FATIGUE BEHAVIOUR OF REINFORCED

CONCRETE BEAMS WITH CORRODED

STEEL REINFORCEMENT

GROUP 11

Priyadarshani S.T.A. RU/E/2008/139

Priyanga Y.M.H.S RU/E/2008/140

Priyankara K.M. RU/E/2008/142

Rajakaruna R.M.O.B. RU/E/2008/143

Rananjaya J.A.W. RU/E/2008/144

OBJECTIVE

Understand the behaviour of reinforced concrete beams with corroded steel reinforcements under high-cycle fatigue loading and the residual mechanical properties of corroded steel reinforcement following fatigue failure of the beam.

The research was done only for the compressive and tension steel bars (not for stirrups)

MATERIALS AND SPECIMEN DETAILS

9 beams (L0 to L8) with one control beam with uncorroded steel was created. All the beams were designed to fail in flexural mode under concentrated mid spam load by providing ample uncorroded vertical shear reinforcement to prevent shear failure

3600 mm

150 mm

300

mm

2T10

2T20

8R @ 150275

mm

3400 mm

MATERIALS AND SPECIMEN DETAILSMaterial CharacteristicsConcrete (G30) 1:2.23:3.08:0.54 (cement : aggregates : sand : water)

by weight

Fatigue stress :- 7.3Mpa

Tore steel bars (Grade HRB335)20mm tension 10mm compression

Chemical compounds :-

C : 0.17%-0.25% Si : 0.4%- 0.8%

Mn : 1.2% - 1.6%P : 0.4% S : 0.04%

Yield strength :- 390 Mpa

Ultimate tensile strength :- 578 Mpa

Elongation :- 25%

Fatigue stress :- 154Mpa

Round steel bars (Grade HPB235)Used as stirrups

Anti-rust paint was used at the location where the stirrups were made to prevent stirrups from corroding.

Six 150*150*150 mm cubes were made for each beam to determine the compressive strength of concrete which was used.

ACCELERATED CORROSION OF STEEL

A special electrolytic pool was constructed (5200*4500*120mm). After the specimens were placed in the pool, a layer of sand was spread. A 4% NaCl solution was sprayed on the sand to retain moisture. Negative electrode was connected to copper wire and placed in the wet sand. Each beam has a separate DC power supply with a voltage limit of 12V Applied corrosion currents were monitored daily using a multimeter with an

accuracy of 0.01mA Corrosion was estimated using Faraday’s low.

∆m :- Mass loss of steel (g) I :- Corrosion current (A) T :- Time (s) F :- Faraday’s constant (96490 C/mol) Z :- Valence (Fe =2) M :- Atomic mass (Fe = 56 g/mol)

Assumption :- The applied current is fully used in the dissolution of iron

∆m

=(Mit)/(ZF)

MATERIALS AND SPECIMEN DETAILSBeam no. L0 L1 L2 L3 L4 L5 L6 L7 L8

Concrete compressive strength at 28 days MPa

32.0

32.2

32.2 33.0 34.5 33.5 32.5 32.4 32.6

Concrete compressive strength at testing

33.4

43.5

44.30

48.5 46.7 47.3 46.2 43.8 46.9

Average mass loss percentage

0 3.25

3.5 4.20 5.5 6.35 8.04 10.17

11.60

Average impressed current density (µA/cm2)

0 40.3

43.25

44.56

54.2 59.17 78.81

94.44

97.63

Corrosion duration (Days)

0 85 104 142 158 167 161 168 185

FATIGUE AND STATIC TESTS Hydraulic fatigue testing machine was used. Auxiliary supports at both end of the beams were used to

prevent shifting of beams. Magnitude of fatigue load was controlled and measured by a

load cell. Frequency of repeated load was 3.5 Hz Applied maximum and minimum fatigue loads are 33 & 7 kN. When the number of cycles reached 10,000, 50,000, 100,000,

300,000, 500,000, 1million, and 2million, a static load with a maximum value equal to the maximum fatigue load was applied.

The deflection at mid span and quarter span of the beams were measured by using 3 displacement transducers.

The vertical displacements at the 2 pivots of the supports were also measured.

When the beam did not fail after 2million cycles, the static load was applied to failure.

FATIGUE AND STATIC TESTS

Auxiliary supports

load cell.

displacement transducers

Beam

TEST RESULTS AND DISCUSSIONS (CRACKS)

No cracks were observed in beam L1 & L2 One of these 2 cracks were observed in

other beams, Lateral surface of the beam parallel to

corroded reinforced steel Bottom surface directly under the corroded

reinforced steel Crack width :- 0.1mm - 0.3mm

TEST RESULTS AND DISCUSSIONS (CONTROL BEAM)

Cycles Flexural stiffness

<50,000 decreased rapidly

50,000 – 1 million reduced slowly

1 million – 1.5 million

beam remained stable.

1.5 million – 2 million

beam began to increase slowly.

Deflection – load ratio versus fatigue cycle of control beam

(L0)

After 2 million cycles, beam L0 still showed good ductile failure characteristics, and failure load was 71.13kN.

The fatigue cycles have no marked effect on the beam’s static performance except for the disappearance of elastic stage due to concrete cracking

TEST RESULTS AND DISCUSSIONS (CORRODED STEEL EX: L6)

Cycles Flexural stiffness

<10,000 decreased rapidly

10,000 – 100,000 Nearly unchanged

>100,000 Decreased slightly

Flexural stiffness of the un corroded and corroded beam decreased during the early loading cycles due to the transverse cracking of concrete beams

Stiffness of tested beam was characterized by 3 stages,Decreasing stiffnessStable responseIncreasing stiffness

L0 has the lower flexural stiffness and lower compressive strength in concreteL5 has the higher flexural stiffness and compressive strength than less corroded beam and maximum compressive strength

Main factor that influence the mechanical properties of steelMass loss percentageFatigue stress magnitude

TEST RESULTS AND DISCUSSIONS

The reduction of steel area due to mass loss under corrosion could result in an increase of fatigue stress in steel reinforcement.

The maximum upper limit of fatigue stress in steel reinforcement could reach 220 Mpa

More the mass loss percentage of steel reinforcement the less the fatigue life of the beam.

Corrosion substantially reduces the fatigue life of the beam.

Beam no.

Average mass loss %

Fatigue life cycles Maximum fatigue stress (Mpa)

L0 0 Larger than 2 million

194

L1 3.25 626,000 200

L2 3.05 707,000 201

L3 4.2 497,000 203

L4 5.5 334,000 205

L5 6.35 326,000 207

L6 8.04 620,000 211

L7 10.17 324,000 216

L8 11.6 89,000 220

AFTER THE FAILURE OF BEAMS UNDER FATIGUE LOADING

The main tensile reinforcements were carefully removed. Cleaned with a brass bristle brush to remove all adhering mortar. Cleaned with an acid solution Residual rust products were removed again by using a bristle brush. The reference weight of a steel bar per unit length was measured

using noncorroded bars. The mass loss percentage of corroded steel reinforcement was

obtained. Cut each steel bar in 5 parts (a,b,c,d,e) and did static material test

to find out fatigue damage for each section.

When the corroded steel reinforcements were removed, severe corrosion pits at the location of fracture of steel reinforcement was observed Result shows that the critical section of corroded RC beam under fatigue

load is at the maximum bending moment region & at the location of most severe corrosion pits in the steel reinforcement

STATIC TESTING OF CORRODED STEEL BARS AFTER FATIGUE CYCLES

Maximum fatigue strength of L0

Ratio of mechanical properties of corroded steel to original steel after fatigue loading

Segment Max fatigue strength (Mpa)

Test

a & e 2 elastic analysis

b & d 114 -

c 194 Crack section analysis

Beam Segment

Ratio of yield strength

Ratio of ultimate tensile strength

Ratio of ultimate strain

L0 a 1.005 1.005 0.702

b 1.031 1.002 0.714

c 1.026 1.007 0.579

L8 a 0.949 0.974 0.516

b 0.795 0.756 0.290

c 0.782 0.621 0.167

TEST RESULTS AND DISCUSSIONS

The increase of fatigue stress in steel bars did not significantly change the tensile strength of steel.

But decrease the ultimate tensile strain. Yield strength of steel was reduced from 389MPa to

305Mpa due to the effect of corrosion & fatigue. Yield strength of corroded steel reinforcement decreased

linearly with the mass loss percentage of decrease. Fatigue stress induces several cumulative damage in

corroded steel reinforcement.

The effect of steel reinforcement corrosion on structural performance should have a significant consideration during the durability design of RC structures