The American Society of Civil Engineers' latest evaluation of the condition of U.S. infrastructure shows improvement, but not by much.

The report card, released on March 19 in a digital format, shows that the state of infrastructure in six sectors—including drinking water, wastewater, solid waste, roads, bridges and rail—improved slightly. Those improvements brought the overall grade to a D+, up from a D four years ago.

The grades in 2013 range from a high of B- for solid-waste infrastructure to a low of D- for inland waterways and levees. None of the category rankings regressed this time.

"The 2013 report card demonstrates that we can improve and modernize our nation's infrastructure," said Patrick Natale, ASCE's executive director. "Where investment was made and projects moved forward, we saw the grades rise."

However, the overall needs for investment are still great, Natale said. According to ASCE, the total investment needed for all infrastructure by 2020 is approximately $3.6 trillion. "Even with the modest progress we've made in the past four years, there remains a significant—some would say staggering—$1.6-trillion gap between the needed investment level and the proposed funding," he said.

To assign the grades, a committee of approximately 30 ASCE engineers reviewed existing data and consulted industry and technical experts. Criteria considered in each sector included the infrastructure's condition, funding, operations, public safety, innovation, resilience and ability to meet demand.

The report card's digital format features a corresponding phone app and dedicated website. The phone app makes the report "much more portable," allowing it to reach a wider audience than the traditional print version, said Greg DiLoreto, ASCE's 2013 president. For the first time, the report card includes information for all 50 states and highlights innovations that ASCE believes are making a difference. DiLoreto, who is also CEO of the Tualatin Valley Water District in Portland, Ore., said that, for his district, raising rates has been key to fueling investment in water projects.