Forensic Evaluation of the Cement Treated Base Failure on SH 36 in Houston (7-2919-2)

The Cement Treated Base (CTB) on SH 36 experienced rapid deterioration after only three years in service. This report presents the field and laboratory studies undertaken to identify the cause of failure. The main cause of the failure was attributed to the process of carbonation where the calcium component from the cement matrix is leached out, resulting in layer disintegration. Contributing factors to this process were the substantial clay contamination of the field sands used which resulted in high suction levels within the CTB; and the pavement design which placed CTB's made from different aggregates on top of one another. This resulted in unusual crack patterns which permitted water to be trapped within the base.|Recommendations are given on how to avoid this problem in the future, including reducing the allowable PI of the fine materials from 10 to 8. A field carbonation test is also proposed and demonstrated on another project which was experiencing construction problems.