Moisture transport in concrete during fire: an NMR study

Abstract

The question how concrete responds to a fire is one of the main questions in fire safety. During a fire, a building material can suddenly be heated up to temperatures well above 1000 oC. At temperatures above 100 oC, water inside the pores will start to boil. Simultaneously, in concrete, but also for example in gypsum, chemically bound water will be released by dehydration of the porous matrix. If the concrete has a low permeability, the vapour pressure inside will increase which can give rise to a sudden (explosive) failure of a material. Numerous heat and mass transfer models have been used to predict the moisture transport and its consequences on the strength and permeability of the concrete. However, these models are only of use if they can be validated. For model validation, quantitative measurements of the evolution of moisture, temperature, and possibly pressure distributions in time are needed. For this purpose, we have developed an NMR setup to measure the moisture transport in heated building materials. This setup makes used of the 1.5 T magnet of a medical MRI scanner, while dedicated gradient coils are used to image the moisture profiles in 1D. The sample is heated with the help of four 100 W halogen lamps, capable of generating a heat flux of 12 kW/m2. The measured combined moisture content and temperature profiles give a unique insight in the moisture transport and dehydration kinetics inside concrete during fire. These measurements give the first quantitative proof for the build-up of a moisture peak due to the vapour pressure build-up. While many moisture transport models predicted the existence of such a peak, these predictions had never been validated with moisture profile measurements during fire tests.