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Photograph of a gap between the garage floor slab and garage foundation wall: garage floor shrinkage and settlement (C) Daniel Friedman Concrete Shrinkage Gaps
How to Identify & Evaluate Shrinkage Cracks in Concrete Floors Along Foundation Walls

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A detailed guide to concrete shrinkage cracks:

How to recognize cracks or separation between the concrete floor slab and building foundation walls: concrete floor slab shrinkage, how to evaluate normal concrete slab shrinkage, and how to recognize when cracks along a foundation wall at the floor indicate a problem. This forms part of our longer concrete cracking article which describes the types of cracks that occur in poured concrete slabs or floors and explains the risks associated with each, thus assisting in deciding what types of repair may be needed.

This article series describes how to recognize and diagnose various types of foundation failure or damage, such as foundation cracks, masonry foundation crack patterns, and moving, leaning, bulging, or bowing building foundation walls.

Types of foundation cracks, crack patterns, differences in the meaning of cracks in different foundation materials, site conditions, building history, and other evidence of building movement and damage are described to assist in recognizing foundation defects and to help the inspector separate cosmetic or low-risk conditions from those likely to be important and potentially costly to repair.



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Shrinkage Cracks in Poured Concrete Slab Floors Along the Foundation Walls

Shrinkage cracks in a floor slab (C) Daniel Friedman

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.

Here we discuss two types of poured (placed) concrete shrinkage cracks: meandering fine cracks that appear in the open area of the floor or perhaps a wall (shown at left) , and straight cracks or gaps that appear at the juncture of the poured concrete floor and the foundation wall against which it abuts (shown below).

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 left where I marked the two ends of two shrinkage cracks with my black marker.

The shrinkage crack that occurs in the open field of a poured concrete floor or slab 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. Shrinkage cracks are discussed separately and in excruciating detail at SHRINKAGE CRACKS in SLABS.

Photograph of a basement floor slab crackWatch out: while usually concrete shrinkage cracks are very fine in width, 1/16" or less across, severe shrinkage cracks due to mixing or temperature/curing problems can be much larger - enough to require more extensive repairs.

And cracks can appear in a floor or wall due to a combination of factors: shrinkage, loading, settlement, some of which may be significant or may combine forces to cause actional damage or leaks.

How to Identify Slab Shrinkage Along a Building Wall

Shrinkage cracks that occur at the juncture of slab to wall (and also within control joints) appear differently than their brothers out in the middle of the floor or wall.

Look at the photo just above 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.

Photograph of a basement floor slab crack

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

Concrete floor settlement visible at wall (C) Daniel FriedmanWatch out: while shrinkage crack openings between a floor slab and its foundation (shown above) 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.

The concrete floor slab in the photo above 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.

In our concrete floor settlement photo shown at left the green arrow points to a concrete pour trace left on the building foundation wall - this is where the top of the slab was located when the floor was first placed or poured.

The red arrow in the photo above points to the present top of the same concrete floor slab - it has settled several inches!

Garage floor settlement (C) Daniel Friedman

A different concrete floor settlement example is in the photo at left. Here the garage floor has broken close to the foundation wall, and the slab has dropped more than 1/2". We suspect improper soil compaction under the slab.

Why would part of the garage floor remain attached to the foundation wall while other floor sections break away? Perhaps the slab was pinned to the foundation wall with re-bar.

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.

Watch out: other more varigated concrete crack patterns may be ascribed to FOUNDATION DAMAGE by MATERIAL or INCLUSIONS producing crack patterns in concrete caused by inclusion of iron sulfide (pyrrhotite) particles in the concrete mix.

Conditions Leading to Garage or Building Interior Floor Slab Collapse

As we can testify from our 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

Foundation damage from slab settlement

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.

This damage occurred as the floor slab broke and began to collapse; tipping slab edges actually pushed out a surrounding concrete block foundation wall.

Watch out: because combinations of forces can be at work on a foundation wall that seems "cracked" or damaged in any way, it is possible to have both old movement that occurred at the time of backfill or construction (wavy mortar that is not broken) and more recent ongoing movement and damage from any of a variety of causes. Here are some clues to watch for:

  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.

Do we need to repair shrinkage cracks in slabs along the Foundation Walls?

Shrinkage cracks between a floor slab and the foundation wall do not need to be repaired except in these instances:

  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.

While shrinkage in poured concrete walls or floor slabs is a normal property of curing concrete, shrinkage cracks can be controlled, or where they have occurred, in some cases repairs are needed. In addition to reading about repairing concrete shrinkage cracks (if crack repair is needed at all) at Shrinkage Crack Repairs also see how we prevent shrinkage cracks in poured concrete floors and walls by reading Cracks at Control Joints in Concrete.

For detailed information about foundation repair methods, including repairs to various kinds of cracks in concrete, see the articles listed below.

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Continue reading at CONCRETE SLAB CRACK REPAIR or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.

Or see FOUNDATION DAMAGE by MATERIAL or INCLUSIONS crack patterns in concrete caused by inclusions iron sulfide (pyrrhotite) in the concrete mix.

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SHRINKAGE CRACKS or GAPS at FOUNDATION WALLS at InspectApedia.com - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.

INDEX to RELATED ARTICLES: ARTICLE INDEX to BUILDING STRUCTURES

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