Experimental Investigation of Cumulative Damage and Self-Healing Properties of Smart Cementitious Composite under Continuous Compression Load

This study investigated the effect of sustained loading on the cumulative damage of a newly developed smart cement-based self-healing composite material (SMA-ECC). SMA-ECC is composed of engineered cementitious composite (ECC) and shape memory alloy (SMA) fibers. A uniaxial compressive test with five predefined loading levels (0%, 30%, 40%, 50%, and 60% of compressive strength) was conducted on SMA-ECC hollow-cylindrical specimens and ECC control hollow-cylindrical specimens. The cumulative damage was mainly determined by changes in the total water absorption of different groups of specimens during three different periods (not loaded, at a predefined loading level, and after unloading). A normalized water content index was proposed to couple the effects of self-healing, sustained loading, and cumulative damage. The test results indicate that the cumulative water absorption of SMA-ECC was 35% lower than that of ECC, which may indicate less irreparable damage. In addition, the self-healing ability of SMA-ECC specimens under different compression load levels was evaluated through normalized water content analysis. SMA-ECC exhibited a 100% repair rate at load levels of 30% and 40%. At a higher load level of 60%, the repair rate of SMA-ECC was 76%. These results collectively emphasize the significant impermeability and self-healing performance of SMA-ECC after unloading.

Cost-Effective Engineered Cementitious Composites with Hybrid PVA and Basalt/PP Fiber: A Study on Compressive, Tensile and Impact Performance

Currently, oil-coated PVA fibers are the most commonly used material in ECC research. However, the high price limits the application of PVA-ECC in practical engineering. In order to reduce the cost, one of the methods is to partially replace the PVA fibers in ECC. In order to demonstrate the feasibility of PVA/BF-ECC and PVA/PP-ECC, polyvinyl alcohol fibers (PVA), basalt fibers (BFs) and polypropylene fibers (PP) were added at 0.5%, 1.0% and 1.5% by volume of PVA in addition to 1% by volume of PVA. Subsequently, tensile, compression and drop-weight impact tests were conducted on single or hybrid fiber concrete. The results showed that the post-peak compression toughness, tensile strength, and initial cracking impact strength of PVA/BF-ECC and PVA/PP-ECC increased significantly with the increase in the volume ratio of BF and PP fibers, while the performance of PVA-ECC materials with the same fiber volume ratio decreased slightly. Therefore, the cost can be reduced by designing hybrid PVA/BF-ECC materials that meet the performance requirements. The experimental evidence presented in this study demonstrates the feasibility and reasonable prospect of the new hybrid PVA/BF-ECC.

Development of Sustainable Engineered Cementitious Composites by Incorporating Local Recycled Fine Aggregate

In this study, sustainable engineered cementitious composites (ECC) exhibiting high tensile strength as well as high tensile strain capacity were successfully developed by incorporating polyethylene (PE) fiber, local recycled fine aggregate (RFA), and limestone calcined clay cement (LC³). The improvement in tensile strength and tensile ductility was attributed to the self-cementing properties of RFA as well as the pozzolanic reaction between calcined clay and cement. Carbonate aluminates were also generated owing to the reaction between calcium carbonate in limestone and the aluminates in both calcined clay and cement. The bond strength between fiber and matrix was also enhanced. At the age of 150 days, the tensile stress-strain curves of ECC containing LC³ and RFA shifted from a bilinear model to a trilinear model, and the hydrophobic PE fiber exhibited hydrophilic bonding performance when embedded in RFA-LC³-ECC matrix, which could be explained by the densified cementitious matrix as well as the refined pore structure of ECC. Moreover, the substitution of ordinary Portland cement (OPC) by LC³ resulted in energy consumption and equivalent CO₂ emission reduction ratios of 13.61% and 30.34%, respectively, when the replacement ratio of LC³ is 35%. Therefore, PE fiber-reinforced RFA-LC³-ECC demonstrates excellent mechanical performance as well as considerable environmental benefits.

Bonding Mechanical Properties between SMA Fiber and ECC Matrix under Direct Pullout Loads

SMAF-ECC material composed of shape memory alloy fiber (SMAF) and engineered cementitious composite (ECC) has good bending and tensile properties, as well as good crack self-healing ability, energy consumption, and self-centering ability. The bond behavior between fiber and matrix is crucial to the effective utilization of the superelasticity of SMAF. The experimental study considered three variables: SMA fiber diameter, fiber end shape, and bond length. The pullout stress-strain curve of SMAF was obtained, and the maximum pullout stress, maximum bond stress, and fiber utilization rate were analyzed. Compared with the straight end and the hook end, the maximum pullout stress of the specimen using the knotted end SMAF is above 900 MPa, the fiber undergoes martensitic transformation, and the fiber utilization rate is above 80%, indicating that the setting of the knotted end can give full play to the superelasticity of the SMAF. Within the effective bond length range, increasing the bond length can increase the maximum anchorage force of the knotted end SMAF. Increasing the fiber diameter can increase the maximum pullout stress and maximum anchoring force of the knotted end SMAF but reduce the utilization rate of SMA fiber. This study provides a reliable theoretical basis for the bonding properties between SMAF and ECC.

Effect of Cementitious Capillary Crystalline Waterproofing Materials on the Mechanical and Impermeability Properties of Engineered Cementitious Composites with Microscopic Analysis

Building structures are prone to cracking, leakage, and corrosion under complex loads and harsh marine environments, which seriously affect their durability performance. To design cementitious composites with excellent mechanical and impermeability properties, Engineered Cementitious Composites (ECCs) doped with ultrahigh molecular weight polyethylene short-cut fibers (PE-ECCs) were used as the reference group. Different types (XYPEX-type from Canada, SY1000-type from China) and doses (0%, 0.5%, 1.0%, 1.5%, 2.0%) of Cementitious Capillary Crystalline Waterproofing materials (CCCWs) were incorporated. The effect of CCCWs on the mechanical and impermeability properties of PE-ECCs, and the microscopic changes, were investigated to determine the best type of CCCW to use and the best amount of doping. The results showed that with increasing the CCCW dosage, the effects of both CCCWs on the mechanical and impermeability properties of PE-ECC increased and then decreased, and that the best mechanical and impermeability properties of PE-ECC were achieved when the CCCW dosing was 1.0%. The mechanical properties of the PE-ECC were more obviously improved by XYPEX-type CCCW, with a compressive strength of 53.8 MPa, flexural strength of 11.8 MPa, an ultimate tensile stress of 5.56 MPa, and an ultimate tensile strain of 7.53 MPa, which were 37.95%, 53.25%, 14.17%, and 21.65% higher than those of the reference group, respectively. The effects of the two CCCWs on impermeability were comparable. CCCW-PE-ECC(X1.0%) and CCCW-PE-ECC(S1.0%) showed the smallest permeation heights, 2.6 mm and 2.8 mm, respectively. The chloride ion diffusion coefficients of CCCW-PE-ECC(X1.0%) and CCCW-PE-ECC(S1.0%) exhibited the smallest values, 0.15 × 10^-12 m²/s and 0.10 × 10^-12 m²/s, respectively. Micromorphological tests showed that the particle size of the XYPEX-type CCCW was finer, and the intensity of the diffraction peaks of C-S-H and CaCO₃ of PE-ECC increased after doping with two suitable doping amounts of CCCW. The pore structure was improved, the surface of the matrix was smoother, and the degree of erosion of hydration products on the fiber surface was reduced after chloride ion penetration. XYPEX-type CCCW demonstrated a more obvious improvement in the PE-ECC pore structure.

Bond Degradation Mechanism and Constitutive Relationship of Ribbed Steel Bars Embedded in Engineered Cementitious Composites under Cyclic Loading

In order to investigate the bond degradation mechanism and constitutive relationship of ribbed steel bars in engineered cementitious composites (ECCs) under cyclic loading, 12 groups of specimens were tested in this paper. The design parameters included ECC compressive strength, ECC flexural toughness, cover thickness, and anchorage length. The results indicated that the degradation of the bond behavior of the ribbed steel bars in the ECCs under cyclic loading was mainly caused by the degradation of the properties of the ECC material itself, concentrating on the development of cracks in the ECC, the extrusion and shear failure of the ECC between the steel bar ribs, and the continuous grinding of the ECC particles on the shear failure surface. The degradation of the bond stress-slip curves under cyclic loading was mainly reflected by the degradation in the ultimate bond strength and unloading stiffness. According to the monotonic loading test results, a monotonic bond stress-slip relationship model was proposed. On this basis, through building the hysteretic rules of the bond stress-slip curves under cyclic loading, a calculation model was proposed to predict the bond stress-slip constitutive relationship between the ribbed steel bars embedded in the ECCs under cyclic loading. Finally, the validity of the proposed model was verified by a comparison between the model curves and the tested curves.

Relationship between Fractal Dimension and Properties of Engineered Cementitious Composites with Different Aggregates

In this study, the effects of different fine aggregates on the properties of polyethylene fiber engineered cementitious composite (PE-ECC) were systematically investigated. The PE-ECCs were prepared with four fine aggregates, respectively. Furthermore, their flowability, compressive strength, and uniaxial tensile properties were studied experimentally and comparatively analyzed by microscopic techniques including X-ray diffraction (XRD), scanning electron microscope (SEM), energy-dispersive spectroscopy (EDS), and mercury intrusion porosimetry (MIP). The results showed that all the different types of fine aggregates exhibited little effect on the flowability of PE-ECC, but a greater effect on the compressive strength, uniaxial tensile strength, and ultimate tensile strain. PE-ECC prepared from untreated desert sand showed the best comprehensive performance, with compressive strength, uniaxial tensile strength, and ultimate tensile strain of 47.92 MPa, 6.26 MPa, and 3.638%, respectively. Moreover, it was found that the ultra-fine particles in the desert sand promoted the hydration reaction of cement and produced more C-S-H gels. The pore structures of ECC prepared with different aggregates exhibited obvious fractal characteristics, and the fractal dimension ranged from 2.8 to 2.9. The fractal dimension showed a strong correlation with parameters including ultimate tensile strain and pore structure, and the larger the fractal dimension, the smaller the ultimate tensile strain, porosity, and average pore size of ECC.

Carbonation Behavior of Engineered Cementitious Composites under Coupled Sustained Flexural Load and Accelerated Carbonation

Engineered cementitious composites (ECCs) belong to a broad class of fibre-reinforced concrete. They incorporate synthetic polyvinyl alcohol (PVA) fibres, cement, fly ash and fine aggregates, and are designed to have a tensile strain capacity typically beyond 3%. This paper presents an investigation on the carbonation behaviour of engineered cementitious composites (ECCs) under coupled sustained flexural load and accelerated carbonation. The carbonation depth under a sustained stress level of 0, 0.075, 0.15, 0.3 and 0.6 relative to flexural strength was measured after 7, 14 and 28 days of accelerated carbonation. Thermogravimetric analysis, mercury intrusion porosimetry and microhardness measurements were carried out to show the coupled influence of sustained flexural load and accelerated carbonation on the changes of the mineral phases, porosity, pore size distribution and microhardness along the carbonation profile. A modified carbonation depth model that can be used to consider the coupled effect of flexural tensile stress and carbonation time was proposed. The results show that an exponential relationship can be observed between stress influence coefficient and flexural tensile stress level in the carbonation depth model of ECC, which is different when using plain concrete. Areas with a higher carbonation degree have greater microhardness, even under a large sustained load level, as the carbonation process refines the pore structure and the fibre bridges the crack effectively.

An Experimental and Analytical Study on a Damage Constitutive Model of Engineered Cementitious Composites under Uniaxial Tension

Engineered cementitious composites (ECC) exhibit ultra-high ductility and post-cracking resistance, which makes it an attractive material in civil engineering. First, a monotonic uniaxial tensile test was performed, considering the effects of polyvinyl alcohol (PVA) fiber volume content and water-binder ratio. Then, the effects of the above variables on the tensile characteristics including the tensile stress-strain relationship, deformation capacity, and fracture energy were investigated based on test results; and when the water-binder ratio is 0.28 and the fiber volume content is 2%, the deformation performance of ECC is improved most significantly. Next, combined with damage mechanics theory, the damage evolution mechanism of ECC in monotonic uniaxial tension was revealed, based on which the damage factor and damage evolution equation of ECC were developed and the expressions of model parameters were proposed. Moreover, the comparison between the proposed model and test results demonstrated the accuracy of the proposed model. Finally, to further verify the feasibility of the proposed model, a finite element (FE) simulation analysis of the tensile performance of high-strength stainless steel wire rope (HSSWR) reinforced ECC by adopting the proposed model was compared with test results and the simulation analysis results by using anther existing model, the "trilinear model of ECC". The comparison shows that the proposed model in this paper can predict more accurately.

Time and Crack Width Dependent Model of Chloride Transportation in Engineered Cementitious Composites (ECC)

This paper aims to develop a chloride transport model of engineered cementitious composites (ECC) that can consider the influence of both exposure time and crack width. ECC specimens with crack widths of 0.1 mm, 0.2 mm and 0.3 mm were soaked into NaCl solution with periods of 30, 60, 90 and 120 days. The free chloride content profile was measured and used for the development of the transport model. Regression analysis was applied to build the time and crack width dependent models of apparent diffusion coefficient and surface chloride content. The results show that the crack width has significant influence on the free chloride concentration profile when it is above 0.2 mm and the time-dependent constant n decreases linearly with the crack width. The chloride transport model was obtained by subscribing the models of apparent diffusion coefficient and surface chloride content into the analytical solution of Fick's second law. The model was further validated with the experimental results, showing a deviation within 20%. The findings of the presented study can enhance the current understanding on the chloride transportation in ECC.

Research on Impact Resistance of Reinforced Concrete Beams Strengthened with Carbon Fiber Reinforced Polymer Grid and Engineered Cementitious Composites

When reinforced concrete structures are subjected to impact loads, they may suddenly yield or fail, or even collapse as a whole. In this paper, the impact resistance of reinforced concrete (RC) beams strengthened with carbon fiber reinforced polymer (CFRP) grid and engineered cementitious composites (ECC) was studied. Drop hammer impact tests were conducted on eight beams, then the finite element model was used to simulate the impact test, finally a simplified two-degree-of-freedom (TDOF) model was proposed for CFRP grid reinforced ECC layer strengthened RC beams under impact loading. The results showed that CFRP grid reinforced ECC layer significantly improved the impact resistance of RC beams. When the ECC and CFRP grid were used, the crack development was inhibited after the concrete cracked in the tensile area, avoiding the brittle damage of concrete beams with one crack to the end. Compared with the control beam, the reaction force of RC beams strengthened with CFRP grid and ECC under impact load increased by 16.2%~34.5%, the maximum mid-span displacement decreased by 16.3%~31.6% and the mid-span residual displacement decreased by 36.02%~49.53%. The finite element model and the proposed TDOF mode were demonstrated to effectively simulate the strengthened beam under impact loading.

Multiscale Investigation on the Performance of Engineered Cementitious Composites Incorporating PE Fiber and Limstone Calcined Clay Cement (LC3)

Limestone calcined clay cement (LC³) is successfully used to fabricate engineered cementitious composites (ECC) exhibiting tensile strength σ_tu of 9.55 ± 0.59 MPa or tensile strain capacity ε_tu of 8.53 ± 0.30%. The high tensile strength of the composites is closely related to the improvement of fiber/matrix interfacial bond strength, and the high ductility is attributed to the enhancement of fiber dispersion homogeneity. For the case of ECC incorporating 50% LC³, the reduction of initial cracking stress σ_tc that favors the growth of the crack in a controlled manner also contributes to the improvement of strain hardening behavior. The composition analysis indicates that carboaluminates and additional hydration products including C-(A)-S-H and ettringite are generated, which contributes to the densification of the microstructure of the ECC matrix. The pore structure is thus remarkably refined. Besides, when ordinary Portland cement (OPC) is partly replaced by LC³, the consumed energy and equivalent CO₂ emission decrease, especially the equivalent CO₂ emission with the reduction ratio attaining 40.31%. It is found that ECC using 35% LC³ exhibits the highest mechanical resistance and ECC incorporating 50% LC³ shows the highest ductility from the environmental point of view.

Experimental Investigation on Bending Behavior of Existing RC Beam Retrofitted with SMA-ECC Composites Materials

In order to realize the self-centering, high energy consumption, and high ductility of the existing building structure through strengthening and retrofit of structure, a method of reinforced concrete (RC) beam strengthened by using Shape Memory Alloy (SMA) and Engineered Cementitious Composites (ECC) was proposed. Four kinds of specimens were designed, including one beam strengthened with enlarging section area of steel reinforced concrete, one beam strengthened with enlarging section area of SMA reinforced concrete, beam strengthened with enlarging section area of SMA reinforced ECC, and beam strengthened with enlarging section area of steel reinforced ECC; these specimens were manufactured for the monotonic cycle loading tests study on its bending behavior. The influence on the bearing capacity, energy dissipation performance, and self-recovery capacity for each test specimens with different strengthening materials were investigated, especially the bending behavior of the beams strengthened by SMA reinforced ECC. The results show that, compared with the ordinary reinforced concrete beams, strengthening existing RC beam with enlarging section area of SMA reinforced ECC can improve the self-recovery capacity, ductility, and deformability of the specimens. Finally, a revised design formula for the bending capacity of RC beams, strengthened with enlarging sections of ECC, was proposed by considering the tensile capacity provided by ECC, and the calculated values are in good agreement with the experimental value, indicating that the revised formula can be well applied to the beam strengthening with enlarging section of SMA-ECC Materials.

Behavior Deterioration and Microstructure Change of Polyvinyl Alcohol Fiber-Reinforced Cementitious Composite (PVA-ECC) after Exposure to Elevated Temperatures

In the case of fire, explosive spalling often occurs in cementitious composites due to dense microstructure and high pore-pressure. Polymer fibers were proved to be effective in mitigating such behavior. However, deterioration of these fiber-reinforced cementitious composites inevitably occurs, which is vital for the prediction of structural performance and prevention of catastrophic disaster. This paper concentrates on the behavior and mechanism of the deterioration of polyvinyl alcohol fiber-reinforced engineered cementitious composite (PVA-ECC) after exposure to elevated temperatures. Surface change, cracking, and spalling behavior of the cubic specimens were observed at room temperature, and after exposure to 200 °C, 400 °C, 600 °C, 800 °C, and 1200 °C. Losses in specimen weight and compressive strength were evaluated. Test results indicated that explosive spalling behavior was effectively prevented with 2.0 vol% polyvinyl alcohol fiber although the strength monotonically decreased with heating temperature. X-ray diffraction curves showed that the calcium hydroxide initially decomposed in the range of 400-600 °C, and finished beyond 600 °C, while calcium silicate hydrate began at around 400 °C and completely decomposed at approximately 800 °C. Micrographs implied a reduction in fiber diameter at 200 °C, exhibiting apparent needle-like channels beyond 400 °C. When the temperature was increased to 600 °C and above, the dents were gradually filled with newly produced substance due to the synergistic effect of thermal expansion, volume expansion of chemical reactions, and pore structure coarsening.

Review of Cementitious Composites Containing Polyethylene Fibers as Repairing Materials

Polyethylene (PE) is an important polymeric material which is widely used in civil engineering. Recently, engineered cementitious composites (ECCs) have adopted PE fibers in structural repairing. ECC with polyethylene fibers (PE-ECC) has excellent tensile properties, ductility, strain-hardening behavior, thermal performance and durability. In this paper, a systematic review of the cementitious composites with PE fibers is summarized to facilitate the application of PE-ECC. The influence of PE fibers on the properties of ECC, such as compressive strength, flexural behavior, shear properties, impact resistance and tensile properties, is presented. Meanwhile, the properties of PE-ECC repaired structures, such as beams, walls and columns, are described. Further, the self-repairing properties of PE-ECC are presented. Finally, some suggestions for future research are provided in order to apply PE-ECC to practical repairing cases. The review exhibits that PE-ECC is of notable significance to the repairing of structures and clarifies its application scope.

Corrosion Behavior of AISI 1018 Carbon Steel in Localized Repairs of Mortars with Alkaline Cements and Engineered Cementitious Composites

The selection of materials for repairs of reinforced concrete structures is a serious concern. They are chosen for the mechanical capacity that the repair mortar achieves. However, several important characteristics have been left aside, such as the adhesion of the repair mortar with the concrete substrate, the electrical resistivity and—hugely important—the protection against corrosion that the repair material can provide to the reinforcing steel. The aim of this work was to study the corrosion behavior of AISI 1018 carbon steel (CS) in mortars manufactured with alkaline cements, engineered cementitious composites (ECC), and supplementary cementitious materials (SCM). Two types of ordinary Portland cement (OPC) 30R and 40R were used. The constituent materials for the mortars with ECC mixture mortars they use OPC 40R, class F fly ash (FA), silica fume (SF) and polypropylene (PP) fibers. The sodium hydroxide (NaOH) and sodium silicate (Na₂SiO₃) were used as activating agents in alkali activated cements. The reinforced specimens were immersed in two different electrolytes, exposed to a 3.5 wt % of NaCl and Na₂SO₄ solutions, for 12 months and their electrochemical behavior was studied by half-cell potential (E_corr) and linear polarization resistance (LPR) according to ASTM C876-15 and ASTM G59-97, respectively. The results obtained indicated that, the mortar they have the best performance and durability, is the conventional MCXF mortar, with OPC 30R and addition of 1% polypropylene PP fiber improves the behavior against the attack of chlorides and sulfates.

Investigation of Fibers Reinforced Engineered Cementitious Composites Properties Using Quartz Powder

In relation to the use of retrofit materials on damaged constructions, application on earthquake-resistant buildings, and for the strengthening and rehabilitation on weakened regions, there is a need for a more superior material than concrete. Application sites include beam-column joints, corbels, link-slabs, deep beams, support regions and dapped-end areas. Fiber reinforced engineered cementitious composites (FR-ECC) can address this issue, because FR-ECC is one of the composite materials that has high strength, ductility and durability. In order to develop FR-ECC, this study was done to investigate the effect of adding quartz powder on the compressive strength capacity and properties of FR-ECC through the use of polyvinyl alcohol (PVA) and steel fibers. The volume fraction of fiber was set to 0%-2%. To support the friendly environment, FR-ECC uses by-product materials such as fly ash and silica fume, with a cement content less than 600 kg/m³. In terms of the experimental investigation on FR-ECC, this work conducted the fresh property tests showing that PVA fibers have quite an influence on ECC workability, due to their hydrophilic behavior. By adjusting the superplasticizer (SP) content, the consistency and high workability of the ECC mixes have been achieved and maintained. The test results indicated that the PVA and steel fibers-based ECC mixes can be classified as self-compacting composites and high early compressive strength composites. Significantly, addition of quartz powder into the ECC mixes increased the compressive strength ratio of the ECC samples up to 1.0747. Furthermore, the steel fiber-based ECC samples exhibited greater compressive strength than the PVA fibers-based ECC samples with the strength ratio of 1.1760. Due to effect of the pozzolanic reaction, the fibers dispersion and orientation in the fresh ECC mixes, so that the cementitious matrices provided the high strength on the FR-ECC samples. During the compression loading, the bulging effect always occurred before the failures of the fibers-based ECC samples. No spalling occurred at the time of rupture and the collapse occurred slowly. Thus, FR-ECC has provided unique characteristics, which will reduce the high cost of maintenance.

Experimental Investigation on the Quasi-Static Tensile Capacity of Engineered Cementitious Composites Reinforced with Steel Grid and Fibers

An engineered cementitious composite (ECC) was reinforced with a steel grid and fibers to improve its tensile strength and ductility. A series of tensile tests have been carried out to investigate the quasi-static tensile capacity of the reinforced ECC. The quasi-static tensile capacities of reinforced ECCs with different numbers of steel-grid layers, types of fibers (Polyvinyl alcohol (PVA) fiber, KEVLAR fiber, and polyethylene (PE) fiber), and volume fractions of fibers have been tested and compared. It is indicated by the test results that: (1) On the whole, the steel grid-PVA fiber and steel grid-KEVLAR fiber reinforced ECCs have high tensile strength and considerable energy dissipation performance, while the steel grid-PE fiber reinforced ECC exhibits excellent ductility. (2) The ultimate tensile strength of the reinforced ECC can be improved by the addition of steel grids. The maximal peak tensile stress increase is about 50–95% or 140–190% by adding one layer or two layers of steel grid, respectively. (3) The ultimate tensile strength of the reinforced ECC can be enhanced with the increase of fiber volume fraction. For a certain kind of fiber, a volume fraction between 1.5% and 2% grants the reinforced ECC the best tensile strength. Near the ultimate loading point, the reinforced ECC exhibits strain hardening behavior, and its peak tensile stress increases considerably. The energy dissipation performance of the reinforced ECC can also be remarkably enhanced by such an increase in fiber volume fraction. (4) The ductility of the steel grid-PVA fiber reinforced ECC can be improved by the addition of steel grids and the increase of fiber volume fraction. The ductility of the steel grid-KEVLAR fiber reinforced ECC can be improved by the addition of steel grids alone. The ductility and energy dissipation performance of the steel grid-PE fiber reinforced ECC can be improved with the increase of fiber volume fraction alone. A mechanical model for the quasi-static initial and ultimate tensile strength of the steel grid-fiber reinforced ECC is proposed. The model is validated by the test data from the quasi-static tension experiments on the steel grid-PE fiber reinforced ECC.

Investigation on the Failure Behavior of Engineered Cementitious Composites under Freeze-Thaw Cycles

This paper investigates the freeze-thaw performance of engineered cementitious composites (ECC) reinforced with polyvinyl alcohol (PVA) fibers, by applying an innovative criterion for judging the specimen’s working state mutation. The ECC materials are prepared into 25 mixtures using the Taguchi method. Then, the fundamental transverse frequency, the flexural performance and the internal strain variation of ECC specimens subjected to freeze-thaw cycles are measured. Unlike the existing studies, this investigation focuses on the failure behavior of ECC materials in the process of freeze-thaw. The Mann-Kendall (M-K) criterion is introduced to detect the ECC specimen’s working state leap feature, leading to the updated definition of frost-induced failure concept. Furthermore, the three-level model for evaluating the freeze-thaw performance of ECC materials is established according to the revealed essential leap feature. Thus, the effect of each individual mix design factor on the frost-induced failure indices is perceived from the signal-to-noise (S/N) ratio analysis and the analysis of variance (ANOVA). Finally, a mix formulation estimated based on Taguchi method is recommended for its optimum resistance against frost-induced failure, which is verified by the confirmation experiment.

Stress⁻Strain Properties and Gas Permeability Evolution of Hybrid Fiber Engineered Cementitious Composites in the Process of Compression

Polyvinyl alcohol (PVA)-steel hybrid fiber reinforced engineered cementitious composites (ECC) characterized by optimal combination of high strength and high ductility were developed recently. These composites exhibit even tighter crack width than normal ECC, showing great potential for lower permeability in cracked state, and consequently improving the durability of ECC structures. In addition, the wide variety of promising applications in underground or hydraulic structures calls for knowledge on the mechanical behavior and corresponding permeability properties of strained ECC under multiaxial stress, as they are essential for structural analysis and durability design. Experimental investigations into the compressive properties and the in-situ gas permeability of PVA-steel hybrid fiber ECC were performed in this study, with special focus on the impact of additional steel fiber content and confining pressure. The test results show that the presence of a low confinement level allows ECC to attain a substantial improvement on compressive behavior but impairs the enhancement efficiency of additional steel fiber. The permeability evolution of strained ECC corresponds to the variation of radial strains, both of which experience little change below the threshold stress but a rapid increase beyond the peak axial strain. Apart from exhibiting low permeability at relatively small strains in the pre-peak stage, ECC can also exhibit low permeability at higher levels of compressive strain up to 2.0%. However, unlike the case in tensile loading, impermeability of cracked ECC in compression would be weakened by additional steel fibers, especially in the post-peak stage. The present research is expected to provide insight into performance-based durability design of structures made of or strengthened with ECC.