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Recently, the push to improve bituminous concrete mixtures has resulted in higher fractions of crushed aggregate. Since coarse aggregate is becoming more difficult to extract as gravel sources are depleted, the cost of supplying crushed aggregate is increasing. Adding too much crushed aggregate can produce two mechanisms for problem mixes. The first arises from the crushing and washing of the manufactured fines portion of the aggregate. Hammermill, impact or cone crushers have a tendency to produce gradations for fine aggregates below the maximum density line when washed. This can lower the voids in mineral aggregate which is important to resist rutting.
A theory is advanced that aggregate water absorption can be used to predict the bonding surface volume available to improve results of heavy traffic mixes. Let the bonding surface volume be defined as the volume around each particle opened by pores. The second mechanism can be described as the propensity for shearing of mastic to aggregate bonds over time when aggregates with low water absorption values are used. Marshall stability tests for a mix using aggregates with generally high values of water absorption are compared to tests for a mix using aggregates with low values of water absorption. The conclusion supports the premise that shearing under load increases as average water absorption values decrease.
asphalt cement, arterial mixes, bituminous concrete, bonding surface volume, surface shearing, stability, voids in mineral aggregate
assistant manager of gravel operations, Borland Construction (1989) Limited, Winnipeg, Manitoba