###### Abstract  
Polymer-modified cement mortar has been increasingly used as a runway/road pavement repair material due to its improved bending strength, bonding strength, and wear resistance. The flexural strength of polyurethane-cement mortar (PUCM) is critical in achieving a desirable maintenance effect. This study aims to evaluate and optimize the flexural strength of PUCM involving nano silica (NS) using a central composite design/response surface methodology (CCD/RSM) to design and establish statistical models. The PU binder and NS were utilized as input parameters to evaluate the responses, such as compressive and flexural strength. Moreover, machine learning (ML) algorithms including artificial neural networks (ANN) and Gaussian regression process (GPR) were used. The PUCM mixtures were prepared by adding a PU binder at 0%, 10%, 15%, and 25% by weight of cement. At the same time, NS was incorporated into the mortar mixes at 0 to 3% (interval of 1%) by cement weight. The results showed that the simultaneous effect of PU binder at the optimal content and NS improved the performance of PUCM. Adding NS to the mortar mixture mitigated some of the strength lost due to the PU binder, which remarkably reduces the strength properties at a high content. The optimized PUCM can be obtained by partly adding 3.5% PU binder and 2.93% NS particles by the weight of cement. The performance of the machine learning algorithms was tested using performance indicators such as the determination of coefficient (R\({}^{2}\)), mean absolute error (MAE), mean-square error (MSE), and root-mean-square error (RMSE). The GPR algorithm outperformed the ANN with higher R\({}^{2}\) and lower MAE values in the training and testing phases. The GPR can predict flexural strength with 90% accuracy, while ANN can predict it with 75% accuracy. m 2023 18 December 2  
concrete properties such as durability, impact resistance, and water absorption capacity, as found in the literature [5; 6; 7], required for engineering structure and maintenance purposes. The damage in civil engineering structures results from several factors, such as corrosion due to aggressive environments, the use of de-icing salts, high chloride levels in the air, the alkali effect, and poor design or construction. Moreover, concrete cracking usually complements steel corrosion, resulting in a decreased load-carrying capacity of the structure [5; 8]. Polymer concrete has been applied to maintain damaged concrete members, such as industrial flooring, underground pipes, and road pavement, where high strength, durability, and a fast setting are desirable [9]. The polymer comprises several thousand repetitive units in their extremely long molecules. These include rubber, Polyethylene (PE), Epoxy Resins, Polyurethane (PU), and other polymer materials [10; 11; 12; 13]. Due to its desirable properties, PU has attracted the attention of researchers among these polymeric materials to carry out studies on PU-based polymer materials [7; 14; 15], a PU-cement composite [5; 6], elastomeric polymer material [16], polymer-modified bitumen [17], coating material [18], grouting materials [19], and polyurethane foams [20; 21]. Compared with ordinary polymer materials, PU demonstrates a high resistance to chemical attacks, good adhesion, fast hardening, and great mechanical properties, capable of enhancing the cementitious material toughness with a low content [5; 22]. It is widely used in pavement structures due to its excellent performance [12; 14]. A PU-cement based composite was obtained by adding polyurethane binder into cementitious material such as concrete and mortar, which helps maintain and rehabilitate engineering structures [23]. The mechanical properties of PU-cement based composite have been studied by [23; 24; 25]. [PERSON] et al. [5] investigated PU-based cement's bonding strength and mechanical behavior as a repair material for existing bridges.