Estimation of Curve Numbers for Concrete and Asphalt

by Ashok Pandit,
Howell H. Heck,

Document Type: Proceeding Paper

Part of: World Environmental and Water Resources Congress 2008: Ahupua'A


All impervious surfaces are assigned a Curve Number (CN) of 98. However, no field or experimental evidence exists (to our knowledge) that shows why the impervious CN was assigned a value of 98. The purpose of this study was to collect the necessary data to determine CN values for concrete and asphalt surfaces. Two 50 ft by 10 ft concrete pavements at slopes of 2.2% and 4.6%, and two 50 ft by 10 ft asphalt pavements at slopes of 2.4% and 5% were constructed to conduct field experiments. A water distribution and collection system was especially designed and constructed to be able to simulate rainfall at variable intensities on the pavements. A total of 64 experiments were conducted on the four pavements and the data were analyzed to determine the respective curve numbers for asphalt and concrete. Many of the experiments were duplicated on a smaller part of the asphalt pavement to avoid paved areas that may have developed construction defects. The effects of pavement slope (2.25 and 4.6%), rainfall intensity (ranging from 0.25 in/hr to 8 in/hr), and rainfall duration (ranging from 0.25 hours to 4 hours) on the respective CN values were examined for both asphalt and concrete pavements. It was determined that the CN for concrete is essentially 100 irrespective of pavement slope, rainfall intensity or rainfall duration. It was also found that the presence of cracks in the concrete pavement can lower the CN values to 98 especially during low intensity rainfall events. The CN values for asphalt ranged from 97 to 100. The CN for the asphalt pavement were always smaller for the steeper slope, however, changes in rainfall intensity and duration did not change the curve number value. It was also found that the presence of just a small porous area in the asphalt pavement, perhaps due to construction defects or weathering, considerably reduced the CN value. The asphalt pavement with the porous area yielded Curve Number values as low as 88. It was found that the defected part of the asphalt pavement was most influential when the rainfall intensity was relatively smaller. Therefore, it is likely that CN values, under normal conditions, may be significantly lower than 98 as it is normal for highways to develop defects due to weathering and other reasons.

Subject Headings: Asphalt pavements | Concrete pavements | Asphalt concrete | Rainfall intensity | Slopes | Runoff curve number | Rainfall duration

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