Concrete testing 5

[Alpharetta]

Have been setting spec for testing (psi and slump) concrete in sidewalks and/or curbing at mid-point per 9-yard truck load. Is this average ratio in the southeast?
Thanks

 

[MRM]

I'm in Michigan and we usually have our field techs obtain a sample for testing in the mid point of the pour, although I'm not sure if it is written anywhere-I still think it's a good idea.

Also, when pumping concrete, we typically take samples from the discharge of the pump halfway through the pour. Sometimes, we take samples at both the pump discharge and the truck discharge points if there is a question regarding the difference in concrete quality as it goes through the pump.

 

[Alpharetta]

MRM, Thanks! Feel I'm in good company then.

 

[OREO25840]

I agree that mid-point would be a good location for concrete psi(strength) test and possibly be ok for slump too. I feel the slump test should be an initial test performed from the first portion( 1/4 cu yd) poured. If slump test failed at mid-point, then you have 4 1/2 cu yds of concrete out of specs.

 

[mhnds]

I want to ask all that replied this thread and other experienced engineers.How many samples do you take?,i think only one sample will not give a representative result.And i wonder what you do when the test results are bad-especially the strength test-,do you break down the partially set concrete?
regards umitk2211

 

[boo1]

The procedure for the slump test is covered in ASTM C-143 from a sample of concrete obtained per ASTM C-172. We sample by pail through the discharging stream of concrete, starting the sampling operation after the first couple of yards then atleast one more in the last 1/2 of the pour.

With the inclusion of admixtures the slump test more correctly is a measure of the fluidity of the mixture and only an indirect measure of the water. In designing concrete mixes by either the American Concrete Institute or the Portland Cement Association method, strength and durability are governed by the water-cement ratio (pounds of water per pound of cement), not by slump. It is possible to meet the same standards for concrete through a wide range of slumps – as long as cement is added in the same proportion as water is added to increase slump. Of course overly-wet mixes should be avoided, of course, because they can lead to finishing problems and problem surfaces.

 

[KRSServices]

At a minimum, I typically specify slump, air, and compressive strength to be tested a minimum of 1 test per 150 m3 of poured concrete or 1 test per day for curbing and sidewalks. Now this will depend on the area and confidence on the concrete supplier because a higher frequency can be specified. Most of my contracts are end product specification based, and it is up to the contractor to ensure adequate testing to provide conformance or deal with rejected product. Typically, the contractor tests once every two or three trucks. 150 m3 is a huge incentive to get it right and accordingly, I have not had a failure due to non-compliant concrete in years. KRS Services

http://www.krs-services.com

 

[MikeDB]

We do air and slump tests at the beginning of each truck load. We also take 3 cylinders from each truck for compression tests at 7 and 28 days. The third cylinder is a spare that could be broken at different time if needed for construction or in case of a bad or questionable break. If all 3 cylinders test below spec, we check with designer to see if actual is adequate, if not we will require testing of the placed concrete.

 

[BigH]

My first try into the fray! AASHTO (and it is downstairs in the office) clearly states the locations within a truck to take sample. As AASHTO is basically, ASTM does too. If I remember it correctly, it is at the 1/3 and 2/3 points - then mix the two "samples" together to form the "test sample" for a "representative sample for cast cylinders (or cubes). But, they state that you may wish to take a sample just for slump after about 1/2 yd (or 1/2m)has been discharged - idea is to be able to reject the truck if too high or slump doesn't slump. Indian standards says to get "test" sample from three locations in truck - I'll confirm.

As for how many tests per pour, see the Indian MOST (Ministry of Surface Transport) - it will surprise you KRS - if I remember right, it is three samples (i.e 3 sets of tests - each set is for three cubes to be taken fro 28day strengths and additional cubes if desired for 7 day). But I will confirm. Of course, they, here take samples of even the lean (10MPa oops - that is 10MPa cube or 8MPa cylinder) concrete!!! used for mud mat!!!

Best to you all.

 

[KRSServices]

BigH,

In the example I provided, all samples are taken in accordance with the procedures prescribed by ASTM standards by the representative geotechnical engineer. My understanding of the thread was mainly in reference to the standards in relation to frequency of testing. I normally specify cylinders (strength) for 7 and 28 day strengths based upon the job. Certainly, local municipal/highway standards will always apply, depending on the contract source, however I have found that in many projects, including Provincial highway projects, an end product specification places the onus of primary testing on the Contractor, with results forwarded to the geotechnical engieer for review and approval. Random testing by the owner, as a quality control and measure for payment, serves to verify the Contractor's numbers. The result is a surprising reduction of project cost to the Owner and a vastly improved product as well because the onus of responsibilty shifts from the Owner to the Contractor.

Local ordinances and regulations shall always be deemed the standard to follow, however, if other methods are desired, I am merely providing my thoughts on the subject.

I truly enjoy the responses to these forums due to the many different ways of performing the seemingly similar tasks.

Take care and all the best in the new year! KRS Services

http://www.krs-services.com

 

[BigH]

Wrote this before KRS latest but will send anyway. I agree that specs should be job oriented but not too many will take the time for something as "mundane" as testing intervals to change the previous spec. Contractor here has to do the testing per the MOST noted below. We view all tests in field and view testing in lab. We also will, from time to time, direct another set of cylinders - oops, cubes -- in the field as a check. On most international jobs I have been posted on, contractor is responsible for testing, RE staff does occasional tests to confirm. Team Leader then does (orders) an occasional test, too. I will still send as earlier written just to bring out some points.
++++++++++++++++

Confirmation of items in my earlier post – and apologies for my feeble mind!!

AASHTO (T141) specifies that sampling be a composite of two or more regularly spaced portions within a specified time frame (less than 15 minutes). The various portions are then mixed together to form a composite. For air and slump, an initial sample “may be taken after at least ¼ cubic yard or concrete has been discharged.” – not the ½ yd that I put in my earlier post – sorry.

Indian Standards (IS 1199) states “At least three approximately equal sample increments totaling 0.02m3 shall be taken from a batch during its discharge . . . Where three sample increments are taken they shall be taken at about the time when ¼, ½ and ¾ of the concrete has been discharged . . .” The samples are then made into a composite sample.

I give the IS so that we may see that different countries do have slightly different procedures. For instance, ASTM/AASTO do slumps in 3 layers; IS is in 4 layers. Normal Indian practice is to use cubes, not cylinders. Need to correct – approximately: 80% cube strength = cylinder strength (i.e., 30MPa cube = 24MPa cylinder). Ratio is somewhat higher at higher strengths (say 90% at cylinder strength >35 MPa but most here just use the 80% value. This is important to correlate specifically if you have a mixed bag as sometimes may happen in SE Asia depending on origin of contractor. One other difference is curing temps after initial 24hr – at AASHTO/ASTM it is 23degC plus or minus 1.7degC (AASHTO T23); in India it is 27degC plus or minus 2degC (IS516). This really takes into account the more typical ambient temperatures in India – probably so not so much ice or chillers are needed.

As for number of tests, MOST (Indian Ministry of Surface Transport Specs for Road and Bridge Works, 3rd Revision) states: The MINIMUM frequency of sampling of concrete of each grade shall be:
1 – 5 m3 in works: 1 sample
6 – 15 m3 in works: 2 samples
16 – 30 m3 in works: 3 samples
31 – 50 m3 in works: 4 samples
>51 m3 in works: 4 samples plus 1 sample for each added 50m3 or part thereof.
Here, the auditors would say that if you had 52 m3 in works, do 5 samples. Note, too, that this is per “works”. If contractor does three bridge parapet walls, say 11m3 each, he would be required to do 3 x 2 samples even of same class concrete.

On another international job – it was 1 sample per day of each mix used, at least 1 sample per 50m3 of concrete placed for reinforced and prestressed concrete, at least 1 sample per 100m3 for mass concrete; at least one sample from each pier cap, column and abutment, at least one set from each precast element. If same grade is used at two or more places on same structure – at least one set subject to above. For small works, say culvert headwalls and do several in day – at least 1 sample for each class. The fallacy in this particular job was that each 0.125m3 was mixed individually in portable mixers – we had, due to extreme remote locations, no ready mix!

KRS: Your 1 sample per 150m3 seems pretty liberal!!! (Point taken that your 1 per 150m3 was for client confirmation). I couldn’t find my CSA book – lent it out so can’t confirm CSA requirements.

Hope this helps – it shows that engineers/country practice/organization practice, once again, vary widely in opinions and requirements for such common and routine items as this.

p.s. (1) Can anyone tell me how I report, in accordance with AASHTO T22, a test result (7 day or 28day) to the nearest 69kPa? (viz., 0.069MPa)? I queried AASHTO and got a snide remark in return.
(2) Does anyone have a handle on maximum flakiness and elongation combined? KRS??? DirtDoc1???? This is becoming a great “audit” concern of ours: specs say 25% combined; we can’t get anything under 30 to 35% (have tried four quarry sources! – rock is not siltstone/slate, or laminated). Mixes are workable; strengths are all in accordance with spec values but worries abound about hand slaps. I have a thread in civil related on this for some details. Thanks.

 

[boo1]

check these links;

http://www11.myflorida.com/statematerialsoffice/QualitySystems/IAProgram/IAChecklist/ASTMC-172.pdf

http://www11.myflorida.com/statematerialsoffice/QualitySystems/IAProgram/IAChecklist/ASTMC-143.pdf

http://www11.myflorida.com/statematerialsoffice/QualitySystems/IAProgram/IAChecklist/ASTMC-39.pdf

 

[Alpharetta]

I want to thank everyone for their responses to my question of concrete testing ratios. Reason I spec tests at middle of truck pour (sidewalks and curbing); I've found "some" contractors feel free to open the water valve after test cylinders taken. And, per one response, projects and contractor history somewhat dictate variances beyond the standards.
Thanks,
Alpharetta