Study on mechanical properties of traffic pavement materials under fatigue loading

. The durability of pavement subgrade materials is the most important indicator for judging their merits as well as their mechanical properties. The durability of pavement subgrade material is mainly reflected in its water stability, because in the actual pavement environment, the subgrade is subjected to the infiltration of groundwater, immersion and the washout of pressure water under the concrete slab. The subgrade is subjected to a series of damage caused by freeze-thaw cycles and wet-dry cycles due to changes in temperature. In view of this, this chapter has targeted. In view of this, this chapter focuses on the frost resistance, water stability, scour resistance, dry shrinkage and sulfate erosion resistance of iron tailings as pavement base material. The study is carried out to simulate the real situation as much as possible to provide more detailed and accurate guidance for the actual engineering application. The study will provide more detailed and accurate guidance for the actual engineering application.


Introduction
The compressive rebound modulus specimen is a cylindrical specimen made by the static compression molding method [1]. The pavement subgrade material is not an ideal elastomer, and the stress-strain relationship is non-linear. Moreover, since the stress distribution in each structural layer of the pavement is not uniform, the modulus is mostly cut-line modulus. The cut line modulus is the slope of the line connecting two points on the stressstrain curve, reflecting the average condition of the stressstrain relationship in a certain working stress range. In pavement materials, these two points are generally chosen as the origin on the stress-strain curve and the stress is half of the ultimate strength or at a specified strai [3,4]n.
In addition, when the strain in the modulus value is the total strain including the rebound and residual strain components, it becomes the deformation modulus; if the strain only includes the rebound strain after unloading, it is called the rebound modulus. Since the elastic analysis theory is used to analyze the stresses and strains in the pavement structure, and the value of the rebound strain is related to the fatigue damage of the pavement, the modulus of resilience is usually used to characterize the deformability of the material in pavement design [5,6].
The curve shown in the figure is the modified rebound modulus curve after excluding the significant deviation points. It can be seen from the figure that the incorporation of fiber significantly reduces the rebound modulus value of the specimen, and when the amount of polypropylene fiber is incorporated, the rebound modulus of the specimen decreases compared with that of the specimen without polypropylene fiber [7,8]. The main reason for this phenomenon is that the initial properties of polypropylene fibers are better and the low temperature brittleness is obvious, compared with the traditional brittle cement stabilized gravel material, the tensile and compressive deformation capacity of the composite material is greatly improved after the addition of polypropylene fibers, and the bending fatigue strength is excellent. The increase in flexibility ensures that the aggregate material can withstand large shrinkage deformation without causing damage [9,10].
The durability performance of pavement base material is the most important index to judge its merit as well as the mechanical performance. The durability of pavement subgrade material is mainly reflected in its water stability, because in the actual pavement environment, the subgrade is subjected to the infiltration of groundwater, immersion and the washout of pressure water under the concrete slab when the vehicle and other loads pass through, humidity and temperature changes caused by freeze-thaw cycles and wet-dry cycles and a series of damage [11,12].
This results in a large temperature difference between day and night in winter. Therefore, the freeze-thaw cycle test standard is proposed for the first time in the newly introduced Experimental Procedures for Inorganic Bond Stabilized Materials for Highway Engineering [13,14]. The specimens of freeze-thaw cycle are cylindrical specimens made by static compression molding method. The specimens were immersed in water for hours and then taken out, frozen at for hours, and then thawed in water tank for hours, so that the cycle times to measure the lateral limitless compressive strength. The ratio of the lateral limit compressive strength of the two groups of specimens was obtained from their freezing coefficients, and the results are shown in the table. It can be seen from the graph that the freezing coefficient of all the ratios are above, and the change of the freezing coefficient of the specimens with different polypropylene fiber doping is not obvious, which means that the freezing stability of each ratio is better and the amount of polypropylene fiber doping has little effect on the freezing stability of the specimens [15,16].

Compaction experiments
Compaction experiments were carried out on cement stabilized iron tailings, lime stabilized iron tailings, cement-lime stabilized iron tailings and cement and biocoagulase stabilized iron tailings respectively, in order to plot the relationship curve of water content and dry density of stabilized soil, so as to determine its optimum water content and maximum dry density and provide compaction standards for engineering design and field construction. The ratio of cement: gravel: iron tailing mass is used as an example to explain in detail the determination method of the best water content and maximum dry density of this ratio. The maximum particle size of the experimental material should be controlled as follows.
Preset a gradient for the water content and calculate the total amount of water to be added to the specimen based on the formula. Spray the water evenly on the test material and mix it well, close and moisten it with a plastic pocket, then add the cement to the test material and mix it to a homogeneous state. When the cement is added to the wonk, the compaction experiment must be carried out within. The test material will be loaded into the compaction cylinder in five times, each compaction before leveling its surface and slightly compacted, and then heavy hammer compaction times. After the last compaction experiment is completed, the test piece should not exceed the top of the barrel, beyond the test piece should be scrapped. After the compaction is completed, the specimen is taken out and weighed with a stripper. After weighing and breaking, the representative sample is weighed in quarters of about grams, and its mass is recorded and put into the oven with the temperature set to dry, and when the sample is weighed twice before and after, the mass of the sample no longer changes, and its water content is measured at this time. From the formula, the dry density of the sample is calculated. With the water content as the horizontal coordinate and the dry density as the vertical coordinate, the relationship curve between dry density and water content is drawn. If the experimental data fails to form a continuous and complete convex curve, additional experiments should be done at this time until the curve.

Effect of cement dose on unconfined
compressive strength The effect of cement dose on the unconfined compressive strength of the specimens was the greatest, so the effect of cement dose on the unconfined compressive strength of the specimens was carried out first. Since the iron tailings are very fine grained, the cement dose experiment was carried out with reference to the standard of cement stabilized fine-grained soil. The national standard only specifies the requirement of day unconfined compressive strength, but in order to better reflect the development pattern of specimen strength, this experiment was conducted simultaneously with day and day specimens of unconfined compressive strength experiment. The experimental results are shown in Fig. With the increase of cement dose, the unconfined compressive strength of the day and sky specimens increased significantly. However, when the cement dose is day lateral limit compressive strength is only, and does not meet the strength requirements of inorganic bond stabilization materials for highway engineering, which is used as the base material for high-speed and first-class roads. And according to engineering experience, the amount of cement to do pavement base material generally does not exceed, too high cement dose is easy to cause cracks in the pavement and thus affect the normal use of the pavement. Therefore it is not desirable to increase the cement dosage only to increase the lateral limitless compressive strength. It is necessary to explore other suitable admixtures or binder materials to improve the unconfined compressive strength. Considering that the unconfined compressive strength of the specimen is close to this strength index when the cement dose is, the cement dose is set as a comprehensive technical and economic factor.

Effect of lime admixture on unconfined
compressive strength Lime is also a common inorganic binder. However, lime alone is used to excite iron tailings specimens that disintegrate after immersion in water, and the specimens have no strength. Therefore, lime and cement were chosen simultaneously as excitation agents for stabilizing iron tailings. The dose of cement was fixed as appropriate and the dose of lime was in turn appropriate. But many cracks appeared. The unconfined compressive strengths measured after days of immersion were and could not meet the requirements of the national standard for the unconfined compressive strength of the specimens in days.The mechanical properties are shown in Table 1. From the above experiments, it was found that the addition of lime did not have the expected results. The reason may be that the plasticity index of iron tailings is low, and the content of clay particles is small, so it is difficult for lime to play an effective role, but affects the strength of the specimen. The effect of crushed stone admixture on the unconfined compressive strength was to improve the gradation by adding crushed stone as aggregate. It can be seen from the figure that the increase of crushed stone dosing helps to improve the unconfined compressive strength, but when the crushed stone dosing exceeds, the unconfined compressive strength decreases. Therefore, the optimum amount of crushed stone is determined. Although the crushed stone is mixed with the amount of unconfined compressive strength of the day to meet the strength requirements of the highway and primary highway base, but because the mining road needs to drive tons of heavy trucks, the strength requirements as close as possible to even reach the upper limit of national regulations. Therefore, at this stage, the strength still can not meet the actual requirements, still need to further explore.

Determination of aggregate mix ratio
The cement dosage has a great influence on improving the lateral limit compressive strength of the specimens, and ranks second among the factors affecting the lateral limit compressive strength of the specimens. However, in practical application, it is found that although the increase of cement dosage can improve the lateral limit compressive strength of specimens, too high cement dosage will lead to cracks in inorganic aggregates after a period of time, which will affect the effectiveness and life of inorganic materials. Moreover, according to the requirements in the national experimental regulations on inorganic aggregate materials. The mechanical properties of the material after optimization are shown in Table 2. Therefore, although the increase of cement dose can significantly improve the lateral limitless compressive strength of the specimen, the cement dose is set as considering the other properties of the material. Crushed stone is mainly added to inorganic aggregates as an aggregate to improve the gradation of inorganic aggregates. Appropriate addition of some crushed stone aggregate can not only improve the gradation, but also increase the lateral limitless compressive strength of the specimens. However, when the amount of crushed stone is increased to, the unconfined compressive strength of the aggregate starts to decrease compared with the ratio of crushed stone. The excessive amount of crushed stone leads to a significant increase of coarse particles in the aggregate and an increase of pore space, which affects the unconfined compressive strength of the specimens. Therefore, the optimum amount of crushed stone is set as the total mass fraction of the aggregate.
In order to improve the service life of pavement materials, on the premise of meeting the compressive strength of road base materials, this chapter focuses on experimental research on the mechanical properties of pavement materials in terms of the short board a strain tensile strength, flexural tensile strength, and compressive rebound modulus to try to achieve a reasonable use of materials.

Split tensile strength test
The tensile strength is the ultimate strength or maximum stress when the material is subjected to axial tension. The tensile strength of a pavement subgrade material is mainly provided by the internal cohesion of the material. The tensile strength of the pavement material is related to the composition of the material, especially the interfacial bonding properties of the bonding material and the bonded material components. The incorporation of alkene fibers does not significantly increase the labor-cracking tensile strength of the aggregates. When the fiber dosage exceeded, the tensile strength of the specimens increased significantly, and the tensile strength of the specimens with the fiber dosage was higher than that of the specimens without polypropylene fiber, which was quite obvious. After that, when the amount of fiber is used, the tensile strength begins to decrease.
The measured day, day labor cracking tensile strength results and the analysis of the results are shown in the table, table. From the data in the above table, it can be seen that when the fiber dosage is, the specimen's, flexural tensile strength reaches, respectively, after the compressive strength starts to decrease with the increase of fiber dosage. Corresponding to the same ratio of specimens, no lateral compressive strength were, that is, the ratio of flexural tensile strength and no lateral compressive strength were, all meet the Sha Aimin in the "road base engineering" proposed the most suitable ratio of flexural tensile strength and no lateral compressive strength should be greater than the conditions. In addition, the ratio of flexural tensile strength to unconfined compressive strength of the specimen is greater than the corresponding ratio of flexural tensile strength to unconfined compressive strength, which indicates that with the growth of time, the growth of flexural tensile strength of the specimen is more significant than the growth of unconfined compressive strength.

Compressive resilient modulus test
The stress-strain relationship of a material under load is customarily reflected by the modulus of stress to strain ratio. The modulus of resilience is one of the key factors affecting the thickness of each layer in the pavement composition, and a small change in the resilience modulus of the soil base can have a large impact on the thickness of the pavement structure, especially in asphalt pavements. And pavement design indicators and other indicators of pavement performance, such as top surface compressive strain, top surface bending and sinking of the soil base and other major parameters are inseparable from the modulus of resilience, are determined by the state of the soil base, so a reasonable and accurate evaluation and selection of the modulus of resilience of the soil base is of great importance.
The loss of strength of the specimens after freeze-thaw cycles is mainly due to the following two reasons. The first is the liquid expansion pressure. Generally speaking, the base material has more pores and is a porous material. When water freezes, it has to expand in volume. When the water content in the pores exceeds a certain critical value, a large pressure will be generated due to the volume expansion effect. When it is freeze-thaw cycle action experiment, it is the internal pore water freezing and swelling effect, the additional internal stress generated will cause extrusion and damage to the pore wall of the pavement material. The second main reason is the penetration pressure. As the water in the capillary of the test piece is a solution containing several soluble salts. When the salt concentration increases force news, the freezing temperature decreases. Therefore, partial freezing of the solution in the micro-pores of the base material will result in osmotic pressure caused by water freezing.
Many domestic scholars' research concluded that the strength and stability of the roadbed are closely related to the moisture content of the roadbed and affect the strength of the pavement structure to a certain extent. Therefore, it is very important to study the water stability of iron tailings as pavement subgrade material. Cylindrical specimens made by static compression molding method for dry and wet cycles. The fixed specimen ratio is cement: gravel: iron tailings: bio-coagulase; fiber admixture were, specimens were divided into two groups after maintenance, one group for dry and wet cycle experiments, the other group as a reference, maintenance of the same time. The test specimens of the experimental group were immersed in water for hours, and then taken out of the air dry hours, so that the cycle times to measure the lateral limit compressive strength.

Conclusion
The addition of polypropylene fiber helps to improve the splitting tensile strength of the aggregate, and the splitting tensile strength of the specimens reaches the highest value when the fiber dosage is, which is higher than the splitting tensile strength of the specimens without the addition of polypropylene fiber.
When the fiber dosage is, the splitting tensile strength of the specimen reaches the highest value, and the compressive strength starts to decrease with the increase of fiber dosage. Corresponding to the same proportion of specimens, no lateral compressive strength, respectively.
When the fiber dosage is,, the ratio of flexural tensile strength to unconfined compressive strength of the specimen days is greater than, which meets the requirement that the most suitable ratio of flexural tensile strength to unconfined compressive strength should be greater than that derived by domestic scholars.
With the growth of time, the growth of flexural tensile strength of all specimens is more significant than the growth of unconfined compressive strength. The incorporation of polypropylene fiber significantly reduced the rebound modulus value of the specimens, and the larger the amount of polypropylene fiber, the more obvious the decrease of the rebound modulus value of the specimens. When the amount of polypropylene fiber was added, the rebound modulus of the specimens decreased compared with that of the specimens without polypropylene fiber.