Concatenate a string to a Column of type string in DataFrame

[satish chandra j]

HI,
I am trying to update a column value in DataFrame, incrementing a column of
integer data type than the below code works

val new_df=old_df.select(df("Int_Column")+10)

If I implement the similar approach for appending a string to a column of
string datatype  as below than it does not error out but returns only
"null" values

val new_df=old_df.select(df("String_Column")+"00:00:000")
                         OR
val dt ="00:00:000"
val new_df=old_df.select(df("String_Column")+toString(dt))

Please suggest if any approach to update a column value of datatype String
Ex: Column value consist '20-10-2015' post updating it should have
'20-10-201500:00:000'

Note: Transformation such that new DataFrame has to becreated from old
DataFrame

Regards,
Satish Chandra