Quantifying the Fatigue Material Properties of UHPFRC with Steel Microfibers at Cracks

Abstract

The addition of steel fibres to concrete in ultra-high performance concrete (UHPC) to form ultra-high performance fibre reinforced concrete (UHPFRC) has been shown to have a great benefit by substantially increasing the flexural capacities and ductilities at the ultimate limit state and reducing crack widths and increasing flexural rigidities at the serviceability limit state. This is because the fibres bridge a crack and consequently allow tensile stresses across the crack. Tests have also shown that tensile cyclic loads applied across a crack can reduce these benefits by allowing the crack to widen through a gradual debonding of the fibres. To quantify the behaviour of UHPFRC post cracking, the fatigue behaviour of steel microfibre concrete at a crack is studied through 33 tensile fatigue tests on precracked UHPFRC and 6 monotonic tests. An approach for processing the results based on the increase in crack width per cycle, that is the incremental set, has been developed and can be applied to any UHPFRC that exhibits debonding. Three distinct cyclic behaviours have been identified and quantified: where there is no incremental set such that there is no quantifiable damage due to cyclic loading; where the incremental set is constant such that there is quantifiable damage; and where there is rapid unstable increase in the incremental set.

A displacement based approach for the analysis, development, and design of reinforced concrete

“…However, the fibers vary in orientation and embedment as illustrated in Figure 17, so an alternative approach is to treat the tension‐stiffening behavior in Figure 17 as a pseudo‐material property. This can be done from testing either individual strands embedded in concrete 60 or from cracked UHPC prisms 61 to derive the properties in Figure 18, both methods being relatively inexpensive. In these tests under displacement control, 61 the concrete first cracks at f ct and then, as the crack is widened, the now cracked region reaches a peak strength of f fi .…”

“…The effects of cyclic loads such as fatigue or earthquake loads can also be quantified or designed for using this displacement approach, just as long as the effects of these cyclic loads on the material properties are quantified through tests. Take for example fatigue, tests have shown 61,65,66 that cyclic loads on fiber concrete cause an increase in slip per cycle, that is incremental set dw/dN in Figure 18. This incremental set has been quantified 61 and can be included directly in the flexural analysis in Figure 11 to quantify the effects of fatigue.…”

“…Take for example fatigue, tests have shown 61,65,66 that cyclic loads on fiber concrete cause an increase in slip per cycle, that is incremental set dw/dN in Figure 18. This incremental set has been quantified 61 and can be included directly in the flexural analysis in Figure 11 to quantify the effects of fatigue. All of this can be done because the displacement based approach directly simulates PI RC behavior.…”

“…This can be done from testing either individual strands embedded in concrete 60 or from cracked UHPC prisms 61 to derive the properties in Figure 18, both methods being relatively inexpensive. In these tests under displacement control, 61 the concrete first cracks at f ct and then, as the crack is widened, the now cracked region reaches a peak strength of f fi . On unloading, the path follows B–A, then on loading it follows A–B then B–C where debonding causes the stress to reduce.…”

Experimental and theoretical analysis of cracking and tension stiffening in UHPFRC under high‐cycle fatigue

“…12,13 Many studies have focused on experimentally quantifying the impact of high-cycle fatigue on the material properties that control tension-stiffening, that is, the stress/strain behavior of the reinforcement (σ r /ϵ r ) and concrete (σ c /ϵ c ) and the bond stress/slip relationship between the reinforcement and concrete (τ/δ). [14][15][16][17][18][19][20][21][22][23] However, relatively little testing has been conducted at the mechanism level. For example, few previous studies have tested reinforced concrete tension elements under high cycle fatigue to observe average strains over the element, local strains at cracks and crack-widths.…”