In Greece, concrete is still the main building material for the construction of the load-bearing structure of buildings. The development of formwork during the 20th century paralleled the development of concrete structures.
The purpose of designing and manufacturing molds is to cast the fresh concrete in the form and dimensions required by the study of each project. The forms safely carry all the loads until the concrete has solidified and gained sufficient strength.
The mold is a temporary structure, which is permanently imprinted in the concrete. It is quickly set up, takes heavy loads for a few hours during concreting, and within a few days is removed for future use on another project. Characteristic of the temporary nature of the construction in question are the joints, supports and adjustments needed to hold the various sections together. Molds are used again and again, many times throughout their lifetime. The main parts of a mold are:

• the coating, which is the surface of the mold that comes into direct contact with the concrete (gives the shape and dimensions of the structural element and forms the form of the final surface of the concrete),
• the joints and supports of the decking,
• the supports of the formwork, so that the loads are transferred through them either to the ground or to the scaffolding.

Characteristics of concrete forms
The necessary characteristics that a concrete mold must have, regardless of its construction material, are the following:

• Strength capable of safely taking on loads and applied pressures. In particular, it must be able to undertake:
a. the total vertical loads, both of the concrete (permanent loads) and of the workers (moving loads), until the concrete reaches its strength,
b. the pressure exerted by the liquid concrete on the vertical walls of the mold, due to the hydrostatic pressure, since this pressure is greater in the lower parts of the walls and especially in the molds of thin and tall columns, while the addition of fluidizers causes a delay in the setting of the concrete , resulting in an increase in its hydrostatic pressure; on the contrary, an increase in the temperature of the concrete accelerates its solidification and therefore limits the horizontal thrusts exerted on the walls.
c. the pressure exerted on the formwork during concreting, taking into account that the reduction of the height from which the concrete is injected has the effect of reducing the horizontal thrusts on the walls of the formwork,
d. the pressure created by the vibration of the concrete during its compaction,
e. wind pressure,
f. accidental actions (e.g. earthquake), which may occur during the period when the concrete has not acquired sufficient strength.
• Sufficient suitable supports. In this way, it maintains its original shape and there is no risk of movement, after the injection of the fresh concrete.
• Tight joints. Fresh concrete leaks are thus avoided.
• Ease of detachment of the formwork, if the formwork is detachable, so that its removal is easy and does not cause damage to the concrete and, by extension, to the form and appearance of the structure.
• Ability to check its sufficient resistance to time and its harmless presence in the construction, if the mold remains permanently in it.
• Use of readily available, reusable materials of low cost and as little as possible of the same weight.
• Rigidity against deformation. Its construction material should not bend, even if it is exposed to the elements.
• Use of the concrete construction material without the risk of altering the composition and appearance of the concrete and without the risk of affecting its strength.
Types of molds and methods of application
Molds are divided into two main categories:
• removable, which make up the majority of applied moulds, and
• permanent, which remain in the construction even after the concrete has hardened. Permanent molds are divided into two subcategories:
– Functionally. They are usually made of steel and create in cooperation with the concrete a composite construction (e.g. composite slab).
– Non-functional (sheet metal, expanded polystyrene, cardboard). They remain invisible inside the concrete and serve only to give the concrete the desired shape of the structural element, while reducing the weight of the structure. Depending on the material of their construction, the molds are divided into wooden, metal, plastic and cardboard molds.
Wooden moulds
Wood has been the first choice material for formwork for decades, as it is cheap, light, can be easily cut to size, assembled on site at the project site and removed without damaging the concrete. However, it has a relatively short life (low reuse rate) and requires frequent cleaning of the wood.
In wooden molds, the lining is made either of boards or of non-stick wood plates (commonly plywood). Plywood is preferred as a sheathing over boards when the concrete is to remain exposed. Decking joints and supports can be either wood, i.e. planks (claps) or battens (slats) or intermediate wooden frames (for clamping column moulds) or metal, i.e. keys or clamps (butterflies), for clamping column moulds. 16/21 nails are used to connect a board to boards, 19/45 nails are chosen to connect a board to a board, while steel dowels are necessary for nailing to concrete. Boards or boards or metal beams can be used as supports for the mold. Boards and battens (the usual cross-section of battens is 8 x 8 (cm)) come from white spruce or pine wood, which has been treated in a wood dryer to obtain high strength and stiffness. The thickness of the boards is always 2.5 cm, while their width varies between 8 and 20 cm and their length between 2 and 5 m. Anti-glued wood panels (plywood) come from the welding of several layers of wood sheets with a transverse direction between two superimposed layers (for greater strength). The number of these sheets is usually an odd number so that the fibers of the outer sheets have the same direction.
Points of caution on how to apply to avoid failures are summarized as follows:

– Decking boards (when plywood is not chosen as decking) must be thoroughly wetted to saturation before concreting so that the concrete mixing water is not absorbed by the wood.
– Sometimes it may be necessary to install joint caps, to seal the molds.
– The plywood of the lining must be reinforced with a waterproofing membrane (special waterproofing coating).
– To facilitate the removal of a formwork, whether it is made of wood, metal or plastic, the inner surface of the formwork can be coated with special chemicals (especially protective oils), which reduce the adhesion between the concrete and the material of the mould. These are colorless substances, which do not create stains and alterations and do not react with concrete. However, these substances then make it difficult for the coating to adhere well to the concrete.
Metal molds
Standardized (industrialized) molds can be made from steel, aluminum and zinc sheets. These metal molds are called metal forms and are applied to foundations, columns (rectangular or square up to 80 x 80 (cm)), walls, beams and slabs. The load-bearing frame is metal, being made of either steel or aluminum, while the skin (the surface that comes into contact with the concrete) is usually plywood. The joining and support of the various elements of these molds is done with screws, bolts, bolts and wedges. The positive characteristics of metal molds are the simplified and fast way of assembling and disassembling the mold elements, the quick installation (which, however, requires the use of a crane due to their high uniform weight), increased strength and rigidity against deformation. Steel molds can be used about 100 to 150 times. It is particularly noted to avoid failures that, if the coating is metal, it must be electrostatically painted, so that the remains of the concrete can be removed more easily by simple washing.
Cardboard molds
A standard mold, mainly used for concreting cylindrical columns, is made of cardboard and is called a paper form. The positives of this type of molds are that they are transported and placed particularly easily and quickly due to their very low weight and create a smooth concrete surface. The advantage over traditional wooden molds is that the use of nails and the cleaning of the wood is avoided. Their dimensions are 3 m long and the diameter 15 – 150 cm. They are made of fully recyclable cardboard, which is waterproof so that the concrete dries properly.
In order to avoid failures, care should be taken during its application not to injure the inside of the formwork by the reinforcement, in which case there is a risk that the cardboard will not withstand the pressure of the fresh concrete during concreting. To avoid this risk, expanded polystyrene is usually placed temporarily at the edges of the rebar until the mold is placed around the perimeter of the rebar.Also, the craftsman who places the mold must keep it in a vertical position as much as possible, so as not to be scratched by the rebar irons. Placing the formwork first and then the reinforcement is not preferred, because then a crane is required to lift the reinforcement.
Plastic molds
These are standardized molds of many dimensions (plastic molds), which can be made from sheets of reinforced plastics. These molds have a relatively long life, are durable, light (11 kg is the maximum piece weight), easy and quick to install, and clean with water under low pressure. Plastic molds are used for the construction of classical slabs, trabecular slabs (ribbed or Zöllner type), walls and columns. In the case of trabecular panels, standardized reusable plastic boat-shaped molds are used, which are nailed one-by-one onto the usually prepared coating, in order to form the characteristic panels. In the case of vertical structural elements, standard plastic sheets are used (there is a wide variety of cross-sectional dimensions of columns), which are connected to each other and covered around the perimeter by a plastic or metal frame, to ensure their rigidity against deformation. The lining is made of either plastic, plywood or metal.
Particular attention must be paid to the method of application to avoid failures, so that the covering thickness of the panel of the plastic forms does not exceed 15 cm and to ensure that they remain in place during concreting. Particular attention should be paid to the fact that plastic forms, like to a large extent those made of plywood, create smooth surfaces on the concrete after their removal and not rough ones, which has the consequence of not achieving good adhesion of the coating on the they.
The ever-increasing acceptance of concrete as one of the most basic building materials on a global scale has led to the need to develop new suitable materials for the manufacture of molds.
• Insulated concrete forms. They are made from expanded polystyrene and are mainly used in masonry. During the injection of the fluid concrete they are used as molds, however after concreting they are not removed, as they offer thermal insulation protection to the construction.
• Fiberglass reinforced plywood system molds. This is a new trend that is often preferred as it forms smooth concrete surfaces that do not require painting. These molds can be used up to 30 to 50 times.
• 3D printing of molds. A particularly innovative solution is the 3D printing of molds using a 3D industrial 3D printer.
• Fabric molds. A mold technology that has been developing rapidly since the beginning of the 21st century is fabric molds. Molds of this type first appeared in the early 20th century, however in recent years the creation of new, highly durable synthetic fabrics has rekindled interest in the production of such flexible molds.
In the future, the construction industry’s need for stronger, lighter and more cost-effective molds is sure to lead to an ever greater restriction on the use of molds made from traditional materials.