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.