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Research Board Compares Notes

Use of a digital camera and laptop computer for a portable aggregate imaging system, the viability of increased use of Reclaimed Asphalt Pavement in highway structures

Tom Kuennan

Use of a digital camera and laptop computer for a portable aggregate imaging system, the viability of increased use of Reclaimed Asphalt Pavement in highway structures, and the usefulness of a green mix made of controlled low-strength materials were among the new research in aggregates technology presented at the 88th annual Transportation Research Board meeting.

Every January more than 10,000 researchers and experts in transportation attend the meeting in Washington, D.C., and aggregates and reclaimed materials research were part of some 3,500 presentations and peer-reviewed research presented there.

Following are summaries of new peer-reviewed research at TRB selected for its interest to the readers of Rock Products.

A digital camera with laptop computeróor a microcomputer with integral cameraówith software will make a useful portable image analysis system for aggregate plants, says Wang, Lane, Lu and Druta in their paper, ìPortable Image Analysis System for Characterizing Aggregate Morphology.î

ìAggregate morphological properties have significant effects on asphalt pavementís performance,î the authors say, ìas they are critical factors influencing the permanent deformation and fatigue characteristics of the pavement. These properties also vary widely with the type and source of aggregates and processing variables.î

In the last decade, the application of image-based evaluation of particle shape, angularity and texture has been widely researched to characterize aggregate morphology. ìYet, the lack of rapid, objective, and quantitative methods for assessment has inhibited its application in the engineering process,î they said. ìHowever, recent advances in technology have produced pocket computers having as much processing power as was available in some desktop computers.î

The authors used this new computing power in developing an inexpensive portable image-analysis system for characterizing aggregate morphology. The system uses an integral pocket computer/high-resolution camera. It also can employ individual components consisting of a digital camera, laptop or desk-top computer.

Digital images of coarse aggregate particles are captured with the camera and analyzed within Matlab software with a macro developed and written for this project, Wang, Lane, Lu and Druta said. Particle morphology was characterized with respect to three parameters: shape, angularity and texture, based on the particle perimeter (outline or edge).

Several coarse aggregate types from 10 various Virginia sources were analyzed, and the reliability of the image processing was statistically assessed.

ìAsymptotic analysis was performed to determine the number of images needed to obtain a statistically stable value for each aggregate parameter,î the authors say.

ìIt was determined that images acquired at close range (2 inches or 3 inches) were needed to provide sufficient resolution to adequately characterize the aggregates. Also, it was found that statistically valid values for the three aggregate parameters could be obtained from 15 particle images of random but stable orientation, thus making the system efficient in characterizing coarse aggregate morphology.î

The authors found that, based on the acquired analysis results, the microcomputer with built-in camera can successfully be used for field evaluation of coarse aggregate. ìThe unified Fourier method can accurately quantify the shape, angularity, and surface texture factors of coarse aggregate profiles with threshold factors being incorporated in Matlab software for image analysis. The Fourier method can also quantitatively rank coarse aggregates consistently with their qualitative ranking.î

Tar Heels Served by ABC with Asphalt Lifts

Aggregate Base Courses keep fresh pavements at optimum pavement condition ratings, with longer ratings associated with thicker asphalt lifts, and should continue to be used, wrote Corley-Lay and Mastin, North Carolina Department of Transportation pavement management unit, in their article, ìPerformance of Aggregate Base Course Pavements in North Carolina.î

ìNorth Carolina has constructed thousands of miles of Aggregate Base Course pavements, in part because of the presence of high-quality aggregate over most of the state,î Corley-Lay and Mastin say. ìRecently questions have been raised about the performance of [ABC] pavements, as well as other pavements commonly used in the state.î

To determine whether the use of ABCs is deserved, the authors used Pavement Management System data to evaluate the performance of three categories of aggregate base pavements based on the thickness of Hot-Mixed Asphalt placed over the aggregate base: thin, mid-range, and thick. For pavements constructed on new location, the Pavement Condition Rating immediately after construction (time 0) should be equal to or close to 100. ìOver time, the PCR should decrease as traffic loading and environment degrade the pavement,î Corley-Lay and Mastin said.

ìEventually, a treatment will be applied to restore the surface or to provide additional pavement structure. This treatment will cause an increase in the PCR.î A PCR of 60 is the lowest PCR when light to moderate rehabilitation can be used to correct pavement defects.

ìSections of pavement and their performance measures were pulled from the PMS by queries,î they say. ìThin sections had asphalt thicknesses less than 3 inches. Mid-range sections had asphalt thicknesses of 5 to 7 inches, and thick sections had asphalt thicknesses of more than 9 inches. Aggregate Base Course thickness is generally 8 inches, although 6 inches may be used on very low-volume roadways. Aggregate thickness of 10 inches is common in the mountain region to avoid frost penetration into the subgrade.î

Performance curves, graphs of pavement condition rating versus time, were generated for each section or group of sections, they say. ìA polynomial curve of form ax2 + bx + c, was fit to each curve and the time to reach a pavement condition rating (PCR) of 60 was calculated. Average, standard deviation, and histograms of the time to reach PCR of 60 was determined for each group.

Time to PCR of 60 was highest for sections located in the coastal plain. The average times to PCR of 60 were 13.67, 15.2 and 14.03 years for the thin, mid-range and thick asphalt sections.î

The authors conclude that North Carolina has had favorable performance from most ABC pavements over a wide range of applications.

Thin HMA sections over ABC were commonly used for local roads and had an average time to a pavement condition rating of 60 equal to 13.7 years. Climate had an effect on the time to failure for pavement with thin HMA over ABC: 16.3 years for the coastal plain, 13.5 years for the Piedmont, and 13.1 years for the mountains.

The mid-range thickness of HMA over ABC pavements had an average time to PCR of 60 of 15.2 years, or a year and a half longer than the thin HMA ABC pavements. The mid-range pavements constructed between 1980 and 1990 had a time to failure of 16.2 years, compared to 13.0 years for those constructed between 1990 and 2000.

ìTraffic levels and freight traffic increased dramatically after 1990 and may have contributed to the reduced life of pavements constructed after the high traffic growth period,î they wrote.

Thick HMA over ABC has a very favorable performance with almost half of all data points showing a PCR of 100 after seven years from construction. ìWhen an overall performance curve was generated for all thick sites, a time to PCR of 60 of 14.03 years was obtained. The thick asphalt layers result in reduced stresses at the bottom of the asphalt layers, slowing the formation of bottom-up fatigue cracking.

Based on the results of this study, North Carolina will continue to construct [ABC] pavements, and will continue to track their performance.î

Canadian Recycling

Impact crushing, followed by mechanistic characterization of the resulting material, is the key to obtaining quality building materials for road bases in Saskatoon, Saskatchewan, write Berthelot, Haichert, Podborochynski, Wandzura, Taylor, Bews, Guenther and Prang in their paper, ìUse of Recycled Asphalt Concrete and Portland Cement Concrete in Road Substructure Construction.î

ìAs part of urban infrastructure renewal, significant amounts of asphaltic and portland cement concrete rubble are being generated in many urban centers,î the authors say. ìUnfortunately, much of these materials being generated are often disposed of in landfills, or are being used as low value backfill material for utility cuts.î

The primary historic limitations to processing and using these concrete materials as higher value road construction materials is production rate and quality of product obtained using conventional crushing equipment, particularly if the stockpiled materials are contaminated with deleterious materials and reinforcing steel, they say.

This project demonstrated impact crushing could efficiently process both RAP and Recycled Cement Aggregate into quality road materials. ìMechanistic laboratory characterization of the processed materials showed these materials to provide engineering properties that exceed that of conventional granular materials when characterized under typical field state conditions.î

The first objective of this study was to evaluate the effectiveness of processing stockpiled RAP and RCA using advanced impact crushing and screening to develop high-quality road substructure materials, the authors said.

ìThe second objective was to perform a mechanistic laboratory evaluation of the end-product recycled concrete materials generated to assess the recycled materials mechanical properties for use as road construction materials,î the authors say. ìThe third objective of this study was to perform a structural asset management survey of a recycled material test section in a road reconstruction application.î

Based on the findings of the laboratory characterization, the processed recycled RAP and RCA provide mechanistic material properties that equaled or exceeded that of conventional granular materials when characterized under realistic field state conditions, they concluded.

ìThe findings ... show considerable promise in an engineered application of a typical road reconstruction. Given increasing input costs for materials and construction energy, it is expected that the use of [these] recycled materials could provide the city of Saskatoon with significant capital infrastructure savings and a more sustainable infrastructure renewal process if a material recycling program be adopted within city of Saskatoon engineering specifications.î

More RAP, Lower Cost

Higher percentages of RAP in pavements in Virginia is justified, said Maupin, Diefenderfer and Gillespie in their paper, ìPerformance and Economic Evaluation of Virginiaís Higher RAP Specification.î

In 2007, the Virginia Department of Transportation decided to allow higher percentages of RAPómore than 20%, in HMA with no change in binder grade. This increase meant rewriting one section of the contract provisions in certain plant-mix overlay schedules around the state, raising the limit on the proportion of recycled material to 30% from the customary 20%.

ìOn specific VDOT paving projects in 2007,î the authors say, ìsix contractors produced a total of 129,277 tons of mix containing 21% to 30% RAP from seven asphalt plants in four districts. This study estimated the effect of increased RAP percentages on performance and relative cost.î

Mix from most of the high-RAP jobs was sampled and tested; mix from roughly comparable jobs containing less than 20% RAP also was sampled and tested for comparison purposes. Price and quantity data from all of the contracts in the 2007 resurfacing schedule were subjected to statistical analysis.

ìLaboratory tests performed on samples collected during production revealed no significant difference between the higher RAP mixes and the control mixes regarding fatigue, rutting, and susceptibility to moisture,î they say. ìBinder, recovered from asphalt mix sampled during construction, was graded to determine the effect of adding higher percentages of RAP.î

There were no construction problems attributed to the use of the mix with the higher RAP percentage. Only slight price adjustments were applied to two of the 10 high-RAP projects, and these adjustments were not due to the higher RAP percentage. Analysis of bid data found that the contract specification allowing the higher RAP percentages had a negative, but statistically insignificant, impact on the bid prices for surface-mix items.

Value engineering proposals received for jobs that were not advertised with the high-RAP specification also confirmed that the use of over 20% RAP could reduce costs in at least some cases.

RCA Pavements endure

Pavements containing RCA studied in 1994 and revisited 12 years later are performing in the field under traffic as well as control pavements, write Gress, Snyder and Sturtevant in their paper, ìPerformance of Rigid Pavements Containing Recycled Concrete Aggregates: 2006 Update.î

Interest in Portland Cement Concrete recycling using RCA was widespread in the mid-1970s, the authors write. ìIncreased interest is expected due to decreased availability of new aggregates, emphasis being placed on green highways, and exceptionably lowered carbon footprint for recycled projects. While most recycled pavements have performed acceptably, some have received national attention for their poor performance.î

To document performance of pavements containing RCA, in 2006, the Federal Highway Administration, through the University of New Hampshire Recycled Materials Resource Center, sponsored research to revisit the 1993 study project sites. The 2006 evaluation provided a better indication of long-term performance trends and further insight into the factors that affect RCA pavement performance.

The 1993 pavements containing RCA, first surveyed in 1994, were constructed in Connecticut, Kansas, Minnesota, Wisconsin and Wyoming. These pavements were resurveyed during the summer of 2006 to update their performance.

ìAlthough the recycled pavements contain higher mortar contents,î the authors write, ìthere was no clear correlation between recycled pavementsí higher total mortar content with cracking distresses in either survey, although one recycled pavement did exhibit more cracking than the control pavement. Overall there was little difference between the 1994 and 2006 surveys.î

Laboratory evaluation of field cores showed 10 of the 16 pavements surveyed were found to have Alkali Silica Reaction, possibly explaining why they were originally recycled. ìEight of these pavements were shown to have significant remaining expansion potential and are expected to continue expanding,î they say, ìall pavements identified with ASR and D-cracking showed field performance equivalent to their controls and pavements without distress.î

The recycled pavements have performed comparably to their controls. For instance, present serviceability rating was similar for the recycled and control sections. Likewise the recycled pavements that incorporated RCA derived from D-cracked and ASR concrete appears to be performing at least equivalent to the original pavements.

Repairs Eschew Cement

High-early strength, controlled low-strength material mixture designs containing no portland cement can be used successfully for green base repairs, say Crouch and Self in their paper, ìA Rapid Green Base Repair [with] CLSM [Controlled low-strength material]î.

These CLSM designs were developed using three different fine aggregates (natural sand, manufactured sand and limestone screenings) for green rapid subgrade repair applications under the direction of the FHWA, and included fly ash and other reclaimed industrial byproducts. CLSM is a highly flowable, self-compacting, cementitious material that is primarily used as a backfill in place of compacted earth fill. CLSM also is known as controlled density fill, flowable fly ash, unshrinkable fill, and soil-cement slurry. Itís made up of water, fine aggregates, and commonly contains fly ash or other industrial byproducts, chemical admixtures and optionally cement.

The compressive strength of CLSM is limited to 8.3 MPa (1,200 psi), which is much lower than that of normal concrete. ìLower limits on strengths can make future excavation an option if desired. CLSM is a safe and economical alternative, [as] it can be self-compacting and highly flowable, thus eliminating the need to enter confined spaces and excavation areas to manually compact fill.î

CLSM is used in lieu of backfill, and the innovative uses are growing every year. Common applications include trench fills, pipe embedment, pavement bases, abandoned underground facilities, structural fills and bridge approach repairs.

ìAn attempt was made to determine if 50% by volume replacement of the fine aggregate with ASTM No.67 stone would allow the modified mixtures to function as emergency green base repair materials. The substitution of coarse aggregate increased the static modulus of elasticity by an average of 69% and 78% at 24 hours and 28 days, respectively. The mixtures achieved a high level of stiffness, with a minimum of 7.9 GPa (1,150 ksi) at one day. Further, the mixtures with coarse aggregate substitution had average California Bearing Ratios of 83% and 605% at 6.5 and 24 hours, respectively.î

No mixture with coarse aggregate substitution had a CBR less than 55 at 6.5 hours and the mixture with limestone screenings as fine aggregate had a base quality CBR of 129%. ìAt 24 hours, the CBRs of the mixtures with coarse aggregate substitution were above 300, far above the typically specified 100 for a good base. Coarse aggregate substitution increased CBR by 324% and 244% on average at 6.5 and 24 hours. Finally, average compressive strengths of the mixtures with coarse aggregate substitution were 0.3 and 3.5-Mpa (43 and 507 psi) at 4 and 24 hours, respectively.î

ìCLSM has also been used in environment-enhancing applications such as protecting groundwater resources, and minimizing problems and damage caused by moving water,î the authors write. ìRapid-setting CLSM has proven to be an ideal backfill material for use on street repairs and infrastructure rehabilitation projects, [especially] situations in which pavement must be opened to traffic as soon as possible. These rapid-setting materials also have low shrinkage, are economical, are not labor intensive, and can be excavatable if desired.î