Any addition of water in concrete further to mixing of concrete leads to reduction in strength unless the aggregates are absorbing more water than calculated at the time of mixing.
after 3 to 4 hours normally water addition will lead to low strength only.
It is always better to break those columns and recast same than doing remedies on it.
If slab work is completed and pouring of concrete is balance, then do cast column first by breaking them and then do cast slab.
If slab is casted, you can still break the columns by supporting the slab but it is not advisable, so you shall seek the help from your structural consultant.
to understand about redosing, you may read following thread to understand it.
What is redosing in RMC and is there any IS recommendation.
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