Effects of binder paste-to-aggregate ratio and polypropylene fiber content on the performance of high-flowability steel fiber-reinforced concrete for slab/deck overlays

2.1 Raw materials

The raw materials used in this experiment included cement, fly-ash, river sand, crushed limestone, steel fiber, PP-fiber, superplasticizer, and water. The ordinary Portland cement of strength grade 42.5 produced by Xinxiang Tianrui Cement Co. Ltd., Henan, China, was used with the properties as summarized in Table 1, which met the specification of China code GB 175 [25]. The class-I fly-ash supplied by Hebi Tongli Building Materials Co. Ltd., Henan, China, was used as the mineral admixture with the properties as summarized in Table 2, which met the requirements of China codes GB/T 1596 [26]. The natural river sand with fineness modulus of 2.65 was gathered from the beach of Tanghe River, Henan, China. The crushed limestone was used in gradation with a maximum particle size of 20 mm, which met the continuous gradation in specification [27], and satisfied the requirement for pumped concrete related to the diameter of pumping pipe and the length of steel fiber [6,10]. The main properties of river sand and crushed limestone are summarized in Table 3. The ingot-mill steel fiber was used with a length of 32.5 mm, an equivalent diameter of 0.8 mm, and a tensile strength over 860 MPa, which was produced by Shanghai Harex Steel Fiber Technology Co. Ltd., Shanghai, China. The PP-fiber was used with a length of 12 mm, a diameter of 0.03 mm, a tensile strength over 400 MPa, and an apparent density of 910 kg/m³, which was produced by Shanghai Chengqi Chemical Technology Co. Ltd., Shanghai, China. The properties of steel fiber and PP-fiber met the specification of China code JG/T 472 [10] and CECS 38 [11]. The superplasticizer was a compound pumping agent with 23.7% water reduction, 16.1% solid content, and 3.21% alkali content, and having a setting time differential of 145 min, a shrinkage rate of 103%, and the compressive strength ratio of 165 and 153% at a curing age of 7 and 28 days, which was produced by Jiangsu Subote New Materials Co. Ltd., Nanjing, China. The properties of superplasticizer met the specification of China code GB 8076 [28].

2.2 Mix proportion

Based on the diagnoses of concrete slab/deck overlay and the specification of pumped concrete in China code JGJ/T 10 [6], the HPSFC was designed to reach a target strength grade of C40 with a cubic compressive strength of 48.2 MPa, and to be pumped a maximum distance around 1,000 m or a maximum height about 200 m with an initial slump of 220–260 mm and a slump-flow of 450–600 mm. The mix proportion of HPSFC was calculated with the absolute volume method, in which the river sand and crushed limestone were counted at the saturated and surface-dry condition, and the steel fiber was counted as coarse aggregate [10]. The fly-ash content was set at 30% for a common amount of admixture, and the water-to-binder ratio could be determined as 0.36 with the prediction formula of compressive strength of concrete [10]. By referencing the previous study, the sand ratio was 52% and the water dosage was 192.4 kg/m³ for a high-flowability concrete with 0.8% volume fraction of steel fiber [24]; the volume ratio of binder paste to aggregates (abbrev. P/A ratio) was selected as 0.48, 0.54, and 0.60. The PP-fiber content was set as 0.45, 0.90, and 1.35 kg/m³ based on the previous study of high-flowability concrete with PP-fiber [18]. Finally, the mix proportions of test HPSFCs are summarized in Table 4. The number followed by letter PA represents the P/A ratio of 0.48, 0.54, and 0.60, and that followed by letter SP represents the PP-fiber content of 0.45, 0.90, and 1.35 kg/m³.

2.3 Test methods

A single-shaft forced mixer was used for mixing the raw materials with a process summarized in Figure 1. The workability of fresh mix was evaluated with the slump and slump-flow and the static segregation rate that measured using test methods specified in China code GB/T 50080 [29]. Some operation process was adjusted by referencing the previous study [30]. The rheological property of fresh HPSFC including the yield stress and the plastic viscosity was measured by the eBT-V concrete rheometer produced by Germany Schleibinger Co. Ltd. The test for rheological property was performed within 60 s, in which the rotating speed uniformly increased from zero to 30 rpm at the first 30 s and then uniformly decreased to zero at the second 30 s.

Figure 1

Mixing process of raw materials for the HPSFC.

After the tests of workability using standard methods, the pumping test was conducted in laboratory using a secondary structure concrete pump that was produced by Henan Yigong Machine & Equipment Co. Ltd. China. Using the automatic air pressure technology of piston structure and pumping the concrete with S-pipe valve, the pump is composited with hopper, piston cylinder, S-pipe valve, and pump pipe [24]. The pump has a maximum pumping ability of 6 m³/h, a working pressure of 10 MPa while a peak-pressure of 15 MPa, a motor power of 5 kW and a filling height of 0.7 m for fresh mix. A pipe of 6 m length with 80 mm diameter was used for cyclic pumping. With a flow of concrete in pump pipe at 0.47 m/s, a test simulated 100 m pumping distance of 220 s.

After the testing for workability, the fresh mix was artificially mixed again and poured into the molds to form the specimens that were required for testing of hardened HPSFC. Test for the early cracking resistance of HPSFC was conducted according to the specification of China code GB 50082 [31]. The steel mold was composited by surrounded U-steel of 63 mm × 40 mm × 6.3 mm on a steel plate of 100 mm × 100 mm × 5 mm. Several short steel bars were welded on the inner surface of U-steel to produce a confinement for the HPSFC specimen. Before casting of HPSFC, a plastic film was paved on the bottom steel plate to free the bottom surface of HPSFC specimen. After HPSFC formed in the mold, the exposure simulation test began with an electric fan blowing to the surface of specimen for 24 h; the crack width and length were detected with an automatic crack viewer produced by Beijing Zhibolian Technology Co. Ltd. Beijing, China. The nominal total area (A _cr) and the crack reduction factor η _cr are calculated as follows; the average of two specimens for a HPSFC was used as the test result in this study:

where ω _i,max is the maximum width of the ith crack, mm; l _i is the length of the ith crack, mm; A _cr,0 and A _cr,f are the nominal total crack areas for the reference specimen and the HPSFC specimen, respectively.

The water penetration resistance of HPSFC is known as a property that relates to the durability, because the corrosive substances are always carried by water penetrated into concrete. It was measured according to the specification of China code GB/T 50082 [31]. The water penetration tester produced by Zhangjiagang Jianyi Equipment Co. Ltd. China, was used. The specimen was a frustum of cone with a top section of ϕ175 mm, a bottom section of ϕ185 mm, and a height of 150 mm. Six specimens as a group were tested on the tester under a water pressure of 1.3 MPa that sustained for 24 h. Then, the specimens were split on a testing machine. The water penetration depth was measured along the bottom edge at 10 points. The average of 10-point test values was used as the water penetration depth of a specimen, and the average of six specimens of a group was taken as the water penetration depth of a HPSFC. According to the specification of China code SL352 [32], the relatively permeability coefficient k _r is calculated as

where h _m is the average water permeable height, cm; T is the sustainable time, s; H is the pressure presented by water head, cm.

The splitting tensile method perpendicular to the bond section was used to measure the bond strength of HPSFC to the existing concrete, while the splitting load was exerted on the bond section of specimens [33,34]. To simulate the rough surface of the existing concrete, the cubic specimens with a dimension of 150 mm of the conventional concrete with a strength grade of C40 were split into two parts on testing machine, and the splitting surface was artificially shaped to expose coarse aggregates and keep a similar roughness. The surface roughness was measured by the sand filling method, and the average rough depth was around 4.5 mm. As shown in Figure 2, the half block with rough surface was placed in the mold, and the HPSFC was cast into the remnant half space to form the specimen for bond test, and the average of five specimens for a HPSFC was used as the test result in this study. Using the test of bond strength, the splitting tensile strengths of conventional concrete and HPSFC were also tested.

Figure 2

Preparation of specimens for the test of bond strength: (a) rough surface of existing concrete, (b) specimen molds, and (c) specimens.

The impact resistance of HPSFC was determined on MTS automatic drop hammer impacting testing machine. According to the specification of China code CECS 13 [35], a steel hammer of 4.5 kg weight automatically drops from a height of 500 mm on the center of a specimen with a dimension of ϕ150 × 63 mm. The numbers of impact at initial cracking on surface of specimen, and that at failure when the cracks extension leads the specimen contacted with at least three of surrounding baffles, were recorded. The impact energy dissipation can be calculated as follows, and the average of six specimens for a HPSFC was used as the test result in this study:

where W is the impact energy dissipation, J; N is the number of impact; m is the weight of hammer, kg; h is the drop height of hammer, m; g = 9.81 m/s².

The cubic compressive strength of HPSFC was determined using a testing machine according to the specification of China code GB 50081 [33]. The dimension of cubic specimens was 150 mm, and three specimens were made for a HPSFC as a trial. As shown in Figure 3, the HPSFC specimens failed with less crushed and linked with fibers across cracks, which kept an entirety without broken.

Figure 3

Test of cubic specimens of HPSFC under compression.

The flexural toughness of HPSFC was determined using a third-point loading test on the beam specimens with a span of 450 mm by a testing machine according to the specification of China code JG/T 472 [10]. The specimen was made with a section of 150 × 150 mm and a length of 550 mm. Three specimens were made for a HPSFC as a trial. As shown in Figure 4, a displacement meter installed at mid-span automatically measures the deflection of test beam under two concentrated loads. Normally, a complete load-deflection curve for each test specimen can be obtained, as the crack at the mid-span segment under pure bending was bridged with steel fibers and PP-fibers [36]. The steel fibers and PP-fibers across crack lost the bond to concrete matrix or broke with the increase of exerted load, and the mid-span deflection sustainably increased under the post-peak load. This leads the HPSFC to have flexural toughness without suddenly fractured. The representative curve of each HPSFC is obtained by dealing with three tested curves of each group [37].

Figure 4

Flexural toughness test for HPSFC.

Using the calculation method of flexural toughness as per China code JG/T 472 [10], the initial flexural toughness ratio R _e,p is computed to represent the pre-peak-load deflection toughness, and the remaining flexural toughness ratio R _e,k is computed to represent the post-peak-load deflection toughness at a given deflection. A larger R _e,p indicates a better enhancement of steel fibers on the flexural performance of HPSFC before it reaches the ultimate flexural strength, while a larger R _e,k represents a great contribution of fibers to the remaining flexural strength and energy absorption capability of HPSFC [36,38]:

where f _f is the flexural strength of HPSFC, MPa; P _max is the peak load at the complete load–deflection curve, N; L ₀ is the span of specimen, mm; b and h are the cross-sectional width and depth of specimen, mm; δ _p is the deflection at the peak load, mm; δ _k is a given deflection at condition of L ₀/k (k = 500, 300, 250, 200, 150), mm; Ω_p is the area enveloped by the curve within the deflection δ _p, N mm; Ω_p,k is the area enveloped by the curve within the deflection from δ _p to δ _k, N mm.

Except for those specimens tested for early cracking resistance, all the specimens were covered with plastic film and placed at laboratory room for 24 h before demolded. The specimens were cured in the standard curing room at a temperature of 20 ± 5°C and a relative humidity over 90%.

3.1 Slump flowability

As summarized in Table 5, the slump and slump-flow of fresh mixes present an expected tendency that increased with the P/A ratio and decreased with the increase of PP-fiber content. This attributes to the increased volume of flowable binder paste with the P/A ratio and the linkage of fibers that arrested the flowing of binder paste. Considered that the initial slump of HPSFC varied from 230 to 255 mm, and the initial slump-flow varied from 470 to 600 mm, the pumping distance could reach 1,600 m when a pump pipe with 125 mm diameter is used as specified in China code JGJ/T 10 [6]. Meanwhile, except for the HPSFC with 0.15% volume fraction of PP-fiber, the other HPSFCs presented a slump loss of 20–30 mm after standing for 1 h. This was within the limit of 30 mm specified in China code GB 50164 [7]. After pumped by the simulation pumping test, the slump reduced to 150–195 mm, while the slump-flow reduced to 310–380 mm, due to the hydration of cement at a high temperature in pump pipe. The losses of slump and slump-flow took place on the HPSFC with the highest P/A ratio of 0.60. This indicates more content of cement hydrated during the pumping to elevate the temperature and then accelerated the hydration of cement to reduce the flowability. However, the flowability still met the requirement of flowability concrete specified in China code JGJ 55 [39].

As summarized in Table 6, the plastic viscosity and yield stress of fresh HPSFC decreased with the increase of P/A ratio and increased with the PP-fiber volume fraction, whatever before or after pumped. With the increase of P/A ratio from 0.48 to 0.54 and 0.60, the plastic viscosity decreased by 16.9 and 64.5% and the yield stress decreased by 63.3 and 93.6%. This is consistent with an obvious increase of the slump-flow than the slump and corresponds to the increased tendency of static segregation at initial and after pumped. Meanwhile, with the increase of PP-fiber volume fraction from 0.05 to 0.10% and 0.15%, the plastic viscosity increased by 64.3 and 145.2% and the yield stress increased by 27.6 and 76.6%. This is consistent with an obvious decrease of the slump than the slump-flow and corresponds to the decreased tendency of static segregation at initial and after pumped.

From the test results of rheological properties of fresh mixes, it can be seen that a dominating effect of the P/A ratio existed on the yield stress, because the skeleton of aggregate blocks the flowing of binder paste [24]. Comparatively, the plastic viscosity of fresh mix increased because more binder paste was bonded on the fine and short-and-thin PP-fibers, while the yield stress of fresh mix became higher because the staggered dispersion of PP-fibers confined the flowable deformation of binder paste.

Compared to the initial rheological properties, the plastic viscosity and the yield stress of fresh mix multiplied after pumped, which were highly influenced by the PP-fiber volume fraction than the P/A ratio. This reveals that different from the independent particles of aggregate, the PP-fibers built an interlinked frame bonded with cement hydrates to confine the flowability binder paste. With the increase of PP-fiber volume fraction, the frame became complete to present a much higher confinement.

3.2 Static segregation

The static segregation rate is commonly used to intuitively estimate the bleeding resistance of fresh mix. As presented in Figure 5 for the test results, the static segregation rate increased by 8.6 and 24.0% with the P/A ratio increased from 0.48 to 0.54 and 0.60. This shows that the segregation rate increased with the P/A ratio, because more free paste leaved in the gaps of aggregates. The segregation of fresh mixes became obvious after pumped with an increment of 145.2, 146.9, and 127.1%, respectively. This indicates that the fresh mixes had a tendency that the paste freely separated from the wrapped aggregates under pumping pressure. When fixed P/A ratio at 0.60, the static segregation rate reduced by 17.3 and 51.0% with the PP-fiber content increased from 0.45 to 0.90 kg/m³ and 1.35 kg/m³. This indicates that the segregation of fresh mix can be obviously reduced by adding more PP-fibers, because the cohesiveness of paste was increased with the mesh binding of PP-fibers. The segregation rate of pumped fresh mixes increased by 127.1, 111.6, and 139.2%, compared to those initial values before pumped. However, the tendency was kept ensuring that the static segregation rate decreased with the increase of PP-fiber content.

Figure 5

Static segregation rate of HPSFC changed with (a) P/A ratio and (b) PP-fiber content.

Generally, the static segregation rate of fresh mixes was 2.93% in maximum. This was far below the limit of 10% specified for pumping concrete in China code JGJ/T 10 [6]. Therefore, the static segregation of fresh HPSFC was not an issue that should be detected, due to the contributions of steel fibers and PP-fibers to binder paste retaining by surface coating and constituents gathering.

3.3 Pumping verification test

The pumpability of fresh mix was observed on the flow state in the hopper, as presented in Figure 6. All fresh mixes presented good cohesive without the water bleeding and the exposure of coarse aggregates and steel fibers, while the formation of lubricating layer benefited to the flow of fresh mix in pump pipe. The pump became easier with the increase of P/A ratio for fresh HPSFC due to the increased volume of binder paste (Figure 6c). An obvious slow-down appeared for the fresh HPSFC pumped into piston cylinder with the increase of PP-fiber content, due to the increased linkage of PP-fibers (Figure 6e).

Figure 6

Pumping status of fresh HPSFC: (a) PA1SP1, (b) PA2SP1, (c) PA3SP1, (d) PA3SP2, and (e) PA3SP3.

4.1 Resistances to early cracking

Table 7 summarizes the test results of the early cracking resistance of HPSFC, where PA1, PA2, and PA3 were the reference concretes without fibers in condition of the P/A ratio at 0.48, 0.54, and 0.60. Obviously, the number of cracks, the maximum crack width, and the nominal total area of cracks were all increased with the P/A ratio for the reference concretes. This indicates that the shrinkage of concrete mainly came from the set cement at early age. With the increase of P/A ratio, the volume of binder paste increased, while that of coarse aggregates decreased, and the higher shrinkage of set cement was less confined by the coarse aggregates. Corresponding to the appearance of more cracks with larger maximum crack width, the A _cr,0 increased by 4.0 and 8.6% for concrete with P/A ratio increased from 0.48 to 0.54 and 0.60. Obviously, this situation could be improved by adding steel fibers and PP-fibers.

For the HPSFC with a PP-fiber content of 0.45 kg/m³, the A _cr,f could be decreased with the decrease of the number of cracks and the maximum crack width. Compared with that of reference concrete, the reduction factor changed from 29.5 to 33.1%. This reveals a better confinement of steel fibers and PP-fibers on the shrinkage of set cement that contributes to the early cracking resistance of HPSFC. With the increase of PP-fiber content from 0.45 to 1.35 kg/m³, the improvement on the early cracking resistance of HPSFC became obvious, which led to almost disappearance of early crack on HPSFC with a PP-fiber content of 1.35 kg/m³. This means that the HPSFC can be produced without early cracking by adding certain PP-fibers.

According to the specification of China code CECS 38 [11], the first, second, and third degree of cracking resistance of HPSFC, respectively, corresponds to η _cr ≥ 70%, 55 ≤ η _cr < 70, and 40 ≤ η _cr < 55. Therefore, the HPSFC with a PP-fiber content of 0.45 kg/m³ did not reach a target of cracking resistance even if at the third degree. Meanwhile, the HPSFC with a PP-fiber content of 0.90 kg/m³ reached the third degree of cracking resistance, while that with 1.35 kg/m³ was best at the first degree. This shows that a higher content over 0.45 kg/m³ was needed to provide sufficient confinement for the early shrinkage of HPSFC.

4.2 Resistances to water penetration

Figure 7 presents the measuring of water penetration depth in HPSFC specimens. The black curve marked the variation of water penetration into HPSFC. As observed from the splitting sections, the HPSFC was dense with well-wrapped and uniformly distributed coarse aggregates and fibers, and no macroscopic pores were observed. The water penetration depth was basically flush without the obvious penetration surrounding the coarse aggregates. This presents an average depth with small dispersion. Obviously, the water penetration depth decreased with the increase of the P/A ratio and the PP-fiber content.

Figure 7

Water penetration depth of HPSFC changed with P/A ratio and PP-fiber content: (a) PA1SP1, (b) PA2SP1, (c) PA3SP1, (d) PA3SP2, and (e) PA3SP3.

As shown in Figure 8, the relatively permeability coefficient k _r obviously decreased with the increase of the P/A ratio and the PP-fiber content. That is to say, the resistance of HPSFC to water penetration increased with the P/A ratio and the PP-fiber content. Because the water penetration always takes place along the pores and defect of concrete matrix especially those existed around the coarse aggregate, this possibility was lightened by sufficient binder paste of HPSFC with higher P/A ratio. The k _r decreased by 42.0% for HPSFC with the P/A ratio increased from 0.48 to 0.54, and that decreased by 14.4% for HPSFC with the P/A ratio increased from 0.54 to 0.60. This indicates that the P/A ratio was insufficient at 0.48 for the HPSFC, and that was well at 0.54, combined with the mechanical properties of HPSFC. Certainly, the P/A ratio of 0.60 was best for HPSFC that presented best performances of resistance to early cracking and of mechanical properties. Compared to that of HPSFC with a PP-fiber content of 0.45 kg/m³, the k _r decreased by 46.7 and 75.0% for HPSFC with a PP-fiber content of 0.90 and 1.35 kg/m³. This attributes to the improvement of PP-fibers on refining macro-pores and eliminating defects in concrete matrix. Most notably, this directly relies on the improvement of the early cracking resistance that eliminated the path of water penetrated into HPSFC, as previous study revealed that the water penetration rate of concrete with micro-cracks increased about 10 times [40].

Figure 8

Relative permeability coefficient of HPSFC changed with (a) P/A ratio and (b) PP-fiber content.

4.3 Bond strength to existing concrete

The splitting tensile strength of conventional concrete was 3.21 MPa. As presented in Figure 9, the splitting tensile strength of HPSFC slightly increased with the P/A ratio and the PP-fiber content. The former attributed to the increased binder paste that was well wrapped on coarse aggregates to eliminate the defects on interface between set binder paste and coarse aggregates. The latter was due to the uniform distributed PP-fibers that improved the tensile strength of set cement that benefits to the entire tensile performance of HPSFC. Results of this study indicate that the splitting tensile failure took place on the bond surface of specimens, because the HPSFC and conventional concrete had higher splitting tensile strength. Observing the bond strength of HPSFC to existing concrete, it presents the increasing tendency with the P/A ratio and the PP-fiber content. This can be attributed to the beneficial effects that sufficient binder paste benefited to fill the pits on the rough surface of existing concrete and enhance the entirety of indented interface between HPSFC and existing concrete, while a plentiful PP-fibers in the binder paste contributed to improve the stress distribution with a decreased shrinkage of HPSFC on the interface. On an average level, the bond strength was 70.1–80.2% of the tensile strength of HPSFC and 76.7–93.7% of the existed concrete.

Figure 9

Bond strength of HPSFC changed with (a) P/A ratio and (b) PP-fiber content.

However, it should be noted that the dispersion of bond strength was obviously larger than that of tensile strength. This indicates that the bond strength was affected by multifactors except for the HPSFC and existing concrete themselves, especially by the actual morphology feature of existed concrete surface although a similar roughness was evaluated by the sand filling method.

4.4 Impact resistance

As shown in Figure 10, similar failure patterns with three blocks that divided by three cracks appeared on tested specimens of HPSFC with different P/A ratio. A deeper crushed pit appeared on the center of specimen for the HPSFC with the highest P/A ratio. This indicates that the HPSFC with a higher compressive strength gave a higher impact resistance. With the increase of PP-fiber content, only a straight crack presented on the specimens, and the secondary crack disappeared on the specimen of HPSFC with the PP-fiber content of 1.35 kg/m³. This reveals that the linkage of PP-fibers in concrete matrix improved the entirety of HPSFC to resist the impact action.

Figure 10

Impact failure patterns of HPSFC: (a) PA1SP1, (b) PA2SP1, (c) PA3SP1, (d) PA3SP2, and (e) PA3SP3.

Figure 11 summarizes the test number of impact on HPSFC at the initial cracking and the failure states. Generally, the number of impacts on HPSFC specimens increased with the P/A ratio and the PP-fiber content; however, a greater dispersion existed on the tested number of impacts on six specimens for a HPSFC. Comparatively, the number of compacts increased remarkably with the P/A ratio. This reveals that the bond of steel fiber in concrete matrix is very important to resist the impact action [24]. Increasing PP-fiber content continues to improve the impact resistance of HPSFC, where the increment reached 13.8 and 20.2% when the PP-fiber content increased from 0.45 to 1.35 kg/m³. Because the number of impacts at the failure only increased by 12.5 to 25.8% from that at the initial cracking, the impact resistance of HPSFC mainly relied on the resistance to initial cracking.

Figure 11

Impact number of HPSFC changed with (a) P/A ratio and (b) PP-fiber content.

5.1 Cubic compressive strength

Test results of cubic compressive strength of HPSFC are presented in Figure 12. Except for the strength of PA1SP1, which was less than the target of 48.2 MPa, others were all over it at a curing age of 28 days. The cubic compressive strength of HPSFC increased with the P/A ratio, while that increased by 31.5, 29.1, and 32.9% at a curing age of 28 days from 7 days, respectively, for the HPSFC with a P/A ratio of 0.48, 0.54, and 0.60. This indicates that the set cement accompanied by hydrates of fly-ash dominated the compressive strength of HPSFC, and enough paste wrapped on aggregates was necessary to build a strong structure of HPSFC. Meanwhile, the compressive strength of HPSFC presented a slight decreased tendency with the increase of PP-fiber content, and that increased by 32.9, 35.1, and 38.1% at a curing age of 28 days from 7 days. This indicates that the compressive strength development of HPSFC was less influenced by the randomly distributed PP-fibers in concrete matrix due to the finite deformation confinement of PP-fibers with lower modulus of elasticity.

Figure 12

Cubic compressive strength of HPSFC changed with (a) P/A ratio and (b) PP-fiber content.

5.2 Flexural toughness

Figure 13 presents the complete load–deflection curves of HPSFC under bending. The peak load increased with the increase of P/A ratio, while an increased tendency of peak load appeared with the increase of PP-fiber content. This can be directly observed on Figure 14, which presents that the flexural strength of HPSFC changed with the P/A ratio and the PP-fiber content. The HPSFC with PP-fiber content of 0.90 kg/m³ presented a highest flexural strength of 5.26 MPa. This is attributed to the bond performance between steel fibers and concrete matrix that strengthened with uniformly distributed PP-fibers [37,41]. The HPSFC with higher P/A ratio provided sufficient binder paste wrapped on coarse aggregates to eliminate the defects between the set cement and the surface of coarse aggregates. This creates a good cross-connected architecture of steel fibers across coarse aggregates with a steadily wrapped bond layer of set cement. Meanwhile, the uniformly distributed short-and-thin PP-fibers improved the macro-structure of concrete matrix, which benefits to reinforcing the bond performance of steel fibers to concrete matrix. This inferential analysis can be supported by the post-peak behavior of load–deflection curve presented in Figure 14. A slowly drop-down appeared on the post-peak decline curve of HPSFC with a higher P/A ratio and a greater PP-fiber content, which presented a well-sustainable deflection under a higher load and a second ascending at a relative larger deflection.

Figure 13

Flexural load–deflection curves of HPSFC with (a) P/A ratio and (b) PP-fiber content.

Figure 14

Flexural strength of HPSFC changed with (a) P/A ratio and (b) PP-fiber content.

As shown in Figure 15, the HPSFC with a PP-fiber content of 0.90 kg/m³ reached the highest initial flexural toughness ratio R _e,p and the remaining flexural toughness ratio R _e,k (k ≤ 250). Meanwhile, the fullness of post-peak decline segment of load–deflection curve ensured a higher R _e,500 that corresponded to the deflection of 0.90 mm. The R _e,500 remained 80.9 to 90.2% of the R _e,p. When the mid-span deflection reached L ₀/200, i.e., 2.25 mm, the R _e,200 remained 42.6 to 60.8% of the R _e,p. This indicates that the HPSFC in this study had better flexural performance with ideal flexural toughness.

Figure 15

Flexural toughness ratio of HPSFC changed with (a) P/A ratio and (b) PP-fiber content.