Center for Light and Environmentally-Friendly Structures ZELUBA®

Research project

Development of an intumescent fire protection coating for ultra-high performance concrete (UHPC)

Ultra-high performance concrete (UHPC) is a very densely textured concrete with a compression strength similar to steel at up to 250 N/mm². It is up to ten times as hard as common concrete. When exposed to fire, UHPC can burst open explosively due to the sharply rising temperature. This behaviour is largely influenced by the mass transfer of water, steam and air through the concrete’s pores. When there is a fire, the pressure in the pores increases. The very high density of UHPC promotes this process and the result can be a premature loss of load-bearing capability. As part of a DFG project run by the iBMB at the TU Braunschweig, methods were sought to minimize bursting due to fire exposure and thus to achieve overall improvement to fire protection for UHPC.

The researchers in the Department of Structural Engineering and Construction had the task of finding out whether the successful high-performance fire protection coatings developed for wood could offer approaches for solutions. The idea was to minimize the temperature increase by using an intumescent fire protection coating. If the temperature increase is slower (especially during the beginning of a fire), then the temperature gradients at the outer areas of a concrete structure could be reduced. This would then allow a reduction of the pressure in the pores which causes the tension and leads to bursting.

It is generally known how intumescent fire protection coatings work. Intumescent coatings react to the temperature increase in the surrounding gas phase when exposed to fire. If a certain temperature is exceeded, a voluminous carbonaceous layer is formed which isolates the substrate below it and protects it from the heat. The formation of the insulating layer is based on a series of temperature-dependent chemical reactions. Fire protection coatings generally consist of binder, blowing agent, char former, an acid catalyst and other additives. Applications of intumescent fire protection coatings include e.g. protecting steel and wood building components. Systems are generally available for concrete components. They are used to enhance fire resistance of historic, under-dimensioned reinforced concrete ceilings. They are not suitable for protecting UHPC because they only react at higher temperatures.

The first step of the development work was screening the commercially available binders which are suitable for mineral substrates. The focus was on aqueous polymer dispersions based on a co-polymer made of vinyl acetate (VAc) and/or acrylate (Acr) and vinyl ester of the versatic acids (VeoVa). The investigated binders differ in their polymer particle size, emulsifier type, pH value and their minimum film-formation temperature. An intumescent base mixture was dispersed into each binder.

© Fraunhofer WKI
Fig. 1: UHPC slab with BB3 fire protection coating following fire exposure in a laboratory
© Fraunhofer WKI
Fig. 2: Laboratory fire results for several fire protection coatings with and without additives, dry application 1.1 kg/m².
© Fraunhofer WKI
Fig. 3: Maximum foaming height and foaming height at 450°C using TMA and T (°C) from the laboratory fire tests after 30 minutes.

The reaction to fire of coated UHPC slabs was evaluated in a laboratory fire test using the “WKI method”. The back side of an unprotected 1cm-thick UHPC sample reaches up to 350°C after 15 minutes. In contrast, the best base formulation and dry application of 1.1kg/m² was able to drop the temperature to 213°C after 15 minutes. The intumescence formed a crack-free, homogenous, compact foam with a white crown and formed an almost pointed dome shape. There were no open hollow spaces to be observed (Fig. 1). All the test specimens maintained unreacted coating under the produced isolating foam.

In the next step, processing or dispersing additives were added to the basic BB3 recipe. With recipe BB21 it was possible to improve the fire protection effect throughout the entire exposure time (Fig. 2). The BB21 sample displayed homogenous foaming without cracks or breaks.

The increase in volume of the fire protection coating (regardless of the substrate) was assessed using thermo-mechanical analysis (TMA). This allows conclusions to be drawn on whether the chosen binder allows intumescence, at which temperature this begins and how high the formulation foams up. The TMA curve also provides information on the stability of the foam.

Fig. 3 summarizes the results from the TMA and the lab trials. The highest foaming level, the most stable foam and the best protection were achieved with fire protection coating BB21.

Two recipes were provided to the iBMB for further analysis in furnace trials. UHPC column segments were tested under standardized fire conditions. The results showed that high-performance fire protection coatings can achieve improvements, but that these improvements currently do not go far enough. Further development work could not be carried out due to budget restrictions. More development work could be the subject of later research projects.