Too much fly ash - never reach design strength? 1

[StructureMan44]

We had petrography performed on a sample taken from a concrete core that came back with below expected strength. The petro results show the fly ash as being 35% of the cementing material instead of 25% as expected. The sample also showed air content slightly above ACI recommendations. I know that fly ash tends to slow the concrete curing. We're trying to reach 75% target strength and are willing to wait beyond 56days.

My question is with beyond the expected fly ash will this concrete eventually reach design strength, just take a little longer? Will the slightly above air content affect long term strength and/or durability?

 

[JedClampett]

If your fly ash content is 35% it should reach strength, just more slowly. We've had mixes where the percentages were reversed (80% fly ash, 20% cement) and strength is never met. But I think you're in the range of high fly ash percentages, not excessive.

 

[TehMightyEngineer]

Just curious, what strength are you trying to reach and how far off are you?

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[SlideRuleEra]

Agree with JedClampett. See "Optimizing the Use of Fly Ash in Concrete", published by the Portland Cement Association. Concrete with up to 60% fly ash should be fine, as long as you can wait for it to cure.

http://www.SlideRuleEra.net

http://www.VacuumTubeEra.net

 

[dik]

I typically spec 25% max for flyash... is there a really good reason for limiting this to 25%? Any problems with topping out at 30 or 35%? and would you change the design for 56 day strength? And using the 28 day strength as 70% of the design strength?

curious mimes want to know...

Dik

 

[StructureMan44]

We were trying to meet 5800psi at 28days however some of our core samples were as low as 3800psi. After reading more it appears we'll ultimately reach design strength (likely higher) just at much slower pace because of the increased fly ash.

 

[hokie66]

I don't think you will ever reach 5800 psi. Is strength your only issue? Durability will also be compromised.

 

[TehMightyEngineer]

I might agree with hokie on that based on gut feeling; you do loose strength by retarding the reaction with fly-ash and the fact that you've only got 65% of your design strength means you'll likely get close but I'm not sure you're going to get the full 5,800 psi.

Professional Engineer (ME, NH, MA) Structural Engineer (IL)
American Concrete Industries

https://www.facebook.com/AmericanConcrete/

 

[BigH]

I've been on projects with 50% added fly ash and have had no problems with strength being met; usually the strength is set at 90 days rather than 28 or 56.

 

[dik]

BigH... if I upped my max % to, say, 35%, with strength at 56 days, there should be no issues; I've used 25% for the last decade and thinking I should make a bit of a change. I know they use greater percentages for concrete strengths in the order of 60 MPa... I've had requests for 30% and have accepted this.

Dik

 

[hokie66]

I don't think that fly ash replacement should be used as an excuse for poor strength gain. If the mix is proportioned to use fly ash as part of the cementitious material, it can be designed to meet the same strength requirement, at 28 days.

There are many advantages to using fly ash, but as with any other component of concrete, quality matters, and the quality of fly ash probably varies more than most.

http://www.cement.org/docs/default-...48-optimizing-the-use-of-fly-ash-concrete.pdf

 

[Ron]

SM44...You asked two questions. First, you wanted to know if 35% fly ash would ever allow adequate strength gain. As a percentage of cement, that amount will significantly retard the strength gain but you'll likely get there. One caveat....not all fly ash is the same. The higher its carbon content, the more it will adversely affect the strength gain and even the ultimate strength.

Your second question was about air content. Excessive air content is not recoverable.....air doesn't get stronger with time! Depending on the air content excess, you might not achieve the strength you want.

The combination of excessive air content and high fly ash content will certainly put you in a marginal range for compliance. Let us know how it goes.

Can you post a copy of the mix design?

 

[structuralsteelhead]

Agree with Ron,...not all flyash is the same,...If in US,...Type F (less Carbon Oxide) or Type C (more carbon oxide), both require different W/C ratios and air entrainment. Also, what is the animal you are making,....foundation?,...massive criteria maybe? Both types of fly ash have different initial and final set times in this structures.

If massive concrete pours,...and depending on purpose, maybe you do not need 75% design strength to proceed with live loading to a limited degree for the structure. Say a 6 foot deep large scale generator foundation, size necessary for tuning of frequencies of the operational system,...not it's deadload only, which may sitting for months before the generator operational. So, maybe foundation concrete testing 2,500 psi is okay to set equipment, but not torque anchors or fill bins, silos, or turn on equipment, etc. We have spec'd 4,000 psi foundations regularly with 35% flyash with good long term results. May have records into the 5,000psi range on the 4,000 mix. Weather conditions/curing?

OP Could use more information on the pour size and purpose and yes mix design including that flyash type