What Is the Reason for Cracks in the Sidewalk?

Crazing

• If surface layers of newly poured sidewalks dry out before the interior dries, the exterior of the sidewalk contracts and cracks. Cracks form as a fine network of hairline faults called crazing. Concrete with higher proportions of Portland cement sets without crazing, according to Colorado School of Mines "Mines" magazine. During winter, water collects in any unsealed cracks and freezes. When the surface warms, the ice expands and forces the crack farther apart. The surface of the pavement could flake away from the crack's sides. As cracks deepen, the ice finds better leverage. Eventually the ice wedge splits the slab in pieces.

Thermal Shifts

• Gaps between the slabs of concrete that make up sidewalks allow portions of the pavement to expand and contract independently. If poured as one long solid slab, the pavement would buckle during hot weather, cracking as heat expansion forced the pavement to lift. Pockets of ice collecting under a solid sheet of pavement cause cracks when warming ice expands over frozen ground, forcing the pavement up in a frost heave. Expansion joints -- the gaps between sidewalk sections -- allow parts of the pavement to expand during summer heat and shift up and down in winter without cracking.

Root Damage

• Woody plants growing near sidewalks often extend feeder roots underneath sidewalks and into any existing cracks. As roots increase in diameter, the lateral pressure lifts and cracks the sidewalk. Roots exert as much as 150 psi of force as fluid inside the woody tissues expands new cell walls, says biologist Virginia Berg of the University of Northern Iowa. Roots can't enter the pavement layer unless an opening already exists, but gaps between the poured concrete and the compacted footing often provide a starting point. Any tiny crack in the structure admits the first feeder roots only a few cells wide.

Poor Maintenance

• Cured concrete absorbs water, and water in the pavement's pores could freeze and create the first weathering damage. Sealing the concrete keeps the pavement interior dry and resistant to freezing damage. A sealed concrete sidewalk that starts out with a firm footing and correct construction should last for many years if properly maintained. Treating an icy sidewalk with salt during the winter melts ice but allows saltwater to penetrate unsealed concrete. In later weather cycles, salt crystals embedded in the concrete absorb water, creating ice pockets inside the sidewalk. Expanding ice shatters the concrete surface. Hairline cracks allow ice wedges to slowly crumble the walkway.