Concrete strength actually decreasing from 28 to 56 days?

[dm3415]

Does any have any insight or experience with concrete actually decreasing in strength from 28 to 56-days? We are having three different entities test cylinders from a precast plant independently and all are coming up with similar results. 28-day strength is around 10-12,000 psi compression and then the 56-day breaks are around 7-9,000 psi. All are ACI approved testing facilities using ACI methods, so there doesn't appear to be any results due to testing error. This is a high-strength high-pozzolan mix and we are at a loss.

 

[briancpotter]

I've never experienced it, but this thread suggests certain admixtures may reduce post-28 day strength. However, it's light on specifics:

http://www.researchgate.net/post/Do_admixtures_reduce_the_strength_of_concrete_after_28_days

Brian C Potter, PE
Simple Supports - The history and practice of structural engineering.
ConstructionPic - Send annotated jobsite photos.

 

[concretemasonry]

Are you sure there is an appropriate ACI standard and testing/sampling procedure? When you get to manufactured products the general standards are woefully inadequate for automated plant controls.

Most independent testing laboratories use the ASTM standards, unless the person requesting tests has different criteria to meet. - What is your specification?

Who specified the pozzolanic and the amount? - This is usually beyond a design engineers knowledge or scope because of process (temperatures, proportions, timing, and curing) for the end product. I assume the samples were grabbed and do not reflect the actual placed concrete strengths.

PCI (Precast Concrete Institute) has some standards that are required to be certified that require long term testing programs (decades) and documentation. Even for mundane planks and Tees they can usually track the production of any product by the number of the production lot or project.

Dick

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

 

[civeng80]

Something thats always lurked in the back of my mind. With all the admixtures they put in today who knows what goes on after the magical 28 dsys.

 

[dm3415]

Concretemasonry: Yes, it's all appropriate testing and sampling (ASTM, ACI, State accreditation, etc), that's not the issue. This is a state lab and two separate highly accredited testing labs, all are independently coming up with similar results. Thousands of other cylinders are tested as part of this project from other sources and there have been no issues. If you have any insight as to why concrete would lose strength such as from ASR or other crystalline formations I would be interested to hear, but the mix design is not up for debate. I appreciate the effort to respond, but it's not the question I was asking.

 

[SLTA]

Sounds like a masters degree thesis in the making... or a published paper.

 

[concretemasonry]

What type of pozzolanics? There is a very wide range of types and sources from around the world. Some do not even work at low temperatures (ambient), but do perform well at higher curing temperatures when in a formed product.

Dick

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

 

[RWW0002]

Were the concrete (and cylinders)heat cured to achieve high early strength? I am wondering about the possibility of DEF (although I would not think this would be an issue in high-pozzolan concrete)

@briancpotter - I think that the link you posted is referring to admixtures affecting the post 28-day strength gain when compared to specimen without admixtures - not a decrease in strength after 28 days.

 

[Ron]

dm3415....reactivity would be my first thought given the descriptions. Check swelling on the specimens. The other thing to look for is with high strength, high pozzolan mixes you might be getting some internal microcracking from loss of moisture. Very dense mixes might not be getting that desired 100% relative humidity curing process in the middle of the speciment.

Do a petrographic examination to determine if you are getting either reactivity or microcracking.

 

[dm3415]

Ron and RWW0002:
The cylinders are not being steam cured, but the actual product is; we are going to core the finished specimens and perform petrographic analysis. ASR mitigation should have been accomplished by the low alkaline Type II cement and the high pozzolans. The source pit is periodically checked for ASR issues, but maybe the pit has a batch of bad agg.

The pozzolans in this case are some small amounts of silica fume and quite a bit of ground granulated blasted slag.

Doing a swell test is a good idea and we'll probably perform that shortly.

thanks

 

[msquared48]

"maybe the pit has a batch of bad aggregate"...

Seems doubtful to me as both the 28 and 56 day breaks would be affected negatively. I would lean toward Ron's thesis...

Mike McCann
MMC Engineering

 

[BigH]

Hate to bring it up (not really as I have seen it happen) - is there a chance that cylinders of different pours got mixed up? . . . and you ended up testing cylinders not at 56 day but at something less than maybe even 28? Such a drastic "loss" of strength seems very suspect. Can you provide a definition of high-pozzalonic? I've used 50% fly ash on one project with good results - are you at that level?

 

[dm3415]

BigH,
No switched mixes are not an issue, this is a plant dedicated to one product with one mix.

 

[TXStructural]

I asked our concrete mix guy and he says he has NEVER seen this happen, in 30+ years. It does sound like a testing anomaly.

 

[dm3415]

@TXStructural
Our experienced group of testers and engineers feel the same way. We are awaiting petrographic analysis now.