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Section 4: Recommended Input Design Values

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Input Values

The following input variables are needed for the AASHTO rigid pavement design procedure:

  • 28-day Concrete Modulus of Rupture, psi
  • 28-day Concrete Elastic Modulus, psi
  • Effective Modulus of Subgrade Reaction, pci
  • Serviceability Indices
  • Load Transfer Coefficient
  • Drainage Coefficient
  • Overall Standard Deviation
  • Reliability, %
  • Design Traffic, 18-kip Equivalent Single Axle Load (ESAL).
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28-day Concrete Modulus of Rupture, Mr

The Mr of concrete is a measure of the flexural strength of the concrete as determined by breaking concrete beam test specimens. A Mr of 620 psi at 28 days should be used with the current statewide specification for concrete pavement design. If the engineer selects an alternate value for Mr, it must be documented with an explanation.

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28-day Concrete Elastic Modulus

Elastic modulus of concrete is an indication of concrete stiffness. It varies depending on the coarse aggregate type used in the concrete. Although the value selected for pavement design could be different from the actual values, the elastic modulus does not have a significant effect on the computed slab thickness. A modulus of 5,000,000 psi should be used for pavement design. The use of a different value must be documented with an explanation.

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Effective Modulus of Subgrade Reaction: k-value

The AASHTO guide allows pavement designers to consider the structural benefits of all layers under the concrete slab. The guide 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 reaction, otherwise known as k-value.

Field performance evaluations of concrete pavement have revealed that the soundness of the 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.

TxDOT-Required Base Layer Combinations

TxDOT recognized this and requires one of the following base layer combinations for concrete slab support:

  • 4 in. of asphaltic concrete pavement (ACP) or asphalt stabilized base (ASB) or
  • a minimum 1 in. asphalt concrete bond breaker over 6 in. of a cement stabilized base.

A k-value of 300 psi/in. should be used in the rigid pavement design procedure with one of the stabilized base layer combinations listed above.

The designer may use a higher k-value, up to 800 psi/in., with the requirement of field verification tests during the construction. Conduct verification tests in accordance with “Method 1: On-site Static Load Test” found in Tex 125-E, “Determining Modulus of Subgrade Reaction (k-value).”

Use a 30-in. diameter steel plate and test on top of the base for every 0.1 mile of roadbed, for each 20 ft. of plan dimensional steel plate width or a portion of that. The k-values obtained from verification tests may be adjusted by the loss of support factor as directed by the Engineer. These k-values should meet the design k-value at all times during construction.

TxDOT requires these stabilized bases because they do not erode over time under truck traffic loading. TxDOT’s general philosophy is to prevent water intrusion and pumping of underlying materials by using bases that are dense graded, non-erosive, and stabilized. Bases that are properly designed and constructed using TxDOT specifications and test methods should provide adequate long-term support.

Where long-term moisture susceptibility of ACP is a concern, using a plan note to increase the target laboratory density (and thus total asphalt content) may be beneficial. To ensure long-term strength and stability of cement stabilized layers, sufficient cement must be used in the mixture.

Selecting Appropriate Strength

Item 276, Cement Treatment (Plant-Mixed) currently designates three classes of cement-treated flexible base, based on 7-day unconfined compressive strength. Classes L and M are intended for use with flexible pavements. Class N, which has a minimum strength as shown on the plans, is intended for use with rigid pavements.

There are several approaches to selecting an appropriate strength (and, indirectly, cement content as a result):

  • successful long-term experience with similar materials
  • laboratory testing using 100% of the retained strength of a conditioned specimen to determine if the design cement content and strength are acceptable
  • laboratory testing using the tube suction test to determine if the design cement content and strength are acceptable.

Use of Bond Breaker

A bond breaker should always be used between concrete pavement and cement stabilized base. There have been several instances across Texas where excessive cracking and premature failures occurred when a concrete slab was placed directly on cement stabilized base. These problems occur because concrete slabs tend to bond directly to cement stabilized 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.

TxDOT recommends a minimum of 1 in. asphalt concrete stress-relieving layer be used between cement stabilized base and the concrete slab. A polyethylene sheet is not recommended for use as a bond breaker, due to construction problems evident from 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 CST-M&P for further assistance, if needed.

Subgrade/Base Widths. The subgrade/base should 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 CST-M&P personnel is required.

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Serviceability Indices

For concrete pavement design, the difference between the initial and terminal serviceability is an important factor. An initial serviceability value of 4.5 and a terminal serviceability value of 2.5 are to be used in the procedure, which results in a difference of 2.0. Different values, if used, must be documented and justified.

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Load Transfer Coefficient

The load transfer coefficient is used to incorporate the effect of dowels, reinforcing steel, tied shoulders, and tied curb and gutter on reducing the stress in the concrete slab due to traffic loading. The coefficients recommended in the AASHTO Guide were based on findings from the AASHO Road Test.

Table 8.1 lists the load transfer coefficient values to be used for rigid pavement design:

Anchor: #i1025435Table 8-1: Load Transfer Coefficients

CRCP or Load Transfer Devices at Transverse Joints

Tied PCC Shoulders, Curb and Gutter, or Greater Than Two Lanes in One Direction

Yes

No

Yes

2.6 for CRCP

2.9 for JCP

3.2

No

3.7

4.2



There is substantial evidence that tied Portland cement concrete (PCC) shoulders improve PCC pavement performance significantly. Therefore, it is strongly recommended that tied PCC shoulders be provided, if possible. In case it is not feasible to provide tied PCC shoulders, the use of a minimum 2-ft. wider outside lane should be considered.

PCC shoulders should be tied to the main lane pavement by tie bars or by the main lane's transverse steel. The joint between the concrete shoulder and the concrete main lane pavement should be a warping or hinge joint, not a construction or expansion joint. The PCC shoulder should have the same thickness and the same base layers as the main lane pavement. This will allow traffic to be routed on the shoulder during future maintenance and construction while reducing the chances of structurally damaging the shoulder. It will also facilitate the construction sequence in most cases.

Tied or monolithic curb and gutter help in reducing edge stresses and serve as a barrier that discourages traffic from riding too close to the edge of the pavement structure. Although tied curb and gutter sections usually contain tie bars, the tie bars are too small in either size or number to transfer load stresses effectively by themselves. Construction joints usually exist between tied curb and gutter and the concrete pavement. This means no aggregate interlock exists. Since the number of edge stresses in the pavement are much less for monolithic curb sections than for tied curb and gutter sections, the use of monolithic curb is recommended when practical.

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Drainage Coefficient

The drainage coefficient characterizes the quality of drainage of the subbase layers under the concrete slab. Good draining pavement structures do not give water the chance to saturate the base 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 base layers. Higher drainage coefficients represent better drainage. The most credit is given to permeable bases with edge drains.

TxDOT has not had much experience with positive drainage systems. As stated earlier, TxDOT's philosophy on this issue is to prevent water intrusion and pumping by using bases that are dense graded, non-erosive, and stabilized. TxDOT has had good performance with such bases and it is believed that the bases recommended earlier in this section provide performance equivalent to a fair level of drainage.

Currently, drainage coefficients in Texas for non-erosive stabilized bases are based on the anticipated exposure of the pavement structure to water. “Table 8-2: Drainage Coefficients” shows the recommended drainage coefficients for Texas. The coefficients are selected based on the annual rainfall in the project area.

Anchor: #CEGJHACGTable 8-2: Drainage Coefficients

Annual Rainfall (in.)

Drainage Coefficients

58 – 50

0.91 - 0.95

48 – 40

0.96 - 1.00

38 – 30

1.01 - 1.05

28 – 20

1.06 - 1.10

18 – 8

1.11 - 1.16

NOTE: Higher drainage coefficients decrease the pavement thickness in the AASHTO procedure.



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Overall Standard Deviation

Overall standard deviation accounts for both chance variation in the traffic prediction and normal variation in pavement performance prediction for a given traffic. 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 considered reasonable for Texas since it is believed that the inputs recommended for Texas are less accurate than the inputs determined at the AASHO Road Test. A value of 0.39 is to be used for rigid pavement design.

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Reliability, %

The reliability value represents a "safety factor" with higher reliabilities representing pavement structures with less chance of failure. The AASHTO Guide recommends values ranging from 50% to 99.9%, depending on the functional classification and the location (urban vs. rural) of the roadway. Based on TxDOT's experience, a reliability of 95% should be used for rigid pavement with more than 5 million design ESALs; a reliability of 90% should be used for rigid pavement with 5 million or less design ESALs. If the engineer decides to use a different value, then it must be documented and justified.

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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 provided by the Transportation Planning and Programming (TPP) Division. This report is requested during the design phase of a project and, upon receipt, should be evaluated for reasonableness.

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 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.

Anchor: #i1025835Table 8-3: Lane Distribution Factor

Total Number of Lanes in Both Directions

LDF

4

1.0

6

0.7

8*

0.6

*Unless field observations show otherwise



The lane distribution factor (LDF) decreases with an increase in the number of lanes of a facility. Highways with two lanes in each direction would have a higher LDF than highways with three or more lanes in each direction. This is because traffic tends to spread out over the available lanes. If there is a legal constraint requiring the trucks to use a particular lane, or avoid using certain lanes, then a review should be made of the appropriate lane distribution factor.

The traffic analysis report also lists a directional distribution of traffic. This value 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.

TPP assumes the directional distribution of heavy vehicles on any project is evenly split in both directions, unless specifically stated otherwise. Typically, the directional distribution shown on the traffic projections from TPP applies only to the 30th highest hourly volume. Therefore, the directional distribution factor listed in the TPP report should not be used to modify the design 18-kip ESALs.

Local conditions may cause the directional distribution of heavy vehicles to be unequal. For example, 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. The reasons and methods used to make the adjustment should be documented.


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