(Adds details on offering, background)
MILAN May 30 (Reuters) - Italy's biggest builder Salini Impregilo said on Friday it was evaluating an equity offering to institutional investors to increase its free float and raise money for possible expansion.
The offering includes a portion of newly issued shares worth up to 10 percent of its current share capital plus existing shares held by its biggest investor Salini Costruttori, a family owned company.
"The proceeds from the capital increase ... are expected to provide additional strength to Salini Impregilo's capital structure and increase the group's flexibility to pursue its 2014-2017 targets," it said in a statement.
Salini Chief Financial Officer Massimo Ferrari said in April Salini Costruttori would reduce its stake in the builder to around 75 percent from 90 percent by the end of the summer.
Increasing the amount of shares that can be freely traded on the market could make the stock more attractive to investors because it makes the investment more liquid.
Salini Impregilo, whose order book of big civil engineering projects includes work to broaden the Panama Canal and build the giant Grand Renaissance Dam in Ethiopia, was formed in 2013 after Salini took over its largest peer Impregilo.
Under a new business plan unveiled in March, the company said it would turn cash-positive by end-2017 and forecast average annual orders of more than 7 billion euros ($9.55 billion) a year. ($1 = 0.7328 Euros) (Reporting by Danilo Masoni, editing by Stephen Jewkes and Susan Thomas)