Slabs-On-Ground - Structural Deterioration And Damage

Problem: Cracking of slab - 4.

Cause: Frost heave.

Frost heave can occur where moisture in the soil beneath the slab-on-ground freezes, expands, and exerts vertical and horizontal pressures on the slab. As a general rule, frost heaving will occur at the perimeter of the slab where frost penetration is more significant. Heaving can result in structural defects and damage to the slab and the supported structure.

Solutions: Prevent soil below the slab from freezing.

• Never pour concrete on a frozen sub-grade. Frozen soil is unstable and may settle unevenly, causing the slab to stress and crack.
• Keep moisture out of the materials that come in contact with the slab.
• Provide heat in the house during construction to prevent freezing of the soil below the slab.
• Prove skirt insulation around the perimeter of the foundation. The effects of skirt insulation on the pattern of isotherms in the soil around a slab-on-ground are shown in Figure 40a and 40b.

Figure 40a - Slab Temperature Effects Without Skirt Insulation

Figure 40b - Slab Temperature Effects With Skirt Insulation

• Insulating vertically and horizontally out from the slab perimeter with an appropriate amount of rigid insulation can maintain soil temperatures above the freezing point in most of North America's cold climates. Insulation at corners of the slab will need to project further to accommodate higher levels of heat transfer.
• Table 1 provides guidance on the amount of, and depth of, insulation required in different climatic areas. Specific amounts and location may need to be engineered. The table assumes the use of moisture-resistant plastic foam insulation, such as extruded polystyrene.

Table 1: Suggested horizontal perimeter insulation for shallow foundations

• Ensure that any perimeter insulation extending over the slab above grade is protected with parging, flashing, or another rigid material (metal, or pressure-treated plywood).