Effect of SAP internal curing on mechanical properties and durability of semi-rigid subgrade in aeolian sand

2.1. Materials

The aeolian sand used in this study is selected from the south of Mao Wusu Desert, with reference to the specification “Aggregate Test Procedure for Highway Engineering” (JTG 3432-2024), it is known that its particle size is distributed between 0.075-0.3 mm by dry sieving method, and the aggregate is selected from limestone crushed stone, and the engineering characteristics such as crushing value, needle flake content and apparent density are tested with reference to the “Test Procedure for Aggregate in Highway Engineering” (JTG 3432-2024), and the test results are shown in Table 1. The cement was ordinary silicate cement P O 42.5, and the fly ash was Class III fly ash, and the test results of its various technical properties are shown in Table 2. The commercially available ionic SAP (No. SAP-AA) and non-ionic SAP (No. SAP-SA), as well as the composite SAP (No. SAP-NA) independently produced by the laboratory, were used in the range of 80-100 mesh, and their performance indicators are shown in Table 3.

2.2. Orthogonal experimental design

According to the relevant requirements of the specification “Highway pavement grass-roots construction technical rules” (JTGT F20-2015) and the results of aggregate sieve test, the skeleton dense structure is selected, the grading gravel is used CF-B-2S, and the synthetic grading of the mixture is shown in Fig. 1. Cement: Aggregate = 17:83, Cement: Fly ash = 1:3, 1:4, 1:5, using aeolian sand instead of 12 % of fine aggregate, SAP is the three types described in 2.1, and its dosage is 0.05 %, 0.1 %, 0.15 % of the mass of the cementitious material, and the additional water intake of SAP is 10 times, 20 times, and 30 times, and the dosage of cement-fly ash dosage, type of SAP, dosage of SAP, and the additional water intake of SAP are taken as the main factors in this orthogonal test. Water diversion as the four influencing factors of this orthogonal test, each factor is selected three levels, orthogonal test factor level table is shown in Table 4.

Fig. 1Mix synthetic grading curve

2.3. Sample preparation and test methods

In this paper, SAP dry mixing method is adopted, and the graded gravel (including aeolian sand) is mixed well with water, and the material is smothered with cling film covering for 6 h, and the SAP particles are mixed well with the cementitious materials before the test, and then added into the pre-wetted mixture and mixed well for the test. The maximum nominal particle size of the aggregate used in this paper is 26.5 mm, so the C method in the “Test Procedure for Stabilising Materials of Inorganic Binding Materials for Highway Engineering” (JTG 3441-2024) was chosen for the compaction test. The unconfined compressive strength test adopts the press static compression molding 150 mm×150 mm cylindrical specimen, dry shrinkage test molding 100 mm×100 mm×400 mm beam specimen, molding to be mold static 24 h±2 h after demolding, the use of plastic bag sealing moved into the standard perishable environment for standard perishable, to be tested when the age is reached.

3.1. Dry shrinkage performance results and analysis

Table 5 shows the extreme difference analysis of 27 d average drying coefficient index of orthogonal test groups. From Table 5, it can be seen that the extreme difference of cement-fly ash (A) is the largest value of 61.85, the extreme difference of SAP dosage (C) is smaller of 40.06, the extreme difference of SAP type (B) is the second largest value of 29.51, and the extreme difference of additional water intake (D) is the smallest value of 23.31. It can be judged that the order of the primary and secondary factors is: cement-fly ash (A) > SAP type (B) > SAP dosage (C) >additional water intake (D), and the order of the primary and secondary factors is: cement-fly ash (A) > SAP type (B) > SAP dosage (C) > additional water intake (D).

Table 6 shows the analysis of variance of the mean drying coefficients for 27 d. Among them, when $F_A$, $F_B$, $F_C$, $F_D$ is less than F_0.1 (2,8) = 9.00, “-“ indicates no significant effect; when $F_A$, $F_B$, $F_C$, $F_D$ is between F_0.1 (2,8) = 9.00 and F_0.05 (2,8) = 19.00, “*” indicates a significant effect; When $F_A$, $F_B$, $F_C$, $F_D$ are between F_0.05 (2,8) = 19.00 and F_0.01 (2,8) = 99.00, “**” indicates a significant effect; when $F_A$, $F_B$, $F_C$, and $F_D$ were greater than F_0.01 (2,8) = 99.00, “***” was used to indicate that the effect was highly significant. Analysis of variance revealed that the effect of cement-fly ash (A) on the 27 d mean coefficient of drying was highly significant, and the effects of SAP type (B), SAP dosage (C) and additional water intake (D) on the 27 d mean coefficient of drying were more significant.

3.2. Unconfined compressive strength results and analysis

Table 7 shows the extreme difference analysis table of 7 d unconfined compressive strength index of orthogonal test group. It can be seen from Table 7 that the extreme difference value of SAP type (B) is the largest 2.12, the extreme difference value of SAP dosage (C) is the smaller 1.40, the extreme difference value of additional water intake (D) is the second largest 0.55, and the extreme difference value of cement-fly ash (A) is the smallest 0.23, which indicates that SAP has a significant effect on the strength of semi-rigid base. It shows that SAP has a significant effect on the strength of semi-rigid base, and SAP type has the greatest effect on the unconfined compressive strength of cement-fly ash-stabilised sandy semi-rigid base. From the size of the extreme difference value of each factor, the order of the primary and secondary factors can be judged as follows: SAP type (B) > SAP dosage (C) > additional water intake (D) > cement-fly ash (A).

Table 8 shows the analysis of variance of 7 d unconfined compressive strength. Among them, when $F_A$, $F_B$, $F_C$, $F_D$ is less than F_0.1 (2,14) = 2.73, “–” indicates no significance; when $F_A$, $F_B$, $F_C$, $F_D$ is between F_0.1 (2,14) = 2.73 and F_0.05 (2, 14) = 3.74, “*” indicates a significant effect; when $F_A$, $F_B$, $F_C$, $F_D$ are between F_0.05 (2,14) = 3.74 and F_0.01 = 6.51, “**” indicates a more significant effect; when $F_A$, $F_B$, $F_C$_,$F_D$ is greater than F_0.01 (2,14) = 6.51, “***” is used to indicate that the effect is highly significant. It was found that the effects of SAP type (B) and SAP dosage (C) on 7 d unconfined compressive strength were very significant.