Influence Factors and Evolution Process of Mechanical Behavior of Waterborne Polyurethane-Modified Concrete

Due to different degrees of microphase separation, functional groups and types of polyols, waterborne polyurethane exhibits different hydrophilicity and dispersibility. The mechanical behavior of waterborne polyurethane-modified concrete (WPUMC) under different waterborne polyurethane (WPU) types, polymer-cement ratios and curing systems were investigated. Research results indicated that the compressive strength of WPUMC I (modified by polyether-polyester hybrid WPU) under different polymer-cement ratios is 7.14%-17.56% and 1.64%-6.32% higher than that of WPUMC II (modified by polyether WPU) and WPUMC III (modified by polyester WPU), while the splitting tensile strength of WPUMC I is 12.92%-21.88% and 2.63%-7.79% higher than that of WPUMC II and WPUMC III, respectively. In addition, the compressive and tensile strength of WPUMC first increase and then decrease with the increase of polymer-cement ratio. Therefore, using strength as the control criterion, the optimal WPU can be determined as polyether-polyester hybrid WPU, while the optimal polymer-cement ratio for WPUMC I is 0.004. Test results of microstructure indicated that, compared with OPC, the critical and threshold apertures of WPUMC I with different polymer-cement ratios are reduced by 33.8%-69.0% and 50.6%-90.5%, respectively. Consequently, the micropores can be filled effectively by adding an appropriate amount of WPU, thus increasing the compressive and tensile strength of concrete. Similarly, the optimal curing system for WPUMC I could be determined as 7d standard-21d indoor natural drying curing, which can provide a reference for the selection of curing system for WPUMC I. Finally, based on damage mechanics and equivalent strain hypothesis, the calculation methods for key mechanical parameters and the stress-strain constitutive relationship of WPUMC I under uniaxial compression were proposed. The proposed constitutive relationship is crucial for the performance design of WPUMC I structures, which can provide a theoretical basis for the structural design and numerical analysis of WPUMC I. Considering the better environmental friendliness of WPU, it will help to further expand the application of WPUMC I.