AgdaPkt 2006-06-05 - Laserfiche WebLink

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6.1E Page 2 Redwood City residents. Several neighboring Cities have recently switched to black-color aggregate, because of the improved appearance over gray-color aggregate. . Slurry made with lighter colored aggregate becomes light grey soon after application, resulting in a pavement that appears aged and worn down. Residents may perceive that the gray aggregate is an inferior or a lesser quality product, based on appearance. Visibility: . Slurry with black-color aggregate provides a darker pavement surface. This provides a better contrast for the white and yellow pavement markings and traffic lane lines, making the markings stand out more, especially at night. . According to a report prepared by the local material supplier, lighter color aggregate roadways improve the effectiveness of streetlights and headlights at night by reflecting more lighting than would a darker surface, which would absorb more of the light. Durability: . Based on observations of past projects, staff believes that the black-color aggregate performs better to resist the problem of "bleeding" when placed over a new chip seal surfacing. Bleeding occurs when the asphalt emulsion rises up through the aggregate, and pools on the surface. We believe that the porous nature of the black-color rock allows more absorption of excess asphalt oil. . To ensure quality, aggregate must meet minimum requirements measured as the Sand Equivalent (SE) and Durability Index (01) of the aggregate. Both the Sand Equivalent and Durability Index values were greater for the black-color aggregate sample.1 . According to a report prepared by the local material supplier, gray-color aggregate is lighter, and therefore reflects heat while darker colored materials absorb heat. The lighter colored material reduces the temperature of the slurry seal coating, and also reduces the temperatures transferred to the underlying pavement layers. Higher temperatures age asphalt products, causing the asphalt to become brittle or stiff more rapidly. 1 SE expresses the concept that most granular soil and fine aggregate are mixtures of desirable coarse particles, and generally undesirable plastic fines. The higher the SE, the less undesirable plastic fines. OJ provides a measure of relative resistance of an aggregate to producing clay-sized fines when subjected to prescribed methods of interpartical abrasion in the presence of water. The City has a report from a testing laboratory in which these values were measured for both a sample of black-color and gray-color aggregate. The table below shows the test results. Test City ReQuirement Black Rock Gray Rock Sand EQuivalent 60 minimum 76 69 Durabilitv Index 55 minimum 81 68 Page 2 of 3