Behavior Of Strain-hardening Cement-based Composites Research Articles

Direct assessment of the shear behavior of strain-hardening cement-based composites under quasi-static and impact loading: Influence of shear span and notch depth

Strain-hardening cement-based composites (SHCC) represent a new frontier for improving the resistance of concrete structures against highly dynamic loading regimes, e.g., in the case of impact. A novel testing device was designed to characterize the shear behavior of such pseudo-ductile cementitious composites, whose dynamic response is extremely complex. The newly developed shear testing device was adapted to investigate the performance of fiber-reinforced, cementitious composites under both quasi-static and impact regimes. In the framework of setup validation and standardization, this article focuses on the investigation of the shear behavior of SHCC specimens and spotlights the influence of two main experimental shear parameters: shear span and notch depth. The purpose-specific shear device was integrated into a hydraulic testing machine and a gravity Split-Hopkinson tension bar (SHTB) for quasi-static and impact shear experiments, respectively. Shear spans of 2 mm and 5 mm were introduced by modifying the test setup. Furthermore, the specimens were shaped through sawn U-notches with varying depths of 3 mm, 5 mm, and 7 mm. The shear response of the SHCC specimens was monitored by means of Digital Image Correlation (DIC), which enabled the accurate derivation of strain fields, cracking behavior, and fracture modes on the specimen surface. The results showed that both shear span length and notch depth regulate the shear/tension fracture propagation. With an appropriate shear specimen shape, the desired dominant shear fracture could be obtained.

An experimental-analytical scale-linking study on the crack-bridging mechanisms in different types of SHCC in dependence on fiber orientation

A scale-linking, experimental study complemented by an analytical model was carried out to investigate the influence of fiber orientation on the crack-opening behavior of strain-hardening cement-based composites (SHCC). Three SHCC compositions were investigated with polyvinyl alcohol (PVA) and ultra-high molecular weight polyethylene (UHMWPE) fibers in combination with normal- and high-strength matrices. The micromechanical experiments with fiber inclinations of 0°, 30°, 45°, and 60° involved fiber embedment in plain and fiber-reinforced specimens. The experimentally derived micromechanical parameters were input into an analytical crack-bridging model to assess the upscaling accuracy of the micromechanical results by comparing the predicted crack-bridging laws to the single-crack opening behavior of equivalent miniature SHCC specimens with controlled fiber orientation. This study yields new insights into the effect of fiber orientation on the crack-bridging properties of different types of SHCC, assesses the link between micromechanical and composite scale properties, offers a solid experimental basis for refining the analytical models, and developing anisotropic materials models for SHCC in dependence on fiber orientation.

Rheological Model to Describe the Cyclic Load-Bearing Behaviour of Strain-Hardening Cement-Based Composites (SHCC).

The mechanical behaviour of strain-hardening cement-based composites (SHCC) under monotonic tensile loading has been the subject of research for many years. The recent research on the SHCC’s performance under cyclic loading has enabled the identification of a wide variety of damage phenomena different to those observed under monotonic loading. The article at hand first summarises the experimental evidence of such phenomena in the context of the material performance observed. On this basis, the mechanisms behind these phenomena are discussed and explained using rheological modelling.

Probabilistic Finite Element Modeling of Textile Reinforced SHCC Subjected to Uniaxial Tension.

The paper presents a finite element investigation of the effect of material composition and the constituents’ interaction on the tensile behavior of strain-hardening cement-based composites (SHCC) both with and without textile reinforcement. The input material parameters for the SHCC and continuous reinforcement models, as well for their bond, were adopted from reference experimental investigations. The textile reinforcement was discretized by truss elements in the loaded direction only, with the constitutive relationships simulating a carbon and a polymer textile, respectively. For realistic simulation of macroscopic tensile response and multiple cracking patterns in hybrid fiber-reinforced composites subjected to tension, a multi-scale and probabilistic approach was adopted. SHCC was simulated using the smeared crack model, and the input constitutive law reflected the single-crack opening behavior. The probabilistic definition and spatial fluctuation of matrix strength and tensile strength of the SHCC enabled realistic multiple cracking and fracture localization within the loaded model specimens. Two-dimensional (2D) simulations enabled a detailed material assessment with reasonable computational effort and showed adequate accuracy in predicting the experimental findings in terms of macroscopic stress–strain properties, extent of multiple cracking, and average crack width. Besides material optimization, the model is suitable for assessing the strengthening performance of hybrid fiber-reinforced composites on structural elements.

Mechanical behavior of strain-hardening cement-based composites (SHCC) subjected to torsional loading and to combined torsional and axial loading

Strain-hardening cement-based composites (SHCC) are a novel class of fiber-reinforced concretes which exhibit high tensile strain capacity prior to failure localization. Although the tensile behavior of SHCC has been a matter of study in numerous research works, the behavior of these composites under other loading modes has scarcely been investigated. The article at hand addresses the mechanical behavior of two types of normal-strength SHCC subject to uniaxial tension, torsion, and combinations of torsional and axial loading. The SHCC under investigation were made with polyvinyl-alcohol (PVA) and ultra-high molecular weight polyethylene (UHMWPE) fibers, respectively. Digital Image Correlation (DIC) was applied to evaluate the multiple cracking process and crack opening modes in conjunction with the axial and torsional loading histories. The study demonstrates the suitability of torsion experiments to assess the multi-axial and shear performance of SHCC, highlights the relation between multiple cracking and transfer capacity for shear forces, and emphasizes the importance of the type of reinforcing fibers on the shear strength and ductility of such composites.

An Experimental Investigation of the Behavior of Strain-Hardening Cement-Based Composites (SHCC) under Impact Compression and Shear Loading

The ductile behavior of strain-hardening cement-based composites (SHCC) under direct tensile load makes them promising solutions in applications where high energy dissipation is needed, such as in earthquakes, impacts by projectiles, or blasts. However, the superior tensile ductility of SHCC due to multiple cracking does not necessarily point to compressive and shear ductility. As an effort to characterize the behavior of SHCC under impact compressive and shear loading relevant to the aforementioned high-speed loading scenarios, the paper at hand studies the performance of a particular SHCC and its constituent, cement-based matrices using the split-Hopkinson bar method. For compression experiments, cylindrical specimens with a length-to-diameter ratio (l/d) of 1.6 were used. The selected length of the sample led to similar failure modes under quasi-static and impact loading conditions, necessary to a reliable comparison of the observed compressive strengths. The impact experiments were performed in a split-Hopkinson pressure bar (SHPB) at a strain rate that reached 110 s−1 at the moment of failure. For shear experiments, a special adapter was developed for a split-Hopkinson tension bar (SHTB). The adapter enabled impact shear experiments to be performed on planar specimens using the tensile wave generated in the SHTB. Results showed dynamic increase factors (DIF) of 2.3 and 2.0 for compressive and shear strength of SHCC, respectively. As compared to the non-reinforced constituent matrix, the absolute value of the compressive strength was lower for the SHCC. Contrarily, under shear loading, the SHCC demonstrated higher shear strength than the non-reinforced matrix.

Tailoring the crack-bridging behavior of strain-hardening cement-based composites (SHCC) by chemical surface modification of poly(vinyl alcohol) (PVA) fibers

With the aim of reducing the affinity of poly(vinyl alcohol) (PVA) fibers to cementitious matrices and allowing the tailoring of crack-bridging behavior in strain-hardening cement-based composites (SHCC), a study was conducted, in which PVA fibers typically used in SHCC were subjected to different types of chemical surface modification. Alkyl chains were covalently bonded to the fiber surfaces via an acid-catalyzed acetalization with butanal diluted in sulfuric acid (H2SO4) and hydrochloric acid (HCl), respectively. X-ray photoelectron spectroscopy and contact angle measurements documented the successful treatment and the hydrophobic character of the modified fiber surfaces when compared to the as-received oiled and non-oiled fibers. The single-fiber pullout experiments showed that the butanal modification performed in H2SO4 yielded a considerably more pronounced reduction in chemical adhesion in comparison to butanal-HCl. However, the butanal-H2SO4 modification yielded no significant improvement in composite ductility, which could be traced back to the relatively low abrasion resistance of the fibers and to the resulting surface damage amplified by the higher extent of slippage. Furthermore, the milder butanal-HCl modification yielded a balanced fiber-matrix adhesion, resulting in SHCC with distinctly superior tensile properties.

Tensile behaviour of strain-hardening cement-based composites (SHCC) with steel reinforcing bars

Strain-hardening, cement-based composites (SHCC) are a group of fibre-reinforced construction materials developed specifically to exhibit pronounced quasi-ductile behaviour and high strain capacity under tensile loading. In structural applications, SH CC are often used in combination with steel reinforcement. The article at hand presents an extensive experimental investigation carried out to analyse the load-bearing behaviour of R/SHCC subject to tensile loading. It includes material characterisation as well as uniaxial tension tests of slab elements with different reinforcement ratios and differing steel bar arrangements. It was shown that the production particularities of large specimens and their size influenced the mechanical performance of the SHCC, including its cracking behaviour, thus leading to a significant reduction of tensile strain capacity of structural elements in comparison with test results on small size specimens. The interaction with steel reinforcement, on the other hand, was found to facilitate multiple cracking and enhance tensile strain capacity during the stage of elastic steel deformations. However, a mutual dependency of SHCC fracture localisation and local plastic deformations of steel bars could be observed in the yielding stage of the steel reinforcement. The experimental results are discussed with respect to their implications for constitutive modelling under consideration of the individual behaviours of SHCC and steel bars as well as their bond interaction.

Coupled temperature and moisture effects on the tensile behavior of strain hardening cementitious composites (SHCC) reinforced with PVA fibers

This paper reports the experimental findings on the tensile behavior of strain-hardening cement-based composites (SHCC). The composites were subjected to the combined effects of elevated temperatures and internal moisture condition. Uniaxial tensile tests on dumbbell-shaped SHCC specimens with in situ temperature control were performed at 22, 60 and 100 °C. In addition, the effect of the internal humidity of SHCC (95, 50, 20 and 0%) coupled to the elevated temperatures was investigated. It was shown that the tensile strength decreases and the strain capacity increases with an increase in temperature. The influence of the internal moisture conditions was more significant in high temperatures. The strain capacity reduced significantly with a decrease in the humidity level. The crack pattern of the SHCC specimens was determined. Furthermore, single fiber pullout tests were performed under the considered high temperatures condition. Finally, the results are discussed based on the thermogravimetry analysis of the PVA fiber, alterations on its microstructure and surface coating.

Dynamic Tensile Behaviour of Strain-Hardening Cement-Based Composites (SHCC)

The paper presents a part of an experimental campaign consisting of quasi-static and impact tensile experiments on three different types of strain-hardening cement-based composites (SHCC) as well as on their constitutive cementitious matrices [1]. The investigation on different SHCC types was intended for analysing the effect of matrix composition and fibre type on the strain rate sensitivity of the composites and for enabling the formulation of material design recommendations for impact resistant SHCC. The dynamic tests were carried out by means of a Modified Hopkinson Bar (MHB) installed in the DynaMat Laboratory in Lugano, Switzerland, which enabled the characterization of the dynamic material behaviour under direct tensile loading in terms of force-displacement relationships.

Fatigue behaviour of strain-hardening cement-based composites (SHCC)

The safe use of strain-hardening cement-based composites (SHCC) in structural and non-structural applications often requires a solid knowledge of the mechanical performance of this novel material under cyclic loading. The article at hand presents the findings of a comprehensive experimental investigation focusing on fatigue behaviour and failure mechanisms of SHCC made with polyvinyl-alcohol fibre and subject to various loading regimes. Uniaxial tests were performed both as tension-swelling tests and alternating tension-compression cyclic tests. While the upper reversal point was controlled – depending on the chosen regime – either by a given deformation increment or load level, the lower reversal point was always controlled by a given load value. The experiments revealed a pronounced decrease in the number of load cycles to failure with increasing upper stress level, smaller applied strain increments and with transition from purely tensile loading to alternating tension-compression regime. Furthermore, the effects of these tests parameters on strain capacity and other material properties of SHCC were investigated. On the basis of the test results, evaluation of the crack patterns and microscopic analysis of the conditions of the fracture surfaces, four various failure modes were identified, each of them typical for particular cyclic loading scenarios.

Rheological characterisation and prediction of pumpability of strain-hardening cement-based-composites (SHCC) with and without addition of superabsorbent polymers (SAP) at various temperatures

The paper at hand presents the results of an investigation into the rheological behaviour of strain-hardening cement-based composites (SHCC) with and without the addition of fresh superabsorbent polymers. The effect of temperature variation in the range between 10 and 30°C is examined in terms of rheology, tribology and pumpability for up to 60min after mixing. The flow behaviour of SHCC is approximated as that of a Bingham fluid. Furthermore, two methods of estimating mixture pumpability, a tribometer and a Sliding Pipe Rheometer (SLIPER), are applied. The results describe the development of the Bingham parameters and slippage resistance as parameters related to pumpability. Additionally the correlation between the rheological properties and the pumpability of SHCC at various temperatures and times after mixing are analysed. Finally, the predictive capacity of the tribometer with respect to the concrete pumpability is validated by measuring the discharge pressure with SLIPER.

Effect of fiber properties and matrix composition on the tensile behavior of strain-hardening cement-based composites (SHCCs) subject to impact loading

The article at hand describes the behavior of high-strength and normal-strength strain-hardening cement-based composites (SHCCs) made of fine-grained matrix and high-density polyethylene fibers under quasi-static and impact tensile loading. The dynamic tension testing of unnotched and notched cylinders was performed using the Hopkinson bar at strain rates of around 150s−1. The responses of the materials under dynamic and quasi-static tensile loading were compared to the corresponding results for normal-strength SHCC made of polyvinyl-alcohol fibers as obtained in previous investigations. To explain the pronounced differences in rate effects on the material performance of various SHCC compositions, cracking pattern and fracture surface conditions were studied. Additionally, strain rate dependent changes in the mechanical behavior of individual fibers and in the fiber–matrix interfacial properties were deduced from single-fiber tension tests and fiber pullout tests, respectively. Altogether, the results obtained provide clear indications as to the decisive parameters for a purposeful material design of impact resistant types of SHCC for use in structural elements or protective overlays.

Cracking Behavior of Strain-Hardening Cement-Based Composites Subjected to Sustained Tensile Loading

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Coupled strain rate and temperature effects on the tensile behavior of strain-hardening cement-based composites (SHCC) with PVA fibers

This paper reports the experimental findings on the tensile behavior of Strain-hardening cement-based composites (SHCC) subjected to elevated temperatures and different strain rates and to combinations of these parameters. Uniaxial tension tests with in-situ temperature control were performed at 22°C, 60°C, 100°C and 150°C. In addition, the effect of loading rate was investigated using the strain rates of 10−5s−1, 3×10−4s−1 and 10−2s−1 at all four temperatures considered. It was shown that tensile strength decreases both with an increase in temperature and with a decrease in the strain rate. The strain capacity increases with decreasing strain rate at temperatures of 22°C and 60°C, but for the temperature of 100°C this material property increases when the strain rate increases. At 150°C the investigated SHCC loses its ductility and no noticeable strain rate effect can be observed. Furthermore, the residual properties of SHCC were evaluated using uniaxial tensile tests at room temperature on the specimens which were previously heated to 60°C, 100°C or 150°C. The residual tests showed that the strength, strain capacity, and work-to-fracture decrease with increasing pre-treatment temperature. However, in comparison with the results of the in-situ tests with elevated in-situ temperatures, the residual tests on SHCC yielded higher tensile strength and lower ductility. These results and possible mechanisms leading to changes in mechanical performance are discussed on the basis of the observed crack patterns on the specimens' surfaces as well as the microscopic investigations of the condition of fibers on fracture surfaces.

Behaviour of Strain-Hardening Cement-Based Composites Under High Strain Rates

In this research project the behaviour of strain-hardening cement-based composites (SHCC) subjected to low and high strain rates was studied. Uniaxial tension tests on dumbbell-shaped SHCC specimens were performed at rates ranging from 10-5s-1 to 50s-1. For the tests performed at strain rates of 10-2s-1 and below, SHCC yielded a moderate increase in tensile strength and simultaneous decrease in strain capacity with increasing strain rate. When tested for higher strain rates from 10 to 50s-1 a considerable increase in tensile strain and strain capacity was measured. Microscopic investigation of the fracture surfaces showed that almost no fibre failure and an average pullout length of 2.5mm were found in the high strain rate test. This observation is in contrast to that of rapid quasi-static testing, where the average fibre pullout length of 300μm was much shorter. Furthermore, the fibres on the fracture surfaces produced in the high rate tests exhibited pronounced plastic deformations. Finally, quasi-static and high-speed tension tests on individual fibres and single fibre pullout tests were performed. While the increase in the tensile strength of the fibre was only moderate in the range of strain rates investigated, a considerable increase in bond strength between fibre and matrix was determined.

Behaviour of Strain-hardening Cement-based Composites (SHCC) under monotonic and cyclic tensile loading

Following the presentation of experimental work in Part I of this article [1] the present part is dedicated to the derivation of constitutive relations for Strain-hardening Cement-based Composites (SHCC) under monotonic and cyclic tensile loading. These constitutive relationships are developed on the basis of a multi-scale modelling approach which considers the determining physical phenomena observed in experimental investigations. The multi-scale model is based on reproducing the fibre-pullout behaviour under monotonic as well as under cyclic loading by a multi-linear approximation, while statistics are used to describe the variation of results as observed in the experiments. Fibre embedment length and inclination serve as main parameters. These responses are further superimposed in order to describe the stress-crack opening behaviour of each individual crack while being loaded, unloaded, or reloaded. The overall stress–strain relationships for material under tensile loading are then derived by considering an increasing number of serial cracks and the contribution of the uncracked matrix. Particular cracking behaviour is adjusted by varying the model parameters. The modelled tensile behaviour of the material is compared with representative results of uniaxial tensile tests performed on the investigated SHCC, and the results of this comparison are discussed.

Behaviour of Strain-hardening Cement-based Composites (SHCC) under monotonic and cyclic tensile loading

This first part of the project at hand presents the results of experiments performed on a Strain-hardening Cement-based Composite (SHCC) in order to investigate the specific behaviour of such materials under monotonic and cyclic tensile loading. The testing programme included uniaxial tension tests on dumbbell specimens, tension tests on single fibres, single-fibre pullout tests , and optical investigations. All mechanical tests were performed under deformation-controlled loading regime. Optical investigations of the SHCC microstructure provided detailed insights into the failure mechanisms observed in the mechanical tests. The results obtained can serve as a reliable basis for the development of corresponding constitutive relationships relevant to SHCC. Since the modelling to be presented in the second part of this paper is based on a multi-scale approach, the experimental results are discussed in particular with respect to the identification and description of the determinant physical phenomena influencing material performance at different levels of observation.

Improved mechanical performance: Shear behaviour of strain-hardening cement-based composites (SHCC)

The retardation of moisture and gas ingress associated with important degradation mechanisms in cement-based composites in general and reinforced concrete or prestressed concrete in particular is an ongoing research focus internationally. A dense outer layer is generally accepted to significantly enhance durability of structural concrete. However, cracking leads to enhanced ingress, unless the cracks are restricted to small widths. Strain-hardening cement-based composites (SHCC) make use of fibres to bridge cracks, whereby they are controlled to small widths over a large tensile deformation range. In this paper, SHCC shear behaviour is studied, verifying that the cracks which arise in pure shear are also controlled to small widths in these materials. The design of an Iosipescu shear test setup and specific SHCC geometry is reported, as well as the results of a test series. A computational model for SHCC, based on finite element theory and continuum damage mechanics, is elaborated and shown to capture the shear behaviour of SHCC.