Concrete Test Sampling 3

[Samwise Gamgee]

The GC has sent an RFI about the concrete test samples . The concrete supplier mentioned industry standards suggests sampling the concrete before pumping it out of the truck.

 

[Aesur]

If the concrete is being "changed" by the pump, then would not sampling at the pump be more accurate?

 

[MTNClimber]

The supplier wants to cover their butt. Their job is to produce a product as requested, not to anticipate the contractor's means and methods of placement. Therefore, they want it tested from their trucks.

If you need a certain slump and air entertainment at the point of placement, you need to collect the sample after conveyance (pump discharge point). If there are mechanical effects to the concrete due to pumping, someone should tell the supplier to adjust the mix design to a pump mix.

 

[Eric C.]

"current industry trends"?? I believe I would need further information and ask the supplier to produce published standards regarding this, eg ASTM, ACI, etc.

Alternatively, you could sample from the truck and from the pump discharge and compare the results.

 

[Ron]

Sampling and testing are for validating the mix design, not the in situ conditions. Sample at the point of delivery, not the point of placement. If you want to see the differences, sample at the truck and at the end of the pump hose.

For validation and reporting for record, the sampling should be at the truck.

 

[Enable]

Ron is correct. Unless stated otherwise in the contract documents sampling is always done at the point of discharge from the truck. Even if you desire samples as close to placement as possible (e.g. end of pump) you'll still take samples at the truck since that's all that the ready-mix supplier is guaranteeing.

See best practices to concrete guide in BC

Unless otherwise written in the contract
specifications or agreed upon at the pre-construction
or pre-placement meetings, sampling for the
purposes of acceptance shall be a grab sample
obtained between 10% and 90% of the load at the
point of discharge. This requirement is not applicable
if the concrete is to be tested for slump prior to the
addition of superplasticizer.

A23.1-14 – Clause 4.4.2 Sampling concrete
Samples of concrete for testing purposes shall be
secured in accordance with CSAA23.2-1C. When the
owner elects to assess the quality of concrete at a
location other than the point of discharge from the
delivery equipment, the owner shall state the point from
which the samples shall be taken.
Note: The point at which the concrete is sampled will
depend on the intended use of the test information.
Where the test data are intended to give information on
the properties of the concrete
(a)as delivered to the site, the concrete should be
sampled at the point of discharge from the delivery
equipment; or
(b) as incorporated into the structure, the concrete
should be sampled as close to the point of final deposit
in the form as is practicable.

 

[MotorCity]

The concrete supplier's job is to deliver the specified product from A to B. What you choose to do with the product is up to you, whether you feed it into a pump truck or dump it in a big pile. I would be curious to see how the properties change after being pumped. The end user of the building doesn't care how the concrete comes out of the truck, he cares about the quality of the concrete that ends up in his building.

 

[MTNClimber]

ASTM C172, Standard Practice for Sampling Freshly Mixed Concrete states under Section 5.2, 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."

 

[BigH]

One of the sticky wickets so to speak is there is a difference as Ron so correctly pointed out is that the concrete cubes taken as per ASTM is to determine the potential strength of the concrete mix delivered - it has no "direct" correlation to the strength of the concrete as placed, vibrated and cured. The samples taken at the mixer truck are maintained one day in the field (within a certain temperature range) and then taken to the lab and cured either in water bath or humidity room for the FULL time period until testing. The concrete in the field is placed - sometimes very poorly (such as moving the ooncrete with the vibrator, placing concrete from the moving face over the face instead of into the face and then there is the issue of proper vibration. Seldom have I seen on the many projects on which I have worked (albeit in Asia - China, Laos, India, Indonesia, Malaysia) that the concrete is vibrated in a proper and organized fashion - ensuring overlapping of each insertion with an adjacent one. Typically, the vibrator operator inserts here - then drops the next insertion 1 m away to his left, then back to his right in yet another spot . . . and many times the vibrator operator fishes. Then is the contractor honouring cold and hot weather concreting requirements? Does the contractor continuously cure or spray water on a hot sunny day 2 or 3 times per day and says he/she is curing?

All this affects the strength and durability of the concrete in place. It would be very difficult for me to try to "blame" the concrete supplier when the workmanship using the supplied concrete is so poorly done. This is why the concrete supplier is happy, I would believe, that the concrete sample is taken immediately out of the truck - oops, there too is a problem in whether the lab technician has sampled properly and cast the cylinders (or cubes in some countries) correctly. All out of the control of the concrete supplier. Most definitely a sticky wicket.

 

[EcoGen]

For quality control and to satisfy ACI requirement, the only thing that matters is the concrete at the point of placement which is after it has been pumped. Since there is some slump and air content loss during pumped, suppliers will have to adjust their mix accordingly depending on pumping procedures. That's not always an easy thing to do. From the Supplier's perspective he only cares about the concrete delivered to the site prior to pumping. That's why he's trying to cover his ass and claim that it's somehow "more accurate" if you test pre-pumping. In either case, it's the GC's responsibility to ensure the concrete at the point of placement after pumping meets project/contract specs. If they really wanted to investigate the effect of pumping on concrete properties, they can add in extra tests at the truck pre-pump too but that shouldn't be used for acceptance.

 

[dik]

The supplier's obligation stops at the point of discharge... all else is man made (with apologies to Kronecker).

Rather than think climate change and the corona virus as science, think of it as the wrath of God. Feel any better?

-Dik

 

[OldDawgNewTricks]

The supplier's legal obligation may stop at the point of delivery, but the general contractor's obligation is whatever the contract documents say. Some specifications might require additional testing (slump, air content, etc.) at the point of placement. The contractor will want the concrete supplier to work with them to design a mix that will meet the specifications and it is in the supplier's best interest to make sure their customer is happy.