BASEMENT WALL CRACKS
|Horizontal, vertical, and diagonal basement wall cracks may be a structural issue, a leaking issue or both, as well as costly to repair. Therefore, it is important to know which cracks are major and which are minor.
The seriousness partially depends on factors like the size of the crack, the location, the shape and direction of the crack. As well for concrete block walls if the blocks have shifted.
THREE TYPES OF BASEMENT WALLS
It is not unusual for cracks to appear in basement walls, regardless of the type you have.
Poured in place concrete walls – these are the most common type of basement walls. They are constructed by pouring concrete into metal or wood forms and then the forms are removed once the concrete has set-up or hardened.
Masonry block walls – built with rectangular block often called concrete block or cinder block. This type of wall is the second most common wall used.
Brick or stone – these types of walls are often found in historic and older homes. Due to the age and construction designs of these walls, they tend to have more cracks and leaks than other walls.
CAUSES OF WALL CRACKS
The majority of basement wall cracks occur because of the following reasons:
Lateral loads and pressures pushing against the basement wall.
Foundation movement resulting from soil or other issues.
Hydrostatic pressures involving wet soils, high water tables and other water issues.
The “Shrinkage” process of the concrete, especially when first poured.
Shrinkage cracks are very common, especially in poured in place concrete walls. They’re a thin, hairline type of crack. They normally do not run continuously or in straight lines but tend to meander and have interruptions in a number of places.
When concrete (basically a mixture of sand, aggregate, cement and water) is first mixed together, more water is added than is required for hydration, the chemical curing process, often thought of as the hardening process. This extra water dissipates or evaporates causing stress in the concrete which causes the concrete to shrink. Thus, shrinkage cracks.
Concrete poured when the temperature is cool will have less shrinkage cracks than concrete poured when it is hot; and overall the more water added when first mixed, the more shrinkage cracking there will be. Approximately a 1/3 of the shrinkage cracks occur in the first 10 days, 1/2 in the first month and the majority will occur within a year.
Engineers generally feel that shrinkage cracks are more cosmetic than structural in nature.
Vertical cracks in a basement wall may be caused by several common problems.
There are two types of vertical cracks that are generally not considered structural issues. They are:
Cold joints and Shrinkage cracks.
Cold joints in a basement wall are not structural cracks
Homeowners often think that a cold joint, which at first glance may look like a crack but it is not. A cold joint is where one batch of concrete is poured and has begun to set up or has hardened and then later another batch of concrete is poured against the previous batch.
The time between the two pours may be several hours, a day, or even weeks or months. Contractors often plan cold joints at specific locations in a concrete wall or slab.
In basement walls cold joints have a propensity to leak unless proper waterproofing has been done
As a general rule horizontal cracks are not caused by settling issues or the footing heaving upward but by issues that create lateral pressure on a basement wall.
Five basic causes of horizontal cracks:
Back filling soil right after the basement has been poured
Loading – weight of equipment or vehicles near the basement wall
Soil loading issues
Moisture loading and hydrostatic pressure
The weight of soil pressing against a basement wall may cause horizontal cracking at times. Evidence of this being an issue would be that the wall is bowed or tilting inwardly. In the case of soil loading, the greatest pressure on a wall is generally in the bottom third area of the wall. Should the home be on a hillside, the uphill side will frequently have more lateral pressure than the downhill side.
Hydrostatic pressure / Water loading
Soil heavily laden with water may exert significant pressure on a basement wall. Not only is hydrostatic pressure one of the main causes for a basement to leak, but it is one of the key factors in creating lateral pressure. High water tables and poor drainage are two of the main culprits; also, sewer and water line leaks may likewise contribute or cause hydrostatic pressure.
Homes that have clay types of soils against their basement wall will often experience significant cracking when the soil gets wet; for these soils expand in volume when they are wet and place tremendous lateral pressures on a basement wall.
Horizontal cracks due to Frost Heaving
Our climate conditions cause the soils next to a basement walls to experience freezing, at times this soil may freeze and thaw several times in a year. Soils that are wet or saturated with water will expand significantly when they freeze.
This freezing process exerts tremendous pressures on basement walls, so much that they may crack, tilt or bow and generally when they crack, it will be a horizontal crack.
Block wall horizontal cracks
Horizontal cracks in a block (masonry) basement wall are common and are caused by many of the same issues and forces as those of a poured in place concrete wall.
Horizontal cracks or bowing are considered structural cracks as the structure has shifted.
Block wall movement or offset usually occurs near the bottom of the wall, this is a more severe structural concern.
Diagonal and stair stepped cracks
Diagonal cracks in a poured in place wall may be more related to shifting soils, settlement, expansive soils or even footing compaction issues. Most are minor and can be injected to prevent leakage.
Stepped cracks in block walls are very common. If the gaps are large or the blocks have shifted, this again is a structural issue.
OUTSIDE CORNER FOUNDATION CRACKS
Most houses with brick veneer have triangular shaped cracks on both sides of at least one corner of the foundation wall. Occasionally the concrete corner will pop off. This is caused when the brick veneer expands and the concrete foundation below contracts, which is normal. Typically there is not a need for engineer review.
BASEMENT AND GARAGE SLAB FLOOR CRACKS
When concrete cures (dries and hardens) it shrinks and wants to crack; this is why you see control joints on sidewalks to provide weak spots where the concrete can crack without affecting the aesthetics, strength or safety. No repairs are needed for the floor cracks.
TYPE OF REPAIRS
Most common repair today is crack injection in poured concrete walls.Average cost is $800 for a crack.
Membrane Installation A membrane can be installed on the exterior but today it is most common to fix leaky block or stone foundations with an interior weeping tile system. The concrete floor is cut out around the perimeter and weeping tile installed. A membrane is installed against the wall and run below to the weeper. The concrete floor is re-poured and the weeper is run to a floor drain or a sump pump.Average cost is $5000 per wall
Bowed block wall straightening
This process requires a series of steel plates to be installed in the ground 10-15 feet from the foundation. A threaded rod is run through steel plates on the inside of the foundation concrete block wall. The nuts on the threaded rod are tightened and the wall is slowly pulled back into a straight position.
Another method to straighten the bowed wall is to install a series of jack posts that are anchored to the floor and slowly tightened so that they push the bowed wall outward to a straight position.
Stabilizing Block Walls
After the wall is straightened or if there are minor horizontal cracks, the block wall should be stabilized. This can be achieved by a method of installing kevlar straps with an epoxy. This process is extremely effective to eliminate any further movement of the wall and can be the selling feature needed if the house has horizontal cracks.
Article used with permission, from Allan Spisak, ACISS
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