Forensic Evaluation of Three Failed Cement-Treated Base Pavements

Three cement-treated base (CTB) pavements constructed around 1990 near Houston, Texas, showed severe pavement deterioration after 3 to 4 years. The unusual distress patterns included wheelpath alligator cracking and severe pumping. A forensic investigation was initiated to determine the cause and to recommend modifications to materials specification, design, and construction practices to avoid future problems. The primary cause was determined to be chemical deterioration that resulted in destruction of the cement matrix. In all cases water was trapped within the CTB layer. In two bases moisture flowed rapidly by capillary action through the CTB; these bases showed the most rapid deterioration. Water flow in one case was attributed to smectite clay contamination of the fine aggregate and in another to a highly absorptive sandstone coarse aggregate. Extensive ettringite was found in one material. In one case deterioration was caused by the growth of two phases exhibiting different morphologies: hydrated calcium aluminum silicate with a radiating needle morphology and ettringite, which showed a bladed morphology in these samples. Practical recommendations to prevent recurrence of the problem were (a) moisture absorption as acceptance testing for coarse aggregate and (b) elimination of designs placing different stabilized materials on top of one another. The laboratory suction test that was developed is recommended as a screening test for molded CTB specimens.