Strain Hardening Cementitious Composites

The article at hand focuses on the influence of wet-dry cycles on the mechanical properties and crack formation in strain-hardening cementitious composites (SHCC) made of limestone calcined clay cement (LC3) and high-performance polyethylene (PE) fibers. Specimens for uniaxial tension tests were produced with 2 vol.% fiber content and preloaded until 1% strain at the age of 28 days. Subsequently, the specimens were weekly exposed to three days of wetting and four days of drying cycles for 12 consequent weeks, followed by further tension tests, this time until ultimate failure. In addition, a series of preloaded specimens were kept under a controlled environment (20 ℃ and 65% RH) in a climate chamber and characterized as well. The mechanical properties of the two curing conditions were compared with each other, 28 days reference specimens, and the respected preloaded samples. The analysis of the mechanical properties showed a pronounced recovery in Young’s modulus, first crack stress, and tensile strength due to the wet-dry exposure. Furthermore, a detailed crack analysis via DIC analysis and optical microscopy revealed the crack closure phenomenon to some extent due to wet-dry cycles. The results indicate that strain-hardening limestone calcined clay cementitious composites (SHLC4) exhibit considerable mechanical performance and self-healing capacity.

One of the remarkable attributes of strain hardening cementitious composites (SHCCs) is the hardening region of the material. Because of this even if the material is suddenly overloaded, the structure may still be functional. One of the major component which drives the hardening behavior of the material is the fiber. The problems with polymer fibers which is the commonly used for making SHCCs is that they have a low melting point. Unlike polymer fibers, basalt fibers have high melting point thus, more resistant to fire. In this work, we investigated usage of basalt fibers in making SHCCs. More specifically, SHCCs developed through a combination of basalt fiber with ultra high volume LCC cement blend (UHVLC3-BF-SHCC). We investigated the effect of composition of ultra high volume of SCM in the binder on the mechanical performance UHVLC3-BF-SHCC. We observe that UHVLC3-BF-SHCC has ~ 1% tensile strain while cracks widths are controlled to very value of 20 microns and below. By improving mechanical properties, UHVLC3-BF-SHCC may be an attractive green SHCC for applications subjected to high temperatures.

In this study, high-strength high-ductility Engineered/Strain-Hardening Cementitious Composites (ECC/SHCC) were developed with the combined use of ultra-high-strength cementitious matrix, artificial geopolymer aggregates (GPA), and ultra-high-molecular-weight (UHMW) polyethylene (PE) fibers. Apart from short-term characteristics, the long-term mechanical properties of GPA-ECC were evaluated by an accelerated aging test. It was found that GPA could behave as “additional flaws” in the high-strength matrix, leading to a better strain-hardening ability of ECC. Compared with fine silica sand ECC (FSS-ECC) whose strength indices increased but both tensile ductility and crack resistance decreased after accelerated aging, GPA-ECC showed improved long-term performances in all aspects. Furthermore, the multiple cracks were found to propagate through GPA in GPA-ECC, and the microhardness analysis revealed that the hardness growth of GPA was slower than that of cementitious matrix during the accelerated ageing test, ensuring the role of GPA as “additional flaws” in improving the long-term performance of the ECC material.

Engineered Geopolymer Composites (EGC), also known as Strain-Hardening Geopolymer Composites (SHGC), are considered more environmentally friendly than their cement-based counterpart. This study for the first time presents EGC with an ultra-high compressive strength (i.e., over 150 MPa) and an ultra-high tensile ductility (i.e., over 9%) simultaneously. The blended use of fly ash (FA), ground granulated blast slag (GGBS), silica fume, alkali activator, and ultra-high-molecular-weight polyethylene fibers led to the successful development of “Ultra-high-strength & ductility EGC (UHSD-EGC)”. The UHSD-EGC were characterized with excellent multiple cracking and strain-hardening features. In addition, it was found that microstructures of FA-rich geopolymer matrix were looser than those with lower FA/GGBS ratios. The findings arising from this study provided a sound basis for developing EGC materials with ultra-high mechanical properties for sustainable and resilient infrastructure.

Reactive magnesia cement (RMC) is an emerging green cement as it can sequestrate substantial CO2 to harden itself. However, the penetration of CO2 in RMC from outside to inside causes a change in microstructure with depth, which influences the fiber/matrix interface bond and fiber-bridging capacity. This work firstly investigated the influence of carbonation degree on interface bond by single fiber pull-out, SEM, FTIR and acid digestion tests, the results demonstrated that the interface bond is positively correlated to the carbonation degree, but high carbonation degree may induce the fiber rupture. Secondly, tensile test was conducted to explore the influence of carbonation degree on tensile performance, the results suggested increase in carbonation degree can significantly improve the tensile performance, and replacing partial PVA fiber with sisal fiber can prominently enhance the tensile performance at early stage. This work is the first time to clarify the relationship between carbonation degree, fiber/matrix interface bond and tensile performance of RMC, which may provide some guidance to the mix design and application of SHMC.

In recent years, research on nanomaterials has been extensively conducted in various areas, including construction fields. The use of nanofibers, such as carbon nanotubes (CNT) and cellulose nanofibers (CNF), has also been proposed for fiber-reinforced concrete and fiber-reinforced cementitious composites (FRCC). The excellent mechanical properties of nanofibers are very potential to improve the FRCC performances. In many previous works, remarkable results have already been reported on the use of nanofibers in FRCC.However, the use of high-performance nanofibers requires high manufacturing costs. Therefore, it cannot be easily introduced into the concrete that is known as an inexpensive building material. In this study, the application of polypropylene (PP) nanofibers is proposed. The PP nanofibers have nano-order diameters and can be provided in mass production at a low cost. Here, the mechanical performances of FRCC using the PP nanofibers and the combinations with other fibers were investigated. As a result, although the simple mixing procedures had problems with dispersibility, the PP nanofibers can be dispersed well using appropriate admixtures. It has been confirmed that the well dispersed PP nanofibers are improving the mechanical properties of FRCC. In addition, the combination use of PP nanofibers and polyvinyl alcohol (PVA) fibers enhancing the ductility.

It is known that the use of natural fibers as reinforcement for composite materials present economic benefits and eco-friendly appeal when compared to man-made fibers. However, even demonstrating excellent mechanical performance due to the strain-hardening behavior, its use for structural applications still presents a gap in the literature. Therefore, the current work discusses the use of natural fiber cement-based composites as external strengthening for concrete structures. For such, curauá natural fibers were used as reinforcement in a cement composite, which was used as a strengthening material for RC structural beams. The beams were strengthened for flexural and shear. Both externally reinforced specimens presented an increase in loading capacity and deflection decrease compared to their respective references, which was associated with a yielding delay on the rebars in a range between 21% to 34%.

In this study, 3D-printable fiber-reinforced cementitious composites with PE fiber contents of 0.75% were characterized. Specimens were extracted from 3D-printed elements and subjected to compression, splitting tensile, and bending tests to evaluate the mechanical characteristics of the printed elements. The results of the compression tests showed that the compressive strength of the specimens cored in the vertical direction was 109 MPa and the coefficients of variation of the compressive strength and Young’s modulus were smaller than those of the mold-cast specimens. Splitting tensile tests were conducted on specimens in which the direction of the interface between each printed layer (layer-to-layer interface) and the direction of crack propagation matched, and the result showed that, at 3.50 MPa, the cracking strength was 30% lower than that of the specimens whose direction of the layer-to-layer interface and crack interface did not match.

The paper presents a reinforcement strategy for 3D printed concrete by a thin, bonded strain-hardening cement-based composite (SHCC) overlay. Concrete structures additively manufactured through extrusion-based 3D concrete printing (3DCP) present orthotropic mechanical behaviour. Tensile strength and ductility across layers are a fraction of those in the extruded direction, i.e. parallel to the layers, due to reduced interfacial bond between printed layers. In addition, structural application of 3DCP may require reinforcement to enhance limited tensile capacity of printed concrete. Strategies for automated 3DCP reinforcement are sought that retain the benefits of 3DCP technology. Here, a thin, bonded SHCC reinforcing overlay is proposed. Automation is envisaged by integration of either spraying technology, or multiple nozzle technology for new, composite structural elements. For retrofitting of existing 3DCP, shotcrete or plaster application is appropriate. For the proof of concept here, thin SHCC overlays were simply plastered onto 3DCP elements. Characterising tensile tests are performed on SHCC dumbbells, and SHCC-3DCP interfacial tensile (pull-off) and shearing adhesive (triplet) tests. From these results composite 3DCP-SHCC specimens were designed to exhibit ductile behaviour by multiple cracking of the SHCC overlay, as opposed to abrupt, brittle delamination of the SHCC overlay from 3DCP substrate. Subsequently, reference 3DCP beams and SHCC reinforced 3DCP beams were prepared and subjected to in-plane and out-of-plane actions. The results validate thin bonded SHCC overlay as an appropriate reinforcement strategy for significantly increased resistance and ductility.

It is considered that the different placing thicknesses in casting of fiber-reinforced cementitious composite (FRCC) can be one of the factors affecting the fiber orientation and distribution, which is considered to be one of the most important influence factors of the bridging performance of fibers. In this study, a water glass solution is used to conduct the visualization simulation of the flow patterns of fresh mortar with short discrete fibers. Water glass has high viscosity, and it is colorless and transparent. The rheology of mortar matrix before mixing the fiber has been inspected using the flow time based on the test method for flowability of grout measured by the funnel. The orientation intensity that expresses the fiber orientation tendency for the principal orientation angle is calculated by counting the orientation angles of the black target fibers in the water glass solution of three different placing thicknesses. The effect of different placing thicknesses on the fiber bridging performance is considered in the calculation of the bridging law using the elliptic function characterized by the principal orientation angle and the orientation intensity. The results show that a smaller placing thickness in casting leads to a greater fiber orientation intensity and better tensile performance based on the bridging law.

Strain hardening cementitious composites (SHCC) are a class of specially designed fiber-reinforced materials with good crack width control capacity and durability. The crack width of SHCC is mainly governed by the mechanical properties of fibers, interfacial properties between fibers and matrix, the fiber volume fraction as well as fiber orientation distribution. To quantify the effect of these parameters on SHCC behavior, micromechanical models have been developed. In previous studies, the fiber properties and parameters governing interfacial behavior have been experimentally obtained with well-established methods. However, the fiber orientation distributions were simply assumed to be 2D or 3D random (or the average between the two), but such assumptions have never been checked against test results. In this study, the experimentally examined fiber orientation distributions in SHCC tensile samples with PVA fibers were presented and compared with theoretical orientation analysis based on random distribution and consideration of the wall effect. The results show that the real fiber orientation forms a clear peak at around 20° while the pure wall effect analysis cannot reflect this phenomenon. A modified orientation analysis considering the simplified mortar flattening effect was then proposed. Large differences in simulated stress-crack width curves are found between the measured distribution and the wall-effect-induced distribution while the distribution obtained by the modified analytical model can greatly reduce the gap. The measured orientation distribution or proposed analytical approach can be used to replace random distributions in future work to reduce systematic simulation errors.

The self-healing of strain-hardening cementitious composites (SHCCs) relies on the penetration of CO2 (or dissolved CO32−) into the cracks; some SHCCs mixed with special binders, e.g., the reactive magnesia cement (RMC)-based SHCC, even rely on carbonation to harden in the first place. As the carbonation degree decreases with matrix depth, it induces depth-dependent fiber-to-matrix interface properties in these scenarios. In this work, we present a new analytical model that captures the effect of depth-dependent carbonation and self-healing of RMC-based SHCC. In this model, the fiber-bridging tensile stress vs. crack width curve is formed by summing the tensile load vs. displacement relationship of individual fibers. On the single-fiber level, the debonding and slip-hardening of the fiber-to-matrix interface induced by a tensile preloading as well as the recovery of the interface properties by self-healing are coherently quantified in a clear kinetic process. On the fiber-bridging level, the experimentally characterized carbonation vs. depth relationship is added to the model. The modeling results can well capture the single-fiber pullout behavior and the fiber-bridging behavior of the self-healed SHCC specimens.

X-ray micro-computed tomography (microCT) is a non-destructive technique that can provide 3D images of the internal microstructure of a composite material. Optimizing the analysis with modern computational tools leads to a higher precision in quantitative analysis and, consequently, to more accurate results. In this scenario, machine learning has been widely used as solutions for complex image processing and analysis tasks. The SHCC microCT images can be considered complex, given the small scale of analysis and the typical resolution of common microCT, as well as the small differences among the material constituents in terms of density and x-ray absorption. The present work brings innovative solutions for fiber and pore quantification in SHCC using Machine Learning. SHCC were tested in an in-situ testing device coupled to a microCT and the material mechanical response was correlated with microstructure changes through an image sequence. The internal displacement and strain were calculated by Digital Volume Correlation (DVC). The strain results were correlated with the initial quantification of the constituent phases of the material.

The properties of the interfacial transition zone (ITZ) between microfiber and cement-based matrix are of primary significance for the overall behavior of strain hardening cementitious composites (SHCCs). However, due to the relatively small diameter of polymeric microfibers (e.g., PVA fiber), it is technically difficult to obtain quantitative and representative information of the properties of the ITZ. In the current study, a new method that is able to quantitatively characterize the microstructural features of the ITZ surrounding a well-aligned microfiber was reported. With the method, the porosity gradients within the ITZs between PVA fiber and cement paste matrices with different water to cement (w/c) ratios were determined. The results show that the matrix surrounding a microfiber were more porous than the bulk matrix. The thickness of this porous region can extend up to 100 microns away from the fiber surface even at a relatively low water to cement ratio (w/c = 0.3). It is thus believed that the method could facilitate the investigation and modification of fiber/matrix bond properties and also contribute to the development of SHCC with superior properties.

Pull-out behavior of single fiber embedded in Porosity Free Concrete (PFC), which is a non-porous ultra high strength matrix, was experimentally investigated. Four types of fibers were used: steel fiber, stainless steel fiber, bundled aramid fiber, and bundled PBO fiber. Two kinds of embedment length (2 mm and 5 mm) were adopted in the test. Regardless of the difference in embedment length, the maximum load was in the order of stainless steel fiber, steel fiber, aramid fiber, and PBO fiber. Regarding the shape of the load-displacement curve, steel fibers, aramid fibers, and PBO fibers showed softening behavior after the maximum load, whereas stainless fibers showed yielding behavior that caused displacement while maintaining the load. It seems that the stainless fiber has good bonding with the ultra-high strength matrix.

Strain-hardening cementitious composites (SHCC) are pseudo-ductile materials with remarkably high tensile ductility and excellent crack control capability. In some promising applications of SHCC (e.g., construction of structural joint between pre-cast reinforced concrete components), good bonding between SHCC and rebars is essential to ensure sufficient stress transfer. However, the present understanding on the bond between high-strength SHCC and steel rebars is limited. This paper presents an experimental investigation on the bond property between deformed rebars and high-strength SHCC under direct tension. High-strength SHCC with compressive strength of 112 MPa, tensile strength of 8.6 MPa, and tensile strain capacity of 5.5% was used. Steel rebars with three different diameters of 20, 25 and 32 mm, as well as three different cover-to-rebar diameter (C/D) ratios of 1, 1.5, and 2 were examined. The effects of rebar diameter and cover thickness on the bond-slip behavior of steel rebars in high-strength SHCC were discussed. The results showed a significantly improved bonding characteristics between SHCC and deformed steel rebars in comparison to concrete. The bond strength generally increases with an increasing C/D ratio but may stay constant beyond a certain cover thickness. The findings of this study can hopefully improve the current understanding of bonding characteristics of SHCC with deformed steel rebars and can facilitate the structural design of reinforced SHCC members.

This study evaluates the crack width in FRP-reinforced DFRCC members and confirms the adaptability of the proposed formula. To achieve these goals, pullout tests followed by uniaxial tension tests were performed on braided aramid FRP (AFRP)-reinforced DFRCC specimens using PVA fibers. The experimental parameters included the cross-sectional area (100 × 100 mm2, 120 × 120 mm2, 140 × 140 mm2) and the fiber volume fraction (0%, 1%, 2%) added to every specimen. First, the pullout test results show that the maximum bond stress increases as the fiber volume fraction increases. The trilinear models are employed for the bond stress–slip relationships to formulate the theoretical calculation of crack width. Finally, the uniaxial tension test results show that crack width increases with increasing fiber volume fraction and decreasing the cross-section of the specimens. The theoretical calculations are compared with the crack width obtained from the uniaxial tension test. In conclusion, most specimens’ theoretical curves showed good adaptability to evaluate crack width from the experiment.

Strain-Hardening Cementitious Composites (SHCC) have become increasingly popular as a material which could be adopted for enhancing the durability of structures. Studies have shown that the cracks formed during short-term tensile loading in SHCCs can be controlled in the range of 50–100 µm as compared to conventional reinforced concrete, where cracks are designed to be around 200–300 µm at serviceability state. Thereby, applying SHCC as an additional protective layer (e.g. permanent formwork) on the concrete tensile face can effectively control the ingress of water and harmful chemicals due to the fine crack size, thus improving the durability of the structure. Although the crack widths are small under short-term loading, imposing a constant sustained load on SHCC over a longer period has shown to increase the crack size beyond acceptable levels (to over 200 µm). In real structures, however, the deformation is limited by design as well as load sharing between structural elements, therefore the crack patterns in such cases will follow a different propagation mechanism to that under constant loading tests. Specifically, for a reinforced concrete member with a SHCC layer on the surface, any tensile creep deformation in the SHCC will redistribute load to the steel reinforcement and/or concrete, whereby the stress on SHCC gradually reduces with deformation. This study aims to investigate the creep of SHCC under realistic conditions by the experimental simulation of different cases. A novel testing set-up is developed to impose a constant moment that is shared between reinforcement and an SHCC layer, so the ensuing crack formation and opening in SHCC under restrained creep conditions is studied. It was observed that the creep strain increase was predominantly due to widening of pre-existing cracks while the number of cracks stays the same. The crack widths increased by about 10 µm on average (20% increase) which is far less than the value reported for constant stress test and stabilised after 1000 h. Such a marginal increase would have a very low impact on the durability performance reported in short-term tests.

Structures are exposed to a variety of quasi-static and dynamic/cyclic loads. For a safe, material-minimized structural design, a comprehensive knowledge of the material behavior under various loading conditions is required. Previous studies showed that Strain-Hardening Cement-based Composites (SHCC), in literature also often called Engineered Cementitious Composites (ECC) are a promising class of materials that exhibits an outstanding mechanical resistance under both quasi-static and cyclic loading regimes. However, a profound understanding of the mechanisms leading to the specific behaviors under cyclic loads is missing.The article at hand presents experimental results from cyclic tension-swelling and alternating tension-compression tests performed on uniaxially loaded, notched dogbone-shaped specimens made of high-strength SHCC with a polyethylene fiber content of 2% by volume. The samples were exposed to harmonic loads with different frequencies, i.e., 1 Hz and 20 Hz for a certain number of load cycles. The chosen stress level in the tension-swelling tests corresponded to 80% of the first crack strength while for the alternating cyclic loading tests 25% of the compressive strength and 80% of the first crack strength were defined as reversal points. In addition, morphological analysis of the fracture surfaces and crack patterns were carried out by means of microscopy in order to determine the degradation condition of each phase, i.e., polyethylene fiber and matrix. Finally, the results were discussed referring to the physical phenomena causing the observed behavior.

Strain hardening cementitious composites (SHCCs), can be designed to exhibit small crack widths. As a result, even after the material is cracked it restricts the flow of deleterious material. This property among many other properties makes it useful to design durable infrastructures. Assessment of durability of SHCCs requires measurements of surface cracks. The process of development of an SHCC mix for any application thus involves the characterization of cracks to understand its durability component at different strain values. Conventionally, this can be done by using digital cameras to document the images of SHCC surfaces during testing and manually analyzing these images to compute different crack characteristics such as width. This process is laborious and time-consuming thus, non-scalable if the number of examples is large. In this work, we designed a novel deep learning model for this purpose called Strain Hardening Segmentation Network (SHSnet). SHSnet has a very high accuracy of 85% while requiring ~ 4M parameters which is one order less than other state-of-the-art networks like U-net. Due to the inherent thinness of the cracks in SHCCs, the amount of examples for training is fairly low (<1%). To this end, we proposed a loss function (PLF) to train the network efficiently. With this, it takes at least 10x less time for computing crack parameters. We further show applications of the SHSnet empowered technique for durability assessment and detecting crack evolution. These results suggest that SHSnet has the potential to facilitate the autonomous characterization of SHCC cracks in many scenarios.

The mechanical response of textile-reinforced aerated concrete sandwich panels was modeled for flexural loading. The core material used in the sandwich composite consisted of plain autoclaved aerated concrete (AAC) and fiber-reinforced aerated concrete (FRAC). The stress skins for the sandwich beams were made out of AR-glass (ARG) textiles embedded in a cementitious matrix. A constitutive material model comprised of a multi-linear tension model for the bottom stress skin and an elastic-perfectly plastic compression model for the top stress skin. The core was modeled using an elastic-perfectly plastic tension and compression model. A detailed parametric study was conducted to determine the effect of the model parameters on the flexural response of the sandwich composite. The model was further applied to simulate the experimental flexural data from the static tests on sandwich composite beams. Flexural strength, stiffness, and energy absorption capacity can be determined for both static loadings. It is observed that textile reinforcement at the tension and compression faces of the beam element results in a ductile behavior using multiple flexural cracking, leading to diagonal tension cracking in the core element. The distributed cracking mechanism in the sandwich composite significantly improves the flexural properties by 5–10 times when compared to the plain aerated concrete specimens which predominantly exhibit single flexural cracks.

In Textile Reinforced Cementitious (TRC) composites, cementitious elements are reinforced with fibre textiles instead of corrosion-prone steel reinforcement. This allows for a considerable reduction of the cross-section of the cementitious elements, and so of the required amount of cement. One of the drawbacks of brittle materials, however, is the occurrence of wide cracks, requiring proper repair. In the presented research, 3D textiles are used as reinforcement in combination with microfibres to design a smart material with self-healing features. The use of 3D textiles has demonstrated a superior flexural behaviour in comparison to 2D textiles, while the short microfibres are known to limit the width of the cracks in the cementitious matrix. These narrow cracks can be healed through the autogenous healing characteristic of the cementitious material.In this research, the synergetic interaction between the 3D textiles, microfibres and cementitious matrix is investigated for the first time. First, the optimal material composition of 3D TRC composites with integrated polypropylene (PP) microfibres of 6 mm (PP6) and 8 mm (PP8) length was explored. This resulted in an optimal fibre content of 0.7 v% for both fibre lengths, allowing sufficient fibre penetration through the 3D textile meshes. Then, the loadbearing behaviour and crack formation of these material compositions were studied by four-point bending tests, monitored with Digital Image Correlation (DIC). The 3D TRC + PP6 samples demonstrated superior flexural properties, and showed slightly more and narrower cracks.

Discrete spacers allow to transversally connect planar textiles and to transform them into pseudo-3D architectures. These 3D textiles can provide the optimal reinforcement in cement composites as the concrete casting process is enhanced and the textile placement within the mould is well-controlled. This research investigated the effect of discrete spacer connections on the flexural properties and cracking behaviour of TRCs and performed a comparison with equivalent 2D TRC systems (same in-plane textiles but without connections) and with knitted 3D textiles. Additional investigated parameters were the material of the reinforcement (glass, carbon) and the density of the textile (fibre volume fraction of the composite). A four-point bending experimental campaign was conducted on four different textile configurations containing (i) no connections (ref), (ii) four spacer connections distributed over the shear zone, (iii) nine spacer connections distributed over the entire length between the supports (both shear and constant moment zone) and (iv) 3D knitted connections. The spacer connections resulted in a pronounced post-cracking stiffness increase and a positive influence on the crack formation of the TRC. Their influence was however dependent on the fibre volume fraction (Vf) of the in-plane reinforcement; configurations with a higher Vf required a higher degree of connections to showing a post-cracking stiffness increase.

SHCCs have been widely reported for their distinct autogenous healing property of the multiple fine cracks because of fiber bridging effect and remarkable precipitation of secondary products in cracks. However, most of previous studies performed autogenous healing tests of SHCC without simultaneous loading, i.e., applying healing environment to specimens after removing the pre-cracking load. Experiment studies on autogenous healing of cracked SHCC under sustained loading, which is closed to serving conditions of real structures, are still limited. In this study, the authors performed autogenous healing experiment of SHCC specimens under sustained bending loading. Small SHCC beams with steel rebar reinforcement at tensile side were prepared. A setup that can offer sustained load was used to apply four-point bending to the beam. Then, the tensile side of the beam, where multiple bending cracks took place, were immersed in water for autogenous healing in more than one month during when the bending load was sustained. For the control group, the same healing operation was applied but the load was unloaded before that. The crack widths were observed using microscopy periodically to study the healing degree. The results indicated that, although all of the specimens exhibited crack healing in certain degrees, the specimens with sustained load showed slower healing process than those unloaded, leading to higher water absorption after the healing operation.

In the present study, the effects of sustained load combined with aggressive exposure on the long term performance of UHPFRC has been investigated. Three different materials have been tested: a reference one, containing a crystalline admixture as promoter of the autogenous self-healing and two further matrices, additionally enhanced with alumina nano-fibers and cellulose nano-crystals. The aim of adding these functionalizing nano-constituents is to work on the micro- and nano-structure of the matrix and enhance the durability in the cracked state. For this reason, specimens (100 × 30 × 500 mm) were pre cracked to a target crack width level. A couple of specimens was self-contained in suitable 4PBT setup to exert a constant flexural sustained load while being exposed to different exposure conditions of tap water, 3.3% chloride aqueous solution, and geothermal water. The study is proposed to elaborate different nondestructive and destructive measurements to evaluate the self-healing efficiency and its impact on the mechanical performance of the specimens, specifically the tensile constitutive response. Direct tensile test, and 4PBT have been used to assess healing efficiency by testing the conditions of the specimens before damage, after damage and after simultaneous mechanical and chemical exposure. Inverse analysis has been applied to the 4PB curves to obtain the tensile constitutive laws before and after crack localization. The obtained results proved that the autogenous self-healing of UHPFRC, as an active process, not only allows to recover the pristine condition of the cracked specimens, but also to achieve higher tensile capacity, attributed to the healing particles precipitating in the distributed cracks in the damaged area.

Due to bridging effect of fibers, the crack widths in strain hardening cementitious composites (SHCCs) are restricted to a small value <100 μ. Crack widths of this small size promotes autogenous healing in SHCCs. Ideally, autogenous healing should lead to noticeable reduction of crack width as well as recovery of strength and stiffness. The former leads to recovery of ‘transport properties’ and latter lead to recovery of ‘mechanical properties’ of damaged material. If we want to rely on healing-induced recovery of mechanical properties (HIRMP) are relied upon in built environment, it is important that this should be assess HIRMP in a practical scenario. Most reliable method for assessment of HIRMP is by comparing stress-strain curves of healed specimen with that of is the pristine condition. Load/stress can not be directly measured in practical scenario which render these methods impractical. Healing induced recovery stems from bonding of damages in materials, which if broken produces acoustic emission (AE) signals. Exploiting this fact, in this work potential of AE technique for assessment of HIRMP was studied. In this study, AE testing was performed during loading of ‘pristine’ SHCCs, which were healed and again during re-loading of these self-healed specimen. AE results were changed to power spectral entropy (PSE) to minimize adverse effect of computational parameters. PSE of healed specimen was compared with that of the ‘pristine’ state to assess HIRMP. The major observations of the study are 1. With increasing HIRMP the healing induced ‘bonding’ increases as a result cumulative PSE increases 2. With increasing HIRMP the density of PSE becomes less skewed (to the right) this is because with increasing average stiffness, lower strain is needed to reach ‘hardening’. With these observations novel damage parameters are developed for practical monitoring of HIRMP.

This study aimed at clarifying the bond behavior of corroded rebar in SHCC. SHCC and ordinary concrete (NC) beams with a bending deflection were exposed to chloride solution in wet-dry cycles to accelerate deterioration. Bending cracks formed by bending the beams had some influence on chloride ingress and resulted in differences in chloride penetration between NC and SHCC. SHCC showed higher corrosion proof performance than NC. Corrosion locations on the rebar corresponded to the positions of bending cracks. A double pull-out test was conducted to investigate the bond behavior of corroded rebar in NC/SHCC. NC beams suffered more longitudinal cracks than SHCC and cover spalling due to bond fracture. Crack dispersion of SHCC did not reduce with the progress of rebar corrosion. With the corrosion mass loss being not more than 1%, the bond fracture energy of SHCC beams did not reduce with the progress of corrosion.

This paper discussed some early applications of SHCC to structures in which the authors were involved, describing their outlines and evaluations to date, while presenting various technical ideas related to SHCC. SHCC has been shotcreted for surface repair of concrete hydraulic structures, since around 2005. Premixed SHCC products have since then been progressively enhanced, with the application methods being improved. It has been continuously adopted, with no defects or significant deterioration over time having been reported. In the construction of emergency bays in Hida Tunnel around 2007, multilayer shotcrete lining was adopted, and SHCC was shotcreted as the protective layer for the lining. The follow-up review of the lining conducted in 2015 confirmed that the multilayer shotcrete lining including the thin layer of SHCC provides the desired performance. SHCC has been applied to the bases of new-type noise barriers as protective mortar along Tokaido Shinkansen since 2013. No problems have arisen regarding the renewed new-type noise barriers and the protective mortar (SHCC) for the bases.

Concrete is by far the most widespread construction material worldwide for buildings and infrastructures. While offering wide range of advantages, concrete structures are vulnerable to impact loading such as collisions, rock fall or explosions. This can be traced back to the intrinsically brittle nature of the material. Against this background, the Research Training Group (RTG) 2250 funded by the German Research Foundation (DFG) focuses on the development of strengthening overlays made of strain-hardening cement-based composites (SHCC) and other quasi-ductile mineral based materials capable of drastically enhancing the impact resistance of existing concrete structures. Multidisciplinary collaborative work is carried out by three renowned research institutions in Dresden with nine departments involved. In this contribution, an overview of the recent achievements in the RTG 2250 work are presented, spanning from the design of new sustainable SHCC as high-ductility matrices for textile-reinforced strengthening layers to the structural performance of such layers under impact loading. The latter is assessed by means of customized real-scale test protocols. Furthermore, some insights into the advanced techniques of data acquisition and management, numerical modeling as well as sustainability and resilience assessment are provided.

This article reports on the repair of an approximately 100-year-old hydraulic structure with different strain-hardening retrofitting materials under special consideration of application technology, labour costs, material properties and durability. The lock structure was characterised in places by very poor concrete quality (compressive strength below 5 MPa) and wide separating cracks (crack width over 2 cm), while the load-bearing capacity did not cause any concern. The aim of the repair campaign was to restore the impermeability of the lock with the minimum possible changes in the cross-section of the existing structure. Depending on the section, the repair was carried out with strain-hardening cement-based composite (SHCC) or with textile reinforced concrete (TRC), whereby TRC was applied in four variants which included different types of grids and numbers of layers. Epoxy resin, SBR and mineral-impregnated carbon fibre yarns were used. None of the areas repaired was smaller than 35 m2.The entire structure was monitored with regard to its deformation behaviour for half a year before the repair and regularly monitored after the retrofitting for up to two years. The observations showed that all applied retrofitting variants can effectively bridge the cracks despite the low layer thicknesses of max. 3 cm and that the permeability of the entire structure decreased considerably.

A case of overlaying on ASR-deteriorated reinforced concrete bridge deck is introduced for structural use in repair of Ultra High Performance-Strain Hardening Cementitious Composites (UHP-SHCC). The material is a type of ultra high strength fiber reinforced mortar. Results of investigation are presented as well, focusing on the restraint stress of substrate concrete as one of the subjects to consider when selecting a repair material. Specifically, the author highlights the “cracking potential” calculated as a ratio of restraint stress, which is measured by pseudo perfect restraint testing, to the cracking strength at the same age. By this method, the cracking potential of the material under study turns out to be 70% at 5 days.

In the expressway in Japan, large-scale renewal projects of the bridge in which various changes were actualized by the progress of the deterioration were advanced. In concrete slab bridges, deterioration due to salt damage or freezing and thawing, or fatigue deterioration due to traffic loads are the majority. In steel slab, the fatigue degradation by heavy traffic has become obvious. From such background, research and development have begun on upgrading bridge decks by utilizing UHPFRC in ways of increasing the thickness of the members, replacing surfaces, and increasing the rigidity. The UHPFRC used in this study which is called “AFt-UHPFRC” is characterized by a matrix densified by ettringite (AFt) formation. In this paper, the authors report the results obtained from the Full-scale construction experiment. In this experiment, fields simulating concrete slab and steel slab were prepared. In addition, the AFt-UHPFRC was casted using construction equipment and its workability and quality were evaluated on the assumption that it would be applied under the two-lane regulation. Specifically, the formability in the condition with the gradient, filling situation around the reinforcing bar, adhesion with the existing slab, orientation of the fiber, etc., were confirmed. In addition, the strength development, shrinkage behavior, pore size distribution, and chloride penetration resistance of casted AFt-UHPFRC were confirmed by this experiment, and the feasibility of the developed method was confirmed.

Public awareness of the seismic performance of civil engineering structures has increased against earthquakes. The authors are developing a seismic retrofit method for RC piers that utilizes ultra high performance fiber reinforced concrete (hereafter, UHPFRC). The placement of UHPFRC in the cover part imparts resistance against the large bending and compressive stresses that occur during earthquakes and prevents crushing of the cover, and we have devised a method to retrofit existing RC piers by using two types of UHPFRC that can be applied in situ. it is possible to plaster UHPFRC onto vertical surfaces by adjusting the mix proportion. The ductility of the pier was evaluated by cyclic loading test on mock-up specimens of a pier base retrofitted by this method. It was confirmed that reinforcing the pier base with UHPFRC that can be applied in situ increased the ductility by approximately 1.4 times and improved the seismic performance. A sprayable UHPFRC was developed and required to allow its application to vertical surfaces by spraying. UHPFRC can be applied in situ to existing piers by plastering or spraying for retrofitting purposes, and it was found that the seismic performance of the pier can be easily improved.