Design and Application of High-Volume Fly Ash Self-Consolidating Concrete with the Incorporation of Nanoparticles
I. INTRODUCTION Fly ash, a byproduct of coal combustion, has been used in portland cement concrete for decades, and is widely accepted in the industry as a replacement for cement in amounts up to approximately 20% by mass. Addition of fly ash reduces the environmental impact of concrete by simultaneously consuming an otherwise unused waste product and by replacing cement, which is the most polluting and energy intensive component of concrete (an estimated 5% of all manmade CO2 emissions are due to cement production). Fly ash also improves the properties of concrete in several ways: improved fluidity during mixing and placing, higher strength, improved dimensional stability, and increased longevity of concrete structures. (a)	(b) Figure 1 - (a) Aerial image of Kingston Ash Slide immediately following spill 12/23/082, and (b) site cleanup as of 8/25/10 In spite of these improvements, challenges associated with the use of fly ash have prevented its cement replacement level from increasing beyond 20%. Currently, only about 40% of the fly ash that is available is used by the concrete industry1; the remainder is stored in large landfill-type enclosures. In 2008, one such enclosure at the Tennessee Valley Authority (TVA)'s Kingston Fossil Plant ruptured and spilled approximately 5.4 million cubic yards of impounded coal fly ash slurry onto the surrounding land and into the adjacent Emory River2. An aerial photograph of the disaster is shown in Figure 1. It was the worst environmental disaster of its kind in United States history. Public opinion is often shaped by disasters of this type, which in this case highlight the seemingly negative impacts of fly ash use. It is incumbent upon leaders of research in infrastructure, construction, and materials science to highlight the benefits of fly ash, to facilitate its maximum use, and to communicate its importance to the wider community. Therefore, the goal of this proposed research is to reduce the environmental impact of concrete construction through innovative materials use and production techniques, resulting in dramatic increases in the percentage of fly ash used in widespread construction. II. BACKGROUND Fly Ash Fly ash is known to enhance many concrete properties. Fly ash particles are smooth, spherical, and relatively small compared to cement particles (fly ash has a diameter of about 10 - 25 μm compared to 10 - 100 μm for ordinary portland cement). This leads to improved fluidity of fresh concrete mixtures for easier and faster mixing and placement. In addition, fly ash has pozzolanic properties - it reacts with calcium hydroxide, an unwanted byproduct of cement hydration, to produce calcium silicate hydrate, which is the desirable cementitious product of cement hydration. This process increases strength at later ages of curing, reduces heat of hydration, and reduces the rate of concrete free shrinkage, resulting in a decrease in thermal shrinkage cracking. Fly ash has also been found to refine the pore structure of concrete and decrease its permeability, which has good implications on durability and long-term strength. Despite its great advantages, however, cement replacement with fly ash is typically limited to 15 - 20% by mass in most commercial use. With the current technology, higher amounts of fly ash can result in slow setting and retardation of early-age strength development3; 4. These effects slow the construction process, which is highly undesirable, and prevent the widespread use of high volumes of fly ash. Efforts to combat the decrease in the rate of early-age strength development due to fly ash include mechanical treatment (grinding), accelerated curing and autoclaving, and chemical activating. However, a materials-based approach will be pursued in the proposed research by incorporating nanomaterials. In a study conducted by Sato and Beaudoin, they discovered that nano-sized CaCO3 (ground limestone) additions of 20% by mass in HVFA cement pastes (50% cement replacement with fly ash) lead to significant accelerations in early hydration5. Microhardness and modulus of elasticity in the early stage of hydration and 28-day compressive strength development were improved, as well. Similar results were obtained in a separate study by Li where additions of nano-SiO2 (nanosilica) in HVFA concrete showed an increase in pozzolanic activity, as well as an increase in both short-term and long-term strength6. In both studies, the underlying mechanisms of the nanomaterials were not fully understood, although it was suggested that the nano-sized particles served as nucleation sites to accelerate hydration. Self-Consolidating Concrete One of the most promising avenues for maximizing the use of fly ash is to incorporate it into self-consolidating concrete (SCC), which requires a relatively large amount of fine particles to produce. SCC flows like liquid in its fresh state, making external vibration and extensive finishing unnecessary. Characterized by its high flowability, passing ability (flows into tight spaces and dense reinforcement), and stability (resistance to segregation and bleeding), these superior properties enhance construction productivity in many ways, including: *	Quicker unloading of ready-mixed trucks and faster casting rates. This results in faster construction. *	Eliminates durability issues due to over-consolidation and poor surface finishes *	Reduction in energy consumption by avoiding the need for external vibration. *	Reduction in labor and equipment costs, which can streamline construction processes. *	Improved aesthetics. Despite its advantages, traditional SCC achieves its superior properties through a high cement content. This results in high material costs, a high energy footprint, as well as performance concerns, including increased creep and shrinkage and cracking due to heat of hydration. Replacing a significant amount of cement with fly ash would address all of these issues. Fly ash can be effective in improving the rheological properties of SCC, i.e. increasing fluidity and segregation resistance. Studies have shown that with proper mix design, high-volume fly ash (HVFA) SCC mixes (cement replacements with fly ash of 30 - 70% by mass) can exhibit comparable workability and compressive strength development to conventional SCC, and even increased durability and decreased drying shrinkage7; 8; 9. Although the use of SCC is becoming more prevalent in pre-cast plants, there is still apprehension when extending it to cast-in-place applications. This is partially attributed to the high lateral pressure SCC exerts on formwork from increased fluidity and faster casting rates compared to vibrated concrete. Figure 2 shows a typical formwork system for a modest sized SCC structure; the complexity of SCC formwork is clear from this photograph. Current codes for SCC require formwork to be designed to withstand hydrostatic pressure, although several studies have shown that the actual pressure on formwork can be less10; 11; 12. In the US, the cost of formwork can be as much as 60% of the total cost of the completed concrete structure in place13. To increase the level of confidence for SCC cast-in-place applications, a strategy to reduce formwork pressure of SCC is necessary. This is possible by increasing the stiffness of SCC while it is in its fresh state, i.e. increasing its green strength. Previous work at the Center for Advanced Cement-Based Materials (ACBM) at Northwestern University (NU) has shown that this may be achieved through small additions of clays (about 1% by mass of cement). Figure 2 - Formwork for cast-in-place SCC application14 In a study on the influence of clays on the rheology of cement paste done at ACBM NU, very small additions (about 1% by mass of cement) were found to significantly increase flocculation strength. Flocculation is the combining of sma
610 4742000 60022741
Infrastructure Technology Institute
L260 Technological Institute
Evanston, IL United States 60208-3109
Corr, David
Shah, Surendra
TRT Terms: Concrete; Environmental impacts; Fly ash; Landfills; Maintenance; Portland cement concrete; Self compacting concrete; Waste products (Materials)
Subject Areas: Bridges and other structures; Highways; Maintenance and Preservation; I15: Environment; I30: Materials;
Source Agency: Infrastructure Technology Institute
Contract Numbers: 610 4742000 60022741
Created Date: May 7 2013 1:01AM