The Economist reports on the need to reinvest in bridges: https://www.economist.com/science-and-technology/2018/08/18/crumbling-infrastructure-is-a-worldwide-problem.
Summary: As the recent tragic collapse of the Morandi bridge in Genoa, Italy shows, we need to make investments in high quality bridges. The Economist pointedly states:
"All around the world bridges built long ago, particularly those using reinforced concrete, are deteriorating. Even back in 1999, a study found that around 30% of road bridges in Europe had some sort of defect, particularly corrosion of their steel reinforcing or pre-stressed tendons. A report from the American Road & Transportation Builders Association in January is even more sobering. It reckoned that 54,259 of that country’s 612,677 bridges are 'structurally deficient.' These problem bridges have an average age of 67 years and are crossed by vehicles 174m times every day. At the present rate of repair and replacement, it will take 37 years to remedy all the problems, says Alison Premo Black, the organisation’s chief economist."
The Economist reports that with these reinforced concrete bridges, the "difficulty is that concrete, or rather the steel used to reinforce it, can fail in a number of ways. Salt, ice and the pounding of weather can cause tiny fractures in the concrete’s surface. As these cracks creep inward, they let in water. Once the water reaches the steel reinforcing or tendons, it corrodes them. This enlarges the cracks, which can cause the concrete to fall apart. That this is happening is evident from rusty streaks on crumbling concrete.... Other factors compound the deterioration of bridges, such as a constant cyclic vibration from traffic," which has been increasing significantly over the decades. "On top of that, extreme weather can take a toll, with heat and cold expanding and contracting the structure, floods eroding away foundations and high winds buffeting the bridge. This is why regular inspections and maintenance are essential." The Economist also reports on new technologies used to inspect bridge integrity including observation drones, electronic vibration sensors, and laser scanning and modeling.
On a forward looking note, the Economist reports that if we were to replace rather than refurbish old bridges (which may be more cost effective than refurbishment and maintenance), "[n]ew structures can also take advantage of advances in engineering. There has been huge progress in materials science, so much so that it is now possible to tinker with the internal structure of substances to make concrete more robust and steel better at resisting rust. Ultra-high-performance concrete is already being made in some countries to toughen buildings against such things as earthquakes and bombs. Apart from just sand and cement, other ingredients are added to these super concretes, such as quartz and various reinforcing materials. In some tests, the addition of plant fibres has been shown to produce markedly stronger concrete." Moreover, "[s]elf-healing concrete is also being explored."
However, it seems these technologically advances bridges may be far more expensive than the previous generation of bridges in inflation-adjusted terms. Just look at the comparison between the old Tappan Zee Bridge which crosses the Hudson River in New York and its replacement, the new Governor Mario M Cuomo Bridge, which opens later this year. The old bridge cost $564m in todays dollars compared with $4bn (though the old bridge only lasted 62 years whereas the new bridge is expected to last a century).