Changes in Other PCC Design Features

There are a number of other design features that go into concrete pavement design, and it is interesting to trace how these have changed during the time of the Interstate pavement construction in the North Central region. Table 3 lists the general practices for each of these design features at various points in time during the Interstate pavement construction (based on information compiled from PCA (16), Nussbaum and Lokken (17), ERES (23), and ACPA (18).

Historically, most agencies have placed transverse contraction joints perpendicular to the centerline of the pavement and at uniform, fixed intervals (e.g., 15 ft apart). Beginning in the 1960s, some agencies began experimenting with joint orientation and layout, often placing the transverse joints at a counterclockwise skew to the centerline (typically offset by 1 to 2 ft per 12- ft wide lane) and at repeated variable spacings (such as 12, 15, 13, and 14 ft). By the 1970s and on into the 1980s and 1990s, this was a standard practice in many states in the North Central region. However, Table 3 shows that the highway agencies in the North Central region returned to the use of perpendicular and uniformly spaced joints for JPCP, based on several performance reports questioning the effectiveness of skewed and variable joint spacings (24, 25).

At the beginning of the Interstate construction, JRCP designs in the North Central region were almost universally doweled, using either 1- or 1.25-in. diameter dowels. However, JPCP designs at this time were often undoweled, the belief being that aggregate interlock would provide effective load transfer because the shorter joint spacings would minimize joint opening.

TABLE 3 Historical Changes in Other PCC Pavement Design Features in North Central Region

Design Feature

JPCP transverse joint

Perpendicular and orientation and layout

Perpendicular and

Skewed and

uniform spacing

variable spacing

uniform spacing

1.5 in. dowels Load transfer

1 to 1.25 in. dowels

1.25 in. dowels

(JRCP designs only)

(JRCP and JPCP)

Hot-poured sealants

Hot- and cold-poured

Joint sealing

Unsealed joints (one

Silicone salants

sealants

state ―experimental)

Unsealed joints Treated or permeable

Granular and greater use

Base type

Granular base

bases (medium- to high-

of treated bases

traffic volumes)

PCC shoulders and/or Shoulder type

HMA and

HMA

experimental PCC

widened PCC slabs

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

By the mid-1970s, the use of dowel bars in the North Central region was almost universally adopted for both JRCP and JPCP designs, and the use of 1.25-in. dowels was fairly standard for the 9- and 10-in. slabs commonly constructed during that era. The more recent trends have been to the use of larger diameter dowel bars (1.5-in. are now quite common). A number of research projects conducted in the period of the 1970s to the 1990s confirmed the need for dowel bars for most PCC pavements subjected to significant truck traffic (24, 25, 26, 27).

Transverse joint sealing practices have evolved considerably in the North Central region. The early Interstate pavements were sealed after construction using either a hot- or cold-poured bituminous sealant. By the mid-1970s, agencies had adopted high-type hot-poured sealants almost exclusively, with one highway agency (Wisconsin) beginning to evaluate unsealed joints (28). In the late 1990s, the use of silicone sealants had become more common, and Wisconsin had adopted a no-seal policy for its new PCC pavements constructed on the Interstate. Moving into the 21st century, several highway agencies in the North Central region have constructed experimental pavements to assess the relative performance of sealed and unsealed joints.

Base types used in the early days of the Interstate pavements in the North Central region were almost universally granular. Performance studies conducted in the 1940s had clearly demonstrated the benefits of a granular base beneath PCC slabs to prevent pumping (29). Because of the experience of other highway agencies in the use of treated base courses, by the mid-1970s several highway agencies in the North Central region had either adopted or were evaluating the effectiveness of treated bases (e.g., aggregate or soil–aggregate mixtures treated with a small percentage of either cement or asphalt) in reducing joint faulting and improving overall performance [see, for example, studies by Minnesota (30), Michigan (31), and Ohio (32)]. In the past decade, many highway agencies in the North Central region have adopted the use of treated or permeable bases for Interstate pavements subjected to significant truck traffic volumes.

The 1956 Federal-Aid Highway Act called for a 10-ft paved outer shoulder, and the standard for the early Interstate pavements was one paved with either a HMA or a bituminous surface course. PCC shoulders were constructed as early as 1957 in Missouri and by several other North Central states (Illinois, Michigan, Nebraska, Iowa, Kentucky, Ohio, and North Dakota) on an experimental basis in the 1960s and 1970s (33, 34). The use of widened PCC slabs (in which the outer slab is paved 1 to 3 ft wider than normal but the traffic lane is still striped at 12 ft) was first used in the late 1970s. Both of these design features are expected to provide performance enhancements by reducing critical edge stresses. Today, widened slabs and PCC shoulders are commonly used in the North Central region on Interstate pavements.

Changes in Other PCC Design Features