I presume that it rains from time to time in your neck of the woods - does this seem to do any harm? Diesel fuel on the ramp would be obvious and it is unlikely to be underneath the centre section of the aircraft - at the front where a GPU would be positioned perhaps.A pushback tug might drop oil but that too would be obvious.What exactly is happening to the concrete? Is it over a large area or small isolated areas and is it so bad that it causes concern?I still think you have something wrong with your concrete.I have worked in the UK,Europe,USA,South and Central America,Far East,India,Pakistan,Central Asia,Caribbean , Africa ,Middle East and places I have forgotten but have never seen ramp damaged by anything from the Aircraft or refuelling equipment. fuel spills on long haul aircraft were common at one time,even Phosphor ester based hydraulic fluid does not seem to cause a problem.Get some kind of concrete expert to have a look. Just one point - do you have a high salt atmosphere in your area as salt can cause concrete degradation.