This year and next, the Federal Highway Administration will direct big bucks toward field testing of two-lift concrete paving, according to a disclosure
Tom Kuennen
This year and next, the Federal Highway Administration will direct big bucks toward field testing of two-lift concrete paving, according to a disclosure at the 86th annual meeting of the Transportation Research Board (TRB) in Washington, D.C., in January. That information was revealed during a panel discussion on long-life concrete pavements targeting design technologies gleaned during an FHWA-sponsored International Technology Scanning Tour in 2006. The panel was one among dozens of presentations and technical papers devoted to ready mixed concrete at TRB, where more than 3,000 scheduled events addressed topics of interest to the public and private sectors responsible for transportation infrastructure. A sampling follows of presentations of interest to the ready mixed industry.
FHWA TO SPONSOR TWO-LIFT CONCRETE PAVING FIELD TESTS
FHWA will underwrite field demonstrations of twin-lift concrete paving technology in a number of states across the country Û so indicated an announcement during a panel on the findings of an International Technology Scanning Tour on Long Life Concrete Pavements. FHWA has a program to demonstrate two-lift concrete paving technology, notes Suneel Vanikar, P.E., panel moderator, and Concrete Group Leader, Office of Pavement Technology, FHWA. We are looking for six to eight projects in various parts of the country, with a maximum funding of about $1 million for each demo, plus additional money for workshops and showcasing.
Two-lift construction involves the placement of two wet-on-wet layers, or bonding wet to dry layers of concrete, instead of the homogenous single layer commonly placed in concrete paving, reports the National Concrete Pavement Technology Center (NCPTC) at Iowa State University, Ames. With two-lift paving, a thick bottom layer contains lesser quality, lower durability or strength, locally available aggregate, or recycled aggregate comprising asphalt, concrete rubble, or local aggregate. A thin top layer consists of high-quality aggregate designed to provide superior resistance to freeze-thaw damage as well as noise reduction and/or improved traction.
While two-lift paving is not new to the United States, today’s emphasis is unprecedented. Between 1950 and 1970, two-lift paving was implemented extensively in many states, including Iowa, Wisconsin, Michigan, Pennsylvania, and Minnesota, to facilitate placement of mesh in Interstate highway construction. Between 1970 and 2000, the U.S. concrete paving industry moved away from a mesh pavement design and significantly shortened the design length of pavement panels, effectively eliminating the need for two-lift paving.
Following the May 2006 international tour, however, interest in twin-lift paving was revived. The goal of the long-life concrete pavement scan tour was to learn more about design philosophies, materials requirements, construction practices, and maintenance strategies involved in constructing and managing portland cement concrete pavements with long life expectancies.
Sponsored by FHWA, American Association of State Highway and Transportation Officials (AASHTO), and the Transportation Research Board’s National Cooperative Highway Research Program (NCHRP), the scan team included representatives from state transportation departments, FHWA, NCHRP, academia, and industry associations.
Among other recommendations, the long-life concrete pavement scan team tentatively identified two-lift concrete placement as a technology to be pursued in this country, in combination with exposed aggregate surfaces for noise reduction and lower-quality aggregates and recycled concrete in the bottom course for economic and environmental needs. Accordingly, Iowa State University’s NCPTC will collaborate with FHWA to facilitate a Two-Lift Concrete Paving Construction program. A framework is under development; and, several state DOTs have indicated interest in implementing the technology. NCPTC’s Tom Cackler is the point person for this activity.
There can be variations in what each state does, Vanikar tells Concrete Products in Washington. We have been fortunate enough to work a deal with our Highways for Life folks so they have agreed in principle that these projects can be funded by FHWA. We are urging representatives of state DOTs to see what the possibilities would be for doing a demo in their states.
This is one of the best deals to come out of Washington, so we want to make sure states take advantage of the opportunity, Vanikar affirms. For example, it could become a state’s Highways for Life project. We want to get something on the ground in the next couple of years, and want to move very fast.
CONCRETE PRISM TEST BEST METHOD FOR JUDGING LITHIUM DOSAGE
To determine the dosage of lithium required to adequately suppress alkali-silica reactivity (ASR)-induced expansion, the best method is the concrete prism test using a 0.04 percent expansion limit at two years, write Folliard, Barborak and Ideker, Concrete Durability Center, University of Texas at Austin; Fournier of the Concrete Technology Program, CANMET-MTL; and Thomas, Department of Civil Engineering, University of New Brunswick, in a paper titled Laboratory Test Methods for Determining the Dosage of Lithium Nitrate Required to Control ASR-Induced Expansion. Their research on the use of lithium as an admixture to control ASR in concrete was mainly FHWA funded with additional support from the International Center for Aggregate Research (ICAR), and the Texas Department of Transportation.
The study involved testing a range of reactive aggregates in combination with lithium nitrate. The concrete prism test (ASTM C 1293), accelerated mortar bar test (ASTM C 1260), and selected modifications of both regimens were used in the research. Additionally, outdoor exposure blocks were cast for a more realistic and accurate evaluation of long-term performance.
Chief among the study’s findings is identification of the best method for determining the dosage of lithium required to adequately suppress ASR-induced expansion: the concrete prism test, using a 0.04 percent expansion limit at two years. A modified version of the accelerated mortar bar test, in which the [Li/Na+K] ratio of the soak solution is matched to that of the interior of the mortar bars, was found to overestimate the benefits of lithium and underestimate the actual amount of lithium nitrate needed to suppress expansion in the concrete prism test and outdoor exposure blocks, the authors report.
Another major finding, they add, is that the dosage of lithium nitrate required to suppress expansion is highly dependent upon aggregate mineralogy. Moreover, the most commonly cited dosage [based on Li/Na+K ratio of 0.74] in technical and product literature is not adequate to control expansion for some aggregates.
Over nearly four decades, the authors contend, little interest in using lithium compounds to control concrete’s alkali-silica reactivity was reflected in only a few studies. [But] in the past 10 years or so, a resurgent interest in lithium-bearing compounds is evident in scientific publications and field applications, they assert. Most of the work done to date on lithium compounds has involved their use as a chemical admixture in mortar and concrete. While a general consensus exists that lithium compounds have shown an ability to reduce expansion by ASR in fresh concrete as long as they are used at appropriate dosages, still undetermined is the specific dosage required for a given aggregate.
Following extensive testing, the authors conclude:
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The dosage of lithium needed to suppress ASR-induced expansion is highly dependent on aggregate mineralogy. Interestingly, aggregates that are extremely reactive Û as evidenced by excessive expansion values in the concrete prism test Û typically tend to require less lithium to suppress expansion than aggregates that are less expansive in the concrete prism test.
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The conventional approach of specifying a standard lithium dosage (e.g., based on a [Li/Na+K] molar ratio of 0.74 is not appropriate and may result in nondurable concrete when using certain aggregates. A range of aggregates evaluated in this research, or in collaborative research efforts, requires substantially more than this commonly specified dosage (a.k.a. the 100 percent manufacturer’s recommended dosage). In fact, one Canadian greywacke aggregate requires greater than 200 percent of the manufacturer’s recommended dosage to adequately suppress expansion.
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A modified version of the accelerated mortar bar test (ASTM C 1260), in which the [Li/Na+K] of the soak solution is matched to that of the mortar bars, tends to overestimate the beneficial effects of lithium-based admixtures, thereby underestimating the actual dosage of lithium needed to suppress expansion in field concrete. In addition, this test is highly sensitive to the alkalinity of cement, further highlighting shortcomings of the testing regimen. Consequently, research is underway to develop an accurate, yet rapid, test method for determining safe levels of lithium sufficient to control ASR-induced expansion.
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Based on the above findings, substantial changes have been made to FHWA guidelines on using lithium-based admixtures. The first set of guidelines, lacking verification by laboratory or field research, was based solely on a review of available literature. The authors’ comprehensive laboratory and field evaluation of lithium nitrate-based admixtures has yielded results leading to substantial changes to the initial recommendations.
Revised guidelines, currently in press, will eliminate the prescriptive approach to specifying lithium dosage, a Îone size fits allÌ method (based on 0.74 molar ratio of Li/Na+K), and withdraw the modified version of ASTM C 1260 (including the Li/Na+K ratio of the soak solution matched to that of the mortar bars) as a viable test method for predicting requisite lithium dosages.
Currently, it is recommended that the concrete prism test be used to determine the dosage of lithium required to suppress ASR-induced expansion, the authors note, with an expansion limit of 0.04 percent at two years.
POTASSIUM ACETATE DEICER USED AT AIRPORTS WILL TRIGGER ASR
Potassium acetate deicer solution, like that used at some airports and airfields, is capable of causing significant alkali-silica reactivity distress in concrete specimens containing reactive aggregates, write Rangaraju and Sompura, Department of Civil Engineering, Clemson University; and Olek, School of Civil Engineering, Purdue University, in their paper, Investigation into Potential of Potassium Acetate Deicer Solution to Cause ASR Using a Modified ASTM C 1293 Test Method. In addition, the authors observe, a secondary reaction product, comprising primarily potassium sulfate phases, was observed in prisms containing both reactive and nonreactive aggregates.
Following recent Federal Aviation Administration investigations into premature deterioration of some airfield concrete pavements exposed to deicing chemicals, a comprehensive research study was undertaken to examine the role of deicing solutions in ASR occurring in mortar and concrete test specimens. Specifically, the researchers evaluated the impact of potassium acetate deicer/anti-icer on concrete specimens with respect to ASR distress. A modified ASTM C 1293 test method was employed in their investigation, wherein concrete prisms containing aggregates of known reactivity were exposed to a 50-percent solution of potassium acetate (KAc) or a normal solution of sodium hydroxide (1N NaOH).
Expansion of the concrete test prisms was monitored periodically, along with changes in their dynamic modulus of elasticity. Further, visual and scanning electron microscopic (SEM) examinations were conducted on polished specimens at the conclusion of the tests. The pH of the deicer soak solution also was monitored to detect any changes caused by its interaction with the specimens.
Based on observation of changes in specimen length, dynamic modulus of elasticity, and soak solution pH during standard and modified ASTM C 1293 tests, plus microscopic examination of concrete prisms, the authors assert:
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Potassium acetate deicer solution is capable of triggering deleterious alkali-silica reaction in concrete specimens containing reactive aggregates.
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The magnitude of expansion observed in concrete prisms in the standard ASTM C 1293 test and modified ASTM C 1293 test reflected the degree of reactivity of aggregate used in the prisms.
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For concrete prisms of given cement alkalinity and reactive aggregate source, exposure to potassium acetate deicer solutions caused more distress (expansion and loss in DME) than exposure to 1N NaOH solution in the modified ASTM C 1293 test. The specific degree of distress, however, depended on the reactivity of the individual aggregate.
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With the exception of the specific alkali ion, the composition of the ASR gel observed in the concrete prisms subjected to the potassium acetate deicer solution was similar to that observed in prisms subjected to 1N NaOH solution.
DEPLETED MUSHROOM SUBSTRATE POTENTIAL AGGREGATE SUBSTITUTE
Depleted mushroom substrate may find its way into portland cement concrete, as researchers in mushroom-producing Pennsylvania attempt to qualify used mushroom growth medium as a fine aggregate substitute. After treatment with quick lime, spent mushroom substrate (SMS) might be recycled as a sand substitute in concretes used for sidewalks, concrete curbs, concrete barricades, sound walls, and other nonstructural applications, write Pang, Liu and Suri, Department of Civil & Environmental Engineering, Villanova University (Pa.), in their paper, Recycling Spent Mushroom Substrate as Fine Aggregates in Concrete.
Although the durability of this kind of concrete needs further investigation, the authors attest, satisfactory results were found in compressive strength tests. They examined methods for treating SMS before mixing as aggregate in order to make the recycling process economical and applicable. A cost-benefit analysis and environmental impact estimation were completed to evaluate the economy, efficiency and effectiveness of recycling the material into concrete mixes.
Disposal of SMS has been a major problem facing farmers, especially in areas where yield far exceeds demand, the authors report. Traditionally, SMS was discarded as waste, creating an environmental nuisance, they say. SMS piles can create detrimental runoff containing aluminum and iron precipitates, as well as phosphorous and nitrogen, which are major sources of pollutants in lakes and streams.
Today, mushroom growers worldwide face increasing regulatory pressure due to environmental legislation, giving rise to the need for a more suitable SMS disposal solution. According to the authors, SMS has been used in general agriculture as a soil remediation agent; in horticulture, as mulch and a component of soil mixes and potting soils; as fill for abandoned strip mines; to inhibit the growth of some fungus; to remedy contaminated water in wetlands; and, by state DOTs, for miscellaneous purposes in highway construction. Therefore, they emphasize, SMS should be treated to suit the specific demand of each outlet.
The moisture content of a SMS product is important where uniform and good mixing is desired. Products with moisture content between 30 and 50 percent typically are suitable for handling and uniform mixing with other materials. Wet SMS (greater than 60 percent moisture content) is heavy and tends to form large clumps; thus, wet product is difficult to handle and does not mix evenly with other materials.
Some previous experiments indicate that concrete mixed with fresh SMS not only fails to set two weeks after mixing, but also releases strong offensive odors, the researchers say. Treating SMS with quicklime before mixing can stabilize its pH and reduce peculiar odors by halting anaerobic digestion and getting rid of ammonia present in the fresh substrate. Results from our experiments demonstrated that concrete could set in a couple of days if SMS is treated with a certain amount of quicklime prior to mixing, although its hardness is relatively weak in the first week.
Five batches of concrete mixed with SMS were produced and tested. Proportioning of ingredients matched that of the control batch, with the exception of a certain amount of sand replaced by SMS. Accordingly, in the five batches, fine aggregate consisted of both sand and SMS. Also, moisture content of the substrate was taken into consideration when water was proportioned, i.e., actual water added to the mixture was less than that of the control batch.
Our study found that Spent Mushroom Substrate has the potential to be economically and massively recycled into concrete, the authors note. The amount of SMS used should be adjusted to fit the required mix design or specified concrete strength. For nonstructual applications, SMS should be treated with either quicklime or cement before mixing into concrete to facilitate concrete hardening and reduce offensive odors. For economic considerations, the amount of quicklime recommended for use is about a third of SMS moisture content by weight.
RESIN-BASED ADMIXTURE CAN SALVAGE MARGINAL AIR VOID MIXES
A Vinsol admixture outperforms a synthetic agent in providing freeze-thaw resistance in marginal air void mixes, write Federal Highway Administration’s Tanesi and Meininger, in Freeze-Thaw Resistance of Concrete with Marginal Air Content. The authors explain, Freeze-thaw resistance is a key durability factor for concrete pavements. Recommendations for air void system parameters normally comprise 6 percent (±1 percent) total air and 0.20-mm (0.008-in.) spacing factor. However, laboratory studies revealed that some concretes lacking these commonly accepted thresholds presented good freeze-thaw resistance.
The researchers evaluated the freeze-thaw resistance of several marginal air void mixes containing two types of air-entraining admixture (AEA) Û a Vinsol resin and a synthetic agent. The study used rapid cycles of freezing and thawing in plain water without deicing salts.
For the specific materials and mix proportions incorporated in the project, marginal air mixes Û i.e., concrete with fresh air contents of 3.5 percent or higher Û presented an adequate freeze-thaw performance when Vinsol resin based air-entraining admixture was used. Two sets of concrete were prepared with varying fresh air contents of 2.5 percent and 4.5 percent. Set 1 and Set 2 differed only in type of air-entraining admixture: Set 1 had Vinsol resin and Set 2 synthetic. For the mixes prepared and specific admixtures used, Vinsol resin mixes exhibited better freeze-thaw resistance, although corresponding air void systems were less satisfactory with respect to spacing factor and specific surface.
The reasons for this unexpected observation are not known, say Tanesi and Meininger. Insufficient data is offered in this study to generalize results for all the Vinsol resin and synthetic air-entraining admixtures and all levels of air content. More research is needed in order to confirm this finding.