Keywords

1 Introduction

The rapid development of China's construction industry also brings a large amount of construction waste, which is dominated by brick and concrete type waste [1]. The use of construction brick waste as recycled aggregate, and the preparation of recycled permeable concrete, on the one hand, can effectively solve the problem of urban construction waste pollution, lack of natural sand and gravel aggregate resources, China's current production of construction waste has reached 40% of the total production of municipal garbage, and with an average growth rate of 8% per year continues to grow [2], on the other hand, it can also alleviate the urban heat island effect and urban flooding phenomenon due to the intensive population activities, and bring significant benefits to the development of the building materials industry [1]. On the other hand, it can also alleviate the urban heat island effect and urban flooding caused by intensive population activities, and bring significant economic, social and environmental benefits for the development of building materials industry [3, 4]. However, due to the porous nature of pervious concrete and the thin bonding layer between aggregates, these proper-ties lead to its low mechanical proper-ties and poor durability, which greatly limits its further popularization and application [5,6,7].

Based on this, many scholars at home and abroad have carried out a large number of related studies around the preparation of pervious concrete with recycled aggregates, in which the addition of mineral admixtures, fiber admixtures, polymer-based admixtures and other external admixtures is considered to be one of the most effective ways to enhance the performance of pervious concrete [8,9,10,11,12,13,14,15,16,17]. Jul et al. [8] and Lutfur et al. [9] investigated the effects of fly ash, EVA emulsions, air-entraining agents and fibers on the permeability, compressive strength and freezing resistance of pervious concrete. The effects of fly ash, EVA latex, air-entraining agent and fiber on permeability, compressive strength and frost resistance of permeable concrete were investigated, and the results showed that the mechanical properties and frost resistance of permeable concrete with different admixtures were improved, but the degree of influence and the mechanism of action were different. Some researchers analyzed the mechanism to improve the mechanical properties and frost resistance of pervious concrete from macro and microscopic perspectives, and gave a reasonable range of mineral admixture mixing, and the results showed that mixing the appropriate amount of fly ash, silica fume, ultrafine mineral powder, nano-SiO2 and other admixtures in pervious concrete can improve the densification of the cementitious slurry, and improve the interfacial strength of the cementitious materials and aggregates as well as the microscopic pore structure[10,11,12,13].The results showed that the mixing of different aggregates can improve the mechanical properties of pervious concrete, but with different degrees of influence and mechanisms. Abdulkader et al. [14] concluded that PET fiber composites have a significant modifying effect on the compressive strength, tensile strength and elastic modulus of pervious concrete. Mahapara et al. [15] pointed out that the addition of polymerized fibers in pervious concrete can better inhibit the drying shrinkage of pervious concrete, and the strength is increased in a certain range. Xu et al. [16] investigated the addition of different types, amounts and lengths of short-cut fibers (basalt fibers, polyvinyl alcohol fibers, glass fibers) in pervious concrete, and the results showed that the permeability coefficient was reduced more significantly, and the basalt fibers were optimal for the improvement of the strength of pervious concrete. Yu et al. [17] studied the modification effect of basalt fibers and carbon fibers on recycled aggregate permeable concrete under the condition of water-cement ratio of 0.3, and the results showed that the admixture of fibers did not have much effect on the porosity and permeability coefficient of the concrete, but it could improve its strength and abrasion resistance.

It can be seen that scholars at home and abroad have fully affirmed the role of adding external admixture materials to enhance the performance of pervious concrete, and have deeply analyzed the influence mechanism of various external admixture materials on the performance of pervious concrete. However, there is still a lack of thematic research on the modification of recycled brick-mixed pervious concrete, which does not match the scale of China's construction industry. Moreover, there is a lack of research on the modification of recycled brick-mixed pervious concrete by adding two different admixtures to improve the performance of pervious concrete, and the strengthening strategy of recycled brick-mixed pervious concrete needs to be studied in depth. Therefore, three questions are raised about the modification of recycled brick-mixed pervious concrete in China:

  • Does the simultaneous addition of two different admixtures have the effect of further strengthening the performance of recycled brick-mixed pervious concrete?

  • How do different admixtures affect the performance of recycled brick-mixed pervious concrete?

  • What is the optimum admixture level for the mechanical and permeability properties of recycled brick-mix permeable concrete?

This paper proposes to mix fly ash and basalt fiber in the waste brick aggregate pervious concrete, through the use of different amounts of fly ash and basalt fiber to prepare recycled pervious concrete, to study its impact on the mechanical properties of recycled pervious concrete, durability and water permeability, and to derive the optimal mixing ratio, so that the mechanical properties and durability of recycled pervious concrete get the maximum improvement. This will provide experimental basis and technical reference for the application of recycled brick permeable concrete.

2 Materials and Methods

2.1 Materials

Recycled aggregate: the aggregate used in this experiment is selected from the construction waste generated by the demolition of a residential area in Hangzhou City, Zhejiang Province, through selective demolition and crushing, the collected concrete and brick mix crushing, sieving out the recycled concrete aggregate and recycled brick mix aggregate with a particle size of 9.5–13.2 mm for the test, the basic physical properties of recycled aggregate are shown in Table 1.

Table 1. Basic physical properties of recycled aggregates
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Cement: The selected strength grade is P-O 42.5 grade cement, its chemical composition and basic properties are shown in Table 2.

Table 2. Performance index of cement
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Fly ash: The experiments were conducted with fly ash from Hangzhou Banshan Power Plant, which was determined to be class II fly ash, the main performance indexes are shown in Table 3, and the chemical composition of fly ash is shown in Table 4.

Table 3. Performance parameters of fly ash
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Table 4. Table of basic chemical composition content of fly ash (%)
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Fine aggregate: fine aggregate selection of particle size 0.35 ~ 0.5mm, fineness modulus 2.3 ~ 3.0 sand.

Fiber: basalt fiber basic properties shown in Table 5.

Table 5. Basalt fiber basic properties
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Water reducing agent: adopt high-efficiency carboxylic acid water reducing agent, and the dosage of water reducing agent is 1% of the dosage of cement.

2.2 Mixing Ratio

Selected aggregate size of 9.5–13.2 mm recycled concrete coarse aggregate and recycled brick mix coarse aggregate, the effective water-cement ratio is 0.25, i.e., the ratio of mixing water and cementitious materials, sand rate is 2%, the mixing amount of recycled brick mix aggregate is 15%, and the mixing amount of fly ash is 0, 10%, 20%, respectively. Basalt fiber dosage was 0.05%, 0.1%, 0.2% respectively. The concrete mix ratios are shown in Table 6. 85% recycled concrete aggregate and 15% recycled brick aggregate were used in the mix ratios, and it has been shown in the literature [18] that when the admixture of brick aggregate is greater than 15%, the mechanical properties and frost resistance of recycled pervious concrete will be significantly reduced.

Table 6. Concrete mixing ratio
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2.3 Methods for Recycled Pervious Concrete

Mechanical properties

According to GB/T50081–2019 “Test Methods for Physical and Mechanical Properties of Concrete”, the flexural strength and compressive strength of recycled pervious concrete are tested, the size of the sample is 100 mm × 100 mm × 100 mm cube, and the loading rate of the universal testing machine is 0.5 kN/s, and the test result is accurate to 0.1 MPa.

Permeability

The method used to determine the effective porosity of permeable concrete in this test is the drainage method [18, 19]. The length, width and height of the specimen were measured accurately with a steel ruler to calculate its volume, then the specimen was immersed in water, and its mass m1 in water was measured after no bubbles were produced on the surface; the specimen was dried in an oven at 60 ℃ for 24 h, and then placed to cool down to room temperature after removal, and its mass m2 was measured; finally, the porosity P of the specimen was calculated according to the following formula.

$$ P = \left[ {1 - \frac{{m_{2} - m_{1} }}{{{\rho V}}}} \right] \times 100\% $$
(1)

where: P is the connected porosity, %; m1 is the mass of the specimen in water, g; m2 is the mass of the specimen after drying in the drying oven for 24 h, g; ρ is the density of water, g/cm3; V is the volume of the specimen, cm3.

The permeability coefficient of the specimen is determined according to the permeability meter test device in CJJ/T 253–2016 Technical Specification for Application of Pervious Concrete with Recycled Aggregate.

Frost Resistance

According to CJJ/T 253–2016 Technical Specification for Application of Pervious Concrete with Recycled Aggregate, anti-freezing test is carried out, the anti-freezing performance of recycled pervious concrete is measured by slow-freezing method, and the mass loss rate after 35 freeze-thaw cycles is recorded, and the mass loss rate is calculated as shown in Eq. (2):

$$ K = \frac{{m_{i} - m_{j} }}{{m_{i} }} \times 100\% $$
(2)

In the formula, K is the mass loss rate after j times of freeze-thaw cycle, %; mi is the mass of the ith freeze-thaw cycle, kg; mj is the mass of the jth freeze-thaw cycle, kg; j > i.

3 Results and Discussion

The effects of different amounts of fly ash and basalt fiber on the water permeability, mechanical properties and frost resistance of recycled pervious concrete were investigated. In order to optimize the comprehensive performance of recycled pervious concrete, it is proposed to mix the two kinds of admixtures on the basis of preparing mixed aggregate pervious concrete to obtain an optimal combination of ratios, so that the comprehensive performance of recycled pervious concrete can be optimized.

3.1 Mechanical Properties

Figure 1 shows the test results of compressive strength and flexural strength of permeable concrete with different fly ash admixtures. As can be seen from Fig. 1, when the fly ash is the same dosage, the compressive strength and flexural strength gradually increase with the increase of fiber dosage. When the dosage of fly ash was 0, 10%, and 20%, respectively, along with the increase of basalt fiber dosage from 0.05% to 0.2%, the compressive strength increased by 5.8%, 9.0%, and 10.0%, and the flexural strength increased by 9.8%, 11.8%, and 12.3%. This is because the basalt fiber in the concrete mixing process, and the cementitious materials in full contact, the fiber like a needle interspersed in the aggregate and the cementitious materials, taking up part of the effective pore space, to improve the mechanical occlusion effect of the interface between the cement paste and the aggregate, can withstand a greater impact when subjected to the load stress, showing better mechanical properties, which, the flexural strength increased due to the crack-blocking effect of concrete, can effectively slow down the cracking of concrete, and can effectively slow down the cracking of concrete. Effect, which can effectively delay the formation and expansion of concrete microcracks [20, 21].

Through horizontal comparison, it can be found that in the case of the same amount of basalt fiber mixing, compressive strength and flexural strength with the increase in the amount of fly ash showing a downward trend, when the basalt fiber mixing amount was 0.05%, 0.1%, 0.2%, respectively, with the increase in the amount of fly ash mixing from 0 to 20%, the compressive strength decreased by 14.0%, 12.4%, 10.6%, flexural strength decreased by 18.7%, 21.8%, 21.8%, 21.8%, 21.8% and 21.8%, respectively. 18.7%, 21.8%, 22.3%. The main reason for this phenomenon is that due to the partial replacement of cement by fly ash, the number of hydration products of concrete in hardening is reduced, coupled with the delayed chemical reaction of the active component of fly ash particles, the interstices of the water film layer around the particles are not filled, and the structural densification is poor, which leads to a reduction in compressive strength and flexural strength.

Fig. 1.
figure 1

Effect of different basalt fiber and fly ash dosage on compressive and flexural strengths

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Figure 2 shows the results of compressive strength and flexural strength tests for 28 and 56 d for some test groups. As can be seen from Fig. 2, the compressive strength and flexural strength increased with the increase of the curing time. And with the increase of fly ash dosage, the increase of compressive strength and flexural strength is more significant. This is consistent with the conclusion that fly ash admixture does not significantly enhance the early strength of concrete specimens, but significantly enhances the late strength of concrete, as described in the relevant literature [22, 23]. This is because the fly ash itself hydration reaction progresses slowly, not play the volcanic ash effect, in the maintenance of 28 d when only part of the fly ash completed hydration, the concrete still exists in part of the internal unhydrated fly ash; with the maintenance time up to 56 d, the active effect of the chemical reaction occurs, so that the concrete is more dense, the strength of the concrete is gradually improved.

Fig. 2.
figure 2

Compressive and flexural strength tests of some test groups under different curing periods

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3.2 Water Permeability

The water permeability of permeable concrete is related to the connecting porosity and water permeability coefficient, the larger the connecting porosity and water permeability coefficient, the stronger the water permeability of permeable concrete. Figure 3 shows the test results of connectivity porosity and water permeability coefficient of permeable concrete with different basalt fiber and fly ash admixtures. From Fig. 3, it can be seen that with the same amount of fly ash admixture, the connectivity porosity and water permeability coefficient have a significant decreasing trend with the admixture of basalt fibers. When the dosage of fly ash is 0, 10%, 20%, respectively, with the basalt fiber dosage increased from 0.05% to 0.2%, the connected porosity decreased by 2.8%, 2.9%, 3.3%, and the water permeability coefficient decreased by 9.8%, 11.8%, 12.3%, respectively. This is mainly due to the basalt fiber blending, along with the mixing of aggregate and cement slurry, the fiber will be wrapped by the cement slurry, the volume gradually increases, filling in the coarse aggregate, so that the pore space between the coarse aggregate gradually shrinks, which leads to the decrease of the connecting porosity and water permeability coefficient.

It can be found through horizontal comparison that when the basalt fiber doping is the same, the connecting porosity and water permeability coefficient show a slight decreasing trend. When the basalt fiber dosage was 0.05%, 0.1%, and 0.2%, respectively, with the increase of fly ash dosage from 0 to 20%, the connected porosity decreased by 0.83%, 1.12%, and 0.76%, respectively, and the permeability coefficient decreased by 4.66%, 3.27%, and 5.10%, respectively. It can be seen that the addition of fly ash has a slight effect on the water permeability of concrete in the early stage, which is because fly ash can reduce the friction between particles and particles, and the addition of fly ash in the appropriate range can improve the fluidity of the concrete mix, which will increase the thickness of the cement paste, however, in the early stage of the concrete fly ash has not yet been fully involved in the reaction, it will block some of the pores inside the concrete, which will cause a slight decrease in the connecting porosity and the water permeability. Slight decrease in water permeability properties.

Fig. 3.
figure 3

Effect of different basalt fiber and fly ash dosage on connected porosity and water permeability coefficient

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Figure 4 shows the comparison of the connected porosity and permeability coefficient of some test groups at 28 and 56 d. It can be found that with the increase of the maintenance time, the connected porosity and permeability coefficient of the seven test groups decreased, among which, the connected porosity and permeability coefficient of the six test groups doped with fly ash decreased by a larger magnitude than that of the blank control group. This is mainly due to the fact that the fly ash did not participate in the reaction at the early stage, but with the increase of the dosage of fly ash and the increase of the age of maintenance, the fly ash exerted its volcanic ash activity and reacted with alkaline substances to produce hydrated calcium silicate and calcium aluminate, which further filled up the pore space and decreased the water permeability performance continuously.

Fig. 4.
figure 4

Connected porosity and permeability coefficient tests for some test groups under different maintenance periods

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3.3 Frost Resistance

Mass loss rate is an indicator of the merit of frost resistance, the larger the mass loss rate, the worse the frost resistance, the smaller the mass loss rate, the better the frost resistance [24].

Fig. 5.
figure 5

Mass loss rate tests: (a) different fly ash dosage and basalt fiber dosage; (b) different test groups and conservation periods

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Figure 5a reflects the effect of different dosages of fly ash and basalt fiber on frost resistance. When the dosage of fly ash was the same, the mass loss rate showed a decreasing trend with the increase of basalt fiber dosage, and the mass loss rates of the three groups of single-doped fibers were lower than those of the blank control group, and the mass loss rate gradually decreased with the increase of fiber dosage. When the fly ash dosage was 0, 10% and 20% respectively, the mass loss rate decreased by 22.4%, 25.3% and 29.5% with the increase of basalt fiber dosage from 0.05% to 0.2%. However, at the same basalt fiber dosage, the mass loss rate tended to increase with the increase of fly ash dosage, indicating the decrease in frost resistance of the specimens, which increased by 60.2%, 51.7%, and 46.1% with the increase of fly ash dosage from 0 to 20% when the basalt fiber dosage was 0.05%, 0.1%, and 0.2%, respectively. Figure 5b shows the comparison of mass loss rate of some test groups at 28 and 56 d. It can be seen from the figure that the mass loss rate of the same type of test group at 56 d decreased dramatically compared with that of the test group with fly ash doping at 28 d. The mass loss rate of the same type of test group at 56 d decreased dramatically. The above data show that the addition of basalt fibers enhances the frost resistance of concrete, and the higher the dosage, the better the frost resistance, the reason is consistent with the mechanism of the fiber to improve the mechanical properties, the basalt fibers play its crack-blocking effect, which can reduce the number and length of plastic cracks, and improve the integrity and continuity of the concrete material [25]. Overall, the addition of fly ash makes the concrete specimens become denser with the growth of the curing age, and the frost resistance gradually improves, and the mechanism of action is consistent with the previous section to improve the mechanical properties.

Comprehensive analysis of the above results shows that the mixing of fly ash and basalt fiber can enhance the mechanical properties of recycled pervious concrete, in which the addition of fly ash will improve the long-term mechanical properties and durability of recycled pervious concrete, but it will make its water permeability decline, and the larger the admixture, the more obvious the decline in the water permeability, therefore, the admixture amount of fly ash is selected as 10%; basalt fiber will make up for the shortcomings of the pre-activation of fly ash, which will enhance the overall permeability of concrete. The addition of basalt fiber will make up for the shortcomings of low activity of fly ash in the early stage, and improve the mechanical properties and frost resistance of permeable concrete, but it will affect the water permeability, so the dosage of basalt fiber is selected as 0.15%, and considering the comprehensive performance of regenerated permeable concrete, the combination of fly ash dosage of 10% and the dosage of basalt fiber of 0.15% is selected as the double admixture.

4 Conclusion

In this study, we investigated the effects of fly ash and basalt fibers on the mechanical properties, water permeability and frost resistance of recycled pervious concrete through compound mixing, and came to the following conclusions:

(1) The mixing of fly ash has a certain effect on the early strength and water permeability of recycled pervious concrete, and with the increase of the curing time, the mechanical properties and frost resistance of the specimens are greatly improved.

(2) The admixture of basalt fiber can significantly improve the frost resistance and mechanical properties of recycled pervious concrete, but the basalt fiber in the mixing process of pervious concrete, will be covered with a layer of cement paste, and the aggregates are more closely connected, reducing the permeable pores, resulting in a reduction in the permeability of recycled pervious concrete.

(3) After the test, it was found that the mechanical properties and water permeability of recycled pervious concrete were best achieved when the mixing amount of fly ash was 10% and the mixing amount of basalt fiber was 0.05%.

(4) There are many factors affecting the physical and mechanical properties of recycled pervious concrete by fiber. Its type, length-diameter ratio, shape characteristics and dosage will affect the performance of recycled pervious concrete. Therefore, the study of the influence mechanism of fiber on recy-cled pervious concrete based on multi-factor coordination is the focus of sub-sequent research.