Fatigue in Concrete Structures
Raquib Ahsan, Ph.D.
Professor
Department of Civil Engineering
BUET
Does Fatigue Occur in Concrete?

Yes. Fatigue occurs both in plain and
reinforced concrete.
Is Fatigue Important for Concrete
Structures?

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To date there have been no fatigue fractures
reported for concrete structures.
It is possible, however, for reinforcement bars
to fail without any outward signs of structural
distress except local cracking of the concrete
(Tilly, 1979).
In the AASHO (1962) road experiment,
fatigue fractures occurred in overload tests
on concrete bridges.
Is Fatigue Important for Concrete
Structures?

Although fatigue has not proved to be a
problem to date for concrete structures, as
structures are becoming more slender, the
traffic volume is increasing, the axel loads are
larger, and the traffic speed limits are higher,
the margin of reserve strength is
progressively being reduced.
Why Doesn’t Concrete Fail in Fatigue?
-ve
bending
+ve
bending
+ve
bending
-ve
bending
Concrete cracked at only
one side of the section.
Concrete cracked at bothe
sides of the section.
How Reinforced Concrete Can Fail?




We design concrete to take only
compression. Cracks under compression are
closed and can take load. Fatigue failure
under compression is ductile.
Reinforcing steel within RC can fail in fatigue.
Reinforcing steel can fail in fatigue when the
RC section is subjected to tension, flexure or
shear.
Fatigue failure of reinforcement is brittle.
Factors that Affect Fatigue Strength of
Reinforcement


Fatigue strength is independent of the grade
of steel.
Stress range that can be sustained depends
on the minimum stress.
Factors that Affect Fatigue Strength of
Reinforcement




Plain bars have higher fatigue strength than
deformed bars.
When bars are welded fatigue strength is
reduced.
Increase in diameter of deformed bars
produces a very pronounced reduction in
fatigue strength.
Corrosion reduces fatigue strength.
Design of RC Structures for Fatigue




AASHTO LRFD 2012 Bridge Design
Specifications defines two load combinations
for fatigue design.
Load combination Fatigue I is related to
infinite load-induced fatigue life.
Fatigue II is related to finite load-induced
fatigue life.
Load factors 1.5 and 0.75 are used for
Fatigue I and II combinations respectively.
Load Calculation for Fatigue Design


The fatigue load is a standard design truck
but with a constant spacing of 30.0 ft
between the 32.0 kip axles.
Maximum and minimum stresses are
calculated using influence lines.
Stress Range Check

For prestressing tendons, the threshold
varies from 10 ksi to 18 ksi when radii of
curvature is less than 12 ft to more than 30 ft
respectively.
Standard Tests




There are standard tests for axial loading and
bending.
ASTM E466 – 07: Standard Practice for
Conducting Force Controlled Constant
Amplitude Axial Fatigue Tests of Metallic
Materials.
ISO 1099:2006: Metallic materials -- Fatigue
testing -- Axial force-controlled method.
ISO 1143:2010: Metallic materials -- Rotating
bar bending fatigue testing
Bending Test for Reinforcement?
When a steel section is subjected
to bending, steel is subjected to
both compression and tension at
the same time.
When a RC section is subjected to
bending, the reinforcing steel is
subjected to only tension or
compression at a time. Thus
bending test for reinforcement has
no significance.
High Cycle Vs. Low Cycle Tests



High-cycle fatigue corresponds to a large
number cycles at lower stresses.
Low-cycle fatigue means that the load is
applied at high stress levels for a relatively
low number of cycles.
ASTM E606-80 provides the Standard
Recommended Practice for Constant
Amplitude Low-Cycle Fatigue Testing.
Fatigue Test for Seismic Application
Stress
Stress
Strain
Stress-Strain range
for HCF



Strain
Stress-Strain range
for LCF
For earthquake induced loading structures are subjected
inelastic stresses.
A low cycle fatigue is important for structures subjected
to earthquake loads
Results of low cycle high stress fatigue tests show that
steel of higher ductility exhibit better fatigue
performance.
Fatigue in Concrete Buildings



Distress in concrete buildings due to high
cycle fatigue is rarely encountered.
Industrial buildings may be subjected to low
stress high cycle fatigue due to vibration from
rotating devices.
Air conditioning plants of spinning mills are
such cases where concrete structure housing
the plant is subjected to long term vibration.
Cracks in Concrete Buildings for Continuous
Vibration
Cracks in a beam of an AC plant of a spinning mill
Experiment on Fatigue in Concrete
Industrial Buildings



Level of vibration at these plants may be
recorded using sensitive accelerometers or
micro-tremor equipment.
From finite element modeling stress and
strain levels of the concrete structures will be
determined.
Stress range will be checked with established
constant amplitude threshold levels.
Summary

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
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Concrete exhibits fatigue. When under
compression fatigue does not reduce load
carrying capacity.
Fatigue of reinforcing steel is important for
RC structures.
Only Axial test is meaningful for reinforcing
steel.
HCF tests do not have any relation to seismic
performance.