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Punching shear in reinforced concrete bubbled slabs: experimental investigation

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Punching shear in reinforced concrete bubbled slabs: experimental investigation

Abstract

The behavior of twelve reinforced concrete bubbled slabs of dimensions (1500×1500 mm) with thickness of (100 or 130 mm) under concentric load was investigated to study the effect of several variables on ultimate load capacity, central deflection of the slab, strains in flexural reinforcement and in concrete and the value of failure angle. The main variables considered are the diameter of plastic bubbles (60 or 90 mm), the concrete compressive strength (30 or 60 MPa) and the position of bubbles (d or 2d) with respect to the column face. The test results showed that, the ultimate load decreased by about (4.41-18%) and (14.7-29.4%) in specimens containing bubbles at (2d) and (d) from the face of the column, respectively compared with the corresponding solid slabs. Also, the ultimate load increased by about (17.6-58.5 %) by using concrete of (60 MPa) compared with specimens with concrete of (30 MPa).
Punching Shear in Reinforced Concrete Bubbled
Slabs: Experimental Investigation
Nazar K. Oukaili 1, Luma F. Husain 2
1 Professor, College of Engineering, University of Baghdad, Iraq;
2 Lecturer, College of Engineering – University of Al- Mustansiriyah, Iraq
Introduction
Flat plates are two-way concrete slabs having uniform depths and
transfer loads directly to supporting columns without the aid of beams
or capitals or drop panels. Flat plates can construct quickly due to their
simple formwork and reinforcing bar arrangements.
Probably the greatest practical problem for these floor systems is
unacceptable punching shear failures around the columns.
Punching shear or two-way shear phenomenon is a localized failure.
It occurs when the column, punches through the slab, and it can be
characterized by the truncated or pyramid failure surface, as shown in
Plate 1.
Plate (1) Punching Shear Failure
One of the most popular solutions to avoid this failure is increasing
the slab thickness, as a result the weight of the structure will be
increased due to the use of large amounts of concrete.
A new solution to reduce the weight of concrete structures and
increase the spans of two-way reinforced concrete slab systems
is referred to BubbleDeck system which was developed in the
1990s in Europe and is gaining popularity and acceptance
worldwide.
The main components of BubbleDeck are reinforcing mesh at
top and bottom, between them there are air bubbles, (hollow
ball), made of recycled plastic. While, lattice girder made of
steel is used between the bubbles. Usually, any grade of concrete
can be used. Generally grade 20 or 30 is used.
Plate (2) BubbleDeck Components
The experimental work of this study consists of a series of tests
carried out on twelve half-scale two-way slab specimens of
dimensions (1500×1500 mm) with total depth of (100 or 130
mm) which were cast and tested in order to evaluate the effect of
several variables on punching shear strength of solid and
bubbled flat plates.
These variables are concrete compressive strength, the slab
thickness, the diameter of the bubbles and the position of
bubbles with respect to the face of the column.
Experimental Program
The diameter of the bubbles used in this study was (60 mm and 90
mm) while the distance between bubbles is (15 mm). The bubbles
were tighten with the flexural reinforcement by using tighten wires.
Plate (3) Specimens with Bubbles
Table 1. Details of the Experimental Specimens
Labeling Type of
Specimen
Total
Thickness
(mm)
Bubble
Diameter
(mm)
Distance
between
Bubbles c/c
(mm)
D/H No. of
Bubbles
Reinforce-ment
Ratio (%)
f c ̍
(MPa)
Location
of
Bubbles
SD1 Solid 100 - - - - 0.73 30 -
SD3 Solid 130 - - - - 1.07 30 -
SD5 Solid 100 - - - - 0.73 60 -
SD7 Solid 130 - - - - 1.07 60 -
BD1 Bubbled 100 60 75 0.6 240 0.73 30 2d
BD3 Bubbled 130 90 105 0.7 128 1.07 30 2d
BD5 Bubbled 100 60 75 0.6 308 0.73 30 d
BD7 Bubbled 130 90 105 0.7 160 1.07 30 d
BD9 Bubbled 100 60 75 0.6 240 0.73 60 2d
BD11 Bubbled 130 90 105 0.7 128 1.07 60 2d
BD13 Bubbled 100 60 75 0.6 308 0.73 60 d
BD15 Bubbled 130 90 105 0.7 160 1.07 60 d
TESTING SETUP
All slabs are simply supported along the four edges. The test was done by
subjecting a universal load on the column. The vertical deflection was
measured at the center and at one third the span at each direction using dial
gages of sensitivity of 0.01mm.
The strain of concrete at compression face and on flexural steel
reinforcement is measured at each load stage using strain gages.
Four strain gages are located at (d and 2d) from the face of the
column on the flexural reinforcement. While six strain gages are
located at (d and 2d) from the face of the column at the
compression face of slab.
Fig. (1) Position of Strain Gages
Plate (4) Testing Setup
TEST OBSERVATIONS
When the load is applied on the slab, the initial cracking of all
tested slabs was first observed in the tension zone of the slab
near one or more of the corners of the column. The first cracking
load initiated at (24.6%-36.8 %) of the ultimate load.
With further loading, cracks increased in number from the
center towards the four edges of the slab.
On the compression face there were no flexural cracks noticed,
only the crack from the column penetration inside the slab.
TESTING OBSERVATIONS
The width of the cracks is measured at each stage of loading.
The cracks are usually increased in number but the width is
constant and its magnitude is about 0.05 mm.
As the load increased and particularly at stages of loading near
ultimate load, the crack width increased and reaches about 1.0
mm. After that sudden failure occurred.
It was observed that, the behavior of bubbled slabs was almost
the same as the behavior of solid slabs. However, the number of
the cracks in bubbled slabs are higher than those noticed in solid
slabs. Plate 5 shows the crack pattern of all the tested specimens.
Plate 5. Crack Pattern
TEST OBSERVATIONS
Table 2. Experimental Results of the Tested Slabs
Slab
designation
Cracking load
(kN)
Ultimate load
(kN)
Deflection at
cracking load
(mm)
Deflection at
ultimate load
(mm)
SD1 55 170 5.43 19.45
SD3 80 250 3,0 17,8
SD5 65 200 4,55 18,7
SD7 95 340 2,93 16,2
BD1 40 140 6,0 24,5
BD3 70 205 3,7 20,45
BD5 40 120 6,73 22,1
BD7 70 190 4,72 20,9
BD9 50 180 5,65 22,1
BD11 80 325 3,7 18,88
BD13 45 170 5,75 20,22
BD15 85 290 4,5 19,75
TEST OBSERVATIONS
Due to the existence of the bubbles, the ultimate loads of bubbled slabs are
lower than that of solid slabs by about (4.41- 29.4 %) as shown in Table 3.
Table 3. Effect of the Existence of the Bubbles
TEST OBSERVATIONS
BubbleDecks
(BD)
Reference slabs (Solid
Decks SD)
Ultimate load
(kN) for BD
Ultimate load
(kN) for SD
Discrepancy
Ratio (%)
BD1 SD1 140 170 17.6
BD3 SD3 205 250 18.0
BD5 SD1 120 170 29.4
BD7 SD3 190 250 25.0
BD9 SD5 180 200 10.0
BD11 SD7 325 340 4.41
BD13 SD5 170 200 15.0
BD15 SD7 290 340 14.7
However, by using high strength concrete, the ultimate strength of the
specimen increased by about (17.6-58.3 %) as shown in Table 4.
Table 4. Effect of Using High Strength Concrete
Slabs with Compressive Strength of Ultimate load
(kN) for (S30)
Ultimate load
(kN) for (S60)
Discrepancy
Ratio (%)
(30 MPa)
(S30)
(60 MPa)
(S60)
SD1 SD5 170 200 17.6
SD3 SD7 250 340 36.0
BD1 BD9 140 180 28.6
BD3 BD11 205 325 58.3
BD5 BD13 120 170 41.7
BD7 BD15 190 290 52.6
Also, when the total thickness increased from 100 mm to 130 mm, an increase
in the total capacity by about (46.4-80.6%) was observed as shown in Table 5,
this is due to the increase in slab overall stiffness. While a decrease in the steel
strain was fixed. Besides, the concrete strain increased.
Table 5. Effect of Increasing the Total Slab Thickness
Slabs with Total Thickness of Ultimate load
(kN) for (H100)
Ultimate load
(kN) for (H130)
Discrepancy
Ratio (%)
100 mm
(H100)
130 mm
(H130)
SD1 SD3 170 250 47.1
SD5 SD7 200 340 70.0
BD1 BD3 140 205 46.4
BD5 BD7 120 190 58.3
BD9 BD11 180 325 80.6
BD13 BD15 170 290 70.6
1. At ultimate the tensile strain of flexural reinforcement and concrete
compressive strain of bubbled slabs is higher than that of solid slabs.
2. The first cracking load initiated at (24.6%-36.8 %) of the ultimate load.
3. There is a reduction in the load capacity of the bubbled slabs compared
with the corresponding solid slabs by about (4.41- 29.4 %).
4. When the total thickness increased from 100 mm to 130 mm, the ultimate
load increased by about (46.4%-80.6%). This is due to the increase in slab
overall stiffness.
5. When the concrete compressive strength increased from 30 to 60 MPa, the
ultimate load increased by about (17.6%-58.5 %).
6. The behavior of bubbled slabs was almost the same as the behavior of solid
slabs. However, the number of the cracks in bubbled slabs are higher than
those noticed in solid slabs.
CONCLUSIONS
The following conclusion can be
drawn:
Thanks
For
Listening
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