Effect of Static and Fatigue Cracking on Concrete Strain Measurements

Three concrete specimen types were tested to failure under static and cyclic loading. Simply supported beams, fully supported beams, and fully supported slabs were constructed from the same concrete mix. Strain gauges were used to help determine the effect static and fatigue cracks had on concrete strain measurements. Static load tests for all specimen types found strain gauges overestimated bending stresses after 70 percent of the static strength of the concrete. Stresses based on cyclic strain measurements were not useful in predicting the bending stresses in the concrete specimens. The strain gauges accumulated permanent deformations almost immediately after cyclic testing started. For all specimen types tested, strain gauges indicated that cracking in concrete occurred in a localized narrow band. Concrete in the cracking plane had increased strain levels, whereas adjacent uncracked regions experienced a reduction in strain levels. Location of the failure crack on each specimen type was essential for interpretation of the strain data. Strain gauges allowed for the determination of the direction of crack propagation in slabs. Cracks began at the bottom edge of the slab and propagated across the bottom of the slab’s centerline. Vertical crack propagation through the slab’s depth occurred for the majority of the slab’s fatigue life. Strain measurements showed concrete slabs’ ability to resist load in directions perpendicular to the maximum stress not seen in fully supported beams and simply supported beams.