When concrete (or any other material) heats up, it expands, and when it cools, it contracts. The stress of this constant variation in size caused by changing temperatures can cause concrete sidewalks to crack. The expansion joints between sections of sidewalk are intended to minimize this kind of cracking by giving the concrete room to expand and contract without exerting pressure on large expanses of concrete surface. Sidewalks may also be cracked by the movement of the underlying soil, which tends to heave upward when it freezes.
If the soil under the sidewalk is not sufficiently compacted when the sidewalk is installed, the soil may settle over time, causing the sidewalk to sink or to be unsupported where the soil has settled away from the underside of the concrete. The flexing caused by this movement of the soil can cause the concrete to crack. Erosion of the underlying soil due to ineffective drainage may also cause the unsupported concrete to buckle or break.
Heavy traffic on a sidewalk or traffic by vehicles heavier than the concrete is designed to support can also lead to cracking of the surface over time. This kind of wear-related cracking may be especially pronounced if the sidewalk is insufficiently supported due to settlement or erosion. In order to avoid this kind of cracking, homeowners should be sure that vehicles are not allowed to drive over sidewalks.
The force exerted by growing tree roots is slow and gradual, but it is tremendously strong. Roots growing under concrete sidewalks can heave the entire surface upward and cause the concrete to buckle and disintegrate. Trees and other plants growing in existing cracks can cause those cracks to enlarge and spread. In order to prevent this type of sidewalk destruction, trees should not be planted close to sidewalks, and concrete pathways should not be constructed close to existing trees.