(Add S&P's and Fitch ratings, expected amount and maturity)
MILAN, July 18 (Reuters) - Italian builder Salini said on Thursday it had mandated banks Banca IMI, Natixis and UniCredit to lead a roadshow for a potential euro-denominated bond.
The bond will likely mature in 2018 and be worth 350 million euros ($458 million), rating agency Standard & Poor's said on Wednesday as it assigned a preliminary 'BB-' rating with stable outlook to the planned issue.
The proposed bond is rated 'BB' with stable outlook by Fitch, the rating agency said.
The roadshow to present the issue begins on July 19 and will visit Italian and international institutional investors, Salini said in a statement.
Salini is due to complete its merger with larger rival Impregilo later this year.
($1 = 0.7639 euros)
(Reporting By Isla Binnie and Francesca Landini)