1. In general, a lot of literature is available on compressive strength of cube or cylinder specimens. However, in practice, reinforced concrete is used in the field, especially for large structures.
2. How can the strength of the concrete specimen be compared to the reinforced concrete? I imagine reinforced concrete would anyway yield only at a much higher compressive stress than the former.
3. Assuming the strength of the steel is nearly constant for a particular set of environmental conditions, can we estimate the compressive strength of the actual in-field structure only by coring parts of the structure and subjecting them to the compressive load test?
You should take a core cutter apparatus and should take a sample from the site or the building who's strength is to be checked.
Dear
Casting of cube or cylinder and curing of such specimen can simulate the field condition of concrete. The reinforcement will usually handled the tensile stress.
There are many NDT by which we can estimate the strength of concrete in the field. It is not necessary that we will get field strength higher than designed value as the variations depend on quality control at site. Depending upon concrete strength the resisting capacity of RCC can be predicted
It seems like the strength (property of material) and the load capacity (property of structure or element) are being mixed up (in questions 1 and 2).
To assess load capacity of structure one could:
a) do direct tests on elements or even on the whole structure
b) calculations - do tests on materials to find their strength and then calculate the estimated load capacity (EN 1992-1-1)
c) or a combination of both
Usually option b is being used (lowest cost).
Question 3:
To assess strength of concrete in an already finished structure you could:
1) use Schmidt's rebound hammer to find the weakest spot (see EN 12504-2)
2) take cored specimens from the weakest spot by drilling (see EN 12504-1), the core should not have steel rods in the direction of testing, so reinforcing has little effect on it (if it does, the combined "strength" value is useless)
3) use ratio 0,85 to convert the in-situ characteristic strength to characteristic potential strength (as used in calculations) (see EN 13791 chapter 6 table 1)
AS3600 indicates that the core will be the : cube strength / 1.15. This calculation will gives you about 87% of the cube strength.Cores to be taken after 28 days and reinforcement will affect the result.
As far as the concrete is concerned, you will only get a comparison. The stress' are different. The cube is not directly equal to a test cylinder compression test. It's like a swiss hammer test, just a comparison.
I agree with Om Prakash Chhangani and Mattias Poldaru
In my opinion,
1. To compare compressive strength of concrete at field with compressive strength obtained at lab by conducting compressive strength on minimum 3 cubes is that both concrete materials must have same quality in terms of proportion, compaction level and curing condition.
2.Normally, actual compression resistance of of concrete in case of RCC member is higher than plain concrete of the same material, due to confinement effect of reinforcement. However, when question checking quality of actual concrete at field in terms of strength arises, we normally check the compressive strength of cubes made from the concrete at field assuming that it is similar to field.
3. compressive strength of core taken from RCC structural member, represents actual compressive strength of concrete. But it is costly process. The accuracy of results depends on cor dimensions, ratio of size of member to core diameter, location of core, core cutting apparatus quality.
4. NDT tests may give test results with variation plus or minus 10 to 25 percent depending upon NDT apparatus used.
It will be better to compare test results as stated 1, 3 and 4 above.
I think that you can measure compression strength using ultrasonic method. You can control this method using Schmidt Hammer. Usually the difference between these two methods is less than 8%.
In testing of compressive strength of concrete in-field, it should be done using the cores taken so there is no reinforcement in it, or not big parts present. If your cores taken from the reinforced structure comprise reinforcement, it is possible that get results with nonsufficient reliable values, different than real compressive strength.
Estimation of compressive strength of concrete, get from the cores or cubes, to be relates to the concrete as material, not to the "compressive strength of the actual in-field structure". The compressive strength of concrete is only an input data for calculation of structure bearing capacity.
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