Fatigue and impact strength of concrete

Fatigue strength

Fatigue is the progressive internal damage and eventual failure of concrete under repeated (cyclic) loading, at a stress level well below the static strength.

• Fatigue strength is the maximum stress range that concrete can carry for a given number of cycles (usually 10 million). For plain concrete it is about 55% of the static compressive strength at 10 million cycles (for flexure about 55% of modulus of rupture) for 0 to maximum stress.
• Results are plotted as an S-N curve (stress ratio against log of number of cycles). Concrete shows no clear endurance limit, but the curve flattens near $10^7$ cycles.
• Failure occurs by growth of microcracks at the aggregate-paste interface, with increased strain and lower stiffness before final rupture.
• Affected by range of stress, frequency, stress reversal, rest periods, moisture, strength of concrete and air entrainment. Important in bridges, pavements, airfield slabs, machine foundations and offshore structures.

Impact strength

Impact strength is the capacity of concrete to resist a sudden, high-rate load, such as a falling weight, pile driving or vehicle collision. It is measured by a drop-weight test (a mass dropped repeatedly on a specimen; number of blows to first crack and to failure are recorded) or by the Charpy/Izod pendulum.

• Under rapid loading, concrete shows higher strength and higher modulus than under static loading (strength may increase by 20 to 40% for very fast rates), but it behaves in a more brittle way, with less strain at failure.
• The impact resistance rises with the strength and toughness of aggregate and with fibres; it falls with higher W/C and lack of reinforcement.
• Fibre reinforcement (steel, polypropylene), polymer modification and strong aggregates greatly increase impact resistance.