Construction Cycle 2 (CC-2) Single Slab
A single concrete slab was placed at the NAPTF on June 2, 2003. This effort was intended to provide experience for the design, placement, and monitoring of the main CC2 PCC test items. The single slab was approximately the same size as those to be used in the test items, 15 ft. by 15 ft. by 11 in. thick. The concrete mix was also similar except that the cementious mix contained 60% class C flash as opposed to 50% for the test items. The slab was placed on an existing 20 ft. by 20 ft. by 9.75 in. thick concrete slab, which previously had been placed on a stabilized base in the medium-strength subgrade area as part of the original CC-1 pavement construction. Strips of construction paper were placed on the surface of the existing slab to act as a bond breaker between the new single slab and the existing slab. The sensors and data acquisition system were a dry run for the CC2 main test items. The single slab was located between stations 542 and 557 on the south side of the NAPTF test area (station is the distance from the origin in feet).
The slab was wet-cured for 28 days using burlap strips, timed soaker hoses, and plastic sheet coverings. All coverings were then removed, and the slab was allowed to dry for 113 days. After this period, the burlap strips, soaker hoses, and plastic coverings were reapplied. The slab was kept continually wet for the next 59 days, after which the data retrieval was discontinued. These alternate wet and dry periods allowed for the acquisition of data relative to characterization of the curling. The sides of the slab were coated with a sealer after the initial 28 day wet curing period. This action simulated a condition of the slab drying while being embedded among other slabs as it would be in an actual test item.
The objectives of the single slab experiment were as follows:
a. Gain experience with the placement of a high fly ash cementious mix concrete
b. Determine the adequacy of the instrumentation system
c. Determine the effectiveness of the monitoring system
d. Characterize the slab curling
e. Develop a procedure to control slab curling
Concrete placement procedures, the instrumentation system, and the monitoring procedures all proved adequate for application to the actual test items.
Curling during the 28 day wet curing period was minimal. Significant upward curling was observed, however, during the 113 day drying period. The average separation of the corners with the base exceeded 200 mils. Differential moisture developed through the slab thickness with the top of the slab drying prior to the bottom. The slab did undergo significant recovery during the 59 day rewetting period. However, an average residual curl of about 70 mils was still observed.
The most significant finding of the single slab experiment was that, if the upward curling of the slabs in the test items were to be held below 20 mils, considered to be a critical range for precluding premature top-down corner cracking, the concrete surfaces would have to be kept continually wet.
The slab was subjected to static loading tests prior to its demolition in preparation for construction of the CC-2 rigid pavement test items, medium strength subgrade.