Design Methods

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.

All too often, pavement design tends to focus only on thickness design. However, there are numerous other aspects of a CRCP that affect its behavior and performance including the reinforcement, thickness, shoulders, support, and concrete-making materials. The design of the CRCP should therefore consider each of these features, and ideally arrive at an optimum design through an iterative process.

To complete the design, a life-cycle cost analysis is sometimes performed. This allows the designer to take into account the costs associated with various pavement design alternatives along with the benefits in terms of increased pavement performance.

Various design methods for determination of slab thickness and the amount of reinforcement required in CRCP have been developed in the past. The two most relevant due to their common use and/or level of validation are briefly discussed below.

AASHTO-86/93 Design Procedure

Although the AASHTO-86/93 Guide procedure does not directly consider one of the primary failure mechanisms in CRCP (punchout development), this procedure has been used for design of CRCP by making similar considerations to those for the design of jointed concrete pavements. In addition, because reinforcement keeps cracks tight in CRCP, a slightly improved load transfer coefficient is typically used, which results in a moderate reduction in thickness for this pavement type under similar traffic and environmental conditions.

The AASHTO-86/93 method also includes design procedures for the selection of reinforcement. These procedures are based on a desired range of crack spacing, maximum crack width, and maximum steel stress. It should be noted that it has been reported that this design procedure tends to underestimate the required steel. A summary of the AASHTO-86/93 design procedure is presented in Appendix B of the CRSI-FHWA CRCP Design and Construction Guidelines document.

AASHTOWare Pavement ME (MEPDG) Design Procedure

Over the last decade, the National Cooperative Highway Research Program (NCHRP) has undertaken a major effort to develop the next generation of pavement design procedure based on mechanistic-empirical methods. This has been conducted under research project 1-37A, and has resulted in the current AASHTO Interim MEPDG. In this design procedure, specific mechanistic-empirical models for prediction of CRCP performance have been developed.(29,53)

The design process begins with the selection of a trial design including layer thicknesses, materials, reinforcement, shoulder characteristics, and construction information. Site-specific conditions including environment, foundation, and traffic are also considered. Performance criteria in terms of punchouts and IRI are then specified, along with the reliability level for each criterion. The MEPDG also has limiting design criteria on crack width (over design period), crack spacing, and crack load transfer efficiency (over design period as well).

The procedure explicitly predicts punchout development as a function of the crack width and load transfer efficiency due to aggregate interlock at transverse cracks. Stresses due to loading are predicted as a function of load transfer effi- ciency, and continuously evaluated and modified through-out the design period. Fatigue damage as a function of the stress level and strength is evaluated and accumulated, and punchout development is subsequently predicted.

IRI is also predicted throughout the design period as a function of the initial smoothness conditions, punchout development, and site-specific conditions. Once the trial design is evaluated, its predicted performance is checked against design criteria at the specified reliability level. If the design requirements are satisfied, the trial design is considered as a viable alternative that can later be evaluated in terms of life-cycle costing. Otherwise, a new trial design is evaluated.

In some cases, this resulted in an under-design, which in turn required expensive maintenance and rehabilitation. As a result, this practice is no longer recommended.(56) Today, typical CRCP thicknesses vary from 7 to 15 in. (178 to 381 mm) depending on the level of traffic and environmental conditions, although most common practice is between 10 and 12 inches (254 to 305 mm).