1. Introduction

2. Materials and Methods

2.1. Testing Materials

1) Cement: Tianshan P·O42.5R cement produced by Xinjiang Urumqi Tianshan Cement Plant, with the actual strength of 44.6 MPa;

2) Natural coarse aggregate (NCA): pebbles in Urumqi, Xinjiang, with particle size of 5 ~ 30 mm and bulk density of 2,700 kg/m³;

3) Recycled coarse aggregate (RCA): it is broken from the abandoned cement stabilized base after the demolition of a military apron in Liugong Town, Changji Prefecture, Xinjiang. The particle size ranges from 5 mm to 30 mm, and the bulk density is 2,600 kg/m³. The detailed performance indicators are outlined in Table 1.

4) Fine aggregate: washing machine-made sand in Xinjiang, with fineness modulus of 3.0 and apparent density of 2,487 kg/m³;

5) DS: taken from the Taklimakan Desert, with the fineness modulus of 0.12, the average particle size of 0.118 (Table 2), the bulk density of 1,334 kg/m³, the apparent density of 2,790 kg/m³, the water content of 0.4%, and the mud content of 0.4%.

6) FA: produced in Xinjiang Kangsheng Lvyuan Building Materials Co., Ltd.

Table 1. Performance indicators of coarse aggregate.

Table 1. Performance indicators of coarse aggregate.

Indicators	Unit	NCA	RCA	Specification
Apparent specific gravity	g·cm-3	2.687	2.613	≥2.50
Water absorption	%	0.97	4.93	≤3.0
Soil content	%	0.8	1.4	≤1.5
Flat elongated particles content	%	6.4	13.7	≤18
Ruggedness	-	4.8	9.5	≤12
Crush value	%	15.7	21.9	≤26

Table 2. Particle size distribution of DS.

Table 2. Particle size distribution of DS.

Grain size	0.16	0.075	<0.075
Cumulative sieve margin/%	11.8	88.2	10.2

3. Results and Discussions

3.1. Unconfined Compression Strength

Table 5 lists the unconfined compressive strength obtained by orthogonal test at 7, 28, and 90 d. Figure 5 explicates the evolution of compressive strength of mixture with age under 6 % cement dosage, which is served as an example herein. The figure signifies that the compressive strength of CSM mixture changes with the curing age. Notably, the growth rate is fast before 28 d but slows down after that day. Consequently, when CSM is functioned as the semi-rigid road base, the curing time should be maintained until the basic strength is formed to avoid the damage of the base caused by the load in the subsequent construction. When the cement content is greater than or equal to 8%, the compressive strength of CSM can satisfy the specification for heavy traffic on the first-class highway.

In the specification [19], the unconfined compressive strength obtained on the 7th day (referred to as 7-d unconfined comprehensive strength) is undertaken as the reference to measure the road performance of cement stabilized base. The specific range analysis on the orthogonal test results is presented in Table 6.

Figure 6. Mean range of each factor at each level.

Figure 6. Mean range of each factor at each level.

It is evident from the Table 6 that different factors exert varying influences on the 7d unconfined compressive strength. Specifically, in the selected four-level change interval, influence of each factor on the 7-d unconfined compressive strength adheres to the following order: cement dosage > FA replacement rate > RCA replacement rate > DS replacement rate. Furthermore, the above figure reveals that the compressive strength of the mixture is higher under the 30% DS replacement rate and 50% recycled aggregate replacement rate.

Range analysis on the test results can’t estimate the test error. On the other hand, variance analysis can identify the differences and error fluctuations among the test results generated by interaction among distinct levels, thus improving the accuracy of the result analysis. Consequently, the reliability of the analysis results can be enhanced by performing the variance analysis on the test results. Table 7 shows the variance analysis of the orthogonal test results of the 7-d unconfined compressive strength.

It becomes evident from the Table 7 that the F test results of the four orthogonal factors are:

a. The cement content: F > F_0.01, proving that the cement content significantly affect the 7-d unconfined compressive strength;

b. The replacement rate of RCA: F_cement > F > F_0.01, indicating that the RCA replacement rate exerts a highly significant effect on the 7-d unconfined compressive strength, which is weaker than thatof cement content;

c. The DS replacement rate: F_0.01 > F > F_0.05, suggesting that the DS replacement rate greatly influence the 7-d unconfined compressive strength;

d. FA replacement rate: F_cement > F > F_RCA > F_0.01, implying that the influence of FA replacement rate on the 7-d unconfined compressive strength is highly significant, showing its degree between cement content and RCA replacement rate.

Based on the results of range analysis and variance analysis, the following order can be concluded when comparing their influence on the unconfined compressive strength of cement stabilized base: cement content > FA replacement rate > RCA replacement rate > DS replacement rate. It signifies that the appropriate amount of RCA and DS can improve the compressive strength of CSM mixture.

3.2. Cleavage Strength

The stipulated 90-d splitting tensile strength of the base is 0.4 ~ 0.6 MPa [19]. range analysis of the orthogonal test results is presented in Table 8. It suggests that the four orthogonal factors influencing the 90-d splitting tensile strength exhibit the consistent order with that of the compressive strength. Specifically, in the selected four-level change interval, order of the influence degree of factors on the 90-d splitting tensile strength is observed as cement dosage > FA replacement rate > RCA replacement rate > DS replacement rate.

The variance analysis on the orthogonal test results of the 90-d splitting tensile strength in Table 9 displays the F test results of the four orthogonal factors, as follows:

a. The cement content: F > F_0.01, proving that the cement content plays a significant role in influencing the 90-d splitting tensile strength;

b. The RCA replacement rate: F_cement > F > F_0.01, indicating that the RCA replacement rate exerts a highly significant effect on the 90-d splitting tensile strength, which is weaker in contrast to that of cement content;

c. The DS replacement rate: F_0.01 > F > F_0.05, signifying that the DS replacement rate significantly influence the 90-d splitting tensile strength;

d. The FA replacement rate: F_cement > F > F_RCA > F_0.01, suggesting that the FA replacement rate extremely impact the 90-d splitting tensile strength, whose degree is between the cement content and the RCA replacement rate.

Table 9. Variance analysis results of the 90-d splitting tensile strength.

Table 9. Variance analysis results of the 90-d splitting tensile strength.

Factor	Cement content	RCA replacement rate	DS replacement rate	FA replacement rate	Error
Square of deviance	0.830	0.175	0.087	0.465	0.049
Degree of freedom	3	3	3	3	15
Estimate of variance	0.277	0.058	0.029	0.155	0.016
F0.01	5.417	5.417	5.417	5.417	—
F0.05	3.287	3.287	3.287	3.287	—
F	17.090	3.606	1.782	9.572	—

Based on the results of range analysis and variance analysis, it can be concluded that the order of influence degree of the four factors on the unconfined compressive strength of cement stabilized base is: cement dosage > FA replacement rate > RCA replacement rate > DS replacement rate. It reveals that increasing the cement content can enhance the binding among mixture particles and the base strength. FA exhibits poor performance, so it can decrease the strength of the base as an alternative material for cement. Meanwhile, FA demonstrates distinct cementitious properties with cement. As a result, it will induce significant decrease in base strength if too much FA is applied to substitute cement. When the RCA replacement rate is 50 %, the optimal effect can be achieved. The analysis unveils that the RCA used in this crushing exhibits better performance, showing little effect on in case of partial use. However, it will still affect the base strength when it is applied to substitute most natural aggregate. The particle size of DS is smaller. Using DS to partially replace fine aggregate can improve the gradation of the mixture and an appropriate amount of DS can fill the pores among the aggregates, making the interior of the base more dense, thereby improving the strength of the cement stabilized base.

The 90-d compressive strength and splitting tensile strength of DRCSM in range analysis were taken separately to analyze the correlation between the compressive strength and splitting tensile strength of CSM by curve fitting.

Figure 7 reflects that the linear fitting equation of the 90-d compressive strength and splitting tensile strength of DRCSM can be expressed as follows:

$$R_i=0.03562+0.08672R_c$$

(4)

The correlation coefficient is $R^2=0.99316$, which suggests a good correlation between the 90-d compressive strength and splitting tensile strength of the mixture.

In the equation: $R_i$——Splitting tensile strength of the specimen/MPa;

$R_c$——Compressive strength of the specimen/MPa;

$R^2$——Correlation coefficient.

The evaluation parameters of CSM mixture stipulated in the specification include 7-d unconfined compressive strength and 90-d splitting tensile strength. Due to similar development law of compressive strength and splitting tensile strength with age in the experimental results, correlation between 7-d compressive strength and 90-d splitting tensile strength of mixture can be further analyzed under the controlled test amount.

Figure 7. Correlation between 90-d compressive strength and splitting tensile strength of mixture.

Figure 7. Correlation between 90-d compressive strength and splitting tensile strength of mixture.

It can be observed from Figure 8 that the linear fitting equation of 90-d compressive strength and splitting tensile strength of DRCSM is expressed as follows:

$$R_i=0.04513+0.14076R_c$$

(5)

Figure 8. Correlation between 7-d compressive strength and splitting tensile strength of mixture.

Figure 8. Correlation between 7-d compressive strength and splitting tensile strength of mixture.

The correlation coefficient is $R^2=0.98567$, implying a good correlation between the 90-d compressive strength and the splitting tensile strength of the mixture. Therefore, the 90-d splitting tensile strength can be estimated according to Equation (5) and the 7-d compressive strength of CSM required by the specification.

3.3. Compressive Resilient Modulus

As presented in Table 10, under the influence of four factors, the compressive resilient modulus of the mixture gradually increases with the prolonging curing age. Taking6% cement content as an example, the 28-d compressive resilient modulus values of the four groups are 1.26 times, 1.19 times, 1.32 times, and 1.18 times of the 7-d values, and the 90-d compressive resilient modulus values are 1.74 times, 1.75 times, 1.75 times, and 1.68 times of the 28-d values. Therefore, compressive resilient modulus of DRCSM increases with the rise of curing age.

The range analysis of the orthogonal test results for the compressive resilient modulus of the mixture is summarized in Table 11. It signifies that the four orthogonal factors on the 90-d compressive resilient modulus exhibit consistent influence degree with the compressive strength. The selected four-level change interval reveals that the influence degree of the factors on the 90-d splitting tensile strength follows the order of cement dosage > FA replacement rate > RCA replacement rate > DS replacement rate.

Furthermore, Table 12 displays the variance analysis results of the orthogonal test results for the 90-d compressive resilient modulus.

Table 12 indicates that the F test results of the four orthogonal factors are as follows:

a. The cement content: F > F_0.01, proving the significant effect of cement content on the 90-d compressive resilient modulus;

b. The RCA replacement rate: F < F_0.05, indicating that the replacement rate of RCA fails to substantially influence the 90-d compressive resilient modulus;

c. The DS replacement rate: F < F_0.05, showcasing that the DS replacement rate does not significantly affect the 90-d compressive resilient modulus;

d. The FA replacement rate: F_cement > F > F_0.01, indicating that the FA replacement rate plays a highly significant effect in influencing the 90-d compressive resilient modulus, but the degree is less than the cement content.

Analysis on results of range analysis and variance analysis suggests that the order of the four factors when influencing the compressive resilient modulus of cement stabilized base is cement content > FA replacement rate > RCA replacement rate > DS replacement rate, with no significant influence of RCA and DS replacement rates. Moreover, with the increase of cement content, the mixture will form more cement during the curing, enhancing the density of internal structure, thus increasing the cohesion, deformation resistance, and compressive resilient modulus of the base. FA possesses poorer mechanical properties than cement, so the base strength will be reduced after substituting cement with FA. Besides, the mineral impurities such as calcium oxide in the FA will hinder the hydration reaction of the cement, decrease the amount and quality of cement in the mixture, thereby reducing the compressive rebound modulus of the base.

Analysis in section 3.2 reveals that if the 90-d compressive resilient modulus can be predicted by the 7-d unconfined compressive strength required by the specification, the test cycle can be greatly reduced, greatly improving the test efficiency. Consequently, the 7-d compressive strength and 90-d compressive resilient modulus of DRCSM were selected to analyze the correlation between them using curve fitting.

As illustrated in Figure 9, the linear fitting equation of the 7-d compressive strength and 90-d compressive rebound modulus of DRCSM can be written as follows:

$$E_c=602.45+343.73R_c$$

(6)

The correlation coefficient is $R^2=0.98822$, which means that the 7-d compressive strength of the mixture demonstrates a good correlation with the 90-d compressive rebound modulus. Therefore, the 90-d compressive rebound modulus can be estimated by the 7-d compressive strength of the specimen through Equation (6).

In the equation: $E_c$——Compressive resilient modulus of the specimen/MPa;

$R_c$——Compressive strength of the specimen /MPa;

$R^2$——Correlation coefficient.

The results of variance and range analyses indicate that the compressive resilient modulus of the mixture is primarily affected by the cement content. Table 13 lists the mean values of the 90-d compressive resilient modulus under the influence of the varying cement contents in the variance analysis. After that, the correlation between the cement content and the 90-d compressive resilient modulus is fitted by curve.

As explicated in Figure 10, the following linear fitting equation is observed between the cement content of DRCSM and the 90-d compressive rebound modulus:

$$E_c=1026.93+345.49C$$

(7)

In the equation: $E_c$——Compressive resilient modulus of the specimen/MPa;

$C$——Cement content in the specimen/%;

$R^2$——Correlation coefficient.

The correlation coefficient is $R^2=0.98822$, suggesting the existence of a good correlation between the cement content of the mixture and the 90-d compressive resilient modulus. Additionally, the above Equation (7) can be adopted to estimate the 90-d compressive resilient modulus under different cement contents.

Figure 10. Correlation between cement content and 90-d compressive resilient modulus of mixture.

Figure 10. Correlation between cement content and 90-d compressive resilient modulus of mixture.

4. Carbon Emission Assessment of Recycled Aggregate

5. Conclusions

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