Experimental Study on Deformation Properties of Basalt Fiber Reinforced Recycled Aggregate Concrete

Abstract

In this paper, the effects of different substitution rates and basalt fiber (BF) contents on the deformation properties of recycled aggregate concrete (RAC) are studied, and the correlation degree of the changing parameters on the deformation properties of basalt fiber reinforced recycled aggregate concrete (BFRAC) is evaluated by gray correlation analysis. A total of 48 cylindrical test blocks are designed in this experiment, the failure modes of the test blocks are observed and analyzed, and performance indicators such as stress–strain curve, peak strain, and elastic modulus are obtained. The results show that the test block mainly suffers longitudinal splitting failure under uniaxial compression. The longitudinal cracks become denser and narrower with the increase in BF content. With the increase in BF content, the stress–strain curve decreases gradually, and the peak strain in the fully recycled aggregate concrete increases. There is no obvious change rule toward the peak strain with the increase in the substitution rate. The peak strain and elastic modulus of most test blocks show a trend of first decreasing and then increasing. At each substitution rate, when the BF dosage is 6 kg/m³, the elastic modulus of the test block is the minimum. Based on gray correlation analysis, the substitution rate has a greater impact on the deformation performance of BFRAC than fiber content. Therefore, an appropriate substitution rate has a better effect on improving the stiffness of test blocks and reducing the deflection of bending members.

1. Introduction

At present, building materials and natural resources are being increasingly wasted, which has become a worldwide problem and is being paid more and more attention [1,2]. In recent years, some scholars carried out research to solve this problem and proposed a variety of new methods to dispose of building materials waste, such as landfills, roadbeds, etc. [3,4]. Among these solutions, reusing waste concrete as a recycled aggregate is a good solution, which solves both economic and environmental problems and is considered to have great market potential [5,6]. However, research results show that there are still difficulties and drawbacks of using recycled aggregates in concrete. Studies have found that the performance of recycled aggregate concrete is lower than that of natural aggregate concrete, and there are also durability problems [7,8,9]. Moreover, compared with natural aggregate, recycled aggregate has higher water absorption and lower density, which reduces the mechanical properties of concrete to a certain extent [10]. Therefore, the use of recycled aggregate is hindered by its poor performance.

In order to improve the mechanical properties of recycled aggregate concrete, Marinkovic [11] showed that under the same mix ratio, the compressive strength and working performance of RAC are not as good as natural concrete, but the amount of cement increased by about 5% based on the RAC mix ratio, which can make up the difference between the two. In order to overcome the defects and ensure a high proportion of recycled aggregates in concrete, domestic and foreign scholars have proposed several methods [12,13] to treat recycled aggregates, expecting to change their mechanical properties. Among them, basalt fibers are one of the most reliable solutions currently on the market. Basalt fiber is a kind of synthetic fiber with excellent performance, which has excellent properties such as light structure, high-temperature resistance, environmental protection, and superior compatibility with RAC [14,15]. Incorporating basalt fiber into recycled aggregate concrete can effectively improve its mechanical properties. Some scholars have carried out related research in this area. Gao Yin [16] found that appropriate BF content can significantly improve the compressive performance of RAC. Fang [17] showed that the bridging effect of BF can effectively inhibit the generation and propagation of cracks in RAC specimens. Lian et al. [18] conducted a full-stage damage study of basalt fiber nano-calcium carbonate concrete during fracture to explain the toughening mechanism of basalt fiber and explore the fiber-strengthening mechanism. In conclusion, adding basalt fiber to recycled aggregate concrete can effectively increase strength and improve crack resistance.

In summary, some scholars have conducted extensive experimental studies on the physical and mechanical properties of recycled aggregate concrete and fiber-reinforced concrete and analyzed the effects of fiber content, fiber length, and substitution rate on the mechanical properties of RAC. There are limited literature reports on the analysis of the deformation properties of RAC and its influencing factors under the conditions of the amount of recycled coarse aggregate and the replacement rate of recycled coarse aggregate. In order to promote the application of BF in recycled aggregate concrete, more in-depth research on the performance of BFRAC is still needed. Therefore, this paper carried out the deformation performance test and parameter sensitivity analysis of RAC under multi-substitution rate and multi-fiber content.

2. Methods

2.1. Test Material

The test was based on the specification “Standard for technical requirements and test method of sand and crushed stone (or gravel) for ordinary concrete” (China GB/T14685-2011) for the actual measurement. Recycled aggregate is composed of stone and old mortar. In the process of crushing, mechanical extrusion, impact, and grinding, and other physical effects on the interface and interior impact the formation of many microscopic cracks, resulting in its strength being weaker than natural aggregate. Because the surface of the cement slurry attached to its surface is rough and porous, the water absorption is higher than that of natural aggregate. The concrete production, when mixed with recycled aggregate, will reduce the fluidity of concrete but improve the adhesion and water retention. The aggregates used in this experiment were natural aggregates and recycled aggregates. The recycled aggregates were obtained from a waste concrete pavement by mechanical crushing. The recycled aggregates were washed and sieved. The basic physical properties of coarse aggregate are shown in

Table 1. Basic physical properties of coarse aggregate.

Coarse Aggregate Type	Grain Size/mm	Apparent Density/(kg/m3)	Bulk Density/(kg/m3)	Water Absorption/%	Water Content/%	Crushing Indicators
Regeneration	5~20	2433	1274	3.5	1.8	12.7
Natural	5~20	2798	1681	0.1	0.7	10.4

. The gradation curves of natural and recycled coarse aggregates are shown in Figure 1. The cement was PO 42.5 grade ordinary Portland cement; the fine aggregate was natural yellow sand; fly ash was grade II fly ash produced by a power plant. The basic performance of basalt fiber is shown in

Table 2. Basic performance of basalt fiber.

Diameter/μm	Mean Fraction of Losses Length/mm	Density /(kg/m3)	Tensile Strength /Mpa	Elastic Modulus/Gpa	Elongation/%
15	18	2650	4500	104	3.1

.

2.2. Design and Preparation of Test Block

2.2.1. Mix Design

The experimental mix ratio is determined on the basis of ⟪Specification for mix proportion design of ordinary concrete⟫(JGJ 55-2011) [19]. The all-factor experimental design method is adopted to control the optimal mix ratio, and the optimal content of basalt fiber can be identified through horizontal experiments. The RAC mix ratio is shown in

Table 3. The mix design of RAC (kg/m³).

Replacement Rate	Water-to-Binder Ratio	Water	Added Water	Cement	Flyash	RA	NA	Sand	Water Reducer
0%	0.39	214	0.0	455.3	88.8	0.0	1196.6	536	2.73
50%	0.39	214	35.6	455.3	88.8	598.3	598.3	536	2.73
75%	0.39	214	53.5	455.3	88.8	897.5	299.1	536	2.73
100%	0.39	214	71.7	455.3	88.8	1196.6	0	536	2.73

.

2.2.2. Preparation of Test Block

In this paper, 48 BFRAC cylinders were designed with the substitution rate (0%, 50%, 75%, 100%) and fiber content (0 kg/m³, 2 kg/m³, 4 kg/m³, 6 kg/m³) as changing parameters, with a group of 3, and a total of 16 groups. The test block is 50 mm in diameter and 100 mm in height. We take FxRx as an example for the nomenclature of the test block. F0 ~ F6 indicate that the basalt fiber content is 0 kg/m³, 2 kg/m³, 4 kg/m³, 6 kg/m³, respectively, and R0 ~ R100 indicate that the substitution rate is 0%, 50%, 75%, 100%.

The production of the specimen was based on the specification “Standard for test method of performance on ordinary fresh concrete” (China GB/T50080-2016). The mixer adopted was a HJW-60 type mixer. First, recycled coarse aggregate and fine sand was added, followed by stirring for 30 s; then cement was added and stirred for 30 s; and finally, we added water, fiber, and water reducer and stirred for 60 s until uniform.

According to “Standards for Test Methods for the Performance of Ordinary Concrete Mixtures” (China GB/T50080-2016), the test block was formed with a standard mold. After pouring, vibrating, compacting, and smoothing, it was left to stand for 24 h. After the test block was removed from the mold, we put it into a standard curing room (20 ± 2 °C, relative humidity >95%) for 28 days to meet the molding quality of the test block according to standard “Standard for evaluation of concrete compressive strength” (China GB/T 50107-2010), to assess the strength of the fabricated blocks as C30.

2.3. Loading Device and System

The cylindrical BFRAC test block was axially compressed to failure using a RMT-150 electro-hydraulic servo testing machine(Institute of Rock and Soil Mechanics, Chinese Academy of Sciences, Wuhan, China). In order to ensure the accuracy of the test and reduce the adverse effects caused by the uneven surface of the test block, the gap between the loading surfaces, and the insufficient stiffness of the pressure testing machine at the initial stage of loading, the test block was preloaded before loading. In preloading, a loading rate of 0.6 MPa/s was used to load to 1/3 of the axial compressive strength of the test block [20], and displacement control was used in formal loading with a loading rate of 0.005 mm/s. The test loading is shown in Figure 2.

3. Test Results and Analysis

3.1. BFRAC Failure Form

When the parameters are changed, the failure form of the test block is shown in Figure 3. In comparison, under different substitution rates, there is no obvious difference in the failure morphology of the test blocks; however, the fiber content has a significant effect. At the initial stage of loading, the surface of the BFRAC test block was crack-free; as the load increased, the test block expanded laterally, and the micro-cracks existing inside the recycled aggregate and between the new and old mortar and the recycled coarse aggregate began to develop, while the BF distributed chaotically inside the test block at this time prevented the expansion of the cracks to a certain extent, making the crack generation and development tend to a relatively stable state. With the further increase in the load, the width and number of cracks increased sharply, the test block longitudinally produced splitting damage, the crack width increased, the internal aggregate was exposed, and the bearing capacity decreased. Thereafter, the external force increased again, the cement paste at the main crack lost the bond between the aggregate and BF, the frictional bite force on the sliding surface was basically exhausted, and finally, the test block completely lost its bearing capacity. It can be seen from Figure 3 that the test block eventually underwent longitudinal splitting failure. With the increase in fibers, the longitudinal cracks became denser and narrower. This shows that when the test block was damaged by longitudinal splitting, BF had the effect of toughening and crack resistance inside the test block, which reduced the generation and expansion of cracks to a certain extent.

3.2. BFRAC Stress–Strain Curve

The stress–strain curves of the test block under different replacement rates of recycled aggregate and fiber content measured in the test are shown in Figure 4. It can be seen that each curve of the test block has an obvious peak point, and the intensity decreases rapidly after passing the peak point. The parameter change has an obvious influence on the curve of the test block. The curve of the descending section of recycled aggregate concrete is steeper than that of ordinary concrete, indicating that the ductility of the test block is reduced after the addition of recycled aggregate. With the increase in fiber content, the descending section of the curve becomes smoother, the lower surface gradually increases, and the whole tends to be full. This is because the tie effect of fibers inside the test block can effectively prevent the generation and development of cracks. Its toughening and crack resistance effect increases the ductility of the test block, making the descending section of the curve relatively gentle.

3.3. Deformation Performance Index of BFRAC Test Block

The deformation performance indicators such as peak strain ε_v and elastic modulus E of the test block under different parameters (take the secant modulus at 0.4 times the peak stress in the ascending section of the axial stress–strain curve of the cylindrical test block [21]) are shown in

Table 4. Deformation performance index of BFRAC test block.

Specimen Number	εv/10−3	E/GPa
F0R0	6.17	9.83
F0R50	6.92	7.13
F0R75	5.47	8.54
F0R100	5.26	8.91
F2R0	5.91	8.32
F2R50	4.79	7.34
F2R75	6.18	8.31
F2R100	6.26	9.58
F4R0	5.04	9.91
F4R50	5.88	9.54
F4R75	5.14	6.93
F4R100	6.68	7.32
F6R0	6.17	5.14
F6R50	5.25	6.42
F6R75	6.02	4.61
F6R100	6.86	3.42

. In order to reduce the error, three replicate test blocks were tested under each design parameter, and the average value was calculated under the premise of valid data.

4. Analysis of the Influencing Factors of Deformation Performance

4.1. BFRAC Peak Strain

4.1.1. Substitution Rate

The effect of substitution rate on the peak strain of BFRAC is shown in Figure 5. It can be seen that with the increase in the substitution rate, the axial peak strain of the test block first increases and then decreases. On the one hand, due to the inherent defects of low stiffness and large porosity of recycled coarse aggregate, which reduces the overall stiffness of RAC, the test block has a large deformation when it reaches the peak bearing capacity. On the other hand, due to the low strength of the recycled coarse aggregate and a large number of old mortars attached to the surface, RAC damage and failure are advanced in the loading process, leading to a gradual decrease in peak strain with the increase in replacement rate. These two factors may account for the above trend.

4.1.2. Fiber Content

The effect of fiber content on the peak strain of BFRAC is shown in Figure 6. It can be seen that with the increase in fiber content, the peak strain in the fully recycled aggregate concrete increases, but the axial peak strain of most of the test blocks has no obvious change law. This is because, in the elastic–plastic stage, there are fewer micro-cracks inside the test block, the overall deformation is small, and the tie effect of fibers does not play a large role in this process.

4.2. BFRAC Elastic Modulus

4.2.1. Substitution Rate

The effect of substitution rate on the elastic modulus of BFRAC is shown in Figure 7. It can be seen that with the increase in the substitution rate, the elastic modulus of most of the test blocks shows a trend of first decreasing and then increasing. This is because the cement base adhered to the surface of the recycled coarse aggregate and the internal cracks affect the elastic modulus of the recycled aggregate concrete. Therefore, with the increase in recycled coarse aggregate, when the fiber dosage is 4 kg/m³, compared with 0% substitution rate, the elastic modulus of the specimen with 100% substitution rate decreases by up to 26.1%.

4.2.2. Fiber Content

The effect of fiber content on the elastic modulus of BFRAC is shown in Figure 8. It can be seen that when the fiber content is 6 kg/m³, the elastic modulus of the test block is the smallest at the same substitution rate among the elastic moduli of all test blocks. When the substitution rate is 0% and the fiber content is 4 kg/m³, the elastic modulus of the test block is the largest, reaching 9.91 GPa. Compared with the fiber content of 0 kg/m³, when the fiber content is 2 kg/m³, 4 kg/m³_, and 6 kg/m³, respectively, the elastic modulus of the test block decreases by about 61.6%. Although BF and concrete are compatible, the incorporation of excessive fibers will reduce the density of the test block, so that the weak interface between the fiber and the concrete matrix is increased and the elastic modulus is reduced.

5. Parameter Sensitivity Analysis

Gray relational analysis is an analysis method that can obtain the influence and correlation degree of each factor on the main behavior by quantitatively analyzing the data of each factor, and can effectively distinguish the main factor and the secondary factor. Therefore, the sensitivity of different parameters to the deformation performance of BFRAC can be obtained by gray relational analysis. The specific steps are as follows:

(a) We select the optimal peak strain and elastic modulus in each test block as the reference sequence X₀(k) and use the substitution rate and fiber content variation parameters as the comparison sequence X_m (k), where k = 1, 2, …, p and m = 1, 2, …, q.

$$x_m(k)=\frac{X_m(k)}{\frac{1}{p}{\displaystyle {\sum }_{m=1}^p{X}_m(k)}}$$

(1)

(b) The calculation method of the gray correlation coefficient ξ_m is shown in Formulas (2)–(5).

$${\mathsf{\xi }}_m\left. [x_0(k),x_m(k)\right]=\frac{\underset{m=1,p}{\min }\underset{k=1,q}{\min }{\mathsf{\Delta }}_m(k)+\rho \underset{m=1,p}{\max }\underset{k=1,q}{\max }{\mathsf{\Delta }}_m(k)}{\mathsf{\Delta }i+\rho \underset{m=1,p}{\max }\underset{k=1,q}{\max }\mathsf{\Delta }i(k)}$$

(2)

$${\mathsf{\Delta }}_m(k)=\left|x_0(k)-x_m(k)\right|$$

(3)

$$\underset{m=1,p}{\min }\underset{k=1,q}{\min }{\mathsf{\Delta }}_m(k)=\underset{m}{\max }(\underset{k}{\max }\left|x_0(k)-x_m(k)\right|)$$

(4)

$$\underset{m=1,p}{\max }\underset{k=1,q}{\max }{\mathsf{\Delta }}_m(k)=\underset{m}{\min }(\underset{k}{\min }\left|x_0(k)-x_m(k)\right|)$$

(5)

where ρ is the coefficient, ρ ∈ [0,1], usually ρ = 0.5 [22].

(c) The calculation method of gray correlation degree r_m is shown in Formula (6), and r_m can evaluate different sequence levels. The closer the r_m is to 1.0, the stronger the correlation between the reference and comparison sequences.

$$r_m=\frac{1}{p}{\displaystyle {\sum }_{m=1}^n{\mathsf{\xi }}_m\left. [x_0(k),x_m(k)\right]}$$

(6)

The correlations between the peak strain and elastic modulus and the changing parameters, respectively, are shown in

Table 5. Correlation between peak strain and elastic modulus, respectively, and variation parameters.

rm	Peak Strain	Elastic Modulus
Replacement ratio	0.6989	0.5617
Basalt fiber content	0.6657	0.5884

. It can be seen that the correlation between the substitution rate and the peak strain of BFRAC is greater than that of the fiber content. The correlation between the fiber content and the elastic modulus of BFRAC is slightly greater than that of the substitution rate, and the correlation degree between the two is relatively similar. Therefore, for the deformation performance of BFRAC, the substitution rate can be considered first. By selecting an appropriate substitution rate, the stiffness can be increased and the deflection of the flexural member can be reduced.

6. Conclusions

(1) Through comparison of the failure patterns, there is no obvious difference in the failure patterns of the test blocks under different substitution rates, but the fiber content has a significant impact. With the increase in fiber content, the longitudinal crack gradually becomes thinner and narrower, which indicates that when the test block is destroyed by longitudinal splitting, BF produces a toughening and cracking effect inside the test block.

(2) The parameter changes have a significant impact on the stress-strain curve of the test block, and the curve of the descending section of the recycled aggregate concrete is steeper than that of ordinary concrete, indicating that the ductility of the test block is reduced after the recycled aggregate is added. With an increase in fiber content, the downward section of the curve becomes more and more gentle, and the lower surface gradually increases, and the overall tendency is full. The toughening and cracking effect of BF increases the ductility of the test block, so that the descending section of the curve is relatively flat.

(3) With the increase in fiber content, the peak strain in fully recycled aggregate concrete increases, and the axial peak strain of most test blocks has no obvious change law. The tension effect of fibers plays a small role in this process. With the increase in the substitution rate, the elastic modulus of most test blocks shows a trend of first decreasing and then increasing. At each substitution rate, when the BF dosage is 6 kg/m³, the elastic modulus of the test block is the minimum.

(4) According to the gray correlation analysis, the substitution rate has a greater impact on the deformation performance of BFRAC than the fiber content. As a result, when measuring the deformation performance of BFRAC, the substitution rate should first be considered, and the appropriate substitution rate can be used to improve the stiffness of the member and reduce its deflection.