Foundations made of poured concrete are the most common ones found in residential construction in the United States.
Concrete has become the material of choice for foundations because it is relatively low cost, strong and durable. Although pouring a foundation is a skill that requires knowledge and experience, it is not as labor-intensive as masonry and doesn’t have to be done by masons.
The only limitation to the use of poured concrete for foundations is proximity to a concrete plant because premixed concrete can only be kept in a liquid state for so long before use. Areas without access to concrete manufacturing still rely on masonry for residential foundations.
To understand how concrete foundations undergo damage, it helps to know how they are constructed.
The first step in building a concrete foundation is to dig a hole. An excavation contractor scoops a giant bowl out of the earth at the building site that is the depth of the foundation and approximately ten feet wider all around.
The foundation contractor then sets forms around the perimeter of the foundation-to-be and pours the footings -- wide, shallow slabs that define the foundation’s shape and size. These footings sit on the undisturbed soil left in place by the excavator and serve as a both a stable platform for the foundation walls and a way to spread the weight of the foundation (and the house) across a wider surface.
Once the footings have cured, the contractor sets forms for the foundation walls with appropriate blocking for doors, windows, beams and other openings. These forms are filled with concrete, which is agitated by heavy-duty vibrating cables during the pour to ensure that any voids or air pockets are filled. The concrete is leveled at the top and hardware is embedded to secure the first level of aboveground construction.
When the concrete walls have cured, the forms are stripped and the soil around the outside of the foundation is backfilled. Construction on the home can then begin.
Even with the strength and durability of poured concrete and the pains taken during its construction, a poured concrete foundation is still vulnerable to damage, which may result in water seeping into the basement.
We have written extensively in an earlier article about the types of seepage problems and how they occur, so here’s a quick summary:
Poured concrete walls can seep water through non-structural cracks in the wall, through the cove joint between floor and wall, over the top of the foundation wall or through patches of porous concrete.
Pressure from the exterior is the culprit in most of these instances of damage because oversaturated soil outside the foundation can cause non-structural cracks that lead to seepage and cause water to enter through the cove joint.
Conversely, the withdrawal of moisture from soil below the foundation, such as during times of drought when trees and shrubs extend root systems in search of water, can cause supporting soil to collapse and the foundation to drop, sink or settle. This will cause seepage problems at the least and may also result in more serious structural damage.
Seepage through or over the wall can be caused by dry spots in the concrete wall that occurred during construction or by improper grading or poorly planned outdoor construction that allows water to seep in over the top of the foundation wall.
So, when these problems occur, what’s the best way to repair them?
Crack Repair – The most common source of seepage in a poured concrete foundation is a non-structural crack in a basement wall. These cracks can appear anywhere in a wall although they often emanate from an opening such as a window, door or utility penetration. They are characterized by their narrow width (less than 1/8”) and the fact that they do not appear in any sort of pattern.
Although it is possible for a single crack to occur, quite often offsetting pressures will create others that may have to be sought out.
Seeping cracks in poured concrete walls can be fixed either on the inside or the outside, with the interior injecion method generally preferred.
To fix a crack on the interior a basement waterproofing professional first cleans the crack of loose aggregate and other debris. He then places tubular plastic injection ports at intervals along the length of the crack and covers them and the crack with a sealer coat of epoxy to hold the ports in place and create a confined space.
He next injects the crack, starting with the uppermost port and working downward, with expanding polyurethane. This material expands to fill the crack completely and even extrude through to the outside, creating a permanent seal that remains flexible to prevent re-opening of the crack from minor foundation movement.
The epoxy and injection ports can be left in place or removed later by the homeowner.
Fixing a crack on the exterior starts with digging a small-diameter hole down to the footings at the site of the crack. The installer fills the hole with granular sodium bentonite clay, which absorbs water from the soil to form a flexible barrier against water on the outside of the wall. When the top of the excavation is backfilled, the repair is invisible.
Fixing Cove Seepage – When the foundation walls are poured on top of the footings, a very small gap exists between the two because wet concrete does not bond to cured concrete. When hydrostatic pressure exists under the foundation, water can be forced through this joint and then through an equally small gap between the wall and the basement floor, creating what is known as cove seepage.
The best permanent way to fix cove seepage is to install interior drain tile, which alleviates the hydrostatic pressure and carries ground water off to a sump pump.
To install interior drain tile, the technician first removes a foot-wide strip of the basement floor around the perimeter and digs out a trench in the soil below down to the base of the footings. A base of washed gravel is then poured in and leveled.
Despite the “tile” in its name, interior drain tile is actually corrugated plastic pipe with a series of perforations that admit water. This pipe is flexible and can be installed in long, continuous lengths and curved smoothly around corners when it is laid on top of the gravel bed. Typically, this pipe is encased in a “sock” of filtration fabric. Both ends are connected to the sump basin.
When the pipe is in, more washed gravel is added on top and leveled and then the concrete floor is replaced. The drain tile requires no maintenance and should function without interruption.
Repairing Wall Seepage— When water enters the basement through or over the wall, the homeowner has the option of either managing it or stopping it.
To manage wall seepage, interior drain tile is again required and is installed the same way up to the very last step. When replacing the concrete floor, the installer leaves a small gap between the edge of the floor and the wall, allowing access to the drain tile. This gap is covered by a plastic cove molding that keeps out debris and allows water to flow.
The homeowner may also choose to install a vapor barrier on the wall to cover the seepage and reduce dampness and humidity.
A permanent fix to wall seepage is done by installing an exterior waterproofing membrane. To do so, the affected wall must be excavated down to the footings, leaving a trench wide enough for the installer to work. Usually, this digging is done by hand but can be done with machinery when conditions permit.
The installer first cleans the foundation wall of dirt, loose concrete and other debris. Then, a thick coating of asphalt-modified polyurethane is applied to the wall with a trowel to ensure complete coverage. This material cures to form an impermeable membrane that holds up to underground pressures and exposure to soil.
Once cured, the membrane may be covered with heavy-duty plastic drainage board that will help protect the membrane and channel water downward. In situations with very high ground water, exterior drain tile may be added to ensure a dry basement. Exterior drain tile is very similar in function to the interior variety except that perforated PVC pipe is used instead of flexible corrugated to better withstand exterior soil movement and pressure. The drain tile is still routed to a sump pump to discharge water away from the house.
Insulating material may also be applied over the membrane to help keep the basement warmer in cold climates. The excavation is backfilled and the installation is complete.
Regardless of the source of water and the recommended method of repair, a homeowner who needs to fix a seepage problem will require the advice and assistance of a professional basement waterproofing contractor. At U.S. Waterproofing, our basement waterproofing experts have injected miles of cracks, applied acres of exterior waterproofing membranes and installed enough drain tile, both interior and exterior, to reach across several states. Why not ask for our free advice?
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