International Concrete Abstracts Portal

ified. Experimental data on the fatigue life of prestressing strands are compared with data on the fatigue life of partially prestressed beams. A close correlation results of a reliability based models of partially prestressed beams in which serviceability and ultimate strength limit states is observed. Global were considered, are presented. They strongly suggest that fatigue in the reinforcement is a controlling limit state that must be seriously accounted for in design.

As part of an ongoing experimental/analytical research effort to evaluate the feasibility of a test method for fracture toughness of concrete, forty-eight plain concrete beams have been tested in bending to failure. All beams were notched and then precracked to different crack length/depth ratios-prior to load-ing to failure. The precracking was done using an electro-hydrodynamic materials testing system and displacement control. The beams which were cracked in fatigue were subjected to one million cycles of sinusoidal loading at 4 Hz. After the cycling was complete on a beam, the crack depth was determined using a compliance calibration technique following which the beam was loaded to failure. A load versus crack-mouth-opening-displacement trace was plotted during this final load run. For each beam tested in fatigue, a companion beam was pre-cracked "statically" by loading in repeated cycles until the crack depth, as measured by compliance calibration, matched that of the fatigued specimen. The studies were made on two different beam sizes in three and four-point bending with two different mix designs. Test results indicate the failure strength and associated maximum stress-intensity of the statically precracked beams to be slightly higher than those precracked in fatigue.

In this investigation, the effect of various load histories on the fatigue behaviour of plain concrete was studied. (1). A total of 462 cylindrical specimens (100x250 mm) were tested statically and dynamically in compression. In addition, a total of 18 cylinders (150x300 mm) plus 132 cubes (100 mm) were tested statically. In addition to fatigue strength, special interest has been paid to the deformation characteristics of the concrete. A total of 140 specimens were tested under constant amplitude loading. Based on the fatigue strength results, empirical ex-pressions between the stress level (S), number of cycles (N) and probability of failure (P), S-N-P relationships, were derived. Using deformation characteristic results, an empirical expression for the total longitudinal strain (&maX) as a function of the cycle ratio (N/NF) and the loading time (t) was derived. This expression can be used to predict the fatigue life from deforma-tions early in the life. A total of 180 specimens were tested under various variable load histories of a given statistical distribution. The effect of both small and large amplitudes on fatigue strength and deforma-tion characteristics was examined. Based on fatigue strength results, the validity and limitations of the Palmgren-Miner hypothesis were examined. The PM hypothe-sis was found to give more or less unsafe predictions of the fatigue life depending on the load histories. Features of the loading histories affecting the accuracy of the hypothesis are discussed and an empirical relationship between loading histogram parameters and the Miner-sum at failure is presented. The empirical expression for the total longitudinal strain de-rived for constant amplitude tests is modified for tests under variable load histories.

Rigid pavement design procedures currently in use con-sider only the modulus of rupture strength of concrete in deter-mining the fatigue life of concrete highway pavements. The effects of entrained air, water-cement ratio, and aggregate type are considered only to the extent they affect the modulus of rup-ture of the concrete. The purpose of this study was to determine the effects of air content, water-cement ratio, and aggregate types on the flexural fatigue strength of plain concrete and to develop fatigue curves that incorporate these effects and thus could be used for design. Fifteen series of concrete were investigated in this study. The variables consisted of air content, water-cement ratio, coarse aggregate type and fine aggregate type. Over 350 beams were sub-jected to flexural one-third point fatigue loading in which the bottom fiber stress varied from essentially zero to a predeter-mined maximum stress. Of the variables investigated, air content and coarse aggregate type were determined to have the greatest effect on flexural fatigue strength. Water-cement ratio also affects the fatigue strength but to a lesser degree. Fatigue strength of concrete was not significantly influenced by the type of fine aggregate used.

Reinforced concrete beams of different sizes and reinforcement arrangements were repeatedly loaded at frequencies of 0.5 and 1 Hz. Curvature and deflection are presented as ratios of the initial values and a unique relation is obtained between each of these ratios and the logarithm of the number of load repetitions. Each ratio increases gradually in a curved form to about 0.5 x l06 repetitions from where it takes a straight line form with a slope varying between 0.2 and 0.225. The long term deflections of beams tested in earlier investigations are predicted with good agreement. A similar expression is given for maximum crack width and predicted long term values are in reasonable agreement with those measured by earlier investigators.