Flexural behavior of two-layer beams made with normal and lightweight concrete layers

In this paper, twelve concrete beams with two different layers of concrete were evaluated as a simply supported beam under four-points loading. The beams assembled of two different types of concrete layers, one of which was normal-weight concrete (NWC) and the other was lightweight aggregate concrete (LWAC). The investigated parameters were the thickness of the lightweight concrete to the overall depth of beams (hLW/h), and the compressive strength of normal and lightweight concrete. Due to the weak lightweight aggregates used, lightweight aggregate concrete exhibits more brittleness and lower stiffness. Therefore, the viability of compensating for this degradation and providing a layer of normal concrete seems to be very interesting in such beams. The behavior of beams was evaluated based on cracking, failure mode, flexural strength, maximum deflection, stiffness, and toughness. The results showed slight variations on the majority of the above-mentioned performance aspects of two-layer beams compared to fully normal concrete beams. While there were great enhancements compared to fully LWAC beams. The variants were mainly attributable to the efficacy of using LWAC in providing lower stiffness and lower tensile strength. The experimental results have been compared to predicted values using the ACI 318-19, with some modifications for the equations to be matched with two-layer beams, the comparison was in terms of the deflection due to service load, moment capacity, and cracking moment.


Introduction
The rapid development of high-rise buildings, larger sizes and longer spans concrete structures necessitated the use of concrete that performed well in terms of strength, toughness, and light weight, the latter being related to the density of concrete. A lower density leads to a reduction in dead loads in structural design and foundations, which allows for a decrease with in horizontal inertia actions on buildings in earthquake regions. In comparison to normal weight concrete (NWC), lightweight concrete (LWC) shows more brittleness and lower stiffness [1], [2]. These variance in mechanical properties of NWC and LWC reveals an idea to combined both concretes in structural composite elements [3], [4]. The structural composite elements are typically made up of two materials: one carries the majority of the flexural loads, and the other is a thick, while the low-weight core increases shear ability and the section's moment of inertia. Such types of composite system are becoming more commonly adopted in the peculiarly industry of conservative infrastructure construction, due to their cost-effectiveness, high strength to weight ratio, excellent durability, good impact resistance, excellent fatigue and corrosion resistance, and design flexibility. Such composite systems are mainly used for structural roofs, walls, floors, and bridge decks. [5].
Over the previous few years, considerable research has been conducted on the action of beams composed of two layers with two different materials. However, despite its incredible potential, its use in developing sustainable composite beams remains limited. lately, some researches on a full-scale two-layer beam and a continuous two-layer beam were performed, which consisted of normal strength concrete in the tensile zone and high strength concrete with steel fiber in the compression zone [6], [7]. Other researchers have been focused on replacing the tension zone with Engineered Cementitious Composite (ECC) to improve the tensile strength of the concrete around the main steel reinforcement [8]- [10]. Mohsin et al. [11] introduced analyses of a new twolayer RC beam, where the compression zone and tension zone were made of high strength concrete and normal strength concrete, respectively. Dybel and Wałach [12] investigated the bond strength development between two concrete layers in an experimental study. The types of normal concrete (NC) to high-performance concrete (HPC) and HPC to HPC, as well as comparisons with NC-to-NC specimens, have been established. The essential objectives of this study is to evaluate the strength and behavior of two-layer beams consisting of normal and lightweight concrete, the parameters have been chosen to investigate the effect of combining these two materials together in the same two-layer beams. The depth of each layer and compressive strength of normal and lightweight concrete were the main parameters of this study.

Experimental program
As shown in Fig. 1 and Fig. 2a twelve two-layer reinforced concrete beams were prepared and tested as simply supported beams under a four-point loading schematic. All the beams having similar cross-section of 140 × 200 mm and total length is 1700 mm. The considered parameters in this research are the thickness of lightweight aggregate concrete layer and the compressive strength of lightweight aggregate concrete in compression and/or tension zones while the compressive strength of normal concrete in compression zone only. The Beams were divided into four groups the first group based on ratio of thickness of LWAC layer to overall depth of the beam h LW /h: 0%, 25%, 50%, 75%, and 100% where the LWAC in the tension zone (TZ). The second and third groups investigate the effect of compressive strength of NWC AND LWAC in the compression zone (CZ), while the last group investigates the compressive strength of LWAC in the TZ.

Preparation of beams
The prepared lightweight aggregate concrete mixtures are considered to be structural LWAC mixtures with an equilibrium density of 1801,1953 and 2031 kg/m 3 , where it meets ASTM C330 and ACI 318-19 for structural LWAC classifications. The mixture components include: ordinary Portland cement Type I, pumice as lightweight aggregate with a maximal particle size of 25 mm and 1.75 of dry specific gravity, normal weight fine aggregate with 2.65 of specific gravity. The absorptions of the lightweight coarse and normal weight fine aggregates were 10% and 1.1%, respectively. The normal concrete mixtures are made with normal coarse aggregate of a maximal particle size of 19 mm and dry specific gravity of 2.66. Table 1 listing the used mixes proportions for six mixtures. A barrel mixer with 0.20 m 3 of capacity have been used for concrete mixing with 0.10 m 3 as a batch volume, which appropriate to cast two beams and six cylinders with100 mm diameter and 200 mm height.
Smooth wood molds were used to cast the beams. The wood molds have been marked with grooves to specify the level of concrete layers as shown in the Fig. 2b. Prior to pouring the concrete, the steel reinforcements were placed in the wood molds then positioned with adequate cover. The beams were cast into two layers, each of which was properly compacted with an electrical vibrator. After 72 hours, the beams were removed out of molds and wrapped with wet blankets and plastic sheets for 28 days of curing. The compressive strength, splitting tensile strengths, and the modulus of elasticity of the tested cylinders after 28 days summarizes in Table 1. deformed steel bars with f y = 620 MPa were used for the longitudinal steel and stirrups. As top reinforcement, two bars with a diameter of 10 mm were used, and two bars of with diameter of 16 mm were used as bottom reinforcement. 10 mm in diameter stirrups were used with 50 mm spacing distributed in shear spans to avoid the shear failure. The top, bottom, and side concrete cover were 33 mm to the center of bars as shown in Fig. 1. The designation of beams included a combination of letters and numbers: For example, (B1N-1L25) B stands for flexural beams, Part 1N is the first normal concrete mix ID ,1L for first lightweight aggregate concrete mix ID and 25 indicate the ratio of lightweight layer thickness to overall depth of beams. Grade 75, (620 MPa) reinforcement steel have been used. All the reinforcing bars met the requirements of ASTM A615. Fig. 3 shows the stress-strain curve of steel reinforcement bars. Table 2 summarizes the specifications of tested beams.

Testing setup
As shown in Fig. 4, the experimental tests were performed as a simply supported beams with 1400 mm clear span under four-point loading, distance between the two loading points was 300 mm. Two steel supports and steel bearing plates with cross-section of 20 mm × 100 mm were constructed in order to withstand the applied load without any deformation that could have an effect on the results. A universal hydraulic testing machine with a portable load cell of a capacity of 470 kN has been used for load application under 50 N/s loading rate. One LVDT was placed vertically at the center of the beam to measure the mid-span deflection. A computerized data logger system was used to collect and analyze the experimental data. Three strain gauges have been used for strain measurement in the reinforcement and concrete. One of strain gauges was installed on the lower part of the main longitudinal reinforcement at mid-span, while the other two strain gauges were installed on concrete surface at 25 mm and 75 mm of beam depth. Fig. 1 shows the locations of the strain gauges and LVDT. During the test, any cracks that formed on the surface of the beam were marked and measured using a digital crack width meter at load increments, both the deflections and strains have been monitored till the beam failure.

Cracking and failure modes
In terms of crack propagation, the behavior of the both control and two-layer beams were similar except for crack spacing, compared to the cracks of control beam, the cracks of the two-layer beams were closer to each other. All of the beams in failed the flexure. The longitudinal tension steel yielded first in all of the beams, followed by concrete crushing at constant moment zone, which is a ductile failure mode, generally known as tension failure. Fig. 5 shows the patterns of cracks generated during the testing of the beams. In tested beams, the first appearance of cracks in the center of the beam where the bending moment is constant, followed by formation of the additional flexural cracks. As shown in the Table 3, the first crack loads for all beams are ranged between 8.7 kN and 12.06 kN, as well as the ratio of ultimate load to first crack load (P u /P cr ) was between 6.7 to 8.9. In this region, as the loading increased, all cracks propagated almost vertically upward.. Also, as the loading increased, inclined cracks formed and propagated upward on both sides of the constant moment zone. The patterns of cracks were comparable; slightly consistent distribution as the lightweight aggregate concrete layer increases. Furthermore, the number, length, and distribution of cracks increase as h LW /h increased. Decreasing the NWC compressive strength in the compressive zone decreases the number of cracks with significant concrete crushing in the compressive zone. The pattern of cracks did not significantly change with increasing the LWAC compressive strength on the compressive or tension layers.

Load-deflection curves
Load-deflection curves for all beams are shown in Fig. 6. These curves are depicted in a way that enables comparison of the effect of layer thickness in the same group of beams, as well as the influence of compressive strength of NWC and LWAC. Clearly, the ultimate strength, and also the ultimate deflection, both decrease as the h LW /h increases. The load-deflection curves tend to indicate how the beams behaved as the load increased up to failure, with such a pre-cracking segment that is almost a straight line. Slope of the curve changes after cracking, indicating a reduction in beam stiffness before the yielding of tension steel reinforcement. After yielding, the beams' strength increased slightly, followed by an approximately flat top segment with a small reduction in strength. even though the existence of a lightweight aggregate concrete layer increased ultimate deflection, the behavior of beams remained ductile, with the exception of fully lightweight aggregate concrete beams and concrete layer at the compression zone with compressive strength 23.2 MPa, where the loaddeflection curve deteriorated after ultimate load. The influence of compressive strength of concrete on the ultimate load and cracking was typical, both of which increased as compressive strength increased. Nevertheless, it is also interesting to realize that the maximum deflection incurred by the beams with ρ =1.72% (which exceeds ρ' max ) smaller, and the corresponding ductility is as low as the companion beams with higher ρ' max . Also, it is important to mention that steel reinforcements limits of the beams with LWAC were designed mainly based on testing NWC beams. Furthermore, this could be due to the small spacing of the stirrups, which provides powerful confinement to the concrete. Table 3 summarizes the key behavior characteristics of all the tested beams. The ultimate load on each beam was normalized to the control beam in its group, and the results are presented Fig. 7 Table 4 summarizes the stiffness characteristics determined for each beam based on the curves illustrated between experimental load and mid-span deflection. The slope of the linear portion of the load-deflection curve before the creation of the first flexural crack is referred to as initial stiffness. On the upward segment of the load-deflection curve, service stiffness is described as the slope of the points corresponding to 50% and 80% of the ultimate load capacity on the load-deflection curve [13]. The stiffness results of each two-layer beam were normalized with respect to the control beam in its group to be compared, and the results are shown in Fig. 9.

Stiffness
For the first group the results reveal that the two-layer beams have no significant changes to each other the initial stiffness, while it is with average value about 40% in compared to beam with full NWC and increased about 46% in compare to beams with full LWAC. While the average decrease in service stiffness was 4.47% in compared with B1N and the average increase was 6.68% in compared to B1L. Decreasing the compressive strength of NWC in compression zone from 49.3 to 33.1 and 25.5 MPa decreases the initial and service stiffness by 3.85%-17.74% and 8.5%-10.73% respectively. Increasing the compressive strength of lightweight aggregate concrete in compression zone from 23.2 to 28.5 and 35.1 MPa increases the initial stiffness by 3.87% and 4.96%, and decreases the service stiffness by 9% and 2.39% respectively. Increasing the compressive strength of lightweight aggregate concrete in tension zone from 23.2 to 28.5 and 35.1 MPa increases the initial stiffness by 0.28% and 1.21% while decreases the service stiffness by 6.20% and 6.16% respectively. In all beams, the influence of LWAC layer was significant in values of initial stiffness, conversely of service stiffness. Figure 9. Normalized initial and service stiffness Table 3 shows the determined displacement ductility index and flexural toughness values of all beams. Ductility is a highly eligible structural property that indicates the structural member's capability to withstand substantial deflections before failure. The ratio of the ultimate deflection to the first yield deflection can be used to calculate the ductility index values, while the flexural toughness values reflect the total area under the load-deflection curve. The displacement ductility results distinctly show that ductility decreases as h/h LW increases, as shown in Fig. 10, which is expected. However, cautious inspection of Fig. 10 Figure 11. Normalized toughness for the tested beams

Load-crack opening behavior
Digital crack width meter was used to read the crack opening in the constant moment zone correspond the load manually as shown in Fig.2b. The achieved load-crack width behavior of each beam is presented in Fig. 12. For all the beams in this study were failed with formation of flexural main cracks. The ultimate capacity of these beams was lower than the load required to produce shear cracks due to an intensive amount of shear reinforcement. The results seem to be in agreement with the experimental cracking behavior and failure modes, which indicates that a higher depth ratio resulted in a consistent distribution and lesser crack opening.

Validation and comparisons
As shown in Figs. 13-15, the experimental results were compared with predicted results using ACI 318-19 Code [14] in terms of cracking moments, service load deflections moment capacities. It is important to point out that there is no availability of papers on constitutive modeling for two-layer beams, which is an interesting subject. The parameters studied and the results presented in this study are unique, and it focus on an area where it has received less attention and is not actually addressed in most design codes. Theoretical cracking moments of the beams M cr (ACI) are determined according to ACI 318-19 Code Eq. 24.2.3.5 [14] as follows: M cr(ACI) = λf r I g y t Where λ is a reduction factor depends of density of structural lightweight concretes, I g is the moment of inertia of gross section of concrete beam, y t represent the distance from extreme fiber of tension to the neutral axis, and f r modulus of rupture which equal f r = 0.62 √f c ' based on ACI 318-19 Code [14]. These equations are based on beams made with same concrete type for both tension and compression zones. While this study there is two deferent types of concrete these equation needs to be modified as follows: M cr(ACI) = (α 1 λ 1 f r1 I g1 +α 2 λ 2 f r2 I g2 ) . y t (2) where α 1 represents h LW /h and α 2 is [1-(h LW /h)], while λ 1 and f r1 are the reduction factor and modulus of rupture for the first layer, λ 2 and f r2 are the reduction factor and modulus of rupture for the second layer. I g1 and I g2 is the inertia moment for the first and second layers respectively. The experimental and predicted cracking moments are around 80% in agreement, as shown in Fig. 13. This significant level of agreement validates the study's testing procedure and results, indicating that the modified equations can be used. Figure 13. Theoretical versus experimental cracking moments The experimental and theoretical deflections at service loads are compared in Fig. 14. Using the test setup shown in Fig. 1, the deflection δs of service loads at mid-span is calculated as: where Ma is the moment at service loads, L is the clear span of beam, a is the projection of distance between load and support, E c is the modulus of elasticity for concrete as specified in ACI 318-19, = 1.5 0.043 √ ′ and = 4700 √ ′ for the lightweight and normal concrete respectively, the effective inertia moment I e calculated using ACI 318-19 Eq.24.2.3.5a [14] whereas: I cr = bc 3 3 Where I cr and I g are the inertia moments for the cracked and gross sections, respectively, The results show an agreement of 86% between the experimental and predicted deflections, which indicates that the modified equations for predicting deflections at service loads are adequate. The experimental and predicted moment capacities are shown in Fig. 15. Sectional analysis as a doubly reinforced section was used to calculate the nominal moment strength (M n ) as illustrated in Fig. 1 Figure 15. Experimental versus theoretical moment capacity.

Conclusions
Based on this study results of this research, the based on the results obtained: • The behavior of the both control and two-layer beams were similar except for crack spacing, the cracks of two-layer beams were closer to each other in comparison to the cracks of control beams all of the beams failed in flexure. • Flexural strength: increasing light weight aggregate layer in the tension zone from 0% to 25%, 50%, 75%, and 100% results in an approximately 0.13%, 0.44% ,3.4%, and 8.65% decreases in the ultimate load capacity of beams, respectively. • Stiffness: The two-layer Beams have no significant changes to each other the initial stiffness, while it is decreased 40% and in compared to beam with full NWC beam and increased 46% in compare to beams with full LWAC beam, while the service stiffness was decreased in average of 4.47% in compared with full NWC beam and increased with an average of 6.68% in compared to LWAC beam. Decreasing the compressive strength of NWC in compression zone decreases the initial and service stiffness. Increasing the LWAC compressive strength in compression zone increases the initial stiffness, while decreases the service stiffness. Increasing the LWAC compressive strength in tension zone increases the initial stiffness while decreases the service stiffness. In all beams, the influence of LWAC layer was more initial stiffness. • Two layer beams the ductility decreased as hLW/h increased compared to B1N, and increased compared to B1L respectively. Decreasing the compressive strength of NWC and LWAC in compression zone decreases the ductility. In the other hand the ductility increased as the compressive strength of LWAC in tension zone increased. • The toughness decreased significantly for all beams as compared to control beam of the same group, implying that two-layer beams have a major influence on toughness. The toughness decreased as h LW /h increased compared to B1N, and increased compared to B1L respectively. Decreasing the compressive strength of NWC and LWAC in compression zone the toughness decreased. The toughness decreased as the compressive strength of LWAC in tension zone increased. • Due to the lack of provisions in the ACI 318-19 Code for predicting the cracking moment, nominal moment capacity, and deflection at service load of two-layer beams, a modification to the equations was proposed.
The modified equations demonstrated about 80% agreement with the results of the experimental study.