Modelling the temperature envelope of concrete hydration in insulated forms at sub-zero temperatures

Abstract

The aim of the thesis is to demonstrate the viability of expanded polystyrene (EPS) insulated concrete form (ICF) and concrete construction in sub-zero [°C] environmental temperatures. The concrete core temperature was experimentally investigated at environmental temperatures between +10°C and -24°C. The thesis is conducted in co-operation with Nordicform Oy, and ICFs manufactured by Nudura. The scope of the thesis is to construct a thermodynamic model of the curing concrete, and to analyze the chemical and thermomechanical properties of construction grade concrete in ICFs. A predictive model of the concrete core curing conditions including an extrapolation of the strength from the time and temperature dependent maturity factor is created in Microsoft Excel. The model allows for prediction of critical environmental conditions that may result in incomplete curing. Heat flow analysis is used to mathematically model the cement hydration enthalpies and the flow of heat to the outside environment through the expanded polystyrene form. Time and temperature dependence of the hydration reactions of the main constituents of Portland cement and water are predicted and fit to experimental sensor data for a curing period of 28 days. Temperature data readings are collected once per hour with a measurement accuracy of ±1 [°C] and maturity-based strength is estimated to an accuracy of 0,1 [MPa]. The unit of study is one standard block of Nudura ICF and its concrete core. Temperature data for the inside of a concrete core at a construction site is collected using a SmartrockTM temperature and maturity sensor made by Giatec Scientific. The hydration reactions of alite (3 CaO · SiO2), belite (2 CaO · SiO2), tricalcium aluminate (3 CaO · Al2O3) and tetracalcium alumino ferrite (4 CaO · Al2O3 · Fe2O3) and their interactions with water and gypsum (CaSO 4·2H₂O) are included. Interactions between the crystal growth of the various hydrate phases and their influence on compressive strength can be suggested. The equations for heat flow, and the Arrhenius and Avrami equations are used. The model is validated by predicting concrete temperatures at a secondary site. In the case study; data from the data indicate concrete core temperatures remain warm enough for concrete to reach its design strength within 28 days. In typical cold weather concrete construction, the EPS ICFs provide adequate insulation to allow for efficient curing. Extrapolating from the model, at environmental temperatures of below -20°C, concrete near an uninsulated, exposed surface such as: in a window opening or at the top of a wall section, may fall below a recommended limit of 4,4°C within the first 120 hours of curing. The design strength of concrete (30 [MPa]) is reached at 41 hours suggesting bulk concrete content is adequately insulated from the elements. In conditions of below 20°C, additional insulation is required to protect open surfaces to ensure design strength is reached.