Pavement Design

Dr. Shen, PHD.,P.E.

June 23rd, 2011

AASHTO GUIDE FOR DESIGN OF PAVEMENTS

S E C T I O N 2 . 4

TABLE OF CONTENTS

INTRODUCTION AND BACKGROUND………………………………… 2

SECTION 1: DRAINAGE AND STRUCTURAL NUMBER……………… 3

SECTION 2: LOAD TRANSFER…………………………………………….19

SECTION 3: LOSS OF SUPPORT…………………………………………...28

1

Introduction and Background: Pavement Structural Characteristics

Major corridors throughout the United States have been suffering from congestion and bottlenecks in recent years. As an example, as the population of the Miami-Dade metropolitan area continues to grow, the existing corridors traveling Eastbound, Westbound, Northbound and Southbound continue to face gridlock. The genetic makeup of most corridors consists of a mixed community of residential, commercial and school zones. Several areas have minimum design standards with narrow sidewalks. The lack of designated bike lanes in combination with narrow sidewalks has created a very adverse environment for all parties involved – the vehicular traffic, the pedestrians and the bicyclists. Due to this along with other factors such as severe budget cuts for 3R projects in the U.S., the pertinence of durable, smooth, cost effective and quality pavement has been increasingly important. The underlying surface of the pavement is perhaps the most critical component that affects the lifespan of the pavement. A thorough understanding of the traffic, environmental factors, and drainage on any pavement structure is critical.

The focus of this report is to review the following section in the AASHTO Guide for Design of Pavement Structures:

Part II Section 2.4: Pavement Structural Characteristics

The section is composed of six (6) pages which includes two (2) full page Figures and six (6) tables. The main topics of discussion in this report will include drainage, load transfer, and loss of support and their pertinent influence on the design of any pavement structure.

2

SECTION 1: DRAINAGE AND STRUCTURAL NUMBER

One of the most important factors in pavement design is drainage. In the past, drainage design did not warrant as much attention as it does in modern day pavement design. A common perception was that if the thickness design was based on saturated conditions, then good drainage was not an essential focal point in final design. This concept was perhaps a valid argument in the past when volume and traffic loadings were far less. However, with today’s vehicular traffic mix both in the United States and on a global scale, the weigh and number of axle loads has increased dramatically. With this increase, the number of axle loads has increased and water damage is pernicious by not only causing a loss in shear strength but also by causing distress such as pumping and degradation of paving materials.

Pictures of pumping and degradation conditions are shown below:

Figure 1: Effects of Pumping due to Pavement Distress

Figure 2: Poor Drainage

Poor drainage can lead to alligator cracking and edge cracks along shoulder.

3

If the supply of water infiltrating the pavement is less than the drainage capacity of the base, subbase, and subgrade, then theoretically speaking, an internal drainage system is not required. However, drainage, temperature conditions and moisture conditions are very difficult to estimate and it is suggested that drainage layers are utilized for all important pavement structures.

Water can be very detrimental to the pavement structure as it filtrates through the pavement surface, shoulders, joints and cracks. In cities such as Miami, Florida, the high water table creates an even challenging situation for the pavement design engineer. Interrupted aquifers and localized springs are moisture conditions which warrant attention in the design process. Some of the damaging effects of water in the pavement include but are not limited to:

1) Differential heaving above swelling soils

2) Asphalt mixture stripping and durability (“D”) cracking of concrete

3) A reduction in strength of unbounded granular materials and subgrade soils

4) The pumping of fines in the base course may occur with loss of support. This occurs with flexible pavements due to moving traffic which generates high hydrodynamic pressure

5) In cold regions if the depth of frost penetration is larger than the thickness of the pavement, frost heave and a reduction in load-carrying capacity can occur during the frost melting period with a high water table

6) Water can cause concrete pavements to have pumping characteristics which induces cracks, faults, and deterioration to the shoulders

The study of pavement drainage must begin by identifying the sources of water entering the

pavement section. A good, competent pavement engineer has a thorough understanding of the

sources of water that infiltrate the pavement.

Figure 3: Diagram of Sources of water infiltrating pavement.

4

Surface Infiltration

Joints and cracks are the single largest and pernicious source of water entering the PCC pavement.

Rising Groundwater

A secondary source of water penetrating the pavement section is the groundwater coming from underneath. In cities such as Miami, Florida, the groundwater table is very high, at nearly 9 feet. Seasonal fluctuations of the water table occur naturally.

Seepage of water

Seepage can pose another significant problem in drainage design. Seepage occurs in sections of roads with flat longitudinal grades or in cut sections where ditches are shallow.

Capillary action

Capillary action can transport water well above the water table. The subgrade is saturated due to this. For sandy soils, typical values for capillary rise are 4 to 8 feet, for silty soils 10 to 20 feet, and for clayey soils in excess of 20 feet. Frost heave action is caused by capillary action. In asphalt concrete, it is the major source of moisture problems.

Vapor movement

5

Temperature gradients can cause the water vapor present in the air voids of the subgrade and pavement structure to migrate and condense. Water vapor is not a dominant source of moisture in the pavement structure.

Pavement drainage design should have the core goal of removing water in a timely manner as it infiltrates the pavement structure. It is critical to seal joints and cracks to prevent unwanted damage to the pavement.

Figure 4: Sources of water infiltration and penetration through pavement support.

A drainable pavement contains various components.

Asphalt or concrete surface pavement A permeable base A separator/filter layer The subgrade

Figure 4: Drainage

Drainage Coefficient (m and Cd)

In addition to the layer coefficients, the SN number includes a drainage coefficient denoted as “m” in flexible pavements. Based on the availability of moisture and the quality of drainage, drainage coefficients m2 and m3 should be applied to granular bases and subbases.

6

Figure 5: Typical Pavement Section – Flexible Pavement

Structural Number (SN)

The pavement structure is identified with a Structural Number (SN). For a given combinations of total traffic (expressed in ESAL’s), terminal serviceability, environment and soil support (MR), one can determine the Structural Number. The SN is converted to actual layer thicknesses (i.e., 4 inches (100 mm) of Hot Mix Asphalt) by utilizing a layer coefficient (denoted as “a” that is indicative of the relative strength of the construction materials in that layer. The “a” layer pavement coefficient is typically denoted as a1, a2, and a3 representing each corresponding layer of asphalt, subbase and subgrade. The layer coefficient is essentially a measure of the relative ability of a unit thickness of a given material to function as a structural component of the pavement. Test roads or satellite sections are used to determine layer coefficients along with correlations of materials properties. However, the material properties, such as the resilient modulus, are the recommended method of determining layer coefficients. The depth (D) of each layer is also considered in the SN equation for the pavement structure.

Thus, the structural number (SN) is a function of the layer thicknesses, layer coefficients, and drainage coefficients and can be computed with the following overall equation:

Figure 6: SN = a1D1 + a2D2m2 + a3D3 m3

7

Subgrade support.  Subgrade support is characterized by the subgrade's resilient modulus(MR).  Intuitively, the amount of structural support offered by the subgrade should be a large factor in determining the required pavement structure.

Unconfined Compressive Strength (psi) 7 day Break TestThe 7-day UCS test results using correlation charts available in the AASHTO Guide for Design of Pavement Structures can be used to obtain a layer coefficient for the cement-treated base material. For 7-day UCS values in the range of 400 to 500 psi, the layer coefficientwill be between 0.16 and 0.18, although many agencies specify a value of 0.20 for cement treated base with this level of strength. The layer thickness may then be determined using standard AASHTO design procedures.

Figures 7 and 8: Concrete Compressive Testing

8

Figure 7: Concrete compressive test apparatus Figure 8: Concrete compressive

test apparatus during Prior to breaking. breaking (failure) phase.

With the concrete compressive test results, one can determine the pavement coefficient “a2” from

the Figure Below:

Figure 9: This is Figure 2.8, Page II-23 in AASHTO DESIGN OF PAVEMENT

STRUCTURES Book:

9

Variation in “a” for Cement-Treated Bases with Base Strength Parameter (3)

Note: Marshall Stability of a test specimen is the maximum load required to produce failure when the specimen is preheated to a prescribed temperature placed in a special test head and the load is applied at a constant strain (5 cm per minute). While the stability test is in progress dial gauge is used to measure the vertical deformation of the specimen. The deformation at the failure point expressed in units of 0.25 mm is called the Marshall Flow value of the specimen.

10

Once the Marshall Stability or/and the Resilient Modulus are known, structural coefficient (a2)

can be determined from Figure 2.9 below. The use of a ruler and a steady hand is needed to ensure accurate results by the pavement designer.

Figure 10: This is Figure 2.9, Page II-24 in AASHTO DESIGN OF PAVEMENT

STRUCTURES book:

Variations in a2 for Bituminous Treated Bases with Base Strength Parameter (3)

Figure 11: Marshall Stability Test Apparatus

11

Drainage Coefficient in Rigid Pavements

The value for the drainage coefficient in rigid pavements is denoted as Cd. Essentially, it has the same effect as the load transfer coefficient which will be described later in this report. As depicted in the figure below, a rise in the drainage coefficient is equal to decrease in J. With this, there is an increase in the W18.

The figure below exhibits recommended Cd values. The values are derived based on the percentage of time the pavement structure is exposed to moisture levels approaching saturation versus the quality of drainage. The quality of drainage is denoted as follows:

Figure 12: The drainage coefficient values for Flexible Pavement are:

12

Thus, all layers below the HMA layer have an assigned drainage coefficient. The total time of exposure to near saturation moisture conditions along with the relative loss of strength within the layer due to characteristics of drainage are depicted in the drainage coefficient.

The range of a coefficient can be as high as 1.4 for quick draining layers that almost never become saturated. On the opposite range, slow draining layers that are often saturated can warrant a coefficient layer as low as 0.40. Generally speaking, a drainage coefficient is imposed to make a specific layer thicker. However, if a medium to complex drainage problem is suspected, the engineer thoroughly investigate the actual drainage problem by using very dense layers in an effort to minimize water infiltration. Another option is to design an actual drainage system for the pavement. In pavement design, it is common to set the drainage coefficient to 1.0 (m=1) which neglects the drainage coefficient parameter in its entirety within the structural number equation.

Similarly, the recommended values of drainage coefficients CD for Rigid Pavements are:

Figure 13: Cd Values for Rigid Pavement

13

Figures 14 & 15: As shown below, flexible pavement can be greatly damaged due to water infiltration.

Figure 14: Cracking from edge failure, frost, and fatigue.

14

Figure 15: Very poor structural performance of pavement in residential road.

Figure 16: Full depth damage Figure 17: Traffic is delineated due to drainage

15

Figure 18: Typical cross section of Flexible Pavement

Figure 19: Flexible pavement and rigid pavement have some distinct differences.

16

Modern Software/Equipment used to Determine Pavement Drainage

With improved technology and modern spreadsheet programs such as MS Excel, pavement design engineers have improved their ability to estimate the drainage characteristics of subsurface drainage materials. An example of this is the program called PDE (Pavement Drainage Estimator) which is an Excel based program. After inputting variables such as pavement dimensions, amount of drainage desired, aggregate properties and rainfall intensity, PDE can compute the required hydraulic conductivity of a pavement base layer. It can also calculate the time required to achieve a certain percentage of drainage. In addition to software, Permeability testing can be performed. In-situ testing of permeability and stability is important for quality assurance/quality control. Developed to determine the hydraulic conductivity of pavement bases in just seconds, the air permeameter test (APT) device has been a major contribution to determining permeability. In an hour, one operator can perform about 50 tests. Several tests of a base layer can ensure uniformity.

The new APT device is the only rapid permeability testing device in the world. This device weighs 40 pounds and can be carried by one person.

17

Figure 20: Permeameter test (APT) device

Drainage within a pavement structure varies depending on the materials implemented in design. Simply put, not all materials drain the same. Below are some photos exhibiting various pavements under a wide range of moisture conditions. Drainage is categorized with a range of Excellent to Very Poor.

Figure 21: (Not all materials drain the same)

Figure 22: Excellent Drainage Conditions

18

Figure 23: Excellent Drainage Conditions

Figure 24: Good Drainage Conditions

19

Figure 25: Poor and Very Poor Drainage Conditions

20

Figure 26: Typical Edge Drain Location for crowned concrete pavement with tied concrete shoulders

As shown in the figure above, edgedrains are provided on both sides of the pavement section since the pavement is crowned. The time to drain is less as the length of the pavement path is significantly reduced. The pavement’s edges have substantial support with the tied shoulders. Geotextiles are also implemented in a good drainage system as denoted in the figure above.

SECTION 2: LOAD TRANSFER

The Load Transfer Coefficient (J)

Used in rigid pavement design, the load transfer coefficient (J) accounts for the ability of a concrete pavement to transfer a load across joints and cracks. The implementation of load transfer devices (such as dowel bars) along with tied concrete shoulders escalates the amount of load transfer and decreases the load transfer coefficient. Aggregate interlock also plays a significant role in determining the value of load transfer. The figure below shows the recommended for values Load Transfer Coefficient:

21

Figure 27: Recommended load transfer, “J’ values.

22

The most common load transfer device is the dowel bar. The use of dowel bars has been a very effective method of reducing the amount of joint faulting when compared to nondoweled sections of similar designs. Egregiously, the diameter/size of the dowel has a great effect on the load transfer performance. The larger the dowel diameter leads to better load transfer and fault control, especially under heavy traffic conditions.

Figure 28: The figure below shows dowel bars prepared to be placed in concrete pavement, at the mid-depth point of the slab.

• Dowel bars are a very important component for increasing load transfer efficiency

• Dowel bars are measured by diameter which is equal to slab thickness multiplied by 1/8 inch. Dowel spacing and length are normally 12 inches or 18 inches.

Studies have shown that nondoweled JPCP slabs generally lead to a larger occurrence of faulting. Permeable bases can reduce this dilemma. Dowel bar coatings are also beneficial. They protect the dowels from hostile effects of moisture.

It is important to note that dowel bar design can be optimized with the use of engineering software such as DowelCAD. A pavement engineer can determine joint responses to varying dowel sizes or investigate the impact of various alternate dowel bar configurations.

23

Figure 29: DowelCAD

The magnitude of deflections in the slabs under loading and the distribution of stresses in the slab are all influenced by the load transfer devices. Proper implantation of load transfer devices enables concrete pavement to behave more beneficially. For example, dowels and other load transfer devices can reduce corner deflections which will reduce pumping and faulting. Differential deflections are also reduced will be reduce crack deterioration found in overlays. Load transfer devices also reduce tensile stresses in concrete, will leads to a reduction in corner breaks and cracking.

The vehicle loads can greatly vary based on the type of vehicle. The damaging effect of larger vehicles such as Loaded 40’ and Loaded 60’ buses have a tremendous impact on the pavement when compared to a typical car as denoted in the figure below.

24

Figure 30: Impact of various vehicles on ESAL

Measuring and Interpreting Load Transfer (J)

In real world applications, project sites are evaluated and determined if a load transfer restoration is appropriate. The degree of deflection load transfer varies from crack to crack and joint to joint along a project at any given time. Daily temperature, moisture fluctuations, and seasonal temperature are all variances for the pavement during its lifetime. A load transfer evaluation should be performed to identify which cracks and joints need load transfer restoration. The Falling Weight Deflectometer (FWD) is a heavy load testing device which can measure the deflection load transfer measurements.

Along with the FWD, other nondestructive testing (NDT) devices are used. The outer wheel path truck loads can be simulated on each side of a joint or crack and deflections can be measured on both sides. Sensors are used to measure the loads and deflections and they are recorded by a computer in a tow vehicle. The FWD is a rapid and efficient operation and in one day, more than a few miles of pavement can be analyzed (a relative relationship of joint spacing). Like most work in construction which requires safety and protection of complex equipment, traffic control must be implanted to protect the testing machine in the pavement lane being diagnosed.

25

Figure 31: The falling weight deflectometer (FWD) is a non-destructive testing (NDT) and non-intrusive device. The FWD has been widely used in pavement engineering to evaluate pavement structural condition.

Material properties and Jointed Pavements

Material properties that contribute to pavement distress include modulus and shear strength, which are greatly influenced by gradation, moisture content, and density. When granular layers are failing or have poor performance permanent deformation such as rutting can occur. Other occurrences include frost heave, corrugations, longitudinal cracking, and depressions.

Failures in rigid pavements from granular layers include cracking, corner breaks, faulting, and pumping. All of these distresses reduce the pavement life and diminish the structural integrity of the pavement. As moisture enters the pavement through cracks and joints there is a loss of load carrying capacity. Subgrade and base strength is reduced along with stiffness and premature pavement failure can occur.

26

Figure 32: Jointed Pavement

Figures 33 & 34: Longitudinal, Transverse Construction Joints; Tie Bars, Dowel Bars

27

Contraction Joints

Contraction joint is to relieve tensile stresses resulting from temperature drops and moisture variations in concrete. An example is shown in the Figure above. This type of joint is used only in plain jointed pavement at the transverse joint.

Figure 35: Dowel Bar Assembly System

Heavy welded wire baskets are often used to erect the dowel bars and the proper elevation, depth, spacing, and alignment. Contractors can use full length or half-length baskets (the full length basket is preferred by most). The wire basket is manufactured in the shop and one end of the dowel bar is tack welded to the basket. Once delivered and placed at the project location, concrete will be paved over the basket. It is extremely important the dowel bars be parallel for the contraction joint to function properly.

28

Figure 36: Photo before paving. Tie bars (laying on base in lower part of picture) will be

placed at the paintmarks during paving.

Figure 37: Photo after paving.

In plain jointed concrete pavements, tie bars are used. If a tie bar is used to connect a lane and a shoulder or two lanes together, the paving machine can be used to mechanically insert the devices into the newly poured concrete. In this case, the tie bars are not positioned and secured like they are in CRCP because there is not an available mat of steel reinforcement. Thus, each tie bar will have to be held in position and supported with the installation of pins and stakes which must be driven into the subbase structure. It is critical that the supports are strong enough to hold the tie bars in place during concrete pouring operations.

29

SECTION 3: LOSS OF SUPPORT

Figure 38: Table 2.7 from AASHTO GUIDE, PP. II-27.

Figure 39: Types of materials for construction. The photos below show unbound granular materials for road pavements, lime treatment on subgrade, and mixing lime into subgrade material

30

Loss of Support and Joint Sealants

Overview

Joints Sealed with Self-Leveling Silicone

A loss of structural support will occur when water enters a pavement’s layers (subgrade, subbase). With heavy vehicular traffic, pumping of fines can occur of these layers. Degradation will often occur as well, leading to loss of support in the structure, joint faulting and pavement settlement.

Since it isn’t always practical to build a pavement that is continuously watertight, many highway agencies utilize joint sealants to prohibit surface water infiltration through joints. In addition they provide a permeable subbase to keep water from entering the pavement.

It is better to use porous asphalt or concrete in lieu of impervious concrete and asphalt when it comes to loss of support. In porous asphalt, the mix has open graded aggregates with fewer fines. Voids are interconnected which results in a porous surface.

Because fines in the mix only serve to make compaction easier while the bigger pieces of crushed aggregate give enough structural stability (even for large trucks and other heavy applications), there is no loss of structural support capacity in porous asphalt compared to impervious asphalt and concrete.

Figure 40: Porous asphalt vs. standard asphalt.

31

Shoulder and Jointing Considerations – Transverse Joints

Cracking is controlled by installing transverse joints. A mid-panel crack is less likely to develop with closer joint spacing. The critical element at joints is load transfer, where it is much recommended to use dowels. In undoweled pavements, aggregate interlock provides support for the load transfer. As slabs contract and the joints get larger, aggregate interlock is lost. As traffic passes over concrete, the concrete experiences movement and the interlock is gradually destroyed. In areas that experience heavy truck traffic and high temperature variations, aggregate interlock is not effective and faulting occurs. The joints can be supported with dowels that are 18 inches long to handle the expansion and contraction. They can be placed in the transverse joints and the mainline joints. Most government agencies have Standard Construction Drawings in which the spacing requirements and transverse joint design are shown.

Expansion and Pressure Relief Joints

Due to seasonal temperature changes, concrete slabs contract. This causes cracks and joints to open allowing incompressible materials into the pavement structure. Due to this, the pavement can grow in length and even create pressure. This induced pressure can be beneficial in small amounts, since a lack of pressure would make joints and cracks open which would reduce load transfer while not creating too much pavement distress. If distresses are discovered, some type of rehabilitation is recommended in terms of maintenance. A blowup would require immediate attention.

Figure 41: Using values of J and Cd to obtain Structural Number of Rigid Pavement.

32

33

This report investigates two vital components of rigid pavement design when determining the overall structural number (SN)

thickness. The load transfer (J) and the drainage coefficient (Cd) are

depicted in the nomograph above.

Figure 42: Rigid Pavements – Steel compensates for concretes poor ability to cope with tension stress

Conclusion

The primary criteria that design engineers must consider and properly investigate to develop a proper pavement structure are the traffic, environmental conditions and soil conditions. The surface course of the pavement, whether it be flexible asphalt pavement or rigid pavement heavily relies on the foundation that it is placed upon. If the subsurface material conditions are inadequate and drainage is not properly accounted for the pavement will not meet its design criteria and will most likely fail prior to its expected service life. The importance of drainage has been greatly emphasized in this report along with the benefits of load transfer devices such as dowel bars which serve to distribute loads across discontinuities such as joints or cracks. Other steel reinforcing mechanisms such as tie bars and longitudinal bars have also been reviewed as a primary solution to eradicate cracking, faulting and other distresses that cause permanent damage to pavements. The loss of support factor has also been discussed in this report as proper and cost effective material selection plays a significant role in pavement design. With stringent government budgets both in the U.S. and on the global scale, highway funding in virtually all nations has been diminishing over time and it is vitally important that pavement engineers design pavements that are durable, long-lasting and efficient. The importance of the pavement structural characteristics are identified and stressed in this report along with the significant roles played by proper drainage, load transfer and loss of support factors in an effort to mitigate pavement distress.

34