Relation of Field Cracking to Layer Stiffness of Cold In Situ Recycled Materials

In 1996 the Alliance for Transportation Research Institute completed a comprehensive evaluation of 45 cold in situ recycling (CIR) pavement sections on New Mexico State Highway and Transportation Department routes built between 1984 and 1990. The results of further investigation into observed correlations between visual condition surveys, as an indicator of field performance, and the CIR stiffness measured on cores are reported. It was observed that CIR projects in New Mexico that are older than 9 years, with measured resilient modulus exceeding 6000 MPa and indirect tensile strength (ITS) exceeding 800 kPa, also exhibited reduced pavement condition index ratings based solely on environmental cracking distress. Reported is a direct comparison with some additional stiffness data and field performance for pavements recycled with different additives. Accelerated aging following Strategic Highway Research Program procedures was used to assess some samples tested. New sections evaluated were constructed by using hot lime slurry, cement, and a cement-fly ash blend. The criteria for cracking threshold observed in CIR materials appeared to be unaffected by addition of hot lime slurry and were quite different from those for layers recycled with cementious materials. It is proposed that criteria noted for resilient modulus and ITS may be used to guide the development of cold recycle laboratory mix design in combination with accelerated aging to simulate hardening under field conditions. By properly selecting the type of additive and the amount, more crack-resistant pavement sections may be constructed. Recommendations are developed for target values of stiffness in recycled materials.