Bryan Barragán

Uniaxial tension test for steel fibre reinforced concrete: a parametric study

Abstract A RILEM Draft Recommendation was proposed in 2001 for obtaining the stress versus crack opening (σ–w) response of steel fibre reinforced concrete through a uniaxial tension test. The present study analyses the robustness of the recommended test through a parametric study. Furthermore, the methodology is extrapolated to cores extracted from cast elements. Also, the effect of the coring direction with respect to the preferential fibre orientation caused by the compaction procedure is examined. The study demonstrates that the test is robust and representative of the material response, and could be used for determining the σ–w relation of the material that may be needed for comparing the performance of different fibres or for providing input for finite element analysis. No significant influence of the characteristics of the specimen or problems of instability due to the loss of control were encountered. There is some relative rotation between the crack faces but its influence on the σ–w response is expected to be negligible. The parameters obtained from the tests exhibit coefficients of variation of up to 30%, which is mainly attributed to the randomness of the number of fibres bridging the crack, considering the relatively small cross-section of the specimen.

Shear Failure of Steel Fiber-Reinforced Concrete Based on Push-Off Tests

Steel fiber-reinforced concrete (SFRC) uses fibers as shear reinforcement, a successful approach to coping with the brittle nature of shear failure. This article reports on a study undertaken to characterize the failure and toughness of SFRC subjected to direct shear loading at the material level. The study used a push-off test on a double-notched prism to quantify the shear stress-displacement behavior of SFRC. The shear stress-slip response (obtained experimentally) can be used to calculate toughness-based parameters, which can be employed in structural design. The authors note that the test can be performed in a stable manner for steel fiber-reinforced concrete, permitting the determination of the pre- and post-peak responses and, consequently, characterizing the shear stress that can be transferred across an open crack. The authors conclude that significant improvements in the ductility of concrete during shear failure and some increase in the shear strength are achieved through the incorporation of steel fibers in both normal- and high-strength concretes.

Long-Term Behavior of Cracked Steel Fiber-Reinforced Concrete Beams under Sustained Loading

The mechanical behavior of steel fiber reinforced concrete (SFRC) has been widely studied. In many cases, fibers are incorporated into concrete to improve the service life of structures by means of the three-dimensional (3-D) crack control capacity of this type of reinforcement. The residual (postcracking) capacity and long-term (creep) behavior of fiber-reinforced concrete is of utmost importance. This paper presents an experimental study on the behavior of SFRC beams, cracked and then subjected to long-term loading. The cracked beams were then placed in creep frames and subjected to different levels of load. The crack opening under constant gravity load was electronically measured over a period of approximately 21 months by means of displacement transducers. The concept of the crack-opening rate under long-term loading is introduced, and the conditions for a long-term stable response are discussed.

EFFECTS OF HIGH TEMPERATURE ON RESIDUAL, MECHANICAL, AND TRANSPORT PROPERTIES OF CONCRETE

This paper presents an extensive analysis of the physical and mechanical properties of normal and high-strength concretes exposed to temperatures up to 700 deg C. Ultrasonic pulse velocity, static and dynamic modulus of elasticity, and strength and deformations under compressive loading were measured. In addition, flexural and splitting tensile strength and the fracture energy were also determined. Other tests for finding the water permeability, water penetration, and absorption were performed on concrete slices in order to analyze the differences in the physical properties of the cover and bulk concrete.

Round-robin analysis of the RILEM TC 162-TDF uni-axial tensile test: Part 1 Test method evaluation

A round robin test programme was carried out on the uni-axial tensile test recommended by the RILEM TC 162-TDF [1]. Both plain concrete and steel fibre reinforced concrete (SFRC) cylinders were tested. Two concrete strengths and three fibre dosages were included in the test programme. A comprehensive statistical analysis was carried out on the results to investigate the robustness of the test method. Investigations were carried out on the intra-lab variations, inter-lab variations and within-mix variations. Inter-lab variations were found to exist but these were not as significant as the inherent material variation. Although there were difficulties encountered during the test execution, the uniaxial test was found to be a robust test. The material parameter obtained from such a test is important in determining the fracture properties of concrete particularly in non-linear fracture studies.

Round-robin analysis of the RILEM TC 162-TDF uni-axial tensile test: Part 2: Fibre distribution

Plain and steel fibre reinforced concrete cylinders were tested uni-axially in tension under closed-looped conditions in a round robin test programme. During the test, three deformation readings around the notched plane were measured. It was found that during the course of the test, rotations were occurring and thus a fibre counting exercise was initiated. This was to investigate the influence of fibre distribution within the cylinder specimens on the observed rotations of the specimens. The results of the investigation could not detect any strong correlation between the fibre distribution and the observed rotations and a more likely source for the variations observed is the boundary conditions of the tests. Significant variations were found in the number of fibres with coefficients of variation in the range of 30% to 50%. Additionally, it was found that toughness is approximately linearly related to the number of fibres in the fracture surface up to a certain limiting value where the toughness appears to plateau in relation to the mumber of fibres.

Variability of the bond and mechanical properties of self-compacting concrete

This main objective of this research is to evaluate the variability of the mechanical properties (compressive strength, modulus of elasticity and tensile strength) and bond strength of the self-compacting concrete (SCC), with 50 MPa compressive strength at 28 days, varying the maximum aggregate size and the SCC fluidity. The tests were made in 15 x 30 cm concrete cylinders and in beams standardized by Rilem-Ceb-Fib (1973). In agreement with the obtained results, can be concluded that the variability of the self-compacting concrete is small for the modulus of elasticity and for the compressive strength, but the tensile strength presented a significant variability due to the failure mode. About the bond strength, the variability was small showing that the self-compacting concrete is reliable and possesses great potential for use in the civil construction.