Other Significant Developments

Other Significant Developments

Several other significant PCC pavement-related developments occurred within the North Central region, as described in the following sections.

Slipform Paving

The slipform paver completely revolutionized the PCC pavement construction industry and greatly increased overall paving productivity. The slipform paver traces its roots to the Iowa State Highway Commission, where highway engineers James Johnson and Bert Myers developed

a prototype device in 1947 and first slipformed a roadway in 1949 (35). The first project was a

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county highway located near Primghar (in the northwest corner of the state) and was paved in two passes because the slipform paver was only 10 ft wide. From the 1950s into the 1960s, the slipform paver moved from an experimental process to a proven technology, even for the mesh- dowel pavements that were commonly constructed in the North Central states in the 1960s (36). By the early 1970s, virtually all highway agencies had adopted the slipform paver for Interstate pavement construction (17).

D-Cracking

D-cracking has been a nemesis of PCC pavements in the North Central region since it was first identified in Kansas in the 1930s. D-cracking is a form of deterioration in PCC pavements associated primarily with the use of coarse aggregates that disintegrate when they become saturated and subjected to repeated freeze–thaw cycles (37). D-cracking is visible as a series of fine cracks generally running parallel to joints, cracks, or free edges in the slab and may be accompanied by spalling, scaling, and staining.

The performance of many PCC Interstate pavements constructed in the North Central region has been hampered by the development of D-cracking. This led to major research efforts in the highway agencies to develop beneficiation techniques to upgrade coarse aggregates for use in pavement construction. One of the most effective methods found to reduce the susceptibility of coarse aggregates is to reduce the maximum size of the aggregate, and consequently many highway agencies adopted a maximum coarse aggregate size of 0.5 or 0.75 in. This practice, however, may lead to a significant increase in the paste requirement and may compromise the structural integrity of cracks and joints relying on aggregate interlock for load transfer (38). In recent years, highway agencies have developed extensive testing programs using a suite of tests to identify D-cracking susceptible aggregates.

Summary of PCC Pavement Design Trends in North Central Region

The following summarizes some of the key trends that have been observed in the design of PCC Interstate pavements in the North Central region:

• Use of longer designs lives (from a nominal 20-year design to 30 years or longer); • Construction of thicker slabs (from 9- and 10-in. slabs to 11- to 14-in. slabs); • Adoption of JPCP designs as the almost exclusive PCC pavement design type (with three

states also constructing CRCP); • Use of shorter JPCP joint spacings (typically 15-ft perpendicular joints that are uniformly spaced); • Adoption of dowel bars for virtually all Interstate pavement construction; • Increased use of treated and permeable bases for most Interstate pavements; • Incorporation of edge support features, including tied PCC shoulders and widened slabs; • Resurgence in production of durable PCC mixtures (including use of combined

gradations, supplementary cementitious materials, and specialized admixtures); • Adoption of initial PCC pavement smoothness specifications; and • Reexamination of PCC pavement surface texturing techniques because of noise concerns

(with some agencies adopting alternative methods, such as random transverse tining, longitudinal tining, and burlap drag).

74 Pavement Lessons Learned from the AASHO Road Test and Performance of the Interstate Highway System