Research on mechanical performances of composite cementitious material system in new bridge expansion joint UHPC material

In the bridge expansion joint repair project, in order to ensure the efficiency of the project, the repair material needs to have better flow properties and higher early strength. The introduction of sulphoaluminate cement will significantly change the properties of UHPC. Compared with ordinary Portland cement, the composition of its cement clinker mineral belongs to another physical and chemical system, which has excellent properties such as early strength, high strength, impermeability and corrosion resistance. Therefore, different groups of UHPC were prepared by changing the content of sulphoaluminate cement, and the fluidity test of each group of slurry and the compressive strength and flexural strength test of 1d, 3d, 7d, 28d were carried out to study the sulfur. The influence of aluminate cement content on slurry fluidity and early mechanical properties in order to obtain the optimal mix ratio of UHPC materials for new bridge expansion joints.


Introduction
With the intensification of domestic expressway traffic and the increase in the number of years of operation, many serious diseases have appeared on bridges and pavements, and there is an urgent need for fast and effective maintenance programs. The traditional repair plan for fast hardening concrete has poor repair effects and a high repair rate due to complex pavement forces, limited curing time, insufficient concrete tensile strength, and poor durability. And because UHPC has the advantages of high strength, high toughness, good wear resistance, good impermeability and good durability, and the adjustment of the components can well meet the construction performance and operating time during the pouring construction process. It is required to quickly condense and harden and increase the strength after the construction is completed, and the traffic can be quickly restored. UHPC is considered to be a new type of cement-based material with ultra-high strength, ultra-high toughness and excellent durability [1] in terms of ultra-high performance concrete. It is used in highway bridge structures, building structures, prefabricated decorative structures, and building repair and reinforcement projects. There are more and more applications in the field, and it has become a research hotspot of cement-based materials at home and abroad [2,3] . It is promising in the application of structural materials and has great development potential. Now in various engineering applications, many experts and their teams are tapping the potential and availability of UHPC [4] . SHAFIEIFAR et  [5] found that the compressive strength of UHPC is three to four times higher than that of ordinary strength concrete, and the modulus of elasticity is twice that of ordinary concrete; therefore, UHPC is considered by the engineering community to be a better repair material for engineering structures. In summary, starting from the material point of view, preparing repair materials with high performance and greater social and economic benefits according to the requirements of different repair projects undoubtedly has important theoretical value and engineering application value.
Under normal circumstances, different systems of cement should not be mixed, but in view of the different advantages and disadvantages of the two kinds of cement, many scholars have developed a strong research interest in the mixed system. Yu Jin et al. analyzed the specific effect of adding sulphoaluminate cement on the hydration process of Portland cement, and studied the effects of sulphoaluminate cement with different particle sizes and content as admixtures on various properties of Portland cement [6,7] . Yang Qing et al. research on ordinary silicate aluminate composite cementing system found that sulphoaluminate cement can significantly shorten the setting time of composite cementing system at a certain amount, and obtain a substantial increase in early strength [8] . Bizzozero et al. studied the development and changes of the microstructure during the hydration process of the composite gelling system, and analyzed the hydration products [9] . Research by Pelltie et al. believes that the two cements can improve the performance of the composite cementing system at a certain ratio [10] . This experiment mainly studied the mechanical properties of ordinary silicate-sulphoaluminate cement composite cementing system in concrete.
Ordinary Portland cement is currently the most widely used building cementing material in the world. It is widely used because of its low cost and stable performance [11] . Since the 21st century, with the introduction of the concept of green development and the continuous changes in the use of cement, sulphoaluminate cement has shown certain advantages. As a low-energy cement, it not only consumes less energy in the production process, but also it has the characteristics of high early strength, rapid strength development, and corrosion resistance in the process of use, which has attracted wide attention from the industry [12] . This experiment mainly studies the effect of ordinary silicate-aluminate cement composite cementing system in concrete on the fluidity and mechanical properties of concrete, and further optimizes the mix ratio of ordinary silicate-aluminate cement composite system.

Materials
The cement used in the experiment is P.O 52.5 grade ordinary Portland cement and 42.5 fast hardening high-strength sulphoaluminate cement. Its basic composition and related performance are shown in the following Table 1 and Table 2

Test ratio design
This experiment mainly studies the influence of the content of sulphoaluminate cement on the performance of UHPC. A unified matrix mix ratio is adopted. The matrix mix ratio is obtained through a large number of previous mix ratio tests and has the best mechanical properties. The specific coordination of the test is shown in Table 3; The concrete test block produced in the experiment is a standard cubic test block with a side length of 100mm, and is cured to a specified age under normal temperature curing.

Flow performance
As can be seen in Fig.1, when the sulphoaluminate cement content gradually increases, the fluidity of the fresh UHPC slurry shows a trend of first rising and then falling. Observing the changes in the graph, it can be seen that when the content of sulphoaluminate cement is below 0.137, the increase in fluidity is more obvious; when the content of sulphoaluminate cement is above 0.137, the decrease in fluidity is relatively slow; When the acid salt cement content is 20%, the fluidity of the slurry is 163mm, and the fluidity of the slurry is better at this time. Fig.1 The influence on the fluidity of New Bridge Expansion Joint UHPC Material Fig.2 show the following results: The compressive strength of UHPC at 1d, 3d, 7d, 28d ages all show a trend of first increasing and then decreasing with the increase of the sulphoaluminate cement content. When the sulphoaluminate cement content reaches 0.068, it reaches the maximum. The maximum compressive strength of 1d is 26.97MPa; the maximum compressive strength of 3d is 78.12MPa; when the age is 7d, the maximum compressive strength is 96.27MPa; when the age is 28d, the maximum is 122.13MPa. When the aluminate cement content is 10%, the early compressive strength of UHPC concrete has the fastest growth rate, and the 3d growth percentage of A2 group reaches 220%. After more than 0.068, the UHPC compressive strength shows an overall downward trend.  Fig.2 The influence on the compressive strength of New Bridge Expansion Joint UHPC Material Fig.3 show that the content of sulphoaluminate cement can enhance the early flexural strength of UHPC, and the overall situation is relatively complicated. When the age is 1d, the UHPC flexural strength first increases and then decreases with the sulphoaluminate cement content, and reaches the maximum value of 6.55MPa when the sulphoaluminate cement content is 0.137. The flexural strengths of ages 3d, 7d and 28d all reach the maximum when the sulphoaluminate cement content is 0.068, which are respectively 13.55Pa, 14.3MPa and 18.13MPa. When the sulphoaluminate cement content is greater than 0.068. Shows a downward trend, and the flexural strength shows a shrinking phenomenon.

Conclusion
Based on the results and discussions presented above, the conclusions are obtained as below: (1) When the content of sulphoaluminate cement reaches 0.137, the fluidity of composite cementing system concrete is the largest.
(2) Combined with the development of compressive and flexural strength, the optimal content of sulphoaluminate cement should be controlled at 0.068~0.137. The blending amount can be formulated into a salt cement-sulphoaluminate cement composite cementing material that is easy to shape, has good fluidity and high early strength. 3) The early compressive and flexural strength of composite cementing system concrete increases first and then gradually decreases with the increase of sulphoaluminate cement content. The composite cementation system with sulphoaluminate cement content of 10%. The compressive and flexural strength of concrete is better than other admixtures, and it has good mechanical properties. In the later stage of strength, the composite cementing system concrete with a cement content of 20% showed a greater decline overall.