Shear performance of prestressed ultra high strength concrete encased steel beams

Construction and Building Materials 52 (2014) 194–201

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Shear performance of prestressed ultra high strength concrete encased steel beams Dali Yao a,c,⇑, Jinqing Jia a, Feng Wu b, Fang Yu c a

State Key Laboratory of Coastal and Offshore Engineering, Dalian University of Technology, Dalian City, Liaoning Province 116024, China School of Civil and Safety Engineering, Dalian Jiaotong University, Dalian City, Liaoning Province 116028, China c School of Architecture and Civil Engineering, Shenyang University of Technology, Shenyang City, Liaoning Province 110178, China b

h i g h l i g h t s  Encasing structural steel improves the shear performance of PUHC beam significantly.  The height of diagonal cracks is restricted to the upper flange of structural steel.  The post-cracking stiffness decreases slightly due to structural steel contribution.  Structural steel prevent the instantaneous failure and keep load descending stably.  The shear ductility is efficiently improved to an average increase of 2.13 times.

a r t i c l e

i n f o

Article history: Received 29 June 2013 Received in revised form 6 October 2013 Accepted 2 November 2013

Keywords: Beam Shear span–depth ratio Degree of prestress Shear ductility

a b s t r a c t Due to the high compressive strength and durability properties of ultra high strength concrete, prestressed ultra high strength concrete beam was used extensively in bridge engineering, but it possessed obvious brittle behavior. Encasing structural steel into it was a good way for alleviating the problem of brittleness. The purpose of this study was to investigate shear performance of prestressed ultra high strength concrete encased steel beams. A total of fifteen prestressed ultra high strength concrete encased steel beams and seven prestressed ultra high strength concrete beams were tested to shear failure under simply supported three-point loading conditions. The primary variables of this investigation included the presence or not of structural steel, shear span-depth ratio, degree of prestress, ratio of stirrup and thickness of web. The shear performance was evaluated based on cracking pattern, load–deflection behavior and shear ductility. Test results showed that prestressed ultra high strength concrete encased steel beams had higher residual shear capacity and post-cracking stiffness as well as by far better shear ductility than prestressed ultra high strength concrete beams. In addition, influence of experimental parameters on the shear performance of prestressed ultra high strength concrete encased steel beams and prestressed ultra high strength concrete beams also was discussed and compared, respectively. Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved.

1. Introduction Prestressed concrete (PC) members have been used in building structures and infrastructure facilities since the 1960s because of their various advantages such as high quality, durable aesthetics, reduced time span of construction and economic efficiency. In particular, prestressed concrete technology has been advanced by the technical growth of ultra high strength concrete manufacture from the development of various admixture agencies. In general, it defines ultra high strength concrete with 28-day uniaxial compressive strength as determined by a standard 150 mm  150 mm test specimens in excess of 100 MPa [1]. In comparison to ordinary ⇑ Corresponding author at: State Key Laboratory of Coastal and Offshore Engineering, Dalian University of Technology, Dalian City, Liaoning Province 116024, China. Tel.: +86 411 13644916093. E-mail address: [email protected] (D. Yao).

strength concrete, ultra high strength concrete exhibits superior compressive and tensile mechanical behaviors, as well as exceptional durability properties [2]. Hence, the use of ultra high strength concrete has become in prestressed cross-sea bridge and prestressed concrete offshore platforms. Up to now, there also are a few studies about flexural and shear performance of prestressed ultra high strength concrete (PUHC) beams [3–5]. Furthermore, studies showed that PUHC beams had higher strength and smaller crack width than prestressed ordinary strength concrete beams. Studies also reported that PUHC beams exhibited higher stiffness, due to ultra high strength concrete having much greater elastic modulus. However, Yao et al. [6] found that PUHC beams had obvious brittle behavior in shear test. The lack of shear ductility results in sudden failure without warning in severe earthquakes, which is a serious drawback [7–9]. Thus, it is very necessary to improve the shear ductility of PUHC beams.

0950-0618/$ - see front matter Crown Copyright Ó 2013 Published by Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.conbuildmat.2013.11.006

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One type of the structural members in the composite construction is the concrete encased structural steel, referred to as ‘‘steel-reinforced concrete (SRC) construction.’’ This type of composite member has been used in Japan for more than 4 decades [10], and has become increasingly popular in building constructions in Taiwan since the JiJi earthquake in 1999 [11]. Because the encasement of structural steel in concrete columns or beams can greatly increase the strength, stiffness and energy absorption capacity of composite members, it has been a common way to improve the ductility of concrete members in seismic zone [12–20]. Therefore, encasing structural steel into PUHC beam could be also a way to alleviate the problem of brittleness on PUHC beams, due to the lack of shear ductility. Unfortunately, according to the literature search, no research results related to shear performance of prestressed ultra high strength concrete encased steel (PUHCES) beams. Hence, it is necessary that the test systematically investigates shear performance of PUHCES beams. The experimental program described in this paper has two objectives: (1) present the results of an investigation comparing the shear performance of PUHCES beams to that of PUHC beams; (2) further investigate the influence of test variables on shear performance of PUHCES beams. To achieve two objectives, twentytwo test beams were tested in this study. Fifteen of test beams were PUHCES beams and the rest were PUHC beams. The experimental parameters included in the study were the presence or not of structural steel, shear span-depth ratio a/d, degree of prestress kp, ratio of stirrup qsv and thickness of web tw. 2. Experimental programs

Table 1 Details of test beams. Beam no. PUHCES-01 PUHCES-02 PUHCES-03 PUHCES-04 PUHCES-05 PUHCES-06 PUHCES-07 PUHCES-08 PUHCES-09 PUHCES-10 PUHCES-11 PUHCES-12 PUHCES-13 PUHCES-14 PUHCES-15 PUHC-01 PUHC-02 PUHC-03 PUHC-04 PUHC-05 PUHC-06 PUHC-07

tw (mm)

1.5 1.5 1.5 1.5 1.5 1.5 1.5 2.0 2.0 2.0 2.0 2.0 2.0 2.0 2.5 1.5 1.5 1.5 1.5 2.0 2.5 1.5

kp

5.5 3.0 8.0 5.5 5.5 5.5 5.5 5.5 3.0 8.0 5.5 5.5 5.5 5.5 5.5 – – – – – – –

0.42 0.42 0.42 0.34 0.00 0.42 0.42 0.42 0.42 0.42 0.34 0.00 0.42 0.42 0.42 0.42 0.42 0.42 0.00 0.42 0.42 0.34

qsv (%) 0.32 0.32 0.32 0.32 0.32 0.22 0.42 0.32 0.32 0.32 0.32 0.32 0.22 0.42 0.32 0.22 0.42 0.32 0.32 0.32 0.32 0.32

Structural steel a

B1 B2b B3c B1 B1 B1 B1 B1 B2 B3 B1 B1 B1 B1 B1 – – – – – – –

840 840 840 840 840 840 840 1120 1120 1120 1120 1120 1120 1120 1400 840 840 840 840 1120 1400 840

bf

bf

ð1Þ

where Aps is the area of prestressing strand; As is the area of non-prestressed tensile reinforcement; fps is the yield stress of prestressing strand; and fy is the yield stress of non-prestressed tensile reinforcement.

tw

60 mm x 10 mm B1

ds hf

ds

h'f

h'f

tw

hf

Twenty-two test beams were made in this study. Fifteen of test beams were PUHCES beams, while the others were PUHC beams. The main experimental parameters considered in the study included the presence or not of structural steel, shear span–depth ratio a/d (1.5, 2.0 and 2.5), degree of prestress kp (0, 0.34 and 0.42), ratio of stirrup qsv (0.22%, 0.32% and 0.42%) and thickness of web tw (3.0 mm and 8.0 mm). All test beams with a dimension of B (width)  H (depth)  L (length) were 160 mm  340 mm  (1200, 1400 and 1600) mm, were tested on a span of ls 840 mm, 1120 mm and 1400 mm. The compressive strength of concrete was 108.2 MPa, which was determined by compression tests on 9 cubic specimens with each side dimension of 150 mm. Each beam had three longitudinal tensile bars of D 20 (diameter 20 mm) and two longitudinal compressive bars of D 18 (diameter 18 mm). Longitudinal tensile bars had 90° hooks at the test beam end to ensure adequate anchorage. The stirrups were symmetrically placed and stirrup of D 6.5 (diameter 6.5 mm). Two different cross section areas of prestressing strand, 139 mm2 and 98.7 mm2 were used. In order to reduce the prestress loss due to the short length of test beam, low shrinkage anchor and a technique of double jacking with the same prestressing forces were used (details of this technique were published in the literature [21]) in this study. The PUHCES beams had three different types of structural steel. To avoid the possible shear splitting failure along the interface between upper flange of structural steel and ultrahigh strength concrete, shear studs of 10 mm diameter  55 mm height were welded on upper flange of structural steel. Table 1 shows the details of test beams. Fig. 1 shows the cross section of structural steel. Fig. 2 shows the cross section of test beams. The degree of prestress kp is defined as the ratio of the force carried by the prestressing strand to the force carried by the total reinforcement at ultimate conditions [22], as shown in the following equation:

ls (mm)

a The dimension of welded steel plate b  h is 60 mm  10 mm in lower flange of structural steel (the dimension of structural steel ds  bf  tw  hf’  hf is 140 mm  80 mm  5.5 mm  9.1 mm  9.1 mm). b The dimension of structural steel d s  b f  t w  hf ’  hf is 140 mm  80 mm  3.0 mm  10 mm  18 mm. c ’ The dimension of structural steel d s  b f  t w  hf  hf is 140 mm  80 mm  8.0 mm  10 mm  18 mm.

2.1. Design of the test beams

kp ¼ ðAps fps Þ=ðAps fps þ As fy Þ

a/d

B2/B3

Fig. 1. Cross section of structural steel. 2.3. Measurement and test scheme Monotonic loading was provided with a 10,000 kN hydraulic servo testing machine. A calibrated load cell was placed between the jack and test beam while linear variable differential transducers (LVDT) were properly installed to measure the displacements at the two supports and mid-span as well as the slip between upper flange of structural steel and ultra high strength concrete during the test, as shown in Fig. 3. Monotonic loading was applied step-by-step up to 85% of the expected ultimate load in a load control manner and then shifted to a displacement control method until the failure of test beam. The combination of the loading methods is to effectively perform the test, while obtaining a full history of failure behavior. The rate of displacement is 0.2 mm/min. An acquisition system automatically monitored load and displacements at pre-selected time intervals throughout the loading history. The test also provided information on the overall behavior of test beams including cracking pattern and crack width.

2.2. Material properties In this test program, the concrete mixture was made with Portland cement type 52.5R, Class-I fly ash, silica fume, water, sand and coarse aggregate. To improve the workability, a polycarboxylic acid–based high-range water-reducing admixture (HRWRA) and D-Gluconic acid sodium salt (D-Gass), were added after extensive trials. Table 2 summarizes the mixture proportions for ultra high strength concrete employed in this test program. The yield stress fy, ultimate stress fu and modulus of elasticity Es of structural steel are given in Table 3. Similarly, Table 4 presents fy, fu and Es values of the longitudinal steel bar, stirrup bar and prestressing strand.

3. Results and discussions 3.1. Shear resistance of PUCHES beams In PC beams, the prestressing strand contribution to the shear resistance of beam is generally derived from the dowel action Vpf [23,24]. As expressed in Eq. (2), the shear resisting force of PC

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shear stud structural steel

340

340

20

100 20

40

prestressing strand 70

100 70

prestressing strand

160

160 Fig. 2. Cross section of test beams (units: mm).

beams VPC can be expressed as the algebra sum of the component shear resisting mechanisms, namely, the contribution from uncracked concrete in compression Vc, the shear force carried by vertical stirrups Vus, aggregate interlock mechanism Va and dowel action of the longitudinal tensile bar Vd, as well as that offered by prestressing strand Vpf.

V PC ¼ V c þ V us þ V a þ V d þ V pf

ð2Þ

In SRC beams, one model that calculates the shear resisting force of beams, VSRC, takes the sum of all the terms, as shown in the following equation [25]:

V SRC ¼ V cl þ V sw þ V sv þ V c0 þ V tf þ V ff

ð3Þ

where Vcl is the shear carried by the uncracked concrete in compression; Vsw is the shear carried by the web of structural steel; Vsv is the shear force carried by the stirrups crossed by the diagonal crack; Vc0 is the shear resistance of the aggregate interlock across the diagonal crack; Vtf and Vff are the shear carried by the dowel action of the longitudinal reinforcement and flange of structural steel, respectively. In PUHCES beams, therefore, the shear resistance is additionally contributed by prestressing strand derived from the dowel action Vpf, contrast with SRC beams. The shear resisting force of PUCHES beams VPUCHES is given in the following equation:

V PUHCES ¼ V cl þ V sw þ V sv þ V c0 þ V tf þ V ff þ V pf

ð4Þ

Fig. 4 shows the vertical equilibrium conditions of the free-body diagram for PUCHES beams. In this condition, the presence or not of structural steel, degree of prestress, ratio of stirrup and thickness of web have significant influence on the shear performance. Thus, these factors are the main focuses in the subsequent discussions. 3.2. Cracking pattern Fig. 5 shows typical crack patterns of PUHC and PUHCES beams. The chronological development of cracks was carefully outlined with a black or red felt tip marker and recorded throughout each test. As expected, during the early stages of loading, fine vertical flexural cracks appeared around the mid-span zone of test beams. With an initial increase in load, new flexural cracks formed away from the mid-span zone. With a further increase, the flexural cracks started to propagate diagonally towards the loading point,

and other new diagonal cracks began to form in shear span zone. In most cases, for PUHC beams, shear failure occurred shortly after a dominant diagonal crack extended to the top fiber, as indicated in Fig. 5(a) and (c), indicating extremely brittleness behavior in shear failure. However, for PUHCES beams, the enhancement in the diagonal crack width was more pronounced after the ultimate load, and as load continues, the diagonal cracks opened significantly near main diagonal crack and finally formed multiple main diagonal cracks (Fig. 5(b) and (d)). This phenomenon indicates that PUHCES beams have better post-peak deformation capacity than PUHC beams. In addition, the height of diagonal cracks of PUHCES beams was obviously lower than that of PUHC beams at the ultimate load state (Fig. 5(a) and (b); Fig. 5(c) and (d)), and it was observed that most of diagonal cracks only extend to position of upper flange of structural steel for PUHCES beams. This shows that the shear capacity of test beam can be improved because structural steel can prevent the decrease of the height of shear compression region.

3.3. Load–deflection behavior In Fig. 6(a)–(c), it can be seen that structural steel has a significant influence on the load–deflection curves of PUHCES and PUHC beams. Before any diagonal crack developed, the stiffness is constant with a linear load–deflection response, regardless of whether structural steel is present or not. After the appearance of diagonal crack, the stiffness of PUHCES beams is slightly reduced, but the stiffness of PUHC beams is significantly decreased. This behavior of beam is altered by structural steel in two ways. Firstly, structural steel can effectively limit the diagonal crack growth for PUHCES beams, resulting in larger effective height. Secondly, structural steel may be capable of providing test beams with the extra stiffness, indicating the residual stiffness of PUHCES beams is higher than that of PUHC beams after the appearance of diagonal crack. The presence of structural steel also has small range increase in the shear capacity, and the average ratio of Fu-PUHCES/Fu-PUHC is approximately 1.16. In addition, it can also be observed that PUHCES and PUHC beams have steep descent stage and gentle descent stage in the post-peak region of load–deflection curves. Furthermore, PUHCES beams have a shorter steep descent stage, while PUHC beams have a longer steep descent stage. Ft is the load of converging point of steep descent stage and gentle descent stage. For PUHCES beams, the average ratio of Ft-PUHCES/Fu-PUHCES is

Table 2 Mixture proportions for ultra high strength concrete. Concrete strength (MPa)

w/b

Cement (kg/m3)

Class-I fly ash (kg/m3)

Silica fume (kg/m3)

Water (kg/m3)

Sand (kg/m3)

Coarse aggregate (kg/m3)

HRWRA (kg/m3)

D-Gass (kg/m3)

108.2

0.23

420

120.0

60

138

495.0

1155.0

9.00

3.00

D. Yao et al. / Construction and Building Materials 52 (2014) 194–201

197

Vcl

stirrup bar Vc0

structural steel Vsw

VPUCHES

prestressing strand longitudinal tensile bar

Vsv Vtf Vff Vpf

Fig. 4. The free-body diagram of PUCHES beams.

Table 3 Material properties for structural steel.

a b c

Type of structural steel

fy (MPa)a

fu (MPa)b

Es (MPa)c

Flange

Web

Flange

Web

Flange

Web

B1 B2 B3

320 325 325

320 325 315

420 465 435

420 435 435

208,000 205,000 205,000

208,000 205,000 205,000

Defined as yield stress. Defined as ultimate stress. Defined as modulus of elasticity.

Table 4 Material properties for steel bar.

a b c

Type of steel bar

fy (MPa)a

fu (MPa)b

Es (MPa)c

Longitudinal tensile bar Longitudinal compressive bar Stirrup bar Prestressing strand (139 mm2) Prestressing strand (98.7 mm2)

370 355 335 1815 1798

616 525 482 1911 1893

206,000 206,000 210,000 195,000 195,000

Defined as yield stress. Defined as ultimate stress. Defined as modulus of elasticity.

Jack Load cell

LVDT Fig. 3. Loading and measuring arrangement of test beam.

approximately 0.90; for PUHC beams, the average ratio of Ft-PUHC/ Fu-PUHC is approximately 0.50, as shown in Table 6. Such a phenomenon demonstrates that PUHCES beams have better residual shear capacity than PUHC beams in the post-peak. This is due to the reason that the presence of structural steel could prevent the instantaneous failure and sustain the load. In both PUHCES and PUHC beams, notice that the post-cracking stiffness of test beams decreases significantly as the shear spandepth ratio (a/d) increases (Fig. 6(a)). This is mainly due to the maximum moment to the maximum shear (M/V) ratio on test

beam. For a certain shear level, the moment would be larger and consequently the effective moment of inertia of the section would be smaller after the formation of the cracks for larger a/d, resulting in a noticeable reduction on the stiffness of test beams [26]. Compared to the PUHC beams respectively, the ultimate load is more than 14% for beam PUHCES-01 (a/d = 1.5), more than 12% for beam PUHCES-08 (a/d = 2.0) and more than 30% for beam PUHCES-15 (a/d = 2.5). This indicates that the effect of structural steel on shear capacity is more predominant in test beam with a higher a/d. Fig. 6(b) shows that the effect of degree of prestress (kp) on the load–deflection curves of test beams. Contrast to the nonprestressed beams (kp = 0) respectively, the stiffness is 1.09 times for beam PUHC-03 (kp = 0.42) and 1.07 times for beam PUHC-07 (kp = 0.34) before cracking, however, the stiffness is 3.99 times for beam PUHCES-01 (kp = 0.42) and 1.95 times for beam PUHCES-04 (kp = 0.34) before cracking. It can be seen that an increase in the degree of prestress has little effect on the stiffness of PUHC beams before the occurrence of diagonal crack, but it is more pronounced for improving the stiffness of PUHCES beams. According to Ref. [27], an increase in the lateral compressive pressure of concrete results in higher initial slopes of stress–strain curve in triaxial compressive experiments for a certain elasticity modulus. Structural steel can provide better lateral restriction for core concrete in PUHCES beams, and the enhancement in longitudinal compressive pressure of core concrete is more pronounced corresponding to a higher degree of prestress. As a result, PUHCES beams with kp of 0.42 have higher stiffness than that of 0.34 and 0.00. Compared to the slopes of descending portion of load–deflection curves of PUHC beams, it can be observed that PUHCES beams with kp of 0.42 and 0.34 behaved more like the nonprestressed beams, which has similar slopes of descending portion of load–deflection curves. This demonstrates that prestress does not seem to have a significant effect on stiffness of PUHCES beams in the post-peak region. In addition, test results also indicate that an increase in the degree of prestress slightly increases the shear capacity of test beams if all other parameters remain the same. The shear capacities of test beams with kp of 0.42 are only 11% (PUHCES beams) and 7% (PUHC beams) higher than that of nonprestressed beams. To evaluate the effect of ratio of stirrup (qsv) on load–deflection curves of PUHC beams (Fig. 6(c)), as exhibits in this plot, the load– deflection response before the failure is barely influenced by ratio of stirrup, and whereas an increase in the ratio of stirrup improves the shear capacity of PUHC beams. This is because stirrup can provide additional capacity and restrict the development of diagonal cracks, due to stirrups crossing diagonal cracks after cracking. This phenomenon is also observed in the test results of PUHCES beams. For PUHCES beams with a/d of 1.5, it can be seen that the slope of the descending portion of load–deflection curves for PUHCES beam with qsv of 0.22% is steeper than that for PUHCES beam with qsv of 0.32%, however, the behavior of PUHCES beam with qsv of 0.42% would be very similar to PUHCES beam with qsv of 0.32%.

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Fig. 5. Cracking patterns of test beams.

Fig. 6. Load–deflection curves of test beams.

In addition, for PUHCES beams with a/d of 2.0, it is interesting to notice that beam PUHCES-13 was made with tw of 5.5 mm and a lower qsv (0.22%), and its ultimate load is the similar as beam PUHCES-09, which used tw of 3.0 mm and a higher qsv (0.32%) from Fig. 6(d). Similar behavior is observed for beam PUHCES-02 (tw of 3.0 mm and qsv of 0.32%) and beam PUHCES-06 (tw of 5.5 mm and qsv of 0.22%) for test beams with a/d of 1.5. These results further suggest that it is quite necessary to adopt thinner web and

higher ratio of stirrup for PUHCES beams, due to thicker web resulting in serious waste. Fig. 6(d) shows that the stiffness of PUHCES beams is almost identical, regardless of the thickness of web. In general, an increase in the thickness of web can improve the shear capacity of PUHCES beams. This behavior of PUHCES beams can be idealized as a truss-arch model [28], web of structural steel and ultra high strength concrete can be seen as an inclined compressive strut.

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F Y

Fu Ff Fy

N

B A

μ =Δ f /Δ y

Δy

O

Δf

Fig. 7. Schematic diagram for definition of shear ductility value l. Table 5 Shear ductility values of PUHCES and PUHC beams. Beam no.

Fu (kN)

D0 (mm)

Dy (mm)

Df (mm)

l

PUHCES-01 PUHCES-02 PUHCES-03 PUHCES-04 PUHCES-05 PUHCES-06 PUHCES-07 PUHCES-08 PUHCES-09 PUHCES-10 PUHCES-11 PUHCES-12 PUHCES-13 PUHCES-14 PUHCES-15 PUHC-01 PUHC-02 PUHC-03 PUHC-04 PUHC-05 PUHC-06 PUHC-07

1164.36 1043.62 1212.19 1147.32 1042.20 1079.83 1239.38 881.68 807.34 1003.84 855.63 846.64 825.87 947.02 870.04 861.79 1129.13 1019.95 958.39 804.49 672.57 975.45

2.26 3.35 3.58 2.75 3.70 2.29 2.39 3.39 3.54 5.51 3.48 4.16 3.00 3.86 6.05 1.99 3.26 3.55 4.10 5.54 7.50 4.78

1.69 2.72 3.10 2.20 2.86 1.57 1.20 2.23 2.20 3.32 2.80 3.25 1.96 1.27 1.69 1.42 2.35 2.29 3.26 3.02 3.07 3.20

8.03 6.76 9.40 9.08 9.94 3.63 6.28 11.06 5.81 10.88 12.07 12.60 5.13 7.13 10.27 2.29 5.07 4.16 7.55 6.52 7.77 5.85

4.75 2.48 3.03 4.13 3.48 2.31 5.23 4.96 2.64 3.28 4.31 3.88 2.61 5.61 6.08 1.61 2.16 1.81 2.32 2.16 2.53 1.83

The lower flange of structural steel and longitudinal tensile bars as well as prestressing strand can balance moment which is produced by the applied load. The load path for the inclined compressive arch is a direct load transfer from the loading point to the support, and the capacity of inclined compressive arch action is greatly influenced by compressive strength of concrete and thickness of web. Hence, using thicker web can transfer more load. 3.4. Shear ductility Based on the above discussion of cracking pattern and load– deflection behavior, it demonstrates that the presence of structural steel can improve post-peak deformation capacity of beam. To

further reflect the deformation capacity for a certain load, a shear ductility value is adopted. The shear ductility value (l is defined as Df/Dy, as shown in Fig. 7, where Fy and Dy are the yield load and corresponding displacement that are determined based on the assumption that the area of hatched region SOAB equals to that of hatched region SYNB, respectively; Ff is the failure load, which is defined as 75% the ultimate load and Df is the corresponding failure displacement [29,30]. The shear ductility values are computed from test results of 22 test beams, and the results are presented in Table 5. Also shown in Table 5 is the ultimate load Fu, the corresponding displacement D0, the yield displacement Dy and the failure displacement Df. Table 6 shows the comparison of load and shear ductility value of PUHCES and PUHC beams. In Table 6, it can be observed that PUHCES beams have by far better shear ductility than PUHC beams. The average value of lPUHCES/lPUHC is approximately 2.13. This indicates that encasing structural steel into PUHC beam is an effective method to enhance shear ductility. It can be readily seen that this result has two significant implications. Firstly, the presence of structural steel can improve the stiffness of beam, leading to reducing displacement (D0) at the ultimate load (Fu). Beams PUHCES-01, PUHCES-08 and PUHCES15 have the D0 of 2.26 mm, 3.39 mm and 6.05 mm, compared with 3.55 mm, 5.54 mm and 7.50 mm for beams PUHC-03, PUHC-05 and PUHC-06. Secondly, the presence of structural steel also prevents immediate failure of beam and allows to the reduction rate of the load at slower in the post-peak region. Fig. 8(a) shows that the effect of shear span–depth ratio (a/d) on shear ductility. It indicates that PUHCES beams have better shear ductility than PUHC beams when a/d increases. For a/d of 1.5, beam PUHCES-01 has a l value of 4.75 and beam PUHC-03 has a l value greater than 1.81. For a/d of 2.5, beam PUHCES-15 has a l value of 6.08, and beam PUHC-06 has a l value greater than 2.53. Similar to the shear ductility of PUHC beams, the shear ductility values of PUHCES beams increase from a/d = 1.5 to a/d = 2.5. It is because that an increase in the shear span-depth ratio results in the failure mode of beam changing from the brittle shear failure to the ductile flexural failure. The effect of degree of prestress (kp) on the shear ductility is illustrated in Fig. 8(b), where it can be seen that it is more prominent on PUHCES beams than PUHC beams. For a/d of 1.5, beam PUHCES-01 (kp = 0.42) had a l value of 4.75, which was 2.62 times that of beam PUHC-03 (1.81). Beam PUHCES-04 (kp = 0.34) had a l value of 4.13, which was 2.26 times that of beam PUHC-07 (1.83). Fig. 8(b) also shows that an increase in the degree of prestress decreases the shear ductility of PUHC beams. However, dissimilar to the PUHC beams, an increase in the degree of prestress increases the shear ductility of PUHCES beams. The shear ductility of PUHCES beams was altered by degree of prestress in two ways. Firstly, unlike the PUHC beams, an increase in the degree of prestress would result in higher stiffness. The importance of this is that lower yield displacement is observed in higher degree of prestress. Secondly, prestress also has an insignificant effect on shear capacity and

Table 6 Comparison of load and shear ductility value. Beam no.

Fu-PUHCES (kN)

Ft-PUHCES (kN)

lPUHCES

Beam no.

Fu-PUHC (kN)

Ft-PUHC (kN)

lPUHC

Fu-PUHCES/ Fu-PUHC

Ft-PUHCES/ Fu-PUHCES

Ft-PUHC/ Fu-PUHC

lPUHCES/ lPUHC

PUHCES-01 PUHCES-04 PUHCES-05 PUHCES-06 PUHCES-07 PUHCES-08 PUHCES-15

1164.36 1147.32 1042.20 1079.83 1239.38 881.68 870.04

1009.05 1047.88 960.75 904.95 1094.79 835.27 761.29

4.75 4.13 3.48 2.31 5.23 4.96 6.08

PUHC-03 PUHC-07 PUHC-04 PUHC-01 PUHC-02 PUHC-05 PUHC-06

1019.95 975.45 958.39 861.79 1129.13 804.49 672.57

570.11 451.41 446.99 437.05 536.67 328.14 410.88

1.81 1.83 2.32 1.61 2.16 2.16 2.53 Ave

1.14 1.18 1.09 1.25 1.10 1.10 1.29 1.16

0.87 0.91 0.92 0.84 0.88 0.95 0.88 0.90

0.56 0.46 0.47 0.51 0.48 0.41 0.61 0.50

2.62 2.26 1.50 1.43 2.42 2.30 2.41 2.13

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Fig. 8. Effect of experimental parameters on shear ductility for test beams.

stiffness of PUHCES beams in the post-peak region. Hence, PUHCES beams have similar the failure displacement, regardless of whether prestress is present or not. Yield displacement values of 1.69 mm, 2.20 mm and 2.86 mm are found for beams PUHCES-01 (kp = 0.42), PUHCES-04 (kp = 0.34) and PUHCES-05 (kp = 0.00), respectively. However, failure displacement values of 8.03 mm, 9.08 mm and 9.94 mm are found for beams PUHCES-01 (kp = 0.42), PUHCES-04 (kp = 0.34) and PUHCES-05 (kp = 0.00), respectively. It implies that an increase in the degree of prestress may have a positive effect on the shear ductility of PUHCES beams. An increase in the ratio of stirrup (qsv) does not only increase the shear capacity, but also it can improve the shear ductility significantly. The effect of ratio of stirrup on shear ductility can be seen in Fig. 8(c), demonstrating that PUHCES beams gain more shear ductility than PUHC beams when qsv increases. In PUHC beams, relationships between shear ductility value and ratio of stirrup are almost linear, and as qsv increases from 0.22% to 0.42%, the shear ductility value increases approximately 34%. This is basically due to tying effect of stirrup by restricting the development of diagonal cracks after cracking. Thus, this will ensure the coordination mechanism of separation body of ultra high strength concrete at both sides of diagonal cracks. Likewise, in PUHCES beams, an increase in the ratio of stirrup can also increase shear ductility. This means that besides having better tying effect of stirrup on ultra high strength concrete at both sides of diagonal cracks, an increase in the qsv also has a great ability in decreasing the slip between structural steel and ultra high strength concrete (in Fig. 9). This indicates that ultra high strength concrete has better coordinative work capacity with structural steel at higher qsv,

especially at the maximum load stage. Furthermore, as qsv increases from 0.22% to 0.32%, shear ductility value increases approximately 105% for a/d = 1.5 and 90% for a/d = 2.0. However, as qsv increases from 0.32% to 0.42%, shear ductility value increases approximately 10% for a/d = 1.5 and 13% for a/d = 2.0. This shows that stirrup insignificantly contributes to improve coordinative work capacity between structural steel and ultra high strength concrete for qsv more than 0.32%. In addition, thickness of web (tw) also has a certain role on the shear ductility of PUHCES beams. The effect of thickness of web on shear ductility is illustrated in Fig. 8(d), where it can be seen that

Fig. 9. Load–slip curves on upper flange of structural steel.

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an increase of tw improves shear ductility of PUHCES beams. For PUHCES beams with a/d of 1.5, beam PUHCES-03 has a shear ductility value of 3.03, which is approximately 1.22 times that of beam PUHCES-02 (2.48). For PUHCES beams with a/d of 2.0, beam PUHCES-10 has a shear ductility value of 3.28, which is approximately 1.24 times that of beam PUHCES-09 (2.64). One explanation of this phenomenon is that it may be easier to result in web buckling for using thinner web than using thicker web in the post-peak region. Table 5 shows that beam PUHCES-02 (tw of 3.0 mm and qsv of 0.32%) and beam PUHCES-06 (tw of 5.5 mm and qsv of 0.22%) have the similar shear ductility (2.48 for beam PUHCES-02 and 2.31 for beam PUHCES-06). Similarly, beam PUHCES-09 (tw of 3.0 mm and qsv of 0.32%) and beam PUHCES-13 (tw of 5.5 mm and qsv of 0.22%) also have the similar shear ductility (2.61 for beam PUHCES-13 and 2.64 for beam PUHCES-09). This indicates that if the same shear ductility is required, thickness of web can be reduced by improving ratio of stirrup. 4. Conclusions To investigate the shear performance of PUHCES beams in comparison to their PUHC counterparts. The investigation includes a total of 22 test beams: fifteen of test beams are PUHCES beams and the rest are PUHC beams. The main test variables are the presence or not of structural steel, shear span-depth ratio, degree of prestress, ratio of stirrup and thickness of web. The focus is cracking pattern, load–deflection behavior and shear ductility. The results of this study have led to the following conclusions: 1. A significant difference is noted between PUHCES beams and PUHC beams in terms of cracking pattern. The height of diagonal cracks of PUHCES beams is lower than that of PUHC beams at the ultimate load state, and most of diagonal cracks only extend to the position of upper flange of structural steel. In addition, PUHCES beams have multiple main diagonal cracks in the post-peak stage. 2. The presence of structural steel has small range increase in the shear capacity, but significantly improves the residual shear capacity in comparison to PUHC beams. For PUHCES and PUHC beams, an increase of degree of prestress, increase of ratio of stirrup and a decrease of shear span–depth ratio improve the shear capacity. An increase in the thickness of web also has a positive influence on the shear capacity of PUHCES beams. Test results suggest that it is more reasonable to adopt thinner web and higher ratio of stirrup at same shear capacity for PUHCES beams. 3. Structural steel has a significant effect on post-cracking stiffness of PUHCES beams. For PUHCES and PUHC beams, the effect of shear span-depth ratio on the stiffness is obvious, while the effect of ratio of stirrup on the stiffness is negligible. An increase in the degree of prestress improves the stiffness of PUHCES beams, but barely affects the stiffness of PUHC beams. In addition, the use of thicker web has an insignificant influence on the stiffness of PUHCES beams. 4. The shear ductility of PUHCES beams is by far better than that of PUHC beams. The average value of lPUHCES/lPUHC is approximately 2.13 times. The thickness of web has a certain role on shear ductility of PUHCES beams. An increase in the degree of prestress decreases shear ductility of PUHC beams, but improves shear ductility of PUHCES beams. An increase of ratio of stirrup and increase of shear span-depth ratio improve shear ductility of PUHCES and PUHC beams. This is found that it can reduce the thickness of web by improving the ratio of stirrup to achieve the same shear ductility.

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Acknowledgments The research described in this paper was financed by the National Natural Science Foundation of China (Grant: 51078059) and ‘‘863’’ Project of the Ministry of Science and Technology of the People’s Republic of China (Grant: 2007AA11Z133). The authors wish to express their gratitude for this financial support. References [1] Pu XC, Wang ZJ, Wang C, et al. Mechanical properties of super high-strength and high performance concrete. J Build Struct 2002;23(6):49–55. [2] Taylor CW, Montoya KF, Jáuregui DV, et al. Feasibility Analysis of Using UHPC in Prestressed Bridge Girders. In: 2011 structures congress, ASCE, Nevada, April 2011. [3] Graybeal BA. Structural behavior of ultra-high performance concrete prestressed I-girders. Rep. FHWA-HRT-06-115, Federal Highway Administration, Washington, DC; 2006. [4] Graybeal BA. Flexural behavior of an ultrahigh-performance concrete I-girder. J Bridge Eng 2008;13(6):602–10. [5] Steinberg E. Structural reliability of prestressed UHPC flexure modes for bridge girders. J Bridge Eng 2010;15(1):65–72. [6] Yao DL, Jia JQ, Yu F, et al. Analysis on the shear ductility of prestressed ultrahigh reinforced concrete beams. J Harbin Eng Univ 2013;34(5):593–8. [7] Ding YN, You ZG, Jalali S. Hybrid fiber influence on strength and toughness of RC beams. Compos Struct 2010;92:2083–9. [8] Xie YL, Ahmad SH, Yu TJ, et al. Shear ductility of reinforced concrete beams of normal and high-strength concrete. ACI Struct J 1994;91(2):140–9. [9] Ahmad SH, Xie YL, Yu TJ. Shear ductility of reinforced lightweight concrete beams of normal strength and high strength concrete. Cem Concr Compos 1995;17:147–59. [10] Wakabayashi M. A historical study of research on composite construction in Japan. In: Proceeding Conference Paper. Composite Construction in Steel and Concrete, ASCE, New York; 1997. p. 400–27. [11] Weng CC, Yen SI, Jiang MH. Experimental study on shear splitting failure of full-scale composite concrete encased steel beams. J Struct Eng 2002;128(9):1186–94. [12] Naito H, Akiyama M, Suzuki M. Ductility evaluation of concrete–encased steel bridge piers subjected to lateral cyclic loading. J Bridge Eng 2011;16(1):72–81. [13] Mirza SA, Lacroix EA. Comparative strength analyses of concrete-encased steel composite columns. J Bridge Eng 2004;130(12):1941–53. [14] Procter AN. Tests on stability of concrete encased I-beams. Consulting Engineer. London, England, February, 56–58, March, 59–61; 1967. [15] Furlong RW. Design of steel-encased concrete beam-columns. J Struct Eng 1968;94(1):267–81. [16] El-Tawil S, Deierlein GG. Strength and ductility of concrete encased composite columns. J Struct Eng 1999;125(9):1009–19. [17] El-Tawil S, Sanz-Picòn CF, Deierlein GG. Evaluation of ACI318 and AISC (LRFD) strength provisions for composite beam-column. J Constr Steel Res 1995;34(1):103–23. [18] Ricles JM, Paboojian SD. Seismic performance of steel-encased composite columns. J Struct Eng 1994;120(8):2474–94. [19] Mirza SA. Parametric study of composite column strength variability. J Constr Steel Res 1989;12(2):121–37. [20] Mirza SA, Hyttinen V, Hyttinen E. Physical tests and analyses of composite steel-concrete beam-columns. J Struct Eng 1996;122(11):1317–26. [21] Yao DL, Jia JQ, Tu BX, et al. Elastic stiffness analysis on ultra-high strength prestressed concrete beam. J. Wuhan Univ Technol (Transport Sci & Eng) 2013;37(1):74–6. [22] Shahawi ME, Batchelor BV. Fatigue of partially prestressed concrete. J Struct Eng 1986;112(3):524–37. [23] Padmarajaiah SK, Ramaswamy A. Behavior of fiber-reinforced prestressed and reinforced high-strength concrete beams subjected to shear. ACI Struct J 2001;98(5):752–61. [24] Park SY, Naaman AE. Shear behavior of concrete beams prestressed with FRP tendons. PCI J 1999;44(1):74–85. [25] Xue JY. Steel–concrete composite structures. Wuhan, China: Huazhong University of Science & Technology Press; 2006. [26] Fathifazl G, Razaqpur AG, Isgor OB, et al. Shear strength of reinforced recycled concrete beams without stirrups. Mag Concr Res 2009;61(7):477–90. [27] Zhao GF. The study of reinforced concrete structures. Beijing: China Machine Press; 2005. [28] Zheng SS, Hu Y, Che SL, et al. Experimental study on the shear capacity of SRHSHPC beams. Eng Mech 2011;28(3):129–36. [29] Yi WJ, Pan BR, Lv YM, et al. Experimental study on the shear failure of reinforced concrete beams with grade HRB500 steel stirrups. China Civil Eng J 2012;45(4):56–62. [30] Yi WJ, Lv YM. Experimental study on shear behavior of high-strength concrete beams with high-strength stirrups. J Build Struct 2009;30(4):94–101.