Accepting cylinder test when fly ash is used in the mix

[rscassar]

Hi guys

What is the general procedure for accepting cylinder tests when fly ash is in the mix. The reason I ask is that I received three tests where all tests were more than 500psi less than the 5000psi specified.

I then questioned the concrete supplier why the concrete tests were so low (I originally thought that additional water was added to the mix). So I requested a 56 day cylinder test and the concrete mix design. The 56 day cylinder test were all above the 5000psi and the mix design indicated that 20% of the cementitious material is fly ash.

On this basis I am willing to accept the concrete but would like to question how others treat fly ash? Is it okay to accept 56 day cylinder tests to ensure compliance of the concrete?

 

[HSIII]

We've had a project here recently (first major one which had to be LEED silver certified) which used 20% to 30% fly-ash in the concrete as a way of "recycling" material. We have had a few cases of failures at the 28-day mark, but passed at the 56-day mark, and the EOR (My company was the QA end of things) accepted the concrete as passing.

What the owner did do, however, was reduce the payment to the concrete subcontractor and supplier (95% of contracted price) due to "insufficient" concrete.

When I asked about this out of curiosity, I was told that while the concrete did sufficiently meet the designed concrete strength eventually, it wasn't "reliable" to meet the strength in a timely manner. My translation - 28 days or sooner is preferrable, while 56 days is marginally acceptable as long as the strength is met.

Anyone else with ideas on this?

 

[Ron]

The addition of fly ash retards strength gain. You've done what should be done in an "after the fact" scenario; however, it would have been better to review the mix design on the front end and require a record of the moving average of strength tests for that particular mix design. While fly ash is counted as a "cementitious" material, it does not have the same hydration nor strength characteristics as portland cement.

For future test results I would require the following...

1. Have the supplier adjust the mix design to give you the strength parameters you requested, since the design was likely based on f'c at 28 days.
2. Have the supplier provide a strength gain curve for that particular mix design.
3. Check the 7-day strength results to see that they are tracking the curve.
4. Have the supplier provide you with the moving average strength results for that mix design and provide a record of service of satisfactory performance of the mix design.

 

[JAE]

Also...keep in mind that ACI 318, section 5.6 provides guidance on when you need to respond to low strengths.

The code generally gives the EOR (and building official) great leeway in judging whether or not a low strength concrete is a concern.

The EOR can, and many times should, return to the calculations to determine if the low breaks are even a concern. The commentary to section 5.6.5 states [red]"Lower strength may, of course, be tolerated under many circumstances..."[/red]

See also this FAQ: faq507-1575

 

[a2mfk]

JAE may have alluded to this- it all depends on the structure and the sequencing. If you are talking about foundations that will not see final full loading for several months, it may be no big deal at all. But if you are doing flat plate, CIP floors and beams, etc., and the vertical schedule is tight, you may need at least some of that early strength.

While under construction, many buildings won't have much of the MEP,misc. dead loads, or the live loads in place for several months. If we are talking about a wood framed house you could probably start framing the next day after you place the concrete foundation (from a loading standpoint).

 

[rscassar]

Thanks JAE. That is a good FAQ and I looked at it before requesting the 56-day breaks and the concrete mix.

I did not specify a high early-age strength and I don't think the readymix supplier would provide fly ash in the mix if this had been requested.

 

[TXStructural]

There are lots of examples where cylinder strength at 56 days (or later) is perfectly acceptable. There is no basis to reduce payment, unless completion of the job was slowed by the low 28 day breaks. The owner was not going to be occupying the structure within the 56 day window, and the concrete supplier should have known that the breaks would be good after 56 days (unless the supplier provided an unproven mix design.)

For example, some of the column mix designs on the Trump Tower in Chicago were based on 56 day breaks. Because the progression of construction did not require full design strength until after that time, it was not only acceptable, but expected.

28 day strength is arbitrary. For old-style portland-only mix designs, 28 days represents a plateau of strength development - the concrete would continue to gain strength at a slow rate, but 28 days was the point at which everyone agreed that the concrete should be ready for use. As we have moved to portland-plus-SCM mixes which develop strength more slowly, 28 days is not the appropriate time frame to evaluate strength.

 

[BigH]

As Ron indicates, fly ash retards strength gain in that the fly ash needs certain chemical reactions of the cement in order to hydrate itself. I think that it is a disservice when specifications that were developed for full cement mixes are used for fly ash mixes as these do not take into account the later strength gain. On a project of which I am aware, the requirement, due to the use of fairly high fly ash contents (30 percent originally) specifies the 90-day strength - not the 28-day strength. In fact the contractor is using 50% fly ash and 50% Type 1 cement and, after many thousands of breaks, no problems have occurred in achieving the required strengths although at 28-days, the strengths were lower than the characteristic value (but the characteristic value is a 90-day value, not 28). With the fly ash being used, the strength gain at 90-days is about 50% above that at 28-days. On a mass concrete, due to the inclusion of unwashed crushed rock fines, the gain approaches 100% gain. Engineers, when specifying the use of fly ash need to modify the acceptance criteria so suit the nature of the concrete being used.

 

[DRC1]

My understanding of the 21 day strength was that at 28 days, it was generally assumed most of the curing had taken place. It is also exactly 3 weeks from the pour date, so if you pour on a Monday, you could get the break on Monday 4 weeks later.
I have always assumed the selection of 28 days, was based on typical strength gain curves and was a practical but somewhat arbitrary date that was selected as a standard. I am going on about this because our DOT is now not accepting concrete that is out of spec, and looking for credits, if the strength is not up to par. They are not recognizing tests beyond the 56 day strength.
So.. I was wondering and this thread looked like a good place to ask, does anyone know the origin of f'c = 28 days, and can you point me toward some documentation?

Thanks

 

[Ron]

I don't have a copy of Neville at home, but checking Sidney Mindess's "Concrete", the 28-day strength criterion was first established as concrete strength was codified in 1921.

In general, test periods were conditioned upon the coarse grind of portland cement in that time period, having sufficient time to cure and gain necessary strength.

The selection of 28 days was a convenience factor, since if placement and sampling occurred on a weekday, testing would also occur on a weekday. 28 days was near the 30 days first thought to be necessary for adequate curing. I don't recall the citation for this little factoid, but I'll see if I can find it. I remember running across this many years ago when preparing for a concrete lecture/presentation.

 

[BAretired]

It seems to me that the 28 day strength of concrete is a rather arbitrary standard for the vast majority of concrete that we pour.

For elements such as high rise building columns, this is not true because the entire column section is required to perform under continuous stress throughout the life of the structure. For the majority of structures which we design, initial strength relates more to durability of the concrete under climatic events such as alternate freezing and thawing than to the probability of overstress in the members.

In the case of flexural members for example, a slight difference in concrete strength equates to no significant difference in strength or deflection of the member. Penalizing the contractor for failure to strictly adhere to strength requirements is usually more politically motivated than technical.

BA

 

[TXStructural]

As a quick reference, check the following website (PCA):

http://www.cement.org/tech/cct_curing.asp

28 days is not really arbitrary, but it only works for traditional portland cement in a basic mix with normal water-cement ratios.
As the portland cement hydrates in concrete with adequate moisture, it reaches almost "full" strength by 28 days. Other cementitious materials develop strength at different rates.

 

[DRC1]

Ron, BAretired and TX structural,

Thanks a lot really appreaciate the help, which is what makes this forum a great resource.
Just for cairification, I meant to say DOT was not accepting strength tests beyond the 28 day test, not the 56 day test.

 

[FixedEarth]

Not all fly ash are created equal. Class C fly ash is more cementitious because of the higher CaO content than Class F fly ash. We usually limit other pozzolans to less than 140 lb of class C fly ash per cubic yard of concrete.

Also in the summer, approximately 50% of your entrained air would be lost. We had a bridge job in the mid 90's-the concrete trucks left the plant with 8% entrained air and on 90 dgree day (humid day too), the job site air content was only 4%--200 lbs of class F fly ash was used in the mix. Had to add liquid air with wate to bring air content to a 6.5% content.

 

[concretemasonry]

Anytime you accept a mix design for acceptance and try to evaluate test results of a different mix that was placed you have some responsibility to justify the approval of the concrete.

Job site cylinders are just a measure of what is delivered and how it tests over time.

Looking at the mix designs approved and comparing them to the actual batching report provides a clue to the ultimate strength on the concrete in place, assuming properly inspected placement according to the specifications and and verification of the curing. If the cylinders are low at the "magical" 28 day mark it then becomes an engineering decision or just cutting cores to verify the in-place strength in comparison to the specified strength.

Different mixes have different time vs. strength properties that may be beneficial or detrimental to a completed project.

After that, the facts are verified and it is up to negotiations on the responsibility and possible credits/penalties.

How did a different mix design that what was accepted get used in the project?

Dick



Engineer and international traveler interested in construction techniques, problems and proper design.

 

[rscassar]

It's not that a different mix was used, the specification requests that the contractor submits the proposed mix design prior to delivery to site, however this submissions is not generally provided. It wasn't until I saw the 28-day strengths were low that I decided to investigate further and request the 56-day cylinder break and the concrete mix.