Concrete floors can be the main floor in slab-on-grade construction or may be a rough, unfinished floor in a basement or crawlspace area. There will be concrete floors below most raised or finished flooring in basement areas as well.
What are some causes of cracking in concrete flooring?
Most concrete floors crack as a result of shrinkage during curing. Floors may settle as a result of inadequate support from the substrate below or from excessive loads. We also learned in an earlier blog that floors may heave as a result of frost below the floor or hydrostatic pressure from a high water-table. Water or efflorescence coming up through the floor indicates hydrostatic pressure below. This means the area below the slab is saturated with water and is under some pressure. A floor that slopes away from a floor drain is an original installation issue. Floors that are hollow below may be the result of poor original construction or sub-slab erosion.
What do the cracks mean?
The implications of shrinkage cracks are usually not significant. Water and or efflorescence may appear at the cracks if there is hydrostatic pressure below. Settled or heaved slabs may indicate structural problems or local problems of little significance. The extent of movement is the best clue as to the severity. Settled and heaved slabs may also be a trip hazard. Water and efflorescence may result in damage to finishes and to the structure. Again, it is a question of the extent and amount of water. Floors that slope away from drains suffer more damage to finishes and structures when floors get wet. Floor drains are typically only found in areas with below-grade concrete floors, such as basements. Hollows below concrete floors may also indicate serious erosion and structural problems.
What do I do about these issues?
Where floors show typical random shrinkage cracking, no action is typically necessary. Where cracks are accompanied by settlement or heaving, the location and direction of the cracks may be important. Does the pattern suggest a sinking foundation or heaving column, for example? Sometimes it can be hard to know if we should be alarmist or reassuring in cases where more than typical movement is suspected. In most cases, it is impossible to be conclusive about whether the movement is ongoing based on a one-time visit. The extent of movement and the age of the building are valuable clues. For example, a ½-inch crack caused by movement in a 1-year-old house is far more likely to be significant than in a 100-year old house. In these situations, we may recommend monitoring the situation if the problem is mild or explain further investigation is warranted if the movement is extensive.
We use a similar approach for evidence of water and/or efflorescence at cracks. Remedial actions may include a sump pump. Hollow spaces below floors should be treated mush like settled or heaved floors. You can’t usually be conclusive about the side and severity of voids below floors. We won’t know whether there is a progressive erosion or movement of the soil below, or whether it’s simply an original construction condition that won’t get any worse. In these situations, further investigation of extensive hollow spaces is warranted.