Browsing by Author "Kahn, Lawrence F."

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    Characterization of elastic and time-dependent deformations in high performance lightweight concrete by image analysis
    (PERGAMON-ELSEVIER SCIENCE LTD, 2009) Lopez, Mauricio; Kahn, Lawrence F.; Kurtis, Kimberly E.
    Image analysis and strain mapping were used to examine the nature of elastic, creep and shrinkage strains in high performance lightweight concrete (HPLC). The strain maps showed non-uniform deformations related to microstructural features. Both average strain and non-uniformity increased with time under testing. Paste-rich regions exhibited higher creep plus shrinkage than the lightweight aggregate (LWA) particles examined herein; it is suggested that LWA could have a role in reducing deformations of the paste. Compared to normal weight high performance concrete (HPC), the paste and LWA in the HPLC exhibited more gradual spatial differences in elastic deformations, creep and shrinkage. It is proposed that this difference results from the lower stiffness of the LWA compared to granite used in the HPC. The results indicate that improvement in elastic property matching between the lightweight aggregate and high performance paste reduces stress concentrations at the aggregate/paste interface and contributes to reductions in deformations of HPLC compared to HPC. (C) 2009 Elsevier Ltd. All rights reserved.
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    Characterization of elastic and time-dependent deformations in normal strength and high performance concrete by image analysis
    (PERGAMON-ELSEVIER SCIENCE LTD, 2007) Lopez, Maunicio; Kahn, Lawrence F.; Kurtis, Kimberly E.
    Deformations in normal strength concrete (NSC) and high performance concrete (HPC) were examined using image analysis to better understand the distribution of strain in these materials as related to their composition and microstructure. Elastic strain, creep and shrinkage were shown to occur non-uniformly throughout the NSC and HPC microstructure and creep and shrinkage strain both increased with time, as expected. The non-uniformity in the strain measurements also increased with time, as a consequence of dissimilarities in time-dependent behavior of the paste and the aggregate. However, compared with NSC, the time-dependent strains in the HPC were lower and showed less variation, suggesting a more uniform microstructure. In both NSC and HPC, high-strain sub-regions were evident in the vicinity of the interfacial transition zone (ITZ), likely as a consequence of the strain mismatch between aggregate and paste. The thickness of the high strain sub-regions along the ITZ in HPC were approximately one half of those in NSC. (c) 2007 Elsevier Ltd. All rights reserved.
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    Effect of internally stored water on creep of high-performance concrete
    (AMER CONCRETE INST, 2008) Lopez, Mauricio; Kahn, Lawrence F.; Kurtis, Kimberly E.
    The effects of internally stored water on long-term deformations of high-performance concrete (HPC) were examined on 130 creep and shrinkage specimens and 750 compressive strength specimens. Creep and shrinkage were monitored for 500 days on sealed and unsealed concrete specimens using either normalweight or lightweight aggregate with different initial moisture conditions. The use of prewetted lightweight aggregate decreased creep by approximately 45% when compared with mixtures with air-dried lightweight aggregate. HPC with prewetted lightweight aggregate experienced creep that was 10% lower than that obtained in a similar HPC with normalweight granite aggregate. It is proposed that the reduction in creep with the prewetted lightweight aggregate is due to hydration enhancement, expansion afforded by internal curing, and by inhibition of water seepage.