The rate and amount of heat generation are important in concrete structures having considerable mass. A rise in temperature accompanies thermal expansion, and non -uniform cooling of mass concrete creates undesirable stresses. Thermal cracking in a concrete structure tends to be wide and propagates through the structure. This naturally has adverse effects on strength, durability and permeability. Moreover, mass concrete structures are cast in many stages with construction joints. Individually constructed segments exhibit different heat source properties and time dependent properties. Therefore, construction stages must be incorporated in a heat of hydration analysis model to truly reflect a real construction process.
Stresses due to heat of hydration are classified as Internal Constraining Stress and External Constraining Stress. The Internal Constraining Stress results in from the restraining effect of volumetric changes due to different temperature distributions within the concrete structure. For instance, at the initial state of hydration, temperature differences between the surface and inner parts result in surface tension. Whereas at a later stage, contracting deformations in the inner parts are greater than those at the surface, thereby resulting in tension stresses in the inner parts. The magnitude of the Internal Constraining Stress is proportional to the temperature difference between the surface and inner parts.
External Constraining Stress is caused by restraining the volumetric change of fresh concrete in contact with subsoil or the substrate of previously cast concrete. The change in concrete heat results in the change of volume, and the restraining effect is dependent on the contact area and stiffness of the external constraining objects.
Heat of hydration analysis can be accomplished through Heat Transfer Analysis and Thermal Stress Analysis. Heat Transfer Analysis entails the process of calculating the change of nodal temperatures with time due to heat source, convection, conduction, etc., which take place in the process of generating heat of hydration of cement. Thermal stress analysis provides stress calculations for mass concrete at each stage based on the change of nodal temperature distribution with time resulting from the heat transfer analysis. The stress calculations also account for time and temperature dependent material property changes, time dependent shrinkage, time and stress dependent creep, etc.
This tutorial demonstrates the process of construction stage analysis and analyzes the results for a foundation structure constructed in two stages or pours.
The tutorial also outlines the procedure of generating a construction stage model for heat of hydration analysis and reviewing the analysis results.