Unbonded Concrete Overlays for the Rehabilitation of Continuously Reinforced Concrete Pavement

North Carolina constructed continuously reinforced concrete pavement (CRC) between 1968 and 1978, with almost all sections located on interstate highways. Thickness of the CRC pavement was reduced from 9 inches (the standard thickness of jointed concrete pavement at that time) to 8 inches to offset the cost of the steel reinforcement. In some sections of CRC, the transverse steel was either reduced or eliminated, again to make the cost comparable to jointed concrete. Traffic, and particularly heavy truck traffic, increased rapidly and pavement distresses developed. In addition to punchouts, longitudinal cracking was observed in sections where transverse steel was reduced or omitted. Selection of a rehabilitation option required a treatment that would minimize the risk of uncovering weak subgrade, while maintaining traffic on the 4-lane divided highways. To date, two interstates have had a total of 5 sections of unbonded overlay. This paper outlines North Carolina Department of Transportation's (NCDOTs) experience in selection, construction and performance of unbonded concrete overlays, highlighting the three sections on I-85. Selection criteria include an evaluation of bridge clearances, the number of mainline structures, the need for horizontal or vertical grade changes to meet current interstate standards, traffic control considerations, and the depth of embankments. The unbonded overlay raises the grade by about 12 or more inches. As a result, costs must be included to reconstruct shoulders, pave ramps, reset guardrail and for pavement removal to meet structures and maintain clearances if near minimum levels. Drainage proved to be an issue on the first project where the bonded overlay was being constructed on one lane while traffic was being maintained on the adjacent lane. Water would back up across the travel lane on the uphill side of superelevations. We solved the problem by changing from a surface course to an asphalt stabilized drainable layer for the separator layer. North Carolina performs pavement condition surveys every year on the interstate system. A group of four pavement coordinators from the Pavement Management Unit are responsible for the interstate surveys. On jointed concrete pavements, a sample of 39 to 41 slabs (1/10th mile) is used for each mile; 100% coverage is used for CRC. Construction of the unbonded overlays resulted in a dramatic “bump” of the Pavement Condition Ratings (PCR). The rate of decline of the PCR is about 1 point per year based on 5 surveys. Most of the decline in PCR to date is associated with damage to the joint seals. In all except the first section of unbonded overlay constructed on I-85, IRI was equal to or better than pre-overlay values. The Division reports a reduction in maintenance costs from per year to per year.