Carlos Slim’s Inversora Carso has made a bid to acquire the stake in the Spanish construction company FCC it doesn’t already own.

Slim, already FCC’s largest shareholder, controls about 36.6 percent of the company directly and indirectly, according to the filing. The offer values the company at €2.88 billion, representing a 15 percent premium to FCC's closing share price last week of 6.59 euros.

The bid came after Slim’s firm surpassed the 30 percent ownership threshold that Spanish regulation requires for buying the whole company, a filing showed last week. Slim's company already owned 27.4 percent of FCC but said its stake had increased to 36.6 percent after it bought into a €709.5 million rights issue which ended last week.

Carso separately requested that FCC study a bid for Cementos Portland Valderrivas SA, the cement company owned by FCC.

There are question marks over whether shareholders will accept the offer for FCC.

FCC has struggled in the last years posting negative results and is heavily indebted. The company  managed to attract Carlos Slim and other investors such as Microsoft co-founder Bill Gates, who owns a 3.92 percent stake in FCC.