Composite Pavements

See FHWA’s CRCP Design and Construction Guidelines for the references included in this page. A new, more comprehensive FHWA CRCP Design, Construction, Maintenance and Rehabilitation manual is currently under development and this page will be updated upon its release in the spring of 2016.

Rigid composite pavements are defined as a concrete pavement (commonly CRCP) overlaid with a thin HMA wearing course. Rigid composite pavements are not new in the US. In fact, many existing CRCP have been rehabilitated with a HMA overlay to improve ride quality and skid resistance, and to provide additional structural support in the past. However, a stricter definition of rigid composite pavements includes an HMA overlay on a newly-placed concrete pavement. Use of this pavement type has recently become common practice in the Australia, the United Kingdom, and other European countries. Their popularity has increased over the years as a means to provide both structural and functional performance.(19,32) Benefits of composite pavements that have been cited include:

  • Improved ride quality and skid resistance.
  • Reduction in tire-pavement noise generation.
  • Reduction in water infiltration.
  • Possible reduction in corrosion of reinforcement in CRCP.
  • Thermal insulation to prevent large temperature changes in the CRCP.

In the UK, a HMA surfacing of at least 4 in. (100 mm) is commonly used on top of what is termed a Continuously Reinforced Concrete Roadbase (CRCR). Overlays are scheduled at later stages during the pavement life. Rigid composite pavements are commonly designed for a 40-year life. Transverse reinforcement in the CRCR is required for ease and consistency of construction. Typical designs range from 6 to 10 in (150 to 240 mm) of CRCR and 4 in. (100 mm) of HMA surfacing.(19) A minimum of 1.2 in. (30 mm) of wearing course is allowed in the UK on top of the CRCP. If a 4-in. (100-mm) wearing course is used, a reduction in thickness of the CRCR is allowed. The structural contribution and thermal insulation provided by the wearing course is accounted for in the CRCR thickness design.

Porous asphalt (PA) can be used as the surface course on CRCR. However, 2 in. (50 mm) of PA surface course on top of 3.5 in. (90 mm) binder course are required, otherwise, 2 in. (50 mm) of PA on top of 2.4 in. (60 mm) of binder course is allowed as long as the CRCR is increased in thickness by 4 in. (10 mm). When a PA surface course is used, it is modified with a polymer or fiber additive.(19) While 0.6% of longitudinal reinforcement with #5 (16 mm) bars on CRCP without wearing course is specified in the UK, 0.4% with #4 (12 mm) bars is allowed for CRCR with a minimum of 4 in. (100 mm) asphalt surface course.

Transverse reinforcement is typically specified with 0.5 in. (12 mm) diameter deformed bars at 24 in. (600 mm) spacings. Although a separation membrane (bond breaker) is used for jointed pavements, this layer is omitted from CRCR to provide a higher level of friction between the con- crete slab and the base. A layer with uniform properties such as a lean concrete or asphalt base is recommended under the CRCR. Whenever a 1.2-in. (30-mm) thin wearing course is used, a bond coat is specified to ensure adhesion between the CRCR and this layer.(19)

It is important to note that while composite pavements provide one solution to improving ride quality and reducing tire-pavement noise, other treatments such as diamond grinding can provide similar benefits for CRC pavement. Diamond grinding may in fact be a more economical solution for improving functional characteristics of new and existing CRCP.