f'c < 2500 psi 1

[Robert216]

I know the ACI 318 states that concrete shall have a minimum f'c of 2500 psi. I have a situation where 28 day, 56 day, and even some cored breaks have come in lower (around 2000 psi). Keep in mind that all concrete is for shallow foundations, no vertical elements. I have checked shear and moment capacities in the footings and everything works ok, even when I set f'c=1500 psi. I also checked epoxied anchorage and "backed into" published values from Hilti for masonry (min. f'c=1500 psi). Is there any other potential problem that I should be aware of? Thanks in advance for any help. Rob

 

[Ron]

Concrete with lower strength will have lower durability. If the concrete is in contact with any groundwater or soil that could be considered deleterious (sulfates, chlorides, etc.) then you could have issues beyond that of strength.

 

[concretemasonry]

It would take real talent to make concrete as low as 2000 psi at 56 days. There is no real cost savings for anyone to make something that low.

Have you investigated the sampling, cylinder preparation, handling, curing and testing of the cylinders?

Dick

 

[JAE]

...and be sure to check out ACI chapter 5 for acceptability of concrete - within 500 psi is considered OK for a single test.

 

[hokie66]

As Ron said, durability would be the most important issue. The concrete probably has too little cement and too much water. Lack of cement will lead to poor protection for the reinforcement.

 

[Robert216]

On this project, there is also a small building where the test cylinders broke around 1500 psi at 28 days. I have investigated the testing to an extent. I spoke with a representative from the testing company and he mentioned that the cracks were very clean. He said that the cracks continued straight through the aggregate. I would expect if the water cement ratio was the problem, the cracks would indicate the paste deboning from the aggregate, leaving a "rough" shear plane.

 

[Robert216]

concretemasonry,

I have at least 36 cylinders that are between 2000 and 2500 psi.

 

[Robert216]

My potential problem is that many of these buildings are already framed. In terms of durability, what would be an appropriate "bottom line" for acceptance (something other than what is in the code).

Thanks

 

[hokie66]

Robert,

Yes, if the failures involved shear planes through the aggregate, you should investigate the aggregates used. Such low strengths should not occur with competent aggregates.

You stated that there are many buildings. What type of buildings? If they are all just single storey structures with only strip footings involved, the concern would certainly be less than with a more significant structure. But you should still try to get to the bottom of why it happened.

 

[BigH]

Ron is correct on durability. However, as when reading AASHTO, when there is a low strength test result, the concrete can be accepted when structural computational checks show that it is acceptable - durability aside.

 

[Robert216]

Thanks for all of the input.

 

[irasamm]

I would immediately check the calibration of the equipment at the testing lab, then the supplier (sounds like you may not have all the cement you are paying for). 1500 psi is very very odd. Start using a 3,000 or 4,000 psi mix and save yourself real money.

 

[concretemasonry]

The observation of the lab that the aggregate was sheared and not pulled out of the so-called "weak" concrete matrix. Descriptions of the failure mode are often overlooked. Photos are very valuable and cheap.

It is hard to comprehend the aggregate breaking and not a lot of aggregate particles pulled out is unusual at such a low indicated cylinder strength. The split aggregate is indicative of reasonably decent concrete matrix strength.

Calibration of the testing equipment in addition to the testing procedure (capping method and accuracy, centering on the platen and rate of loading) should be looked at. Also, look at the means to allow the upper plate to a rotate if the capping is not parallel between the top and bottom.

Dick

 

[dicksewerrat]

Take some of your samples to another lab.

Richard A. Cornelius, P.E.

http://WWW.amlinereast.com

 

[cvg]

have some cores taken and deliver them to another lab

 

[DRC1]

There used to be a time when 2,000 was a design mix and design mixes of 2,500 are still available. Although if this was a redi-mix with a target of 3,000 or better, something is wrong. Talk to the supplier. Also, and I may be wrong, but I always thought the lower the strength, the better the duribility; which was why old time designers tried to match the required strenth to the mix strength.

 

[Ron]

DRC1....the HIGHER the strength, the better the durability, provided you don't get screwy with the shrinkage.


In general if you design a mix for durability, the strength will follow.

 

[hokie66]

Old wives tales die hard.