Strain-hardening Cement-based Composites Research Articles

Effect of shear reinforcement and external strengthening with strain-hardening cement-based composites (SHCC) on the impact resistance of reinforced concrete beams

The paper presents a series of impact experiments performed on reinforced concrete (RC) beams with and without stirrups and additionally without and with lateral strengthening layers made of two types of strain-hardening cement-based composites (SHCC). The impact tests were performed in an advanced drop tower facility with accelerated steel projectiles. Four impact velocities were applied, ranging from 17 m/s to 30 m/s, corresponding to kinetic energies from 2.1 kJ to 6.4 kJ. The difference among the 20 mm-thick, normal-strength SHCC layers with 2 % fiber volume content consisted in the type of reinforcing fiber: polyvinyl alcohol (PVA) or ultra-high molecular weight polyethylene (UHMWPE). The RC beams with no shear reinforcement yielded shear failure with displaced punching cones under all impact velocities. The stirrups notably increased their impact resistance and damage extent, allowing multiple impact loads without structural collapse. The lateral strengthening layers of SHCC substantially increased the load-bearing capacity, diminished the damage extent, and increased the damage tolerance under repeated impact events of the RC beams both with and without stirrups, proving to be a promising strengthening solution for existing structures prone to severe mechanical loading.

Micromechanical study on polypropylene-bicomponent fibers to improve mechanical interlocking for application in strain-hardening cement-based composites

Polyproplylene (PP) fibers find application in strain-hardening cement-based composite to enable the formation of multiple fine cracks for high energy absorption. Often, however, the mechanical performance of such composites suffers from insufficient fiber-matrix interaction. In the research at hand, bicomponent PP single fibers with rough surfaces for improved mechanical interlocking are produced using an IPF (Leibniz-Institut für Polymerforschung Dresden e. V.), an in-house designed and built, laboratory-scale piston fiber spinning device. The melt-spun fibers consist of a shell component composed of PP and various volume percentages of different inorganic particles of calcium carbonate (CaCO3), aluminum oxide (Al2O3), and of a core component made of the same polymer as in the shell. The bicomponent fibers, with shell diameters between 20 and 45 μm, were characterized by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX) to understand their morphology and to study the fiber surfaces after composite failure. Tensile strength and Young's modulus of the fibers were evaluated using tension tests. Furthermore, the single fiber pullout (SFPO) test was used to investigate the interfacial interaction between fiber and a cement-based matrix. Significant improvements in fiber-matrix bonding were achieved due to the rough surface employed in connection with the particles incorporated in the outer shell. Still further, the fiber's strength, attained using an offline step in the drawing process, contributes to enhanced energy adsorption under dynamic pullout loading. To evaluate the performance of the newly developed bicomponent fibers, they were compared to a self-spun monocomponent PP fiber and a commercial PP fiber. This comparison revealed slip-hardening induced by increasing surface roughness for enhanced mechanical interlocking and plastic polymer deformation in the fiber-matrix contact zone.

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.

Modeling the Tensile Behavior of Fiber-Reinforced Strain-Hardening Cement-Based Composites: A Review.

Strain-Hardening Cement-Based Composites (SHCCs) exhibit high toughness and durability, allowing the design of resilient structures. Despite the exceptional properties of SHCC and the current modeling techniques, the widespread use of the composite is limited. One limiting factor is developing and validating analytical models that could be used for optimizing mixes and designing structural elements. Furthermore, the composite mechanical response is complex and depends on several phenomena, such as fiber pullout, fiber orientation and distribution, size effect, fiber content, group effect, embedding length, fiber dimensions, and matrix strength. In this context, this research presents the state-of-the-art on the micro- and mesomechanisms occurring in SHCC during cracking and robust techniques to predict its tensile behavior accounting for such phenomena already proved experimentally. The study is relevant for designers and the scientific community because it presents the gaps for the research groups to develop new investigations for consolidating SHCC, which is a material to produce resilient structures.

Self-healing behavior of high-strength strain-hardening cement-based composites (HS-SHCC) blended with limestone calcined clay cement (LC3)

Self-healing is an important characteristic of cement-based composites, which is highly related to the safety and service life of construction materials under load and variable environmental conditions. This study investigated the self-healing behavior of ordinary Portland cement- (as Ref) and Limestone Calcined Clay Cement (LC3)-based high-strength, strain-hardening cement-based composites (HS-SHCC) under different exposure conditions. Experimental results showed that wet-dry cycles were favorable for crack healing in both Ref and LC3 HS-SHCC due to the abundant moisture for cement rehydration. However, the new-formed hydration products were weak and fiber bridging in HS-SHCC was not re-built. Thus, the healed pre-cracks were easy to crack again during re-loading. Between Ref and LC3 HS-SHCC, Ref composites showed a favorable self-healing behavior in terms of flexural strength improvement after wet-dry cycles, whereas LC3 composites presented desirable self-healing performance after freeze–thaw cycles due to the discrepancy in reaction products and microstructure. In summary, Ref and LC3 HS-SHCC presented desirable but different self-healing performances under various exposure conditions.

Mechanical Behavior of Strain Hardening Cement Based Composite Concrete

Abstract: In this research work we had to enhance the compressive as well as tensile strength of concrete with retrofitting method using the SHCC (strain hardening cement based composite). There are two critical issues with this type of cement replacement i.e. the changes in physical properties of concrete with respect to compressive, tensile strength and cost analysis of alternatives. The first stage of this research looks at the change in physical properties of concrete when cement is replaced by PVA fiber with respect to compressive strength and tensile strength. The result from this research shows a linear diminution in strength with linear increase in the relative %age of PVA fiber to cement. A traditional cost analysis looks only at the first or construction cost without considering the long-term cost to palliate for the increased global warming emission resulting from the manufacturing process of Portland cement. Experiments were performed on concrete mixtures with varying PVA fiber contents in concrete i.e. 0%, 1%, 2%, 3%, 6% and 9%. For SHCC the cubes specimens of size (150mmx150mmx150mm) with mild steel rods were used to early setting and hardening. We have to check the compressive strength at 3days, 7days and 28days and Tensile Strength at 28 days after the curing of sample.

Design and optimization of free-form surfaces for modular concrete 3D printing

Although they are very efficient structures, concrete shells have lost popularity due to the complexity of the traditional construction process using cast-in-place concrete. A key concept to overcome the labor-intensive formwork in situ is the segmentation of the shells into prefabricated parts. In order to avoid individual formworks during prefabrication as well, the authors rely on extrusion-based 3D printing of strain-hardening cement-based composite (SHCC). The goal is a highly automated, scalable, and adaptable flow-prefabrication of modules controlled by a holistic digital design process. Such the creation of modular free-form shell structures can be accelerated significantly, resulting in structures comparable with gridshells. Starting with the problem statement and the elaboration of the technology used, the main contribution of this research is the development of geometrical methods for modularization based on given production conditions. The challenge lies in the free-form geometry discretization with respect to the structural analysis and within the defined constraints such as planar quads, no edge torsion, and minimal material consumption. Methods of discrete differential geometry for circular PQ (planar quad) mesh generation are combined with Response Surface Methodology (RSM) for multi-objective optimization of the global parameterized shape. The results were illustrated in a study case where the geometrical and structural production parameters of starting and final shell are compared.

Textile-Based 3D Truss Reinforcement for Cement-Based Composites Subjected to Impact Loading Part I: Development of Reinforcing Structure and Composite Characterization

Concrete is extremely vulnerable against impact loading due to its low tensile strength and pronounced brittleness. The application of thin strengthening layers, containing Textile Reinforced Concrete (TRC) and Strain-Hardening Cement-based Composites (SHCCs) in a ductile cement-based composite, is a promising solution to enhance the impact resistance of existing concrete structures. Three-dimensional (3D) textile structures exhibit numerous advantages over two-dimensional (2D) ones, most importantly higher shear, bending and energy absorption capacity, hence, appear to be instrumental in providing sufficient reinforcement to the target strengthening layers. However, design variability and optimization possibility of available 3D textile reinforcement are restricted. This paper presents the development of novel textile-based 3D truss reinforcement that can overcome these limitations. On the basis of woven 3D cellular structures, innovative pyramidal 3D truss reinforcement with favorable load-bearing capacity as well as notable energy absorption capability is developed and successfully realized. To investigate the feasibility and efficacy, cement-based composite consisting SHCC and newly developed pyramidal 3D truss reinforcement is prepared and tested under high-speed tensile loading as well as transversal impact loading. The experimental results show that woven 3D truss reinforcement is highly compatible with SHCC, and significantly enhances its impact resistance. Furthermore, SHCC reinforced with novel pyramidal 3D truss structure remarkably outperforms that with 2D carbon reinforcing structure approved for commercial use.

Experimental modal analysis of RC beams strengthened with SHCC subjected to shear under impact strain rates

Strain Hardening Cement-based Composites (SHCC) holds great potential to be a remarkably effective material for shear strengthening of reinforced concrete beams in case of impact scenarios due to its great energy dissipation capability through the formation of multiple very fine cracks (∼100 μm of width). The effectiveness of this technique is investigated in the paper at hand through the observation of the changes in the dynamic properties in terms of natural frequencies, mode shapes, and damping ratios. Considering a non-destructive free vibration excitation of the specimens before the experiment (undamaged/intact state) to their condition after the destructive impact of an accelerated steel projectile. In this sense, two types of reinforcement configurations and two types of strengthening layers were investigated: reinforced concrete beams with and without stirrups were strengthened with SHCC made with polyvinyl alcohol (PVA) and ultra-high molecular weight polyethylene (UHMWPE) and tested under increasing levels of kinetic energy varying between 2.1 kJ and 6.4 kJ by using an accelerated impactor inside a drop tower facility. The parameter variation enabled conclusions with respect to the influence of the stirrup reinforcement and the SHCC strengthening contributions in the case of dynamic scenarios.

3D printing and assembling of frame modules using printable strain-hardening cement-based composites (SHCC)

Despite all their advantages, load-bearing concrete shell structures with double curvatures are not frequently in use. The main reason is the complexity of their construction. In such a context, this article starts with a brief, critical review of existing technologies while their pros and cons are highlighted. Against that background, the authors propose a new approach for the highly automated fabrication of gridshell structures from variable modules reinforced with textile meshes. To demonstrate the feasibility of such a new technology, a demonstrator called ConDIT 1.0, a sphere-like shell structure composed of several frames, was designed and built. The frame modules were fabricated automatically using extrusion-based 3D printing and a printable, strain-hardening cement-based composite (SHCC). This article presents the design of ConDIT 1.0, the mechanical material characterization of printed SHCC, the technology of module production, the results of geometry verification for print modules using 3D scanning, and the procedure for the demonstrator’s assembly.

A testing device to investigate the properties of strain-hardening, cement-based composites (SHCC) under impact shear loading

Strain-hardening, cement-based composites (SHCC) are a novel class of fiber-reinforced concretes which exhibit high tensile strain capacity and crack control prior to failure localization, making them suitable for application as strengthening layers or as main materials for construction elements exposed to severe loading and environmental conditions. As opposed to their tensile properties, the shear performance of SHCC has been only scarcely investigated. In the context of structural strengthening against dynamic actions such as impact loading, the paper at hand describes the development, implementation, and testing of a shear testing setup for ductile cementitious composites. The development of the testing setup had as its target its integration in a gravity-driven Split-Hopkinson tension bar (SHTB) and required both finite element modeling and experimental testing. The numerical modeling aimed at a detailed parameter study of the device geometry, boundary conditions, and notch configuration of the material specimens as well. The experimental program implied both quasi-static and impact shear experiments on normal-strength SHCC made with ultrahigh-modulus polyethylene (UHMWPE) fibers. Both the numerical and experimental analyses demonstrated the inapplicability of one-dimensional wave analysis in deriving specimen shear response. This was solved by applying high-speed optical measurements with subsequent digital image correlation (DIC), which also allowed the accurate evaluation of the fracture modes related to load history. The study highlighted the pronounced influence of loading and boundary conditions on the specimens’ response and demonstrated the necessity of complementary numerical analyses to derive the constitutive relationships of SHCC. Furthermore, it serves as basis for further device optimization in respect of structural and inertial effects.

Mechanisms of high frost scaling resistance of SHCC

This study aimed at evaluating the scaling resistance of strain-hardening cement-based composite (SHCC) exposed to freezing and thawing cycles. RILEM-CDF test was conducted to clarify which parameters of the composition of SHCC have an impact on the scaling resistance.The investigation was made to determine the influence of the following parameters on the scaling: water cement ratio of the mortar matrix, fiber volume, air volume, and the mixing ratio of non-binding materials, namely limestone powder, quartz sand, and sand. The water cement ratio was set to 0.3, 0.4, and 0.55. The volumetric ratio of fiber and air ranged from 0 to 1.2% and 0.8 to 11.0%, respectively. The CDF test was conducted on trowel-finished surfaces for 60 cycles, and the scaling mass was measured every 6 cycles. The specimen surfaces were observed after the test. Samples were taken out from the center part of the test specimens and pore size distributions were measured.The results indicated a generally high scaling resistance of SHCC, its scaling mass being lower than that of ordinary mortar with the same water cement ratio. Even an SHCC mortar matrix without fiber suffered less scaling than the ordinary mortar having the same water cement ratio. The scaling amount reduced proportionally to the particle size of the non-binding materials, i.e., the finer the non-binding material, the lower the scaling amount, presumably because finer particles make the microstructure denser. The fiber apparently also improved the resistance against the progress of scaling by bridging cracks.

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.

The use of X-ray microtomography to investigate the shear behavior of hybrid fiber reinforced strain hardening cementitious composites

This paper presents an experimental investigation on the shear response of strain-hardening cement-based composites (SHCC) using X-Ray microtomography (microCT). For the developed composites, PVA and micro steel fibers were used in a constant fiber volume fraction of 2.0 %. The hybrid composites were produced replacing PVA by steel fibers in 25 %, 50 % and 75 % fractions. The shear behavior of the composites was studied by performing Iosipescu shear tests. The crack pattern evolution along the tests and the displacement field in the notch region were analyzed using the Digital Image Correlation (DIC) technique. Information about the porosity, fiber distribution and alignment were obtained using microCT for each specimen tested . The results show that the hybridization increases the crack opening but has little effect on the shear strength. The results also show that PVA fibers affect the alignment of the steel fibers and increase the porosity of the composites. • PVA fibers increase the porosity and affect the alignment of steel fibers. • The inclusion of steel fibers increases the crack opening. • Steel fibers tend to concentrate in the notch region. • The largest pores do not affect the cracks pattern in the notch region.

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.

Increasing the Fatigue Resistance of Strain-Hardening Cement-Based Composites (SHCC) by Experimental-Virtual Multi-Scale Material Design.

Strain-hardening cement-based composites are a promising class of materials for a wide variety of applications due to their considerable tensile strength and pronounced ductility caused by the development of multiple fine cracks. Nevertheless, the safe use of such composites requires sound knowledge of their mechanical behaviour under different types of loading, particularly under fatigue loading, while considering distinct influences like initial crack width and fibre orientation. To deepen this knowledge, single-fibre pull-out tests on PVA-fibres from a cementitious matrix were carried out to gain information about the micro-mechanical and degradation processes of the fibre. It could be shown that the fibres tend to rupture instead of being pulled out under quasi-static loading. When changing the loading regime to alternating loading, this failure mechanism shifts to pull-out. By varying the experimental parameters such as initial crack width, inclination angle or compressive-force level a clear influence on the fibre’s crack bridging capacity could be observed associated with effects on the degradation processes. Based on the data obtained, a micro-mechanical numerical model was developed to support the assumptions and observations from single-fibre pull-out tests and to enable predictions of the performance of the material on the microscale under cyclic loading.

Diffusion coefficient and infinitesimal shrinkage strain of a Strain-Hardening Cement-Based Composite (SHCC) determined by inverse analysis of experiments

The moisture transport and drying shrinkage properties of a certain type of Strain-Hardening Cement-Based Composite (SHCC) are investigated. This material is characterized by the usage of short high-modulus polymeric fibers as reinforcement of a cementitious mortar matrix with comparatively small grain sizes. The drying-induced mass loss and length change of circular cylindrical specimens with a radius of 25 mm have been measured. By inverse analysis of the experiments utilizing an evolutionary optimization algorithm, the material-specific transport and drying shrinkage parameters were identified. A sensitivity analysis revealed that the model assumptions concerning the mechanical material behavior, incorporating damage and creep, had only a small influence on the present inverse analysis results. It was also concluded that for the investigated type of material and the chosen geometry, the directly measured steady-state specimen contraction appropriately describes the material-specific drying shrinkage behavior.

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.

A multi-scale approach for simulation of capillary absorption of cracked SHCC based on crack pattern and water status in micropores

Strain-hardening cement-based composites (SHCC) under cracked condition exhibit distinct behaviour of capillary absorption because of multiple fine cracks and complicated crack patterns. Past studies for estimating capillary absorption of cracked SHCC mainly focused on evaluating the results by total gravimetric weight. In addition, the average values of crack width and crack spacing were often used to characterize crack patterns. In this study, a multi-scale approach for numerical simulation of the capillary absorption of cracked SHCC is proposed by considering the detailed crack patterns such as the width and location of each crack. Water flows in bulk matrix and multiple cracks are simulated using two individual transport equations. Water absorbed from a crack to its adjacent matrix is treated as the mass exchange of the two equations, taking into account pore structure and water status in the bulk matrix as well as the damage level of the crack surfaces. Analysis is firstly carried out for SHCC with uniform crack width and crack spacing, showing consistent results with test data of previous studies in which the total absorption increased with decreasing crack spacing. Analysis for SHCC with non-uniform crack widths and locations, furthermore, exhibit significant water penetration at crack zones. Other factors, such as relative humidity for pre-drying, specimen height, water-to-binder ratio, and fibre content, are also discussed in the analysis. This approach enables the estimation of water profile of cracked SHCC, so can be used for further crack-linked process, for example, chloride penetration or self-healing.

On the use of limestone calcined clay cement (LC3) in high-strength strain-hardening cement-based composites (HS-SHCC)

High-strength strain-hardening cement-based composites (HS-SHCC) demonstrate excellent mechanical and durability properties. However, high cement content typical to HS-SHCC results not only in high carbon footprint, but also in excessive hydration heat and severe autogenous shrinkage. In this investigation, Limestone Calcined Clay Cement (LC3) was used to produce sustainable HS-SHCC. The LC3 substitution resulted in higher energy consumption during mixing and in shorter setting times of the fresh, plain matrices. Although the LC3 substitution slightly reduced the compressive strength, the formation of highly polymerized C-A-S-H gel and abundant ettringite benefited the flexural strength of the plain matrices. Additionally, single-fiber pullout experiments showed that the use of LC3 led to increased fiber-matrix bond strength and pullout energy. Finally, the replacement of Portland cement by LC3 resulted in HS-SHCC with similar mechanical performance to the reference composite, indicating a high potential for using LC3 in high-performance cement-based composites.