Acceptable Deviation in Concrete Cylinder Strength 4

[ali07]

Hi Guys,

How much reduction of cylinder strength is acceptable for 28 days strength value. Should we wait for 56 days strength. Thanks.

 

[frv]

I guess I'm confused.. what do you mean reduction?

Did your 28 day breaks come up short of specified?

Is this for beams, columns or something else?

 

[theonlynamenottaken]

Assuming you meant all of your cylinders were understrength...

The structural engineer of record should answer this. This could mean waiting to introduce new loads or removing existing loads such as facade, live, equipment, etc. OR, it could mean that mix was inadequate, bad, etc. In either case the S.E.o.R. should be discussing this with the personnel in charge of the mix at the concrete plant. A quick review of the mix documentation may reveal that the wrong type of portland cement was used... After reviewing the mix constituents you may decide that waiting for additional curing is the most appropriate course of action.

Worst case: the structure may need repair, retrofitting, reinforcing, or replacement.

 

[JAE]

The following is our in-house guideline that (I believe) follows ACI 318 Chapter 5:

Acceptance of Concrete
Per Chapter 5 of the ACI Building Code Requirements for Structural Concrete
(ACI 318)

A strength test is defined as the average strengths of TWO cylinders tested at 28 days.

A strength test is acceptable if BOTH of the following are met:
1. Every arithmetic average of any three consecutive tests (6 cylinders) equals or exceeds f’c.
2. No individual strength test (average of two cylinders) falls more than 500 psi. below the required strength, f’c.

If either of the above two requirements are not met, contractor should take steps to increase the average of future concrete tests.

If Item 2 above is not met, then the following should also be followed:

1. With the lower f’c derived from the strength tests above, the structural engineer should review the design of the affected portion of the structure to determine if the lower f’c is acceptable. If the lower f’c is acceptable, then no further efforts are required and the concrete can be accepted. Steps still should be taken by the contractor to increase the compressive strength of the concrete for future mixes.
2. If the lower f’c is found to be critical to the performance of the structure, then further testing should be performed using drilled core samples.

Core samples should be obtained – three at a time, for each strength test that falls below 500 psi. If the concrete in the structure will be dry under service conditions, cores shall be air dried for 7 days before the test and tested dry. If the concrete in the structure will be more than superficially wet under service conditions, cores shall be immersed in water for at least 40 hours and be tested wet.

The concrete can be accepted if BOTH of the following are met:
1. The average of the three cores is equal to or greater than 85% of f’c.
2. No single core is less than 75% of f’c.

If the cores don't meet the required levels the following are all options to consider:
1. Perform a load test of the structure in the area under consideration (per ACI Chapter 20)
2. Provide additional structural framing to strengthen the portion of the affected structure to meet the required load carrying capacity.
3. Accept the concrete if acceptable to the owner.
4. Reject the concrete and remove and replace the portion that is considered below strength.

 

[msucog]

some eor will wait for the 56day if the 28 day breaks are marginal. jae looks like he hit it on the aci acceptance criteria. i'd core (likely at the contractor's expense) the field concrete prior to considering ripping anything out. i typically see the cores come up a little higher than cylinders (the two compressive strengths are different so don't be surprised if they come up different).

 

[ali07]

Concrete in question is basically not much required for strength but it is more likely act as an architectural exposed concrete. Does it has different acceptance criteria.
Thanks JAE for the information. It is acceptable to wait for 56 days strength.

 

[hokie66]

If it is architecturally exposed concrete, I would be even more concerned about the bad breaks, as durability is now the issue.

 

[ali07]

Yes my concern is also the same.Is there any requiremnt for acceptance for exposed architectural concrete if it is reinforced or unreinforced, as structurally i don't reqd that much strength. We are receiving 22.3Mpa against 25Mpa at 28 days which is about 10.8% less than the specified at that age for exterior exposed reinforced concrete pier.

 

[hokie66]

Sounds like a good case for requiring a high performance coating system to protect the architectural concrete. 25MPa concrete is probably not adequate for architectural concrete anyway, but that depends on your environment.

 

[dik]

A star for ya... pretty much what I follow, but nicely put. I've copied it for future reference.
ali07...

Regarding strength gain at a later date, I've included part of a recent technical memo I prepared...

... For example, the writer often has difficulty accepting concrete that has failed to meet the design strength... It matters little that the strength gain at 56 days or 90 days matches the specified 28 day strength. The writer’s experience is that a concrete with expected properties has, in past, been achieved by concrete having the specified strength; the same properties may not be provided by a concrete that has the same strength achieved at a later date. Although it has achieved the specified strength, albeit at a later date, it may not meet the design intent or requirement.

 

[dik]

Re Architectural concrete... different strengths, ie. lack of quality control, may have an impact on the colour of the finished concrete...

Dik