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Article

Preparation of Recycled and Multi-Recycled Coarse Aggregates Concrete with the Vibration Mixing Process

by
Fa Yang
1,
Yunshi Yao
1,2,*,
Xinxin Wang
1,
Jin Wei
1 and
Zhongxu Feng
1,2
1
Key Laboratory of Highway Construction Technology and Equipment of Ministry of Education, School of Construction Machinery, Chang’an University, Xi’an 710064, China
2
Detong Intelligent Technology Co., Ltd., Xuchang 461000, China
*
Author to whom correspondence should be addressed.
Buildings 2022, 12(9), 1369; https://doi.org/10.3390/buildings12091369
Submission received: 30 July 2022 / Revised: 24 August 2022 / Accepted: 31 August 2022 / Published: 2 September 2022
(This article belongs to the Section Building Materials, and Repair & Renovation)
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Abstract

:
The reuse of construction and demolition waste has become vitally important because of the requirements of environmental protection and sustainable development. The vibration mixing process is a new technology and novel method that improves the performance of concrete by optimizing the mixing process without changing constituents. This study discusses the 100% recycled and multi-recycled coarse aggregates concrete using the vibration mixing process and investigates the fresh and hardened properties. The results show that the recycled and multi-recycled concrete using the vibration mixing process experienced obviously higher compressive strength than that of non-vibration mixing process. At 120 days, the compressive strength of all mixtures surpassed the target strength (40 MPa), except for the multi-recycled concrete with the non-vibration mixing process. More importantly, the compressive strength of the recycled and multi-recycled concrete using vibration mixing was larger than the previous-generation concrete using non-vibration mixing. Another interesting finding was that the coefficient of variation of compressive strength using vibration mixing was smaller and the concrete quality was better and more stable. The splitting tensile strength of recycled and multi-recycled concrete was also investigated and discussed. In addition, the results of t-tests show that vibration mixing has a significant influence on the compressive and splitting tensile strength of recycled and multi-recycled concrete. It is expected that the vibration mixing process could be a more efficient way to help the wide application of recycled and multi-recycled concrete.

1. Introduction

1.1. Recycled Concrete

Because of its high strength, good durability, and convenient application, etc., concrete is widely used in construction, roads, bridges, and other fields. In China, with the acceleration of urban modernization, the demand for concrete continues to grow. According to the National Development and Reform Commission, the total production of commercial concrete had reached 3.29 billion cubic meters in 2021, and the demand for sand and gravel is also remarkable. The annual production of sand and gravel is about 19.7 billion tons, accounting for about 36% of the world’s production [1]. With continuous mining, the gravel, as a nonrenewable resource, is on the verge of being exhausted, and its price is rising accordingly. In light of this, whether from the point of environmental protection or production cost, it is urgent to find a sustainable development path.
In most countries, behind the huge demand for concrete, a large amount of construction and demolition waste (CDW) is generated every year due to the upgrade of infrastructure or the service life of buildings. Unfortunately, this CDW is disposed of in landfills, which not only wastes limited land resources and pollutes the environment but also buries the huge potential value in the CDW.
Despite people gradually realizing the huge potential, the reuse of construction waste varies from country to country. In some developed countries, such as Japan, the highest recycling rate can reach 90% [1,2]. However, due to the internal defects of recycled aggregates, most of them are used on some occasions with lower performance requirements. On the contrary, in developing countries and underdeveloped areas, the recycling rate is very low. In China, only 30–40% of the construction waste can be recycled [3]. Given this, it is imminent that more comprehensive and systematic research on construction waste recycling be carried out.
Aa lot of research has been carried out. The previous literature has studied the properties of recycled aggregates [1,4,5,6], and the workability [6,7], porosity [8,9,10], mechanical properties [8,9,11,12,13], the interfacial transition zone (ITZ) [14,15], failure mechanism [16], fatigue limit [17], long-term performance [7,9], and durability [12,18,19,20] of recycled coarse aggregates concrete. Most researchers have reached the same conclusion: as the replacement ratio increases, the performance of recycled concrete decreases, but there are also different voices. The results show that when the replacement ratio is less than 30% up to 50%, even if the performance of recycled aggregates is low, the impact on the performance of recycled concrete is negligible. Particularly in terms of sustainability, the 50% recycled coarse aggregates formula appears to be optimal, despite its environment-related costs being higher than those for the reference and 100% recycled coarse aggregates concrete [21]. In addition, many scholars have tried to compensate for the adverse effects of recycled aggregates on the properties of recycled concrete by secondary processing of recycled aggregates [22], changing the mixing approach [23,24,25], improving the curing condition [26,27], reinforcing fiber [28], and adding admixtures [29,30,31,32,33], etc. These meaningful studies provide a better choice for the diverse applications of recycled concrete.
While a lot of research focuses on recycled concrete, some scholars realize that when buildings made of recycled concrete reach the end of their service life, needing to be transformed and upgraded, the problem of how to deal with waste recycled concrete will also arise, which is as thorny as the current problem of recycled concrete. Therefore, there are some preliminary attempts at studying multi-recycled concrete, which means that recycled concrete is repeatedly recycled to produce second-generation, third-generation, and even higher-generation recycled concrete. Some innovative results have been achieved in this field.
Marie [34], Huda [35], and Selesa [36,37] all stated that the compressive strength of multi-recycled concrete is slightly higher than control concrete or surpasses the target strength. Thomas [38,39] indicated that after three recyclings, the volume of adhered mortar is 80% of the aggregate, and demonstrates that it is only possible to recycle the concrete a finite number of times. Abreu [40] also found that the mechanical performance of multi-recycled coarse aggregates concrete will tend towards a final value representative of the property’s stabilization. However, Silva’s results [41] showed that in most cases, it was not possible to establish that three recycling cycles were enough to stabilize the properties. Zhu [42] found that the durability of recycled coarse aggregates tends to deteriorate with increasing recycling cycles, but concrete utilizing recycled coarse aggregates can be designed as structural concrete with a life span of at least 50 years.
From the above research, it can be seen that the study of multi-recycled concrete has not yet reached a consistent conclusion. Meanwhile, only the performance of the multi-recycled concrete was evaluated, and almost no one has focused on how to improve and enhance this performance. Can the performance of multi-recycled concrete be improved by changing the mixing process, adding admixtures, or other methods, such as using recycled concrete? The answer does not seem to be unique. This paper will try to answer this question.

1.2. Vibration Mixing Process

The mixing process, as the most critical step in the concrete production, has a crucial impact on the performance of concrete. To better improve the mixing quality and the micro-uniformity of concrete, Feng et al. [43,44] have been devoted to research on vibration mixing technology. Vibration mixing is a new technology and process that improves the performance of building materials by intensifying the mixing process without changing constituents. The new technology can make the mixture subject to high-frequency vibration while mixing. By effectively transferring vibration energy to the mixture, the uniformity and performance of the fresh and hardened concrete will be improved. In recent years, there have been more and more studies on the vibration mixing process.
Xiong [45] indicated that the workability of high-strength, lightweight aggregate concrete (HSLWAC) can be further improved and the bond strength between lightweight aggregates (LWA) and cement matrix can be enhanced by using the vibration mixing process.
Zhao [46] pointed out that the slump, compressive strength, and density of the different concrete were improved using vibratory mixing process, which benefited from fewer internal defects and denser structure.
According to Zheng [47], the vibratory mixing method can make the steel fiber easier to distribute uniformly in the concrete; as a result, mechanical properties can be improved accordingly.
Zhang [48] found that the vibration mixing process can improve the microstructure of the interfacial transition zone (ITZ) in concrete, optimize the morphology of cement paste, and make the porosity and pore distribution more reasonable.
It is well known that the performance deterioration of recycled concrete is caused by the weak interfacial transition zone and unreasonable pore structure. Previous research has proved that the vibration mixing process can improve the ITZ, promote hydration reaction, increase the bond strength between aggregate and mortar, and optimize the void structure in mortar, etc., thereby enhancing the performance of concrete. Therefore, it will be meaningful to study the preparation of multi-recycled concrete with the vibration mixing process. To the authors’ best knowledge, there has been no research to focus on the influence of the vibration mixing process on multi-recycled aggregates concrete.

2. Research Objective

The objective of this study was to investigate the effect of the vibration mixing process on the comprehensive performance of multi-recycled concrete produced from 100% replacement of recycled coarse aggregates. For this purpose, the non-vibration mixing process, that is, conventional compulsory mixing, will also be considered. After planning, the experimental campaign was organized into three phases.
The first phase consisted of the production of all the recycled coarse aggregates. Three types of coarse aggregates were used in this research:
NCA: natural coarse aggregates purchased from local stone companies.
RCA1: recycled coarse aggregates. obtained from normal concrete. After curing in a laboratory for 120 days, the normal concrete specimens were crushed with the help of a jaw crusher, and then went through several cleaning and screening processes to generate the aggregates.
RCA2: multi-recycled coarse aggregates. It was obtained from the first recycled coarse aggregates concrete. Like RCA1, after cured in a laboratory for 120 days, the first recycled coarse aggregates concrete specimens were crushed, and then went through several cleaning and screening process to generate these aggregates.
In the second phase, to make a better comparison between normal concrete (C), recycled concrete (RC1), and multi-recycled concrete (RC2), five different concrete mixtures were produced:
C: C-nv (normal concrete with non-vibration mixing process), as a reference group, was made only with NCA.
RC1: RC1-v (recycled concrete with vibration mixing process) and RC1-nv (recycled concrete with non-vibration mixing process) were produced with 100% substitution of NCA with RCA1.
RC2: RC2-v (multi-recycled concrete with vibration mixing process) and RC2-nv (multi-recycled concrete with non-vibration mixing process) were produced with 100% substitution of NCA with RCA2.
Figure 1 presents a schematic diagram of the production process of different generations of recycled concrete.
Finally, the last phase consisted of testing the properties of aggregates and the performance of the five mixtures.

3. Materials and Methods

3.1. Materials

In this research, four constituents—cement, water, and fine and coarse aggregates (natural and recycled)—were used to produce different types of concrete. CONCH Portland cement (P.O 42.5) and the tap water were used in all groups. The fine aggregates were class medium natural sand with a maximum particle size of 5 mm. The fine aggregates and natural coarse aggregates were purchased from Shaanxi Lishan stone. The RCA1 and RCA2 was obtained from previous generation concrete (cured in a laboratory for 120 days) with a design strength of 40 MPa. To avoid the influence of admixture interaction on the results, no admixtures were used in any of the concrete.

3.2. Concrete Composition

To establish a good comparison between the vibration mixing process and the non-vibration mixing process, all the five concrete mixtures were based on the same composition, that is, the constituents’ contents and the size distribution of aggregates were maintained. The design compressive strength of concrete was 40 MPa at 28 days of standard curing. The slump was set at 80 ± 10 mm. Because of the higher water absorption of RC1 and RC2, an extra part of water must be added to meet the design slump. Previous research indicated that recycled coarse aggregates absorbs 70% to 90% of full potential during the first 10 min [40,49]. As a result, the water compensation was set to 80% of the full water adsorption in this study. Furthermore, the substitution of NCA with recycled coarse aggregates was made by volume. The composition of all concrete mixes is presented in Table 1.

3.3. Two-Stage Mixing Approach (TSMA)

To reduce the impact of recycled aggregates with high water absorption, the two-stage mixing approach (TSMA) is adopted in this investigation. TSMA [25] divides the mixing process into two parts and also splits the required water into two parts proportionally which are added at different times. During the first stage of mixing, the use of half of the required water for the aggregate to be saturated with water. In the second stage of mixing, the remaining water is added to complete the cement-hydration process [24]. Almost all results confirm that the mechanical properties and durability of recycled coarse aggregates concrete are found to be improved by adopting the TSMA in the published literature [50,51,52,53]. The production of recycled fine aggregates concrete using TSMA shows similar sorts of benefits [54]. Some studies have also modified the two-stage mixing approach and proposed new methods, such as TSMAp1, TSMAp2, TSMAs, TSMAsc, and adopting identical mortar volume method in conjunction with the TSMA [50,51,52]. The use of the TSMA can develop a denser old cement mortar by filling up the old pores and cracks and a stronger interfacial zone, and thus the approach opens up a wider scope of recycled concrete applications. The TSMA of producing natural concrete and recycled concrete is shown in Figure 2.

3.4. Vibration Mixer

A twin-shaft vibration mixer (Figure 3) was used in the experiment. It mainly consists of a mixing drive device and a vibration drive device. The mixing drive device drives the mixing shaft, mixing arm, and mixing blade to rotate in the normal mode, so as to force the mixture to circulate in the mixer. In the vibration mode, the mixing drive device still works normally, but the vibration driving device at the other end will force the mixing shaft to rotate eccentrically at a high speed through the eccentric structure, so the mixing shaft will produce periodic vibration. In short, the mixing shafts rotated in the normal mode, while they eccentrically rotate in the vibration mode to produce vibration, thereby realizing vibration mixing.
Due to the high-frequency vibration during the mixing process, the number of collisions, squeezing, and rubbing between the different constituents increases, which breaks the viscous connection between the mixtures, greatly reduces the internal friction between the constituents, and facilitates in changing the cement particles from agglomerated state to uniformly distributed state (Figure 4). With the increasing content of cement involved in the hydration reaction, the content of mortar also continues to increase, which can make aggregates be fully wrapped by the mortar, all of which is very helpful to improve the performance of concrete.
In the experiment, the vibration mixing process and non-vibration mixing process can be manually adjusted through the changeover switch.

3.5. Tests

The properties of natural and recycled aggregates were tested, including particle density, water content, and water adsorption for 24 h according to Chinese standards [55,56]. Fresh state tests included slump [57] and air content [58]. Hardened state tests included compressive strength [59] at 3, 7, 14, 28, and 120 days and splitting tensile strength [60] at 28 and 120 days.
To improve the accuracy and repeatability of the experiment, each group of concrete was produced twice. Take three samples for each strength index each time. The average of six values was recorded as the strength of concrete. Several 100 mm cube samples were cast to test the compressive strength and splitting tensile strength of concrete. All specimens were cast in standard molds and compacted. All of them were removed from the molds after 24 h and placed in a standard curing room (20 ± 2 °C, RH > 95%) until the tested ages [61].

4. Properties of Aggregates

4.1. Physical Properties

The density, water absorption, and moisture content of different coarse aggregates are listed in Table 2. It can be seen that as the number of recycling iterations increases, the apparent density, oven-dried density, and saturated surface dry density of the aggregates significantly decreases, while the moisture content and water absorption increase. The apparent density of NCA was 2739.81 kg/m3, which was the highest among the considered coarse aggregates. The apparent density of RCA1 and RCA2 was approximately 6.33% and 11.35% smaller than that of NCA, respectively. The oven-dried density and saturated surface dry density followed the same trend. This is mainly attributed to the increasing adhered mortar.
The water absorption value and moisture content of aggregates can indirectly reflect its porosity. The absorption capacities of RCA1 and RCA2 were 7.80% and 11.20%, respectively, but the absorption capacity of NCA was only 0.97%, which was much lower than any of the recycled aggregates. The same trend can be seen in the moisture content. The moisture content of RCA1 and RCA2 was 4.58% and 7.15%, respectively, and NCA was 0.07%, which would be ignored. The phenomenon was mostly due to the large number of voids and microcracks contained in the attached mortar. This is also one of the main reasons for the deterioration of the stiffness of recycled aggregates.

4.2. The Morphology of Recycled Aggregates

The poor morphology of recycled aggregates is the key reason for the degradation of recycled aggregates properties. Figure 5 shows several typical recycled coarse aggregates. Figure 5a,b illustrate that the recycled aggregates has only a small amount of attached mortar, and their morphology and size are relatively close to natural aggregates. Figure 5c,d reveal that there are some obvious defects on the surface of the recycled aggregates, such as cracks and large bubbles. Figure 5e shows that the volume of mortar is much larger than natural aggregates, and the surface is very rough. Figure 5f indicates a “special recycled coarse aggregate” composed entirely of fine aggregates and mortar. The existence of microcracks, pores, and other damages makes the performance of the recycled aggregates worse, which will also become the weak part of the recycled concrete. Moreover, as the number of repetitions increases, the frequency of these poor performance aggregates is higher in this research.

5. Results and Discussion

5.1. Fresh Concrete Properties

5.1.1. Slump

The variations in slump are presented in Figure 6. It shows that the slump value of all mixers is within the range of the target slump of 70–90 mm (between the red line and the green line), except for RC1-v. Moreover, the values of RC1-nv, RC1-v, RC2-nv, and RC2-v were a little bit higher than C-nv (78 mm), increasing by 7.69%, 19.23%, 5.13%, and 11.54%, respectively. The difference from Huda [35] is that a decreased trend with the increasing number of repetitions was not observed. In this research, to ensure the consistency of the effective water-cement ratio, a certain amount of extra water was added to compensate for the higher water absorption of the recycled aggregates, but when the slump was tested, this part of the water was not completely absorbed by the recycled aggregates. Therefore, the slump of recycled concrete is a bit larger than natural concrete.
It is worth noting that—whether using the vibration mixing process or not—the slump value of RC1 is correspondingly higher than RC2. It is mostly due to the adhered mortar and surface roughness of RCA2 being higher, which makes the absorption rate of RCA2 faster. Therefore, the amount of water in the cement mortar is less and the mortar has a greater consistency when testing the slump.
In addition, the slump value of concrete using the vibration mixing process is a little bit higher than that using the non-vibration mixing process, whether RC1 or RC2. It is the expected result, and also consistent with Xiong’s results [45]. The slump of RC1-v and RC2-v is 10.71% and 6.10% higher than that of RC1-nv and RC2-nv. It could be attributed to the function of vibration which would effectively destroy cement agglomeration and release more cement particles to participate in the hydration reaction. As more hydration reactions occur, the mortar content in the fresh concrete increases, so the fluidity of concrete is improved. Another reason is vibration can reduce the “internal friction” of concrete, and the flow resistance of fresh concrete will reduce. These can also explain that the slump of RC1-v slightly exceeds the design upper limit (red line). All of the above illustrates that the vibration mixing process can be used to enhance the workability of concrete.

5.1.2. Air Content

The results of the air content are shown in Figure 6. It can be found that an increasing trend with the number of repetitions in the case of the non-vibration mixing process. The air content of RC1-nv and RC2-nv was 1.7% and 2.2%, which were significantly higher than C-nv. Correspondingly, the air content of RC2-v (2.7%) is higher than that of RC1-v (2.1%) and also surpasses C-nv. The same result is found in the present study [35,37]. It can be explained that the adhered mortar contains a lot of pores and microcracks. In addition, the content of attached mortar increases from RC1 to RC2, which leads to positive growth in the air content.
Another result is found from Figure 6 that the vibration mixing process can increase the air content of concrete. The air content of RC1-v and RC2-v increased 23.5% and 22.7% respectively, compared to RC1-nv and RC2-nv. This phenomenon can be accounted for the following reasons: (1) During the mixing process, a water film is formed on the surface of the mortar to prevent gas from entering its interior, but under the action of vibration, the water film ruptures, and gas will be more easily introduced into the mortar. (2) Simultaneously, the high-frequency vibration can refine the bubbles entrapped into the mixture, so that more large bubbles are dispersed into more and smaller bubbles which can be easily and stably existing in the concrete. This fact has been proved by Koch’s experiment [62]. The result can also explain the increase in a concrete slump with the vibration mixing process. As the air content increases, more and smaller bubbles act as balls in the mortar, thereby improving the fluidity of the concrete.

5.2. Hardened Concrete Mechanical Properties

5.2.1. Compressive Strength

The compressive strength of different mixtures at the age of 3, 7, 14, 28, and 120 days are presented in Figure 7. It can be seen that the compressive strength of all types of concrete has a similar trend with the increase of the curing age. Moreover, the growth rate of compressive strength slows down as the curing age increases. As Figure 7 depicted, using the vibration mixing process or not, the 100% replacement rate of multi-recycled coarse aggregates leads to a drop in the compressive strength with the increasing number of repetitions. The same results were obtained from other research [35,42]. More specifically, the compressive strengths of RC1-nv and RC2-nv were 34.9 MPa and 35.7 MPa at 28 days which was slightly lower than the C-nv (41.0 MPa). The compressive strength of RC1-v and RC2-v were 38.9 MPa and 38.8 MPa which was 5.12% and 5.37% smaller than that of C-nv, respectively. The compressive strength of fourfold-recycled concrete had not reached the target value (40 MPa) at the age of 28 days. These results can be attributed to the lower density (including apparent density, oven-dried density, and saturated surface dry density) and more attached mortar of RCA1 and RCA2. Furthermore, during the crushing process, the forced crushing method will produce some microcracks on the surface or inside recycled coarse aggregates (Figure 5e,f). These will have adverse effects on the strength of recycled concrete.
To investigate the effect on the mechanical properties of recycled concrete using the vibration mixing process, the comparison between RC1-nv and RC1-v or RC2-nv and RC2-v was made. A clear trend can be seen from Table 3 that the compressive strength of concrete using the vibration mixing process is higher than that of the non-vibration mixing process at the age of 28 days. The compressive strength of RC1-v and RC2-v is 11.46% and 8.68% higher than that of RC1-nv and RC2-nv, respectively. It can be attributed to the additional vibration frequency, which can effectively destroy the agglomeration of cement particles and increase the amount of cement participating in the hydration reaction, which means the effective water-cement ratio is lower. It plays a positive role in the interfacial transition zone. In addition, the high-frequency vibration during the mixing process can break the viscous connection between the constituents of the mixture and greatly reduce the friction as well. In this case, the uniformity of the mortar will be improved and the aggregates will be more easily distributed evenly and reasonably. This is another reason why the vibration mixing process is beneficial for strength of concrete.
The mechanical properties of different mixtures under long curing age were also investigated. At the age of 120 days, using non-vibration mixing process, the compressive strength decreases with the number of recycling cycles, but the value of RC1-v and RC2-v exceeds or approaches the compressive strength of C-nv and is also higher than RC1-nv and RC2-nv. Specifically, the strength value of RC1-v (46.7 MPa) and RC2-v (42.9 MPa) was 16.17% and 15.63% higher than that of RC1-nv and RC2-nv, respectively. The compressive strength of all mixtures exceeds the design value 40 MPa, except for RC2-nv. The value of RC2-nv is 37.1 MPa, which failed to achieve the target strength even at the age of 120 days. The measured value decreases as follows: RC1-v > C-nv > RC2-v > RC1-nv > RC2-nv. The strength value of RC1-v is the highest, even RC1-v is a bit higher than C-nv and the value of RC2-v also exceeds RC1-nv. This may be attributed to the function of vibration during the mixing process being able to refine the void structure. When more and smaller bubbles are evenly distributed in the mortar, the internal stress of the concrete is effectively released, thereby reducing the possibility of microcracks and internal defects.
To verify that the strength difference of concrete using vibration and non-vibration mixing process is not caused by experimental error, a paired-sample t-test of the strength was performed. The results are illustrated in the Table 4. It can be found that at the 0.05 level, the compressive strength of RC1-nv and RC1-v, RC2-nv, and RC2-v showed significant differences, regardless of age: 28 days or 120 days. This means the vibration mixing process has a significant positive effect on the compressive strength of recycled and multi-recycled concrete.

5.2.2. Relative Variation

The relative variation in compressive strength from 28 days to 120 days is shown in Figure 8. It can be seen that the strength growth rate of concretes under the vibration mixing process is significantly higher than that using the non-vibration mixing process. The values of RC1-v and RC2-v are higher than that of RC1-nv, RC2-nv respectively, and all exceeds C-nv. The value of RC1-v is the biggest, which agrees with the compressive strength at the age of 120 days. It is well known that as time goes by, the hydration reaction has been going on, but the hydration rate will become slower and slower. The hydration products are not only related to the temperature but related to the internal water content of the concrete (this point can well explain that the relative variation of all recycled concrete is greater than that of C-nv, except for RC2-nv). At the same time, the vibration mixing process makes more cement particles continue to participate in the hydration reaction and produce more hydrated calcium silicate (C-S-H) gels and calcium hydroxide (Ca(OH)2), which forms a better bonding between aggregate and mortar. This is beneficial to the growth of concrete strength from 28 days to 120 days.
In addition, the relative variation of RC2-nv is almost negligible, only 3.9%. This can also explain that the strength of RC2-nv does not meet the design requirement even after 120 days.

5.2.3. Coefficient of Variation

The coefficient of variation is an indicator that reflects the degree of dispersion of a set of data. Its value represents the relative amount of standard deviation relative to the mean. For the compressive strength, the coefficient of variation reflects the uniformity of concrete quality. A larger value indicates that the quality of the concrete is fluctuating and unstable.
The coefficients of variation of compressive strength of RC1 and RC2 at 3, 7, 14, 28, and 120 days are shown in Figure 9a,b. A valuable result can be obtained by combining the two pictures. For recycled and multi-recycled concrete produced by different mixing processes, the coefficient of variation shows different changes with the increase of curing age. From Figure 9a, the coefficients of variation of RC1-nv and RC1-v change synchronously, but the value of RC1-v is smaller. And Figure 9b shows that the value of RC2-nv is larger and fluctuates with the increase of curing age while RC2-v keeps decreasing. Especially, the coefficient of variation of RC1-v and RC2-v is smaller than that of RC1-nv and RC2-nv after 14 days. This all proves that the quality of concrete produced by the vibration mixing process is better and more stable.

5.2.4. Splitting Tensile Strength

The splitting tensile strength of all concrete at the age of 28 and 120 days are presented in Figure 10. It can be observed that, unlike the compressive strength, the splitting tensile strength of concrete does not significantly decrease with the increasing number of repetitions. On the 28th day, there is a marked increase in the splitting tensile strengths from C to RC2. Compared with C-nv (3.63 MPa), the values of RC1-nv, RC1-v, RC2-nv, and RC2-v were increased by 4.41%, 8.82%, 7.16%, and 17.08%, respectively. This phenomenon matches some studies [9,35] that indicated that the splitting tensile strength of recycled concrete was higher than the natural concrete.
When curing for the age of 120 days, the splitting tensile strength of RC1-nv and RC2-nv are significantly lower than that of the other three concrete. However, it can be clearly observed that the values of RC1-v and RC2-v approach or even exceed the reference concrete. In addition, the growth rates of RC1-nv and RC2-nv from 28 days to 120 days are smaller. Possible reasons are as follows. Since the vibration mixing process improving workability, the cement paste with high fluidity is more likely to infiltrate into the pores and microcracks contained in the recycled aggregates, which can make up for these defects (Figure 11). Moreover, the number of collisions and rolling of the aggregates has also increased, making the cement paste more fully warp the periphery of the aggregates and improving the cohesiveness of concrete. Another important reason is that the vibration mixing process improves the fluidity of the concrete (this has been proved by the slump results), which makes the concrete more compact and increases the bond between aggregate and mortar. Therefore, it is beneficial to reduce the thickness of ITZ and form an impregnation effect [45]. All of the above have a positive effect on the fusion of new and old mortar and the enhancement of bond strength.
The paired-sample t-test of the splitting tensile strength was performed. The results are illustrated in Table 5. At the 0.05 level, the splitting tensile strength of concrete using vibration and non-vibration mixing process was significantly different at the age of 120 days.

6. Conclusions

The following conclusions are drawn from this study:
The increasing attached mortar leads to a drop in the density of aggregates and a significant increase in water adsorption and moisture content from NCA to RCA2.
By adding additional water, the slump of all concrete can meet the design value, and there is no loss over regenerations. Among them, RC1-v and RC2-v are 10.71% and 6.10% higher than RC1-nv and RC2-nv, and RC1-v is the largest. The air content of fresh concrete increased with the increasing number of repetitions. Moreover, the air content of RC1-v and RC2-v is 23.5% and 22.7% larger than that of RC1-nv and RC2-nv, respectively.
The compressive strength of all investigated concrete has a similar trend from 3 days to 120 days. At 28 days, the strength of multi-recycled concrete decreases as the number of repetitions increases. Particularly, the value of RC1-v and RC2-v is 11.46% and 8.68% higher than RC1-nv and RC2-nv, respectively. At the 120th day, the compressive strength of all mixtures exceeds the design value 40 MPa except for RC2-nv. The compressive strength decreases as follows: RC1-v > C-nv > RC2-v > RC1-nv > RC2-nv. Compared with the non-vibration mixing process, the recycled concrete using the vibration mixing process has higher compressive strength, smaller coefficient of variation, and better quality.
The splitting tensile strength is markedly increased with increasing recycling at 28 days. When curing to the age of 120 days, the values of RC1-v and RC2-v are significantly higher than that of RC1-nv and RC2-nv, and both exceed the reference concrete.
The significance of strength differences of concrete using vibration and non-vibration mixing process was also investigated. The results of paired-sample t-test showed that at the 0.05 level, the vibration mixing process has a significant effect on the compressive strength and splitting tensile strength of recycled and multi-recycled concrete. It can be substantiated that the vibration mixing process is a more effective way to improve the performance of recycled and multi-recycled coarse aggregates concrete.

Author Contributions

Conceptualization, F.Y. and Y.Y.; methodology, Y.Y. and J.W.; software, F.Y.; validation, X.W. and J.W.; investigation, X.W.; resources, Z.F.; data curation, F.Y.; writing—original draft preparation, F.Y.; writing—review and editing, F.Y. and Z.F.; supervision, Z.F. All authors have read and agreed to the published version of the manuscript.

Funding

The authors gratefully acknowledge the support of the National Natural Science Foundation of China (51208044) and the Natural Science Foundation of Shaanxi Province, China (213025170173). All the authors of the following references are much appreciated.

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Conflicts of Interest

The authors declare no conflict of interest.

References

  1. Tam, V.W.; Soomro, M.; Evangelista, A.C.J. A review of recycled aggregate in concrete applications (2000–2017). Constr. Build. Mater. 2018, 172, 272–292. [Google Scholar] [CrossRef]
  2. Tam, V.W.; Tam, L.; Le, K.N. Cross-cultural comparison of concrete recycling decision-making and implementation in construction industry. Waste Manag. 2010, 30, 291–297. [Google Scholar] [CrossRef] [PubMed]
  3. Yang, H.; Xia, J.; Thompson, J.R.; Flower, R.J. Urban construction and demolition waste and landfill failure in Shenzhen, China. Waste Manag. 2017, 63, 393–396. [Google Scholar] [CrossRef]
  4. De Juan, M.S.; Gutiérrez, P.A. Study on the influence of attached mortar content on the properties of recycled concrete aggregate. Constr. Build. Mater. 2009, 23, 872–877. [Google Scholar] [CrossRef]
  5. McGinnis, M.J.; Davis, M.; de la Rosa, A.; Weldon, B.D.; Kurama, Y.C. Strength and stiffness of concrete with recycled concrete aggregates. Constr. Build. Mater. 2017, 154, 258–269. [Google Scholar] [CrossRef]
  6. Kovler, K.; Roussel, N. Properties of fresh and hardened concrete. Cem. Concr. Res. 2011, 41, 775–792. [Google Scholar] [CrossRef]
  7. Manzi, S.; Mazzotti, C.; Bignozzi, M.C. Short and long-term behavior of structural concrete with recycled concrete aggregate. Cem. Concr. Compos. 2013, 37, 312–318. [Google Scholar] [CrossRef]
  8. Thomas, C.; Setién, J.; Polanco, J. Structural recycled aggregate concrete made with precast wastes. Constr. Build. Mater. 2016, 114, 536–546. [Google Scholar] [CrossRef]
  9. Kou, S.-C.; Poon, C.-S.; Etxeberria, M. Influence of recycled aggregates on long term mechanical properties and pore size distribution of concrete. Cem. Concr. Compos. 2011, 33, 286–291. [Google Scholar] [CrossRef]
  10. Thomas, C.; Setién, J.; Polanco, J.; De Brito, J.; Fiol, F. Micro-and macro-porosity of dry-and saturated-state recycled aggregate concrete. J. Clean. Prod. 2019, 211, 932–940. [Google Scholar] [CrossRef]
  11. Xiao, J.; Li, W.; Fan, Y.; Huang, X. An overview of study on recycled aggregate concrete in China (1996–2011). Constr. Build. Mater. 2012, 31, 364–383. [Google Scholar] [CrossRef]
  12. Le, H.-B.; Bui, Q.-B. Recycled aggregate concretes—A state-of-the-art from the microstructure to the structural performance. Constr. Build. Mater. 2020, 257, 119522. [Google Scholar] [CrossRef]
  13. Pradhan, S.; Kumar, S.; Barai, S.V. Multi-scale characterisation of recycled aggregate concrete and prediction of its performance. Cem. Concr. Compos. 2020, 106, 103480. [Google Scholar] [CrossRef]
  14. Del Bosque, I.S.; Zhu, W.; Howind, T.; Matías, A.; De Rojas, M.S.; Medina, C. Properties of interfacial transition zones (ITZs) in concrete containing recycled mixed aggregate. Cem. Concr. Compos. 2017, 81, 25–34. [Google Scholar] [CrossRef]
  15. Djerbi, A. Effect of recycled coarse aggregate on the new interfacial transition zone concrete. Constr. Build. Mater. 2018, 190, 1023–1033. [Google Scholar] [CrossRef]
  16. Liu, Q.; Xiao, J.; Sun, Z. Experimental study on the failure mechanism of recycled concrete. Cem. Concr. Res. 2011, 41, 1050–1057. [Google Scholar] [CrossRef]
  17. Thomas, C.; Setién, J.; Polanco, J.; Lombillo, I.; Cimentada, A. Fatigue limit of recycled aggregate concrete. Constr. Build. Mater. 2014, 52, 146–154. [Google Scholar] [CrossRef]
  18. Thomas, C.; Setién, J.; Polanco, J.; Alaejos, P.; De Juan, M.S. Durability of recycled aggregate concrete. Constr. Build. Mater. 2013, 40, 1054–1065. [Google Scholar] [CrossRef]
  19. Guo, H.; Shi, C.; Guan, X.; Zhu, J.; Ding, Y.; Ling, T.-C.; Zhang, H.; Wang, Y. Durability of recycled aggregate concrete—A review. Cem. Concr. Compos. 2018, 89, 251–259. [Google Scholar] [CrossRef]
  20. Evangelista, L.; De Brito, J. Durability performance of concrete made with fine recycled concrete aggregates. Cem. Concr. Compos. 2010, 32, 9–14. [Google Scholar] [CrossRef]
  21. Vymazal, T.; Hrabová, K.; Teplý, B.; Kocáb, D. Sustainability quantification of concrete with recycled aggregate. AIP Conf. Proc. 2020, 2210, 020018. [Google Scholar] [CrossRef]
  22. Lu, B.; Shi, C.; Cao, Z.; Guo, M.; Zheng, J. Effect of carbonated coarse recycled concrete aggregate on the properties and microstructure of recycled concrete. J. Clean. Prod. 2019, 233, 421–428. [Google Scholar] [CrossRef]
  23. Liu, K.; Yan, J.; Hu, Q.; Sun, Y.; Zou, C. Effects of parent concrete and mixing method on the resistance to freezing and thawing of air-entrained recycled aggregate concrete. Constr. Build. Mater. 2016, 106, 264–273. [Google Scholar] [CrossRef]
  24. Tam, V.W.; Tam, C.M.; Wang, Y. Optimization on proportion for recycled aggregate in concrete using two-stage mixing approach. Constr. Build. Mater. 2007, 21, 1928–1939. [Google Scholar] [CrossRef]
  25. Tam, V.W.; Gao, X.; Tam, C.M. Microstructural analysis of recycled aggregate concrete produced from two-stage mixing approach. Cem. Concr. Res. 2005, 35, 1195–1203. [Google Scholar] [CrossRef]
  26. Fonseca, N.; De Brito, J.; Evangelista, L. The influence of curing conditions on the mechanical performance of concrete made with recycled concrete waste. Cem. Concr. Compos. 2011, 33, 637–643. [Google Scholar] [CrossRef]
  27. Thomas, C.; Setién, J.; Polanco, J.; Cimentada, A.; Medina, C. Influence of curing conditions on recycled aggregate concrete. Constr. Build. Mater. 2018, 172, 618–625. [Google Scholar] [CrossRef]
  28. Horňáková, M.; Lehner, P. Analysis of Measured Parameters in Relation to the Amount of Fibre in Lightweight Red Ceramic Waste Aggregate Concrete. Mathematics 2022, 10, 229. [Google Scholar] [CrossRef]
  29. Barbudo, A.; De Brito, J.; Evangelista, L.; Bravo, M.; Agrela, F. Influence of water-reducing admixtures on the mechanical performance of recycled concrete. J. Clean. Prod. 2013, 59, 93–98. [Google Scholar] [CrossRef]
  30. Kou, S.-C.; Poon, C.-S.; Agrela, F. Comparisons of natural and recycled aggregate concretes prepared with the addition of different mineral admixtures. Cem. Concr. Compos. 2011, 33, 788–795. [Google Scholar] [CrossRef]
  31. Pereira, P.; Evangelista, L.; De Brito, J. The effect of superplasticisers on the workability and compressive strength of concrete made with fine recycled concrete aggregates. Constr. Build. Mater. 2012, 28, 722–729. [Google Scholar] [CrossRef]
  32. Matias, D.; De Brito, J.; Rosa, A.; Pedro, D. Mechanical properties of concrete produced with recycled coarse aggregates—Influence of the use of superplasticizers. Constr. Build. Mater. 2013, 44, 101–109. [Google Scholar] [CrossRef]
  33. Pereira, P.; Evangelista, L.; De Brito, J. The effect of superplasticizers on the mechanical performance of concrete made with fine recycled concrete aggregates. Cem. Concr. Compos. 2012, 34, 1044–1052. [Google Scholar] [CrossRef]
  34. Marie, I.; Quiasrawi, H. Closed-loop recycling of recycled concrete aggregates. J. Clean. Prod. 2012, 37, 243–248. [Google Scholar] [CrossRef]
  35. Huda, S.B.; Alam, M.S. Mechanical behavior of three generations of 100% repeated recycled coarse aggregate concrete. Constr. Build. Mater. 2014, 65, 574–582. [Google Scholar] [CrossRef]
  36. Salesa, Á.; Pérez-Benedicto, J.A.; Colorado-Aranguren, D.; López-Julián, P.L.; Esteban, L.M.; Sanz-Baldúz, L.J.; Sáez-Hostaled, J.L.; Ramis, J.; Olivares, D. Physico–mechanical properties of multi-recycled concrete from precast concrete industry. J. Clean. Prod. 2017, 141, 248–255. [Google Scholar] [CrossRef]
  37. Salesa, Á.; Pérez-Benedicto J, Á.; Esteban, L.M.; Vicente-Vas, R.; Orna-Carmona, M. Physico-mechanical properties of multi-recycled self-compacting concrete prepared with precast concrete rejects. Constr. Build. Mater. 2017, 153, 364–373. [Google Scholar] [CrossRef]
  38. Thomas, C.; De Brito, J.; Cimentada, A.; Sainz-Aja, J. Macro- and micro-properties of multi-recycled aggregate concrete. J. Clean. Prod. 2020, 245, 118843. [Google Scholar] [CrossRef]
  39. Thomas, C.; De Brito, J.; Gil, V.; Sainz-Aja, J.; Cimentada, A. Multiple recycled aggregate properties analysed by X-ray microtomography. Constr. Build. Mater. 2018, 166, 171–180. [Google Scholar] [CrossRef]
  40. Abreu, V.; Evangelista, L.; De Brito, J. The effect of multi-recycling on the mechanical performance of coarse recycled aggregates concrete. Constr. Build. Mater. 2018, 188, 480–489. [Google Scholar] [CrossRef]
  41. Silva, S.; Evangelista, L.; De Brito, J. Durability and shrinkage performance of concrete made with coarse multi-recycled concrete aggregates. Constr. Build. Mater. 2021, 272, 121645. [Google Scholar] [CrossRef]
  42. Zhu, P.; Hao, Y.; Liu, H.; Wei, D.; Liu, S.; Gu, L. Durability evaluation of three generations of 100% repeatedly recycled coarse aggregate concrete. Constr. Build. Mater. 2019, 210, 442–450. [Google Scholar] [CrossRef]
  43. Yao, Y.; Feng, Z.; Chen, S. Strength of concrete reinforced using double-blade mixer. Mag. Concr. Res. 2013, 65, 787–792. [Google Scholar] [CrossRef]
  44. Feng, Z.; Zhao, L.; Zhao, W.; Wang, W.; Yao, Y. Advanced Theory and Equipment of Production Concrete; China Communications Press: Beijing, China, 2016. [Google Scholar]
  45. Xiong, G.; Wang, C.; Zhou, S.; Jia, X.; Luo, W.; Liu, J.; Peng, X. Preparation of high strength lightweight aggregate concrete with the vibration mixing process. Constr. Build. Mater. 2019, 229, 116936. [Google Scholar] [CrossRef]
  46. Zhao, K.; Zhao, L.; Hou, J.; Zhang, X.; Feng, Z.; Yang, S. Effect of vibratory mixing on the slump, compressive strength, and density of concrete with the different mix proportions. J. Mater. Res. Technol. 2021, 15, 4208–4219. [Google Scholar] [CrossRef]
  47. Zheng, Y.; Wu, X.; He, G.; Shang, Q.; Xu, J.; Sun, Y. Mechanical properties of steel fiber-reinforced concrete by vibratory mixing technology. Adv. Civ. Eng. 2018, 2018, 9025715. [Google Scholar] [CrossRef]
  48. Zhang, L. Research on Vibratory Mixing Mechanism and Industrial Application of Concrete; Chang’an University: Xi’an, China, 2013. [Google Scholar]
  49. Montero, J.; Laserna, S. Influence of effective mixing water in recycled concrete. Constr. Build. Mater. 2017, 132, 343–352. [Google Scholar] [CrossRef]
  50. Tam, V.-Y.; Gao, X.-F.; Tam, C. Comparing performance of modified two-stage mixing approach for producing recycled aggregate concrete. Mag. Concr. Res. 2006, 58, 477–484. [Google Scholar] [CrossRef]
  51. Kisku, N.; Rajhans, P.; Panda, S.; Nayak, S.; Pandey, V. Development of durable concrete from C&D waste by adopting identical mortar volume method in conjunction with two-stage mixing procedure. Constr. Build. Mater. 2020, 256, 119361. [Google Scholar] [CrossRef]
  52. Tam, V.W.; Tam, C.M. Diversifying two-stage mixing approach (TSMA) for recycled aggregate concrete: TSMAs and TSMAsc. Constr. Build. Mater. 2008, 22, 2068–2077. [Google Scholar] [CrossRef] [Green Version]
  53. Kisku, N.; Rajhans, P.; Panda, S.; Pandey, V.; Nayak, S. Microstructural investigation of recycled aggregate concrete produced by adopting equal mortar volume method along with two stage mixing approach. Structures 2020, 24, 742–753. [Google Scholar] [CrossRef]
  54. Sivamani, J.; Renganathan, N.T. Effect of fine recycled aggregate on the strength and durability properties of concrete modified through two-stage mixing approach. Environ. Sci. Pollut. Res. 2021, 1–14. [Google Scholar] [CrossRef] [PubMed]
  55. Chinese Standard JGJ 52-2006; Standard for Technical Requirements and Test Method of Sand and Curshed Stone (or Gravel) for Ordinary Concrete. Ministry of Construction of the People’s Republic of China: Beijing, China, 2006.
  56. Chinese Standard GB/T 25177-2010; Recycled Coarse Aggregate for Concrete. Quality Supervision Inspection and Quarantine of the People’s Republic of China and Standardization Administration of the People’s Republic China: Beijing, China, 2010.
  57. EN 12390-2:2019; Testing Fresh Concrete-Part 2: Slump Test. BSI: London, UK, 2019.
  58. EN 12390-7:2009; Testing Hardened Concrete—Part 6: Air Content-Pressure Methods. BSI: London, UK, 2009.
  59. EN 12390-3:2002; Testing Hardened Concrete—Part 3: Compressive Strength of Test Specimens. BSI: London, UK, 2002.
  60. EN 12390-6:2009; Testing Hardened Concrete—Part 6: Tensile Splitting Strength of Test Specimens. BSI: London, UK, 2009.
  61. EN 12390-2:2009; Testing Hardened Concrete—Part 2: Making and Curing Specimens for Strength Tests. BSI: London, UK, 2009.
  62. Koch, J.A.; Castaneda, D.I.; Ewoldt, R.H.; Lange, D.A. Vibration of fresh concrete understood through the paradigm of granular physics. Cem. Concr. Res. 2019, 115, 31–42. [Google Scholar] [CrossRef]
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Figure 1. Flow diagram of the evolution process of recycled concrete.
Figure 1. Flow diagram of the evolution process of recycled concrete.
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Figure 2. Mixing procedures of all the investigated mixes.
Figure 2. Mixing procedures of all the investigated mixes.
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Figure 3. The twin-shaft vibration mixer.
Figure 3. The twin-shaft vibration mixer.
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Figure 4. Schematic diagram of the phenomenon of cement agglomeration destroyed by vibration mixing process.
Figure 4. Schematic diagram of the phenomenon of cement agglomeration destroyed by vibration mixing process.
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Figure 5. The morphology of different types of recycled aggregates.
Figure 5. The morphology of different types of recycled aggregates.
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Figure 6. The slump and air content of all considered concrete (The red and cyan lines are the upper and lower limits of the slump design value, respectively).
Figure 6. The slump and air content of all considered concrete (The red and cyan lines are the upper and lower limits of the slump design value, respectively).
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Figure 7. The compressive strength of different concrete at 3, 7, 14, 28 and 120 days.
Figure 7. The compressive strength of different concrete at 3, 7, 14, 28 and 120 days.
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Figure 8. The relative variation the compressive strength.
Figure 8. The relative variation the compressive strength.
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Figure 9. The coefficient of variation under two mixing processes. (a) RC1; (b) RC2.
Figure 9. The coefficient of variation under two mixing processes. (a) RC1; (b) RC2.
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Figure 10. The splitting tensile strength of various concrete mixes at 28 and 120 days.
Figure 10. The splitting tensile strength of various concrete mixes at 28 and 120 days.
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Figure 11. The impregnation effect in ITZ under the vibration mixing process. (a) Recycled coarse aggregate. (b) ITZ using non-vibration mixing process. (c) ITZ using vibration mixing process.
Figure 11. The impregnation effect in ITZ under the vibration mixing process. (a) Recycled coarse aggregate. (b) ITZ using non-vibration mixing process. (c) ITZ using vibration mixing process.
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Table
Table 1. Composition of the concrete mixers (kg/m3).
Table 1. Composition of the concrete mixers (kg/m3).
ComponentsSize (mm)NCRC1RC2
Natural coarse aggregate 4.75–9.5457.6--
9.5–16480.9--
16–19171.6--
19–26.534.32--
Recycled coarse aggregate4.75–9.5-428.7405.7
9.5–16-450.1425.9
16–19-160.7152.1
19–26.5-32.130.4
Fine aggregate 602602602
Cement 439439439
Water 215215215
Compensation water 017.625.6
w/c 0.490.530.55
w/ceffective 0.490.490.49
Table
Table 2. Properties of coarse aggregates.
Table 2. Properties of coarse aggregates.
PropertiesNCARCA1RCA2
Apparent density (kg/m3)2739.812566.512428.80
Oven-dried density (kg/m3)2735.232424.272252.74
Saturated surface dry density (kg/m3)2759.302593.942486.63
Water absorption, 24 h (%)0.977.8011.20
Moisture content (%)0.074.857.15
Table
Table 3. Compressive strength and coefficients of variation.
Table 3. Compressive strength and coefficients of variation.
Compressive Strength (MPa)3 dCv7 dCv14 dCv28 dCv120 dCv
C-nv20.30.05129.80.04034.10.07341.00.05744.90.061
RC1-nv15.20.08426.80.07931.90.09134.90.08040.20.061
RC1-v17.10.07326.30.08633.00.08738.90.07146.70.051
RC2-nv12.00.05624.00.07929.80.05735.70.07637.10.062
RC2-v11.80.06923.50.06428.90.04238.80.02842.90.032
Table
Table 4. Results of paired-sample t-tests for compressive strength.
Table 4. Results of paired-sample t-tests for compressive strength.
AgeConcreteDescriptive StatisticsTest Statistics
NMeanSDSEMMediant StatisticDFProb > |t|
28 dRC1-nv634.8522.7911.13934.844−4.61750.0058
RC1-v638.9042.7631.12838.660
Difference6−4.0522.1500.878−3.593
Overall1236.8783.3890.97836.778
RC2-nv635.7822.7241.11235.804−2.83050.0367
RC2-v638.7771.1000.44938.593
Difference6−2.9952.5921.058−3.810
Overall1237.2792.5240.72937.734
120 dRC1-nv640.1852.4420.99740.483−3.97250.0106
RC1-v646.7742.3750.97047.265
Difference6−6.5894.0641.659−7.220
Overall1243.4794.1371.19443.291
RC2-nv637.1212.3180.94637.354−4.67350.0055
RC2-v642.9041.3860.56642.858
Difference6−5.7833.0311.238−6.106
Overall1240.0123.5271.01840.605
Table
Table 5. Results of paired-sample t-test for splitting tensile strength.
Table 5. Results of paired-sample t-test for splitting tensile strength.
AgeConcreteDescriptive StatisticsTest Statistics
NMeanSDSEMMediant StatisticDFProb > |t|
120 dRC1-nv63.8320.1480.0613.817−5.39750.0030
RC1-v64.3450.2040.0834.348
Difference6−0.5130.2330.095−0.552
Overall124.0880.3170.0924.035
RC2-nv64.0580.3430.1403.986−3.53250.0167
RC2-v64.4420.3240.1324.457
Difference6−0.3840.2660.109−0.288
Overall124.2500.3760.1094.327
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Yang, F.; Yao, Y.; Wang, X.; Wei, J.; Feng, Z. Preparation of Recycled and Multi-Recycled Coarse Aggregates Concrete with the Vibration Mixing Process. Buildings 2022, 12, 1369. https://doi.org/10.3390/buildings12091369

AMA Style

Yang F, Yao Y, Wang X, Wei J, Feng Z. Preparation of Recycled and Multi-Recycled Coarse Aggregates Concrete with the Vibration Mixing Process. Buildings. 2022; 12(9):1369. https://doi.org/10.3390/buildings12091369

Chicago/Turabian Style

Yang, Fa, Yunshi Yao, Xinxin Wang, Jin Wei, and Zhongxu Feng. 2022. "Preparation of Recycled and Multi-Recycled Coarse Aggregates Concrete with the Vibration Mixing Process" Buildings 12, no. 9: 1369. https://doi.org/10.3390/buildings12091369

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