Uncovering the Real Purpose of Fire Testing in Concrete Tunnel Linings

If you’re not familiar with fire design of concrete tunnel linings, you might assume that fire tests are conducted to validate that the lining has adequate strength capacity for a specified fire load (e.g., hydrocarbon or Rijkswaterstaat [RWS] curves). However, in most tests, this is not the case—at least not directly.

Fire tests are performed for two main reasons

1. To validate that the thermally affected parameters assumed and resultant thermal gradients through the concrete lining reflect the values used in the design. From our experience, having witnessed numerous fire tests, we’ve found that the thermal behavior and the associated thermal affected parameters (e.g., reduction in strength and stiffness) of concrete linings tend to align well with the recommended values provided in Eurocode 1 and 2 (EN 1991-1-2, EN 1992-1-2). Thus, for typical tunnel lining concrete mixes, the fire test samples generally pass this performance requirement.
2. To validate the assumed spalling behavior of the concrete material, in particular, “explosive” spalling, which is known to occur in concrete under certain conditions. Unlike the thermal behavior (see above), spalling performance can vary considerably based on the mix design. Accurately estimating likelihood and, most importantly, the depth and extent of explosive spalling via engineering numerical models is not possible
   under the time frames typically given to designers. We typically rely on previous experience (published data) to determine a reasonable spall depth to be used in our designs. Explosive spalling, for this discussion, is generally defined as spalling that occurs early in the design fire (first couple of minutes). It is typically observed in tests with hydrocarbon (HC) design fire (or similar) that have a very rapid initial temperature increase with time. Thus, in our experience, the main reason to perform fire testing is to validate the spall depth assumption for the adopted mix design.

Why Full-Scale Fire Testing Isn’t Always Necessary

With current commercially available structural numerical analysis software (e.g., Strand7, SAP2000, etc.) and computing power, engineers can assess the likely performance of tunnel linings during (and/or post) a specified design fire. Full-scale testing of tunnel linings is generally not warranted. Fire testing is needed for design fires with high thermal energy outputs (HC, HC-Mod, RWS, etc.) to verify concrete mix design spall risk assumptions. However, the testing should be viewed as another material test (e.g., like compressive strength) rather than a test that specifically validates the structural design of the tunnel itself. There will be certain situations that may require full-scale fire testing, but these are situations where extreme fire scenarios are expected, or structural failure consequences are catastrophic. Otherwise, fire testing, generally in accordance with EFNARC 2006, will be adequate for most underground structures.