Unified approach to characterize the strength of cement stabilized macadam subjected to different loading modes

https://doi.org/10.1016/j.conbuildmat.2020.120143Get rights and content

Highlights

  • The strength variation rules of different loading modes under two mixing methods and different loading rates were revealed.

  • Under two mixing conditions, the strength of different loading modes was uniformed characterized.

  • The validity and completeness of strength parameters of semi-rigid base asphalt pavement structure design were improved.

Abstract

Cement stabilized macadam base is the primary stress diffusion and bearing layer of semi-rigid asphalt pavement, therefore its mechanical behavior directly affects pavement performance. The indirect tensile strength, unconfined compressive strength, and bending strength are the essential parameters for evaluating the bearing capacity of cement stabilized macadam. In the design of a pavement structure, the conventional mixing strength values of a test sample are analyzed and calculated at a fixed loading rate. By using this procedure, the influence of the mixing method and the different loading rates on the strength values are ignored. Besides, the strength obtained by using the fixed loading rate with different loading modes has significant differences. As a result, the artificial randomness of the strength parameter in the design of a semi-rigid asphalt pavement structure increases. In an attempt, to improve the effectiveness and completeness of the strength parameters of cement stabilized macadam under different loading modes, mechanical tests were conducted on samples prepared with both vibration mixing and conventional mixing methods. This procedure included, indirect tensile strength test, unconfined compressive strength test, and four-point bending strength test under different loading rates. The results of these strength tests each group of samples (vibration and conventional mixing method) were compared. Besides, all samples were subjected to 6 different loading rates when conducting each loading mode. These experimental values were analyzed. The loading rate and the strength value have a power function relationship and the correlation of strength rates was revealed. The results demonstrated that the vibration mixing method increases the indirect tensile strength, unconfined compressive strength, and four-point bending strength by 13%, 14%, and 10%, respectively. A unified strength model (S/S0 = 0.9841 + 0.16856ln (0.01019 + v/v0) under two mixing methods and various loading modes were established by using the variation between loading rate ratio and strength ratio. In the unified model, the strength under vibration mixing and conventional mixing methods, and the strength among different loading rates can be converted, which improved the test efficiency. The established unified model solves the problem of uncertainty and uniqueness of strength parameters under different loading modes. The accuracy of the semi-rigid asphalt pavement structure resistance design was effectively improved, thereby providing theoretical guidance for the construction of semi-rigid base asphalt pavement engineering.

Introduction

Cement stabilized macadam base asphalt pavement provides excellent driving comfort [1], durability [2], [3], [4], water stability [5], [6], [7], time-efficiency, and easiness of repairing. For these reasons, the mentioned base is the primary structural type for highway construction [8] and represents more than 90% of the total pavement structure in Chinese roads [9], [10], [11], [12]. Cement stabilized macadam is used in the base layer of the asphalt pavement due to its high strength, modulus of elasticity, and integrity [13], [14], [15]. Besides, it can effectively reduce the strain of the roadbed, induced by vertical loads from vehicles [16], [17], [18], [19]. However, durability issues related to cementitious materials can deteriorate the service life of the cement stabilized macadam. Two of the most severe durability issues affecting the cement stabilized macadam are fatigue cracks and shrinkage cracks of cement materials [18], [20], [21], [22]. These damages arise because of the influence of temperature, environmental condition, vehicle loadings, and other factors on the base of asphalt pavement. In recent years, the boom of the construction industry has caused an increase in heavy traffic load in China. From this situation, higher requirements are needed for the structural reactance of cement stabilized macadam base.

Various laboratory mechanical tests have been proposed for characterizing the cement stabilized macadam to evaluate its tensile, compressive, and bending properties [23]. For instance, unconfined compression test [24], [25], [26], [27], triaxial test [28], [29], tension test [30], [31], [32], [33], bending test [16], [24], etc. In the Specification to Design of highway Asphalt Pavement (JTG D50-2017) [34], the bottom tensile stress of the semi-rigid base layer serves as a critical index during the design process. China Highway Engineering Inorganic Bond Stabilizing Materials Test Procedure (JTG E51-2009) [35] stipulates that the indirect tensile test and unconfined compressive strength test should be performed using a 150 mm × 150 mm cylindrical sample at a single loading rate equal to1 mm/min. Besides, the 150 mm × 150 mm × 550 mm girder sample is used for the four-point bending test at a fixed loading rate equal to 50 mm/min. The unconfined compression test and the triaxial test evaluate the samples in a one-dimensional stress state and three-dimensional complex stress state, respectively. The indirect tensile test examines the samples in a two-dimensional stress state (transverse and longitudinal). The four-point bending test characterizes the samples in a two-dimensional stress state, in which the regions above and below the neutral axis are subjected to compression and tension respectively. Hence, it is possible to conclude that these strength properties are obtained in different stress states and it is difficult to compare one another. Therefore, a unified evaluation method is urgently needed for the characterization of the strength obtained from different loading modes.

For this reason, road researchers have spent great efforts to study the mechanical performance of cement stabilized macadam. Deng et al. [36] combined the unconfined compression test and the indirect tensile test to analyze the effect of various cement contents and different curing periods on the strength of cement stabilized macadam. The results showed that the strength rose with the increase in cement content and curing periods. Sun et al. [24] evaluated the influence of the lime-fly ash replacement rate on the mechanical performance of the cement stabilized recycled lime-fly ash macadam. They found that the increase of the lime-fly ash replacement rate reduced the direct tensile strength, compressive strength, and flexural tensile strength in various degrees. Liu et al. [37] carried out the indirect tensile tests and the unconfined compressive strength tests on cement stabilized macadam with different gradations. The results showed that the samples with medium-sized gradation had a higher strength. Liu et al. [38] conducted the indirect tensile tests and the unconfined compressive test to evaluate the strength index of polyester fiber reinforced cement stabilized macadam. They reported that the strength increased by about 7% with the optimization of polyester fiber content at the curing period of 90 days. Yan et al. [39] examined the effect of adding fly ash from urban garbage incineration on the mechanical performance of the cement stabilized macadam. The results indicated that the maximum replacement ratio of cement in the type of mentioned macadam should be below 25%, to maintain its unconfined compressive and indirect tensile strength.

In addition, some studies have shown that vibration mixing has a significant influence on the improvement of mechanical properties of cement stabilized macadam. Dong et al. [40] researched the effects of vibration mixing on compressive strength, drying shrinkage, and microstructure of cement stabilized macadam. The results showed that it was possible to reduce the usage of cement by utilizing vibration mixing to reach the same strength, besides the drying shrinkage also decreased with the same procedure. Zhao et al. [41] studied the effect of vibration mixing on the compressive strength of cement stabilized macadam. It was found that the samples elaborated by using vibration mixing could achieve higher strength than those samples made from conventional mixing. Zhang et al. [42] determined the equivalent correlation between the labor refitted vibration mixing method and the large-scale vibration mixing method at the construction site through the unconfined compression test. They reported that the optimal mixing time for the laboratory vibration mixing is slightly greater than that one related to the large-scale vibration mixing at the construction site under the same conditions. The study of the vibration mixing method effect on the strength and durability of cement stabilized macadam provides a theoretical reference for industrial application. In recent years, due to the great advantages of vibration mixing in terms of strength and durability, this method has been widely applied in the construction industry, so the researchers related to the vibration mixing test at the laboratory level represent a significant guide for engineering practices.

At present, the main aim of researchers is to improve the strength under some specific loading modes. While, the studies on the strength under other loading modes are limited, which leads to the unscientific durability design of the cement stabilized macadam structure. Numerous researchers have been also conducted studies to develop a unified model for strength evaluation. Pinto et al. [43] improved the traditional maturity method for the overall strength prediction of the concrete materials for all ages, which unified the sensitivity at different temperatures before setting and during hardening. Sui et al. [44] proposed a new uniform nonlinear strength criterion based on the Lade-Duncan model, which could reflect better the nonlinear strength of geotechnical materials and the influence of intermediate principal stress. Also, under the condition of three-dimensional stress, the criterion could be applied to soil and rock materials. Han et al. [45] built a compressive strength prediction model for concrete, based on the compressible filling model. It was reported that more accurate predictions can be achieved with the proposed model by considering the effects of stress, age, and fly ash content under low-level continuous load. Wei et al. [46] proposed a unified constitutive model for concrete columns with different shapes (circular, square and rectangular), which significantly expands the range of parameter space and enhances its consistency for a clearer description of the strain–stress relationship. Al-Rousan et al. [47] established an analytical model to predict the stress–strain characteristics of circular concrete columns under axial compression, which was reinforced with CFRP-confined inner longitudinal bars and transverse tie bars. A new design criterion was proposed based on the research results, which could effectively determine the ultimate confined axial stress of the reinforced concrete columns. Although most of the above researches were to predict the strength of other engineering materials (Concrete, rock, etc.), they can be taken into consideration as a solid foundation for the unified characterization on the strength of cement stabilized macadam with different loading modes. Moreover, the current investigation on the uniform characterization of the strength with various loading modes is quite limited.

The main purpose of this study is to enhance the effectiveness and completeness of the strength parameters of cement stabilized macadam. In an attempt to achieve this target, indirect tensile tests, unconfined compressive tests, and four-point bending tests at different loading rates were conducted for samples prepared with both vibration mixing and conventional mixing. Based on the relationship between strength ratio and loading rate ratio, a unified strength model was established by considering numerous loading modes and mixing methods. The uniqueness and uncertainties for the strength parameters with various loading modes were first solved with the definition of a unified model. Then the relationship between the strength parameters related to samples elaborated by using vibration mixing and conventional mixing was unified, and then the refinement level of the structural resistance design for the semi-rigid asphalt pavement was improved.

Section snippets

Materials and specimen preparation

To compare the test results, the same materials were respectively used to elaborate the samples with vibration and conventional mixing methods. Those samples were cured under the same conditions.

Test results and analysis

To study the rate characteristics of cement stabilized macadam strength under different loading modes, six different loading rates were set: 5 MPa/s, 10 MPa/s, 20 MPa/s, 30 MPa/s, 40 MPa/s, and 50 MPa/s.

Conclusion

The main purpose of this study is to improve the effectiveness and completeness of the strength parameters. The variation of strength and loading rate was obtained under different mixing methods and loading modes. Furthermore, it was unified the law through which the strength of different loading modes varies with the loading rate under two different mixing modes. Based on the above research results, the following conclusions can be drawn:

  • (1)

    Comparing the conventional and vibration mixing method,

CRediT authorship contribution statement

Songtao Lv: Conceptualization, Methodology, Formal analysis, Investigation, Resources, Supervision, Project administration, Funding acquisition. Yanpeng Guo: Conceptualization, Methodology, Validation, Formal analysis, Writing - original draft, Writing - review & editing, Funding acquisition. Chengdong Xia: Conceptualization, Methodology, Validation, Formal analysis, Investigation, Resources, Writing - original draft, Writing - review & editing, Visualization, Supervision, Project

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgment

The research was financially supported by the National Natural Science Foundation of China (51578081), Postgraduate Scientific Research Innovation Project of Hunan Province (CX20200812), and the Inner Mongolia Autonomous Region Transportation and Transportation Department Transportation Projects of Science and Technology (HMJSKJ-201801). This team of researchers would like to thank the reviewers, and editors for their advice on this paper.

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