Concrete Basement Construction
Joseph M. Zulovich
Department of Agricultural Engineering
Concrete is the single most popular material used to construct residential basements in Missouri. A properly constructed concrete basement will provide adequate support for the home above it and can be used to economically expand the family living area. A good concrete foundation does not crack or leak water. This publication outlines recommended practices for proper construction of a concrete basement.
Concrete is a mixture of Portland cement, sand and gravel (often called aggregate). Portland cement is the glue that causes the mixture to harden or consolidate into a durable construction material. This hardening is caused by a chemical reaction between the cement and the water used in mixing the concrete.
The most important single factor in determining the quality of a particular concrete mix is the ratio between water and cement. The more water used, the sloppier, weaker and more apt to leak the resulting concrete will be. The recommended ratio of water to cement is 6 gallons of water per sack (one sack = 1 cubic foot) of cement. Concrete mixed with 6 gallons of water per sack will be 40 percent stronger than the same mix made with 8 gallons per sack.
Concrete for your basement will probably be delivered to the site by a local ready-mix company in a truck-mounted mixer unit. When you call for concrete, the supplier will need to know when you want the concrete delivered, how much you need, and what type of mix you want.
Do not call for concrete delivery until your form work is completed and ready to be filled. Concrete can only be held for two to three hours after mixing before it starts to harden. Also, you will be charged extra for wait time if the truck is not unloaded within a reasonable time after delivery. (Reasonable usually means about 6 minutes per cubic yard.)
Concrete is sold by the cubic yard. Carefully estimate how many cubic yards you will need to fill your form work, then add about one-fourth of a yard to make sure you do not run short. One cubic yard of concrete will pour 80 square feet of a 4-inch-thick floor, 40 square feet of an 8-inch-thick wall, or about 30 linear feet of 8 x 16-inch footing.
A satisfactory mix for basement walls, footings and floors is a six-sack mix containing 6 gallons of water per sack and a maximum aggregate size of 1 inch.
Basement walls should be a minimum of 8 feet high. This will provide adequate head room if you wish to use the space as a living area. Homes with conventional wood, metal or plastic siding usually have 8-inch-thick basement walls. Homes with brick veneer require 10- to 12-inch-thick walls to provide a supporting ledge for the brick.
Footings provide a level foundation for setting wall forms and pouring basement walls. They should bear on undisturbed soil below normal frost penetration level. On walk-out basements, footings have to be extended to a deeper than normal depth on the walk-out or exposed side. Footings are usually constructed at a thickness equal to wall thickness and a width equal to twice the wall thickness.
Concrete has excellent strength for bearing but very little strength in tension or bending. This means that walls will frequently crack due to pressures exerted by the soil around them. To prevent this, reinforcing steel is placed in the concrete to provide added strength. The exact amount, size and placement of reinforcing steel depends on the particular characteristics of the soil in your area. Table 1 lists the minimum reinforcing steel required for 8-foot-high basement walls.
Minimum reinforcing steel required for 8-foot-high basement walls
1Standard bar size is specified in 1/8-inch increments (a #3 bar is 3/8-inch in diameter).
Placement of the steel is illustrated in Figure 1. These minimum steel requirements are adequate for most Missouri locations.
Components of a well-constructed basement
Reinforcing steel represents a relatively small portion of the total basement cost. However, a lack of reinforcing steel can lead to reduced performance and major leakage problems throughout the life of the home.
No matter how well the basement is constructed, it will leak if the groundwater table outside is too high. A drain tile located at the base of the wall prevents the water level outside of the home from building up, causing pressure on the wall and then leakage. In areas that are normally wet, a second drain tile is sometimes placed under the basement floor on the inside wall.
Drain tiles should be 4 inches in diameter and slope to a screened outlet downhill from the house. If this is not possible, pitch the tile to a sump located in the basement and install an automatic pump to remove accumulated water.
Tiles should be covered with a 12-inch-deep layer of clean gravel so that water flows freely into the tile. Cover the gravel with a layer of building paper or hay to keep dirt from sifting into it when the walls are backfilled.
A liquid asphalt compound is usually applied to exterior walls below the ground level to seal up small openings in the concrete surface. The best results are achieved with sprayed-on hot compounds. However, brushed on cold materials can do a satisfactory job.
Rubber sheet materials and special clay sealing compounds are available for exterior treatment of basement walls when extreme moisture conditions exist.
Current energy costs have made it economical to insulate basement walls. To do this at construction time, apply a layer of expanded polystyrene foam insulation to the exterior. (Do not use bead-type foam board for this purpose because it is not water-tight.) In some cases, polystyrene can be placed directly in the concrete forms before pouring the concrete. This is an excellent practice because concrete bonds very well with polystyrene. The insulation should be 2 inches thick at a depth of 2 feet below the normal backfill height. A thickness of 1 inch should be used on the lower portion of the wall.
You will need to protect expanded polystyrene from mechanical damage on exposed portions of the wall. This can be done by plastering it with a standard mortar mix or by covering it with a layer of pressure-treated plywood.
Do not use asphalt compounds on walls covered with polystyrene. The solvents in the asphalt often dissolve the polystyrene foam.
Replace the soil around the outside of the basement in 12-inch-thick layers. Be careful not to push large rocks against the concrete or insulation. The final layer of backfill should be graded or sloped away from the foundation so that surface water is carried away rapidly.
Many builders prefer to do backfilling after the house subflooring is in place. This provides extra strength in case too much earth is pushed against the wall too rapidly.
Place a 2-inch-thick layer of aggregate between the wall footings below the floor. This practice provides for an easily leveled surface and a medium through which water can flow if drainage is needed.
A 4-inch thickness of concrete is adequate for residential basements unless some unusually high loading is anticipated. Use the same mixture as for the walls and footings.
Floors can be reinforced with #4 bars spaced 18 inches apart in both directions or with 6 x 6-inch welded-wire reinforcing mesh.