Fatigue strength of steel fibre reinforced concrete in flexure S.P. Singh a, * , S.K. Kaushik b a Department of Civil Engineering, Regional Engineering College, Jalandhar 144011, India b Department of Civil Engineering, University of Roorkee, Roorkee 247667, India Received 2 March 2000; accepted 2 September 2002 Abstract The paper presents a study on the fatigue strength of steel fibre reinforced concrete (SFRC). An experimental programme was conducted to obtain the fatigue-lives of SFRC at various stress levels and stress ratios. Sixty seven SFRC beam specimens of size 500  100  100 mm were tested under four-point flexural fatigue loading. Fifty four static flexural tests were also conducted to determine the static flexural strength of SFRC prior to fatigue testing. The specimens incorporated 1.5% volume fraction of cor- rugated steel fibres of size 0:6  2:0  30 mm. Concept of equivalent fatigue-life, reported for plain concrete in literature, is applied to SFRC to incorporate the effects of stress level S, stress ratio R and survival probability L R into the fatigue equation. The results indicate that the statistical distribution of equivalent fatigue-life of SFRC is in agreement with the two-parameter Weibull distri- bution. The coefficients of the fatigue equation have been determined corresponding to different survival probabilities so as to predict the flexural fatigue strength of SFRC for the desired level of survival probability. Ó 2002 Elsevier Ltd. All rights reserved. Keywords: Fatigue; Equivalent fatigue-life; Steel fibre reinforced concrete; Stress level; Stress ratio; Flexure 1. Introduction Considerable interest has developed in the fatigue strength of concrete members in recent years. There are several reasons for this. Firstly, the use of high strength materials require that the concrete members perform satisfactorily under high stress levels. Hence, the study of the effects of repeated loads on bridge slabs and crane beams is a matter of concern. Secondly, different con- crete systems such as prestressed concrete railroad ties and continuously reinforced concrete pavement slabs are often used. The use of these systems demand a high performance product with an assured fatigue-life. Thirdly, there is a new recognition of the effects of re- peated loading on a member, even if it does not cause a fatigue failure. There may be inclined cracks in the prestressed concrete beams at lower loads due to fatigue loading and the static load carrying capacity of the component material may be altered. Many researchers carried out laboratory fatigue ex- periments to investigate the fatigue behaviour of plain as well as steel fibre reinforced concrete since FeretÕs pioneer tests [1]. Oh, [16,21] studied the distribution of flexural fatigue-life of plain concrete for various stress levels and found that it follows the two-parameter Weibull distribution. Following equations have been used in the past by the researchers to study the fatigue of concrete: S ¼ a  b logðN Þ ð1Þ S ¼ 1  bð1  RÞ logðN Þ for 0 6 R 6 1 ð2Þ S ¼ C 1 ðN Þ C 2 ð3Þ where a, b, C 1 and C 2 are experimental coefficients. Many researchers [2,9,14] used Eq. (1) which represents the relationship between stress level S , and number of cycles to failure N . Eq. (2) used by researchers [12–14,16] is a modified form of the Eq. (1) which takes into ac- count the effect of minimum fatigue stress ðf min Þ in the form of stress ratio R, ðR ¼ f min =f max Þ. Aas-Jakobsen [7] obtained the value of b in Eq. (2) equal to 0.064 for compression fatigue of concrete. However, Tepfers et al. [12] recommended the value of b as 0.0685. Oh [16] * Corresponding author. Tel.: +91-181-293301x229; fax: +91-181- 2991120. E-mail address: spsingh_recjal@hotmail.com (S.P. Singh). 0958-9465/$ - see front matter Ó 2002 Elsevier Ltd. All rights reserved. doi:10.1016/S0958-9465(02)00102-6 Cement & Concrete Composites 25 (2003) 779–786 www.elsevier.com/locate/cemconcomp