Material enhancements of newly developed stiff type polyurea for retrofitting of concrete structures

Recently, polyurea (PU) has been receiving great interest from the construction industry as a structural retrofitting and strengthening material due to its simple application of spraying on the structural member surface without creating an epoxy bonding interface. In this study, based on knowledge gained from developing flexible-type PU (FTPU) in preceding studies, stiff-type PU (STPU) is developed by varying the prepolymer/hardener ratio of FTPU. A mechanical property evaluation of STPU is performed through tensile strength, percent elongation, pull-off, and Shore hardness tests. Furthermore, a durability property evaluation of STPU is performed through acid environmental and ultraviolet (UV) exposure tests. In addition, concrete carbonation exposure and freeze–thaw tests are performed for STPU. The experimental results show that STPU has a higher tensile strength and lower elongation percentage than FTPU with excellent mechanical and durability properties. Using the optimal mix proportion of STPU, the surfaces of concrete specimens sprayed with STPU are tested under uniaxial compression loading. The uniaxial test results show that the strengthened concrete specimens have maximum load ratios 1.14–1.20 times those of the non-strengthened concrete specimens. Based on the test results, STPU can be used as a retrofitting and strengthening material for concrete members to resist seismic and extreme loading (e.g., impact, blast).