Releasing agents are coatings applied on the formwork surface, prior to concreting, in-order to facilitate easy removal the form work 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. Finished concrete surface will reflect all the defects on the ‘form’ surface, such as vibrating poker called as ‘burns’, varying properties in the form-face material, marks of uneven water absorption in timber.
Before pouring the concrete, loose wire & debris should be cleaned out by using a compressed air hose. It should be ensured that steel particles are removed as it will rust & tamper the final finish of the concrete.
Functions of Releasing Agents
- Most important function of Releasing Agents is to permit easy removal of shuttering. Apart from this some major functions and advantages of using releasing agents includes:
- Reduce the incidence of blowholes
- Deliver the recommended surface finish for the concrete member cast.
- Reduce the loss of water from the concrete due to absorption in case of timber forms
- Reduce leakage of water and moisture during curing of concrete.
- They protects the form work and make the reuse of shuttering possible.
- Helps in reducing cracks due to internal restrains.
Types of Release Agents with Merits
There are lot of suppliers and various types of release agent available in the market. Out of these oils with surfactants, chemical release agents & mold cream emulsions are most useful. So Site engineer should know which one to use and what is fit for the project. The merits of various type of releasing agents are summarized below.
Chemical Release Agent
These are light oils base on volatile oils which dry on the surface leaving a thin coating on the shuttering which is water resistant and protects the form during rains. Advantages and facts of Chemical Agents are as below
- It can be used for all types of formwork.
- It doesn’t leave any oily film after it is dry, giving a safer surface to walk on unlike an oily film.
- Release agent does not stick on operatives’ footwear preventing accidental coating on rebars.
- More rate of coverage than conventional oils.
- It is expensive for a given volume but can be economical based on the application method.
Mould Cream Emulsion
Except for steel formwork, cream emulsions can be widely used for all types of shuttering. It has to be mixed thoroughly before using and dilution is not allowed.
- Recommended for absorbent surfaces such as timber form.
- It provides high quality finishes.
- It can be spray applied
- Unlike chemical releasing agents it cannot be used for concreting under freezing
- Shelf life may be limited.
Neat Oil with surfactant
These are general-purpose release agent for all types of shuttering including steel formworks.
- Over application may cause staining of concrete.
- Not water resistant and oil film may be washed by heavy rain.
- Repeated or progressive use reduces the finish and leaves color changes on finished surface.
- New and old forms should not be used together.
Forms made from painted timber and plastics having a glossy surface produce an appearance that changes with the number of uses. After subsequent uses these forms produce a less quality surface on the concrete.
Some plastic-faced plywoods give a similar effect. The first few uses of these materials can sometimes produce a very hard, dense, and almost black surface to the concrete.
Common faults at Construction Site
One of the common mistake while using releasing agents is that it is applied in excess quantity causing negative outcomes. Most recommended method of applications is by an airless spray for thin coats.
- Thicker oils may be applied by brush or cloth and spread as far as possible, all excess being removed with a cloth.
- The use of barrier paints produces a hardwearing surface and may extend the life of timber or plywood forms.
- If paint is not used, three coats of mould oil should be applied before the form is used for the first time.
- Some barrier paints are not suitable for use with certain tropical hardwoods, and the manufacturer’s advice should be sought on this point.
- To avoid contamination of reinforcement, the release agent should be applied before the forms are erected, but then it may be necessary to protect the forms from the weather.