Shear strength of recycled aggregate concrete beams without shear reinforcement

Steel fibers have recently become increasingly popular for strengthening reinforced concrete members. This research contains a study of shear strength under the effect of replacing 50% of natural coarse aggregate (gravel) with recycled aggregate (crushed brick) as well as the effect of steel fibers for natural aggregate concrete and recycled aggregate concrete. Five concrete beams with dimensions of 2000 mm in length, 300 mm in depth, and 200 mm in width were tested to evaluate the effect of recycled aggregates and to evaluate the efficiency of steel fibers in improving the shear capabilities of concrete beams. Beams are designed to fail due to shear stresses (without shear strengthening). The specimens were reinforced with four 4" X 16" steel bars at the bottom and two 2" X 10" steel bars as top reinforcement. Three volumetric fractions of steel fibers were used: 0%, 0.5%, and 1%. The samples were subjected to a two-point loading test. The beam samples have a ratio of shear span to effective depth (a/d) equal to 2. The results showed that replacing 50% of natural aggregate with recycled aggregate decreased ultimate shear strength, but adding steel fibers by 1% by volume resulted in an increase of 31.8% in shear strength for a recycled concrete beam. For ordinary concrete, the final shear strength increased by 18.3% and 30.8% when steel fibers were added by 0.5% and 1%, respectively. Moreover, the results also showed that the failure mode of all samples is the shear failure mode.


Introduction
Globally, the main indicator of the civilized development of countries is urban development.With the increase in construction development, the percentage of waste for building materials increases, whether old or damaged materials from manufacturing new materials.Figure 1 shows the waste resulting from construction operations.There are huge amounts of waste resulting from construction and demolition operations, and these percentages vary internationally according to the country's development and its capabilities in developing recycling programs.The percentage of construction and demolition waste in the United States is 25% of solid waste [1].While the percentage of waste exceeds 48% in South Korea, which reaches 60% of the solid waste percentage, approximately about 7.5 million tons annually [2].In Iraq, the amount of waste is about 100 million tons annually [3].Most of this waste is illegally disposed of by stacking it on public roads in the open air, and this is the cause of increasing environmental and health problems [4].Environmentally friendly researchers worked to recycle and benefit from these wastes by using them in construction again, thus reducing the percentage of damaged materials and reducing the costs of their disposal and the consumption of natural resources in construction.Many researchers have worked on recycling various building materials.They work on iron recycling [5].Tahseen and Hassan worked on wheel recycling for the production of the plastic coupler [6].And others focused their studies on wood recycling [7].Many other researchers worked on recycling concrete blocks and focused their study on the features of concrete produced from recycled aggregates, as well as studying the properties of the latter as well.Basim reused discarded plastic bags and damaged aggregates in the manufacture of asphalt mixes [8] .
One of the most important characteristics is the study of shear resistance, which is the most dangerous type of failure because it occurs suddenly without any preliminary indications since concrete is a brittle material and weak in shear resistance.Many studies found that concrete produced from 100% recycled aggregates was weak in compressive strength and durability [9][10][11].Hansen and Boegh proved that the produced concrete from recycled aggregate has a modulus of elasticity lower by 15 to 30 percent, and the shrinkage increases by a rate ranging from 40 to 60 percent in comparison to ordinary concrete [12].Yamato et al. demonstrates that the compressive strength, freezing, and thawing resistance of concrete produced from recycled aggregate (RA) is less compared to that of natural aggregates (NA), while the percentage of dry shrinkage increases with the increase in the percentage of recycled aggregates [13].
After the studies showed that the use of recycled aggregate weakens the properties of the produced concrete, the researchers worked on studies to treat the recycled aggregate before using it to strengthen the qualities of the produced concrete.Ismail and Ramli treated the recycled aggregates using hydrochloric (HCI) acid and calcium metasilicate (CM) solution [14].Katkhuda et al. used hydrochloric (HCI) acid with sodium metasilicate solution for treating recycled aggregates [15].Dhaheer investigated the properties of clay bricks treated with polyvinyl alcohol (PVA) experimentally [16].Katz uses a silica fume solution to treat the recycled aggregate [17].One of the most important reasons that cause weakness in concrete produced from recycled aggregates is the high absorption of water.Therefore, Rahal worked on treating recycled aggregates and using them in a saturated state with a dry surface to avoid the problem of high absorption of mixing water [18].All the above researchers that worked on the treatment of recycled aggregate found a slight increase in improving the mechanical properties of the produced concrete compared to the concrete produced from untreated recycled aggregate.
After studying the treatment of recycled aggregate, the researchers found other ways to increase the strengthening of the produced concrete, such as using fibers and other materials that improve the mechanical properties of the produced concrete.Afroughsabet et al. show that adding steel fibers increased the mechanical characteristics of recycled concrete substantially [19].Muhaned and Sallal proved that using the steel fiber interaction with steel shear could improve the shear strength of reinforced concrete [20].Zhang et al. indicated that the use of steel fibers in concrete produced from recycled aggregates has a good effect, because of its good mechanical properties, high durability, and resistance to high temperatures [21].Kumar et al. studied the effect of steel fibers on the shear strength of deep concrete beams, the results showed that the shear strength increases with the increase of steel fiber content [22].Dhaheer used steel fibers to improve the properties of concrete produced from recycled red bricks, the results showed that the best performance of concrete is when the fiber content is 1% of the total volume [16].Many Previous studies dealt with recycling materials, benefiting from them, and introducing them into a second life cycle.However, there are few studies on recycling bricks and studying the shear behavior of concrete produced from it after processing the recycled bricks and strengthening the concrete itself.

Figure 1.
Waste percentages of building materials [4].With the increase in the percentage of waste from construction and demolition operations and damaged materials, this work sought to recycle these materials and utilize them again to achieve environmental, economic, and social benefits together.It also aims to study the strength of shear characteristics of concrete models produced from 50% recycled bricks (RBC)after being treated and to test the effects of steel fibers volume fraction Vf with (RBC), without stirrups.as the addition of steel fibers works to enhance the mechanical characteristics of reinforced concrete.

Experimental program
Five reinforced concrete beams were tested until failure.The specimens were tested to evaluate the influence of adding steel fiber and lightweight recycled aggregate instead of natural aggregates on the shear strength of specimens.The recycled aggregate is recycled crushed bricks.Table 1 recorded the specifics of the test specimens.The nomenclature of the beam has two sections, the first letter (N and C) refer to the type of coarse aggregate (normal and crushed).The second letter refers to steel fiber volume fractions.The first three specimens included three different volume percentages of steel fiber (Vf=0,0.5, and 1%), these specimens used a natural coarse aggregate.The last two specimens have two different steel fiber volume fractions (Vf=0, and 1%) these specimens used crushed bricks as a coarse aggregate.Crushed bricks were used instead of 50 % natural aggregates.The coarse aggregates in these specimens are 50% natural aggregates and 50% crushed bricks.
The specimens were designed with compressive strength of concrete near 50MPa.Figure 2 shows the details of the test specimens, The specimens have the same cross-sectional dimensions (200, and 300 mm), length 2000mm, and effective depth (d=262 mm).The bottom flexural reinforcement is four Φ16 bars, while the top reinforcement is two Φ10 bars that are used in the compression zone, this type of reinforcement is used to fix the three stirrups that are used.The bottom longitudinal bars were hooked upwards behind the supports.Three Φ 10 stirrups are used in the middle and above the supports.These stirrups are used to fix the top and bottom bars.For all specimens, no stirrups were used within the shear span.The shear span to the effective depth ratio (a/d) was 2. The specimens were examined as a simply supported beam with two equal concentrated loads applied to the beam.

Material properties
The cement used in the concrete mix is sulfur resistant type; this type of cement was used as it is of better quality and to avoid the effect of salts present in the raw materials.The fine aggregate is natural fine aggregate within Zone III of Iraqi Specifications areas of fine aggregate gradation.The natural coarse aggregate was used after grading it, in three sizes (10, 4.75, and 2.36 mm). Figure 3 shows the size of the coarse aggregate.Crushed bricks were used after grinding them, sieve analysis was done for them, and three sizes were worked out (10, 4.75, and 2.36 mm), figure 4 shows the size of the crushed brick.Then it was cured in water and used as a saturated dry surface.Hook-end steel fiber was used with a diameter of 0.5 mm, length of 30 mm, and aspect ratio of 60, as shown in figure 5.The Hyperplastic PC175 was used to enhance the workability of the mixture.The properties of steel bars are presented in table3 according to the specification ASTM 996M-05.Two types of normal concrete were used to casting the specimens the first used natural coarse aggregate while the second used mixed coarse aggregate (50% natural and 50% recycled aggregate).It should be noted that different percentages of steel fiber are used.Details of concrete mixtures were listed in table 2.

Testing procedure
The test was conducted in the Structural Laboratory of the Civil Department at the University of Al-Qadisiyah.All specimens in this study used the universal hydraulic testing device with a 1000 kN capacity.The specimens were tested over an 1800 mm clear span with two-point loading with shear span to effective depth ratios (a/d) is 2. A digital dial gauge was used to record mid-span deflection.Figure 6 shows the preparation for the test for one of the samples.

Results and Discussion
After completing the tests, the experimental results were obtained for five reinforced concrete beams without shear reinforcement (stirrups).Table 4 lists the test's significant findings.The results include compressive strength, first crack load, ultimate load, and failure mode of each specimen.The compressive strength of cubes was tested in accordance with BS 1881-116.In this section, the effect of steel fiber and the aggregate type used in the specimens on the shear strength will be discussed.Figure 7 shows the cracks of the beams after the test.From what has been observed during the practical examination of the specimens, at the beginning of the loading, vertical hair cracks appear in the flexural region, i.e. in the middle of the specimen.With the increase in loading, the development of cracks increases in this region.In addition, cracks begin to appear in the flexural-shear region, as they appear vertically and then take the slop.At the final stages of loads, diagonal cracks appear in the shear region, where they are between the support and the load applied, as shown in figure 7. From this figure, the number of cracks increases with the increase in the percentage of steel fibers.It can be noticed that the number of cracks for NS1 is higher compared to NS0.5 and NS0.Also, when comparing CS1 with CS0, the same behavior of cracks appeared.When the first crack appears, the concrete loses its ability to resist tensile strength, as it is a brittle material, but adding steel fibers leads to a significant improvement in increasing the resistance because the fibers transform the concrete from a brittle material into a flexible material that is able to absorb the applied stresses.
All specimens' failure mode is a shear failure.In these specimens (concrete beams without shear reinforcement), the highest percentage of steel fiber is 1% of the volume it was not sufficient to change the failure pattern from shear to flexure.To achieve this, it is necessary to increase the percentage of steel fiber, but from a practical point of view, high rates of steel fiber cannot be used because it affects the workability of ordinary concrete.

Effect of steel fiber
The specimens adopted to study the effect of steel fiber percentage on shear strength are (NS0), (NS0.5), and (NS1).The load-deflection curves of these models are indicated in figure 8, which shows the relationship between applied loads with mid-span deflection.After analyzing the results, it was found that the deflection at the ultimate load of the NS1 was more than the deflection of the NS0.5, and NS0.This means that there is a positive effect of steel fiber because the deformations are more visible in the specimens that have a height steel fiber percent.4 and figure 8 show that specimen NS1 has a more ultimate shear load in comparison with the NS0.5, and NS0.When compared to the reference specimen NS0, both specimens NS0.5, and NS1 showed the ultimate shear load capability has increased by 18.3% and 30.8%, and in the first crack load of 21% and 40%, respectively.During the test, it can be noticed that increasing the percentage of steel fibers led to a delay in the development of cracks and reduced their width.Where the steel fiber binds or stitches both sides of the crack and works to convert the load placed into the concrete, as the two hooked ends of the steel fiber increase the strength of its bonding with the concrete and prevent premature slip failure.All this demonstrates the efficiency of the steel fiber in strengthening concrete to resist shear, this is mainly due to the random distribution of discontinuous steel fibers inside the concrete matrix.

Effect of recycled aggregate
To study the effect of aggregate quality on the shear strength of concrete, two types of aggregates natural coarse aggregate (gravel) and lightweight aggregate (crushed brick) were used.The two types of aggregate have the same particle size and gradation ratios.The effect of crushed aggregate on the loaddeflection curves is shown in figure 9.The results listed in table 4 showed that replacing 50% of the 1232 (2023) 012029 IOP Publishing doi:10.1088/1755-1315/1232/1/0120299 natural coarse aggregate (gravel) with lightweight aggregate (crushed brick) decreased the ultimate shear strength.The maximum shear capacity and first shear crack of the CS0 decreased by about 9.6% and 21.5%, respectively when compared with NS0.The improvement method by using steel fiber showed that the addition of fibers significantly improved the shear strength.The ultimate shear strength of the CS1 beam increased by 31.8% and the first shear crack increased by 41.6% when compared with CS0.The use of steel fibers increases durability and strengthens the bond between the concrete particles.

Conclusion
In this paper, a set of concrete beam without shear reinforcement were prepared to study the effect of both steel fibers and recycled aggregates.The results of the experimental test showed that replacing 50% of the natural aggregate with recycled aggregate led to a decrease in the ultimate strength, but the addition of steel fibers played a key role in improving the structural behavior of the concrete.This is primarily due to the efficiency of the steel fibers.From the main results of this study, the following conclusions can be drawn:  The ultimate load and first crack load of the specimens with a recycled aggregate decreased by 9.6%, and 5.0%, respectively, compared to the specimens with natural aggregate concrete for specimens without steel fiber. The addition of steel fibers by 1% by the volume, led to an increase in the ultimate load by 30.8%, 31.8% for the specimens with natural aggregate concrete, and recycled aggregate. Shear collapse is the mode of failure for all beams.The addition of steel fiber leads to an increase in the number of cracks (flexural cracks) and less width of cracks. Adding steel fiber by 0.5% increases the first shear load by 74% for natural aggregate concrete, while adding steel fiber by 1% increases 127% and 132% for natural aggregate concrete and recycled aggregate concrete, respectively.

Figure 7 .
Figure 7. Crack pattern of test specimens.

Figure 8 .
Figure8.Effect of steel fiber on load-mid span deflection.The findings of in table 4 and figure8show that specimen NS1 has a more ultimate shear load in comparison with the NS0.5, and NS0.When compared to the reference specimen NS0, both specimens NS0.5, and NS1 showed the ultimate shear load capability has increased by 18.3% and 30.8%, and in the first crack load of 21% and 40%, respectively.During the test, it can be noticed that increasing the percentage of steel fibers led to a delay in the development of cracks and reduced their width.Where the steel fiber binds or stitches both sides of the crack and works to convert the load placed into the concrete, as the two hooked ends of the steel fiber increase the strength of its bonding with the concrete and prevent premature slip failure.All this demonstrates the efficiency of the steel fiber in strengthening concrete to resist shear, this is mainly due to the random distribution of discontinuous steel fibers inside the concrete matrix.

Figure 9 .
Figure 9.Effect of aggregate type with and without steel fiber.

Table 1 .
Details of models

Table 3 .
Properties of reinforcement bars.

Table 4 .
The results of the test specimens.