Summing the Value of HH: MM: SS In SSRS

Hello!
Now I have a field whose T-Sql looks like:

Declare @sdate datetime = '2014-07-10 17:02:20.170'
 Declare @edate datetime = '2014-06-10 17:10:23.170'
 SELECT cast(DATEDIFF(hour,@sdate,@edate) as varchar(10)) + ':' + Cast(DATEDIFF(minute,@sdate,@edate) % 60 as varchar(10)) + ':' + Cast(DATEDIFF(second,@sdate,@edate) % 60 as varchar(10)) AS Elapsed

I want to total this field in SSRS. I used

=TimeSpan.FromTicks(Sum(Fields!Elapsed.Value))

but get errors that the Aggregate value is not supported by the data type. How do I make this work?

Thank you

Zionlite