huangyhg

concrete proportioning - field experience basis
in looking at aci318-02, i am a bit confused with regards to an item shown in the flowchart of figure r5.3.
the item reads:
"field record of at least ten consecutive test results using similar materials and under similar conditions is available"
this seems odd to me.
isn't the point of allowing a concrete facility the use of the field experience approach to give them a method that permits them to utilize their past record as long as those records are based on "similar materials and under similar conditions"? the way i read this item, it says only ten of them need to meet this criteria. what about the other five to twenty tests (based on 15 to 30 total)?
what am i missing here, folks?
thanks in advance for any insight you can offer.
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this is a question of statistics and standard deviation. it is discussed in detail in the "notes on aci 318-99" (i only have the 318-99 version but i'm sure 2002 version is out there).
ichibansan...
what this means is that if you have enough historical data you can get to the point where you can calculate the standard deviation and use the criteria of table 5.3.2.1....then you have to validate your selection of the standard deviation with an additional 10 consecutive tests. if you can validate your selection of "s" with these 10 values, then you can use that as your average strength requirement. if you can't, then you must go to table 5.3.2.2 and use the appropriate overstrength factor.
most of the larger concerns use quality control for their concrete batching. they use their test cylinders to compile a running average of the test strengths of the concrete. for good control and delivery, the coefficient of variation comes down which means that a few pounds less of cement can be used in the mix. for a large concern, a few pounds here and there add up.
best tincan

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