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AIN SHAMS UNIVERSITY
FACULTY OF ENGINEERING
STRUCTURAL ENGINEERING DEPARTMENT
FIFTEENTH INTERNATIONAL CONFERENCE ON STRUCTURAL AND GEOTECHNICAL ENGINEERING
Advances In Construction Techniques
PPHC SLABS STRENTHENED WITH DIFFERENT TYPES OF
CONCRETE TOPPINGS
KHALED MOHAMED HEIZA
Professor of Reinforced Concrete Structures , bridges and Vice Dean Faculty of
Engineering, Menoufia University, Egypt
E-mail: khheiza@yahoo.com, khheiza@gmail.com
AHMED NABIL
Assistant professor at Department of Civil Engineering, Faculty of Engineering,
Menoufia University, Egypt
NAGEH MELEKA
Professor of Reinforced Concrete Structures at Department of Civil Engineering,
Faculty of Engineering, Menoufia University, Egypt
ABSTRACT
Usage of precast prestressed concrete hollow core (PPHC) slabs has been extensively spread
out in roofing and flooring systems around the globe due to benefits of mass production and
fast site construction. In this flooring system, prestressed precast hollow core slabs are used
together with or without a cast in place concrete topping. A few research works focused on
studying the effect of using concrete topping on enhancement of the structural integrity of the
PPHC system. This paper presents an experimental study conducted to investigate the effects
of using different types of cast in place (CIP) concrete toppings on the behavior of PPHC
slabs. Ten typical full scale specimens of PPHC slabs were prepared in precast concrete plant
under quality control program; specimens were classified into five groups. Group (1) includes
two control specimens without topping, Group (2) contains two specimens with ordinary
reinforced concrete topping, Group (3) involves two specimens with ordinary reinforced
concrete topping connected to the top surface of the slab using steel anchors, Group (4)
comprises two specimens with fibrous concrete topping, and finally Group (5) includes two
specimens with ferrocement topping. All test specimens were tested under static line loading
until failure. Cracking patterns, failure modes, cracking and ultimate failure moment
capacities, and moment- deflection relationship have been illustrated, discussed, and analyzed
in this study. Adding concrete topping to the PPHC slabs enhances flexural behavior directly
under static line loading, as the cracking moment resistance was increased by about 6% to
31% and failure moment was increased by about 11% to 41% by using concrete toppings.
KEYWORDS
Hollow core slabs; Precast concrete; Prestressed concrete; Fibrous concrete; Ferrocement;
Concrete toppings; Steel anchors.
1 INTRODUCTION
Prestressed concrete hollow-core slabs have been widely used throughout the world in
concrete and steel structures [1, 2]. They were developed in the 1950s when long-line
prestressing techniques evolved with the extrusion method, which allow production of
inexpensive and easy-to-handle PPHC slabs [3]. PPHC slabs can be defined as precast,
prestressed concrete members with continuous voids provided to reduce weight and,
therefore, cost and, as a side benefit, to use for concealed electrical or mechanical runs [4, 5].
They are primarily used in floors and roofs of residential, commercial, industrial and
institutional buildings[6]. PPHC slabs have good fire resistance, sound insulation properties,
and are capable of spanning long spances with relatively shallow depths. Common depths of
prestressed hollow-core slabs range from 150 to 500 mm which can achieve 20 m of span on
roofs [7].
There are a number of situations where it may become necessary to increase the structural
performance of concrete members either to incorporate and satisfy the codes modifications or
to allow for any change of use and the associated changes in superimposed loads [8,9]. In
recent years, the cement-base bonded overlay technique has been used to strengthen and
enhance the structural performance of precast concrete slabs by adding a thin layer of cast in-
situ reinforced concrete to the existing slab[10]. The primary purpose of this technique in
PPHC slabs is to overcome the camber caused by prestressing, which results in an uneven
floor surface. Besides creation of a semi-rigid diaphragm that connects hollow core slabs
units together. This concrete layer consequently improves the load-carrying capacity and
stiffness of the slab by increasing its thickness [11, 12, 13, 14]. The main purpose of the
current study is identifying the extent to which different types of CIP concrete topping placed
over surface of precast concrete hollow-core units improves the flexural behavior of the slabs.
Experimental study for the effect of concrete topping for PPHC were performed in references
[15 through 18]
2 EXPERIMENTAL PROGRAM
2.1 Test specimens
The aim of experimental work carried out in this study is to investigate the effect of using
different types of structural concrete topping (screed) with and without using connecting steel
anchors on structural behavior of PPHC slabs subjected to static line loads. For this purpose,
ten full scale PPHC slabs were prepared. All specimens have a length of 4,100 mm, a depth
of 160 mm and a width of 1,200 mm. Experimental program was done in two stages: first
stage in which specimens were prepared, was carried out at Modern Concrete factory and
lab facility at Sadat city (local precast manufacturer), while loading and tests were at the
second stage, which was developed at the Prof. Mounir Hussein's Reinforced Concrete
laboratory of the Civil Engineering Department of Menoufiya University. The test specimens
classifications and naming convention are listed as shown in Table 1.
Geometry, cross section, and details of test groups are illustrated in Figs. 1 - 5.
Fig. 1: Details for control specimen of
Group 1
Table 1: Paper size and margins
Group
Slab Code
Concrete topping
Steel Anchors
Group 1
C1
C2
No topping
No anchors
Group 2
TRC1
TRC2
Ordinary RC topping
No anchors
Group 3
TRCA1
Ordinary RC topping
Anchors at shear span
TRCA2
Ordinary RC topping
Anchors at full span
Group 4
TSFC1
Steel fiber concrete topping
No anchors
TGFC2
Glass fiber concrete topping
No anchors
Group 5
TFERO1
TFERO2
Ferrocement topping
No anchors
Fig. 2: PPHC slabs with ordinary RC
toppings without steel anchors - Group 2
Fig. 3 PPHC slabs with ordinary RC
toppings with steel anchors - Group 3
2.2 Material properties
2.2.1 Concrete materials
a. Ordinary concrete
Unit weight of concrete used is 24 kN/m3. Concrete used for toppings in Groups 2 and 3 are
normal strength concrete with cube compressive strength of 25 MPa.
b. Fibrous concrete
Two types of fibrous concrete are used as toppings, glass fiber and steel fiber concrete. 6 mm
length measuring 13 microns in diameter monofilament glass fibers chopped from type E of
glass are used to produce glass fiber concrete. The fibers are extremely fine, single filaments;
fibers are coated with Silane based to improve initial dispersion and bond. 0.26% of cement
weight was added to the mixture according to manufacturer bulletin [15].
Steel fiber concrete was prepared using high tensile undulated steel fibers made of cold
drawn wire. Corrugated steel fibers are 55 mm in length and 0.8 mm diameter with aspect
ratio l/d: 68.75, fibers wire tensile strength is 1000 N/mm². 5.7% of cement weight was added
to the mixture according to manufacturer data sheet [16].
c. Ferrocement
The ferrocement laminates are reinforced using steel meshes locally produced and available
in the market on commercial scale. Mesh grid size is 15x15 mm with 1 mm wire diameter.
For concrete mortar, water/cement ratio used was 0.4, and the selected sand/cement ratio was
2.0.
2.2.2. Reinforcement materials
Nine uncoated bright steel 7-wire P.C. strand (9.3 mm nominal diameter with nominal area of
52 mm2) low- relaxation strands were used. Average ultimate tensile strength was found to
be 1,860 MPa and modulus of elasticity was 200 GPa respectively.
Physical and mechanical properties for used materials are listed in Tables 2 and 3.
Fig. 4: Details of PPHC slab with fibrous
concrete toppings - Group 4
Fig. 5: Details of PPHC slab with
ferrocement toppings - Group 5
25
Welded
2.3 Test Setup
Test specimens are loaded statically using two line loads. The load was applied on the slab
using two similar-sized steel I-sections in the loading frame, at approximately third points.
The hollow-core slabs are supported on two stiff steel I-sections. The distance from the center
line of the support to the end of PPHC slab is 5 cm giving a clear span of 4.0 m. A 500 kN
hydraulic jack was used to apply the load gradually with a constant increment. The complete
detailed setup for testing of the hollow-core slabs is shown in Fig. 6.
2.4 Measuring Devices
The deflections at the center of the slabs and the center line of applied load were measured by
three mechanical and electrical dial gauges (25 and 50 mm) attached to the bottom of the
slabs. Demec points were attached to the concrete surface to measure concrete strain. The
slabs were loaded gradually (5 kN per increment) until failure .The cracking load, failure
progression and the cracks developed in the slab were recorded. The testing equipment and
the test setup are shown in Figs.7 and 8.
Topping
Material
Thickness
(mm)
Compressive
strength of topping
concrete (MPa)
(MPa)
Ordinary
concrete
50
25
Steel fiber
concrete
50
32
Glass fiber
concrete
50
36
Ferrocement
25
25
Reinforcement
Material
Yield
stress
(MPa)
Ultimate
strength
(MPa)
Diameter
(mm)
Mesh
spacing
(mm)
Prestressing
strands
-----
1860
9.3
----
10 mm welded
mesh
360
520
10
200
Steel anchors
240
350
8
----
Welded wire
mesh
Proofing
stress
450
700
1
15
Table 2: Topping materials properties
Table 3: Reinforcement materials properties
Fig.6: 3D illustration for loading
setup
Load transfer
beams
Dial gauges
PPHC
Test specimen
500 kN
hydraulic
jack
2000 kN loading
frame
2.5 Results and discussion
2.5.1 Cracking and failure loads
Figure 9 shows comparison between cracking and failure loads for the tested specimens.
Cracking load was recorded upon emersion of first crack in slab soffit. Whereas failure load
was determined as slab resistance to load decreased significantly. Cracks propagation in all
tested slabs followed the similar conventional flexural patterns in simply supported slabs.
Initiation of flexural cracks on PPHC specimens was observed to occur at middle third of the
span directly below the applied load. As the loading increased, new cracks were formed on
either side of the loading point. Control specimens C1 and C2 were first cracked on 80 kN
with failure loads of 135 kN for both specimens.
Applying traditional concrete topping in Group 2 leads to a direct enhancement in load
carrying capacity and consequently improvement in flexural resistance of the slab. Cracking
load resistance increased to reach 95 kN with a gain of about 19 %. The increase in failure
load for the same group was about11% as the failure load reached 150 kN.
In the third group which includes traditional concrete topping in addition to steel anchors, the
improvement in cracking capacities were about 25%, and 31% for specimens TRCA1 and
TRCA2 respectively as the cracking load touches 100 kN, and 105 kN. Whilst raise in failure
loads were about 26%, and 41% for loads 170kN and 190 kN respectively.
In the fourth group, in which two types of fibrous concrete topping were used, the cracking
loads were 95 kN and 90 kN for TSFC1 and TGFC2 with a total gain of about 19% and 13%
respectively. On the other hand, the failure loads for this group were 180 kN and 170 kN with
about 33% and 26% enhancement in ultimate capacities for TSFC1 and TGFC2
respectively.
In The last group which includes hollow core slabs with ferrocement topping, the cracking
load was 85 kN for both TFERO1 and TFERO2 with a slight gain of about 6%, on the
contrary the failure loads reached 175 kN and 180 kN with about 30% and 33%
enhancement in ultimate capacity. Figure 10 also shows cracking and failure moments for the
all tested specimens.
4.10 m
25 mm Mechanical
Dial Gauge
500 kN
Hydraulic Jack
2000 kN
Loading
Frame
Load
Transfer
Beam
25 mm Mechanical
Dial Gauge
Demec Points
50 mm Digital
Dial Gauge
PPHC slab
1.33m
1.33m
1.33m
0.05
0.05
Fig.7: Schematic of test setup and
instrumentations
Fig.8: Test setup and instrumentation at lab
Support beam
Load transfer beam
500 kN Hydraulic Jack
2000 kN Loading Frame
50 mm Digital
Dial Gauge
25 mm Mechanical
Dial Gauge
25 mm Mechanical
Dial Gauge
2.5.2 Failure modes and crack patterns
The hollow core slabs may fail by many modes, flexural failure modes may be represented by
concrete cracking at top due to prestress transfer, concrete cracking at bottom, and rupture of
prestressing strands, crushing of concrete at top or excessive deflection under loads. While
shear failure modes may be seen by bond slip failure of strand, flexural shear failure, or web
shear failure [2,17,18], see Fig. 11.
In this study, flexural and shear modes were observed in the tested specimens as concrete
cracking at bottom, flexural shear failure, and web shear failure. Crack patterns and some
failure modes are presented in Figs.12 - 19.
Fig. 9: Comparison of cracking and failure
loads for all tested PPHC specimens
Fig. 10: Comparison of cracking and failure
moment for all tested PPHC specimens
Fig.11: Flexural, flexure-shear, and
web-shear cracks in HCS [17]
Fig.12: Crack patterns, crack distribution
and failure mode for control specimen C1
Fig.13: Crack patterns, crack distribution
and failure mode for specimen TRC1
2.5.3. Moment- deflection behavior
Observation of initial flexural cracks on PPHC slabs was occurred at slabs mid span directly
below the applied loads. When the cracks were visually detected for the first time, they
usually extended from bottom into almost half of the depth of PPHC slab. As the loading
continued, new cracks appeared on either side of the loading point. Cracking was reflected
through a change in slope of the load–deflection curve, after concrete cracking; strands
started resisting the applied load until specimen failure. A typical under-reinforced behavior
for bending stresses was clearly noticed on moment deflection curves for tested specimens.
The moments versus mid-span deflections responses of the PPHC slabs are listed hereunder
in the following subsections.
Fig.14: Crack patterns, crack distribution
and failure mode for specimen TRCA1
Fig.15: Crack patterns, crack distribution
and failure mode for specimen TRCA2
Fig.16: Crack patterns, crack distribution
and failure mode for specimen TSFC1
Fig.17: Crack patterns, crack distribution
and failure mode for specimen TGFC2
Fig.18: Crack patterns, crack distribution
and failure mode for specimen TFERO1
Fig. 19: Some cracks modes and slippage of
concrete toppings in tested
specimens
Fig. 20: Moment-deflection curves at mid-span
for specimens TRC1, TRC2 in Group 2
a. Effect of using traditional reinforced concrete toppings without steel anchors
Figure 20 shows moment deflection curves for the third group; in which traditional reinforced
concrete toppings were used, results were compared to the control specimens. In pre-cracking
and post cracking stage deflection behavior shows lower deformability and higher stiffness.
In pre-cracking stage, the highest deflection was 8.21 mm for C2 at a cracking moment of
about 107 kN.m, Specimens TRC1 and TRC2 show deflections of 8.41 and 9.24mms
respectively at cracking moments of about 53 kN.m for both specimens. In the post cracking
stage, the highest deflection was 57.78 mm for C2 at a failure moment of 90 kN.m,
Specimens TRC1 and TRC2 show deflections of 72.48 and 72.4 mms respectively at a failure
moment of 100 kN.m for both units.
b. Effect of using traditional reinforced concrete toppings with steel anchors
Figure 21 shows moment deflection curves for the third group; in which traditional reinforced
concrete toppings were used in addition to steel anchors located in the shear span for
specimen TRCA1 and in full span in specimen TRCA2. These results were compared to the
control specimens. In pre-cracking and post cracking stage, deflection behavior shows
excellent performance as there was full composite action till failure in TRCA2, while
specimen TRCA1 shows composite action only in pre-cracking stage. Both units reflected
lower deformability and higher stiffness. In pre-cracking stage the highest deflection was
8.21 mm for C2 at a cracking moment of about 53 kN.m, Specimens TRCA1 and TRCA2
showed deflections of 6.72 and 10.58mms at cracking moments of about 53 kN.m and 70
kN.m respectively. In the post cracking stage, the highest deflection was 57.78 mm for C2 at
a failure moment of 90 kN.m, Specimens TRCA1 and TRCA2 show deflections of 85.17 and
71.5 mms at a failure moments of about 113 kN.m and 127 kN.m respectively.
c. Effect of using fibrous concrete toppings
Moment–deflection curves for the fourth group are shown in Fig. 22. Two types of fibrous
concrete toppings were used in this group. The results were compared with the control
specimen. In pre-cracking and post cracking stages, deflection behavior showed lower
deformability and higher stiffness. In pre-cracking stage, the highest deflection was 8.21 mm
for C2 at a cracking moment of about 53 kN.m, specimens TGFC2 and TSFC1 showed
deflections of 9.5 and 9.29 mms at cracking moments of about 60 kN.m and 63 kN.m
respectively. In the post cracking stage, the highest deflection was 57.78 mm for C2 at a
failure moment of 90 kN.m, specimens TGFC2 and TSFC1 showed deflections of 95.7 and
127.5 mms at a failure moments of about 113 kN.m and 127 kN.m respectively.
d. Effect of using ferrocement toppings
Ferrocement concrete toppings were used in Group 5 as a thin topping to enhance flexural
capacity of PPHC slabs; Fig. 23 shows moment deflection curves for PPHC slabs with
ferrocement topping. The results of this group were compared with the control specimen.
Both specimens reflected lower deformability and higher stiffness. In pre-cracking stage, the
highest deflection was 8.21 mm for C2 at a cracking moment of about 53 kN.m, Specimens
TFERO1 and TFERO2 showed deflections of 13.55 and 13.88 mms at cracking moments of
about 57 kN.m for both.
Fig. 21 : Moment-deflection curves at mid-span
for specimens TRCA1, TRCA2 in Group 3
Fig. 22: Moment-deflection curves at mid-
span for specimens TSFC1, TGFC2 in Group
4
In the post cracking stage, the highest deflection was 57.78 mm for C2 at a failure moment of
90 kN.m, specimens TFERO1 and TFERO2 showed deflections of 99.32 and 89.92 mms at a
failure moments of about 120 kN.m and 117 kN.m respectively.
Figure 24 shows comparison between moment deflection curves for second and third groups
compared to control specimens. Concrete topping with steel anchors in the third group shows
the best performance in stiffness, deformability, cracking resistance and failure load as shown
below.
Comparison between moment deflection curves for fourth and fifth groups is shown in Fig.
25 which shows better performance in deformability and higher cracking moment resistance
for fibrous concrete .The two groups are equal in failure loads. Finally, moment deflection
curves for all tested specimens are shown in Fig.26.
Fig. 24: Comparison between
momentdeflection curves at mid span for
Groups 2 and 3
Fig. 25: Comparison between moment-
deflection curves at mid span for
Groups 4 and 5
Fig. 23: Moment-deflection curves at
mid-span for specimens TFERO1,
TFERO2 in Group 5
Fig. 26: Comparison between moment-
deflection curves at mid span for all
groups
3 CONCLUSIONS
Full scale tests for ten PPHC slabs were carried out to investigate the extent to which a
concrete topping placed over the top surface improves the structural behavior of the slabs.
The following conclusions can be drawn:
1. Adding different types concrete topping to the PPHC slabs enhances flexural behavior
directly under static line loading, as the cracking moment resistance was increased by
about 6% to 31% and failure moment was increased by about 11% to 41% by using
concrete toppings.
2. Using steel anchors to connect the concrete toppings to PPHC slabs has great effects
on slab performance, it leads to less deformability and higher stiffness as it prevents
slippage between PPHC slab and concrete topping especially for case of full span
anchors. Deflection at failure for specimens without using anchors was about 72 mm
at failure load of 150 kN, while deflection for the same load in case of using full span
anchors was about 31 mm. Full composite action was occurred by using steel anchors
in full span rather than shear span. Cracking moment and ultimate failure moment
capacities were enhanced by about 10% and 27% for full span anchors compared with
specimens using RC toppings without anchors.
3. Fibrous concrete topping without using any internal conventional reinforcement
achieved higher cracking resistance, ultimate resistance with less deformability and
higher stiffness. Cracking moment and ultimate failure moment capacities were
enhanced by about 12% and 26% for glass fiber topping and 19% and 33% for steel
fiber topping respectively compared with control specimens.
4. Reducing total slab thickness and load by using thin layer of ferrocement topping
was also an effective technique, as it enhances the ultimate moment capacity
considerably. Cracking moment capacity enhanced by about 6% and ultimate moment
capacity improved by about 33%.
4 ACKNOWLEDGMENTS
The authors wish to express their special thanks to Modern Concrete Company for their
technical and financial support.
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