Performance of Retrofitted Self-Compacting Concrete-Filled Steel Tube Beams Using External Steel Plates

Self-compacting concrete-filled steel tube (SCCFST) beams, similar to other structural members, necessitate retrofitting for many causes. However, research on SCCFST beams externally retrofitted by bolted steel plates has seldom been explored in the literature. 0is paper aims at experimentally investigating the retrofitting performance of square self-compacting concrete-filled steel tube (SCCFST) beams using bolted steel plates with three different retrofitting schemes including varied configurations and two different steel plate lengths under flexure. A total of 18 specimens which consist of 12 retrofitted SCCFST beams, three unretrofitted (control) SCCFST beams, and three hollow steel tubes were used. 0e flexural behaviour of the retrofitted SCCFST beams was examined regarding flexural strength, failure modes, and moment versus deflection curves, energy absorption, and ductility. Experimental results revealed that the implemented retrofitting schemes efficiently improve the moment carrying capacity and stiffness of the retrofitted SCCFST beams compared to the control beams.0e increment in flexural strength ranged from 1% to 46%. Furthermore, the adopted retrofitting schemes were able to restore the energy absorption and ductility of the damaged beams in the range of 35% to 75% of the original beam ductility. Furthermore, a theoretical model was suggested to predict the moment capacity of the retrofitted SCCFST beams. 0e theoretical model results were in good agreement with the test results.


Introduction
Concrete-filled steel tube (CFST) members have been used over the world for high-rise buildings, bridges, and other structures involving very large applied moments, particularly in zones of high seismic risk [1][2][3][4][5].CFST members provide several merits over the conventional tubular or reinforced concrete members, including high strength and stiffness, fire resistances, large energy absorption capacities, and favourable ductility [6,7].
In the last years, self-compacting concrete (SCC) arrived as a revolution in the field of concrete technology.SCC was originated in Japan in the eighties and then spread to other countries [8][9][10].SCC is a high-performance concrete that can discharge under its weight and fill in the formwork without any compaction efforts and without showing defects due to segregation and bleeding.Hence, reduces the noise level and construction time and cost, whereas still achieving good compaction [11].Since the steel tubes have confined sections, SCC has a high potential to be the most appropriate concrete type in CFST members to secure that the best compaction is provided without any further precautions [12].e adoption of SCC in CFST members can enhance the constructional quality of concrete, hence ensuring the performance of these composite members.e past experimental research revealed that the behaviour of CFST members filled with SCC is similar to those filled with normal concrete [13,14].
Similar to other structural members, CFST members may necessitate retrofitting or upgrading for various causes.For instance, CFST members may require being repaired because of degradation due to environmental factors, fire, ageing, and earthquake accidents.Also, they may request strengthening in order to increment the load-carrying capacity or to recover the desired structural behaviour due to design faults, constructional errors, updating of design standard, and modification in the practical usage [15][16][17][18][19][20][21].
Hence, CFST members may need to be retrofitted or substituted to compensate for the inadequate capacity.
Recently, the applications of FRP composites in civil structures have been more acceptable due to their higher strength, higher stiffness, and low weight compared to steel plates [20,21].Nevertheless, there are some disadvantages regarding the implementation of FRP materials.For example, the cost of these materials is still relatively high besides the need for skilled labour [17,22,30].Furthermore, there are uncertainties regarding the long-term performance of these materials and the bonding between the composite materials and steel [20,26,27].On the other hand, the implementation of external steel plating is a prevalent successful retrofitting method that was widely experienced in practice [22,27].e advantages of this method include the convenience and relative simplicity of application, the wide availability and low prices of the steel material, and the ductile behaviour and high deformation capacity of the steel plates compared to the composite materials which characterized by brittle nature [17,22].In this research, the external steel plating method was used which is expected to increment or recover the flexural strength and stiffness of the SCCFST beams, reducing their service-load deflection and behaving in a ductile manner.
Search in the available literature revealed that many research works had been done on the strengthening and repair of CFST beams.However, most of these investigations have been performed on normal CFST beams, which externally retrofitted by CFRP laminates (e.g., [19][20][21]27]).On the contrary, studies that examined the behaviour of SSCFST beams retrofitted with bolted steel plate are too scarce.However, there are many studies that have been conducted on reinforced concrete beams strengthened by steel plates (e.g., [16-18, 22, 30-33]).
Al Zand et al. [21] presented an experimental study on the strengthening performance of circular and rectangular CFST beams using unidirectional CFRP sheets.e results demonstrated that the moment carrying capacity, energy absorption capacity, and flexural stiffness of the strengthened beams considerably enhanced with the increase of CFRP layers.Sundarraja and Prabhu [27] experimentally examined the suitability of CFRP fabrics in the external strengthening of square CFST beams.Results indicated that the flexural strength and stiffness of the strengthened beams incremented when the number of CFRP layers was increased except in the case of beams strengthened by partial wrapping.On the other hand, the ductility index decreased as the number of CFRP layers increased.
Peng et al. [17] studied the behaviour of damaged rectangular reinforced concrete (RC) beams strengthened by bolted steel plates considering the effect of corrosion.e test results showed that adopting bolted steel plates in strengthening enhances the strength and ductility of the retrofitted beams.Also, it was found that the maximum load of strengthened damaged beams increased as the steel plate thickness increased.Aykac et al. [22] experimentally investigated the flexural strengthening and repair behaviour of rectangular RC beams retrofitted using external bolted steel plates.e results revealed that the ductility of the tested beams increased as the plate thickness decreased.e experiments proved that the anchorage of steel plates to the beams using steel bolts was a useful technique for obtaining sufficient strength and ductility and preclude the premature peeling failure of the plates in beams strengthened by thick solid plates.

Research Significance and Objectives
In view of the literature above, it can be concluded that research in the field of SCCFST beam retrofitted with external bolted steel plates is still rarely found in the literature.Furthermore, up to the best knowledge of the authors, no any previous study was carried out on the retrofitting of SCCFST beams with external steel plates.erefore, it is evident that the research in this field is still in the primary stage and far from sufficient, especially if the differences in mechanical properties between the steel plates and the FRP composites were taken into consideration.More efforts are required in this field to fill this gap in the literature, to provide a better understanding of the flexural behaviour of such retrofitted composite beams, and to provide novel experimental information for the engineering practice.erefore, the primary aims of this study are threefold: first, to generate novel new test data on the retrofitting of SCCFST beams with bolted steel plates; second, to investigate experimentally the flexural performance of retrofitted SCCFST beams regarding moment resistance, failure modes, ductility, and moment versus deflection curves; and finally, to examine the effect of the different parameter on the flexural behaviour of retrofitted SCCFST beams, such as different steel plate arrangements and lengths including full and partial retrofitting schemes.

Self-Compacting Concrete (SCC).
In this research, selfcompacting concrete (SCC) was adopted as infill material due to its superior workability.e mix design is prepared to attain a targeted compressive strength of 50 MPa with a water to binder (cement and fly ash) ratio of 0.33.e adopted SCC mix was prepared with a binder material consisting of the Portland cement PC (CEM I 42.5R) and fly ash (class F). e river gravel was used as a coarse aggregate with 12 mm maximum particle size.While, the fine aggregate was a mixture of river sand and crushed sand with a maximum size of 4 mm and 2 mm, respectively.Finally, superplasticizer (Glenium 51) was adopted to obtain the required workability of the SCC mix.e mix proportions of the SCC are given in Table 1.
In order to assess and check the workability properties of the adopted SCC, the fresh characteristics of SCC were examined by slump flow and V-funnel tests, according to the EFNARC committee recommendations [34].
e slump flow test represents a substantial check for the flowability of 2 Advances in Materials Science and Engineering SCC in unconfined zones, segregation resistance, uniformity of the fresh SCC mix, and the consistency to satisfy the specification.On the other hand, the V-funnel test describes the viscosity and deformability of SCC. e workability characteristics of the fresh SCC mixture used in this research are listed in Table 2, together with the limitations of the EFNARC committee [34].
ree self-compacting concrete cylinders (100 × 200 mm) were cast and cured in water for 28 days and tested at the same time of the corresponding beam specimens as shown in Figure 1. e concrete cylinder samples were tested according to the ASTM C39/C39M-14a standard [35].At the test day, the average cylinder compressive strength of concrete was determined as 59.48 MPa.

Steel Tubes.
is study includes 18 beam specimens divided into three groups.All the steel tubes were made from three cold-formed square hollow sections with 6 m length.
e nominal dimensions of the square steel tube sections were 100 × 100 × 3 mm.ree coupons from each tube were cut from the three faces of the hollow steel sections to determine the actual material properties as shown in Figure 2.
Tensile coupon tests were performed and tested according to the recommendations of ASTM E8/E8M-15a [36].e average yield and ultimate tensile stress for the steel tubes are listed in Table 3.

Steel Plates and Bolts.
Rectangular steel plates with nominal thicknesses of 3 mm were used for the retrofitting of the SCCFST beams.e steel plates had a width of 94 mm and fabricated with two different lengths, 925 mm and 300 mm, respectively.Similar to the steel tubes, three coupons were taken from the adopted steel plates to conduct the tensile coupon according to the recommendations of ASTM E8/E8M-15a [36].
e average yield and ultimate tensile stress for the steel plates are listed in Table 4.
All of the steel plates were drilled with an electrical rotary drill to produce two rows of circular holes distributed in    Advances in Materials Science and Engineering a staggered manner.All holes were performed with a suitable diameter and the center to center distance between holes was 125 mm and 50 mm, in the longitudinal and transverse directions, respectively.Figure 3 depicts the perforated steel plates.Two types of bolts were utilized in this work.Grade 8.8 and 12.9 threaded bolts were used to fasten the steel plates to the bottom and top faces of the SCCFST beams.e details and mechanical properties of these bolts are presented in Table 5. Figure 4 shows photos of typical anchor bolts adopted for retrofitting.

Specimen Preparation.
e three cold-formed steel tubes of six meters long were cut and machined to the desired length.e total span of each beam specimen is 950 mm with an effective span of 750 mm between the end supports.e inside faces of the beam specimens were brushed and cleaned out.e bottom ends of the hollow steel tubes were capped with suitable steel base plates, the SCC was poured from the top without applying any compaction, and the steel tubes were kept in a vertical position for a curing period of 28 days.After that, the SCC surface at the top end of the beam was smoothed with epoxy coating and welded with a steel base plate to form complete composite beams.Holes with a suitable diameter were predrilled through the steel flanges and the concrete core of the SCCFST beams.ese predrilled holes were executed at the locations where the retrofitting steel plates were required to be anchored.e inside of the predrilled holes was cleaned carefully using air to get rid of any dust or loose parts.Epoxy resin with highstrength properties was used to fill the constructed holes partially.en, the predrilled steel plates were attached to be coinciding with the beam flanges.Finally, the steel bolts were installed in the holes and fastened tightly to hold and anchor the steel plates firmly to the beam flanges.Figure 5 depicts the different stages of drilling and anchoring of steel plates to the SCCFST beams.

Beams and Retrofitting Scheme Details.
A total of 18 beam specimens of square sections were prepared for testing in this study.e 18 beams consist of 12 SCCFST beams retrofitted with steel plates with two different lengths and three distinct retrofitting schemes, three SCCFST beams without any strengthening (control beams) for comparison purposes, and three hollow steel tube beams.e 12 retrofitted SCCFST beams were also divided into two branches.6 SCCFST beams prepared as intact (not preloaded) beams.
ese beams were strengthened with steel plates and bolts (grade 8.8 bolts) and then tested without any preloading.
ese beams aim at exploring the influence of drilling holes in the body of the SCCFST beams on both the steel tube and concrete.On the other hand, the remaining 6 SCCFST beams were considered as retrofitted (preloaded) beams.at is to say, these beams were initially preloaded up to a certain level, to simulate the damage condition.After that, the retrofitting techniques including the steel plates and bolts (grade 12.9 bolts) were applied to these beams before their testing.
e beam specimens were classified into three different groups A, B, and C. ese groups represent three different retrofitting schemes.Each group consists of five beams, namely, one hollow beam, one control beam, two intact beams, and two retrofitted beams.
e dimensions and details of each beam specimen are listed in Table 6.Regarding group A beams, the retrofitting technique was achieved using external single steel plates applied to the bottom face of the SCCFST beams.On the other hand, beams of group B were retrofitted using external steel plates anchored to the top and bottom surfaces of the SCCFST beams.Finally, double steel plates were fastened to the bottom face of the SCCFST beams in group C.
Moreover, in each group, the retrofitting technique was utilized using two different steel plate lengths: full retrofitting length in which the steel plate was attached to the full length of the SCCFST beams and partial length in which the steel plates   Table 6: Dimensions and details of the beam specimens.

Group name Beam designation
Beam dimensions

mm) Details
Group A (single bottom plate) Group B (top and bottom plates) Note: H, B, t: beam depth, width, and thickness; B p × t p × L p : steel plate width, thickness, and length.
Advances in Materials Science and Engineering connected to only one third (middle third) of the beam length.Figure 6 illustrates the details of the various retrofitting schemes adopted to the SCCFST beam specimens.e following labelling system was used to designate each beam specimen in Table 6.For instance, HB and CB indicate hollow steel tube and control (unstrengthened) SCCFST beams, respectively.RB-PL-SB and RB-FL-SB represent damaged beams (RB) retrofitted by single bottom (SB) steel plates with partial length (PL) or full length (FL), respectively.Whereas, IB-PL-SB and RB-FL-SB refer to the strengthened intact beams (IB).Similarly, SCCFST beams in groups B and C have the same designations of the above beams, except the symbols (TB) in group B and (DB) in group C which refer to retrofitting with top and bottom (TB) steel plates and double bottom steel (DB) plates, respectively.

Test Procedure.
All the beam specimens were tested under pure bending moment up to failure.All beam specimens had a span length of 950 mm (an effective span of 750 mm) and were placed in a simply supported condition as shown in Figure 7. e 500 KN capacity testing machine was used to conduct the test.e beams were tested under the four-point loading method providing a constant bending zone of 250 mm. e force was applied using a displacement control method with a loading rate of 1 mm/min.e implementation of displacement control method allowed the experiments to be continued into the post-peak stage and enabled better control when recording the postultimate behaviour [37].Deflections along the beam span were measured by three linear variable displacement transducers (LVDTs).One was placed at the midspan of the specimen; the other two were placed under concentrated loads with the shifting of 50 mm from the left and right of the concentrated load positions as shown in Figure 7.
e above testing procedure was applied directly to the hollow, control, and intact SCCFST beams.However, for the retrofitted SCCFST beams, a preloading stage was implemented to simulate the damage condition.To achieve this goal, SCCFST beams were preloaded before retrofitting.e preloading stage was done using the same loading setup as explained above.In the beginning, the beams were loaded until reaching a moment capacity of 90% of the ultimate moment capacity recorded for the corresponding control beam.Regarding beams of group A (single bottom steel plate retrofitting scheme), the typical failure mode of all the intact and retrofitted beams with different retrofitting lengths (beams retrofitted partially or entirely along their length) was outward local buckling of the top compression steel flange as shown in Figure 8(a).Similar to the control beams, outward local buckling was observed either close to the midspan section or near the locations of the applied loads within the constant bending zone.In addition to the local Finally, considering beams of group B (top and bottom steel plate retro tting scheme), beams of this group revealed a characterized di erent failure mode compared to their counterparts in other groups.e major di erence was that no local buckling was recognized at the top steel ange of all the beams, as shown in Figure 8(b).is may be attributed to the existence of the top retro tting steel plate.Furthermore, another interesting observation was recorded.An upward local buckling appeared at the top retro tting steel plate of the beam (RB-FL-TB) which was retro tted along its full length, as shown in Figure 9(f).

Test Results and Discussion
is phenomenon was unique in this beam specimen.Nevertheless, regarding the tension failure modes, all beams in this group (B) were exhibited similar failure mechanism as that recorded for their counterparts in the other groups, as depicted in Figures 8(b), 9(f), and 9(g).Finally, it can be concluded that the failure modes of beams in group B were to some extent featured from those noticed in other groups.e adoption of anchored top steel plates, fastened to the top ange of the SCCFST beam, was an e ective retro tting technique.is technique was able to change the predominated local buckling mode of CFST beams and hence lead to relatively increase the exural strength of retro tted beams compared to the other retro tting schemes (group A and C).
Figure 10 shows the de ection curves along the length of typical control beam and typical retro tted beams for each group versus di erent bending moment levels.From these gures, it was detected that the curves are symmetrical, and the de ection curves take the shape of half sine wave curves.

Moment (M) versus Midspan De ection (D m ) Curves.
e moment (M) versus midspan de ection (D m ) curves of all the tested beams are depicted in Figures 11(a)-11(c) for the three retro tting scheme groups A, B, and C, respectively.Also, Figure 12 represents the typical moment versus midspan de ection curves of the preloaded beams compared to the control beams.
All the tested control SCCFST beams (CB-A, CB-B, and CB-C) exhibited an elastic behaviour during the initial loading stage, followed by inelastic behaviour with a gradual reduction in exural sti ness until reaching the ultimate moment capacity.en, the control beams showed a plastic behaviour with large deformation capacity and associated with almost constant exural strength up to the initiation of bottom steel rupture.e reduction in moment capacity at the failure point was less than 4% of the ultimate moment capacity.
is behaviour re ects the superior ductile behaviour of the SCCFST beams.

Advances in Materials Science and Engineering
Figure 12 indicates that the preloaded beams followed exactly the same path of the control beams up to their predetermined loading stage.
is response re ected the ability to conduct a healthy comparison between the retro tted beams and their counterpart control beams.e preloaded beams were loaded up to an average bending moment of 15.322 kN•m which represents about 90% of the average ultimate moment achieved by the control beam.
In general, all the intact and retro tted SCCFST beams behaved in an elastic behaviour followed by an inelastic response with a gradual reduction in exural sti ness up to the ultimate moment capacity. is performance is close to that showed by the control beams.However, some di erences in behaviour were detected from the moment versus midspan de ection curves, as will be described in the next sections.
All the intact SCCFST beams followed the similar elastic and inelastic behaviour of the control SCCFST beams with almost similar exural sti ness.However, the major di erence was in the plastic behaviour.e intact specimens showed limited plastic zone compared to the control beams.
is may attribute to the reduction in the cross-sectional area of the bottom tensile anges of the beams and strengthen steel plates and some de ciency that may occur in the concrete core due to the predrilled holes.Intact beams (IB-PL-SB, IB-PL-TB, and IB-PL-DB) which are partially strengthened showed similar exural strength and sti ness compared to that obtained for the control beams.However, beams (IB-FL-SB, IB-FL-TB, and IB-FL-DB) which are fully strengthened (steel plates applied over the full beam length) showed some greater exural strength compared to both control beams and partially strengthened beams.
Generally, all the retro tted SCCFST beams exhibited similar elastic and inelastic response to those of the control SCCFST beams.However, the retro tted beams revealed greater exural sti ness.e partial retro tted beams (RB-PL-SB, RB-PL-TB, and RB-PL-DB) showed close ultimate exural strength to those obtained by the control beams and their counterpart intact beams (IB-PL-SB, IB-PL-TB, and IB-PL-DB).However, these beams also had a limited plastic zone and hence lower ductility compared to the control beams, similar to their intact counterpart beams.e fully retro tted beams (RB-PL-SB, RB-PL-TB, and RB-PL-DB) exhibited larger exural strength and sti ness compared to all other tested beams in this experimental work.
is potential enhancement in exural strength and stiness was more booming in retro tted beams that belong to Furthermore, the fully retrofitted beams (RB-PL-SB, RB-PL-TB, and RB-PL-DB) behaved in a more ductile mode compared to all other retrofitted and intact beams.However, the ductility of these beams was still lower than those obtained for the control beams.Finally, it can be concluded that all the retrofitted beams were able to restore and even to develop enhanced behaviour in terms of flexural strength and stiffness compared to their original undamaged condition.

Moment Capacities.
e ultimate experimental bending moments (M u ) for all beam specimens in all groups and percentage of increase in moment capacity of the retrofitted beams compared to the control beams are tabulated in Table 7 and depicted in Figure 13.
For group A beams (Figure 13(a)), the percentage increase in the ultimate moment of intact SCCFST beams ranged from about 0% for beam IB-PL-SB to 9.70% for beam IB-FL-SB.On the other hand, retrofitted beams achieved a percentage increment in ultimate flexural strength ranged from 0.92% for beam RB-PL-SB to 26.80% for beam RB-FL-SB compared to the control beam.It is clear that the maximum advantage of retrofitting the intact SCCFST beams was achieved by using steel plates along the full beam length.However, a slight increase in the ultimate moment was obtained from beams retrofitted with steel plates along the partial beam length.is may attribute to the fact that steel plates with full length covered the full span of intact beams and hence increase both the moment capacity and stiffness.While, for beams with a partially strengthened span, the steel plates were attached only to the pure bending zone and it was not extended to the shear span.
For group B beams (Figure 13(b)), the percentage enhancement in the ultimate moment due to the strengthening of the SCCFST intact beams was varied from 3.14% for beam IB-PL-TB to 22.29% for beam IB-FL-TB.Regarding the retrofitted SCCFST beams, the percentage improvement in the ultimate moment capacity due to the retrofitting of the SCCFST damaged (preloaded) beams was varied from 5.21% for beam RB-PL-TB to 45.55% for beam RB-FL-TB.Similar to group A, it showed that the maximum benefit of upgrading was obtained by retrofitting the beams along their full length compared to those retrofitted along their partial length.It is worth to note that, that intact and retrofitted beams in this group (group B) attained the highest ultimate moment capacity compared to the beams in other groups (groups A and C) as shown in Figure 13(d).
erefore, it can be stated that the best enhancement in ultimate flexural strength was gained due to the adopting of group B retrofitting technique (top and bottom steel plates).
For group C beams (Figure 13(c)), the percentage increase in the ultimate moment due to the strengthening of SCCFST intact beams ranged from 2.54% for beam IB-PL-DB to 21.59% for beam IB-FL-DB.However, the percentage increment in the ultimate moment due to the retrofitting of SCCFST damaged beams ranged from 1.00% for beam RB-PL-DB to 35.72% for beam IB-FL-DB.Similar to groups A and B, it is evident that the maximum avail of retrofitting of SCCFST beam was achieved by adopting full retrofitting scheme rather than partial retrofitting schemes.Furthermore, the retrofitting scheme used in this group (double bottom plates) provides a moderate improvement in ultimate flexural strength compared to the other retrofitting schemes adopted in groups A (single bottom plate) and B (top and bottom plates).
Finally, it can be indicated that all the adopted retrofitting schemes were effective in restoring the flexural strength of the damaged beams and even enhance the ultimate flexural strength, especially for beams retrofitted along their full span length.Moreover, among the Advances in Materials Science and Engineering three di erent upgrading schemes, it was found that the retro tting method of damaged beams using top and bottom steel plates was the most e ective method.

Ductility Capacities.
Ductile performance of beams implies the ability to absorb as much energy as possible before failure.e ductility of the tested beams was evaluated by the modulus of toughness (MOT), which represents the entire area under the load-de ection curves.Furthermore, the relative ductility index (RDI) of the tested beams was calculated which represents the ratio of ductility achieved by the retro tted and intact beams to the ductility of the control beams.e results of the relative ductility index and modulus of toughness are listed in Table 8 and illustrated in Figure 14.
For group A beams, the RDI of intact beams ranged from 0.41 for beam IB-PL-SB to 0.53 for beam IB-FL-SB which referred to about 53% (average value) reduction in ductility compared to the control beam.However, for the retro tted beams, the RDI ranged from 0.35 for beam RB-PL-SB to 0.70 for beam IB-FL-SB which indicated that the retro tted beams were able to restore about 35% to 70% of the original ductility of the control beam.
For group B beams, the RDI of intact beams varied from 0.49 for beam IB-PL-SB to 0.57 for beam IB-FL-SB which referred to about 47% (average value) reduction in ductility compared to the control beam.On the other hand, for the retro tted beams, the RDI ranged from 0.43 for beam RB-PL-TB to 0.75 for beam IB-FL-TB which implied that the retro tted beams were able to recover about 43% to 75% of the original ductility of the control beam.It is worth noting that the retro tted beams in this group developed the highest ductility compared to their counterparts in other groups.
is pointed that, the retro tting scheme adopting in this group (top and bottom steel plates) demonstrated the most e cient retro tting technique.
For group C beams, the RDI of intact beams was about 0.50 (average) which refereed to about 50% lower ductility compared to the control beam.However, regarding the retro tted beams, the RDI ranged from 0.35 for beam RB-PL-DB to 0.63 for beam RB-FL-DB which indicated that the retro tted beams were capable of getting back about 35% to 63% of the original ductility of the control beam.However, retro tted beams of this group attained the lowest ductility relative to beams in other groups.
Finally, similar to the moment capacities, it is obvious that the full retro tting scheme is much e cient than the partial 12 Advances in Materials Science and Engineering retrofitting scheme regarding the improvement in ductility and energy absorption capacity of the retrofitted beams.

Theoretical Model for the Ultimate Moment Capacity
e American specification AISC 360-10 [38] provides a theoretical model for predicting the ultimate moment capacity of normal concrete-filled steel tube (NCFST) members based on the plastic stress distribution method.Furthermore, Lai et al. [3] reported more details about the calculations of the moment capacities for the NCFST members based on the original model described by AISC 360-10.However, the main limitation of this model is that it was developed to predict the moment capacity of NCFST beams without external retrofitting steel plates.at is to say, the AISC 360-10 model did not take into consideration the effect of the external retrofitting steel plates.It is expected that the existence of external steel plates will increase the ultimate moment capacity of the CFST beams.However, this increase cannot be accounted for by using the AISC 360-10 model.So that, the adoption of the AISC 360-10 model will result in a considerable underestimation (conservative) in predicting the ultimate flexural strength of the retrofitted SCCFST beams using external steel plates.
In this study, a new theoretical model based on the AISC 360-10 [38] model and Lai et al. [3] calculation equations were developed.e new model is developed in such a way to take into consideration the effect of external retrofitting steel plates on the prediction of the ultimate moment capacity of SCCFST beams.e developed model utilizes the plastic stress distribution method as described in the AISC 360-10 specification to compute the plastic moment (M p ) capacity of the beam cross section as shown in Figure 15.
is plastic moment strength represents the nominal flexural (M n ) capacity of retrofitted rectangular SCCFST beams with compact sections using external steel plates.e plastic stress method assumes that the steel and concrete materials have a rigid plastic behaviour.is approach considers that the steel stress in tension and compression is equal to the yield stress (f y ), while the concrete stress is equal to 0.85 f c ′ in compression and equal to zero in tension.In this model, the effect of retrofitted steel plates was taken into consideration by considering that the external steel plates at the compression and tension sides can reach their yielding stress (f y ) in a similar way to that assumed for the steel section.Hence, it ensures that the SSCFST beam and the retrofitted external steel plates can achieve a ductile behaviour.erefore, the tension or compression forces obtained from the bottom or top external steel plates can be predicted based on the yielding tensile stress of the steel plates as follows: where F p is the force in the external steel plates and f yp and A gp are the yielding stress and cross-sectional area of the external steel plates, respectively.e above equation was found to be convenient for computing the ultimate moment capacity of SCCFST beams retrofitted with external steel plates using grade 12.9 bolts (bolts with the higher strength).However, for SCCFST beams retrofitted with external steel plates using grade 8.8 bolts (bolts with the lower strength), another condition was considered for the steel plates on the tension side (bottom steel plates).For steel plates in tension with grade 8.8 bolts, the most control criteria are the shear strength of the fixing bolts rather than the yielding strength of the steel plates.So that, the expected tension forces obtained from the bottom external steel plates can be predicted based on the shear strength of the fixing bolts as follows: where F b is the force in the external bottom steel plates and f nv , A b , and N b are the shearing stress, cross-sectional area, and number of the bolts, respectively.Figures 15(a)-15(c) illustrate the stress blocks and forces of the concrete and steel at the ultimate loading stage for the three different retrofitting schemes, single bottom plate, top and bottom plates, and double bottom plates, respectively.e depth of the neutral axis (a p ) from the compression face is computed by achieving axial force equilibrium over the cross section.
e plastic moment capacity (M p ) is computed as the summation of bending moments caused by the concrete and steel forces around the neutral axis location (a p ). e obtaining equations for calculating a p and M p are given below for the three different retrofitting schemes: (a) For retrofitted SCCFST beams using external single bottom steel plate (group A) with grade 12.9 bolts, a p and M p can be calculated as follows: (b) For retrofitted SCCFST beams using external single bottom steel plate (group A) with grade 8.8 bolts, a p and M p can be calculated as follows: (c) For retrofitted SCCFST beams using external top and bottom steel plates (group B) with grade 12.9 bolts, a p and M p can be calculated as follows: (d) For retrofitted SCCFST beams using external top and bottom steel plates (group B) with grade 8.8 bolts, a p and M p can be calculated as follows: (e) For retrofitted SCCFST beams using external double bottom steel plates (group C) with grade 12.9 bolts, a p and M p can be calculated as follows: Advances in Materials Science and Engineering (f) For retrofitted SCCFST beams using external top and bottom steel plates (group C) with grade 8.8 bolts, a p and M p can be calculated as follows: Finally, regarding the SCCFST beams retrofitted using partial length steel plates for the three different retrofitting schemes (groups A, B, and C). e experimental test results revealed that partial length retrofitting scheme (beams retrofitted along their pure bending zone only) has an inconsiderable effect on the ultimate moment capacity.erefore, the plastic moment capacity (M p ) of these SCCFST beams is calculated based on M p calculated according to the previously mentioned equations multiplied by a reduction factor of 0.91 and 0.8, for beams in group A with bolt grades 8.8 and 12.9, respectively.For beams in group B, the reduction factors are equal to 0.84 and 0.72.However, for the group C beams, the reduction factors are 0.89 and 0.74, for beams retrofitted by steel plates with bolt grades 8.8 and 12.9, respectively.ese reduction factors were estimated based on the experimental test results obtained from this study.

Test Results against the eoretical Model and Design
Code.
e predicted flexural capacities (M ua ) of the retrofitted SCCFST beams utilizing the aforementioned developed theoretical model and the American specification AISC 360-10 method (M uc ) are compared with the current experimental results (M ue ) as shown in Table 9.Furthermore, the mean value and the standard deviation (SD) of the ratios M ua /M ue and M uc /M ue are listed in Table 9.
e results in Table 9 revealed that the developed model in this study could predict the ultimate moment capacity of retrofitted SCCFST beams accurately.On the other hand, the results indicated that the AISC 360-10 model underestimates (conservative) the ultimate moment capacity of these beams.
is underestimation is expected because the AISC 360-10 model did not take into consideration the effect of external steel plates on the flexural strength of retrofitted SCCFST beams.However, the new theoretical model which is suggested in this study includes the influence of these bolted steel plates.
For instance, the theoretical model was able to predict the moment capacity of retrofitted SCCFST beams of group A with an average value of 0.960 and with a standard deviation (SD) of 0.074.On the other hand, the AISC 360-10 code underestimated the moment capacity by an average of about 21% and with SD equal to 0.078.Similarly, regarding the retrofitted beams of group B, the theoretical model predicted the moment capacity with an average value of 0.973 and with a SD of 0.084.However, the AISC 360-10 code underestimated the moment capacity by an average of about 25% and with SD equal to 0.078.Finally, the theoretical model gave an average estimation of about 0.983 and with a SD of 0.096 for retrofitted beams within group C. While, the AISC 360-10 specification again underestimated the moment resistance of retrofitted beams by about 23%.
It is worth to note that the AISC 360-10 method can predict the moment capacity of SCCFST beams retrofitted with partial length steel plate and with lower bolt strength, more accurately than beams retrofitted with full-length plates and with higher strength bolts.For example, the AISC 360-10 method estimated the flexural strength of beams IB-PL-SB, IB-PL-TB, and IB-PL-DB, with an average underestimation of 17%.However, regarding beams RB-FL-SB, RB-FL-TB, and RB-FL-DB, the level of conservation was about 38%.Finally, comparison results indicated that the moment capacity of beams retrofitted using partial length steel plates could be computed by neglecting the effect of the bolted steel plates.It is clear that when the SCCFST beams retrofitted with steel plates along their partial length (along with the constant bending zone span) only, the enhancement in ultimate moment capacity is indeed insignificant.So that, both the theoretical model of this study and the AISC 360-10 model can be used to estimate the moment resistance of such retrofitted beams.

Conclusions
is paper highlights the results of an experimental study investigating the flexural performance of SCCFST beams retrofitted using externally bolted steel plates with three different schemes including varied configurations.e moment capacities, ductility, failure modes, and momentdeflection curves of beam specimens were reported and discussed.Based on the experimental results and observations, the following conclusions can be stated: (i) e typical failure modes of the fully retrofitted SCCFST beams were characterized by outward local buckling of the beam top steel flange, ruptures of some of their anchoring bolts within the beam shear span, delamination and separation of the attached retrofitting plates, and finally rupture failure of the tension bottom flanges.(ii) Different failure modes were observed for SCCFST beams which retrofitted by top and bottom steel plates (group B), compared to their counterparts in other groups.e basic difference was that no local buckling was recognized at the top steel flange of all the beams.Furthermore, an upward local buckling appeared at the top retrofitting steel plate of the beam RB-FL-TB.(iii) e moment versus midspan deflection curves of the control and retrofitted SCCFST beams in all groups showed elastic behaviour at the first stage of loading.en, the curves follow an inelastic path with a gradual reduction in stiffness until reaching the maximum bending capacity.(iv) Test results revealed that the percentage increase in the ultimate moment capacity due to the retrofitting of SCCFST beams using the different three schemes ranged from 1.00% to 46.00%.It is worth to note that retrofitting scheme of group B beams (top and bottom bolted steel plates) was the most efficient technique regarding the flexural strength enhancement.(v) e relative ductility index (RDI) results indicated that all the retrofitted beams were able to restore about 35% to 75% of the original ductility of the control beams.However, the most effective retrofitting method regarding ductility recovering was the retrofitting scheme of group B (top and bottom bolted steel plates).(vi) It was found that the full retrofitting scheme (steel plates applied along the entire beam length) is much efficient than the partial retrofitting scheme (steel plates applied along constant bending zone only) regarding improving the ultimate moment resistance, stiffness, and ductility capacity of the retrofitted beams.(vii) A new theoretical model based on the AISC 360-10 method was developed.e new model is developed in such a way to take into consideration the effect of external retrofitting steel plates on the prediction of the ultimate moment capacity of SCCFST beams.e comparison results revealed that this model could predict the ultimate moment capacity of retrofitted SCCFST beams accurately with a total average of 97%.(viii) Finally, it can be concluded that all the adopted retrofitting schemes were effective in restoring the flexural strength and ductility of the damaged beams and even enhance the ultimate flexural strength, especially for beams retrofitted along their full span length with top and bottom plates.

4. 1 .
Failure Modes.All the tested composite beam specimens reached their ultimate moment capacity with no lateral movement signs of the cross section or any other instability form.e typical failure modes of all the SCCFST beams are shown in Figure8.

Figure 6 :Figure 7 :
Figure 6: Retrofitting schemes of SCCFST beams.(a) Bottom flange view.(b) Side view.(c) Groups A and C retrofitting schemes.(d) Group B retrofitting scheme.(e) Double bottom steel plates.(f) Top and bottom steel plates.

Figure 11 :Figure 12 :
Figure 11: Moment versus midspan de ection curves.(a) Beams of group A. (b) Beams of group B. (c) Beams of group C.

Table 1 :
Mix proportions and properties of self-compacting concrete (SCC).

Table 3 :
Properties of steel tubes.

Table 4 :
Properties of steel plates.

Table 5 :
Properties of bolts.

Table 7 :
Ultimate moments and percentage of increase in moment capacity.

Table 8 :
Modulus of toughness and relative ductility.

Table 9 :
Comparisons of experimental moment capacities against the theoretical model and design code values.