SQl Query to calculate Total in HH:MM Format
===

I have a table(production) which is having a column with name TimeSpent, the datatype of this column is varchar and it stores data in HH:MM format example 10:23,14:59,11:00 etc.I want to write a query in sql which will sum this and give me a total in HH:MM Format only.
I tried some thing like this but getting error 'Conversion failed when converting date and/or time from character string.':- 

    select CAST
	(
	(SUM (datepart(hh, convert (varchar, timespent, 108))) +
	(sum(datepart(mi, convert (varchar, timespent, 108)))/60) ) AS VARCHAR(2)
	)
	+ ':' +
	CAST
	(
	sum(datepart(mi, convert (varchar, timespent, 108))) - 60 * (sum(datepart(mi, convert (varchar, timespent, 108)))/60)
	 as VARCHAR(2)) from production

