Concrete Cylinder Sampling While Placing Reinforced Concrete Columns 2

[twhite18]

The question has come up about where to take the samples for compressive testing when casting reinforced concrete columns. These columns are tall and require a hose approximately 20 ft. long to be snaked through the steel reinforcement in order to place the concrete. The obvious answer is to take the concrete at the end of the hose, however form work and tight column reinforcement make this difficult for contractors to do. Does sampling from the end of the truck rather than the end of the hose pose a change in the conditions of the concrete as it will be present in the field? i.e. changes in air content from going through the long hose, changes in slump? If so, how does a contractor mitigate where they can take the samples for compression testing from?

 

[Ron]

Age-old debate!!

The mix design should be tested from the back of the transit mixer at the time of delivery. If you accept or reject the delivery, it should be done at this point.
If you want to see how the concrete changes, then add testing at the end of the hose.

The concrete purchaser is buying the concrete delivered by the transit mixer. That is the mix design. The mix design cannot accommodate all variables that will be used in placement.

 

[twhite18]

Ron,
Thanks for your response, The only issue that I have with taking the testing from the back of the mixer is that it does not completely represent the concrete that is being placed in the columns. We may see changes in entrained air content from truck to placement going from 7% at the truck to 4% at the end of the hose. If the performance of the column is in question wouldn't it be best to use results from the end of the hose?

 

[concretemasonry]

Kudos to Ron-

Keep in mind that the concrete is just a commodity that is supplied in a truck to the site must meet the project specifications. Not all "trucks" are mixers (difference between central mix and transit mix), but most usually function as agitators and a delivery means for larger projects. - Some suppliers feel concrete is the most expensive technical method to supply/sell aggregate.

A good ready-mix supplier will require a signed notation on the "time snapped" delivery ticket of any alteration of the site modified approved mix, such as water or excessive unloading times. I know of some suppliers that provide digital cameras for drivers to document unusual site conditions and practices to determine liability in case of problems. Columns are a real problem if they are tall and require the maximum rate of vertical placement rates to maintain quality after access situations in the real world.

The contractor/purchaser of the concrete is then responsible. Once the material (concrete) is down the chute, it is the responsibility of the purchaser/contractor. It gets more complex if it is a combination truck and pumper for smaller volume projects with access problems.

Take the sample directly from the truck to determine the original properties of the concrete. - I worked on many concrete inspections (aggregate and mix designs) for 3 years while in engineering school and it was the best experience I had 30 years (and now more) ago.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[Ron]

twhite18....the sampling and testing of plastic concrete at the site is NOT intended to represent the in situ concrete. It is only to check the delivery of the concrete in accordance with the mix design. Nothing more, nothing less.

If you want to check the in situ properties of the concrete, take cores and do petrography.

Testing the concrete at the end of the hose does not represent the concrete in the column either. The samples are taken using standardized procedures and standardized sample sizes. The curing conditions in the column will likely be vastly different than the standardized curing applied to the test specimens.

If you want to see the effect of the pumping on the mix design, then sample at both places, but don't expect the result of either to be representative of the in place concrete.

 

[bb29510]

Testing at hose or truck as been a n argument for years, Contractor always force the at hose because a mix will lose slump traveling through the pump. I always like to think, and it might be wrong, that the concrete belongs to the producer at truck and contractor at hose, so if it bad, who pays. ACI is not clear, they claim, at point of discharge, which could be both. Now actual strength different from hose to truck is minimum, unless you are pumping thirty stories up a side of a building. I think this would classify as a judgement call.

ICC Master Special Inspector, Structural Masonry, Reinforced Concrete, Soils, Structural Bolted Joints, Structural Welding, AWS CWI D1.1

 

[concretemasonry]

If someone is really concerned, write two specifications for properties one at the truck discharge and another for sampling at the pump discharge (and hope for small sample to be representative). This means the cost of dual testing will increase the ultimate cost. The slump and 7/14/28 day compressive strengths are just indicators in the end.

To be expedient, fair and proper to all, the specification for concrete properties should identify the point of sampling. This is not a direct cost the the design/specifications and can easily be an option in a "boiler plate" specification.

Most major concrete suppliers usually know more about concrete mix properties in the end since they see a wide range of mixes than most others. They usually also do pumping or network with pumping subcontractors to learn a wide range of factors from different conditions. I know of several concrete suppliers that hire technicians from a testing lab to do some routine daily roving testing at various plants to provide valuable history. Meeting a specification is not a problem with a good facility and controls.

The strangest problem is when a specification requires lightweight concrete and not specify any specific aggregate or pre-soaking/saturation of the lightweight and expects consistency. Pumping CAN drastically affect the amount of surface moisture for pumping or finishing.

Dick



Engineer and international traveler interested in construction techniques, problems and proper design.

 

[darthsoilsguy2]

In USA.

With absolutely no specifications.... Normal structural concrete is tested at the truck and lightweight structural concrete is tested at the point of placement. That's ACI 301 default settings.

But.... since most projects have specs... there are usually contradictory information concerning material sampling.

Truck side: ACI 301, ASTM C94, ASTM C172
Placement side: Most specifications will specify the properties of the placed materials and the contractor having the duty to deliver those materials. There is good language in ACI 301 to use to justify sampling at the end of the pump when it becomes a concern any time during the project.

As much as neither test site "represents in-situ properties", it is pretty obvious that defending concrete with a 2% air test at the end of the pump as "inconclusive" with regards to in-situ air content is a position that won't win any respect.