Section 3: Rigid Pavement Design Process for CRCP
Anchor: #i10074953.1 TxCRCP-ME Design Program
CRCP design consists of two elements: slab thickness design and steel reinforcement design. The first national CRCP design procedures for slab thickness were developed with information from the AASHO Road Test and were included in the 1972 AASHTO Interim Guide for Design of Pavement Structures (AASHTO, 1981). However, the AASHO Road Test only included jointed concrete pavement sections and not CRCP sections. Distresses in jointed concrete pavements (CPCD) are quite different from the distresses observed in modern CRCP. In CRCP, transverse cracking is normal behavior and does not contribute to degradation of serviceability. The behavior of CPCD and CRCP and their effect on pavement performance is quite different from each other, so the use of the AASHO Road Test data for the development of CRCP design procedures is not rational. In some sense, state DOTs reverse-engineered the AASHTO design equations for CRCP design by selecting reasonable values for selected input variables. In 1986 and 1993, extensive revisions were made to the 1972 Interim Guide, and newer versions of the design guides were published. However, very little effort was made to improve the CRCP design portion, except that steel design equations were incorporated.
The department has used the AASHTO 93 Guide for the design of CRCP, and it has served the department well for the design of CRCP, despite its limitations. In March 2004, the NCHRP 1-37 report and the mechanistic-empirical pavement design guide software (MEPDG) were released. In 2005, the department initiated Research Project 0-4714-1 to evaluate the MEPDG for potential implementation. The study recommended, for various reasons, not to implement the MEPDG as a replacement for the design methods used at that time.
In 2007, the department initiated a research study, 0-5832, to develop its own mechanistic-empirical CRCP design procedures that would model the performance of the department’s typical concrete pavement structure and performance. Three-dimensional analysis was conducted for in-depth analysis of mechanistic behavior of CRCP, including the interactions between longitudinal steel and concrete. The project produced a simple Microsoft Excel spreadsheet to perform the design.
The following Figure 8-3 summarizes the CRCP design process.
Figure 8-3. CRCP Pavement Design Process.
Anchor: #i10075313.2 TxCRCP-ME Design Input Values
The following input variables are needed for the TxCRCP-ME pavement design procedure:
3.2.1 Project Identification
The district is the only required input in this section. By selecting the district, the program will determine the environmental conditions for this pavement design. All other information is not required to complete the thickness design, but they are required for the final design submittal.
3.2.2 Design Life (year)
For rigid pavements, the initial pavement structure shall be designed and analyzed for a performance period of 30 yr. A performance period other than 30 yr. may be utilized with justifications.
3.2.3 Number of Punchouts per Mile
Provide a number of punchouts per mile that is considered the terminal condition of CRCP you are designing. Traditionally, 10 per mile has been the number used for CRCP design. For a higher class of highway where the number of punchouts may be minimized, contact MNT – Pavement Asset Management for further assistance.
3.2.4 Design Traffic
The traffic projections for a highway project (in terms of ADT and one-way total 18-kip ESALs) are obtained from the traffic analysis report provided by the Transportation Planning and Programming Division (TPP). This report is requested during the design phase of a project and, upon receipt, should be evaluated for reasonableness.
Input the one-way total 18-kip ESALs from the TPP traffic analysis report into the design worksheet. The worksheet will calculate the design lane ESALs based on inputted number of lanes in one direction.
Local conditions may cause the directional distribution of heavy vehicles to be unequal. An example is a location near a major quarry adjacent to a highway with otherwise modest levels of truck traffic. If the designer is aware of local conditions that may result in unequal distributions of heavy trucks, TPP should be informed of this condition when requesting traffic projections, and the reported 18-kip ESALS for pavement design should be adjusted.
3.2.5 Thickness of Concrete Layer (in.)
Input a trial concrete slab thickness; the worksheet will predict number of punchouts per mile for the design life. Adjust the slab thickness or other inputs until the predicted number of punchouts per mile meets the requirement in “B. Design Parameters.” Input the trial thickness in 1/2-in. increments.
3.2.6 28-Day Modulus of Rupture (psi)
The Modulus of Rupture (Mr) of concrete is a measure of the flexural strength of the concrete as determined by breaking concrete beam test specimens. Use a 28-day Mr of 570 psi. If the engineer selects an alternate value for Mr, it must be documented with an explanation. Also, if a higher Mr is used, it should be required in the plan to use a higher concrete strength than what is required in Item 360.
3.2.7 Soil Classification of Subgrade
Select the soil classification of subgrade from Unified Soil Classification system in Table 8-2. Selecting the appropriate soil classification will assist in determining the composite k-value of the support layers. The soil k values used in the TxCRCP-ME program are used for thickness design purposes only and shall not be used for field verification of subgrade design during construction.
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Soil Description
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Soil Classification System |
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|---|---|---|
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USCS |
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Coarse Grained Soils |
Gravel |
GW or GP |
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Coarse Sand |
SW |
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Fine Sand |
SP |
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Granular Materials with High Fines |
Silty Gravel |
GM |
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Silty Sandy Gravel |
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Silty Sand |
SM |
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Silty Gravelly Sand |
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Clayey Gravel |
GC
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Clayey Sandy Gravel |
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Clayey Sand |
SC
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Clayey Gravelly Sand |
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Fine Grained Soils |
Silt |
ML or OL
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Silt / Sand / Gravel Mix |
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Poorly Graded Silt |
MH |
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Plastic Clay |
CL |
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Moderately Plastic Elastic Clay |
CL or OL |
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Highly Plastic Elastic Clay |
CH or OH |
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3.2.8 Base Layer Requirements
Select ATB for asphalt treated base, HMA for hot-mix asphalt, or CTB for cement treated base.
Field performance evaluations of concrete pavement have revealed that the use of a durable, stabilized, and non-erodable base is essential to the long-term performance of concrete pavement. If the base underneath the concrete slab does not provide good support, long-term pavement performance will be severely compromised, regardless of the concrete slab thickness.
The department recognized this and requires one of the following base layer combinations for concrete slab support:
- Anchor: #UJGYIGET
- 4 in. of hot-mix asphalt (HMA) or asphalt treated base (ATB), or Anchor: #ATEPJYBK
- a minimum 1 in. hot-mix asphalt bond breaker over 6 in. of a cement treated base; use Item 276, Class L.
Approval from the Pavement Analysis & Design Branch of MNT – Pavement Asset Management is required for use of base layers other than those listed above.
Selection of CTB Class
To ensure long-term strength and stability of cement treated layers, sufficient cement must be used in the mixture. Item 276, “Cement Treatment (Plant-Mixed),” currently designates three classes of cement treated flexible base, based on 7-day unconfined compressive strength. Class M is intended for use with flexible pavements. Class L is intended for use with rigid pavements. Class N can be used if the district has successful long-term experience with other strengths.
Use of Bond Breaker
A bond breaker should always be used between concrete pavement and cement treated base. There have been several instances across Texas where excessive cracking and premature failures occurred when a concrete slab was placed directly on cement treated base. These problems occur because concrete slabs tend to bond directly to cement treated bases. This increases the chances for cracks in the base to reflect through the overlying slab. This also increases tensile stresses in the concrete slab due to temperature and moisture changes, resulting in higher chances of additional cracking.
The department recommends a minimum of 1 in. asphalt concrete stress-relieving layer be used between cement treated base and the concrete slab. A polyethylene sheet is not recommended for use as a bond breaker, due to construction problems evident from past experience.
The subgrade is usually stabilized or treated with lime or cement to facilitate construction as well as to provide additional support to the pavement structure. Large volume changes in the subgrade resulting from moisture variations or other causes can cause the deterioration of concrete pavement. These volume changes in the subgrade should be minimized by appropriate means. Contact the Geotechnical, Soils and Aggregates Branch of CST for further assistance.
The subgrade/base must be designed 2 ft. wider than the concrete slab on each side to accommodate slipform pavement equipment.
If the engineer elects to use a "drainable base," then coordination with the Geotechnical, Soils and Aggregates Branch of CST personnel is required. Refer to Chapter 2, Section 7, for an example of a typical drainable base system.
3.2.8.1 Base Thickness (in.)
Input the proposed base layer thickness. For cement treated bases, ignore the 1-in. thick bond breaker.
3.2.8.2 Modulus of Base Layer (ksi)
Input the modulus of elasticity of the base layer. Typical values of ATB vary from 100 ksi to 400 ksi. Use a value of 400 ksi for HMA and ATB.
The modulus of elasticity for cement treated bases (CTB) varies from 100 ksi to 700 ksi. Use a modulus of 500 ksi for cement treated bases.
Composite k-value will be calculated by the TxCRCP-ME design program based on the inputs of thickness of the stabilized base, elastic modulus of stabilized base, and subgrade soil classification.
Research project 0-5832 developed the composite k-value table by the following process:
- Anchor: #PSADWDJQ
- Concrete stresses due to wheel loading were estimated by two-dimensional Finite Element Model analysis for a wide range of soil and base conditions. In the modeling, soil stiffness was characterized by modulus of subgrade reaction (k) and that of base by modulus of elasticity. Anchor: #FWDPYJDB
- A factorial was developed for various subgrade k and base modulus of elasticity and thickness. Concrete stress was estimated for each cell of the 106 factorial (specific combination of subgrade k and base modulus). Anchor: #SKMQDUEJ
- For each cell, the “equivalent” k value was derived from FEM analysis that would provide the same concrete stresses. Anchor: #SHGJRQUQ
- A table, called “k-Table” was developed and included in the TxCRCP-ME program.
For a factorial mentioned in step 2 above, the following levels were selected:
- Anchor: #QKGHSPFS
- Subgrade k: 7 levels (25, 50, 100, 150, 200, 250, and 300 psi/in.). Anchor: #MENSFDGJ
- Base thickness: 5 levels (2 in. – 6 in. with 1 in. increment). Anchor: #YEQWHGAY
- Base modulus: 34 levels (50 ksi to 100 ksi with 10 ksi increment, 100 ksi to 1,000 ksi with 50 ksi increment, 1,000 ksi to 2,000 ksi with 100 ksi increment).
A total of 1,190 combinations were analyzed, and the k-Table was developed.