Open Journal of Civil Engineering, 2013, 3, 26-32
http://dx.doi.org/10.4236/ojce.2013.33B005 Published Online September 2013 (http://www.scirp.org/journal/ojce)
Copyright © 2013 SciRes. OJCE
Experiment al Behavior of Partially Prestressed High
Strength Concrete Beams
Shady H. Salem, Khalid M. Hilal, Tarek K. Hassan, Ahmed S. Essawy
Department of Structural Engineering, Faculty of Engineering, Ain Shams University, Cai ro, Egypt
Email: shady.salem@bue.edu.eg
Received July 2013
ABSTRACT
In the last few decades, prestressed concrete has been rapidly used in bridge engineering due to the enormous develop-
ment in the construction techniques and the increasing need for long span bridges. High strength concrete has been also
more widely spread than the past. It currently becomes more desirable as it has better mechanical properties and dura-
bility performance. Major defect of fully prestressed concrete is its low ductility; it may produce less alarming signs
than ordinary reinforced concrete via smaller deflection and limited cr acking. Therefore, partially prestressing is consi-
dered an intermediate design between the two extremes. So, combining high strength concrete with partial prestressing
will result in a considerable development in the use of prestressed concrete structures regarding the economical and
durability view points. This study presents the results of seven partially prestressed high strength concrete beams in
flexure. The tested beams are used to investigate the influence of concrete compressive strength, prestressing steel ratio
and flange width on the behavior of partially prestressed beams. The experimentally observed behaviors of all beams
were presented in terms of the cracking load, ultimate load, deflection, cracking behavior and failure modes.
Keywords: Partially Prestressed; High Strength Concrete Beams; Serviceability Behavio r; Failure Modes
1. Introduction
In the last few decades, prestressed concrete has been ra-
pidly used in many fields of structural engineering, espe-
cially in the field of bridge engineering due to the enor-
mous development in the construction techniques and the
increasing n eed for long span bridg es. Now more than 50
percent of bridges all over the world are constructed us-
ing prestressing techniques [1]. High strength concrete
has been also more widely spread than the past due to the
enormous development in the material technology and
the greater demand for high strength concrete which
leads to a better quality control for concrete [2]. There-
fore, it currently becomes more desirable due to its better
mechanical properties and durability performance.
Fully prestressed concrete may result in a less signifi-
cant camber at service load than the specified design one.
The camber can also increase due to creep of the con-
crete with respect to time [3]. Another major defect of
fully prestressed concrete is its low ductility, where it is
stiffer than the ordinary reinforced concrete so it may
produce less alarming signs than ordinary reinforced
concrete via smaller deflection and limited cracking [4].
Therefore, partially prestressing is considered an inter-
mediate design between the two extremes [3]. For these
reasons, partially prestressed became a desirable struc-
tural solution worldwide. High strength concrete was
also believed to have lower flexural ductility than ordi-
nary concrete. However, some researches had proved that
the ductility of the high strength concrete was still in-
creasing when flexural stresses were applied [5]. So,
combining high strength concrete with partial prestress-
ing will result in a considerable development in the use
of prestressed concrete structures regarding the econom-
ical and durability view points.
This paper presents an experimental investigation to
assess the behavior of partially prestressed high strength
concrete beams. The effects of various parameters have
been investigat ed. These parameters ar e the concrete com -
pressive strength, prestressing steel ratio and the concrete
flange width. In the remaining sections of this paper, the
experimental program will be thoroughly discussed. The
results will be enumerated and comparisons to highlight
behavior characteristics will be presented.
2. Experimental Program
2.1. Specimens’ Details
The experimental program was conducted on seven par-
tially prestressed concrete beams with total length of
4800 mm. The beams were simply supported with 4500
mm clear span and 150 mm projection at each end. All
S. H. SALEM ET AL.
Copyright © 2013 SciRes. OJCE
27
the beams were 150 mm wide and 250 mm deep for all
the five rectangular sections. The web of the T-section
specimens was similar to the rectangular ones while
flange widths were 350 and 550 mm. All the beams were
reinforced using closed stirrups of 10 mm diameter with
spacing 100 mm for the first 1400 mm and 200 mm for
the rest of the beam. Also all the beams were reinforced
using two ordinary longitudinal reinforcement of 10 mm
diameter as a top and bottom reinforcement with 25 mm
cover. The end zone was reinforced by extra stirrups for
the first 300 mm with stirrup spacing of 50 mm. Three
square stirrups were used around each tendon to act as
anti-bursting reinforcement. Figure 1 shows the speci-
men’s typical reinforcement details while Table 1 gives
the specimen details.
2.2. Material Properties
Three different concrete mixes where designed so as to
reach 40, 70, 90 MPa cubic compressive strength at 28
days. Numerous trial batches have been developed to
reach the required strength. Finally, the concrete mix-
tures were produced at a concrete central batch plant.
The average 28-days cube compressive strength for the
three batches were 42, 95 and 114 MPa. The prestressing
strands of 1860 N/mm2 tensile strength, 1670 MPa yield
strength and 200,000 MPa elastic modulus were used.
All conventional reinforcement whether the longitudinal
or transverse reinforcement were also of nominal tensile
strength of 400 MPa.
2.3. Instrumentation
Figu re 2 illustrates the instrumentation u sed with the test
setup. Four linear variable displacement transducers
(LVDT) were used to monitor the deflection across the
length of the beams. The concrete top surface strain and
the bottom reinforcement strains were measured using
electrical strain gauges. All the readings of the LVDT’s
and the strain gauges were automatically recorded via a
data acquisition system at each loading step using Lab-
View program. The crack propagation was also moni-
tored during the test up to failure.
2.4. Test Setup and Loading Procedure
The beams were tested under quasi-static load using four
point loading scheme. The applied loads were 1400 mm
far from the supports leaving 1800 mm of a constant mo-
ment zone. Figure 3 shows a photograph of the test setup.
The load was applied up to 80% of the calculated crack-
ing load then released. This cycle is repeated one more
time. After that, another two cycles were achieved by
reaching the first visible crack(s) then releasing. Finally
the load was reapplied incrementally to failure. The aim
of the first two cycles was to ensure the behavior lineari-
ty before cracking, while the other two cycles were to
study the behavior of the beams after stiffness reduction
due to cracking. Figure 4 illustrates the schematic load-
ing path for the concrete beams.
3. Experimental Results
3.1. Test Observations
The cracking behavior of the tested partially prestressed
beams was examined within the constant moment zone.
Generally, it was observed that all cracks initiated at the
Figure 1. Specimen’s typical reinforcement details.
Table 1. Specimen detail s.
Specimen
Code
Cross Section
Type
Number of
Prestressing Steel Cables
Prestressing Steel
Ratio (%)
Concrete Compressive
Strength (MPa)
Study
Parameter
R-0.264-70 R 1 (12 mm) 0.264 85 Control
R-0.264-40 R 1 (12 mm) 0.264 47 Concrete compressive strength
R-0.264-90 R 1 (12 mm) 0.264 101 Concrete compressive strength
R-0.373-70 R 1 (12 mm) 0.373 85 Prestressing reinforcement ratio
R-0.528-70 R 2 (14 mm) 0.528 85 Prestressing reinforcement ratio
T-0.264-70* T 1 (12 mm) 0.264 85 Flange width (b = 350)
T-0.264-70** T 1 0.264 85 Flange width (b = 550)
S. H. SALEM ET AL.
Copyright © 2013 SciRes. OJCE
28
Figure 2. Instrumentation used to monitor the behavior during testing.
Figure 3. Test setup.
Time
Ultimate
load
80% of the
Cracking load
First visable
Cracking load
Figure 4. Schematic loading path for the concrete beams.
constant moment zone started perpendicular to the center
line of the beam. It was also observed that the stirrups
acted as crack initiators for most of the flexural cracks.
On the other hand, Table 2 shows cracking load (Pcr),
ultimate load (Pult) and their corresponding mid-span
deflection in addition to the failure mode for each of the
tested beam. Figure 5, however, depicts photograph of a
typical concrete crushing failure mode.
3.2. Load-Deflection Behavior
It was noticed that the load-deflection re sponse of all the
tested beams acted linearly during the loading and un-
loading of the first two cycles prior cracking, while it
appeared in a parabolic manner after cracking which
might be encountered due to the non-linear behavior of
the stress-strain behavior of the prestressing strands [1].
Table 2. Beam capacities and mode of failure.
Specimen
code Pcr
(MPa) cr
(mm) Pult
(MPa) ult
(mm) Failure
mode
R-0.264-70 26.9 6.7 61.3 80.2 Concrete
crushing
R-0.264-40 24.9 10.5 59.7 109.4 Concrete
crushing
R-0.264-90 27.4 5.7 65.2 127.0 Concrete
crushing
R-0.373-70 30.4 7.4 73.7 65.3 Concrete
crushing
R-0.528-70 35.2 9.5 84.6 77.8 Concrete
crushing
T-0.264-70* 23.6 4.4 67.8 146.6 Concrete
crushing
T-0.264-70** 35.1 4.7 60.1 58.1 Prestressing
rupture+
+ The load ing stopped at a safe load level bas ed on the u ltimate load c alcu-
lations before expected prestressing strand rupture
Figure 5. Typical compression failure due to concrete crush-
ing at the face of applied load.
Figure 6 shows the typical load versus central deflection
behavior encountered in the prestressed beams during the
first four cycles while Figure 7 illustrates the effect of
the studied parameters on the load-deflection behavior.
From the given graphs in Figure 7, it can be concluded
that concrete compressive strength significantly affects
the deflection behavior prior to cracking. It should also
be noted that after cracking, high strength concrete exerts
a slightly higher stiffness than normal strength concrete
S. H. SALEM ET AL.
Copyright © 2013 SciRes. OJCE
29
0.0
5.0
10.0
15.0
20.0
25.0
30.0
35.0
0.05.0 10.0
Load (kN)
Deflection (mm)
Fir st C ycle
Seco nd Cycl e
Thir d Cy cl e
Fo urth Cy cl e
Figure 6. Magnified load-deflection behavior at the first
four cycles for beam (R-0.264-70).
0.0
10.0
20.0
30.0
40.0
50.0
60.0
70.0
-50.0 0.0 50.0100.0 150.0
Load (kN)
Deflection (mm)
R-0.264-70
R-0.264-40
R-0.264-90
(a)
0.0
10.0
20.0
30.0
40.0
50.0
60.0
70.0
80.0
90.0
-20.0 0.0 20.0 40.0 60.0 80.0100.0
Load (kN)
Deflection (mm)
R-0.264-70
R-0.373-70
R-0.528-70
(b)
-10.0
0.0
10.0
20.0
30.0
40.0
50.0
60.0
70.0
80.0
-50.0 0.0 50.0100.0 150.0 200.0
Load (kN)
Deflection (mm)
R-0.264-70
T-0.264-70*
T-0.264-70**
(c)
Figure 7. Load-deflection behavior for the tested beams
under several studied parameters; (a) Beam with different
concrete compressive strength; (b) Beam with different
prestressing steel ratio; (c) Beam with different compres-
sion flange width.
which agrees with other researchers who studied the be-
havior of high strength concrete beams subjected to pure
flexure with no axial load [6]. Increas ing the prestres sing
steel ratios didn’t increase the beams’ stiffness prior to
cracking but significantly increase its’ stiffness after
cracking. As the flange width increase, the stiffness of
the tested beams prior to cracking increase due to the
change in the gross inertia, while after cracking, the ef-
fect of the flange width was not concluded as the loading
of the largest flange width stopped before failure so as
not to reach the prestressing strand rapture. But it can be
observed that the flange width slightly influence the load-
deflection response after cracking.
3.3. Crack Pattern
The cracking behavior was observed along the constant
moment zone at different loading stages from first
cracking to ultimate load. Figure 8 shows the propaga-
tion of the average crack width with respect to the ratio
of applied to ultimate load. From these graphs, the effect
of the studied parameters can be easily detected. As the
concrete compressive strength increases, the average
crack width increases at the same load level and vice
versa. It was also observed that the effect of prestressing
reinforcement ratio on the average crack width is neglig-
ible at the same normalized load level (if the load level
was measured as a ratio of the ultimate load for each
beam). In other words, at 50% of the ultimate load for
any beam, the average crack widths were approximately
equal but as the prestressing steel ratio increases the ul-
timate load increases. So at a certain load, as the pre-
stressing steel ratio increases the average crack width
decreases. It was also observed that the average crack
width increase as the compression flange width increase.
This finding can be encored to the higher location of the
neutral axis so as to balance with the tension force in the
reinfor cement at a given top load level. Figure 9 shows
the average crack propagation height with respect to the
ratio of applied to ultimate load. It was observed that the
concrete compressive strength and the effective flange
width affect the average the crack height the same way of
affecting the crack width, which could be a result of the
slight increase of the neutral axis above beam bottom
fibers for high strength concrete compared to the higher
increase in the normal strength concrete. On the other
side, the prestressing steel ratio was directly proportional
to the average crack height. Figure 10 shows the relation
between the average crack spacing versus the ratio of
applied to ultimate load. From this figure, it could be
concluded that at early loading stages, the concrete com-
pressive strength is in a directly proportional relation
with the average crack spacing while it become of insig-
nificant effect at the final loading stages. It was also
S. H. SALEM ET AL.
Copyright © 2013 SciRes. OJCE
30
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
00.5 11.5
Average crack width (mm)
P
applied
/P
ultimate
R-0.264-70
R-0.264-40
R-0.264-90
(a)
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
00.5 11.5
P
applied
/P
ultimate
R-0.264-70
R-0.373-70
R-0.528-70
(b)
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
00.5 11.5
P
applied
/P
ultimate
R-0.264-70
T-0.264-70*
T-0.264-70**
(c)
Figure 8. Average crack width at different loading levels; (a)
Beam with different concrete compressive strength; (b)
Beam with different prestressing steel ratio; (c) Beam with
different compression flange width.
concluded that as the prestressing steel ratio increas es th e
average crack spacing increases. Finally, the average
crack width is directly proportional to the flange width.
In other words, as the compressive flange width increases,
the cracks tend to widen and propagate rather than form-
ing new cra c ks.
0
5
10
15
20
25
00.511.5
Av er age crack height (mm)
P
applied
/P
ultimate
R-0.264-70
R-0.264-40
R-0.264-90
(a)
0
2
4
6
8
10
12
14
16
18
20
00.5 11.5
P
applied
/P
ultimate
R-0.264-70
R-0.373-70
R-0.528-70
(b)
0
5
10
15
20
25
00.5 11.5
Av er age crack height (mm)
P
applied
/P
ultimate
R-0.264-70
T-0.264-70*
T-0.264-70**
(c)
Figure 9. Average crack height at different loading levels;
(a) Beam with different concrete compressive strength; (b)
Beam with different prestressing steel ratio; (c) Beam with
different compression flange width.
3.4. Ultimate Strain
The top compressive strain in the tested beams was mo-
nitored from the beginning of loading up to failure. Fig-
ure 11 shows the influence of changing the concrete
compressive strength and the prestressing steel ratio on
S. H. SALEM ET AL.
Copyright © 2013 SciRes. OJCE
31
0
10
20
30
40
50
60
00.2 0.4 0.6 0.811.2
Av er age crack spacing (mm )
P
applied
/P
ultimate
R-0.264-70
R-0.264-40
R-0.264-90
(a)
0
5
10
15
20
25
30
35
00.2 0.40.6 0.811.2
Av er age crack spacing (mm )
P
applied
/P
ultimate
R-0.264-70
R-0.373-70
R-0.528-70
(b)
0
5
10
15
20
25
30
35
40
45
00.5 11.5
Av er age crack spacing (mm )
Papplied/Pultimate
R-0.264-70
T-0.264-70*
T-0.264-70**
(c)
Figure 10. Average crack spacing at different loading levels;
(a) Beam with different concrete compressive strength; (b)
Beam with different prestressing steel ratio; (c) Beam with
different compression flange width.
the ultimate compressive strength. It is obvious that the
ultimate compressive strain is directly proportional to
concrete compressive strength, while it is inversely pro-
portional to the prestressing steel ratio. From this figure,
it could be concluded that the ultimate compressive strain
value specified for normal strength concrete might be
conservative for high strength concrete. Moreover, the
0
0.0005
0.001
0.0015
0.002
0.0025
0.003
0.0035
0.004
Top compressive strain x10-6
(mm/mm)
R-0.264-90
R-0.264-70
R-0.264-40
(a)
0
0.0005
0.001
0.0015
0.002
0.0025
0.003
Top compressive strain x10-6
(mm/mm)
R-0.264-70
R-0.373-70
R-0.528-70
(b)
Figure 11. Ultimate compressive strain with respect to dif-
ferent parameters. (a) Beam with different concrete com-
pressive strength; (b) Beam with different prestressing steel
ratio.
prestressing steel ratio influences the ultimate strain for
the high strength partially prestressed concrete beams,
which can be encored for the high ductility achieved in
prestressed concrete beams using higher prestressing
steel ratio.
4. Conclusions
From the observations of the experimental work reported
here above for partially prestressed concrete beams, the
following conclusions could be drawn:
Increasing the concrete nominal strength from 42 to
114 MPa led to an increase at the cracking load by
10% while its’ corresponding deflection decreased by
45%. Also the ultimate load increased by 9.2% whe-
reas its corresp on ding defl e c t ion increased by 16%.
As the concrete compressive strength increases the
average crack width decreases while the average crack
height increases at the same load level.
The prestressing steel ratio had a negligible effect on
the average crack width at early load levels compared
to the ultimate load. However, the ultimate load sig-
nificantly increases with increasing the prestressed
steel ratio. So at the same load as the prestressed steel
ratio increased the average crack width decreased.
S. H. SALEM ET AL.
Copyright © 2013 SciRes. OJCE
32
The compression flange width is directly proportional
to the average crack height and width. While it is in-
versely proportional to the average crack spacing.
The ultimate compressive strain in concrete increased
by 42% by increasing the concrete compressive strength
from 42 to 114 MPa achieving 0.0035 mm/mm.
The prestressing steel ratio had a great influence on
the ultimate compressive strain, as doubling the pre-
stressing steel ratio led to a 36% reduc tion in the con-
crete ultimate strain.
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