Semi-rigid base asphalt pavement is the most typical and widely used pavement structure, which has the characteristics of high-cost performance, strong bearing capacity, comfortable driving, and convenient maintenance. The main pavement structure form of highway construction.
In the semi-rigid base pavement structure, the thickness of the asphalt surface layer is relatively thin, generally, 4 to 18 cm, which is used as a functional layer, and the thickness of the semi-rigid base is generally not less than 20 cm, which is used as the main bearing layer. The sub-base serves as the secondary load-bearing layer. That is to say, the base layer plays a major load-bearing role in our pavement structure, so the base material must have sufficient strength, modulus, and fatigue resistance, as well as sufficient, scour resistance in rainy areas.
The cement-stabilized crushed stone material is composed of coarse aggregate, fine aggregate, cement and water. After indoor compaction or on-site rolling, the cement gradually undergoes a hydration reaction to form cement stone, and the coarse and fine aggregates are cemented together to achieve the design strength, forming a cement-stabilized crushed stone material. In order to obtain a base that meets the strength requirements, is not prone to cracks, and has good fatigue resistance, in addition to material technical requirements and mix design, the uniformity of construction is very important. The uniformity of the cement content, the uniformity of the cement content, and the uniformity of the compaction degree will affect the speed and degree of the hydration reaction of the material, and affect the distribution of the local stress. Once the inhomogeneity reaches a certain level, it will cause the stress concentration to exceed the strength of the material. , thereby causing cracking, in other words, the effect of construction inhomogeneity on the cracking of cement-stabilized materials is even greater than the strength of the material.
In a certain project, technicians carried out gradation design, focusing on controlling the pass rate of key sieve holes of 19mm, 9.5mm, 4.75mm, 2.36mm, 0.6mm, 0.075mm within the gradation range, and the pass rate of 31.5mm 100%, the 26.5mm sieve is allowed to have no more than 3% of the particle content to ensure the smoothness of the gradation curve. Since the fine aggregate of the project is made of machine-made sand, the dust removal effect is good, and the pass rate of the fine aggregate below 0.6mm is very low. Different from the asphalt mixture, which can be supplemented with mineral powder below 0.6mm, when the fine powder of the cement stabilized crushed stone raw material is insufficient, the pass rate of the synthetic gradation below the 0.6mm sieve hole will be low, and the lack of filler will cause the material to fail. dense. Therefore, combined with the sufficient supply of local fly ash, this project uses 5% fly ash as a filler to replace part of the fine aggregate, and the addition of an appropriate proportion of fly ash effectively supplements the part below the 0.6mm sieve hole It solves the problem of lack of fine powder filler in the original grading.
Fly ash contains a large amount of active silica and aluminum oxide, which can undergo a secondary hydration reaction similar to the pozzolanic reaction with calcium hydroxide in the cement hydration product to generate hydrated calcium silicate and hydrated aluminate. Calcium, thereby promoting further hydration of cement, can improve the later strength of the material. The unhydrated fly ash particles are filled in the voids of the mixture as fillers to make the voids of the mixture smaller, improve the material density, and increase the mechanical properties and durability.
According to the cement dosage of 4.5%, the above gradation is subjected to compaction tests under different moisture contents to obtain the maximum dry density and optimum moisture content. According to the aggregate apparent density, fly ash density and cement density, the material can be calculated. The theoretical maximum dry density, combined with the optimal moisture content, can calculate the porosity of the mixture at the maximum dry density state, as shown in the table. It can be seen that after using 5% fly ash to replace part of the fine aggregate, the water porosity of the mixture decreases significantly, and the porosity decreases from 6.5% to 2.4%, indicating that when the fine aggregate lacks fine powder below 0.6mm mesh size When a certain proportion of fly ash is used instead, the voids can be effectively filled, thereby ensuring the compactness of the mixture under the maximum dry density state, which is beneficial to improve the mechanical properties and durability of the material.
Volume Characteristics of Mixtures at Maximum Dry Density
|Base mix||Aggregate synthetic apparent density (g/cm3)||Fly Ash Density (g/cm3)||Cement density (g/cm3)||Optimum moisture content/%||Maximum dry density (g/cm3)||Theoretical maximum dry density (g/cm3)||Water porosity /%||Volume ratio of water/%||Void ratio/%|
|Fly ash free mix||2.844||3.11||5.7||2.277||2.855||20.2||13.8||6.5|
|Mixed with 5% fly ash||2.810||2.24||3.11||6.4||2.308||2.822||18.2||15.8||2.4|