How to Identify & Evaluate Shrinkage Cracks in Concrete Floors Along Foundation Walls
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Shrinkage cracks in poured concrete are easily recognizable and can be distinguished from other types of cracks that occur later in the life of a foundation wall or floor slab.

What is unique about shrinkage cracks in concrete is that when they occur in the field of a poured concrete wall or floor, they usually appear to be discontinuous, as shown at Shrinkage Cracks in Slabs. The crack will meander along in the concrete, taper to a stop, and then continue beginning in a parallel line to the first crack, meandering again through the concrete. This is characteristic of concrete (or mud) shrinking while giving up its moisture.

How to Identify Slab Shrinkage Along a Building Wall

Look at the photo at the top of this page and again here at left. You can see a small gap, about 3/16" wide, which runs along the entire floor slab where it meets the building foundation wall. The gap is rather constant in width, and if you look closely you can see concrete remains on the foundation wall where the floor was touching the wall at the time it was poured.

A poured concrete slab shrinks away from its perimeter - the building's foundation walls. You will see this shrinkage of even a perfect concrete floor slab with no visible cracks elsewhere in the field of its surface if the floor was poured inside of an existing foundation. Look for the gap between the edges of the slab and the foundation wall? Look also for the stains or concrete debris on the wall at the slab level? These confirm that at the time the slab was poured it was touching the wall.

The crack shown at page top is a normal condition where a poured concrete slab is installed.

Serious Floor Slab Settlement May be Indicated by Examining Floor shrinkage Cracks at the Foundation Walls

While shrinkage crack openings between a floor slab and its foundation wall are normal, there is at least one case in which you can detect important floor slab settlement in this location. In the photo above the slab has pulled away from the foundation wall. But the original concrete debris left sticking to the foundation wall is at the same height as the top of the floor slab itself. This slab was probably poured on top of the foundation wall footings, and the chance that the floor would have settled down in this location is slim. The only difference in height between the original wall/slab contact point and the cured slab would be the slight shrinkage in the thickness of the slab itself.

So when will we see a settlement problem in a concrete slab at a foundation wall? Some concrete slabs are poured atop deep backfill inside of a foundation. This is particularly common when an attached garage is built adjacent to a house on a sloped lot. When the rear of the garage (opposite the entry door) is over a hill that sloped down away from the garage entry, the builder had to place extra fill inside the garage foundation before pouring the slab.

Conditions Leading to Floor Slab Collapse

As I can testify from my first construction job (raking fill dirt level inside of foundation walls in Dam Neck Virginia in 1963), the builder often fails to compact the fill-soil. Pouring a slab atop of soft fill can lead to serious slab settlement, settlement cracking, and in extreme cases, a dangerous collapse of the floor slab. A floor slab can collapse under these conditions:

1. The floor slab or portions of it were poured on significant amounts of poorly compacted fill-soil
2. The floor slab was poured on fill over the foundation wall footings, and the slab was not pinned to the foundation wall
3. The floor slab is not resting on the foundation wall footings throughout all of its perimeter
4. Water from roof or surface runoff has passed under the floor slab, adding to soil compaction there
5. The floor slab was poured without sufficient (or any) reinforcing steel
6. The weight of vehicles contributes to cracking of the slab

A case was reported to us of a car actually falling through the garage floor slab.

Common Signs Warning of a Floor Slab Collapse

Slab push out foundation cracking: Our photo at above left shows how a concrete garage slab has pushed over the foundation wall easily seen from outside.

Wavy mortar foundation cracking: Our photo at above right shows the same condition detected in a home's basement. But the right-hand photo has another nice clue. The un-broken but wavy mortar in the joint between the concert blocks tells us that this wall was pushed inwards at the time of new construction, before the mortar had hardened. Perhaps if the builder had waited for her block foundation wall to cure before pouring the garage slab (or had reinforced the block wall) this crack would not have occurred.

1. Significant downwards movement of some cracked, uneven slab sections, shown by comparing the present location of the slab upper surface with the concrete marks left along the foundation walls when the slab was originally poured.
   
   
2. Significantly cracked and uneven floor slabs over an area where several feet of fill would have been placed inside the foundation walls before pouring the slab
   
   
3. Horizontal cracks, bulging, or leaning visible along the outside of a concrete block foundation wall near the height of the floor slab (caused by lateral pressure of earth loading as the slab sinks downwards, pushing the soil out). (As in the two photographs shown above.)

Tip for detecting voids in fill under a concrete slab: Try dragging a heavy chain over the floor of a garage or in any location where you suspect the slab was poured over deep fill. The sound of the chain will change significantly if it passes over a void in the slab.

1. Water entry: Shrinkage cracks in a concrete slab or floor wherever they occur, including along a foundation wall might need to be repaired to avoid water leakage from below the slab. Of course you should also be taking other steps to direct water away from the building as well since sealing a floor crack is a "last-ditch" band aid effort to address water entry and it's likely to fail in the long run.
   
   
2. Radon entry: Shrinkage cracks in a concrete slab or floor wherever they occur, including along a foundation wall might need to be repaired to stop radon gas from entering the building.