Support Properties

Support for a CRCP includes the native subgrade, any subgrade treatments, and base(s)/subbase(s).  Drainage of the composite support system might also be considered.

Subgrades

The performance of any pavement – including CRCP – is affected by the support provided by the subgrade. Subgrades that provide uniform support and that are not affected by moisture variations result in better performing pavements than those with shrinking and swelling changes due to moisture variations. To take advantage of the support capabilities of a subgrade, the designer should provide adequate drainage and stabilization of the subgrade materials as required. In addition, it is convenient to divide the project in sections with similar support characteristics for pavement design purposes. The use of gradual transitions between cuts and fills are needed, especially in bedrock areas or at bridge approaches, to reduce stresses under the slab due to differential support. It has been noted that even in areas that exhibit uniformly poor support (as opposed to intermittent support), CRCP has demonstrated superior performance.

Base(s)/Subbase(s)

Pumping of support layer material through CRCP cracks and joints is a common mechanism contributing to punchchout formation. The erosion caused by pumping action may also result in increased pavement deflections that can lead to spalling at the cracks. The use of a base/subbase layer constructed with non-erodible, impermeable materials is typically specified on CRCP subjected to heavy traffic loads to minimize pumping. In addition to controlling pumping, the base/subbase layer provides a stable platform during construction and may serve other purposes such as controlling frost action and controlling shrink and swell of the subgrade due to moisture changes.(1)Principles of Pavement Design (ISBN: 978-0471977803)

Friction between CRCP and an untreated base/subbase is low compared to a treated base/subbase. Thus, crack spacing and width will be larger if an untreated aggregate base/subbase is used as opposed to a treated (e.g., asphalt, cement) base/subbase. Although unbound granular base/subbase materials have been used for low-volume traffic roads, typical base types used under most CRCP include non-erodible asphalt treated, cement treated, and lean concrete bases/subbases. These have shown to provide better control of pumping on heavily-trafficked highways. When a cement treated or lean concrete base/subbase is used, a thin layer of HMA is needed to reduce the potential of erosion and to provide adequate friction to produce proper crack spacing and widths. No attempt should be made to reduce the friction between the CRCP and the HMA layer.

Some performance problems have been reported in the past with the use of open-graded positive drainage systems, and the designer should take into account such risks. For example, problems observed with open-graded drainable bases/subbases under CRCP include lime-stabilized subgrades pumping into the permeable base/subbase material. Undesirable early-age cracking and poor performance of CRCP on cement-treated permeable bases/subbases has also been reported due to the high restraint (bond) at the CRCP/base interface.(2)Performance of Subsurface Pavement Drainage (NCHRP Project 1-34)(3)Open Graded Drainage Layer Performance in Illinois (FHWA/IL/PRR 147) To help mitigate these problems, some States have recommended the construction of a 25-mm (1-in) dense-graded HMA interlayer directly atop the base/subbase, and beneath the CRCP.

The structural support that the base/subbase layer provides to the pavement depends primarily on its thickness and stiffness (elastic or resilient modulus). A stabilized base/subbase is typically 4 to 6 in. (100- to 150-mm) thick as used under CRCP. A minimum base/subbase thickness of 4 in. (100 mm) is required for constructability considerations. Greater base/subbase thicknesses should be provided when unstabilized materials are used and/or to control frost action or shrink-swell subgrade conditions. In these cases, a well-graded granular non-frost susceptible material may be used for the additional required frost susceptible depth.

Drainage

While much less than for jointed pavements, water infiltrating through cracks and joints in a CRCP may contribute to moisture-accelerated damage because of erosion and loss of support underneath the pavement slab. While ensuring tight transverse crack widths may reduce the infiltration of water, the use of erosion resistant stabilized bases may be warranted, especially for pavements exposed to high levels of precipitation and/or high traffic volumes.

References   [ + ]

1. Principles of Pavement Design (ISBN: 978-0471977803)
2. Performance of Subsurface Pavement Drainage (NCHRP Project 1-34)
3. Open Graded Drainage Layer Performance in Illinois (FHWA/IL/PRR 147)