Concrete Foundation Construction
A concrete foundation used to support a structure in an area where the ground freezes. A footing is required below the frost line and then poured concrete walls are added on top. The footing is wider than the wall, providing extra support at the base of the foundation. A T-shaped foundation is placed and allowed to cure; second, the walls are constructed; and finally, the slab is poured between the walls.
In Ontario a basement foundation is typically built by excavating down to around 8 feet, then constructing a floor and walls for the resulting space. First, concrete footings are poured to support the walls. Next, the walls, usually made of poured concrete, are erected. Finally the concrete slab floor is poured. The result is an additional room you can use for storage area or to extend your living space.
Types of Concrete
Regular concrete for construction is produced by following the mixing instructions that are commonly published on packets of cement, typically using sand or other common material as the aggregate, and often mixed in improvised containers. The ingredients in any particular mix depends on the nature of the application. Regular concrete can typically withstand a pressure from about 10 MPa (1450 psi) to 40 MPa (5800 psi), with lighter duty uses such as blinding concrete having a much lower MPa rating than structural concrete. Many types of pre-mixed concrete are available which include powdered cement mixed with an aggregate, needing only water.
Typically, a batch of concrete can be made by using 1 part Portland cement, 2 parts dry sand, 3 parts dry stone, 1/2 part water. The parts are in terms of weight – not volume. For example, 1-cubic-foot (0.028 m3) of concrete would be made using 22 lb (10.0 kg) cement, 10 lb (4.5 kg) water, 41 lb (19 kg) dry sand, 70 lb (32 kg) dry stone (1/2″ to 3/4″ stone). This would make 1-cubic-foot (0.028 m3) of concrete and would weigh about 143 lb (65 kg). The sand should be mortar or brick sand (washed and filtered if possible) and the stone should be washed if possible. Organic materials (leaves, twigs, etc.) should be removed from the sand and stone to ensure the highest strength.
Types of Foundation Cracks
Cracks in your concrete foundation come in many shapes and sizes. It is not unusual for foundation cracks to form within the first year of new construction, often near windows, corners of the walls or floors or in the longer sections of your foundation. Most are due to foundation settling and are usually not a cause for concern. If cracks become larger of water is seeping through, then it is time to call an expert right away.
- Horizontal cracks in a masonry unit (block or brick) present a higher risk of collapse than vertical cracks in the same walls, depending on the amount of dislocation. Common causes uses of horizontal foundation cracks include damage from heavy equipment near the wall, frost heave, hydraulic pressure from wet soils or excessive back filling. If you have small horizontal cracks on a straight wall that is not bowing yet, it is important to closely monitor those cracks to see if the damage worsens.
- Vertical cracks usually indicate footing settlement and may also need repair but usually present less risk of sudden catastrophic collapse unless the amount and nature of movement have separated structural connectors or supports such as posts or column footings. Vertical foundation cracks are very common and can be seen in most households. They can occur due to concrete tension within the first few years of construction.
- Diagonal cracks usually cause more concern than vertical cracks because they indicate differential settlement that can lead to serious structural issues in your home. When parts of your home settle more quickly than others, or sections of your home settle while other sections remain stable, it can cause uneven (or differential) settlement.
- Note: Vertical cracks which are found on both sides of a home could indicate a broken footing. To repair this problem requires excavating down to footing which is an expensive repair.
Shrinkage cracks are usually uniform in width or (less common) vee-shaped, wider at top and diminishing or stopping before reaching the bottom of the foundation wall (where attachment to footing may tend to hold foundation wall materials in place). A wall crack which continues into the floor is likely to involve the building footings and may be a settlement crack of more structural importance.
Concrete shrinks as it cures. In poured concrete, shrinkage cracks may be non-uniform if wall components are held by footings/framing; very often there are minor shrinkage cracks which are hairline, random, intermittent, multiple, and meandering in the concrete, forming discontinuous cracks in the wall.
Shrinkage cracks occur as concrete cures, appearing more frequently and larger if the mix was improper and where control joints were omitted. Omission or pattern of placement of steel reinforcement may also be a factor in crack formation and location.
Poured concrete shrinkage cracks: usually shrinkage cracking is due to conditions at original construction: poor mix, rapid curing, possibly other conditions. Shrinkage cracks are less likely to require structural monitoring and repair in poured concrete as they would be expected to continue after initial curing.
Concrete block foundation walls shrink as they cure. They rarely expand much on exposure to moisture and temperature variations. In concrete block walls shrinkage cracks are likely to be uniform in width and usually occur towards the center of a concrete masonry unit (CMU) wall. The wall is stronger at the building corners.
Water Entry in Concrete
POURED CONCRETE WALLS typically leak along the joint between the floor and the wall. High hydrostatic pressure outside the foundation can cause water to seep through solid concrete walls. It will also force water into the basement along the crack between the floor and the walls. Cracks that form in walls also provide pathways for water to enter the basement.
In poured concrete foundations, cracks and pipe penetrations are also common entry points for seepage. These openings can be sealed from the interior. Epoxies, which are strong adhesives, or urethanes can be pressure injected into the openings, thus penetrating the foundation through to the exterior and cutting off the path of the seepage.
In masonry foundations, interior sealers will not provide permanent protection from water infiltration where hydrostatic pressure is present. However, interior sealers are good for preventing high atmospheric humidity inside the basement from absorbing into the porous masonry and causing spalling. Spalling is a condition where constant high humidity or moisture breaks down masonry surfaces, causing deterioration and shedding of the concrete surfaces.
Most water entry problems into basements can be controlled by ensuring roof drainage water and surface water is drained away from your homes foundation. You must have a minimum five-degree slope away from your home, on all sides. Measure six feet away from the foundation, the ground should be three inches lower than the ground next to the house.
Efflorescence is the white chalky powder that you might find on the surface of a concrete or brick wall. It can be a cosmetic issue, or it can be an indication of moisture intrusion that could lead to major structural and indoor air quality issues. A home inspector should understand what efflorescence is in order to recognize potential moisture problems.
Efflorescence (which means “to flower out” in French) is the dissolved salts deposited on the surface of a porous material (such as concrete or brick) that are visible after the evaporation of the water in which it was transported. The moisture that creates efflorescence often comes from groundwater, but rainwater can also be the source. Efflorescence alone does not pose a major problem, but it can be an indication of moisture intrusion, which may compromise the structural material.
Foundation Inspection Experience
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