The U.K.-based engineer's board had agreed to talks with Apollo after its latest $2.1-billion purchase offer made in April, but neither company would say why discussions ended on an official deal.

As uncertainty drags on concerning federal funding levels for major infrastructure projects, stakeholders are looking more intensely than ever into what kinds of private capital are available—not only from entities traditionally interested in infrastructure, but also from companies such as Facebook.

A “shark tank” panel of private equity investors at the American Society of Civil Engineers innovation awards celebration in June shed light on how they evaluate prospects, what red flags they look for and how innovation is likely to advance in an industry whose customers are dominated by public sector owners predisposed to “say no.”