Experience in post-crack performance testing of fiber-reinforced concrete (FRC) and shotcrete (FRS) using beams has demonstrated that variability both at first crack and in the post-crack range is high. When variability in performance is high it is important to carefully consider the within-set standard deviation in order to distinguish significant from insignificant differences between sets of data. It is also important to consider the characteristic distribution of results and use an appropriate probability distribution function (PDF) to estimate the standard deviation. The present investigation has sought to identify whether the Normal probability distribution function is the most appropriate model describing variability in cracking and post-crack performance parameters derived using beam tests.
The post-crack performance of fiber-reinforced concrete (FRC) is widely reported to exhibit a high degree of variability. Several reasons exist to explain this problem, most notably the small area of crack surface involved in most beam tests. Of less widely recognized significance to variability is the relationship between the sample standard deviation and the population standard deviation for this material. Standard statistical theory can be used to estimate the relationship between the variance in a sample of observations and the variance in the population from which the observations are drawn, but this critically depends on the observations being drawn from a population displaying a normal distribution. Moreover, the observations must also exhibit covariant stationary characteristics. In this investigation, it is shown that neither of these assumptions are necessarily true for FRC specimens produced using standard mixing procedures. Instead, the distribution of post-crack performance parameters obtained from standard beam and panel-based tests exhibit inconsistent characteristics that possibly require an empirical approach to effective estimation of the population standard deviation.
ASTM C1550 has been used for post-crack performance assessment of fiber reinforced concrete (FRC) and shotcrete (FRS) for several years and has proven to be an excellent tool for quality control testing due to the very low level of within-batch performance variability typically obtained using this test method. Experience in the field indicates that the positions of the three radial cracks generated in a test vary between specimens and concerns have been raised by users of this test that specimen performance may change as a result of this variation in crack location. The significance of this aspect of specimen behavior has been investigated to determine the characteristic range of crack rotations experienced in C1550 specimens as a result of variations in crack location and the magnitude of their corresponding influence on post-crack load resistance.
In the traditional PMV model, the calculation of human skin temperature is only related to the level of human activity. In fact, human skin temperature is related to many factors such as evaporative heat dissipation, radiation and convective heat transfer of human skin. Therefore, it is necessary to introduce human thermal physiological model to modify the skin temperature in PMV model. In addition, considering the impact of human activities on human metabolic rate, the formula of calculating human metabolic rate obtained from Pandolf was introduced to improve the original PMV model. Finally, the model is validated and analyzed with published experimental data. The results show that the improved model can accurately predict the thermal sensation of human body under the test condition, and the predicted thermal sensation is closer to the experimental value than the original PMV model.
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