One of the challenges the U.S. faces today is keeping infrastructure functional, safe, and productive. While politicians debate how to deal with the issue, American infrastructure continues to remain substandard in many areas.

Now, a chart Goldman Sachs put together shows how the biggest infrastructure problems may be yet to come. Let's look at why that is and how you can position your portfolio to profit from the current infrastructure situation.

50-year bridges
According to Goldman Sachs economist Alec Phillips, the "theoretical design life of a bridge is about 50 years." The Federal Highway Administration takes the same position for existing bridges but notes that newly constructed bridges could have a design life of 75 years or greater. However, when considering the life of currently existing infrastructure, the 50-year estimate is more relevant.

Using the 50-year estimate, the chart from Goldman Sachs on the age of current bridges becomes more alarming. According to its data, nearly 40% of all bridges are between 26 and 50 years old, including nearly 70% of interstate bridges. With a 50-year design life estimate, within a couple of decades a large percentage of America's bridges will have exceeded their design life.

Older bridges
Not every bridge is replaced at the 50-year mark. About 20% of all bridges are 51 to 75 years old, while another 10% are even older than that. But just because they haven't been replaced doesn't mean they don't need work. Among the bridges in existence today, the Department of Transportation classifies 25% as "functionally obsolete" or "structurally deficient."

So while we're unlikely to be seeing mass bridge collapses, the pressure to update American infrastructure is likely to grow in urgency over the next couple of decades.

Political climate
Repairing the nation's infrastructure is not going to be an easy task since it will require a large amount of money and likely entail significant government contributions in one form or another. There have been frequent pushes for a comprehensive infrastructure solution with some ideas coming from both sides of the aisle.

Recently, President Obama pushed again for a solution to America's infrastructure problems. In his State of the Union Address, he advocated for taking money saved from tax reform "to create jobs rebuilding our roads, upgrading our ports, unclogging our commutes – because in today's global economy, first-class jobs gravitate to first-class infrastructure." He then called on Congress to pass transportation and waterway bills and said he would speed up the permitting process.

But getting agreement in Washington is easier said than done. With the branches of government currently divided between two parties, large scale action on infrastructure may take longer to happen.

In the longer term, inaction may cease to be a viable option if substandard infrastructure becomes a larger problem. At this point, the federal government could see even more pressure to fix infrastructure. Even if the federal government refuses to act, alternative solutions may be set up at the state level whether they are through direct spending, an infrastructure bank, or toll roads to fund repairs and upgrades.

With such a large portion of bridges passing their theoretical design life in the next couple decades, repairing or replacing them will become in increasingly urgent and could generate a boost to infrastructure-based companies over the next couple decades.

Where to invest: Infrastructure ETFs
For long-term investors, the necessity to repair the nation's infrastructure creates a few areas of opportunity for investment. Each contains its own level of risk, and investors should balance each against their own investing style.

Finding an ETF for U.S. infrastructure only is pretty difficult, since most infrastructure ETFs have either a worldwide or emerging-markets focus. But for U.S.-focused investors, the PowerShares Dynamic Building and Construction Portfolio ETF (NYSEMKT:PKB) provides more domestic-focused investments.

Although this ETF is not a pure play for large-scale infrastructure, since many of its holdings are in consumer construction companies such as Home Depot and Lowe's, this ETF is probably the most U.S.-focused of any of the infrastructure ETFs.

But the U.S. is far from the only country in need of infrastructure. Emerging markets are often in desperate need of new infrastructure to serve their quickly growing economies and rising middle class. The markets of China and India are making particularly large infrastructure investments in everything from bridges to mass transit.

For those looking for a more global approach to ETFs to capture gains in these areas as well, there are several other options for investing in global infrastructure.

One of the largest infrastructure ETFs by assets is the iShares Global Infrastructure ETF (NASDAQ:IGF)This ETF can work for broad-based infrastructure investment, but it's limited in its access to the repair of roads and bridges. As its name implies, this ETF is global rather than U.S.-focused and has only three U.S. companies in its top 10 holdings. Additionally, many of its holdings are energy or resource related. For investors bullish on these areas, iShares Global Infrastructure ETF could be a good pick, but it's far from a pure play for rebuilding American infrastructure.

For those able to handle more volatility, the Market Vectors Steel ETF (NYSEMKT:SLX) invests in worldwide steel companies. With steel being a material of choice for many large construction projects, this ETF could benefit from a rise in infrastructure construction.

However, steel is not only used for bridges, and many other cyclical factors make steel prices volatile. For this reason, only investors with a higher risk tolerance should consider this ETF.

Rebuilding our bridges
It seems that most people want a change from the status quo on infrastructure, and politicians in charge right now can't seem to agree on how to solve the issue. But in the next couple of decades, the need to address the issue is likely to build.

For long-term investors who want to profit from this trend, infrastructure ETFs are options to consider among investments poised to benefit.