Concrete strength actually decreasing from 28 to 56 days?
Concrete strength actually decreasing from 28 to 56 days?
(OP)
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.






RE: Concrete strength actually decreasing from 28 to 56 days?
http://www.researchgate.net/post/Do_admixtures_red...
Brian C Potter, PE
Simple Supports - The history and practice of structural engineering.
ConstructionPic - Send annotated jobsite photos.
RE: Concrete strength actually decreasing from 28 to 56 days?
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.
RE: Concrete strength actually decreasing from 28 to 56 days?
RE: Concrete strength actually decreasing from 28 to 56 days?
RE: Concrete strength actually decreasing from 28 to 56 days?
RE: Concrete strength actually decreasing from 28 to 56 days?
Dick
Engineer and international traveler interested in construction techniques, problems and proper design.
RE: Concrete strength actually decreasing from 28 to 56 days?
@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.
RE: Concrete strength actually decreasing from 28 to 56 days?
Do a petrographic examination to determine if you are getting either reactivity or microcracking.
RE: Concrete strength actually decreasing from 28 to 56 days?
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
RE: Concrete strength actually decreasing from 28 to 56 days?
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
RE: Concrete strength actually decreasing from 28 to 56 days?
RE: Concrete strength actually decreasing from 28 to 56 days?
No switched mixes are not an issue, this is a plant dedicated to one product with one mix.
RE: Concrete strength actually decreasing from 28 to 56 days?
RE: Concrete strength actually decreasing from 28 to 56 days?
Our experienced group of testers and engineers feel the same way. We are awaiting petrographic analysis now.