Structural behavior of hollow-core reinforced self-compacting concrete beams

This paper presents an experimental investigation on the structural performance of hollow-core reinforced self-compacting concrete beams and performs an optimization analysis to select the optimum hollow-core beam section, as well as perform a sustainability analysis. The experimental program includes constructing and testing five beams with different longitudinal hollow-core diameters created by using recycled plastic pipes, as well as a solid beam, used as a reference specimen. The results show that it can reduce the concrete from self-compacting concrete beams with percentages from 5.4 to 14.2 with a decrease in the first crack load from 9.1 to 22.7% and the ultimate strength from 2.3 to 10.5% respectively compared to the reference solid beam. The optimization analysis shows that the beam of 46 mm diameter hollow-core is the optimum selection in the concrete volume reduction of 11.1%, cracking load, and ultimate load reduction of 13.6% and 9.3% respectively among all the other beam specimens. While the sustainability analysis reveals that, using longitudinal voids of diameters from 32 to 52 mm leads to a decrease in the embodied energy with percentages from 5.4 to 14.2% and carbon dioxide emission with percentages from 5.4 to 14.1% respectively. Increasing the longitudinal void diameter makes hollow-core self-compacting concrete beams more ductile and exhibits large deflections before failure occurrences. Tests on hollow-core reinforced self-compacting concrete beams are conducted. Recycled plastic pipes are used to create hollow-core in the beams and to reduce ineffective concrete. Reducing ineffective concrete contributes to sustainability with maintaining a good ratio of beam strength. Tests on hollow-core reinforced self-compacting concrete beams are conducted. Recycled plastic pipes are used to create hollow-core in the beams and to reduce ineffective concrete. Reducing ineffective concrete contributes to sustainability with maintaining a good ratio of beam strength.


Introduction
Using longitudinal voids in reinforced concrete beams is one of the methods to reduce the concrete from the tension zone to get lightweight beams. The hollow-core concrete beams have many advantages over conventional solid beams; however, the voids reduce the concrete of the beams, resulting in decreasing the dead loads that in turn leads to; reducing the total cost, rapid construction, getting long-span beams, with maintaining a good ratio of the beam strength. Also, reducing the concrete from the structural members contributes to the sustainability process since it can preserve the environment due to the reduction in the consumption of natural resources, reducing carbon dioxide (CO 2 ) emission and embodied energy. Furthermore, longitudinal voids can be used to run mechanical and electrical equipment [1].
In structural members that are subjected to a positive bending moment, two stress zones arise in its crosssection; compression zone at the top, and tensile zone at the bottom. Due to the weakness of concrete in carrying tension stresses, steel reinforcement is usually provided in the tension zone where concrete acts as strain transferring media to steel reinforcement. Therefore, it is possible to remove part of the concrete from the tension zone in which the concrete does not significantly affect the strength of the beam using longitudinal voids which are created by placing plastic pipes [2].
There are many studies dealt with the structural behavior of hollow-core beams were presented in recent years, the following are the most important contributions in this field; Alnuaimi et al. [3], found that the rectangular longitudinal openings contributed to the performance of hollow beams and could not be neglected especially as the combined load of the shear, moment, and torsion were presented. Alshimmeri and Al-Maliki [4], Parthiban and Neelamegam [5], Dhinesh and Satheesh [6], Kumbhar and Jadhav [7] investigated the impact of rectangular longitudinal voids on the flexural performance of hollow reinforced concrete (RC) beams, it was found that the existence of rectangular hollow decreased the ultimate strength and increased the deflections for the hollow beams having the same properties. Varghese and Joy [8] investigated the effect of hollow-core position across the beam depth on the flexural performance of RC beams, the results revealed that the optimum ratio of the circular hollow-core center to the beam depth was 0.53. The effect of the transverse openings on the RC hollow beam under cyclic, bending, and torsional loadings was studied by Jabber et al. [9], it was found that the transverse openings of hollow beams showed a reduction in the beam strength, although the small openings had a neglected effect on the strength of hollow beams, the outcomes also showed that the strength of ultra-high-performance concrete (UHPC) beams for twisting was twice that of high strength concrete (HSC) beams. Satheesh and Nyodu [10] presented a study on the structural performance of hollow RC beams using circular Polyvinyl Chloride (PVC) pipes at various depths along the lateral direction, their results revealed that the strength of the beam was high in the centroid region compared to other regions. Manikandan et al. [11] investigated the structural performance of hollow sandwich RC beams with different void shapes, the hollow-core introduced by using expanded polystyrene foam in the tension zone of RC beams, the results of this investigation revealed that the structural behavior of RC beams with circular hollow-core was more efficient than RC square hollow beams. Varghese and Joseph [12] presented a comparison between the flexural behavior of RC beam of hollow-core created by polyurethane and expanded polystyrene, the experimental results showed that polyurethane was a more effective replacement as it offers better damping properties. Sariman and Nurdin [13] compared the ultimate load of RC T-beam with hollow RC T-beam in which the longitudinal voids were created from plastic wastes; the study revealed that the ultimate strength of each tested beam did not give any significant difference. el-kassas et al. [14] presented a study on the impact of longitudinal voids on the performance of deep beams made of high strength concrete, it was concluded that increasing the longitudinal void size led to a decrease in ultimate load, the longitudinal void shape had a slight effect on the performance of these beams. Abbass et al. [15] investigated the behavior of hollow high strength concrete beams under bending loads, the outcomes revealed that the square hollow beams with volume decreasing of percentages of 16% and 28% had a ductility higher than that of the beam without hollows, the hollow beam with decreasing of 44% had the ductility close to that of the solid beam.
From all the previous studies, it can be noted that: using different circular longitudinal voids and its effect on the structural characteristics of the concrete slender beams. Whereas, the previous studies either used longitudinal voids of a fixed cross-sectional or the change was made using longitudinal voids of square or rectangular shape. 2. The previous studies did not investigate beams made of self-compacting concrete (SCC), except one research that studied this topic on deep beams, not slender beams. However, SCC has different characteristics from conventional concrete in that, it is comprised of a smaller maximum size and lower content of coarse aggregate [16], this causes a decrease in the coarse aggregate interlock in SCC, resulting in weakness in the transfer of internal shear forces thus the structural behavior of SCC members are different from that of conventional concrete. 3. The previous studies did not present a sustainability analysis on this type of beams to know the ability of this type of beams in reducing the consumption of natural resources and decreasing CO 2 emissions and embodied energy.
Therefore, this research provides an experimental investigation to study the effect of different longitudinal hollow-core diameters created by using recycled plastic pipes on the structural performance of hollow-core reinforced SCC slender beams and performs an optimization analysis to select the optimum hollow-core beam, as well as making a sustainability analysis on this type of beams to know the ability of this type of beams in reducing the consumption of the natural resources and decreasing CO 2 emissions and embodied energy.
The rest sections of this article are arranged as follows: Sect. 2 introduces the experimental scheme of the tested beams. In Sect. 3, the ingredient materials of each beam are presented. Section 4. describes the testing procedure of the beams. The experimental results and discussion are reviewed in Sect. 5, and the conclusions are presented in Sect. 6.

Experimental scheme
The experimental scheme of this research consists of constructing and testing six simply supported beam specimens made of self-compacting concrete (SCC). The beams have identical dimensions (1000 mm length, 150 mm height, and 100 mm width). Five of these specimens have longitudinal voids created by using recycled plastic pipes with outer diameter of 32 mm, 36 mm, 40 mm, 46 mm, and 52 mm to eliminate the concrete with percentages of 5.4%, 6.8%, 8.4%, 11.1%, and 14.2% from the volume of the beam, as well as a solid beam (without longitudinal voids) used as a reference. The percentage of eliminated concrete depends on the size (diameter) of the pipe. The size of the pipe has been chosen so it can be placed in the tension zone at a level so that the distance from the lower level of any pipe to the upper level of the flexural rebar is greater than 15 mm to provide enough space for the penetration of the fresh concrete in the areas below the pipe. Table 1. lists the details of the beams and the experimental program. The layout, cross-sections of the beams, and side view in one of the hollow-core beams are shown in Fig. 1.

Concrete ingredients and mix quantities
SCC mix was used to cast the beams of this research. It was designed based on the European specification of selfcompacting concrete (EFNARC) [16]. The components of the mix included: Portland cement Type I conformed with IQS No.5-1984 specification [17], powder of limestone complied with the European specifications for SCC [16], crushed gravel and sand complied with IQS No.45-1984 specification [18], Super-plasticizer (Glenium 51) complied with ASTM-C494 requirements [19], as well as tap water. Part of the cement was replaced by limestone powder, with a ratio of 1:0.76 on a mass basis, this ratio was chosen based on the trial mixes that were carried out to choose the appropriate SCC mix. The ingredients of this mix are illustrated in Table 2.
To ensure that the concrete of this study is SCC, the standard experimental tests of fresh SCC were done (L-box, V-funnel, T50 cm slump flow, and Slump flow) [16]. Table 3 shows that the results of these tests satisfied the of EFNARC requirements [16]. The compressive strength (fc') of each beam casting batch was determined according to ASTM-C39 requirements [20]. Figure 2 shows the fresh SCC tests. H1: The height of the concrete at the end of horizontal section of the L-box.
H2: The height of the remaining concrete in the vertical section of the L-box.

Steel reinforcement
The tested beams of this study were reinforced longitudinally by 2ϕ10 mm deformed steel bars of 492 MPa yield stress in the beam tension zone, and with 6 mm@50 mm as shear reinforcement (stirrups). Also, 2ϕ5mm smooth steel bars were placed in the compression zone to fix the stirrups as illustrated in Fig. 1. The tensile test results of the bar 10 mm complied with ASTM -A615 [21].

Pipes
In this research, the longitudinal voids along the beams were constructed by using recycled plastic pipes. The materials of these pipes do not chemically react with the components of concrete or steel reinforcement. Table 4 illustrates the properties of the plastic pipes. The density of the pipes was obtained according to ASTM D792 [22], Young modulus and ultimate tensile strength were obtained according to ASTM D638 [23], and the linear expansion coefficient according to ASTM D696 [24].
These pipes had enough strength to support the load of fresh concrete through the stage of pouring. All these pipes were placed in the tension zone below the neutral axis of the beams at a distance of 79 mm from the bottom fibre of the beam to the center of the pipe. This level was chosen so that the distance from the lower level of any pipe to the upper level of the flexural rebar is greater than 15 mm to provide enough space for the penetration of the fresh concrete in the areas below the pipe. It is worth noting that the reason for placing the pipes at the tension zone below the neutral axis belongs to the fact that the concrete in this zone does not significantly affect the strength of the beam. The position of the neutral axis (c) was calculated according to ACI code [25] as follows: Fig. 1 a Layout of the hollow-core concrete beams. b Cross-sections of the solid beam (B1) and hollow-core concrete beams (B2 to B6). c Side-view in one of the hollow-core beams

Failure modes
The beam tests revealed that all the beams failed in flexure mode. The general behavior of these beams may be described as follows: at the small load increments, initial small cracks formed in the middle region between the two-point loads at the tension zone, these cracks referred to the cracking load. With the progress of the loading, the cracks expanded and propagated upwards, and other cracks formed in this region as well as in the shear regions between the load and support on each side. In this stage, the cracks in the hollow-core beams extended faster than that of the solid beam. Later, more loading made the cracks of the middle region extend faster upward and some of these cracks extended to the compression zone leading to the failure of the beam. Figure 4 shows the cracks at the failure of the tested beams. This figure reveals that the existence of the longitudinal voids in the beams B2, B3, B4, and B5 of diameters 32 mm,36 mm,40 mm, and 46 mm made the cracks concentrated in a narrower middle region, referring to a concentration of tensile stresses in that region of the beam. Also, the cracks spread to a larger area when using the longitudinal void of diameter 52 mm. This behavior can be explained by that the existence of a longitudinal void in a beam affects the distribution of the  components as well as their distribution within the beam concrete.

Cracking and ultimate loads
The results of the cracking and ultimate loads are listed in Table 5, also Figs. 5 and 6 illustrate the influence of longitudinal void diameter on the cracking load and ultimate load respectively. In general, it may be noticed that the cracking load is more affected by the existence of the longitudinal voids than the ultimate strength. However, eliminating the concrete with percentages 5.4%, 6.8%, 8.4%, 11.1% and 14.2% from the volume of the beam concrete, to decrease the crack load by 9.1%, 13.6%, 18.2%, 13.6% and 22.7%, while the ultimate strength decreases with percentages 2.3%, 3.5%, 5.8%, 9.3% and 10.5% respectively. The reason for these reductions in cracking and ultimate loads belongs to the existence of the longitudinal voids, these voids lead to a reduction in the moment of inertia of the beam section; which is, in turn, leads to a reduction the flexural rigidity resulting in a decrease the cracking and ultimate load. Also, it can be noted that the reduction in the cracking load of hollow-core beam B5 is similar to that of the hollow-core beam B3, this behavior can be explained by the same way previously mentioned, which is that the existence of longitudinal voids in the beam affects the pattern of the aggregate distribution within the beam concrete, and both the size of the longitudinal voids and the pattern of the distribution of the aggregate affect the pattern of crack formation and its extension, and this effect does not necessarily increase with the increase in the size of the longitudinal voids, but rather depends on the proportion of the size of the longitudinal voids with the size of the concrete components and their distribution within the beam concrete.
In comparison with the results of previous research, it can be found that Alshimmeri and Al-Maliki [4], showed that the reduction ratios in concrete with percentages of 7.4% and 14.8% using square and rectangular longitudinal voids, led to decreases in the first crack load by 20%, and 33% and the ultimate load by 50% and 55% respectively. It can be observed from this comparison that the effect of the existence of circular longitudinal voids on the cracking load and the ultimate load is less than the effect of the existence of square and rectangular longitudinal voids. This difference can be attributed to the occurrence of stress concentrations at the corners in square or rectangular longitudinal voids, while the distribution of stresses is more uniform with the existence of circular longitudinal voids.

Selection of optimum hollow-core beam
To select the optimum hollow-core beam in this research in terms of structural performance and decreasing in concrete volume, the relationships between the decreasing in the beam concrete volume and the load decreasing at cracking and ultimate load on the one hand, and the diameter of the longitudinal voids on the other hand are presented in Figs. 7 and 8. It can be noted from these figures that the curves of decreasing in concrete volume close to that of the cracking and ultimate load in the beam of 46 mm diameter hollow-core, which gives the optimum decreasing in the concrete volume with a less decreasing in cracking and ultimate load by percentages 13.6% and 9.3% respectively among all the other beam specimens, as the hollow-core are required to satisfy the economic and environmental considerations with preserving the structural beam performance. It can be noted from these figures that the optimum decreasing of concrete is more affected by the least decrease in cracking load as the ultimate load decreases steadily when the percentage of eliminated concrete increases. This behavior can be attributed to the fact that the beam strength before the first cracking load depends on concrete and steel together, so removing part of the concrete from the tensile zone affects the first crack load. While after the first crack, the cracks increase and the contribution of concrete in resisting the applied loads reduce with the increase in the role of steel in resisting the applied loads. Therefore, removing part of the concrete have less impact on the ultimate strength of the beam.

Load-deflection behavior
The load-deflection relationship of the beams is shown in Fig. 9. The figure reveals that the load-deflection curves have three stages; the first represents elastic stage up to appearance first crack, in this stage, the load is directly proportional to deflection, the second starts beyond the cracking load until tensile steel yield, in this stage the deflection increases faster than the first stage but it is still linear, and final stage begun beyond the yielding of tensile steel until the ultimate deflection, in which the deflection increases rapidly with a small increment in load. Also, it can be noted from this figure that the slope of the elastic portion is approximately identical for all the curves of the beams with different cracking loads, and the effect of increasing the hollow-core diameter becomes significant after the cracking load. This behavior may be belong to the fact that the existence of longitudinal voids at a distance of 79 from the bottom face of the hollow-core beam makes them somewhat far from the high stresses that are generated in the bottom face at the beginning of the loading before reaching the cracking load, so it can be seen that the effect of the longitudinal voids is very small, but when the cracks occur and reach to the borders of the voids the effect becomes very clear. But as the loading progresses the stresses at the lower level of longitudinal voids increase causing different cracking loads depending on the size of the longitudinal void in addition to the distribution pattern of the coarse aggregate, so there are differences in the cracking load. Furthermore, the figure reveals that the load-deflection becomes less stiff after cracking load with increasing the hollow-core diameter, however at any particular load level, the deflection increases with increasing the hollow-core diameter. The reason for this behavior belongs to the reduction in the flexural rigidity with increasing the longitudinal void diameter due to the reducing the moment of inertia. Table 6 shows that the ultimate deflection increases by about 3.84%, 5.84%, 8.18%, 6.68%, and 9.02%, while the decreases in deflection values at the cracking load is very slight, this leads to increasing the ductility with percentages of 9.03%, 8.42%, 10.82%, 14.11 and 14.47% respectively as compared with the reference solid beam. Thus, increasing the longitudinal void diameter makes hollow -core SCC beams more ductile and exhibits large deflections before reaching the ultimate load. This characteristic is very useful for concrete members since it makes concrete show earlier caution before the failure and prevents catastrophic collapse. These results are compatible with the findings of previous researchers Al-Maliki et al. [1] and Alshimmeri and Al-Maliki [4].

Sustainability analysis of hollow-core concrete beam
Sustainability can be defined as "It is the ability to preserve the environment and natural resources through responsible interaction when performing every work or activity in life" [26]. The construction of buildings requires large quantities of natural resources represented by construction materials. Providing and producing the construction materials require energy and lead to pollution and waste. Minimizing the construction materials reduces the consumption of natural resources, embodied energy (EE), pollution, and wastes. In the hollow-core beams, reducing part of the concrete by placing pipes in the tension zone leads to a reduction in the total self-weight of the beam which is, in turn, leads to reducing the consumption of natural resources, reducing CO 2 emission and embodied energy. This process contributes to sustainability. There are two methods to calculate CO 2 emissions and embodied energy, these methods are "Alcorn" method [27][28][29] and "GaBi" method [30]. Each method presents factors for each construction material to determine CO 2 emission and embodied energy by multiplying the weight of the material by its factors. The factors of each method differ according to the fuel used in the production of construction materials, inputs for the manufacture of each material. In the Alcorn method the quantities of materials are multiplied by factors of New Zealand embodied energy and embodied CO 2 , while in the "GaBi" method, the embodied energy and embodied CO 2 emissions are obtained from the GaBi LCA software [30], which are established on European materials. In this research Alcorn method is used to determine CO 2 emissions, and embodied energy from the hollow-core beams, in this method, the weights of the beam construction materials (cement, sand, and gravel) are multiplied by factors to determine CO 2 emission and embodied energy as listed in Table 7. The quantities of beam materials of the solid beam and hollow-core concrete beams are listed in Table 8, the calculations of the embodied energy and CO 2 emission based on the ALCORN method are listed in Table 9.
It can be noted from Tables 8 and 9 that the hollow-core concrete beams serve sustainability by the reduction of the construction materials as well as decreasing the energy required for extracting and manufacturing of these raw

Conclusions
Hollow-core RC beams have many advantages over conventional solid beams represented by the lighter self-weight, which leads to lower cost, faster construction, longer spans, with maintaining a good percentage of the beam strength, in addition to its participation in the sustainability process. In this research influence of hollow-core diameter on the structural performance of hollow-core reinforced SCC beams has been investigated experimentally, as well as making a sustainability analysis on this type of beam. This investigation reveals the importance of using longitudinal voids in the reinforced SCC beams for reducing the consumption of natural resources, reducing carbon dioxide emissions and embodied energy with maintaining a good percentage of structural performance. From the results of this investigation, it can be concluded that: Reducing the concrete from SCC beams with percentages from 5.4 to 14.2% using longitudinal voids created by using recycled plastic reduces the first crack load between 9.1 and 22.7% and the ultimate strength between 2.3 and 10.5% respectively as compared with the solid beam.
The existence of longitudinal voids in the tension zone of reinforced SCC beams has a neglected effect on the load-deflection relationship before the cracking load and the effect has begun to increase with increasing the loading after the cracking load.
The load-deflection curve becomes less stiff with increasing the longitudinal void diameter, and the ultimate deflection increase compared to the solid beam.
The beam of 46 mm diameter hollow-core and of 11.1% eliminated concrete gives the optimum decreasing in the concrete volume with a less decrease in cracking and ultimate load by 13.6% and 9.3% respectively as compared with the other beam specimens. Increasing the longitudinal void diameter makes hollow-core SCC beams more ductile and exhibits large deflections before failure occurrences.
The existence of longitudinal voids does not have an effect on the failure mode of SCC beams.
Using longitudinal voids of diameters from 32 to 52 mm leads to a decrease in the embodied energy with percentages from 5.4 to 14.2% and carbon dioxide emissions with percentages from 5.4 to 14.1% respectively.
Future works of this study may include flexural and shear performance of hollow-core reinforced made of high-strength concrete beams, or other concrete types, flexural and shear performance of hollow-core RC continuous beams, numerical analysis to study the effect of more parameters on the structural behaviour of hollow-core RC beams. Also, it is possible to study the effect of the longitudinal voids in an oval shape.

Funding
The authors did not receive support from any organization for the submitted work.

Conflict of Interest
The authors declare that they have no conflict of interest.
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