Concrete Volume Change for Dworshak Dam

Measures required to control volume change and prevent detrimental cracking in mass concrete were determined by test and analysis. This straight gravity dam 717 ft high with a base width of over 500 ft is being built with no joints parallel with the axis. Contraction and shrinkage of mass concrete, when resisted at rigid foundations and old lift joints, result in the development of tensile stresses which, if in excess of the tensile strength, can cause cracking. Stresses induced by restrained contraction occur in the mass as peak internal temperatures are reduced to mean regional temperatures, and at unprotected surfaces, when sharp climatic temperature drops occur. Stresses induced by restrained shrinkage in mass concrete usually occur from autogenous self generated volume change. Principal volume change controls adopted for Dworshak included proper selection of mix materials and design, pre-cooling mix materials, post-cooling placed concrete, and installation of insulation over external surfaces prior to cool weather.