Less than 200 years ago travelers throughout North America utilized dirt paths as transportation routes. Today’s travelers ride along and across a number of different surfaces, from asphalt, bricks, and concrete, to bridges with wood and steel decks. And, of course, there are still a number of dirt and gravel roads out there too.

As a pavement alternative, continuously reinforced concrete pavement (CRCP) became prevalent in highway design in the United States with the construction of the Interstate Highway System in the 1960s and 1970s. In fact, many segments of CRCP highway constructed at that time are still in use, outperforming the original design life projections.

The reputation of CRCP as a favored alternative for subgrade conditions stems from its ability to stand up to heavy traffic and unfavorable construction conditions, such as poor soil conditions and excessive moisture., It’s also favored for its effectiveness in adverse climates, with the ability to withstand very hot and very cold temperatures. Because of CRCP’s improved durability, higher life expectancy and reduced maintenance requirements, it provides long-term value, competitive with other pavement types with respect to performance and life cycle cost.

What is CRCP?
CRCP is constructed with steel reinforcing bars along a continuous piece of pavement, making it free of contraction joints, unlike traditional highway pavements. This "jointless" construction makes CRCP less likely to develop open transverse cracks because the reinforcing bars control the crack width, and promote evenly transferred axle loads. Since the transverse cracks are held closed by the steel, the structural integrity of the pavement is not reduced. The result is a continuous, smooth-riding surface capable of withstanding heavy commercial traffic and adverse environmental conditions. The smooth surface also provides improved ride quality and improved fuel efficiency.

Jointed plain concrete pavement (JPCP) is also constructed in a continuous fashion, but without reinforcement. Saw cuts are used to create individual concrete slabs that are tied together by dowel bars. Load transfer across the joint relies on aggregate interlock and the dowel bars to reduce vertical movement under load applications. Water that gets into joints and cracks can sometimes soften the foundation soils. The stress of transferring large loads from one slab to another, as heavy vehicles cross the joints, can cause additional distress in the pavement if the wet subgrade is yielding. As a result, the slab movement may ultimately require portions of the highway to be replaced.

The advantages of using CRCP—better road integrity and reduced maintenance costs— have made this type of pavement the preferred choice for many highway projects in a number of Midwestern and southern states. Poor subgrade soils and temperature extremes are prominent in these areas of the United States, making CRCP more attractive because of its ability to withstand these conditions.

Other applications for CRCP include airport runways, railway track beds, and warehouse flooring. In all of these situations, the pavement is exposed to very heavy repetitive loads over long periods of time.

A Case Study for CRCP
The Idaho Transportation Department (ITD) constructed its first and only segment of CRCP on Interstate 15 (I-15) in southeastern Idaho in the early 1970s. Many trucks travel this section of I-15 between Salt Lake City and Interstate 90. The pavement performed well and without significant maintenance for more than 20 years until the mid-1990s.

The 9.5-mile segment of CRCP on I-15 is four-lane rural freeway that starts about 20 miles north of the Idaho/Utah line. Elevation of the segment changes from nearly 5,300 feet to 5,574 feet, and ends at an elevation of 4,800 feet. Grades along this stretch of highway generally range from 3.5 to 5 percent, and the pavement surface is exposed to studded snow tire and chain abrasion during the winter months.

Substantial embankment fills of up to approximately 45 feet in height exist in the southern half of the project. A sand and gravel drainage blanket was placed at the base of the embankments to reduce pore pressures from artesian springs that underlie this area. The southern half of the project crosses a mountain summit and is primarily constructed on clayey soil that can be seasonally moist in some areas due to the springs.

Over the years, two significant landslides occurred within a half-mile section of the highway embankments. After the second slide in 1983, horizontal slope drains and siphon wells were installed to reduce the buoyant force from groundwater. The original embankment fills were replaced with pumice, which is nearly half the original weight.

These significant environmental and construction challenges made the 9.5-mile segment in Idaho an ideal place to use CRCP instead of traditional pavement designs because of poor subgrade soils, heavy truck traffic, and excessive subgrade moisture due to natural springs, snow and ice.

Designing an Effective CRCP Surface
While CRCP requires a more significant investment up front, over the long run it is more cost effective because it requires fewer repairs over time than JPCP. Due to its reinforcing steel bars, CRCP allows for thinner concrete slabs to be utilized. Using less concrete helps offset some of the higher initial construction costs of CRCP.

For the I-15 project, both travel directions were similarly designed in the late 1960s and constructed in 1971 and 1972. The design pavement section for the travel lanes consisted of eight inches of CRCP, over four inches of cement treated base (CTB), over compacted subgrade.

The project design called for No. 6 reinforcing bars, a depth of concrete cover between 2.5 and 3.5 inches, and 9-inch center-to-center spacing (0.61 percent steel). Construction joints for the CRCP included an additional six-foot piece of No. 6 bar spliced to every third longitudinal bar. The reinforcing steel is designed to hold transverse cracks together, and eliminate the need for standard transverse contraction joints.

The original construction joints were constructed using a sleeper slab that supported the adjacent overlying slabs. The lower third of an I-beam was set transversely into the middle of the sleeper so that the top flange would be flush with the pavement surface. A foam strip was placed next to the web to create a collapsible space for expansion of the CRCP. The underside of the top flange was greased to eliminate a bond between the steel and concrete. When the maintenance overlay was constructed, a bituminous elastomeric mortar was used above the top flange of the beam to match the new overlay.

While this type of joint has been utilized for years, caution should be exercised in specifying a sufficiently sized beam to withstand flexural loads from heavy truck traffic. Heavy commercial traffic can greatly impact the upper web-to-flange welds. Longitudinal joints should also be tied with deformed bars to reduce movement along that plane.

CRCP Distress Can Occur
While the longevity and performance benefits realized from CRCP provide significant advantages, the solution is not infallible. Since its original construction date, the section of I-15 completed with CRCP has undergone some maintenance and rehabilitation, including repairs to punchouts.

With the challenging soil and weather along this section of I-15, the subgrade support became unstable. When there is loss of subgrade support, the spacing of transverse cracking typically approaches two to three feet. Longitudinal cracks at the ends of the flexure area form a rectangular distress pattern. The block created by the adjacent transverse cracks eventually fatigues and shears the reinforcing bars, creating a punchout. The block of concrete drops slightly and creates a localized depression. The preferred repair method for this distress is to saw shallow transverse and longitudinal cuts outside the observed distress, without cutting the existing steel. The concrete is removed full depth by chipping, and is replaced with a standard highway mix of Portland cement concrete.

Even with the current phase of rehabilitation, the long-term performance and cost-effectiveness over the life of the CRCP solution illustrate the advantages of this type of construction.

CRPC Remains An Effective Solution
The design of CRCP today resembles the design made popular in the 60s and 70s. In fact, some aspects, such as terminal joint design, hasn’t changed significantly in the last 40 years. CRCP and JPCP work well when applied in the appropriate situations.

CRCP is used many places in the United States for highway construction. Even though it might cost more up front, it saves money in the long haul because it usually requires less maintenance. Its "jointless" design reduces cracking and withstands adverse conditions, including:

• • Heavy traffic volumes
  • High percentages of truck traffic
    Extreme temperatures
  • Poor subgrade soils
  • Excessive subgrade moisture

In addition to the monetary benefits of using CRCP, it provides a smoother ride and reduces traveler inconvenience by minimizing road maintenance. The reduced number of joints also improves the user experience through reduced road noise. CRCP has many proven benefits, making it an attractive alternative for highway design in many areas. SLDT