Analysis of the influence of test method and properties of steel fiber addition on concrete under the three-point flexural tensile Análisis de la influencia del método de prueba y las propiedades de la adición de fibra de acero sobre el concreto bajo la tracción por flexión de tres puntos

The characterization of steel fiber reinforced concrete with dry consistency require greater inspection the account of the particularities in the process of mixing, molding and densification, thus this paper aims to propose a methodology combined of densification by manual and mechanical means and the mechanical characterization by the three-point flexural testin prismatic dry cast steel fiber-reinforced concrete specimen. In a qualitative way, the densification-combined methodology provided a smooth surface on the specimen appearance and a reduction of voids due to trapped air. In the flexural test we verified slip-softening and slip-hardening, respectively, to lower and higher reinforcement indexes and theoretical volumes. By the analysis of variance of the results and the database we found out that there was no significant statistical difference to the analyzed residual strength for the combination of the test method (RILEM), consistency (plastic), type (hooked end) and form (glued), when the comparison is made by quadruple association.


INTRODUCTION
The mechanical behavior of the steel fiber-reinforced concrete depends on the concrete matrices and the steel fiber properties (geometry, length, shape, tensile strength, elastic\modulus, effective volume, distribution and orientation) [1]. Some authors in previous studies report that the raising of the fiber volume improves the mechanical response from the resistant behavior after the peak load [2][3][4][5][6][7][8][9]. However, it is necessary the existence of an appropriate fiber dosage, in this context [10] verified that the volume incorporation of long steel fibers of 30 kg/m³ (0.4%), in volumetric fraction) to the concrete provided improvements to the load and the flexural strength comparing to the reference concrete. Another studies indicate substantial mechanical response from the steel-fiber reinforced concrete at a fiber volume range from 20 kg/m³ (0.26%, in volumetric fraction) to 45 kg/m³ (0.60%), in volumetric fraction) [11][12][13] reports that hooked end steel fibers and the straight ones correspond to 67% and 9%, respectively, in terms of commercialization, they have the shape factor between 20.4 and 152 in which the shape factors between 45 and 63 are equivalent to 50% of the commercialized fibers.
The post-peak behavior evaluation in the steel-fiber reinforced concrete occurs through an experimental flexural test [14]. The flexural test of the steel fiber reinforced concrete has some normative recommendations to the behavior evaluation, among them, International Union of Laboratories and Experts in Construction Materials, Systems and Structures [RILEM] with three-point tests. Another objective of this study is to evaluate the behavior of the dry-cast concrete produced with materials from Belem do Pará and with steel fibers randomly oriented (loose or cast), with factor shapes (65 or 80), theoretical volume (0.25% or 0.38% or 0.50%) and reinforcing index (16.25 or 26.0 or 32.5) and (20.0 or 32.0 or 40.0), respectively, obtained from the product of the shape factor by the theoretical volume, when it is characterized by the flexure test, according International Union of Laboratories and Experts in Construction Materials. Systems and Structures [15]. The parameters utilized in this study are based on experimental campaigns from the literature about dry-mix concrete, for example [16][17].

EXPERIMENTAL METHODOLOGY
The developed experimental methodology seeks to obtain mechanical characterization from the steel fiber reinforced concrete executing the flexure test by International Union of Laboratories and Experts in Construction Materials. Systems and Structures [15] to get the flexure strengths in the limit of proportionality (LOP) (fct .L ), equivalents (feq ,1 or feq ,3 ) and residuals (fr ,1 or fr ,2 or fr ,3 or fr ,4 ).

MATERIALS AND METHODS
In the experimental procedure, there was a physical characterization. dosage and the quantity of prismatic specimens with dimensions of 150 mm x 150 mm x 550 mm, the results are presented in the Tables 1, 2 and 3, respectively. This research  uses a reinforcing index parameter, as shown in the  Table 2, which corresponds to the product of the theoretical volume by the shape factor from the steel fiber, in order to approach in a more comprehensive  [15]. The displacement control occurred by means of a LVDT (Linear Variable Displacement Transducer), which was supported on a "yoke" device. We present the schematic representation of the assay by International Union of Laboratories and Experts in Construction Materials, Systems and Structures [15], in Figure 1. Aiming to do a better evaluation of the steel-fiber reinforced concrete there was a database consolidation presented in C -cement CP II Z -32 RS; S -sand; P -pebble; e w/c -water/cement ratio.

RESULTS AND DISCUSSION
In a qualitative way, we verified that energy from an efficient vibration and a methodology combined of manual and mechanical densifying process provided a satisfactory composite compaction, which provided a smooth surface and the elimination of voids by trapped air in the prismatic concrete specimens of steel-fiber reinforced concrete. This condition caused less porosity, considering we observed a little bubble formation when they were immersed in water for curing. In the mechanical characterization, obtained results for flexure strength in the limits of proportionality, equivalent and residual. The results are presented in Table 5 Table 5. This behavior in the post-cracking phase is denominated as slip-softening, which was observed by [20-23, 25, 26-28]. According to [29], the steel fibers regularly present a slip-softening behavior due to the fact that     the breaking of the fiber-matrix interaction occurs by friction in the tension transfer from the matrix to the fiber. According to [30], this can happen by virtue of the fiber's high elastic modulus and the low damage degree from the fiber in the matrix, allowing the fiber to become intensely anchored, what guarantee higher residual strength in levels of minor displacements. We observed in the prismatic specimen that the propagation of the crack aperture was complicated by the steel fibers GSF or LSF reinforcing index (26 or 32 or 40), which resisted until the occurrence of their disruption. While in the prismatic specimens with steel fibers with GSF or LSF reinforcement index (16.25 or 26 or 32.5) there was a slip during crack opening. When prismatic specimens are submitted to higher levels of displacements there is a greater level of fiber slip and its pullout resistance is conditioned to the concrete matrix integrity, where there may be faults in the fiber-matrix interface anchor region. These failures can occur by matrix's micro fissures in the regions around the fibers as there is an interception of the crack propagation in the matrix by the fibers [30]. However, a slip-hardening behavior was observed for the specimens at the theoretical volume of 0.50% for GSF-IR-40 and LSF-IR-32.5, as well as at the theoretical volumes of 0.38% and 0.50% for LSF-40, when residual strength (fr ,1 ) was inferior to that achieved by (fr ,4 ). This behavior was verified by [20][21][22][23][24], which may occur due to defibrillation in the fibers, which tends to increase the post-cracking resistant capacity as the displacement increases.
Through the analysis of variance (ANOVA) to the evaluation whether exists or no meaningful statistical difference among the mean values of residual strengths (fr ,1 or fr ,3 ) when they are compared individually or in 4-way analysis of the variable categories, as example they were compared when the Test Method is RILEM (RM) and the Consistency is plastic (p) and the Shape is hooked end (HE) and the state is Glued (G) (RM-PHE-G). When the meaning differences are found in ANOVA, aiming more specific conclusions between the means, we applied the Tukey's and Fisher's test, which are presented in Tables 6 to 9.
It was verified there was no meaningful statistical difference among the medium results of residual strength when individually compared to the categories of variables: Test Method (p = 0.223), Consistency (p = 0.818), Shape (p = 0.164) and State (p = 0.484), as presented in Table 6. Similarly, there wasn't meaningful statistical difference, when compared the categories of variables: Test Method-Consistency-Shape-State (p = 0.389) in four-way analysis, according Table 7.
The residual strength (fr ,1 ) has better results to the Test Method (RM), Consistency (plastic), Shape (hooked end) and State (glued), when compared individually such as in 4-way. This way, it was obtained higher results of residual strength (fr ,1 ) with 4-way analysis to Test Method (RM) combined to Consistency (P), Shape (HE) and State (G) and Test Method (EN) combined to Consistency (D) and Shape (HE) and State (G), according to Table 6 Table 8. Similarly, there wasn't meaningful statistical difference (p = 0.212), when compared Test Method, Consistency and Shape (p = 0.025) in four-way analysis, as shown in Table 9. Higher values of residual strength were acquired (fr ,3 ) to Test Method (EN), Consistency (dry), Shape (hooked end) when they were individually compared. From the analysis of the variances, the residual strength (fr, 3 ) is statically the same, when combined the categories of variables: (i) Test Method (RM) and Consistency (P) and Shape (HE) and State (G) and (ii) Test Method (EN) and Consistency (P) and Shape (HE) and state (G). In this stage there is a stress transfer process in the matrix-fiber interaction with the steel fiber working as a tool to realize the tension transfer [29]. There is a transfer of friction in the fiber-matrix interaction region, when there is the beginning of the steel fiber displacement process in the matrix [31].

CONCLUSIONS
In the experimental study with materials from the Metropolitan Region of Belém showed that the methodology combined of manual densifying and the     mechanism used in this research regarding the reinforced concrete resulted in a smooth surface appearance and, in a qualitative way, allowed the reduction of trapped air voids, influencing the porosity reduction.
And although there were better results from residual strengths of specimens reinforced with steel fibers in the loose way comparing to the ones glued. It can be observed that the load versus displacement of the flexural tests indicate slip-softening behavior in the theoretical volume (0.25% and 0.38% and 0.50%) for glued steel fiber with reinforcing index (16.25 and 26 and 32.5), and slip-hardening at the theoretical volume 0.50% for the glued steel-fiber with a reinforcing index equals to 40, and theoretical volume (0.38% and 0.50%) for steel fiber in the loose way with reinforcing index of (32.5 or 40). It was verified that there was crack propagation until rupture from the steel fibers with length of 60 mm in both loose and glued-types with reinforcing index (26 or 32 or 40). There was no breaking but sliding of the 35 mm length steel fibers in both loose and glued type with reinforcing indexes (16.25 or 26 or 32.5) during the opening of the cracks.
The analysis of variance of the database contributes to indicate that the residual forces (fr. 1 or fr. 3 ) for the combination of Test Method (MR), Consistency (plastic), Shape (hooked end) and State ), when compared individually or by 4-way analysis, except for fr. 3, when compared to the State category individually and specifically for RM-P-S-L.