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The Right Tool for the Job - Part I

The Right Tool for the Job - Part II

The Rise of the Interlayer 

THE RIGHT TOOL FOR THE JOB

                Part 1 of a 2 part series on utilizing the right asphalt rehabilitation method, at the right time, for the right road.

 

In today’s harsh economic and political times, City Engineers, Public Works Directors, County Engineers and Iowa DOT Design Engineers are constantly challenged to do more with less.  They shoulder the yeoman’s task of getting more life out of antiquated city streets, over-loaded county roads, and aging state highways.  Asphalt is the answer.  This two part series will describe the multiple asphalt rehabilitation methods available to engineers and will help provide insight on which rehabilitation method is the “Right Tool for the Job.”

The Straight Overlay

The Straight hot-mix asphalt (HMA) overlay is the simplest, most cost-effective method to rehabilitate an aging PCC road.  Contractors often refer to these jobs as “blow and go” work.  The straight overlay project generally involves four steps:  1. Clean and prep the existing road to receive the overlay.  2.  Surface patch with asphalt any weakened, spalled, or defective areas in the PCC road.  3.  Place the leveling or intermediate course.  The straight overlay may use a thin leveling course to smooth out the existing road, strengthen any weak areas, and to keep inflections and irregularities from reflecting through the surface course.  Or, a traditional intermediate course of 1.5” - 2.5” can be used alone, or in conjunction with the leveling course, to add structural strength and improved rideability to the roadway.  4.  Place the surface course.  The final lift of asphalt provides additional strength, increased longevity, and improved ride to the final pavement.  The straight overlay is most effective on PCC pavements in “good” to “fair” condition and without too much slab movement. It is also extremely useful on thin asphalt roads needing additional structural strength.

The Mill and Fill.

The “mill and fill” utilizes asphalt’s ease of rehabilitation and recyclability to the fullest.  This process includes milling off 1” - 4” from the existing asphalt or composite roadway and placing one or two lifts of asphalt back.  The milled asphalt will be recycled into the new roadway mix, thereby reducing the cost, environmental impacts, and use of natural resources.  It is a win-win-win for the agency, the contractor, and the environment.  The milling process has become highly refined over the past twenty years and now allows the contractor to cut the existing grade for both slope and smoothness.  The milled surface creates a great bonding platform for the new lifts of asphalt.  If possible, it is best not to mill down to the old PCC when utilizing this method on a composite pavement.  Leaving one inch or more of the old pavement will improve the ride and performance of your new pavement.  This rehabilitation method works best on composite or aging full-depth asphalt roads and streets.  The Perpetual Asphalt Pavement  utilizes the “mill and fill” process to rejuvenate the wearing surface of the road, while leaving the underlying asphalt base intact.

The Asphalt Interlayer

The asphalt interlayer (AI) is a specially designed one-inch lift of hot-mix asphalt placed over a PCC pavement.  The AI utilizes highly polymerized asphalt cement (PG 64-34), a fine-aggregate gradation and a high asphalt cement content, (generally 8% - 10%) to create an impermeable, highly flexible layer between the PCC road and the traditional HMA asphalt overlay.  This product has two effects:  1.  It retards the reflective cracking of the HMA overlay by absorbing the movement of the PCC panels and,  2. When surface cracks appear in the asphalt overlay, the impermeable nature of the AI keeps additional moisture from penetrating and further deteriorating the underlying PCC street or highway.  Due to the high polymerized asphalt cement content the asphalt interlayer can be 30% - 50% higher in cost than traditional HMA, however, the increased performance has many city, county, and IDOT engineers looking at this product.  Recent demonstrations in Dubuque, O’Brien County, and I-35 near Mason City, have shown that there is value to local and state agencies.  This rehabilitation method is best used on higher volume PCC city streets, and nearly any county or state highway that has reasonable structure remaining in the PCC road.  Do not use the Asphalt Interlayer as a “leveling course.”  If the PCC road has poor ride quality and/or significant inflections, consider placing a scratch or leveling course of traditional HMA prior to placing the interlayer.  The Asphalt Interlayer requires a minimum of a 3” HMA overlay and 4” is recommended. The IDOT specification DS-12054 can be found by clicking here .

These three rehabilitation methods form the beginning of the myriad of options the asphalt brings to the engineers’ “tool box”.  In Part II of this series, found in the Winter 2015 edition of the Iowa Asphalt Report, we will examine cold-in-place recycling, crack and seat, rubblization, and full-depth reclamation

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RIGHT TOOL FOR THE JOB

Part 2 of 2 part series

 

This is the second in a two-part series on finding the right rehabilitation method for a particular street or highway project.  Part 1 in the series (available by clicking here or by viewing above) described the Straight Overlay, the Mill and Fill, and the use of the Asphalt Interlayer rehabilitation methods.  This article will evaluate the use of cold-in-place recycling, crack and seat, rubblization and full-depth reclamation.

 Cold-in-Place-Recycling (CIPR) 

The use of cold-in-place recycling or CIPR is a great Iowa success story.  The process was originally adopted by some Eastern Iowa County Engineers to prolong the lives of their hot-mix asphalt (HMA) pavements.  However, this process has been so successful in prolonging the life of the low volume HMA and composite pavements that it has been brought forward successfully by the Iowa DOT for use on several high-volume, high truck traffic roadways.

Cold-in-Place Recycling utilizes 3-4 inches of existing HMA in-situ to create a flexible bond breaker between the existing roadway and a new HMA overlay.  The process requires the use of a specialized contractor that brings in a CIPR “train”.  The train consists of an HMA milling machine that cuts the roadway to the desired depth, the millings are conveyed to a crusher where they are sized and, if necessary, crushed further; the millings are then coated or foamed with a hot asphalt emulsion and then conveyed back on the road and placed through an asphalt paver.  The CIPR material is compacted with both rubber tire and steel drum rollers, and is opened to traffic.  The CIPR material is allowed to cure for approximately 10-14 days or until the CIPR material reaches 2.5% moisture content.  The CIPR is overlaid with 3-5” of traditional HMA.

Although the cold-in-place recycling rehabilitation method adds additional time and costs to a project, it does offer two distinct advantages:  1. It is an extremely “green” construction method, all materials are recycled in situ with only additional asphalt cement added to the CIPR materials.  2.  The CIPR process has proven over time to add 3-5 years of performance life to roadways by delaying, or eliminating, reflective and thermal cracking.  A definitive treatise on CIPR was written by APAI Field Engineer, Royce Fichtner, PE entitled “Cold-in-Place Recycling:  The Mystery Unveiled” for the Summer edition of the Iowa Asphalt Report

Crack and Seat, Rubblization and Modified Rubblization

In addition to the traditional crack and seat, and full rubblization processes, a new hybrid of this process has been added known as a “Modified Rubblization” or “Break and Seat”.  All three processes are used to rehabilitate failing PCC pavements by turning the rigid PCC pavement into a flexible base for the HMA overlay.  The difference between these processes lies in the severity of the cracking process, the selection of appropriate pavements, and the depth of the final HMA overlay.  The Crack and Seat process utilizes a specialized concrete breaker to fracture the concrete roadway into approximately 36” squares.  Crack and seat is best used on structurally “good” PCC roads with limited panel movement.  Its primary purpose is to help limit or retard reflective cracking through the HMA overlay.  To limit damage to utilities and curb lines, it is recommended to saw cut along curbs, manholes and water valves.  Once a short stretch of the roadway has been cracked, a water truck sprays the area and as the street begins to dry, the cracking pattern will become evident.  It is recommended that the engineer and the representative of the cracking contractor have a discussion on any changes that may be needed before proceeding with the cracking of the remainder of the project.  All roadways react differently and it is imperative to listen to the cracking experts.

The road is then “seated” by driving a heavy roller over the cracked roadway.  The crack and seat specification suggests utilizing a 50 ton roller for this process, but experience has shown that a 20-30 ton roller is more than adequate to seat the pavement.  Any failed areas will need to be excavated and replaced with an HMA patch.  The roadway is then cleaned and overlaid with 3”-5” of new HMA.

Rubblization should be utilized on projects with severe deterioration of the PCC, major faulting of the PCC panels and / or “D” cracking.  It is also recommended that rubblization should not be used on urban roadways unless they are of rural design.  Rubblization breaks a poorly performing PCC roadway into an interlocking grid of 4” nominal sized PCC platform to build a new HMA roadway.  Once the road has been rubblized it is compacted using a Z-grid roller to seat the material in place.  Recent rubblization projects in Iowa have included the use of 2”-3” of ¾” roadstone interlayer to provide a construction platform for the asphalt overlay. (Click here to review the Fall 2012 Iowa Asphalt Report for more information on Rock Interlayer.) Rubblization projects typically receive 5”-8” of HMA over the rubblized roadway depending upon the traffic loading.

Modified rubblization has become increasingly popular in Iowa over the past ten years.  It blends the principles of both crack and seat and full rubblization in reducing reflective cracking by cracking the roadway to approximately 12”-18” squares while reducing the risk of catastrophic failures that may occur during full rubblization projects.  This process also may incorporate the use of the rock interlayer to reduce reflective cracking and provide a construction platform.  Typical modified rubblization projects use 4”-8” of new HMA overlay depending upon traffic loading.

Full-Depth Reclamation.

Full Depth Reclamation, or FDR, is an excellent rehabilitating method for roads that have been hodge-podged together or for roadways with moderate to severe subgrade issues.  This method utilizes a reclaimer or pulverizer to blend the existing road, aggregate base and sometimes dirt subgrade to create a strong and homogenous base course in which to build a new roadway.

Many low volume roads in Iowa began as gravel roads, were seal-coated for many years and may eventually have been overlaid with a thin lift of asphalt.  As traffic volumes increased, the road was patched and spot overlays were used to strengthen the road. However, the time has come to create a stronger base to support the increased traffic volumes.  FDR would be an excellent choice for this roadway and still receive the value of the materials already in place on the road. Reclaimers are generally able to reclaim or blend materials from a depth of 8” to 18”.  Depending on the depth of the blend, the existing roadway materials, and the subgrade stability; different additives can be blended into the reclaimed materials.  Dry cementitious materials such as fly-ash, C-stone, lime screenings, or Portland cement can be incorporated in the reclaimed material to make a stable and homogenous subgrade.  Asphalt emulsions or foamed asphalt can be added in conjunction with the cementitious materials, or if the existing roadway section to be reclaimed is mainly bituminous materials, it may be the only additive needed to reach stabilization.  FDR is also extremely useful when widening roadways to increase the roadway top and increase the safety of the road. Once the roadway has been reclaimed, it is shaped using a motor grader to achieve proper slope and is compacted with a steel drum roller, or, if needed, a sheep’s foot roller may be added. The prepared road base is then overlaid with a structurally designed depth of hot mix asphalt dependent upon traffic volumes.

Summary

This series has examined the myriad of ways in which asphalt or composite (asphalt over PCC) pavements may be rehabilitated.  The ease, the speed, and the environmental advantages allow asphalt roads the privilege of being “The Right Choice” for your next roadway project.  For more information and/or consultation on any of these rehabilitation methods, please contact the Asphalt Paving Association of Iowa at 515-233-0015 or at apai@apai.net .

 

 

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THE RISE OF THE INTERLAYER

 

The use of a crack-relief interlayer prior to an asphalt overlay has expanded exponentially in Iowa over the past several years. This article will focus on the use of the asphalt crack relief interlayer to deter PCC reflective cracking, and on the use of a rock interlayer for crack relief.

Asphalt will always be the best product to rehabilitate Iowa’s failing Portland cement Concrete (PCC) pavements, yet there has been one inherent flaw, reflective cracking of the PCC saw cuts. The reflective cracking allows water to penetrate through the overlay and into the already deteriorated PCC pavement, thus causing shorter life spans of the asphalt overlay.  In the past, the Iowa Dept. of Transportation (IDOT) has typically overlaid PCC pavements with a four-inch asphalt overlay.  The straight overlay theory is that you get one year per inch of asphalt before the cracks reflect through the overlay. The PCC reflective cracking greatly reduces the lifespan of an asphalt overlay. The IDOT has begun to address the reflective cracking issue by increasing the use of alternative rehabilitation methods including: crack and seat, cold-in-place recycling and PCC rubblization (see IAR Fall 2011 issue). The newest method to be reexamined is placement of a one-inch asphalt interlayer over the failed PCC followed by an asphalt overlay.

 
Heartland Asphalt paving Asphalt Interlayer on I-35

The asphalt interlayer was first introduced by Koch Materials in the 1990’s as a proprietary product called StrataTM.  StrataTM was engineered hot-mix asphalt placed one inch thick using polymerized asphalt cement and 3/8” minus aggregates.  Several test sections were placed in Iowa including Highway 9 outside of Decorah, IA and on 1st Ave (Bus. Hwy 151) in Cedar Rapids, IA.  Unfortunately, the StrataTM product was sold off to another company and not properly promoted due to several company acquisitions and mergers.  However, the ability to create the mix design for an asphalt interlayer and utilize the correct polymer asphalt cement, is well within the abilities of today’s APAI contractors. The use of the interlayer may be advantageous to all levels of agencies. Scott Schram, IDOT Bituminous Engineer had this to say about the use of the asphalt interlayer by the IDOT, “PCC pavements can be found in roughly 80% of the lane miles in our state; the majority of which cannot be seen by the driver. If you drive on one of these roads today, you’ll find the underlying PCC pavement was originally constructed an average of 37 years ago.  The traditional strategy has been an HMA overlay. However, freeze/thaw cycling, poor drainage, and deteriorating slabs each contribute to their movement. When these slabs move, cracks develop in the surface of the overlay. Cold-in-place recycling has proven quite effective in reducing these reflective cracks, but this approach is not practical in every situation. The DOT has successfully experimented in the past with crack delay systems, but the strategy has never been routinely applied. Funding constraints necessitate cost-effective solutions.”

 
APAI Field Engineer Royce Fichtner measures Asphalt Interlayer

The placement of the asphalt interlayer is done using traditional asphalt paving methods with a few slight variations.  The asphalt is plant-mixed, hauled to the site, and placed one-inch thick using an asphalt paver.  Contractors have opted to not use the traditional belly-dump trucks and windrow pick-up machines and the mat is rolled using the static mode of the steel-drum rollers.  The asphalt interlayer is then overlaid with conventional asphalt at a minimum of three inches thick. In a recent open house on I-35 in Cerro Gordo Co., IDOT, City, County, and Consulting engineers were able to witness the plant mixing and paving of the asphalt interlayer first-hand by Heartland Asphalt of Mason City, IA. “We had never placed an asphalt interlayer prior to this I-35 project. With the exception of running the mix a little hotter to accommodate the polymer asphalt cement and the one inch thickness, it was nearly identical to placing traditional asphalt. We did opt to use straight trucks dumping directly into the paver.” said George Jessen, President of Heartland Asphalt.  ”We were able to use our belly-dumps and pick–up machine for the intermediate layer over top of the interlayer. I was extremely happy with this operation and I hope we see asphalt interlayers used more often.”

  IDOT Design Engineer Chris Brakke talked about why the asphalt interlayer was selected for the Cerro Gordo project,” “We had done some test projects in the past where we used an interlayer on portions of an overlay project.  Some of the results were promising, but the routine use of interlayers had not occurred.  At the conception stage for the I-35 project, we were looking for rehabilitation options.  Cold-in-place recycling had not been used at that traffic level, and a more significant structural rehab was not deemed necessary because the existing overlay had performed well.  So other ways to reduce reflective cracking were discussed, and we decided to use an asphalt crack relief interlayer as part of the new HMA overlay”

 Although the use of the interlayer is gaining momentum with the IDOT, the use of the interlayer has proven to be even more important in the municipal arena. “The use of the asphalt interlayer in a municipal setting, where curb line exposure is at a premium, may be a real cost savings for cities.”  Chuck Finnegan, President of the L. L. Pelling Co. , was the project manager on the 1st Ave. project in Cedar Rapids, IA done over 15 years ago using Koch’s StrataTM product, “Even today, you can see a distinct difference in the amount of cracking that has come back through the original overlay in 1999, both in the number of cracks and the size. Very few go completely across the pavement, and those that do, are very narrow, minimal reflective cracks. This has been a tremendous success story for the Cedar Rapids Engineering Department and the cities’ taxpayers.”

 The City of Dubuque, with the technical expertise of River City Paving, was able to incorporate the asphalt interlayer on its Central Avenue Resurfacing Project (21st to 32nd) in the summer of 2011. "During the planning of this project we were able to identify nearly $390,000 in direct savings and countless dollars in savings for the merchants along the project through the use of the asphalt interlayer instead of milling down to the original PCC and replacing every joint." Said Jon Dienst, Civil Engineer for the City of Dubuque . "We think we will get better performance for our taxpayers with less hassle for the traveling public and our local merchants. The use of the asphalt interlayer has been a Win-Win for the City of Dubuque."

The asphalt interlayer’s highly elastic properties retard the reflective cracking of the PCC pavements through the asphalt thereby prolonging the life of the rehabilitation.  The asphalt interlayer replaces one inch of the typical four-inch overlay and comes at a slightly higher premium due to the additional lift construction and the higher costs of the polymer asphalt cements, but both IDOT and municipal agencies see the value.  ”If the cost of a 4-inch overlay with a PG 64-28 binder is compared to a 3-inch overlay (PG 64-28) with a 1-inch interlayer (PG 64-34), the increase in binder cost is nearly 13%. It is expected this cost can be recovered from improved performance and deferred maintenance, which were demonstrated in past trials.” says Schram.

 
Completed Central Ave. Project in Dubuque

Getting longer life for asphalt overlays is a goal of both the contracting agency and the APAI asphalt contractor.  The use of the asphalt interlayer is both fast and cost-effective. Specifying a full-depth asphalt pavement from inception will insure longer performing asphalt overlays, but for rehabilitating failing PCC pavements, the asphalt interlayer may be just the tool for prolonging pavement life.

 Sometimes great innovation comes about by accident.  The use of the rock interlayer as a crack relief layer is just such an example.  As the story goes, Gary Bishop, Davis County Engineer had a rubblization project that was going poorly, with dump trucks making deep ruts, even though two- thirds of the road had lateral drains installed the previous year.  The rubblization was changed to a light crack and seat to achieve stability in the failing PCC pavement without success.  Expensive and costly repairs were needed to prepare this first half of the project before it could be pavedTo alleviate the structural failures facing his project, Bishop decided to place a rock layer over the second half of the failed PCC and a pavement rehabilitation innovation was born.

 Several years later, Bishop was again faced with a badly faulted PCC roadway.  The use of rubblization was discussed and dismissed due to fears of pavement failures and potential costs. The use of a straight asphalt overlay was discussed, but concerns about the faulted PCC reflecting through made Bishop think back to his previous rock layer project.  When going back to look at that roadway, he observed that the roadway was performing exceptionally well including the PCC section that had not been rubblized. That section of roadway only had a two-inch rock layer placed before the asphalt overlay.  Bishop decided to forgo any rubblization on the new project and just place the 1 ½ inch rock interlayer over the PCC and add the four-inch hot-mix asphalt (HMA) right over the rock.

When Bishop was asked about the choice to use the rock interlayer, and the success that was achieved through the use of this innovative technique, he replied, “I view use of the interlayer as a tool to be used in the proper locations, without rubblization or crack and seat, and where the PCC is deteriorated, but stable.  Widening of the pavement has been achieved with asphalt millings with rejuvenator on several projects and road stone on one project.” Bishop was also quick to give credit to long-time Benton Co. Engineer and former APAI field engineer, Jerry Petermeier, “Jerry had a theory that the rock interlayer would provide both structure and a buffer for the heaving of the PCC slabs”, said Bishop, “He was right all along.”

 The rock interlayer is generally a ¾” road stone placed 1”-3” thick over a failing PCC pavement that has received a full or modified rubblization, crack and seat, or over a badly faulted PCC pavement.  The rock is placed wet through an asphalt paver and then static rolled.  The resulting rock layer is surprisingly strong and durable under construction traffic and provides a crack relief layer between the failing PCC and the asphalt overlay.

 Current Benton Co. Engineer, Myron Parizek, put the rock interlayer to use on W24, a busy county road running south from Highway 30 to the City of Norway. The road had been overlaid with four inches of hot mix asphalt (HMA) in 2000 when it was “gifted” to the county by the Iowa Dept. of Transportation (IDOT). By 2005, the PCC slabs had reflected through the asphalt overlay so bad that it was nearly impassable in a truck. Parizek placed subdrains in the fall of 2007 and debated on how to rehabilitate the road. He settled on milling off the 4” HMA to use as RAP in the new mix, performing a “modified” rubblization (breaking the slabs into one-foot sections) to relieve the stress in the PCC slabs, placing three inches of choke stone and overlaying the road with five inches of new HMA. “I was not a full believer in rock interlayer process going into this project”, said former LL Pelling Project Manager, Bill Rosener, “I was afraid the rock interlayer would move while we were putting the asphalt down. It didn’t. Driving the same road a month ago, it was very smooth and I was only able to count five reflective cracks after some very hard recent winters.”

 
Mathy Construction places Rock Interlayer on Scott Co. Y4E in 2011

Scott County Engineer, Jon Burgstrom, has had similar successes using the rock interlayer for both rubblized and non-rubblized projects.  In 2004, Burgstrom rubblized twelve miles of County Road Y68 (Old Highway 61), placed a two-inch rock interlayer and seven inches of asphalt. "It's been about 10 years now, and while there are a few cracks, we have not had to do anything major to the road. It looks good and is performing well" said Burgstrom regarding the project.  Burgstrom utilized the rock interlayer again in 2011 on County Road Y4E by placing 1.5 inches directly on a badly faulted PCC pavement. He then placed five inches of asphalt over the rock interlayer. "By using only rock we were able to avoid major patching,” said Burgstrom.  ”It seems that with every crack and seat job with the trucks running on it something goes to pot and you have to do expensive patching before the overlay. As costs seemed to be similar, we decided to give the rock only, without the crack and seat, a try. While the cost of rock overran, not having to do any patches saved a lot of money. We learned a lot and will do it again. The rock inner layer also increased smoothness since it was placed with a paver. Today the project rides very good and we are very happy with it."

   
A Fort Dodge Asphalt crew places a rock interlayer A Fort Dodge Asphalt paving crew paves over
on Webster Co. road P59 in 2010. the rock interlayer on Webster Co. road P59 on
the same day.

When using the rock interlayer in a rubblization project, the rock can provide additional stabilization and structure during the construction process.  During the Highway 69 Rubblization Open House held in late July, Manatts General Manager, Duane Hassebrock, talked about the importance of the rock interlayer following the rubblization of some badly deteriorated PCC, “After removing the old asphalt overlay, the PCC was in such bad shape that the rubblization had to be modified or we were going to see catastrophic failures in the base.  By placing the rock layer right behind the breaker, we were able to bridge some areas that otherwise would have needed to be cored out and replaced.”

The use of rock interlayer has become standard procedure on Iowa county road and Iowa Department of Transportation (IDOT) rubblization projects. “The use of a “choke stone” layer to smooth irregularities was standard practice on rubblization projects in the past,” said Chris Brakke, Iowa DOT Pavement Design Engineer.  “On projects in the last 7 years, our Districts have implemented the use of the rock or RAP interlayer.  Typically, these are 3” thick, and in addition to improving the construction platform, there appears to be a performance benefit even at this early age.  Because of this, we have now begun including the interlayer in the structural design of the rehabilitation.”

A final benefit of the rock interlayer is smoothness.  The opportunity to correct crown and ride is not missed by engineers and contractors.  “Any time we get the opportunity to improve ride, we’ll take it,” said Hassebrock.  “The rock interlayer over the rubblization gave us a much better platform to build our overlay compared to overlaying the rubblized PCC.”

 The Rise of the Interlayer is a success story for the Iowa Asphalt Industry in our quest to improve the duration of PCC rehabilitations.  From municipal streets to Interstate overlays, the use of the Asphalt Interlayer has proven an effective method for retarding PCC reflective cracking through asphalt overlays.  By introducing the use of the Rock Interlayer over rubblized, crack and seat, and straight PCC overlay projects, it has proven to be a beneficial construction platform and a cost-effective structural layer.  Continued success of current projects, coupled with stronger knowledge and acceptance of both the asphalt interlayer and the rock interlayer by all agencies, may prove to be another tool for engineers in their quest to rehabilitate failing PCC roadways. There is a better alternative for all agencies: build full-depth, perpetual asphalt pavements!

 

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