Alternatively
DECLARE @YEAR int
DECLARE @MONTH int
DECLARE @DAY int
SET @YEAR = 2013
SET @MONTH = 5
SET @DAY = 20
SELECT RIGHT('0000'+ CONVERT(VARCHAR,@Year),4) + RIGHT('00'+ CONVERT(VARCHAR,@Month),2) + RIGHT('00'+ CONVERT(VARCHAR,@Day),2)
Gives
20130520