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New Markets for Old Products.


Local materials - not costly products shipped from great distances - should be used for bases if they can be modified or used in deeper lifts, said aggregates researchers at the 89th Transportation Research Board meeting in January. Despite the recession, more than 10,000 delegates from around the world attended TRB.

If local aggregate can be brought up to spec, they can be used at great savings for bases over higher-quality materials brought in from a distance, said Biraj Gautam, Marino Engineering Associates, Urbana, Ill., and Deren Yuan and Soheil Nazarian, Center for Transportation Infrastructure Systems, University of Texas-El Paso, in their paper, "Optimum use of Local Material for Roadway Base and Subbase."

Base materials that meet specifications are becoming more difficult to secure in many regions of the United States, the authors write. "As a result, higher-quality materials have to be hauled long distances," they say. "This act would significantly increase the costs associated with roadway construction and subsequent maintenance and rehabilitation."

At the same time, low-quality or out-of-specification materials are usually available from local sources. "If through appropriate treatment of the materials or/and structural design, the optimum use of local materials can be permitted, the construction can be accelerated and significant monetary benefits can be realized," they say.

Under many current specifications, a material can be considered low quality for reasons such as inadequate gradation, plasticity and strength. "In many cases, the local base supplies miss the specifications by small margins," the authors write. They presented a test protocol for the use of low-quality, flexible base materials based on the test results of materials from eight local pits in Texas, documenting how a low-quality material can be used on low-volume roads and still get a quality foundation layer.

The authors found that:

Permanent deformation of the base layer in a pavement with thin surfacing controls rutting.

Current pavement design algorithms (FPS19 or AASHTO 2003) should be used with care for the base courses of lower-quality materials. These algorithms tend to provide base thicknesses that may exaggerate the rutting of bases. More advanced analysis is recommended to ensure the stability of low-quality bases.

The use of low-quality materials as subbase seems feasible and advantageous. The top 6 to 8 inches of the base layer in a low-volume road with thin surfacing seems to contribute the most to rutting. As such, for a base course thicker than 12 inches, the use of low-quality material as subbase, especially for strong subgrades, is recommended.

The quality of most low-quality materials can be significantly improved by treating them with a small amount of calcium-based additives (1% to 2%). In this study, for all cement-treated base materials (1% cement) and most of the lime-treated base materials (1% lime), both unconfined and confined strengths met the minimum requirements in TxDOT Item 247.

For most materials, adjustment in gradation did not significantly improve their quality.

Most of the untreated materials did not achieve adequate modulus. The resilient modulus of treated materials was significantly higher.

INSTEAD OF SENDING limestone screenings to the landfill, they should be put to use in larger quantities in road construction, benefiting aggregate producers as well as their customers, according to "An Investigation of the use of Limestone Screenings in Roadway Construction," by Tyson D. Rupnow, Louisiana Transportation Research Center, and Vernon R. Schaefer and David J. White, department of civil, construction and environmental engineering, Iowa State University.

The general problem with limestone screenings is that the high-fines content makes the screenings unfit for use in concrete production or in roadway aggregates, they say. "However, if the screenings are combined with a stabilizer that can provide some beneficial cementing, useful benefit of the screenings can be made. Potential stabilizers include cement and other waste products such as fly ash or cement kiln dust. Thus, there exists an opportunity to use limestone screenings as another product with which to generate revenue by applying it to roadway construction."

The authors investigated the utility of stabilized limestone screenings in roadway construction. Three objectives were pursued: conduct laboratory studies of the proposed stabilized mixtures of limestone screenings and cement kiln dust (CKD) and Class C fly ash; document construction operations; and conduct field tests of the stabilized roadway to evaluate effectiveness of the stabilized limestone screenings as structural layers in the roadway test sections.

For this study, limestone screenings were a byproduct of crushed limestone production with a maximum particle size of about 9.56 mm and about 25% fines passing the Number 200 sieve. Manufactured sand was crushed limestone sand with a maximum particle size of about 9.56 mm and about 10% fines passing the Number 200 sieve.

Compaction, unconfined compression, freezing and thawing durability, and wet and dry durability tests were performed to determine if limestone screenings could be stabilized and used as a structural layer in road construction. Falling weight deflectometer (FWD) tests were used to compare stiffness differences. And temperature data was used to determine the number of freezing and thawing cycles each test section underwent. Laboratory compaction and unconfined compression test results confirm that limestone screenings can be stabilized for use as a structural layer.

Freezing and thawing and wet and dry durability test results show that CKD is not an acceptable stabilizer due to poor durability performance. Class C fly ash and CKD mixtures were acceptable. Test Section 1 (30% CKD) most likely failed due to freezing and thawing. Visual observations showed good performance of Test Section 2 (15% fly ash and 15% CKD), and the two control sections with no rutting or pothole formation.

The authors concluded that:

Combinations of CKD and Class C fly ash significantly increase the unconfined compressive strength of the stabilized mixtures, leading to a final product that can withstand daily quarry truck traffic.

The moisture and strength curves show that exceeding the optimum moisture content for strength leads to dramatically reduced unconfined compressive strengths. This proves that construction operations should use the optimum moisture content based on strength rather than the optimum moisture content based on density.

Freezing and thawing and wet and dry durability testing show that CKD-stabilized limestone screenings is not a viable construction alternative. Adding Class C fly ash with CKD significantly increased the durability of the mixtures.

The documented construction operation showed that it is viable to stabilize limestone screenings in a cost-effective and timely manner, and was very effective in producing a structural layer constructed of stabilized limestone screenings.

FWD results proved that stabilized limestone screenings can perform as a structural layer in road construction, and visual observations confirmed a well-performing structural layer constructed of stabilized limestone screenings.

The control sections performed nearly equal to Test Section 2 indicating that stabilization may not be required if the traffic volume is substantially low.

RECLAIMED-ASPHALT-PAVEMENT aggregate containing steel slag expands less than virgin steel-slag aggregate, enhancing its potential use in pavement base courses, according to "Evaluation of the Expansive Characteristics of Reclaimed Asphalt Pavement (RAP) and Virgin Aggregate Used as Base Materials," by Derya Deniz, Erol Tutumluer, and John S. Popovics, department of civil and environmental engineering, University of Illinois at Urbana-Champaign.

A viable solution for desposing of large quantities of RAP is to incorporate them into base and subbase applications for highway construction. The authors compare the expansive properties of RAP materials, especially the ones including recycled steel-slag aggregates, with those of the virgin aggregates, to evaluate their potential use as pavement base materials.

Seventeen RAP materials and virgin aggregates collected in Illinois were tested for their expansive characteristics in the laboratory following the ASTM D4792 test method. The specimens in California Bearing Ratio test molds were submerged into a high-alkali cement-water solution and kept soaked at 70[degrees]C to accelerate hydration reactions.

Some steel-slag aggregates showed considerably high-expansion potentials, up to 6.2% swell, when compared to virgin aggregates, such as siliceous gravel and crushed dolomite, which had minor or almost no expansion.

The RAP materials, which often had lower densities, exhibited more initial settlement or contraction before any kind of expansion with time. Two RAP materials, surface RAP with 92% steel-slag aggregates and steel-slag RAP, gave the maximum expansion amounts of 1.69% and 1.46%, respectively.

"Although the RAP materials had much lower tendencies to expand when compared to the virgin steel-slag aggregates, the use of RAP materials containing high percentages of steel-slag aggregates may have to be avoided in the pavement substructure layers depending upon the level of expansion," the authors say.

Nonporous steel-slag aggregate gave the maximum expansion as 6.18%, mainly due to free lime hydration and evidenced by tufa-like precipitate formation observed in the test setup, they write. It was followed by porous-surfaced steel-slag aggregate, surface RAP with 92% steel-slag aggregates and steel slag RAP (compacted with standard Proctor hammer) with expansion amounts of 4.14%, 1.69% and 1.46%, respectively.

"A clear conclusion from the expansion test results was that RAP materials had much lower tendencies to expand when compared to high-expansion potentials of especially the virgin steel-slag aggregates," the authors say.

"Since steel-slag surface texture is often rougher than other natural aggregates, their friction properties are superior and they have significantly improved adhesion ability with asphalt binder. Therefore, the significant differences found between the expansion values of the virgin and RAP steel-slag aggregates may depend on an effective asphalt coating around the aggregate, which prevents any significant ingress of water into the aggregate."

Based on ASTM D2940, which limits expansion values to 0.5% at seven days when tested in accordance with Test Method D 4792, the authors also conclude that steel slag from District 4, SMA RAP, steel-slag RAP, surface binder RAP with 60% steel-slag aggregates, and surface RAP with 92% steel-slag aggregates (almost) may be used as pavement base course aggregates.

"On the other hand, porous and nonporous steel-slag aggregates should never be used in the bases or subbases without any proper curing that satisfies the limitation specified by ASTM D2940," they write.

FOR PAVEMENT BASES and subbases, the mechanical and hydraulic characteristics of virgin-aggregate mixtures with recycled materials generally improve when coarser, recycled materials are added, say D.H. Kang, S.C. Gupta, and Andry Z. Ranaivoson, University of Minnesota-St. Paul, John Siekmeier, Minnesota DOT, and Ruth Roberson, Minnesota Pollution Control Agency, in their paper, "Hydraulic and Mechanical Characteristics of Recycled Materials and Aggregate Mixtures."

To better understand the functionality of these recycled materials, their study evaluated water retention, hydraulic conductivity, resilient modulus, and shear strength characteristics of 17 mixtures of four recycled materials with aggregates, relative to 100% virgin aggregate.

Recycled materials tested were RAP, recycled concrete material (RCM) usually called recycled concrete aggregate, fly ash and foundry sand.

"The shape of the water retention and thus pore size distribution curves of recycled mixtures were nearly similar to that of 100% aggregates," the authors write. "Saturated hydraulic conductivity of these mixtures was higher than that of 100% aggregates. This suggests that drainage characteristics of recycled materials mixtures with aggregates will be similar or better than that of 100% aggregates."

Both addition of RAP (coarser than aggregates), RCM (same as aggregates) and fly ash plus RAP (very fine and coarser than aggregates) to aggregates increased resilient modulus values, they report. At any given RAP content, increasing the fly ash content from 5% and 15% in fly ash RAP aggregate mixture slightly lowered the resilient modulus values.

The addition of foundry sand (fine material) to aggregate decreased resilient modulus values relative to that of 100% aggregates. "These results suggest that the stiffness of RAP, RCM, and fly ash mixtures of aggregates will be similar or better than of 100% aggregates," they write.

The addition of RAP, RCM, and fly ash plus RAP to aggregates generally increased cohesion of the mixtures. Except for three mixtures, friction angles of recycled material mixtures (38[degrees]F to 49[degrees]F) were similar to that of 100% virgin aggregates. However, adding foundry sand to aggregates did not improve the shear strength of the mixtures.

They conclude that fly ash, RAP, and RCM mixtures will be good substitutes for virgin aggregates as base and subbase materials. "However, in-situ testing of RAP, RCM, and fly ash mixed in with aggregates is recommended before field implementation of above findings," they write.

Tom Kuennen is a freelance writer and frequent contributor to Rock Products.
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Author:Kuennen, Tom
Publication:Rock Products
Article Type:Conference news
Geographic Code:1USA
Date:Apr 1, 2010
Next Article:Back in Business.

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