Experimental Behavior of Partially Prestressed High Strength Concrete Beams

doi:10.4236/ojce.2013.33B005

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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)

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: [email protected]

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.

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.

Figure 2. Instrumentation used to monitor the behavior during testing.

Figure 3. Test setup.

Load

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.

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.

0.2

0.4

0.6

0.8

1.2

1.4

1.6

1.8

00.5 11.5

Average crack width (mm)

applied

ultimate

R-0.264-70

R-0.264-40

R-0.264-90

(a)

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

00.5 11.5

Av er age crack w idth (mm)

applied

ultimate

R-0.264-70

R-0.373-70

R-0.528-70

(b)

0.2

0.4

0.6

0.8

1.2

1.4

1.6

1.8

00.5 11.5

Av er age crack w idth (mm)

applied

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.

00.511.5

Av er age crack height (mm)

applied

ultimate

R-0.264-70

R-0.264-40

R-0.264-90

(a)

00.5 11.5

Av er age crack height (mm)

applied

ultimate

R-0.264-70

R-0.373-70

R-0.528-70

(b)

00.5 11.5

Av er age crack height (mm)

applied

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.

00.2 0.4 0.6 0.811.2

Av er age crack spacing (mm )

applied

ultimate

R-0.264-70

R-0.264-40

R-0.264-90

(a)

00.2 0.40.6 0.811.2

Av er age crack spacing (mm )

applied

ultimate

R-0.264-70

R-0.373-70

R-0.528-70

(b)

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

0.0005

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)

0.0005

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.

• 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.

REFERENCES

[1] M. Collins and D. Mitchell, “Prestressed Concrete Struc-

tures,” Prentice H all, Englewood Cliffs (NJ, USA), 1991.

[2] ACI Committee, 363, “State-of-the-Art Report on High

Strength Concrete,” (1992 (Reaproved 1997)), pp. 1-37.

[3] A. A. Abdelrahman, “Serviceability of Concrete Beams

Prestressed by Fiber Reinforced Plastic Tendons,” Ph.D.

Thesis, University of Manitoba, Manitoba, 1995.

[4] Y. Zandi, Y. Akgün and A. Durmuş, “Investigating the

Use of High Performance Concrete in Partially Prestress-

ed Beams and Optimization of Partially Prestressed Ra-

tio,” Indian Journal of Science and Technology, 2012, pp.

2991-2996.

[5] H. Manalip, M. Pinglot and M. Lorrain, “Behavior of the

Compressed Zone of Reinforced and Prestressed High

Strength Concrete Beams,” ACI Journal, Vol. 149, 1994,

pp. 209-226 (SP149-12).

[6] S. W. Shin, K. Ghosh and J. Moreno, “Flexural Ductility

of Ultra-High-Strength Concrete Members,” ACI Struc-

tural Journal, Vol. 86, No. 4, July/Aug., 1989, 39 p.