Ultimate compressive strength of Enveloped Laminar Concrete panels

Construction and Building Materials 27 (2012) 375–381

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Ultimate compressive strength of Enveloped Laminar Concrete panels E.V.M. Carrasco ⇑, E.V. Rodrigues, G.O. Ribeiro, G. Queiroz, F.A. De Paula Department of Structural Engineering, School of Engineering, Federal University of Minas Gerais, Belo Horizonte, Brazil

a r t i c l e

i n f o

Article history: Received 20 July 2010 Received in revised form 7 July 2011 Accepted 18 July 2011 Available online 12 August 2011 Keywords: Experimental analysis Ultimate strength Panel Precast Enveloped Laminar Concrete

a b s t r a c t An experimental research of Enveloped Laminar Concrete (ELC) panels subjected to axial compression loading is presented. ELC panels are characterized by the use of steel U-sections surrounding a concrete panel reinforced by steel gratings welded to these U-sections. The experimental program consisted of evaluating 17 ELC panels with different slenderness ratios under the action of centered compressive loads. The experimental setup allowed for the execution of fixed-end supports and uniform load distribution over the width of the models. Typical failure modes and the influence of the panels’ slenderness on their load-bearing capacity are discussed. The estimated load bearing capacities obtained from semiempirical equations for structural concrete wall panels developed by other researchers and specified in technical standards were compared to the experimental results. In most of the analyzed cases, the strength of the ELC panels is higher than that of other wall panels, which is mainly due to the contribution of steel U-sections in the composed structural system. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction Enveloped Laminar Concrete (ELC) panels consist of precast concrete panels reinforced with steel gratings, encased by Ushaped steel profiles. The gratings are welded to the inner part of the U-shaped sections. These panels are generally slender and are classified as wall panels due to their geometry. Fig. 1 illustrates fabricated ELC panels. Several studies have focused on the structural behavior and strength of wall panels subjected to compression due to their wide potential for application in the civil engineering sector. The types of construction technologies usually analyzed have been reinforced concrete wall panels and sandwich panels. ELC panels, however, differ from the types studied so far mainly because they are encased by a surrounding steel frame. This steel frame contributes to increase their load-bearing capacity and allows them to be welded together, creating several alternative compositions of structural systems based on these panels. The ELC panel submitted to axial compressive load works as a special steel–concrete composite column, since the web and the flanges of the steel U-shaped profiles do not undergo local buckling and the concrete also is prevented against instability as a compressed plate supported in longitudinal edges. The ultimate strength of steel–concrete composite columns, regardless the global instability, is given by the strengths of the steel shapes, the concrete and the steel reinforcement. The increase of strength of this ⇑ Corresponding author. Address: Avenida Dom João IV, 584, 2Direito, PC 4810534, Guimaraes, Portugal. Tel.: +55 31 34091997; fax: +55 31 34091973. E-mail address: [email protected] (E.V.M. Carrasco). 0950-0618/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.conbuildmat.2011.07.034

structural composition when compared to usual concrete panels is due to the strength provided by the steel shapes. Bearing in mind the global instability, the load-bearing capacity is further improved as shown in experimental studies [14–16], about the structural behavior of unbounded steel plate brace encased in reinforced concrete panels. They demonstrated that the smaller clearance between the steel profile and the concrete is a key feature in the steel encased profile performance. Another important feature of the ELC panel is the increased ductility as long as the steel profile yielding occurs before the concrete crushing, which is defined by deformation limit of 0.002. Dan et al. [13] in their experimental research about composite steel–concrete shear walls observed that panels with encased steel profiles have a higher ductility than the common reinforced walls. The main objective of this study was the evaluation of ultimate strength of ELC panels. Further this, the gain in the strength of these panels is compared to the strength of other reinforced concrete panels. This comparison was performed based on equations proposed by several researchers available in the literature. An experimental program was developed at Federal University of Minas Gerais, Brazil, by testing 17 ELC models subjected to axial compressive loads using approximately fixed-end conditions without lateral constraints along the height of the panels. 1.1. Studies of structural wall panel load-bearing strength Leabu [6] is considered a pioneer in the investigation of the behavior of reinforced concrete wall panels. Based on the admissible stress state of concrete, he proposed the following equation to estimate the ultimate strength of panels under pure axial loading:

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h i Pu ¼ 0:2f c A 1  ðh=40tÞ3

ð1Þ

where Pu is the panel’s ultimate axial compressive load, fc is the compressive concrete strength determined by means of cylindrical test specimens, A is the panel’s full unreduced cross-section area, h is its height, and t is its thickness. Oberlender and Everard [8] experimentally investigated 54 reinforced concrete wall panels with different slenderness ratios subjected to centered and eccentric axial compressive loads. These researchers found that h/t < 20 ratios caused crushing of the concrete, while h/t P 20 ratios led to flexural compressive failure. As a result of this experimental program, the aforementioned researchers proposed the following equation to determine the panels’ ultimate axial load capacity:

Pu ¼ 0:6/ f c A½1  ðh=30tÞ2 

Fig. 1. ELC panels. Table 1 Experimental mechanical properties of the concrete. Concrete

fc (MPa)

Ec (MPa)

Group 1 Group 2

24.78 20.80

14,362 12,199

Table 2 Nominal mechanical properties of the steel. Steel

Yield strength (MPa)

Tensile strength (MPa)

Es (kN/cm2)

Q196 grating U-shaped profiles

600 280

660 370

20,500 20,532

ð2Þ

where / is a strength reduction coefficient. Pillai and Parthasarathy [9] carried out 18 tests and made a major contribution to the determination of the ultimate strength of reinforced concrete wall panels. These authors observed that a low h/t ratio usually led to failure by crushing, while panels with a h/t ratio higher than 20 underwent flexural compressive failure at mid-height. As a result of this study, they proposed the following equation to estimate the results of panels with h/t < 30:

Pu ¼ 0:57/ f c A½1  ðh=50tÞ2 

ð3Þ

Another important work for the study of the behavior and design of wall panels was developed by Kripanarayanan [5]. This researcher found a significant increase in the load-bearing capacity

Fig. 2. ELC specimen details: assemblage, welding and concrete molding.

Fig. 3. Transversal section of the ELC specimen.

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E.V.M. Carrasco et al. / Construction and Building Materials 27 (2012) 375–381 Table 3 Designations and geometric characteristics of the panels. Panel (Group1)

h

b

t

h/t

Steel

Panel (Group2) 2

U sections (cm ) ELC1 ELC2 ELC3 ELC4 ELC5 ELC6 ELC7 ELC8

40 60 80 100 120 140 160 180

60

5

8 12 16 20 24 28 32 36

3.86

h

b

t

h/t

2

Q196 grating (cm )

1.96

M1a M1b M1c M2a M2b M2c M3a M3b M3c

40 40 40 100 100 100 160 160 160

50

5

8 8 8 20 20 20 32 32 32

Steel U sections (cm2)

Q196 grating (cm2)

3.86

1.57

Fig. 4. Experimental setup for testing ELC panels.

of the panel when the amount of reinforcement lay within the range of 0.75–1.0% of the cross section. Moreover, it was found that

the h/t ratio in the design equation of the ACI 318:1971 code did not provide a realistic estimate of the loading capacity of panels,

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which is why the inclusion of a k factor related to the panel’s effective buckling length was recommended. As a result of that research, the following equation was proposed, which is currently specified by the ACI 318:2008 code [1]:

Pu ¼ 0:55/ f c A½1  ðkh=32tÞ2 

ð4Þ

Zielinski et al. [12] performed real scale tests on five 2.75 m high reinforced concrete panels and concluded that the steel’s contribution to their strength was very significant. Based on their research and considering maximum eccentricities of up to 1/6 of the panel’s thickness, they proposed the following expression to estimate the panels’ ultimate compressive load: 2

Pu ¼ 0:55/½Afc þ ðfys  fc ÞAs   ½1  ðh=40tÞ 

ð5Þ

where fys is steel yielding strength. Several studies on the strength of reinforced concrete wall panels were conducted by Saheb and Desayi [10]. They performed tests on 24 models and their studies included the effect of parameters such as the h/t ratio, the h/b ratio, and vertical and horizontal reinforcement rates. As a result, they proposed two ultimate strength equations for wall panels without lateral constraints, considering b as the panel’s width, and for h/b < 2:

Pu ¼ 0:55/½fc A þ ðfys  fc ÞAs ½1  ðkh=32tÞ2 ½1:2  ðh=10bÞ

ð6aÞ

and h/b P 2:

Pu ¼ 0:55/½fc A þ ðfys  fc ÞAs   ½1  ðkh=32tÞ2 

ð6bÞ

Doh [4] conducted experimental research on 18 reinforced concrete wall panels under uniformly distributed compression. The panels were produced with normal and high strength concrete. Based on this research, an equation was proposed to determine the ultimate axial compressive load of panels, expressed by:

/Pu ¼ 2:0/ fc0;7 bðt  1:2e  2ea Þ

ð7Þ

where b is the panel’s width (mm), t is its thickness (mm), e is the load eccentricity (mm), and ea is the additional eccentricity due to panel displacements (mm), given by:

ea ¼ ðkhÞ2 =2500t

ð8Þ

where k is the panel’s effective length factor, h is its height (mm) and t is its thickness (mm).

Benayoune et al. [2,3] studied the structural behavior of 12 concrete sandwich panels with truss-type shear connectors subjected to eccentric and centered compressive loads. A semi-empirical expression was proposed to model the ultimate compressive load of panels, incorporating the contribution of the reinforcing steel, which is given by:

Pu ¼ 0:40f c A½1  ðkh=40tÞ2  þ 0:67As fys

ð9Þ

2. CLE Specimen details and materials 2.1. Concrete Two groups of ELC panels have been designed and molded. Group 1 was aimed at analyzing the load-bearing capacity of the panels as a function of slenderness with one specimen for each slenderness ratio. Group 2 differed from Group 1 in that it presented three similar specimens for each evaluated slenderness ratio in order to offer more reliable results and a smaller crosssection area in order to ensure rupture at lower loads than those applied in Group 1. The composition of the concrete mixture for the panels consisted of CPII E 32 cement, grade zero crushed stone as coarse aggregate, stone powder as fine aggregate, and water. The mixture proportion was 1:2:3 (in weight), with a water/cement ratio of 0.60. The characteristic compressive strength expected for the concrete at 28 days was 20 MPa. The concrete’s elasticity modulus (Ec) and compressive strength (fc), which are listed in Table 1, were determined experimentally from 10 cm  20 cm cylindrical test specimens, according to the Brazilian NBR 5739 standard [7], using an EMIC DL 30000 universal testing machine. The strains were measured using clip gauges. 2.2. Steel The panels were reinforced with Q196 welded steel grating (1.96 cm2 of steel area per meter of grating), made of CA-60 steel rebars with 5 mm nominal diameter, spaced vertically and horizontally at 10 cm intervals. The gratings were arranged in two layers symmetrically placed with respect to the median plane of the panel, with 1 cm of concrete cover on both sides of the panel. The reinforced concrete frame was made of cold formed U-shaped

Fig. 5. (a) Test assembly. (b) Lateral constraint of the upper load-distribution I beam.

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Fig. 6. Upper support.

Table 4 Experimental ultimate axial loads and ultimate load ratios. Pue

a

Pu/Pue

Panel

(kN)

Eq. (1)a

Eq. (2)a

Eq. (3)a

Eq. (4)a

Eq. (5)a

Eq. (6)a

Eq. (7)a

Eq. (9)a

ELC1 ELC2 ELC3 ELC4 ELC5 ELC6 ELC7 ELC8 M1a M1b M1c M2a M2b M2c M3a M3b M3c

632 731 627 684 494 626 689 541 501 507 526 503 534 558 435 465 325

0.23 0.20 0.23 0.21 0.28 0.22 0.19 0.22 0.21 0.20 0.20 0.19 0.19 0.18 0.21 0.19 0.27

0.68 0.57 0.63 0.53 0.66 0.45 0.34 0.32 0.60 0.60 0.58 0.50 0.47 0.45 0.37 0.35 0.50

0.66 0.57 0.65 0.58 0.77 0.59 0.51 0.61 0.59 0.58 0.56 0.55 0.52 0.50 056 0.53 0.75

0.63 0.53 0.58 0.50 0.63 0.44 0.34 0.35 0.56 0.55 0.53 0.47 0.45 0.43 0.38 0.35 0.51

0.73 0.62 0.70 0.62 0.81 0.60 0.50 0.57 0.66 0.65 0.63 0.60 0.56 0.54 0.56 0.53 0.75

0.82 0.67 0.72 0.59 0.73 0.51 0.39 0.41 0.73 0.72 0.70 0.56 0.53 0.50 0.45 0.42 0.60

0.88 0.74 0.83 0.72 0.92 0.67 0.54 0.59 0.82 0.81 0.78 0.72 0.68 0.65 0.63 0.59 0.84

0.59 0.50 0.57 0.50 0.67 0.50 0.43 0.51 0.53 0.53 0.51 0.50 0.47 0.45 0.49 0.46 0.66

Adapted equations.

steel profiles of (50.8  25.4  2) mm. Table 2 lists the mechanical properties of the steel for both mentioned components, where Es is the steel’s longitudinal elasticity modulus.

Table 5 Equations versus authorship.

2.3. ELC details The main features of the ELC assemblage are presented in Figs. 2 and 3. First of all, the steel U-channel profiles are cold cut and welded in opposite corners of the frame, which define the ELC panel perimeter. In the next stage, the ends of each steel grating bar are welded to the inner face of the U-channel and the other two corners of the frame are welded. After this, in the molding stage, the steel frame is fixed in a horizontal table and filled with concrete. After 4 h, the steel frame allows the ELC panel to be moved upright, releasing the molding table.

Eqs.

Refs.

Eq. Eq. Eq. Eq. Eq. Eq. Eq. Eq.

Leabu [6] Oberlender and Everard [8] Pillai and Parthasarathy [9] Kripanarayanan [5] Zielinski et al. [12] Saheb and Desayi [10] Doh [4] Benayoune et al. [2,3]

(1) (2) (3) (4) (5) (6) (7) (9)

All the panels were loaded similarly and the behavior of each panel was observed carefully during the loading process.

4. Results and discussion 3. Experimental program

4.1. Failure modes Seventeen ELC panels were fabricated for the centered compression studies, whose designations and characteristics are presented in Table 3. Panels ELC1 to ELC8 constituted Group 1, while panels M1a to M3c corresponded to Group 2. A loading system was designed for the compression tests. These tests were performed in the Experimental Structural Analysis Lab at the School of Engineering of UFMG (LAEES/UFMG). The experimental setup enabled the execution of fixed end supports and uniform load distribution throughout the width of the panels. Fig. 4 shows a schematic diagram of the setup and describes each of its components, while in Fig. 5 are presented pictures of the general test assembly and the detail of lateral constraint of the upper load-distribution I beam. The rigid reaction frame consisted of two I columns (400  250  16  12.5) mm with stiff steel bases. The columns were connected with screws to a transverse I beam (400  250  16  12.5) mm that could be adjusted to the height of the panels to be tested. The loading system, with a maximum loading capacity of 1000 kN, consisted of a hydraulic actuator and a cylinder unit connected to a universal hinge.

The ELC panels showed typically ductile ruptures. In general, the concrete failed by catastrophic rupture accompanied by delamination. Failures occurred predominantly at the upper and lower ends of the panels due to crushing of the concrete, while models ELC5, ELC6, ELC8 and M3c underwent flexural compressive failure at mid-height as presented in Section 4.4. 4.2. Analysis of ultimate loads in terms of fixed-end supports In order to compare the experimental values of ultimate loading of the ELC panels against their respective values provided by Eqs. (1)–(9), two adaptations were required. The first was the consider-

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ation of fixed-end supports with the inclusion of the effective length factor (k), while the second was the exclusion of the strength reduction coefficients (/). Therefore, the panels’ original lengths (h) were substituted for their respective effective lengths (kh) where necessary. A k factor of 0.65 was adopted for supports constrained against rotation. Details of the upper support are shown in Fig. 6, where it can be noted the square steel bars that prevent rotation and lateral displacement to the panel. The lower support was built following the same features. The results were analyzed in terms of the effective height-to-thickness ratio (kh/t).

800 700 600

P u (kN)

500 400 300 200 100

4.3. Comparison of the results

0

5

10

15

20

25

kh/t Exp. results - Stage 1 Oberlender e Everard* Kripanarayanan* Saheb e Desayi* Benayoune et al.*

Leabu* Pillai e Parthasarathy* Zielinski et al.* Doh*

* Adapted equations Fig. 7. Comparisons of experimental results of Group 1 with results predicted based on Eqs. (1)–(9).

600 500

P u (kN)

400 300 200 100 0 5

10

15

20

25

kh/t Exp. results - Stage 2 Oberlender e Everard* Kripanarayanan* Saheb e Desayi* Benayoune et al.*

Leabu* Pillai e Parthasarathy* Zielinski et al.* Doh*

* Adapted equations Fig. 8. Comparisons of experimental results of Group 2 with results predicted based on Eqs. (1)–(9).

From the modifications proposed in Eqs. (1)–(9), the ultimate axial compressive loads of the ELC panels were calculated, based on their mechanical and geometric properties, considering the load eccentricity (e) equal to zero. Table 4 lists the results of the experimental ultimate compressive loads (Pue) and the ratios between ultimate loads (Pu/Pue). The correspondence between the equation number and the authorship is shown in Table 5. The results listed in Table 4 were related to the kh/t ratios of the ELC panels on the comparative graphs presented in Figs. 7 and 8. An analysis of Figs. 7 and 8 reveals that not all the equations approximate the experimental results of the ELC panels, which are represented by the dots (Groups 1 and 2). In these two figures, also note that the strength of the structural ELC system is higher in all the cases analyzed and that the predicted ultimate load that was closest to the experimental results was the one predicted by Eq. (7)a adapted from Doh’s [4]. Kripanarayanan’s [5] Eq. (4) provides important information for an analysis of the results. This expression is similar to the design equation specified by the ACI 318 code [1]. A comparison of the values resulting from this adapted equation and those of the ultimate load of the ELC panels indicates that the results obtained through this equation, devoid of its strength reduction coefficient, are very conservative. This fact is explained by Sanjayan and Maheswaran [11], who state that the failure of standards to estimate the load-bearing capacity of panels is due to the limited number of tests on models with dimensions other than those studied. Therefore, estimates obtained by this equation under geometric conditions and with strengths of materials different from those originally researched are imprecise. 4.4. Influence of effective height-to-thickness ratio on load-bearing capacity Based on the ultimate loads of the ELC panels, it was found that their ultimate strength decreases nonlinearly as the kh/t ratio increases. However, the steel encased panels used in the present

Fig. 9. Typical failure of panels with kh/t less than 19 – Specimens ELC1 and M1a.

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Fig. 10. Typical failure of panels with kh/t greater than19 – Specimens ELC8 and M3c.

Fig. 11. Typical failure of panels in the transition zone with kh/t close to 19 – Specimens M2a and M2b.

study showed a slower reduction in strength with the increase in kh/t than those reported in the literature. This is due to the contribution of the steel U-sections to the panels’ strength and flexural stiffness. Most of the ELC panels with h/t ratios higher than 20 presented flexural compression failure in the section close to mid-height, while panels with a h/t ratio below 20 presented rupture by crushing of the concrete at the top or base, as observed in the literature. The failure modes are presented in the pictures shown in Figs. 9–11. These results are close to those obtained by Refs. [8,9], who reported flexural compression failure in panels with a h/t ratio equal to or higher than 20 and rupture by crushing of panels at a h/t ratio lower than 20. 5. Conclusions Based on a comparative analysis of the experimental results and those obtained using equations for structural wall panels available in the literature it was possible to analyze ELC panels from the standpoint of their compressive strength. The strength of the structural ELC system determined experimentally was found, in most of the analyzed cases, to be higher than the load-bearing capacity estimated from the aforementioned equations. The explanation for this fact seems to be attributable to the contribution of the Uchannel steel sections to the load-bearing capacity, in the case of ELC panels. The analysis of the results also reveals that the ultimate strength of the ELC panels decreased nonlinearly with the increase in kh/t and the steel encased panels showed a slower reduction in strength in comparison to the estimated strength obtained from the application of the adapted equations reported in the literature. Acknowledgements The authors acknowledge the following Brazilian institutions CAPES, CEMIG and FAPEMIG for their financial support.

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