Fatigue and Static Testing of Concrete Slabs

Fully supported concrete slabs (FSS) were constructed and tested under static and cyclic loading in the laboratory. Deflection, strain, and load data were acquired for each slab test. The results of the FSS fatigue tests were plotted as the stress ratio (bending stress in the slab divided by the concrete modulus of rupture) versus the number of cycles to failure. FSS had a resistance to fatigue 30 percent higher than simply supported beams (SSB). For the same number of cycles to failure, concrete slabs could sustain a stress level 30 percent higher relative to beams. The concrete modulus of rupture from an SSB was found not representative of the true strength of an FSS. FSS tests under static load had a flexural strength 30 percent higher than the concrete modulus of rupture from a SSB test configuration. When the actual slab strength under static loading was accounted for in the stress ratio, the fatigue curves for concrete beams and slabs were essentially identical. Strain gauges indicated that plastic deformation in the slab occurred almost immediately with cyclic loading. Strain gauges also showed that partially cracked slabs resisted the load through cantilever action. Deflection measurements showed that crack initiation and propagation occurred in the slab before fatigue failure. Crack propagation through the slab thickness was found to consume most of the slab’s fatigue life.