Strain Hardening Cementitious Composites Research Articles

Recycling polyethylene terephthalate wastes as short fibers in Strain-Hardening Cementitious Composites (SHCC)

As an important portion of the total plastic waste bulk but lack of reuse and recycling, the enormous amounts of polyethylene terephthalate (PET) solid wastes have led to serious environmental issues. This study explores the feasibility of recycling PET solid wastes as short fibers in Strain-Hardening Cementitious Composites (SHCCs), which exhibit strain-hardening and multiple cracking under tension, and therefore have clear advantages over conventional concrete for many construction applications. Based on micromechanical modeling, fiber dispersion and alkali resistance, the size of recycled PET fibers was first determined. Then the hydrophobic PET surface was treated with NaOH solution followed by a silane coupling agent to achieve the dual purpose of improving the fiber/matrix interfacial frictional bond (from 0.64 MPa to 0.80 MPa) and enhancing the alkali resistance for applications in alkaline cementitious environment. With surface treatment, recycling PET wastes as fibers in SHCCs is a promising approach to significantly reduce the material cost of SHCCs while disposing hazardous PET wastes in construction industry.

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.

Experimental determination of crack-bridging constitutive relations of hybrid-fiber Strain-Hardening Cementitious Composites using digital image processing

This study explored the potential benefits of employing hybrid polyvinyl alcohol (PVA)/steel fibers in Strain-Hardening Cementitious Composites (SHCCs) at the single-crack level. Due to the multiple-cracking nature of SHCCs with tight crack spacing, direct measurement of the crack-bridging constitutive relation for a single crack is challenging, as any reasonably sized physical gauge length would cover a number of subparallel cracks, therefore masks the true value of crack opening. In this study, a new test method using Digital Image Processing to capture the opening of a single crack in a double edge-notched specimen under tension was proposed. Compared to the commonly-used single-crack tension method, the proposed method obtained a more satisfactory crack-bridging relation for SHCCs. With the test results from the new method, it was found that hybrid-fiber SHCCs have the potential to show smaller crack widths than PVA-SHCCs, and the hybridization of PVA/steel fibers results in a positive synergetic effect at the single-crack level under uniaxial tension. In addition, the pseudo-strain-hardening indices were used to evaluate the potential of achieving tensile strain-hardening of hybrid-fiber SHCCs. The findings of this study can support future designs of SHCCs for the experimental determination and model calibration of the crack-bridging relations, and provide a better understanding of fiber hybridizations in SHCCs.

Tensile performance and impact resistance of Strain Hardening Cementitious Composites (SHCC) with recycled fibers

Strain Hardening Cementitious Composites (SHCC) are fiber reinforced composites exhibiting strain hardening and multiple cracking behaviors. The Polyvinyl Alcohol (PVA) fibers used in SHCC are expensive for normal civil engineering applications, so one kind of Polyethylene terephthalate (PET) fibers recycled from waste plastics are used in this study as a substitute of PVA fibers. Uniaxial tensile test was carried out on SHCC samples made with PVA or PET fibers where the PET-SHCC was found to behave poorer than the PVA-SHCC due to the weaker bond between PET fibers and matrix. Nevertheless, in the modified Charpy impact test which was designed to evaluate the impact energy absorption ability of SHCC, the PET-SHCC dissipated a great amount of energy comparable to that by PVA-SHCC, indicating that PET fibers can provide excellent impact resistance to cementitious composites. To reveal the mechanism of impact energy absorption, a physical model was developed to simulate the impact test, which can well support the experimental observations. Based on above findings, a hybrid mix of 1 vol% PVA fibers and 1 vol% PET fibers is recommended for practical applications to provide adequate tensile performance and excellent impact resistance with eco-friendly ingredients at low cost.

Self-healing behaviour of multiple microcracks of strain hardening cementitious composites (SHCC)

Strain Hardening Cementitious Composites (SHCC) exhibit multiple crack characteristics when subjected to a bending or tensile load. In this paper, SHCC specimens were preloaded under three-point bending to introduce multiple microcracks and exposed to different conditions to assess their self-healing. Observation and analysis of crack characteristics, water absorption tests by a gravimetric method and neutron radiography were conducted to study the self-healing behaviour of microcracks in SHCC under three conditions (water fog, water/dry cycle, Ca(OH)2 solution/dry cycle). The results indicate that self-healing is a slow process, even in the presence of liquid water or calcium hydroxide solution. The crack sealing degree decreases with the increase in crack width, and only fine cracks of 10–20 μm can be healed completely. Ca(OH)2 solution promotes the healing process due to the enhancement of the pozzolanic reaction of fly ash. Water capillary absorption can be significantly reduced by crack sealing. The correlation between the crack sealing degree (crack closure) and water absorption reduction can be described well by a linear function.

Tensile performance of sustainable Strain-Hardening Cementitious Composites with hybrid PVA and recycled PET fibers

Strain-Hardening Cementitious Composite (SHCC) is a type of advanced construction material that can enhance the resiliency and durability of structures. However, the high cost of the constituents limits the wide application of SHCCs. To reduce the material cost and improve the sustainability, this study explores the potential of replacing commonly-used polyvinyl alcohol (PVA) fibers by recycled polyethylene terephthalate (PET) fibers. The potential of fiber hybridization was first evaluated using micromechanical modeling, and the ultimate tensile strain of hybrid-fiber SHCCs was estimated using a semi-empirical method. Then the tensile performance of SHCCs after standard curing and accelerated aging was experimentally evaluated, and the crack pattern development with increasing tensile strain was recorded. Satisfactory mechanical performance can be achieved even when 50% of PVA fibers are replaced by recycled PET fibers with surface treatment. In addition, using recycled PET fibers in SHCCs can significantly reduce the material cost and environmental impact.

Nacre-inspired composite design approaches for large-scale cementitious members and structures

Nacre boasts a remarkable mechanical property profile, despite being composed 95% of a chalk-like mineral. Previous research has reported the features and deformation mechanisms responsible, inspiring many synthetic, biomimetic versions of nacre. However, due to processing challenges associated with nacre's complex structure, this composite design inspiration has not been applied on a scale useful for civil engineering applications. This research investigates the utility of nacre's composite design on a scale relevant to infrastructure by exploiting the “distributed microcracking” behavior of strain-hardening cementitious composites (SHCCs). With the goal of emulating nacre's deformation mechanisms, rather than simply structural mimicry, various design approaches within the constraints of construction practices are introduced and evaluated in direct tension and beam bending. An alternating layering scheme, using a geotextile mesh and a high strength SHCC, is seen to produce promising behavior, significantly outperforming the high strength SHCC in its monolithic form. Advantages are seen in tension, compression, and particularly flexure. The deformation mechanisms responsible for this enhanced performance, and those of alternate approaches, are discussed.

Use of high strength Strain-Hardening Cementitious Composites for flexural repair of concrete structures with significant steel corrosion

Steel corrosion is a major cause of deterioration for reinforced concrete structures, leading to significant area loss of rebars that affects structural safety. The most common repair approach is to splice additional reinforcements to the corroded rebars, which is time consuming and costly, as a large volume of sound concrete beyond the corroded part must be removed to provide sufficient lap lengths. The present study explores a new repair technique using high strength Strain-Hardening Cementitious Composites (SHCC) in the aforementioned situation. With SHCC compensating for the area loss of rebars, splicing additional reinforcements or removing a large amount of concrete is not necessary. In this paper, the design of high strength SHCC was first discussed. Then, rebars with reduced area were embedded inside SHCC blocks and tested under direct tension. Further, beams containing rebars with reduced area and patched with SHCC were tested under four-point bending. The test results at different scales verify the feasibility of the proposed repair technique which is more efficient and less costly. The findings of this study can support future repair applications using SHCC.

Performance of Strain hardening cementitious composite as strengthening and protective overlay in flexural members

Strain-hardening cementitious composites (SHCC) are advanced type of cement-based composite materials having superior crack control and tensile properties. Owing to such characteristics, SHCC can be used for strengthening and crack-width control of structural members. This paper presents a study on the flexural response of reinforced concrete (RC) beams with different overlays of SHCC. The work consists of RC-SHCC overlay beams, in which SHCC overlays of different thicknesses (15% and 30% of beam height, plus cover) and reinforcement ratios (0% and 0.4%) were cast at the bottom of the RC beams. The performance of the RC-SHCC overlay beams was compared with control RC beams having concrete overlays of similar parameters. A series of eight laboratory-scale control and composite beam specimens were tested under four-point bending test. From the experimental results, it was observed that RC-SHCC overlay beams showed improved flexural capacity and crack control as compared to that of control beams. The beams with unreinforced SHCC overlays showed significant improvement at service stage, while beams with reinforced SHCC overlays showed significant improvement at peak stage. The SHCC overlay beams without reinforcement have showed improved ductility as compared to control beams with concrete overlays. Additionally, the SHCC overlays performed as a protective layer for controlling the crack widths in the composite beams.

Tensile properties of strain-hardening cementitious composites containing polyvinyl-alcohol fibers hybridized with polypropylene fibers

Partially replacing polyvinyl-alcohol (PVA) fibers with polypropylene (PP) fibers in strain-hardening cementitious composites (fiber hybridization) modify certain mechanical properties of these materials. The hybridization based on the introduction of low-modulus hydrophobic polypropylene fibers improves the ductility and the strain-hardening behavior of the cementitious composites containing polyvinyl-alcohol fibers of different types (PVA-SHCC). Pull-out tests indicate that adding PP fibers increases the energy capacity of the hybrid composite with respect to the material containing only PVA fibers under tensile loading, and PP-fiber geometry (i.e., section shape and length) is a key factor in enhancing the strain capacity.

Study of steel corrosion in strain-hardening cementitious composites (SHCC) via electrochemical techniques

This paper investigates corrosion of steel bar in reinforced strain-hardening cementitious composites (R/SHCC) through electrochemical techniques. Linear polarization resistance is engaged to determine corrosion rate of steel bar while electrochemical impedance spectroscopy is employed to study corrosion kinetics by probing the electrode and interface reactions. An equivalent circuit model is used to quantitatively interpret the impedance data of steel corrosion. Results show that polarization resistance Rp of R/SHCC is much higher than that of R/M and thus corrosion rate of steel bar in R/SHCC is much reduced. The Rp of R/SHCC is about two orders of magnitude higher while the corrosion rate of R/SHCC can be two orders of magnitude lower than that of R/M after 137 h accelerated corrosion. Rp of specimen is dominated by the charge transfer resistance Rct before cover cracking and governed by both Rct and cover resistance Rc after cover cracking. R/SHCC specimen possesses a much higher Rct and Rc than R/M due to high tensile ductility and damage tolerance of SHCC material together with tight crack with in SHCC cover.

3D neutron tomography of steel reinforcement corrosion in cement-based composites

Corrosion of steel reinforcement is one of the major durability problems for reinforced concrete structures. The steel corrosion in both ordinary mortar and strain hardening cementitious composites (SHCC) has been for the first time visualized by means of neutron tomography in three dimensions with high resolution. It has been found that the steel rebar starts corroding from the bottom interface between the steel and cementitious matrix. The corrosion products mainly exist around the cracks. Microcracking SHCC has higher resistance against steel rebar corrosion than cracked ordinary mortar.

Improved numerical approach for evaluating the behavior of reinforced concrete members with flexural strengthening using strain hardening cementitious composite

Strain hardening cementitious composite (SHCC) is an attractive construction material used for reinforced concrete (RC) strengthening, and a simplified method has been previously proposed for evaluating the behavior of SHCC flexural strengthening RC member (Zhang, Engineering Fracture Mechanics 2014; 121–122: 11–27), whereas the load carrying capacity of strengthened RC member is over-estimated using the simplified method. In this study, a numerical approach improved from the aforementioned method is presented for accurately evaluating the behavior of SHCC flexural strengthening RC member, considering the influence of the tensile strain capacity of SHCC, presence of localized concrete cracks and varying SHCC layer thickness, in which a zero-span tensile model with fictitious material is adopted to obtain the average tensile behavior of SHCC layer for strengthening RC member with crack. The effectiveness of the improved numerical approach is confirmed thorough the comparison between experimental and numerical investigations.

Capacity Magnification by Imposing Composite Action of Precast Concrete Flanged Girders

This paper presents an experimental study in order to verify the efficiency of different connecting techniques in improving the horizontal shear resistance of composite concrete flanged beams. The studied composite beams consisted of precast concrete webs interconnected with precast concrete flanges. Different connecting techniques had been considered; namely, plain/reinforced Ultra- High-Performance Strain-Hardening Cementitious Composites (UHP-SHCC) shear pockets, UHP-SHCC shear buckets in the flange accommodated extended steel dowels from the web, unbonded internal steel fasteners and intermediate shear intrusions. Eight composite flanged beams incorporating different connecting techniques along with three reference beams; monolithic, successively cast flanged beam and unconnected web/flange beams were prepared and tested up to failure under two cycles of non-reversal repeated loading. Furthermore, the push-off test method was employed in order to determine the horizontal shear resistance of the used connecting techniques. Each technique came up with advantages and disadvantages. Therefore, it is a matter of compromise to decide the most appropriate technique according to the necessary criteria (ultimate capacity or ductility). Finally, the horizontal shear resistances were calculated based on the experimental findings and compared with the stipulated resistances in the ECP 203-2007 and the ACI 318-14 design codes. It was found that the design codes’ values showed conservative results.

Shear stress transfer model for evaluating the fracture behaviour of SHCCs for RC shear strengthening

This paper presents a numerical shear stress transfer model for investigating the shear fracture behaviour of strain-hardening cementitious composites (SHCCs) for strengthening reinforced concrete (RC) structures. The model focuses on the transferred shear stress on the crack surface of the SHCC, taking account of the contributions from both the contact stress of the matrix and the bridging stress of the fibres. The numerical results demonstrate that the proposed shear stress transfer model can appropriately evaluate the shear fracture behaviour of SHCCs for RC strengthening, in which the fibre bridging effect on the crack surface of the SHCC has much influence on the shear fracture behaviour of the SHCC strengthening layer.

Mechanical properties and self-healing evaluation of strain-hardening cementitious composites with high volumes of hybrid pozzolan materials

Abstract The study investigates the self-healing performance and mechanical behavior of strain-hardening cementitious composites (SHCCs) incorporating high volumes of blended ground-granulated blast-furnace slag and fly ash (GGBS-FA SHCCs). The prepared SHCC specimens are pre-cracked by imposing a 0.5% or 1% tensile strain at the age of 180 days, and then exposed to lab-controlled dry, water immersion, or natural weathering conditions for 90 days. Multiple performance evaluation approaches are employed, including compressive tests, tensile tests, resonant frequency measurements, scanning electron microscopy, and energy dispersive x-ray analysis. In comparison with the performance of SHCCs without GGBS, blending GGBS and fly ash in SHCCs is shown to improve the tensile and compressive strengths at 7, 28, and 90 days, while the strain capacity and the multiple narrow cracking pattern of SHCCs are weakened. Furthermore, the SHCCs with blended GGBS and fly ash have enhanced self-healing performance compared to the SHCCs incorporating only fly ash.

Evaluation of strain-hardening cementitious composites for RC seismic strengthening

A numerical approach for evaluating the behaviour of strain-hardening cementitious composites (SHCCs) for reinforced-concrete (RC) seismic strengthening is presented. The method takes account of the seismic tensile strain capacity of SHCCs for RC strengthening, which is affected by the presence of cracks within the RC and varying layer thickness of the SHCC, the shear transfer behaviour of SHCC layers on the crack surface, which is contributed to by both the contact effect and the fibre bridging effect on the crack surface, and the seismic compressive constitutive law of SHCCs. In the numerical approach, a zero-span tensile model is adopted for reflecting the tensile strain capacity of SHCCs for RC strengthening, and a shear stress transfer model is applied for reflecting the respective contributions of both contact stress and fibre bridging stress for shear stress transfer behaviour of the SHCC on the crack surface. The effectiveness of the numerical approach is confirmed through the comparison between the experimental investigation and numerical analysis, in which the numerical result is coincided with the experimental one.

Shear strengthening of damaged reinforced concrete beams with Hybrid Composite Plates

This paper aims to evaluate experimentally the potentialities of Hybrid Composite Plates (HCPs) technique for the shear strengthening and repairing of reinforced concrete (RC) beams. HCP is a thin plate of Strain Hardening Cementitious Composite (SHCC) reinforced with Carbon Fiber Reinforced Polymer (CFRP) laminates. For this purpose, an experimental program composed of four Rectangular and five T-cross section beams was executed to assess the strengthening efficiency of HCPs technique. There were two control beams without any type of shear reinforcement, and seven beams strengthened with HCPs. The HCPs were applied to the lateral faces of the beams by using a combination of epoxy adhesive and mechanical anchors. The bolts were applied with a certain torque in order to increase the concrete confinement. To have a better understanding of the shear behavior of SHCC material, Iosipescu shear tests were carried out, and the results were used to derive their fracture mode II parameters.To further explore the potentialities of the HCPs technique for the shear strengthening, the experimental tests were simulated numerically by a FEM-based computer program considering the material properties obtained experimentally. After demonstration of the good predictive performance of the numerical model, a parametric study was executed to investigate the influence of some parameters on the load carrying capacity of the strengthened beams, namely: i) Use a mortar instead of the SHCC; ii) application of different applied torque level to the mechanical anchors; iii) different bond condition between HCPs and concrete substrate.

DUCTILITY ENHANCEMENT OF RC BEAMS STRENGTHENED WITH STRAIN HARDENING CEMENTITIOUS COMPOSITES

Ultra High-Performance Strain Hardening Cementitious Composites (UHP-SHCC) is a newly developed construction material, has large advantages on large strain capacity as well as high compressive and tensile strength, which is useful for strengthening or repair concrete members. An important obstacle needs to be studied, is the strain localization occurs in the UHP-SHCC strengthening layer around the substrate concrete cracks, which severely limits the ductility of the strengthened beam. In the present study, five specimens were tested experimentally, one as a control and four strengthened with variable reinforcement ratios embedded in the strengthening layer. The recorded tests showed that it is sufficient to use 1.2% additional reinforcement ratio embedded in the strengthening layer for beams strengthened with UHP-SHCC to eliminate the observed early strain localization and to gain adequate ductility. Another important conclusion is the strengthening of RC structures using an unreinforced UHP-SHCC layer may lead to a brittle failure.

STRENGTHENING OF RC BEAMS SUBJECTED TO CYCLIC LOAD USING ULTRA HIGH-PERFORMANCE STRAIN HARDENING CEMENTITIOUS COMPOSITES

Ultra High-Performance Strain Hardening Cementitious Composites (UHP-SHCC) is a composite material comprising a cement-based matrix and short fibers with outstanding mechanical and protective performance having advantages as large strain capacity as well as high compressive and tensile strength, which is useful for strengthening or repair concrete members. In the present study, five specimens were tested experimentally, one as a control and four strengthened with 40 mm thickness of UHP-SHCC layer attached from tension side with variable reinforcement ratios embedded in strengthening layer. Cyclic loading was applied to all specimens. The test results showed the importance of reinforcing the UHP-SHCC to eliminate the observed early strain localization and to gain adequate dissipated energy under cyclic loading. It is also proved that using an unreinforced UHP-SHCC layer for strengthening may lead to a brittle failure especially in case of cyclic load.