Mixture proportioning and microstructural aspects of high-performance concretes
Abstract
Traditionally concretes are proportioned for high compressive strength, even when the emphasis is on achieving high durability. Although, this approach results in concretes that have high strengths, the associated durability is not certain. Concretes proportioned using this approach typically contain high cement factors which can lead to other potential problems such as thermal cracking, excessive shrinkage, and creep besides being expensive. In the present study, a new concept termed "Porosity Transformation" was developed to proportion high-performance concretes (HPC), to produce concretes that have improved properties without having to use high cement factors. The proposed approach was verified by preparing and testing a series of experimental concretes. To evaluate the effectiveness of the proposed approach, a set of reference concretes, designed using conventional procedure (ACI 211) were simultaneously prepared and tested. From this research, it was found that concretes proportioned using the porosity transformation approach exhibited superior mechanical and durability properties compared to concretes designed using conventional procedure, at about similar cement contents. Another aspect that was investigated in the present study was the "percolation phenomenon" due to interfacial transition zone. An experimental program based on a inter-aggregate spacing was devised to evaluate the influence of percolation effect on selected properties of high-performance and conventional concretes. From this study it was found that, for concretes with normal range of inter-aggregate spacings (60-100 microns), the effects of percolation phenomenon were not apparent in any of the properties tested for either the high-performance or the conventional concretes. Finally, in the present study, the alkali-silica reactivity of undispersed silica fume agglomerates in high-performance cementitious systems was investigated. This study was conducted to identify the state of condensed silica fume in cement pastes and mortars and then determine the mechanism and the factors influencing the associated alkali silica reaction. It was found from this study that significant fraction of condensed silica fume remains undispersed, even in high-sheared cement pastes and mortar systems. Further, these agglomerates were found to actively participate in an alkali silica reaction. Agglomerate size and its morphology were found to have a distinct influence on the degree of distress. Also, the degree of deterioration was found to be inversely related to the w/b ratio of the specimens.
Degree
Ph.D.
Advisors
Olek, Purdue University.
Subject Area
Civil engineering
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