Types of Cracks in Concrete and Typical Causes

Introduction

Cracks can be broadly classified as either active or dormant. Active cracks show some change in direction, width or depth over a measured period of time while dormant cracks remain unchanged. If left unrepaired, both active and dormant cracks provide channels for moisture penetration, which can lead to future damage. For guidance on patching dormant cracks, see “Repairing Cracks in Concrete by Injecting Epoxy Resin”.

References

• “Identifying Cracks in Slabs”. Concrete Construction. Web. July 31, 2014.

• Interval, V. “Concrete Cracks: An Overview of Types of Cracking/Deterioration and Their Implications.” Wikispaces Classroom. Web.

• Portland Cement Association. “IS-177 Concrete Slab Surface Defects- Causes, Prevention, Repair”. Concrete Information. 2001.

Types of Cracks

The severity of a crack can be characterized in terms of its direction, width, and depth; cracks may be longitudinal, transverse, vertical, diagonal or random. Different risks for cracking exist for cured versus uncured concrete, and for reinforced concrete. Breakages occur through thermal, chemical or mechanical processes causing shrinkage, expansion or flexural stress. Below is a list of types of concrete cracks, and some of their possible causes:

1. Plastic-shrinkage cracking: Cracks that run to the mid-depth of the concrete, are distributed across the surface unevenly, and are usually short in length.

   • Most often occurs while concrete is curing, due to the surface of the concrete drying too rapidly relative to the concrete below.

2. Crazing/Map cracking/Checking: A web of fine, shallow cracks across the surface of the concrete.

   • Also occur during curing due to the surface of concrete drying faster than the interior concrete, but the surface drying occurs at a lesser depth.

   • Because this type of cracking is limited to the surface, it does not usually pose serious structural problems.

3. Hairline cracking: Very thin but deep cracks.

   • Due to settlement of the concrete while it is curing.

   • Due to their depth, these cracks can allow for more serious cracking once the concrete is hardened.

4. Pop-Outs: Conical depressions in the concrete surface

   • Occurs when a piece of aggregate near the concrete surface is particularly absorbent, causing it to expand and pop out of the surface of the concrete.

5. Scaling: Small pock marks in the concrete surface, exposing aggregate underneath.

   • Once cured, if concrete does have an adequate finish to prevent water penetration, water that seeps into the concrete will expand when it freezes, pushing off pieces of the concrete surface.

   • Scaling can also be caused by delamination, which occurs when too much water (due to insufficient curing) or air (due to insufficient vibrating) remains in the concrete when it is finished. The water and air rise to the top and form pockets below the surface. These pockets may form blisters or which may break open to create scaling.

6. Spalling: Surface depressions that are larger and deeper than scaling, often linear when following the length of a rebar.

   • Also caused by pressure from under the surface of the concrete.

   • Most often occur due to improperly constructed joints or the corrosion of rebar in the concrete

   • Corrosion creates pressure as rust forms, which can push away large chunks of concrete, and expose the corroded metal below.

   • Spalling that exposes corroded metal can be particularly problematic because the corrosion is likely to accelerate due to exposure to air and water.

7. D-Cracking: Cracks that runs roughly parallel or stem from a concrete joint and are deeper than surface cracks.

   • Due to moisture infiltration at the joint.

8. Offset cracking: Cracks where the concrete on one side of the crack is lower than the concrete on the other side.

   • Due to uneven surfaces below the concrete, such as subgrade settlement or pressure from objects such as tree roots, previously-placed concrete, or rebar.

9. Diagonal corner cracking: Cracks that run from one joint to its perpendicular joint at the corner of a slab

   • The corners of concrete slabs can be prone to curling(due to differences in temperature at different depths in the curing concrete) or warping (due to differences in moisture evaporation at different depths in the curing concrete). The dryer or colder level of concrete will shrink more and create cracks as the concrete dries.

   • Because the warped or curled-up corners often have some empty space below them, they are also prone to cracking after curing due to weight overload causing the corner to snap downward into the empty space.