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Title:Microstructure characterization of foamed cement and concrete
Author(s):Das, Arnesh
Advisor(s):Lange, David A.
Department / Program:Civil & Environmental Eng
Discipline:Civil Engineering
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):air voids, foamed cement, air-entraining admixture, SEM
Abstract:Freeze-thaw deterioration of concrete is a major durability issue in cold climates. Air-entraining admixtures are used to create a network of fine bubbles that enhance frost resistance. Experience has shown that fresh air content alone cannot predict the effectiveness of the entrained air system. Instead, parameters such as bubble spacing factor measured on polished sections are more useful for assessing freeze-thaw durability. This study is motivated by concerns about how vibration and handling of fresh concrete may affect air void parameters. Fresh concrete mixtures were cast into both crosstie and cylinder molds which were consolidated using a rod vibrator and table vibrator respectively. The consolidating effect was characterized based on acceleration measurement in the fresh mixtures during vibration and the hardened air-void parameters prescribed by ASTM C457 were also estimated using an automated flatbed scanner method. There was no significant change in the spacing factors of the concrete mixtures subjected to vibration, indicating the robustness of the air-void system. The results also showed strong correlation on the air content and spacing factor between the tie and cylinder samples. This further validates the performance based approach investigated in this study. With regard to foamed concrete, the mechanism of freeze-thaw deterioration is more complicated as there may be a large amount of free water present in the comparatively larger air voids. This makes it necessary to understand the movement of water in foamed cementitious materials. The water absorption characteristics of foamed cement mixtures of densities ranging from 0.4 g/cm3 to 1.6 g/cm3 were investigated. An air-entrained cement and a pure cement paste mixture were also used for the study. Three different water absorption tests were conducted and it was found that amount of water absorbed decreased as density increased from 0.4 g/cm3 to 0.9 g/cm3 but then increased on further increase in density. As density changed from 0.9 g/cm3 to 1.2 g/cm3, the material transformed from an open cell to a closed cell. For the open cell materials, because the voids are interconnected, water flowed through them whereas in the closed cell materials, water flow took place through the capillary pores. This could also be well explained from the SEM images of the samples. It was also concluded from this study that the size of voids governs the absorption of water for the lower density samples while for the higher density samples, only void content matters. Chemical admixtures are an important constituent of concrete and analysis of presence of their traces in concrete is important from the point of view of forensic science. The reliability and efficiency of few techniques, namely energy-dispersive X-ray spectroscopy, inductively coupled plasma optical emission spectrometry, CHN analysis and X-ray fluorescence were studied. The composition of foamed and air-entrained cement samples was analyzed before and after a ‘cleaning’ process (samples were cleaned in water/alcohol in an ultrasonic booth). Because SEM-EDS provides spot composition, it could qualitatively detect presence of traces of the foaming admixtures but their quantification was not possible. ICP-OES and CHN analysis could indicate the presence of the admixtures to some extent but XRF analysis was not found to be useful enough, even for qualitative purpose. In addition to these techniques, surface analysis methods such as X-ray photoelectron spectroscopy and secondary ion mass spectroscopy could be used in order to better qualify the presence of the foaming and air-entraining admixtures.
Issue Date:2018-05-17
Rights Information:Copyright 2018 Arnesh Das
Date Available in IDEALS:2018-09-27
Date Deposited:2018-08

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