Pavement Final Project Raj Naidu Wednesday


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Pavement Final Project Raj Naidu Wednesday

  1. 1. Pavement Design Dr. Shen, PHD.,P.E. June 23rd, 2011AASHTO GUIDE FOR DESIGN OF PAVEMENTS SECTION 2.4
  3. 3. Introduction and Background: Pavement Structural CharacteristicsMajor corridors throughout the United States have been suffering from congestion andbottlenecks in recent years. As an example, as the population of the Miami-Dade metropolitanarea continues to grow, the existing corridors traveling Eastbound, Westbound, Northbound andSouthbound continue to face gridlock. The genetic makeup of most corridors consists of a mixedcommunity of residential, commercial and school zones. Several areas have minimum designstandards with narrow sidewalks. The lack of designated bike lanes in combination with narrowsidewalks 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 cutsfor 3R projects in the U.S., the pertinence of durable, smooth, cost effective and qualitypavement has been increasingly important. The underlying surface of the pavement is perhapsthe most critical component that affects the lifespan of the pavement. A thorough understandingof 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 ofPavement Structures:Part II Section 2.4: Pavement Structural CharacteristicsThe 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 lossof support and their pertinent influence on the design of any pavement structure. 2
  4. 4. SECTION 1: DRAINAGE AND STRUCTURAL NUMBEROne of the most important factors in pavement design is drainage. In the past, drainage designdid not warrant as much attention as it does in modern day pavement design. A commonperception was that if the thickness design was based on saturated conditions, then good drainagewas not an essential focal point in final design. This concept was perhaps a valid argument in thepast when volume and traffic loadings were far less. However, with today’s vehicular traffic mixboth in the United States and on a global scale, the weigh and number of axle loads has increaseddramatically. With this increase, the number of axle loads has increased and water damage ispernicious by not only causing a loss in shear strength but also by causing distress such aspumping and degradation of paving materials.Pictures of pumping and degradation conditions are shown below:Figure 1: Effects of Pumping due to Pavement DistressFigure 2: Poor DrainagePoor drainage can lead to alligator cracking and edge cracks along shoulder. 3
  5. 5. 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 estimateand 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 pavementsurface, shoulders, joints and cracks. In cities such as Miami, Florida, the high water tablecreates an even challenging situation for the pavement design engineer. Interrupted aquifers andlocalized springs are moisture conditions which warrant attention in the design process. Some ofthe 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 shouldersThe study of pavement drainage must begin by identifying the sources of water entering thepavement section. A good, competent pavement engineer has a thorough understanding of thesources of water that infiltrate the pavement.Figure 3: Diagram of Sources of water infiltrating pavement. 4
  6. 6. Surface InfiltrationJoints and cracks are the single largest and pernicious source of water entering the PCCpavement.Rising GroundwaterA secondary source of water penetrating the pavement section is the groundwater coming fromunderneath. 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 waterSeepage can pose another significant problem in drainage design. Seepage occurs in sections ofroads with flat longitudinal grades or in cut sections where ditches are shallow.Capillary actionCapillary action can transport water well above the water table. The subgrade is saturated due tothis. 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. Inasphalt concrete, it is the major source of moisture problems.Vapor movementTemperature gradients can cause the water vapor present in the air voids of the subgrade andpavement structure to migrate and condense. Water vapor is not a dominant source of moisturein the pavement structure.Pavement drainage design should have the core goal of removing water in a timely manner as itinfiltrates the pavement structure. It is critical to seal joints and cracks to prevent unwanteddamage 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 5
  7. 7. Figure 4: DrainageDrainage 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.Figure 5: Typical Pavement Section – Flexible Pavement 6
  8. 8. Structural Number (SN)The pavement structure is identified with a Structural Number (SN). For a given combinationsof 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” thatis indicative of the relative strength of the construction materials in that layer. The “a” layerpavement coefficient is typically denoted as a1, a2, and a3 representing each corresponding layerof asphalt, subbase and subgrade. The layer coefficient is essentially a measure of the relativeability of a unit thickness of a given material to function as a structural component of thepavement. Test roads or satellite sections are used to determine layer coefficients along withcorrelations of materials properties. However, the material properties, such as the resilientmodulus, are the recommended method of determining layer coefficients. The depth (D) of eachlayer 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, anddrainage coefficients and can be computed with the following overall equation: Figure 6: SN = a1D1 +a2D2m2 + a3D3 m3Subgrade support. Subgrade support is characterized by the subgrades resilient modulus(MR).Intuitively, the amount of structural support offered by the subgrade should be a large factor indetermining the required pavement structure. 7
  9. 9. Unconfined Compressive Strength (psi) 7 day Break TestThe 7-day UCS test results using correlation charts available in the AASHTO Guide for DesignofPavement Structurescan be used to obtain a layer coefficient for the cement-treated basematerial. 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 cementtreated base with this level of strength. The layer thickness may then be determined usingstandard AASHTO design procedures.Figures 7 and 8: Concrete Compressive Testing Figure 7: Concrete compressive test apparatus Figure 8: Concrete compressive test apparatus during Prior to breaking. breaking (failure) phase. 8
  10. 10. With the concrete compressive test results, one can determine the pavement coefficient “a2” fromthe Figure Below: Figure 9: This is Figure 2.8, Page II-23 in AASHTO DESIGN OF PAVEMENT STRUCTURES Book: Variation in “a” for Cement-Treated Bases with Base Strength Parameter (3) 9
  11. 11. Note: Marshall Stability of a test specimen is the maximum load required to produce failurewhen the specimen is preheated to a prescribed temperature placed in a special test head and theload is applied at a constant strain (5 cm per minute). While the stability test is in progress dialgauge is used to measure the vertical deformation of the specimen. The deformation at the failurepoint expressed in units of 0.25 mm is called the Marshall Flow value of the specimen.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 toensure 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 10
  12. 12. Drainage Coefficient in Rigid PavementsThe value for the drainage coefficient in rigid pavements is denoted as Cd. Essentially, it has thesame effect as the load transfer coefficient which will be described later in this report. Asdepicted 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 thepercentage of time the pavement structure is exposed to moisture levels approachingsaturation versus the quality of drainage. The quality of drainage is denoted as follows:Figure 12:The drainage coefficient values for Flexible Pavement are:Thus, all layers below the HMA layer have an assigned drainage coefficient. The total time ofexposure to near saturation moisture conditions along with the relative loss of strength within thelayer due to characteristics of drainage are depicted in the drainage coefficient. 11
  13. 13. 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 12
  14. 14. Figures 14& 15: As shown below, flexible pavement can be greatly damaged due to waterinfiltration.Figure 14: Cracking from edge failure, frost, and fatigue.Figure 15: Very poor structural performance of pavement in residential road. 13
  15. 15. Figure 16: Full depth damage Figure 17: Traffic is delineated due to drainageFigure 18: Typical cross section of Flexible Pavement 14
  16. 16. Figure 19: Flexible pavement and rigid pavement have some distinct differences.Modern Software/Equipment used to Determine Pavement DrainageWith improved technology and modern spreadsheet programs such as MS Excel, pavementdesign engineers have improved their ability to estimate the drainage characteristics ofsubsurface drainage materials. An example of this is the program called PDE (PavementDrainage Estimator) which is an Excel based program. After inputting variables such aspavement dimensions, amount of drainage desired, aggregate properties and rainfall intensity,PDE can compute the required hydraulic conductivity of a pavement base layer. It can alsocalculate 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 importantfor quality assurance/quality control. Developed to determine the hydraulic conductivity ofpavement bases in just seconds, the air permeameter test (APT) device has been a majorcontribution to determining permeability. In an hour, one operator can perform about 50 tests.Several tests of a base layer can ensure uniformity. 15
  17. 17. The new APT device is the only rapid permeability testing device in the world. This deviceweighs 40 pounds and can be carried by one person.Figure 20: Permeameter test (APT) deviceDrainage 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 variouspavements under a wide range of moisture conditions. Drainage is categorized with a range ofExcellent to Very Poor.Figure 21: (Not all materials drain the same) 16
  18. 18. Figure 22: Excellent Drainage ConditionsFigure 23: Excellent Drainage Conditions 17
  19. 19. Figure 24: Good Drainage ConditionsFigure 25: Poor and Very Poor Drainage Conditions 18
  20. 20. Figure 26: Typical Edge Drain Location for crowned concrete pavement with tied concreteshouldersAs shown in the figure above, edgedrains are provided on both sides of the pavement sectionsince the pavement is crowned. The time to drain is less as the length of the pavement path issignificantly 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 TRANSFERThe Load Transfer Coefficient (J)Used in rigid pavement design, the load transfer coefficient (J) accounts for the ability of aconcrete pavement to transfer a load across joints and cracks. The implementation of loadtransfer devices (such as dowel bars) along with tied concrete shoulders escalates the amount ofload transfer and decreases the load transfer coefficient. Aggregate interlock also plays asignificant role in determining the value of load transfer. The figure below shows therecommended for values Load Transfer Coefficient: 19
  21. 21. Figure 27: Recommended load transfer, “J’ values. 20
  22. 22. The most common load transfer device is the dowel bar. The use of dowel bars has been a veryeffective method of reducing the amount of joint faulting when compared to nondoweled sectionsof similar designs. Egregiously, the diameter/size of the dowel has a great effect on the loadtransfer performance. The larger the dowel diameter leads to better load transfer and faultcontrol, 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 protectthe dowels from hostile effects of moisture.It is important to note that dowel bar design can be optimized with the use of engineeringsoftware such as DowelCAD. A pavement engineer can determine joint responses to varyingdowel sizes or investigate the impact of various alternate dowel bar configurations. 21
  23. 23. Figure 29: DowelCADThe magnitude of deflections in the slabs under loading and the distribution of stresses in the slabare all influenced by the load transfer devices. Proper implantation of load transfer devicesenables concrete pavement to behave more beneficially. For example, dowels and other loadtransfer 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 cornerbreaks and cracking.The vehicle loads can greatly vary based on the type of vehicle. The damaging effect of largervehicles such as Loaded 40’ and Loaded 60’ buses have a tremendous impact on the pavementwhen compared to a typical car as denoted in the figure below. 22
  24. 24. Figure 30: Impact of various vehicles on ESALMeasuring and Interpreting Load Transfer (J)In real world applications, project sites are evaluated and determined if a load transfer restorationis appropriate. The degree of deflection load transfer varies from crack to crack and joint to jointalong a project at any given time. Daily temperature, moisture fluctuations, and seasonaltemperature are all variances for the pavement during its lifetime. A load transfer evaluationshould be performed to identify which cracks and joints need load transfer restoration. TheFalling Weight Deflectometer (FWD) is a heavy load testing device which can measure thedeflection load transfer measurements.Along with the FWD, other nondestructive testing (NDT) devices are used. The outer wheel pathtruck loads can be simulated on each side of a joint or crack and deflections can be measured onboth sides. Sensors are used to measure the loads and deflections and they are recorded by acomputer in a tow vehicle. The FWD is a rapid and efficient operation and in one day, more thana few miles of pavement can be analyzed (a relative relationship of joint spacing). Like mostwork in construction which requires safety and protection of complex equipment, traffic controlmust be implanted to protect the testing machine in the pavement lane being diagnosed. 23
  25. 25. Figure 31: The falling weight deflectometer (FWD) is a non-destructive testing (NDT) andnon-intrusive device. The FWD has been widely used in pavement engineering to evaluatepavement structural condition.Material properties and Jointed PavementsMaterial properties thatcontribute to pavementdistress include modulus andshear strength, whichare greatly influenced by gradation, moisture content, and density. When granular layers arefailing or have poor performance permanent deformation such as rutting can occur. Otheroccurrences include frost heave, corrugations, longitudinal cracking, and depressions.Failures in rigid pavements from granular layers include cracking, corner breaks, faulting, andpumping. All of these distresses reduce the pavement life and diminish the structural integrity ofthe pavement. As moisture enters the pavement through cracks and joints there is a loss of loadcarrying capacity. Subgrade and base strength is reduced along with stiffness and prematurepavement failure can occur. 24
  26. 26. Figure 32: Jointed Pavement Figures 33& 34: Longitudinal, Transverse Construction Joints; Tie Bars, Dowel Bars 25
  27. 27. Contraction JointsContraction joint is to relieve tensile stresses resulting from temperature drops and moisturevariations in concrete. An example is shown in the Figure above. This type of joint is used onlyin plain jointed pavement at the transverse joint.Figure 35: Dowel Bar Assembly SystemHeavy 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 fulllength basket is preferred by most). The wire basket is manufactured in the shop and one end ofthe 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 forthe contraction joint to function properly. 26
  28. 28. Figure 36: Photo before paving. Tie bars (laying on base in lower part of picture) will beplaced 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 ashoulder or two lanes together, the paving machine can be used to mechanically insert thedevices into the newly poured concrete. In this case, the tie bars are not positioned and securedlike they are in CRCP because there is not an available mat of steel reinforcement. Thus, eachtie bar will have to be held in position and supported with the installation of pins and stakeswhich must be driven into the subbase structure. It is critical that the supports are strong enoughto hold the tie bars in place during concrete pouring operations. 27
  29. 29. SECTION 3: LOSS OF SUPPORTFigure 38: Table 2.7 from AASHTO GUIDE, PP. II-27.Figure 39: Types of materials for construction. The photos below show unbound granular materialsfor road pavements, lime treatment on subgrade, and mixing lime into subgrade material 28
  30. 30. Loss of Support and Joint SealantsOverview Joints Sealed with Self-Leveling SiliconeA 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. Degradationwill often occur as well, leading to loss of support in the structure, joint faulting and pavementsettlement.Since it isn’t always practical to build a pavement that is continuously watertight, many highwayagencies utilize joint sealants to prohibit surface water infiltration through joints. In additionthey 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 itcomes to loss of support. In porous asphalt, the mix has open graded aggregates with fewerfines. Voids are interconnected which results in a porous surface.Because fines in the mix only serve to make compaction easier while the bigger pieces ofcrushed aggregate give enough structural stability (even for large trucks and other heavyapplications), there is no loss of structural support capacity in porous asphalt compared toimpervious asphalt and concrete. 29
  31. 31. Figure 40: Porous asphalt vs. standard asphalt.Shoulder and Jointing Considerations – Transverse JointsCracking is controlled by installing transverse joints. A mid-panel crack is less likely to developwith closer joint spacing. The critical element at joints is load transfer, where it is muchrecommended to use dowels. In undoweled pavements, aggregate interlock provides support forthe load transfer. As slabs contract and the joints get larger, aggregate interlock is lost. Astraffic passes over concrete, the concrete experiences movement and the interlock is graduallydestroyed. 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 dowelsthat are 18 inches long to handle the expansion and contraction. They can be placed in thetransverse joints and the mainline joints. Most government agencies have Standard ConstructionDrawings in which the spacing requirements and transverse joint design are shown.Expansion and Pressure Relief JointsDue to seasonal temperature changes, concrete slabs contract. This causes cracks and joints toopen allowing incompressible materials into the pavement structure. Due to this, the pavementcan grow in length and even create pressure. This induced pressure can be beneficial in smallamounts, since a lack of pressure would make joints and cracks open which would reduce loadtransfer while not creating too much pavement distress. If distresses are discovered, some typeof rehabilitation is recommended in terms of maintenance. A blowup would require immediateattention. 30
  32. 32. Figure 41: Using values of J and Cd to obtain Structural Number of Rigid Pavement. 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. 31
  33. 33. Figure 42: Rigid Pavements – Steel compensates for concretes poor ability to cope withtension stressConclusionThe primary criteria that design engineers must consider and properly investigate to develop aproper pavement structure are the traffic, environmental conditions and soil conditions. Thesurface course of the pavement, whether it be flexible asphalt pavement or rigid pavementheavily relies on the foundation that it is placed upon. If the subsurface material conditions areinadequate and drainage is not properly accounted for the pavement will not meet its designcriteria and will most likely fail prior to its expected service life. The importance of drainage hasbeen greatly emphasized in this report along with the benefits of load transfer devices such asdowel bars which serve to distribute loads across discontinuities such as joints or cracks. Othersteel reinforcing mechanisms such as tie bars and longitudinal bars have also been reviewed as aprimary solution to eradicate cracking, faulting and other distresses that cause permanent damageto pavements. The loss of support factor has also been discussed in this report as proper and costeffective material selection plays a significant role in pavement design. With stringentgovernment budgets both in the U.S. and on the global scale, highway funding in virtually allnations has been diminishing over time and it is vitally important that pavement engineers designpavements that are durable, long-lasting and efficient. The importance of the pavementstructural characteristics are identified and stressed in this report along with the significant rolesplayed by proper drainage, load transfer and loss of support factors in an effort to mitigatepavement distress. 32