What Is A Cracked Slab?

Slang term or technical term

It is a slang term that seems to rest on the assumption that, if there is a crack in a slab foundation, the foundation has failed and foundation repair is mandatory. If you assume that it is a technical term, you are making a mistake. It is rare to see the term “cracked slab” used by a Professional Engineer. From an engineers perspective, all concrete cracks, so the term “cracked slab” is redundant.

Why all concrete cracks

The reality is this: any slab of concrete larger than 100 square feet will develop cracks for a number of reasons.

As concrete cures, it shrinks. Separations develop wherever where the rates of shrinkage differ. An example is where the grade and stiffening beams are. These beams are a minimum of 24-inches deep while the field areas are only 4-inches thick. The field areas cure much faster than the beams. 

If any sewer plumbing penetrates the slab there is likely to be a separation because the PVC plastic sewer pipe does not shrink while the slab concrete does. 

Cracks develop as the slab cures and shrinks. When the slab shrinks it has to slide on the plastic vapor barrier due to friction. The friction force is proportional to the distance the slab moves. Once the friction force exceeds the tensile capacity of the concrete, a crack will develop. This type of crack actually has a name: restraint-to-shortening.

The larger aggregate will create concrete cracks because rocks do not shrink like concrete.  

What is aggregate interlock?

Aggregate interlock is where the aggregate, meaning sand and rock, overlay each other so they are “locked” in place. The fact that the aggregate is interlocked means that the concrete cannot fail in shear without breaking the aggregate.   

What is vertical displacement?

The term vertical displacement refers to a situation where the concrete surface is visually at a higher elevation on one side of the crack than the other side. If there is vertical displacement then some degree of shear loss has probably occurred.

You rarely see vertical displacement in a slab foundation. It is common to see it in flatwork – specifically driveways and sidewalks. The reason is that flatwork usually has little or no steel reinforcement.   

Is the term “cracked slab” a scare tactic?

I have no doubt that many pier salesmen use the term cracked slab precisely because the average person equates cracks with failure.

Here is a test 

If a pier salesman intimates that your slab foundation has failed because it is cracked, ask these questions:

  1. If a crack in the concrete means my foundation has failed and needs repair, will the crack still be there after the repair job is finished?
  2. If he or she answers yes, then you know you are dealing with a less than honest person. If he or she include says: no, then by their own admission, the foundation still needs to be “repaired.”The bottom line: Cracks are a performance indicator, not a repair indicator.

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