Release agent for concrete – demoulding without damage
A release agent is used to ensure that concrete surfaces can be stripped cleanly and without defects. The use of release agents is particularly important for exposed concrete surfaces, but it is also necessary in sensitive areas such as recesses, edges or undercuts. In addition to protecting the concrete surface, protecting the formwork from damage is also of economic importance. A well-maintained formwork treated with release agents lasts longer and ensures the qualitative requirements for the concrete elements to be produced.
Release agents form a protective film between the fresh concrete and the formwork surface. This avoids adhesion of the two materials. The release agents used must be matched to the intended use and in particular to the type of formwork. Here it is necessary to distinguish whether the formwork is wooden or steel, absorbent or non-absorbent, or horizontal or vertical. Carefully prepared release agents appropriate to the intended use ensure that the finished concrete surface is not damaged by the removal of the formwork. Release agents are differentiated according to the substances used:
- Solvent- or water-based mineral oils and vegetable oils
- Waxes and varnishes
Recently, vegetable-based release agents in particular have become increasingly important. In contrast to the mineral oil-based release agents, which in many cases consist of a mixture of medical white oil and solvent, the vegetable oil-based release agents are environmentally friendly and pose little risk to health. The disadvantage of this new generation of formwork oil is its high cost and temperature sensitivity. In addition, these release agents must be continuously agitated or stirred in the storage tank to prevent flocculation and settling of the emulsified material.
Independent of whether release agents are used on the construction site or in the precast plant, the products must meet certain requirements:
- Concrete and formwork must separate easily and cleanly so that neither the concrete surface nor the formwork is damaged.
- After stripping the formwork, there must be visually perfect concrete surfaces with a dense surface and minimal pores.
- No setting defects must be visible on the concrete surface.
- The release agent must protect the formwork from corrosion and damage.
- The order must be simple and economically feasible.
- The concrete release agent should be as environmentally friendly and health-neutral as possible.
Heated formwork is often used in the precast plant. As a result, the release agent must also be largely insensitive to temperature. On the construction site, water resistance and accessibility after application are additional factors.
Absorbent or non-absorbent – decisive for the selection of the release agent
Depending on whether absorbent or non-absorbent formwork is used, the release agent must be selected accordingly. Absorbent formwork materials include wood formwork, while non-absorbent formwork includes steel formwork, tempered wood and plastic formwork, and structural matrices.
- For absorbent wood formwork, a combination of release agents and cement paste has proven effective, especially for new formwork. Adhering cement and concrete residues must be removed before using already used formwork. Due to the increasing surface sealing older wooden formwork can be used with a thin layer of formwork oil.
- Non-absorbent formwork can not absorb moisture or large quantities of release agents for concrete. Therefore, the application must be very thin. Puddle formation must be avoided, as this leads to increased pore formation, color disturbance and destruction of the concrete surface.
- For non-absorbent heated formwork, a temperature-resistant release agent must be used that does not volatilize or show chemical reactions when heat is generated.
- Structural formwork made of rubber or silicone rubber is self-releasing and usually does not require a release agent, at least when new. If a formwork oil is used, it must not cause the material to swell or dissolve.
Careful working – clean separation
For perfect results and easy removal of the formwork from the concrete surface, the concrete release agent must be applied properly. The spraying process, in which the selected release agent is sprayed on at high pressure, has proved successful. A flat spray nozzle provides ideal results. On the one hand, this ensures an even and thin application of the release agent, on the other hand, the product is used sparingly and economy in application is guaranteed. A ball valve with integrated filter prevents leakage and dripping and saves material. In addition, care must be taken to work cleanly. During the mounting of the formwork, dirt particles can get into the release agent layer. These, in turn, later cause stains or discoloration on the concrete surface.