Abstract

This study investigates the efficacy of using lightweight self-compacting concrete (LWSCC) in steel-concrete composite beams, focusing on the role of headed stud connectors in shear connection. The research aimed to evaluate the behavior and shear strength of M16 and M20 headed stud connectors within LWSCC. To this end, six push-out test samples and six simply supported steel-LWSCC composite beams were fabricated and tested, with variables including the degree of shear connection (DSC) and stud diameter. The results demonstrated that an increase in DSC significantly enhanced the ultimate load capacity, service load, and stiffness of the composite beams, with marked improvements observed in the beams with M16 connectors (94%, 95%, and 122%, respectively) and those with M20 connectors (43%, 43%, and 20%, respectively). Furthermore, increasing the stud diameter from 16mm to 20mm resulted in a 38% increase in shear strength of the connectors and notably improved the mechanical characteristics of the beams. The study also found that while an increased DSC reduces deflection at ultimate and service loads due to heightened stiffness, a larger stud diameter has only a marginal effect on deflection. This research highlights the potential of LWSCC in enhancing the structural performance of steel-concrete composite beams, particularly with optimized shear connections, offering valuable insights for advancements in construction materials and methods.

Highlights : 

• Enhanced Load Capacity: Increase in the degree of shear connection significantly boosts ultimate load capacity and stiffness in composite beams.
• Stud Diameter Impact: A 38% rise in shear strength is achieved by increasing stud diameter from 16mm to 20mm.
• Deflection Reduction: Higher degree of shear connection effectively minimizes deflection, though stud diameter has minimal impact.

Keywords : Lightweight Self-Compacting Concrete, Steel-Concrete Composite Beams, Shear Connection, Headed Stud Connectors, Structural Performance.

Introduction

Composite beams are frequently utilized in modern constructions and bridges, which consist of steel beams supporting concrete slabs that are joined to represent a single unit. Regarding stiffness and strength, composite beams from steel and traditional concrete outperform steel and reinforced concrete beams. The improvement in the structural properties of these composite beams relies heavily on the quality and effectiveness of the shear connectors connecting the steel beam and concrete slab, along with the overall behavior of both components. [1.4]. Although many researchers investigated the structural behavior of composite beams made of steel-concrete taking into consideration the effects of various loading states as well as the impact of various material parameters and various shear interaction methods [5,13], Research on composite beams made with lightweight self-compacting concrete is still limited. Utilizing lightweight concrete for composite beams can be achieved to reduce their weight considerably. Therefore, in order to construct a lighter structure, it is feasible to reduce the size of the steel sections or expand the spans of the beams to reduce construction costs, which led to shallower and smaller foundations. Even though self-compacting concrete has been around for years, its use in building construction has increased due to its cost savings and environmental efficiency from its ability to spread and fill the structure's formwork even with congested reinforcement without mechanical vibration. So, lightweight concrete that has self-compacting properties (LWSCC) will be a very attractive material for constructing composite beams used in different types of structures.

LWSCC characterizations have been intensively studied in recent years to provide more design data. [14,17]. LWSCC is primarily produced by utilizing lightweight aggregates, which can be further classified into two categories: natural aggregates and artificial aggregates. [18]. Utilizing self-consolidating concrete in LWSCC is intended to minimize the quantity of cementitious materials to the greatest extent feasible. [19]. Furthermore, the utilization of different cementation materials in the fabrication of LWSCC, like silica fume and fly ash, could improved the connections between the lightweight aggregate and the cement matrix, which was a weak point in conventional vibrated lightweight concrete. [20].

In this research focuses on investigate the structural performance of composite beams manufactured of steel and LWSCC. The chosen mixture for LWSCC involved the utilized of a light expanded clay aggregate (LECA) as the coarse aggregate. Compared to other lightweight aggregates, a LWSCC produced from this type of lightweight aggregate may have higher workability and durability [21,23]. Six beam specimens were fabricated with three various degrees of shear connection to study the impact of shear connector diameter with shear connection on those composite beams' behavior. Headed stud shear connections are the most commonly used type of shear connectors. They were selected to connect the steel beams to the LWSCC slabs. In addition, to assess the effectiveness of the stud shear connectors implemented in LWSCC, six push-out samples were investigated.

Materials and Methods

Test Specimens

Push - out Test Samples

Six push-out samples were constructed to evaluate the load-slip relationship and shear capacity of the stud shear connections used in LWSCC. Fig. 1 illustrates the details of the push-out specimens, where two (500 × 500 × 150 mm) LWSCC slabs were attached to a 500 mm length of HE200B steel column by using two stud connectors with 90 mm long. The LWSCC concrete slabs were reinforced with twin layers of 10 mm steel reinforcement bars arranged horizontally and longitudinally. The push-out test specimens were divided into two groups according to size of the shear connector used. The first group, POT1, consisted of three samples [POT11, POT12, and POT13], in which M16×90 stud headed connectors were utilized to link the concrete slab to the HE200B steel column. The second group, POT2, comprised the remaining three specimens, symbolled [POT21, POT22, and POT23], which were fabricated by using M20×90 stud shear connectors. Table 1. explains the specifications of the push-out test specimen.

Figure 1.Details of push-out test specimens

Composite Beam Specimens

Six composite steel–LWSCC beams were fabricated for testing under a three-point loading effect. Each composite beam has IPE140 European steel beams that attached to LWSCC slabs having a width of 400 mm and thickness of

120 mm. Headed studs were utilized as shear connectors. Stud shear connectors were welded to upper flange of steel beams and subsequently embedded in the LWSCC slabs. However, the concrete slabs were reinforced by two layers of 10 mm steel reinforcing bars spaced 100 mm in each direction, as shown in Fig. 2. Three different degrees of shear connection were utilized to assess their impact on the performance of such composite beams that were tested at sagging bending moment action. Table 2 shows details regarding the specimens of the composite beams.

Figure 2.Details of composite beam specimens

Materials

To reach the goals of the present study, a LWSCC mix with a target cube compression strength of about 30 MPa has been developed utilizing light expanded clay aggregate (LECA) as a lightweight coarse aggregate combined with normal density coarse (gravel) and fine (sand) aggregates, cement, superplasticizer (SP), limestone powder (LP), and water. The details of the adopted concrete mixture and the properties of fresh concrete, the results, which were assessed based on the EN 206-9: 2010 requirements, are displayed in Table 3.

The particle size distributions of the adopted aggregates (LECA, gravel, and sand) are shown in Table 4. with a maximum particle size of about 9.5 and 12.5 mm for LECA and gravel, respectively, and about 2.36 mm for sand. Table 4. illustrates the coarse and fine aggregate physical characteristics, which were evaluated according to BS812-110: 1990 specification.

For each batch, six 150x150x150 mm standard concrete cubes were cast during the manufacturing of push-out and beam specimens to evaluate density and compressive strength (f_cu) of the specimens’ concrete slabs, see Table 2. Table 5. demonstrates, in accordance with ASTM A370-14, the mechanical properties of steel reinforcement, steel beams, and headed studs.

Loading and t est procedure

A TORSEE universal testing machine was employed for testing the beam and push-out samples. The tests involved subjecting the samples to monotonic loading at a loading rate of approximately 0.5 tons per minute. The simply supported composite beam specimens were tested using three points loads. To measure the applied load, a 75-ton load cell was used, whereas two linear variable differential transducers (LVDTs) were utilized to determine the vertical slip that occurs between the concrete slabs and the steel beam of the push-out test samples and a tested composite beam specimens' mid-span deflection and end slip for each load increment, as shown in Fig. 3.

Figure 3.Beam and Push-out Test Specimens under Loading

Results and Discussion

Push-Out Tests

Modes of Failure

After the maximum load was reached, each of the samples was exposed to further loading until one or both of slabs were separated from steel beam. As displayed in Fig.4 during testing of push-out test samples, two failure types were noticed. The first mode of failure occurred in the three specimens (POT11 to POT13), which have 16mm headed studs, where the headed studs had been cut off at the root and remained lodged in the concrete slabs with small slip. These samples' studs were cut off at the base that they remained inserted in the concrete slabs, which exhibited small slip values (for slender studs). It's because of the stress distribution on the headed stud shank but stress concentration near the base, as a result of concrete being restricted by a steel flange, a connector, and reinforcement led to the studs and LWSCC having high strength which resulted the weld becoming a weak point. However, the tests of the second group of push-out specimens (POT21 to POT23) that having 20mm headed studs show another mode of failure, which started with a small crushing of concrete in the regions near the studs followed by the failure of the slabs and finally the shearing off of the studs at the ultimate applied load. This failure took place at a significant slip compared with results of other specimens (POT11to POT13) Table 6. shows the testing findings for the tested specimens.

Figure 4.Failure Modes for Push-out Test Samples

It is obvious from the findings in Table 6. that the increase in diameter of stud from 16 mm to 20 mm developed a considerable increase in the shear strength of headed studs of around 38 %. It can to concluded that a stud shear strength inserted in LWSCC is roughly proportional to a diameter of the headed studs.

Load–Slip Relationships

Studying the mechanical behavior of stud shear connectors mainly depends on a load-slip curve. Fig.5. illustrate a load-slip behavior for the tested samples.

Figure 5.load-slip curves for the tested push-out specimens

It can be observed that each curve has a similar tendency, but the vertex and endpoint positions differ greatly. In each of the load-slip curves, three different steps could be seen, beginning at a linear phase of up to 39-58 % of ultimate load later, as the load approached the ultimate load, a nonlinear phase with increasing slip happened. After reaching ultimate load, the samples were exposed to additional deformation as well as a decrease in load, this represents the load-slip curve's third stage. It seen that the tested sample's shear stiffness, which represents the slope of the load-slip curve's linear part, was significantly positively affected by increasing stud diameter. It could be observed that all the load-slip curve's growth rate, height, and traverse extension height considerably increase with increasing stud diameter. Hence, the increasing of the stud diameter could significantly increase the static behavior of the stud in terms of ductility, shear capacity, and shear stiffness. This might be related to the failure mode in specimens, which is started with concrete crushing and is followed by the stud failure producing an increase in the ultimate slip and ductility if a comparison was made between the behavior of push-out specimens with 16mm and 20mm stud diameters. Therefore, it can be expected that the increase of headed stud diameter in such composite construction may enhance their ductility behavior and stiffness.

Steel-LWSCC Composite beams.

Failure Modes and Ultimate Strength

A unique mode of failure was observed in all the beam specimens which were examined in the influence of three-point loading, which failed in flexure in absence of any indication that there had been a failure in the connection between the steel beams and concrete slabs. This may be related to the efficiency of headed studs that are included in the LWSCC as a shear connection, which is one of the main objectives of the present study. It was observed, and as expected, that the observed flexural failure mode began by yielding steel beam followed by the concrete flange crushing due to increase the compression in the midspan of the tested beams. Crack patterns were present was flexural shear cracks outside of mid span region of tested samples and flexural cracks at the mid span of the tested specimens. There was no separation observed (deponding) between concrete slab and steel beam for all tested samples. In addition, it can be observed from Table 7. that the first cracking load and ultimate load increased with increasing a degree of shear interaction (DSC) for each testing specimen. From Table 7., when comparing the experimental results for the steel-LWSCC beam specimens [L050(16)S, L075(16)S, and L100(16)S], and [L050(20)S, L075(20)S, and L100(20)S], it is apparent that the ultimate strength capacity increased with increasing a stud diameter from 16mm to 20mm by a ratio ranging from 8% to 32% for specimens that have the same DSC as increasing the stud diameter increases the ultimate load. Fig. 6 and Fig. 7 demonstrate the modes of failure of steel-LWSCC composite beams.

Figure 6.Failure modes and crack patterns for steel-LWSCC beam specimens with 16mm stud diameter

Figure 7.Failure modes and crack patterns for steel-LWSCC beam specimens with 20 mm stud diameter

Load-Deflection Relationships

In general, the midspan deflection of all tested beams increases by an approximate constant small rate with increasing applied load up to the load level at which the first visible crack appeared. After the appearance of the first visible crack, the midspan deflection increases at a significantly increasing rate until the load reaches its failure level. As shown in Fig. 8, it could be noticed from the test findings for six specimens of composite beams that Load deflection curves are composed of two parts, parts that are linear and nonlinear. The first part of the curves represents the linear elastic relationship of the tested specimens and covered the region up to the first crack load. On the other hand, the second part clarifies the tested specimens' nonlinear behavior when applied load exceeds yield load, that the stiffness of the tested beams, which represents the slope of the curves' linear part, was gradually degraded then, the beams began to collapse, and there was a rapid increase in deflection accompanied by a decrease in load readings after reaching to their ultimate capacities.

Figure 8.The load midspan deflection for tested composite beam specimens

In light of the test results for three steel-LWSCC composite beam specimens [L050(16)S, L075(16)S, and L100(16)S] and based on Table 7, It was observed that the elastic range persisted until the load approximated (89%, 72%, and 74%) of ultimate load at DSC (50%, 75%, and 100%) respectively. As can be observed from Table 8 and Figs. 8 and 9 that the deflection values could be significantly affected by a degree of shear connection. Thus, it's possible to deduce that the values of a midspan deflection at the ultimate load and service load were decreased with increasing the DSC due to the increase of the composite beam stiffness. The cause of the composite beam's increased stiffness as the shear connection degree increases is related to the behavior of a concrete slab and steel beam of composite beams, which deform as one unit with increase the DSC and reduce the relative slip between them.

Figure 9.The deflection relationship for the specimen with different degrees of shear interaction and 16 mm stud diameter

The analysis of test findings and the load midspan deflection relationships for tested beam specimens of three composite beams L050(20)S, L075(20)S, and L100(20)S, as shown in Table 7 may leading to note that the elastic range in such beams was continued till applied load reaching to approximately (74%, 72%, and 66%) of the ultimate load at DSC (50%, 75%, and 100%) respectively. The values of deflections increased as the DSC decreased as shown in Fig. 8 and Fig. 10 but their values were approximately equal to the values of deflection of the specimens with stud 16 mm at the same DSC because the two groups have the same degree of shear interactions. In addition, the impact on load carrying capacity is clear for those specimens, whereas its load carrying capacity is greater when compared with the tested specimens that have a stud size 16 mm. Because the stud's shear capacity is proportional to its diameter.

Figure 10.The deflection relationship for the specimen with different degrees of shear interaction and 20 mm stud diameter

Load-Slip Relationships

According to experimental findings of tested composite beam samples in present work with the adopted different degrees of shear interaction and under sagging bending moment as illustrated in Table 9. an end slip is always existent even with those specimens having full interaction. It should be noted that the stud connectors are flexible and slip between the slab and the beam is inevitable.

Figure 11.Depicts the load-slip curves of all composite beam specimens used in this study

Based on Fig. 11, it can be seen that each curve from these curves of all tested beam specimens has a similar trend but there are big differences in their vertex and endpoint positions. This manifested that the steel concrete composite beams with studs give the same phases of deformation. As represented in the above curves, three different behavioral stages can be observed, which started with a linear stage followed by a nonlinear stage with increasing slip when the load approached the ultimate load. After the ultimate load was reached, the specimens were subjected to further deformation together with a decrease in load, which represents a third phase of load slip curve. With the highlight on the relationships between the end slip and the ultimate load for all tested beam samples as listed in Table 9. and Figs.11 and 12., it clearly appeared that when the degree of shear connection increases from 50% to 100 %, the measured end slip decreases due to increase shear stiffness of tested beams with increase of a DSC.

Figure 12.The end slip relationship for the beam specimens with different DSC

It can be observed from Table 9, when comparing the experimental results for composite beam specimens of the L050(20)S, L075(20)S, and L100(20)S with beam specimens of the L050(16)S, L075(16)S, and L100(16)S, that the increase of the diameter of stud shear connector from 16 mm to 20 mm lead to increase the shear capacity for each stud around (32%, 11.6%, and 7.5%) for DSC (50%,75%, and 100%) respectively, the stiffer connection will accept more load, whereas the slip is relatively small, due to the enhancement of the stud effective shear area.

Conclusions

1. It is possible to construct a structural lightweight concrete having low density with high self-compacting characteristics utilizing expanded clay aggregate (LECA)
2. 4- The increase of stud diameter from 16 mm to 20 mm in push-out tested specimens developed a considerable increase in the shear strength of headed studs of about 38 %. Moreover, It was observed that the increase of stud connectors diameter in the steel-LWSCC composite beams improve significantly the overall mechanical characteristics of such structural members.
3. The stiffness, service load, and the ultimate load capacity of the tested steel-LWSCC composite beam specimens were significantly increased with the increase of degree of shear interaction for all the tested beams.
4. With different degrees of shear interaction, the ultimate load carrying capacity of the steel-LWSCC composite beams was increased with the increase of the size of studs shear connector from 16mm to 20mm.
5. In general, the values of the deflection for tested beam specimens at the ultimate load and service load decrease with increasing the degree of shear interaction due to the increase of the composite beams' stiffness.
6. The increase of the diameter of the stud shear connectors in the tested composite beam have a slight effect on the deflection of such beams especially with the increase of degree of shear interaction.

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