LARIMER COUNTY RURAL AREA ROAD STANDARDS

Chapter 5 – Geotechnical Standards and Pavement DesignSection 5.6 Rigid Pavement Design5.6.2 AASHTO Design ApproachAll rigid pavement designs shall be prepared in accordance with AASHTO PavementDesign Procedures. Computer generated printouts and/or other design calculations mustbe included with the design submittal. Nomographs of the AASHTO parameters mayalso be used. If used, copies of the nomograph determinations must be included with thedesign submittal.The design of rigid pavements is a function of structural quality of the subgrade soil (Rvalue),traffic (ESAL), and the strength of the concrete (working stress). In comparisonto the strength of the concrete slab, the structural contributions of underlying layers tothe capacity of the pavement are relatively insignificant. Therefore, the use of thickbases or sub-bases under concrete pavement to achieve greater structural capacity isconsidered to be uneconomical and is not recommended. However, road sections withlandscape medians may require a drainage layer section to provide positive drainagefrom the median to an acceptable outlet. In all cases, subgrade shall be stable asdetermined by proof-rolling requirements.The following procedure should be used in determining the structural number andthickness of the pavement being designed:1. Determine roadway classification and corresponding ESAL. The predictedvolumes in the traffic impact study must be used whenever they exceed theminimum ESAL values given in Table 5-2.2. Determine the serviceability index of the roadway classification from Table 5-2.3. The Reliability Factor for design of all concrete pavements shall be 90%.4. The Standard Deviation for design of concrete pavements shall be between 0.30and 0.40.5. Determine the structural numbers using AASHTO pavement design software.Nomographs of the AASHTO parameters may be used. If used, copies of thenomograph determinations must be included with the design submittal.6. Determine the slab thickness. A minimum thickness of 5 inches must beprovided.7. The design must reference any mitigation measures required when the subgradecontains swelling soils. Design reports recommending permeable layers in thepavement system must present the measures to be used to ensure adequatedrainage of such layers, and to maintain segregation of the layers from theswelling soils. Design must reference any mitigation measures required when thesubgrade contains sulfates.5.6.3 Material StandardsAll sources for the mined or manufactured materials must be annually approved by theCounty as having met the appropriate materials performance specifications as certifiedby an independent testing laboratory. This approval is a condition of using thosematerial sources for public improvement construction.Larimer County Rural Area Road Standards Page 5-8October 22, 2007