Fatigue of concrete under compression

Abstract

The compressive strength of concrete decreases as an element is subjected to cycles of loading. In a typical fatigue test for the concrete compressive strength, a concrete specimen (typically a cylinder) is loaded between a lower and upper stress limit. These limits are expressed as a fraction of the concrete compressive strength, and can be written as Sminfck and Smaxfck. The value of Smin and Smax are thus between 0 and 1. The upper limit for Smax in experiments is typically 0,95 and Smin can be as low as 0,02. Experiments in which alternating tensile and compressive stresses are applied can also be executed, but this loading case is not considered in the current study. The result of fatigue tests on concrete cylinders in compression is the so-called Wöhler-curve, or S-N curve. In this graph, a (linear) relation is found between the logarithm of the number of cycles N and the maximum fraction of the static compressive strength Smax. In the codes, different expressions are given for the relation between N and Smax. The codes that are studied in this report are the Dutch Code NEN 6723:2009 (Code Committee 351 001 "Technical Foundations for Structures", 2009), the Eurocode suite for concrete: NEN-EN 1992-1-1+C2:2011 (CEN, 2011a) with the Dutch National Annex NEN-EN 1992-1-1+C2:2011/NB:2011 (Code Committee 351 001, 2011a) and NEN-EN 1992-2+C1:2011 (CEN, 2011b) and the Dutch National Annex NEN-EN 1992- 2+C1:2011/NB:2011 (Code Committee 351 001, 2011b), the new Model Code 2010 (fib, 2012). Some expressions from the literature are considered as well, such as the proposal by Hans Bongers (Snijders, 2013) and an expression suitable for higher strengths concrete (Kim and Kim, 1996). The expression for concrete under compression subjected to cycles of loading from NEN-EN 1992-1-1+C2:2011 is more conservative than previously used expressions in the Netherlands. Therefore, different expressions are given in the National Annex NEN-EN 1992-1-1+C2:2011/NB:2011. The S-N relationship given in the Dutch National Annex consists of two equations: the first branch is valid for N ? 106 cycles and the second branch for N > 106 cycles. The transition between these two expressions is not smooth, but instead causes a jump in the Wöhler-curve. Because of this anomaly in the current code provisions, it is necessary to propose a new expression for concrete under cycles of compressive loading. Moreover, the proposed expression should be valid, yet not overly conservative, for high strength concrete. The current Eurocode NEN-EN 1992-1-1+C2:2011 is limited to concrete class C90/105. The fib Model Code goes up to C120. The goal of this report is to develop an expression that is valid up to C120. To check the quality of the proposed expression, it should be compared to experimental results. For this purpose, a database of experiments on (ultra) high strength concrete tested in compressive fatigue is developed first, and then used to validate the new proposal for concrete under cycles of compression.