C 172 Sampling - Number of Samples Per Truck and 7 vs 28 Day Break
C 172 Sampling - Number of Samples Per Truck and 7 vs 28 Day Break
(OP)
This question had probably been answered before, but I could not find it in the discussions. C172 seems to indicate that only one sample per truck (revolving drum)is required, but that sample must be made up from two portions, sampled no longer than 15 minutes apart. If this is the case, then is it safe to assume that this would be a 28 day break (since the 7 day break is for information only)? If you want 7 day breaks, then would you have to take two samples per truck....or could you alternate - one truck on 7, the next on 28? There are also standards that call for testing every 150 cuyd, but I would assume this is from batch plants and not from trucks? If it applies to trucks, then alternating 7/28 would certainly provide adequate sompling since each truck is about 10 yards....but I am not sure how the 150 cuyd testing frequency relates to truck and C 172. And....yes....this area is not my expertise! I appreciate your responses.





RE: C 172 Sampling - Number of Samples Per Truck and 7 vs 28 Day Break
http://precast.org/wp-content/uploads/2011/05/Tech...
http://www.concreteconstruction.net/Images/Testing...
I think the reference to "sample" in C172 is not the same as "cylinder". The number of cylinders needed is usually specified in the project contract documents - not the ASTM standard.
RE: C 172 Sampling - Number of Samples Per Truck and 7 vs 28 Day Break
RE: C 172 Sampling - Number of Samples Per Truck and 7 vs 28 Day Break
RE: C 172 Sampling - Number of Samples Per Truck and 7 vs 28 Day Break
RE: C 172 Sampling - Number of Samples Per Truck and 7 vs 28 Day Break
so the procedure is:
truck pulls up
run some concrete into the forms
stop and run some into the wheelbarrow
continue filling the forms
stop again and more into the wheelbarrow
technician than uses the combined sample in the wheelbarrow to run the air, temp, slump tests and form the cylinders.
wait for 15 trucks and then make more cylinders
typically do slump on all trucks
RE: C 172 Sampling - Number of Samples Per Truck and 7 vs 28 Day Break
As for the sampling for record purposes, the common method is to take a sample of at least 3 cylinders (6x12" in the US). Most testing labs base their pricing on a set of 3 cylinders. Usually 1 specimen is tested at 7 days for informational purposes (as noted), that can give a hint as to the 28 day strength of the material. If there are admixtures, ground granulated blast furnace slag (ggbfs), fly ash or other variations to a "classical" concrete mix, the 7-day estimate of 28-day strength can be wildly wrong! For Type 1 portland cement, coarse and fine aggregate, and water....the 7-day strength should be about 70 percent of the 28-day strength. Most changes to the mix design will reduce that 70 percent estimate...I've seen as low as 45% for GGBFS. There are exceptions in the other direction as well.
For a 3 sample set of cylinders, one is usually tested at 7 days and then two are tested at 28 days (and averaged for the "record" strength per ACI 318 as noted). Additional samples can be taken and often 1 additional sample is taken and held in reserve in case there is a strength issue at 28 days. The reserve cylinder is usually tested at 56 days in such cases.
There is no good predictor of strength while the concrete is still plastic. The slump test is not a good predictor of strength, mostly because of the variations in slump that can be achieved with admixtures that do not adversely affect the strength. The slump test is not an indicator of water-cement ratio for the same reasons and the water-cement ratio is the better predictor of concrete strength; however, there is no convenient means of measurement of the water-cement ratio in the field. Using the batch ticket (assuming accuracy) and checking to see if water was added at the site can give you a reasonable idea of W-C ratio; however, it is not accurate because of varying absorption of the aggregates and other batching issues.
RE: C 172 Sampling - Number of Samples Per Truck and 7 vs 28 Day Break
In my early days as a concrete inspector and looking back, a bunch of cylinders in a pick-up are looking for problems and strapping to pallets is not really practical because of the equipment needed. I ran into one bridge contractor that paid extra (in some way) to have the cylinders in a protected manner.
Dick
Engineer and international traveler interested in construction techniques, problems and proper design.
RE: C 172 Sampling - Number of Samples Per Truck and 7 vs 28 Day Break
RE: C 172 Sampling - Number of Samples Per Truck and 7 vs 28 Day Break
Something that we used to do but I have not seen done in quite some time, is to have a qualified technician on site whose task is to record the trip ticket info on every truck, run a slump test on every truck (twice if water is added at site) and sample at the required interval. On a large placement, that is one busy guy! Sometimes requires more than 1 technician. This process is invaluable for record purposes and for troubleshooting in the event of a problem.
But, alas,...no one wants to take the time or spend the money for such anymore!
RE: C 172 Sampling - Number of Samples Per Truck and 7 vs 28 Day Break