Mechanical Properties of Nano-CaCO3 and Basalt Fiber Reinforced Concrete: Experiments and Numerical Simulations

DIAO Mushuang, REN Qianhui, SUN Xinjian, ZHAO Yawei, WEI Chengpeng

(State Key Laboratory of Plateau Ecology and Agriculture, School of Water Resources and Electric Power, Qinghai University, Xining 810016, China)

Abstract: In this study, the compressive, split tensile, and flexural strengths of concrete with nano-CaCO3 only were compared with those of concrete with nano-CaCO3 and basalt fibers through field experiments,and the underlying mechanisms were analyzed by the Scanning Electron Microscope (SEM) techniques. On the mesoscale, a damage model of concrete was established based on the continuum progressive damage theory, which was used to investigate the influence of different lengths and contents of fibers on the mechanical properties of concrete. Then, the experimental and numerical simulation results were compared and analyzed to verify the feasibility of model. The results show that nano-CaCO3 can enhance the compressive strength of the concrete, with an optimal content of 2.0%. Adding basalt fibers into the nano-CaCO3 reinforced concrete may further enhance the compressive, split tensile, and flexural strengths of the concrete; however, the higher content of basalt fiber can not lead to higher performance of concrete. The optimal length and content of fiber are 6 mm and 0.20%, respectively. The SEM result shows that the aggregation of basalt fibers is detrimental to the mechanical properties of concrete. The numerical simulation results are in good agreement with the experimental results.

Key words: concrete; nano-CaCO3; basalt fiber; mechanical property; microscopic characterization;numerical simulation

1 Introduction

Concrete is one of the most important building materials in the construction of large hydraulic structures, but due to its inherent shortcomings such as poor ductility and brittleness, cracks often appear in the concrete structure under load, which may lead to a risk of fracture[1,2]. In the dam structure, the complexity of the load and working environments makes this phenomenon more serious. Because of the extreme natural conditions (e g, drought and cold), large temperature difference between day and night, and high radiation in the Qinghai-Tibet Plateau, cracks and fractures have been found in many dam structures in recent years[3].Therefore, ordinary concrete, which contains a great amount of initial structural defects and cracks, can no longer meet the safety and durability requirements of the dam structure. It is worth noting that, with the development and applications of high-performance concrete materials, these problems are being gradually solved.

Nanomaterials refer to the materials with a particle size between 1-100 nm, which are known as “the most promising materials in the 21st century”[4]. The performance superiority of nanomaterials is mainly manifested in the size effect, quantum effect, surface effect andetc. Thus, a growing number of scholars have begun to study the possibility of adding nanomaterials into cement-based materials to form composite concrete materials to acquire an enhanced performance[5,6]. At present, the inorganic nanomaterials that have been added into concrete mainly include nano-SiO2and nano-CaCO3. The related experimental studies[7-9]have shown that the addition of nano-SiO2can accelerate the hydration rate of cement, shorten the setting time, reduce the porosity, and improve the early compressive strength of concrete. Compared with nano-SiO2, nano-CaCO3can also enhance the performance of concrete; moreover, owing to the easy preparation and low cost of nano-CaCO3, its applications are wider in engineering practice. The results of Qian[10]showed that the addition of nano-CaCO3could increase the compressive strength of the slurry. It was also found that the crystal form of cement hydration products could be refined through crystal nucleation, and nano-CaCO3could be filled into microscopic pores of the cement base material to improve the microstructure of the concrete. Shaikh[11]found through experiments that, when the addition amount of nano-CaCO3was 1%, the compressive strength of the concrete would increase along with decreased porosity, increased water resistance and increased chloride ion permeability and diffusivity. Other studies on nano-CaCO3reinforced concrete[12-14]also showed that the enhancement of concrete performance was closely associated with the nucleation effect, chemical activity, micro-aggregate filling effect and other properties of nano-CaCO3. At present, there are limited studies on the enhancement of concrete above C50 by adding nano-CaCO3. Compressive strength is the most basic mechanical performance indicator of concrete, which is usually used as a basic parameter to determine the strength grade of concrete.Therefore, comprehensive research on the evaluation of the compressive strength of concrete is essential.

In recent years, many scholars have attempted to improve the toughness and mechanical properties of concrete by adding fiber materials[15,16]. Denget al[17,18]examined the fracture parameters and fatigue properties of carbon fiber reinforced concrete through the threepoint bending experiment, and the results showed that the critical crack opening displacement of carbon fiber concrete was significantly higher than that of plain concrete and carbon fibers could significantly improve the fracture resistance of the concrete under bending fatigue load. Bencardinoet al[19]investigated the fracture characteristics and behaviors of polypropylene fiber reinforced concrete and found that polypropylene fibers could improve the structural integrity and durability of the concrete. Kazemi[20]conducted an experiment to test the three-point bending notch beam fracture performance by adding steel fibers into the concrete at different volume ratios and concluded that steel fibers could improve the fracture toughness and the effective crack length of the concrete. The raw material of basalt fiber is a kind of volcanic rock, belonging to an inorganic non-metallic material. It is known as the “green industrial raw material” of the 21st century. As a new concrete reinforcement material, basalt fiber is characterized by a number of advantages such as excellent mechanical properties, outstanding thermal properties and stable chemical properties, compared with other types of fibers[21-23]. Therefore, research on the mechanical properties of basalt fiber concrete is of an important theoretical significance and practical value, and plays a critical guiding role in resolving the strength defects of hydraulic concrete in the Qinghai-Tibet area.

On the mesoscale, basalt fiber concrete can be classified as a multiphase composite material. Its failure process is extremely complicated, while experiments can only study its mechanical properties from a macroscopic perspective. Therefore, it has become an effective method to establish a basalt fiber concrete meso-material model using numerical simulation to examine the meso-structure and macro-mechanical properties of the concrete in an integrated manner. So far, only a small number of scholars have studied the mechanical properties of fiber reinforced concrete using numerical simulation. For example, Zhanget al[24]pointed out that the variation of the fracture strength factor could be used to indicate the increase in the fracture strength caused by the fibers. Enfedaqueet al[25]established the fracture toughness cohesion model of glass fiber concrete using ABAQUS. Caoet al[26,27]applied the concrete elastoplastic damage model to examine and analyze the failure behaviors of fiber reinforced concrete under different addition amounts. Owing to the development of computer technology, more meso-models have been established to study the damage evolution of fiber reinforced concrete under loading conditions[28-30], but there has been no comprehensive experimental research and numerical simulation on the establishment of hydraulic basalt fiber reinforced concrete models. This study aimed to combine experiments with the finite element method so as to examine the mechanical properties of basalt fiber reinforced concrete through multi-scale coupling analysis.

2 Experimental

2.1 Raw materials

The cement used in the experiment was P·O 42.5 ordinary Portland cement; the performance indicators of cement were shown in Table 1. The fly Class II fly ash was used as mineral admixture. The coarse aggregates were obtained locally in Qinghai with a particle size of 5-20 mm and a saturated surface dry density of 2 660 kg/m3. The fine aggregates adopted quartz sands, which were mixed according to the followingratio: 10-30 mesh : 30-50 mesh : 50-100 mesh = 4.5: 4.5 : 1; the fineness modulus after mixing was 2.38.The high-efficiency polycarboxylic acid water reducing agent made by Jiangsu Sobute New Materials Co.,Ltd was adopted. The water was simply obtained from tap water. The nano-CaCO3was supplied by Jiangxi Bairui Calcium Carbonate Co., Ltd.; the testing results of its performance indicators were shown in Table 2.The chopped basalt fibers manufactured by Zhengzhou Dengdian CBF Co., Ltd. were used, with lengths of 3,6, and 9 mm respectively; the performance indicators were shown in Table 3.

Table 1 Performance indicators of cement

Table 2 Performance indicators of nano-CaCO3

Table 3 Performance indicators of basalt fiber

Table 4 Mix proportions of NC/kg

2.2 Mix proportion

In this experiment, nano-CaCO3was used to replace partial cement according to the mass percentages of 0.0%, 1.5%, 2.0%, 2.5%, 3.0%, and 3.5%, respectively. The corresponding specimens prepared were indexed as PC, NC-1.5%, NC-2.0%, NC-2.5%, NC-3.0%, and NC-3.5%, respectively. The mix proportions of nano-CaCO3concrete (NC) are shown in Table 4.

The experimental results showed that the reinforced concrete had relatively better mechanical properties when nano-CaCO3was mixed at the proportion of 2.0%. Therefore, when the influence of basalt fiber on the mechanical properties of concrete was explored,the proportion of nano-CaCO3was fixed at 2.0%. The basalt fibers were added at the volume percentages of 0.05%, 0.10%, 0.15%, 0.20%, 0.25% and 0.30%to replace coarse aggregates, and the corresponding specimens were indexed as NCBC-X-0.05%, NCBC-X-0.10%, NCBC-X-0.15%, NCBC-X-0.20%,NCBC-X-0.25%, and NCBC-X-0.30% (where X represents the length of basalt fiber), respectively. The mix proportions of the nano-CaCO3and basalt fiber reinforced concrete (NCBC) are shown in Table 5.

2.3 Mechanical property experiment

Compressive strength tests were performed on NC specimens at the ages of 3, 7, and 28 d respectively.All the specimens were designed to be 150 mm×150 mm×150 mm cubes. Each group of compressive strength test was repeated for 3 times, and the average value was taken for analysis. Compressive, split tensile,and flexural tests were performed on NCBC specimens at the age of 28 d. The specimens used for compressive and split tensile tests were designed to be 150 mm×150 mm×150 mm cubes. The specimens used for flexural strength test were designed to be 100 mm×100 mm×400 mm blocks. Moreover, samples were collected from the center part of the specimens after damage and were soaked in anhydrous ethanol solution for fur-ther analysis. A sample of about 5 mm was selected for SEM observation.

Table 5 Mix proportions of NCBC/kg

3 Results and discussion

3.1 Test results of NC compressive strength

According to the results of the compressive tests on the concrete specimens with different ages and nano-CaCO3contents, it can be found that the failure process of NC is similar to that of ordinary concrete.During the loading process, the specimens have a peeling-off phenomenon first at the surface area; as the load increased, cracks are gradually formed until penetrating the entire specimen. The failure modes of NC are shown in Fig.1.

Fig.1 NC compressive failure modes

Fig.2 Increase percentage of compressive strength

The compressive strength of the concrete specimen can be calculated by Eq.(1):where:fccis the compressive strength of the concrete specimen, MPa;Fis the failure load, N;Ais the pressure-bearing area of the specimen, mm2. The calculation results are shown in Table 6 and Fig.2.

Table 6 NC compressive strength/MPa

According to Table 6 and Fig.2, it can be seen that the compressive strength of concrete could be improved by adding nano-CaCO3at different ages. Specifically, for the ages of 3 and 7 d, the specimens mixed with 2% of nano-CaCO3achieve the largest increase of compressive strength, which are 3.93% and 5.21%,respectively. For the age of 28 d, the specimen mixed with 2.5% of nano-CaCO3achieves the largest increase of compressive strength, which was 8.42%. Therefore,the higher nano-CaCO3content can’t lead to higher mechanical properties of concrete. By considering the actual application situation comprehensively, the addition content of nano-CaCO3should be controlled at 2.0% to achieve a satisfactory improvement effect.

3.2 Test results of NCBC compressive strength

The compressive failure modes of NCBC are shown in Fig.3. By comparing Fig.1 and Fig.3, it can be clearly seen that the specimens mixed with basalt fibers have a relatively integral shape after failure;there is a lower level of peeling-off at the surface area,and the number of longitudinal cracks are reduced.Therefore, the addition of basalt fibers can improve the anti-deformation ability of concrete.

The results of compressive strength calculated according to Eq.(1) are shown in Table 7 and Fig.4.

Fig.3 NCBC failure modes: (a) NCBC-3 mm-0.20%; (b) NCBC-6 mm-0.20%; (c) NCBC-9 mm-0.20%

Table 7 NCBC compressive strength/MPa

Fig.4 Increase percentage of compressive strength

According to Table 7 and Fig.4, the compressive strengths of NCBC specimens with different fiber lengths basically increase first and then decrease with the increase of the basalt fibers contents. The basalt fibers with the length of 3, 6 and 9 mm achieve the largest increase percentage of compressive strength (i e,15.17%, 14.03%, and 7.77% respectively) at the addition amount of 0.20%, 0.20% and 0.15% respectively.With the increase of the fiber length, the increase percentage of compressive strength gradually decreases.With the increase of the basalt fibers contents, the compressive strength of some specimens does not increase,and even decreases to a certain extent. Specifically, the compressive strength of NCBC-6-0.30% decreases by 4.25%, that of NCBC-9-0.25% decreases by 0.18%,and that of NCBC-9-0.30% decreases by 4.48%. Based on a comprehensive analysis, it is found that the improvement effect is relatively excellent when the basalt fibers content is 0.20%.

3.3 Test results of NCBC split tensile strength

During the loading process, a crack first appeared around the longitudinal center of the specimen. With the increase of the load, the crack gradually expands until it penetrates the entire specimen. There are basically no differences in the failure process and failure mode between the specimen with and without basalt fibers.

The split tensile strength of the concrete can be calculated by Eq.(2):

where:ftsis the split tensile strength of the concrete specimen, MPa;Fis the failure load, N;Ais the pressure-bearing area of the specimen, mm2. The specific results calculated by Eq.(2) are shown in Table 8 and Fig.5.

Table 8 NCBC split tensile strength/MPa

Fig.5 Increase percentage of split tensile strength

According to Table 8 and Fig.5, the split tensile strengths of NCBC specimens with different fiber lengths basically increase first and then decrease with the increase of the basalt fibers contents. The basalt fibers with the length of 3, 6 and 9 mm all achieve the largest increase percentage of split tensile strength (i e,6.07%, 7.52% and 7.28% respectively) at the addition amount of 0.20%. When the content of basalt fibers is less than 0.20%, the split tensile strengths of all specimens show an increasing trend; when the content exceeds 0.20%, the increase percentage begin to decrease;when the content is equal to 0.30%, the split tensile strength of the concrete specimen mixed with basalt fibers is lower than that of the specimen without basalt fibers. Specifically, the split tensile strength of NCBC-3-0.30% decreases by 1.46%, that of NCBC-6-0.30%decreases by 4.85%, and that of NCBC-9-0.30% decreases by 3.64%. Based on comprehensive analysis, it is found that the improvement effect is relatively good when the addition amount of basalt fibers is 0.20% for all the three fiber lengths.

3.4 Test results of NCBC flexural strength

During the loading process, the specimen would make a “bang” sound and break from the middle part upon failure. There are basically no differences in the failure process and failure mode between the specimen with and without basalt fibers. Both failure modes belong to instantaneous failure.

The flexural strength of the concrete can be calculated by Eq.(3):

where:ftis the flexural strength of the concrete specimen, MPa;Pis the failure load, N;lis the span (l=3h), mm;bis the cross-section width of the specimen,mm;his the cross-section height of the specimen, mm.The results calculated by Eq.(3) are shown in Table 9 and Fig.6.

Fig.6 Increase percentage of flexural strength

Table 9 NCBC flexural strength/MPa

According to Table 9 and Fig.6, it can be seen that adding basalt fibers into nano-CaCO3concrete can increase the flexural strength. For 3 and 6 mm basalt fibers, the flexural strength of the specimen increases first and then decreases with the increase of the basalt fibers contents. The specimens achieves the largest increase percentage of flexural strength when the addition content is 0.20%, which is equal to 4.95% and 6.60% respectively. For 9 mm basalt fibers, the flexural strength shows a fluctuating trend with the increase of the addition content of basalt fibers, and the specimen achieves the largest increase percentage of flexural strength when the addition amount is 0.20%, which is equal to 7.84%. Based on comprehensive analysis, it is found that there is a relatively good improvement effect on the flexural strength when the addition content of basalt fibers is 0.20%.

3.5 NCBC SEM test results

3.5.1 NCBC microscopic morphology

The microstructures of the concrete specimens with different fiber lengths and addition amounts observed by SEM are shown in Fig.7. Fig.7(a) corresponds to the specimen with neither nano-CaCO3nor basalt fibers. It can be seen that there are cracks and voids on the surface of the specimen, and the overall structure is relatively loose, with a dispersed distribution of hydration products. Fig.7(b) corresponds to the specimen of NCBC-3-0.20%. It can be seen that the overall structure is relatively dense, and the number of cracks and voids are significantly reduced. This is mainly attributed to the filling and crystal nucleus effects of nano-CaCO3, that is, nano-CaCO3can fill the interface cracks to reduce the porosity, and the crystal nucleus effect of nano-CaCO3can improve the degree of hydration reaction, accelerate the hydration of C3S,and increase the content of C-S-H gel in the hydration products. After adding basalt fiber into concrete, there is no chemical reaction between basalt fiber and cement mortar, and it is directly embedded into the cement matrix. There are a large amount of tiny protrusions on the surface of the fibers, which can produce an interlocking effect with the cement matrix and increase the strength of the interface between the fiber and the cement. Thus,the fibers have a strong effect on the concrete. During the loading process, when the concrete matrix is damaged, the basalt fibers dispersed in the matrix will be pulled out or broken. Due to a relatively high tensile strength, the toughening and cracking effect of basalt fibers will consume a part of energy, leading to an increase in the mechanical performance of the concrete.Combined with the analysis of macroscopic mechanical test results, it can be found that nano-CaCO3and basalt fibers play their respective roles in the concrete and promote each other to improve the compressive, split tensile, and flexural strengths of the concrete.

Fig.7 Microstructure of (a) ordinary concrete; (b) NCBC

3.5.2 The influence of the content and length of basalt fibers on the concrete

The microstructures of each group of NCBC specimens are shown in Fig.8. By comparing Fig.8 (c)with Fig.8 (d), it can be found that the aggregation phenomenon appeares more obviously with the increase of basalt fibers. Meanwhile, due to the hydrophobicity of the basalt fibers, there are gaps at the interface between the fibers and the cement mortar. If the addition content is too high, basalt fibers would increase the initial defects of the concrete, and then affect the mechanical properties of the overall structure of the concrete.Therefore, in the compressive and split tensile tests,the strength of the concrete would not increase all the way with the increase of the addition amount of basalt fibers, but would decrease to a certain extent. In the flexural strength test, since the failure surface area is relatively small and the cracks propagated in a single direction, the crack resistance of basalt fiber is higher than the initial defect. Thus, when the addition amount is 0.30%, basalt fibers could still exert an improvement effect on the flexural strength of the concrete. However,the test results also show that, when the fiber content exceeds 0.20%, the increase percentage would begin to decrease gradually, that is, the flexural strength would not increase all the way with the increase of the fiber content.

By comparing Fig.8 (b), Fig.8 (d), and Fig.8 (f),it can be seen that the length of fiber determines the degree of dispersion of the fibers distributed in the concrete. The longer the length, the worse the degree of dispersion. The main reason is that the basalt fiber itself has a “clumping” phenomenon, and some fibers will not be dispersed after mixing. Therefore, the improvement effect of 9 mm fiber is weaker than that of 3 and 6 mm fiber. In the compression test, NCBC-3-0.20% achieves the largest increase of strength. This is because the specimen is uniformly stressed during the loading process, and the cracks could be evenly distributed throughout the specimen. The 3 mm fiber,which has the best dispersion performance, can fully exert the effect of toughening and crack resistance so as to maximize the increase of compressive strength.In the split tensile and flexural tests, the failure modes of the specimens show a longitudinal cracking pattern from the middle part, with only one large crack. Due to a relatively short length of the 3 mm fiber, the fibers distribution along the crack propagation path could not fully exert their improvement effect. Comprehensive comparison test results show that basalt fiber with a length of 6 mm is more stable than other lengths to improve the mechanical properties of concrete.

Fig.8 Microstructure of (a) NCBC-3-0.20%; (b) NCBC-3-0.30%; (c)NCBC-6-0.20%; (d) NCBC-6-0.30%; (e) NCBC-9-0.20%; (f)NCBC-9-0.30%

4 Numerical simulation of NCBC mechanical properties

4.1 Material model

At the mesoscale, NCBC is a multi-phase composite material composed of the cement mortar (including nano-CaCO3), aggregates, interface, and basalt fibers. Specifically, the cement mortar was considered as the matrix phase. The aggregates wrapped by the interface and the basalt fibers were taken as the inclusion phase. In this study, different length-diameter ratios and volume ratios were used to simulate the length and addition amount of basalt fibers. According to the Eshelby inclusion theory and the Mori-Tanaka homogenization method, the progressive damage model of basalt fiber concrete was established using the nonlinear multi-scale composite material and structure modeling software DIGIMAT.

4.1.1 Elastic mechanical parameters

Prediction was performed on the constitutive model of the basalt fiber concrete using DIGIMAT.Firstly, the elastic mechanical parameters of the matrix phase and the inclusion phase must be determined. The elastic mechanical parameters of cement mortar (matrix phase) and aggregates (inclusion phase) were obtained from the experimental results. When the thickness of the interface was within the range of 100 μm, the concrete would usually have the characteristics of low strength, low elastic modulus, and high permeability[31].Since the interface is a very thin layer of material, its mechanical properties are difficult to determine through experiment. Thus, the interface parameters given by Stock[32]were used for calculation. Specifically, the elastic modulus of interface was taken as 0.4 times of the elastic modulus of cement mortar, and the thickness was taken as 0.008 times of the aggregate particle size.The elastic mechanical parameters of basalt fibers adopted the testing indicator parameters. The summary of elastic mechanical parameters is shown in Table 10.

Table 10 NCBC elastic mechanical parameters

4.1.2 Progressive damage model

1. The 3D Hashin failure criterion and failure indicators

2. Damage evolution

The damage evolution was analyzed using the Matzenmiller-Lubliner-Taylor (MLT) model, which assumes that various phases of concrete obey the Weibull distribution. The damage evolution process is actually the process of the release of material strain energy. In this process, the materials will get softened. Its macroscopic manifestation is shown as the degradation of elastic modulus and the decrease of load-bearing capacity. In this paper, the softening form adopted anexponential form. The definition of damage variables directly affects the damage evolution mechanism of the materials. In this paper, the functionφ(f)is used to describe the relationship between the failure indexfand the damage variableD, that is, each damage variable is defined by the functionφ(f). The specific expressions are shown in Table 12.

Table 11 The 3D Hashin failure criterion

Table 12 Damage variables of the MLT model

The expressions of the damage evolution functionφ(f)are shown in Eqs.(4) and (5), whereαandβare material response parameters. In the numerical simulation of this study,αwas set to 1, and the constitutive relationship was established by changing the value ofβ. The change ofβcan lead to Weibull softening of the stress-strain curve of the material. Thus, the value ofβcan be estimated according to the position of the maximum stress. When the value ofβapproaches to infinity,the model will be transformed into an instantaneous damage model. After trial and error, the value ofβwas determined as 0.5.

4.2 NCBC finite element model

The methods for establishing the compressive,split tensile and flexural models of NCBC are similar. The model parameters used for the same working condition remained the same. Specifically, the model establishment process is as follows:

4.2.1 Importing of the material model

A progressive damage model considering the meso-characteristics of nano-CaCO3and basalt fiber concrete materials was obtained based on the 3D Hashin failure criterion and the MLT damage evolution model. DIGIMAT was coupled with ABAQUS, and the subroutine was used as the carrier to import the progressive damage model into ABAQUS as material properties for subsequent calculation, so as to simulate the basic mechanical properties of the concrete.

4.2.2 Gridding of the geometric model

The dimensions of the compressive, split tensile and flexural specimen models remained consistent in terms of the dimensions of test specimens. As shown in Fig.9, hexahedral elements were used to grid the geometric model. The concrete components were considered as (C3D8R), while the remaining components were considered as (R3D4) in discrete rigid bodies.After calculation, the concrete model in Fig.9 (a) generated a total of 10 255 elements and 11 148 nodes; the concrete model in Fig.9 (b) generated a total of 19 215 elements and 20 200 nodes; the concrete model in Fig.9 (c) generated a total of 4 270 elements and 5 261 nodes.

Fig.9 (a) Compression test and numerical model; (b) Split tensile test and numerical model; (c) Flexural test and numerical model

4.2.3 Component connection and calculation

The model was set with two contacts,i e, the contact between the lower surface of the pressure plate and the upper surface of the concrete specimen, and the contact between the upper surface of the backing plate and the lower surface of the concrete specimen. The normal direction was considered as “hard” contact, and the tangent direction was considered as “rough”. The backing plate was set as a fixed constraint. The pressure plate was set to exert a displacement load in the normal direction. The model was solved by adopting the incremental-iterative strategy using the Newton-Raphson method.

4.3 Comparison between NCBC tests and numerical simulation results

Fig.10 (a) Compressive test results and simulation results; (b) Split tensile test results and simulation results; (c) Flexural test results and simulation results

Fig.10 presents the experimental values and the numerical simulation results of the compressive, split tensile and flexural strengths of the concrete under each group of working conditions. It can be seen from Fig.10 that the numerical simulation results calculated according to the progressive damage theory are similar to the experimental values, and the error range is basically within 10%. In the flexural strength tests, NCBC-9-0.15% has a relatively larger deviation. This is probably because that the basalt fibers were not evenly distributed during the mixing process due to human factors and machine factors, leading to a decrease in the test results. Comparatively, in the numerical simulation, all the mesoscopic parts of the materials were evenly distributed. The influence pattern of basalt fibers on the mechanical properties of concrete is basically consistent between numerical simulation and experimental results. Based on comprehensive analysis, numerical simulation can effectively simulate the various working conditions that cannot be accurately achieved by experiment, and can compensate for the loopholes and defects of the experimental study to a certain extent under the premise of ensuring the accuracy of results.Therefore, it is an economical and effective approach to examine the basic mechanical properties of concrete using numerical simulation.

5 Conclusions

The basic mechanical properties of the nano-Ca-CO3and basalt fiber reinforced concrete are examined through experiment and numerical simulation. Then,the mechanism of NCBC is analyzed from the perspective of microstructure, and the following conclusions are drawn:

The compressive strength tests showed that, with the increase of the nano-CaCO3contents, the increase percentage of compressive strength of the 3, 7 and 28 d specimens increased first and then decreased. The optimal content of nano-CaCO3is 2.0%. The basic mechanical tests of NCBC showed that, with the increase of the addition amount of basalt fibers, the compressive and split tensile strengths increased first and then decreased, while the flexural strength gradually increased with a decreasing growth rate. The optimal length and content of the basalt fibers are 6 mm and 0.20% respectively.

SEM tests of NCBC show that nano-CaCO3and basalt fibers play the roles of filling, nucleation, toughening and cracking resistance respectively in the concrete matrix, leading to an increase in the compressive,split tensile, and flexural strengths of concrete. However, the content of basalt fibers should not be too high;otherwise the phenomenon of “clumping” will occur,which is not conducive to the improvement of mechanical properties of concrete.

A material model was established based on the progressive damage theory, and was used as the material properties of the concrete for numerical simulation.The results show that the numerical simulation results are in good agreement with the experimental results.