![]() ![]() Failure to take into account the climatic conditions may lead to inadequate or reduced pavement performance. Therefore, appropriate procedures for design and material selection need to be selected to withstand high precipitation and freezing winter temperatures. Wet-freeze climates can result in various concrete pavement distress mechanisms such as thermally-induced cracking, freeze-thaw deterioration, accelerated cracking due to loss of support, frost heave, and material degradation. The local wet-freeze climate makes the requirements for Michigan's pavement system different from many other regions. For the purposes of this paper, a best practice is a procedure that has been shown by field-validated research or experience to produce improved results and that is established or proposed as a standard suitable for widespread implementation. Table 1.The research presented in this paper aims to identify best practices of design and materials for concrete pavements in wet-freeze climates similar to the Michigan State. II-9) provides some recommended levels (reproduced in Table 1). ![]() Recommended Values of Z R and S oĮach agency that uses the 1993 AASHTO Guide design equation can choose their own levels of reliability to use, however the 1993 AASHTO Guide (Table 2.2, p. However, if the logarithm on the left side of the equation is removed through algebraic manipulation, it can be seen that the quantity (Z R ´ S o) would then be multiplied with the resulting equation. At first glance it may seem unusual that the quantity (Z R ´ S o) is added to the right side of the equation rather than multiplied. Therefore, the quantity (Z R ´ S o) is always negative and will serve to decrease the predicted number of 80 kN (18,000 lb.) ESALs that a particular design can accommodate. The right side of the above equation is then augmented with S o (to account for input value variability) and Z R (to establish a confidence level that a certain design will perform as intended) to obtain the final form of the equation: The more these values vary, the higher the value of S o.įor example, the basic 1993 AASHTO Guide flexible design equation is: What these two brief examples are expressing is that structural design input values can vary from those initially chosen and the equation must account for this somehow. Similarly, pavement design factors may turn out to be different than estimated. However, actual traffic may turn out to be 2,500,000 ESALs over 20 years due to unanticipated population growth. For instance, traffic may be estimated at 2,000,000 ESALs over 20 years. ![]() This variable defines how widely the two basic design inputs, traffic and performance, can vary. S o = combined standard error of the traffic prediction and performance prediction.Then, the reliability is 95 % (100 % – 5 %) and the corresponding Z R value is -1.645 (see 1993 AASHTO Guide, Table 4.1, p. This is the same as stating that there should be a 95 % chance that the design does last the specified number of years (e.g., 20 years). For example, a designer may specify that there should only be a 5 % chance that the design does not last a specified number of years (e.g., 20 years). The standard normal table value corresponding to a desired probability of exceedance level. The reliability factor is comprised of two variables: All variability the pavement structural design process is then accounted for in the “reliability” factor. Basically, a pavement structure is designed using the most accurate input data available data are not manipulated or inflated (nor are conservative values used) to compensate for their estimated variability but rather the best value is used. (Note: design period is defined as the time from initial construction or rehabilitation to its terminal serviceability index.) ReliabilityĪASHTO uses the reliability concept to account for design uncertainties. “The reliability of the pavement design-performance process is the probability that a pavement section designed using the process will perform satisfactorily over the traffic and environmental conditions for the design period.” (AASHTO, 1993 ) ![]()
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