0.002 mm