Fire protection of reinforced concrete structures.

To ensure the bearing capacity of existing concrete structures, they are sometimes reinforced during renovations using steel strips, bonded (carbon) reinforcement, etc. The question then arises: what in case of fire? Should these reinforcements be fire protected?

Strengthening of concrete structures is commonly used in renovations and the need for it usually finds its cause in:

Various systems exist on the market for reinforcing concrete structures. For example, one can use steel plates that are attached to the existing concrete structures with an epoxy adhesive and/or mechanical anchoring. Here it is sometimes necessary to additionally attach them mechanically. In addition, fiber composite epoxy adhesive systems that are glued to the concrete structure to be reinforced are often used.

An engineering firm can determine, based on the accidental load combination according to the Eurocode, whether the reinforcement strips should contribute to the load-bearing capacity of the concrete element (i.e., they should not dislodge) in case of fire. Indeed, in some cases the reinforcement strips do not need to contribute to the load-bearing capacity in case of fire, but the original concrete element may still need additional protection to absorb the internal forces of the accidental load combination in case of fire. 

1. The concrete structure is sufficiently fire-resistant
2. The existing concrete structure must be additionally fire protected
3. The reinforcement should be fire protected

Based on EN 1992-1-2 (Eurocode 2) it can be verified whether the concrete structure is sufficiently fire resistant for a certain time. For this one can use the tabulated values for floors, walls, columns and beams. As a function of the required duration of time, the tables include, on the one hand, minimum concrete thicknesses to meet the compartmentation requirement (separation capacity - EI). On the other hand, a minimum reinforcement axis distance is also imposed to ensure the load-bearing capacity (R). In practice, it will therefore be necessary to check whether the concrete element is sufficiently thick and whether the reinforcement axis distance is sufficiently large to guarantee the good behavior of the concrete structure in case of fire.

If point 1 shows that the real reinforcement axis distance is smaller than included in the tabulated values of EN 1992-1-2, one will have to apply additional fire protection. Among other things, this protection will have to ensure that the reinforcing steel does not heat up too much. To protect the concrete, one can, for example, use sheet materials, fire-resistant paint, spray plaster, etc. The protective material used is tested according to standard EN 13381-3. This will show in what thickness the fire protection material must be applied to compensate for the lack of concrete.

A unidirectional bearing existing concrete floor with a thickness of 140 mm should achieve fire resistance REI 120. Research shows that the reinforcement axis spacing is only 10 mm. Based on Table 5.8 from EN 1992-1-2, this floor should have a minimum thickness of 120 mm and the reinforcement axis spacing should be at least 40 mm. In this case, the floor is sufficiently thick but the reinforcement axis spacing is not respected. In fact, 30 mm of concrete is too short. This means that the protection, which is applied, must have a minimum equivalent concrete thickness of 30 mm.

If the reinforcement strips also contribute to absorbing the load in the event of fire, they will also need to be fire protected. Here, fastening will play a crucial role. 

When the concrete structure is reinforced with steel strips that are mechanically fastened and bonded, it may be sufficient in the event of fire that only the steel strip remains mechanically fastened. In that case, the steel strip and its attachment must be fire protected. The protection thickness will then be a function of the profile factor and the critical steel temperature. The critical steel temperature can be calculated or one can use a value in absence from EN 1993-1-2 (Eurocode 3), namely 540°C for beams, 500°C for columns and 350°C for prestressed elements.

If the bonded reinforcement strips are to perform their function even in the event of fire, it will be necessary to ensure that the temperature of the adhesive does not exceed its temperature resistance (glazing temperature of the adhesive (Tg)). This is usually located between 40°C and 80°C. At higher temperatures, the adhesive will soften and the bond will lose its strength, preventing the transfer of forces to the reinforcement strips. In order to prevent the temperature at the adhesive layer from rising too high in the event of a fire, fire-resistant protection must be provided. Fire-resistant boards are often used for this purpose. 

Although there is no specific standard for testing reinforcement strips, tests according to EN 1363-1 'Fire resistance tests - Part 1: General requirements' and/or the EN 1365 series can nevertheless be used to verify that the interface temperature between the sheet material and the reinforcement strips remains sufficiently low, i.e. below the glass transition temperature at a fire exposure according to the ISO 834 fire curve.