Rigid Pavement Design
Section 1. Overview
Definition of Rigid Pavement
Portland Cement Concrete (PCC) pavements are commonly referred to as rigid pavements. This classification is based on rigid pavement behavior. Behavior of a pavement is defined as the immediate response of a pavement to a load. Rigid pavements respond to a wheel load as a very stiff material (concrete) over much softer materials (subbase and subgrade). The rigid pavement develops significant bending moments and uses these bending moments to act as a beam to spread the wheel load over a large area of the subbase and subgrade.
Rigid Pavement Types
There are two types of concrete pavements that are commonly used:
Continuously Reinforced Concrete Pavement (CRCP). CRCP contains both longitudinal and transverse steel. CRCP does not contain transverse joints except at construction joints. The function of the longitudinal steel is not to strengthen the pavement, but to force the pavement to crack within a certain desirable crack spacing (usually from three to eight feet) and to keep those cracks tightly closed. The function of the transverse steel is to keep longitudinal joints and cracks closed. If the steel serves its proper function and keeps cracks from widening, aggregate interlock is preserved and concrete stresses at the cracks due to traffic loading are reduced.
Jointed Plain (Non-Reinforced) Concrete Pavement. Jointed plain concrete pavement does not have reinforcing steel and has transverse joints spaced at regular intervals. The transverse joints are used to control temperature induced stresses in the concrete. Longitudinal joints are used to facilitate construction and control cracking. Pavements of this type will have smooth dowels at the joints for load transfer. The joint spacing is kept at a constant 15 feet for CPCD standards.
Performance Period
For rigid pavements, the initial pavement structure shall be designed and analyzed for a minimum performance period of 30 years.
Section 2. Approved Design Method
AASHTO Rigid Pavement Design Procedure
The AASHTO Rigid Pavement design procedure is the only method used to design rigid pavements. It is available in automated or nomograph form. Automated procedures include the AASHTO DARWin ® program and the TSLAB86 program.
The AASHTO guide also contains design procedures for rehabilitation of rigid pavements, including asphalt concrete overlays or PCC overlays of existing rigid pavements. Contact the district pavement engineer or the Construction Division Pavements Section for further assistance.
Section 3. Rigid Pavement Design Process
Figure 3-1 summarizes the rigid pavement
design process.
Figure 3-1. Rigid Pavement Design Process
Section 4. Recommended Design Values
Input Values
Input values for the following variables are needed as listed below for the AASHTO rigid pavement design procedure for a new construction or reconstruction project:
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Mean Concrete Modulus of Rupture, Mr
Mr of concrete is a measure of the flexural strength of the concrete as determined by breaking concrete beam test specimens. Mr design values for the concrete are used in conjunction with the AASHTO rigid pavement thickness design procedure to determine the design thickness of the proposed concrete pavement. A Mr of 650 psi is to be used with the current statewide specification for concrete pavement and standard detail drawings to calculate the concrete pavement thickness.
Concrete Elastic Modulus
The AASHTO design equation also requires a value for the concrete elastic modulus. This value varies depending on the coarse aggregate type. Although the value selected could be significantly different from the actual values, the elastic modulus does not have a significant effect on the final thickness generated by the equation.
The current recommended concrete elastic modulus values are:
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4,000,000
psi for concrete containing crushed limestone |
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5,000,000
psi for concrete containing siliceous river gravel. |
Effective Modulus of Subbase/Subgrade Reaction
k-value. The AASHTO guide allows pavement designers to take into account all layers to be placed under the concrete pavement. It also allows designers to consider the effect of loss of support of the underlying material due to erosion or deterioration.
The slab support is characterized by the modulus of subgrade/subbase reaction, otherwise known as the k-value. It can be measured in the field by applying a load equal to 10 psi on the subgrade/subbase combination using a 30-inch diameter steel plate. The k-value is then calculated by dividing 10 psi by the measured deflection (in inches) of the layers under the plate.
A k-value of 300 pci is to be used in the rigid pavement design procedure when one of the stabilized subbases layer combinations is specified. However, there is one exception to this requirement. Two inches of ACP will be allowed as a subbase under the following conditions:
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The project is an urban or
county road project and |
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The
concrete pavement will be one inch thicker than is required by the AASHTO
rigid pavement design procedure and |
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The
30-year 18-kip ESAL estimate is less than 3,000,000. |
Hence, a k-value of 140 pci is to be used in the rigid pavement design procedure when two inches of ACP is specified.
The pavement engineer can also assist in selecting a k-value for any subbase combination.
Currently, the modulus of subgrade/subbase reaction has a minimum effect on the concrete pavement thickness when using the AASHTO procedure. However, future revisions to concrete pavement thickness design may give more credit to subbase support than is given now, especially to non-erosive subbases.
Rigid Pavement Subbase Requirements. For new and reconstruction projects, we require one of the following stabilized subbase layer combinations for rigid pavement support:
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four inches of ACP or
asphalt stabilized subbase or |
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a
one-inch asphalt concrete bond breaker over six inches of a cement stabilized
subbase. |
The same exception to this requirement applies as in the k-value case. Two inches of ACP will be allowed as a subbase under the following conditions:
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The project is an urban or
county road project and |
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The
concrete pavement will be one inch thicker than is required by the AASHTO
rigid pavement design procedure and |
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The
30-year 18-kip ESAL estimate is less than 3,000,000 |
We requires such stabilized subbases since they do not erode over time under truck traffic loading. The general philosophy used is to prevent water intrusion and pumping of underlying materials by using subbases that are dense graded, non-erosive, and stabilized. In addition, the longitudinal steel requirements for CRCP and transverse steel requirements for CRCP and CPCD were developed using the subbases listed above.
A bond breaker
should always be used between concrete pavement and cement-stabilized subbases.
There have been several instances where excessive cracking and premature failures occurred when concrete pavement was placed directly on such subbases. These problems occur because concrete pavements tend to bond directly to cement-stabilized subbases. This increases the chances for cracks in the subbase to reflect through the overlying pavement. This also significantly increases tensile stresses in the concrete pavement due to temperature and moisture changes, which promotes cracking.
Usually, the subgrade is also stabilized or treated with lime or cement for facilitating construction.
Subbase Widths. The subbase should be designed two feet wider on each side than the concrete pavement width to accommodate the pavement equipment.
Serviceability Indices
For concrete pavement design, the difference between the initial and terminal serviceability is the most important factor. It is recommended that an initial serviceability value of 4.5 and a terminal serviceability value of 2.5 be used in the procedure, which results in a difference of 2.0.
Load Transfer Coefficient
The load transfer coefficient is used to indicate the effect of dowels, reinforcing steel, tied shoulders, and tied curb and gutter on reducing the stress due to traffic loading. The coefficients recommended in the AASHTO guide were based on findings from the AASHO Road Test.
The use of load transfer devices, tied shoulders, and tied curb and gutter would result in lower stresses in the pavement. The AASHTO procedure recognizes this by generating a lower concrete pavement thickness when such devices are used.
Table 3-1 lists
the recommended load transfer coefficients for Texas.
Table 3-1. Recommended Load Transfer Coefficients
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Load transfer devices at transverse joints or cracks, or dowels
at joints
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Tied PCC shoulders, curb and gutter, or greater than two lanes in
one direction
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- |
Yes
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No
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Yes
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2.9 |
3.2 |
No
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3.7 |
4.2 |
Drainage Coefficient
The drainage coefficient characterizes the quality of drainage of the subbase layers under the concrete pavement. Good draining pavement structures do not give water the chance to saturate the subbase and subgrade; thus, pumping is not as likely to occur.
The AASHTO guide provides a table of drainage coefficients based on the anticipated exposure of the pavement structure to moisture and on the quality of drainage of the subbase layers. Higher drainage coefficients represent better drainage. The most credit is given to permeable subbases with edge drains.
We have not had much experience with positive drainage systems. As stated earlier, the general philosophy used is to prevent water intrusion and pumping by using subbases that are dense graded, non-erosive, and stabilized. Since the department has had vast experience with such subbases, it is believed that the subbases recommended earlier in this section provide performance equivalent to a fair level of drainage.
Currently,
drainage coefficients for non-erosive stabilized subbases
are based on the anticipated exposure of the pavement structure to water. Table
3-2 shows the recommended drainage coefficients. The coefficients are selected
based on the annual rainfall in the project area.
Table 3-2. Recommended Drainage Coefficients
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Annual Rainfall (inches)
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Drainage Coefficient
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58 - 50 |
0.91 - 0.95 |
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48 - 40 |
0.96 - 1.00 |
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38 - 30 |
1.01 - 1.05 |
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28 - 20 |
1.06 - 1.10 |
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18 - 8 |
1.11 - 1.16 |
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NOTE: Higher drainage coefficients decrease the pavement thickness in the AASHTO procedure. |
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Overall Standard Deviation
This value represents the variability of the input values used. It is recommended that a value of 0.39 be used for thickness design.
The AASHTO guide recommends values in the range of 0.30 to 0.40, with 0.35 being the overall standard deviation from the AASHO Road Test. Higher values represent more variability; thus, the pavement thickness increases with higher overall standard deviations. A value on the high end of the range is used since it is believed that the inputs recommended for Texas are less accurate than the inputs determined at the AASHO Road Test.
Reliability, %
The reliability value represents a "safety factor, " with higher reliabilities representing pavement structures with less chance of failure. We strongly recommends that higher reliabilities be used for pavements where the consequences of failure would be detrimental. As a result, higher reliabilities produce thicker concrete pavements in the AASHTO procedure.
It is assumed that higher reliability levels would produce facilities that require less maintenance over their design lives, thus causing fewer traffic delays. Therefore, higher reliabilities are provided in critical high traffic areas where traffic delays need to be minimized.
Table 3-3 shows
the recommended reliability values. The input values for reliability are based
on the average daily traffic (ADT) per lane and whether or not the facility is
a controlled access facility.
Table 3-3. Recommended Reliability Values
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Projected ADT Per Lane
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Controlled Access Freeways
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Other Highways
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Recommended Reliability, %
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N/A |
<15,000 |
85 |
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<15,000 |
15,000 - 20,000 |
95 |
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15,000 - 20,000 |
21,000 - 25,000 |
99 |
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>20,000 |
>25,000 |
99.9 |
Design Traffic 18-kip ESAL
The AASHTO guide requires a prediction of the number of 18-kip ESALs that the pavement will experience over its design life.
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. This report is requested during the design phase of a project. Traffic for a 30-year design period should be used.
In addition, the predicted 18-kip ESALs is multiplied by a lane distribution factor (LDF). This factor represents the percentage of the total one-way 18-kip ESALs that could be expected in the design lane. The design lane, in this case, is the lane that will carry the most traffic. Usually, it is assumed that the outer lane of a highway with two lanes in each direction carries the most traffic. For a three-lane facility, the middle lane is assumed to carry the most traffic. Traffic distribution in urban areas is somewhat more complex due to merging and exiting operations, but the same assumptions could apply.
Table 3-4 shows the current recommended LDF
values.
Table 3-4. Recommended LDF
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Number of Lanes in Both Directions
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LDF
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£ 4 |
1.0 |
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6 |
0.7 |
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³ 8 |
0.6 |
The LDF decreases with an increase in the number of lanes of a facility. So, a highway with two lanes in each direction would have a higher LDF than a highway with three or more lanes in each direction. This is because traffic tends to spread out over the available lanes.
The traffic analysis report also lists a directional distribution of traffic, which indicates the percent distribution of the design hourly volume in each direction of a highway facility. This value is used for capacity analysis and applies to all vehicles in the design hourly volume. However, we assume that the directional distribution of heavy vehicles on any project is evenly split in both directions. Therefore, the directional distribution factor listed in the report should not be used to modify the design 18-kip ESALs.