Protection of concrete structures in tunnels and infrastructure applications against spalling and collapse at high temperature vehicle fires.
There is an on-going debate on how to best perform efficient and economical protection of a concrete structure in a tunnel either;
- Because the risk assessment has changed, e.g. greater traffic volumes or more hazardous goods are allowed in relation to the original risk assessment (HRR & Temperature);
- Or that the selected concrete composition, for example high temperatures achieved by a major vehicle fire.
The more conservative approach to solving the above problems is typically a constructive protection of the concrete structure by applying fire insulation with a possible final decorative/protective finish.
This is in most cases a major intervention in the original visual appearance and associated with either a total or limited closure of the tunnel for a longer period of time.
For a number of years, it has been debated whether a Fixed Fire Fighting System can provide a similar or better solution constructively, technically and economically?
The time may have come to regain past times rejection of a Fixed Fire Fighting System as a solution to the above issues because;
- Previously "teething" on the systems installed for more than 10 years or longer ago have been resolved;
- New vendors have entered the market, resulting in a more competitive market place;
- New technologies and innovations being launched to the FFFS market.
There has been intensive research in the last 10 years in FFFS and performed countless full scale tunnel fire tests by many manufacturers in the market and with very positive results, e.g. reduction of HRR and lowering of temperatures in case of a major vehicle fire.
One of the more recent full scale tunnel fire tests carried out, proved to be very effective against structural concrete protection and offered instantaneous lowering of the temperature at the outside and inside of the concrete structure.
Criteria to be met in the tests were NFPA 502 (to demonstrate that there is minimum spalling, which may lead to progressive tunnel collapse).
- Ceiling surface temperature shall not exceed 380 °C;
- Temperature of steel reinforcement within the concrete should not exceed 250 °C.
The test was carried out using concrete slabs mounted above the fuel package and at the ceiling area. In the concrete slab was inserted thermocouple`s at different depths 0, 25 and 40mm see fig. 3. Concrete slab.
The concrete slab was exposed to a fire of RHR 68 MW upon activation and the installed FFFS reduced the fire to 30 MW or lower (the potential RHR of the fuel package was +200 MW) see fig. 1. Rate of Heat Release.
Source: TUNPROTEC® is a registered brand within VID Fire-Kill, a global supplier of low-pressure water mist systems and components.
As above concrete slab temperature curve show, provides a Fixed Fire Fighting System a very effective structural protection and temperature reduction against a progressive tunnel collapse see fig. 2. temperature curve, showing values far below the criteria's as set by NFPA 502.
Author: Johnny Jessen, Sales Manager – Infrastructure Systems, VID Fire-Kill ApS.
Source images: TUNPROTEC® a registered brand within VID Fire-Kill, a global provider of Low Pressure Watermist Systems and components.