failing concrete cylinders

[eastendeng]

I've got a large concrete parking lot pour. The concrete is supposed to be 4,000psi with fiber reinforcement and general specs in accordance with ACI 330.1-03. One slump test and one set of cylinders were taken for each 50yds. Cylinders had a 7 day ave of 2,160psi and 28 day ave of 3,355psi. the concrete supplier took their own QA samples and I'm trying to see their results for comparison. the subgrade compaction was confirmed within acceptable density and moisture limits. What is the acceptable mitigation? removal and replace? Wait for signs of failure then replace? combination?

 

[concretemasonry]

What was the specification for the concrete and what mix design was submitted and approved? The use of being "green" and using ash has thrown in a few wrinkles into the historic data that standards are influenced by.

If fly ash or similar materials were used, the strength gain would be later than 28 days.

The slump test is really just an indicator of the free water in the mix at the time. One set of cylinders seems a little light when it comes to cylinder making by one technician, curing and handling and no room for future verification to hang your hat on.

Dick



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

 

[Ron]

Agree with CM on the delayed strength gain with fly ash or GGBFS cement. Don't pull the trigger yet!

 

[dBasement]

I would be pulling the trigger. It is most likely that the specs read 4000psi at 28 days, not 256 or whenever the supplier is comfortable. As a result, the concrete should be considered failed. Should it be ripped out? Probably not, but certainly either a performance bond or significant reduction in payment is warranted. Where these sorts of things go sideways is if there is any discrepancy or deviation whatsoever in the test method or handling of the test cylinders.

84% of design strength is quite low and that is the AVERAGE. If it was a set or even two at that range I'd be less concerned, but that is a significant reduction in strength and will likely affect the long-term performance of the mix.

 

[eastendeng]

thanks for the replies and info. No fly ash was used. The slab is being poured in several sections and the second section, with the same (supposedly) mix, had a 7 day strength of over 4k. It seems clear to me that the mix was off. I'm not as concerned as to the why right now as I am with the remedy. The slab is a retail parking lot with typical traffic in the area in question. I'm leaning toward an agreement with the conc plant for regular monitoring and a fix if/when needed. Am I being too naive?

 

[dBasement]

Probably. Cash talks. You should be compensated for the potential lessened life of the pavement. There is unlikely anything will happen in five or ten years. But a 30 year life might be reduced to 25 (just pulling a number out)

 

[TomDOT]

In 5 years when it fails early, and the guy you had an agreement with has moved to a different company (or the plant has been bought out by someone else) - it's gonna be a PITA to find your documentation and get them to actually follow through.

If you have cash, you can just pay anyone you want to fix the problem.

 

[eastendeng]

Came to terms with the supplier. The issue was a too much admix that has substantially delayed curing (56 day break at between 3k and 3.6k). Thanks for the input.