The offer for Hochtief, which will remain open until midnight Wednesday, would - if successful - make ACS the largest public works company in Europe and the third largest worldwide.
The ACS initiative, which initially failed to attract much support from Hochtief shareholders, got a boost in mid-December when Southeast Asset Management of the United States, which has a 4.84 percent stake in Hochtief, agreed to sell two million shares to ACS.
ACS earlier this month raised its offer by 12.5 percent, proposing 1.8 of its shares for one Hochtief share rather than 1.6. But management at the German group continues to maintain that the bid undervalues the company.
ACS, already the leading shareholder in Hochtief, has said its initial target is to secure a 30 percent stake in its German rival on its way to full control.