C 172 Sampling - Number of Samples Per Truck and 7 vs 28 Day Break 1

[UW1981]

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.

 

[JAE]

Check out these resources to see if they will help:

http://precast.org/wp-content/uploads/2011/05/TechNote_Concrete_Sampling_and_Testing.pdf

http://www.concreteconstruction.net/Images/Testing Fresh Concrete in the Field_tcm45-342628.pdf

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.

 

[UW1981]

JAE, thank you for the response....I have the second article you provided, and appreciate the link to the first. Your comment regarding "sample" and "cylinder" does put it all into better perspective. I suspect that if I want a 7 and 28 day samples from each truck, I would need to take two cylinders per truck (since we are not performing any other lab tests other than compressive strength).

 

[cvg]

a test consists of breaking two cylinders and averaging the results. so one test = two cylinders. so for 7- and 28- day tests, you need at least four cylinders. Extra cylinders can be use for 1- or 3-day tests to track the strength gain better. I would never alternate trucks. you really want to correlate strength gain to a particular truck or batch and that is hard to do if you are alternating.

 

[UW1981]

CVG - would it make any difference if all of the concrete was coming from the same batch plant (consistent mix design) - about 60 miles away. I have seen in other codes (ACI 318) that require testing every 150 cubic yards (assuming you have more than 5 tests for the class of concrete poured). I understand testing every truck if the concrete is coming from different plants. In short, I don't understand the basis for onet test (two cylinders) per truck (basically, one 28 day test for every 10 cuyd) from C172, and then basically one test (again, two cylinders) for every 15 trucks under ACI 318.....and I am sure this is due to my ignorance of these codes. In any event, I appreciate any and all enlightenment!

 

[cvg]

I guess I didn't meant to imply one test per truck, only that both the 7-day and 28-day cylinders should be pulled from the same truck. the combined sample should be pulled from the same truck to form all the cylinders.

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

 

[Ron]

The point of sampling and testing the concrete is to check the validity of the mix design and the concrete that was delivered to the site. For that reason, you typically sample at some interval of the total concrete placement/delivery, usually one test for every 100 to 150 cubic yards of delivery or one test for each mix design/supplier if less than the stated interval. You do not need more than one "test" per truck for two reasons...first you are taking a composited sample from the truck and assuming the transit mixer is in good condition, then the concrete delivered by a single truck should be relatively consistent in properties. If there is a suspicion that this is not the case, then subject the supplier to a National Ready Mix Concrete Association (NRMCA) audit, which includes a review of the aggregate and cement storage, batching process, controls, admixtures and the transit mixer...which includes climbing inside and empty transit drum to measure and assess the wear on the blades, etc.

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.

 

[concretemasonry]

It is always good to have and extra cylinder or two. Often there may be questions/problems with the transportation, handling, preparation and testing that most problems indicate lower strengths that required. an extra sample can make things easier since it may be used for additional verification. For 28 day, it can be kept with the official sets and can be broken if desired (cheap insurance and good verifier).

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.

 

[BigH]

Ron - one of our "biggest" fights on site is that the testing lab(s) want to do a spot sample for cubes (or cylinders depending on the country and specifications) rather than composite samples. BS, now EN tells one how to take a sample - both a spot and a composite but does not "specify" which to use when . . . I now have the lab on our site at least letting the first bucket go and take slump on the second skip but do get composite samples . . . Trials and tribulations of actual site work . . .

 

[Ron]

BigH...agree. Most testing labs do concrete sampling on a "per set" basis, so they want to get in and out quickly....not good. I am firmly of the opinion that there are very few test functions that can be charged "per test" and to maintain quality, the time has to be spent.

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!

 

[BigH]

We have full time testing as you described Ron on my job now and the last one in Indonesia. Every transit mixer a slump was taken and cylinders or cubes at specified volume intervals depending on the size of the placement. One good reason for a site-specific laboratory.