3.1 Failure modes
According to the failure mode of the benchmark concrete as shown in Fig. 2 (a), it can be seen that the specimen exhibits sudden failure during the loading process, with a sudden decrease in load without obvious signs, and showed obvious brittle failure characteristics. During it failed, many small cracks suddenly appeared and continuously penetrated, then intertwined and developed rapidly. The specimen bulged to the side, and eventually many concrete fragments falled off, with obvious crack distribution and severe overall failure. According to the failure modes of specimen with single fibers as shown in Fig. 2.(b), it can be seen that compared to the benchmark concrete, the number of cracks increased, the loading time slightly lengthened, and the degree of concrete block peeling decreased, that exhibited certain plastic failure characteristics.
As hybrid fibers were added to low heat cement concrete, the specimens under compression were significantly more complete and their bearing capacity continued to increase. In the initial stage of loading, the specimens made a slight noise due to the sound produced by the slight displacement between the fibers and concrete during the synergistic effect. Compared with the benchmark group and the single fiber group, cracks appeared around the specimens. Most specimens did not have large fragments falling off, and only damage occured at the edges and corners around the specimens. The overall integrity was good, as shown in Fig. 2 (c), and the comparison diagram of the cracking interface between benchmark concrete and HFRLCC is shown in Fig. 3.
According to Fig. 4 (a), when the tensile load reached its maximum, a large macroscopic crack suddenly appeared at the center of the benchmark concrete specimen, and rapidly expanded and penetrated the entire specimen. Finally, the specimen was split in half. Before the benchmark concrete is subjected to tensile failure, there is no sign of sudden failure, and the the process of failure is short, that shows obvious brittle failure characteristics.
From Fig. 4 (b) - (c), it can be seen that the tensile failure states of single fiber reinforced concrete and hybrid fiber reinforced concrete are relatively similar. The specimens show obvious middle cracks, followed by small cracks around the middle cracks. However, during the failure process under tensile force, the number of small cracks in hybrid fiber reinforced concrete specimens continued to increase, and the crack propagation was slower. At the same time, the middle cracks continued to widen, and the tensile time increased, that showed mode of plastic failure. The comparison diagram of the cracking interface between benchmark concrete and HFRLCC under tensile force is shown in Fig. 5.
3.2. Mechanical analysis
As shown in the Fig. 6, for the condition of a certain AF volume fraction, the compressive strength of the specimens first increases and then decreases with the increase of RF volume fraction. When the volume fraction of AF is 0.00% and 0.10%, the compressive strength of specimens decreases at the 0.45% volume fraction of RF; When the volume fraction of AF is 0.20% and 0.30%, the compressive strength of specimens shows a decreasing trend at the 0.30% volume fraction of RF. When the volume fraction is 0.2% for AF and 0.15% for RF, the HFRLCC strength reaches its maximum value, which is 40.2% higher than the benchmark group. Except for the A0R3 group and the A3R3 group, all other groups have a positive enhancing effect on the compressive strength of low heat cement concrete. The influence of fibers on the splitting tensile strength is more significant. When the volume fraction of AF is 0.00% and 0.10%, the splitting tensile strength of specimens decreases at the 0.45% volume fraction for RF. When the volume fraction of AF is 0.20% and 0.30%, the splitting tensile strength of specimens shows a decreasing trend at the 0.30% volume fraction for RF. When he volume fraction is 0.2% for AF and 0.15% for RF, the HFLCC strength reaches its maximum value, which is 70.6% higher than the benchmark group. Except for the A0R3 group and A3R3 group, all other groups have a positive enhancing effect on the splitting tensile strength of low heat cement concrete. In sum, with the increase of fiber volume fraction, the compressive and splitting tensile strength first increase and then decrease. When the fiber volume fraction exceeds a certain level, it even has an adverse effect on the strength of concrete. This also reflects that hybrid fibers have a dual function on the mechanical performance of specimens, that is, they play a unique bridging role, inhibiting the generation and development of cracks, while excessive fibers introduce new weakening areas, reducing the mechanical properties..
3.3. Microstructures
3.3.1.The bond distribution of fibers in low heat cement concrete
The bonding effect between fibers and concrete mortar is closely related to the performance of concrete.The fibers and the concrete are combined and synergistically subjected to external loads through bonding effect. The reinforcement and crack resistance effect of fibers for concrete largely depends on the bonding strength between fibers and mortar. Formulas (1) and (2) refer to the hydration mechanism of low heat cement. The C-S-H gel and calcium hydroxide (CH) crystal generated by the cement hydration reaction are the direct source of the bonding effect between the fiber and the concrete.
2C2S+H2O=C3S2H4+Ca(OH)2 (1)
2C2S+H2O=C3S2H4+3Ca(OH)2 (2)
During the process of cement hydration, these dense hydration products are stacked and wrapped together, gradually filling the pores and spaces originally occupied by water in the concrete. After the concrete hardens, it cooperates with other materials inside the concrete to bear stress, which is the fundamental reason and guarantee of concrete strength. The distribution of hydrates in concrete at different magnifications is shown in Fig. 7 (a) and (b), Therefore, analyzing the distribution and bonding of fibers and hydration products in different groups has significant significance. As shown in the Fig. 7 (c) and (d), whether it is AF or RF, a large amount of hydration products are attached to the fiber surface, proving that them has good bonding performance with the mortar, that is also the fundamental guarantee that hybrid fibers can play a reinforcing role in low heat cement concrete .
3.3.2. Interfacial transition zone (ITZ)
ITZ is a weak area in concrete, mainly manifested at the interface between aggregates and mortar, as well as between fibers and mortar, where damage in concrete often first occurs [34].
As shown in Fig. 8 (a), there are some small cracks in the low heat cement concrete microstructures, and the interface transition zone between the cement matrix and aggregates is relatively loose. The bonding performance is affected to a certain extent, and the existence of weak interface areas is further verified. Compared to other areas, the ITZ of HFRLCC between fiber and mortar is relatively loose, and cracks appear earlier and more frequently than other areas, as shown in Fig. 8 (b). However, the development of cracks is also limited, and the pores and cracks inside the matrix are reduced compared to the reference group. The main reason is that the ITZ between the fibers and mortar is a weak stress zone, and the failure of the concrete often occurs first in these weak areas, with complex stress distribution. When external loads act on the concrete, microcracks are prone to occur near the fibers. By bonding effect to bridge the cracks and limit their further expansion, fibers can delay the failure of the mortar. As the load further increases, microcracks continue to expand until the fibers are pulled out or fractured from the concrete matrix, that loses their ability to resist cracking. In addition, the agglomeration phenomenon caused by excessive fibers leads to the inability of fibers to bond better with the mortar, resulting in an increase in defects in the ITZ in the fiber slurry interface.
3.3.3. The failure mode of fibers in low heat cement concrete
The failure modes of AF and RF in HFRLCC are shown in Fig. 9. where the failure mode for AF are shown in Fig. 9 (a-c), while for RF in Fig. 9 (d). The two types of fibers have similar failure modes, which are reflected in different stages, mainly including pull-out, tensile failure, and torsional failure.
Based on the differences in size and elastic modulus between AF and RF, the initiation and propagation of cracks at different scales can be effectively suppressed, and the composite effect of the two fibers can enhance the strength of low heat cement concrete. On the one hand, in the early stage of damage HFRLCC, single RF, single bundles with smaller diameter and single AF have the bridging effects between microcracks. The bonding and bridging effect between fibers and mortar makes them bear some of the load, limiting the further expansion of microcracks. As the load further expands, they are gradually pulled apart and lose their effectiveness, while basalt fibers continue to play a crack resistance role, Until the bridging stress exceeds the load-bearing capacity of AF, AF are pulled out or broken from the concrete mortar, and lose their effect of crack resistance. On the other hand, when HFRLCC undergoes splitting and tensile failure, hybrid fibers undergo tensile, pull-out, and torsional failure, bearing partial loads that will consume load energy, resulting in an increase in the strength of HFRLCC.
3.4. Phase composition
As showing in Fig. 10, compared to the chemical composition of low heat cement, the peaks corresponding to C2S, C3S, and C3A in the hydration products of the specimens significantly decrease, accompanied by the formation of C-S-H crystals. Many crystals are present in A0R0, including quartz (SiO2), portlandite (Ca(OH)2) and C-S-H. These minerals have been often found in previous studies of the hydration of cement. In addition, calcite (calcium carbonate) was found in A0R0, mainly due to incomplete hydration of some cement during the hydration process. Compared to A0R0, after adding fibers, there was no significant change in the XRD pattern, only the peaks corresponding to calcite, C-S-H, and quartz showed certain differences, especially the calcite content was significantly reduced and the C-S-H content was significantly increased. During the concrete mixing process, the fiber surface uniformly distributed in the matrix will adsorb a certain amount of water, which will maintain more moisture compared to ordinary concrete[35]. This effect is conducive to the rapid progress of cement hydration reaction, promoting the generation of C-S-H and consuming calcite. This is also similar to previous findings, especially for A0R2, the above peak is more pronounced than that of A2R0, that is, hydrogen bonding and chemical bonding of Si-OH are easily formed between RF and mortar. Hydrophilic groups in the brucite fiber structure (such as - OH (hydroxyl)) form hydrogen bonds when they encounter water, and water in the concrete matrix is adsorbed on the fiber surface, causing hydration reaction to occur, More hydration products are adsorbed on the surface of RF, effectively strengthening the ITZ structure at the fiber interface[36]; The peak changes in the hybrid fiber group are more pronounced, indicating that AF and RF can synergistically promote hydration reactions, generate more calcite and C-S-H, and thereby improve the mechanical properties of the specimens.
3.5. Pore defects
The mercury intrusion method is based on the functional relationship between the amount of mercury injected into the pores of concrete and the corresponding pressure, in order to obtain porosity, specific surface area, and pore size distribution. Based on the influence of pore size on strength, Wu [37] combined many studies to divide pores in concrete into harmless pores, less harmful pores, harmful pores, and more harmful pores, with concrete pore sizes of < 20nm, 20-100nm, 100–200 nm, and > 200nm, respectively. Low porosity, small pore size, and multiple circular pores are prerequisites for excellent macroscopic properties. Analyzing pore structure parameters can provide theoretical basis for improving the macroscopic properties of HFRLCC.
This section prepared the benchmark group, two groups of optimal single fiber group, and optimal group of hybrid fibers samples to explore the changes in pore structure parameters of low heat cement concrete. Table 5.1 shows the pore structure characteristic parameters of difference groups after mercury intrusion testing. According to the test results, compared with the benchmark concrete group, the porosity of the single basalt/brucite fiber and hybrid fiber groups has been improved to a certain extent, but the most probable pore size has slightly decreased and is distributed in the range of less harmful pores. Among them, the most probable pore size of the hybrid optimal group is 32.40nm, which is smaller than the other groups. In addition, compared with the benchmark concrete group, the total pore volume of other groups decreased, but the total pore area increased, indicating that although the total porosity slightly increased, the internal pore structure of the concrete was refined.
Table 5
The characteristic parameters of pore structure
Specimen | Porosity(%) | The most probable pore size (nm) | Total pore volume (ml/g) | Total aperture area (m2/g) |
A0R0 | 13.96 | 50.40 | 0.0707 | 6.715 |
A2R0 | 14.58 | 42.72 | 0.0662 | 7.395 |
A0R2 | 14.67 | 41.47 | 0.0651 | 7.164 |
A2R1 | 14.82 | 32.40 | 0.0606 | 8.051 |
Table 5.2 shows the pore size distribution of HFRLCC. According to the test results, compared with the benchmark concrete, the number of harmless and harmful pores increased by 30.11% for A2R0, 28.67% for A0R2 and 42.72% for A2R1, respectively. Moreover, the pore size distribution of HFRLCC is mostly concentrated between 5-100nm, with fewer pore sizes exceeding this range. In addition, the overall pore size distribution shows a left shift trend. As shown in Fig. 11, the number of harmful and harmful pores has decreased, improving the internal pore structure of the concrete. The optimal hybrid group has the best degree of improvement.
Table 6
The volume distribution of pore structure
Specimen | Pore volume distribution(%) |
<20 (nm) | 20–100 (nm) | 100–200 (nm) | >200 (nm) | ≤ 100(nm) | >100(nm) |
A0R0 | 10.82 | 46.20 | 16.10 | 26.88 | 57.02 | 42.98 |
A2R0 | 17.64 | 56.55 | 11.66 | 14.15 | 74.19 | 25.81 |
A0R2 | 18.02 | 55.35 | 15.37 | 9.31 | 73.37 | 26.63 |
A2R1 | 19.35 | 62.03 | 9.95 | 8.67 | 81.38 | 18.62 |
The reason for the above phenomenon is that the addition of fibers affects the interface structure of concrete, leading to a slight increase in porosity. However, compared with the benchmark group, the total pore volume of other groups decreased, but the total pore area increased. The internal pore structure of the concrete is refined, and the hybrid fibers group shows the best improvement effect. This indicates that the pore size distribution of HFRLCC is good, with high density. An appropriate amount of hybrid basalt-brucite fibers are dispersed in low heat cement concrete, and the evenly distributed basalt fibers combine with the mortar to form a cement slurry matrix, which is well connected to the aggregate and suppresses crack development. Moreover, AF overlap with each other to form a three-dimensional spatial stress structure, that fill the internal pores and improve the density of the concrete. In addition, RF has good compatibility with the mortar, excellent interface bonding, dense surface hydrates, and the small RF can compensate for the small pores in the concrete, improve the compactness of the concrete. The synergistic effect of hybrid fibers has refined the internal pore structure of concrete, increases the number of harmful pores, and thus achieved the effect of toughening and crack resistance.
3.6. Enhancement mechanism
Based on mechanical analysis in section 3.2, the strength of most single fiber reinforced concrete has been improved, while the hybrid fibers group has a better strengthening effect on low heat cement concrete, and the effective fiber "bearing network" and the optimization effect on pore filling have reached the optimal level at an appropriate dosage. Based on the theory of composite materials [38], the enhancement mechanism consists of two parts: On the one hand, uniformly distributed AF form a three-dimensional disordered spatial network with a good load-bearing structure inside the mortar. The crack resistance effect can to some extent suppress the initiation and development of cracks, improve the pore structure of the concrete. In addition, AF combine with mortar to form a cement slurry matrix, which is well connected to the aggregate. At the same time, based on the excellent water dispersion and splitting properties of RF, the length to diameter ratio of the fibers has been improved, achieving uniform distribution in low heat cement concrete, and thus compensating for small pores in the concrete matrix, limiting the propagation of small cracks. Moreover, hydrogen bonding and chemical bonding of Si-OH between the fibers and cement are prone to occur, strengthening the transition zone structure at the fiber interface and improving the compactness of the concrete matrix. On the other hand, when the volume fraction of AF is too large, agglomeration phenomenon is severe, forming large voids during the concrete mixing process, and the bonding ability with mortar is reduced, the crack resistance ability is reduced, and thus the strength of concrete is reduced. At the same time, as the volume fraction of RF continues to increase, the dispersion and splitting effect of water reducer and water on it weakens. In addition to filling and suppressing microcracks, the presence of undispersed fibers, excess fine fibers, and larger diameter fiber bundles that have not been split actually leads to an increase in weak interfaces inside the concrete. Therefore, at an appropriate dosage, the effective fiber "load-bearing network" and the optimization effect on pore filling achieve the best, and the elastic modulus advantages between fibers complement each other. RF with low elastic modulus limit the occurrence and expansion of micro cracks in low heat cement concrete in the early stage of concrete hardening, refine the internal pore structure, and AF with high elastic modulus inhibit the expansion of micro cracks in concrete in the later stage of hardening, improving the pore structure, Thereby enhancing the mechanical strength of concrete.