Double Edge Wedge Splitting Research Articles

The optimal design of double-edge wedge-splitting tensile test for ultra high performance concrete

The double-edge wedge-splitting (DEWS) test is a new indirectly tensile test, those of stress distribution similar with the uniaxial tension state. However, there are no standard rules regarding the splitting specimen and groove dimensions suitable for the UHPC tensile properties. In this paper, DEWS test, conventional splitting test, and direct tensile test were performed to measure the tensile strength and tensile stress vs. crack opening response of UHPC. The digital image correlation (DIC) technique was utilized to detect the crack propagation paths and fracture modes. Two specimen shapes, three groove angles and three fiber types are utilized to this investigation. The experimental results of 18 DEWSs under tensile loading, including peak strength, tensile stress-COD curves, fracture modes and properties, and the crack paths, are presented. The results show that the UHPC tensile performance used with cubic specimen is less sensitive to groove angle than in the cylinder. An important finding is that DEWS test used with 45° grooved cube specimen presents the lowest variation in peak strength and stress-COD curve for UHPC tensile behaviors. After comparison, the tensile strengths using DEWS test and direct tension test are essential equal in first-cracking and peak strength. The crack paths and fracture mode are basically the same inside the DEWS and direct tension specimens. Accordingly, the DEWS test used with 45° grooved cubic specimen is suggested for measuring the UHPC tensile property.

Evaluation of tensile failure behavior of ultra high performance concrete under double-edge wedge splitting and direct tension loadings

UHPC tensile strength is often measured indirectly through splitting tests and flexural tests substituted for direct tensile tests. However, the tensile strain-hardening and multiple cracking performance of UHPC is highly sensitive to the testing techniques and the specimen shape used. In this paper, the double-edge wedge splitting (DEWS) test and direct tensile test were performed to evaluate the tensile strain-hardening properties and damage evolution of UHPC. The digital image correlation (DIC) technique was utilized to detect the cracking opening displacement (COD) and evaluate the damage evolution in the tensile stress vs. crack opening response. The results show that the tensile strength between DEWS and direct tensile tests in UHPC strain-hardening and strain-softening stages had a high correlation value of 0.98. Moreover, the difference in cracking strengths in the DEWS and direct tensile tests was less than 5 %, and those of peak strengths were fewer than 5.23 MPa. An improvement in relative COD increment was observed in DEWS and direct tensile tests before 90 % of the peak load at the post-peak branch. Results indicate that the DEWS testing is an efficient way to evaluate the UHPC tensile failure properties.

3D DEWS digital parametric modeling and manufacturing for obtaining the post-cracking parameters of SFRC

A pullout analytical model is introduced to simulate the Double Edge Wedge Splitting (DEWS) test to obtain the tensile post-peak response of steel fiber reinforced concretes. One of the main barriers to the widespread utilization of FRC as a structural system is the anisotropy of the material. This augments the uncertainty of the material behavior (fibers random distribution and orientation in the bulk concrete), increases the dispersion of the test results, and requires the performance of characterization tests to check the residual tensile stresses at specific crack opening intervals. The characterization tests proposed in the literature (e.g., Barcelona, Montevideo, Wedge Splitting Test, EN 14651) are time-consuming and relatively complex, demanding careful preparation and hydraulic presses with data acquisition systems. This paper aims to contribute to the discussion of alternatives for FRC characterization procedures by the introduction of a pullout analytical model to predict FRC constitutive law based on DEWS tests. The pullout analytical model is calibrated with pullout tests considering straight and hooked-end steel fibers with different strengths, embedment lengths, inclinations, and diameters. A 3D digital model reproducing the experimental DEWS samples is developed, considering the fibers’ randomness distribution, orientation, and wall effect. The analyses are compared with four different experimental tests. The results define a linear post-peak tensile constitutive law, considering the mean residual stress values at different COD values. The model was able to reproduce the specimens’ production procedures and the fibers’ distribution and orientation correctly. The results demonstrated a good agreement with the experimental data.

Integrated approach for quality control of fibre-reinforced sprayed concrete for tunnel lining

The quality control of fibre-reinforced sprayed concrete (FRSC) for tunnel linings has evolved significantly in recent years. One of the main innovations concerns the control of FRSC in the lining using small-scale tests of cores extracted from the tunnel instead of the traditional approach, based on the characterization of panels sprayed on moulds. This study combines experimental and numerical investigations to evaluate if constitutive models derived from small-scale tests, such as the double punch test (DPT) and double-edge wedge splitting (DEWS) test can be used to predict the behaviour of larger panels in the round panel test (RPT) ASTM C1550. Results confirm the link between the new and the traditional approaches. Findings demonstrate that small-scale tests can be considered as a viable method to evaluate the mechanical behaviour of FRSC both in regular quality control programs and for verifying an existing tunnel lining condition.

Technical and environmental potentialities of recycled steel fiber reinforced concrete for structural applications

The use of recycled materials and industrial by-products as sustainable constituents of cement-based materials could be an environmentally and technically promising solution for application to structural elements. In the present work, the technical and environmental impact of using recycled steel fibers as an alternative to industrial steel fibers for concrete reinforcement was assessed at material level. Numerical simulations were performed to derive the post-cracking constitutive laws of the developed Recycled Steel Fiber Reinforced Concrete (RSFRC) and Industrial Steel Fiber Reinforced Concrete (ISFRC) by inverse analysis of experimental results obtained from three-point notched beam bending tests (3PNBBT), round panel tests supported in three points (RPT-3ps) and double edge wedge splitting tests (DEWST). These simulations were able of fitting with high accuracy the experimental results and consequently to derive the tensile stress-crack width relationships of RSFRC and ISFRC that was used to numerically simulate the bending response of a T-cross section steel RSFRC beam failing in shear.The environmental impact of the incorporation of RSF in concrete in comparison with ISFRC was evaluated using Life Cycle Assessment methodology. The reduction of the environmental impact of the production of RSFRC compared to ISFRC with the same concrete strength class is demonstrated.

Effect of high temperature on the mechanical properties of concrete reinforced with different fiber contents

This study evaluates the effect of fiber content on the thermo-mechanical properties of steel fiber reinforced concretes (SFRC) and contributes to the definition of design-oriented parameters for SFRC after temperature exposure, which are limited in literature and current guidelines. The mechanical behavior of samples subjected to high temperatures (25 °C ≤ T ≤ 750 °C) and produced varying the fiber contents were analyzed. The tensile properties were determined using the Double Edge Wedge Splitting (DEWS) test, which proved to be a viable methodology to evaluate the post-crack behavior of SFRC exposed to high temperatures. The rate of degradation in compressive and tensile strength was reduced for higher contents of steel fibers after exposure to elevated temperatures. The reductions in compressive strength and the elastic modulus were more severe than those prescribed by European guidelines. Additionally, the tensile properties reduced significantly for T ≥ 450 °C. Also, a temperature-related reduction rate in the post-crack tensile properties was parameterized for all the fiber content samples, in which only 20% of fFts and 30% of fFtu room temperature values were retained after exposure to 600 °C, which are higher values comparing the European guidelines.

Alternative small-scale tests to characterize the structural behaviour of steel fibre-reinforced sprayed concrete

The flexural beam test EN 14488-3 is considered a reference for the characterization of the post-cracking tensile response of fibre-reinforced sprayed concrete (FRSC). However, the geometry of the specimens is a complication in case of characterising specimens extracted from the structure. The double punch test (DPT) and double-edge wedge splitting (DEWS) test are two alternative approaches that have been introduced to simplify the assessment of the post-cracking response of fibre-reinforced concrete. These tests could allow the characterisation of samples extracted from sprayed test panels or even existing structures. The objective of this experimental study is to demonstrate the possibility to correlate the flexural beam test EN 14488-3, the DPT and DEWS tests, and verify the capacity of these tests in predicting the constitutive model of FRSC. The results indicated high correlations (R2 above 0.98) between tests. The constitutive models obtained from the alternative tests were close to those determined employing the flexural beam test. Both the DPT and DEWS test can replace the EN 14488-3 test in systematic quality control or for the confirmation of design parameters of FRSC structure. However, the DEWS test requires more time to prepare the specimens, being more appropriate to parameterize the constitutive models of the FRSC. The DPT, due to its simplicity of execution, becomes more practical for quality control of FRSC. Specimen with different size subjected to the DPT show a similar residual strength pattern.

Characterization tests for predicting the mechanical performance of SFRC floors: identification of fibre distribution and orientation effects

In the context of the evaluation of the load-bearing capacity of a steel fibre reinforced concrete (SFRC) elevated slab recently built in northern Italy, this paper presents the study addressing the effects of fibre distribution and orientation. An extensive experimental programme was carried out in a collaboration between Politecnico di Milano and Universitat Politècnica de Catalunya. The programme included mechanical tests on four shallow beams and six notched standard beams. Additionally, uniaxial tensile tests (UTTs), double edge wedge splitting tests (DEWSTs) and double punching tests (DPTs) on 192 drilled core samples extracted from the shallow beams were performed. Inductive tests, measuring the self-induction change that occurs when a SFRC sample is placed inside a coil, were performed on all samples subjected to DPTs to assess fibre distribution. This paper compares direct and indirect tensile tests for the definition of the FRC post-cracking constitutive law, highlighting advantages and disadvantages of each test type. A comparison between standard and non-standard mechanical tests is also presented. Mechanical and non-destructive tests on drilled core samples extracted in different directions and at different locations have allowed the evaluation of the effects of fibre distribution and fibre orientation which provide an estimation of the possibility of using these tests as simplified tests for production control.

Characterization tests for predicting the mechanical performance of SFRC floors: design considerations

The paper presents an experimental program carried out to check the load bearing capacity of a steel fibre reinforced concrete (SFRC) floor in northern Italy. The extensive mechanical characterization focused on the suitability of 3 non-standardized test methods for quality control and tensile constitutive curve assessment was performed, this consisting of: uniaxial tensile test (UTT), double edge wedge splitting test (DEWST) and double punching test (DPT) to characterize the post-cracking mechanical properties of the material. The joint experimental programme, carried out at the Politecnico di Milano and at the Universitat Politècnica de Catalunya, included the flexural characterization of four shallow beams (1.5 × 0.5 × 0.25 m3) and six standard notched beams (0.55 × 0.15 × 0.15 m3). All the samples were produced from the same batch and with the same SFRC mix which was applied for the floor. After that, 192 cores were drilled from the shallow beams and subjected to either UTTs, DEWSTs or DPTs. The stress level, the scatter and the constitutive curves derived from the non-standardized tests were identified and analysed. The calculated constitutive curves were used to predict the behaviour of the shallow beams.

Fracture toughness parameters to assess crack healing capacity of fiber reinforced concrete under repeated cracking-healing cycles

This paper is the continuation of a research focused on the assessment of the crack-sealing capacity of Steel Fiber Reinforced Concrete (SFRC) with crystalline admixtures subjected to repeated cracking-healing cycles. In the first study, the work was focused on the quantitative evaluation of the crack-sealing performance by means of image analysis. To this purpose, crack sealing effectiveness was evaluated as a function of the presence of crystalline admixtures, maximum crack opening, duration of the healing period, exposure conditions (immersion in water, exposure to open-air exposure or wet/dry cycles), fiber orientation and number of cracking and healing cycles. The outcomes of the self-sealing phenomenon were analyzed defining a crack-sealing index calculated from images taken by means of a digital microscope both at the beginning and at the end of each healing exposure period. In this paper, it has been tried to move a step further, correlating the Sealing Index (crack closure in %) with parameters obtained from fracture toughness tests on specimens subjected to repeated cracking-healing cycles, with the aim of quantifying the retention and/or recovery of mechanical properties along the testing path. This is meant to simulate a real structural service scenario, in which a healed crack may reopen and be allowed enough time to re-heal, this repetition of events being likely to occur several times during the structure service life. To this purpose, equivalent tensile stresses (obtained from absorbed energy per unit fracture surface) were determined from nominal tensile stress vs. crack opening displacement curves obtained from a dedicated testing methodology, namely the Double Edge Wedge Splitting (DEWS) tests, and their evolution along the cracking and healing cycles was assessed. Results showed that, an increase of the Sealing Index, i.e. a more effective sealing of the cracks, also results into a slight increase of SFRC toughness performances as a consequence of both the through-crack matrix continuity reconstitution as well as of a likely improved bond between fibers and matrix. The method proposed in the paper can be further employed to build up a data-base in order to establish, through suitable meta-analysis procedure, sound correlation between parameters representative of crack self-sealing and material performance recovery (self-healing).

Evaluation of post-cracking behavior of fiber reinforced concrete using indirect tension test

This paper presents an experimental investigation on the applicability of the novel double-edge wedge splitting (DEWS) testing technique to evaluate the mechanical properties of fiber reinforced concretes (FRCs) with low fiber content. This study aims to validate the DEWS test for the structural evaluation of FRCs with softening behavior, which is also considered by the design codes for structural applications. In this case, the occurrence of post-peak instability is critical. This instability could compromise the residual strength evaluation, especially for the serviceability limit state (SLS). The experimental studies focused on three main aspects affecting the DEWS test response: effect of test displacement rate, effect of fiber content, and orthotropy of FRCs. Two different groups of FRCs (reinforced with steel and macrosynthetic fibers) were mechanically evaluated, both at the SLS and ultimate limit state. The variation of the test displacement rate had no significant impact on the FRC post-cracking pattern. The DEWS test could detect behavioral variations induced by altering the fiber content and fiber orientation on both composites. This variation was more evident for higher crack opening levels because the post-peak instability may compromise the DEWS results in the SLS.

A methodology to assess crack-sealing effectiveness of crystalline admixtures under repeated cracking-healing cycles

This paper analyzes the autogenous and stimulated self-sealing capacity of steel fiber reinforced concretes, with and without crystalline admixtures, under repeated cracking and healing cycles. To this purpose, the performance under cracking and healing cycles was investigated on 150 × 150 × 50 mm3 specimens, cracked by means of an indirect tensile test called Double Edge Wedge Splitting (DEWS) test. Two concrete mixes (with and without crystalline admixtures) and three healing exposure conditions were investigated: water immersion, open-air exposure and wet/dry cycles. Initially, the specimens were cracked up to a crack opening of 0.25 mm and were then subjected to the different aforementioned exposure conditions for 1, 3 and 6 months. At the end of each period, the specimens were cracked again and were subjected to the different exposure conditions for an additional 1 or 2 months, repeating the cracking and healing procedure up until a total duration of one year. The crack closure was analyzed using image processing methods. The results show that, for the same healing period, the specimens immersed in water reached the largest crack closures. In addition, it was observed that the crystalline admixture may favor long-term self-sealing capacity under repeated cracking and healing events.

Performance of concrete with and without crystalline admixtures under repeated cracking/healing cycles

The present paper analyses the repeatability of self-healing capacity in fibre reinforced concretes, both autogenous or enhanced by the addition of crystalline admixtures. The behaviour of the investigated concrete mixes was analysed by means of the “Double Edge Wedge Splitting (DEWS)” test, whose configuration and specimen geometry (150x15x50 mm) allow to predetermine the failure plane and the relative orientation of fibres. Initially, specimens were pre-cracked up to a crack width of 0.25 mm, then they were subjected for 1, 3 and 6 months to different exposure conditions, including water immersion, open air exposure and wet/dry cycles After that, specimens were subjected to repeated cracking and healing cycles, alternatively for 1 and 2 months, and up to an overall duration of one year. The crack closure index was evaluated using photogrammetric methods. The results highlighted the potentials of crystalline admixture in guaranteeing a better persistence of the healing performance in the long term and under repeated cracking and curing cycles.

On the reliability of the design approach for FRC structures according tofibModel Code 2010: the case of elevated slabs

AbstractThis paper focuses on the reliability of the design approach proposed in the fib Model Code for Concrete Structures 2010 for estimating the ultimate capacity of fibre‐reinforced concrete (FRC) elevated slabs on the basis of different tests for material characterization. The fracture properties of the material are determined through three‐point bending tests on notched beams and through double edge wedge splitting (DEWS) tests carried out on cylinders cored in the full‐size test structure. As a case study, an FRC elevated flat slab 0.2 m thick is considered which consists of nine bays (panels) measuring 6 × 6 m (overall size 18.3 × 18.3 m) and is supported by 16 circular concrete columns. The ultimate bearing capacity of the slab determined experimentally is compared with the design value predicted by means of a procedure based on limit analysis following fib Model Code 2010. The results show that the method proposed in fib Model Code 2010 using the characteristic values and the classification is reliable. Even if the tests are affected by a significant standard deviation and the two experimental campaigns with three‐point bending tests give a significant difference between class ”5c“ and class ”3e“, the structural test results in a loadbearing capacity that is always larger than the predicted one, which considers a safety coefficient for the materialγF= 1.5.

Double edge wedge splitting (DEWS): an indirect tension test to identify post-cracking behaviour of fibre reinforced cementitious composites

Experimental identification of the tensile constitutive behaviour of fibre reinforced cementitious composites represents a foremost step in design approaches for structural elements made with this kind of advanced construction materials. To this purpose, international standards and guidelines recommend either direct tension or/and bending tests, coupled with an inverse analysis procedure, both featuring pros and cons which have been exhaustively highlighted and discussed in these last years. The tremendous developments of self compacting fibre reinforced concretes have furthermore highlighted the need of duly taking into account, within the aforementioned identification procedure, the likely flow-induced alignment of fibres with the applied tensile stress. In this paper a novel experimental testing technique, called double edge wedge splitting test, has been presented and validated, also by means of nonlinear finite element analysis. The test, as it will be shown, is able to yield directly, i.e. with no need for back analysis, the tensile stress versus crack opening “constitutive relationship” of the material, though performing an “indirect” test, i.e. by applying a compressive load to the specimen. Both the simplicity of the indirect tensile testing technique and the straightforwardness of the identification procedure surely represent a distinctive and extremely attractive characteristic of the proposed test. Furthermore, the test features a compact specimen geometry, which can be even easily “cored” from existing structures, and a “notch preordained” fracture plane, which can be aligned to any desired angle with respect to the expected flow-induced fibre orientation. This makes the proposed test suitable for a thorough characterization of the influence of fibre orientation on the material constitutive behaviour, as it may be required by a casting process tailored to the intended applications.