1 Introduction
Fueled by urbanization and population growth, the construction industry has expanded tremendously in recent decades worldwide. Concrete is now the second most commonly used construction material worldwide after water and is the main material used in civil engineering. The estimated 25 billion tons of concrete manufactured annually represents and per capita rate of concrete consumption of more than one cubic meter per person per year [
1,
2]. Concrete has been criticized as a non-eco-friendly material because of the severe environmental impacts, including carbon dioxide (CO
2) emissions and natural aggregate consumption, associated with its manufacture. Celik et al. [
3] stated that the production of cement, which is the main component of concrete, accounts for around 7% of global human-caused CO
2 emissions. As part of an overall strategy to reduce CO
2 emissions, waste by-products materials such as fly ash (FA) and rice husk ash (RHA) may be used in concrete as environmentally sustainable alternatives to Portland cement.
The advancement of modern civil engineering in recent decades has led to the innovation of traditional concrete production and the development of concretes such as high-performance concrete (HPC), ultra-high-performance concrete, and high-performance fiber-reinforced concrete (HPFRC) that perform significantly better than conventional concrete [
1,
4,
5]. In general, HPC is designed to be superior to conventional concrete in terms of workability, engineering properties, and resistance to chemical attacks [
6,
7]. Moreover, the high cement content, low water-to-cement ratio, and correct aggregate packing in HPC give this type of concrete superior mechanical properties and durability. Supplementary cementitious materials (SCMs), including industrial by-products such as FA and agricultural wastes such as RHA, may be used as a partial replacement of cement in HPC to both help reduce CO
2 emissions and further enhance the physico-mechanical properties and durability [
8–
10]. However, incorporating SCMs has been associated with the pozzolanic activity that may result in the HPC having a structure that is more brittle than conventional concrete [
11,
12]. In detail, the inclusion of SCM as cement substitution in concrete specimens results in the fewer formation of hydration products (i.e., calcium-silicate-hydrate (C-S-H) gel) at the early ages due to the low pozzolanic activity, and thus the concrete’s brittleness is higher [
13]. At later ages, the mechanical strength enhancement in concrete incorporating SCMs due to the higher degree of pozzolanic reaction inhibits the formation of cracks, resulting in sudden failure under loading [
14]. In addition, the generation of more hydration products under a higher degree of pozzolanic reaction creates denser concrete and consequently increases the modulus of elasticity (higher brittleness) that are more susceptible to tensile failure than the concrete with a looser structure and a lower modulus of elasticity [
15].
The relative brittleness of HPC compared to conventional concrete has limited its utilization in commercial applications [
16,
17]. Incorporating fibers into HPC has been proposed in several studies to improve this issue [
18–
23]. While no improvement in compressive strength or modulus of elasticity has been demonstrated in fiber-enhanced HPC (high-performance fiber-reinforced concrete or HPFRC), significant improvements have been demonstrated in terms of material tensile strength, fracture toughness, ductility, and crack-width control [
7,
16,
23]. The findings of several studies indicate that the inclusion of steel fiber (SF) in HPC may decrease brittleness and change the failure mode in the concrete structure [
24–
27]. For example, Usman et al. [
24] demonstrated that SF, while having a minor positive effect on compressive strength, significantly improved ductility and post-peak behavior in HPC. Li et al. [
27] examined the effect of fiber content on the mechanical properties of concrete and reported that the compressive, splitting tensile, and flexural strengths of HPFRC all improved with increased volume fractions of both steel and polypropylene (PP) fibers, with these results attributed to the contribution of fibers to preventing crack propagation in the hardened concrete. Furthermore, Afroughsabet and Teng [
18] investigated the time-dependent creep and shrinkage properties of hybrid fiber (HF)-enhanced HPC, finding that the added fibers decreased drying shrinkage and the creep coefficient significantly and that the fiber volume fraction had an insignificant influence on drying shrinkage.
Although several important investigations of the effect of fiber incorporation on the mechanical properties of HPFRC have been conducted, few studies have been published on the durability of HPFRC incorporating SCMs. Moreover, no studies in the literature have applied the densified mixture design algorithm (DMDA) to the design of HPFRC mixtures. Therefore, this study was designed to investigate the influence of different fiber types (PP, SF, and HF) and different fiber-volume fractions on the mechanical properties and long-term durability performance of HPFRC designed using the DMDA and incorporating FA and RHA. The mechanical properties of the specimens were evaluated in terms of compressive strength, splitting tensile strength, and flexural strength. In addition, tests for surface electrical resistivity (ER) and chloride ion penetration (RCPT) were conducted to assess durability and drying shrinkage in the specimens. Furthermore, the workability of the fresh concrete mixtures was also evaluated using slump, slump-flow, flow-time, and unit-weight experiments.
2 Experimental program
2.1 Materials
The cementitious materials used in this study were type-I ordinary Portland cement (OPC) from Taiwan, China, type-F FA from a power plant in Taiwan, China, and RHA from Vietnam. The physical properties and chemical compositions of these materials are shown in Tab.1 and Tab.2, respectively.
Crushed aggregate from a local quarry in Taiwan, China with a maximum particle size of 19 mm, density of 2646 kg/m3, and water absorption of 0.7% was used as the coarse aggregate. Crushed sand sourced from China with a maximum particle size of 4.75 mm, density of 2640 kg/m3, water absorption of 1.2%, and fineness modulus of 3.0 was used as the fine aggregate. All of the aggregates used in this study were in dry form. The particle size distributions of both the fine and coarse aggregates were calculated in accordance with ASTM C33 and plotted as shown in Fig.1.
Two types of fibers, including PP fiber (Fig.2(a)) and SF (Fig.2(b)), were used in this study. The two were hybridized as HF (50% PP and 50% SF). As shown in Fig.2, the SF fibers were 30 mm in length and hooked at both ends, with an aspect ratio (L/D) of 50–60, specific gravity of 7.8, and Young’s modulus of 200000 MPa. The PP fibers were a commercial 25/38 type of 3M Scotchcast polyolefin fiber, measuring 25 mm in length and 0.38 mm in diameter with a specific gravity of 0.91 and an elastic modulus of 2647 MPa. The tensile strengths of the SF and PP fibers were 1345 and 275 MPa, respectively.
A type-G superplasticizer (SP) that was brown in color with an approximately 43% solid content, pH of 6–8, and specific gravity of 1.1 was used to achieve the desired slump depth of (250 ± 10) mm for all of the concrete mixtures. Local tap water was used as the mixing water.
2.2 Mixture design and concrete proportions
The mixture proportions of the HPFRC used in this study were designed in accordance with the DMDA, with all of the concrete ingredients packed into a very dense concrete structure to create a high physical density concrete [
28]. By using the finer particles to fill the void among larger particles (as described in Fig.3), the DMDA minimizes concrete porosity to increase concrete density [
29]. To reduce cement and water consumption and improve the long-term durability of the resulting concrete, the focus of the DMDA is on using pozzolanic materials and SP. As described in Fig.3, FA was only used to fill the voids among the fine aggregate system (through the
α test), not to partially replace cement. Thus, FA plays the role of both filler and pozzolanic material in the DMDA system. Then, the FA-fine aggregate mixture obtained at a fixed ratio of
α was used to fill the voids between coarse aggregate particles (through the
β test). It is noted that the amounts of FA, fine aggregate, and coarse aggregate corresponding to the highest density of blended aggregates are determined after conducting the
α and
β tests, and the calculation of fiber content was not included in DMDA. The standard operating procedure for determining the proper mixture proportion by DMDA has been detailly described by the Hwang research group [
28,
30,
31]. Hwang and Hung [
28] have proved that concrete produced using the DMDA mix design method has been demonstrated to be safer, more durable, more workable, more economical, and more ecologically sustainable than concrete produced using the method recommended by the American Concrete Institute.
In this study, all of the HPFRC mixtures were designed using a constant water-to-cementitious-material proportion of 0.32. Hwang et al. [
32] previously demonstrated that concrete incorporating 20% RHA exhibited comparable compressive strength value to the no RHA specimen and the inclusion of up to 20% RHA did not adversely affect the mechanical strength and durability performance of concrete. In addition, based on the preliminary trials, OPC was partially replaced by RHA at a level of 20% by mass in this study. The quantities of other concrete ingredients (i.e., OPC, FA, sand, stone, water, and SP) were calculated using the calculation procedures previously described by Hwang’s research group [
28,
30,
31]. To investigate the influence of adding different fiber types and fiber volume fractions on both the mechanical properties and long-term durability performance of HPFRC, the fiber was calculated (by the total volume of concrete) and added to the mixtures.
A total of seven concrete mixtures were produced, as shown in Tab.3. Five of these mixtures were prepared using different volume fractions of PP fiber, ranging from 0.4% to 1.6% by volume and denoted as PP00 (reference specimen), PP04, PP08, PP12, and PP16. The results of preliminary trials showed that the PP12 specimen exhibited the highest efficiency of fiber content on compressive strength among the five concrete specimens. Therefore, two concrete mixtures (denoted as SF12 and HF12) were prepared using SF and HF (0.6% PP + 0.6% SF) with a fixed fiber dosage of 1.2% by volume.
2.3 Mixing procedures and specimen preparation
A laboratory mixer pan was used to mix all of the concrete mixtures, as follows. After mixing all of the cementitious materials together at a slow speed and while the mixer was still operating, two-thirds of the SP-dosed water was poured into the mixer. After mixing for a further one minute, fine and coarse aggregates were added, and mixing continued for another five minutes. Finally, all of the fibers and the remaining SP-dosed water were added to the mixer, which continued to operate for a further three minutes to achieve a homogenous concrete mixture. Next, the mixture was divided into two groups, with the first used to immediately determine the fresh concrete properties and the second poured into molds and cast as required to perform mechanical property and durability tests. In the second group, the finished concrete specimens were prepared in different sizes specifying for each test method as described in Subsection 2.4. After being covered by plastic film, these concrete specimens were left in the laboratory for 24 h. The concrete specimens were then de-molded and cured in the limewater at (23 ± 2) ºC until the designated age, except for the drying shrinkage test specimens (cured at a temperature of (23 ± 5) ºC and relative humidity of 50% ± 1%).
2.4 Test methods
To investigate the effect of fiber content as well as fiber type on the properties of fresh concrete, the slump, slump flow, slump flow time, and unit weight were measured in accordance with ASTM C143. With regard to the mechanical properties, compressive and splitting tensile strengths were tested on Ø100 mm × 200 mm cylindrical specimens at 3, 7, 14, 28, 56, and 91 d of curing age in accordance with ASTM C39 and ASTM C496, respectively. In addition, prismatic specimens of 150 mm × 150 mm × 550 mm were used to determine flexural strength in accordance with ASTM C78. For the drying shrinkage test, the lengths of concrete prism specimens of 75 mm × 75 mm × 285 mm maintained at a temperature of (23 ± 5) ºC and relative humidity of 50% ± 1% were measured at 1, 3, 7, 14, 28, 56, and 91 d of curing age in accordance with ASTM C490. Moreover, the effects of fiber content and type on concrete durability were evaluated using the ER and RCPT tests. ER tests were conducted on the Ø100 mm × 200 mm cylindrical specimens at 3, 7, 14, 28, 56, and 91 d of curing age in accordance with AASHTO T277, while RCPT tests were carried out on Ø100 mm × 50 mm cylindrical specimens at 28, 56, and 91 d of curing age in accordance with ASTM C1202. All of the reported results represent the average of the results for three specimens of each mixture at each age.
3 Results and discussion
3.1 Fresh properties of concrete
The results of the fresh properties of all specimens incorporating SCMs of different fiber contents and types are presented in Tab.4. The SP content increased, as fibers were used to achieve the target slump depth of (250 ± 10) mm. Moreover, increasing the volume fraction of PP fiber led to a decrease in concrete workability. Thus, even with the increase in SP content, slump flow in all of the fiber-enhanced specimens was lower than that in the reference concrete specimen (PP00), independent of fiber type. For example, the slump flow of reference concrete was 570 mm, while that of specimens containing 0.4%–1.6% PP fiber content ranged from 560 to 430 mm, respectively. The random distribution of fibers in the spatial network composite reduced the flowability of the fresh HPFRC because of adhesion between the fibers and the cement matrix [
33,
34]. This finding is consistent with those of prior studies [
21,
34]. Because fiber content was held constant at 1.2%, slump flow in specimens containing PP fiber was lower than that in specimens containing SF and HF. For example, slump flow was 510 mm in PP12 and 540 and 520 mm, respectively, in SF12 and HF12. In addition, the density of the fresh mixtures was found to be the same in PP-fiber enhanced mixtures as the reference mixture, regardless of PP-fiber content. Furthermore, the difference in fresh unit weight between the mixtures containing PP fibers and the reference concrete was minimal (< 1%). However, the inclusion of SF increased the fresh unit weight of the HPFRC mixture. Thus, the fresh unit weights of the HPFRC mixes incorporating SF and HF were higher than that of the reference concrete. This may be explained by the specific gravity being significantly higher in SF than in the other concrete components (see the Materials section).
3.2 Compressive strength
The compressive strength values for all of the mixes at 3, 7, 14, 28, 56, and 91 d of curing age are shown in Fig.4. Compressive strength in all of the HPFRC specimens increased significantly with the curing time because of continuous hydration and the pozzolanic reaction of FA and RHA. Compressive strength values ranged between 32.1 and 41.8 MPa at 3 d and between 56.5 and 78.1 MPa at 91 d, with these variances reflecting differences in fiber content and type. At 91 d, the specimens containing SF had the highest compressive strength value, with the compressive strength of SF12 22.8% higher than the reference specimen. The physical packing and long-term pozzolanic reaction of FA and RHA achieved using the DMDA mix design method may have created a denser structure, enhanced connectivity between the fibers and the binder matrix, and thus increased the compressive strength of the concrete continuously over curing time [
35,
36].
The effects of fiber content and type on compressive strength are presented in comparison with the reference specimen (PP00) in Fig.5. As shown in Fig.5(a), incorporating PP fiber increased compressive strength slightly at 3 d, while specimens with < 1.2% of PP fiber exhibited compressive strength values that were comparable (< 5% difference) to the reference specimen at 91 d. Meanwhile, the compressive strength of PP16 was 11.2% lower than that of PP00 at 91 d. These results imply that adding an appropriate amount of PP fiber content should have a negligible impact on compressive strength. The high volume fraction of PP fiber may induce additional defects in the composite by increasing air void volume through poor fiber dispersion [
20]. This reduction in compressive strength in specimens with PP fiber incorporation has also been reported in previous studies [
37,
38]. However, other researchers have demonstrated that adding PP fiber to concrete increases compressive strength due to the limiting effect of fibers on crack propagation [
1,
16]. Therefore, the incorporation of fibers may reduce or increase compressive strength in concrete because of their complex influences, which include increasing air voids, reducing compaction, and arresting crack propagation, among others [
9].
Fiber type, especially SF, had a consistently significant and positive effect on compressive strength (see Fig.5(b)). At the same content level (1.2%), compressive strength improved more significantly in the mixture with SF fiber than with PP fiber. For example, the compressive strengths of the SF (SF12) and HF (HF12) specimens were, respectively 22.8% and 7.2% higher than that of PP00 at 91 d of curing age. This may be explained by SF having greater strength and modulus of elasticity than HF [
1,
16]. Thus, SF is more effective at bridging macrocracks, imparting a higher compressive strength value than PP.
3.3 Splitting tensile strength
The development of splitting tensile strength in all of the mixes at 3, 7, 14, 28, 56, and 91 d of curing age is shown in Fig.6. Splitting tensile strength in all of the specimens significantly increased with curing time, with fiber-enhanced specimens further boosting splitting tensile strength and specific effects differing by fiber content and type. Similar to the compressive strength results, the highest splitting tensile strength was observed in the SF specimens, which reached 9.7 MPa at 91 d. As mentioned in the previous section, the continuous increase in concrete strength may be attributable to the increase in fiber binding strength and internal structural densification, especially at the later ages of concrete, achieved by using the DMDA mix design [
35,
36].
The effect of fiber content and type on splitting tensile strength is shown in Fig.7. As expected, splitting tensile strength in the specimens increased with fiber volume fraction (see Fig.7(a)), with 0.4%, 0.8%, 1.2%, and 1.6% PP fiber concrete specimens reporting 91-d splitting tensile strengths that were 1.03, 1.27, 1.21, and 1.40 times higher than the reference specimen. This improvement may be attributable to the strong bond between the PP fibers and the matrix as well as to the limiting effect of PP fibers on crack propagation due to the bridging action of fibers. This finding is consistent with those of previous studies [
16,
37,
39]. On the other hand, as shown in Fig.7(b), the SF and hybridized (SF and PP) fiber specimens exhibited better splitting tensile strength performance than the PP fiber specimens at all testing ages. In particular, splitting tensile strengths in the SF (SF12), HF (HF12), and PP fiber (PP12) specimens were approximately 67%, 40%, and 21% higher, respectively, than the reference specimen at 91 d of curing. The relatively better performance of SF may be attributed to the higher stiffness and modulus of elasticity of SF compared to PP [
37,
40]. In addition, PP fiber is naturally hydrophobic, which inhibits its adherence to cementitious materials [
38,
41]. Thus, the interfacial bonding between the cement matrix and PP fiber may be lower than that between the cement matrix and SF. As a consequence, the SF and HF fiber-enhanced specimens had better splitting tensile strength than the PP fiber-enhanced specimens.
3.4 Flexural strength
The flexural strength of all of the specimens at various curing ages is presented in Fig.8, while the effect of fiber content and type on flexural strength is shown in Fig.9. Most fiber-enhanced specimens exhibited flexural strengths higher than the reference specimen, attributable to the abovementioned fiber bridging effect and effectiveness of the DMDA mixing design, especially at long-term ages. The flexural strength values of the reference specimen at 28, 56, and 91 d of curing age were 5.3, 5.5, and 6.4 MPa, respectively. The maximum flexural strength value at all curing ages tested was obtained for the SF-enhanced specimens. Similar to the splitting tensile strength results, as shown in Fig.9(a), the incorporation of PP fiber enhanced flexural strength, with higher PP fiber volume fractions associated with greater flexural strength. For example, the flexural strength values in the specimens with 0.4% and 1.6% PP fiber were, respectively, 7.3% and 54.7% higher at 91 d than in the reference specimen. This finding concurs with observations reported in a previous study [
9].
Notably, flexural strength in the SF and HF-enhanced specimens was significantly better than in PP-enhanced specimens (see Fig.9(b)). In particular, the respective flexural strength values for the SF, HF, and PP-enhanced specimens were 1.63, 1.48, and 1.33 times higher than that for the reference specimen at 91 d. The interfacial bonding between the cement matrix and SF could be enhanced significantly due to the additional formation of hydration products from the pozzolanic reaction at later ages. Consequently, the bridging effect and crack-arresting mechanism of the SFs were more effective. Therefore, the improvement in flexural strength of the SF specimen compared to the reference specimen at 91 d was higher than that at 28 d. Specimens containing SF fibers may be more effective in preventing cracks due to the anchoring effect of their hook-shaped tails and higher stiffness as well as provide better modulus of elasticity than PP-enhanced specimens. As a result, incorporating SF may be more effective than incorporating PP fiber in terms of improving flexural strength in concrete.
3.5 Drying shrinkage
The shrinkage probably reduces the lifespan of the concrete structure due to the formation of cracks. Zhang et al. [
42] demonstrated that the autogenous shrinkage mainly takes place in the early hydration period of the cement and can be clearly evaluated by ring test, using the digital image correlation method. However, it is noted that this test was not carried out in this study due to the limitation of equipment. Instead, drying shrinkage, which reflects the volume change in the hardened cementitious matrix over time, reflects the loss of water under drying conditions with a relative humidity level of < 95%. In the context of concrete, drying shrinkage is related to crack formation, which seriously influences durability in concrete structures [
43]. The results of drying shrinkage for all of the mixes up to 91 d are shown in Fig.10, and the effects of fiber content and type on drying shrinkage are presented in Fig.11. Overall, the HPFRC specimens were less affected by drying shrinkage than the reference specimen for all curing ages, fiber content levels, and fiber types. In addition, the reduction effect of the fibers on drying shrinkage was higher during 0–14 d of curing age than at later curing ages.
The results of Fig.11(a) show that increasing PP fiber content level was associated with lower rates of drying shrinkage (i.e., less length change in specimens). For instance, at a curing age of 91 d, drying shrinkage was 13.7%, 18.3%, 23.3%, and 28.9% less than the reference specimen, respectively, for specimens with 0.4%, 0.8%, 1.2%, and 1.6% PP fiber. This finding is in agreement with previous studies [
44,
45]. Furthermore, as shown in Fig.11(b), the specimens with SF recorded the highest reductions (up to 30.1%) in drying shrinkage as compared to the reference specimen. In addition, the hybridized (SF and PP) specimens exhibited less drying shrinkage than the PP-only specimens. Therefore, based on the above discussions, HPFRC specimens may exhibit relatively less drying shrinkage because of the fiber-facilitated bond strength between the aggregates and cement matrix and/or the limiting effect on crack propagation. This finding is in good agreement with other researchers [
18,
43].
3.6 Surface electrical resistivity
ER is an important characteristic of concrete durability that is directly related to the chloride-induced corrosion of reinforced concrete [
46]. ER in concrete has demonstrated a good relationship with the corrosion rate of reinforcing steel bars embedded within concrete. According to the guidelines of the American Concrete Institute [
47], very high and low corrosion rates for steel bars in reinforced concrete should be, respectively, less than 5 kΩ·cm and within 20–100 kΩ·cm in ER, respectively. Meanwhile, the corrosion rate for steel bars is very low, with an ER value of > 100 kΩ·cm. The ER results for all of the specimens at various testing ages are shown in Fig.12. In general, ER increased remarkably with curing time. The addition of fibers led to a decrease in ER independent of both fiber content and type compared to the reference concrete. After 91 d of curing time, the maximum and minimum ER values observed for PP00 and HF12 were 55.1 and 25.8 kΩ·cm, respectively, which corresponds to a low corrosion rate for the reinforcing steel bar in the concrete classification.
The effects of fiber content and type on ER in comparison to the reference specimen are shown in Fig.13. The reduction effect of the incorporated fibers on ER declined notably after 28 d of curing, which may be attributed to the pozzolanic reaction between FA and RHA, which subsequently enhanced the microstructure of the specimens at later ages [
48,
49]. However, further investigation into the microstructure of specimens is needed to strengthen this explanation. In addition, as can be seen in Fig.13(a), the higher the PP fiber volume fraction, the higher the reduction in ER. For example, the 0.4% and 1.6% PP specimens had, respectively, 2.2% and 10.0% lower ER values at 91 d than the reference specimen. This finding is consistent with Nili and Afroughsabet [
12]. On the other hand, the SF specimens exhibited significantly reduced ER values, as shown in Fig.13(b). In particular, at 91 d of curing age, while the 1.2% PP specimen had an ER value that was 7.4% less than the reference specimen, the ER value of the 1.2% SF specimen was 51.0% less than the reference specimen. Similar results were observed for the hybridized (SF and PP) specimen (HF12). Afroughsabet and Ozbakkaloglu [
16] reported that the ER value of concrete is strongly affected by the electrolytes in the structural composite. Thus, in this study, SF acted as a conductive element in the concrete specimens, resulting in significantly lower ER values observed in the specimens containing SF. Besides, the addition of SF to the concrete can also affect its pore structure. Niu et al. [
50] reported that the interface of the fiber-paste matrix has a high porosity and a more porous network structure. Hence, this interface is a very sensitive area and microcracks can easily occur more than in other areas of concrete. As a result, the ER of concrete incorporating SF could also be decreased.
3.7 Chloride ion penetration
The RCPT results for all of the specimens are presented in Fig.14, while the effect of fiber content and type on chloride ion penetrability in comparison with the reference specimen is shown in Fig.15. Generally, the RCPT of all of the specimens decreased over curing time, with the inclusion of fibers leading to an increase in RCPT. The highest and lowest charges passed were found in the SF specimen and the reference specimen, respectively, with values of 1132.0 and 819 C at 91 d of curing age. At 91 d, in accordance with ASTM C1202, the specimens with 1.6% PP fiber (PP16) and 1.2% SF (SF12) were identified as having low chloride ion penetrability, while all others were identified as having very low chloride ion penetrability (Fig.14).
Furthermore, increasing the percentage of PP fiber added may be expected to increase the number of connected fibers and lead, as shown in Fig.15(a), to an increase in chloride ion penetrability, with higher levels of PP fiber content resulting in higher chloride ion penetrability. For example, increasing the PP fiber content from 0.4% to 1.6% increased RCPT by approximately 12.2% to 37.1% above the value of the reference specimen at 91 d. This trend was also reported by Zhang and Li [
51]. On the other hand, adding SF increased the RCPT significantly more than adding other fibers, as shown in Fig.15(b). For example, the respective RCPT values for SF12 (1.2% SF) and PP12 (1.2% PP) were 38.2% and 22.7% more than the reference specimen at 91 d. This finding may be explained by the presence of SF in specimens increasing the inner conductive elements, resulting in a higher probability of reinforcement corrosion compared to specimens containing PP fiber [
1]. Moreover, Afroughsabet et al. [
1] mentioned that microstructural properties such as the size, distribution, and interconnection of microcracks and pores were the main characteristics that affect the result of RCPT of concrete. As already mentioned in Section 3.6, the SF inclusion could lead to the weakness of the fiber-paste matrix interface [
50]. Consequently, it may also contribute to the increase in RCPT values of concrete containing SF.
A linear relationship between ER and RCPT was identified in the HPFRC specimens, with a correlation coefficient (
R2) of 0.79 (see Fig.16). Rahmani et al. [
52] demonstrated that the prediction equations may be reasonably reliable, as
R2 is ≥ 0.7. Therefore, the derived equation may be used to represent the correlation between ER and RCPT in HPFRC independent of fiber content and type. As shown in Fig.16, reducing chloride ion penetrability decreased the ER of specimens, which implies that the resultant concrete is more durable in terms of resistance to steel-bar corrosion.
4 Conclusions
In this investigation, the influences of PP, SF, and HF fibers and of different PP fiber volume fractions on the mechanical properties and durability performance of HPFRC designed using the DMDA with the incorporation of FA and RHA at various content levels were analyzed and evaluated. The significant conclusions based on the experimental results include.
1) Adding fiber to HPC (i.e., creating HPFRC) decreases flowability in the fresh mixtures independent of fiber type (i.e., PP, SF, and HF). To maintain a slump value consistent with the reference concrete specimen, the volume of SP must be increased when the fiber volume fraction is increased.
2) The effect of PP fiber on compressive strength is negligible at fiber content levels < 1.2%. The HPFRC specimen with 1.6% PP fiber exhibited 11.2% less compressive strength at 91 d than the reference specimen, while the respective compressive strengths of the concrete reinforced with SF and HF increased by 22.8% and 7.2% over that of the reference specimen.
3) The splitting tensile and flexural strengths of HPFRC both increase as the PP fiber volume fraction increases, regardless of testing age. Moreover, SF and HF are both more efficient than PP in terms of improving splitting tensile and flexural strength performances.
4) Adding PP fiber reduces shrinkage effectively, especially at higher levels of PP fiber content. The highest reduction in drying shrinkage (as much as 30.1% below the reference specimen) was obtained for specimens incorporating SF. Furthermore, drying shrinkage in the HF-enhanced concrete was significantly less than in the PP-enhanced concrete.
5) The presence of conductive elements such as SF in HPFRC specimens containing FA and RHA decreases ER significantly. Furthermore, increasing the PP fiber volume fraction decreased the ER of the concrete specimens significantly through 28 d of curing age, after which this effect was ameliorated.
6) The inclusion of PP fiber affects the chloride ion penetrability of concrete adversely to an increment ranging from 12.2% to 37.1% at 91 d compared to the reference specimen. Furthermore, a remarkable increase in RCPT was identified when SF or HF was added to the HPFRC specimen due to the conductivity effect of SF.
In this study, the change in the microstructure of concrete specimens has not been clearly identified. Therefore, to strengthen the explanation of results, it is recommended that further experiments such as porosity measurement or scanning electron microscopy analysis should be done. Moreover, autogenous shrinkage of HPFRC specimens should be further considered to evaluate the development of cracks at the early ages.
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