2008 – Thermal modelling of the mass concrete in the Hinze Dam spillway raise

Scott Jones, David Hughes, Orville Werner

Abstract: As part of the 15 m raise of Hinze Dam, the existing 33 m high mass concrete spillway structure will be raised an additional 12.5 m. This will be achieved using conventional mass concrete placed on the top and downstream side of the existing spillway to form a new monolithic structure. Heat generated by the hydration of the cement and fly-ash will raise the peak temperature in the body of the new concrete relative to the stable and relatively uniform temperature within the existing concrete. The early, comparatively rapid volume expansion (and subsequent slow contraction) of the new concrete is externally restained along the interface and there is a potential for tensile stains to develop along the interface that are large enough to cause cracking through the body of the composite dam and potentially compromise the interface bond. The temperatures and thermal gradients induced are a function of the mix design, particularly the amount and thermal properties of cement used, and the sequencing of construction.

Two-dimensional transient coupled thermal-structural finite element (FEM) analyses were used to predict thermal deformations and stresses within the body of the spillway in the weeks and months after placement. Laboratory measured mechanical and thermal properties of the concrete and local boundary climatic data were input to the analyses. The measured adiabatic calorimeter curve showed that the fine grind and chemical composition of the local South East Queensland cement produced a rapid generation of heat which magnified potential thermal expansion issues with the early-age concrete.

Creep, shrinkage, viscous-elastic behaviour, and the increase in modulus of elasticity with age influence the degree to which expansion and contraction of the concrete are converted into stress. These variables, were either accounted for directly in the elastic FEM model, or were taken into account in the interpretation of the results.

This paper presents the assumptions, methods, and criteria used in the FEM analyses; the results of the material testing program; and the results and conclusions drawn from the analyses. A discussion on the concrete mix design trials recently completed on site is also included.

Keywords: Adiabatic Temperature, Creep, Mass Concrete, Placement Temperature, Pre-cool