Structural Design

Thickness design for Roller Compacted Concrete pavements employs the same basic strategy as for conventional concrete pavements. The strategy requires keeping the pavement’s flexural stress and fatigue damage caused by wheel loads within allowable limits.  In the structural design, the pavement thickness is a function of the expected loads, concrete strength, and characteristics of supporting base and subgrade.

The tensile stress at the bottom of the concrete slab is affected by the applied load, tire pressure and spacing, concrete thickness and stiffness, and the characteristics of the supporting materials. In Roller Compacted Concrete structural design, the pavement thickness is increased or the strength of the concrete is increased until the stress ratio is reduced sufficiently to provide for adequate fatigue performance.

Since the critical stresses in Roller Compacted Concrete are flexural, fatigue due to flexural stress in used for thickness design. Research has shown that Roller Compacted Concrete exhibits fatigue behavior very similar to that of conventional concrete. Therefore, the same design methodology used for conventional concrete can be applied to Roller Compacted Concrete pavements. The pavement thickness is a factor of the subgrade support, the Roller Compacted Concrete flexural strength and the anticipated vehicle loading.

For structural design purposes, Roller Compacted Concrete pavements fall into two main categories—heavy-duty industrial pavements and pavements carrying mixed-vehicle traffic. For heavy-duty industrial pavements, which carry heavy industrial vehicles such as loaders or container haulers, the design may be based on the expected number of load repetitions of the single heaviest vehicle, and other vehicles that are significantly lighter can be ignored. This is the approach used by the PCA procedure and the USACE procedure. For pavements carrying mixed-vehicle traffic, design tools suitable for undoweled conventional concrete pavements—such as the ACI tables or American Concrete Pavement Association’s StreetPave computer program—can generally be used with satisfactory results. Some engineers use WinPAS computer program as a check on the StreetPave program or ACI tables.

Users of these computer programs should be aware that the fatigue curve traditionally used in Roller Compacted Concrete pavement design is more conservative than the fatigue curve traditionally used in conventional concrete pavement design.  For this reason, when designing Roller Compacted Concrete pavements using conventional pavement design software, such as WinPAS or StreetPave, it is recommended that the default reliability level be increased by 5 percent to achieve results comparable to those of traditional Roller Compacted Concrete pavement design software such as RCC-PAVE.

RCC-PAVE program is based on PCA’s manual procedure.  It can be used to design a new pavement or analyze an existing pavement. Users can select interior or edge loading and have the options of standard vehicle loading or user-defined loading. Similar to the manual procedure, the software was developed mainly for heavy-duty pavement applications.

The StreetPave design program allows the engineer to design conventional concrete and Roller Compacted Concrete pavements. The calculation in StreetPave follows the PCA procedure with a few exceptions. Probably the most significant exception is the use of a variable fatigue curve, based on the desired reliability and the allowable percentage of cracked slabs. The StreetPave design program uses the conventional concrete fatigue curve. The RCC-PAVE program uses a more conservative fatigue curve. To achieve the same result using both programs, it is not uncommon to increase the reliability by 5 percent when using the StreetPave program to provide additional conservatism. When using StreetPave, the option for undoweled joints should be selected. The user should be aware that for a pavement thickness of 8 in. or more, StreetPave will automatically require dowels. Therefore, for more than 8 inch thick pavements, ACI tables 5-6 and 5-7 should be used.

For more detailed information on Roller Compacted Concrete structural design, please check out Guide for Roller-Compacted Concrete Pavements, Section 5: Structural Design of Roller Compacted Concrete Pavements (National Concrete Pavement Technology Center, 2010) or Structural Design of Roller-Compacted Concrete for Industrial Pavements (Portland Cement Association, 1987).

Photos courtesy of Andale Ready Mix Central/Andale Paving, Inc.