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Advanced High-Performance Materials for Highway Applications: A Report on the State of Technology

Chapter 7, Aggregate Materials

Synthetic Aggregates

Description

Synthetic aggregates are manufactured using industrial waste material or by-products. These materials may be used as replacement for aggregates in AC or PCC.

Applications

There are three groups of synthetic aggregates.

  1. Group 1 is created from waste product that is heated in a blast furnace or rotary kiln to temperatures between 1,000 and 1,500 °F (538 and 816 °C), and then turned into pellets. Waste materials generally used in this process include sewage sludge, incinerated sewage sludge, pulverized fuel ash, oil sands, slag and other solid waste materials. High amounts of natural gas and electricity are needed to fuel the kilns and furnaces to eliminate bacteria and produce the pellets. The pellets/extrusions are cooled, sized, crushed, and graded to meet the job specification. Smokestack pollution can be a by-product. The cost for this group is comparable to production of mineral-based expanded shales and clays. The strength ranges from low to medium, except for the slag pellets, which fall in the high range. The synthetics in this group weigh about the same as standard mineral aggregate. The limitations can be the size of the pellets. Slag aggregate cannot be used where the irons may leach from the concrete when water is present, or in esthetic uses where slag would stain or leach from the concrete.
  2. Group 2 combines only non-cementitious fly ash with a binder, which is then pressed or extruded into pellets. In Group 2, the pellets/extrusions are cooled, sized, crushed, and graded to meet a job specification. The cost of this production can vary with the supply of ash and energy, and their primary market is road-base and lightweight concrete. This technology uses less energy than Group 1 blast furnace or kiln processes. The pellet size limits the size of the aggregate.
  3. Group 3 combines recycled products such as non-cementitious fly ash, bottom ash, mine tailings, recycled plastics, recycled glass and other recycled materials. This group's primary market is lightweight concrete where high compressive and tensile strength is of primary concern, such as in skyscrapers, bridges, buildings, cultured stone, and roofing materials. It uses both fly ash varieties (non-cementitious and cementitious) and bottom ash (the major waste products from coal plants), and it can be mixed by formula to include plastics or other recyclable materials. The combined materials are pressed or extruded into a solid block. The solid block is then crushed with a standard rock crusher into a lightweight aggregate. All standard aggregate sizes are available. The aggregates are high-strength, lightweight - with weights ranging from 10 to 50 lb/ft3 (160 to 801 kg/m3), and can be used in multiple industries. The ultra-lightweight concrete weighs 95 lb/ft3 (1522 kg/m3), with compressive strength of 6,000 lbf/in2 (41.4 MPa), and a tensile strength of 11 percent of the compressive strength.

The synthetic aggregate products can be produced and stockpiled during the construction off-season for use during the next construction season.

Benefits

The primary benefit of synthetic aggregates is that industrial waste products are productively used. These products can also serve as replacement for more expensive aggregates or local aggregates of marginal quality.

Costs

Costs are reported to be comparable to natural aggregates. Many of the processes for producing synthetic aggregates are patented, and costs may vary by the process used.

Current Status

Synthetic aggregates are widely used in nonhighway applications and also for light-weight concrete for transportation structures. However, there has been very little application of synthetic aggregates, except for slag aggregates, in pavement construction.

For More Information

Western Research Institute: The Synag Process for producing ash-based pelletized lightweight aggregates. http://www.netl.doe.gov/technologies/coalpower/ewr/coal_utilization_byproducts/utilization/wri.html.

David Shulman. 2005. "Synthetic Aggregates - A Look at the Three Groupings of Synthetic Aggregates and the Potential Uses for Each," Pit & Quarry Magazine, August 1.

Manufactured Aggregates Using Captured CO2

Description

A process - The Calera Process - is under development by Calera Corporation to manufacture calcium and magnesium carbonate using mineralized CO2 captured from power plant flue gas to create aggregates that can be used to produce concrete.

Applications

The manufactured aggregates can be used as partial or total replacement of natural aggregates used in paving and structural concrete.

Benefits

Two important benefits are expected from this process. The first is the availability of good quality aggregates at locations where sound aggregates may be in short supply. The second is the sequestering of CO2 produced by coal-powered plants..

Costs

Costs estimates are not available as full-scale production has not started.

Current Status

A pilot manufacturing plant to produce calcium and magnesium carbonate aggregate is under development (as of early 2009).

For More Information

Cecily Ryan, Terence Holland. 2009. "Next Generation Paving Materials Using Mineralized CO2 Captured from Flue Gas," abstract submitted for the International Conference on Sustainable Concrete Pavement Technologies, held in Sacramento, California, September 2010, organized by the Federal Highway Administration.

Engineering News-Record, February 18, 2009, "New Green-Concrete Process Combines Seawater, Flue Gas."

Materials That Allow Internal Concrete Curing

Description

Internal curing is the process by which the hydration of cement occurs because of the availability of additional internal water that is not part of the original mixing water. This additional water is typically supplied by using relatively small amounts of saturated, lightweight, fine aggregates (LWAs) or by the addition of super-absorbent polymers (SAPs) in the concrete (Bentz, Lura, and Roberts 2005). Once the original mixing water is used up, additional water is drawn from the LWA or SAP to promote more complete hydration of the cementitious materials. The amount of additional water available is dependent on both the volume and the absorption capacity of the aggregate (Cleary and Delatte 2008).

Internal curing is especially beneficial in low water-to-cementitious material ratio (w/cm) concrete (say, below ~0.42) because of the increased potential for autogenous shrinkage, defined by the American Concrete Institute (20I0) as the "change in volume produced by continued hydration of cement, exclusive of effects of applied load and change in either thermal condition or moisture content" (p. 79). With more agencies moving towards lower w/cm concrete (for strength and durability reasons), there is an increased potential for early-age cracking to occur, particularly if inadequate curing methods are used during construction (Lam 2005). Furthermore, internal curing may be more necessary with concretes that use supplementary cementitious materials (fly ash, slag cement, or silica fume), another common feature of today's highway concrete mixtures.

When LWA is used to provide internal curing, a portion of the fine aggregate is replaced with the LWA. The amount of LWA that should be used is a function of type, size, degree of moisture pre-conditioning of the LWA, and type and amount of binder (Cleary and Delatte 2008). Some initial guidance is available on determining the amount of partial replacement of the fine aggregate in a concrete mixture with LWA (Bentz, Lura, and Roberts 2005).

Applications

The applications for internal curing concrete include most transportation facilities, including bridges, parking structures, highway and street pavements, parking lots, and overlays.

Benefits

There are a number of purported benefits associated with internal curing, including the following (Bentz, Lura, and Roberts 2005; Cleary and Delatte 2008):

  • Reduced early-age shrinkage, particularly in concretes with low w/cm.
  • Increased concrete strength (compressive, flexural, and tensile strength).
  • Reduced permeability.
  • Increased durability.
Costs

No cost data are currently available for this concrete produced using either LWA or SAP.

Current Status

A number of laboratory studies have been conducted evaluating concrete produced using either LWA or SAP (Lam 2005; Cleary and Delatte 2008). These laboratory studies have demonstrated the effectiveness of internal curing in promoting more complete cement hydration. At the same time, LWA has been used in the construction of bridge decks and in residential paving in North Texas for several years (Villarreal and Crocker 2007). A number of agencies continue to evaluate the merit and potential benefits that could be reaped from internal-curing concrete.

For More Information

American Concrete Institute. 2010. http://www.concrete.org/Technical/CCT/FlashHelp/ACI_Concrete_Terminology.pdf, p. 79.

Bentz, D. P., P. Lura, and J. W. Roberts. 2005. "Mixture Proportioning for Internal Curing." Concrete International, Vol. 27, No. 2. American Concrete Institute, Farmington Hills, MI.

Cleary, J., and N. Delatte. 2008. "Implementation of Internal Curing in Transportation Concrete." Transportation Research Record: Journal of the Transportation Research Board 2070. Transportation Research Board, Washington, DC.

Lam, H. 2005. Effects of Internal Curing Methods on Restrained Shrinkage and Permeability. PCA R&D Serial No. 2620. Portland Cement Association, Skokie, IL.

Villareal, V. H., and D. A. Crocker. 2007. "Better Pavements through Internal Hydration." Concrete International, Vol. 29, No. 2. American Concrete Institute, Farmington Hills, MI.

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Updated: 05/22/2012
 

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