Using Fly Ash In Concrete
Ready-mix concrete provides a high degree of versatility in construction however there are instances where the standard portland cement mixes alone may not yield optimal performance.
Fly ash is sometimes used as a partial cement replacement in situations where higher than usual durability or strength or lower than usual permeability is required. It works by reducing the amount of water required for a specified workability which in turn leads to increased strength, reduced permeability and reduced drying shrinkage.
What is fly ash?
Fly ash is fine powder that is produced in coal fired power stations as part of the combustion process. Fly ash is made up of different chemical compounds depending on the type of coal used and where in Australia the fly ash is sourced from but generally it is greater than 50% silicone dioxide.
High quality fly ash has a low carbon content which means that it requires less water to create a bonding effect than standard cement. This means that fly ash mixtures require less water than standard mixes which is the key property that leads to their enhanced strength and durability. The carbon content in fly ash is related to the result of the loss on ignition test with use cases specified in AS 3583.3.
When should I use a fly ash mix?
Fly ash can yield the following benefits when used in concrete:
Reduced water requirement for set workability
Increased set strength in later age
Improvement in pumpability/workability
Increased resistance to various concrete degradation issues including alkali silica reaction; chloride ion penetration (a cause of concrete cancer); sulphate attack; acid attack
Decreased permeability
Reduced creep and drying shrinkage
Improved definition of off-form finishes
Some issues with using fly ash concrete
When using fly ash mixes, the following limitations need to be kept in mind:
Pre-7 day strengths will be lower than standard mixes
Setting times will be longer than standard mixes in cold environments
A higher level/dosage of air-entrainment agent will be needed
Carbon particles may float out
Australian Standards to keep in mind when using fly ash
1. Fly ash
The following standards set out requirements for the use of fly ash in Australia:
AS 3582.1–1998 (amended January, 1999) Supplementary cementitious materials for use with portland and blended cement - Fly ash
AS 1379–1997 (amended January 2000) Specification and supply of concrete
In particular, AS 3582.1 specifies the required properties for the three grades of fly ash which should be kept in mind when ordering your concrete mix:
Note: this information is for reference only and was extracted from the Concrete Institute of Australia Current Practice Note on fly ash. Please insure it is up to date with current standards at the time you use a fly ash product. The data contained here is an excerpt, you should consult the full standard before relying on any information contained here.
AS 3582.1 also recommends methods of demonstrating that you used to ensure compliance with the standard including sampling and the use of a suitable test certificate for reporting
test results.
2. Curing
All concrete needs to be cured to develop its potential strength and durability. Failure to allow for adequate curing will adversely affect the performance of your concrete.
Australian Standard AS3600 requires that:
Exposure classifications A1 or A2 must be continuously cured for at least three days, or, if cured by accelerated methods, cured until the average compressive strength of the concrete is at least 15MPa
The following exposure classifications must be cured for at least 7 days or where cured by accelerated methods, cured until the average compressive strength of the concrete is at least 20 MPa (exposure classification B1); 25 MPa (exposure classification B2); 32 MPa (exposure classification C); 40 MPa (65 MPa concrete)
In addition, for special class concrete, the minimum cement content and the cement type shall be specified
3. Formwork stripping
AS3600 specifies the following stripping times for concrete based formwork:
[a] Reinforced slabs continuous over supports.
[b] Slabs and beams not supporting structures above.
Note: this information is for reference only and was extracted from the Concrete Institute of Australia Current Practice Note on fly ash. Please insure it is up to date with current standards at the time you use a fly ash product. The data contained here is an excerpt, you should consult the full standard before relying on any information contained here.
This information should be read in conjunction with the respective conditions of clauses 19.6.2.4 and 19.6.2.5 of AS 3600. Clauses 19.6.2.7 and 19.6.2.8 should be read for “Stripping of forms and removal of supports from soffits of prestressed slabs and beams” and “Control tests” respectively.
Need more information?
Further information can be found here:
http://www.concreteinstitute.com.au/getattachment/Resource-Centre/CIA-Library/Current-Practice-Notes/CPN-25---Fly-Ash-and-its-Use-in-Concrete/Fly-Ash-and-its-Use-in-Concrete.pdf.aspx
Australian Standard AS3600-2009 Concrete structures
Alternatively, submit a quote specifying details of your project and we'll arrange for suppliers to contact you to discuss the suitability of using fly ash in your project.
