Performance Evaluation of Recycled Aggregate Concrete Enhanced with Hooked Steel Fibers: Fresh and Hardened Properties

3.1. Materials

The materials employed in this investigation were conventional Ordinary Portland Cement, sand from local quarries, and crushed gravel, all in accordance with ASTM specifications. Mixing and curing were carried out using tap water. A high-range water reducer, Glenium 51, was used to maintain the workability of the mixtures without increasing the water content.

3.1.2. Steel Fibers

Hooked steel fibers were added to compensate for the strength loss associated with RCA. The study began with no fiber and increased the content up to 1.0% in stages of 0.2%. Six fiber contents were examined. The fibers were 30 mm in length and 0.5 mm in width, with an aspect ratio of 60. The tensile strength was at least 1000 MPa, while the modulus of elasticity was over 200 GPa (Table 3, Fig. 1).

Table 3.

Property	Value	Unit	Standard Reference
Length (L)	30	mm	ASTM A820 Type I
Diameter (d)	0.5	mm	ASTM A820 Type I
Aspect Ratio (L/d)	60	–	Calculated
Tensile Strength	≥ 1000	MPa	Manufacturer Spec
Modulus of Elasticity	> 200	GPa	Manufacturer Spec

3.2. Concrete Mix Proportions

Superplasticizer dosage and water–cement ratio were maintained constant for all batches. To increase the fiber volume fraction, more fibers were added. Therefore, to compensate, the amounts of cement, sand, and gravel had to be adjusted. The total batch volume was kept constant for all blends. Full proportions are given in Table 4.

3.3. Workability Test

To determine the effect of steel fibers, the workability and consistency of fresh concrete were evaluated using slump tests according to ASTM C143/C143M [15]. The basic control mix and fiber-reinforced mixes were tested, providing a suitable basis for comparison of slump values with increasing HSF concentration. Concrete was placed in three lifts into the cone and rodded with a standard tamping rod to ensure proper consolidation. All aggregates were conditioned to SSD before mixing to reduce water demand and avoid batch-to-batch variations, since RCA tends to absorb more water than natural aggregate. A similar observation was reported by Kong et al. [1] in their research, which highlighted that aggregate moisture content must be carefully controlled in fiber-reinforced mixes to obtain accurate workability results.

3.4. Tests on Hardened Concrete

All specimens were compacted using a mechanical vibrator to remove air pockets and ensure adequate concrete penetration into the molds. The castings were permitted to stand for 24 h, followed by demolding. Then each specimen was labeled and placed directly into clean water. The main method to maintain hydration and promote strength development was to keep them submerged throughout the curing process.

3.5. Compressive Strength Test

The compressive strength test was carried out using 150 × 150 × 150 mm cubes, prepared in accordance with BS EN 12390-3:2019 [16]. After casting, the cubes were covered with polyethylene sheets to prevent rapid moisture loss and to maintain the necessary conditions for proper concrete curing.

After 24 hours, the cubes were removed from the molds and immediately placed in a tank of clean tap water. They were kept submerged until the age of testing, 28 days. This was done to provide sufficient time for the concrete to hydrate and gain strength. Figure 2 shows the prepared specimens for testing hardened concrete properties, including compressive strength, flexural strength, and other mechanical evaluations.

3.6. Test of Flexural Strength

Flexural strength was determined by the third-point loading technique in accordance with ASTM C78 [17]. This method clearly demonstrates concrete's ability to withstand bending stress. It is commonly utilized for this purpose. Prism specimens of 150 × 150 × 500 mm were cast from the identical mixtures used throughout the investigation. After casting, the prisms were covered and kept wet to prevent surface drying in the initial hours. After 24 hours, they were removed from the molds and placed in a water-curing tank, along with the compressive-strength cubes, under the same curing conditions. Figure 3 depicts a typical specimen undergoing testing and the loading configuration.

However, it is known that fiber-reinforced concrete has a limitation in ASTM C78 [17]. It measures the load at first crack but does not go beyond that; therefore, post-crack behavior, energy absorption, and toughness are not captured. A significant portion of the contribution of fibers, especially hooked steel fibers, is reflected in these post-fracture characteristics. Additional information can be obtained using ASTM C1609, which measures the full load–deflection response and provides toughness indices related to fiber performance after cracking. Since the equipment was available, it was not employed in this investigation. Future studies in this area should include both ASTM C78 [17] and ASTM C1609 to obtain a comprehensive understanding of the effects of fiber reinforcement on ductility and energy absorption in RCA concrete.

The splitting tensile strength was tested on cylinders 150 mm × 300 mm in accordance with ASTM C496-04 [18]. The cylinders were cast from the same concrete batches as all other mechanical tests, so there was no variation in mix quality between specimen types. They were cast, covered to retain moisture, and left for 24 hours. Then they were demolded and transferred to the curing tank, where they were kept fully submerged in clean water until the 28-day mark. The testing setup and application of the splitting load to the specimens are shown in Fig. (4).

4.1. Fresh Concrete Results

Slump tests were carried out to evaluate the effect of hooked steel fibers on the workability of fresh concrete. It should be noted at the outset that this portion of the testing only used NCA-based blends. The RCA mixes were intentionally excluded because the aggregates were in SSD condition and would have introduced water absorption as an additional variable; thus, the effect of fibers alone on workability could not be isolated.

The baseline was the control mix with no fibers (18 cm, as indicated in Table 5 and Fig. 5). At 0.2% HSF, the slump decreased slightly to 17.65 cm, a drop of ~1.94%, which is small enough that it does not cause any practical problems for workability at that dosage.

The trend continued in one direction, with fiber content increasing. With 0.4% HSF, the slump was reduced to 17.2 cm, corresponding to a 4.44% reduction. Increasing the HSF to 0.6% further decreased it to 15.85 cm, resulting in an 11.94% decline. A more noticeable reduction occurred at 0.8% HSF, where the slump decreased to 14.5 cm (19.44% decrease from the control). The most significant change was observed at 1.0% HSF, where the slump decreased to 9.45 cm, approximately half the previous value, and representing a 47.50% reduction.

There is a simple explanation for this continuous reduction. A higher fiber content in the mix leads to increased internal friction between fibers and, consequently, greater resistance to flow. This is further exacerbated by the hooked shape, which increases the tendency of fibers to interlock with each other and with the aggregate, making the mix stiffer and more difficult to place, particularly in the presence of dense reinforcement.

To interpret these findings in practice, the slump data presented here reflect only the effect of HSF, as RCA was not included in this particular test. The SSD preconditioning used elsewhere in this research prevents excessive water demand from RCA; however, its exclusion from the workability test implies that the observed slump behavior can be attributed exclusively to fiber addition, without any influence from aggregate moisture behavior.

4.2. Results of Hardened Concrete

The fresh property findings were previously presented, and the emphasis is now placed on the behavior of the hardened concrete. In this phase of the investigation, compressive, flexural, and splitting tensile strengths for all mixtures, including both RCA and HSF at the tested amounts, are reviewed. The aim was to blend both elements in the same mixes to determine how their combined presence affects mechanical performance and whether fiber-reinforced RCA concrete may be a structurally feasible solution.

4.3. Compressive Strength Results

This section discusses the compressive strength performance of RCA and HSF mixtures. What is assessed here is whether the fibers can effectively compensate for the strength decrease that comes with using recycled material. All cube specimens studied in this phase contained both RCA and HSF, which means the findings reflect the interaction between the two factors rather than what each does on its own. The results of the overall compressive strength are presented in Table 6 and Fig. 6.

▪ 0% RCA Replacement

The NCA reference mix without fibers had a compressive strength of 40 MPa. With the addition of HSF, that figure increased gradually at each step:

• 0.2% HSF increased it to 41.7 MPa, 4.25% above the control.

• 1.0% HSF achieved a compressive strength of 47.8 MPa, which is approximately 19.50% higher than that of the mixture without fibers.

Also, here, the trend observed in the NCA mixes is found: HSF significantly improves internal cohesion and load resistance regardless of aggregate type.

▪ 25% RCA Replacement

The unreinforced compressive strength significantly decreased to 39.4 MPa after the replacement of 25% of the natural aggregate by RCA. Adding fibers brought it back up very consistently.

• 0.2% HSF had 41.05 MPa, which is a 4.19% increase over the unreinforced RCA mix.

• 1.0% HSF increased it to 46.85 MPa, an 18.91% increase.

The strength reduction from RCA was small at this replacement level, and the fibers had no difficulty compensating for it.

▪ 50% Replacement RCA

In this case, the unreinforced baseline declined more significantly to 34.4 MPa, demonstrating the larger effect of RCA at higher replacement levels. However, the fibers retained their performance:

• 0.2% HSF increased the strength to 35.65 MPa (+3.63%).

• 1.0% HSF increased it to 41.0 MPa, an increase of 19.19%.

Acceptable for a mix that lost 50% of the natural aggregate.

▪ 75% Replacement RCA

The lowest baseline of the four groups was no fibers at 31.6 MPa. Here, too, it was the fibers that caused the difference:

• 0.2% HSF resulted in 32.6 MPa (3.16% increase).

• 1.0% HSF reached 37.45 MPa, 18.51% increase.

Heavy RCA use decreases strength, but the data suggest it does not have to be a structural limitation. A similar trend is observed for all levels of RCA shown in Table 6 and Fig. 6. Higher fiber content leads to higher compressive strength, even at 75% replacement. RCA increases porosity and decreases the paste/aggregate bond. The fibers counteract this by bridging cracks and distributing load more evenly throughout the matrix. The overall picture favors the use of RCA at high replacement levels, provided that the fiber dosage is correctly matched.

4.4. Flexural Behaviour of RCA–HSF Concrete

In this section, the performance of RCA and HSF combinations in terms of flexural strength is discussed. The full picture is in Table 7 and Fig. 7. Moreover, as with compressive strength, all specimens here contained both RCA and HSF, so the findings reflect their interaction rather than what each does on its own.

▪ 0% RCA Substitution

The reference mix without RCA and fibres has a value of 5.25 MPa. Moreover, this is where the addition of fibre made all the difference:

• Gain of 18.10% with 0.2% HSF at 6.20 MPa.

• 1.0% HSF increased it to 9.55 MPa, an increase of 81.90%.

Such a rise clearly indicates the significant effect HSF has on the flexural performance of conventional mixtures, primarily via crack bridging and enhanced tensile response in bending.

▪ 25% RCA Replacement

The addition of 25% RCA reduced the unreinforced baseline marginally to 5.15 MPa. Fibres brought it back up effectively:

• The HSF strength was 6.08 MPa at 0.2%, an 18.06% improvement.

• It increased to 9.22 MPa at 1.0% HSF (an increase of 79.03%).

This is close enough to the NCA reference values to show that fibre reinforcement more than compensates for what RCA reduces at this replacement level.

▪ 50% RCA Replacement

The unreinforced baseline saw a higher decline here to 4.10 MPa, which means RCA had a greater effect at this replacement level. However, fibres managed to recover a significant portion of that:

• 0.4% HSF increased strength to 5.50 MPa, an increase of 34.15%.

• 1.0% HSF achieved 6.83 MPa, an increase of 66.58%.

The fibre reinforcement restored the flexural performance to a competitive range, even when half of the natural aggregate was replaced.

▪ 75% RCA Replacement

The unreinforced mix at this level had a 3.45 MPa compressive strength, the lowest baseline among all groups. HSF still increased it:

• 0.2% HSF increased it to 3.87 MPa (gain of 12.17%).

• 1.0% HSF increased it by 57.39% to 5.43 MPa.

Fibre reinforcement from the weakest point in the dataset still increased the flexural strength to a level that can be used structurally.

Table 7 and Figure 7 together indicate the same consistent trend observed in compressive strength: higher HSF content results in improved flexural performance, and this holds across all RCA levels used in the study. The lower quality of recycled aggregate and the weaker bonds formed generally deteriorate the flexural behavior of RCA. However, the fibres counteract this by controlling cracks and enhancing matrix toughness.

The most notable finding is that the 75% RCA mixtures, with the addition of fibre at an appropriate dosage, achieved flexural strengths equal to or higher than those of the unreinforced control. This is an important finding for the potential use of significantly recycled concrete in structural applications.

4.5. Splitting Tensile Strength Results

▪ Replacement of 0% RCA

For the simple control mix without fibres, the splitting tensile strength was measured at 3.45 MPa. Adding HSF made a significant difference:

• 0.2% HSF increased it to 4.16 MPa (20.58% increment).

• 1.0% HSF reached 7.00 MPa, an increase of 102.90%.

This is more than twice the unreinforced value, demonstrating how effectively HSF can redistribute stress and bridge cracks under tensile loading.

▪ Replacement of 25% RCA

Partial application of RCA resulted in a slight reduction in the unreinforced baseline to 3.3 MPa. Fibres recovered and improved this value:

• 0.4% HSF reached 4.55 MPa, an increase of 37.88%.

• 1.0% HSF increased it to 6.53 MPa, i.e., a 97.88% enhancement.

Even with some RCA present, the contribution of fibres to tensile strength was extremely substantial.

▪ RCA Replacement 50%

At 50% RCA replacement, the unreinforced baseline decreased to 2.85 MPa, showing a more pronounced influence of the recycled aggregate. Still, the inclusion of fibre produced excellent recovery:

• 0.4% HSF reached 4.26 MPa, a rise of 49.47%.

• 1.0% HSF increased it by 72.63%, from 2.85 MPa to 4.92 MPa.

The strength loss from 50% RCA was significant, but the fibres bridged much of that gap.

▪ 75% RCA Replacement

The minimum control strength for this test was 2.57 MPa in the 75% RCA group. Even here, the fibres made a meaningful improvement:

• 0.6% HSF increased it to 3.55 MPa, a gain of 38.13%.

• 1.0% HSF gave 3.99 MPa, which is an increase of 55.25%.

The increases were lower than those of the lower RCA groups, which is expected with higher amounts of recycled aggregate, but the rising trend still remained.

Table 8 and Figure 8 show the same trend for all groups investigated, i.e., HSF improves splitting tensile strength for any RCA content in the mixture. The reduction in tensile strength due to recycled aggregate is attributed to reduced bond quality and increased porosity, but the fibres compensate for this by hindering crack propagation and providing more uniform stress distribution in the matrix. The results highlight the interaction between RCA and HSF rather than their individual effects, showing that fibre-reinforced recycled concrete has potential for durability in applications where tensile performance is critical, with both RCA and HSF present in all tested mixes.