The rigid pavements in the first construction cycle (CC1) at the NAPTF developed corner cracking during the early stages of traffic testing. At this point the pavements were considered to be inadequate for normal life cycle testing since failures attributed to non-structural mechanisms could affect the test data obtained for development of structural failure criteria.
NAPTF rigid pavements were scheduled for reconstruction and testing in 2003, and it was essential for the new rigid sections to perform as required, i.e., by experiencing defined structural failure before secondary failures occur. Several primary factors were believed to be responsible for the slab curling that caused CC1 corner cracking (McQueen, 2002):
• Use of a shrinkage prone mix during the original construction;
• Intermittent wetting and drying of the slab surface during curing;
• Unfavorable temperature gradient between subbase and ambient temperatures during placement and initial cure period;
• Unfavorable aspect ratio for slab curling (i.e., slabs too large for thickness); and
• Since the slabs were constructed indoors, an extended period (1 year) elapsed with little additional moisture available to the concrete.
Prior to full construction of the CC2 test items, a PCC test strip was constructed to study the effects of slab size and mix design on pavement curling during early age of the concrete. This study was motivated by the observation that most of the concrete slabs constructed in Construction Cycle 1 (CC1) had exhibited significant curling and premature corner breaks under traffic. Factors such as concrete mix, slab size, and curing procedure were examined for their potential in mitigating curling. Experience gained was to be applied to the placement and curing of the new concrete test items over the medium strength subgrade. A PCC test strip consisting of 12 slabs was designed to address the following concerns:
• Comparison of PCC mix designs, slab sizes 15 ft. and 20 ft. (4.5 meters and 6.0 meters) and curing methods to minimize the risk of corner cracking;
• Evaluation of removal and replacement methods;
• Measurement of pavement response related to curling; and
• Traffic for failure model.