** Shares of Summit Materials down 2% at $50.45 premarket
** Privately-held concrete maker Quikrete to acquire SUM in a cash deal valued at $11.5 bln
** Quikrete's $52.50/shr offer represents about 29.2% premium to SUM's closing price on Oct. 23, a day before Reuters reported the talks
** Up to last close, SUM up 0.20% YTD
(Reporting by Anshuman Tripathy)
((Anshuman.Tripathy@thomsonreuters.com))