International Concrete Abstracts Portal

Results are presented from a study carried out at the Building Research Establishment, United Kingdom, on the effect of rate of‘compressive loading on the fatigue characteristics of plain concrete. Specimens made with a typical normal weight aggregrate, gravel, and a manufactured lightweight aggregate, Lytag, were tested at one of two constant rates of stressing and unstressing, 0.5 MN/m2s and 50 MN/m2s. Using as a basis the static strength of the concrete at a standard stressing rate (0.25 MN/m2s), two distinct S-N curves were obtained for each type of.concrete. When related to the static strength at the same stressing rate as in the corresponding fatigue test, the results fell onto a single curve for each type of concrete. In studying the development of axial strains in the concrete during a fatigue test, it was found that the rate of strain increase per cycle of load was constant for most of the life of the specimen. It is shown that a strong correlation exists between this strain rate and the fatigue life of the specimen, and that this can be used to determine relationships between the strain rate and the true level of loading. Using these re1ationships'S-N curves are produced with very little scatter of results.

As usual with other materials the constant-amplitude test (Wdhler) is widely adopted as the criterion for the fatigue sensitivity of concrete. In this test the number of load repetitions Ni the material can stand before failure occurs, is determined. In general stresses vary in a more erratical and random way. As for concrete from tests no relation is known between this kind of loading and the service life of a structure, Miner's rule is adopted for predicting this life on basis of constant-amplitude tests. According to this rule failure will occur if the following condition is satisfied: where c is the number of stress cycles during the service life. As the number of stress repetitions N. is a stochastic quantity, Miner's rule will also be stochastic. An experimental verification of the rule is therefore complicated. So a theory is giventose-parate in Miner's rule the possible uncertainty in the rule from the influence of the dispersion in N. With the aid of this theory test results of about 220 variable-amplitude tests have been verified. Miner's rule proved to be very accurate to predict the life-time of the test cylinders. At the moment the test program is continued with random loading tests. The results of that will soon be available.

The paper presents a short summary of the state of the art with respect to fatigue strength of reinforced and prestressed concrete structures. The simplification made in ordinary design evaluations are shortly discussed. Offshore concrete structures are exposed to an environment which is different from that compared with ordinary land structures. The special feasures of this difference will be discussed. Specially, the nature of the environmental loads causes a random load application in the structure. The paper will discuss how this may be tackled in design. Different methods used in evaluation the fatigue strength of offshore concrete structures are discussed and a proposal is made, on how to formulate a design criteria for offshore concrete structures with respect to fatigue strength evaluation.

SP-75 The eighteen papers included in this volume report on international research into the fatigue of concrete structures. Among the topics are fatigue in structures subject to cyclic loading in offshore and Arctic environments; hydraulic fracturing effects of water; marine corrosion and fatigue strength; the validity of Miner's hypothesis; and methods of predicting crack widths and fatigue loading.

Structural concrete is widely used in marine environments, but a relatively recent development has been its use in structures such as oil production platforms, ships, wave energy devices etc., where fatigue loading can be significant. It is well known that the effect of a corrosive environment on structural steelwork is to reduce its fatigue life, and this paper describes work in progress to determine whether or not the same is true for structural concrete, both reinforced and prestressed. Reinforced and prestressed concrete beams are being tested in unidirectional bending, and in reverse bending, in jackets containing sea-water, at slow cycling rates (about 0.17 Hz)which approximate to sea-wave frequencies. The sea-water is pH and temperature controlled and is continuously circulated from a storage tank. Control specimens are tested at higher frequencies (3 to 5 Hz) and these show the expected reduction in fatigue endurance, compared withtests in air. However, the wave-frequency test results show that deposits are formed in the flexure cracks after 3 to 4 days of cyclic loading, and this has the effect of increasing, rather than decreasing, the fatigue lives of the beams - certainly when the bending is unidirectional. Under reverse bending this effect is not yet confirmed, although the crack-blocking is observed to take place. Electron-microscopy of the failure surface is being utilised to establish the mechanism by which corrosion fatigue failure occurs under these conditions.