Development of Material Requirements for Portland Cement Concrete Pavements at the U.S. FAA National Airport Pavement Test Facility

The National Airport Pavement Test Facility (NAPTF) is located at the Federal Aviation Administration’s (FAA) William J. Hughes Technical Center. The facility was constructed to obtain data on pavement performance that can be used to investigate the relative effects of fourand six-wheel aircraft gear loads, and to develop reliable failure criteria that can be used for development of mechanistic design procedures for airport pavements. In order to meet these basic objectives, prototypical test pavements were constructed consistent with actual in-service pavements. A basic requirement for the test pavements was to ensure that structural failure would occur at the desired location in the controlling pavement layer. For rigid test pavements, failure was defined in terms of structural cracking initiating at the joints at the bottom of the Portland cement concrete (PCC) layer. The thickness designs for the concrete slabs were predicated on these structural cracks forming as a result of the full-scale applied loading. Therefore, cracks or failures attributed to nonstructural mechanisms needed to be avoided in order to obtain the basic data for development of structural failure criteria. Although the pavements were constructed to exacting standards, corner cracks developed soon after full-scale gear loads were applied to the slabs. This form of cracking was not anticipated, since the NAPTF pavements were housed in a building and large temperature gradients in the slabs were not expected. In fact, temperature data acquired before and during application of the gear loads of less than ½ degree F per inch depth indicated that the corner cracking could not be due to slab warping from temperature gradients within the slabs. The crack mechanism is believed to be primarily due to differential drying shrinkage between the top and bottom of the slabs, possibly exacerbated by unfavorable temperature differences within the slabs during curing. Since the NAPTF rigid pavements are scheduled for reconstruction and testing in 2002, it is essential for the new rigid sections to perform as required, i.e., by experiencing defined structural failure before secondary failures occur. For the new rigid pavements, shrinkage tests were performed on concrete mixes batched with different aggregate types, sizes and proportions, including the mix used for the original construction. The primary purpose of these tests was to screen out potentially shrinkage-prone aggregates and to size the coarse aggregates for the next construction cycle. This was followed by detailed mix designs on several candidates, using mix optimization techniques to minimize shrinkage and maximize workability, and plant trials to further optimize the mix design. After selecting the candidate mix, a test strip was placed to investigate the effect of the slab size, concrete properties, and curing procedures on curling of the test slabs and corner cracking. The test strip slabs were instrumented to measure concrete strains, temperature, joint performance, and curling. Slab size, material requirements, and curing procedures for the full scale rigid test items planned for the next construction cycle will be based on the performance of, and data collected on, the test strips.

Authors: Roy D. McQueen, P.E., Jeffrey Rapol, Civil Engineer, Robert Flynn, Civil Engineer