Journal of Advanced Concrete Technology
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Volume 12 , Issue 8
Showing 1-4 articles out of 4 articles from the selected issue
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Scientific paper
  • Myoung Sung Choi
    Volume 12 (2014) Issue 8 Pages 239-249
    Released: August 06, 2014
    JOURNALS FREE ACCESS
    The objective of this study is to numerically investigate the effects of coarse aggregate size on the pumpability of concrete. In actual construction of super-structures, the coarse aggregate size plays a determining factor on concrete pumping. Thus, depending on the coarse aggregate sizes, the properties of lubrication layer which is the major role facilitating concrete pumping, were numerically simulated using shear-induced particle migration analysis. Moreover, the particle shape effects according to the coarse aggregate sizes were also considered in numerical simulation to precisely simulate the actual flow conditions. For verification, 1,000m long full-scale tests with three different coarse aggregate sizes (10, 20 and 25mm) were conducted and compared with numerical results. It could be found that when concrete is being pumped, the rheological properties of concrete and lubrication layer highly depend on the coarse aggregate size, which consequently influences the flow condition of pumped concrete, whereas the thickness of lubrication layer remained almost constant irrespective of the coarse aggregate sizes.
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  • José L. Granja, Miguel Azenha, Christoph de Sousa, Rui Faria, Joaquim ...
    Volume 12 (2014) Issue 8 Pages 250-265
    Released: August 08, 2014
    JOURNALS FREE ACCESS
    The use of embedded relative humidity (RH) sensors for assessing the internal humidity in concrete is widely spread, dully backed by existing standards. Even though the approaches adopted in the literature seem to have several differences between each other, few or none research works were found to focus on the comparison of performance of sensors and methods for RH measurement. In view of this, several sets of experiments comparing the performances of different sensors and monitoring procedures will be presented in this paper, discussing the main findings and providing recommendations for the strategies to be adopted in what concerns the measurement technique. The main points addressed in this work are: (i) comparisons between readily available systems for RH measurement in concrete, as well as custom measurement strategies reported in the literature; (ii) issues related to calibration procedures and re-calibration necessity; (iii) relevance of the existence of an interface porous material between the embedded sensor and the measurement spot in concrete; (iv) importance of the size of the embedment body into which the RH sensor is inserted; (v) equivalence of results obtained when the probe is constantly inserted into the embedment body, or placed inside it at discrete instants.
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  • Umberto Costa, Teresa Mangialardi, Antonio Evangelista Paolini
    Volume 12 (2014) Issue 8 Pages 266-278
    Released: August 27, 2014
    JOURNALS FREE ACCESS
    The effectiveness of blended cements (three pozzolanic cements - two with natural pozzolana and one with coal fly ash – and one blast-furnace cement) in counteracting alkali-silica reaction (ASR) was assessed by using both the ASTM C1567 accelerated mortar bar expansion test and the accelerated concrete prism expansion test at 38℃ and 100% RH. A low-alkali Portland cement was also tested as ASR inhibitor. The results of the mortar and concrete expansion tests were analyzed through a kinetic-based model (KAMJ model) to evaluate the expansion rate constant, k, and the Avrami exponent M. These two kinetic parameters were taken as efficacy parameters for each type of inhibitor. The results of the concrete prism expansion tests were also analyzed through an innovative methodological approach and a third efficacy parameter, such as the potential minimum alkali contribution by the inhibitor to concrete (Lim), was evaluated. It was found that the values of ln(k)mb =-6.0 for mortar bars and ln(k)cp = -7.7 for concrete prisms were appropriate efficacy criteria for discriminating between deleteriously expansive and non-deleteriously expansive mortar or concrete mixes, respectively. In the case of concrete mixes, a good relationship between the efficacy parameter Lim and the kinetic parameter Mcp was found, thus demonstrating the suitability of Mcp as a criterion for ranking not expansive concrete mixes. With respect to the kinetic parameters ln(k)cp and Mcp, the efficacy parameter Lim appears to be of greater technological interest, the last being strictly related to the composition of the concrete mix.
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  • Ippei Maruyama, Ai Sugie
    Volume 12 (2014) Issue 8 Pages 279-288
    Released: August 30, 2014
    JOURNALS FREE ACCESS
    This paper describes new insights on the influence of aggregate size on drying shrinkage of concrete through numerical simulation. Using a two-dimensional finite element method, it is demonstrated that it is impossible to reproduce the impact of aggregate size on concrete drying shrinkage by simply considering crack opening in the mortar matrix. From this result, it is deduced that the effect of aggregate size on concrete shrinkage is influenced by aggregate surface area. The application of a constitutive law that considers the characteristics of ITZ to coarse aggregate elements successfully reproduced the damage and shrinkage strain of concretes with different aggregate sizes. A certain range of virtual ITZ pore thickness in the model was found to allow simulation of the real phenomenon. It is concluded that ITZ characteristics should be taken into account when considering aggregate behavior in drying concrete.
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