Effect of Humidity on the Thermal Properties of Normal- and High-Strength Concrete at Elevated Temperatures

Abstract

Understanding how concrete properties vary at high temperatures in certain environmental conditions is critical for assessing realistic fire performance of concrete structures. Environmental humidity is an important factor that governs moisture within concrete, and any variations in moisture can influence the properties of concrete. This paper presents results from an experimental study conducted to investigate the influence of humidity conditions on thermal conductivity and specific heat of normal-strength concrete (NSC) and high-strength concrete (HSC) at elevated temperatures. Thermal characteristics were measured using the transient plane source technique at temperatures ranging from ambient to 750°C under relative humidity (RH) conditions of 6%, 50%, and 100%. Furthermore, experimental and numerical studies were conducted by exposing humidity-treated concrete elements to standard fire to evaluate the influence of measured thermal properties on the temperature progression in the concrete. The test results demonstrate that fully saturated humidity conditions (RH 100%) significantly affect the thermal properties of concrete relative to dry conditions. In fully saturated conditions, the thermal conductivity of NSC and HSC increased by 31.88% and 31.02%, respectively, at ambient temperature compared with dry concrete; however, at elevated temperatures, it varied in the range 3.16% to 31.88% for both concretes. Similarly, the specific heat increased by 29.01% for NSC and 41.40% for HSC at ambient conditions, and varied from 5.24% to 48.37% over the measured temperature range. The temperature evolution obtained from the fire test and heat transfer analysis in dry concrete indicates that the maximum temperature variations during fire exposure are approximately 10%–20% higher for NSC and 21%–25% higher for HSC than for saturated concrete. The test data were utilized to establish simpler correlations for thermal property variations as a function of temperature and humidity. The proposed correlations can be used to determine the thermal evolution in concrete structures exposed to elevated temperature for different RH conditions.