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NATIONAL ASPHALTPAVEMENT ASSOCIATION

Information Series 132

Rubblization

Design and Construction Guidelines on Rubblizing and Overlaying PCC Pavements with Hot-Mix Asphalt

National Asphalt Pavement Association5100 Forbes Boulevard Lanham, Maryland 20706-4407

888-468-6499 (toll free) 301-731-4748 301-731-4621h [email protected] www.asphaltpavement.org

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This publication is designed to provide information of interest to NAPA members and is not to be considered a publication of standards or regulations. The views of the author expressed herein does not necessarily reflect the decision making process of NAPA with regard to advice or opinions on the merits of certain processes, procedures or equipment.

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© 2006 National Asphalt Pavement Association

Information Series 132

Printed 1/06

NATIONAL ASPHALT PAVEMENT ASSOCIATION

NAPA Building ■ 5100 Forbes Blvd. ■ Lanham, MD 20706-4407 Toll Free: 888-468-6499 ■ Tel: 301-731-4748 ■ Fax: 301-731-4621

www.hotmix.org ■ [email protected]

RubblizationDesign and Construction Guidelines

on Rubblizing and Overlaying PCC Pavements with Hot-Mix Asphalt

ByDale Decker, P.E.

CONTENTS

1. Introduction __________________________________________________________ 5

2. Reflection Cracking ____________________________________________________ 6

3. Project Evaluation _____________________________________________________ 8 Evaluation of the Existing Structure ____________________________________ 8 Distress Survey ____________________________________________________8 Existing Pavement Structure __________________________________________8 Subsurface Conditions ______________________________________________9 Non-Destructive Testing _____________________________________________9 Drainage _________________________________________________________9 Project Evaluation Report ____________________________________________9

4. HMA Overlay Thickness Design _________________________________________ 10 Level I Approach ___________________________________________________ 11 Identify PCC Thickness _____________________________________________11 Select Design Traffic Category _______________________________________11 Select Subgrade Soil Category _______________________________________11 Compute Existing Subbase Layer Structural Number ______________________ 11 Select Appropriate HMA Overlay Thickness Table and Read Overlay _________ 14 Level II Approach ___________________________________________________ 16 ESAL: Design Traffic Value __________________________________________ 16 Mr: Design Subgrade Modulus _______________________________________ 16 SNsb: Subbase Layer Structural Number _______________________________ 16 Level II Graphical Solution for Thickness Design ________________________ 16 Level III PerRoad Layer Elastic Analysis ________________________________ 23 Inputs __________________________________________________________23 Traffic ___________________________________________________________23 Structure ________________________________________________________23 Analysis _________________________________________________________23

5. Rubblization Equipment _______________________________________________ 25 Multi-Head Breaker _________________________________________________ 25 Resonant Frequency Breaker ________________________________________ 25 Other Equipment ___________________________________________________ 26

6. Construction Operations for Rubblization ____________________________________27 Surface Preparation of the Existing Pavement ______________________________27 Drainage _____________________________________________________________27 Location of Utilities and Underground Structures ___________________________27 Rubblization Recommendations __________________________________________27 Compaction after Rubblization ___________________________________________28 Troubleshooting Rubblizing Operations ____________________________________28 Quality Control Issues for Rubblization ____________________________________29

7. Placing the HMA Overlay __________________________________________________30

8. User Benefits to PCC Rubblization and Conclusion ____________________________31 Rubblization Advantages ________________________________________________31 Rubblization Performance _______________________________________________31 Conclusion ___________________________________________________________31

Bibliography _______________________________________________________________32

6 NATIONAL ASPHALT PAVEMENT ASSOCIATION

RUBBLIZATION • IS 132 7

RubblizationDesign and Construction Guidelines

on Rubblizing and Overlaying PCC Pavements with Hot-Mix Asphalt

joint or crack. The reflection cracking problem must be addressed in the HMA overlay design phase if long-term performance of the overlay is to be achieved.

The best way to control reflection cracking in an HMA overlay over a PCC pavement is to fracture the slabs prior to placement of the HMA overlay. “Slab fractur-ing” techniques have proven to be an excellent method for preparation of the PCC pavement prior to overlay with HMA. NAPA’s publication Guidelines for Use of HMA Overlays to Rehabilitate PCC Pavements (IS-117), provides an exhaustive review of all slab fractur-ing techniques. The information presented in IS-117 is based on a comprehensive national study performed by PCS/Law in 1991. Slab fracturing can be accomplished by crack/seat, break/seat, and rubblization processes.

Rubblization can be used to eliminate or significantly reduce reflection cracking in HMA overlays placed on PCC. This process is normally achieved by rubblizing the slab into fragments, resulting in destruction of the existing slab action of the PCC pavement. Temperature and/or reinforcing steel, if present in the PCC pavement, is generally fully debonded from the concrete by this approach. The rubblization process is applicable to all types of PCC pavements.

This publication is intended as a companion publica-tion to IS-117. Its objective is to provide updated design and construction guidelines specific to the PCC rubbliza-tion process. A procedure is presented for determining the required thickness of an HMA overlay placed over rubblized PCC slabs, based on the mechanistic empirical design procedures.

1. Introduction

Rehabilitation of existing pavements is one of the greatest pavement priorities facing local, state, and fed-eral transportation agencies. The use of hot-mix asphalt (HMA) overlays presents a long-term and economical solution to the pavement rehabilitation challenge. HMA overlays increase the structural capacity of the existing pavement system and improve the long-term functional pavement performance including ride, noise reduction, splash and spray, friction, and general appearance.

In many respects, the rehabilitation of pavement systems is a more complex engineering task than the design of new pavement systems. Pavement reha-bilitation requires significant engineering judgment in the evaluation process. The engineer must define the problem, develop potential problem solutions and then select the preferred solution. Rehabilitation of PCC pavements can be accomplished by concrete pavement restoration (CPR), reconstruction and by resurfacing. Due to the expense, time and traffic delay involved in CPR and reconstruction, resurfacing of PCC pavements with an HMA overlay is a very appealing option for many agencies.

However, existing, worn-out PCC pavements pres-ent a particular problem for rehabilitation due to the likelihood of reflection cracking when an HMA overlay is used. Horizontal and vertical movements occur-ring within the underlying PCC layer cause reflection cracking. Reflection cracking can occur at any PCC


2. Reflection Cracking

Reflection cracking can occur in an HMA overlay over any joint or crack in the PCC pavement. The current state-of-the-technology does not provide accurate meth-ods to predict the occurrence and growth of reflection cracks. The National Cooperative Highway Research Program (NCHRP) Project 1-41, Selection, Calibra-tion, and Validation of a Reflective Cracking Model for Asphalt Concrete Overlays began in 2003 to select, calibrate, and validate a model for incorporation in the future AASHTO design guide. Figure 2-1 schematically illustrates reflection crack distress in an HMA overlay placed over a joint or crack of an existing PCC slab. Figure 2-2 illustrates the mechanism through which the crack develops and propagates in the HMA layer.

Stresses and strains at the bottom of the HMA overlay are caused by horizontal movement of the PCC slabs due to temperature changes, moisture changes, and vertical movement caused by traffic loads. (2.2A). These stresses and strains at the bottom of the HMA overlay will eventu-ally cause the development of a microcrack at the bottom of the HMA overlay (2.2B). With time, this microcrack will grow and eventually reflect upwards to the surface of the HMA overlay (2.2C and D). As temperature and loading cycles continue, multiple cracks will form and

eventually result in significant deterioration of the HMA surface (2.2E and F). Figure 2.3 illustrates a distressed reflection crack area in an HMA overlay over an existing PCC pavement.

A variety of techniques have been used over the years in an attempt to eliminate reflection cracking in HMA overlays. These approaches include: sawing and sealing

Figure 2.1 Reflection crack distress

Figure 2.2 Growth mechanism associated with reflection cracking

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the HMA overlay; use of thick HMA overlays; instal-lation of crack relief layers (including stress absorbing interlayer materials); use of modified asphalt HMA overlay materials; and slab fracturing prior to HMA overlay.

Of the slab fracturing techniques, rubblizing has proven to be one of the most economical and successful ways to eliminate reflection cracking. The underlying principle of this approach is to significantly reduce the effective slab length of the PCC pavement by fracturing the slab into small fragments and destroying the bond between reinforcing/temperature steel and concrete. The reduction of the effective slab length will result in minimal horizontal movements at joints and cracks due to temperature and moisture changes. This greatly minimizes the tensile and shear forces normally occur-ring at the bottom of the HMA overlay.

Slab fracturing is tempered/balanced by the need to conserve structural support. The modulus of a fractured PCC pavement(E

PCC) is a measure of structural support

and is an important parameter in the design of HMA overlays on rehabilitated PCC systems. The greater the degree of slab fracturing and steel-concrete debond-ing achieved in the construction process, the lower the modulus E

PCC, and hence structural support. Thus, the

effective modulus of a fractured slab is a function of the nominal fragment size or crack spacing actually achieved in the slab fracturing process.

Figure 2.4 illustrates the relationship of the fractured slab modulus (EPCC) to both func-tional distress caused by reflection cracking and structural requirements of the HMA over-lay. As the fractured PCC modulus decreases (slab becomes more intensely fractured), the likelihood of having reflection cracking problems in the HMA overlay is significantly reduced. However, as the fractured PCC modulus decreases, the structural capacity of the fractured PCC slabs also decreases, re-quiring a thicker HMA overlay. The ultimate goal is to reduce the E

PCC value to a minimum

or critical value such that reflection cracking will not occur, but not so low a value that the capacity of the fractured slab is reduced to a point where an excessive HMA overlay thick-ness is required.

Rubblizing has been used extensively by many states in the last 20 years. In general, field performance of HMA overlays on rubblized slabs has been found to be good to excellent.

Figure 2.3 Reflection cracking in HMA overlay of PCC pavement (courtesy Antigo Construction, Inc.)

Figure 2.4 Influence of PCC fractured modulus upon structural and reflection crack failure

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The field-proven success and growing use of this rehab approach indicates that this technique no longer needs to be considered as research or experimental in nature.


3. Project Evaluation

Every rough, worn-out PCC pavement may not be a candidate for rubblization with an HMA overlay. A structural evaluation of the existing pavement including considerations for traffic, subgrade, and environmental conditions must be performed. Condition surveys of the existing pavement are important to understand the types, severity, and extent of distresses and their likely causes.

It is vital to understand the soil and moisture conditions for the pavement system prior to making a decision on the rehabilitation type. These steps are impera-tive to determine if the specific pavement is an appropriate candidate for rubblization. However, most PCC pavements can be rub-blized in an appropriate manner and overlaid with HMA.

Evaluation of the Existing Structure

As with any pavement overlay project, it is necessary to know the existing condition of the pavement. It may be that existing conditions are so poor that nothing short of removal and replacement is appropriate. These are decisions that must be made by the design engineer, given an appropriate engineering evaluation of the project.

The key elements of the evaluation are:• Perform visual condition survey to define the type, amount and severity of distresses. • Cracking (corner, mid-slab, fatigue, etc.)• Amount and type of patching• Joint deficiencies• Surface defects• Miscellaneous distresses• Evaluate existing pavement structure• Layer types (materials and strengths)• Layer thickness• Drainage• Shoulder condition• Determine soil conditions.• Soil types• Bearing value (modulus)• Moisture condition

Distress SurveyIn order to evaluate long-term performance of the

pavement system, it is critical that the pre-construction condition be known. Using the Distress Identification Manual for the Long-Term Pavement Performance Project (FHWA-RD-03-031), evaluate the condition of the existing pavement. Each type of distress should be identified, along with the relative extent and severity of the distress. The LTPP document describes the following distresses for PCC pavements:

Existing Pavement StructureThrough a process of coring and/or trenching, evaluate

the existing pavement structure. The thickness of each existing layer, the material type, and condition should be determined. These data are important for the design of the new pavement system.

A sampling plan must be developed that will provide an appropriate overview of the pavement section to be rehabilitated. As a minimum, two core samples should be taken randomly per lane mile. Core locations should be in representative cut and fill locations and staggered between lanes. Any areas of obvious structural distress should be evaluated.

The condition of the pavement shoulder must also be evaluated if traffic will be routed onto it while adjacent

Jointed PCC distresses CRC* distressesCracking Cracking Corner breaks “D” cracking “D” cracking Longitudinal cracking Longitudinal cracking Transverse cracking Transverse cracking Surface defectsJoint deficiencies Map cracking/scaling Joint seal damage Polished aggregate Spalling of joints PopoutsSurface defects Miscellaneous distresses Map cracking/scaling Blowups Polished aggregate Transverse joint deterioration Popouts Lane-to-shoulder drop offMiscellaneous distresses Lane-to-shoulder separation Blowups Patch deterioration Faulting of transverse joints Punchouts Lane-to-shoulder drop off Spalling of longitudinal joints Lane-to-shoulder separation Water bleeding and pumping Patch deterioration Longitudinal joint seal damage Water bleeding and pumping

* continuously reinforced concrete


lanes are under construction. The shoulders will need to be able to carry the traffic loading during the construc-tion process.

As an example of evaluation criteria, Wisconsin DOT considers the rubblization process when one or more of the following conditions are met:• Greater than 20% of the concrete pavement joints are in need of repair• Greater than 20% of the concrete surface has been patched• Greater than 20% of the concrete slabs exhibit slab breakup distress• Greater than 20% of the project length exhibits longitudinal joint distress greater than 4” wide.

Subsurface ConditionsAfter the coring or trenching has been completed,

testing of the subsurface materials, base, subbase and soil should be performed to determine the structural adequacy of the foundation material. Field tests such as dynamic cone penetrometer (DCP) and field California Bearing Ratio (CBR) have been used to characterize the materials. Laboratory testing may be performed on undisturbed samples for fine-graded materials or re-compacted materials for coarser materials to determine modulus values, CBR or R-Value. Moisture content of the in-situ materials should also be determined. From the field data, typical values for the project can be de-veloped.

The Illinois Department of Transportation (IDOT) recommends splitting the top 12 inches of the subgrade into two equal layers, determining the DCP for each layer, and using the average of the two values to deter-mine the type of rubblization method to be used. The selection of rubblization method will be discussed later in this publication.

The soil condition survey will provide the designer with data to make decisions regarding the rehabilitation process. If very soft subgrades are noted, it may be nec-essary to limit the extent of the rubblization or in some cases, change the processing to another rehabilitation technique such as Break and Seat or Crack and Seat.

Non-Destructive TestingFalling Weight Deflectometer (FWD) testing is a

non-destructive testing tool for evaluating pavement structures. Modulus values for different layers may be calculated from deflection data. Many locations may be tested with FWD equipment in a short time, giving a more complete picture of material properties along the project length.

Another non-destructive tool that may be used in evaluating pavement sections is ground penetrating radar (GPR). GPR is useful in determining variations in layer thicknesses and depths and locations of under-ground utilities. Fluctuations in soil moisture may also be detected with GPR.

FWD and GPR should always be used in conjunction with coring and sampling of materials. This is important to gain what is often referred to as “ground truth” to cali-brate the systems. However, the amount of coring and sampling can be significantly reduced while increasing the amount of useful data.Drainage

Surface and subsurface drainage for the project should be evaluated. For surface drainage, look for areas that allow water to pond next to the roadway. Also evaluate the cross slope of the existing pavement to determine if corrections are necessary. If edge drains are present, they should be evaluated to determine they are not clogged and operating properly. If edge drains are not present, the site soil conditions should be evaluated to determine if adding edge drains would be beneficial.Project Evaluation Report

To aid in the preparation of plans and specification, additional information should be included in a project evaluation report including comments on the mate-rial conditions at the time of sampling, clearances for overhead items for the project, location of utilities and culverts in the pavement, location of any buildings within 50 feet of the pavement to be rubblized, and the location and condition of any underdrains in the pavement.

Rubblized concrete base with HMA overlay.


4. HMA Overlay Thickness Design

The overlay thickness design process in this publica-tion is based on mechanistic empirical design principles, whereas the procedure used in IS-117, Guidelines for Use of HMA Overlays to Rehabilitate PCC Pavements was based on the structural capacity deficiency approach.

The difference between these approaches are that the AASHTO guide relies on empirical correlations with past performance and models developed from experience or observations of past performance. In this procedure a structural number is determined based on traffic and soil properties. The thickness of the various pavement layers is then determined by layer coefficients and thickness for the different materials used in the pavement structure.

While this procedure has served us well for many years, it cannot accurately account for traffic loadings and material properties beyond the observed conditions used to develop the models. Mechanistic empirical de-signs are based on engineering properties of the materi-als and their calculated responses to loading. Stresses and strains may be calculated at various depths in the pavement structure. These stresses and strains can then be related to performance based on empirical relation-ships. The advantage to this procedure is that we can calculate pavement responses to different loading situ-ations and material properties and relate this response to performance.

The design procedure recommended in this publica-tion was developed using the PerRoad software which is available from the Asphalt Pavement Alliance for the design of Perpetual Pavements. The PerRoad software is a layer/elastic software that can calculate stresses and strains in different pavement layers. Of key interest to pavement designers are the horizontal tensile strain at the bottom of the asphalt layer and the vertical compressive strain at the top of the subgrade. The horizontal tensile strain at the bottom of the asphalt layer has been shown to be related to alligator or fatigue cracking in the HMA. Vertical compressive strain at the top of the subgrade is related to permanent deformation deep in the pavement structure.

NAPA’s IS-117 describes three different levels of engineering evaluation, Level I, Level II, and Level III. The concept is that the level of engineering effort for the evaluation process should be consistent with the relative importance and the cost of the project. An increasing engineering evaluation effort is required from Level I

to Level III. As a result, the procedure recommended in this publication uses a similar classification. For a Level I approach the designer would: • Estimate subgrade support values (resilient

modulus, Mr) based on soil classifications,

other test values such as CBR and R-value, and charts

• Estimate base structural number• Estimate traffic based on general road classifica-

tionsFor the Level II approach, more precise data would

be collected through field investigations and laboratory testing to determine subgrade support, base structural number, and traffic loading. For Level III design, mecha-nistic empirical design procedures such as the PerRoad analysis design software and would be used to determine the overlay thickness.

Level I is the most direct and simplest solution to the determination of an HMA overlay thickness. Sim-plifying assumptions are made to establish “typical” overlay variables. Subgrade support and traffic are expressed in subjective categories rather than requiring the selection of a specific value. This simplified Level I approach leads to a set of tables to provide a recom-mended overlay thickness for different combinations of design conditions. In general, a Level I analysis would be expected to generate a thicker, more conservative HMA overlay.

Level II requires an enhanced engineering effort to select appropriate input values for the variables used in the HMA overlay thickness determination. The engineer must select specific design values for:

• Subgrade support

• Resilient modulus (Mr)

• Design traffic repetitions

• Axle loadings (ESAL)

• Structural layer coefficients

• Existing subbase layers (asb

)

The solution for the Level II approach is accom-plished through a set of graphs developed using data from the PCS Law Study and PerRoad software.

The Level III overlay thickness determination repre-sents the most detailed solution approach. This solution requires the use of the PerRoad software to determine the required overlay thickness. The use of this software requires input of structural values, modulus values, and Poisson’s ratio for each pavement layer. Other inputs include traffic as a load spectra and failure criteria. Us-


Table 4.1 Level I Design Traffic Categories

Traffic Category Low Medium Heavy Very Heavy

Design ESAL Value < 5 x106 5 x106 – 107 107 – 20x107 > 20x107

ing this software will also allow the user to design the overlay as a Perpetual Pavement

Level I ApproachThe Level I overlay thickness determination involves

five major steps. They are:

Step 1: Identify PCC thickness

Step 2: Select appropriate traffic category

Step 3: Select subgrade soil category

Step 4: Compute existing subbase layer structural number

Step 5: Select appropriate HMA overlay table and read overlay thickness

Step 1: Identify PCC ThicknessInformation on the original PCC pavement type as

well as thickness can generally be determined from his-torical records. It is always wise to confirm the as-built thickness of the PCC with core test results. Step 2: Select Design Traffic Category

General estimates of the future equivalent 18,000 pound single axle load (ESAL) repetitions for the overlay life period are used for Level I. Table 4.1 indicates the four general traffic categories used and typical design traffic ranges for each category. The engineer must select one of these four traffic categories (low, medium, heavy, and very heavy) to proceed with the HMA overlay analysis.Step 3: Select Subgrade Soil Category

The engineer is required to assess the existing sub-grade support within one of four subgrade soil groups (poor, medium, good, and excellent). Figure 4.1 presents information to assist the engineer in the selection. The four subgrade support categories and typical ranges of resilient modulus, California Bearing Ratio, resistance value and soil classification groupings for both AASHTO

and unified soil classifications systems are included in the figure.Step 4: Compute Existing Subbase Layer Structural Number

For each subbase layer under the existing PCC pave-ment, the structural contribution of these layers must be evaluated by computing the combined SN

sb value.

Subbase layers are generally of two major types:Treated subbase Cement treated Asphalt treated Lime treatedUnbound granular Crushed stone Sand/gravelFor the unbound granular subbase materials, the

engineer must make an appropriate decision regarding the drainability (after the overlay has been placed) of these materials. This is accomplished by subjectively categorizing the anticipated drainage condition into one of three categories (excellent, fair, or very poor).

For the treated subbase materials, the engineer must evaluate the general condition of the stabilized layer prior to the rubblization process. The two categories are good-fair and poor-very poor. The severity of deteriora-tion such as cracking in a cement-treated subbase and the amount of moisture damage in an asphalt-treated base are issues that will determine the condition category.

The structural number (SN) is determined for each subbase layer and added together to produce the com-bined SN for the subbase layers. Figures 4.2 through 4.5 allow the designer to determine a SN value for each of the material types. The user must identify each sub-base layer present; determine the layer thickness (h) for each layer; determine the SN for each layer from the appropriate figure; and add the individual SN values to determine the combined SN

sb.

(PCS/Law, Guidelines and Methodologies for the Rehabilitation of Rigid High-way Pavements Using Asphalt Concrete Overlays, 1991.)


Figure 4.1 Typical subgrade soil categories

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Figure 4.2Asphalt treated base

Figure 4.5Sand/gravel base

Figure 4.3Cement treated base

Figure 4.4Crushed stone base

Asphalt Treated Base

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Good to Fair Condition Poor to Very Poor Condition

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Figure 4.6 illustrates the use of the thickness tables. The example shown is for the following conditions:

PCC Type: JPCP (Jointed Plain Concrete Pavement)Fracture Mode: RubblizationPCC Thickness: 8.0 inchesDesign Traffic Category: Heavy TrafficSubgrade Soil Category: Medium Subgrade SupportTotal Subbase SN: SN

sb = 1.2

HMA Overlay Thickness = 7.0 inches

Step 5: Select Appropriate HMA Overlay Thickness Table and Read Overlay Thickness.

From steps 1 through 4, the design engineer has identified the PCC type and thickness, the design traffic category, the subgrade soil classification, and the total subbase SN. The next and final step is to determine the overlay thickness required. For the Level I analysis, the overlay thickness is obtained from a series of tables for different pavement types and conditions.

Figure 4.6HMA overlay thickness example

Table 4.2 (see page 15) provides overlay thickness design information for all concrete pavements including jointed plain concrete pavements (JPCP), jointed rein-forced concrete pavements (JRCP), and continuously reinforced concrete pavements (CRCP). The values in the tables already incorporate recommended minimum HMA overlay thickness as a function of traffic. The minimum values shown in these tables are different

than the values given in IS-117. Ten years of experience since the publication of IS-117 have shown that, in spite of the fact that the minimum values may have satisfied theoretical structural requirements, some of the minimum values in IS-117 were too thin. General industry opinion today is that the minimum overlay thickness should be 5 inches with some exceptions for low-volume roads.

Required HMA Overlay Thickness (inches)

Existing Structural PCC slab number Medium Traffic Heavy Traffic thickness subbase

H(pcc) Total Subgrade Soil Category Subgrade Soil Category

(in.) SNsb Poor Med Good Exc Poor Med Good Exc

7 0 10.00 7.50 6.00 6.00 12.00 9.00 7.00 7.00

7 0.4 10.00 7.50 6.00 6.00 12.00 9.00 7.00 7.00

7 0.8 9.50 6.50 6.00 6.00 11.00 8.00 7.00 7.00

7 1.2 9.00 6.00 6.00 6.00 10.50 7.00 7.00 7.00

7 1.6 8.50 6.00 6.00 6.00 9.50 7.00 7.00 7.00

7 2.0 7.50 6.00 6.00 6.00 9.00 7.00 7.00 7.00

8 0 9.00 6.50 6.00 6.00 10.50 8.00 7.00 7.00

8 0.4 9.00 6.50 6.00 6.00 10.50 8.00 7.00 7.00

8 0.8 9.00 6.00 6.00 6.00 10.50 7.00 7.00 7.00

8 1.2 8.00 6.00 6.00 6.00 9.50 7.00 7.00 7.00

8 1.6 7.00 6.00 6.00 6.00 9.00 7.00 7.00 7.00

8 2.0 6.00 6.00 6.00 6.00 8.00 7.00 7.00 7.00

9 0 7.50 6.00 6.00 6.00 9.50 7.00 7.00 7.00

9 0.4 7.50 6.00 6.00 6.00 9.50 7.00 7.00 7.00

HMA Overlay Thickness = 7 inches


7 0 8.50 6.00 5.00 5.00 10.00 7.50 6.00 6.00 12.00 9.00 7.00 7.00 15.50 12.50 9.50 8.00 7 0.4 8.50 6.00 5.00 5.00 10.00 7.50 6.00 6.00 12.00 9.00 7.00 7.00 15.50 12.50 8.50 8.00 7 0.8 8.00 5.00 5.00 5.00 9.50 6.50 6.00 6.00 11.00 8.00 7.00 7.00 14.50 11.50 8.00 8.00 7 1.2 7.00 5.00 5.00 5.00 9.00 6.00 6.00 6.00 10.50 7.00 7.00 7.00 14.50 10.50 8.00 8.00 7 1.6 6.50 5.00 5.00 5.00 8.50 6.00 6.00 6.00 9.50 7.00 7.00 7.00 13.5 9.50 8.00 8.00 7 2.0 5.50 5.00 5.00 5.00 7.50 6.00 6.00 6.00 9.00 7.00 7.00 7.00 13.5 8.00 8.00 8.00

8 0 7.00 5.00 5.00 5.00 9.00 6.50 6.00 6.00 10.50 8.00 7.00 7.00 14.00 11.50 8.50 8.00 8 0.4 7.00 5.00 5.00 5.00 9.00 6.50 6.00 6.00 10.50 8.00 7.00 7.00 14.00 11.00 8.00 8.00 8 0.8 7.00 5.00 5.00 5.00 9.00 6.00 6.00 6.00 10.50 7.00 7.00 7.00 14.00 10.50 8.00 8.00 8 1.2 6.50 5.00 5.00 5.00 8.00 6.00 6.00 6.00 9.50 7.00 7.00 7.00 13.50 9.50 8.00 8.00 8 1.6 5.50 5.00 5.00 5.00 7.00 6.00 6.00 6.00 9.00 7.00 7.00 7.00 13.00 9.00 8.00 8.00 8 2.0 5.00 5.00 5.00 5.00 6.00 6.00 6.00 6.00 8.00 7.00 7.00 7.00 12.00 8.00 8.00 8.00

9 0 6.00 5.00 5.00 5.00 7.50 6.00 6.00 6.00 9.50 7.00 7.00 7.00 13.00 10.5 8.00 8.00 9 0.4 6.00 5.00 5.00 5.00 7.50 6.00 6.00 6.00 9.50 7.00 7.00 7.00 13.00 10.00 8.00 8.00 9 0.8 6.00 5.00 5.00 5.00 7.50 6.00 6.00 6.00 9.50 7.00 7.00 7.00 13.00 9.50 8.00 8.00 9 1.2 5.00 5.00 5.00 5.00 6.50 6.00 6.00 6.00 8.50 7.00 7.00 7.00 12.50 8.50 8.00 8.00 9 1.6 5.00 5.00 5.00 5.00 6.00 6.00 6.00 6.00 8.00 7.00 7.00 7.00 12.00 8.00 8.00 8.00 9 2.0 5.00 5.00 5.00 5.00 6.00 6.00 6.00 6.00 7.00 7.00 7.00 7.00 11.00 8.00 8.00 8.00

10 0 5.00 5.00 5.00 5.00 6.50 6.00 6.00 6.00 8.50 7.00 7.00 7.00 12.00 9.50 8.00 8.00 10 0.4 5.00 5.00 5.00 5.00 6.50 6.00 6.00 6.00 8.50 7.00 7.00 7.00 12.00 9.50 8.00 8.00 10 0.8 5.00 5.00 5.00 5.00 6.50 6.00 6.00 6.00 8.50 7.00 7.00 7.00 12.00 8.50 8.00 8.00 10 1.2 5.00 5.00 5.00 5.00 6.00 6.00 6.00 6.00 7.50 7.00 7.00 7.00 11.5 8.00 8.00 8.00 10 1.6 5.00 5.00 5.00 5.00 6.00 6.00 6.00 6.00 7.00 7.00 7.00 7.00 11.00 8.00 8.00 8.00 10 2.0 5.00 5.00 5.00 5.00 6.00 6.00 6.00 5.00 7.00 7.00 7.00 7.00 10.00 8.00 8.00 8.00

11 0 5.00 5.00 5.00 5.00 6.00 6.00 6.00 6.00 7.50 7.00 7.00 7.00 11.00 8.50 8.00 8.00 11 0.4 5.00 5.00 5.00 5.00 6.00 6.00 6.00 6.00 7.50 7.00 7.00 7.00 11.00 8.50 8.00 8.00 11 0.8 5.00 5.00 5.00 5.00 6.00 6.00 6.00 6.00 7.00 7.00 7.00 7.00 10.00 8.00 8.00 8.00 11 1.2 5.00 5.00 5.00 5.00 6.00 6.00 6.00 6.00 7.00 7.00 7.00 7.00 10.00 8.00 8.00 8.00 11 1.6 5.00 5.00 5.00 5.00 6.00 6.00 6.00 6.00 7.00 7.00 7.00 7.00 9.00 8.00 8.00 8.00 11 2.0 5.00 5.00 5.00 5.00 6.00 6.00 6.00 6.00 7.00 7.00 7.00 7.00 8.50 8.00 8.00 8.00

12 0 5.00 5.00 5.00 5.00 6.00 6.00 6.00 6.00 7.00 7.00 7.00 7.00 10.00 8.00 9.50 8.00 12 0.4 5.00 5.00 5.00 5.00 6.00 6.00 6.00 6.00 7.00 7.00 7.00 7.00 10.00 8.00 8.50 8.00 12 0.8 5.00 5.00 5.00 5.00 6.00 6.00 6.00 6.00 7.00 7.00 7.00 7.00 10.00 8.00 8.00 8.00 12 1.2 5.00 5.00 5.00 5.00 6.00 6.00 6.00 6.00 7.00 7.00 7.00 7.00 10.00 8.00 8.00 8.00 12 1.6 5.00 5.00 5.00 5.00 6.00 6.00 6.00 6.00 7.00 7.00 7.00 7.00 9.00 8.00 8.00 8.00 12 2.0 5.00 5.00 5.00 5.00 6.00 6.00 6.00 6.00 7.00 7.00 7.00 7.00 8.00 8.00 8.00 8.00

Table 4.2All concrete pavements – fracture mode: rubblizationRequired HMA Overlay Thickness (inches)

Existing Structural PCC slab number Low Traffic Medium Traffic Heavy Traffic VERY HEAVY TRAFFIC thickness subbase H(pcc) Total Subgrade Soil Category Subgrade Soil Category Subgrade Soil Category Subgrade Soil Category (in.) SNsb Poor Med Good Exc Poor Med Good Exc Poor Med Good Exc Poor Med Good Exc


Level II ApproachThe solution to the HMA overlay thickness deter-

mination used in the Level II approach is an enhanced engineering analysis of the Level 1 tabular solution pro-cedure. In the Level II approach, the engineer is required to determine or select specific design input values for the following variables:

ESAL: Design traffic valueh

PCC = PCC slab thickness (inches)

Mr: Design subgrade modulus

SNsb

: Subbase layers structural numbersESAL: Design Traffic Value

The design traffic for Level II analysis is based upon the expected equivalent single axle loads (ESAL) antici-pated during the design period for the overlay. ESAL is a widely used and accepted industry standard for quan-tifying traffic loads. The values used in the graphs are million equivalent single axle loads (MESAL).Mr: Design Subgrade Modulus

The subgrade support is characterized by the resilient modulus parameter, M

r. It is difficult to use lab results

of resilient modulus tests directly into the solution pro-cedure. Correlations between conventional subgrade design parameters such as CBR and R-value to the resilient modulus value of subgrade soils have been established. Figure 4.3 illustrates these correlations. However, some agencies are developing experience and confidence in performing resilient modulus testing. If lab or field (FWD) estimated resilient modulus data are available that represent a cross section of materials for the project, they may be used in lieu of the correlation to other properties. It is recommended that the correlation be used to verify the lab test properties.SNsb: Subbase Layer Structural Number

The structural number of the subbase (SNsb

) is the sum of the structural number for each layer of subbase. This structural number is determined by multiplying the structural layer coefficient of the material (a

sb) by the layer thickness in inches.

Detailed guidance for the selection of these val-ues is contained in the AASHTO Guide for the Design of Pavement Structures. For unbound granular layers, it is important to adjust the a

sb by

the AASHTO drainage coefficients, msb

. For treated subbase layers, difficulties arise in

the selection of an appropriate design asb

value. The engineer must evaluate the probable loss of structural capacity in the original (as-built) pave-ment layer due to subsequent damage incurred

during the previous performance life of the pavement. It is likely that some additional damage will occur to the stabilized layer during the rubblization process. As a result, typical values of a

sb for cement and asphalt treated

materials used in new construction must be reduced ac-cordingly to compensate for possible loss of strength. If specific information is not available, the engineer can use the procedure discussed for Level I to determine the Structural Number of the subbase layers.

Level II Graphical Solution for Thickness Design

The graphical solution to overlay thickness design provides a simple method with minimal input require-ments. To determine the overlay thickness:• Select the appropriate chart based on the thickness of

the rubblized concrete and structural number of the subbase (SN

sb).

• Draw a vertical line upward from the subgrade modu-lus value until it intersects the traffic value.

• Draw a horizontal line from this intersection to the y-axis, and read the overlay thickness required.

Figure 4.7 illustrates the use of the graphical solution.

The example shown is for the following conditions:PCC type: JPCP (jointed plain concrete pavement) Fracture mode: RubblizationPCC thickness: 8.0 inchesDesign traffic: 50 MESALSubgrade modulus: 7 ksiTotal subbase SN: SN

sb = 1.2

HMA Overlay Thickness = 8.5 inches

Figure 4.7Example Level II design

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Figure 4.8Level II overlay design charts 7” rubblized PCC

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Figure 4.12Level II 0verlay design charts 11” rubblized PCC

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Level III PerRoad Layer Elastic AnalysisThe Level III design requires the use of PerRoad

software which is available for download from the As-phalt Pavement Alliance at www.asphaltalliance.com. PerRoad is a mechanistic-based procedure for the design of flexible long-life or Perpetual Pavement structures. The procedure was developed at the National Center for Asphalt Technology (NCAT) at Auburn University in conjunction with the Asphalt Pavement Alliance (APA).

The design software utilizes layered elastic theory to compute critical pavement responses under axle load spectra. Monte Carlo simulation is used to model the uncertainty corresponding to material, loading, and construction variability. The program can be used as a design and analysis tool to assess the likelihood that critical pavement responses will exceed a threshold set by the analyst. Additionally, transfer functions may be used to determine a damage accumulation rate for pave-ment responses exceeding the threshold.

The following is a description of some of the inputs required for the software. A detailed description of the required inputs and how to run the software is available from the Asphalt Pavement Alliance web site.

TrafficTraffic loading is input by load spectra, which breaks

down the traffic loads by axle types and axle weights. Load spectra may be obtained from FHWA W4 tables or from default values provided with the software.Structure

Material properties for two to five layers (includ-ing the subgrade) can be analyzed with this software. Material property inputs include resilient modulus (M

r),

Poisson’s ratio, and variability. Material properties for each layer can also be changed for up to five seasons to account for variations due to temperature and moisture throughout the year. Typical values of resilient modulus and Poisson’s ratio for a variety of materials, includ-ing rubblized concrete, may be found in the PerRoad Guide.

A key element of this software is that performance criteria are used to calculate accumulated damage from the traffic. While the user can select any performance criteria desired, typically horizontal tensile strain at the bottom of the asphalt and vertical compressive strain at the top of the subgrade are generally used for flexible pavements. For Perpetual Pavements another key ele-ment of the performance criteria is limiting strains. A limiting strain is a strain value which if not exceeded is

assumed to result in no damage to the pavement struc-ture. For HMA pavements the recommended limiting strain criteria are:

Fatigue: Horizontal tensile strain at bottom of asphalt = 70 x 10-6

Rutting: Vertical compressive strain at top of subgrade = 200 x 10-6

Transfer functions are used to equate strain levels to damage. A number of transfer functions have been proposed by researchers. Transfer functions used in the software are:

1 k2

Fatigue: Nf = k

1 [ —]

1 k2

Rutting:: Nf = k

1 [—]

v

Nf = Number of load cycles to failure

k1, k

2 = Constants

t = Tensile strain at the bottom of the asphalt layer

v = Vertical strain at the top of the subgrade layer

Constants used in the transfer functions to develop the Level II design graphs were:

Fatigue: k1 = 2.83 x10-6

k2 = 3.15

Rutting: k1 = 6.03x10-8

k2 = 3.87

These values were determined for pavements at the Minnesota Road Research Project that showed fatigue and deep rutting distresses, and they may vary according to soil, climate, materials and traffic.Analysis

After all the data have been entered for traffic loads and structure, the analysis may be performed. Either a deterministic or probabilistic analysis may be run.

If the deterministic analysis is selected, the program will run through the seasons and loads that have been input. If the limiting strain values have been exceeded, the program will indicate that the structure does not meet the criteria and what the worst case pavement response is. This only indicates that the limiting strain criteria have

t


been exceeded by one or more loads for the seasonal material strength value(s). It does not necessarily mean that it should not be considered a Perpetual Pavement. However, it may be used as a quick check before run-ning a probabilistic analysis to see if the limiting strain criteria are greatly exceeded.

A probabilistic analysis needs to be run to truly evalu-ate the pavement structure. For the probabilistic analysis

the program randomly selects values within the moduli and thickness variability inputs to develop a range of outputs. This type of analysis presents a risk assessment of the probability that a given threshold value will not be exceeded as well as an indication of the rate of damage from loads causing the criteria to be exceeded. For infor-mation on criteria for evaluating pavement performance using this program, refer to the PerRoad Guide.

Contractors, engineers, and agency personnel examine a test pit at a rubblization project.


5. Rubblization Equipment

Rubblization requires the use of highly specialized equipment to break the concrete down to a specified maximum particle size. There are two basic types of self-contained, self-propelled devices for rubblizing PCC pavements.

Multi-Head BreakerThe multi-head breaker (MHB) has 12 to 16 1,200

to 1,500 pound drop hammers mounted laterally, either in pairs offset between two rows, or singularly in-line. Both hammer configurations ensure continuous breakage from side to side. The hammers are attached to a hy-draulic lift cylinder which can be operated independently of the others to provide for custom breaking patterns. Each hammer (or pair of hammers) develops between 1,000 and 8,000 foot pounds of energy depending upon the drop height selected and cycles at a rate of 30 to 35 impacts per minute. The drop height of each hammer (pair of hammers) and the distance between impacts can be adjusted during production to control the amount of breaking energy that is transferred to the PCC pavement. The eight-foot wide machines carry at least 12 hammers eight inches in width. Wing additions can be attached to each side for a total breaking width of up to 14-16 feet. Due to individual control of each lifting cylinder, breaking can be as narrow as one foot or increased in increments to as wide as 16 feet. The MHB is capable of rubblizing a full lane width of the pavement in a single pass.

Figure 5.1 illustrates the MHB fitted with hammer pairs. This MHB is manufactured by Badger State High-way Equipment Inc., Antigo, Wis.

Figure 5.2 illustrates the MHB fitted with single hammers. This MHB is manufactured by Specialties Company, LLC, Indianapolis.

Resonant Frequency BreakerThe resonant frequency breaker (RFB) is a self-pro-

pelled device that utilizes high-frequency, low-amplitude impacts with a foot force of 2,000 pounds. The foot is located at the end of a pedestal that is attached to a beam and counter weight. The force applied to the pavement is achieved by vibrating the large steel beam connected to the foot. The foot is moved along the concrete surface at the front of the machine. The breaking principle is that low-amplitude, high-frequency, resonant energy is delivered to the concrete slab, resulting in high tension at the top. Since concrete has low tensile strength, the slab fractures on a shear plane at approximately 45o

through the pavement. The foot, beam size, operating frequency, loading pressure, and speed of the machine can be varied.

Using the RFB, the breaking begins at the outside free edge and proceeds accross the pavement. The breaking pattern is approximately 8 inches wide, thereby requir-ing approximately 18 to 20 passes to break a 13-foot wide lane. The RFB is generally required to operate at a maximum amplitude of one inch to avoid disruption of base and prevent damage to underground structures. The RFB encroaches up to three feet onto the adjacent lane to rubblize near the centerline of the pavement. When the

Figure 5.1Multi-head breaker (courtesy of Antigo Construction, Inc.)

Figure 5.2Multi-head breaker (courtesy of Specialties Company, LLC)


pavement foundation is weak, flotation tires are used to spread the weight of the 60,000-pound machine.

Figures 5.3 and 5.4 illustrate two models of RFB, manufactured by Resonant Machines Inc., Tulsa Okla-homa (www.resonantmachines.com).

The RFB can also be fitted with high flotation tires, thereby allowing operation on pavement sections that are thinner or have soft subgrades. Note the difference in tire widths between Figure 5.2 and 5.3.

Figure 5.4Resonant pavement breaker(courtesy of RMI)

Figure 5.3Resonant pavement breaker with flotation tires(courtesy of RMI)

Other EquipmentOther types of pavement breaking equipment have

been used to rubblize PCC pavements and are discussed in NAPA’s IS-117. However, recent experience in the U.S. has shown that the multi-head breaker and resonant frequency breaker are the two most common approaches for rubblizing PCC.


6. Construction Operations for Rubblization

This chapter will present guidelines for construction operations for the rubblization process. The topics include:

Surface preparationDrainage issuesLocation of utilities and underground structuresSelection of rubblization equipmentRubblization recommendationsCompaction after rubblizationTroubleshooting rubblizing operationsQuality control issues for rubblization

Surface Preparation of the Existing Pavement

Experience has shown that any existing HMA overlay should be removed prior to rubblizing the PCC. A clean surface allows better transmission of the rubblization en-ergy into the underlying concrete. Due to possible devia-tions in the surface of the existing concrete from faulting and variations in the milling operation, short, thin layers (< 1/2 inch) of HMA may be present and should present no problems with the rubblization process.

Any loose materials on the surface should be removed. This might include loose patching material, joint fillers, and expansion material. Some agencies wait until after the rubblization to remove any loose materials, in case the rubblization process creates any additional materials that should be removed. Full-depth concrete joint repair is not necessary prior to rubblization.

The engineer must evaluate an appropriate course of action relative to patches on the existing PCC surface. If the patch is PCC, it can be rubblized along with the rest of the pavement. If the patch is HMA and is relatively small and sound, it can remain in place. If the HMA patch is large and unsound, it should be replaced with HMA. If the patch is small and unsound (< one square yard) the patch should be replaced with HMA or ag-gregate (#57 is often used).

Before rubblizing begins, all load transfer devices in the existing PCC pavement adjacent to the PCC that will remain intact must be severed with a full-depth saw cut. This process isolates the rubblized area. Sawing jointed pavements at an existing joint has proved to be successful. Examples of this situation are exit ramps

that are not to be rubblized and the beginning and end of the project.

DrainageWhen underdrain systems are required, they should

be installed and functioning before rubblizing begins. In areas of weak subgrade or high water table, the drainage system should be functioning as far in advance of the rubblizing as possible to allow for the subgrade to be as stable as possible. The drainage system also serves to remove rainwater from the rubblized concrete layer, base layer and subgrade during construction. During the rubblizing operation, a steady flow of water in the drainage system is often observed.

Location of Utilities and Underground Structures

Underground utilities and structures must be clearly marked prior to rubblization of the pavement. Special attention should be given to identifying any covers or shutoffs that are not exposed at the surface. When necessary, the breaking energy should be reduced in the proximity of sensitive utilities to avoid damage. The rubblization specification may also allow the contractor to remove the pavement over and around utilities and backfill with aggregate.

Rubblization RecommendationsAs a general rule, the smaller the crack spacing and/

or fragment size achieved, the greater the likelihood that reflection cracking will be eliminated in the HMA overlay. Of course, the smaller the fragment size, the lower the structural layer coefficient; therefore, a greater thickness of overlay is required.

Typical rubblizing specifications require the break-ing of the concrete down to specified maximum particle dimensions while giving the engineer the discretion to di-rect or allow larger maximum particle dimensions. These specified particle dimensions are what can be expected when rubblizing over a fair to good base/subgrade. The particle sizes that can be achieved are directly related to the condition of the base/subgrade, the slab thickness, seasonal variations, reinforcement type, and the opera-tion of the rubblization equipment.

A firm and stable base/subgrade will allow for the production of smaller particle sizes than when working over a less firm and stable base/subgrade. Engineering judgment must be used when evaluating the rub-blizing process, keeping in mind that the intent of


rubblizing is to produce a structurally sound base which prevents reflective cracking by obliterating the existing pavement distresses and joints. The intent is not to meet a gradation requirement.

It must be remembered that the rubblized layer must provide a working platform for paving operations and a stable foundation for the pavement overlay. If isolated areas of weak subgrade exist, appropriate actions must be taken to repair these areas. In many cases this can require overexcavating the soft subgrade and replacing it with compacted aggregate base and/or HMA. In some cases, adjustment of the rubblization process is appropri-ate. It may be necessary and appropriate to modify the rubblization process to introduce less energy into the PCC pavement, thereby producing larger particle sizes which conserve more structural support.

Traffic should not be allowed on the compacted rub-blized slab, due to the risk of “unseating” the particles. There may need to be an exception for cross traffic at intersections, if speeds can be kept low . The amount and weight of construction traffic on the rubblized sur-face should also be minimized, especially in areas with weak subgrade.

Light to moderate rainfall does not affect the rub-blization operation. Work might need to be stopped for safety reasons if heavy rain and/or lightning occur. Rubblized PCC drains well, especially if the edge drains are functioning properly. Therefore, paving operations can usually begin shortly after the rain has stopped. If, however, the rubblizing is being performed over a moisture-sensitive subgrade, the rubblizing/paving operations should be coordinated to minimize exposure of the subgrade to excessive moisture. Generally, it is recommended that the HMA overlay be placed over a properly prepared rubblized PCC pavement within a 24-hour period after the compaction process.

Some contractors use water to control dust until the HMA overlay is placed. The quantity of water used is relatively low and should not affect the subgrade.

Consideration must be given to the effect of vibration and impact on buildings in close proximity to the project where rubblization is being performed.

Rubblization has been successfully performed on PCC pavements ranging from local roads to interstate high-ways to airfield pavements. Using either MHB or RFB, the contractor must determine an optimum operation to successfully rubblize the slabs. It may be necessary to use multiple pieces of equipment to accomplish adequate fracturing of the PCC. As an example, when rubblizing thick airfield pavements, a guillotine breaker followed

by a rubblizer has in some cases been used, followed by the rubblizer. Multiple rubblizing devices are also used to improve productivity of the operation.

Regardless of the equipment used, typical production for the rubblization operation is approximately one lane mile per work day per machine.

Compaction after RubblizationThe purpose of compacting the rubblized pavement

surface is to ensure adequate seating of the rubblized segments and to provide a compacted surface upon which the HMA overlay can be placed. A vibratory roller is normally used to compact and prepare the rubblized surface for placement of the HMA overlay.

The MHB and RFB have slightly different compaction requirements. For the RFB, a 10-ton tandem vibratory steel wheel roller is used in low amplitude and high- frequency settings. This is a typical HMA roller. The operation is very similar to compacting about 2 inches of HMA. Some agencies use an 8- to 10-ton pneumatic roller to smooth the surface prior to placement of the overlay. Louisiana requires the pneumatic roller to make one pass after the initial pass with the vibratory roller. Two additional passes of the vibratory roller are made after the pneumatic pass.

For the MHB , the first compaction passes are per-formed with a vibratory roller that has been fitted with a “Z” or Elliott grid. The purpose of the Z grid is to further pulverize the broken concrete particles at the surface. This piece of equipment is shown in Figure 6.1. Some agencies then require a pass with a 10-ton pneumatic roller. A vibratory roller with a smooth drum is used for two final passes.

In either compaction scenario, it may be necessary to reduce the vibratory amplitude to prevent damage to the subgrade or underground utilities. This is particularly true in areas of weak or wet subgrade.

Observation of the compaction process is an effec-tive method to determine the stability of the rubblized layer. Proof rolling with a loaded tandem-axle truck after compaction is a quick and effective way to determine the stability of the rubblized layer. This would only be done if the engineer has concerns about the stability of the rubblized section after the compaction is completed.

Troubleshooting Rubblizing OperationsRubblization is not for every pavement. As an ex-

ample, it may not be possible to properly rubblize thin PCC pavements (less than 6 inches) on poor subgrade with high moisture content. Other slab fracturing tech-


niques such as crack/seat, break/seat (as described in IS-117), or reconstruction may be more appropriate in such conditions.

For some PCC pavement conditions, both the RFB and MHB may apply too much load to the pavement. Examples have been observed where the rubblizing device has broken through the structure. This may be the result of an inappropriate project evaluation, or of changed conditions since the time of the evaluation.

With the RFB device, if the vibratory response in the pavement is too much for the existing PCC, the foot can punch through the concrete or cause extreme vibration in the equipment. In this case, either reduce the rub-blization effort, or locate the limits of the weak zone and remove and replace.

Quality Control Issues for RubblizationThe quality control/quality assurance (QC/QA) pro-

cess used to ensure that proper fracturing has occurred in the rubblization process is a very important consideration for the ultimate success of the project.

Relying on a visual surface crack survey to enforce specifications may not be entirely reliable, as surface cracks may not be a true indication of the effectiveness of the rubblization process. A more effective method is to excavate test holes to verify the efficiency of the rubblization process as shown in Figure 6.2. At the beginning of the project, test holes or trenches may be excavated to confirm the process. Once the engineer has verified the specification requirements are being met, the digging of test holes is not usually continued throughout the project. The test holes are usually about 3 feet square and are dug during the first day of rubblizing. Trenches

Figure 6.1Roller with “Z” grid(courtesy of Antigo Construction, Inc.)

would be excavated across the whole width of the lane. The test holes/trenches must be repaired with replace-ment material and compacted.

A typical specification requires less than 9 to 10 inch size particles on the surface and a maximum of 12 inch particle size in the lower half of the broken pavement. However, Arkansas State Highway and Transportation Department (AHTD) requires that the maximum particle size be 8 inches with the majority of particles being in the 1 to 3 inch range (AHTD specifies the RFB only). If the rubblization process does not achieve these maximum particle size requirements, the contractor must repeat the process, use other equipment to achieve the requirements or remove and replace the materials. Experience has shown that segments of 12 to 18 inches in the lower half of the slab do not adversely affect the effectiveness in eliminating or reducing reflection cracking.

Reinforcing steel in the rubblized pavement can be left in place unless any steel is exposed on the surface as shown in Figure 6.3. Any exposed steel should be removed by cutting at or below the surface. The steel should be removed from the site.

Figure 6.2Test hole to check effec-tiveness of rubblization(courtesy of RMI)

Figure 6.3Exposed steel needs to be cut and removed(courtesy of Antigo Construction, Inc.)


7. Placing the HMA Overlay

The paving of an HMA overlay on a rubblized and compacted PCC surface is very similar to paving on a prepared crushed aggregate base. Care must be taken to maintain the compacted condition of the rubblized surface up to the time of paving. A vibratory steel roller may be used to recompact the rubblized surface if local and/or construction traffic has loosened the rubblized surface. Of course, the best plan is to not allow traffic on the rubblized surface.

Tack or prime coats should not be used on the rub-blized surface to avoid pick-up of the rubblized surface by construction equipment.

The first lift of HMA on the rubblized surface should be placed with a tracked paver. Experience has shown that a rubber-tired paver may cause movement of the rubblized PCC, causing a reduction in smoothness of the HMA overlay. After placement of the first lift of HMA, any type of paver may be used.

After the rubblization process is completed, asphalt overlays are placed to accommodate the structural requirements established in Chapter 4 for the traffic, subgrade, and environmental conditions. As with most paving operations, traffic can be placed on the intermedi-ate HMA lifts to accommodate construction scheduling. This situation presents a different set of circumstances

than for a conventional overlay that is being placed on a pavement that is already carrying traffic.

Through the rubblization process, the PCC has been reduced to the approximate load carrying capacity of unbound base course. As a result, the road builder must note the thickness of the first lift of HMA that is to be placed on the rubblized surface. The first lift of HMA must be thick enough to adequately cover the rubblized PCC surface and carry traffic temporarily until the addi-tional lifts are paved. The number of large trucks and the type of base determine the minimum thickness needed to carry traffic. If the project includes making cross slope corrections with the first lift, there could be a variable thickness of the lift placed. If this is the case, attention must be paid to maintaining an adequate thickness not only at the centerline, but also at the edge of the pave-ment. In areas of low base/subgrade support, additional thickness of the HMA or removal and replacement of soft soils may also be required.

NAPA recommends that traffic not be allowed on the rubblized surface until the minimum HMA thickness (5 inches for low and medium traffic, 6 inches for heavy traffic and 8 inches for very heavy traffic) for the type of facility has been placed.

If the thickness of the HMA overlay is decreased when approaching a bridge or overpass, rubblizing should stop at the point where the thickness of the overlay begins to decrease. Removing and replacing bridge approaches with full-depth asphalt is recommended to prevent reflec-tive cracking in these areas. The length for the transition necessary to provide a smooth transition to the bridge

varies depending on grade. The designed thickness of the overlay should be maintained over all rubblized areas.

If a yielding subgrade is identified in the operation, it is highly recommended that the soft material be removed down to stable material and backfilled with approved fill to the bottom elevation of the concrete. Fixing the problem by removing and replacing the yielding materials is always the best course of action. Placement and operation of edge drains prior to rubblization will help evacuate the water from the top of the subgrade and strengthen the soil.

Figure 7.1Paving on rubblized surface


8. User Benefits to PCC Rubblization and Conclusion

Agencies are looking for quick, cost-effective means to rehabilitate PCC pavements. Across the U.S., concrete pavements are in need of rehabilitation to meet both structural and functional requirements.

Rubblization is a process whereby the existing worn-out PCC pavement is converted into a high-quality aggregate base. This rubblized base layer is the perfect starting point to build a perpetual hot-mix asphalt pave-ment. More information on rubblization and Perpetual Pavement design, including free Perpetual Pavement design software, may be found at www.asphaltalliance.com.

Rubblization AdvantagesThe advantages that rubblization offers to the owner/

agency include:

■ Elimination of reflection cracking

■ Improvement in smoothness with the placement of HMA as the new surface

■ Elimination of Alkali Silica Reactivity (ASR) and D-cracking problems with the existing PCC

■ Dramatic decrease in construction time relative to PCC reconstruction

■ Improved maintenance of traffic

■ Reduction in length of time traffic is in a two-way situation compared to PCC reconstruction and/or PCC overlay

■ Reduction in cost versus reconstruction of PCC pave-ment

■ Rubblization can be done in 1/5 the time at 1/3 the cost of reconstruction

■ Reduction in cost versus Concrete Pavement Restora-tion (CPR)

■ Increase in service life of the HMA overlay

■ Improved public relations due to decrease in construc-tion time and work zone delays

Rubblization PerformanceThe excellent performance of pavements that have

been rubblized has been well documented. The PCS/Law study involved 118 actual pavement sections throughout the U.S. Performance of the rubblized sec-tions was better than any other rehabilitation technique. The Ksaibati report (see Bibliography) was done for the Florida Department of Transportation as an evalu-ation of the rubblization process. The study conducted a nationwide survey of DOTs to determine the use of rubblization. Twenty-one states reported experience with rubblization. The report states “...it is clear that most states are highly satisfied with rubblization as a good means for eliminating reflected cracks. Only a few states indicated problems with rubblization, mainly due to weak subgrade.”

As a specific example, the Illinois 10-year study indicates that performance has been better with rubbliza-tion than with other techniques which they tested side-by-side. Arkansas has over 300 miles of rubblization either completed or in progress with similar excellent results. Michigan, Wisconsin, Louisiana, and Nevada have reported the same excellent performance results. The bottom line is that rubblization has been successfully used in many states.

ConclusionAs agencies continue to look for cost-effective

methods to rehabilitate PCC pavements, it is clear that rubblization offers an excellent tool to the pavement engineers. The existing PCC pavement is quickly re-habilitated into a long-life HMA pavement very quickly and with minimal disruption to the traveling public. In addition, costs are kept to a minimum. The process is no longer experimental. Rubblization of PCC with an HMA overlay works and can provide excellent pavement performance.


Bibliography

NAPA: Guidelines for Use of HMA Overlays to Rehabili-tate PCC Pavements, (IS-117), 1994.

Illinois Department of Transportation: Guidelines for Rubblizing PCC Pavement and Designing a Bituminous Concrete Overlay, 2001.

IDOT: Special Provision for Rubblizing PCC Pavement, 2001.

IDOT: Construction Memorandum No. 01-40, Rubblizing PCC Pavement and Placing a Bituminous Concrete Overlay, 2001.

Wolters, R.O. and Thomas, J.M., Best Management Prac-tices for Rubblizing Concrete Pavements, 2003.

Interview with Gary Fitts, Asphalt Institute Field Engineer, San Antonio, Texas.

Interview with Jay Hensley, Asphalt Consultant, Pea Ridge, Arkansas.

Interview with Marshall Thompson, Professor Emeritus, University of Illinois, Champaign, Illinois.

Interview with George Shinners, Antigo Construction Inc., Wisconsin.

Interview with Phil Kirk, Resonant Machines Inc., Okla-homa.

Distress Identification Manual for the Long-Term Pave-ment Performance Project, SHRP-P-338.

New York State Department of Transportation: Rubbliz-ing Existing Portland Cement Concrete Pavement, EI 96-030, 1996.

Michigan Asphalt Pavement Association: HMA Overlay Design Study for Rubblization of PCC Slabs, Report No. 3066, Harold Von Quintus, 2001.

Wisconsin DOT: Standard Specifications Section 335 Rubblized Pavement.

WisDOT: Facilities Development Manual, Chapter 14, Section 25, Subject 15, “Concrete Pavement Rubbliza-tion,” 2002.

WisDOT: Construction and Materials Manual, Chapter 5, Section 5.5, Rubblizing Concrete Pavement, 2003.

Badger State Highway Equipment, MHB Badger Breaker.

Arkansas State Highway and Transportation Department (AHTD: Standard Specifications Section 513 “Rub-blizing Portland Cement Concrete Pavement.”

Louisiana DOT: Standard Specifications Section 734 “Rubblizing Portland Cement Concrete Pavement.”

Boyer, Bob, and Goree, Ronnie, “Rubblizing Concrete Pavement in the United States,” International Construc-tion Magazine, 2000.

Harmelink, Donna, Hutter, Werner and Vickers, Jeff, “In-terstate Asphalt Demonstration Project NH 0762-038 (Rubblization)”, Colorado Department of Transporta-tion, Research Report No. CDOT-DTD-R-2000-4.

Fitts, G.L., “Performance Observations of Rubblized PCC Pavements,” Second International Symposium on Maintenance and Rehabilitation of Pavements and Technological Control, Auburn, Ala., 2001.

Heckel, L.B., “Rubblizing with Bituminous Concrete Overlay – 10 Years’ Experience in Illinois,” Illinois Department of Transportation, Physical Research Re-port No. 137, April 2002.

American Concrete Pavement Association, “Rubblizing of Concrete Pavements: A Discussion of its Use,” Technical Information Report, 1998.

Thompson, M.R., “Hot-Mix Asphalt Overlay Design Concepts for Rubblized Portland Cement Concrete Pavements,” Transportation Research Record No. 1684, TRB, 1999.

Bemanian, Sohila, and Sebaaly, Peter, “Cost-Effective Rehabilitation of Portland Cement Concrete Pavement in Nevada,” Transportation Research Record No. 1684, TRB, 1999.

Ksaibati, Khaled, Miley, William, and Armaghani, Jamshid, “Rubblization of Concrete Pavements,” Transportation Research Record No. 1684, TRB, 1999.

Galal, K.A., Coree, B.J., Haddock, J.E., and White, T.D., “Structural Adequacy of Rubblized Portland Cement Concrete Pavement,” Transportation Research Record No. 1684, TRB, 1999.

Rubblization, Asphaltopics, Ontario Hot Mix Producers Association, December 2000, Toronto, Canada.

APPROXIMATE CONVERSION TO SI UNITS

Symbol When You Know Multiply By To Find Symbol

LENGTHinches inches 25.4 millimeters mmft feet 0.305 meters myd yards 0.914 meters mmi miles 1.61 kilometers km

AREAin2 square inches 645.2 millimeters squared mm2

ft2 square feet 0.093 meters squared m2

yd2 square yards 0.836 meters squared m2

ac acres 0.405 hectares hami2 square miles 2.59 kilometers squared km2

VOLUMEfl oz fluid ounces 29.57 milliliters mLgal gallons 3.785 liters Lft3 cubic feet 0.028 meters cubed m3

yd3 cubic yards 0.765 meters cubed m3

NOTE: Volumes greater than 1000 L shall be shown in m3.

MASSoz ounces 28.35 grams glb pounds 0.454 kilograms kgT short tons 0.907 megagrams Mg

(2000 lb)

TEMPERATURE (exact))F Fahrenheit 5(F-32)/9 Celsius )C

temperature temperature

APPROXIMATE CONVERSION FROM SI UNITS

Symbol When You Know Multiply By To Find Symbol

LENGTHmm millimeters 0.039 inches inm meters 3.28 feet ftm meters 1.09 yards ydkm kilometers 0.621 miles mi

AREAmm2 millimeters squared 0.0016 square inches in2

m2 meters squared 10.764 square feet ft2ha hectares 2.47 acres ackm2 kilometers squared 0.386 square miles mi2

VOLUMEmL milliliters 0.034 fluid ounces fl ozL liters 0.264 gallons galm3 meters cubed 35.315 cubic feet ft3m3 meters cubed 1.308 cubic yards yd3

MASSg grams 0.035 ounces ozkg kilograms 2.205 pounds lbMg megagrams 1.102 short tons(2000 lb) T

TEMPERATURE (exact))C Celsius 1.8C + 32 Fahrenheit )F

temperature temperature)F

)F 32 98.6 212-40 0 40 80 120 160 200

-40 -20 0 20 40 60 80 100)C 37 )C

*SI is the symbol for the International System of Measurement.

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