Design Criteria

Design criteria are pre-established parameters (controls and limits) that are required when designing a pavement structure. Design criteria for CRCP are often no different than other concrete pavements; however, some differences do exist and are worth noting. This section will highlight some of those differences.

Limiting Criteria on Crack Spacing, Crack Width, and Steel Stress

The AASHTO-86/93 Guide recommends controlling crack spacing to fall within 3.5 to 8 ft (1.1 m to 2.4 m).(1)Guide for Design of Pavement Structures (AASHTO 86/93) However, the new CRCP design procedure described in the AASHTOWare Pavement ME (MEPDG) does not provide recommendations on the control of minimum crack spacing due to the numerous factors that affect this variable including reinforcement percentage. A maximum average crack spacing of 6 ft (1.8 m) is however recommended. While it does not specify a minimum crack spacing, the AASHTOWare Pavement ME (MEPDG) does instead recommend designing for small crack widths to ensure long-term performance.(2)Guide for Mechanistic-Empirical Design of New and Rehabilitated Pavement Structures (NCHRP 1-37A)

The AASHTO-86/93 Guide limits the crack widths to 0.04 in. (1 mm) to avoid spalling.(3)Guide for Design of Pavement Structures (AASHTO 86/93) However, crack widths of 0.024 in. (0.6 mm) have been found to be more effective in reducing water penetration, and thus minimizing corrosion of the steel, maintaining the integrity of the support layers, and ensuring high load transfer efficiency.(4)Design of CRCP for Highways (NCHRP Project 1-15) The AASHTOWare Pavement ME (MEPDG) predicts and requires a maximum crack width of 0.020 in. (0.5 mm) over the entire design period.(5)Guide for Mechanistic-Empirical Design of New and Rehabilitated Pavement Structures (NCHRP 1-37A) Regardless of what procedure is used, the use of corrosive deicing salts should be taken into consideration when selecting the crack width criterion.

Reinforcement design has to consider possible fracture and/or excessive plastic deformation. To accomplish this, the stress in the reinforcement is usually limited to a reasonable percentage of the ultimate tensile strength to not only avoid fracture, but to limit the amount of plastic deformation.(6)Guide for Design of Pavement Structures (AASHTO 86/93)(7)Limiting Criteria for the Design of CRCP (FHWA/TX-79/21+177-17) The below table shows the maximum allowable working stress for steel with yield strength of 60 ksi (420 MPa) that is recommended by the AASHTO-86/93 Guide.

Allowable steel working stress

It can be noted that in some cases, the AASHTO-86/93 Guide allows a working stress above the yield strength that could result in a possibility of some plastic deformation.(8)Guide for Design of Pavement Structures (AASHTO 86/93)(9)Limiting Criteria for the Design of CRCP (FHWA/TX-79/21+177-17) As a result, it should be noted that slightly wider crack openings may result when permanent deformation is allowed.

With respect to the use of limiting criteria in general, it should be noted that CRCP design is continuing to evolve. Motivation for revised thinking is born out of the new design process inherent with the AASHTOWare Pavement ME (MEPDG). More importantly, however, it is the result of observations of in-service CRCP performance. While design of CRCP in the past might have required the conventional limiting criteria for crack spacing, crack width, and steel stress, these might not be as relevant today. The pavement designer is encouraged to seek out the latest guidance in this evolving practice before taking exception to what could be overly-conservative designs resulting from the current design standards.

Structural Performance

In mechanistic-empirical design procedures such as those developed for the AASHTOWare Pavement ME (MEPDG), structural performance for CRCP is typically expressed in terms of allowable punchouts per unit of distance (i.e., punchouts/mile) before rehabilitation is needed. The figure to the right conceptually illustrates the structural performance level in terms of punchouts as a function of time or load applications. Recommended threshold values for the allowable number of punchouts are commonly expressed as a function of the functional highway classification or traffic level. The limit that is selected is also a function of the design reliability (risk).

The AASHTOWare Pavement ME (MEPDG) recommends a maximum of 10 medium- and high-severity punchouts per mi (6 punchouts/km) for interstates and freeways, 15 punchouts per mi (9 punchouts/km) for primary highways, and 20 (12) for secondary highways. The American Concrete Pavement Association (ACPA) recommends a maximum of 10 punchouts per mi (6 punchouts/km) for Average Daily Traffic (ADT) greater than 10,000 vehicles/day, 24 punchouts per mi (15 punchouts/km) for ADT between 3000 and 10,000 veh./day, and 39 punchouts per mi (24 punchouts/km) for ADT below 3000 veh./day.(10)Guide for Mechanistic-Empirical Design of New and Rehabilitated Pavement Structures (NCHRP 1-37A)(11)The Concrete Pavement Restoration Guide: Procedures for Preserving Concrete Pavements (ACPA TB020P)

Functional Performance

Functional performance in terms of IRILike structural performance, functional performance thresholds are commonly defined based on the functional highway classification or traffic level. The figure to the right conceptually illustrates the functional performance level in terms of international roughness index (IRI) as a function of time or load applications. The AASHTOWare Pavement ME (MEPDG) recommends a maximum IRI of 175 in./mi (2.7 m/km) for interstates and freeways, 200 in./mi (3.2 m/km) for primary highways, and 250 in./mi (4 m/km) for secondary highways. The ACPA recommends a maximum IRI of 158 in./mi (2.5 m/km) for ADT greater than 10,000 vehicles/day, 190 in./mi (3.0 m/km) for ADT between 3,000 and 10,000 veh./day, and 220 in./mi (3.5 m/km) for ADT below 3,000 veh./day.(12)Guide for Mechanistic-Empirical Design of New and Rehabilitated Pavement Structures (NCHRP 1-37A)(13)The Concrete Pavement Restoration Guide: Procedures for Preserving Concrete Pavements (ACPA TB020P)  In the AASHTOWare Pavement ME (MEPDG), the threshold value is selected based on the design reliability (risk).

References   [ + ]