~34% fly ash cement replace and the strength gain curve show no significant change? 3

[hetgen]

Hi,

We queried a GC to verify the strength curve for a proposed 40MPa (5.8ksi) concrete mix which has a 33.6% fly ash replacement ratio and we received the curve shown below. The curve show no apparent delay in strength gain, is this normal?

Considering the high fly-ash replacement ratio we were expecting slow strength gain which could delay the striping time, the concrete is to be used for a mid-rise building with multiple flat slabs.

Mix:
W/C = 0.433 |Cement = 294 | fly-ash = 149 |Aggregte = 1538 | Sand = 132

Many Thanks.

.

 

[JLNJ]

So they are telling you that a mix with with 443 pounds of cement, of which 1/3 is fly ash, will yield a 60-day of 7700 psi?

I think you are right to be skeptical. How much test data did they give you?

 

[Ron]

The curve shown gives 70 percent of design compressive strength at 7 days. That is typical of conventional portland cement concrete with no admixtures. With fly ash in the percentage you have in the mix, you can expect about 45 to 50 percent of design strength at 7 days. I would not believe the curve you have been given.

 

[hetgen]

@ JLNJ They didn't provide us any data besides the strength gain curves of various mixes of different strength that we require for the project.

All the strength gain curves do not show any apparent delay in strength, the graph below is for a 30MPa (4350 psi) mix design with ~30% fly-ash replacement ratio, as shown at 7-days it gets to ~85% strength?

Mix:
W/C = 0.539 |Cement = 244.1 | fly-ash = 104.6 |Aggregte = 1615(22mm->990+6.7mm->625)| Sand = 156

 

[Ron]

These graphs are not for concrete with fly ash or GGBFS!

 

[oldestguy]

Hey, before you complain, check out the kind of fly ash. Some fly ash, depending on coal source, has a lot of calcium oxide. That has significant cementing capabilities.

This is one possible source of info.

https://www.fhwa.dot.gov/pavement/recycling/fach03.cfm

 

[BridgeSmith]

OG, I don't think anyone was arguing that fly ash would result in lower ultimate strength, only that the strength gain would be slowed by using fly ash at the 34% level. That assertion is supported by a statement near the end of the article you linked to: "Lower early strength. Fly ash concrete mixes typically result in lower strengths at early ages."

 

[IDS]

As further back-up to oldestguy's comment:

The strength development of this particular FA concrete was observed to be
similar to that of an equivalent Portland cement concrete at standard curing
temperature (20 °C) up to 32 days. From this age onwards the strength
continues to develop and is higher as the level of FA increases.

http://www.flyash.info/2009/145-elsageer2009.pdf

The flyash.info site is based at the University of Kentucky:

http://www.flyash.info/

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[IDS]

Another quote from the linked paper, just to emphasise that this behaviour is not typical, but it does happen:

"The strength development under standard (20 °C) curing condition for PC and FA
concrete is shown in Figure 3. At the standard 20 °C curing temperature, the early
strength of this particular FA concrete was not significantly affected by the standard
curing condition as indicated by earlier studies 3, 4, which reported a lower strength
gain of FA concrete at early ages. From an age of 32 days, the strength of FA
concretes continued to develop and was higher as the level of FA increased. This
finding is consistent with that of the earlier studies 3, 4."

Doug Jenkins
Interactive Design Services

http://newtonexcelbach.wordpress.com/

 

[hetgen]

Thanks @ oldestguy & IDS, it was an interesting read.

I have read many reported and design codes stating fly-ash equate to low strength gain, but this report clearly shows that is not the case, I really don't know which reports represent what I'm dealing with. We have decided to ask the GC to conduct early strength test (7-days) before we use the mix for a suspended slab.

The clip below is from the report for a 70MPa (10.15ksi) concrete trail mix, with a 30% fly-ash replacement the strength gain @ 3 days is the same as one would expect from PC concrete. I'm curious a bit as to why they didn't show a similar graph for 7-days strength, but 3-days strength gain is convincing enough.

Many Thanks

 

[epoxybot]

Shouldn't the GC be asking the Ready-Mix supplier to supply something on their letter head stating the mix designs "typical properties"?

 

[Ron]

hetgen....keep in mind that since you are dealing with form removal and the potential of compromising the structure, it wouldn't be good to look at the best case scenario as depicted in the reference papers, but a more predictable case scenario of historical performance. While fly ash sources can be quite variable, in my area we have not seen the performance indicated by the referenced documents, but a somewhat slower strength gain.

Consider having multiple sets of test specimens made and test both field cured and lab cured specimens. Neither replicates the in-place condition of concrete in the structure, but will give you an idea of the mix performance.

Let us know how it goes!

 

[StrucDesignPE]

I agree with OG, I think it depends on the class of fly ash being used. Class C fly ash can have greater hydraulic properties than other classifications of fly ash due to its chemical makeup (ie higher calcium). This can result in concrete that has equivalent or greater strength gain in the 1 to 28 day time period as an equivalent control mix. I suggest you look at PCA's "Design and Control of Concrete Mixtures" to get some further insight. I also suggest you determine what class of fly ash is being used in the mix.

 

[DTGT2002]

I would not think it to be unreasonable to ask for raw cylinder test data.

There are always outliers to ponder, we want to put ourselves on the safe side, if not the painfully safe side until proven otherwise - like how long til the silo of good stuff runs out and the next batch is more like the industry is used to? Or was the bin stocked wrong and its just subbing cement with...cement.


I have seen people use a sample strength gain curve from a textbook as their reference curve before. Yes, we should not assume the world and contractors are clueless, but when we see something contrary to the prevailing understanding of our science, seeking a little more info is not harmful.

The additional cylinders and testing of the materials as delivered is certainly a good and logical step no matter what, even more so if there is no good way to seek out data without stomping some toes. But surely the supplier wants to promote the adequacy of their product.


Daniel

 

[hetgen]

We just got the 7 days strength result for a 40MPa (5.8ksi) concrete, the result matches what the supplier claimed would be.