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This paper presents results from an experimental investigation in which thirteen rectangular reinforced concrete beams were subjected to cyclic loading within service conditions. One beam was designed to measure the variation in steel stress and bond stress along the beam length, particularly between cracks and in the anchorage zones near the beam ends. The variables in the other twelve beams included the area of concrete concentric with each tension rebar and the ratio of compression reinforcement to tension reinforcement. Resulting bond deterioration is analyzed and its effect on bond stress distribution as well as crack development is illustrated and discussed.

The results show that (a) the flexural fatigue life of normal concrete beams is higher than the fatigue life of layered beams utilizing dense concrete or latex modified concrete at lower stress levels but is lower than that of hybrid layered beams at higher stress levels and (b) essentially the same fatigue strength was obtained for beams containing only a 2-inch layer of either dense concrete or latex modified concrete at the surface as for beams containing these concretes full depth.

Bond-slip failure between a smooth steel bar and con-crete was studied in micro scale. Both theoretical and experimen-tal work are presented. The theoretical analysis was perfomed as a nonlinear fi-nite element analysis, based on fracture mechanics. Roth a virgin loading stageanda subsquentloadingstagewasmdelled. In the experimental part a total of ten pull-out tests were performed, both with monotonic loading and cyclic loading. As far as the load-slip relations are concerned, the specimens were much softer at virgin loading compared to subsequent repeated loading. This is suggested to be caused by the phencmenon that the concrete, when it was cast, did not completely grow into the irregularities of the steel. The gaps, which consequently arised, were determined to be of the same magnitude as the depth of the irregularities, and were causing a soft performance for shear movements at virgin loading. This softness also caused the shear stresses in the contact zone to be quite uniformly distributed along the anchorage length. At subsequent repeated loading, as far as shear movment in the same directionas the virgin loading is concerned, all gaps were filled up, which resulted in a much stiffer performance.

Nine fiber-reinforced-concrete spheres were subjected to external pressure loading; four spheres were tested to failure by static loading, and five by low-cycle fatigue loading. The state of stress in the wall of the spheres was multiaxial, varying from biaxial on the inside surface to triaxial elsewhere. The average triaxial state of stress was 0 1 = a2 and 0 3 = 0.3 CT . 1 The fatigue data show a substantial difference in behavior compared to that of previous work on confined concrete. Changing the stress levels, c 9 from 0.70 to 0.50 changed the cycles to failure from 10 to 346, respectively. However, a better parameter to describe fatigue behavior was the stress-to-strength ratio, (qcyc/f;' which varied from 1.55 to 1.08, respectively. Under triaxial compression, it appeared that cyclic loading in which all principal stresses cycle was a considerably more severe condition than cyclic loading in which only one principal stress cycles while the other two principal stresses remain constant.

Fifty concrete beams reinforced with 24 mm deformed bars in the tension zone, were subjected to sinusoidal load fluctuations at 6.7 Hz in air, 3 percent sodium chloride solution and natural sea water. Total numbers of cycles at failure varied between 10' and lo7 for calculated stress ranges in the steel between 100 MPa and 280 MPa. Two types of tension reinforcement were compared; one was a hot-rolled 230 Grade deformed bar, and the second a cold.-worked 410 MPa Grade deformed similar chemical composition. bar with a As practised in some countries, cold-working by twisting was found to reduce the fatigue endurance of the deformed reinforcement in concrete beams tested both in air and sea water. The detrimental effect of sea water or sodium chloride solution gaining access, via concrete cracks, to bars subjected to fatigue loading was confirmed. In sea water the influence of cyclic loading on the hot-rolled series was different to that on the cold-worked series; for the latter series a decrease in slope of portion of the S-N curve was observed, which may represent a fatigue limit within lo7 cycles, whereas for the former no such change in slope exists. A reduction of fatigue endurance was observed for tests in a 3 percent sodium chloride solution compared with data for beams loaded in natural sea water. when A fractographic investigation was conducted on typical failure surfaces of bars subjected to tests in concrete,in air, and in sea water.