Ternary Concrete Mixes Offer High Performance
Reclaimed industrial byproducts play role in high-performance and ultra-high-performance mixes
By Tom Kuennen, Contributing Editor
Ternary and quaternary portland cement concrete mixtures – those incorporating three or four products into their cementitious components – are providing transportation infrastructure owners more durable, longer-lasting structures while incorporating industrial byproducts that in an earlier day were landfilled.
Ternary concrete mixtures include three different cementitious materials, including portland cement, and reclaimed industrial byproducts like ground, granulated blast furnace slag, silica fume or fly ash, to achieve high-compressive strengths, low permeability and corrosion resistance, resistance to sulfate and alkali-silica reactivity deterioration, and reduction of thermal cracking.
These mixes comprise today’s High Performance Concrete (HPC), the mix design of choice for precast, prestressed bridges, beams and decks, and for some portland cement concrete pavements.
The importance of ternary concrete mixes is such that the National Concrete Pavement Technology Center (CP Tech Center) at Iowa State University has made them a high priority for research, with work continuing in two phases in 2011 (see below).
A ternary mix has three cementitious components in the mix, according to Tim Cost, P.E., a senior technical service engineer for Holcim. “In general, [a ternary mix] includes portland cement and two of the following – slag cement, fly ash and silica fume – and often allows a higher total portland replacement.”
How can three components be better than one or two? “There are practical limits and potential side effects from individual supplementary cementitious materials (SCMs),” Cost said at a recent presentation to the Alabama Concrete Industries Association. “Ternary blends allow the designer to best optimize mix properties for the project, for design and construction.”
This includes optimization of a concrete mix for strength, permeability, durability, heat of hydration and plastic properties, including finishing, pumping, segregation and bleeding. They also may result in higher total cement replacement in a mix, with improved mix economics and implications for sustainability, he says.
Indeed, ternary mix components fly ash, slag cement and silica fume are 100-percent recycled content. Cost says they reduce the clinker content of concrete, thus lowering associated CO2 emissions and embodied energy, and reduce landfill disposal of industrial byproducts.
HPC, also referred to in the literature as “durable concrete,” is an engineered concrete made up of the classic components of water, portland cement, and fine and coarse aggregates, but with a twist. With HPC, materials and admixtures are carefully selected and proportioned to realize high early strengths, high ultimate strengths and high durability beyond conventional concrete.
Reclaimed industrial byproducts are integral to HPC mixes. HPC describes a set of specialized concrete mixes that provide added durability for concrete structures, ease of placement and consolidation without affecting strength, long-term mechanical properties, early high strength, and longer life in severe environments, all the while using less material. As such, HPC might potentially permit fewer girders in a design, with reduced maintenance, extended life cycle, and if designed well, enhanced aesthetics.
HPC was a component of the Strategic Highway Research Program (SHRP), specifically the Concrete and Structures section. Under SHRP, four types of HPC were developed:
Very Early Strength (VES, 2,000 psi at 6 hours),
High Early Strength (HES, 5,000 psi at 24 hours),
Very High Strength (VHS, 10,000 psi at 28 days), and
Fiber-reinforced HES, with steel or poly fibers added to control shrinkage cracks.
Originally, HPC was targeted at bridge sub- and superstructures, but since then HPC has migrated into pavements, and even precast pavement panels. More information about HPC for bridges and pavements is available from http://knowledge.fhwa.dot.gov/cops/hpcx.nsf/home.
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