AUTOCLAVED CELLULAR CONCRETE, THE FUTURE OF FLY ASH

ABSTRACT

Autoclaved Cellular Concrete (ACC, also known as Autoclaved Aerated Concrete, or AAC) is a lightweight building product with high insulating value which can be manufactured using 60-75% fly ash by weight. Fly ash with carbon content up to 12% may be used in the mix, thereby allowing high volume use of ash from sources that do not meet ready-mix concrete specifications. This paper will discuss the process by which TVA  evaluated the potential market and awarded a contract for the first ACC plant in North America to be located at a fossil plant site and the first to use fly ash as the siliceous component.

ACC is a lightweight building material which can be manufctured using water, cement, aluminum, and up to 70% fly ash by weight. Manufctured in blocks or panels, ACC provides high insulation value, is fireproof, is lightweight (some mix designs float in water) and, based on total installed cost, is less expensive to use than conventional concrete block or lumber. ACC is a widely used building product in Europe. Until recently, the abundance of inexpensive wood products in the United States has proviede little incentive for the development of new building technologies; however, rapidly dwindling timber reserves and high prices for quality construction lumber make ACC an attractive substitute for lumber construction products. 

THE STRATEGY

1. Maximize the quantity of coal combustion by-products diverted from disposal into beneficial reuse.

2. Minimize the cost of by-product handling and disposal at the fossil plants.

3. Maximize revenue through by-product marketing.

Long-term planning for ash disposal resources requires utilities to know three to ten years in advance what impact by-product utilization will have on manpower, equipment, maintenance, and the development of future disposal capacity. Expanding TVA's core by-product marketing business by locating multiple year-round, high-volume users of non-specification by-products within sight of coal-burning power plants addresses each of the three goals of increasing utilization, reducing coast, and generating revenue.

MARKET STUDY

As a follow-up to the pilot plant demonstration, TVA conducted a market study which assessed various investment scenarios for construction of ACC manufacturing facilities in the Tennessee Valley. Under contract to TVA, LAW Engineering prepared a Feasibility Study and Market Survey for Autoclaved Cellular Concrete to evaluate regional demographics and construction activities, and the potential Autoclaved Cellular Concrete (ACC) market share for manufcturers in the vicinity of TVA plants. The final report also presented financial analyses for startup of a small ACC plant and additional cases for a larger plant. The key assumption for the analyses is:" ACC materials are not intended to be used as direct replacement materials in the present construction methods. ACC is a special alternative construction method and must be evaluated using total installed cost not on material cost alone."

LWA surveyed key contacts in the manufacturing and construction industries in order to gauge the receptivity of the US market to ACC. interviews were conducted with:

1. Two European ACC manufacturers

2. A regional vice president of construction for a major US home builder 

3. A major, quality masonry contractor

4. A small, custom, high end home builder

5. Several induviduals who are building or considering building with ACC block.


BARRIERS

Based on these interviews, the study identifies several barriers to residential and commercial markets for ACC:

1. Reluctance toward change and little interest in new products by architects and devolopers.

2. No interest in new products by buyers, homeowners,residential developers,commercial developers.

3. Homebuyers' perception of ACC may not be favorable in the short term; ACC may not be viewed as a "quality material."

4. Resistance to change by trades involved in residential and commercial construction.

5. Costs of the "learning curve" while working with a new product.

6. Investors may not be able to lose money in short run as ACC becomes accepted by industry.

7. Start up costs associated with promoting and teaching industry to build with ACC.

8. Lower R-value per inch of ACC compared to wood frame with insulation.

9. Possible building code barriers: manufacturers may need to "sell" product to public agencies.

10. ACC is designed to replace wood, fiberglass insulation, concrete block and precast concrete tilt-ups: materials that industry is generally satisfied with now.

RESULTS

The results of the financial analyses, using a captial investment of $25 million for a plant producing 5.3 million cubic feet annually and with a 50%/50% block/panel product mix, indicated a Return on Investment (ROI) in the range of 11 to 24%. These results are consistent with returns projected by European ACC companies for a plant in the US (20 to 35%).

BASED ON THE  SITING AND FINANCIAL ANALYSES, THE FOLLOWING RECOMMENDATIONS WERE MADE: 

1. Locate an ACC plant within the largest lotential market area based on minimizing transportation costs to major market locations.
 
2. Locate an ACC plant at a dry fly ash handling facility to avoid potential costs and problems associated with wet handling facilities.

3. Because the project return on investment will probably be low to nonexistent for the first several years, potential investors should be willing to look longer term for return on investment.

4. A significant barrier to ACC's use is the price driven approach of American construction.

5. A significant barrier to ACC's use is the lack of American design codes. Since this study was conducted, TVA has obtained rights to use the ACC codes developed by EPRI for EPRI Blocks.

6. Because the majority of building permits (70%-80%) are granted in the five major metropolitan areas of Tennessee, the focus for ACC should be on achieving  acceptance in one or two large cities first, then spreading out to other smaller city permitting agencies.

Next: Green Building Materials Industry Report 2013-2019