Continuously Reinforced Concrete Pavement (CRCP)

Continuously reinforced concrete pavements (CRCP) is a type of concrete pavement that does not require any transverse contraction joints. Transverse cracks are expected in the slab, usually at intervals of 1.5 - 6 ft (0.5 - 1.8 m). CRCP is designed with enough embedded reinforcing steel (approximately 0.6-0.7% by cross-sectional area) so that cracks are held together tightly. Determining an appropriate spacing between the cracks is part of the design process for this type of pavement.

CRCP designs generally cost more than JPCP or JRCP designs initially due to increased quantities of steel. However, they can demonstrate superior long-term performance (typical design service lives are 30-40 years) and cost-effectiveness. A number of state highway agencies choose to use CRCP designs in their heavy urban traffic corridors where traffic over the service life of the pavement can be on the order of tens of millions of equivalent load repetitions. CRCP also makes a good candidate for resurfacing with asphalt concrete due to its tight crack widths and minimal vertical movement between adjacent joints due to restraint from the steel which reduces the frequency and severity of reflective cracking.

Basics of CRCP

CRCP utilizes reinforcing steel to effectively eliminate transverse joints in favor of very tightly maintained cracks. The reinforcing steel is used in combination with other design properties such as slab thickness and concrete materials to prevent traffic and environmental distresses. The inclusion of the reinforcing steel results in cracks that have a shorter spacing than the joints in jointed plain concrete pavement due to increased restraint from the steel. Transverse cracks in CRCP are typically 1.5 - 6 ft. apart. The steel holds the cracks extremely tight which helps promote load transfer through aggregate interlock (as well as through the steel itself). Other benefits of tight cracks include minimal faulting, reduced potential for water penetration, and it is more difficult for harmful chlorides and other corrosive agents to reach the steel. In short, tight cracks increase the service life and durability of CRCP.

• Continuously Reinforced Concrete Pavement Placement (Photos courtesy of Concrete Reinforcing Steel Institute (CRSI))
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  CRCP placement on Dan Ryan Expressway in Chicago, Illinois

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  Placement of reinforcing steel on a curve

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  Close-up of reinforcing steel

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  Paving CRCP

Early Crack Development

Typical (Very Tight) Crack in CRCP

Concrete pavement cracks at an early age to help relieve stresses that develop due to shrinkage, volumetric changes, and restraint. The normal progression of cracking initially occurs at intervals of 40 to 80 feet and then progresses to 18 to 20 feet. JPCP controls the cracks by using joints to create a weakened plane to control the developing cracks. The reinforcing steel of CRCP creates extra restraint which causes cracks to develop more quickly and at a shorter interval. While the resulting crack spacing is shorter than JPCP, the crack progression is very similar, as cracks develop typically midway between two other cracks. This helps relieve the building tensile stresses. Most of the cracks form before the pavement is opened to traffic, but cracking can continue through the rest of the pavement's life. The crack widths typically remains very thin (typically less than 0.02 inches) which helps promote load transfer through aggregate interlock.

Design

As with any pavement, there are numerous design considerations when creating a CRCP design. The pavement structure and associated thicknesses, concrete materials, reinforcing requirements, and climate can all play an important role in the long-term performance of CRCP. The main design philosophy for CRCP is maintaining tight crack widths in order to reduce risk of punchouts, spalling, and corrosion of the reinforcing steel.

Thickness Design

ACPA's StreetPave software does not cover the design of CRCP as StreetPave is built for the design of JPCP. There are two main design methods for CRCP. The first design method is the AASHTOWare Pavement ME Design (formerly the MEPDG). This is the state of the are design for CRCP in the United States as it utilizes data from long-term pavement performance (LTPP) sections where performance data has been monitored over a few decades in numerous types of climactic environments. The other main method of design is the AASHTO 1993 design. The AASHTO 1993 design is the last iteration of design methods based on the AASHO road test in the late 1950's. This design method is empirical being based on observations and measurements at the AASHO road test as opposed to a mechanistic-empirical design which relies on the mechanics of materials in association with performance observations. Extra background on design methods can be found on the concrete pavement thickness design page. While it might seem that CRCP thicknesses should be thinner than JPCP or JRCP due to their relatively high steel contents, this is not the case. The main purpose of the steel is not to add tensile or flexural capacity to the concrete but rather to maintain tight crack widths hence why the steel is not placed near the bottom of the pavement. Typically, CRCP thicknesses are similar to those of JPCP or JRCP.

Other Design Considerations

Some additional design considerations of CRCP are very similar to other concrete pavements (with the main substitution being reinforcement design instead of joint design). The typical considerations of edge and base support are very important to CRCP just as they are to JPCP. The biggest difference in design considerations is the reinforcing steel. The amount of steel required can be dependent on the climate. Unlike dowel bars, which are placed in the middle third of the slab,the steel in CRCP is typically between one third and one half the slab thickness as measured from the pavement surface. Steel that is placed too close to the surface will be more susceptible to corrosion while steel placed too deep will not be effective in maintaining tight crack widths. The concrete is intended to bond to the reinforcing steel to help provide resistance to the cracks widening. The FHWA recommends a bond area of 0.03 square inches per cubic inch of concrete. This too is a difference from the dowels in JPCP as bonding with the dowel bars can actually lock up the joints.

Detail of a Wide Flange Steel Beam Terminal Joint

While typical transverse joints are not used, a specialty joint known as a wide flange beam joint can be used as an edge treatment at the end of a CRCP section or when encountering a structure. This type of joint allows for the concrete to fluctuate with weather changes; providing sufficient room to expand and contract. Another end treatment is the use of anchor lugs which restrict the movement of the end of the slab. This end treatment is not as common as the wide flange beam joints. The joints page holds more details on these and many other types of joints.

In order to reduce the occurrence of punchouts or longitudinal joint issues along the pavement edge, tied concrete shoulders are typically used with CRCP. The use of tied concrete shoulders reduces the stresses and deflections due to heavy wheel loads at the edge as a result of load transfer between the outside lane and the shoulder. This is a change from past construction practices where asphalt shoulders were commonly used.

A non-erodible base layers is crucial to obtaining a good-performing CRCP. Therefore, asphalt concrete is typically used to separate the subbase/base from the CRCP. The asphalt concrete provides sufficient friction to ensure a good cracking pattern and is also much less erodible than a non-stabilized granular material which leads to better support conditions.

Construction

CRCP Hand Placement

Transverse Bar Assembly

There are a number of special considerations that need to be made when paving CRCP as opposed to JPCP. Most of these considerations are the result of the reinforcing steel. Since the steel is so important to the design and performance of CRCP, it is very important that it is placed at the proper level and that it stays where it is placed while the concrete is placed around it. Consolidation around the steel is extremely important to promote bond between the concrete and the steel to provide long term performance and keep the developing cracks tight. As with JPCP and all other concrete pavement types, curing is important to producing quality CRCP.

The most common method for steel placement is installing the steel and placers by hand before paving. The steel is held in place at the proper vertical location by chairs. Transverse bars are also used to help hold the steel in place and make sure that any longitudinal cracks that develop are held tight. The system of steel reinforcement, chairs, and transverse steel bars must be secure enough to ensure that it does not move as the concrete is placed around the steel either by hand or by a slip-form paver. The system must also allow consolidation of the concrete to ensure proper performance. The reinforcing steel bars are lap-spliced together to build the length of the roadway. The laps are typically staggered or skewed to prevent any compaction issues. One key to ensuring good consolidation and good performance is the use of a proper concrete mixture produced at a steady production rate that is vibrated properly. Limiting the heat of hydration and keeping concrete temperatures between 50 and 90 degrees F has also shown good performance and a consistent cracking pattern. The HIPERPAV program can shed light on the effects of temperature changes on cracking patterns and what changes may need to occur.

One development in steel placement is the use of transverse bar assemblies or TBAs. These are used as supports for the reinforcing steel and can speed installation time significantly. TBAs provide transverse restraint to the steel reinforcement, while holding the reinforcement at the proper height and allowing it some longitudinal movement.

Resources

• Continuously Reinforced Concrete Pavement Manual - Guidelines for Design, Construction, Maintenance, and Rehabilitation
• Continuously Reinforced Concrete Pavement Performance and Best Practices - TechBrief
• Continuously Reinforced Concrete Pavement: Extending Service Life of Existing Pavements
• Continuously Reinforced Concrete Pavement - FHWA Technical Advisory

Other Types of Concrete Pavement

• Jointed plain concrete pavement (JPCP)
• Jointed reinforced concrete pavement (JRCP)
• Pervious concrete pavement
• Roller-compacted concrete (RCC) pavement