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? 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 E provides the Standard Recommended Practice for Constant Amplitude Low-Cycle Fatigue Testing.

Fatigue Test for Seismic Application 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. Stress Strain Stress-Strain range for HCF Stress-Strain range for LCF Stress Strain

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 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.