We always hear contractors say: “concrete just cracks by itself and there is nothing you can do about it”.  That statement is false.  The fact is that concrete does not crack until you give it a specific reason to crack.  Every crack tells a story.  Sometimes the moral of the story is obvious, like Aesop’s Fables.  Sometimes additional investigation is required. In some cases, cracking is only an aesthetic concern.  In other cases, the cracking may be a prelude to complete collapse of the structure.

The purpose of structural concrete is to transfer loads from one location to another.  We can start at the top, and work our way down.  Any problems we find will be located where the loads are coming from, or the structure in which loads are transferred, or where the loads are transferred to.

The following can be used as a checklist of items to consider when concrete is cracked:

A complete investigation will consider the possibility that the direction and/or the magnitude of loads was not determined correctly. This could include load combinations of shear, bending, compression, tension, and torque. Every type of structure has specific design standards for determining the correct loading. You will need a Structural Engineer to determine whether there was a problem with the correct determination of loads.
The loads are transferred within the concrete structure itself, including the reinforcing embedded in the concrete, if any. There are a large number of causes of defects within concrete. Examples include using the wrong type of concrete, the wrong curing procedure, insufficient cross-sectional area, insufficient joints, the wrong type or amount of reinforcing steel, and the incorrect placement of steel within the structure. Every possible situation which can go wrong within the concrete structure is covered by industry standards. If you see a crack in concrete, it is usually because somebody ignored the applicable standards.
The concrete structure uses a foundation to transfer loads into some other place, such as the soil under the structure. Foundations can include piles, slabs, footings, or other configurations. Every soil and foundation type has industry-standard methods for determining the allowable loading. There should never be any experimentation in the design of foundations for habitable structures. They should always be designed with a suitable factor of safety. The soil can be compressed within the load path or displaced to the side of the load path. The foundation must be designed to limit the forces which are placed on subsurface soils. Soils can also move for reasons which are not related to any structures. However, the concrete within those structures can only resist a certain amount of movement before structural problems develop. You may need a Geotechnical Engineer or Geologist if you suspect there is a problem with over-loading of the soil, or excessive soil movement of any type.

Try This:

The damage you can see right now is rarely the end result. You should immediately document the existing conditions, before additional damage is done.  Obtain photographs from all possible angles, along with something to provide scale, such as a millimeter scale or coin next to the cracks.  Add reference marks so that follow-up photos can be obtained with the exact same framing in subsequent photos.  The next location where the concrete is going to crack will probably be an extension of an existing crack.  Use a permanent marker to make a tiny but unique mark just beyond the end of the current cracking. Photograph the unique mark with the camera in macro mode and the date displayed.

As is the case with the majority of foundation and slab failures, the most proximate cause is usually human error.  An investigation may reveal the responsible person or entity, and the industry-standard guidance they chose to ignore. You may have noticed that none of this has anything to do with the age of the structure.

Resources for Further Research

Almost everything we need to know about the design and installation of concrete structures is already known.  The following examples of standards for concrete design provide a good idea of the type of information which is available. However, it should be noted that you will need the assistance of a Structural Engineer or perhaps other professionals if your situation requires further interpretation of these standards:

  • American Concrete Institute (ACI) Quality Management System for Concrete Construction (ACI 121R).
  • American Concrete Institute (ACI) Guide to Durable Concrete (ACI 201.2R).
  • American Concrete Institute (ACI) Causes, Evaluation, and Repair of Cracks in Concrete Structures (ACI 224.1R).
  • American Concrete Institute (ACI) Specification for Testing Ready-Mixed Concrete (ACI 311.6).
  • American Concrete Institute (ACI) Building Code Requirements for Structural Concrete (ACI 318).
  • Portland Cement Concrete (PCA) Diagnosis and Control of Alkali-Aggregate Reactions in Concrete (ISBN 0-89312-146-0).
  • US Army Corps of Engineers EM 1110-2-2002 Evaluation and Repair of Concrete Structures.