1. Introduction
The global use of concrete has exceeded 4 billion m
3/year, and the use of sand and gravel materials has exceeded 6 billion to 8 billion tons/year [
1,
2]. Mining is used to satisfy such a large demand for sand and aggregates. However, mining consumes many natural resources and causes environmental pollution and ecological damage [
3,
4,
5]. In addition, the demolition of old houses, factories, and other buildings for various reasons generates a large amount of waste concrete. The annual production of construction and demolition waste in China reaches 1.8 billion tons and is increasing year by year [
6,
7,
8]. The most common practice at present to treat waste concrete is to pile it up or place it in landfills [
9,
10,
11,
12].
The mining method not only occupies a large land resource but also causes environmental pollution. On the contrary, waste concrete can be crushed and ground to obtain recycled aggregate, which can be partially or completely replaced by recycled aggregate to prepare recycled concrete as per previous research [
13,
14]. To a certain extent, it alleviates the pressure of natural aggregate extraction and solves the land occupation and environmental pollution problems caused by waste concrete, thus realizing the green and sustainable development of waste concrete [
15,
16,
17]. However, compared with natural aggregates, waste concrete aggregates have the disadvantages of high water absorption, high porosity, low apparent density, and a high crushing value, resulting in the low strength and poor durability of the prepared recycled concrete. Therefore, it is crucial to improve the performance of recycled concrete [
18,
19,
20].
Researchers have delved into two primary approaches for modifying recycled aggregates, aiming to ameliorate their detrimental impact on new concrete. The first approach involves employing biological, chemical, and mechanical techniques [
21,
22,
23,
24], among others, to mitigate the adverse effects of the recycled aggregate’s subpar quality. This includes strategies like eliminating the residual adhesive mortar from the aggregate’s surface and enhancing the adhesive mortar’s performance. The second approach entails enhancing the overall quality of synthesized recycled concrete directly by incorporating supplementary materials, such as polymers and mineral admixtures [
25,
26,
27,
28,
29,
30]. Heat treatment stands out as a favorable approach for dissolving weakly bonded mortar because of its straightforward application and cost efficiency. However, this method is contrary to the low-carbon objective, as it triggers additional energy consumption. An alternative method, mechanical treatment, is widely employed in detaching bonded mortar from the original natural aggregate, yielding premium recycled aggregate [
19]. This technique employs mechanical force to crush and dislodge the bonding mortar. Yet, given the collision and grinding inherent in the process, there is a propensity for detrimental impacts on the structural integrity of recycled aggregate particles. This may lead to alterations in the geometric attributes of these particles, thereby potentially introducing new microcracks [
31,
32,
33,
34]. Subsequently, acid prepreg technology was developed; it has the ability to effectively improve the quality of recycled aggregates by eliminating bonding mortar. However, usually after treatment with hydrochloric or sulfuric acid, the chloride and sulfate content of the aggregate will increase, which, in turn, affects the durability of the concrete. In recent years, there is more and more research on carbonization to improve the properties of recycled aggregates because carbonization can increase the density of cementified materials. Liang et al. [
35] studied the carbonization behavior of concrete containing carbonized recycled aggregate in different environments, and the results showed that a CO
2 curing treatment improved the physical and chemical properties of recycled aggregate (such as significantly reducing the water absorption of RA). The recycled concrete prepared by adding carbonized fine and coarse recycled aggregate has higher compressive strength and lower carbonization depth, and the carbonization depth decreases more obviously [
36,
37,
38].
Other enhancements involve using various fibers, such as steel fibers, polyvinyl alcohol fibers, carbon fibers, and so forth, to improve the strength and toughness of recycled concrete [
39,
40,
41]. Steel fibers have a strengthening and toughening effect on recycled aggregates, but they are heavy. Carbon fiber is expensive. Basalt fiber, as a kind of green material, has a high tensile strength, a high modulus of elasticity, a low price, and enhanced toughening [
16,
42,
43,
44]. Basalt fiber has thereby recently been considered a potential reinforcement material to strengthen concrete prepared by recycled aggregates. Li et al. [
45] studied the influence of different fiber contents on the mechanical properties of fiber-reinforced concrete (HFRC), especially the shear strength and toughness, and obtained the optimal fiber content in HFRC. The results show that the compressive strength of HFRC is higher than that of plain concrete when the content of basalt fiber is less than 0.15%. Algin and Ozen et al. [
46] studied the application of basalt fiber in the production of self-compacting concrete (SCC) to determine the influence of fiber addition on the working and mechanical properties of concrete. The results show that, when the fiber length is 12 mm and the fiber content is 0.1%, the compressive strength can be improved. At present, most studies consider only the length of fiber or the content of fiber and other single factors for fiber-reinforced recycled concrete. It is necessary to use basalt fiber with different contents (0.1~0.3%) and different lengths (6~18 mm) to determine the optimum content and length of fiber. It is necessary to consider the influence of many factors on the slump, compressive strength, splitting tensile strength, and bending strength of recycled concrete. Zhang et al. [
42] first proposed the composite effect based on the fiber reinforcement theory. The effects of the mixture of the regenerated coarse aggregate replacement rate and BF content on the failure mode, mechanical properties, and interfacial transition zone properties of regenerated concrete were studied by conducting a macroscopic performance analysis and microscopic test. The results show that the basic mechanical properties of recycled concrete with different BF contents are better than ordinary concrete when the replacement rate of recycled aggregate is 50%.
We conducted an experimental study on the basalt fiber-reinforced recycled aggregate concrete. The novelty lies in the use of a mathematical model established by the Taylor formula and the associated surface trend map in analysis.
4. Conclusions
This work provides a feasible approach to strengthening recycled aggregate concrete. The basalt fiber as an environmentally friendly fiber in a small dosage effectively lifts the strength of concrete prepared with recycled aggregates. Through the study of basalt fiber-reinforced recycled concrete, the following conclusions can be drawn.
Basalt fibers reduce the fluidity of recycled concrete, increase the friction between the cement matrix, and have a certain enhancement and toughening effect on the cubic compressive strength of recycled concrete and the splitting tensile strength.
Regarding the same fiber admixture, the increase in the replacement rate of recycled coarse aggregate shows a more stable upward trend. Under the same fiber dosage, the increase in the replacement rate of recycled coarse aggregate results in a stable upward trend. The fiber in a dosage of 0.5% makes the compressive strength of concrete prepared with 100% recycled aggregates comparable to that of natural aggregate concrete, whilst achieving a c.a. 90% splitting tensile strength.
For the splitting tensile strength, the fluctuation of the splitting tensile strength of recycled concrete at a 0.1% fiber dosage tends to flatten out, and basalt fibers enhance the splitting tensile strength of concrete with a larger recycled aggregate substitution rate obviously.