An experimental evaluation to determine the required thickness of passive fire protection layer for high strength concrete tunnel segments

•30 mm layer thickness above 50 mm concrete cover limits temperature of steel below 160 °C and concrete surface below 300 °C.•Exposure of protected high strength concrete to ZTV-ING fire load is investigated.•Concrete cover is thermally ineffective when layer thickness is greater than 55 mm.•Fire damage into the protection layer initiates through the shrinkage cracks.•Design thickness must include the depth of shrinkage cracks if they are inevitable.

The Eurasia Tunnel is a part of the Bosporus Strait Highway Crossing Project connecting the Anatolian and European shores of Istanbul. This paper evaluates the use of high strength concrete for the segments for the bored part of the tunnel under the effects of design fire conditions. High strength concrete enables the attainment of high early strength levels required for segment handling and increases the segment durability compared to segments designed by ordinary strength concrete. However, in the event of a tunnel fire, the increased concrete strength of the segments increases their likelihood of spalling compared to segments fabricated by ordinary strength concrete. Therefore, determination of the passive fire protection requirements must take into account the strength of the protected concrete. This paper will present and include a discussion of the results of small-scale fire exposure tests conducted to determine the required thickness of passive fire protection layer for the evaluated high strength concrete tunnel segments.