Permanent Deformation and Breakage Response of Recycled Concrete Aggregates under Cyclic Loading Subject to Moisture Change

Abstract

Natural granular materials widely used in building and infrastructure development consume a considerable number of natural resources. To avoid depleting natural granular materials, recycled concrete aggregates (RCA) from construction and demolition waste are now commonly used as an alternative. The mechanical behavior of RCA used in road construction is highly influenced by field conditions such as traffic load and moisture variation. The particle breakage of RCA influences long-term pavement performance because it changes the RCA grading over time. However, the effect of moisture content (m.c.) on mechanical behavior and particle breakage during compaction and cyclic loading is often neglected. The aims of the study were to investigate the mechanism of permanent deformation development due to moisture change, the breakage response of RCA in this process, and the assessment of existing permanent deformation prediction model. The results showed that initial m.c. effectively controlled the deformation of RCA; the permanent axial strain (ε_pa) was enhanced, and the breakage of coarse fraction (19~9.5 mm) under cyclic loading was reduced at higher levels of m.c. Based on the experimental results, a modified model for predicting ε_pa was proposed, incorporating a deviation factor induced by m.c. The model fitted the experimental data well, suggesting that it is useful to have a quantitative estimation of ε_pa of RCA with different m.c. under cyclic loading.

1. Introduction

The best option to manage the vast amounts of construction and demolition waste (CDW) generated by demolishing and renovating old infrastructure and natural disasters is reuse and recycling. With a rapid increase in CDW generation, CDW recycling and sound waste management, including appropriate recycling laws and legislation, are greatly needed all over the world [1]. The use of recycled materials as well as industrial by-products in construction and infrastructure development has a number of benefits, including reducing the use of natural resources and carbon dioxide emissions, as well as promoting sustainable development, environmentally friendly waste management, and urban greening.

Many previous studies have investigated CDW, scrap construction materials, and industrial by-products as road base and subbase materials. However, most of them were focused largely on the mechanical properties, hydraulic conductivity, and gas transport parameters [2,3,4]. Concrete waste is one of the key components generated by CDW, and the use of RCA in geotechnical engineering applications directly contributes to the increase in CDW recycling [5]. Many factors, such as the variety of material qualities produced by non-homogeneous sources and inconsistency and the mixing of other components (asphalt, bricks, and glass), however, affect the material qualities of recycled materials, indicating that the use of RCA as a granular material must be investigated carefully [6,7].

Residual or permanent deformation (PD) associated with granular materials is an indicator of failure [8,9,10,11,12,13,14]. PD is a critical indicator in choosing the required amount of pavement materials and their depths in road pavement design because it can be used to predict the rutting of roads. There has been limited research on the assessment of PD, particularly the impact of moisture content (m.c.) and breakage or the degradation of material during service life. The occurrence of particle breakage under dynamic loading is also a crucial factor to investigate when assessing the long-term durability of granular materials. The particle size distribution of granular materials varies due to particle breaking during the loading process, which has a substantial impact on mechanical and hydraulic properties. It was reported [15] that the rate of particle breakage increased in granular materials due to the angularity of grains and increased the confining pressure. Moreover, other factors such as particle size distribution, void ratio, and the shape and toughness of individual particles greatly affected the particle breakage of RCA. Till now, researchers have mainly focused on examining the properties of RCA and assessed its applicability as a road base and subbase material [16,17,18,19,20]. However, only a few studies have looked at the effects of the m.c. of RCA on mechanical behavior and particle breakage under cyclic loading, even though a road pavement system is exposed to variations of m.c. under field environmental conditions.

Keeping in view the previous research and the gaps observed, this study, therefore, aimed (i) to investigate the permanent deformation mechanism subject to moisture change, (ii) to assess the particle breakage characteristics of RCA with different levels of m.c. and to identify the particle size which most contributes to breakage content by incorporating a coloring technique for detailed breakage analysis, and (iii) to assess the existing permanent deformation model for RCA with varying levels of m.c.

2. Materials and Methods

2.1. Material

In this research, recycled concrete aggregate (RCA) was acquired from a recycling factory in Japan (Figure 1). The RCA had a specific gravity of 2.59. Whereas the absorption of particles larger than 2.36 mm was 5.9%, that of particles smaller than 2.36 mm was 15.7%. The sample was sieved and graded from 19.0 to 0.075 mm (Figure 2) based on particle size distribution (PSD) RM-30, as advised by the Japan Road Association [21].

2.2. Testing Methods

2.2.1. Preparation of Samples

In this study, the tested specimens for static and cyclic triaxial tests were compacted in the same manner as the compaction test to keep the compaction condition uniform in the tested specimens for laboratory tests. The samples were prepared at target moisture contents in three layers using the modified proctor energy. One of the aims of the study was to assess the breakage behavior of RCA. Therefore, the material for each layer was equally distributed based on the desired particle size distribution to ensure uniformity in each layer. This research focused on the behavior of RCA from the compaction stage to the cyclic loading stage. Therefore, the process adopted for each test is detailed below.

2.2.2. Compaction Test

The compaction test was conducted with a modified proctor energy of 2700 kJ/m³ [22,23]. The sample was compacted using a two-way split mold and a rammer, and the specifications of compaction are shown in

Table 1. Specifications of equipment used for the compaction of specimen.

Mold		Rammer
Height (cm)	15	Drop height (cm)	45
Diameter (cm)	7.5	Weight (kg)	4.5

. Figure 3 depicts the measured compaction curve. The tested RCA specimen had a maximum dry density of 1.88 g/cm³ and an optimal moisture content (OMC) of about 10%.

2.2.3. Static and Cyclic Triaxial Tests

Figure 4 shows a schematic illustration of the cyclic triaxial system used in the tests. The confining pressure was generated by pressurizing the air in the cell and spreading it through water, with a transducer attached to the cell monitoring it continuously. A double-action pressure cylinder installed on top of the system was used to apply cyclic stress. A wave generator and an electro-pneumatic (EP) transducer were used to convert electric signals into pressure. A linear variable differential transducer (LVDT) was mounted to the vertical shaft on top of the triaxial cell to detect the vertical deformation of the specimen. A low-capacity displacement transducer (LCDPT) was used to measure the volumetric deformation using the water level differences in the double cell. All the transducers including load cell and cell pressure were connected to a data logging system.

A strain-controlled static triaxial test was performed under a drained condition to measure the strength and deformation characteristics of tested RCA. The strength and deformation parameters of the RCA specimen were measured using a strain-controlled static triaxial test in a drained condition. A stress-controlled cyclic triaxial test, on the other hand, was carried out in a drained condition. The specimen was sealed in a rubber membrane with O-rings at the top and bottom before being placed on the pedestal of a triaxial cell for static and cyclic loading tests after compaction. The inner and outer cells were placed and filled with water to the required level. The specimen was consolidated by applying cell pressure (σ₃).

Table 2. Test conditions used for cyclic triaxial test.

Test Conditions
Compaction energy [kJ/m3]	2700
Moisture content [%]	5.4, 6.9, 9.5, 11.56, 15 (15% for compaction only)
Frequency [Hz]	0.2
Number of cycles [Nos.]	4000
Confining pressure [kPa]	40, 70, 100
Max. cyclic deviatoric stress [kPa]	450, 580, 850
Min. cyclic deviatoric stress [kPa]	45, 58, 85

shows the summary of test conditions in this study. A function generator was used to convert electric signals through an EP transducer corresponding to the selected maximum and minimum deviatoric stresses (q). The stress states utilized in this study, i.e., axial, and confining stresses, were chosen based on the usual load sustained by pavements, as proposed by previous researchers [24]. To ensure constant contact between the load assembly and the specimen, constant contact stress (10% of the maximum deviatoric stress) was applied. Then, 4000 load cycles (N = 4000) were applied under constant confining pressure. The tested specimen was carefully retrieved after completing the desired number of loading cycles, then oven-dried and sieved to evaluate the particle breakage.

2.2.4. Coloring Technique to Characterize Particle Breakage

To evaluate the particle breakage characteristics during compaction and cyclic loading, fluorescent synthetic resin paints were used to color the coarser portions of investigated materials ranging from 19 to 2.36 mm. As a result of the difficulties in coloring the finer fraction (<2.36 mm), this fraction was kept in its original color. The test flow from the coloring of particle grains to the particle tracking of colored grains after the test is illustrated in Figure 5. After sieving each particle size, particles that had shifted from one to another color scheme were identified by color and weighed. With this methodology, it was possible to observe the particle movement from one fraction size to the next size range, and the actual breakage or mass reduction (mass reduction/accumulation in %) can be calculated.

3. Results and Discussion

3.1. Particle Breakage after Compaction

The measured particle size distribution (PSD) curves before and after compaction are exemplified in Figure 6. Qualitatively, the PSD curves after compaction shifted with different tendencies from less to more depending on the initial moisture content (m.c.) of tested samples. In the samples with lower m.c., the shift after compaction was more comparable to that of higher m.c. samples. This behavior indicates that particle breakage decreased with the increase in m.c.

To examine the effect of m.c. on the overall particle breakage quantitatively, Marsal’s breakage index (B_g) was adopted in this study [25,26]. The B_g is a quantitative breakage index and is defined as the percentage by weight of the solid phase that has broken:

$$B_g=\Sigma (\Delta W_k>0)$$

(1)

$$\Delta W_k=W_{ki}-W_{kf}$$

(2)

where W_ki is the percentage of the sample weight retained before the test in each sieve size and W_kf is the percentage of the sample weight retained after the test in each sieve size. The calculated B_g values are shown as a function of m.c. in Figure 7. It can be observed that the B_g values decreased linearly with increasing m.c. This implies that direct and close contacts of particles enhanced the surface interparticle friction, resulting in more particle breakage due to the impact from particle to particle in drier conditions (lower m.c.). On the other hand, water between particles acts as a lubricant between them, and a cushion effect among particles lessened particle breakage more in a wetter condition (higher m.c.).

To track particle breakage directly, test results from the coloring technique are shown in Figure 8. In the figure, the sieved samples after compaction are divided into two fractions: a coarse fraction ranging from 19.0 mm to 2.36 mm (Figure 8a) and a finer fraction ranging from 2.36 mm to 0.075 mm (Figure 8b). The mass reduction or accumulation was calculated using the following equation in Equation (3):

$$Mass reduction/accumulation \left(\%\right)=[\frac{\left(Initial mass ofsize range – Final mass ofsize range\right)}{Initial mass ofsize range}]\times 100$$

(3)

Figure 8a clearly shows that coarser fractions such as 19.0, 9.5, and 4.75 mm were broken (i.e., mass reduction). It can be observed that there was no clear decreasing trend with the changing m.c. and that the particle size ranging from 9.5 to 4.75 mm contributed more to particle breakage in drier conditions (lower m.c.). The mass accumulation of finer particles (2.36~0.075 mm in Figure 8b), on the other hand, did not show considerable breakage, and the mass of fine particles increased due to the accumulation of broken particles from the larger sizes of coarse fractions. This behavior agrees with the findings of a previous study [27]. In addition, the mass accumulation of fine particles decreased with increasing m.c., meaning that water hampered the interparticle friction and reduced particle breakage.

3.2. Static Triaxial Test

Static triaxial tests were performed at three different confining pressures (σ₃ = 40, 70, and 100 kPa) prior to evaluating the mechanical behavior of RCA specimens during cyclic loading. The test results are shown in Figure 9. With increasing σ₃, the peak strength of the tested specimen increased and became approximately 1400 kPa at the deviation stress (q) of σ₃ = 100 kPa. The specimen failed after reaching over ε_a = 3.5% for the tested conditions at σ₃ = 70 and 100 kPa. The tested specimen at σ₃ = 40 kPa, on the other hand, showed more strain resistance and reached failure at q = 600 kPa soon after ε_a = 5%. The peak strength obtained from the static triaxial test was used to determine the maximum and minimum cyclic deviatoric stresses (σ_dmax and σ_dmin) for assessing the mechanical behavior of tested RCA during the cyclic triaxial tests.

The volumetric strain (ε_v) of investigated specimens first showed modest compression before becoming more dilative towards the end (Figure 9b). Aqil et al. [19] and Tatsuoka et al. [28] concluded that a compacted RCA specimen exhibited adequate strength in comparison to that of a graded gravelly soil. Kayani [29] reported that better inter-particle contacts between adjacent stiff aggregates increased the tangent stiffness and compressive strength. Further studies are needed to understand the effects of crushing a mortar layer surrounding the core particles and inter-particle contact points on the mechanical properties of RCA during triaxial compression loading.

3.3. Cyclic Triaxial Test

RCA specimens with varying m.c. were used in cyclic triaxial tests at N = 4000 to determine long-term performance and particle breakage characteristics. Figure 10 shows the maximum and minimum deviatoric stresses (σ_dmax and σ_dmin) applied for the cyclic triaxial tests. The σ_dmax was taken as about 65% of the maximum strength obtained in the static triaxial test [24], and 10% of the maximum deviatoric stress was taken as σ_dmin to ensure constant contact between the loading rod, top cap, and tested specimen. The illustration in Figure 11 shows the explanation of strain development during cyclic loading. In this study, the permanent strain was calculated at the unloading stage of each designated loading cycle. According to a given number of loading cycles up to N = 4000, the accumulated permanent axial and volumetric strains (ε_pa and ε_pv) of the specimen were measured at three confining pressures (σ₃) of 40, 70, and 100 kPa.

3.3.1. Permanent and Volumetric Axial Strains

The measured ε_pa and ε_pv as a function of N at σ₃ = 40, 70, and 100 kPa are shown in Figure 12. Higher ε_pa values (Figure 12a,b,e) were observed for the tested specimens with higher m.c. The ε_pa-N curves, and therefore, the plastic response, tended to stabilize (plateau) with repeating N (at the end). In particular, the tested specimens with lower m.c. (5.4% and 6.9%) gave lower ε_pa compared to the specimens with higher m.c. (9.5% and 11.56%). This behavior illustrates the state of RCA energy absorption in each stress–strain loop, as well as its ability to withstand minor permanent deformations at lower m.c. The stability of ε_pa with the increasing number of loading cycles in an RCA sample shows that the plastic shakedown limit has been reached [30]. At lower m.c., the ε_pa value of RCA sample suggests that the plastic limit has been reached.

In addition, all tested RCA in this study exhibited a resistance to collapse, while the ε_pa increased with m.c. under cyclic loading. This demonstrates that the tested RCA has a load-bearing capacity equivalent to a high-quality granular pavement material in accordance with other conventional pavement materials [31].

The ε_pv-N relations were also dependent on the m.c. of tested RCA (Figure 12b,d,e). The ε_pv of the specimen with m.c. = 11.6% (over OMC) showed generally dilative behavior under cyclic loading up to N = 4000. For the tested specimens with lower m.c. (5.4%, 6.9%, and 9.5%), on the other hand, the ε_pv curves portrayed a compressive nature. The results might be attributed to the inter-particle contacts of aggregates at different m.c. conditions. For the specimen at higher m.c., it is supposed that the water escapes during drainage along with the settlement, increasing the interparticle contact and contributing to its dilative nature. For the specimen at lower m.c., on the other hand, the material matrix still has space for particle readjustment under cyclic loading, causing compressive behavior.

To clarify the effects of m.c. on ε_pa, the measured ε_pa values at N = 4000 were plotted as a function of m.c. and are shown in Figure 13. The ε_pa increased nonlinearly with increasing m.c. at each σ₃, and a large increment in ε_pa can be found at an m.c. close to OMC (~10%). This suggests that the m.c. had a significant effect on the accumulation of permanent deformation under cyclic loading, consequently affecting the softening of RCA.

3.3.2. Permanent Deformation Model

To express measured permanent axial strains from cyclic loading tests, a permanent deformation (PD) model by [32] was adopted:

ε_pa = a(σ_d/p_o) bN_c

(4)

where ε_pa is the permanent axial strain, σ_d is the axial deviatoric stress, p_o is the normalizing stress (taken as 1 psi or 1 kPa), N is the number of loading cycles, and a, b, and c are model parameters obtained by a regression analysis of experimental data in this study. The PD model in Equation (4) was chosen because it incorporates the effect of applied stress compared to other predictive models [33], and the model can consider the effect of N regardless of the magnitude of loading.

The original PD model in Equation (4) reveals the effects of applied stress, but N did not consider the variation of the m.c. of tested specimens. Thus, the model did not predict well and deviated from the measured data. In this study, therefore, we newly introduced a deviation factor (R_w) that incorporates the effect of the m.c. of tested specimens, as given in the following expression:

R_w = me^αw

(5)

where m and α are the regression parameters from experimental data, and w is the moisture content as a percentage (= m.c.). Incorporating Equation (5) into the original model in Equation (4), a modified PD model equation can be expressed as:

ε_pa = a(σ_d/p_o) ^bN^cR_w

(6)

Note that a, b, and c in Equation (6) are the model parameters in the original model in Equation (5) (a = 0.28, b = 0.2, c = 0.095).

The estimation of R_w in Equation (5) is shown in Figure 14. As shown in the figure, Equation (6) captured the effect of the m.c. of tested specimens in this study well, and the values of regression parameters, m and α, became 0.28 and 0.16, respectively. The performance of the modified PD model in Equation (6) was compared to experimental data. The prediction of existing model equation and its comparison with the modified equation is shown in Figure 15. The capability of the existing model is to predict similar trends at all moisture contents. However, the modified PD model showed reasonable agreement with the experimental data at different σ₃ conditions, suggesting it would be useful to have a quantitative estimation of the ε_pa of RCA with different m.c. values under cyclic loading. Conversely, further research is recommended to verify the applicability of the modified equation due to the considerable variability of recycled and natural materials.

3.3.3. Particle Breakage after Cyclic Loading

The calculated B_g values (Equation (1)) after cyclic loading tests are shown as a function of m.c. in Figure 16. In the figure, a linear regression of the B_g-m.c. relationship at the compaction stage of sample preparation is also given. It can be observed that the B_g values decreased with increasing σ₃ and that the B_g values became higher at lower m.c., compared to those at higher m.c. at > 9.5% (close to OMC = 10%). The results seem to be in accordance with a previous study by [34]. The study indicated that the elevated σ₃ decreased/weakened particle breakage and that less particle breakage (degradation) existed at the optimum degradation zone. As well as the tested results of particle breakage from compaction tests (Figure 7), the existing moisture is assumed to have created a cushion effect and/or a sliding surface surrounding the particles, resulting in small particle breakage of the tested RCA. Comparing the B_g values after cyclic loading to those at the compaction stage, it is interesting that there was no significant difference between the tested specimens with m.c. = 5.4% and 6.9% at σ₃ = 100 kPa. This means no further particle breakage occurred during cyclic loading. The difference, on the other hand, became large for the specimen tested at σ₃ = 40 kPa throughout the whole m.c. range, indicating that particle breakage was enhanced during cyclic loading.

The calculated mass reduction/accumulation for tested RCA with different m.c. are shown in Figure 17. Like the results from compaction tests (Figure 8), the mass reductions of coarser fractions (19.0~9.5, 9.5~4.75, and 4.75~2.36 mm) were observed under cyclic loading, and the reductions were in the approximate range of 10~25%. In particular, the coarser fraction of 9.5~4.75 mm had a higher mass reduction compared to other fractions at all other σ₃ (Figure 17a–c). The effect of m.c. on particle breakage of coarse fractions was not clearly shown. The mass accumulation of the finer fraction of 2.36~0.075 mm, on the other hand, increased by approximately 10~20% (Figure 17d) and did not show any significant dependence on m.c.

4. Conclusions

This study examined the long-term durability of RCA as a road base and subbase material from the viewpoints of permanent deformation under cyclic loading and particle breakage characteristics. The main findings of this study led to the following conclusions.

(1)

The compaction moisture content has a significant effect on particle breakage. The results of both Marsal’s breakage ratio and mass reduction/accumulation showed that the particle breakage of RCA decreased with increasing moisture content.

(2)

The coloring technique introduced in this study showed that a coarse fraction of 9.5~4.75 mm was more breakable than the other fractions, mainly resulting in mass accumulations of fine fractions, suggesting a weak link in the particle size range, which needs to be adjusted.

(3)

The results from cyclic loading tests indicated that the moisture content of tested specimens had a considerable effect on the accumulation of permanent strain, and the increase in moisture content of tested specimens caused a decrease in stiffness, and consequently, an increase in permanent deformation in pavements.

(4)

Overall, the breakage and permanent deformation show opposite trends to each other.

(5)

Based on the experimental results, a model for evaluating a permanent axial strain was amended by newly incorporating it into a deviation factor induced by moisture content. The model fitted the experimental data well, suggesting that it would be useful to have a quantitative estimation of the permanent axial strain of RCA under cyclic loading.