|
|
|
|
Frequently Asked Questions How much Fly Ash should be used in concrete?
The amount of fly ash, like most pozzolans, in a concrete mix is described as a "percentage of cementitious materials". This percentage is obtained by dividing the mass of fly ash in a cubic meter of concrete by the sum of all cementing and fly ash materials the mix. 20% fly ash is a very commonly used amount of fly ash for most mixes utilizing fly ash. Slightly higher percentages are common in lower and higher strength mixes and mass concrete often has fly ash percentages above 30%. Early strength application concretes typically use little to no fly ash.
There have been demonstration projects done in which up to 50% and greater of the cementing materials in a cubic meter of concrete has been fly ash. Lehigh Cement does not promote or endorse such high uses of fly ash but encourages continued investigation into the appropriateness of higher uses of fly ash in highly engineered and monitored concrete applications. ^ top Does Fly Ash change the look of my concrete?
No. In typical quantities fly ash does not change the colour or look of concrete.
This said, as fly ash makes concrete easier to place and finish, fly ash can sometimes help the work crew get the smooth, solid look to the concrete that is typical desired.
This photograph shows two concretes made with and without fly ash. Can you see any difference?
^ top Do I need to do anything special for Fly Ash concrete?
No. Fly ash concrete looks, handles and places very similarly to cement-only concrete. In most instances it is a little easier to work with fly ash concrete and this is one of its benefits. fly ash concrete does set a little slower than cement-only concrete and anyone working with it should be aware of this. ^ top When should I specify Fly Ash concrete?
There are very few instances where fly ash concrete should not be requested. Concrete with 20% fly ash is almost always better concrete than a similar strength concrete made with Portland cement alone. If there are questions about a specialty concrete application and the appropriateness of specifying fly ash contact our Fly Ash Division or a materials engineering firm for advice. ^ top Is Fly Ash typically used in concrete?
Yes. It is estimated that 95% of the concrete produced in the Lower Mainland of BC has fly ash in it. Typical fly ash quantities are 20% of the cementing materials. 20% fly ash concrete has been used for many years in British Columbia. ^ top How will Fly Ash affect entrained air in concrete?
Fly ash is a very fine material. As with any fine material, its addition to concrete will, for a given air entrainment, cause an increase in the required air entraining admixture dosage. Further, a commonly accepted theory states that the residual carbon found in many fly ashes also drives up the air entrainment requirement of fly ash concrete.
Field experience shows that fly ashes are very easy to work with from an air entrainment dosage standpoint. Typically with fly ash, once the air entrainment dosage for a given mix has been determined, it varies very little from fly ash load to fly ash load although seasonal and other causes of dosage requirement variation still exist. ^ top What is the difference between the types of Fly Ashes?
The Canadian Standards Association (CSA) as per its Standard A-3000 (Cementitious Materials Compendium), designates fly ashes as Type F, CI or CH based on the calcium oxide (CaO) content of the fly ash. Type F has a CaO content of less than 8%, Type CI between 8% and 20%, and Type CH greater than 20%.
Concrete made with Type CI fly ash may have a slightly earlier setting time and somewhat higher early strength than concrete made with Type F fly ash. There have been some studies showing that concretes made with Type F fly ashes having better sulphate resistance. There are documented cases of pros and cons to each. Depending on the individual fly ash, there may be no difference in long-term performance of concretes made with Type CI or Type F fly ash.
Lehigh Cement markets Type CI fly ash in the Lower Mainland, Vancouver Island and parts of the Interior of BC and Type F fly ash in parts of the Interior and Northern BC. Type CH fly ash is not available in BC.
The standards (ASTM) used in the United States have only Class F and Class C designations for fly ash. These designations are somewhat different than those used in Canada. ^ top Why is Fly Ash use good for the environment?
Fly ash typically replaces some of the Portland cement in a concrete mix.
Portland cement, while inherently being a very environmentally beneficial product in that it is the primary binder in concrete, which is a highly environmentally beneficial building material, does have environmental impacts associated with its manufacturing. This of course can be said of any modern building material from wood to concrete to steel.
Fly ash is a by-product of thermal power generation. It is typically disposed of as a waste in a landfill if it is not used in some other way. ^ top Why should I specify Fly Ash concrete?
Concrete is the best building material available to man. Fly ash makes this excellent material even better!
Fly ash improves the engineering properties of concrete and lowers the environmental impact of any project it is used on, typically without impacting the costs associated with the job. Why would you not specify fly ash concrete?
^ top
|
