# Durability Investigation of Fiber-Reinforced Functionally Graded Concretes in Cold Regions
## 2 Test Methods and Proportioning Model  
In this study, the bridge pile caps' concrete design strength was 35 MPa, while the pier concrete design strength was 40 MPa. The cement used was P-O42.5 Portland cement, the fineness was 2.7%, and the cement hydration exotherm was 233 kJ/kg. The fly ash used was quasi-grade fly ash. S95 grade mineral powder was used. The gravel was selected from 5-20 mm continuous-graded granite gravel. The distribution area required medium and coarse river sand. The water-reducing agent used was polycarboxylate. The hydration heat deceleration material was a cement hydration heat control material--a pale yellow and odorless polysaccharide substance with a molecular weight of 500-3000. The fibers included polypropylene fibers and imitation steel fibers, along with the steel-like fiber shown in Figure 1, which was made of synthetic resin as raw material, processed via a unique process and surface treatment. This product is an alternative to steel fiber, possessing some characteristics of synthetic fiber. Compared with steel fiber, it has corrosion resistance characteristics, easy dispersion, easy construction, and causes no damage to the mixing equipment. The steel-like fiber has a diameter of 0.9 mm, a length of 40 mm, a breaking strength of 500 MPa, an elastic modulus of 6.5 GPa, and elongation at break of 20 mm. The principle of dense skeleton accumulation ensures the anti-freezing and anti-cracking mass concrete design. The goal was to achieve the maximum reduction in the heat of hydration to meet the requirements of the mechanical properties and service environment according to the performance requirements of mass concrete in different structural parts of the bridge. The basis of the elemental mixture composition was \"Ordinary Concrete Mixing Performance Test Method\" GB/T 50080-2016. Therefore, six groups of concrete proportions were selected to study the concrete durability performance, as shown in Table 1. The number and shape of the experimental samples used are shown in Table 2.  
The preparation, maintenance, and mechanical property tests of each group of concrete specimens in this paper were carried out following the \"Test Methods for Mechanical Properties of Ordinary Concrete\" (GB/T50081-2002). The shrinkage performance test depicted in Figure 2, along with the early crack resistance test, the water resistance test, the chloride ion penetration test, the frost resistance test, and the carbonation resistance test, were all carried out following the steps in the \"Standard for Long-term Performance and Durability Test of Ordinary Concrete\" (GB/T50082-2009). The test results were evaluated according to the \"Concrete Durability Test Evaluation Standard\" (JGJ/T 193-2009). All experiments were completed in the State Key Laboratory of Silicate Building Materials, Wuhan University of Technology. \begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline
**Number** & **Cement** & **Fly Ash** & \
\begin{tabular}{c} **Mineral** \\ **Powder** \\ \end{tabular} & **Sand** & **Gravel** & **Water** & **Additives** &
\begin{tabular}{c} **Externally** \\ **Doped Fiber** \\ \end{tabular} \\ \hline \#1 & 250 & 170 & / & 792 & 1043 & 152 & 4 general water reducers; & / \\  & & & & & & 4 composite anti-freezing and & \\ \#2 & 180 & 140 & 80 & 760 & 1090 & 145 & shrinkage-reducing polycarboxylic & / \\  & & & & & & & acid water reducers & \\  & & & & & & 4 composite anti-freezing and & 0.75 monofilament & \\ \#3 & 210 & 80 & 80 & 760 & 1090 & 145 & shrinkage-reducing polycarboxylic & polypropylene fiber + 2.7 steel-like fiber + 30 expansion agent & \\ \#4 & 290 & 140 & / & 780 & 1060 & 148 & 4 general water-reducing agents; & / \\  & & & & & & 4 composite anti-freezing and & 0.75 monofilament & \\  & & & & & & & shrinkage-reducing polycarboxylic & polypropylene fiber + 2.7 steel-like fiber \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Baseline concrete mix ratios for different strength classes and parts (kg/m\({}^{3}\)). Figure 1: Steel-like fiber.