Estimated Insitu Strength 1

[discopants]

anyone know of research or had experience of the following.

C60 concrete, incl 20kgs m/silica supplied to a project. Cubes at 7 days 55-60kn so no problem there. Client is unhappy with workmanship and wants to know strength of insitu concrete. Cores drilled at 9 days, these will be crushed at 14 days. Little or no curing on site


What strength should we expect from these cores ?

 

[Zambo]

Do the cores look sound - no honeycombing? If so I expect you will have strengths of +70MPa. If you inspect the surface of the cast concerte there could well be shrinkage cracks. RCPT tests, if required, are likely to fail if no curing.

 

[henri2]

ASTM C 42-99 Test Methods for Obtaining and Testing Drilled Cores and Sawed Beams Note-1, recommends the concrete to be at least 14 days old before cores are taken. After cores are obtained, depending on the environment the concrete (the cores were obtained from) will be exposed to, curing will last between 2 up to 7 days prior to testing.

The core strength at 14 days may not be much more than the cube strength at 7 days for a variety of reasons which include inter-alia: microcracking introduced by coring operation, aspect ratio of cores, MSA, compresive strength, types of cements and admixture used, etc

BTW, what size cubes and cores are we talking about...150 mm cubes and 150 mm x 150 mm cores?

 

[BigH]

Please note that cube strength and cylinder strength are not 1:1. It is usually used that 0.8*cylinder = 1*cube, but, as some have pointed out, this is dependent on strength. If I remember a thread from the past correctly, the British have some new correlations in this area. You can check out Neville's book on Concrete Technology (with a guy named Brooks) or his Properties of Concrete - he has formulations for this. Others would be Shetty or Gambhrir. I would doubt you take 150mm cores but most likely will take 100mm diameter.

 

[henri2]

Big H,

You are right, 3.75 inch (4-inch nominal)diameter is what's commonly used, at least in this country. It's good up to 1-1/2 inch MSA. Since it is HPC I'm sure the MSA will be less than 1 inch, perhaps 1/2 inch.

I read that C60 is an HPC designation. Is this classification a new thing? Never heard of it until I read discopants post.

 

[Ussuri]

C25 C30 C35 C40 etc are UK designations for concrete as per BS8110 and BS5328. They are sometime written in terms of C40/20 for instance, which means a 40N/mm2 cube strength at 28 days with 20mm nominal size aggregate.C60 would then have a design strength at 28 Days of 60N/mm2. That said 60N/mm2 concrete is very high.


The designation has been changed by BS EN 206 to harmonise the UK and Europe. In the UK we specify using cube strength however on the continent they use cylinder strength. As a result we must now specify both cube and cylinder strength for concrete, such as C32/40 (cylinder strength first)

 

[henri2]

Ussuri,

Thanks a lot.

So the C32/40 is based on the assumption that the cylinder strength is 80% of the cube strength, or was that just an example?

Last question. What are the standard dimensions for concrete cylinders?

 

[Ussuri]

I think 80% is an average figure, cylinder strength is usually between 70-90% of cube strength, but the code uses 80% in its designations(eg C40/50, C20/25, C28/35) . The difference is something to do with the stress state developed within the test specimen under loading. In the cube test this difference leads to an increased compressive resistance.

Th standard for concrete testing (BS EN 12390) states cubes can be 100,150,200,250 or 300mm and cylinders are 100,113,150,200,250,300mm diameter with the overall length equal to twice the diameter. In my experience 150mm cubes and 150mm diameter cylinders are most common.

 

[Zambo]

Henri2,

I am currently working on a project with 80N/mm2 concrete, this is not unusual nowadays.

The term HPC can be used to refer to concrete with imporved strength or durability characteristics. The threadstarter's 60N/mm2 concrete could be considered HPC due to the strength and the use of silica fume.

 

[henri2]

Zambo,

I live in Los Angeles county...earthquake territory and many of the local building codes do not allow use of f'c values of that magnitude for RC frames designed to resist seismic forces.

 

[Bbird]

To answer the original post

Concrete is permitted to have an in situ strength lower than the cube or cylinder strength, to account for the fact that it is not possible to replicate the compaction and curing of the cube or cylinder specimens in the actual work.

ACI 318 recommends for strength test that 3 cores are to be extracted to test. Strength is considered acceptable with average 85% of the cylinder strength and no one to fall below 75%.

Concrete Society publication CSTR11 accepts in-situ strength can be 80% of the cube strength.

 

[discopants]

just to let you know how this worked out

again we knew the concrete was ok, the 150mm cubes told us this at 7 days so from a suppliers point of view I was quite relaxed about that, it was the workmanship that was in question. The client just wanted reassurance as to the strength of the insitu concrete.

my concerns were other peoples aspirations for the core results and their interpretation of the results. That's why I was was worried about the cores being cut at only 9 days, the concrete had no curing and had been baked at 40 degrees. All parties met and agreed the criteria we were looking for and Concrete Society publication CSTR11 was at the "core" of our agreement.

The cores were 100mm due to the amount of reinforcement. We tested one immediately, dry at 9 days it gave 22kn. The others were placed in water, tested at 14 days and gave strengths between 38 - 45 kn. This was acceptable to all and we're happy the cubes will give us in excess of 70kn at 28days.

The job goes on

 

[BigH]

discopants - just a thought. You said that from the cubes (QC) you knew the concrete was okay. I beg to differ. You knew that the concrete delivered to the job was consistent. It had the potential of being okay. But due to the workmanship (likely the lack of curing), the owner was worried that the concrete in "HIS" structure was not okay. You did the cores - the 9 day strength seems low to me. You got better core strengths at 14 days due to the saturation in the water - additional water added for hydration - that might not be available in the structure as it was "baked" - as you said. If you are all happy, well, fine. But, I suggest that you take cores of the structure at 28 days in accordance with the accepted practice and test them for strength. Then see if they are 85% of the specified strength. Again, just a thought.
Ussuri - I use 80% for practical reasons. M.S. Shetty in his book (Concrete Technology, Theory and Practice) has a table that gives cube strengths to cylinder strengths for a range of strengths. It is apparent from his data that the higher the strength of the mix, the closer the ratio of cylinder/cube will be to 1. For example, for cube strengths over 30MPa to 54MPa, he has ratio values ranging from low of about 0.9 to 0.96. The arithmetic differnce between the two being generally 3 to 4 higher in the cube value. For std cube strenghs of 9MPa to 25MPa, the characteristic ratio is about 0.8, again with the arithmetic difference being in the order of 3 to 5. There was a thread about a year ago where the differences of cubes to cylinders was extensively discussed.
to all.

 

[Ron]

BigH is exactly right! The testing is done only to show that the design mix, as purchased, is what it was designed to be. It doesn't mean that the in-situ strength is good or bad. It's not intended to represent the in-place concrete, just the mix design.

...to further BigH's point, it has the POTENTIAL but if marginal on potential, consider that you might be even more marginal (or worse) in place.

 

[discopants]

of course I was speaking from a suppliers point of view and my liabilities.

dsuring the pre-test meeting the client, the consultant and even the architect agreed on a figure they'd be happy with from these results and they surpassed that figure.

As far as I'm concerned the 9 day result was irrelevant, it goes against all the relevant standards but it was an eye opener for all involved.

The concrete meets the spec which was 60kn at 28days under lab conditions, contractors need to treat new concrete with a little more tlc in the early days. A lot of good lessons have been learned in this case and all involved were a breath of fresh air to deal with because normally the supplier is guilty no matter what the evidence *little yellow bloke winking if I knew how to post them *

 

[Ussuri]

discopants

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