Are you asking how one sets the strength criteria or how one checks that the criteria is met? That is, how do you select the 28 day compressive strength for a design or how do you check that the material used meets the selected 28 day strength?
Answer #1: You pick 3,000; 3,500; 4,000 etc. based on the size of the pile that each choice will result in vs. the cost of that size pile.
Answer #2: You take standard test cylinders of the concrete at the time of pouring the pile and test them to see if they meet the specification.
I had previously designed piles with concrete 28 day strength of 28MPa and then tests on site revealed that the concrete used had only 20MPa strength. Now I need to determine either that is adequate or that I need to modify the piles in some way. Some suggested remedies would be appreciated.
seerc
you state that tests ON SITE revealed a reduced strength.
Cubes stored and tested on site will often give a low reading because the conditions are not equivalent to the en masse, in situ conditions of the actual structure.
The actual strength of the piles can be determined by removing the top meter of some of the piles to reach sound material and taking cores for testing in a laboratory.
The strength of concrete in tension piles affects
- the bond between rebar and concrete. This will not be a problem if the rebar is sufficiently long to achieve full bond strength.
- the resistance of the pile to corrosive groundwater. The sub soil survey should provide info on this.
Core testing is one method and the other would be to perform a load test on some of the in place piles. In fact, most pile installations I've been involved with have load test proceedures in the specifications. There is also usually a unit price for the performance of a load test.
I would be concerned if the concrete really does have 20MPa strength, if you have relied on the presence of the reinforcement (eg for bending capacity or for added compression strength). Reduced strength could well lead to insufficient corrosion protection, and reduction in pile capacity some years away. Even with 'pure' groundwater you could still get rusting reinforcement in the upper regions of the soil where a supply of oxygen could be available.
A typical load test will almost certainly show that your pile/soil system will carry the intended axial load at the time of the test, but will not tell you anything about the loss of reinforcement protection.
I think you need to concentrate on the concrete itself. Perhaps you could get some help from corrosion protection specialists. They might have some cunning electrical resistivity test which would tell them something about the effectiveness of the concrete cover (I don't know whether there is such a test, but it could be worth enquiring).
