Abstract
The response of high-performance concrete (HPC) to rapid heating due to exposure to fire with air temperatures exceeding 700°C is analyzed. The analysis focuses on coupled thermal-mechanical-transport processes at the mesoscale in the first 10 minutes of exposure during which heating rates on the order of 10 °C/minute and temperature gradients on the order of 15 °C/cm are involved. The driving forces for damage considered are thermomechanical stresses and internal pore pressure resulting from the expansion of water within the material. The HPC is modeled as a two-phase composite consisting of a cementitious matrix and a population of quartz aggregate particles. Mesostructures with aggregate sizes of 400-1600 μm and aggregate volume fractions of 10-30% are considered. To capture the development of stresses and pore pressure, the cementitious matrix is modeled using a coupled thermal-mechanical-transport formulation and the aggregate is modeled using a thermal-mechanical formulation. Simulations show that the composition of the mesostructures significantly influences the time and spatial distribution of damage. Materials with smaller aggregate sizes and the lower effective permeability are found to exhibit more rapid property degradation. The time to failure and depth of thermal spall are quantified as functions of structural variables. This framework and the mesostructure-response relations obtained serves as a tool for the design of HPC that are more resistant to fire-induced damage.
Highlights
IntroductionChemical transformations occurring during heating include the evaporation of free water; the dehydration and subsequent evaporation of bound water in calcium silicate hydrates (C-S-H), ettringite, and gypsum; the dehydroxylation of calcium hydroxide; and the decarbonatation of calcium carbonate [1]
Concrete is a porous, reactive, composite material
Chemical transformations occurring during heating include the evaporation of free water; the dehydration and subsequent evaporation of bound water in calcium silicate hydrates (C-S-H), ettringite, and gypsum; the dehydroxylation of calcium hydroxide; and the decarbonatation of calcium carbonate [1]
Summary
Chemical transformations occurring during heating include the evaporation of free water; the dehydration and subsequent evaporation of bound water in calcium silicate hydrates (C-S-H), ettringite, and gypsum; the dehydroxylation of calcium hydroxide; and the decarbonatation of calcium carbonate [1]. The constituents of concrete have differing coefficients of thermal expansion which lead to short-range thermal stresses. Large thermal gradients arising from the low thermal conductivity of the material lead to long-range stresses. The combination of chemically and thermally induced stresses when concrete is exposed to fire can lead to property degradation, damage, and failure. The response of concrete to rapid heating is complex and multi-physics in nature and the thermal, mechanical and transport processes governing the response are coupled
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