Enhancing the flexural performance of ultra-high-performance concrete using long steel fibers

Significance Statement

The ultra-high-performance concrete is promising for use in construction sector due to its high tensile and flexural performance coupled with its high compressive strength. Despite these advantages, its high commercial cost which is mainly due to high price of steel fiber and superplasticizer along with additional cost for heat curing remains a big challenge.

In order to curb this challenge, researchers have tried to reduce the required amounts by using different types of steel fibers. It is also important to analyze factors that influence the tensile and flexural performances of ultra-high-performance concrete as they have been considered by previous studies to be major influencers.

Research by Professor Young-Soo Yoon from Korea University, Dr. Doo-Yeol Yoo from Hanyang University and Dr. Su-Tae Kang at Daegu University investigated the flexure performance of an ultra-high-performance concrete UHPC beams according to their fiber length with the consideration of fiber distribution characteristics such as fiber orientation, fiber dispersion, and number of fibers in a unit area. The study is published in journal, Composite Structures.

From test results, it was seen that all series exhibited deflection-hardening behavior generating a higher load carrying capacity after first cracking. Beams with longer fiber length had higher load carrying capacity and deflection capacity as minute effect on post-peak ductility was seen.

After investigations on load carrying capacity and toughness at points of limit of proportionality and modulus of rupture MOR and other deflection points L/600(d0.5), L/150(d2), L/100(d3) and L/75(d4), results show that the fiber length of 19.5mm exhibited the highest load and toughness and magnitude of load. Toughness decreases with a decrease in fiber length except at point of proportionality. The insignificant difference observed in the limit of proportionality was due to the fact that first cracking behavior is more influenced by the matrix strength rather than fiber bridging which is consistent with findings from Yoo et al. Int. J. Dam Mech 2015.

Binary image analysis results on fiber orientation and dispersion showed that for all test series, fibers near the wall of the mold tends to be more aligned in the flow direction than those being further from the wall as a result of wall effect. Poorer fiber dispersion was obtained near the end of the specimens at the point of placing concrete than the center of specimen which is due to alignment of fibers parallel to cutting plane as a result of zero velocity of flow at the end wall.

Lowest coefficients of fiber orientation, dispersion and lowest number of fibers in a unit area were obtained near the end of the specimen for all test series. Coefficients of fiber dispersion increased with increasing flow distance up to 80mm and similar values thereafter. However, higher packing density and fiber orientation coefficient was obtained along the flow distance when shorter fiber length was used. This result means that shorter fiber length tends to be aligned parallel to flow direction than a longer fiber length.

Finite element analysis showed flexural strengths lesser than the experimentally obtained values for beams with fiber length of 13mm. However, using a discrete crack approach with the suggested tri-linear tension-softening curve, fairly well predicted the experimental load-deflection responses of the ultra-high-performance concrete beams with various fiber length.

This study finding on flexural performance of ultra-high-performance concrete beams according to their fiber length provides another avenue of reducing high cost associated with ultra-high-performance concrete.

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Abstract

In this study, the flexural performance and fiber distribution characteristics of ultra-high-performance concrete (UHPC) were investigated according to the fiber length. To do this, three different fiber lengths having an identical diameter were used. Enhancements in flexural strength and energy absorption capacity were observed when longer fibers (or higher aspect ratios of fiber) were used, whereas insignificant effect of fiber length on the first cracking properties (i.e., first cracking strength and corresponding deflection) was obtained. Fiber length had a little influence on the degree of fiber dispersion, but a significant influence on the fiber orientation. A higher fiber orientation coefficient along the flow distance was obtained when shorter fibers were used. A finite element analysis incorporating previously suggested material models was performed and verified by comparing the analytical results with the present experimental data.