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• How to distinguish settlement cracks vs. shrinkage cracks in concrete slabs
• Photographs of types of poured concrete slab cracks

This article describes How to distinguish Settlement Cracks vs. Shrinkage Cracks in concrete slabs - a division of our article on How to Identify and Evaluate Settlement Cracks in Slabs in Poured Concrete Slabs or in concrete floors in basements, crawl spaces, or garages.

This website 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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Settlement Cracks Compared With Shrinkage Cracks in Poured Concrete Slabs

What about the cracks in these two photographs? They had been filled, and looked pretty significant. At first we thought it might be a settlement problem, but that was probably wrong. We think these are shrinkage cracks too, from a really bad pour, probably with way too much water in the mix.

First: look at the left-hand photo - that's a close up of some of the cracks in this floor where they were not filled-in. This is a classic discontinuous concrete shrinkage crack pattern.

Second: look at the right-hand photo. It's hard to see, but there was no obvious difference in the slope nor level of the concrete throughout the basement. If we were looking at settlement rather than shrinkage, or settlement combined with shrinkage cracks in the concrete, there ought to be some vertical dislocation or sloping among the broken up sections of slab.

It is useful to keep in mind that diagnosing slab or any other building cracks or movement can be tricky since there may be more than one contributor to the problem.

These cracks may not be a structural concern, but there are cases where a serious hazard can be present, such as garage floor cracking when the floor was poured over soft, loose, inadequately-compacted fill and where the floor slab was not pinned to the garage foundation walls.

Soil settlement under a garage floor, perhaps aggravated by groundwater which can increase soil settlement, can lead to first hollowing-out of space below the floor and second, sudden collapse of the floor structure.

How to find voids and settlement under a concrete slab

It sounded a bit goofy when we heard this, but we tried it and it works: we use a heavy chain, dragging it across the garage floor and listening to changes in the sound it produces, to find areas of significant soil voids below the floor. The pitch of the chain noise drops significantly when passing over a void below the concrete.

There are more sophisticated instruments that can also penetrate slabs and the earth to check for various conditions, and of course, before a professional slab-jacking firm starts pumping grout or inserting pins to lift a settled slab, the consultant will explore the soils below the slab by drilling through it (or perhaps other means).

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