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Title: The proposed hybrid flaws model of interfacial transition zone in HPC at normal and elevated temperatures
Other Titles: Gao xing neng hun ning tu zai zheng chang he gao wen xia qi jie mian guo du qu zhi hun jie lie wen mo xing ti yi
The proposed hybrid flaws model of interfacial transition zone in HPC at normal and elevated temperatures
Authors: Abid, Nadeem
Department: Dept. of Building and Construction
Degree: Doctor of Philosophy
Issue Date: 2005
Publisher: City University of Hong Kong
Subjects: High strength concrete -- Cracking
High strength concrete -- Testing
Notes: CityU Call Number: TA440.A24 2005
Includes bibliographical references.
Thesis (Ph.D.)--City University of Hong Kong, 2005
x, [348] leaves : ill. ; 30 cm.
Type: Thesis
Abstract: This thesis examines the deterioration mechanism in high performance concrete (HPC) subjected to elevated temperatures. This has mainly been illustrated by proposing a model of interfacial transition zone (ITZ) in HPC applicable to both normal and elevated temperatures. HPC mixes incorporating metakaolin (MK), pulverised fuel ash (PFA) and ordinary Portland cement (OPC) were used in the investigation. Metakaolin is a relatively new pozzolanic material for concrete and is being increasingly investigated by many researchers all over the world. However, it is realized that there is a lack of published literature on its elevated temperature performance. Moreover, there is hardly any published study available on the microstructure of HPC describing the character of ITZ and hardened cement paste (hcp) under elevated temperatures. The studies in this thesis try to address these issues by using Scanning Electron Microscope (SEM) and Image Analysis (IA) in addition to mechanical, permeability and stiffness properties investigations of HPC under normal and elevated temperatures. Variables of the test program include partial replacement of cement with MK from 5% to 20%, PFA from 20% to 60%, temperatures from 27°C to 800°C and two types of cooling methods. The microstructure of ITZ in HPC is investigated by the analysis of SEM images of fractured concrete specimens. These images show the presence of both ‘flaw’ and compact areas in ITZ of concrete at normal and elevated temperatures. The flaws are categorized into Texture and Orientation (TO) flaws and Local Band (LB) flaws. TO flaws are associated either to the surface roughness of aggregate (texture flaws) or to the close-gap between two or more aggregate particles (orientation flaws). LB flaws are associated with local insufficient compaction of concrete and they can occur at areas in ITZ where the conditions for TO flaws are not present. The distribution of the number of TO and LB flaws and the variations in their porosity at various temperatures enabled the author to propose ‘The Hybrid Flaws Model of ITZ in HPC at Normal and Elevated Temperatures’. According to the proposed model, the character of ITZ in HPC changes gradually from a discrete or discontinuous flaw zone at normal temperature to a continuous flaw zone at elevated temperatures. For normal to low range elevated temperatures (27°C-200°C), there are more number of TO flaws than LB flaws. There are compact areas between flaws which make the flaws discrete. For moderately elevated temperatures (200°C-400°C), there is no specific increase in the number of TO flaws but there is an increase in the number of LB flaws and LB flaws outnumber the TO flaws. The increased number of LB flaws is due to the effect of elevated temperature. The porosity of both types of flaws in this temperature range is higher than the porosity at temperature range of 27°C-200°C. Some of the new LB flaws join with the original TO and LB flaws and the discreteness of flaws reduces. Thus the ITZ at this temperature range shows semi-discrete or semi-continuous flaws. At the high range of elevated temperatures (400°C-800°C), LB flaws appreciably outnumber TO flaws. In this range of temperatures, the porosity of flaws further increases and the microstructure of ITZ shows almost continuous flaws. This is a highly disintegrated state of concrete and is manifested in other properties of concrete at elevated temperatures. Mechanical properties tests on compressive strength show that HPC concrete is slightly weaker than the corresponding mortar at temperatures of 400°C or above. This is because of the property losses in ITZ of HPC due to elevated temperatures. Chloride permeability results shows that HPC is less permeable than corresponding mortars at normal temperatures. However, relative permeability of HPC with respect to mortar increases with the increase of temperature and at temperatures of 600°C or above HPC is more permeable than mortar. This shows the changing contribution of ITZ of HPC to allow more charge to pass through ITZ at elevated temperatures than at normal temperature. SEM study using image analysis of the hcp of mortar specimens shows that there is an increase in the pore area fraction with a simultaneous decrease in calcium hydroxide (CH) and hydrated paste (HP) area fractions of hcp with the increase of temperature. The strength loss and high permeability at elevated temperatures can be ascribed to the changes in HP and pore area of hcp with temperature. A relatively new method ‘stiffness damage test’ (SDT) is performed to assess the damage to HPC due to elevated temperatures by measuring Damage Index (DI), Chord Modulus (Ec), Unloading Stiffness (Eu), Non Linearity Index (NLI) and Plastic Strain (PS). These five parameters are sensitive to temperature change and gave a good idea of the damage mechanism in various HPC mixes tested. Variation in NLI with temperature in SDT shows that the ductility of HPC increases with the increase of temperature but this occurs simultaneously with losses in other properties of HPC. The results from mechanical properties studies for the effect of cooling method show that the additional loss in compressive and tensile strengths due to quick cooling is high in the range of 300°C to 500°C. This is because the effect of thermal shock in this range of temperatures is higher than at temperatures below 300°C or above 500°C. HPC with MK shows better performance than other type of concrete mixes at normal temperature. At elevated temperatures above 400°C, all HPC mixes show appreciably more loss in properties relative to the values at normal temperature. PFA mixes exhibit better performance than other types of mixes at elevated temperatures. HPC specimens do not suffer appreciable loss in compressive strength up to 400°C under slow cooling. However, there is sharp loss in other properties like tensile strength, water sorptivity, chloride permeability and stiffness of HPC at temperatures above 200°C.
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