Subgrade

• Subgrade is existing soil underneath the sidewalk. This soil should be compacted and graded evenly to create a smooth, consistent surface to support the concrete. A poorly graded subgrade could cause significant problems later. If your local soil has a high percentage of clay, wet the clay before building to reduce the chance of soil swell while the concrete is curing. Expanding clay soil can crack and otherwise damage your sidewalk.
  
  

Sub-base

• This layer of granular material helps provide a cushion for the sidewalk. Some people install sidewalks directly on the subgrade, but this practice increases the chance of cracks and problems later. The subgrade, usually made from sand or very small gravel, helps reduce the impact of expanding and contracting soil on the sidewalk itself. The National Research Council Canada recommends compacting this sub-base to about 95 percent density and making sure it is a consistent depth throughout.
  
  

Freeze Protection

• When installing a sidewalk in cold temperatures, protect the concrete from freezing for five days or more after pouring. If ambient temperatures are close to the freezing point, cover the concrete with polyethylene sheeting. When temperatures are colder, use two sheets of polyethylene, separated by straw or other insulation, to prevent the concrete from freezing.
  
  

Air-entrained Concrete

• Air-entrained concrete is a special type of concrete with uniform air bubbles in it. These air bubbles protect the concrete from cracking due to freeze and thaw conditions as well as de-icing chemicals and sulfates. Air-entrained concrete is also more easily workable than regular concrete. This material is a good choice for sidewalks, and according to the National Association of Home Builders, some building codes require it. If you live in an area with severe winters, choose this type of concrete for your sidewalk.
  
  

Trees

• Tree roots can cause serious problems with a concrete sidewalk. As they grow under the sidewalk, they lift up the slab, causing an uneven surface and potential cracking. Keep at least two sidewalk widths between any tree and the edges of the sidewalk. Some species are safer for sidewalks: Oaks and maples usually have deep roots and are less likely to lift your sidewalk than species with shallower root systems.