Date of Award
Doctor of Philosophy
Civil, Construction, and Environmental Engineering
An adequate air void system in concrete is critical to its freeze-thaw (F-T) durability. With an effective air void system, concrete can survive longer in a wet climate with F-T cycles. Over the years, researchers have focused on fresh air content and the hardened air void system, but little attention has been paid to changes in the air void system between those two states. Stability of air void system and its changes during handling, finishing, and even testing can influence the concrete performance.
This dissertation focuses on the air void system between when fresh air content is tested in practice and when the concrete is hardened. Three stages were investigated: 1). the stage after mixing and during transport/handling (at rest and not disturbed); 2). during pressure meter testing; 3). after internal vibration.
To study air voids during handling and transport, the first objective was to study the compatibility of the admixtures and cement that would be used in concrete mixing, and modified a method to evaluate compatibility to address critical details of the test procedure. Then 60 combinations of admixtures and cementitious binders were tested using this method to develop a pass/fail limit for compatibility of the materials. Using this limit, eight selected concrete mixtures were evaluated for air void system stability. The data indicated a good correlation between stability of air void system in concrete and compatibility of mixture ingredients.
The second topic reported in this dissertation is the variation of air void system in a Super Air Meter (SAM). The SAM uses cycles of pressure to assess the air-void spacing factor in fresh concrete. However, the mechanism behind this prediction and the changes in air volume inside the device under different pressures is not well understood. By comparing hardened air void analysis results of mortar and concrete subjected to different pressure stages, the tendency for air to dissolve under pressure in concrete and mortar was studied. Moreover, by analyzing fresh concrete pressure results and the hardened concrete air void system, correlation between results from SAM and spacing factor was found.
The third topic was the variation in an air void system within a concrete sample consolidated by internal vibration. This topic was divided into two parts: understanding the mechanism for air void system variation; and distribution of aggregate and air voids in such concrete. It was noted that there was an increase in air content in part of the samples excessively consolidated by internal vibration. Work was conducted to understand the factors that may affect this type of variation in an air void system. For aggregate and air void distribution, digital image processing was used to evaluate aggregate distribution in vibrated concrete. It was found that excessive vibration will affect aggregate distribution and cause segregation in the system. In terms of air content and air void distribution, it was found that the air increase after vibration regardless of the types of admixtures.
Wang, Xin, "Air entrainment and variation of air void system in fresh concrete" (2018). Graduate Theses and Dissertations. 16894.