placing foundation wall concrete
When placing foundation wall concrete, the first consideration is access. Often due to excavations and site logistics, concrete walls happen to be in locations difficult for the concrete truck’s chute to reach. If this is the case, the placement operation will often require a concrete pump truck or concrete conveyor.
Before the concrete is placed in the wall, it’s important for the workers to double check that the walls are square, straight, and plumb. Also the top, middle, and bottom of the wall should be braced with ‘kickers’ which is often lumber kicked back at a 45 degree angle to the ground or a weighted object. This resists the hydrostatic and lateral pressure of the concrete. Studs, walers, and strongbacks are for wall alignment only and are not sufficient by themselves in resisting the pressure of wet concrete. The wall ties help resist the forms from blowing out, but also are not in themselves enough to resist the strength of wet concrete. Concrete is a very heavy building material weighing approximately 4,000lbs per cubic yard, wall bracing is critical.
As the concrete is placed inside the wall forms, vertical drops more than 4-6 feet should be hindered by a funnel sock attached to the chute. This will reduce the possibility of segregation of the aggregate, sand, and cement paste.
If the amount of concrete to be poured is more than what a standard concrete ready mix can hold (9-10 cubic yards) it is important to have enough continuous concrete trucks scheduled. This will allow all of the walls concrete to be placed with similar consistency and temperature. If one lift of concrete placed in a wall sets for too long before the next lift is placed, a ‘cold joint’ can develop, which can exhibit discoloration and a wall that is not entirely monolithic.
As the lifts of concrete are placed in the wall, it is common (particularly for architectural or exposed walls) to vibrate the concrete for each lift which aids in a smoother wall surface and reduces ‘honeycombing’ once the walls are stripped. Also if there is rebar in the wall, the vibrator helps the concrete form consistently around the deformed surface of the rebar which is a critical part of the structural integrity of the concrete/rebar combination. The tool used for vibrating the concrete is commonly referred to as a pencil vibrator and is comprised of an electric motor and a cord leading to a pencil shaped metal end piece which does the vibration work.
Once the concrete has reached the top of the wall forms it is struck or screeded off then floated with a darby to provide a smooth finish. A jointer tool may be used along the edge of the wall to provide a smooth and neat edge. The top of the wall is sometimes troweled to remove ridges, valleys, burrs, or voids. It is important for the top of the wall to be level and true as it is the basis for the material that will soon be placed on it, whether it’s a wood plate, block, or brick. The problem of a non-level concrete foundation wall will only compound as other materials are built on top of it.