Effect of Rubber Aggregates on Early-Age Mechanical Properties and Deformation Behaviors of Cement Mortar

Abstract

Rubberized cement-based materials are widely utilized because of their good ductility, impact resistance, and fatigue resistance. This research investigated the effect of the rubber aggregates content, particle size of rubber aggregates, and water–cement ratio on the early-age mechanical properties and deformation behaviors of mortar through laboratory tests, and strength reduction coefficient fitting models were established according to the testing results. The results show that the compressive strength growth rate of cement mortar is about 15% slower than that of flexural strength. The existence of rubber aggregates lowers the strength increase rate of mortar. The reduction coefficient of strength decreases with increasing rubber aggregates content and increases with the age of mortar. Increasing rubber aggregates content and decreasing particle size of rubber aggregate can lower the autogenous shrinkage in the initial stage, but the autogenous shrinkage of the later stage increases as the rubber aggregates content increases, with a turning point between 30 h and 50 h. After 3 days, the dry shrinkage of mortar accounts for about 70–80% of the total shrinkage, and it increases with higher rubber aggregate content, smaller particle size of rubber aggregates, and higher water–cement ratios.

1. Introduction

In recent years, on account of the rapid expansion of the automotive industry, the global annual production of waste tires has exceeded 200 million tons, of which China’s waste tire output accounts for more than 30% [1]. Waste tires are non-biodegradable waste. If only landfilled, they can cause significant harm to the environment and natural resources. Additionally, these wastes are highly sensitive to temperature, so the accumulation of large quantities of waste tires heightens the risk of spontaneous combustion, seriously impacting the environmental safety and health, and facilitating the spread of pests and diseases. Therefore, the treatment and recycling of waste tires have become an important environmental protection industry, with the market value expected to exceed USD 9 billion by 2025 [2,3]. How to efficiently and economically apply these tire waste materials has become a major issue facing the world [4,5].

In response to the above issues, civil engineering practitioners have proposed an effective solution. Existing research indicated that rubberized cement-based materials can be formed by adding rubber aggregates from waste tires into cement-based materials as aggregates to replace all or part of the aggregates [6]. This approach not only protects natural resources and reduces the environmental risks associated with waste tires, but also realizes the effective reuse of resources, offering good economic benefits. Meanwhile, due to the excellent elasticity, toughness, seismic resistance, and damping properties, the addition of rubber aggregates effectively enhances the deformability, impact resistance, fatigue resistance, and freeze resistance of cement-based materials [7,8,9]. Moreover, rubber aggregates can absorb various stresses generated within the cement-based materials, inhibit concrete shrinkage deformation, and prevent or slow down cracking caused by microcracks or shrinkage. Of course, the addition of rubber aggregates also produces some negative impacts, such as the formation of relatively weak interfacial transition zones in the cement-based materials, thereby diminishing their strength [10,11,12]. However, with ongoing research, measures such as surface modification or pretreatment of rubber particles have effectively addressed these negative impacts, expanding the potential application of rubberized cement-based materials in civil engineering [13,14,15,16].

At present, research on the rubberized cement-based materials strength has reached a generally consistent conclusion: as the rubber aggregates content increases, the concrete strength significantly decreases, but the toughness increases. However, conclusions on the influences of the rubber aggregates particle size on strength vary. Topcu [17] concluded that coarse rubber particles decreased the mechanical strengths of concrete more than fine rubber particles, while Cao [18] held the opposite view. Sukontasukku [19] found that concrete with graded rubber particles had better compressive strength. In terms of deformation, Kaloush [20] indicated that the temperature shrinkage coefficient of rubberized cement concrete decreased by 30% with a fourfold increase in rubber aggregates content. Zhou [21] pointed out that rubber aggregates could obviously increase the shrinkage deformation of concrete. However, Raghvan [22] clarified that a 5% rubber particle content could reduce the plastic shrinkage of mortar, thus decreasing the crack width from 0.9 mm in the control specimen to 0.14–0.6 mm. In addition, based on the existing research results, the researchers deeply analyzed the relationship between rubber aggregates’ content and material strength. Liu [23] determined the correlations between the strengths of rubberized cement concrete and those of the control group through the analysis of experimental data. Kang [24] and Long [25] pointed out that the strength reduction coefficient of rubber roller-compacted concrete has a good linear relationship with the rubber aggregate content, which could be obtained by a linear fitting method. However, some researchers believed that this relationship should be fitted non-linearly. Khatib [26] and Ghaly [27] proposed that the compressive strength reduction coefficient of rubberized mortar (RM) should be a polynomial. Overall, the current research on rubberized cement-based materials mainly focuses on the service life performance, with less emphasis on early-age performance.

However, the early-age performance of cement-based materials cannot be ignored. The early age of cement-based materials is the key period for the formation of performance, which will influence the development trends of later performance [28,29,30]. The early-age performance evolution of cement-based materials is closely related to the hydration process, but the development rates of various properties differ, which are primarily influenced by the mix proportion, water–cement ratio, age, etc. It was also reported that early-age mechanical properties were influenced by curing conditions and additives; air curing conditions impeded the strength gain of ordinary concrete, while elevated-temperature curing conditions improved the early-age strength, and the greater impeding effect was observed in fly ash concrete [31]. Some researchers believed that in the early age, the tensile strength of concrete increased faster than that of compressive strength [32]; conversely, other studies found that the tensile strength of concrete increased more slowly [33]. In the early-age stage, cement-based materials undergo not only the development of mechanical properties but changes in volume, primarily including autogenous shrinkage, drying shrinkage, and thermal deformation. The autogenous shrinkage of traditional Portland cement concrete generally ranges from 40 × 10⁻⁶ to 100 × 10⁻⁶, equivalent to a temperature decrease of 4 to 10 °C. It is significantly impacted by the fineness and chemical properties of cement. The drying shrinkage of ordinary Portland cement concrete ranges from 200 × 10⁻⁶ to 1000 × 10⁻⁶, approximately 10 times that of autogenous shrinkage [34,35]. The amount of aggregate is a crucial factor influencing shrinkage potential. Increasing the amount of aggregate under the same water–cement ratio can reduce the shrinkage of concrete. Thermal deformation is the dimensional change caused by concrete temperature variations [36,37]. According to the thermodynamic properties of materials, the magnitude of concrete temperature deformation also depends on the thermal expansion coefficient of concrete. Zahabizadeh [38] found that concrete had a high initial coefficient of thermal expansion, which then gradually declined to a local minimum. After the minimum value, the coefficient of thermal expansion remains basically unchanged or increased slightly.

Compared to ordinary cement-based materials, the presence of a large proportion of rubber aggregates alters the mix proportion, aggregate composition, and microstructure of cement-based materials, inevitably affecting its properties [39,40]. Especially for the early age, this effect is particularly obvious. However, the current research on rubber cement-based materials mainly focuses on the service period, and there are few studies on the evolution of properties in early age. The early-age performance of rubber cement-based materials not only affects the service life performance, but also serves as the primary basis for controlling non-load-bearing cracks during the early curing period, determining the technical parameters for the construction of rubber cement-based materials. Therefore, it is necessary to study the evolution patterns of the early-age mechanical performance and deformation behavior of RM and to conduct a thorough analysis of the impact of rubber aggregates on the reduction in early-age mechanical properties of mortar.

This paper investigated the impact of rubber aggregate content, particle size of rubber aggregates, and water–cement ratio on the early-age performance of RM through laboratory experiments. Meanwhile, based on the existing models, the relationships between the strength reduction coefficients and rubber aggregate content at different ages were established. Through this research, it can effectively reveal the development tends of the early-age behaviors of RM, and provide support for predicting the early-age mechanical properties of RM. At the same time, those findings can provide a basis for the application of RM and the further research of rubberized cement concrete.

2. Raw Materials and Testing Methods

2.1. Raw Materials and Mixing Proportion

P.I 42.5 Portland cement produced by Fushun Cement Co. Ltd. in Fushun City, China, was used as the cementitious material. Its properties meet the specification of GBT8076-2008 [41] and its chemical composition is listed in

Table 1. Chemical composition of P.I 42.5 Portland cement.

SiO2	CaO	Al2O3	Fe2O3	SO3	MgO	f-Ca	Na2Oeq	Loss of Ignition	Cl
20.58	63.32	5.03	3.38	2.06	2.01	0.68	0.55	1.76	0.018

. The surface-modified rubber aggregates, produced by Sichuan Jinmoer Environmental New Materials Co. Ltd. in Chengdu City, China, were employed. The technical parameters of the rubber aggregates are presented in

Table 2. Technical parameters of rubber aggregate.

Properties	Specification
Organic matter (%)	67.21
Inorganic content (%)	32.79
Contact angle (°)	0

. To investigate the impact of rubber aggregate particle size, rubber aggregates with particle sizes of 20 mesh, 50 mesh, and 100 mesh were selected. The specific particle size distributions are shown in the Figure 1. River sand served as the fine aggregate, whose technical parameters are shown in

Table 3. Technical parameters of river sand.

Properties	Specification	Standard
Fineness modulus	2.9	JTG 3432-2024 [42]
Bulk density	1439 kg/m3	JTG 3432-2024 [42]
Apparent density	2580 kg/m3	JTG 3432-2024 [42]
Mud content (by mass)	1.78%	JTG 3432-2024 [42]

.

Ordinary mortar (OM) was used as control group in this study, whose mixing proportion was water: cement: sand = 1: 2.5: 3.98. The RM was prepared by incorporating rubber aggregate as a volume-equivalent replacement for sand. According to the research objectives, the experiment was designed using the control variable method. To investigate the impact of rubber aggregate content, rubber aggregate content levels were set at 10%, 20%, 30%, 40%, 50%, and 60%; to investigate the impact of rubber aggregate particle size, rubber aggregates with the particle size of 20, 50, and 100 mesh were used; to investigate the impact of water–cement ratio, water–cement ratios were set at 0.38, 0.40, and 0.42. The specific mixing proportions of cement mortar are detailed in

Table 4. Mixing proportions of cement mortar.

Test Number	Water-Cement Ratio	Cement	Water	Sand	Rubber Aggregate	Rubber Aggregate Type
OM	0.40	1.00	0.40	1.59	0.00	B
RM-10	0.40	1.00	0.40	1.43	0.07	B
RM-20	0.40	1.00	0.40	1.27	0.14	B
RM-30	0.40	1.00	0.40	1.11	0.20	B
RM-40 *	0.40	1.00	0.40	0.96	0.27	B
RM-50	0.40	1.00	0.40	0.80	0.34	B
RM-60	0.40	1.00	0.40	0.64	0.41	B
RM-0.38	0.38	1.00	0.38	0.94	0.27	B
RM-0.42	0.42	1.00	0.42	0.97	0.27	B
RM-20 Mesh	0.40	1.00	0.40	0.96	0.27	A
RM-100 Mesh	0.40	1.00	0.40	0.96	0.27	C

* RM-40 was also recorded as RM-0.40 and RM-50 Mesh.

.

2.2. Test Method

2.2.1. Specimen Preparation

Based on the requirement of GB/T 17671-2021 [43], the mortar was mixed using a cement mortar mixer. First, the measured water and cement were added to the mixing bowl and stirred at low speed (self-rotation 140 r/min, revolution 62 r/min) for 30 s. Then, the river sand and rubber aggregate were added and stirred at high speed (self-rotation 285 r/min, revolution 125 r/min) for 30 s. Afterward, the mixing was stopped for 90 s, and the mortars adhering to the bowl wall and blades were scraped back into the bowl. The mixer was then restarted for an additional 60 s of high-speed mixing to form RM or OM.

For the mechanical property tests, according to GB/T 17671-2021 [43], detachable steel molds were used to form prismatic specimens with cross-sections of 40 mm × 40 mm × 160 mm. During molding, the air temperature was maintained at 20 °C ± 2 °C, with a relative humidity greater than 50%. The vibration table was operated for 120 s ± 5 s. For cement mortar specimens at an age of 1 d, demolding was performed within 20 min of testing, and then the demolded specimens were kept covered with a wet cloth until the time of testing. For mortar specimens aged more than 1 d, demolding was performed within 20 to 24 h of molding, and the demolded specimens were cured in a water tank at 20 ± 1 °C until the specified ages (3, 7, 14, and 28 d) for testing.

For the deformation behavior tests, prismatic specimens of 40 mm × 40 mm × 160 mm were used, as required by JGJ/T70-2009 [44]. Copper pins were placed at both ends of the specimen, with the pins extending 8 ± 1 mm from the specimen ends. The specimens were cast in molds under standard conditions of 20 ± 2 °C air temperature and 60% ± 5% relative humidity. After casting, the specimens were cured under the same conditions for 12 h before demolding.

2.2.2. Mechanical Properties

Based on the requirement of GB/T 17671-2021 [43], the flexural strength of the cement mortar was obtained by conducting flexural tests on the prismatic specimens of 40 mm × 40 mm × 160 mm. Subsequently, the compressive strength of the cement mortar was obtained using the compressive tests on the specimens from the flexural strength tests, as presented in Figure 2. The flexural and compressive strengths of mortar were tested by a fully automatic strength testing machine, whose loading rates were 50 ± 10 N/s and 2400 ± 200 N/s, respectively. The test results of flexural strength and compressive strength were the average of 3 and 6 parallel specimens, respectively, with accuracy to 0.1 MPa. To clearly characterize the evolution of the early-age mechanical properties, the proportion of strength was proposed. It was defined as the ratio of the strength increment of different ages to the 28-day strength, and was calculated according to Equation (1):

$$\begin{array}{cc}{K}_t={\displaystyle \frac{f_t-f_{t-1}}{f_{28}}}\times 100\%& (1\le t\le 28)\end{array}$$

(1)

where K_t is the strength proportion at t age; f_t is the strength at age t, Mpa; f₀ = 0; and t is the age of cement mortar, which is 1, 3, 7, 14 to 28 d.

2.2.3. Deformation Behaviors

According to the requirements specified in JGJ/T70-2009 [44], prismatic specimens with copper pins at both ends were used for the deformation behavior test. The specimens were first cured for 12 h after molding. Then, the specimens were installed on a vertical mortar shrinkage instrument for shrinkage test, with a 20 ± 2 °C air temperature and 60% ± 5% relative humidity. For autogenous shrinkage tests, the specimens were sealed with a plastic membrane immediately after demolding, while the specimens for total shrinkage tests were not sealed. The deformation of mortar was monitored by a digital dial gage, as shown in Figure 3. In detail, after demolding, the specimen was installed on the mortar shrinkage instrument, and then rotated and adjusted to stabilize the digital micrometer reading. The digital micrometer was then set to zero as the start time of the test, and the micrometer reading was recorded every 2 h thereafter. The shrinkage of the mortar was calculated according to Equation (2), and the mean result from three parallel specimens was reported, with an accuracy of 10 × 10⁻⁶.

$$\epsilon =-{\displaystyle \frac{L_t}{L-L_d}}$$

(2)

where ε is the shrinkage of mortar; L_t is the reading of the dial gage, mm; L is the length of the specimen, mm; and L_d is the length of two copper pins embedded in the mortar specimen, mm.

3. Results and Discussion

3.1. Early-Age Mechanical Properties

3.1.1. Flexural Strength

(1)

Effect of rubber aggregate content

Figure 4 presents the evolution laws of early-age flexural strengths of cement mortar with varying contents of rubber aggregates and the flexural strengths proportions at varying ages. The data indicate that adding rubber aggregates into cement mortar can reduce its flexural strength, but the extent of this reduction gradually slows down as the rubber aggregate content increases. From the evolution law of flexural strength, it is evident that the increase in mortar flexural strength mainly occurs before 7 d. This is because the early hydration of cement is relatively fast. At 7 days, a high degree of hydration has been reached, and a relatively complete bond structure has formed between aggregates and cement stones. The 3-day flexural strength can reach more than 70% of that at 28 days, and the flexural strength at 7 days can exceed 85% of that at 28 days. A comparison of testing results between RM and OM demonstrates that the flexural strength of OM increases fast during the first 3 days. In detail, at 1 day and 3 days, the flexural strengths reach 52% and 90% of the 28-day strength, respectively, after which the strength increase significantly slows down. The existence of rubber aggregates not only significantly reduces the mortar strength but also reduces the growth rate of flexural strength during the first 3 days. This trend becomes more obvious with higher rubber aggregate content. For example, for RM-60, its 1-day and 3-day flexural strengths are 31.67% and 55.19% of those of OM, respectively. At the same time, 1-day and 3-day flexural strengths of RM-60 are only 25% and 75% of that of RM-60 at 28 days, which are 27% and 15% lower than those of OM, respectively.

(2)

Effect of rubber aggregates particle size

Figure 5 presents the evolution trends of early-age flexural strengths with varying particle sizes of rubber aggregates and the flexural strength proportions with varying ages. The data indicate that the flexural strength decreases with the decrease in the particle size of rubber aggregates. This is because the smaller rubber aggregates have larger specific surface areas, leading to more weak rubber–cement stone interfaces. This is analogous to the reduction in flexural strength caused by increased rubber aggregate content, as both result in a greater number of weak interface transition zones. Specifically, the decrease in particle size is mainly manifested in the increase in particle size distributions between 75 and 200 μm. The filling effect of these particles on the influence of mortar strength is much smaller than the weak interface effect caused by the increase in specific surface area. Regarding the proportion of flexural strength at different ages, it can be evident that at 1 day, the proportion of flexural strength of RM-100 Mesh is only 30%, whereas the one of RM-20 Mesh rubber is 47%. At 7 days, the proportion of flexural strength of RM-100 Mesh reaches only 83%, while the ones of RM-50 Mesh and RM-20 Mesh exceed 91%. This indicates that reducing particle size of rubber aggregates can slow down the development rate of flexural strength.

(3)

Effect of water–cement ratio

Figure 6 displays the evolution laws of early-age flexural strengths with varying water–cement ratios and the flexural strength proportions with different ages. As presented in Figure 6, increasing the water–cement ratio can reduce the flexural strength of RM. Regarding the proportion of flexural strength, it is evident that, unlike the effects of particle size and content of rubber aggregate, changes in the water–cement ratio have minimal impact on strength growth during the first 3 days.

3.1.2. Compressive Strength

(1)

Effect of rubber aggregate content

Figure 7 presents the impact of rubber aggregate content on compressive strength of mortar. It exhibits that the evolution trends of compressive strength of mortar differ from those of flexural strength. At 1 day and 3 days, the compressive strengths of mortar only reach about 30% and 70% of that at 28 days, which is approximately 20% lower than the proportion of flexural strength. The existence of rubber aggregates will reduce the compressive strength of the mortar. As the rubber aggregate content increases, this rate of decrease gradually diminishes. For example, the 1-day and 3-day compressive strengths of RM-60 are only 24.57% and 36.01% of those of OM, respectively. From the proportions of compressive strength, it is evident that the 1-day strength is only 35% of that at 28 days, and the effect of rubber aggregate content is minimal. However, between the ages of 1 day and 3 days, the strength development accelerates with increasing rubber aggregate content. After 7 d, the overall growth in compressive strength gradually diminishes with the increase in the rubber aggregates content. At 60% rubber aggregate content, the compressive strength increased little after 7 days, with a strength ratio of only 10%. This is because the addition of excessive rubber aggregates leads to the introduction of a good deal of pores. Meanwhile, the hydration of cement slows down after 7 days, and the strength growth formed by hydration is much weaker than the influence of pores, so the strength increase is very slow after 7 days.

(2)

Effect of rubber aggregate particle size

Figure 8 displays the impact of rubber aggregate particle size on mortar compressive strength. The data indicate that the compressive strength reduces as the particle size of rubber aggregates reduces. This is because smaller rubber particles introduce more additional pores, reducing the effective compressive area and increasing the weak interface zones, which is similar to the effect of increased rubber content. However, after 7 days, the RM with a smaller rubber aggregate particle size shows a greater increase in compressive strength compared to RM with higher rubber aggregate content. This suggests that finer rubber aggregates, despite introducing weak interfaces, also contribute a filling effect. This effect is more pronounced in compressive strength than in flexural strength. From the strength proportion, it can be evident that the proportion of compressive strength of RM-100 Mesh is only 25.59%, while the one of RM-20 Mesh reaches 37.51%. The proportion of compressive strength at 7 days can reach around 85%. This indicates that the different particle sizes have a minimal impact on the development of compressive strength, showing similar development rates.

(3)

Effect of water–cement ratio

Figure 9 presents the impact of the water–cement ratio on the compressive strength of mortar. As depicted in the figure, growth in the water–cement ratio will reduce the early-age compressive strength of RM. Additionally, it slows down the development of compressive strength in mortar before 3 days, but results in gradual increases after 7 d. According to the proportion of compressive strength, at a water–cement ratio of 0.42, the compressive strength proportion at 3 days is only 59%, while it is 74% for a water–cement ratio of 0.38. After 7 days, the compressive strength of RM-0.42 can still increase by 30%, whereas that of RM-0.38 can only increase by 22%.

3.1.3. Influence of Rubber Aggregate Content on Flexural Strength Reduction

In line with the existing research on the strength reduction factor of rubberized cement-based materials and combining the flexural strengths of RM and OM at different ages, it was found that the Khatib’s multiple polynomial model better reflected the influence of the rubber aggregate content on the flexural strength reduction factor, as shown in Equation (3). Khatib also pointed out that the exponent m in the equation reflects the degree of downward curvature of the curve. A larger m value indicates greater sensitivity of the mortar strength reduction. The fitting parameter b represents the impact of rubber aggregate content on the strength reduction rate. A larger b value indicates a greater reduction rate of the mortar strength as the rubber aggregate content increases. The parameter a is the intercept, representing the predicted strength reduction factor of RM-100 (100% rubber concrete) based on the experimental data [26].

$$R_c=a+b{(1-\phi )}^m$$

(3)

where R_c is the strength reduction coefficient; a, b, and m are fitting coefficients, and a + b + m = 1; φ is the rubber aggregate content, which ranges from 0 to 100%.

Figure 10a shows the influence of rubber aggregate content on the flexural strength reduction coefficient at different ages, and Figure 10b~f show the fitting relationships between the flexural strength reduction coefficients of mortars at different ages and the rubber aggregates content based on Khatib’s model. The fitting parameters are listed in

Table 5. Fitting parameters of flexural strength reduction coefficient and rubber aggregate content at different ages.

	Age (d)	1	3	7	14	28
Parameters
a		0.1921	0.5661	0.6233	0.6387	0.6589
b		0.8079	0.4339	0.3767	0.3613	0.3411
m		1.8001	5.7608	5.7050	5.1958	4.9330

. The data demonstrate that the flexural strength reduction coefficient of RM decreases and increases as the rubber aggregates content and age increase, respectively. Moreover, as the content of rubber aggregates increases, the influence of age on the flexural strength reduction coefficient is more obvious. According to the fitting relationship, as the age increases, the b value gradually decreases, the a value gradually increases, and the m value initially increases and then decreases. The decrease in the b value indicates that the increase in age slows down the change rate which the flexural strength reduction coefficient reduces as the rubber aggregates content increases. At the 1-day age, the b value is 0.8079. After that, as the content of rubber aggregate increases, the reduction coefficient decreases almost linearly. The change in m value indicates that as the age increases, the sensitivity of the mortar strength reduction to rubber aggregate content initially increases and then decreases; in particular, the m value at 1 day is only 1.8001. The parameter of a increases continuously as the age increases, confirming that the strength growth of RM is greater than that of OM after 3 days.

3.1.4. Influence of Rubber Aggregate Content on Compressive Strength Reduction

The correlations between the compressive strength reduction factor of RM and rubber aggregate content at varying ages are depicted in Figure 11, whose fitting parameters are detailed in

Table 6. Fitting parameters of compressive strength reduction coefficient and rubber aggregate content at different ages.

	Age (d)	1	3	7	14	28
Parameters
A		0.1613	0.3843	0.3481	0.3460	0.3442
B		0.8387	0.6157	0.6519	0.6540	0.6558
M		2.3176	6.5556	5.7455	5.4962	5.9038

. In terms of the relationships depicted in the figures, the compressive strength reduction factor of the mortar gradually decreases as the content of rubber aggregates increases. However, unlike the evolution of the flexural strength reduction factor, the compressive strength reduction factor generally decreases at low rubber aggregate content and increases at high rubber aggregate content as the age increases. According to the fitting relationship, as the age increases, the fitting parameters a and m first increase significantly and then decrease slowly, while the fitting parameter b first decreases significantly and then increases slowly. This indicates that as the age increases, the sensitivity of the cement mortar’s compressive strength reduction to rubber aggregate content initially rises and then declines. Meanwhile, increasing rubber aggregates content causes the reduction rate of compressive strength to first decrease significantly and then increase slowly. The variations in the a values also confirm that within 3 days to 28 days, RM exhibits a slower growth rate in compressive strength compared to OM.

3.1.5. Flexural–Compressive Strength Ratio

The flexibility of mortar characterizes its resistance to deformation, which can provide the resistances to shrinkage and cracking of mortar. The flexural–compressive strength ratio is a commonly used flexibility index for cement-based materials, and is calculated according to Equation (4). A higher value of this ratio indicates better flexibility of cement-based materials [45].

$$\eta ={\displaystyle \frac{f_{\mathrm{f}}}{f_{\mathrm{c}}}}$$

(4)

where η is the flexural–compressive strength ratio; f_f is the flexural strength of mortar, MPa; and f_c is the compressive strength of mortar, MPa.

(1)

Effect of rubber aggregate content

The impact of rubber aggregate content on the evolution trends of the flexural–compressive strength ratio is illustrated in Figure 12. As the content of rubber aggregates increases, the overall flexural–compressive strength ratio of mortar gradually increases, that is, rubber aggregates can increase the flexibility of mortar, making it have stronger resistance to deformation. Over time, the flexural–compressive strength ratio of cement mortar with less than 50% rubber aggregate content decreases rapidly at first and then gradually stabilizes; and when the rubber aggregate content is more than 50%, the flexural–compressive strength ratio decreases first and then increases, reaching its minimum at 3 days. This indicates that for the cement mortar with low rubber aggregate content, its flexibility decreases with age. This trend is primarily attributed to the fact that the compressive strength increases more than the flexural strength in the later stage. However, for cement mortar with high rubber aggregate content, the compressive strength basically stops increasing after 3 days. This leads to a pattern where the flexural–compressive strength ratio initially decreases and then increases.

(2)

Effect of rubber aggregate particle size

Figure 13 presents the effect of rubber aggregate particle size on the evolution of the flexural–compressive strength ratio. As the age of mortar increases, the impact of rubber aggregate particle sizes on the evolution of the flexural–compressive strength ratio is basically consistent. In detail, the flexural–compressive strength ratio decreases rapidly before day 7 and the rate of decrease slows down significantly after day 7. Meanwhile, the flexural–compressive strength ratio gradually increases as the particle size of rubber aggregates decreases. That is to say, the decrease in rubber aggregate particle size enhances the flexibility and resistance to deformation of the mortar, allowing it to withstand deformation and even cracking caused by various loads and making it suitable for more complex and high-resilience structures.

(3)

Effect of water–cement ratio

Figure 14 illustrates the impact of the water–cement ratio on the evolution of the flexural–compressive strength ratio. Before day 7, the flexural–compressive strength ratio increases with the increase in the water–cement ratio. However, after 7 days, the variation pattern of the flexural–compressive strength ratio with the water–cement ratio is exactly the opposite. This indicates that in the early stages, a high water–cement ratio is beneficial to improve the flexibility of mortar, while in later stages, the increases in the water–cement ratio will reduce the flexibility of mortar and weaken its resistance to deformation.

3.2. Early-Age Deformation Behavior

3.2.1. Deformation Separation

In an unconfined state, the types of deformation of cement-based materials at early ages mainly include autogenous shrinkage, dry shrinkage, and temperature deformation [34,36]. According to the deformation mechanism, it can be seen that autogenous shrinkage and dry shrinkage have similarities, but the development modes of them are different and temperature-dependent. At an early age, obtaining the total deformation of cement-based materials is generally straightforward, but it is difficult to distinguish the dry shrinkage, autogenous shrinkage, and temperature deformation from the total deformation due to their simultaneous occurrence. Currently, it is more common to use the superposition principle to separate the total deformation. In the superposition principle, it is assumed that the three deformations are independent of each other, and the total deformation is the algebraic sum of the temperature deformation, drying shrinkage, and autogenous shrinkage, as shown in Equation (5). At the same time, it was assumed that autogenous shrinkage in the sealed condition is the same as that in the unsealed condition, and the temperature deformation is unaffected by the drying condition [46].

$${\epsilon }_{\mathrm{total}}={\epsilon }_{\mathrm{a}}+{\epsilon }_{\mathrm{d}}+{\epsilon }_{\mathrm{t}}$$

(5)

where ε_total is the total deformation of cement-based materials; ε_a, ε_d, and ε_t are the autogenous shrinkage, dry shrinkage, and temperature deformation of the mortar, respectively.

In this research, the total deformation and autogenous shrinkage were tested under unsealed and sealed conditions, respectively. According to the assumption of the superposition principle, the drying shrinkage (ε_d) of mortar was calculated as the difference between total deformation (ε_total) and autogenous shrinkage (ε_a) of the specimens. The shrinkage was defined as the ratio of mortar specimen deformation to its effective length. Taking the deformation of RM-50 Mesh as an example, the separation of the early-age deformation is shown in Figure 15. The starting age of the test was 12 h after casting. Figure 15 illustrates that the total deformation and autogenous shrinkage of the RM decrease first and then increase, which means that both the total deformation and the deformation of the sealed specimen show expansion first and then contraction. Through the deformation separation, it is found that the early-age drying shrinkage of RM is only due to contraction deformation, and the expansion in the total deformation of cement mortar at early ages is not related to the drying shrinkage.

Furthermore, mortar specimens having dimensions of 40 mm × 40 mm × 160 mm were tested; thus, the heat generated by cement hydration could dissipate quickly, which had a relatively small impact on the total deformation. Additionally, to eliminate the effect of temperature deformation, the time corresponding to the expansion peak of total deformation was usually taken as the starting point in the deformation separation process. Then, the testing results of deformation behavior were normalized, as presented in Figure 16. The results show that after removing the impact of early-age temperature, the total deformation of cement mortar is mainly composed of dry shrinkage and autogenous shrinkage, with autogenous shrinkage accounting for only about 20% of the total deformation. Meanwhile, as the age of mortar increases, the proportion of autogenous shrinkage fluctuates greatly in the early age and gradually stabilizes after 36 h.

3.2.2. Autogenous Shrinkage

(1)

Effect of rubber aggregate content

As presented in Figure 17, the data illustrate the impact of rubber aggregate content on the autogenous shrinkage of mortar. Specifically, the early-age autogenous shrinkage of OM is mainly concentrated before 80 h. During this period, the hydration is intense, causing obvious self-drying and rapid development of autogenous shrinkage. In the initial stage, a higher rubber aggregate content can slow down the hydration process, resulting in a noticeable reduction in autogenous shrinkage. However, in the later stage, the existence of rubber aggregates increases porosity, so its autogenous shrinkage gradually increases. This turning point is between about 30 h and 50 h. At 7 days (168 h), the autogenous shrinkage of RM-60 is 1.85 times that of OM. According to the proportion of autogenous shrinkage, over time, the proportion of autogenous shrinkage of OM first rapidly decreases and then decreases gradually, reaching 26.42% at 7 days. In the initial stage, the proportion of autogenous shrinkage rapidly decreases with increasing rubber aggregate content. Since the content of rubber aggregate exceeds 50%, the proportion of autogenous shrinkage gradually increases with the development of age, which is exactly opposite to the change in OM. When the rubber aggregates content is between 10% and 40%, the proportion of autogenous shrinkage fluctuates between 20% and 30% overall. After 4 days, the proportion of autogenous shrinkage shows a slow decreasing trend. This demonstrates that the autogenous shrinkage development rate gradually decreases compared to that of drying shrinkage as the age increases.

(2)

Effect of rubber aggregate particle size

As presented in Figure 18, the data illustrate the influences of rubber aggregate particle size on autogenous shrinkage. The autogenous shrinkage shows a pattern of initially decreasing and then increasing with reducing the particle size of rubber aggregates. The turning point is between about 50 h and 70 h. This differs from the impact of particle size and content of rubber aggregates on strength. The development of mortar strength is a function of the degree of hydration, whereas shrinkage deformation is related not only to strength but also to the evolution of the mortar’s pore structure. At 7 days, the autogenous shrinkage RM-100 Mesh is 1.33 times that of RM-50 Mesh, while the autogenous shrinkage RM-20 Mesh is 0.81 times that of RM-50 Mesh. The proportion of autogenous shrinkage of RM-20 Mesh gradually decreases with age, while the proportion of autogenous shrinkage of RM-100 Mesh exhibits a pattern of rapid growth followed by decline. At 7 days, when the particle size of rubber aggregates reduces, the proportion of autogenous shrinkage gradually decreases. The detail proportions of autogenous shrinkage of RM-20 Mesh, RM-50 Mesh, and RM-100 Mesh are 19.54%, 21.65%, and 23.93%, respectively.

(3)

Effect of water–cement ratio

As presented in Figure 19, the data demonstrate that the autogenous shrinkage of RM gradually decreases with increasing the water–cement ratio throughout the early-age period. At 7 days, the autogenous shrinkage of RM-0.38 is 1.19 times that of RM-0.40, while the autogenous shrinkage of RM-0.42 is 0.76 times that of RM-0.40. The proportion of autogenous shrinkage gradually decreases with increasing the water–cement ratio. As the curing age of mortar increases, the proportion of autogenous shrinkage shows a decrease in the initial stage. Meanwhile, the reduction effect becomes more pronounced with lower water–cement ratios. At 7 days, the proportions of autogenous shrinkage of RM-0.38, RM-0.40, and RM-0.42 are 28.74%, 21.65%, and 16.16%, respectively.

3.2.3. Dry Shrinkage

(1)

Influence of rubber aggregate content

Figure 20 illustrates the impact of rubber aggregate content on early-age dry shrinkage. It will be observed that the dry shrinkage increases when the age increases. Meanwhile, increasing rubber aggregates content can intensify the dry shrinkage of mortar, but the increasing magnitudes gradually decrease. At 7 days, the dry shrinkage of RM-60 is 2.08 times that of OM. The proportion results indicate that the dry shrinkage proportion of OM develops quickly in the initial stage. After 24 h, the dry shrinkage growth rate slowed down significantly, reaching 73.58% at 7 days. The addition of rubber aggregate significantly changes the evolution trend of dry shrinkage. This is because the presence of rubber aggregates slows down the hydration rate of cement, resulting in less initial autogenous shrinkage and predominantly drying shrinkage. In contrast, ordinary concrete hydrates quickly, and the shrinkage due to moisture reduction is mainly attributed to the consumption of water by hydration. In detail, the proportion of dry shrinkage increases with increasing rubber aggregate content, but it decreases with the age development. Moreover, increasing rubber aggregate content will contribute to a more obvious trend of this decrease in the early stage. Once the rubber aggregate content is greater than 50%, the proportion of dry shrinkage rapidly decreases before day 4 and gradually stabilizes after day 4.

(2)

Influence of rubber aggregate particle size

Figure 21 shows the impact of rubber aggregate particle size on dry shrinkage. Unlike the impact of rubber aggregate content on drying shrinkage, the development of the drying shrinkage remains relatively constant with decreasing rubber aggregate particle size in initial stages. After 20 h, the dry shrinkage gradually increases with reducing the particle size of rubber aggregates. At day 7, the dry shrinkage of RM-20 Mesh, RM-50 Mesh, and RM-100 Mesh is 500.00 × 10⁻⁶, 542.85 × 10⁻⁶, and 635.71 × 10⁻⁶, respectively. From the dry shrinkage proportion, the reduction in the particle size of rubber aggregates initially causes an increase in the dry shrinkage of mortar, followed by a gradual reduction. The turning point occurs approximately between 50 h and 70 h. In particular, when the particle size of rubber aggregates reaches 100 mesh, the proportion of dry shrinkage rapidly decreases in the first 4 days and gradually stabilizes after day 4.

(3)

Influence of water–cement ratio

As presented in Figure 22, the testing results demonstrate the impact of the water–cement ratio on the drying shrinkage. Unlike the evolution pattern of autogenous shrinkage, the drying shrinkage gradually increases with increasing the water–cement ratio. At 7 days, the drying shrinkage of RM-0.38 and RM-0.40 is 0.82 and 1.09 times that of RM-0.40, respectively. The proportion results indicate that the drying shrinkage proportion gradually increases as the water–cement ratio of mortar increases. With the development of the mortar age, the proportion of drying shrinkage first increases and then gradually decreases, peaking between 12 h and 18 h. In particular, for RM-0.38, the peak effect is significant.

4. Conclusions

In this research, OM served as the control group to investigate the early-age mechanical properties and deformation behavior of RM. Moreover, the influence of rubber aggregate content on strength reduction was analyzed. The conclusions are as follows.

(1)

The early-age development rate of mortar flexural strength is higher than that of compressive strength. The flexural strength at 7 days can attain 85% of that at 28 days, whereas the compressive strength at the same period only reaches 70%. The existence of rubber aggregates significantly reduces the increasing rate of compressive strength after 7 days and flexural strength within 3 days. In addition, the reduction in the rubber aggregates particle size mainly affects the strength development at the initial stage, while the water–cement ratio exerts a more consistent effect in the whole testing stage.

(2)

The R_c values of flexural and compressive strength decrease with increasing the content of rubber aggregates. As the age increases, the R_c value of flexural strength gradually increases, while the R_c value of compressive strength increases at a high rubber aggregate content and decreases at a low rubber aggregate content. Increasing rubber aggregates content, decreasing particle size of rubber aggregates, and increasing water–cement ratio all lead to gradual increases in the flexural ratio of mortar, that is, the flexibility of mortar gradually improves.

(3)

Reducing the particle size of rubber aggregates and increasing rubber aggregate content both significantly decrease the autogenous shrinkage at the initial stage. Within 7 days, the drying shrinkage and proportion of drying shrinkage rise with the increases in curing age, rubber aggregate content, and water–cement ratio, as well as with the reduction in the rubber aggregate particle size.

(4)

This study only investigated the early-age mechanical properties and deformation behavior of RM from a macroscopic perspective. Subsequent research will further analyze the mechanisms underlying these changes at the microscopic level and explore the early-age performance development of rubberized concrete and its correlation with RM.