The words “concrete” and “cement” are often used interchangeably but they are different, with one being a component of the other.
The most common form of cement is known as Portland cement, which is made by heating various components of calcium, silicon and other materials, often derived from limestone, in a kiln and grinding it into powder. The material formed can be used as mortar or grout but, when mixed with aggregate, a granular material like gravel, it becomes concrete.
Cement and concrete, of a sort, were first discovered by the ancient Romans but the process was lost for some time after the fall of the Roman Empire. It was rediscovered in Europe in the 18th Century and was first introduced to the United States in the mid-19th Century.
Concrete quickly became a preferred building material, particularly for large commercial structures because of its strength and ease of use. Poured concrete began to replace masonry foundations during the 20th Century but really took off during the postwar building boom of the late 1940’s through the early 1960’s.
Homebuilders began using poured concrete widely for both belowground and slab foundations and today, poured concrete is the standard for residential foundation construction in most of the U.S.
Building a poured concrete residential foundation starts with an excavation, digging a bowl in the earth the depth of the foundation and about ten feet wider all around. Footings for the foundation, wide, shallow concrete slabs that describe the perimeter of the foundation, are poured first to sit on solid, undisturbed soil.
After the footings have cured, forms are built and the foundation walls are poured, often using rebar to reinforce corners or other areas likely to experience stress. Hardware is also set into the top of the foundation walls to secure the wooden sill plate as the first step in building the aboveground structure.
When walls have cured and the forms have been removed, the soil is backfilled against the foundation and aboveground construction can begin.
Poured concrete creates a strong, monolithic foundation that can be counted on for stable support of the home and significant resistance to water and movement. It is not, despite its strength, perfect and can suffer problems of both seepage and structural damage.
Poured Concrete Foundation Problems – Seepage Issues
Any foundation can admit water through cracks in the basement floor or through the cove joint because water can be pushed through these openings by hydrostatic pressure. Also, all foundations can suffer seepage over the top of the foundation wall when the yard outside has been improperly graded to leave a negative slope where water runs back toward the house instead of flowing away.
Exterior improvements such as decks or patios that have not been installed properly can also cause water to be held against the foundation or run toward it.
Missing or clogged drains in window wells will also result in seepage in a poured concrete basement.
Two causes of seepage are unique to poured concrete foundation walls:
Wall Cracks – Poured concrete foundation walls are always under pressure from surrounding soil and can move a little or a lot. Oversaturated soil around the foundation that is caused by heavy rain or snowmelt will expand due to the absorption of water. It then pushes against the foundation wall and can cause enough movement to create non-structural cracks in the wall.
Sinking or settling of the foundation can also cause wall cracks.
As mentioned earlier, a poured concrete foundation rests on soil left undisturbed by the original excavation. Depending on the content of the soil, the weight of the structure can cause it to compress and allow the house to move downward, known as settling or sinking.
When drought occurs, plants and trees located close to the foundation will extend their root systems into new areas of soil in search of water. If these roots draw moisture out of the soil beneath the foundation, the soil quickly compacts to fill the voids left by the withdrawal of water. The foundation will then sink to the level of the newly compacted soil and cause cracks to occur in the walls.
The cracks can occur anywhere in the wall and often emanate from windows, doors, utility entrances and other openings in the concrete. When water is present outside the wall, lateral pressure from the saturated soil will force it through the crack and into the basement.
Porous Concrete – Generally speaking, poured concrete, although by nature slightly porous, is highly resistant to water when properly poured and cured. A solid concrete wall should hold ground water outside the basement on its own.
However, sometimes even highly competent foundation contractors will suffer a small lapse in pouring a foundation. Sometimes a “dry spot” will occur in the concrete mixture due to a portion of the concrete being insufficiently mixed. Or, because mechanical vibrators are used to settle concrete mix in the foundation forms, a slight omission may result in a portion of wall that has insufficient cement bonding the aggregate together, leaving patches of porous concrete.
These issues, along with simple ageing of the foundation, don’t create structural problems but can ultimately allow the affected wall to seep water into the basement.
There are, however, other problems that can cause structural damage in a poured concrete foundation.
Poured Concrete Foundation Problems – Structural Damage
Some of the same conditions that can cause seepage in a poured concrete foundation can, if continued or amplified, also create structural damage.
Settlement – The damage caused by a foundation that has dropped, sunk or settled can be extensive and affect the entire home. With the settlement that often occurs in newly built homes, the damage may be slight, often limited to drywall cracks and jammed or sticking doors and windows. When the problem worsens, homeowners may see cracking of exterior stone and brickwork and building elements such as chimneys and additions may begin to separate from the rest of the structure.
Entire sections of the foundation and the home it supports may drop as much as several inches when sinking or settlement occurs, and the damage will extend throughout the building and compromise its structural integrity.
To repair significant structural damage caused by sinking or settling, a contractor must raise the foundation back to level and put systems in place to stabilize it there and ensure the long-term stability of the home.
Lateral Pressure – The same lateral pressure that in limited amounts created seepage in the basement can also cause structural damage if the pressure intensifies or continues for a long period of time.
When oversaturated soil expands and presses against the wall with enough force to create seeping cracks, it’s not a huge leap to its creating larger structural cracks and pushing the wall inward. When this happens to poured concrete walls, cracks will usually appear in a pattern of one vertical crack in the center and two angled cracks across the upper corners. Invisible from inside are two more vertical cracks on the outside corners where the damaged wall has begun to separate from the adjacent ones.
In most cases, the wall will remain anchored at the bottom and begin to tip inward from the top, compromising the structural integrity of the wall and the entire foundation. This displacement is known as “rotation.” Left unrepaired, this wall movement will cause significant damage to the home; repairing it can require either stabilizing the wall in place or completely replacing it.
Fixing these displaced walls requires stabilizing them in place to prevent further movement and ensure the integrity of the structure. In extreme cases, the house may have to be temporarily shored up and the damaged wall demolished and replaced.
Because of the potential for significant damage to the home, planning a repair of structural damage requires the expertise of a skilled engineer to determine the best type of repair and the materials and placement that will do the most to ensure future stability. Foundation repair contractors should rely on this type of engineering data to assess the damages and propose the most effective repair methods and materials.
Although seepage problems in a poured concrete foundation are generally less severe and threatening to a home’s stability than structural damages, both must be taken seriously to avoid bigger problems in the future.
When a homeowner discovers seepage problems in a poured concrete foundation, he or she will need the advice and services of a professional basement waterproofing contractor; those with structural damage to their foundation will need the help of a foundation repair expert. At U.S Waterproofing, we have permanently repaired seepage problems in thousands of poured concrete basements and make use of the best methods of repair on either the interior or exterior of the structure. Our foundation repair team makes use of engineering data and sophisticated materials and techniques to stabilize concrete foundations permanently and cost-effectively. Why not ask U.S. Waterproofing for a free consultation?