Concrete Sampling Guidlines 2

[sponcyv]

I am searching for guidelines that denote the specific time/location in which samples of concrete shall be taken for quality control measures. In my case, the concrete samples were taken when the concrete was transferred from the mixing truck to a bucket for transportation to the pour location. There were no samples of concrete taken at the pour location. I have found some information that states that the sampling should be taken at the point of discharge. The discharge in my opinion should be at the pour location.

It would be ideal to find a guideline that dealt directly with transporting concrete from one location to the next without constant aggitation specifically from a shoreline to a pour location in a body of water. I am thinking there is a guideline out ther put out by a department of transportation.

Thanks

 

[LobstaEata]

The following is from ACI Committee 311 for your review along with several ASTM references.

Placement inspection

1. Conditions
a. Coordination of concrete delivery;
b. Protection against sun, rain, hot or cold weather conditions;
c. Lighting and power.
2. Field tests of concrete
a. Use of specified mixture;
b. Field water additions (minimum 30 drum revs);
c. Sampling freshly mixed concrete, ASTM C 172;
d. Slump, ASTM C 143;
e. Temperature of freshly mixed concrete;
and C 1064 (maximum and minimum as specified);
f. Air content (pressure or volumetric), ASTM C 231,
C 173;
g. Unit weight, ASTM C 138;
h. Yield, ASTM C 138;
i. Cylinder specimens, ASTM C 31 (identification,
mixture, location, date);
j. Discharge of ready-mixed concrete truckload before
300 revs or 90 min, ASTM C 94; and
k. Initial curing of cylinder specimens, ASTM C 31.
3. Conveyance of concrete
a. Nonreactive materials;
b. Prevention of segregation and loss of materials;
c. Prevention of contamination;
d. Condition of conveying equipment (smooth surfaces,
no holes, and cleanliness); and
e. Use of drop-chutes or funnel hoses to contain freefall.
4. Placement and Consolidation of Concrete
a. Precautions taken for hot or cold weather conditions;
b. Preparation of contact surfaces;
c. Ability of conveying method to place concrete in all
areas of the placement;
d. Mortar bedding (use of starter mixture);
e. Prevention of segregation (no chuting or dropping
against forms or reinforcement);
f. Depth of layer (maximum limit);
g. External vibration (spacing to prevent dead spots)
h. Internal vibration (depth of insertion, spacing, time,
vertical insertion, no movement of concrete by vibration)
(Vibrators to be equipped with rubber heads
when consolidating concrete around epoxy water
reinforcing bars.);
i. Even layering around openings and embedments;
j. Removal of bleed water; and
k. Removal of temporary ties and spacers.

Hope this is helpful.

 

[cedent]

My understanding is that the purpose of the sampling is to ensure that the client is getting what was ordered/specified. The actual in-place strength is dependent upon the contractor's placement and curing operations. This way, when a problem arises you have the appropriate information to determine if it is a mixing/transporting problem or a placement/curing problem.

 

[sponcyv]

LobstaEata,

I located this list in ACI 311.4R, but it looks like it is just a checklist. I kind of thought that it may be an outline from another publication where each subject is elaborated. It references ACI 304, but the heading of the outline "Inspection of batch plants and truck mixers before or during construction" doesn't match the heading for 304. I guess I am a little confused by this checklist.

Thanks

 

[conceng]

It sounds like your concrete was sampled correctly.

ASTM C 172 "Standard Practice for Sampling Freshly Mixed Concrete" is the standard for measuring the concrete quality as-delivered. The following is an excerpt from the standard:

"5.2 The procedures used in sampling shall include the use of every precaution that will assist in obtaining samples that are truly representative of the nature and condition of concrete sampled as follows:"
NOTE 2—Sampling should normally be performed as the concrete is delivered from the mixer to the conveying vehicle used to transport the concrete to the forms; however, specifications may require other points of sampling, such as the discharge of a concrete pump.

ASTM C 172 goes on to describe sampling from different types of mixers or delivery vehicles.

I understand your concern that the process of moving the concrete from the truck chute to it's final point of placement can significantly alter the engineering properties of the concrete. That is why ASTM added the Note 2. Sampling from somewhere other than the discharge chute should be discussed and agreed upon before the concrete placement. All strength specimen samples should be taken in the same manner.

In your situation, I would recommend testing at both locations to determine the changes caused by the placing procedures. One can always test at the end of the truck chute for initial acceptance and perform regular QC testing and casting strength specimens at the point of placement.

We usually default to obtaining samples at the point of placement, unless safety or accessibility is an issue.

Gregory A. Johnson, P.E.

http://www.midwesttestinglabs.com

 

[sponcyv]

conceng,

It appears that the ball was dropped because none of the parties brought up the issue of sampling at both locations. This being said, on whose shoulders does the brunt of the blame fall.....EOR, QC Inspectors, Concrete Suppliers, Concrete Sub, General Contractor, all of the above? The guidelines seem to pull everyone into this in my opinion.

Thanks

 

[conceng]

In my opinion there is plenty of shoulders to take the blame. Did the EOR specify additional testing requirements over the standard? Did the QC inspectors ask if anyone was interested in the differences? Did the concrete supplier ask where the concrete would be sampled from and measured properties accepted from? A good concrete sub should care and bring it up, but he may not. The GC generally doesn't care, just that project gets done.

I'm a firm believer in pre-pour conferences where these things can/should be discussed. I was involved with one project where they were pumping air entrained concrete and the concrete supplier initiated a pre-pour conference to discuss the sampling location. Typically they would assume approximately 2% air loss knowing the mix and pump orientation that was going to be used. The concrete producer understands the in-place air properties are most important and wanted to provide that.

The owner/agency agreed to sample at the point of placement. The concrete trucks showed up and the agencies testing technicians decided to sample from the end of the truck chute. The air was out of spec on the high side and the cylinders did not meet the specified compressive strength. After a lot of coring and many man hours, the placement was accepted, but at a far greater cost than if the technicians had just sampled from where they were supposed to.

Gregory A. Johnson, P.E.

http://www.midwesttestinglabs.com

 

[sponcyv]

conceng,

I agree. Thanks for you input.

 

[Ron]

As conceng noted, the sampling was done at the proper location. The point of discharge is the back of the transit mixer.

Keep in mind that this record sampling is done to check the mix design only, not to check the in-place conditions. The only questions answered by a proper sampling program are "Is the compressive strength consistent with the mix design?; Is the slump (and air content, other plastic concrete parameters) consistent with the mix design?; Was the concrete delivered within a reasonable timeframe?; etc.) Even sampling the concrete at some other location (end of pump hose, bucket discharge, etc) does not reflect the in-place condition...it only reflects the change in the concrete from the discharge of the transit mixer to the point of placement.

If you want to check the in-place condition, do it by other means (cores, correlated nondestructive testing, load test, etc.) You can sample at both locations as noted, but the sample for record would be the one from the transit mixer discharge chute.

 

[sponcyv]

Ron,

16.1, 16.4, and 17.2 of ASTM C94 all state that samples shall be procured at the time of placement.

 

[Ron]

sponcyv...time of placement does not mean location of placement. Read paragraphs 4 and 11 of C94 as well as C172. They are referring to the discharge chute of a transit mixer.

Time of placement is considered to be time of delivery.