Releasing agents are coatings applied on the formwork surface, prior to concreting, in-order to facilitate easy removal the formwork or shuttering. If the appearance of the concrete surface is of significance, it is important that care is taken with the finish surface of the shuttering. Release agents provide the critical barrier between a molding surface and the substrate, facilitating separation of the cured part from the mold. Without such a barrier in place, the substrate would become fused to the mold surface, resulting in difficult clean-up and dramatic loss in production efficiency. Even when a release agent is used, factors such as irregular applications or improper release agent choice may have a dramatic effect on the quality and consistency of the finished product. In the concrete construction industry, form release agents prevent the adhesion of freshly placed concrete to the forming surface, usually plywood, overlaid plywood, steel or aluminum. In this application, there are two types of release agents available: barrier and reactive.
Release agents should have a reasonably long and stable storage life and should not be susceptible to damage from extreme tempera t u re changes or from rough or repeated handling. Care should be taken to ensure that release agents are stored in accordance with the manufacturer ‘s recommendations, particularly with regard to temperature extrenes Before using the release agents should be checked for sediment. To ensure uniformity it may be necessary to stir them adequately. Care must also be taken to ensure that they do not become contaminated. Release agents containing volatile solvents must be stored in airtight containers to prevent a change in concentration. Release agents should not be diluted at the jobsite unless specifically permitted by the manufacturer. Some oils have a critical emulsifier content and dilution makes the emulsion unstable and causes poor performance.
Agents commonly used
There are several agents which are in practise and are used according to the requirements.
Neat Oils – Neat oils are usually mineral oils; they tend to produce blowholes and are not recommended for use for the production of high quality concrete surfaces. They are used in the storage of formwork and for concrete which will be hidden below ground.
Neat Oils With Surfactant – Neat oils with the addition of a small amount of surface activating or wetting agent minimise blowholes and have good form penetration and resistance to climatic conditions.
Mould Cream Emulsions – Emulsions of water in oil tend to be removed by rain, but minimise blowholes and are good general purpose release agents, except they are not very suitable on steel formwork.
Water-Soluble Emulsions – Emulsions of oil in water produce a dark porous skin on the concrete which is not durable. They are not recommended for good class work and are seldom used.
Chemical Release Agents – These are small amounts of chemical suspended in a low viscosity oil distillate. The chemical reacts with cement to produce a form of soap at the interface. Recommended for all high quality work, they should be applied lightly by spray to avoid retardation. Increased cost is compensated for by better coverage compared with the oil-based materials. They generally have good weathering resistance. Certain of the proprietary chemical agents are of the ‘drying type’ and so are particularly suited to use in dusty, dry climates, on soffit formwork, and in pre-stressed concrete applications.
Paints, Lacquers, Waxes and Other Surface Coatings – These are not strictly release agents but are sealers which prevent release agents being absorbed into the form face. Wax treatments also come into this category. They are all particularly useful where it is necessary to avoid uneven porosity with consequent colour variations in the concrete surface and to give increased usage of the formwork.
Other Specialised Release Agents – These are various types of release agents not listed in Categories 1 – 6. They include chemical systems, silicones and vegetable oils for such applications as concrete forms, special heated formwork systems, spun pipe systems, etc..
Materials for release agents
Absorbent Surfaces – With many untreated timbers or plywoods, particularly softwood species, the release agent is absorbed according to the density of the material, giving a grain patterned finish. This is most marked on the spring and summer growth rings found in softwood plywood. This problem can be overcome by giving one of the following pre-treatments before use:- ◆ One or more full coats of the normal release agent. ◆ A suitable barrier paint or varnish (on a dry surface, preferably factory applied). ◆ A suitable proprietary formwork wax.
Before concreting for the first time, and for all subsequent pours, a normal application of the chosen release agent should be given. Patches of barrier paint or varnish may wear off with use and generally cannot be renewed because of the absorption of release agent into the form face. This will result in a blotchy appearance to the concrete surface.
Non-Absorbent Surfaces – If the surface to which it is applied is virtually impervious, the release agent may tend to migrate and dry up; this will occur where an emulsion is used. This frequently happens, for instance, on bridge decks, where the time between application and concrete placing may be prolonged. The condition is also aggravated by drying winds, strong sunlight and rain. Chemical release agents are recommended in these circumstances, or possible neat oils with surfactant. One coat of release agent should be applied as near as possible to the time of the first pour and prior to each subsequent pour. Very smooth flat surfaces may require initial ‘ageing’ to roughen the surface to assist retention of the release agent. The use of a pre-treatment wax (Category 6) will also improve the finish from the initial cast of the form.
Steel – It is recommended that a chemical release agent be used on steel, although for lower quality work, a neat oil with surfactant can be used. When used in conjunction with accelerated curing and heating systems, the addition of de-watering and rust inhibitors to these products will make them more suitable. New steel moulds may cause spalling for several uses until ‘worn-in’ and the use of a formwork wax or oil initially will help.
Particle Board (Wood Chipboard)– Generally the material should be treated in the same way as pre-coated plywood.
Wood-based Sheet Materials – The choice of release agent depends on the pre-treatment and surface condition of the sheet. Sheets are generally available in the following four conditions:- ◆ Sanded but otherwise untreated (the user may subsequently apply sealants to the face and edge surface). ◆ Sanded and treated with a release agent. ◆ Face and edge sealed (e.g.. 1 or 2 coats polyurethane). ◆ Surface overlaid with phenolic or melamine resin films, factory bonded and edge sealed.
Plastics (Particularly Trough and Waffle Moulds) – It is recommended that a good quality chemical release agent be used. It should be applied by absorbent cloth or sponge and particular attention should be paid to removal of any excess present on the base of the mould which has run down the sides.
Aluminium – Chemical release agents have been successfully used in conjunction with aluminium but these are generally special versions and details should be obtained from manufacturers. Pre-etching of panels is essential to give uniformity of concrete colour. There is increased adhesion between the aluminium and cement due to the affinity of aluminium oxides and oxides formed in the cement paste. Thus careful selection and use of release agent is of particular importance.
Liners (Plastic or Rubber) – When a proprietary liner is supplied, the manufacturer may recommend a suitable release agent. If this is not documented, refer to the manufacturer of the release agent and conduct a trial on the material, checking for swelling.
Concrete Moulds – The master moulds should be free of all surface blowholes and blemishes, and when fully cured, sealed either by a coating or form wax treatment. They should then have a compatible release agent applied for each cast.
Tips on Choosing the form release agent
One valuable standard for evaluation and selection of a release agent is prior experience. However, the safest approach is to evaluate different commercial release agents under actual use conditions, either on a test panel or on a nonarchitectural portion of the concrete on the project. In addition, information should be obtained from the manufacturer of the release agent about the kind of form surface for which the product is intended and the proper method of application. In making the selection the following may have to be considered;
- Compatibility of the release agent with the form material or form sealer; that is, whether the release agent softens the plastic from face.
- Final surface requirements. If surfaces are to be plastered or painted, the form contact area should be treated with materials that don’t leave oily or waxy residue that interfere later with adhesion of plaster or paint. Some contractors consider it sufficient to wet the forms with water if surfaces are to be plastered. If the striped surface is slightly rough the plaster will adhere better.
- Durability of the final surface. The release agent should not cause the concrete surface to soften and dust. Moreover, it should not impede wetting of surfaces that are to be water cured nor should they otherwise hinder the proper functioning of curing compounds
- Discoloration and staining. On forms for architectural concrete, regardless of the kind of concrete finish, a 1 00-percent non staining form release agent free from pigments should be used. It will prevent uneven coloring of the concrete. The type of release agent used is of less importance, howe ve r, for exposed aggregate concrete because the discoloration usually does not penetrate to any great depth.
- Time period before stripping.
- Environment of the cast concrete e.
- Uniformity of performance of the release agent.
Benefits of Releasing Agents
Releasing agents mean coatings which are provided on the formwork surface, before starting of concreting to allow smooth detachment of the formwork or shuttering. Consideration should be given on the finish surface of the shuttering as it reveals all the flaws on the ‘form’ surface like vibrating poker known as ‘burns’, changeable properties in the form-face material, marks of irregular water absorption in timber. There are several benefits of it.
- It allows smooth elimination of shuttering.
- Minimize the occurrence of blowholes.
- Supply the recommended surface finish for the concrete member cast.
- Minimize the loss of water from the concrete caused by absorption in timber forms.
- Minimize seepage of water and moisture throughout curing of concrete.
- They safeguard the formwork and help in reusing them if possible.
- Minimize the cracks because infernal restraints.
Application of formwork release agents
Release agents should be applied to a clean form before the reinforcement has been placed to reduce the likelihood of inadvertently applying it to the reinforcement. If the release agent does come in contact with reinforcement it should be wiped clean before placing the concrete. When applying for a release agent it is best to follow the manufacturer’s instructions. When too much form release is used, it is not only wasteful and inefficient, but it leads to a number of other associated problems with the finished product. He who holds the wand determines the amount of material being applied, so proper training is crucial. The actual cover thickness will depend on the application method and viscosity of the product, which is related to the ambient temperature. Typically, the colder it is in the plant, the thicker, or more viscous, the release agent will be. The warmer it is the plant, the thinner, or less viscous, it will be. Different measures can be taken during the application process to account for changes in material temperature throughout the year.