Sidewalk cracks: which kinds matter and what each points to

The short answer

Most early, random cracks in a concrete sidewalk are not a structural problem in themselves. The National Ready Mixed Concrete Association (NRMCA) says all concrete tends to crack and that completely crack-free concrete cannot be produced. It says most random cracks that appear at an early age, although unsightly, rarely affect the structural integrity or the service life of the concrete.

Three kinds do need attention: a crack with a step across it, a panel broken into pieces, and a crack whose direction points to movement in the soil underneath. The table on this page sorts what you see into where it points. The repair methods are explained on the pages linked below.

Why concrete cracks at all

NRMCA explains that concrete expands and contracts with moisture and temperature, and deflects under load. Cracks can occur where the design and construction did not allow for that movement. It says the majority come from improper design and construction practice, such as poor jointing, poor subgrade preparation, concrete that is too wet, poor finishing, rapid drying, and inadequate curing.

Joints are the planned answer. NRMCA calls them "simply pre-planned cracks," and the Federal Highway Administration's (FHWA) sidewalk maintenance guide says any length of concrete will eventually crack, so methods are used to control it. NRMCA adds that wire mesh and reinforcement cannot prevent cracking, although well-placed reinforcement can reduce crack width.

Surface cracks that are mostly about appearance

Crazing. NRMCA defines crazing as a network of fine, random cracks on the surface, caused by shrinkage of the paste-rich top layer. It says these cracks are rarely more than 1/8 inch deep, generally show up within the first week after placement, and do not affect the structural integrity of the concrete. The American Concrete Institute's terminology calls them craze cracks, and the glossary carries the definition.

Plastic shrinkage cracks. NRMCA says these appear in fresh concrete soon after it is placed, while it is still plastic. They are usually roughly parallel, about 1 to 3 feet apart, shallow, and generally stop short of the slab's edges. It says they rarely impair strength, but can let in aggressive chemicals that affect durability and can become weak points where later cracking starts.

Cracks that grow. NRMCA also says that as concrete dries over time it shrinks further, so more cracks may form and existing cracks may widen. A crack that was hairline last year and is open now is worth a note in the walkway inspection checklist.

Cracks with a step across them

A crack becomes a trip-hazard question when one side sits higher than the other. The American Concrete Institute calls this faulting: displacement of a slab along a joint or crack. FHWA says heaving of concrete panels is a common problem that causes a significant number of tripping hazards.

For measurement, the 2010 ADA Standards for Accessible Design, as published by the U.S. Access Board, set three steps in section 303:

• 303.2: changes in level up to 1/4 inch high may be vertical.
• 303.3: changes from 1/4 inch to 1/2 inch must be beveled with a slope not steeper than 1:2.
• 303.4: changes greater than 1/2 inch must be ramped, and comply with section 405 or 406.

Those figures do not tell you whether the Standards govern a particular walkway, or whether a crack meets them. If someone has already reported the spot, follow the trip-hazard response plan before choosing a repair.

Which repair fits a stepped crack depends on which side moved and whether the slab is whole:

• The high side is stable and the slab is sound. FHWA rates grinding and horizontal cutting most suitable as a permanent repair for displacements of a quarter inch to a half inch, and as a temporary repair from a half inch to 1 inch. See sidewalk grinding.
• The low side has sunk. FHWA says mudjacking only works on sunken sidewalks, and that the cause of the settlement should be found and dealt with first. The National Concrete Pavement Technology Center's guide, written for highway pavement, says the ideal slab jacking project has localized settlement and is generally free of cracking. A sunken panel with a crack through it is a question for the site visit. See concrete leveling.
• The panel is in pieces. See the next section.

Panels broken into pieces

FHWA's section on crack repairs addresses this. It says sealants, however strong their bond, will not hold two sidewalk segments together; they only keep water out of the void. It says that if cracks are creating a tripping hazard, or soon will, replacement is the better and necessary option. It also says many sidewalk failures are caused by poor subgrade or tree roots, and that a replacement that does not address them will have a shortened life. See sidewalk replacement.

Cracks that point to the soil underneath

FHWA says that apart from cracking and gaps, which can be just a surface condition, the problems it lists are structural, the result of deformation forces. A slab can rise, sink or tilt because of expansive native soil, freeze and thaw, thermal expansion of the concrete, or uneven compaction of the subgrade. Raised panels are also commonly caused by tree roots. When a whole slab drops below its original grade, FHWA says the forces behind it are more accurately explained by shrinkage of the soil underneath as it loses moisture. The guide to why a sidewalk panel sinks sets out the causes of a sunken panel and the clues each one leaves.

The direction of a crack is a clue:

• Along the walkway. FHWA says longitudinal cracks usually run in the middle third of the sidewalk and can extend through several joints. It links them to uneven movement, including clay soils that swell more at the edges or more at the center.
• Across the walkway. FHWA says transverse cracks come from uneven subgrade compaction, especially where vehicles cross the sidewalk at a driveway.
• Fine hairline cracks running side by side. FHWA calls these alligator cracking: typically fine, longitudinal hairline cracks running parallel to each other, with few or no connecting cracks. It says that if alligator cracking comes from poor sub-base drainage or serious structural damage, sealing is not a good option.

NRMCA names one freeze-thaw crack of its own. D-cracks come from freeze-thaw damage to some porous aggregates; they start at joints at the bottom of exterior slabs and typically appear later in a slab's life. The sources opened for this page do not say how common any of these causes are in North Texas.

Open cracks with no step

A crack can be wide without being uneven. FHWA treats sealing as prevention only. It rates sealing most suitable for cracks a quarter inch or wider but less than a half inch, and least suitable for cracks of more than a half inch. It says agencies rarely use crack sealing on sidewalks, and that at best it buys time and defers more costly repairs.

FHWA also describes how some public programs treat a cracked sidewalk with no tripping hazard. It would not trigger a spot repair, and it would be picked up in a rotating replacement program instead.

What a city may use

Cities that run repair or cost-share programs write their own criteria. Allen's Sidewalk Replacement Program, for example, lists criteria that include surface spalling a quarter inch deep or more covering 50% or more of the area, sections heaved or depressed by an abrupt change in grade of "40% or more," or creating an unsafe condition as the City's Director of Community Services determines, and sections broken with pieces missing. The City's page writes the grade figure with "[2 inches vertical in 10 inches horizontal]" beside it, which works out to 20%, not 40%, so ask the City which figure it applies. Those criteria are for its cost share on owner-occupied homes, not a rule for every walkway. The Allen page covers what the City says owners must do.

Who must repair a sidewalk along the street depends on the city. The Dallas guide and the Fort Worth guide quote each city's own code. Check with the city that the address is in before you hire anyone or wait for a city crew.

Next steps

Walk the property with the inspection checklist and record each crack's direction, width, and any step across it. Then use the method comparison to sort the locations, and the inquiry preparation guide to gather what a contractor needs. When you are ready, ask about your project.