|FHWA > Engineering > Pavements > HIF-10-002 > Chapter 7, Aggregate Materials|
Advanced High-Performance Materials for Highway Applications: A Report on the State of Technology
Chapter 7, Aggregate Materials
Synthetic aggregates are manufactured using industrial waste material or by-products. These materials may be used as replacement for aggregates in AC or PCC.
There are three groups of synthetic aggregates.
The synthetic aggregate products can be produced and stockpiled during the construction off-season for use during the next construction season.
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 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.
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
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.
The manufactured aggregates can be used as partial or total replacement of natural aggregates used in paving and structural concrete.
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 estimates are not available as full-scale production has not started.
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
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).
The applications for internal curing concrete include most transportation facilities, including bridges, parking structures, highway and street pavements, parking lots, and overlays.
There are a number of purported benefits associated with internal curing, including the following (Bentz, Lura, and Roberts 2005; Cleary and Delatte 2008):
No cost data are currently available for this concrete produced using either LWA or SAP.
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.
PDF files can be viewed with the Acrobat® Reader®