Improvement of Performance of Ultra-high Performance Concrete Based Composite Material Added with Nano Materials

. UHPC is a kind of composite material characterized by ultra high strength, high toughness and high durability. It has a wide application prospect in engineering practice. But there are some defects in concrete. How to improve strength and toughness of UHPC remains to be the target of researchers. To obtain UHPC with better performance, this study introduced nano-SiO2 and nano-CaCO3 into UHPC. Moreover, hydration heat analysis, X-Ray Diffraction (XRD), mercury intrusion porosimetry and nanoindentation tests were used to explore hydration process and microstructure. Double-doped nanomaterials

Human society is developing constantly, and the human living space is also becoming smaller.The development trend of living space is high-rise and large span.The residential buildings with concrete structure are often corroded by the external environment [1,2]Therefore, the performance of concrete materials must be improved.Ultra-high performance cement matrix composite material (UHPCC) has many excellent properties, which has been accepted and applied in engineering practice, and is obviously superior to ordinary concrete in the subsequent testing.

The trial process 1.1 Raw materials
Cement: P•Ⅱ,42.5R,ordinary Portland cement, density of 3.4g / cm3 Fly ash: Grade I fly ash of Nantong thermal power Plant, with a specific surface area of 400m2And / kg, density of 2.0 g / cm3 The specific components of the above materials are shown in Table 1.

Results and analysis
In this study, the hydrostatic properties of UHPCC concrete were examined at different curing ages, as shown in Figure1 As shown:

Numerical simulation of the concrete flow field 2.1 Grid division and solution method
The generation of the grid is a pre-processing process in the numerical simulation of the flow field.
Meshing of the model is the premise and a key step for numerical calculation.The quality of the grid will not only affect the accuracy of the calculation results, but also seriously distort the results and get the wrong simulation results.
In this paper, the grid generation software Gambit2.1.6 is used for pre-processing.After a large number of model calculation, it proves that the 3 D tetrahedral hybrid grid is more suitable for the requirements, the grid domain is too sparse to accurately describe the characteristics of the flow field, and the calculation error is large.When the grid number reaches a certain degree, the grid can reflect the actual situation and reduce the calculation speed.The grid model is shown in Figure 2:

Simulation results and discussion of turbulence degree
Turbulence is an important indicator to measure the strength of a material [4] , Is an important aspect

The development status of structural reliability analysis method
At present, the theoretical research on time-invariant reliability both at home and abroad has been relatively mature, while the time-varying reliability theory is more complicated due to more consideration factors, which needs to be further studied.Therefore, many scholars have conducted a series of studies on the time-varying reliability of reinforced concrete structure.And Czarnecki et al [4] It is found that the resistance of reinforced concrete structure changes significantly with time.A time-varying reliability model of reinforced concrete structure is put forward and predicts the service life of the structure.Frangopol [5] The degradation performance of the structure and the prior knowledge of the structure.Madsen [6] A new method for structural time-varying reliability and sensitivity analysis is proposed.Similarly, many domestic scholars are also actively exploring to promote the development of structural time-varying reliability analysis.Cheng Shoushan et al [7] Through the analysis of the structural resistance decay process caused by steel corrosion, the reliability evaluation and prediction method of the future state of the structure are established.

Parameters of the degradation model of reinforced concrete resistance 4.1 Determination of the resistance parameters
The uncertainty of material performance mainly refers to the various variability caused by durability degradation, such as carbonization of concrete, section loss caused by corrosion of steel bar, rust swelling and cracking of concrete protective layer and mechanical performance degradation of corroded steel bar.Considering the uncertainty of the calculation mode, the resistance of the RC structure can be expressed by the following formula:

) (t P
Where the --resistance stochastic process; r K --Calculate the pattern uncertainty random variables.resistance.With the current experimental means and research conditions, it is relatively easy for us to obtain the statistical characteristics of some parameters, and then through mathematical methods, we can obtain the uncertainty statistical law of the final model, so as to solve the problem that cannot be directly statistical.

Statistical parameters of resistance to corrosion and bending members
Bending members are the most commonly used structural components in the service reinforced concrete structure.After many years of research, the bearing capacity calculation of corrosion bending members has been relatively mature.According to the relevant knowledge of concrete, the bending bearing capacity of the rectangular section positive section of the reinforced concrete structure corrosion member can be obtained from the following formula: Y Among them, the positive section bearing capacity of the --structure (kN•m); o v f f , --Axial compressive strength (MPa); 1  --Coefficient, its value according to the Code for Design of Concrete Structure; b l, --Section width, section effective height (mm); se B --Equivalent section area of tensile reinforcement (mm 2 ) 。 According to the above analysis, it is easy to know that through the parameter analysis of the model and starting from the uncertainty of the model parameters, we can finally obtain the relevant uncertainty statistical parameters of the resistance of the corroded bending members of the reinforced concrete structure.

Conclusion
The conclusion can be drawn from the above test and the simulation results (1) UHPCC concrete materials in different circumstances, its compressive and folding strength change trend is the same, the strength is proportional to the age, at the same time.At a certain age, nanometer CaCO was added3With the increase of incorporation, its compressive and folding strength will also be significantly improved, if nano CaCO3If the amount is too high, the intensity of the increase is limited, and even the intensity will decrease.
(2) can be seen from figure 3, in the different radial position turbulence degree difference is obvious, its distribution and the tangential velocity distribution has a close relationship, distribution "hump" shape, in the center and the wall is small, and between the value, but the symmetry of the turbulence distribution, it also reflects the asymmetry of the flow field, related to the material size.

Figure 2 A
Figure 2 A. Numerical simulation of the grid model of the flow field characteristics, which is mainly related to the imported Reynolds number, and ultimately affects the material separation efficiency.The simulation of turbulence is shown in Figure3: Distribution curve along the radial turbulence direction in Figure 3 Advances in Engineering Technology Research ICISCTA 2023 ISSN:2790-1688 Volume-7-(2023)

Z
In this equation, --affects the associated random variable of resistance partial derivative is taken at the mean value.It can be seen from the above derivation process that in order to get the statistical characteristics of the final resistance, we can first determine the statistical characteristics of some basic parameters included in the Advances in Engineering Technology Research ICISCTA 2023 ISSN:2790-1688 Volume-7-(2023)

Table 1
Composition of cement and fly ash Table (%)