Date of Award
Master of Science
Civil, Construction, and Environmental Engineering
Air void clustering is a phenomenon in concrete in which air bubbles accumulate around the coarse aggregate. It is considered as a major cause of reduction of concrete strength.
This thesis focuses on the effect of different variables on air void clustering and its contribution to the performance of concrete. Six variables were considered in the study, including cement type (low alkali cement and TIL cement), fly ash (fly ash A and B), coarse aggregate type (lime stone and river gravel), chemical admixture type (admixture 1 and 2) , mixing water temperature (70℉ and 90℉), and retempering (with and without). A total of 64 mixtures were prepared. The slump, unit weight and air content were tested on the fresh concrete. For hardened concrete, compressive strength at 7 and 28 days was determined. Air void structure including spacing factor and specific surface, and air void clustering were evaluated using rapid air and imagine analysis respectively. To study the temperature influence on air void clustering in more details, additional mixtures with all concrete materials heated to 90℉ before mixing were made. Statistical method including stepwise linear regression and neural network were utilized for data analysis to investigate relationships among variables, fresh and hardened properties of concrete.
This study found that air void clustering was observed in mixtures with and without retempering; thus retempering might not be the single cause for air void clustering, though it does exacerbate the severity of air void clustering. Fly ash type and coarse aggregate type had significant effects on air content and air void clustering. However, cement type and admixture type only had significant effects on air void clustering. The higher temperature increased air content, air void clustering, and strength loss of concrete due to air void clustering. Although retempering and more air content could increase air void clustering around the aggregate, they still had a limited function on growth of air void clustering.
Clustering sensitive index (CSI) was first proposed to describe the sensitivity of concrete to clustering. The smaller value of CIS indicated higher sensitivity of concrete to air void clustering with retempering. Air void clustering causing strength loss was observed and it was possibly happened more easily in an air void system with low spacing factor and high specific surface. The model established by the neural network had a good fit with experimental data, and it reveals clearly the negative effect of air content and air void clustering on compressive strength.
Sun, Wen, "Air void clustering in retempering concrete and its contribution to compressive strength" (2019). Graduate Theses and Dissertations. 17574.