fatigue behaviour of reinforced concrete beams with corroded
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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
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