Zhang Yongbo, Zhao Zhijun
Shanxi Chengda Highway Survey and Design Co., Ltd. Taiyuan Station of Daqin Railway Co., Ltd
Abstract: Taking a municipal road as an example, the road model was constructed by using ABAQUS finite element software, and the change trend of principal stress under porosity and surface thickness was analyzed, and the optimal porosity and surface thickness of steel slag permeable concrete under different rainfall intensities were analyzed. The analysis results show that the elastic modulus of steel slag permeable concrete is basically consistent with that of ordinary concrete, but the former is slightly smaller than the latter. In order to ensure that the maximum principal stress meets the requirements of flexural strength and the thickness of the permeable layer meets the requirements of permeability, the porosity of the steel slag permeable concrete is finally determined to be 17%, and the thickness of the pavement structure layer is 22cm.
Keywords: steel slag permeable concrete; mechanical properties; pavement structure;
About author:ZHANG Yongbo (1995—), male, assistant engineer, engaged in highway and bridge design.
0 Introduction
With the continuous increase of the amount of steel slag piled up in the steel production process, it occupies a lot of land, and the volatile infiltration of chemical substances in the steel slag will pollute the environment. The steel slag meets the relevant requirements of road materials in terms of crushing value and grading, and it is feasible to use steel slag as aggregate to prepare steel slag permeable concrete for road pavement engineering. At present, the research on steel slag permeable concrete mainly focuses on the mechanical properties of the material itself and the improvement of concrete properties by steel slag addition. In this paper, ABAQUS software is used to construct a finite element model of steel slag permeable concrete for a municipal road, and the pavement structure design is carried out with the surface thickness and concrete porosity as the main variables.
1. Raw material and performance test
1.1 Raw Materials
P.O42.5 cement, S95 slag powder and grade II fly ash and steel slag powder of a nearby power plant are used in a stone yard near the project; 5~25mm continuous graded coarse aggregate and mixed medium sand fine aggregate prepared by coarse sand and fine sand in accordance with the ratio of 1:1; The properties of steel slag powder, fly ash and mineral powder are detailed in Table 1. The apparent density and bulk density of steel slag were 3 720 kg/m3 and 1 920 kg/m3, the needle flake content was 1.8%, the water absorption rate was 1.21%, and the crushing value was 6.9%. The apparent density and bulk density of ordinary crushed stone are 2 665 kg/m3 and 1 400 kg/m3, the needle flake content is 6.4%, the water absorption rate is 0.96%, and the crushing value is 6.6%. The performance of fine aggregates is shown in Table 2.
1.2 Test Methods and Results
The mechanical properties of steel slag permeable concrete were tested, and the density, slump, setting time and gas content of concrete were tested in accordance with the "Standard for Test Methods for Performance of Ordinary Concrete Mixtures" (GB/T50080-2002). According to the "Standard for Test Methods for Physical and Mechanical Properties of Concrete" (GB/T50081-2019), the mechanical properties such as compressive strength, flexural strength, splitting tensile strength and compressive modulus of steel slag permeable concrete were tested; The durability of steel slag permeable concrete, such as impermeability, frost resistance, and chloride ion permeation resistance, was tested by the gradual pressurization method, quick-freezing method, and electric flux method in the "Standard for Test Methods for Long-term Performance and Durability of Ordinary Concrete" (GB/T50082-2009) [1]. According to the test results, the adjusted steel slag permeable concrete mix ratio is cement: mineral powder: fly ash: steel slag powder: gravel: steel slag stone: medium sand: steel slag sand: water: admixture = 230: 55: 70: 15: 800: 270: 420: 350: 182: 5.18.
Table 1 Admixture properties Download the original image
Table 2 Fine aggregate performance Download the original image
When the water consumption is the same, because the water absorption rate of steel slag sand is large, the free water used for the hydration process in the unit volume is less, so the amount of water reducer for steel slag permeable concrete is larger, and the concrete is easy to harden after water consumption, so the coagulation time of steel slag concrete is short. The flexural strength of steel slag concrete can be increased by 30% in 28 days. The modulus of elasticity and axial compressive strength under static compression are also higher than those of ordinary concrete, indicating that its strain capacity under stress is better than that of ordinary concrete [2].
2 Permeable concrete pavement design
A municipal road project is located in a high temperature and rainy area, considering that the number of steel mills in the city is large, facing a large steel slag treatment pressure, so it was decided to use steel slag permeable concrete materials in municipal road construction. The road permeable concrete layer is divided into three layers, upper, middle and lower, and the porosity of each layer is calculated according to the corresponding formula [3].
2.1 Model Building
The roadbed is silty clay, which needs to be replaced, and the roadbed silty clay is taken out and filled with high strength and good performance, and is compacted in layers. The test results show that the permeability rate of the ore waste subgrade soil used is 5.0×10-4cm/s, the porosity of the graded gravel base is 23%, the elastic modulus of the base is 400MPa, and the Poisson's ratio is 0.35.
Combined with the actual situation of steel slag permeable concrete pavement, the vehicle axle load loading conditions are determined according to the light traffic grade, and the cumulative axle load times are 3.0×104 times, and the standard axle load is selected as a double wheel set single axle with an axle load of 100kN; The contact surface is approximately regarded as a rectangle and two semicircles, in order to reasonably simplify the contact area of the wheel, and the size of the loading surface is 32.234×22.2cm, and the contact area is 715.576cm2. The size of the permeable pavement model constructed by applying 3D modeling technology is 5.0×5.0×2.7m, the transverse inner edge and longitudinal sides of the plane layer are extended by 2.7m in turn, and the design thickness of the concrete surface layer is 25mm, and the model structure is shown in Figure 1.
Fig.1 Dimensions of permeable pavement structure (unit: mm) Download the original drawing
In order to optimize the bending performance of the elements and facilitate the calculation and storage, the incompatible mode enhancement technique and the first-order linear interpolation block elements are used to optimize the bending performance of the elements. Finally, the number of 67,336 elements was determined, and the finite element model is shown in Figure 2, where the x, y, and z axes represent the formation direction, transverse direction, and depth direction, respectively.
Fig.2. Finite element model Download the original drawing
2.2 Pavement structure parameters
There are many design parameters of steel slag permeable concrete pavement structure, and in this paper, each parameter is converted into parameters related to porosity and pavement thickness, and the results are shown in Table 3 and Table 4. According to the results in the table, it can be seen that when the porosity of steel slag permeable concrete is 25%, the flexural strength of the pavement structure is lower than the flexural strength of at least 2.0MPa specified in the specification, so after the porosity is removed, the porosity of steel slag permeable concrete is mainly studied to be 15%, 17%, 19%, 21% and 23%.
Table 3 Values of pavement structure design parameters under different porosities Download the original image
Table 4 Values of pavement structure design parameters under different pavement thicknesses Download the original drawing
According to the change trend of temperature stress and load stress of steel slag permeable concrete of different thicknesses, when the porosity is 15%, the main stress within the thickness range of the permeable concrete surface layer can meet the flexural strength requirements of the pavement structure (see Fig. 3), but at the porosity level, the thickness of the surface layer is thin and cannot reach the thickness of the water storage requirements, and only when the thickness of the structural surface layer is 24cm and above, can it meet the water storage requirements. Therefore, for the steel slag permeable concrete pavement with 15% porosity, the surface layer thickness of 24cm is selected. When the porosity is 17%, only when the thickness of the surface layer exceeds 22cm, the maximum principal stress can meet the requirements of flexural strength, and can also meet the thickness of the water storage, so the thickness of the surface layer of 22cm will be taken (see Fig. 4). When the porosity of steel slag permeable concrete is 19%, 21% and 23% respectively, the thickness required increases with the increase of porosity, but the total permeable layer thickness can meet the requirements of water storage thickness, so the maximum principal stress and flexural strength become the main aspects affecting the design of pavement structural layer. For the porosity of 19%, 21% and 23%, the minimum surface thickness should be 26cm, 30cm and 34cm respectively. In summary, the porosity of steel slag permeable concrete should be 17%, and the thickness of the surface layer should be 22cm.
Fig.3. Thickness of each surface layer with a porosity of 15% Download the original image
2.3 Pavement design process
According to the relevant specifications and engineering experience, the cumulative axle load of the highway is 3.0×104, the base layer is 30cm thick permeable graded gravel, the thickness of the structural layer is preliminarily determined to be 26cm, and the elastic modulus and other parameters are calculated according to the porosity. According to the ABAQUS finite element analysis, the maximum load under the standard axle load can reach 1.724MPa, the temperature gradient is 87°C/m, and the maximum temperature warping stress is 1.746MPa. The maximum principal stress obtained under the combined fatigue action of temperature and driving load is 3.16MPa, which is slightly less than the limit value of flexural strength of 3.23MPa, so the structural strength meets the design requirements, and the permeability test can be carried out for the thickness of the structural layer.
Fig.4. Thickness of each surface layer with a porosity of 17% Download the original image
The mean porosity of steel slag permeable concrete pavement is determined according to equation (1):
In equation (1): V is the average porosity of steel slag permeable concrete pavement (%); H1 is the thickness of the cushion (cm); H2 is the thickness of the base layer (cm); H3 is the thickness of the surface layer (cm); V1 is the porosity of the cushion (%); V2 is the porosity of the base layer (%); V3 is the porosity of the surface layer (%).
Substituting the value of the engineering parameters into equation (1) can obtain that the average porosity of the pavement is 21.1%, and the thickness of the steel slag permeable concrete pavement considering the rainfall intensity and rainfall duration is calculated according to equation (2):
In equation (2): Htx is the thickness of the pavement (cm); i is the rainfall intensity (mm/h); q is the mean permeability of the subgrade (%); t is the duration of rainfall (h); The meaning of the remaining parameters is the same as above.
The required thickness of the permeable pavement calculated by equation (2) is 49.49cm, and the design thickness of the pavement is 56cm, so the design thickness meets the requirements of structural strength and permeability.
2.4 Porosity and surface layer thickness
The average rainfall intensity of the whole line is 52.88mm/h, and the thickness and porosity of the pavement structure layer under different rainfall intensities are analyzed on the basis of other conditions. According to the above design process, the thickness of the surface layer that meets the maximum principal stress requirements of the pavement and the requirements of water storage capacity when the rainfall intensity is 40mm/h, 50mm/h, 60mm/h and 70mm/h is studied in detail, and the most suitable thickness of the structural layer is obtained by comparison. The results are detailed in Table 5.
According to the results in Table 5, it can be seen that when the rain intensity is 40mm/h, the porosity is 15%, 17%, 19%, 21% and 23% of the overall thickness of the pavement to meet the requirements of water storage is within 28.7~34.4cm, only the thickness of the base layer can meet the requirements of water storage, so the thickness of the base layer can be appropriately reduced on the basis of meeting the structural strength requirements, and the design thickness of the surface layer is determined according to 15cm, and the porosity of the surface layer steel slag permeable concrete is 15%. When the rain intensity is 50mm/h, the actual climate and environment, rainfall intensity and design requirements of the project are basically the same, the stress has a greater impact on the thickness of the pavement, the design thickness of the surface layer should be controlled at 20cm, and the porosity of the surface layer steel slag permeable concrete is 17%. When the rainfall intensity is 60mm/h, the rainfall intensity is larger than the design value, and the road water storage capacity has a main impact on the pavement thickness, so the design thickness of the surface layer is controlled at 25cm, and the porosity of the surface layer steel slag permeable concrete is 21%. When the rain intensity is 70mm/h, the steel slag permeable concrete pavement structure requires the use of concrete materials with a porosity of 23% and a pavement thickness of 36cm. However, the thickness value of this surface layer is too large, which is both economical and impractical. Therefore, under the rainfall intensity, auxiliary drainage facilities must be set up outside the steel slag permeable concrete pavement, and the pavement design should be carried out according to the 50 mm/h rain intensity after sharing about 20% of the rainfall intensity [4].
Table 5 Calculation results of porosity and surface layer thickness Download the original image
3 Conclusion
Based on the analysis of this paper, it can be seen that the mechanical properties and durability of steel slag permeable concrete are close to those of ordinary concrete, but the density is larger, the setting time is slightly shorter, the compressive strength is slightly higher, and the strength development in the later stage is more stable. With the increase of age, the growth rate of strength in the later stage is better than that of ordinary concrete. The thickness of the steel slag permeable concrete pavement structure requires the flexural strength, the maximum principal stress and the thickness of the total permeable layer to meet the permeability requirements, and the thickness of the steel slag concrete pavement structure layer determined for the specific road section is 22cm, and the concrete porosity is 17%. When the actual rainfall intensity is small, the thickness of the base layer can be appropriately reduced, and when the rainfall intensity reaches more than 70mm/h, the treatment method of simply increasing the thickness of the pavement structure layer is obviously undesirable, and the auxiliary drainage facilities must be considered.
bibliography
[1] Zhou Taihua, Chen Zhengzheng, Zhao Chenyang. Study on permeability and mechanical properties of steel slag permeable concrete[J].Jiangxi Building Materials,2022(6):69-71.)
[2] Hu Liangpeng, Chen Xudong, Zhu Xiangyi, et al. Analysis of mechanical properties and discrete element simulation of permeable concrete of steel slag with different aggregate particle sizes[J].Comprehensive Utilization of Fly Ash,2022(1):69-75.)
[3] Yang Hanqing, Chen Xudong, Chen Chen, et al. Numerical simulation and structural design analysis of steel slag permeable concrete pavement[J].Concrete,2022(1):177-184.)
[4] Kang Xinhuai, Ma Fei, Li Xiqin, et al. Discussion on the relationship between permeability coefficient and porosity of permeable concrete with different admixtures[J].China Building Materials Science and Technology,2017(6):51-53.)
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