Agenda - Council - 02/11/2020 - Laserfiche WebLink

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balanced by the egress of moisture in the form of water vapor) wasn't true, there would be substantially more stripping problems. The current investigation is concerned with stripping of generally older pavements with chip seals applied `late' (>7 years) in the pavement's service life. So, it is interesting to note that stripping almost universally occurs from the bottom up, or at the interface of two layers outward, so, based on this, it is logical to be concerned with the application of a chip seal over an HMA pavement, as the new pavement is now a "lower layer". In addition, section 7 (surface treatments) of the Texas DOT Pavement Design Guide (5) recommends testing HMA mixtures for susceptibility to stripping before the application of a chip seal because, "A surface treatment will generally seal off the vertical escape of moisture migrating upward out of a pavement which can set up accelerated stripping in the existing HMA layer beneath the seal". The sealing off of the vertical escape of moisture will not increase the pavement's stripping susceptibility if there isn't moisture, in other words if the moisture can escape through the sides (edge drains), or if the moisture is intercepted with a permeable base layer. Thus this implies that the stripping mechanisms of the pavement were already in -place, merely accelerated by the presence of the chip seal surface treatment. Texas currently, as of 2011, has two standard methods for testing moisture susceptibility of HMA mixtures: 1. Placing a 200g sample of prepared HMA, no more than one aggregate thick in a beaker which is immersed in boiling oil 2. Modified Lottman Test which involves comparing the Tensile Strength Ration of moisture conditioned specimens with that of non -conditioned specimens. Aschenbrener and McGennis () reported on using the boiling water test and seven versions of the modified Lottman test (AASHTO T 283) to predict the stripping of materials extracted from twenty sites of known field (stripping) performance. They reported that two levels of severity for conditioning laboratory samples correlated well with what was observed in the field; the most severe conditioning cycle included a 30-minute vacuum (610 mm HG) saturation a 15 hour freeze, followed by a 16 hour soak in a high temperature water bath, the milder conditioning consisted of: 55-80% saturation and no freeze, followed by a 16 hour soak in a high temperature water bath. They recommended the severe conditioning for mixtures placed under high traffic, high temperature, high moisture and possibly freeze conditions and milder laboratory conditioning for mixtures placed on low traffic sites. They went further, stating that it is critical that the conditioning in the laboratory (vacuum saturation, freeze, hot-water soak) be equal, or greater than the severity expected in the field. The authors did not recommend the boiling test as it is a very severe test that does not consider important factors of: gradation, void structure, or permeability, all of which influence field performance related to moisture susceptibility. B-8