concrete footers
A concrete footer is a building element required to transfer the live and dead loads of a building or structure down to the soils below. Based on soil conditions and/or the engineer’s specifications, a concrete footer may be earth formed or installed with fixed-forms. Footer installation can be one of the dirtier and messier building tasks as it takes place often in the bottom of a hole or trench, surrounded by, dirt, mud, buried debris, and often accumulated underground water. Footers must be designed wide enough, thick enough, strong enough, and with enough reinforcing to resist the loads for the life of the building above. Concrete itself is a material high in compressive strength but low in tensile strength, making it more susceptible to shear cracking, breaking, and failure under loading. For this reason, horizontal and vertical steel reinforcement bars are placed inside the concrete, combining tensile and compressive strength making a structural mass far superior than either element alone.
Also, the type of soil on which a footer rests will play into the design as well. Sandy and gravely soils tend to have higher density and compactibility which makes them more suitable for resisting building loads under a footer. Loose and unstable clays can become problematic for concrete footings and may require removal and replacement with engineered fills, aggregate, or controlled density fill. In colder climates, the footing must be placed below the frost line (the max depth frost will creep below grade in the cold season). This will guarantee no heaving or foundation movement will occur with frequent freezing and thawing. Concrete for a footer is often placed directly from the chute of the concrete truck or through a funnel sock attached to the chute. This funnel sock breaks the fall of the concrete which keeps segregation of the mixtures from occurring. Once rough placed the top of the footer is often screeded, floated, and sprayed with curing compound. Because there are typically no aesthetic requirements to concrete footers, the surface requires little or no finishing, only a straight and level surface free of ridges, valleys, and burrs.
AFter footing placement, foundation walls can typically be formed and poured as early as the next day, but depending on the mix and weather conditions it may take several days before the wall above can be built. As a general rule, it is expected that a concrete mix will reach 25% of its design strength in 24 hours, 50% in 3 days, 75% in 7days, and 100% at 28 days. Using this as a general guideline, and after calculating the weight of concrete and rough wall elements, one can usually be fairly certain a footer is ready after 1-3 days to start receiving loads from the next stages of construction.
A more viable concern however is backfilling of the foundation walls, which must reach sufficient strength to resist the lateral and hydrostatic pressure of backfilled earth and ground moisture. It’s important to note that all foundations settle a small amount, but when differential settlement exists (different parts of the foundation settle different amounts) it can become a real problem. This can crack drywall, pierce the thermal/moisture protection of the exterior, and cause doors and windows to fail.
Grade beams are a style of footing/foundation wall combination which utilizes earth or manual formed walls to create a reinforced beam which is carried all the way up to grade.
