# Dispersion and Pressure Sensitivity of Carbon Nanofiber-Reinforced Polyurethane Cement
## 1 Introduction  
Two-component non-aqueous reactive polyurethane (PU) is mainly composed of polyols and isocyanates, which are used as cementitious materials. Adding cement into PU leads to polyurethane cement (PUC), which has the advantages of high compressive strength, excellent fluidity, environmental friendliness, low weight and easy construction. Therefore, PUC has been gradually applied for the reinforcement of highways and bridges and in the construction industry [1; 2; 3; 4; 5]. With the progression of science and technology, people are demanding higher functionality of building structures, in hopes of improving the carrying capacity and deformation capacity of structures through a strengthening process while monitoring and diagnosing the health of the structure in real-time [6; 7; 8; 9]. In the past, the cement base was the main research object in the domain of intelligent materials and polymer base was rarely used as a reinforcement material [10; 11]. By adding carbon nanofiber (CNF) to PUC reinforcement materials, a carbon nanocarbon fiber polyurethane cement material (CNFPUC) could be made and thus, the fluidity of the mixture could be improved, with the advantages of low density, high specific modulus, high specific strength, high conductivity, thermal stability, compact structure and so forth [12; 13; 14]. Not only is CNFPUC used as a structural reinforcement material, it also has the conductive properties necessary to satisfy engineering application requirements. By monitoring the resistivity change in CNFPUC, it is possible to understand the internal damage development process of CNFPUC and achieve self-induction of stress and strain. Moreover, by utilizing the super bond ability of concrete and CNFPUC and the plane cross-section assumption of a reinforced rear beam and deformationcoordination of the columns, the functions of self-perception, self-diagnosis and self-monitoring of components can be realized, which may allow for early prediction of damage and defects, thereby reducing the loss of life and property of the masses [15; 16]. Owing to the high specific surface area and aspect ratio of carbon nanofiber, the Vander Waals force between CNF leads to fiber winding and agglomeration, resulting in poor dispersion and the production of weak areas in the matrix [17; 18]. However, the conductive effect of CNFPUC is influenced by the worst section in terms of the CNF dispersion or the section having the least amount of fibers due to poor fiber dispersion, which may critically reduce the effectivity of CNF and electrical conductivity in the PU Matrix [19]. Thus, the sensitivity of CNFPUC as a functional material is weakened. The degree of uniform dispersion of CNF is a key factor for the sensitivity of CNFPUC as a functional material. So far, with the exception of scanning electron microscope (SEM) observation used for estimating the dispersion effect, there is no uniform criterion to evaluate the dispersion and its effect on the material. Currently, the main methods for evaluating the dispersion of carbon nanofiber in the cement matrix include: the fresh mix method, hardening specimen resistance method and hardening specimen section morphology method. The fresh mix slurry method involves taking approximately the same quantity of samples from different parts of the fresh mix, calculating the coefficient of variation of the carbon fiber and analyzing the dispersion of carbon fibers in the matrix. This method can quantitatively evaluate the dispersion uniformity of carbon fibers in different mixtures by using the dispersion coefficient. However, it can only reflect the degree of fiber dispersion in the mixture before molding and cannot reflect the dispersion of carbon fibers in hardened specimens; therefore, it is not suitable for the dispersion discrimination of a polymer matrix.