reduction of shear resistance in bubbledecks with

REDUCTION OF SHEAR RESISTANCE IN BUBBLEDECKS WITH OPENINGS Nazar Oukaili1 and Hammad Merie2 1

Professor, College of Engineering, University of Baghdad, Iraq Assistant Lecturer, College of Engineering, University of Kirkuk, Iraq

2

Abstract: BubbleDeck systems use hollow recycled plastic spheres in conventional slabs to reduce the structural self-weight. The most critical issue in the bubbled slab is the slab–column connection, where the concentration of loads can lead to punching shear failure. There is often a need to install new services that required creation of openings in the vicinity of columns for utilities and ducts. The existence of the opening reduces the punching shear capacity. Experimental and theoretical studies for ten two-way BubbleDecks, (2000x2000x230) mm dimensions with sphere diameter of (180) mm, were conducted. The main objective of the study is to investigate the punching shear capacity of the BubbleDeck slab with square openings. Parameters which were considered are: the size of opening and the position of opening relative to column. The test results showed that, there is a reduction in the total capacity of the BubbleDecks with openings compared with the control BubbleDeck without opening. This reduction may reach (45%) depending on the bubbles location relative to the punching shear critical section. Based on finite element method, the numerical analysis by ANSYS program was performed. Also, the punching shear capacity for experimental BubbleDecks was determined according to ACI 318-14 and Eurocode 2.

Keywords: BubbleDeck, critical section, openings, punching shear, plastic spheres.

i.

Introduction BubbleDeck slab is a biaxial concrete floor system which first time developed in Europe. High-density polyethylene hollow sphere replace the ineffective concrete in the centre of the slab, thus decreasing the dead weight and increasing the efficiency of the floor. This system consists of hollow plastic spheres cast into the concrete to create a grid of voids from inside the slab; this makes the slab column connection weaker in punching and the shear resistance of BubbleDeck slab is limited compared to the solid slab. Studies performed in Denmark at Denmark's Technical University and Darmstadt University in Germany also in Holland at Eindhoven University have shown that the shear strength of BubbleDeck slab can conservatively be taken as the shear strength of a solid slab of the same depth, determined by (Eurocode 2 2004), multiplied by a reduction factor of 0.6 (BubbleDeck Design Guide 2008). For design purposes of BubbleDecks, the designer shall first compare the applied shear with the shear capacity of the bubbled slab. If the applied shear is less than the shear capacity, no further checks are required; if it is greater, the designer shall remove the spheres around the column and then recheck the shear in the newly solid section (BubbleDeck UK 2008). Structure may need openings of various size, shape and position for situating cable channels, water and gas pipes, ventilation gaps and fireextinguishing systems. Designers must compensate the shortage in strength due to the negative effects of creation of these openings. Openings will reduce the punching shear capacity of the slab-column

connection that is because the opening takes away part of the volume of concrete which affects the overall stiffness of slab. In flat reinforced concrete slabs, the influence of an opening depends on its relative to the column location. From structural viewpoint, they best located away from the column location, preferably in middle strip where the moment is comparatively small. Unfortunately, architectural and functional consideration usually cause them to be located close to the column in column strip, in such situation , the load transfer mechanism to support columns would be changed substantially, leading to considerable reduction in load carrying capacity of the slab (Haidong 2006). The American building code (ACI 318 2014) suggests that if an opening is located closer than ten times the slab thickness from the concentrated load or reaction area, a portion of critical perimeter enclosed by straight lines projecting from the centroid of column, concentrated load or reaction area and tangent to the boundaries of the opening shall be considered ineffective. The (ACI 318-14) code defined the critical perimeter for rectangular or square column as the rectangular or square critical perimeter at a distance of 0.5 𝑑 from the face of column, where 𝑑 is the effective depth of the slab, see Fig. 1(a). The European code (Eurocode 2 2004) adopted rules for considering the effect of openings on the punching shear strength of solid flat slabs. Eurocode 2 suggested reduce the shear perimeter in a manner similar to that in (ACI 318-14) but, for loaded areas situated near openings, if the shortest distance between the perimeter of the loaded area and the edge of the opening does not exceed 6𝑑, that part of the control perimeter contained between lines radiating from the center of the loaded area and tangent to the extremities of opening is considered to be ineffective and shall be subtracting, see Fig. 1(b). It is worth mentioning that, if the dimension of the opening in the radial direction ℓ1 is greater than the dimension ℓ2 in the transversal direction, ℓ2 is replaced by (ℓ1 ℓ2)1/2. For rectangular or square columns, Eurocode 2 defined the punching shear critical perimeter as a zone with rectangular or square configuration with rounded corners which is located at a distance 2𝑑 from the column face. Up to this moment there is no information available about the behavior of BubbleDeck slabs with openings due to the lack of research on this type of structural members, but since these slabs are one of flat slabs types, therefore, the same calculation method used for punching shear strength for solid slabs with openings may be applied to BubbleDeck slabs with openings with some suggested formulation on their treatment of punching shear capacity. Borges, Melo & Gomes (2013) investigated experimentally and theoretically thirteen reinforced concrete flat plates with and without openings or/and shear reinforcement. The dimensions of the slabs were (3000×3000×200) mm. The openings (one or two) were located adjacent to the shorter sides of rectangular supports and had widths equal to those of the supports. Authors concluded that for the relatively small openings considered, the provision of continuous bars adjacent to openings - to replace the areas of reinforcement - seems to be an adequate approach to flexural design. They suggested that the shear stress used for the concrete in the American building code (ACI 318) could be increased for the assessment of strengths at the edges of zones with shear reinforcement (outer control perimeter). Salman (2013) exposed sixteen reinforced concrete flat plates of (1000×1000×70) mm dimension to monotonic static concentrated load. One specimen was without opening while the other fifteen had various arrangements of openings around the column. Due to presence of opening the test results showed a decrease in punching shear capacity which ranged between (11%) and (29%) comparing with the control solid slab. The value of reduction was depending on the size and location of these openings with respect to the column. Reshma & Binu (2016) investigated theoretically, using finite element software ANSYS 14.5, the effect of GFRP strengthening strips with various orientations on improving the load carrying capacity of BubbleDeck slabs. Results were compared with the punching shear strength of solid slabs. Author

concluded that the GFRP strips increased up to (20%) the load capacity of the Bubbled slabs compared to the reference specimen. The main objective of this study is to investigate the punching shear capacity of the reinforced concrete BubbleDeck slabs with square openings. Parameters which will be investigated are the size, the position, the distance of opening relative to column and the location of bubbles relative to the punching shear critical section, where the critical section is considered to be 2𝑑 measured from the face of column (Eurocode 2 2004).

(a) AIC-318.

ii.

(b) EC2 Fig. 1 Punching shear perimeter near an opening.

Experimental program

a. Characteristics of the tested slabs To evaluate the influence of openings on the punching shear strength of BubbleDeck slabs, two-way BubbleDeck reinforced concrete slab specimens were designed with the dimensions of (2000x2000x230) mm with sphere diameter of (180) mm. Parameters which will be investigated are the size, position, distance of opening relative to column and the location of bubbles to the critical section, where the critical section is considered to be (2𝑑) from the face of column according (Eurocode 2 2004), where (𝑑) is the effective slab depth, where two sizes of square opening (300 and 450) mm, two positions of opening relative to column (in front and at corner) and two location distances of opening from column (0 and 200) mm were considered. Accordingly, ten specimens were fabricated; one of these specimens was solid reinforced concrete slab without opening, which was considered as a reference specimen (S0). Another reference specimen was BubbleDeck slab without opening (B0) where the bubbles distributed over the whole area excluding the area under column. While the other eight specimens were BubbleDeck slabs with openings that were divided to two groups. Group (I) includes four specimens, where the bubbles distributed over the whole area excluding the zone under column. While, Group (II) comprised four specimens in which the bubbles distributed over the area which existed outside the punching shear critical section. The designation of specimens can be explained as follows: the first symbol (B) indicates the BubbleDeck specimen. The second symbol declares the opening edge dimensions (i.e., 1 = (300x300) mm or 2 = (450x450) mm). Meanwhile, the third and fourth symbols show the location distance from the edge of opening to the face of column (i.e., 0 = 0 mm or 2 = 200 mm) and the position of the opening relative to the column

(i.e., F = in front or C = at corner), respectively. The last symbol denotes the distribution scheme of the plastic bubbles (i.e., I = bubbles over the whole area except the zone under column or O = bubbles distributed over the area outside the punching shear critical section only), see Table 1 and Fig. 2.

Table 1. Experimental details of BubbleDeck specimens. Group's number

Slab's symbol

Reference

S0

Reference

B0

I

II

Opening edge dimensions (mm)

Opening position relative to column

Distance of opening from column (mm)

-

-

-

B10FI

300x300

in front

0

B10CI

300x300

at corner

0

B12FI

300x300

in front

200

B20FI

450x450

in front

0

B10FO

300x300

in front

0

B10CO

300x300

in front

0

B12FO

300x300

at corner

200

B20FO

450x450

in front

0

b. Material properties The concrete mix was prepared to fabricate experimental specimens using cement Type I, coarse aggregate of (10) mm maximum size and fine sand. The cement : sand : aggregate proportions by weight were 1 : 1.13 : 2.70 with a water/cement ratio equal to 0.62. To estimate the concrete compressive strength for each experimental BubbleDeck, three concrete cylinders with (150x300) mm dimensions were made and tested according to (ASTM C39-86 2002). The average of three tests was recorded as (fc′ ). All specimens were reinforced with bottom reinforcement layer of (12) mm diameter bars, (fy = 568.12 MPa), and top reinforcement layer of (6) mm diameter bars distributed at (100) mm c/c. All BubbleDecks had no shear reinforcement. The column was reinforced with 4Ø16 mm, (fy = 569.67 MPa), and 2Ø12 mm longitudinal bars. The transverse reinforcement of the column was performed as closed stirrups of Ø10 mm bars, (fy = 623.96 MPa), distributed at (150) mm c/c. The plastic spheres were made by embodying high density polypropylene (HDPE) from recycled plastic with diameter (180) mm, where the minimum distance between bubbles are (20) mm.

Fig. 2 Experimental specimens.

c. Experimental setup All experimental bubbled slabs during testing were simply supported along the four edges. In both directions, supports allowed angular movement in one end and both horizontal and angular movement of the specimen at the other end, and, hence simulated simply supported scheme. All slabs were loaded using one concentrated load applied at midspan. The test was conducted using closed loop ram with (1000) kN capacity actuator, see Fig. 3. The reaction forces were not measured. The vertical deflection was measured at three locations (at center and two at quarter of the span in each direction) using three mechanical dial gauges of (0.01) mm sensitivity. The strain of concrete at compression face was recorded at each loading stage using six electrical strain gauges, type (PL-60-11-5L) of (60) mm base length, which are located at (d) and (2d) from the face of column in diagonal and orthogonal directions. Also, the strain of steel reinforcement was measured using four strain gauges, type (FLA-6-11-5L) of (6) mm base length, which installed at the same locations which mentioned above in both orthogonal directions. Specimens were subjected to monotonically increasing load up to failure using a load control test. All measurements, such as slab deflections, strains in concrete and steel were recorded twice, immediately after the application of the load and after 10 minutes later. Each load stage consisted (20) kN load increment.

Fig. 3 Testing of experimental specimens.

iii.

Experimental results and discussion The primary goal of this study is to determine the ultimate load of the tested specimens and the influence of the size, position, distance of opening relative to column and the location of the hollow plastic spheres relative to the punching shear critical perimeter, respectively. In this study, the service load is considered to be 65% of the ultimate load. The observed cracking and failure loads and the values of deflection at the center of the panel at service load and at failure are shown in Table 2. Obviously, the load carrying capacity of the refernce bubbled slab (B0) was lower than the load capacity of the reference solid slab (S0) by (37%). Also, it was noticed that the creation of an opening in the vicinity of the loaded area (i.e., the column) affected negatively the shear capacity of the BubbleDecks. Therefor, the punching shear strength of bubbled slabs with openings decreased compared to the BubbleDeck slabs without openings (B0). This drop in strength depended on the size, position, and distance of the opening relative to the column, by (29%, 38%, 25% and 46%) for the (B10CI, B10FI, B12FI and B20FI), respectively, when bubbles distributed over the whole area even inside the critical perimeter. Whereas, when bubbles distributed over the area outside the critical section only, this reduction was by (4 %, 15%, 0% and 25%), for the (B10CO, B10FO, B12FO and B20FO), respectively. In specimens with opening, it was noted that, the perimeter of failure zone was decreased compared to corresponding specimen without opening. Accordingly, the failure angle increased. On the other hand, the perimeter of failure zone was increased as the bubbles were shifted out of the critical perimeter. So, the failure angle decreased. Fig. 4 shows the cracks in tension face and inside the transverse edges of the opening and punching shear failure of experimental specimens. For all specimens the first crack appeared around the periphery of the column at the tension face of the slab and inside the transverse edges of the opening. At each loading stage, the cracks were monitored and marked. As the applied load increased, other cracks were formed at the central region of the slab and extended towards the edges of the specimen. At ultimate load, punching shear failure occurred suddenly. It is worth to mention that the cracking load consisted (33%) of the ultimate capacity of the BubbleDeck without opening (B0). On the other hand, the cracking load of BubbleDeck slab with openings, when bubbles distributed over the whole area except the zone under column, reached (29%, 27%, 33% and 23%) of the load carrying capacity of (B10CI, B10FI, B12FI and B20FI), respectively. Meantime, when bubbles distributed over the area outside the critical section only, the cracking load

consisted (30%, 29, 38% 28%) of ultimate load for specimens (B10CO, B10FO, B12FO and B20FO), respectively. The deformability of the BubbleDecks mainly depends on the stiffness of the slab, the arrangement of supports and the type of the applied load. Also, the dual effect of the location of bubbles, relative to the punching shear critical perimeter, and the existance of opening may play major role in droping the overall stiffness of such structural members due to the removal of part of the concrete volume. Therefore, as a result of the reduced stiffness of the BubbleDeck slabs with or without opening, the value of midspan deflection at service and ultimate loads was higher than the deflection the solid slab. Due to the dual effect which mentioned above, the value of deflection for BubbleDecks with opening, when the bubbles covered the entire area except the zone occupied by loaded column, was higher than the deflection of the BubbleDeck specimen without opening (B0), at the same loading level. Accordingly, the deflection increased by (39%, 50%, 16% and 56%) for the (B10CI, B10FI, B12FI and B20FI), respectively, at the service load of each specimens compared to reference specimen (B0). BubbleDecks with opening, when the bubbles covered the area outside the critical perimeter, are existing, almost, under the effect of opening only on droping the overall stiffnes. Mainly, that domainant effect which revealed to the opening presence and has the major influence on the behavior of bubbled slabs. Accordingly, the response of the specimens (B10CO), where the opening was at corner of the column and the bubbles distributed outside the critical perimeter, was identical to the reference specimen (B0). The same character was noticed for specimen (B12FO), where the opening was at a distance of (200) mm from the face of column and the bubbles distributed outside the critical perimeter, was similar to the behavior of the reference specimen (B0). Obviously, the effect of opening, with the same above mentioned characteristics, (i.e., size and location), is equivalent to the presence of bubbles inside the control perimeter. Meanwhile, in comparison with specimen (B0), the deflection decreased by (16% and 29%) for specimens (B10FO and B20FO), respectively. This fact indicates that the major effect on midspan deflection is the location of bubbles relative to the critical section rather that the location and the size of opening.

Fig. 4 Slab failure due to punching shear.

Table 2. Cracking and ultimate loads and their corresponding deflections. Slab's symbol

Concrete compressive Strength (MPa)

Ultimate load 𝑃𝑢.𝑒𝑥 (kN)

Cracking load 𝑃𝑐𝑟 (kN)

𝑃𝑐𝑟 ( ) 𝑃𝑢.𝑒𝑥 × 100%

Reduction of ultimate load (%)

Deflection at ultimate service load (mm)

Deflection at ultimate load (mm)

S0

26.6

760

200

26

-

12

20.4

B0

25.5

480

160

33

0

8.5

16.5

B10CI

26.3

340

100

29

29

9.8

13.7

B10FI

26.3

300

80

27

38

9.4

12.2

B12FI

25.7

360

120

33

25

9.4

13.5

B20FI

27.4

260

60

23

46

7.6

13.0

B10CO

28.7

460

140

30

4

8.6

14.8

B10FO

28.2

410

120

29

15

9.7

14.4

B12FO

29.5

480

180

38

0

9.3

14.7

B20FO

27.2

360

100

28

25

8.6

15.2

The influence of the size, the position and the distance of opening relative to column, on the midspan deflection for all specimens is shown in Fig. 5, Fig.6 and Fig. 7. While, Fig. 8, shows the influence of shifting hollow plastic spheres outside the punching shear critical section. The slope of all curves are almost identical at the beginning of loading, and before the formation of first crack, of all BubbleDecks. Then after the formation of cracks, the deflection increased progressively as the load monotonically increasing up to failure that associated with the increasing of the cracks numbers and the decreasing of the load-deflection curve slope depending on the size, position, distance of opening relative to column and the location of bubbles relative to the critical section.

Fig. 5 Influence of opening size on the load-deflection response.

Fig. 6 Influence of opening position on the loaddeflection response.

Fig. 7 Influence of opening location on the loaddeflection response.

Fig. 8 Influence of shifting bubbles outside the critical section on the load-deflection response.

The load-steel strain relationship, in one direction, at a distance (d) from the face of the column is shown in Fig. 9, Fig. 10 and Fig. 11, where the maximum normal strain in flexural reinforcement was monitored. It can be seen that the BubbleDecks with opening (B10CI, B10FI, B12FI and B20FI), when the distribution of bubbles was inside the critical section, led to increase the steel normal strain at the same loading level by (33%-60%) compared to the reference specimen (B0). That is depending on the size, the position and the distance of opening relative to the column face. Meanwhile, when bubbles distributed outside the critical section for specimens (B10CO, B10FO, B12FO and B20FO), the progress of the steel normal strain was increased by (22%-58%) at the same loading level, compared to reference specimen (B0), see Fig. 12. Also, it was noticed that, for each BubbleDeck, the magnitude of steel normal strain at failure is less than the yield strain (2841×10-6). This fact ensures that the failure in all specimens resulted due to punching shear.

Fig. 9 Influence of opening size on the steel strain.

Fig. 10 Influence of opening position on the steel strain.

Fig. 11 Influence of the opening location on the steel strain.

Fig. 12 Influence of shifting bubbles outside critical section on the steel strain.

The max strain in the extreme concrete fibers at compression face of tested specimens occurred at a distance (d) from face of column. Fig. 13, Fig. 14 and Fig. 15, show the influence of the size, the position and the distance of opening relative to column on the load-concrete strain relationship. It should be noted that for the BubbleDecks with opening (B10CI, B10FI, B12FI and B20FI), the increase of the concrete strain at same loading level was ranged between (46%) and (75%) compared with the reference specimen (B0). Meanwhile, for specimens with opening (B10CO, B10FO, B12FO and B20FO) the increase of the concrete compressive strain at same loading level was ranged between (7%) and (58%) compared with reference specimen (B0), see Fig. 16.

Fig. 13 Influence of opening size on the concrete compressive strain.

Fig. 14 Influence of opening position on the concrete compressive strain.

Fig. 15 Influence of opening location on the concrete compressive strain.

Fig. 16 Influence shifting bubbles outside critical section on the concrete compressive strain.

From the analysis of the experimental results above, the effect of the opening size can be noted on the punching shear strength. So, increasing the size of the opening led to reduce the ultimate strength, and increased the deflection and strain in steel and concrete. Likewise, the distance between the opening edge and the column face affected the punching shear strength that, as closer the opening was, to the column face, as more the reduction was for the ultimate strength, and as more the increasing in midspan deflection and strain in steel and concrete. The arrangement of openings position relative to the column also affected the punching shear strength, where opening located at front of column led to decrease the ultimate strength, increase the deflection and strain in steel and concrete, more than opening located at the corner of column.

iv.

Theoretical analysis In order to determine theoretically the punching shear strength for the BubbleDeck slabs with or without openings, three methods were adopted: The first method for calculation of punching shear strength (𝑷𝒖𝟏 ) is according to the American building code (ACI 318 2014), where the shear capacity of the BubbleDeck slab will be considered equivalent to the shear capacity of the solid slab of the same cross-sectional depth. This assumption can be interpreted by the fact that, the (ACI 318) nominates the punching shear critical perimeter with limited width of (𝒅⁄𝟐) from the face of the loaded area (i.e., the column). This width is very small, in comparison with hollow plastic spheres diameter, especially in slabs with small cross-sectional depth. The second method is according to (BubbleDeck Design Guide 2008) recommendations, where the shear strength (𝑷𝒖𝟐 ) of BubbleDeck slab will be calculated typically as for a solid slab of the same cross-sectional depth using (Eurocode 2 2004) and then the result will be multiplied by a reduction factor of (0.6). The third numerical analysis for determination the shear capacity (𝑷𝒖𝟑 ) of BubbleDecks is according to finite element method using (ANSYS 14.0) software program. To simulate the reinforced concrete BubbleDeck slabs with or without opening, nonlinear materials behavior, as it relates to steel reinforcing bars and concrete, will be considered. Accordingly, eight-node solid element, (Solid65) was used to model the concrete, a (Link 8) was used to model the steel reinforcement and eight-node solid element (Solid45) was used to model the steel plates at the supports and applied load location in the slab models. All specimens were represented, the solid and BubbleDeck slabs with or without openings of the same dimensions, by the software program which created the solid slab, solid spheres and plates as volumes. The model was (2000x2000x230) mm dimensions for solid slab, (300x300x500) mm dimensions for column which located at the center of slab and (300x300x20) mm dimensions for loading plate. Solid spheres with radius of (90) mm were created and then moved to the correct positions inside the block of the solid slab. Then subtract command was used to remove all the solid spheres from the solid slab to form voids in the center of cross-section. In order to model the reinforcement steel in its correct location, the total thickness of the specimens was divided into three layers, where the top and bottom layers were selected to be at the level of steel reinforcement mesh. The boundary conditions need to be applied at points where the supports and loadings exist. For displacement boundary condition, the slab was modeled to be simply supported along four edges. The load was represented in the same approach as applied in the experimental work. The total applied load was equally shared by number of nodes which located at the top end of the column. The comparisons between experimental and theoretical results are given in Table 3. From this table it can be noticed that, the theoretical result of punching shear strength calculated according to the method adopted by (ACI 318 2014) and the finite element method is smaller than the experimental result for solid slab (S0) by (12%) and (3%), respectively. Whereas, for the BubbleDeck slab without openings (B0), the (ACI 318) and the finite element methods overestimated the shear capacity by (28%) and (4%), respectively, while the (BubbleDeck Design Guide 2008) approach underestimated the load capacity by (19%). It should be noted that, the method adopted by the (ACI 318 2014) for predicting shear capacity for solid slabs overestimated the shear capacity for BubbleDecks with opening by (39% - 46%), when the bubbles distributed over the entire area except the zone under column, and (21% - 29%) when bubbles distributed over the area which located outside the critical section only. Obviously, reduction factor multiplier should be suggested to conservatively use the (ACI 318) method for predicting the punching shear capacity for BubbleDecks. This factor should consider the location of bubbles relative to the

critical perimeter. The differences between the experimental and theoretical results of the punching shear capacity for BubbleDeck slabs with opening which calculated according to the (BubbleDeck Design Guide 2008) is ranged between (-5%) and (+3%), when the bubbles distributed over the entire area except the zone occupied by the column. This range of discrepancy explains the suitability of the proposed reduction factor of (0.6) when the bubbles arranged inside the critical perimeter. Meantime, (BubbleDeck Design Guide 2008) recommendations underestimated the shear capacity for BubbleDecks with opening, when the hollow plastic spheres distributed outside the critical section only, by (20% - 32%). This range of discrepancy indicated the large conservativism of the reduction factor of (0.6) when the bubbles arranged outside the critical perimeter. The finite element method simulated the performance of the BubbleDecks with opening in acceptable range of discrepancy, where the difference between the experimental and the theoretical ultimate load is between (-8%) and (+16%).

Table 3. Comparison between experimental and theoretical ultimate punching load. Slab's symbol

Experimental ultimate load 𝑃𝑢.𝑒𝑥 (kN)

Theoretical Ultimate Load 𝑃𝑢1 (kN)

𝑃𝑢.𝑒𝑥 − 𝑃𝑢1 ( ) 𝑃𝑢1 × 100%

𝑃𝑢2 (kN)

𝑃𝑢.𝑒𝑥 − 𝑃𝑢2 ( ) 𝑃𝑢2 × 100%

𝑃𝑢3 (kN)

𝑃𝑢.𝑒𝑥 − 𝑃𝑢3 ( ) 𝑃𝑢3 × 100%

S0

760

681

+12

-

-

735

+3

B0

480

667

-28

403

+19

500

-4

B10CI

340

562

-40

330

+3

305

+11

B10FI

300

508

-41

305

-2

315

-5

B12FI

360

592

-39

349

+3

380

-5

B20FI

260

484

-46

274

-5

225

+16

B10CO

460

613

-25

360

+28

475

-3

B10FO

410

545

-25

327

+25

375

+9

B12FO

480

680

-29

401

+20

520

-8

B20FO

360

453

-21

272

+32

330

+9

v.

Conclusions Bases on the findings of the experimental and theoretical investigation, the following conclusion can be drawn:

1. Experimental results revealed that the load carrying capacity of the BubbleDeck slab without opening, in case the bubbles distributed over the entire area except the zone under column, was lower than the load capacity of the solid slab by (37%). 2. The creation of an opening in the vicinity of the loaded area, (i.e., the column), affected negatively the shear capacity of the BubbleDecks, led to decrease the ultimate load compared to the BubbleDeck slab without openings by (25%-46%), in case bubbles distributed over the whole area even inside the critical section and by (0%-25%), in case bubbles distributed outside the critical section only. This drop in strength depended on the size, the position, and the distance of the opening relative to the column. 3. The cracking load performed (33%) of the ultimate capacity of the BubbleDeck without opening. On the other hand, for specimens with opening, where the bubbles covered the entire area except the zone under column, the cracking load reached (23%-33%) of the failure load. Meanwhile, for specimens with opening, where the bubbles covered the area outside the critical perimeter only, the cracking load consisted (28%-38%) of the load carrying capacity. 4. The deformability of the BubbleDecks depends on the stiffness of the slab, the arrangement of supports and the type of the applied load. Also, the dual effect of the location of bubbles, relative to the punching shear critical perimeter, and the existance of opening may play major role in droping the overall stiffness of such structural members due to the removal of part of the concrete volume. 5. The creation of an opening in BubbleDecks led to decrease the cracking resistance and increase the number of cracks. Accordingly, the deflection at service load increased by (16%-56%), in case bubbles distributed over the whole area even inside the critical section. 6. For the BubbleDecks with opening, when the bubbles covered the area outside the critical perimeter, especially when the opening was at the corner of the column or at a distance from the face of column, the effect of opening with the same above mentioned characteristics, (i.e., size and location), is equivalent to the presence of bubbles inside the control perimeter. Meanwhile,when opening was created in front and adjacent to the column, the deflection decreased by (16% -29%). This fact indicates that the major effect on midspan deflection is the location of bubbles relative to the critical section rather that the location and the size of opening.. 7. In BubbleDecks with opening, when the location of bubbles was inside the critical section, the normal strain was increased at ultimate service load by (33%-60%) and (46%-75%), for steel in tension zone and concrete in compression zone, respectively, compared with BubbleDeck slab without openings. Meanwhile, in BubbleDecks with opening when the location of bubble was outside the critical section, the normal strain was increased at ultimate service load by (22%-58%) and (7%-58%) for steel in tension zone and concrete in compression zone, respectively, compared with reference BubbleDeck. 8. In BubbleDeck slabs with opening the perimeter of failure zone decreased compared to BubbleDecks without opening. Accordingly, the failure angle increased. On other hand, the perimeter of failure zone increased as the hollow plastic spheres were shifted out of critical section. Accordingly, the failure angle decreased.

9. Increasing the size of the opening led to reduce the ultimate strength, and increased the deflection and strain in steel and concrete. Also, the distance between the opening edge and the face of column affected the punching shear strength. As closer the opening was to the column face as more the reduction was for the ultimate strength and as more the increase of midspan deflection and strain in steel and concrete. 10. The arrangement of opening position relative to the column also affected the punching shear strength. Opening located at front of column led to decrease the ultimate strength, increase the deflection and strain in steel and concrete more than opening located at the corner of column. 11. The theoretical result of punching shear strength calculated according to the method adopted by (ACI 318 2014) and the finite element method is smaller than the experimental result for solid slab by (12%) and (3%), respectively. Whereas, for the BubbleDeck slab without opening, the (ACI 318) and the finite element methods overestimated the shear capacity by (28%) and (4%), respectively, while the (BubbleDeck Design Guide 2008) approach underestimated the load capacity by (19%). 12. The method adopted by the (ACI 318 2014) for predicting shear capacity for solid slab overestimated the shear capacity for BubbleDecks with opening by (39% - 46%), in case bubbles distributed over the entire area except the zone under column, and (21% - 29%) when bubbles distributed over the area which located outside the critical section only. 13. Reduction factor multiplier should be suggested to conservatively use the (ACI 318) method for predicting the punching shear capacity for BubbleDecks. This factor should consider the location of bubbles relative to the critical perimeter. 14. The differences between the experimental and theoretical results of the punching shear capacity for BubbleDeck slabs with opening which calculated according to the (BubbleDeck Design Guide 2008) is ranged between (-5%) and (+3%), in case bubbles distributed over the entire area except the zone occupied by the column. Meantime, (BubbleDeck Design Guide 2008) recommendations underestimated the shear capacity for BubbleDecks with opening, when the hollow plastic spheres distributed outside the critical section only, by (20% - 32%). 15. The range of discrepancy between experimental results and theoretical results, calculated according to the (BubbleDeck Design Guide 2008) recommendations, explains the suitability of the proposed reduction factor of (0.6), when the bubbles arranged inside the critical perimeter, and indicates the large conservativism of this factor, when the bubbles arranged outside the critical perimeter. 16. The finite element method simulated the performance of the BubbleDecks with opening in acceptable range of discrepancy, where the difference between the experimental and the theoretical ultimate load is between (-8%) and (+16%).

References

American Concrete Institute, “Building code requirements for structural concrete (ACI 31814)”, ACI Committee 318, 2014, Michigan. ASTM Designation C39-86, “Compressive strength of cylindrical concrete specimens”, Annual Book of ASTM Standards, American Society for Testing and Materials, 4(4), 2002, Pennsylvania. Borges, L, Melo, G. & Gomes, R., “Punching shear of reinforced concrete flat slab with opening”, ACI Structural Journal, 110(4), 2013, pp. 1-10.

BubbleDeck Design Guide, “BubbleDeck Design Guide for Compliance with BCA Using AS3600 and EC2”, Kyng consulting pty ltd., Ref: DG-V.1.2, 2008, Australia and New Zealand. BubbleDeck UK, “Voided Flat Slab Solutions”, Technical Manual & Documents, 2008, White Lodge. Eurocode 2, “Design of Concrete Structures”, Part1-1, General Rules and Rules for Building, European Committee for Standardization, 2004, Brussels, Belgium. Haidong, Z., “Strengthening of RC Slabs with Opening Using CFRP System”, Ph.D. Thesis, Department of Civil Engineering, National University of Singapore, 2006. Oliveira, D., Gomes, R. & Melo, G., “Punching shear in reinforced concrete flat slab with hole adjacent to the column and moment transfer”, Ibracon Structures and Materials Journal. 7(3), 2014, pp. 414-467. Reshma M. & Binu P., “Punching Shear Strength Development of Bubble deck Slab Using GFRP Stirrups”, Proceedings, International Conference on Emerging Trends in Engineering and Management (ICETEM), 2016, pp. 1-6. Salman, T., “Strengthening Effects Around Opening in Reinforced Concrete Flat Plates Subjected to Concentrated Loads”, Ph.D. Thesis, University of Baghdad, 2013.