Concrete drying kinetics: development of an accelerated drying protocol in fire testing

The fire behavior of concrete is a critical aspect in the context of civil engineering works for which, in most cases, fire characterization tests are required. Concrete spalling is strongly dependent on the hydric state and material permeability. The fire resistance test standard NF EN 1363-1 stipulates that at the time of testing, the condition of the specimen should be representative of its normal service condition (mechanical strength and water content), considered to be obtained at equilibrium after storage in an ambient atmosphere. A conditioning period of at least three months is required. Nevertheless, the concrete drying process is a very slow mechanism so testing after only a few months may underestimate the fire resistance due to higher spalling susceptibility associated with high water content. Concrete fire testing is expensive and time\space-consuming, generally requiring a qualified workforce, adapted manutention equipment, and the storage of both full or intermediate scale specimens in controlled spaces for several months. This work proposes a novel experimental protocol that optimizes the specimen's storage time before fire resistance testing while keeping a representative hydric and mechanical state of its normal service condition. The temperature and relative humidity conditions are chosen in a range in which no significant microstructure changes modifying concrete behavior occur. An extensive experimental campaign on four concrete formulations including drying experiments (ambient and accelerated) and fire tests with different timeframes depending on the storage type is carried out. This protocol is based on the modeling of drying at moderate temperature. According to the predictive study, target water content could be reached under several drying conditions, which shows that ad hoc solutions can be proposed. The reproducibility of the accelerated drying protocol is important to ensure large application in both research and industrial laboratories. Keywords: concrete drying kinetics, water content, concrete spalling to fire, microstructure, ...