Precast concrete elements are often made in a controlled environment and later transported to the job site. In general, is the process as follows:
Concrete elements can either be casted in horizontal (most used) or vertical forms. In horizontal casted elements the border of the form is put in place often on top of a formwork table also called a concrete bed. Wooden borders or flamingo cubes are put in place to shape the concrete structure like we want (space for windows, doors or similar).
For wall (often for sandwich walls with insulation made for building fronts) can have special requirements for a surface design to obtain a certain architectural design, a unique profile in a wooden material or similar can be placed on the surface of the bed (will be the front surface of the element in the end) to shape it according to the specifications.
The rebar is often manufactured internally, either by hand or by a robot in small production for itself. The rebar is manufactured according to the specifications for each concrete element.
The prestressed elements are manufactured by pulling out wires between the two ends of the table/lane where the concrete is gonna be casted. The casting lanes/clamping tables are usually from 50 meters up to around 160 meters long. One very long concrete element is then casted and later cut in the required sizes.
Products often made with prestressing:
The manufactured rebars are placed in the form and connect in the joints either with a tread or a welded joint. Other parts are also now placed in the form to secure space for windows, doors or maybe electric installations
For normal slap reinforced concrete is the concrete often poured on top from a container lifted by a crane and afterwards vibrated by handheld devices or bed/table controlled vibrators. The vibration secured all concrete to be evenly distributed and compact in all corners and areas of the concrete element. It is important not to vibrate too much to avoid areas with an increased ratio of aggregates.
The surface will be smoothened with some kind of tree or metal frame after the vibration. After some hours the concrete surface will often be polished (glitting), to make a nice even and smooth finish.
Over the past 20-30 years the use of SCC (self-compacting concrete) has been in rapid growth and is normally what is used in most regions when it comes to the casting of normal concrete elements today. What is important when using SCC concrete is to avoid heavy vibration, as it can damage the mix.
For sandwich, elements are manufactured very similar to the normal slap reinforced elements, other than the concrete is casted in two stages. First, is the front part casted, then insulation is attached to the back of the first casted part, and then the back part of the sandwich is casted on top of the insulation.
For prestressed elements, it is a bit different. An extruder pours a very dry concrete directly into its final shape / no requirement for forms on the side on top of the prestressed cables.