Experimental Study on the Mechanical Properties and Permeability of Cement-Stabilized Permeable Recycle Aggregate Materials

Abstract

This paper designed cement-stabilized permeable road subgrade materials. Construction demolition waste with recycled aggregate replaced natural aggregate in cement-stabilized materials to utilize recycled resources for construction solid waste. This paper tests the compressive strength, water permeability, bending strength, and compressive resilience modulus of cement-stabilized permeable recycled aggregate materials under different cementitious additive ratios. The results show that at a recycled aggregate proportion of 30% in cement-stabilized permeable recycled aggregate material, the 7-d unconfined compressive strength exceeds 3.5 MPa, and the permeability coefficient surpasses 3.5 mm/s, which can meet the roadbed requirements in China. The incorporation of recycled aggregates significantly reduces the mechanical properties and water permeability of cement-stabilized permeable recycled aggregate materials, while cementitious additives improve the mechanical properties. Specifically, red brick, old concrete, and ceramics in recycled aggregates weaken the mechanical properties of the skeleton structure of cement-stabilized permeable recycled aggregate materials, and the compressive strength, bending strength, and compressive resilience modulus decrease with the recycled aggregate content. Cementitious additives can fill the micro-pores of the interface transition zone of cement-stabilized permeable recycled aggregate materials to improve the cementation strength between aggregates.

1. Introduction

As part of urbanization in China, many old houses have been demolished, and more buildings have been built, generating significant construction demolition waste (CDW). Statistically, 500–600 tons of CDW is generated per 10,000 m² of buildings during construction, and 7000–12,000 tons of CDW is generated per 10,000 m² of old buildings demolished [1,2]. The current stock of CDW in China is about 20 billion tons, with 2 billion tons generated each year. The CDW accounts for 40% of the total municipal waste and will inevitably pollute natural environments if disposed of improperly. However, the primary disposal method for CDW in China is in landfills, and the utilization rate of CDW resources is less than 5% [2]. In comparison, the comprehensive utilization rate of CDW in Europe and the United States has reached 90% [3]. With the Chinese government’s emphasis on environmental management and the increasing price of ore resources, recycling CDW will be an important task.

Many researchers have studied recycling CDW. Methods include crushing CDW into recycled aggregate (RA) and analyzing the mechanical properties and long-term stability of RA applied in concrete, road-base, and cement-stabilized materials. In general, studies on the application of CDW have become ubiquitous in civil engineering. The research mainly includes (a) the effect of the gravel type, properties, and content on the cementitious material performance [4,5,6], (b) the porosity and pore structure properties of RA permeable cementitious materials [7,8,9], and (c) the relationship between the mechanical properties and permeability of RA permeable cementitious materials [10,11].

In recent years, many cities around the world have been affected by natural environmental problems, including urban flooding, water pollution, and heat island effects [12,13,14]. These environmental problems are because many roads were hardened during city construction, and most of the roads built in the past were impermeable structures, causing poor drainage and flooding in heavy rainfall. Therefore, China hopes to alleviate these environmental problems through sponge engineering in cities.

The fully permeable road, consisting of permeable pavement and a permeable road base structure layer, plays a vital role in sponge cities. Previous studies have shown that the porosity of fully permeable roads typically ranges from 15–30%, and the water permeability can usually reach 200 L/(m² × min) [15,16,17]. The high porosity of fully permeable roads allows the passage of both water and air. Previous research results show that fully permeable roads have advantages over traditional impermeable roads: (a) Fully permeable roads can replenish groundwater resources, restoring urban water ecology [18]; (b) fully permeable roads can divert urban surface runoff, alleviating urban flooding [19]; (c) the road pore structure absorbs noise generated by vehicle movement [20,21]; and (d) reducing urban heat island effects and improving urban ecological environments [22]. Therefore, research on permeable roads has become a topic of interest for many scholars.

It worth noting that the majority of the current research on permeable road materials revolves around pervious concrete, which undoubtedly boasts commendable mechanical properties and water permeability [23,24,25,26]. Nevertheless, the cement content in previous concrete typically reaches a staggering 20%, and the employed aggregates predominantly consist of natural aggregates (NA), which fail to promote the sustainability of road engineering. Consequently, the reduction in cement consumption and the integration of RA from CDW in permeable road materials hold considerable environmental and economic merits. Minimizing the quantity of cement employed in engineering can effectively mitigate CO₂ emissions and alleviate the environmental repercussions of the greenhouse effect. Implementing RA on permeable roads not only resolves the disposal of CDW but also enhances the utilization of recycling resources derived from CDW [27,28,29].

NA has generally been used to make permeable roads, while recycled construction waste aggregates have rarely been used [30,31]. The main reason is the complex composition of recycled construction waste aggregates, which usually contain red bricks, old concrete, and crushed stone. Compared with NA, red bricks and old concrete have poor mechanical properties. Red bricks more easily disintegrate and break after water immersion than old concrete and NA [32]. Therefore, the rational design of cement-stabilized permeable RA materials that meet road use and permeability requirements is challenging and valuable. Such materials can improve the recycling rate of recycled construction solid waste aggregate and effectively improve the urban ecological environment [33].

In this study, RA from CDW is used to partially replace natural aggregates in order to produce cement-stabilized permeable recycled aggregate materials (CPRAM). A total of nine cementitious ratios were designed and utilized in CPRAM based on the orthogonal design theory. The CPRAM specimens underwent unconfined compressive testing, 4-point bending (4PB) testing, and permeability testing. The influence of cementitious materials and the percentage of RA on the mechanical properties and permeability of CPRAM specimens was analyzed. The research findings will serve as a reference for the engineering application of CPRAM.

2. Experimental Design

2.1. Materials and Methods

2.1.1. Aggregates

Here, NA and RA were used for CPRAM. The type of NA is natural crushed limestone aggregate, and the RA is from the Yunzhong Company of China (Changsha, China). This RA contains multiple recycled material types, including wood blocks, ceramic, old concrete, pebbles, and red bricks, as shown in Figure 1. The percentage of each component recycled material in CDW was determined, as shown in

Table 1. Measured component materials of RA.

Size (mm)	Crushing Stone (%)	Red Bricks (%)	Old Concrete (%)	Other (Wood Blocks, Glass, and Mortar, etc.) (%)
19–26.5	42.52	13.97	34.62	8.90
9.5–19	28.01	19.19	44.90	7.90
0–9.5	62.43	2.74	29.68	5.14

. The apparent specific gravity, water absorption, and crushing values were tested. The technical indicators are summarized in

Table 2. Technical indicators of RA and NA.

Aggregates	Size (mm)	Apparent Specific Density (g/cm3)	Crushing Value (%)	Water Absorption (%)	Flakiness Content (%)
RA	19–26.5	2.55	26.4	7.59	8.9
	9.5–19	2.63	25.7	6.72	9.5
	4.75–9.5	2.66	23.9	6.01	12.5
NA	19–26.5	2.72	21.8	0.35	9.4
	9.5–19	2.72	20.2	0.75	12.1
	4.75–9.5	2.76	23.7	0.41	6.2

, which comply with Chinese standards (JTG/T F20-2015) [34].

2.1.2. Cementitious Additives

Cementitious materials used in CPRAM include P.O 42.5 Portland cement, fly ash, blast furnace slag, and silica fume. The P.O 42.5 Portland cement was purchased from Hunan Nanfang Cement Group Co., Ltd., Changsha, China. Fly ash was purchased from Platinum Refractory Materials Co., Ltd., Changsha, China. The blast furnace slags were purchased from Longze Water Purification Materials Co., Ltd., Zhengzhou, China. The silica fume was purchased from Anhui Wanwei Updated High-tech Material Industry Co., Ltd., Caohu, China.

Briefly, fly ash and blast furnace slag represent a type of environmentally-friendly cementitious substance that is produced and repurposed from industrial byproducts [35,36]. These materials possess the advantageous qualities of being cost-effective and exhibiting enhanced hydration properties. By partially replacing cement as a cementitious component, they can effectively diminish the production expenses of CPRAM. In addition, the silica fume is a nanomaterial with good hydration activity [35,37], which can effectively enhance the mechanical properties and service life of CPRAM.

Analyzing the chemical composition of cement, fly ash, blast furnace slag, and silica fume utilizing X-ray Fluorescence (XRF) instruments, the density and major chemical compositions of cement, fly ash, blast furnace slag, and silica fume are reported in

Table 3. Chemical compositions of cement, fly ash, blast furnace slag, and silica fume.

Type	Cement (%)	Fly Ash (%)	Blast Furnace Slag (%)	Silica Fume (%)
Density (g/cm3)	3.1	2.9	2.3	2.7
SiO2	20.53	55.37	34.74	97.1
Al2O3	4.44	30.11	13.65	0.20
CaO	4.20	4.38	0.78	0.08
Fe2O3	58.40	3.54	33.16	0.57
SO3	2.03	1.27	7.27	0.80
MgO	3.28	1.11	2.17	0.32

.

2.2. CPRAM Design

The aggregate gradation was adjusted to give the CPRAM materials good permeability. The NA and RA are used as aggregates for CPRAM. There are three combinations of NA and RA in the mixtures with RA contents of 0%, 30%, and 60%. The grading of the aggregate composition types in the mixtures is shown in Figure 2, together with the design limits.

The preliminary experimental results provide the design limits, including the step-by-step filling tests for multi-aggregates and void ratio tests of aggregate and cementitious material mixtures with different designs. The design porosity of RPCAM was set at 22%, and the water-cement ratio was 4.1% based on preliminary experimental results. The CPRAM is characterized by high porosity and low strength compared with conventional concrete materials. To improve its mechanical properties, a total of nine cementitious material proportions were designed based on the orthogonal design method [38,39]. In this research, RA, fly ash, blast furnace slag, and silica fume were considered for the design of CPRAM mixtures, and three different utilization ratios for these materials to optimize the design of CPRAM mixtures. The proportions of the cementitious materials and the composition of the aggregates in the CPRAM mixture are shown in

Table 4. Percentage of aggregates and cementitious materials.

Type	RA (%)	NA (%)	Total Percentage of Cementitious Materials (%)	Cement (%)	Fly Ash (%)	Blast Furnace Slag (%)	Silica Fume (%)
0%–1	0	100	10	10	0	0	0
0%–2	0	100	10	8.10	1	0.5	0.40
0%–3	0	100	10	6.80	2	1	0.20
30%–1	30	70	10	8.80	0	0.5	0.20
30%–2	30	70	10	7.50	1	1	0
30%–3	30	70	10	8.60	2	0	0.40
60%–1	60	40	10	7.60	0	1	0.40
60%–2	60	40	10	9.30	1	0	0.20
60%–3	60	40	10	8	2	0.5	0

.

2.3. Sample Preparation

The CPRAM sample preparation process includes four steps. First, the load weight and water consumption of the CPRAM specimens during compaction were determined based on considerations of the water-cement ratio, porosity, aggregate density, and cementitious materials density, adding quantitative water to the NA and RA, mixing them, and placing them into plastic bags for smothering. Second, mix a portion of the cementitious materials and water in the NA and RA mixtures. Third, pour the CPRAM mixture into a container and immediately compact and shape the CPRAM mixture using the gravity static compaction method, the loading rate set at 1 mm/min, and cease the process of loading once the CPRAM mixture has reached the designated volume capacity. Each CPRAM of mixture design types includes 42 cylindrical Φ 150 × h 150 mm samples and six beam specimens with dimensions of 100 × 100 × 400 mm for each mixture to measure the compressive and bending strengths. Fourth, de-mold the CPRAM sample after resting for one day and place it into plastic bags in the standard maintenance room.

2.4. Methods

2.4.1. Compressive Stress Test

The ultimate compressive stress of the CPRAM specimens is tested under unconfined compressive stress according to JTG E51-2009 [40], and the specimen size was Φ 150 × h 150 mm. The CPRAM cylindrical specimens cured for 7, 28, and 90 d were tested. Unconfined compressive strength tests were performed after soaking six groups of parallel samples in water for 24 h to determine the pavement material strength. The loading rate of the compressive stress test was set to 1 mm/min. The test results are calculated according to:

$$R_c=\frac{P}{A},$$

(1)

where R_c is the unconfined compressive strength (MPa), P is the axial pressure (kN), and A is the area of the top surface of the specification (m²).

2.4.2. Four-Point Bending (4PB) Stress Test

The 4PB stress tests measured the ultimate stress strain of CPRAM specimens according to the JTG E51-2009. The bending stress-strain was determined with a 100 × 100 × 400 mm specimen when cured for 90 d. The 4PB stress test was conducted after soaking six groups of parallel specimens in water for 24 h, using the MTS-Landmark test machine. The loading rate is set as 1 mm/min. The test results are calculated according to:

$$R_s=\frac{PL}{b^{2}h},$$

(2)

where R_s is the bending strength of the specimen (MPa), P is the ultimate stress (N) at the specimen failure time, L is the distance between two pivot points (mm), b is the specimen width, and h is the specimen height (Figure 3).

2.4.3. Water Permeability Test

The CPRAM cylindrical specimens with 7 and 28 d curing ages underwent water permeability testing, and the specimen size was Φ 150 × h 150 mm. The CPRAM sample was soaked the day before maintenance completion, and water permeability tests were conducted after 24 h of soaking. The water permeability test process is shown in Figure 4 [23], and the water permeability coefficient is calculated as:

$$k_T=\frac{L\times Q}{A\times h\times t},$$

(3)

where k_T, L, Q, A, h, and t indicate the water permeability of CPRAM specimens at a water temperature of T °C, height of the CPRAM specimens, overflow water in time t s, surface area of the upper CPRAM specimen surface, water level difference between the drainage outlet and overflow outlet, and time consumed by the overflow water [41,42,43].

2.4.4. Freeze-Thaw Cycle Test

The mechanical properties evolution of CPRAM specimens were analyzed by freeze-thaw cycle test according to JTG E51-2009. The test started after curing the CPRAM specimens for 28 d, six parallel specimens with each mix design were tested, and the specimen size was Φ 150 × h 150 mm. The temperature cycled between −18 and 20 °C. The freezing time was 16 h, and the thawing time was 8 h in each cycle, with a total of five cycles. The CPRAM specimens were soaked in water for 24 h after completing the five cycles. An unconfined compressive strength test was performed using a pavement material strength meter with a compressive stress loading rate of 1 mm/min. Three parallel specimens without freeze-thaw cycle tests were also tested as a comparison.

The test results are calculated as:

$$B=(1-\frac{R_{DC}}{R_C})\times 100\%$$

(4)

where B is the strength loss rate of the CPRAM specimens after freeze-thaw cycle tests (%), R_DC is the unconfined compressive strength of the CPRAM specimens after freeze-thaw cycle tests (MPa), and R_C is the unconfined compressive strength of comparison CPRAM specimens (MPa).

2.4.5. Dry-Wet Cycle Test

The dry-wet cycle tests analyze the mechanical properties of the CPRAM specimens. The CPRAM is usually subjected to a cyclic process of wetting by rainfall infiltration and dewatering via drying through services in the road subgrade. When CPRAM undergoes wet and dry cycles, its structure deforms slightly, and the mechanical properties change [44]. Here, the CPRAM specimens were soaked in tap water and dried at room temperature with water loss to simulate the dry-wet cycle process.

The dry-wet cycle tests began after curing the CPRAM specimens for 28 d, six parallel specimens for each design mix were tested, and the specimen size was Φ 150 × h 150 mm. The drying time was 12 h, and the wetting time was 12 h in each cycle, with a total of 15 cycles. The CPRAM specimens were soaked in water for 24 h after completing all 15 cycles. An unconfined compressive strength test was performed using a pavement material strength meter with a compressive stress loading rate of 1 mm/min. Three parallel specimens without the dry-wet cycles were tested as a comparison.

2.4.6. Compressive Resilience Modulus Test

Tests pertaining to the modulus of compressive resilience were executed in accordance with JTG E51-2009, aiming to scrutinize the capacity of CPRAM samples to withstand deformation across assorted levels of stress. The samples were soaked the day before the curing 90 days was completed, and compressive resilience modulus tests were conducted after 24 h of soaking. The loading stress for the dynamic compressive resilience modulus test was obtained by multiplying the 90 d unconfined compressive strength by the stress level factor. All six stress levels (0.1, 0.2, 0.3, 0.4, 0.5, and 0.6) were used in the tests according to the JTG E51-2009. The loading stress utilized in the test to assess the compressive resilience modulus is derived by multiplying a stress factor with the 90-day unconfined compressive strength of the CPRAM specimen. The stress loading waveform of the dynamic compressive resilience modulus test is the Haversine type, as shown in Figure 5b.

3. Test Results and Analysis

3.1. Unconfined Compressive Strength

The unconfined compressive strengths for the different CPRAM specimens are shown in Figure 6. The 7-d unconfined compressive strength for the nine CPRAM specimen types is greater than 3 MPa, and the 7-d unconfined compressive strength of the CPRAM specimens all reached 3.5 MPa except for the 60%-1 and 60%-3. The JTG/T F-20-2015 [34] shows that most CPRAM mixtures can be used as subgrade in heavy-traffic and first-grade highways. Therefore, the mechanical properties of CPRAM mixtures also meet the requirements of heavy-traffic highways when the RA ratio reaches 60%.

The correlation between the mechanical properties and cementitious types of CPRAM specimens is investigated. The influence regularity of each component was judged based on the range in the orthogonal experiments to analyze the law for each component of the cementitious material on the mechanical properties, as shown in

Table 5. Range analysis of the 7-d compressive strength orthogonal test.

Type	RA (%)	Blast Furnace Slag (%)	Fly Ash (%)	Silica Fume (%)	7-d Compressive Strength (MPa)
0%-1	0	0	0	0	3.87
0%-2	0	0.5	1	0.40	4.14
0%-3	0	1	2	0.20	3.92
30%-1	30	0	1	0.20	3.94
30%-2	30	0.5	2	0	4.14
30%-3	30	1	0	0.40	4.36
60%-1	60	0	2	0.40	3.20
60%-2	60	0.5	0	0.20	4.14
60%-3	60	1	1	0	3.19
K1	11.93	11.01	12.37	11.20
K2	12.44	12.42	11.27	12.00
K3	10.53	11.47	11.26	11.70
k1	3.98	3.67	4.12	3.73
k2	4.15	4.14	3.76	4.00
k3	3.51	3.82	3.75	3.90
Range	0.64	0.47	0.37	0.27

,

Table 6. Range analysis of the 28-day compressive strength orthogonal test.

Type	RA (%)	Blast Furnace Slag (%)	Fly Ash (%)	Silica Fume (%)	28-d Compressive Strength (MPa)
0%-1	0	0	0	0	4.55
0%-2	0	0.5	1	0.40	4.85
0%-3	0	1	2	0.20	4.95
30%-1	30	0	1	0.20	4.53
30%-2	30	0.5	2	0	4.49
30%-3	30	1	0	0.40	4.57
60%-1	60	0	2	0.40	3.56
60%-2	60	0.5	0	0.20	4.56
60%-3	60	1	1	0	3.65
K1	14.35	12.64	13.77	12.69
K2	13.59	13.99	13.03	14.13
K3	11.86	13.17	13.00	12.98
k1	4.78	4.21	4.59	4.22
k2	4.53	4.66	4.34	4.71
k3	3.95	4.39	4.33	4.33
Range	0.83	0.45	0.26	0.48

and

Table 7. Range analysis of the 90-day compressive strength orthogonal test.

Type	RA (%)	Blast Furnace Slag (%)	Fly Ash (%)	Silica Fume (%)	90-d Compressive Strength (MPa)
0%-1	0	0	0	0	5.31
0%-2	0	0.5	1	0.40	5.63
0%-3	0	1	2	0.20	5.88
30%-1	30	0	1	0.20	5.36
30%-2	30	0.5	2	0	5.33
30%-3	30	1	0	0.40	5.49
60%-1	60	0	2	0.40	5.02
60%-2	60	0.5	0	0.20	5.45
60%-3	60	1	1	0	5.05
K1	16.82	15.69	16.25	15.69
K2	16.18	16.41	16.04	16.69
K3	15.52	16.42	16.23	16.14
k1	5.61	5.23	5.42	5.23
k2	5.39	5.47	5.35	5.56
k3	5.17	5.47	5.41	5.38
Range	0.44	0.24	0.07	0.33

. In Table 5, the K value equals the sum of the strength test for each factor at one level. The k value equals K divided by the number of samples at one level for one factor. The Range value equals the maximum value of k minus the minimum value.

In Table 5, the order of the influence of each factor on the compressive strength is determined based on the Range value. A higher Range value indicates a more significant effect for a given factor on the compressive. The magnitude of the Range values of the four cementitious material factors are ranked as Range (RA) > Range (Fly ash) > Range (blast furnace slag) > Range (Silica fume). Therefore, the rank of the four cementitious material types indicates the 7-d unconfined compressive strength is most affected by the RA content of CPRAM specimens. The K values of the four cementitious material factors are ranked as K₂ (RA) > K₁ (RA) > K₃ (RA), K₂ (Fly ash) > K₃ (Fly ash) > K₁ (Fly ash), K₁ (blast furnace slag) > K₂ (blast furnace slag) > K₃ (blast furnace slag), and K₂ (Silica fume) > K₃ (Silica fume) > K₁ (Silica fume). This indicates that the greatest 7-d unconfined compressive strength values for CPRAM specimens were obtained when RA = 30%, Fly ash = 1%, blast furnace slag = 0%, and Silica fume = 0.4%.

In Table 6, the magnitude of the Range values for the four cementitious material factors are ranked as Range (RA) > Range (Silica fume) > Range (Fly ash) > Range (blast furnace slag). Therefore, the rank of the four cementitious material types indicates the 28-d unconfined compressive strength is most affected by the RA content of CPRAM specimens. The K values of the four cementitious material factors are ranked as K₁ (RA) > K₂ (RA) > K₃ (RA), K₂ (Fly ash) > K₃ (Fly ash) > K₁ (Fly ash), K₁ (blast furnace slag) > K₂ (blast furnace slag) > K₃ (blast furnace slag), and K₂ (Silica fume) > K₃ (Silica fume) > K₁ (Silica fume). This indicates that the greatest 28-d unconfined compressive strength values for CPRAM specimens are obtained when RA = 0%, Fly ash = 1%, blast furnace slag = 0%, and Silica fume = 0.4%.

In Table 7, the magnitude of the Range values of the four cementitious material factors are ranked as Range (RA) > Range (Silica fume) > Range (Fly ash) > Range (Blast furnace slag). Therefore, the 90 d unconfined compressive strength is most affected by the RA content of CPRAM specimens. The K values of the four cementitious material factors are ranked as K₁ (RA) > K₂ (RA) > K₃ (RA), K₃ (Fly ash) > K₂ (Fly ash) > K₁ (Fly ash), K₁ (blast furnace slag) > K₃ (blast furnace slag) > K₂ (Blast furnace slag), and K₂ (Silica fume) > K₃ (Silica fume) > K₁ (Silica fume). This indicates that the greatest 90-d unconfined compressive strength values of the CPRAM specimens were obtained when RA = 30%, Fly ash = 0.5%, blast furnace slag = 0%, and Silica fume = 0.4%.

The unconfined compressive strength test results indicate that the CPRAM specimen’s mechanical properties are correlated primarily with the RA content. The effects of the three cementitious materials on the mechanical properties are significantly related to the curing age. For low curing ages, Fly ash has the greatest impact on the CPRAM specimen strength, and silica fume has the least influence on the 7-d unconfined compressive strength. As the curing age increases, the effect of silica fume on the unconfined compressive strength becomes the most important, and the influence degree on the unconfined compressive strength exceeds that of the Fly ash and blast furnace slag. As a nanomaterial, silica fume effectively fills micropores at the interface between aggregates in CPRAM mixtures during curing [37]. The bonding strength between the aggregates is also significantly increased during curing, enhancing the mechanical properties of the CPRAM materials [45]. As listed in Table 3, the Ca²⁺ and Si⁴⁺ content is lower in the Fly ash than the blast furnace slag and silica fume, so the Fly ash might not be able to produce more Ca-based and Si-based high-strength hydrate gels to increase the strength compared to the blast furnace slag and silica fume, resulting in a weaker effect of Fly ash on the mechanical properties of CPRAM specimens than that of silica fume and blast furnace slag [46].

The Fly ash in cementitious materials did not enhance the compressive strength, and the early and late mechanical properties decreased with the Fly ash content. This is because increasing the amount of Fly ash reduces the amount of blast furnace slag, cement, and silica fume in the CPRAM mixture. The strength effect provided by the fly ash on the CPRAM specimens is lower than for the blast furnace slag, cement, and silica fume. Therefore, the curing effect of cementitious materials in the CPRAM mixture diminishes as the Fly ash content increases.

3.2. Bending Strength

The ultimate 4PB bending stress of CPRAM specimens is investigated, and the bending strength test results are shown in Figure 7. It is well known that the components of CPRAM mixtures are complex, so the mechanical properties are also highly dispersive. The data in Figure 7 indicates that the bending strength of CPRAM specimens containing RA is more dispersive than those without RA, and dispersion in the bending strength increases with the RA content. When the RA content of CPRAM specimens is 0%, the bending strength of the specimens ranges from 1.47 to 1.79 MPa, with a fluctuation of 9.8%. When the RA content is 30%, the bending strength of the specimens ranges from 1.28 to 1.67 MPa, with a fluctuation of 13.2%. Lastly, when the RA content is 60%, the bending strength of the specimens ranges from 0.92 to 1.72 MPa, with an upward and downward fluctuation of 30.3%. The average ultimate bending stress for CPRAM specimens decreases with the RA content. This is attributed to the large wood blocks, ceramic, old concrete, and red bricks in the RA. Additionally, there is variability in the RA constituents, causing more variable forms of aggregate composition in the CPRAM mixture.

Figure 8 shows the stress-strain curves for the nine CPRAM mixture types under the 4PB stress. The peak of the stress-strain curve for specimens with a 60% RA content decreased, and the up segments generally flattened compared to the 0 and 30% RA content CPRAM specimens. The stress-strain curve of the 0 and 30% groups became steeper, especially for the up segment. This confirms the RA content in the CPRAM mixture could significantly affect the strength and stiffness of CPRAM specimens.

3.3. Water Permeability

The water permeability and effective porosity of CPRAM specimens are investigated. As shown in Figure 9a, the water permeability of CPRAM specimens is greater than 0.5 mm/s, so the CPRAM reaches the permeable road requirements according to the CJJ/T135-2009 [47]. Figure 9b shows that the effective porosity of CPRAM specimens ranged from 20 to 26%, and the effective porosity decreased with the RA content. This is due to the apparent density of wood blocks, ceramic, old concrete, and red bricks in RA being smaller than the NA. Thus, the more construction solid waste a CPRAM mixture contains in the same quantity, the larger its volume will be. When the CPRAM mixtures are compacted to the same volume, the pore volume will decrease, as will the porosity. As the porosity of the specimen decreases, the continuity porosity also decreases, so the water permeability diminishes [41].

3.4. Compressive Strength after Freeze-Thaw Cycle Tests

The compressive strength is investigated after freeze-thaw testing for CPRAM specimens with different mixture designs. Figure 10 shows that the compressive strength after five freeze-thaw cycles decreases significantly compared with the control groups. The compressive strength loss rate increases with the RA content. This indicates that the RA weakens the freeze-thaw resistance properties of CPRAM specimens.

The wood blocks, ceramic, old concrete, and red bricks have more capillary pores compared with the NA (limestone crushed stone) [48,49,50]. During freeze-thaw cycle testing, water enters the capillary pores of the wood blocks, ceramics, old concrete, and red bricks. As the temperature decreases, water in the capillary pores condenses into ice, the material produces frost-heave deformation, and the capillary pores gradually fracture and break to form microcracks. The internal structure of the specimen is damaged, and the mechanical properties weaken. Therefore, materials with more capillary pores produce additional capillary pores during frost-heave deformation [51,52,53].

3.5. Compressive Strength after Dry-Wet Cycle Tests

The compressive strength is investigated after dry-wet testing for CPRAM specimens with different mixture designs. Figure 11 shows that the compressive strength after 15 dry-wet cycles did not change significantly compared with the control group. The compressive strength loss rate increased with the RA content. It is noted that the compressive strength loss rate of CPRAM specimens after dry-wet cycles is much lower compared with the freeze-thaw test. This indicates that the CPRAM specimens have good dry-wet resistance properties.

The pure dry-wet cycle has minimal impact on the mechanical properties of CPRAM specimens due to its small effect on the deformation [44]. It is noted that the aggregate type in CPRAM specimens significantly impacts its dry-wet resistance. For example, the mechanical properties of red bricks in RA are substantially weakened after soaking in water. Therefore, the compressive strength loss rate of CPRAM specimens decreases with the RA content after dry-wet cycle tests.

3.6. Dynamic Compressive Resilience Modulus

The dynamic compressive resilience modulus of CPRAM specimens with different mixture designs is investigated. Figure 12 shows that the compressive resilience modulus increased with the stress level, and the compressive resilience modulus increased with lower RA contents. Thus, the RA can weaken the deformation resistance of CPRAM specimens under loads. The factors affecting the deformation resistance of CPRAM specimens include the cementitious type. In the 0% RA content CPRAM specimens, the dynamic compressive resilience modulus is much lower than the CPRAM specimens containing other cementitious additives. This is because cementitious additives can fill the interface transition zone of aggregates, strengthening the mechanical properties of CPRAM specimens [37].

4. Discussion

CPRAM is a road subgrade material with environmental significance and versatility. These materials can achieve permeability and water storage functionality in roads; thus, using CPRAM materials in urban road subgrades can alleviate urban flooding. The compressive strength and water permeability of CPRAM specimens achieve the requirements for road subgrade use in China. The CPRAM specimens were made by designing various cementitious additive ratios. The effects of various cementitious additives on the mechanical properties of CPRAM were investigated, and = optimal ratios were proposed based on the experimental test results. However, this study neglected the effects of many factors on the mechanical properties and water permeability of CPRAM specimens. These include the effect of aggregate on the internal pore structure characteristics, and the effect of cementitious additives on pore characteristics in the aggregate interfacial transition zone. Many previous studies have concluded that the pore structure characteristics of road subgrade materials are closely related to mechanical properties. The aggregate component types of RA from CDW were very heterogeneous, which is a larger contributor to the very discrete mechanical properties and the complexity of the pore characteristics is the complexity of CPRAM specimens. Therefore, the pore structure characteristics of CPRAM specimens will be the focus of future studies.

It should be noted that CPRAM exhibits inferior mechanical properties compared to previous concrete. However, the utilization of a substantial quantity of RA from CDW in CPRAM not only reduces engineering costs but also serves environmental conservation purposes. Given the mechanical characteristics of CPRAM, its applicability is more suitable for urban roads with light traffic loads. Briefly, there are three primary causes attributed to the unfavorable mechanical characteristics of CPRAM. (a) CPRAM exhibits a heightened level of porosity, which compromises the mechanical properties of its skeletal structure. (b) The mechanical properties of the RA present in CPRAM are inferior to those of NA, which consequently leads to a decline in the overall mechanical properties of CPRAM. (c) While the utilizes cementitious ratio of previous concrete is 20%, CPRAM employs a cementitious ratio at 10% proportion. Therefore, employing physical and chemical techniques for enhancing the mechanical properties of RA without altering the porosity and cementitious ratio is the optimal approach to bolster the overall mechanical properties of CPRAM.

5. Conclusions

This paper designed CPRAM with nine types of cementitious additives based on orthogonal design theory and investigated the compressive strength, bending strength, water permeability, and compressive resilient modulus. Improved cementitious additives ratios are proposed, and the main conclusions are listed as follows:

(1)

The mechanical properties and water permeability of CPRAM mixed with large RA ratios can satisfy the road subgrade requirements of China. Incorporating RA weakened the mechanical properties and water permeability of CPRAM, and the weakening degree increased with the RA content.

(2)

The laboratory test results of CPRAM specimens with nine proportions of cementitious ratio show that the cementitious additives improve the mechanical properties of CPRAM. The effect of fly ash on the compressive strength of CPRAM is larger for low curing ages compared with silica fume and blast furnace slag, and the effect of silica fume on the compressive strength of CPRAM is larger for late curing ages compared with fly ash and silica fume.

(3)

CPRAM is poorly adaptable to low-temperature environments. The compressive strength of CPRAM specimens significantly reduces after freeze-thaw cycles, and the compressive strength reduction degree increases with the RA content. CPRAM is better adaptable to pure dry-wet environments as the compressive strength of CPRAM does not show a clear decreasing trend.

(4)

The incorporation of silica fume greatly strengthens the mechanical properties of CPRAM. That is, filling the micropores of the CPRAM interface transition zone improves the mechanical properties.