Pseudo Strain-hardening Behavior Research Articles

폴리에틸렌 단일섬유를 혼입한 SHCC로 휨 보강된 콘크리트 보의 균열손상 제어 및 휨 성능

Required performance for repair materials are strength, ductility, durability and bonding with the substrate concrete. Various kinds of fiber-reinforced cement composites (FRCCs) have been developed and used as repair materials. Strain-hardening cement based composites (SHCC) is one of the effective repair materials that can be used to improve crack-damage tolerance of reinforced concrete (RC) structures. SHCC is a superior FRCC that has multiple cracking characteristic and pseudo strain-hardening behavior. The expansive admixture, which can be used to reduce shrinkage in SHCC materials with less workability by controlling interfacial bonding performance between SHCC and substrate concrete. For the application of SHCC as a repair material to RC structures, this study investigates the flexural performance of expansive SHCC-layered concrete beam. Test variables include the replacement levels of expansive admixture (0 and 10%), repair thickness (30 and 40 mm), and compressive strength of SHCC (30, 70 and 100 MPa). Four point bending tests on concrete beams strengthened with SHCCs were carried out to evaluate the contribution of SHCC on the flexural capacity. The result suggested that expansive SHCC materials can be used for repairing and strengthening of concrete infrastructures.

Direct assessment of tensile stress-crack opening behavior of Strain Hardening Cementitious Composites (SHCC)

The process of designing Strain Hardening Cementitious Composites (SHCC) is driven by the need to achieve certain performance parameters in tension. These are typically the pseudo-strain hardening behavior and the ability to develop multiple cracks. The assessment of the tensile load-deformation behavior of these materials is therefore of great importance and is frequently carried out by characterizing the material tensile stress–strain behavior. In this paper an alternative approach to evaluate the tensile performance of SHCC is investigated. The behavior of the material in tension is studied at the level of a single crack. The derived tensile stress-crack opening behavior is utilized to analyze and compare the influence of various composite parameters on the resulting tensile behavior. The deformations occurring during tensile loading are furthermore examined using a digital image-based deformation analysis technique to gain detailed insight into the crack formation, propagation and opening phases.

압축강도와 팽창재 대체에 따른 폴리에틸렌 합성섬유로 보강된 변형 경화형 시멘트 복합체의 역학적 특성

변형경화형 시멘트 복합체(SHCC)는 직접인장 상황에서 FRCCs에 비해 우수한 변형경화 특성을 갖는 재료이다. 하지만 SHCC는 일반적인 콘크리트에 비해서 시멘트비율이 높은 부배합 이며, 이에 따라서 자기수축이 큰 특성을 갖는 재료이다. 따라서 시멘트 복합체 내에 팽창재를 대체함으로써 수축저감을 통한 성능향상을 기대하였다. 이 연구에서는 각 강도별 SHCC의 배합에 팽창재를 대체함에 따른 역학적특성을 평가하고자 하였다. 시험결과 설계기준 압축강도 70MPa 배합이 압축, 인장, 휨, 시험에서 우수한 역학적 특성을 나타내었으며, 균열 특성에서는 팽창재를 대체한 SHCC가 균열분산 및 연성에서 우수한 특성을 나타내었다. In order to improve the dimensional stability and mechanical performance of cement-based composites, the effect of an expansive admixture based on calcium sulphoaluminate (CSA) on the shrinkage and mechanical properties of strain-hardening cement-based composite (SHCC), which exhibits multiple cracks and pseudo strain-hardening behavior in the direct tension, is investigated. Polyethylene fibers reinforced SHCC mixtures with three levels (30, 70, and 100MPa) of compressive strength were compared through free shrinkage, compressive strength, flexural strength, and direct tensile strength measurements. The SHCC mixtures were cast with and without replacing 10% of Portland cement content with CSA admixture. According to test results, CSA admixture is effective in reducing shrinkage of SHCC material. SHCC mixture with CSA admixture exhibited a little higher strength than companion mixture without CSA admixture.

Freeze-thaw influence on the flexural properties of ductile fiber-reinforced cementitious composites (DFRCCs) for durable infrastructures

Abstract Ductile fiber-reinforced cementitious composites (DFRCCs) are innovative cementitious materials characterized by multiple cracking and pseudo strain-hardening behavior under static flexure . This paper investigates the effects of freeze-thaw cycles, and water-to-binder ratio (W/B) as well as reinforcing fiber combination on flexural properties and cracking procedure of DFRCC prismatic specimens. The DFRCC materials used in the present study are reinforced with hybrid polyvinyl alcohol (PVA) and ultra-high molecular weight polyethylene (PE) at the 1.5% volume fraction. These DFRCC materials were tested for modulus of rupture (MOR), relative dynamic modulus of elasticity, and change in mass. The test results for freezing and thawing actions within 300 cycles indicate that freeze-thaw cycles have little effect on the MOR of the DFRCC materials, whereas freeze-thaw cycles have a negative effect on multiple cracking behavior and deformation capacity of DFRCC prismatic specimens under flexural loadings. The results of durability tests show that the DFRCC specimens remain durable after 300 cycles of freezing and thawing actions.

압출성형 ECC 패널의 섬유분포 특성과 휨 성능

이 연구는 변형률 경화거동을 나타내기 위한 압출성형된 ECC를 제조하기 위하여 사용되는 조성물의 특성, 제조 방식, 배합 조건, 양생 조건에 관한 검토를 수행하였으며, 섬유의 분포 특성이 압출성형 ECC의 휨거동에 미치는 영향을 파악하고자 하였다. 이를 위하여 이론적/실험적 연구를 수행하여 압출성형 ECC를 생산하기 위한 기본 배합 및 제조 공정을 제시하였으며, 이미지 프로세싱 기법을 이용하여 섬유 분포 특성을 파악하였다. 실험 결과, 최적의 압출성형 ECC 패널의 배합비를 물-매트릭스 비, 시멘트, ECC 파우더, 그리고 규사미분의 비율로 제시하였다. 또한 섬유 분포 특성은 배합에 따라 달라지며 이러한 섬유 분포 특성에 따라 휨거동에 차이가 발생하는 것으로 나타났다. 즉, 섬유 분산성이 좋을수록, 상보에너지( )와 최고 가교 응력(σ0)이 클수록 휨 인성이 크게 나타났다. 이는 ECC 배합의 차이가 섬유 분포 특성에 차이를 줄 뿐만 아니라, 마이크로역학 특성에도 변화를 주기 때문이다. 섬유 방향성의 경우, 실험체별로 크게 차이가 발생하지 않았으나 섬유의 분포가 3차원보다 2차원에 가깝게 배열되는 것으로 나타났다. 그러나 섬유 방향성에 대한 확률 밀도 함수는 2차원으로 가정한 경우와 매우 다른 양상을 보이는 것으로 나타났다. 따라서 원하는 성능(초기균열 강도 및 인성)을 얻기 위해서는 배합과 섬유 분포 특성을 고려하여야 하는 것으로 나타났다.

Fracture and tensile properties of polyvinyl alcohol fiber reinforced cementitous composites

Experiments were carried out to design polyvinyl alcohol (PVA) fiber reinforced cementitous composites (PVA-FRCCs) holding high ductility and energy consumption ability. Besides, the properties of each ingredients in composites, mixing method and technology for fresh mixture were described in detail. Then, the pseudo-strain-hardening (PSH) behavior was investigated in uniaxial tension test. As a result, the maximum ultimate tensile strain can reach 0.7 percent. On the other hand, the single edge notch (SEN) thin sheet specimens were employed to gain the normal tensile load via crack mouth opening displacement (CMOD) curves, which can show obvious PSH behavior. In addition, the curves can be divided into four zones whose fracture toughness calculation methods were discussed. The wedge splitting (WS) test method can be applied to discuss the fracture toughness. Moreover, fracture energy of SEN and WS specimens were both approximately evaluated.

PROPOSAL AND VERIFICATION OF EVALUATION METHOD FOR SHEAR STRENGTH AND DEFORMATION CAPACITY OF DAMPER USING HIGH PERFORMANCE FIBER REINFORCED CEMENTITIOUS COMPOSITE

The influence of ultimate tensile strain (ePSH) fluctuation during the Pseudo Strain Hardening (PSH) behavior on the shear strength and deformation capacity after the flexural yielding of damper with High Performance Fiber Reinforced Cementitious Composite was examined by FEM parametrical analysis. Subsequently, the evaluation methods for shear strength and deformation capacity after flexural yielding, which can give proper consideration to fluctuation of ePSH, were proposed, and their applicability was verified. As a result of comparing the applicability of these evaluation methods with parametric analysis, it was shown that the improvement of the deformation capacity after the shear strength along with the increase of ePSH and flexural yielding could be well evaluated. Furthermore, as a result based on comparison between the test results in the past research and the evaluated value, this proposal equation evaluated all the test results in the past research to be safe.

Diverse Application of ECC Designed with Ground Granulated Blast Furnace Slag

In the recent design of high ductile engineered cementitious composites (ECC), optimizing both processing and mechanical properties for specific applications is critical. This study employs a method to develop useful ECC produced with slag particles (slag-ECC) in the field, which possesses different fluid properties to facilitate diverse types of processing (i.e., self-consolidating or spray processing). Control of rheological modulation was regarded as a key factor to allow the performance of the desired processing while retaining the ductile material properties. To control the rheological properties of the composite, the basic slag-ECC composition was initially obtained, determined based on micromechanics and steady-state cracking theory. The stability and consequent viscosity of the suspensions were then mediated by optimizing the dosage of the chemical and mineral admixtures. The rheological properties altered through this approach were revealed to be effective in obtaining ECC-hardened properties, represented by pseudo strain-hardening behavior in uniaxial tension, allowing the readily achievement of the desired function of the fresh ECC.

Fatigue enhancement of concrete beam with ECC layer

The pseudo strain-hardening behavior of Engineered Cementitious Composites (ECC) is a desirable characteristic for it to replace concrete to suppress brittle failure. This widespread use of ECC in the industry is, however, limited by its high cost. To achieve higher performance/cost, ECC can be strategically applied in parts of a structure that is under relatively high stress and strain. In this paper, layered ECC-concrete beams subjected to static and fatigue flexural loads were investigated by experiments. The static test results showed that the application of a layer of ECC on the tensile side of a flexural beam increased its flexural strength and the degree of improvement increased with the thickness of ECC applied. In addition, the layer of ECC enhanced the ductility of the beam and the failure mode changed from brittle to ductile. Under four-point cyclic loading, the ECC layer significantly improved the fatigue life of the beam. Moreover, in comparison to plain concrete beams, layered ECC beams could sustain fatigue loading at a larger deflection without failure. The great improvement in fatigue performance was attributed to the effectiveness of ECC in controlling the growth of small cracks. The experimental findings reflect the feasibility of using ECC strategically in critical locations for the control of fatigue crack growth.

Flexural strengthening of reinforced concrete beams with carbon fibers reinforced polymer (CFRP) sheet bonded to a transition layer of high performance cement-based composite

Resistance to corrosion, high tensile strength, low weight, easiness and rapidity of application, are characteristics that have contributed to the spread of the strengthening technique characterized by bonding of carbon fibers reinforced polymer (CFRP). This research aimed to develop an innovate strengthening method for RC beams, based on a high performance cement-based composite of steel fibers (macro + microfibers) to be applied as a transition layer. The purpose of this transition layer is better control the cracking of concrete and detain or even avoid premature debonding of strengthening. A preliminary study in short beams molded with steel fibers and strengthened with CFRP sheet, was carried out where was verified that the conception of the transition layer is valid. Tests were developed to get a cement-based composite with adequate characteristics to constitute the layer transition. Results showed the possibility to develop a high performance material with a pseudo strain-hardening behavior, high strength and fracture toughness. The application of the strengthening on the transition layer surface had significantly to improve the performance levels of the strengthened beam. It summary, it was proven the efficiency of the new strengthening technique, and much information can be used as criteria of projects for repaired and strengthened structures.

高靱性セメント系複合材料を用いたダンパー部材のせん断耐力と変形能に影響を及ぼす要因に関する実験および解析的検討 : 高靱性セメント系複合材料を用いたダンパー部材のせん断耐力と変形能に関する研究(その1)

High performance fiber reinforced cementitious composite (HPFRCC) shows pseudo strain hardening (PSH) behavior under tensile stress. The purpose of this study is to verify the influencing factors on shear strength and deformation capacity of dampers using HPFRCC. In this study, the structural tests using HPFRCC dampers are conducted to obtain the basic data on shear behavior and investigate the application of FEM analysis to these experiments. On that basis, the shear resistance properties of dampers using HPFRCC are examined based on the results of experiments and analysis. It is concluded that the resistance force of the shear reinforcement and the tensile resistance force of the HPFRCC independently activate in the shear resistance properties of dampers using HPFRCC. In addition, the boundary point between PSH zone and strain softening zone in HPFRCC tensile stress-strain relationship is an important factor to control the brittle failure.

Flexural performance of layered ECC-concrete composite beam

The pseudo strain-hardening behavior of fiber reinforced engineered cementitious composites (ECC) is a desirable characteristic for it to act as a substitute for concrete to suppress brittle failure. The use of ECC in the industry is, however, limited by its high cost. To achieve higher cost/performance ratio, ECC can be strategically applied in parts of a structure that is under relatively high stress. In this paper, layered ECC-concrete beams subjected to flexural load are investigated from both theoretical and experimental aspects. Four-point bending tests are performed on beam members with ECC layer at its tensile side. The application of ECC layer leads to increase in both the flexural strength and ductility, and the degree of improvement is found to increase with the ECC thickness. A semi-analytical approach for modeling the flexure behavior of layered ECC-concrete beams is also developed. In the model, the stress–crack width relation of both concrete and ECC are employed as fundamental constitutive relationships. The model and experimental results are found to be in good agreement with one another. Simulation with the model shows that when the ECC thickness goes beyond a certain critical value, both the flexural strength and ductility (reflected by crack mouth opening and crack length at ultimate load) will significantly increase. The critical ECC thickness is hence an important design parameter, and it can be determined with the theoretical approach developed in the present work.

Practical Design Criteria for Saturated Pseudo Strain Hardening Behavior in ECC

Engineered Cementitious Composites (ECCs) have recently demonstrated their high performance with pseudo strain hardening (PSH) behavior in civil engineering structures and buildings. These materials incorporate low cost fibers such as Polyvinyl Alcohol fibers, which often rupture in composites. Such fiber rupture type ECCs tend to have inferior and unsaturated PSH behavior compared with those incorporating properly designed pull out type fiber. The present study focuses on presenting practical design criteria to achieve saturated PSH behavior in fiber rupture type ECCs. These criteria are proposed based on two performance indices, which are measures of energy exchange during steady state flat crack propagation and stress level to initiate micro-cracks. The latter performance index necessitates a new cracking strength prediction theory, which is proposed in the current study. Finally the cracking strength theory is justified using tensile test data, and the criteria are proposed based on the data in terms of these two indices.

New Developments in Analytical Modeling of Mechanical Behavior of ECC

Finite element method in conjunction with an appropriate material model may serve as a suitable tool to analyze the structural performance of Engineered Cementitious Composites (ECCs) - short fiber reinforced composites, which exhibit pseudo strain-hardening behavior and multiple cracking under tension. Several such models are reviewed and some new formulations are proposed. The new model represents a composite in multiple cracking state as an equivalent continuum with identical macromechanical properties. The constitutive law of the equivalent continuum is obtained as a relationship between overall stress and strain of a representative volume element (RVE). The RVE is modeled as a solid element intersected by fiber-bridged matrix cracks. In order to relate the relative displacements of crack faces to the bridging tractions, a generalized model of crack bridging is derived. A relationship between stress and crack density is also discussed. The resulting constitutive law is suitable for implementation in FEM, yet maintains a transparent link to a composite microstructure.

New Micromechanics Design Theory for Pseudostrain Hardening Cementitious Composite

The micromechanics design theory has realized random short fiber-reinforced cement composites showing pseudostrain hardening (PSH) behavior with over 5% of strain capacity under tension. Nevertheless, this existing theory currently is limited to specific constituent properties, which does not account for chemical bond and fiber rupture. This article presents a new design theory that eliminates this restriction, achieving fiber rupture type PSH-random short fiber-reinforced cement composites with high-performance hydrophilic fibers like polyvinyl alcohol fibers. Uniaxial tensile tests are conducted employing polyvinyl alcohol fiber composites, the results of which support the validity of the proposed theory. Furthermore, parametric study employing the proposed theory quantitatively evaluates the effects of composite's micromechanics parameters, such as bond strength and fiber strength, on composite performance. This parametric study reveals that continuously increasing the degree of fiber rupture (fiber rupture intensity) enhances the strength performance of composites but not energy performance. However, an optimum rupture intensity exists for maximizing energy performance, which is critical for PSH behavior. The consistency between theoretical predictions and experimental results consequently demonstrates that the proposed theory can be utilized practically as a powerful and comprehensive tool for PSH composite design.

On the shear behavior of engineered cementitious composites

Abstract Results of an experimental investigation of structural response of shear beams made of a special class of cementitious composites, referred to as engineered cementitious composites (ECCs), are reported. ECCs are designed with tailored material structure and have been shown to exhibit pseudo strain-hardening tensile behavior. The improved performance in shear over conventional plain, fiber-reinforced, and wire mesh reinforced concrete is demonstrated. It is suggested that ECCs can be utilized for structural applications where superior ductility and durability performance are desired.

Effect of fiber length variation on tensile properties of carbon-fiber cement composites

Unlike other fiber types, carbon fibers tend to possess a distribution of fiber lengths even before mixing into a cementitious matrix. During mixing, some amount of fiber rupture may be expected. Along with other fiber parameters, fiber length distribution may be expected to play an important role in governing the composite properties. This paper describes the results of an experimental program which measures the length distribution of carbon fibers both before and after mixing. A database of composite tensile stress-strain behavior is established for a number of commercially available carbon fibers in a cementitious matrix. Special emphasis is placed on pseudo strain-hardening behavior and the associated ultimate tensile strength beyond first cracking. A theoretical model of this ultimate strength which explicitly account for fiber length distribution is developed and shown to account for the reduced tensile strength of experimentally observed results reasonably well. Results of this research suggest that for the purpose of brittle cementitious matrix reinforcement, carbon fibers with higher strain capacity may be more important than high modulus typically demanded for polymer matrix composite reinforcement.