| RCC is placed in Fayetteville Shale Play Area to accommodate heavy truck traffic resulting from the natural gas extraction boom. Different thicknesses and sub-bases were specified for comparison along the two-mile demonstration. PHOTO: Arkansas State Highway & Transportation Department |
By Tom Kuennen
Roller-compacted concrete (RCC), followed by diamond grinding to smooth the surface, is the smart choice for rebuilding rural roads damaged by energy development truck traffic in Arkansas, according to new research presented at the 93rd annual meeting of the Transportation Research Board earlier this year in Washington, D.C.
The 2014 TRB conference drew nearly 12,000 transportation professionals. More than 4,500 presentations in nearly 800 sessions and workshops were conducted during TRB week, covering all modes of transportation on behalf of government, industry and academia delegates. For nearly six decades TRB has been held among a group of hotels along Connecticut Ave. near Woodley Park and the National Zoo, but next year it will move to the Walter E. Washington Convention Center north of Mt. Vernon Square.
Last month our TRB report focused on precast/prestressed research; see FRP Hybrids Make Encore Appearance at TRB, April 2014, pp 39-41, or visit http://concrete.epubxp.com/i/289704/40. Following are summaries of TRB technical papers involving ready mixed or cast-in-place concrete. For more information, visit www.trb.org.
Roller-Compacted Concrete Makes Grade on Rural Roads
New methods of extracting oil and natural gas in areas unaccustomed to the heightened activity have led to deteriorated roads from loads for which they were never designed, and owning agencies strapped to pay for the repairs. To this end, roller-compacted concrete (RCC) is the right choice for reconstructing truck-damaged two-lane roads in remote areas of Arkansas, according to Stacy Williams, Ph.D., P.E., University of Arkansas-Fayetteville, in her peer-reviewed paper, Construction of Roller Compacted Concrete Pavement in the Fayetteville Shale Play Area.
“Due to recent heavy truck traffic related to natural gas exploration efforts in Arkansas, the Fayetteville Shale Play Area has experienced significant pavement distresses that typical maintenance activities cannot sufficiently address,” she says. “RCC pavement was selected as a potential solution for failing pavements in this area because it can be constructed more quickly than conventional concrete pavements and provide adequate structure for truck traffic.”
Two sections of RCC pavement were constructed on a rural two-lane highway, she writes. Section I had 6 inches of full-depth cement-treated base reclamation, topped by 7 inches of RCC. Section II had an 8-in. RCC overlay of the existing pavement, including level-up. Safety edge was used on both sections, as was diamond grinding to improve smoothnessa point of criticism for RCC surfaces. The RCC mixture contained a nominal maximum aggregate size of ¾-inch, and was designed to meet a minimum 28-day compressive strength of 5,000 psi. Although some difficulties were encountered during construction, the final product was considered successful.
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| RCC is placed in Fayetteville Shale Play Area to accommodate heavy truck traffic resulting from the natural gas extraction boom. Different thicknesses and sub-bases were specified for comparison along the two-mile demonstration. PHOTO: Arkansas State Highway & Transportation Department |
Low initial strengths in Section I were believed to be caused by a combination of low temperatures and the presence of fly ash in the mix. Fly ash was removed from the mix design before Section II construction began, and the strength characteristics were much improved. Diamond grinding was employed to ensure surface smoothness, resulting in an average IRI value of 69.5. After six months, a distress survey was performed, revealing minimal cracking. Minor surface distresses were noted, including popouts and deterioration at construction joints.
The project took less time and money than conventional reconstruction of a typical two-lane rural road. Construction of both miles took nearly one month, where reconstruction would have taken most of a construction season. The cost for the project was $1.9 million for the two miles, compared to the average $3 million per mile the Natural State spends for reconstructing a two-lane rural roadway. Ultimately, the project saved Arkansas approximately $2 million per mile compared to conventional asphalt reconstruction, and the RCC pavement should be able to withstand the increased weight loads from the heavy vehicle traffic.
“After equipment and plant issues were corrected, the RCC process proved to be an efficient solution for providing significant structural enhancements to an otherwise deteriorated roadway,” Williams affirms. “Low strengths were believed to be caused by both low nighttime curing temperatures and the presence of fly ash.”
Laboratory-prepared specimens that were cold cured exhibited low strength levels, indicating that cooler temperatures adversely affected strengths. “Fly ash was also believed to be partially responsible because the strength values of Section II were much improved after fly ash was removed from the mix design, while nighttime temperatures remained similar,” Williams writes. “Without adequate early strength and the ability to quickly apply traffic to the roadway, one of the major advantages of RCC is negated. Thus, further investigation is needed to establish the interaction of fly ash and curing temperature for RCC.”
Williams recommends that a) restrictions be placed on RCC paving such that a temperature of 50°F be required as a minimum for placement, with a minimum predicted nighttime temperature of 40°F; b) fly ash should be used in RCC paving mixes unless further research determines the particular conditions or curing temperatures in which it would perform adequately; and, c) diamond grinding should be employed if the RCC is to be used as the wearing course.
Virginia Finds PCC Overlays Viable Options for CRCP
Despite their higher cost and longer curing time, portland or hydraulic cement concrete overlays are a cost-effective option to hot mix asphalt for surfacing continuously reinforced concrete pavements, say Michael M. Sprinkel, P.E., Celik Ozyildirim, Ph.D., P.E., and Shabbir Hossain, Ph.D., P.E., Virginia Center for Transportation Innovation and Research, and Mohamed Elfino, Ph.D., P.E., Chung Wu, Ph.D., P.E., and Affan Habib, Ph.D., P.E., Virginia Department of Transportation, in their TRB paper, Concrete Pavement Overlays on U.S. 58.
“Asphalt overlays are typically used to extend the life of continuously reinforced concrete pavement (CRCP) because they can be placed in one or more layers while traffic uses the adjacent lane, and they can be opened to traffic in a short time,” the Virginia team writes. “Hydraulic cement concrete overlays have also been used to extend the life of CRCP but have often not been considered an alternative to asphalt because of the higher cost and longer curing time.”
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| During concrete overlay along U.S. 58 in Virginia, certain factors impacted workability of mixture for edges: high slumps make edge difficult to form, while insertion of tie bars also affects the edge. |
That’s changing due to favorable cost differentials. “As the price of asphalt continues to increase, HCC overlays are becoming more competitive on an initial cost basis and more economical on a life cycle cost basis,” note Sprinkel, Ozyildirim, Hossain, Elfino, Wu and Habib. “Bonded HCC overlays placed on I-295 and I-85 in 1995 are still in service after 18 years, providing a longer life than most asphalt overlays. The overlays are in much better condition than the heavily patched pavement sections located immediately before and after the bonded overlays.”
To further study the opportunity, Virginia DOT undertook 2012 rehabilitation of a 4.8-mile CRCP on U.S. 58 in Southampton County using two sections, a 4-in.-deep bonded concrete overlay, and a 7-in.-deep unbonded concrete overlay with a 1-in. asphalt separation layer. The existing four-lane, divided primary highway is an 8-in.-thick CRCP placed over a 6-in. cement treated aggregate layer. Saw cutting was used to form joints at 6- x 6-ft. panels for the unbonded overlay, and tie bars were used along the centerline of the pavement and along both shoulders.
A concrete overlay was placed on the shoulders of the unbonded overlay, and asphalt was placed on the shoulders of the bonded overlay. Two layers of asphalt with a total thickness of 5 inches were placed on a 9.75-mile section of the eastbound lane of U.S. 58, which provided cost information that was used to compare the alternatives. After evaluation of the projects, Sprinkel, Ozyildirim, Hossain, Elfino, Wu and Habib conclude:
Construction of the concrete overlays was successfully executed on time. Concrete was of high quality with good strength and low permeability. The bonded overlay is well-bonded.
As constructed, ride quality was good for the unbounded section (IRI = 56 to 71 in./mile) and fair for the bonded section (IRI = 73 to 93 in./mile); the ride quality for both was much better than the original pavement.
Concrete overlays can be economically constructed for a 30-year design life. Using the initial cost of materials in-place, the bonded and unbonded overlays were approximately same at an average of $33 to $35 per sq. yd. The unit cost of patching of concrete pavements is more than six times the cost of the concrete overlay. Unbonded concrete overlays should be used when more than about 10 percent of the CRCP must be patched.
On the other hand, bonded concrete overlays should be considered when the CRCP is in good condition and needs negligible patching. Bonded and unbonded concrete overlays are a competitive alternative to asphalt overlays.
High Volume Fly Ash Mixes for Tennessee Bridge Decks
High-volume PCC with at least 50 percent of portland cement replaced with Class C fly ash outperforms the existing Tennessee spec of 25 percent maximum Class C fly ash in mixes, say L. K. Crouch, Ph.D., P.E., Aaron Crowley, E.I.T., Daniel Badoe, Ph.D., Tennessee Technological University-Cookeville, and Heather P. Hall, P.E., Tennessee DOT, in their TRB paper, A High Volume Fly Ash Concrete Mixture for Tennessee Bridge Decks.
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| Tennessee DOT Class D and HVFA Mixture Designs TABLE: Crouch, Crowley, Badoe and Hall |
“High volume fly ash (HVFA) portland cement concrete was developed to compete with TDOT Class D PCC,” the authors note. “HVFA PCC is PCC with at least 50 percent of portland cement replaced with Class C fly ash. A higher PC replacement rate would greatly increase the use of an industrial byproduct, thus making more efficient use of natural resources. However, performance and economy cannot be sacrificed for environmental concerns,” they emphasize, stressing sound engineering benefits as opposed to environmental sustainability.
HVFA PCC has lower total cementing materials and water contents than TDOT Class D PCC, and is similar in material costs below a placement temperature of 85°F and cheaper at a placement temperature of 85°F and above.
A literature search and series of tests followed. “The results of the research show first that HVFA PCC meets all TDOT 604.03 Class D PCC property requirements,” the authors conclude. “Second, HVFA PCC is statistically superior to TDOT Class D PCC in compressive strength (at seven to 182 days), static modulus of elasticity (28 to 182 days), hardened concrete absorption (28 to 182 days) and rapid chloride permeability (at 91 days). Finally, above 85°F, hot HVFA PCC hardened properties and rapid chloride permeability are statistically superior to hot TDOT Class D PCC (at seven to 182 days).”
ILLINOIS ENGINEERS: LIMESTONE, OTHER ADDITIONS DON’T ALTER CONCRETE PERFORMANCE
Concrete batched with cement and 5 percent by weight of limestone and/or inorganic processing additions has similar performance in terms of strength and durability as conventional concrete, say Mustapha A. Ibrahim and Mohsen A. Issa, University of Illinois-Chicago, Mustafa Al-Obaidi, HBM Engineering Group, LLC, Chicago, and John Huang, Abdul Dahhan and Carmen Lopez, Illinois Department of Transportation, in their peer-reviewed, 2014 Transportation Research Board Meeting paper, Effect of Limestone and Inorganic Processing Addition[s] on the Performance of Concrete for Pavement and Bridge Decks.
Typically, inorganic processing additions (IPAs) include limestone, fly ash, bottom ash, slag, cement kiln feed, cement kiln dust, and calcined byproducts. As with other countries, in an effort to suppress carbon dioxide emissions toward a more environmentally sustainable concrete, the United States has been moving toward further acceptance of limestone and other additions as replacement for cement in concrete mixes.
“The addition of limestone and alternative raw materials to cement to reduce CO2 emissions in the cement production and concrete industries has been used in Europe for decades, with quantities up to 35 percent replacement of cement by weight,” the authors write. “The Canadian Standards Association (CSA A3000) recently approved the addition of limestone in cement up to 15 percent by weight.”
Current ASTM C150/AASHTO M 85 and ASTM C465/AASHTO M 327 specifications state that the maximum limestone and IPA of cement is limited to 5 percent by weight, the writers say. Now, they add, Illinois DOT is pushing forward in its efforts to modify the ASTM specifications to approve the use of limestone and IPA with more than 5 percent replacement of cement by weight.
“[IDOT] is making several changes to concrete mix designs, using revisions to [ASTM] cement specifications [to] enable use of more sustainable materials for concrete pavements, overlays and bridge decks,” the authors write. “If this modification is approved, it will have both an environmental and economic impact on the concrete industry in the United States. The addition of more than 5 percent limestone and IPA to cement, as proposed by IDOT, will mitigate some environmental problems by reducing the amount of raw materials burned to produce cement and to reduce the carbon footprint by at least 3 to 4 percent of total CO2 emissions.”
Twenty-four concrete mixes with different cementitious combinations and aggregates were developed for the study. Each cement source was batched in a concrete mixture by replacing 30 percent of the total binder content with supplementary cementitious materials (SCM), fly ash or slag. Also, each cementitious combination was batched with fine aggregates (either natural or combined sand) and coarse aggregate (crushed limestone).
“The results of this study showed that increasing the amount of limestone and IPA in cement in quantities exceeding 5 percent by weight, and the increase of IR to 1.5 percent had negligible effect on the strength and durability properties of concrete,” Ibrahim, Issa, Al-Obaidi, Huang, Dahhan and Lopez write.