Pavement Selection

DelDOT Road Design Manual

Chapter Nine

PAVEMENT SELECTION

This chapter discusses the general criteria, procedures and responsibilities for structural 9.1 DESIGN RESPONSIBILITY design of highway pavements. In addition there is information given on the various types of The design of pavement structures has some pavements, pavement rehabilitation techniques, jointly shared responsibilities between the Mate- and other factors that enter into pavement design rials and Research (M&R) Section and the re- and final pavement selection. sponsible Project Development section. Howev- er, the primary responsibility for structural de- For the purposes of uniform and consistent sign and final recommended pavement sections design practices, the Department has adopted the is that of the M&R Section. criteria and procedures as set forth in the AASHTO Guide for Design of Pavement Struc- 9.1.1 SOIL SURVEY/PAVEMENT tures 1993. This chapter briefly reviews the con- EVALUATION REQUEST cepts and criteria used. Reference should be made to the AASHTO Guide for more detailed There are several elements in the design information on design procedures, if needed. process that need to be accomplished before a pavement section recommendation can be re- The design procedures include the determina- quested from the M&R Section. The project tion of total thickness of the pavement structure handoff package will describe the project scope. as well as the thickness of the individual com- If the intent is to construct new pavement or re- ponents using input parameters specified in the place the existing pavement then a soil survey, DARWIN 3.01 computer program. Provision is pavement design and pavement type recommen- made for the design of equivalent alternate dation will have to be requested. pavement sections, with the selection primarily a function of availability of materials, comparative The following should be available when re- costs, constructibility, and availability to traffic. questing borings for a soil survey from Del- DOT’s Geotechnical Engineer and/or corings of The discussion and explanatory material pre- the existing pavement from DelDOT’s Pavement sented here are intended to give the designer a Design Engineer: general understanding of pavement design concepts, alternative paving treatments, and a basic • preliminary surveys, understanding of the information contained in a soil survey and pavement design report.

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• survey plans with the location of the investi- For projects designed by a consultant, the de- gation, including the road name and state sign consultants must: maintenance road number, and any prede- termined locations marked, • Develop the subsurface investigation plan to have drilling and inspection services pro- • existing right-of-way verified, vided under their direction. • Right-of-entry to trespass, if needed, • Notify the DelDOT Project Manager of the • Purpose of the investigation in order to de- scope of the subsurface investigation plan termine what pertinent information is re- and provide a boring plan sheet(s) to the quired from the cores or the borings. Department. The DelDOT Project Manager will forward the boring plan to the Materials • If a boring is required, the depth of the bor- & Research (M&R) Geotechnical Engineer ing. If the designer is unsure of depth to sample, contact the Geotechnical Engineer • Specify to M&R what soil testing is re- for guidance. quested. • Inform the Geotechnical Engineer at least 24 Coring and boring requests are typically hours in advance when drilling and soil processed within 30 days. If a coring and boring sampling is to begin and provide the name request is received simultaneously, they will be and phone number of the consultant who is processed concurrently. If the boring request is to receive the M&R soil test information. received after the coring request has been completed, it will be treated as a different request • Arrange to have the soil samples and copies and will be processed within 30 days of receipt. of the field logs brought to the M&R lab within 24 hours after obtaining samples. For a pavement design, the following information is required with the request: The Geotechnical Engineer shall:

• Design year traffic data (AADT) • Perform, or arrange for, all soil testing required for the project as requested by the de- • Design year truck percentages sign consultant. • Weight group pattern of trucks • Furnish completed boring logs and laborato- • Directional split ry test summaries to the design consultant with a copy to the DelDOT Project Manag- • Existing pavement structure (if applicable) er. • Subsurface investigation report (if applica- • Provide the DelDOT Project Manager regu- ble) lar weekly updates on the progress of the • Description of any existing pavement dete- drilling/testing programs. rioration • Inform the DelDOT Project Manager when Copies of any existing corings, borings, or sub- problems arise and when the testing pro- surface condition reports should be provided to grams are completed. the Pavement Design Engineer when sending the Many projects are initiated with the intent of pavement design request. The Pavement Design extending the service life of the existing pave- Engineer may elect to have additional corings, ment section. Rather than rebuilding the entire borings, or other investigations performed to pavement, improvements are made in the riding ensure the existing conditions are known and quality, skid resistance and limited structural considered when performing the pavement de- improvements through various rehabilitation sign. All information, including the design, will methods. For projects of this type a pavement be forwarded to the designer.

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evaluation is requested. A pavement evaluation sary, the M&R Section may perform an econom- includes an extensive pavement survey noting ic and life cycle cost analysis or other studies in pavement conditions, drainage and major dis- making the final determination and recommen- tress problems. The initial evaluation will de- dation for pavement type and section. For many termine which rehabilitation method(s) should projects, only one pavement design recommen- be considered. Based upon the alternatives being dation is made. considered, it may be necessary to conduct detailed measuring and testing, including coring The Soil and Pavement Design Report should and sampling, and accurately determining struc- be evaluated thoroughly and considered as the tural clearances to any overhead structures. The design progresses. In the design process signifi- final report received by the designer will contain cant changes in the proposed profile or align- most if not all this collected field data and will ment can dramatically affect the pavement de- include the recommended rehabilitation method. sign and need to be discussed with M&R, per- Much of the field data collected can be a very haps even before such decisions are finalized. In valuable tool in designing the project. addition the proposed construction sequencing, methods of construction and maintenance of

traffic plan can dramatically affect whether the 9.1.2 SOIL AND PAVEMENT DESIGN recommended pavement materials can be placed REPORT in a timely manner and meet the in-place per- The M&R Section performs the soil sampling formance quality necessary for the intended ser- and pavement coring. This field data is tested in vice life. the laboratory and the results documented in the form of a Soil Survey. This report includes the 9.1.3 PAVEMENT SELECTION soil profile data summary showing the sample location, depth, soil profile description, soil As mentioned in Section 9.1.2, there may be classification and any remarks. The soil samples several choices of structurally equivalent pave- are tested and a summary of this analysis is in- ments. The choice of pavement, particularly on cluded in the report. reconstruction and new construction is a major decision and needs to be approved prior to pro- Based upon the subgrade soil characteristics, ceeding with the design. the report may provide recommendations for muck excavation, limits of special fill, need for The factors considered in making the final underdrains, grade adjustments, embankment decision on pavement type are quite varied from construction, the use of geotextiles or other spe- empirical to subjective and may include several cial construction considerations. of all of the following:

An important part of the report is the pave- • Project scope⎯as initiated, ment design portion. Using the AASHTO Guide, the soil survey data, and past experience, the • Cost to construct or rehabilitate the pave- recommended pavement type and thickness by ment, components is included in the report. Pavement • Available project funding, sections for shoulders and turn lanes are also included. • Construction sequencing as it relates to controlling through and local traffic, The report may provide an alternate design • Construction sequencing as it relates to serv- including at least one rigid pavement and one or ing commercial areas, more flexible alternatives. Project Development and M&R will meet and mutually agree on which pavement to use on the project. If neces-

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• Construction sequencing as it relates to • Grinding⎯Patterns cut into a concrete pave- maintaining quality control of pavement ment with closely spaced diamond blades to construction, restore pavement smoothness and skid resistance. • Availability of work areas for the paving equipment, • Grooving⎯Patterns cut in asphalt or concrete pavements to promote surface drainage to • Projected future traffic control and other reduce wet weather hydroplaning. costs to perform maintenance, restoration or rehabilitation • Micro-Surfacing⎯A polymer modified cold- mix paving system consisting of a mixture • Minimum desirable service life, of dense-graded aggregate, asphalt emul- • Performance of similar pavements under sion, water, and mineral fillers. similar soil conditions and traffic loadings, • Modulus of Subgrade Reaction (k)⎯A value • Geotechnical design problems created by the used in rigid pavement design that is the ra- depth of pavement structure, which could tio of the load in pounds per square inch [in increase drainage costs, kilograms per square mm] on a loaded area of the subgrade or subbase divided by the • Effect on underground utilities, and deflection in inches [mm] of the subgrade • Effect on existing vertical clearances. soil or subbase, psi/in [kPa]. Typically, k is adjusted for potential loss of support due to 9.2 PAVEMENT TERMINOLOGY subbase erosion. • Open Graded Mix⎯A special mix, con- The pavement section is treated as a structur- taining aggregate that resists polishing, al element consisting of several different mate- placed on the surface course to drain surface rials of varying depths and supporting strengths. water, improve skid resistance, and reduce Knowledge of the following definitions and the hydroplaning. terminology as shown on Figure 9-1 is needed to understand the pavement design and rehabilita- • Rigid Pavement⎯A pavement structure that tion concepts in this chapter. distributes loads to the subgrade, having as one course a Portland cement concrete slab • Base Course⎯the layer or layers of speci- of relatively high bending resistance. fied or select material of designed thickness • Pavement Milling⎯The use of carbide cut- placed on a subgrade to support a surface ting teeth mounted on a rotary drum to chip course off as much 3 to 4 inches [75 to 100 mm] of • Bituminous Concrete⎯A designed combi- asphalt concrete surface. nation of dense graded mineral aggregate fil- • Pavement Structure⎯A combination of ler and bituminous cement mixed in a cen- subbase, base course and surface course tral plant, laid and compacted while hot. placed on a subgrade to support the traffic • Bonded Overlay⎯An overlay of concrete load and distribute it to the roadbed placed over a Portland cement concrete • Recycling⎯Salvaging and processing por- pavement. tions of existing pavement for use in con- • Flexible Pavement⎯A pavement structure struction of new pavement structures. of bituminous concrete that distributes loads • Resilient Modulus (MR)⎯A measure of the to the subgrade and depends on a firm con- essential properties of untreated subgrade tinuous subgrade, aggregate interlock, par- soils through determination of dynamic elas- ticle friction, and cohesion for stability. tic modulus under conditions that represent a

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reasonable simulation of the physical condi- integrating effects of climate. The top layer tions and stress states of subgrade materials of flexible pavements is sometimes called beneath flexible pavements subjected to “wearing course.” moving loads. • Ultra-Thin-Whitetopping⎯An overlay of • Roadbed Material⎯The material below the concrete less than 4 inches [100 mm] thick subgrade in cuts and embankments and in placed over an asphalt base, usually 2 to 3.5 embankment foundations, extending to such inches [50 to 90 mm]. depth as affects the support of the pavement • Whitetopping⎯A concrete overlay of an structure. asphalt pavement of 4 inches [100 mm] or • Roadbed⎯The graded portion of a highway more. between top and side slopes prepared as a foundation for the pavement structure and 9.3 PAVEMENT DESIGN shoulders. FACTORS • Select Material⎯A suitable native material obtained from a specified source such Pavement design methodology for both new as a particular roadway cut or borrow area pavements or rehabilitation of existing pave- having specified characteristics to be used ments consider several, if not all, of the follow- for a specific purpose. ing factors:

• Subbase⎯The layer or layers of speci- • Pavement design life, fied or select material of designed thickness placed on a subgrade to support a base • Pavement performance, course (or in the case of rigid pavements, the • Traffic volume and vehicle class, Portland cement concrete slab). • Roadbed soil, • Subgrade⎯The top surface of a roadbed soil upon which the pavement structure and • Materials of construction, shoulders are constructed. • Temperature changes, • Superpave⎯An asphalt pavement with a la- • Drainage, boratory design mix that provides superior performance. • Reliability, • Surface Course⎯One or more layers of a • Life-cycle costs, and pavement structure designed to accommo- • Shoulder design date the traffic load, the top layer of which resists skidding, traffic abrasion, and the dis-

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Figure 9-1 Pavement Terminology

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Of these factors, the most influential factors loss of serviceability are traffic, age, and envi- in determining a pavement’s required structural ronment. strength are the characteristics of the underlying roadbed material, the projected traffic volumes The structural performance of a pavement re- and the percentage and weight of vehicles in the lates to its physical condition; including occur- traffic mix using the facility over the expected rence of cracking, faulting, raveling, or other design life of the pavement. The following sec- conditions which would adversely affect the tions briefly discuss the factors considered. load-carrying capability of the pavement or would require maintenance. 9.3.1 PAVEMENT DESIGN LIFE A pavement’s safety performance primarily relates to its ability to provide adequate skid re- Each pavement design has a selected design sistance during its service-life but also can be life. Roadway cross section elements and other affected by its ability to maintain a smooth and components of the project such as the pavement rut free surface. Age, traffic, physical properties structure are expected to remain structurally of materials used to construct the pavement and sound for a designated period of time defined as environmental conditions influence a pave- design life. Although the roadway cross section ment’s safety performance. may become operationally obsolete or the pavement distressed and in need of restoration or rehabilitation, they have not reached the end of 9.3.3 TRAFFIC their design life but rather have reach the end of a condition defined as service life. Not until they Traffic volumes using a facility, in particular need complete replacement are they considered the number and weight class of trucks, is a major to have reached the end of their design life. factor in determining how strong a pavement structure must be. In the design procedure, traffic data is reduced into axle loads, axle configu- 9.3.2 PAVEMENT PERFORMANCE ration, and number of applications of these loads. The result is a design number representing The goal of a pavement design is to produce the damage done to the pavement caused by the a pavement that when placed will perform func- effect a single axle carrying a load on the pave- tionally and structurally while maintaining its ment over its design-life. For the design calcula- safety characteristics for at least the selected tions, traffic data is converted into 18-kip [80 service life. kN] equivalent single axle loads or ESAL’s. Functional performance of a pavement identi- Since it is one of the more important design con- fies how well a pavement will serve the user. siderations, accurate traffic data will ensure that The characteristics identified are riding comfort a pavement’s design life will be attained and the and ride quality. This concept is called servicea- pavement sections selected will not be over or bilty-perfomance and provides a means to meas- under designed. ure functional performance. In the pavement design procedure, this factor is expressed in 9.3.4 ROADBED SOIL terms of the present serviceability index (PSI). PSI is a measurement of roughness and distress The pavement structure rests on a graded and of a pavement during the service life of a pave- compacted roadbed either of suitable natural ment. Therefore, a reliable method of measuring material or on specified imported material. The roughness and maintaining and updating histori- roadbed soil has measurable material characte- cal performance data is an integral part of pave- ristics that are used in the pavement design. This ment design. The major factors influencing the measurement is defined as a soil’s resilient modulus (MR) and is a measure of the elastic

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property of soil. The resilient modulus is used area. directly for designing flexible pavements but Underdrains (a system of perforated pipes to must be converted to a modulus of subgrade collect and transmit the water to an outfall site) reaction (k-value) for the design of rigid pave- are recommended for use on all roadway ments or composite pavements. The resilient projects to adequately address drainage and re- modulus is also a soil property used in analyzing moving water from the roadbed. If site condi- multilayered material systems for predicting tions indicate that underdrains may not be re- roughness, cracking, faulting, rutting, and other quired, contact the M&R Section to initiate fur- potential distresses. ther investigation. The value of a roadbed’s resilient modulus is The soil and pavement condition survey will dependent on how well the roadbed soil is normally identify roadbed drainage problem placed in conformance with the specified com- areas or soils highly susceptible to expansion or paction parameters. For most projects, the ma- loss of strength with increase in water content. terial is to be placed in accordance with the When either of these conditions exist, the M&R Standard Specifications with no special treat- Section may recommend additional work and/or ment. However, the Soil and Pavement Design materials to address the existing conditions. Report may indicate that there is anticipated dif- Another type of material encountered in con- ficulty with the existing roadbed soil meeting structing roadbeds is classified as cohesionless the design M value. For soils that are exces- R (sandy) soil and is much more difficult for the sively expansive the report may recommend contractor to place and compact; it is readily these soils be covered by select material suffi- displaced under the load of the equipment. To ciently deep enough to reduce or eliminate the stabilize this type of soil it may be necessary to expansive affect of the natural material. Other blend granular material or add a suitable admix- solutions may include the adding of an admixture. Wet clay soils may also be encountered. ture to reduce the water content or the use of a Because of high moisture content this type of geotextile. soil is unstable and cannot be compacted. Long One of the more difficult soils encountered periods of dry weather and exposure to the air on projects are those having a large organic con- are required to reduce the water content. To re- tent. These materials are extremely compressi- duce the time necessary to reuse these materials, ble, unstable and frequently non-uniform in the recommendation may be to add a suitable properties and depth. These soils are the most admixture that hastens drying or cover the area complicated and expensive to deal with in order with a more suitable select material. Removing to provide an adequate roadbed. Small, shallow the material and replacing it with suitable ma- or localized deposits are most often excavated terial allowing construction to continue is an and replaced with suitable material. Deeper and option. The material may be used in areas that more expansive areas involve more detailed geo- don’t require compaction or moved to an availa- technical design, more complicated construction ble site for air-drying and reuse at a later time. techniques and costs. Treatments other than complete removal are more time dependent al- 9.3.5 PAVING MATERIALS lowing for the slow consolidation and removal of excess moisture. Methods available include Depending upon materials that comprise a surcharge embankments for preconsolidation of pavement, the pavement structure is identified as the underlying material usually involving sand either a flexible or rigid pavement. Combining drains which allow the water to rise to the sur- these two types of paving materials in a pave- face and be removed. The M&R Section is re- ment structure as a subbase or surface course sponsible for identifying and designing the most results in a composite pavement. economical method treating this type of problem

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9.3.5.1 FLEXIBLE PAVEMENTS 9.3.5.2 RIGID PAVEMENTS

Flexible pavements consist of a prepared Rigid pavements consist of a prepared roadbed with a subbase of graded aggregate or roadbed, a layer of subbase material and a Port- bituminous concrete and a bituminous concrete land cement concrete pavement slab. The subbase with a surface course. Properly preparing a base may be either stabilized or unstablized. Al- uniform roadbed compacted to the prescribed though concrete pavements can span failed sub- density is especially important for providing the grade and subbase areas easier than flexible design support value necessary for flexible pavements, they are still susceptible to damage pavement to perform as designed. and failure from excessive moisture in the support structure. The subbase course usually consists of a compacted layer of granular material or an un- The subbase of a rigid pavement structure treated graded aggregate. If additional support consists of one or more compacted layers of strength is needed or the roadbed soil is ques- granular or stabilized material placed between tionable either of these materials may be treated the roadbed and the rigid slab. The subbase with an admixture. In some instances, the sub- functions to: base may be recommended to be a freely drain- • Provide uniform, stable, and permanent sup- ing-highly permeable material to provide a port, means for water to migrate from under the pavement structure to the side slopes or to an • Increase the modulus of subgrade reaction, underdrain system. The subbase also prevents • Minimize the damaging effects of frost ac- intrusion of fine-grained roadbed soils into the tion, base course, minimizes the damaging affects of • Prevent pumping of fine-grained soils at frost action, and provides a working platform for joints, cracks, and edges of the rigid slabs, construction equipment. and • Provide a working platform for construction The base course is a specified depth of bitu- equipment. minous concrete that is primarily designed to The pavement slab is composed of Portland provide the structural strength needed to support cement concrete, longitudinal tie steel, load and distribute the projected traffic loads. transfer devices between slabs, and joint sealing materials. The surface course is a bituminous concrete mixture placed as the upper course and is usual- Joint sealing is a critical element in the long- ly constructed on a base course. The surface term performance of a rigid pavement. Proper course provides some structural strength. How- joint sealing prevents infiltration of water under ever, the major functions of the surface course the slab that reduces the support strength of the are to provide a smooth riding surface that res- subgrade and reduces pumping action between ists distress, minimizes the amount of water that slabs caused by the transfer of moving traffic may penetrate the lower more porous layers, loads between joints. The vertical movement of provides and maintains its skid resistance for the slabs eventually erodes the subgrade material selected service life. To meet these require- through any unsealed joints allowing a slab to ments, the surface course mix must have the op- break or settle. timum gradation of aggregate and percent of bituminous binder to prevent raveling, provide There are two types of joint sealants in use, durability, resist fracture, and remain stable un- liquid sealants and preformed elastomeric seals. der traffic use and adverse climate changes. Liquid sealants include a wide variety of materials including hot-poured rubber, elastomeric compounds, and polymers. The materials are

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placed in the joint in a liquid form and allowed that rapidly deteriorate the pavement. to set. Preformed seals are extruded neoprene seals having internal webs that exert an outward 9.3.7 DRAINAGE force against the joint face. The size is determined by the amount of joint movement antic- Keeping the pavement subgrade and soilbed ipated. dry is a major design consideration. Excessive moisture combined with increasing traffic and Transverse joints are needed in a rigid slab to load applications will inevitably lead to prema- form cracks at desired locations that can be con- ture pavement distress. Water can enter the trolled and sealed. Joints can be keyed, butted or pavement structure from many directions includ- tied. Transverse joints are sawed or formed to a ing a permeable pavement surface, unsealed or depth of one quarter to one third the slab thick- poorly sealed joints, surface cracks, high water ness. Timing the saw cutting operation to the tables, and even local springs. If water is trapped curing of the concrete is critical. within the pavement structure, pavement performance will be affected through loss of sup- Load transfer devices are used between slabs port due to erosion of any granular material and and usually consist of smooth round steel do- loss of material strength. wels. The dowels should distribute the load stresses without over stressing the concrete sur- Addressing the problem areas that allow wa- rounding it, offer little restraint to longitudinal ter to enter the pavement structure is difficult to movement of the joint, be mechanically stable prevent and expensive to correct. In fact it is a under the loads and load frequencies expected shared responsibility. The pavement designer during the design period, and should be resistant should recommend pavement designs that use to corrosion from moisture and road salts. dense non-permeable surface courses to reduce surface infiltration, specify underlying material Materials used in the design of shoulders can courses that freely drain, and a pavement struc- be either flexible or rigid. Differences in materi- ture that is strong enough to resist the effects of al types and the subbase combined with unex- the traffic loads and water. The roadway design- pected wheel loads along the pavement edge can er must ensure proper pavement crown, draina- cause joint problems. With proper care and at- ble grade lines, proper ditching and other ade- tention, this potential problem can be mini- quate drainage systems to remove water quickly. mized. Solutions include widening the full depth pavement slab, using tied concrete shoulders, properly sealing the joint, and ensuring compa- 9.3.8 RELIABILITY tibility between subbase materials. Reliability is a method to determine the prob- ability that a particular pavement design will 9.3.6 TEMPERATURE CHANGES perform as desired during its design life. In selecting the appropriate level of reliability, the Temperature changes affect (1) the creep pavement designer relates the projected level of properties of asphalt concrete, (2) thermal- usage to the risks involved if a thinner pavement induce stresses in asphalt concrete, (3) contrac- section is recommended. tion and expansion in Portland cement concrete, and (4) freezing and thawing of the roadbed soil. For facilities with higher importance to the Temperature differences between the top and transportation system a pavement design consid- bottom of concrete slabs create uneven stresses ers the traffic disruption caused by closing or on the slab and can be of concern. Temperature restricting traffic flow due to higher levels of and a poorly drained pavement structure or sub- distress, maintenance, and rehabilitation asso- grade, although normally not a concern in Dela- ciated with an inadequate or a marginal initial ware, can combine to create freeze-thaw cycles

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pavement structure. For lesser facilities, it may In performing the analyses, the AASHTO pro- be perfectly acceptable and even economical to cedure requires the pavement designer to pro- use a reduced thickness. vide input in several categories:

9.3.9 LIFE-CYCLE COSTS • Design variables, • Performance criteria, Life-cycle costs are costs and benefits that accrue during a pavement’s complete life cycle. • Material properties for structural design, These costs include the initial construction costs, • Structural characteristics, and maintenance costs, rehabilitation costs, resurfacing costs, maintenance of traffic costs, salvage • Reinforcement variables. or residual value and user costs. 9.4.1 DESIGN VARIABLES As a part of the pavement selection decision, particularly when more than one pavement de- A set of design criteria is established for each sign or rehabilitation procedure is proposed, an project including the pavement’s expected ser- economic comparison may be needed. Two me- vice life (performance period) and projected de- thods are detailed in the ASSHTO Guide to design life (replacement). The traffic projection for termine life cycle cost comparisons, net present the cumulative expected 18-kip [80 kN] equiva- worth and equivalent uniform annual cost. Whi- lent single axle loads (ESAL) during the analy- chever method is used, it is essential that the sis is determined. The level of reliability is se- analysis periods be of equal length. lected and any detrimental environmental factors identified. There is a heirachy of application of these variables with the most important road- 9.3.10 SHOULDER DESIGN ways assigned the most stringent and detrimental values to a pavement’s performance. The inclusion of a shoulder adjacent to the main pavement structure improves pavement performance. The AASHTO guide does not pro- 9.4.2 PERFORMANCE CRITERIA vide a design method for determining the shoulder section. The M&R Section recommends a Performance of a pavement is measured by shoulder section that is compatible with the pro- its serviceability to the expected users. The con- posed mainline pavement section, has good concept is to design a pavement, which at the end of structibility and has performed well in the past. the proposed performance period will still have a Shoulders are usually designed to carry 10 per- predefined minimum level of serviceability cent of the projected Average Daily Traffic (PSI). The terminal level of serviceability is se- (ADT). lected based on the lowest index the user will tolerate, or as defined in a pavement management strategy before rehabilitation, resurfacing 9.4 DESIGN FOR NEW or reconstruction becomes necessary. DelDOT CONSTRUCTION OR typically uses a terminal PSI of 2.5 or 3.0, which RECONSTRUCTION varies based on functional classification and use.

Constructing a pavement section is one of the 9.4.3 MATERIAL PROPERTIES FOR most costly items on new construction and re- STRUCTURAL DESIGN construction projects. In making a selection on the type of pavement to construct it is usually For the design of flexible pavements, roadbed necessary to analyze alternate pavement types materials are characterized based on their effec- and combinations of various support materials. tive elasticity or resilient modulus, MR. Their

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resilient modulus is determined for periods of percent of time during the year the pavement stress and moisture conditions simulated for the structure would be subjected to moisture levels primary moisture season. approaching saturation.

For rigid pavement designs, an effective For rigid pavements, another structural factor modulus of subgrade reaction (k-value) is devel- used is its load transfer ability. Rigid pavements oped. The k-value is directly proportional to have the ability to distribute loads across discon- roadbed soil resilient modulus. However, the tinuities, such as joints or cracks and small voids effective design k-value is also dependent upon under the slab. Load transfer devices, aggregate the effects of the characteristics of the subbase. interlock, and the presence of tied longitudinal joints and concrete shoulders influence this val- Another important input is the compressive ue. strength of the materials composing the pavement structure. In the design of rigid pavements, Another structural factor applied to rigid the modulus of rupture (flexural strength) is de- pavement designs is a loss of support value. This termined using the mean value as tested at 28 factor accounts for the potential loss of support days. This value is specified in the Standard arising from subbase erosion and/or differential Specifications and verified as being consistently vertical movement. met or exceeded through laboratory records. 9.4.5 REINFORCEMENT VARIABLES In flexible pavement design, the layer coefficient method is used. Each structural layer is Rigid pavements may be reinforced with assigned a layer coefficient value which is the joints, unreinforced with joints or continuously relationship between the Structural Number reinforced with no joints. The steel reinforce- (SN) and thickness of the layer. (See Section ment is used to control transverse cracking. 9.5.5.) It is an empirical means to represent the relative ability of the material to function as a Joint spacing is particularly important in the structural component of the pavement. performance of plain jointed pavement. The spacing is usually much closer than in reinforced 9.4.4 PAVEMENT STRUCTURAL pavements to control cracks due to temperature CHARACTERISTICS and moisture enduced stresses. The slab spacing selected must also minimize joint movement Drainage is an important element in the ulti- thus protecting the aggregate interlock value of mate performance of all pavements. The design- the joint. DelDOT has a standard spacing of 20 er assigns a factor that represents the expected feet [6 m] for 12 inch [305 mm] Portland cement quality of a project’s drainage system to minim- concrete pavement. (See the Standard Construc- ize moisture intrusion into the pavement struction Details.) ture.

For flexible pavements, the layer coefficients 9.5 STRUCTURAL DESIGN FOR are modified to reflect the expected quality of FLEXIBLE PAVEMENTS drainage and percent of time during the year the pavement structure would normally be exposed A flexible pavement structure may consist of to moisture levels approaching saturation. three layers, designated as a subbase course, a base course, and a surface course. A flexible For rigid pavement design, the level of drai- pavement system distributes the load by particle- nage is addressed through the use of a drainage to-particle-contact by interlocking, friction, and coefficient. This coefficient represents the quali- cohesion through its thickness. The surface ty and effectiveness of drainage systems and the

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course usually consists of a binder course and a 9.5.2 BASE COURSE surface course. The base course is the portion of the flexible The concept of all pavement design proce- pavement structure immediately beneath the sur- dures is to determine the required structural face course. The base course is the primary load- thickness based on the projected traffic loading. spreading layer. It has to be strong enough to Using this as a basis, the most economical and withstand the shear stresses produced by the constructable combination of materials is deter- wheel loads, and be incompressible and rigid mined. For flexible pavements, this consists of enough to distribute the load over the underlying determining the required thickness of the pave- materials. ment’s subbase, base, and surface courses. Base course material typically consists of a Graded Aggregate Base Course or Bituminous 9.5.1 SUBBASE COURSE Concrete Base Course. A graded aggregate base The subbase course is the portion of the flex- course may be crushed stone, crushed slag, ible pavement structure between the subgrade crushed or uncrushed gravel and sand, or other and the base course. The subbase insulates the combinations of these materials. These materials base and surface courses from frost penetration, may also be used treated with suitable stabilizing provides a drainage medium, and a layer resis- admixtures such as Portland cement or asphalt. tant to erosion and erosion of fine material into Base course specifications are generally more the subgrade. The subbase usually consists of a stringent than for subbase materials in require- compacted layer of granular material, which ments for strength, plasticity, and gradation. may be either treated or untreated. Subbase materials are either a Soil Cement Base Course, 9.5.3 SURFACE COURSE Graded Aggregate Base Coarse or Borrow Type A. The surface course of a flexible pavement consists of a wearing course and a binder course This course has a less stringent specification which are mixtures of mineral aggregates and requirement for strength, plasticity, and grada- bituminous materials, placed as the upper tion. If roadbed materials are of high quality, the courses and usually constructed on a base pavement design report may recommend that the course. In addition to providing a portion of the subbase layer be omitted. If the roadbed mate- pavement structural support, the surface course rials are relatively poor quality, the design pro- must also be designed to resist the abrasive cedure will indicate that a substantial thickness forces of traffic and weather. They are designed of pavement is required. In this case, alternate to be dense enough to minimize surface water designs are usually provided with and without penetrating the pavement. The proper aggregate the use of a subbase. The selection of an alter- selection will provide a non-polishing-skid- nate may then be made on the basis of availabili- resistant surface, resist rutting and provide a ty and relative costs of materials suitable for the smooth and uniform riding surface. The success base and subbase. By using less expensive mate- of a surface course depends to a considerable rials in the lower layer of a flexible pavement degree on obtaining a laboratory mixture with structure, the use of a subbase course is often the the optimum gradation of aggregate and percent most economical solution to construction of of bituminous binder. Open graded mixtures that pavements over poor roadbeds. provide good surface drainage and skid- resistance are available for use on high-speed, high-traffic facilities.

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9.5.4 STRUCTURAL NUMBER (SN) designer must use past experience, consider cost effectiveness and construction and maintenance The structural number is an abstract number constraints in order to avoid an impractical de- expressing the structural strength of pavement sign. required for a given combination of the effective resilient modulus, MR, of the roadbed soils, the The design procedure allows for doing a life total equivalent 18-kip [80 kN] single-axle loads cycle cost analysis based upon planned rehabili- (ESALS), the design serviceability loss, and the tation. This allows the designer to analyze the standard deviation and reliability factors. The tradeoffs between thickness designs of the initial AASHTO Guide provides a nomograph for de- pavement structure and any subsequent overlays. termining this value. The required SN must be converted to an actual thickness of surfacing, The procedure allows for considering the ad- base and subbase by means of appropriate layer verse effects of changing environmental condi- coefficients representing the relative strength of tions. The objective is to perform an iterative the material to be used for each layer. process to determine when the combined serviceability loss due to traffic and environment A design equation is used to solve for the to- reaches the terminal level. tal required SN for the entire pavement structure. By solving the equation with the effective 9.5.5 LAYER COEFFICIENTS resilient modulus value representative of the roadbed soil, an SN for the entire pavement The layer coefficient expresses the empirical structure is obtained represented by the general relationship between the SN and thickness, and equation: is a measure of the relative ability of the material to function as a structural component of the total SN = a1 D1 + a2 D2 m2 +a3 D3 m3 pavement structure. Where: To design a flexible alternative, the structural a1, a2, a3 = layer coefficients representatives number over the roadbed soil is computed. Then, of surface, base, and subbase course, respective- the structural numbers for the subbase and the ly; base layers are determined. Using the differenc-

D1, D2, D3 = actual thickness, in inches [mm], es between these values, the maximum thickness of surface, base and subbase courses, respective- of any layer can be computed. ly (Open-graded surface courses are excluded from this calculation.) and, The SN for any combination of courses is determined with the design equation, the layer m2, m3 = drainage factors modifying base and coefficients and the proposed thickness of each subbase layer coefficients. course. Alternate designs can be prepared by The nomograph allows the pavement design- varying the thickness. The computed SN should er to determine the SN and provides a means equal or exceed the required SN determined through assigning a reliability factor (R) to in- from the nomographs in the AASHTO Guide. corporate some degree of certainty into the va- lidity of the design process. In addition, it allows The layer coefficients per 1 in [25 mm] of for assigning a standard deviation factor, which material have been established for various types accounts for the variance in the projected traffic and classes of flexible pavement, base course, capacities and their reliability. and subbase.

The SN equation does not have a single solution since many combinations of layer thickness will satisfy the equation. However, the pavement

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Figure 9-2 When temporary pavements are needed de- Layer Coefficients signers should closely coordinate with the M&R Section in the development of a practical pave- Material Type Layer Coefficient ment design based on knowledge of local conditions and engineering judgment. Type C Surface Mix 0.40 Figure 9-3 Type B Binder 0.40 Lift Thickness Course Type of Minimum Maximum Bituminous Con- 0.32 Material Lift Thickness Lift Thick- ness crete Base Course Type C 1-¼ inches 2 inches Soil Cement 0.20 Surface Mix [30 mm] [50 mm] (9.5 mm)

Graded Aggregate 0.14 Type B 2-¼ inches 3 inches Base Course Binder/Base [60 mm] [75 mm]

Select Borrow 0.08 Bituminous 3 inches 6 inches Concrete [75 mm] [150 mm] Base Course 9.5.6 MINIMUM LIFT THICKNESS Graded 4 inches 8 inches Although the equations allow for a great Aggregate [100 mm] [200 mm] Base Course number of thickness variations, there are the practicalities of constructing and maintaining a Soil Cement 4 inches 6 inches facility, which must be considered. Depending [100 mm] [150 mm] upon the material being placed, there are minimum and maximum limits in the placement Select 4 inches 8 inches depth that are practical for the available equip- Borrow [100 mm] [200 mm] ment to compact and are economical. Open Graded 1 inch 1 inch [25 mm] [25 mm] Minimum lift thickness for hot-mix is recommended to be three times the nominal aggregate size in the mix. Figure 9-3 shows the prac- 9.6 DESIGN FOR RIGID tical maximum and minimum lift thickness PAVEMENTS (compacted) that are to be applied to the materials normally used in constructing a flexible Rigid pavements consist of a Portland cement pavement section. concrete slab on a subbase course. The design procedure consists of developing an effective 9.5.7 TEMPORARY PAVEMENTS modulus of subgrade reaction based on subbase treatment and thickness, determine the slab It is not practical to attempt to follow the thickness, allowing for any stage construction, formalized AASHTO procedures for design of adjusting for adverse environmental conditions, temporary pavements such as needed for detours determining type of joints, joint sealant, and the during construction. Variations in speed and required reinforcement. ease of placement as well as the anticipated required service life of the detour significantly affect the economic justification for the structural design.

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9.6.1 SUBBASE⎯EFFECTIVE MODULUS the reliability factor to be achieved, the standard OF SUBGRADE REACTION deviation, the design serviceability loss, the concrete elastic modulus, the concrete modulus of Before the slab thickness can be determined, rupture, the load transfer coefficient, and the it is necessary to determine the strength (mod- drainage coefficient. ulus), of the material on which the slab will be supported. This is done by determining an effec- 9.6.3 JOINTS tive modulus of subgrade reaction (k), of the soilbed and subbase. Joints are a very important part in assuring a rigid pavement will perform as intended. They The effective k-value is dependent upon sev- allow for the stresses created by the expansion eral factors including the roadbed soil resilient and contraction of the concrete during curing modulus, the type of subbase, the thickness of and during seasonal temperature changes. They subbase, the potential of loss of support due to also are used to facilitate construction. erosion of the subgrade and in northern New Castle County whether there is rock underlying The Department’s Standard Construction De- the proposed pavement. tails for Construction provide details for locating and constructing the required rigid pavement The subbase used in a rigid pavement struc- joints. The following is a general discussion on ture consists of one or more compacted layers of the need, use and treatment of joints. granular material, graded aggregate or a stabilized material such as bituminous concrete. This material is placed between the subgrade and the 9.6.3.1 JOINT TYPES rigid pavement. The subbase provides several very important functions: The three types of joints used in constructing a rigid pavement are expansion, contraction and * Provides uniform, stable and permanent construction. Expansion joints provide space for support, the pavement to expand, preventing buckling of the slabs. * Increases the effective modulus of subgrade reaction (k), Contraction or weakened plane (dummy) * Prevents pumping of fine-grained soils at joints provide relief for the tensile stresses joints, cracks and edges of the pavement, caused by the effects of temperature, moisture and friction. Without these joints to control * Reduces cracking and faulting, and cracking, the slabs would crack randomly. * Provides a working platform for construction equipment, especially the paver. Construction joints are required to facilitate construction. Although used at the end of a 9.6.2 PAVEMENT SLAB THICKNESS day’s pour, they are particularly dictated by the width of the paving machine and the pavement After developing the effective k-value the thickness. process of selecting the optimum slab and subbase thickness can begin. Past experience, eco- Joints may be developed by sawing, forming, nomics, equipment limitations, ease of construc- or with inserts. When sawing joints, timing is tion, and other subjective factors influence the very important to prevent uncontrolled cracking final recommended section(s). and will vary during the day depending upon the slab temperature, curing conditions, and the The AASHTO Guide provides a nomograph concrete mix. which provides the slab thickness based on in- putting the k-value, the estimated future traffic,

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9.6.3.2 JOINT GEOMETRY allows for moisture intrusion into the subgrade which is the leading cause for distress in the Joint geometry refers to the spacing and slab. layout of the joints. Stresses leading to cracking are temperature Transverse and longitudinal contraction joint and moisture related contraction of the slab. spacing is dependent upon local conditions of These stresses are resisted by the subbase as materials (coarse aggregate type) and the envi- friction and shear between it and the slab. The ronment, whereas expansion and construction result is tensile stresses that are minimum at the joints are dependent on layout and construction mid-point of the slab. To resist these stresses and methods. limit the crack width, reinforcement is installed. The AASHTO Guide provides methods for de- Contraction joints are spaced to prevent in- signing the necessary reinforcement for both termediate natural cracking due to thermal jointed reinforced concrete pavement and conti- changes and subbase friction created by the nuously reinforced concrete pavements. movement of the slabs. The type of joint sealant and slab thickness affects their spacing. 9.7 PAVEMENT DESIGN FOR Expansion joints are usually used at struc- REHABILITATION OF tures and where pavement types change. They EXISTING PAVEMENTS are expensive, complex to construct, and have not performed well in the past. Therefore, they are used only where absolutely necessary. 9.7.1 REHABILITATION CONCEPTS

The spacing of construction joints is a func- Bituminous concrete pavements deteriorate tion of the daily construction activities and because of climatic conditions, age, and traffic. equipment. They are used when equipment Transverse and longitudinal shrinkage stresses breaks down and at the end of the day’s pour. occur due to temperature changes. Over time, Longitudinal construction joints are placed at material problems can develop causing surface lane edges to maximize pavement smoothness problems with stripping, raveling, weathering and minimize load transfer problems and bleeding of the asphalt. Repeated traffic loadings eventually cause fatigue cracking al- The width of the joint is a function of the slab lowing moisture into the subbase causing loss of length, movement due to opening and closing by subgrade support leading to pavement cracking temperature cycles and concrete shrinkage. or failure. More movement expected at the joint affects the quality and cost of the joint sealant used. For composite pavements with both concrete and asphalt as components of the pavement The depth of a joint is selected to ensure that structure, the most prominent problem is reflec- the slab will crack where intended. tive cracking from joints and cracks in the concrete base. This is caused by a combination of underlying slab movement due to temperature 9.6.4 REINFORCEMENT DESIGN changes and heavy loads crossing the joints and cracks. The primary resulting distress is spalling Concrete pavements inherently crack. De- of the asphalt as well of the concrete if severe pending upon the slab length and depth selected, enough. it may be necessary to provide steel reinforcement within the pavement slab. The purpose of Concrete pavements react differently depend- the reinforcement is not to prevent cracking but ing whether or not they are reinforced. Over to control the crack width. Excessive cracking time and load applications each reacts different-

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ly. The only reinforcement in non-reinforced combination of the following alternatives are pavements is tie bars across the longitudinal considered: joints to keep the slab from separating and dowels at the transverse joints to transfer loads 1. Resurfacing to provide structural capacity across the joints. If dowels are not used, the de- and/or serviceability either using concrete or sign depends only upon aggregate interlock to asphalt, transfer these loads. With loss of subgrade sup- 2. Replacing or restoring malfunctioning port, cracking of the slab can occur at almost joints, any location. However, more common problems occur at joints. Once the joints are no longer free 3. Pavement subsealing prior to resurfacing or to move, spalling, buckling, and random slab as a part of concrete restoration, cracking can result. If joint sealant is lost, ag- 4. Grinding rigid pavements to restore smooth- gregate interlock is lost, or cracks become too ness. wide, pumping of the subgrade within the travel lane and the shoulder can occur. This leads to 5. Removing and replacing deteriorated mate- erosion of support material and faulting and rials, cracking 6. Reworking or strengthening bases and sub- ases, Reinforced concrete pavements perform much like non-reinforced pavements. They 7. Recycling existing material, usually have longer joint spacing and internal 8. Improving the subdrainage or adding under- reinforcement to resist the larger tensile stresses. drains, There are fewer joints to relieve stresses; when 9. Joint and crack sealing, their free movement is restricted, rapid faulting of the pavement can occur. In addition, normal 10. Full depth pavement repair, shrinkage, thermal curl, and load applications 11. Partial depth pavement repair, and cause cracks in the slab that over time grow in width, allowing moisture and road salt to infil- 12. Cracking and seating. trate. Corrosion of the reinforcing mesh occurs. As the loads are repeated, pumping begins lead- M&R and other team members develop the ing to faulting and spalling of the pavement. most effective strategy through a detailed pavement evaluation. This evaluation is normally Continuously reinforced concrete pavements prepared as a part of the Department’s long term deteriorate under heavy truck loading and are pavement rehabilitation program and will be a also adversely affect by moisture under the slab. part of the project initiation data. The three steps This type of pavement is more complicated to for determining the preferred strategy are to (1) construct and deterioration will occur due to in- determine the cause of the pavement distress, (2) adequate consolidation, poor vertical steel develop a list of possible solutions that cure and placement, and inadequate steel overlap. hopefully prevent reoccurrence, and (3) recommend the preferred solution. The preferred solu- Because of the need to maintain the pavement tion includes an analysis of funding, traffic con- systems now in place, several rehabilitation trol problems, minimum desirable service life, strategies have been developed to address the utility conflicts, clearances to overhead struc- various problems that may affect a pavement's tures, available materials and equipment, con- performance. The objective is to extend the tructibility, future maintenance and reliability pavement’s service life. The alternative methods based on past performance. of rehabilitating a pavement range from a simple overlay to complete removal and replacement. When developing a rehabilitation strategy a

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9.7.2 TYPES OF DISTRESS Depressions are localized pavement surface areas that are slightly lower than the surrounding Although furnished at the project initiation pavement. Depressions are most noticeable dur- stage, each project usually will have a project ing and after a rain. If deep and large enough, scoping meeting and preliminary field review depressions may cause hydroplaning or an un- prior to beginning detailed design. The field re- pleasant ride. Depressions may be initially built view will include checking the pavement for into the pavement by the paving operation or as condition and any damage that may have oc- a result of settling of the surface support struc- curred since the last pavement survey to confirm ture. the proposed strategy is still valid. The following discussion is a brief description of the vari- Rutting is a surface depression in the wheel ous conditions the designer may observe de- paths. Usually, there is uplift along the sides of pending upon the type of pavement being eva- rutted areas. Rutting is the result of permanent luated. consolidation or lateral movement of any of the pavement layers or subgrade due to traffic loads. Rutting may also occur because of plastic 9.7.2.1 ASPHALT PAVEMENTS movement of the asphalt due to high tempera- Asphalt pavements usually have the follow- tures, poor design mix or inadequate compaction ing major distress conditions: (1) alligator or during construction. fatigue cracking, (2) longitudinal and transverse In addition to the major distresses, the pave- cracking, (3) depressions, and (4) rutting. ment survey may indicate surface corrugation Alligator or fatigue cracking is caused by re- areas, joint reflection cracking, lane and shoul- peated traffic loadings. They are a series of in- der drop off, lane and shoulder separation, patch terconnecting cracks caused by fatigue failure of deterioration, polished aggregate, potholes, rave- the bituminous concrete surface. The crack starts ling, and weathering. at the bottom of the asphalt surface or the stabi- If necessary, the M&R Section may deter- lized base due to high tensile stresses and propa- mine that field observations are not adequate or gates to the surface as a longitudinal crack. After need to be supplemented to identify underlying repeated load applications, a network of these problems. In that case nondestructive testing cracks form that look like chicken wire or the (NDT) will be conducted. NDT is used to: skin of an alligator. This type of cracking does not occur in asphalt overlays over con- • Evaluate the in-situ (in-place) structural ca- crete⎯only in high load areas and is considered pacity of the pavement, a major structural distress. Pattern cracking not in high load areas is called block cracking. • Evaluate the capacity of joint and load transfer, and Longitudinal cracks are parallel to the pave- • Detect the presence of voids under the ment’s centerline or paving laydown direction. pavement. They may be caused by: (1) a poorly constructed paving lane joint; (2) shrinkage of the bituminous concrete surface due to low temperatures or 9.7.2.2 CONCRETE PAVEMENTS hardening of the asphalt; or (3) a reflective crack Jointed concrete pavement may have the fol- caused by cracks beneath the surface course, lowing distresses: (1) pumping, (2) longitudinal including cracks in Portland cement concrete. cracking, (3) spalling of transverse or longitu- Transverse cracks are perpendicular to the dinal joints, or (4) Alkali-Silica-Reactivity pavement’s centerline; they are caused by (2) or (ASR). (3) above and are not usually load-related.

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Pumping is the ejection of material through sorbs it, and causes expansion and cracking of joints or cracks, caused by the deflection of the the concrete. Once an ASR pavement has been slab under moving traffic. In some poorly identified, one possible solution is to apply li- drained pavements particularly in superelevated thium treatments. Lithium treatment will not sections, water may bleed through the joints and repair the concrete, but will slow the further cracks after a rain or continuously if large progression of ASR. Normally the pavement amounts of water are present. will have to be removed and replaced. Overlay- ing ASR concrete with a standard hot-mix over- Longitudinal cracks are caused by a combina- lay may trap moisture in the slab and accelerate tion of heavy load repetition, locking of load the ASR causing premature failure of the over- transfer devices, thermal and moisture stresses, lay. However, hot-mix overlays with water and curing shrinkage stresses. Cracks that are proofing properties may minimize further dete- spalling and/or faulting are considered a major rioration for several years. M&R will recom- structural problem. mend the most suitable solution when encoun- tering ASR pavements. Spalling of cracks and joints is the cracking, breaking or chipping of the slab edges within 2 In addition, other pavement distresses that ft (0.6 m) of the joint or crack. Spalling usually may be observed in a field review are blow ups, does not extend the full depth of the slab, but corner cracks, depressions, durability “D” crack- intersects the joint or crack at an angle. Spalling ing, lane and shoulder drop-offs, lane and shoul- is a result of one or a combination of the follow- der separation, patch deterioration, popouts, and ing: (1) excessive stresses at poorly sealed or staining of the pavement due to subgrade drai- cleaned joints and cracks which allow incom- nage problems. pressible material to accumulate preventing the pavement from expanding, (2) disintegration of Concrete pavements can also display rough- the concrete, (3) weak concrete overstressed by ness caused by irregularities in the pavement repeated loading, and (4) a poorly designed or surface that adversely affect the ride quality, placed load transfer device. safety, and vehicle maintenance costs. Rough- ness is measurable based on the multi-frequency Continuously reinforced concrete pavements of waves, wavelengths and amplitudes. Rough- usually show punchout and patch distress. Pun- ness can be built into the pavement when con- chout is the loss of aggregate interlock between structed or develop over time due to traffic, cli- closely spaced cracks. The cracks fault, spall and mate, and other factors. Equipment is used to under load applications the steel reinforcement measure the roughness and a profile developed ruptures causing concrete pieces to punch down. showing the vertical movement between the trai- This type of distress is considered a major struc- ler axle and body. The results are reported in tural problem. Due to the difficulty in patching in/mile or m/km for the International Roughness continuously reinforced pavements the failure of Index (IRI). The roughness survey identifies previously constructed patches can be antic- areas where severe roughness exists and needs ipated. correction. Data provided can be used in developing a PSI which estimates the user’s subjec- ASR is another type of distress commonly tive assessment of the pavement condition. Sur- found in Delaware. The pavements exhibiting veys taken before and after a project can be used the most severe ASR were generally constructed to document the benefits of the work to the trav- in the 1980’s. ASR is the reaction between the eling public. alkalis (sodium and potassium) in Portland cement and certain siliceous aggregates. The prod- uct of this reaction is a thermodynamically me- tastable. The gel in the presence of water ab-

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9.7.3 DRAINAGE SURVEY 6. Are joint and crack sealants in good condition and preventing surface water infiltra- As emphasized throughout this chapter, the tion? presence of moisture is a primary cause of dis- 7. Are there signs of pumping, such as pave- tress or failure of all pavements. Therefore, a ment discoloration or the presence of fine drainage survey is an important component of material at joints or pavement edges? pavement evaluation. Moisture conditions are caused externally by the climatic conditions and Recommending drainage improvements to internally by the properties of the materials the pavement structure can be a very expensive composing the pavement structure. The severity item and should be carefully evaluated and do- of damage caused by excessive moisture will cumented. influence the decision on which rehabilitation strategy to select. 9.7.4 RESTORATION Since moisture problems can exist in any layer of the pavement structure, more than visual Restoration of a pavement includes the observations may be needed. Cores and even work required to return the pavement’s level of nondestructive deflection testing may have to be serviceability for a designated time period. Fre- conducted. It is necessary to determine which quently, some level of restoration is performed material is responsible for the moisture-related prior to an overlay or resurfacing. Work under- damage and if an economical rehabilitation to taken to restore pavements is quite varied and correct the problem is to be initiated. Not identi- includes: fying and correcting the problem could lead to a failed project. A valuable tool in this evaluation • Full-depth repair of jointed concrete pave- is the as-built plans. ment • Full-depth repair of continuously reinforced In addition to determining if the pavement concrete pavement, structure is freely draining and moisture resistant, the entire roadway section should be eva- • Patching with bituminous mixtures, luated including: • Partial-depth spall repair, 1. Are the ditchlines free of standing water? If • Slab stabilization and slab jacking, not, how high does it stand and will it infil- trate the pavement structure? • Diamond Grinding, grooving and cold milling, 2. Are ditchlines and pavement edges clear of the type of growth that would indicate ex- • Pressure relief joints, cessive moisture? • Load transfer restoration, 3. After a rain, is water standing in the joints or • Joint and crack sealing, cracks. Is there pumping, is there standing water adjacent to the pavement or on the • Surface treatments, shoulder? • Subdrainage, and 4. If there are drainage outlets including un- • Shoulder improvements. derdrains, are the outlets clear, at the proper elevation, and working? 9.7.4.1 FULL-DEPTH REPAIR 5. Are drainage inlets clear and cross slopes adequate to remove the water from the One of the most expensive restoration alter- pavement surface? natives used on all types of pavements is full- depth repair. To limit the repair areas and the

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potential of overruns, the specific distress or dis- lution. tresses to be addressed should be clearly established both during the design and construction 9.7.4.2 PARTIAL-DEPTH REPAIR phases. Depending upon the severity, distresses that may lend themselves to full-depth repair are Some distresses within the upper third of the blow-ups, corner breaks, durability “D” crack- slab in concrete pavements lend themselves to ing, ASR, excessive spalling, and loss of load partial-depth repairs. Partial-depth repairs may transfer. Intermediate working cracks may also or may not be more cost effective than full-depth have to be repaired by full-depth replacement or depending upon the size, location, number, ma- a working joint. Not addressing areas that need terials used, lane closure time and production full-depth repair prior to an overlay could result limitations. in continued deterioration and premature failure of the overlay. The distresses applicable to partial-depth repair are: The major considerations to ensure satisfac- tory performance using this rehabilitation ap- 1. Spalls due the use of certain types of joint proach are: inserts, 1. Joint design, 2. Spalls caused by joint movement locking due to failed sealant and subsequent intru- 2. Selection of the repair areas and their boun- sion of incompressible material, daries, 3. Spalls caused by misplaced dowels or other 3. Preparation of the repair area, load transfer devices, and 4. Placement and finishing of the repair ma- 4. Localized areas of scaling. terial, 5. Joint sealing material and its installation, The M&R Section should be contacted for and details on partial-depth patches as they require the use of better designs and construction tech- 6. Curing time and traffic control. niques to be successful. Usually the full-depth repair material is the same as the adjacent pavement. However, fund- 9.7.4.3 SLAB STABILIZATION AND SLAB ing, traffic control or other reasons may require JACKING that concrete pavement be repaired with bituminous material. Using materials of different Although not that frequently used, voids un- physical properties and characteristics can cause der pavements can be filled, restoring the sup- several problems. Differentials in expansion and port strength of the pavement structure. The loss contraction lead to pushing, shoving and hump- of support is caused by erosion of the subbase ing requiring frequent milling to maintain ridea- and/or subgrade by pumping or, in severe cases, bility. The bituminous patch is more compressi- movement of freely flowing water. Slab stabili- ble and may allow excessive opening of remain- zation does not increase a pavement’s structural ing joints in the concrete pavement resulting in strength, correct depressions or faulting and oth- spalling, pumping, and faulting. In overlaying er distresses. the patched pavement, there may be an increase in reflective cracking from the underlying joints A slab that has settled may be raised through The patch design may not have an equivalent a process called slab jacking, also known as structural strength. The difference in initial cost mudjacking. This procedure which injects water- and possible future maintenance problems may soil-cement slurry (i.e. grout or mud) through not make using dissimilar materials the best so- holes drilled into the pavement slab under high pressure lifting the pavement back into its origi-

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nal profile is also used infrequently. A process 3. Improving the friction resistance of asphalt using expanding polyurethane foam may also be pavements, used. Another alternative is the injection of a 4. Removing asphalt overlays on concrete flowable fill that consists of a mixture of fly ash pavements, and water that expands and hardens as it dries out. If not correctly done, all three of these me- 5. Providing a good bonding surface on con- thods can crack the slab. crete for overlaying, 6. Removing material as part of a recycling 9.7.4.4 DIAMOND GRINDING, GROOVING project, and AND PAVEMENT MILLING 7. Restoring pavement smoothness. Diamond grinding is used to restore a con- All three of these restoration techniques are crete pavement’s smoothness using closely cost effective, relatively quick and proven to spaced diamond-impregnated blades. Diamond perform well but do not improve the structural grinding is effective in: (1) removing joint and integrity of the pavement. crack faulting, (2) remove wheel path ruts, (3) correct joint unevenness caused by slab war- page, (4) removing areas of roughness and (5) 9.7.4.5 PRESSURE RELIEF JOINTS restoring transverse drainage. Diamond grinding does not correct the problem that created the As concrete pavement ages, there can be a net distress and may have to be use in conjunction increase in its length. These increases can result with other rehabilitation techniques. Subsealing, from: poorly sealed joints and cracks filled with slabjacking, full-depth, and partial depth repairs incompressible material, pumping of the base should be completed before grinding. material into the joints and cracks, or the use of expansive or reactive aggregates in the initial Grooving of concrete pavements is used to mix. reduce hydroplaning by improving surface drainage and improving surface friction on curves or Because pressure relief joints can adversely polished aggregate surfaces. affect contraction joints, they are not usually installed unless there are major blow-up prob- Pavement milling is used to remove asphalt lems or the pavement has expanded to the point surfaces as much as 3 to 4 inches [75 to 100 of shoving bridge abutments. These type of mm] in depth using carbide-cutting teeth joints provide no load transfer and are also sub- mounted on a rotary drum. If used on concrete ject to closing through the intrusion of incom- pavements, pavement milling leaves an extreme- pressibles, eventually becoming part of a pave- ly rough surface and spalled joints. Therefore, ment distress problem. unless an overlay is included in the work, it should not be used on concrete. Pressure relief joints are full-depth and full- width saw cuts at mid-slab and are 2 to 4 inches Pavement milling is a very effective rehabili- [50 to 100 mm] in width. The joint must be cut tation technique addressing several problems across all lanes within the same time period to including: relieve the compressive stresses and is usually done at night under reduced traffic flow. 1. Restoring the curb line of asphalt pavements, 9.7.4.6 LOAD TRANSFER RESTORATION 2. Restoring cross slope and correcting inlet drainage problems, Poor load transfer between joints can cause spalling, rocking, pumping, faulting, and corner breaks. Doweled joints are effective in providing

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load transfer but under repeated load applica- pen at joints and sealing is not as tightly con- tions can work loose and lose this capability. trolled. If the field survey indicates the width of cracks are large enough to cause large move- Whether or not joints and cracks have lost ments, then the cracks are sealed and treated their load transfer ability, is determined through similar to joints. field observation and measurements conducted in the cooler parts of the year or day. The Fall- 9.7.4.8 SURFACE TREATMENTS ing Weight Deflectomator (FWD) may be used to test for load transfer capability at joints. The use of surface treatments or seal coats is a very effective way to rehabilitate. The tech- There are two techniques used to restore load nique is to apply asphalt and/or aggregate to a transfer, both of which require the installation of roadway surface at a depth of less than 1 inch dowels. Slots can be cut in the pavement, if the [25 mm]. adjacent pavement is sound, and the dowels installed. More frequently and, usually a better Surface treatments are classified by their alternative, load transfer is restored through full- composition and include: depth repair. Asphalt Chip Seal (Surface Treat- 9.7.4.7 JOINT AND CRACK SEALING ment)⎯one or more applications of asphalt and stone chips. One of the easiest, cost effective and proven techniques in maintaining serviceability and res- Open-Graded Friction Course⎯ the asphalt toring concrete and asphalt pavements is sealing and aggregate mix is designed to be freely flow- and resealing joints and cracks. Inadequate or ing to remove surface water thus reducing hy- failed sealant allows free water and incompres- droplaning. In addition, it provides some im- sibles to enter the pavement structure causing provement in skid resistance. erosion of pavement support and blow-ups. Micro Surfacing⎯a process using a moving Joint sealants are constantly being improved pug mill which mixes latex rubber with an as- and may be field poured⎯self-leveling, hot- phalt emulsion, aggregates and other additives poured, cold-poured, preformed compression that is placed on the surface. seals, or field-poured⎯nonself-leveling sealants. Slurry Seal⎯a diluted emulsion mixed with The performance of a sealant depends pri- a sand-size aggregate in a special mixer, and marily upon the proper preparation of the joint squeegeed onto the pavement. The thickness is to receive the sealant. The factors include the usually less than one half inch [12 mm]. shape of the reservoir created for the sealant, the bonding ability of the sealant to the sidewalls, 9.7.4.9 SUBDRAINAGE IMPROVEMENTS the surface preparation, the cleanliness of the surface, the dryness of the surface and the prop- The long-term presence of water within the erties of the sealant. In addition, it is necessary pavement structure is largely responsible, direct- to predict the amount of movement expected in ly and indirectly, for many of the distress and the crack or joint to customize the type of sea- performance problems, which are found in the lant to fit the distresses encountered and tech- pavement systems. A pavement survey and niques to be implemented. evaluation includes a very detailed study of the drainage and subdrainage within the entire Cracks occur randomly and are irregular in roadway cross section. Considerable design ef- dimension and direction. Usually cracks do not fort and cost are involved when correcting this experience the faulting and movement that hap-

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type of problem. Unless the rehabilitation pro- to construct new pavements. The primary pur- gram involves complete pavement reconstruc- pose is to conserve natural resources. In some tion, improvements are limited to design and cases, there may also be a net cost savings. construction of longitudinal drains. Any modifi- cations to or additions of transverse drains or 9.7.5.1 RECYCLING RIGID PAVEMENTS drainage blankets would be limited to projects involving complete pavement replacement. Eventually, all pavements reach the end of However, the addition of longitudinal perforated their useful and/or structural life and must be underdrains to collect and outlet excess moisture reconstructed. This occurs when there is little or in the pavement structure is a feasible and cost no structural life left due to extensive cracking, effective option. extensive slab settlement or heave, extensive joint deterioration requiring excessive full-depth 9.7.4.10 SHOULDER IMPROVEMENTS repair, extensive concrete deterioration due to poor durability, and failure to meet geometric Shoulders are evaluated much like the main- design standards. line pavement and are include in the distress survey, drainage survey, traffic survey, structur- Two methods of recycling are: to break the al evaluation, and subgrade and materials evalu- pavement slabs into smaller sections and leave ation. Shoulders provide not only safer traffic them in place (rubblizing) as a base for resurfac- flow but also give lateral support for the main- ing; or to break and remove the pavement slabs line pavement structure. to an area for crushing and reuse. The crushed pavement material may be reused as aggregate Shoulders can be rigid or flexible and distress in untreated dense graded aggregate bases for similar to mainline pavements. Many of the Portland cement concrete surfacing, asphalt con- same rehabilitation techniques are used. Review- crete surfacing, fill, filter material, and as a drai- ing the existing cross section can be very valua- nage layer for edge drains. ble. If the initial shoulder pavement design was not compatible with the mainline pavement Each method of recycling has its own cost structure, it may be a major contributor to its and feasibility studies that need to be conducted failure. before making a selection. Both methods involve the use of vibratory, hydraulic, pneumatic, or Two of the most common problems which diesel chisels or hammers to demolish the exist- occur are lane/shoulder joint separation which ing pavement. Common factors to consider are: allows water into the subbase, and blockage of is the resulting recycled material actually reusa- water draining out of the mainline subbase ble; are the underlying soils adequate to support which is usually of more granular and higher an upgraded pavement; and are there shallow quality. The material used in shoulder construc- utilities (older gas, water or sewer lines such as tion also may be of a different thickness. All of vitrified clay or cast iron) that are sensitive to these affect the interaction between the two the equipment pounding and vibration and may pavement structures. rupture during the demolishing process.

Other distresses found in shoulders are pump- The pavement may be demolished, removed, ing, fatigue cracking, lane/shoulder drop-off, crushed and used for other construction purpos- frost heaving and differential shoulder support. es. This involves hauling the material to a crushing plant and having an electromagnet remove any reinforcement. The crushed material and 9.7.5 RECYCLING salvaged steel would be available for reuse. The Recycling is the term used to describe the coarse-aggregate-sized particles resulting from process, which uses existing pavement materials the crushing process have a good shape and an-

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gularity, high absorption and may have a lower tion of PCC pavements. Concrete pavement specific gravity than virgin aggregates. For reuse restoration (CPR) techniques, such as diamond of this material the concerns are the possible grinding, patching or sealing, should be ruled contaminants in the rubble including reinforcing out before the use of rubblization is specified. steel, dowel bars and baskets, admixtures, chem- Pavement Management and M&R should be ical substances such as deicing salts, sea salt, oil, consulted in the selection of reconstruc- joint sealant material and soil. tion/restoration technique.

Rubblization cannot be used over a subgrade 9.7.5.1.1 RUBBLIZATION demonstrating widespread instability or of poor Rubblization is initially the most expensive condition. Many concrete distresses result from pavement recycling method, however, a selec- poor subgrade support conditions. Rubblizing a tive use of it can be very effective in cost and pavement destroys the concrete slab’s bridging construction time. action, causing problems to become more pro- nounced. The poor support condition could be When rubblizing plain or reinforced concrete due to poor soils, poor drainage or high moisture pavement, the rubble is leveled and left in place content. If the problem is widespread, rubbliza- as the base for a new pavement. Maintaining tion cannot be used. Thorough subgrade inves- traffic is a consideration as there must be enough tigation is essential for the successful applica- area for the equipment to operate as well as a tion. Contact M&R for subgrade investigation. protective barrier. Even on a four lane facility this will require a median shoulder strong Treat rubblized PCC pavement as a subbase. enough to temporarily carry full traffic loads or Although rubblization provides the benefits of possibly a detour. an in-place recycle opportunity and an inter- locked stone base, it has challenges as well. As The rubblized surface essentially remains at for all subbase materials, gradation and density the same elevation as the existing pavement. are two important factors, but the control of Therefore, matching existing curbs, drainage these two factors is more difficult since it is in- structures, intersecting roadways, and driveways place recycling. is more complicated and time consuming. In addition, it may become difficult to meet height The pavement breaker may be powerful, but standards for guardrail and other appurtenances, as energy dissipates through the depth of the as well as maintain vertical clearances for struc- slab, it produces smaller pieces at the top and tures. larger pieces at the bottom. A soft subgrade or the reinforcement in the slab only compounds Rubblization is for total reconstruction. Rub- the difference in sizes. The recommended re- blization reduces the structural value of a PCC quirement is for the top pieces to be 3” maxi- pavement to a stone base. It requires a thick mum and 12” maximum at the bottom. Accep- overlay, typically 11 inches [280 mm] of asphalt tance of a larger size will increase the probabili- or 10 to 12 inches [255 to 305 mm] of concrete. ty of future reflective cracking. A good density It is a major reconstruction technique and there- is achieved through the interlocking and good fore it should be used only when the pavement compaction. has reached the end if its service life, as indi- cated by severe deterioration, ASR, or severe With good control of gradation and density, it freeze and thaw damages, etc. is reasonable to expect a good fatigue resistance performance of an asphalt overlay, which is a Rubblization can be used when other con- major controlling factor for a flexible pavement crete pavement restoration methods will not service life. work. Thoroughly evaluate the existing condi-

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Rubblization is a balancing act. The size of passes for a full lane width with a production rubblized concrete can be controlled. A larger rate of about ½ miles per shift per day. size will provide a stronger support (thus a better structural value), but it increases the likelihood The multi-head breaker may cause damages of reflective cracking (thus a reduced service to the subgrade. the resonant breaker may pro- life). The designer needs to balance these two in duce concrete size too small at the top to meet the pavement design. A long term performance the design requirement. Unless the equipment evaluation to validate this design value may be selection is specified on the plan, the specifica- necessary. tion will allow the use of either breaker.

Install a drainage system. An adequate sub- A concrete overlay can be used. Rubblization grade drainage system is essential for rubbliza- was developed to eliminate reflective cracks in tion and future performance of the pavement. the asphalt overlay. Engineers still have an op- Rubblization cannot be successfully done over a tion of using concrete overlay, considering cost, wet subgrade. The drainage system should be in service and construction time. Consult M&R for place at least two weeks prior to the rubbliza- pavement design options. tion. In a special case, if the drainage system cannot be installed prior to actual rubblization, Soft spots need to be repaired. Original PCC then a time limit should be specified to have the pavement could bridge the soft spot and this soft drainage system installed immediately following spot will show up after rubblization. Any de- rubblization. pression one inch or greater in depth from the immediate surrounding area should be examined Use a test section, not a second pass of the to see if it is due to poor underlying subgrade breaker. A second pass over a rubblized area before the application of filler aggregate as re- does not enhance the quality of rubblization, and quired by the specification. A bearing capacity it could cause more damage. Test sections failure during rubblization could cause depres- should be done to calibrate all rubblization va- sion on one area and heave in other areas. riables (machine related-velocity, frequency, pressure or force, shoe size and conditions re- The repair of soft spots is necessary not only lated-concrete condition or state of distress, for a long term performance of the pavement but thickness and subgrade conditions). The objec- also for a good working platform for paving op- tive is to achieve the required sizes of rubbliza- erations. tion both at the top and bottom of the PCC pavement for a good service life. Pavements with delamination type cracks should not be rubblized. Horizontal cracks hind- Selection of Rubblization Equipments and er the rubblization process by absorbing energy Production Rates - Among the different types of and decreasing the effective depth of rubbliza- equipment for breaking the pavement, two fre- tion. quently used types are resonant pavement breakers (a low impact, low-amplitude, high frequen- Survey and set a new profile. Although rub- cy vibration to the slab) and multiple head blization does not significantly change the exist- breakers (12 to 16 drop hammers mounted later- ing grade, simply specifying a few inches over ally in pairs with half of the hammers in a for- the existing grade may not be adequate. The ward row and the remainder diagonally offset in existing PCC pavement may not have adequate a rear row). The multi-head breaker rubblizes a cross slope, or it is distressed due to poor sub- full lane width in a single pass with a production grade which might have resulted in an irregular rate of about one lane mile per shift per day, profile. Comparing survey results with the orig- while a resonant breaker may take up to 20 inal geometry could provide clues on distress or subgrade conditions.

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The design service life should be 20 years or course problems such as gradation, moisture and more; this should be noted on the plans. Rubbli- density. Correction of profile grade problems zation with an overlay is major reconstruction. can be made with this material. It should not be used as a short-term fix. 9.7.5.4 HOT-MIX RECYCLING OF 9.7.5.2 SURFACE RECYCLING OF BITUMINOUS PAVEMENTS BITUMINOUS PAVEMENTS Hot-mix recycling is the process in which all Surface recycling is the process that either or some of the pavement structure is removed, reworks or removes and replaces a limited por- reduced to the required size, and mixed hot with tion, usually about one inch [25 mm], of the as- added asphalt cement at a central plant. phalt surface. The distresses addressable through this process include: raveling, flushing or bleed- This process is used to correct surface rough- ing, low surface friction, weathering, poor drai- ness, cracking, rutting, surface friction, raveling, nage profile, shallow rutting, minor corrugations inadequate structure, and inadequate traffic ca- and block cracking. It may also be used to cor- pacity. It should be remembered that the under- rect problems in the profile grade line. lying cause for structural inadequacy would not be corrected using this process. The major advantage for selecting this rehabilitation method is the minimal amount of work 9.7.6 RESURFACING involved. There are two processes. The first is hot surface recycling involving heating, scarify- The most common type of rehabilitation for ing, remixing, and repaving recycled material. existing pavements is resurfacing. Resurfacing Other surface recycling methods include hot can correct many common distresses and add pavement removal using a heater-planer or cold additional strength to the pavement structure. milling using a rotary drum equipped with closely spaced carbide teeth. Problems encountered with overlay projects include inadequate thickness to correct surface The material to be reused has a rejuvenating problems when the problem is actually structur- agent and soft asphalt added to restore the al, inadequate repair of the deteriorated areas, pavement consistency, viscosity or penetration. unanticipated increasing traffic loadings, and not DelDOT does not typically use this method for addressing reflective cracking. recycling surface courses.

9.7.6.1 TYPES OF OVERLAYS AND THEIR 9.7.5.3 IN-PLACE RECYCLING OF FUNCTIONS BITUMINOUS PAVEMENTS Overlays can be of asphalt or Portland ce- In-place recycling is a process in which the ment concrete. There are several variations of pavement surface is ripped up or pulverized to a these overlay techniques, which are designed for depth greater than 1 inch [25 mm]. The material specific applications. is cold worked and reused as an aggregate base. The recycled material may be further streng- The most commonly used overlay is dense- thened by the addition of admixtures such as graded, hot-mixed asphalt concrete, which may asphalt, lime, cement or fly ash. The recycled be used on existing asphalt or Portland cement material will perform similar to new stabilized concrete pavements. material upgrading the structural capacity, correcting surface distresses and mixture problems Portland cement concrete overlays are de- in the asphalt pavement, and correcting base signed specifically for the type of existing

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pavement and may be unbonded, partially Portland cement concrete overlays of existing bonded or fully bonded. rigid pavement may involve extensive pre- overlay repairs, are more difficult to construct In analyzing overlay alternatives, the design- and have added initial traffic control costs. er should look at how extensive are the proposed These overlays may be fully bonded, partially pre-overlay repairs, structural adequacy of the bonded and unbonded. existing pavement, subdrainage, existing distress conditions to be corrected, future traffic load- Fully bonded concrete overlays require that ings, traffic control problems, clearances to the existing pavement be in good condition and overhead structures, and overall costs. that a complete and permanent bond be achieved. Fully bonded overlays are thinner and Asphalt overlays of rigid pavements are used range from 2 to 4 inches [50 to 100 mm] in both to correct surface problems and to improve thickness. the structural strength. Thicknesses greater than 2.5 inches [65 mm] are needed to provide im- Partially bonded concrete overlays require provements in strength. The greater the overlay repair and/or replacement of damaged slabs. thickness, the higher the possibility of rutting if Surface cleaning by sweeping and either water compaction is not controlled but a significant blasting or sand blasting is necessary to achieve reduction in reflective cracking can be expected. as much bond as possible. These overlays are usually between 5 to 7 inches [25 to 175 mm] in Reflective cracking occurs in overlays due to thickness. thermal cracks in flexible pavements and joints in rigid pavements. Low temperatures, tempera- Unbonded concrete overlays are used to im- ture cycles and traffic loads cause movement in prove the structural capacity of a rigid pavement the existing pavement leading to stresses reflect- in poor condition. A bond breaking and leveling ing through the overlay. course is placed between the existing pavement and overlay to assure there is no bonding and to Another method to reduce reflective cracking absorb any movement in the base slab to prevent in asphalt overlays of rigid pavements is to frac- cracking of the overlay. Unbonded overlays are ture the underlying slabs into pieces 2 to 3 feet thicker and range from 8 to 10 inches [200 to [600 to 1000 mm] in size. After cracking the 250 mm] in thickness. slab, a heavy roller is used to ensure the slabs are fully seated before the asphalt overlay is All types of overlays normally require a fair- placed. ly detailed design analysis. The analysis involves determining the initial construction and Reflective cracking leads to increased infil- pavement information, a pavement distress sur- tration of water into the pavement structure that vey, existing layer properties, future traffic anal- rapidly deteriorates the overlay. This creates ysis, existing pavement subgrade properties, potholes and other distresses. Treatments to mi- overlay design properties, determining effective nimize reflective cracking include the use of structural capacity, future overlay structural ca- reinforcing fabrics, stress-relieving interlayers of pacity, desired remaining service-life, and re- rubber-asphalt with aggregate chips, crack- quired overlay thickness design. arresting interlayers of large aggregate bituminous material to blunt the cracks, and extensive pre-overlay repairs including determining existing joint and major crack locations, sealing them and saw cutting after paving.

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