Shrinkage cracks in poured concrete are easily recognizable and can be distinguished from other types of cracks that occurlater in the life of a foundation wall or floor slab. The photograph of cracks above were taken of shrinkage cracks in a concrete slab floor in a home built in 2006.The cracks in this case ranged in width (measured across the crack) from 'hairline' (less than 1/16') to about3/32' in the basement floor slab of this particular home. They may appear larger. What is unique about shrinkage cracks in concrete is that they usually appear to be discontinuous, as shown inthis photo. The crack will meander along in the concrete, taper to a stop, and then continue beginning in a parallelline to the first crack, meandering again through the concrete. This is characteristic of concrete (or mud) shrinkingwhile giving up its moisture. You can see the shrinkage of even a perfect concrete floor slab with no visible cracks in the field of its surface if the floor was poured inside of an existingfoundation. Look for the gap between the edges of the slab and the foundation wall? Look also for the stains or concrete debris on thewall at the slab level? These confirm that at the time the slab was poured it was touching the wall. |
The photo at left shows some concrete shrinkage cracks that are larger than usual. Note that these cracks begin at building foundation wall inside projection corners - a condition that probably creates stress points as the slab cures. If you click on and enlarge the photo you can see clear evidence that the cracks are discontinuous, multiple, and very roughly parallel in this area - good evidence that they were caused during the concrete curing process and not by a later event which 'broke' the slab. When we see combined slab shrinkage and slab settlement condition we suspect that the concrete pour not only allowed for excessive shrinkage, or perhaps shrinkage without control joints, but also the pour was made on top of poorly prepared soils. For example if a slab is poured on poorly-compacted soil, after shrinkage cracking occurs, we may see uneven settlement among sections of the cracked slab. Settlement cracking following shrinkage cracking may also occur. In both of these cases we anticipate that significant horizontal dislocation in a poured concrete slab should be small unless steel reinforcement was omitted or was improperly installed. Since some contractors use a fiber-reinforced cement and may omit steel reinforcement in floor slabs, this condition may occur. |
Settlement cracks in a conventional concrete floor slab which has been poured inside a separate foundation wall (and oftenresting at its edges on the building's foundation wall footings) are usually not connected to the foundation wall and are not supportingany structure [except possibly Lally columns, discussed next]. These cracks may not be a structural concern, but there are caseswhere a serious hazard can be present, such as garage floor cracking when the floor was poured over soft, loose, inadequately-compactedfill and where the floor slab was not pinned to the garage foundation walls. Soil settlement under a garage floor, perhaps aggravatedby groundwater which can increase soil settlement, can lead to first hollowing-out of space below the floor and second, suddencollapse of the floor structure. We have used a heavy chain, dragging it across the garage floor and listening to changes inthe sound it produces, to find areas of significant soil voids below the floor. The pitch of the chain noise drops significantlywhen passing over a void below the concrete.] |
Frost heaves or expansive soils damage to building floor slabs can range from minor to extensivein buildings depending on soil and weather conditions, site preparation, and slab construction details, as weelaborate here. The photograph above shows a rather straight crack across a garage slab near the garage entry door.What is happening here and why is this particular crack straight if it's a frost or soil heave crack? In freezing climates building foundations include a footing which extends below the frost line. Thisis true for both the occupied space as well as garages. When a concrete slab is poured either abuttingthe top of such a foundation, or poured extending over the edges of such a foundation, there isrisk of cracking across the concrete at the interior edge of the buried footing. |
Basement and garage floor random heave and crack patterns: Cracked and heaved concrete or settled concrete can occur in more random patterns in any concrete floor where there has been frost heaving, soil contraction/expansion, or simple soil settlement, as shown in this photograph. |