Why does concrete crack?
Concrete shrinks as it dries and cures. On average, a concrete slab shrinks 1/16th inch for every ten linear feet. This may not seem much, but what this shrinkage does is produce significant internal stress within the slab.
This stress or force is considered a tension-type force as the concrete is trying to pull itself apart much as you pull on two ends of a piece of newspaper.
Pull hard enough and the paper tears.
How Foundation Cracks Develop
Because of the way concrete shrinks, cracks in poured concrete walls can be found in almost all residential foundations. Most often, they occur in corners, from corners of windows, pour lines, holes for service lines or rod ties. Apart from the natural shrinkage process other causes for cracking includes:
- improper concrete mix (high water content),
- rapid concrete curing,
- improper expansion control joints,
- premature removal of concrete forms (2 days instead of 7),
- compressible soil,
- premature backfilling,
- improper compaction of fill soil,
- expansive clay soils,
- uneven moisture around the foundation,
- plumbing leaks,
- poor drainage,
- hydrostatic water pressure.
In the same way all concrete shrinks, all houses settle. A regular 3-bedroom single family home weighs around 60-70 tons. Because of this and variations in soil properties, not every point on a foundation settles uniformly, which may cause racking doorframes and cracking walls. Good construction prevents differential settlement and minimizes the overall settlement.
Over time, even minor cracks can grow larger and cause big headaches, such as loss of structural integrity
Types of Foundation Cracks
Hairline cracks typically develop within the first month and most foundations cracks appear within the first year. In most cases, the common wall cracks pose no structural concerns of foundation failure. The biggest problem they cause is water leakage.
In a poured concrete foundation wall, the typical foundation crack will run vertically or at an angle. These cracks are normally caused by either shrinkage or settlement.
Such non-structural foundation cracks can be easily repaired by polyurethane foam or elastic epoxy resin injection.
Clay soils can be highly expansive, and in the Niagara Peninsula and Golden Horseshoe clay makes up most of the soil substrate below the top soil. As this type of soil gets repeatedly wet and dry, it expands and shrinks, exerting lateral pressure on the foundation walls, and vertical pressure on basement and garage slabs, paths and driveways etc.
Horizontal cracks along the midpoint of the wall indicates the wall is subjected to lateral pressure, which could bow and structurally damage the wall.
Cracks more than 1/4-inch wide, horizontal cracks in walls, cracks with misaligned edges or continuing movement may require professional assessment.
You can monitor the crack’s length and movement by marking off the ends of the crack and making several “alignment marks” across the crack at various points.
Foundation repair contractors can fix failed foundations:
- Slab jacking involves pumping a cement grout through small holes in the concrete slab
- Piering involves strategically placed mechanical jacks for lifting the settled beam to grade.
When to use Crack Injection?
Crack injection is designed for cracks in poured concrete walls. It does not work on concrete block walls because of their hollow cores. It is usually applied from the inside, which avoids digging on the outside of the walls, but an alternative repair method would be to excavate and patch the foundation crack on the exterior. This is more expensive as it involves excavation, but if the crack is also a by-product of poor water management at the foundation wall this can be rectified at the same time.
It is suitable for hairline cracks, as well as wide
cracks (up to ½”).
Crack injection works by filling the entire length and depth of the crack. It permanently stops water leaks and also, prevents water from getting into the concrete and deteriorating it further. No drilling is needed.
Our standard injection kits use polyurethane, which forcefully expands to fill the entire crack and even a void on the outside. If structural strength is definitely required, use our epoxy crack injection kits, which is more exacting. Epoxy does not expand and tends to run out of the back of the crack. A ¼” wide crack will need twice as much epoxy as a 1/8” crack.
When to use an elastic epoxy joint sealant & crack filler?
Epoxy’s strength and its bond to concrete are stronger than the concrete itself. An epoxy crack filler does not necessarily fill the full depth of the crack but provides a strong surface for heavy traffic. Narrow cracks have to be routed out first with a grinder before pouring in the epoxy filler. Common uses:
- Cracks in concrete block walls: – mix with sand for troweling into the cracks.
- Wide cracks in concrete slabs, first fill partially with sand.
- Cracks and spalls in factory or warehouse floors, driveways or garages – stronger than patching.
How NOT to Repair Foundation Cracks
The typical homeowner will try to fix concrete cracks with caulk. But this is only a superficial repair. Water will fill the inside of the crack and cause efflorescence, which will eventually loosen the caulk. In a couple of years, the caulk will start peeling.
Disappointed, the homeowner will pull off the caulk, chisel out the crack, and fill it with hydraulic cement. But hydraulic cement does not bond well to concrete – it needs an inverted V-groove to hold it in. And it is very rigid. As the concrete continuously moves, shrinks and expands, the rigid “rod” will get loose. Efflorescence (“white powder”) will start coming through around it, soon followed by drops of water. After a couple years, water will start seeping around it.