Journal of Highway and Transportation Research and Development Vol.4,No.2(2009)57

Experimental Research on Fatigue Behavior of Reinforced Concrete Reams Strengthened with HPFL under

Overloading Conditions *

SHANG Shouping (IP!^^)1* * ,GAO Faqi (itj^in )2, LIU Wei (#IJ$|)1,LUO Yexiong {^^MY

(I. College of Civil Engineering, Hunan University, Changsha Hunan 4IOOX2, China;

2. Central and Southern China Municipal Engineering Design & Research Institute. Wuhan Hubei 43001 O.China)

Abstract: In order to study the fatigue behavior of RC beams strengthened with High Performance Ferrocement Laminates (HPFL) under

overloading conditions, static and fatigue experiments were conducted on two control beams and nine strengthened beams. The failure

mode, fatigue life, deflection and material strain under overloading conditions were analyzed. The result shows that (1) fatigue failure of

the beams subjected to overload starts with steel rupture at the bottom and the fatigue life is only between 327 000 and 668 000 while fa-

tigue life of strengthened beams is greater than two million times in case of not overloading; (2) compared with the control specimen, the

fatigue life of strengthened beams is obviously extended and increased with the increase of steel mesh consumption; (3) after the same

number of cycles, the deflections, the strains of concrete and steels of four strengthened beams are lower than those of the control speci-

men. Debonding at the interface of HPFL and concrete is not observed because of shear pins planted at the end of the beams.

Key words: bridge engineering; experimental research; strengthening; overload;fatigue behavior

0 Introduction

At present, the phenomenon of large overloading trucks

running on highway bridges is very severe. One reason is

that the road traffic transportation industry develops vigorous-

ly in China. The axle loads of transport vehicles have im-

proved greatly and traffic flow has been increasing year by

year. On the other hand, a mass of old bridges have gradual-

ly become unsafe due to low standard load code and lack of

maintenance and repair, especially those small and medium-

sized span bridges built before the 1 970 s and 1 980 s .

Overloading may produce wide cracks in reinforced concrete

bridge structures, leading to corrosion of reinforcement and

reduction of durability. In addition, overloading increases the

fatigue load level and amplitude, intensifies bridge damage

and may result in fatigue failure1 . The idea of rebuilding

the abovementioned old and dangerous bridges is neither sci-

entific nor realistic. However,it is an effective way to recov-

er the bearing capacity and traffic capacity and to improve

the service life by repairing them with appropriate reinforce-

ment technologies^ ~ .

High Performance Ferrocement Laminate (HPFL) is a

new type of inorganic composite material. This material is

mainly composed of Portland cement, mineral admixtures of

HPC, concrete admixture and a small amount of organic fiber

mixed with water and sand. HPFL has a series of advan-

tages , such as high strength, small shrinkage and good bond-

ing properties with concrete. Concrete components strength-

ened with HPFL have an effective increase in bearing capac-

ity , stiffness, ductility, flame proof and crack resistance. The

section sizes of components increase very little and this rein-

forcement technique has a good economic benefit . Com-

pared with the CFRP reinforcement technique , HPFL

strengthening method enjoys advantages of easy construction,

economical and practical. Some scholars have researched

bending, shearing, high temperature resistant and fatigue per-

formances of beams strengthened with HPFL. Moreover, axial

compressing, eccentric compressive and seismic behaviors of

HPFL strengthened columns were studied systematical-

ly . With the purpose of safely applying this method to

the reinforcement of old bridges, especially to those medium

or small bridges, a fatigue experiment of beams strengthened

Manuscript received July 12,2008

* Supported by Hunan Provincial Project for Science and Technology Development (No.06SK4057)

* E~ mail address : sps @ hnu. en

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58 Journal of Highway and Transportation Research and Development

with HPFL under overloading conditions is carried out.

1 Experimental scheme

1.1 Design of specimens All specimens share with a common section size of 150

mm by 300 mm. The designed strength grade of concrete

used in this trial is C25. The span and clear span are 2 400

mm and 2 200 mm. Each beam is designed with longitudinal

bars 3<J>18,hanger bars 2<£8 and stirrups <2>6@100. Mate-

rial performance testing results are as shown in Tab. 1.

Tab. 1 Property of materials

/„e(MPa) /cu(MPa) /y(MPa) /my(MPa)

38.1 29.0 378 329

Note: /m^-cube compression strength of composite mortar; /cu-cube com-

pression strength of concrete; fy- yield strength of longitudinal bars; fmy- yield

strength of steel mesh.

1.2 Strengthening patterns

Technical data are shown in Tab. 1. Beams are rein-

forced with IF shape with a 20 mm layer thickness. The mix

proportion of HPFL is designed as cement: sand: admixture:

water = 1 ■" 2 ■" 0.17 •' 0.3. The admixture is HPPO II type for

anti-cracking mortar. In order to be closer to the actual con-

ditions , static load is applied on all specimens to crack them

with a width of 0.02 mm. To increase the bonding forces,

shear dowels are bonded in the upper triangular area on both

sides of the beam ends (see Fig. 1) . The shear dowels are

made of hot-rolled bars with a 4 mm diameter,600 mm deep

and 120 mm spacing. The exposed portions are folded up to

L-shaped hook with a length of about 30 mm. Specimens

grouping is shown in Tab. 2.

Transverse steel mesh Lognitudinal steel mesh Shear pins d> 6 <f>6@100 '>' 6@50

• 4 V /- • • • • • • •

j 2 200 mm . V

Fig. 1 Strengthened specimen

Tab. 2 List of beams

Specimen number A0 Al Bl B2 B3 B4 Cl C2 C3 C4

Strengthened control strengthened control strengthened

pattern beam beam beam beams

Loading mode static static fatigue fatigue fatigue fatigue fatigue fatigue fatigue fatigue fatigue

Spacing of

steel mesh/mm - 50x100 - 30x60 30x60 50 x 100 50 x 100 50 x 100 50 x 100 50x100 50x100

Number of shear pins _ 5 - 5 7 7 5 7 5 3 -

1.3 Loading scheme As fatigue test of specimens B0 ~ B4 is conducted un-

der overloading condition, the loading level and amplitude

value are higher. In accordance with clause 5.7.3.4 of

AASHTO ,the steel bar stress should not exceed 60% of

its yield stress in serviceability limit states. So the upper

limit of fatigue loading is taken as Pmax = 0.8 Py = 0.8x

130= 104 kN, overloading 33% . The lower limit of fatigue

loading is fmin = 44 kN, which is determined by the princi-

ple of no relative bounce between the spreader beam and the

actuator. The fatigue stress ratio is p = 0.42 and the stress

amplitude of longitudinal bars is about A/y = 160.5 MPa for

unstrengthened beams. The stress amplitude of longitudinal

bars of beams strengthened with HPFL is about 136 ~ 149

MPa, 11-24 MPa higher than the requirement 125 MPa in

the Specification for Design of Concrete Structures

(GB50010-2002).

The fatigue loadings of specimens Cl ~ C4 are lower.

The upper limit is defined as Pmax = 0.5Py = 64 kN and the

lower limit as Pm;n = 30 kN. The stress amplitude of longitu-

dinal bars in beams Cl ~ C4 is estimated to be 76 MPa, low-

er than the prescribed limit 125 MPa in Chinese specifica-

tions .

Fatigue experiment is carried out by means of fatigue

testing machine PMS-500 in Bridge Engineering Lab of Hu-

nan University. The test is under constant amplitude loading

with a 0.8 Hz frequency. Before fatigue test, 2 cycles of

static test of loading and unloading is performed with an up-

per limit of Pmax. The purpose is to eliminate gaps in the

loading system and to check that the Data Acquisition System

is working correctly. Five-stage loading from 0 to Pmlix and

four stage unloading from Pmax to 0 are conducted. When

the fatigue test is carried on, the upper and lower limit of

loading is kept stable, with an error less than 5% of the up-

per limit load. After predetermined number of times, shut

down the equipment and unload to 0. Then a cycle of static

loading test is performed to measure deflection and strain.

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SHANG Shouping.et al: Experimental Research on Fatigue Behavior of Reinforced Concrete ReamsStrengthened with HPFL- 59

Survey the maximum crack width and observe crack exten-

sion at the interface. When the fatigue damage occurs, note

the number of cycles and failure characteristics. Static load

test is done for those specimens that still not fail after 2 mil-

lion cycles.

Tab. 3 Main experimental result

Reaction frame Displacement „ . ,

transducers Spreader beam

I VEZI IE Actuator

Displacement transducers

100 800

Displacement Concrete pier_ transducers

300 I 300 L 300 I 300 I "* 2400

soo 100

Fig. 2 Experimental equipment (unit: mm)

2 Test result and analysis

2.1 Experiment phenomenon and failure mode Main test result is shown in Tab. 3. The control speci-

men B0 fails after 32.7 million cycles, with brittle bending

failure of a longitudinal bar fracture on the bottom and con-

crete crush. By contrast with B0 at the same number of cy-

cles, Bl ~ B4 have a significant reduction in deflection, con-

crete and steel strain and the crack width. The number of

cracks on mortar surface is larger and the space is closer.

Fatigue failure happens suddenly without obvious signs. Four

strengthened beams are destructed by steel and steel mesh

tensile failure, but the damage is much less severe than that

of BO. One of longitudinal bars breaks on the bottom of Bl

and B2 while two rupture in B3 and B4. Debonding at the

interface of HPFL and concrete is not observed due to shear

pins planted at the end of the beams.

Fatigue failure does not occur after 2 million cycles for

Cl ~ C4. In the subsequent static load test,Cl - C3 fail in

the manner of steel and steel mesh yielding on the bottom

followed by concrete and composite mortar crush. Entire

debonding of U-shaped HPFL appears at the interface, owing

to no shear pins planted at the end of the beam.

2.2 Fatigue life Tab. 3 indicates that specimens B0 ~ B4 have lower fa-

tigue lives, ranging from 320 000 to 670 000. Suppose we

make 2 million cycles as the failure criterion, the fatigue

lives of control beam and strengthened beams are only equiv-

alent to 16.4% and 18.9% ~ 33.4% of that. For the RC

beams whose design life is 50 years, the fatigue lives of the

Specimen

number

Fatigue

lives( x 104) Failure modes

A0 -

Al -

B0 32.7

Bl 62.8

B2 66.8

B3 43.9

B4 37.8

Cl >200

a >200

a >200

a >200

longitudinal bars yielded, concrete crushed

longitudinal l>ars and steel mesh yielded, con-

crete and composite mortar crusruxl

1 longitudinal bar ruptured, concrete crushed

2 longitudinal l>ars and 2 mesh steels rup-

tured , concrete crushed

1 longitudinal bars and 2 mesh steels rup-

tured , concrete and composite mortar crushed

2 longitudinal l>ars and all mesh steels rup-

tured , concrete cruslu*l

2 longitudinal bars and 3 mesh steels rup-

tured, concrete crushed

failed under static test after 2 million cycles,

concrete crushed

failed under static test after 2 million cycles,

concrete crushed

failed under static test after 2 mi 11 inn cycles,

concrete crushed failed under static test after 2 million cycles,

dclxmding at the interface of HPFL and con-

Fig. 3 Fatigue failure pattern of beams

control beam and strengthened beams are perhaps only 8

years and 9-17 years. This is mainly because the stress

level and stress amplitude of longitudinal bars exceed the de-

fined limiting values under over loading conditions. Steel

rupture leads to much lower life expectancy. Specimens,

Cl ~ C4, subjected to a normal level cyclic loading, all ex-

hibit fatigue lives more than 2 million cycles.

HPFL strengthened beams Bl ~ B4 all enjoy various

degree of longer fatigue lives than the control specimen B0.

Fatigue lives of Bl and B2 increase by 301 000 and 341 000

respectively, both more than double B0. Compared with B0,

B3 and B4 have increasing amplitude of 34% and 16% in

fatigue lives. The fatigue lives of Bl and B2 are both longer

than those of B3 and B4, showing that the fatigue life extends

with the amount of HPFL. This can be attributed to the co-

operation work between HPFL and the beams. The steel

mesh at the bottom reduces the stress level and stress ampli-

tude of longitudinal bars. Decrease of the steel stress ampli-

tude makes the crack growth slow. Therefore, the fatigue life

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60 Journal of Highway and Transportation Research and Development

of strengthened beams could be extended significantly.

Fig. 4 illustrates the steel S~N curves regressed ac-

cording to the experimental result. By summarizing the pre-

vious researches on beams subjected to fatigue loading at

home and abroad, the conclusion shows that most of the

beams fatigue failure begins with steel rupture. This is in

good agreement with our experimental result. Thus, it is the

key to study the fatigue behavior of steel bars. The relation

expressions between the fatigue life and steel stress ampli-

tude are regressed as follows:

]gN= 14.812 5-4.235 71gAa, (1)

jvhere, N is cycles to failure; A<7 is stress range within ten-

sile reinforcing steel.

Correlation coefficient = -0.957; the standard devi-

ations = 0.045.

2.20

2.3 Fig. 4 Regression of fatigue experiment result

Deflection With reference to Fig. 5 we can see that the deflections

at midspans of the beams increase obviously with cycles. B0

deflection increases in linear with cycles before 200 000 cy-

cles , much faster than those of strengthened beams. Deflec-

tion of strengthened beams was basically in linear growth be-

fore 20 000 cycles. After 20 000 cycles the changes were

very small and rapid growth took place after 50 000 cycles.

HPFL strengthened beams always had smaller deflections

compared with that of the companion beam B0. When 200

-■-B0 -T-B3 .-•-Bl -4-B4

£ 6.0 /■^B2 ^ £ / ^^ £ 5.0 s^'^ - 4.0 :

& 3 0 <U u 2.0 lr

2.4 T--*—T—;—* | 2.2- ?2.0; yi^^ -*-c\ r, 1.8 J -»-C2 = 1 6 f -*-C3 Q 1.4: i ^c4

0 10 20 30 40 Cycles(X104)

(a) B0-B4

50 50 100 150 200 Cycles(X104) (b)Cl~C4

000 cycles reached, B0 deflection was as high as 6.3 mm

and B2 reached up to 4.3 mm, 32% less than B0. Bl, B3

and B4 were all deflected about 3.1 mm, only equivalent to

49% of B0. This is primarily due to cooperation work of-

HPFL and prior beams, resulting in fatigue stiffness increase

and deflection reduction.

2.4 Strain

Data in the Fig. 6 are strain result obtained from fa-

tigue test. After the same number of cycles the concrete

strain of the control beam is visibly higher than that of

strengthened beams, proving that HPFL restricts crack devel-

opment very well. The neutral axis moves upward slower and

the mechanical behavior is improved significantly. The strain

of concrete and steels are also greatly reduced because HPFL

cooperates with beam well. The steel mesh help longitudinal

bars of non-strengthened beams reduce stress by participating

in sustaining tension. The more amount of the steel mesh,

the better effect.

-1 200 -1 100 -1 000

-900 -800 -700:

-600

-■-B0-T-B3 I -»-Bl-«-B4

-.0^— -i*

5 10 15

Cycles* X104) (a) B0-B4

20 5 10 15 Cycles(XK)4)

(b) B0-B4

20

50 100 150 Cycles( X104) (c)Cl~C4

50 100 150 Cycles( X 104) (d)Cl~C4

200

Fig. 5 Deflection- cycle curves

Fig. 6 Strain-cycle curves

The experimental result reveals that the strain distribu-

tion of concrete is approximately linear. Before longitudinal

bars ruptured, the concrete at the edge of compression zone

is in elastic phase, with its stress less than the compressive

strength. So the average strain distribution still accord with

flat section assumption in spite of repeated loadings, being

nearly triangular distribution along the section height.

3 Conclusions

(1) All beams strengthened with HPFL are subjected to

fatigue failure in overload situation. The failure mode is lon-

gitudinal bar and steel mesh fracture followed by concrete

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SHANG Shouping, et al; Experimental Research on Fatigue Behavior of Reinforced Concrete ReamsStrengthened with HPFL······ 61

and cement-mortar crush.

(2) Under the condition of upper fatigue load exceed­

ing by 33 % and stress amplitudes beyond regulations by

11 - 24 MPa, the fatigue lives are only 327 000 - 668 000.

But the fatigue lives of HPFL strengthened beams are all

more than 2 million cycles under safe loads.

( 3) Compared with the control beam, the fatigue Ii ves

of strengthened beams are obviously extended, increased with

the increase of steel mesh consumption. The deflections of

strengthened specimens, the strain of concrete and steels of

four strengthened beams are lower than those of the control

specimen.

(4) It is an effective way to avoid debonding at the in­

terface of HPFL and concrete by planting shear pins at the

end of the beams.

From the analysis above it is concluded that damage of

bridges due to overweight trucks is speeding. Bridges

strengthened with right amount of HPFL can keep fatigue

stress and stress amplitude from exceeding permitted limit

values, with fatigue life more than 2 million cycles.

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