Flexural fatigue durability of selected unreinforced structural lightweight concretes

Abstract

Synthetic lightweight aggregates are being extensively used in the construction of highway pavements, bridges, and buildings. For its efficient use in such structures, an evaluation of the fatigue durability of lightweight concrete is required. In this research study, a reproducible flexural fatigue test was developed. Flexural fatigue response in the form of a relationship between stress level and the number of cycles of loading to concrete failure of three lightweight concretes and one regular weight concrete was evaluated. Also, relationships between flexural fatigue durability, freeze-thaw durability, aggregate absorption, and aggregate freeze-thaw weight loss were established. Fatigue tests were conducted on unreinforced concrete prism specimens of three lightweight and one regular weight concretes. Two of the lightweight concretes and the regular weight concrete were tested in a moist condition, and one lightweight concrete was tested in a dry condition. In the fatigue tests, all specimens were subjected to repeated sinusoidal stress cycles at the rate of 697 cpm. The fatigue stress levels were determined as a percentage of moduli of rupture (center point loading) of identical prism specimens. All concretes were cured for a minimum period of 27 days in the moist room after removal from the mold. Following are the important conclusions that were derived from the research study: 1. The resistance of unreinforced structural concrete to flexural repeated load was dependent on the applied stress amplitude, the variation of log of fatigue life being inversely proportional to the applied stress up to 10 million repetitions of the load for all the concretes tested. 2. Type of coarse lightweight aggregate definitely affected the flexural fatigue behavior of unreinforced structural concrete. 3. Dry concrete exhibited longer fatigue life than wet concrete. 4. Lightweight concrete which was durable in the flexural fatigue test was also found to be durable in one freeze-thaw test. 5. High values of aggregate absorption were associated with poor fatigue durability of the resulting concrete. 6. High values of lightweight aggregate freeze-thaw loss were indicative of poor fatigue durability of the resulting concrete.