DEFINITION:
Flyash is defined in Cement and Concrete Terminology (ACI Committee 116) as "the finely divided residue resulting from the combustion of ground or powdered coal, which is transported from the firebox through the boiler by flue gases." Flyash is a by-product of coal-fired electric generating plants.
Two classifications of flyash are produced, according to the type of coal used. Anthracite and bituminous coal produces flyash classified as Class F. Class C flyash is produced by burning lignite or subbituminous coal. Class C flyash is preferable for the applications presented in the Green Building Guide and is the main type offered for residential applications from ready-mix suppliers.
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CONSIDERATIONS:
Flyash is one of three general types of coal combustion byproducts (CCBP's). The use of these byproducts offers environmental advantages by diverting the material from the wastestream, reducing the energy investment in processing virgin materials, conserving virgin materials, and allaying pollution.
Thirteen million tons of coal ash are produced in Texas each year. Eleven percent of this ash is used which is below the national average of 30 %. About 60 - 70% of central Texas suppliers offer flyash in ready-mix products. They will substitute flyash for 20 - 35% of the portland cement used to make their products.
Although flyash offers environmental advantages, it also improves the performance and quality of concrete. Flyash affects the plastic properties of concrete by improving workability, reducing water demand, reducing segregation and bleeding, and lowering heat of hydration. Flyash increases strength, reduces permeability, reduces corrosion of reinforcing steel, increases sulphate resistance, and reduces alkali-aggregate reaction. Flyash reaches its maximum strength more slowly than concrete made with only portland cement. The techniques for working with this type of concrete are standard for the industry and will not impact the budget of a job.
This section also addresses wall-form products. Most of these products have hollow interiors and are stacked or set in place and then filled with steel-reinforced concrete creating a concrete structure for a house.
Some wall-form materials are made from EPS (expanded polystyrene) which is a lightweight non-CFC foam material. There are also fiber-cement wall-form products that can contain wood waste. The EPS/concrete systems offer high insulating qualities and easy installation. The fiber-cement blocks offer insulating qualities as well. Some EPS products also have recycled content.
Commercial
Status Implementation
Issues
Cementitous Structure
Flyash Concrete
Recycled Content Block
Concrete Finish Floor
Concrete Interior Wall
Legend
Satisfactory
Satisfactory in most conditions
Satisfactory in Limited Conditions
Unsatisfactory or Difficult
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COMMERCIAL STATUS
TECHNOLOGY:
Flyash used in concrete is a mature technology. Thirty percent of the flyash in the US is recycled into making concrete. The use of flyash concrete in structural applications such as wall-forms is standard technology. The use of recycled-content block, in particular fiber-cement, as part of a structural foundation system using flyash concrete is still early in development.
SUPPLIERS:
Approximately 60-70% of central Texas ready-mix suppliers offer flyash concrete. Some suppliers provide it automatically, others give a choice. Recycled-content fiber-cement block should become more available as a regional distributor has been established. EPS wall-form materials are locally and regionally available.
COST:
Flyash concrete is the same price as ordinary concrete without flyash. EPS wall-form products provide a cost-effective wall. Fiber-cement wall-form cost approximately $3.50 per square foot of wall surface.
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IMPLEMENTATION ISSUES
FINANCING:
Available.
PUBLIC ACCEPTANCE:
There is a small segment of the population that is fearful of flyash being inferior or unhealthful. U.S. EPA information indicates there is not a health threat, especially in the portions found in ready-mix products and with western coal (which is the primary source of local flyash).
A concrete finish floor may sound less desirable aesthetically to some persons. However, coloring, scoring, and texturing techniques can be very attractive.
Wall-form products should be well-received.
REGULATORY:
Flyash concrete meets applicable codes. Products making use of flyash concrete must indicate having met applicable ASTM test requirements. This information will be provided by the supplier.
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GUIDELINES
1.0 Specification for flyash
Flyash for use in portland cement concrete shall conform to the requirements of ASTM C 618, Standard Specification for Flyash and Raw or Calcined Natural Pozzolan Class C Flyash for use as a Mineral Admixture in Portland Cement. Specifically, it shall conform to all requirements of Table 1 and Table 2 as outlined therein.
The concrete supplier shall furnish a notarized certificate from the flyash marketer at the time of submittal of concrete mix designs for approval indicating conformance with these requirements. Also, a copy of the most recent chemical analysis shall be provided.
At no time during the course of the project will a change of flyash source (plant) be permitted without the prior written consent of the Engineer or Architect. For sulfate environments, only Class F flyash will be permitted and under no circumstances will Class C flyash be used.
2.0 Flyash use.
Class F flyash will typically require an air entraining agent to be added. Class C flyash will not.
Standard concrete procedures can be employed.
3.0 Flyash concrete in poured concrete permanent wall-forms
The use of these systems eliminates the need for conventional framing on exterior walls.
Expanded polystyrene (EPS) wall-forms
Some feature interlocking features and stack like blocks. Some are in rigid panels on interior and exterior connected by metal or steel ties.
EPS blocks are typically stacked as exterior walls. Rebar is placed in the cores vertically and horizontally. The cores are poured full of concrete from the top.
Manufacturers claim R-values of R-30 or greater.
Specify that the foam is protected from insects. Insects will not eat the foam but will nest in it. Borate treatment is preferable.
Urethane block wall-form products are also available. These contain CFC's/HCFC's.
Fiber-cement wall forms.
Can use waste wood; will not burn; insect resistant; will not support condensation.
Approximately R-12 ratings in 9 inch block.
Hollow cores are filled with steel reinforced concrete.
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