Researchers from New Mexico State University have been conducting large-scale tests on bridge girders built with a new ultra-high performance concrete (UHPC) that has significantly stronger structural properties and a longer life span than regular concrete
The new concrete provides three significant benefits over traditional concrete which allow it to increase its design life:
- Increased compressive strength
- Very dense microstructure
- Steel fibres that improve post-cracking strength
It is estimated that the unique mix – a closely guarded secret – would allow bridges built with this UHPC to last up to 150 years compared with the 50-year lifespan of traditional concrete bridges.
So far three large-scale flexural tests have been conducted on three 16-foot long, pre-stressed UHPC girders.
The team designed and built a bespoke structural frame for the tests, utilizing recycled bridge girders donated from the US Department of Transportation.
Unique digital image correlation (DIC) technology has been used to monitor the structure. DIC equipment takes pictures of a grid drawn on the surface of the beam and uses this to track the formation and propagation of cracks throughout the testing. It can cover large areas and detect minute changes, and enabled the team to track regions prone to cracking as the beams were loaded on to the frame, as well as capture shear behavior closer to the end of the beams near the supports.
The testing was to study the flexural behavior of the UHPC at a large-scale level to evaluate design procedures that can aid in the future development of standardized design codes. Previously, tests had been done on only small-scale rectangular beams.
"Large-scale testing provides much more accurate data. Hopefully, these tests will help to prove that UHPC can be designed both accurately, and efficiently, using simplified methods," said Andrew Giesler, a graduate student in civil engineering from New Mexico State University.
Currently, no bridge design specifications for concrete of this strength exist in the United States. UHPC is significantly stronger in compression than normal strength concrete, and has strengths exceeding 22,000 pounds per square inch (psi). The compressive strength of average concrete ranges from 4,000 to 6,000 psi.
Giesler hopes that through these large-scale tests, he will be able to present data that will aid in the development of new specifications for the design of UHPC bridges.
A high-strength concrete, cast-in-place bridge was used for one of the team's tests. In a real-life scenario, this deck serves as the surface that vehicles drive on. If the beams are designed correctly, the deck and the bridge beam lock together to make the girder stronger.
"We wanted to investigate how a composite lower-strength concrete deck would influence the flexural behavior of a UHPC girder. We wanted to see if it would serve any practical purpose from a structural performance standpoint," Giesler explained.
A full-scale test is scheduled for later in the year.