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Fly Ash Concrete
by Betsy del Monte

In the name of sustainability, many construction practices and attitudes are being reconsidered. What used to be thought of as an unusable material is proving to have some real advantages...and challenges. Here are some questions, and answers about using fly ash in concrete.  

Why are concrete and the cement used to make it targets for increasing sustainability?  

  • Concrete is second only to water as most consumed substance on earth, and demand continues to increase dramatically.

  • Cement manufacturing operations are responsible for almost 8% of global emissions of carbon dioxide, a significant greenhouse gas.

  • For every ton of cement manufactured, about 6.5 million BTU’s of energy are consumed -- about one ton of carbon dioxide is released.  

What is High Fly Ash Concrete?  

Fly ash is defined in cement and concrete terminology 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."  In other words, fly ash, a by-product of coal-fired electric generating plants, is the dust left over after the coal burns. 

Fly ash can be used in place of cement to create concrete for construction. Two classifications of fly ash are produced, according to the type of coal used. Class C fly ash is produced by burning lignite or sub-bituminous coal.  Anthracite and bituminous coal produces fly ash classified as Class F. 

How can using Fly Ash in place of cement help?  

Using coal fly ash conserves energy by reducing the demand for typical pavement materials such as lime, cement and crushed stone, which take energy to produce. If all the fly-ash generated each year were used in producing concrete, the reduction of carbon dioxide released from cement production would be equivalent to eliminating 25% of the world’s vehicles.  

  • Every ton of ash reused in cement products equates to nearly a ton of CO2 savings.

  • Each ton of fly ash used to replace a ton of cement saves the equivalent of nearly one barrel of imported oil.

  • Every ton of coal combustion product that is recycled is a ton not deposited in our landfills.

How Does Fly Ash Affect Concrete?  

  • Fly ash creates stronger concrete, but strength may develop more slowly than all portland-cement concrete.

  • Fly Ash is easier to place, pump, work and finish.

  • Fly Ash can be mixed in proportions which make summer concrete finishing easier, due to slower set.

  • Care can be taken to mix proportions which don't affect winter finishing. 

  • Concrete will require less water, resulting in less shrinkage and cracking. 

A CASE STUDY  

Where has fly ash bee n used locally? 

The RadioShack Corporate Headquarters, in Fort Worth, recently received a LEED Silver certification.  This project was designed by HKS, and constructed by The Beck Group, with Walter P. Moore as the structural engineer. Concrete was provided by Latimore.

At the end of the project, there was over 1,500,000 sq.ft. of elevated slab formwork, 82,000 cu.yds. of concrete poured, and almost 14,000 cu.yds./ month placed during peak periods.  

First steps to determining the feasibility of using fly ash concrete on a project.  

The project team set a goal to develop and use concrete mixes with a minimum of 51% fly ash content, in order to qualify for the two LEED points dealing with recycled materials. First, the design team researched case studies from other projects, received feedback from industry experts, and involved local ready-mix suppliers/installers. It quickly became apparent that fly ash concrete was readily available from most concrete suppliers.  

It was also necessary to collect and evaluate performance data on fly ash concrete. Testing was done first in the lab, with 65 different mix designs. The testing focused on the variables of set time and strength gain. The lab results showed that:

  • Ultimate design strength was as high if not much higher.

  • Early strength target is achievable.

  • Use of admixtures were a must for early strength and set.

  • The mixes were less forgiving, and stressed the importance of the Quality Control process.

  • Class ‘F’ fly ash preferred over class ‘C’, primarily for color.

  • Slag use not feasible due to cost/supply constraints.

Next, testing went to the field where selected mix designs in truck loads of 4 cubic yards, were poured, finished and tested by casting multiple test panels, slabs and columns. Subcontractor crews were asked to work different mix designs. All pours were tested for workability, set time and strength development time. The field tests showed that:

  • 51% fly ash mixes have a different “feel” and will require different finishing techniques.

  • Additional wait time to initial set, less working time available between set and troweling.

  • Mixes are pumpable; with higher pressures on equipment.

  • Greater potential for surface cracking; requires wet curing and evaporative retarder re-application.

  • More difficulty in controlling slump.

  • Additional Quality Control measures were required at plant and job site.

  • Minimal to no cost increase for ready-mix material (savings in cement offset by cost of admixtures).  

Subcontractors that participated in field testing were surveyed to get opinions and recommendations on issues such as concrete placement and workability, slab finishing techniques and crew productivity.  They were also asked to comment on any contributing factors to cost increases. The survey resulted in the following comments:

  • Will require different finishing techniques & bigger crews.

  • Working time after initial set is reduced considerably.

  • Pour sizes may have to be reduced to get quality slabs.

  • Surface cracking concerns on hot or windy days; wet curing method will probably be required. Evaporative retarder helped.

  • Doesn’t hold a “broom” finish well (parking garage slabs).

  • Quality control of material will be very critical.

Proposed Solutions to these concerns:

  • Place/Finish Crew Productivity Loss

  • Additional Evaporative Retarder use

  • Wet Curing of Slabs

  • On-site QC Control & Material Testing

  • Concrete heating for cold weather pours

Estimated costs for above =  >$500,000  

Potential Schedule Impact:  

  • Strip forms 3 days after slab pour vs. 2 days

  • Adds 3 weeks to schedule of each building

  • Smaller slab pours, more mobilizations by place/finish crews

  • Slower formwork cycling

  • Wet curing slabs, extended formwork cycling time  

Team Conclusions 

The RadioShack team considered all these issues before deciding how to proceed with the concrete. They realized that the cost impact for 51% fly ash throughout was greater than had been anticipated.  While some areas of the structure (piers and foundations) would be less affected, because of the large project scale, there were higher risks for cost, schedule and quality. The risk of major schedule impact was a real possibility, but difficult to measure.  It was realized that the quantity of fly ash needed for one LEED credit point was attainable; but the additional point could be better earned in some other ways. 

The FINAL decision  

Fly ash can be a very effective way to substitute for some of the cement in any project’s concrete, but care must be taken when it is used in higher percentages.  If the schedule has some flexibility, sufficient or superior strengths can be obtained. All these factors should be considered to be able to use the maximum amount of replacement fly ash in each project.  

At RadioShack, the final decision was made to:  

  • Utilize HVFA concrete in selective areas of project, that will not impact cost / schedule:

  • 51% fly ash mix used in all piers

  • 30% fly ash mix used in other foundations

  • 20% fly ash mixes used in slabs and columns

One LEED credit point achieved for fly-ash use (not 2)  

23% average of total project concrete volume

For more information on RadioShack, click here.

Betsy del Monte is an architect with 24 years of experience, and the 2006 Chair of the USGBC North Texas Chapter. She has worked on a multitude of projects ranging from schools and churches to million-square-foot commercial buildings. As a principal with Beck, she leads both integrated teams of architects and contractors, and architecture-only projects.

For additional information regarding this article, please contact Betsy at betsydelmonte@beckgroup.com. All comments are the sole responsibility of the author.

This article was originally posted 6/27/06.