For information on falsework (to support the concrete until it has achieved sufficient strength) for in situ construction see main entry Falsework and subsequent sub-headings.
Special care must be taken when concrete is cast in winter. If the water in the young concrete is allowed to freeze, the concrete will be damaged to such an extent that it is unfit for use. Even if the temperature does not drop below zero, the concrete will not gain strength as quickly as it would during the warmer months.
Curing is the process of preventing the loss of moisture from the young concrete whilst maintaining a satisfactory temperature regime. The purpose of minimising moisture loss is to achieving a high level of hydration of the cement in the surface layer of the concrete and thus improving durability. As soon as possible, the top surface of the freshly-cast concrete should be covered, for example with a layer of polythene, to prevent drying out. An alternative approach is to cover the surface with wet hessian, but steps must be taken to ensure that this in turn, does not dry out and tend to remove moisture from the concrete. What ever form of protection is used, care must be taken to make sure that the surface of the concrete is not marked
Post-tensioning is the process by which tendons (either steel strands or high tensile steel rods) are placed through preformed holes on the concrete and tensioned, once the concrete has gained sufficient strength. Construction requires the installation of some form void former, at the required location, rigidly fixed so that it does not move during the concreting process. Generally this is a permanent liner to the void (either steel or polyethylene) though various forms of removable void former are available.
Horizontal slip-forming is a method of constructing continuous concrete elements, such as kerbs, drainage channels and safety barriers. It involves the extrusion of the concrete in situ rather than casting it in short lengths. (The principles are the same as for vertical slip-forming, a technique for the rapid construction of tall concrete structures).
The structural components (columns, beams, slabs, walls, cladding) in industrial and commercial buildings, car parks and other applications may be cast in situ or are obtainable as precast units from a number of manufacturers.
This section relates to commercial and industrial insitu concrete building construction.
It is often not possible to complete a job at one go, for example because of the size or complexity of the structure or because of limited materials or manpower. When work resumes it will be necessary to place fresh concrete on or against the previous pour, that will have already hardened. The resultant contact surface is known as a construction joint or daywork joint. Such joints must be formed carefully so that they transmit the required forces and, where appropriate, do not leak. Firstly the surface of the hardened concrete must be clean and free from laitance (the relatively soft surface layer) and any dust or debris. Then the fresh concrete must be place and compacted so that it bonds properly to the surface of the previously place concrete.
It is often not possible to complete a job at one go, for example because of the size or complexity of the structure or because of limited materials or manpower. When work resumes it will be necessary to place fresh concrete on or against the previous pour, that will have already hardened. The resultant contact surface is known as a construction joint or daywork joint. Such joints must be formed carefully so that they transmit the required forces and, where appropriate, do not leak. Firstly the surface of the hardened concrete must be clean and free from laitance (the relatively soft surface layer) and any dust or debris. If it is too hard, hand-held tools or abrasive blasting will be necessary. (Note that the use of hand-held tools should be limited for Health and Safety reasons.) Care must be taken to avoid dislodging aggregate. Then the fresh concrete must be place and compacted so that it bonds properly to the surface of the previously place concrete.
There are many situations in which a connection is required between a precast concrete element and in situ concrete. A typical example is the connection between precast concrete bridge beams and the in situ concrete deck slab (see also Civil engineering/bridges/precast bridge beams). So that the precast and in situ concretes act compositely, it is necessary to ensure an adequate transfer of load (mainly in shear) between the two. This is achieved by the surface of the precast concrete being roughened (generally as one stage of the production process) and having some reinforcing steel projecting through the interface. (The latter prevents separation of the surfaces as well as carrying some of the shear load.) Guidance on the surface preparation and the amount of reinforcement required for a particular application is given in the appropriate design standards.