Mechanical Properties
As can be seen from Fig. 5, the addition of basalt fiber improves the initial compressive strength of concrete, and the initial compressive strength of BFRC1, BRFC2 and BFRC3 groups increases by 8.2%, 11.8% and 3.9%, respectively, compared with group C. The incorporation of basalt fiber elevates the compactness and cohesiveness of hydration products of concrete, reduces the defects caused by early shrinkage of concrete, and contributes to the improvement of concrete strength (Dias & Thaumaturgo, 2005; Monaldo et al., 2019). Excessive incorporation of basalt fiber, however, will increase the disorderly distribution of fiber on the one hand, resulting in fiber agglomeration (Branston et al., 2016), where the defect rate of concrete will be increased as well. The excessive incorporation of basalt fiber reduces the content of concrete cementitious material per unit volume, as a result, the positive effect of BFRC3 group is inferior to those of BFRC1 and BFRC2 groups. It can also be seen (Fig. 6) from the mechanical experiment of 0 cycles that the addition of basalt fiber improves the failure mode of concrete specimens, meanwhile, the cracks of basalt fiber concrete specimens are reduced compared with ordinary concrete. Even under stress, the integrity of basalt fiber concrete specimens is still higher. Under single-side salt-freezing–drying–wetting cycle, the compressive strength of concrete begins to decrease with the increase of cycle times. After 5 cycles, the loss rate of compressive strength in group C, BFRC1, BFRC2 and BFRC3 were 2.4%, 1.2%, 0.6% and 1.1%, respectively. After 10 cycles, the loss rates of compressive strength in group C, BFRC1, BFRC2 and BFRC3 were 8.0%, 4.1%, 3.6% and 7.5%, respectively. After 15 cycles, the loss rates of compressive strength in group C, BFRC1, BFRC2 and BFRC3 were 21.3%, 11.3%, 6.9% and 15.4%, respectively. After 20 cycles, the loss rates of compressive strength in group C, BFRC1, BFRC2 and BFRC3 were 33.9%, 24.6%, 15.5% and 27.3%, respectively. The compressive strength loss rate of basalt fiber reinforced concrete in each group is lower than that of ordinary concrete under single-side salt-freezing–drying–wetting cycle. On the one hand, the addition of basalt fiber improves the compactness of concrete and reduces the erosion content and depth of sulfate ion; on the other hand, the three-dimensional network constructed by basalt fibers and the high elastic modulus of it can offset part of the tensile stress caused by icing pressure (Musa & Yang, 2006), capillary osmotic pressure (Powers, 1954) and the expansion stress caused by sulfate crystallization in the single-side salt-freezing–drying–wetting cycle.
The compressive strength at different single-side salt-freezing–drying–wetting cycles.
Concrete compression failure pattern.
In terms of splitting tensile strength (Fig. 7), compared with that of group C, the initial strength of BFRC1, BFRC2 and BFRC3 groups increased by 11.5%, 17.3% and 28.8%, respectively. The improvement rate of BFRC3 group was the highest, which was different from the factor of compressive strength. The addition of basalt fiber played its reinforcing and toughening role, and the higher the fiber content, the more obvious the effect. After 5 cycles, the loss rates of splitting tensile strength in group C, BFRC1, BFRC2 and BFRC3 were 7.8%, 3.4%, 1.6% and 1.5%, respectively. After 10 cycles, the loss rates of splitting tensile strength in group C, BFRC1, BFRC2 and BFRC3 were 17.3%, 8.6%, 3.3% and 9.0%, respectively. After 15 cycles, the loss rates of splitting tensile strength in group C, BFRC1, BFRC2 and BFRC3 were 23.1%, 15.5%, 8.2% and 19.7%, respectively. After 20 cycles, the loss rates of splitting tensile strength in group C, BFRC1, BFRC2 and BFRC3 were 30.8%, 22.4%, 14.8% and 22.4%, respectively. It can be concluded that basalt fiber under single-side salt-freezing–drying–wetting cycle environment also has a positive effect of reducing the concrete splitting tensile strength loss rate. And the single-side salt-freezing–drying–wetting cycle has little effect on basalt fiber and its three-dimensional network, so after 20 cycles, basalt fiber reinforced concrete splitting tensile strength loss rate is lower than the compressive strength loss rate. Although the splitting tensile loss rate of BFRC3 group was greater than that of BFRC1 and BFRC2 groups, the splitting tensile strength of BFRC3 group and BFRC2 group were 5.2 MPa after 20 cycles, which was better than that of BFRC1 group.
The splitting tensile strength at different single-side salt-freezing–drying–wetting cycles.
Microstructure
Hydration process of Portland cement is a complex chemical reaction, the hydration products of which include calcium hydroxide crystal, C–S–H gel, ettringite and so on. Among them, C–S–H gel is the main product of hydration of Portland cement. In the fully hydrated cement slurry, C–S–H gel accounts for about 70%, which is principally due to the strength of cement. Various hydration products are related to each other to form a spatial network, and the shape of the spatial network has an important role to play in the performance of concrete. ITZ area refers to the interface transition area between coarse and fine aggregate and cement slurry in concrete. With the high porosity and the low hardness, ITZ area contains a lot of hydration product CH (calcium hydroxide crystal) of low density, consequently, the mechanical properties of ITZ are worse than the physical properties of cement mortar, making it the weak link of cement-based composites (Ollivier et al., 1995).
As can be seen from Fig. 8-C(0), hydration products of ordinary concrete are randomly distributed. Pores, micro-cracks and other defects can be clearly observed in the interface, where some hydration products are not closely connected. In the two marked ITZ areas, we can see that there are obvious cracks at the bond between hydration products and aggregate, which are not closely connected. Compared with ordinary concrete, it can be seen in Fig. 8-BFRC1(0) and Fig. 8-BFRC2(0) that after the addition of basalt fiber, the interface pores of hydration products become smaller and no obvious micro-cracks are observed, which is beneficial to the improvement of mechanical properties and durability of concrete. The natural compatibility of basalt fibers and cement-based composites are the main reasons for basalt fibers to blend and wrap with hydration products. However, due to the small fiber content, the ITZ area of BFRC1(0) is lower than that of BFRC2(0), yet still better than that of BFRC3(0). In BFRC2(0) region I, basalt fibers can be observed to cross the pores; in region II and III, basalt fibers can be observed to bridge the two ends of the ITZ areas, which greatly improves the tightness and stability of the ITZ area. In BFRC3(0), the evident disorder distribution of basalt fibers was observed, and the large agglomeration of basalt fibers increased the possibility of defects. A large number of basalt fibers passed through the pores and ITZ area, denoting its importance in cracking resistance and toughening. However, the content of hydration products at the interface is low (which is related to the decrease of cementitious materials per unit volume caused by the increase of the content of basalt fibers), the basalt fibers cannot be fully wrapped, and larger pores can be observed. According to the above analysis, it can be concluded that the proper incorporation of basalt fiber is beneficial to the improvement of mechanical properties and durability of concrete, and the microstructure of initial hydration product interface of each group of concrete is consistent with its initial macro-mechanical properties index.
The SEM of concrete subjected to 28-day standard curing.
After 20 cycles of salt-freezing–drying–wetting (Fig. 9), the obvious point-like and short columnar gypsum crystals were observed at the interface of hydration products of concrete in each group. This is because at low temperature, the TSA-type destruction (Musa & Yang, 2006) is the major type of sulfate erosion, and the main erosion product is the expansive gypsum crystal. Under the action of frost heaving force and expansion stress, the pores on the interface of hydration products of concrete begin to increase, and the pores of group C are the obviously presented. Basalt fiber reinforced concrete has the phenomenon of increasing pores and porosity, but we could see the partition in the fiber through pores, bridging, basalt fiber formation of the three-dimensional network and good elastic modulus part. It can be made to slow down the freeze pressure (Powers, 1954) and capillary osmosis (Powers & Helmuth, 1953) the expansion stress of tensile stress, sulfate crystallization. BFRC2 group has the highest compactness and integrity. Due to the small amount of basalt fiber in BFRC1 group, basalt fiber in pores and cracks become less, in which case basalt fiber cannot exert the partition and bridge function. In BFRC3 group, a large amount of basalt fibers can be observed exposed at interface. Lacking of adequate hydration products with coordination, there are a lot of basalt fibers between the pores. Despite that the framework of the basalt fiber network is obvious, the compactness and integrity of it are still poor. On comparing the initial state with three groups of basalt fiber reinforced concrete interface we observed, the basalt fiber surface parcel of hydration products marginally reduce [TSA damage will cause C–S–H gel decomposition (Liu et al., 2015)], basalt fibers and bondability of hydration products began to reduce, more directly exposed in the interface of basalt fiber.
The SEM of concrete subjected to 20 times of single-side salt-freezing–drying–wetting cycles.
After 20 times of single-side salt-freezing–drying–wetting cycles (Fig. 10), we can observe basalt fibers with a little micro-cracks and crystal surface, but the overall form is still intact, indicating that single-side salt-freezing–drying–wetting cycle has less effect on the basalt fibers, thus, basalt fibers in this kind of work environment can still keep the excellent toughness and crack resistance performance.
The SEM of basalt fiber subjected to 20 times of single-side salt-freezing–drying–wetting cycles.
Pore Structure
MIP test was used to study the influence of different cycles on the pore structure of concrete in single-side salt-freezing–drying–wetting cycle. The research on pore structure mainly focuses on porosity (Fig. 11), pore size distribution (Figs. 12, 13, 14, 15) and critical pore size (Fig. 16). Porosity is closely related to the compressive strength of concrete, the lower the porosity of concrete, the denser the structure, the higher of the compressive strength; pore size distribution and critical pore size are closely related to concrete permeability and durability. It can be implied from Fig. 11 that the addition of basalt fiber will reduce the original porosity of concrete, which is consistent with the studies of some scholars (Jiao et al., 2019). However, it was found that the BFRC2 group had the lowest original porosity, and that of BFRC3 group was higher than that of BFRC1 and BFRC2 groups, which was related to excessive fiber incorporation and large amount of fiber disordered agglomeration, increasing the possibility of defects. The porosity of concrete in each group began to increase with the development of single-side salt-freezing–drying–wetting cycle. The tensile stress caused by freezing pressure (Powers, 1954) and capillary osmotic pressure(Powers & Helmuth, 1953), along with the expansion stress caused by sulfate crystallization, increased the porosity of concrete. After 20 cycles, the porosity of concrete in group C, BFRC1, BFRC2 and BFRC3 increased by 9.8%, 4.1%, 4.7% and 7.5%, respectively. It can be seen that the addition of basalt fiber reduces the increase of concrete porosity under single-side salt-freezing–drying–wetting cycle. That is because, on the one hand, the incorporation of basalt fiber improves the compactness of concrete, improves the content and depth of concrete resistance to sulfate ion erosion; on the other hand, the toughness and crack resistance of basalt fiber reduces the stress concentration and the damage caused by it.
The porosity of concrete at different single-side salt-freezing–drying–wetting cycles.
The pore size distribution of concrete at different single-side salt-freezing–drying–wetting cycles (C).
The pore size distribution of concrete at different single-side salt-freezing–drying–wetting cycles (BFRC1).
The pore size distribution of concrete at different single-side salt-freezing–drying–wetting cycles (BFRC2).
The pore size distribution of concrete at different single-side salt-freezing–drying–wetting cycles (BFRC3).
The critical pore diameter of concrete at different single-side salt-freezing–drying–wetting cycles.
Some scholars (Wu, 1979) divided the pores into harmless pore (< 20 nm), less harmful pore (20–100 nm), harmful pore (100–200 nm) and multi-harmful pore (> 200 nm) in accordance with the pore size. In the initial state, the pore size of concrete is mainly concentrated in the two ranges of harmless pore and less harmful pore. The total proportion of harmless pore and less harmful pore in group C, BFRC1, BFRC2 and BFRC3 were 88.1%, 93.4%, 94.6% and 89.7%, respectively (Figs. 12, 13, 14,15). The increase of small pores will exert a positive effect on the compressive strength of concrete. Because large pores affect the fluidity of concrete, basalt fiber is difficult to function as the way it should be. The percentages of harmful pore and multi-harmful pore in group C, BFRC1, BFRC2 and BFRC3 were 3.0%, 2.5%, 2.2% and 3.4%, respectively. The percentages of harmful pore in group C, BFRC1, BFRC2 and BFRC3 were 8.9%, 4.1%, 3.2% and 6.9%, respectively. The addition of basalt fiber reduces the proportion of more damage pore and less damage pore of concrete and effectively improves the durability of concrete. With the advance of single-side salt-freezing–drying–wetting cycle, the concrete pore diameter gradually becomes larger, and the proportion of harmful pore and multi-harmful pore begin to increase. On the one hand, the increase of the proportion of harmful pore and multi-harmful pore accelerates the invasion of sulfate ions, and the erosion content and depth begin to increase. On the other hand, more unfrozen water moves to the frozen region and transforms itself into ice crystals, increasing the frost heaving force. After 20 cycles, the percentages of harmful pores in group C, BFRC1, BFRC2 and BFRC3 increased by 4.7%, 2.9%, 2.7% and 2.7%, respectively, while the percentages of multi-harmful pores in group C, BFRC1, BFRC2 and BFRC3 increased by 8.5%, 4%, 3.4% and 5.8%, respectively. The increase of harmful pores and multi-harmful pores directly reduce the durability of concrete. Basalt fiber mixed with effective in reducing the concrete under the environment of single-side salt-freezing–drying–wetting cycle aperture coarsening, the increase of harmful pores and multi-harmful pores were decreased significantly than those of normal concrete (the effect on multi-harmful pore is more obvious). Hence, it is beneficial to improve the concrete under the environment of single-side salt-freezing–drying–wetting cycle durability performance.
The pore in concrete is a connected and randomly distributed pore structure system. The critical pore diameter (Fig. 16) is the largest pore diameter to connect the larger pores, which can reflect the connectivity of pores. The physical meaning is: if the pore diameter is greater than the critical aperture, it cannot be connected to each other; if the pore diameter is equal to or less than the critical aperture, it can be connected other pores. Therefore, in cement-based material pore structure system, the smaller the critical pore size, the better the impermeability and durability. The critical pore diameter of concrete in each group under different cycles is shown in Fig. 16. Under 0 cycles, the critical pore diameter of group C, BFRC1, BFRC2 and BFRC3 are 116.7 nm, 79.1 nm, 60.4 nm and 82.6 nm, respectively. The addition of basalt fiber effectively reduces the critical pore diameter of concrete. As the cycle went on, the critical pore size of concrete in each group increased continuously. After 20 cycles, the critical pore sizes of group C, BFRC1, BFRC2 and BFRC3 were 205.5 nm, 133.3 nm, 115.4 nm and 165.6 nm, respectively. The increase of critical pore size weakens the permeability resistance of concrete, which is not conducive to the stability of concrete in service under the condition of single-side salt-freezing–drying–wetting cycle. The growth rate of each major cycle of concrete in group C increased steadily. The growth rate of concrete in group BFRC1 and BFRC2 increased significantly after 10 cycles, while that in group BFRC3 increased significantly after 5 cycles. It is shown that although basalt fiber has the effect of toughening and cracking resistance, it still needs the synergistic effect of cementing material and appropriate amount of basalt fiber for the best effect. In each cycle, the critical pore diameter corresponding to impermeability and durability of concrete is consistent with its macroscopic performance index.
Relationship Between Pore Structure and Mechanical Properties
Figs. 17 and 18 demonstrate the relationship between porosity and mechanical properties of concrete. With the increase of porosity, groups of concrete compressive strength, splitting tensile strength decreases, indicating that under the single-side salt-freezing–drying–wetting cycle concrete porosity and the concrete compressive strength, splitting tensile strength has certain relevance. The correlation coefficients between porosity and compressive strength and splitting tensile strength of concrete in group C, BFRC1, BFRC2 and BFRC3 are all above 0.9, indicating that the correlation between porosity and compressive strength and splitting tensile strength is significant. The correlation coefficient between porosity and splitting tensile strength of concrete in each group is lower than that between porosity and compressive strength, indicating that the influence of porosity on splitting tensile strength of concrete is slightly lower than that of compressive strength under single-side salt-freezing–drying–wetting cycle. In terms of pore size distribution (Figs. 19, 20, 21, 22, 23, 24, 25, 26), the compressive strength and splitting tensile strength of concrete in each group increase with the increase of harmless pores and less harmful pores, while the compressive strength and splitting tensile strength decrease with the increase of harmful pores and multi-harmful pores, which is consistent with the findings of previous studies. The pore size range with the highest correlation of the compressive strength of concrete in each group is multi-harmful pore (> 200 nm), which also indicates that multi-harmful pore has the greatest influence on the compressive strength of concrete. The pore size range of C, BFRC1, BFRC2 and BFRC3 concrete with the highest correlation of splitting tensile strength is 20–100 nm, > 200 nm, < 20 nm and < 20 nm, respectively, displaying a disorder state, which may be related to the three-dimensional network constructed by basalt fiber, however, the specific reasons need further study. The correlation coefficients between critical pore diameter and compressive strength and splitting tensile strength of concrete in group C, BFRC1, BFRC2 and BFRC3 are all above 0.9 (Figs. 27, 28), indicating that the correlation between critical size diameter and compressive strength and splitting tensile strength is also very significant.
Relationship between porosity and compressive strength.
Relationship between porosity and splitting tensile strength.
Relationship between pore size distribution and compressive strength (C).
Relationship between pore size distribution and compressive strength (BFRC1).
Relationship between pore size distribution and compressive strength (BFRC2).
Relationship between pore size distribution and compressive strength (BFRC3).
Relationship between pore size distribution and splitting tensile strength (C).
Relationship between pore size distribution and splitting tensile strength (BFRC1).
Relationship between pore size distribution and splitting tensile strength (BFRC2).
Relationship between pore size distribution and splitting tensile strength (BFRC3).
Relationship between critical pore diameter and compressive strength.
Relationship between critical pore diameter and splitting tensile strength.
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