What are the guidelines for slump test?if we have a slump of 50mm then we will work or not?

According to Greek Concrete Technology Standards: 

in case of crushed aggregates (for concrete composition) the minimum acceptable is 50mm. In case of natural aggregates the minimum acceptable is 30mm. And there are special cases for which lower slump may be acceptable (pre-cast, large volume structures, floors, etc.).

Before considering if you should use a 50mm concrete, I think that you should first search for your project's specifications (do the designs include a minimum concrete workability? the concrete composition certainly has a specified workability; is it as specified by the consultant/standards? European Standards tolerances (EN-206-1) for slump rates 50-90 are 20mm...). Once you know that 50mm is acceptable according to project designs/concrete composition design/national standards, then you have every right to ask this question...we all know that for 50mm, the casting/compaction work would be more demanding & risky. Then, if I were you, I would ask: am I authorized (by standards/consultants etc.) to use admixtures mixed during site delivery? if yes, problem solved, but be aware of admixture quantities and proper mixing on truck...personally, I wouldn't recommend to cast e.g. a building column with concrete of 50mm slump and ordinary compaction equipment...80-90mm would be my goal in such case...