Question about concrete strength
Question about concrete strength
(OP)
Im currently working on a post tentioned bridge project. It seems that most of our cylinders thus far have failed. We are using AAA concrete which should be breaking at a minimum of 5000 lb to suit the DOT but it seems that most are breaking at 4200 to 4700 and some as low as 3500 lbs. At this point even the cores which were tested broke less than 4500 lbs. My question is, What could cause such a decline in strength ? I wish i had the mix design on hand to disclose but i dont. I was just looking for some basic conclusions ... Thanks in advance ... Jack.





RE: Question about concrete strength
Seriously, there are all sorts of reasons why concrete doesn't come up to strength. I assume the cylinders have been made and stored in accordance with standard requirements etc, and are of the appropriate age.
If so, you really need access to the mix design and batch certificates, so that you can check all materials are correct (appropriate source of aggregates, appropriate proportions of fine and coarse aggregate, adequate cement content, water/cement ratio is OK, use of admixtures, etc, etc).
Also, what temperature were the cylinders stored at? Concrete curing can be much slower at lower temperatures.
If you can't get satisfactory evidence that everything is OK with the concrete, you would need to seriously look at condemning the concrete, and definitely talk to the designer, because this is a significant shortfall of strength, and could give rise to serious structural issues in a post-tensioned structure, especially at the anchorages.
RE: Question about concrete strength
RE: Question about concrete strength
It seems that the cylinder handling was done correctly and they were stored accordingly.Also,the batch sheet shows that it was mixed according to design.
What i dont understand is this, How can there be such an inconsistancy between the concrete one day and the concrete the next ? Everyone seems to think it may be the materials used but if that were the case ALL of the cylinders would fail but as it stands now one may pass and then one may fail...
At this point it seems that it will be coming down, We are just waiting for the higher ups to give the word.
RE: Question about concrete strength
I don't think the variations are that unusual. They're more noticable when you're looking at low results.
RE: Question about concrete strength
a yield of 27 cf/cy at the low slump, design strength at 28 days with the high slump, and with everything passing spec at the first standard deviation from the bell shaped curve. Sometimes you get the bad batches on the wrong end of the curve. However for you to have this many falling below spec at one time is statistically improbable. I would look first to the batch plant and check the mix proportions/weights. Sometimes the scales get off and you get a bad mix. Second, if this is a transit mix, check for water added after the trucks left the plant. Also check delivery times. A lot of things can go wrong.
Best
RE: Question about concrete strength
RE: Question about concrete strength
RE: Question about concrete strength
One common reason for slow strength gain is the used of ground granulated blast furnace slag cement as part of the cementitious material. At percentages greater than about 30 percent of the cement, retarded strength gain will likely result. Similarly, fly ash added to the mix will retard the strength gain if used to substitute for part of the cement. Both are allowable, but you have to know the repercussions.
The first move is to obtain the mix designs and determine if either GBFSC or fly ash was used. The look at the field sampling reports and see if water was added. If there were no limits placed on the water cement ratio, ultimate strength can be achieved to the detriment of slow strength gain and reduced durability.
Consistency of the delivered mix should be monitored by a competent testing laboratory. Each truck should be checked for slump and air content, and appropriate samples taken at reasonable intervals for compressive or flexural strength testing. Variations in mix consistency can be spotted immediately, with resulting correction also done at that time. Inconsistency in the delivered mix is usually a quality control problem of the supplier...often a result of improper moisture control of the aggregates or improper batching procedures.
RE: Question about concrete strength
RE: Question about concrete strength
RE: Question about concrete strength
The first thing to happen in response to shortages is the concrete suppliers skimp on the cement, or the cement producers begin pulling product from the clinker too soon and send out 'hot' loads. I'm not accusing your supplier of doing this now, but it has happened a lot in past periods of high demand. But, the first thing I would do is check out the cement used on the project - see if it has varied in fineness and/or set time, particularly over the past 4-6 months - your state DOT should be randomly testing cement samples. Another quick check can be done at the plant's aggregate piles - check to see if stone is dirty. Dirty stone has a much greater effect on high strength mixes than more frequently specified mixes.
In the testing business, if you know your equipment and procedures are in compliance, it's a good market for concrete testing right now. We all can expect to see concrete problems as long as cement shortages exist. It's just the nature of the industry.
RE: Question about concrete strength
RE: Question about concrete strength
Hope that is the reason, if your supplier is any good he'll open his doors and have nothing to hide.
RE: Question about concrete strength
Good Luck with your quest. Hope this helps, Fred
Fred J. Croen, RSM
Engius, LLC
Boston, MA
www: engius.com
RE: Question about concrete strength
The National Ready Mixed Concrete Association (301-587-1400) puts out a Quality Control Manual. Section 3, Certification of Ready Mixed Concrete Production Facilities, has an excellent check list for doing batch plant inspections. I’d suggest getting a copy of this document and performing a complete point-by-point batch plant inspection at your earliest opportunity. Additionally, inspect each truck that the supplier wants to use and only approve the latest model trucks with the cleanest drums. I have this doc in pdf format but I don't know how I could get it to you.
Techmaximus
RE: Question about concrete strength
Fred J. Croen, RSM
Engius, LLC
Boston, MA
www: engius.com
RE: Question about concrete strength