|dc.description.abstract||Early-age cracking is a recurring problem in many mass concrete structures. Currently agencies tend only to limit the maximum temperature difference that may develop in a mass concrete structure in an attempt to mitigate this distress. A maximum temperature difference of 35°F is most often used irrespective of the materials used or placement conditions. Although early-age cracking has been documented since the early 20th century, explorations into this problem have just recently begun in the U.S. The primary objective of this thesis is to evaluate the early-age cracking tendency of mass concrete.
The research presented in this thesis involved implementation of one match-cured rigid cracking frame, one isothermal rigid cracking frame, and a match-cured free shrinkage frame to explore early-age cracking mechanisms of mass concrete. The rigid cracking frames were used to evaluate the development of restrained stresses due to thermal and autogenous deformations. The free shrinkage frame was used to evaluate the thermal and autogenous deformations under zero stress conditions. The laboratory testing program was designed to evaluate the effects of placement temperature, ambient temperature, cement type, supplementary cementing materials, air entrainment, and water-to-cementitious ratio on the cracking tendency of mass concrete mixtures.
The laboratory testing program revealed that the heat generated during hydration greatly affects the restrained stress development of concrete. Measures such as variations of placement and ambient temperature as well as use of supplementary cementing materials were found to be the most effective means of reducing heat generation in mass concrete thus reducing restraint stresses.
The behavior of concrete under isothermal conditions was also investigated. The isothermal cracking frame was held at a constant temperature for the duration of the test. Water-to-cement ratio was found to be the most significant variable controlling the magnitude of stress development associated with autogenous shrinkage. The use of supplementary cementing materials to partially replace portland cement was found to mitigate autogenous shrinkage deformations.||en_US