50-year study validates bonded steel reinforcement of concrete beams
Amy J. Born | January 04, 2021Fifty years ago, Empa (Swiss Federal Laboratories for Materials Science and Technology) began an experiment to determine the durability of epoxy resin as an adhesive for attaching a steel plate to a concrete beam. Today, one of the six identical test subjects remains intact. The other five succumbed to static fracture and fatigue tests when their load limits were exceeded. The final specimen, a reinforced concrete beam, manufactured in 1970, with a steel plate bonded to its underside, has held up to a constant load of just over six tons.
"After 50 years under 87 percent of the average breaking load, the epoxy resin bonding shows no weaknesses. Bonded steel plate reinforcements have thus passed the long-term test," said engineer Christoph Czaderski, the most recent supervisor of the experiment. He confirmed that the long-term tests show “practically no slip” in the adhesive joint.
Empa engineers Christoph Czaderski (right) and Robert Widmann measure the concrete beam 50 years after the start of the experiment. Source: Empa
The experiment has been conducted at Empa’s largest testing laboratory at the Dübendorf site. It began in response to an issue of unusual cracks forming on a number of prefabricated shed roof elements made of reinforced concrete in a new industrial building in Kreuzlingen, Switzerland. The cracks were discovered by a painter in the late 1960s and the damage was repaired by gluing thin steel plates to the concrete surface in order to strengthen it. Empa studied this new technique to learn whether retrofitting the roof, rather than replacing it, could be successful. The tests conducted included static loads up to failure, a fatigue test and long-term investigation initially intended to last a few months.
Half a century later, the beam disproved the engineers initial concern — that over time creeping in the adhesive joint would reduce the reinforcing effect of the bonded steel plate. This is significant because demolishing and replacing structures such as buildings and bridges that are aging and withstanding higher demands is cost prohibitive and would produce more CO2. A simple, inexpensive reinforcement method, is a welcome solution for society, the economy and the building owners, according to Czaderski.
Empa’s “Structural Engineering” lab has been developing and researching reinforcement methods using epoxy resins, carbon fiber reinforced plastics and shape memory alloys for some time. These unique tests have contributed to adhesive reinforcement methods becoming state of the art, giving engineers the confidence in this construction method.