Analysis of the Alignment of Micro-Steel Fibers in Admixture-Based Self-Compacting Concrete (MSFR-SCC) using NDT and Evaluation of Its Effect on the Modulus of Rupture

Self-compacting concrete (SCC) is a particular type of concrete that flows and becomes consolidated under its own weight. Various have been conducted to enhance its performance. Key developments have been the addition of fibers and replacement of the mineral admixtures; several studies have focused on varying these factors in different proportions to produce a high performance concrete. However, there is no standard mix design for self-compacting concrete containing admixtures. Therefore, optimization of the mix design is very important in order to save time and to develop an economical SCC, but there has been no detailed study of mix design and optimization of materials, and no research into the alignment and orientation of the steel fibers. Along with the uniform dispersion of fibers, few steps have been taken to optimally align the micro steel fibers, with ultrasonic pulse velocity used to justify their distribution and alignment. These factors were taken into consideration in this study. The results show that the addition of steel fibers reduces the rheological properties of the concrete by 40%-55% compared to the control mix (MC1), although all the mixtures (MC1-MX4) justified the guidelines set by EFNARC (European Federation of National Association Representing for Concrete). All the specimens failed under shear. The addition of hooked-end micro steel increased the flexural strength compared to the control mixture and a 60% increase in MX4 and 75% increase in MA7 were observed compared to MC1. The fibers were aligned along the direction of flow and the alignment properties was justified by flexural strength and UPV. A regression equation was finally developed between flexural strength and UPV for future development, which predicts the behavior of micro steel fiber reinforced self-compacting concrete (MSFRSCC).

Flexural strength; Micro steel fiber; Orienting; Self-compacting concrete; UPV

Figure 1 Factors influencing the orientation and dispersion of fiber-reinforced concrete

Despite the fact that higher proportions of fibers gave better flexural quality, the workability of SFRC (steel-fibre reinforced concrete) was observed to be negatively influenced by the aspect ratios and proportions. Therefore, the aspect ratio is the most important constraint to accomplishing ideal workability and quality. In addition, the density of steel fibers is higher in the composite materials; because of this fibers could become isolated at the base of the mold during vibration, causing uneven scattering and affecting the homogeneity of the blend (Gettu et al., 2005). Hence, by taking advantage of the rheological properties of SCC in a fresh state, which fills formwork without vibration, the steel fibers can be added to the mix to produce micro steel fiber-reinforced self-compacting concrete (MSFRSCC), with a more uniform fiber dispersion in a highly workable mixture. Since the aspect ratio of steel fibers is directly proportional to their tensile strength and elastic modulus, hooked-ended micro steel fibers were used in this study (Won et al., 2013; Lee et al., 2015). SCC requires more cementitious material, which can be counterbalanced by using mineral admixtures such as fly ash and micro silica fume. Fly ash enhances workability because of its spherical-shaped particles, while micro silica fume helps in early strength (Eddhie, 2017). No study has been conducted on aligning or orienting the fibers. Hence in this study, along with the uniform dispersion of fibers, few steps were taken to align the micro steel fibers. Flexural strength and ultrasonic pulse velocity were used to justify the distribution and alignment of fibers. Initially, the mix design of self-compacting concrete was developed using the Nan Su method.

1.2.   Application of Non-Destructive Testing (NDT) to Fiber-Reinforced SCC (FRSCC)

European standard EN 14721 characterizes various testing methods for the metallic fiber content of concrete. However, these methods are insufficient for evaluating current study. Numerous other NDT and destructive logical techniques have been proposed. Non-destructive testing plays an essential role in distinguishing and recognizing deformities and cracks in numerous modern applications; for example, concrete structures, asphalts and metal testing (Wankhade & Landage, 2013). Non-destructive testing methods are used to classify and make detailed studies of the existing structure without disturbing it, so these techniques play a vital role in evaluating the mechanical characteristics of concrete structures (Tsioulou et al, 2017). The mechanical properties and stress pattern of FRC mainly depend on the alignment and dispersion of fibers in the matrix during the mixing and placing of the concrete. The introduction of self-compacting concrete into the market increased interest in studying and optimizing the distribution and orientation of fibers using the improved rheological properties of SCC. The commonly used NDT methods include CT scan - X-ray computed tomography (Pujadas et al., 2014), which provides detailed photographic images of the different composite materials present in concrete and helps to study their orientation and alignment. However, this method consumes more time and is economically inefficient. Electrical resistance tomography (ERT) is based on the injection of a current and fluctuation in the voltage received from object boundaries. The microwave non-destructive testing technique - ultrasonic and acoustic -is used to gauge the time required for the ultrasonic wave to go through the testing sample. By knowing the time and distance travelled, the wave speed through the material can be calculated, which helps to validate the sample accordingly (Gebretsadik, 2013). Table 1 shows the main advantages of fiber-reinforced self-compacting concrete over fiber-reinforced normally vibrated concrete.

The following conclusion has been derived in this study: (1) Increase in steel fires affects the flow, filling and passing ability of MSFRSCC. Addition of steel fibres increases the flexural strength of the concrete and resists the development of micro cracks that prevents the concrete from brittle failure and increases the strain hardening property; (2) The average flexural strength the contribution of SCC with aligned steel fibres (MA5, MA6 and MA7) was 8.5% more when compared with MX2, MX3 and MX4 for same quantities of fibre content, since of the fibres were aligned and they were artificially placed axial to loading direction; (3) From UPV results it was clearly seen that the inclusion of steel makes the concrete denser which makes the propagation of waves through the concrete easier in MX2, MX3 and MX4 when compared to MC1. The values of UPV were 4%-6% predominant in MA5, MA6 and MA7 when compared to MX2, MX3 and MX4 for the same quantities of steel fibres because of the uniform orientation of steel fibres; (4) Hence, the attempt made to align the steel fibres is successful. This is validated with the help of UPV testing and by flexural strength; and (5) The linear regression equations that were developed will help to predict the behavior of Micro Steel-Fibre Reinforced Self Compacting Concrete (MSFRSCC) according to their flexural strength and velocity.

Hence by making the use of rheological properties, using 0.25% of fibres for densely reinforced structure and using 0.5% and 0.75% for normally reinforced structure with mineral admixtures can make the construction highly efficient towards feasibility and strength criteria.

The authors gratefully acknowledge Vellore Institute of Technology, Vellore for their support through Seed Grant fund to carry out Research.