Deflection Hardening Research Articles

Electrically cured ultra-high performance concrete (UHPC) embedded with carbon nanotubes for field casting and crack sensing

The effects of incorporating carbon nanotubes (CNTs) into ultra-high performance concrete (UHPC), thereby forming UHPC/CNT composites, were investigated in terms of electrical curing efficiency, mechanical properties, and crack sensing capability. The addition of CNTs significantly decreased the electrical resistivity of the UHPC, allowing effective electrical curing at low voltage; improved mechanical properties through bridging, pore filling, and calcium-silicate-hydrate (C-S-H) stiffening effects; and favorably influenced the deflection hardening and multiple cracking behavior under flexural stress. Furthermore, the developed UHPC/CNT composites subjected to compressive or flexural stress showed significant crack sensing capability due to the obtained low electrical resistivity. A dramatic fractional change in the resistivity (FCR) of the UHPC/CNT composites can represent the failure under compression or first cracking under flexure. Therefore, it was experimentally verified that the UHPC/CNT composites can extend the applications of UHPC materials especially for on-site casting and structural crack sensors for UHPC-based structures.

Tensile and flexural behaviour of recycled polyethylene terephthalate (PET) fibre reinforced geopolymer composites

This paper presents the effect of recycled Polyethylene terephthalate (PET) fibre as reinforcing fibre for geopolymer composite. Tensile and flexural behaviour of PET fibre reinforced ambient cure geopolymer (ACG) composite is studied and compared with those of counterpart cement and cement-fly ash (CFA) composites reinforced by the same fibre. The results are also compared with commercially used polypropylene (PP) fibre reinforced composites. Results show that the compressive strength of PET fibre reinforced ACG composite is higher than its counterpart cement and CFA composites. The compressive strength of all three composites reinforced by PP fibre is slightly higher than that of PET fibre composites. The increase in fibre volume fraction from 1% to 1.5% shows reduction in compressive strength in all three composites reinforced by both fibres. The PET fibre reinforced ACG composite exhibits strain hardening and deflection hardening behaviour in uni-axial tension and three-point bending, respectively. However, in PET fibre reinforced cement and CFA composites no such behaviour is observed. Microstructure observation in terms of scanning electron microscope (SEM) images show no significant damage of PET fibres in geopolymer composite. However, in cement and CFA composites adherence of matrix and damage of PET fibres surface in terms of scratches are observed in SEM observation indicating higher interfacial bond of PET fibre with cement and CFA matrix than with geopolymer matrix.

Deflection hardening behavior and elastic modulus of one-part hybrid fiber-reinforced geopolymer composites

Deflection hardening behavior and elastic modulus of one-part hybrid fiber-reinforced geopolymer composites

Role of matrix structure and flaw size distribution modification on deflection hardening behavior of polyvinyl alcohol fiber reinforced engineered cementitious composites (PVA-ECC)

The multiple cracking and deflection hardening performance of polyvinyl alcohol fiber reinforced engineered cementitious composites (PVA-ECC) under four-point flexural loading have been investigated. Matrices with different binder combinations and W/B ratios (from 0.44 to 0.78) providing satisfactory PVA fiber dispersion were specially designed. Effect of pre-existing flaw size distribution modification on deflection hardening behavior was comparatively studied by adding 3 mm diameter polyethylene beads into the mixtures (6% by total volume). Natural flaw size distributions of composites without beads were determined by cross sectional analysis. The crack number and crack width distributions of specimens after flexural loading were characterized and the possible causes of changes in multiple cracking and deflection hardening behavior by flaw size distribution modification were discussed. Promising results from the view point of deflection hardening behavior were obtained from metakaolin incorporated and flaw size distribution modified PVA-ECCs prepared with W/B=0.53. The dual roles of W/B ratio and superplasticizer content on flaw size distribution, cracking potential and fiber-matrix bond behavior were evaluated. Flaw size distribution modification is found beneficial in terms of ductility improvement at an optimized W/B ratio.

Mechanical autogenous recovery and crack sealing of natural curauá textile reinforced concrete

This paper investigates the autogenous sealing capability of curauá Textile Reinforced Concrete (TRC). Curauá is a high-performance natural fiber with excellent tensile strength and water absorption capacity. The curauá TRC mechanical behavior was evaluated through direct tensile and four-point bending tests. The autogenous mechanical recovery was analyzed by means of repeated bending and short-time creep tests. The specimens were submitted to three different environmental conditions (RH of 55%, cycles of sprayed water and 12-h water immersion) and their influence on crack sealing was evaluated. The TRC presented strain and deflection hardening behavior with a mean crack width of 50 µm. The combination of the hydrophilic nature of curauá fibers, the matrix with high pozzolanic content and narrow cracks, contributed to the autogenous sealing of the cracks and mechanical recovery of the specimens. Even under repeated and short-time creep tests, the curauá TRC did not lose its strength or strain capacities; and the mean crack width remained close to the ones submitted to the monotonic test. When submitted to sprayed water, all cracks were partially sealed after only four cycles.

Mechanical strength and flexural parameters analysis of micro-steel, polyvinyl and basalt fibre reinforced magnesium phosphate cement mortars

Magnesium phosphate cement composites exhibits higher compressive strength with high brittleness in nature that minimizes the scope of this material for flexural ductility performance. The study targeted to enhance the flexural qualities along with compressive strength of MPC compound through adding the three different fibres such as micro-steel fibre, polyvinyl alcohol fibre and basalt fibre. Inclusion of each fibre was followed a consecutive dosages system such as 0.6%, 0.8%, 1% and 1.2% of the gross mass of binders and aggregate. The compressive strength, three point flexural strength, four point flexural strength and splitting tensile strength tests were considered to explore the flexural characteristics and modulus of elasticity of the prepared fibre reinforced magnesium phosphate cement mortars with the passage of age. The experimental results pointed out the improvement about 20% to 43%, 15% to 25% and 50−72% in the TPFS, FPFS and STS than control specimens at 28d, whereas only around 5% to 10% progresses were noticed in CS for the inclusion of fibres from 0.6% to 1% in FRMPC combinations. In addition, the outcomes of flexural parameters analysis from load-displacement curves exposed the well development for mixing of 0.8−1% MSF and 0.8% PVAF and BF in the representative MPC compositions that displayed the deflection hardening and deflection softening behavior suitably. Moreover, the L-D curves of TPFS showed the advanced MOE because of less stress distribution intensity in whole cubical samples with low deformations in comparison to other two flexural tests. The study also suggested some power equation based empirical expressions for predicting the strength properties at early ages and correlation equations for flexural tests including MOE in relation to CS results. The analytical findings and recommended correlation equations might be applied in the practical applications of FRMPC related aspects.

Fiber reinforced alkali-activated stone wool composites fabricated by hot-pressing technique

Cementitious composite that has short molding time and high mechanical performance is favorable in pre-cast concrete industry. In this context, this study reports the use of hot-pressing technique to fabricate PVA fiber reinforced composites using alkali-activated stone wool (a waste from building insulation). Eight different mixtures were developed by varying the pressing time and temperature in comparison to the conventional oven-cured alkali-activated material. The mechanical performance of all compositions was evaluated under bending and compressive loadings. Life cycle assessment was used to investigate the greenhouse gas emission and embodied energy of the developed composites. The results reveal that PVA fibers greatly enhanced the mechanical performance of all reinforced mixtures with deflection hardening behavior and improvement in compressive strength. The hot-pressing technique lowered CO2 emission and saved energy. Finally, a multi-criteria ranking method suggests hot-pressed PVA fiber reinforced cementitious composite, manufactured at 120 °C for 2 h, is the best composition attaining balance among energy spent, mechanical properties, and CO2 footprint.

Flexural load-deflection performance of polyvinyl alcohol fiber reinforced cementitious composite beams

PVA-FRCC (polyvinyl alcohol-fiber reinforced cement composite) materials have superior tensile strain hardening characteristics, which can significantly improve the bearing and deformation capacities of the structure. The load-deflection performance of the PVA-FRCC reinforced beam is investigated in this study. Four-point bending experiment and theoretical analysis are conducted on the PVA-FRCC beams with different PVA fiber volume mixing ratios and reinforcement ratios. The experimental results show that the PVA-FRCC beams conform to the assumption that plane sections remain plane and the steel bars have the deformation compatibility with the PVA-FRCC material. The load-deflection curves obtained by the experiment show that the PVA-FRCC beams exhibit obvious deflection hardening and high performance of ductility compared with the RC beams, and the ultimate deflections are about 2.4–3.1 times that of the RC beams. In the theoretical part, the force versus deformation model of the PVA-FRCC beam is proposed. Based on the geometry compatibility equations, the constitutive equations and the force equilibrium equations, the theoretical expressions of the load-deflection curve at the mid-span of a PVA-FRCC beam subjected to flexural loading at different working stages is developed and validated by the experimental results.

Study on self-monitoring of multiple cracked concrete beams with multiphase conductive materials subjected to bending

The multiphasic conductive admixtures like macro steel fiber (SF), nano carbon black (NCB) and carbon fiber (CF) are incorporated into concrete to improve the conductivity and mechanical properties. In this study, the mechanical and conductive properties of the concrete beam are explored by the hybrid use of different conductive admixtures. The deflection hardening (multiple cracking) behavior of concrete beam with different fiber content, monitoring of single and multiple cracks of diphasic (SF+NCB) and triphasic (SF+CF+NCB) conductive concrete beam through resistance measurement is investigated. Moreover, the relationship between fractional change in resistance (FCR) and crack opening displacement is established to study the influence of different types and amounts of electric conductive admixtures on self-sensing behavior of concrete beam. Results reveal that the deflection hardening (multiple cracking) behavior is obtained with 70 kg m−3 SF content. The single and multiple cracks monitoring is illustrated through load-time and FCR-time relationship. In addition, FCR improved linearly with an increment of NCB with triphasic conductive admixtures.

Development of strain hardening cementitious composite (SHCC) reinforced with 3D printed polymeric reinforcement: Mechanical properties

Cracking in concrete needs to be limited for esthetical and durability reasons. Currently, this is commonly done by using steel rebars in the structure or fiber reinforcement in the material. With certain fiber types and micromechanical design, it is even possible to create cement-based materials with steel like (i.e. quasi-plastic) properties – so called strain hardening cementitious composites (SHCCs). In this paper, an alternative approach for creating SHCC – through use of additive manufacturing to create polymeric reinforcement meshes – is proposed. Different designs are manufactured, casted in the cementitious matrix, and tested in four-point bending and uniaxial tension. It was found that, with proper designs, it is possible to create cementitious composites with deflection hardening or strain hardening properties. Furthermore, with proper design, multiple cracking behavior of conventional SHCC can be replicated. In addition, numerical simulations were performed using the Delft lattice model. Four point bending tests on mortar bars reinforced by two different mesh designs were simulated and the results show good agreement with the experiments. This research shows great potential of using additive manufacturing for creating SHCCs with customizable properties.

Performance characteristics of micro fiber-reinforced geopolymer mortars for repair

Abstract Fiber reinforced geopolymer materials are recently being considered as prominent sustainable repair materials for concrete retrofitting/repair due to their good bonding properties. The addition of fibers to geopolymers materials may considerably improve their mechanical properties but they also effect on physical properties such as setting, workability, density, porosity. It is crucial to understand these characteristics of micro fiber reinforced geopolymer mortars before optimizing the micro fiber for geopolymer mortars for repair. In this study, the effects of micro fiber type and fiber volume fraction on the physical characteristics and on flexural behavior are investigated. In addition, the influence of different heat curing durations on the properties of micro fiber reinforced geopolymer mortars is also examined. Six types of micro fibers (high-strength steel, waste steel wool, polyvinyl alcohol, polypropylene, polyester & carbon) at low fiber volume fractions of 0.5%, and 1.0% are considered to study the physical properties (workability, and total porosity) and flexural behavior of fly ash-based geopolymer mortars. High strength steel fibers imparted the least change in workability whereas with polyester fibers the flow was the lowest. Porosity as well was highest for polyester fiber reinforced mortars followed steel wool and polypropylene. Significant improvements in the flexural behavior and compressive strength due to micro fiber reinforcement are noted particularly in steel and polyvinyl alcohol fibers. Deflection hardening was observed for mortars with 1% polyvinyl alcohol fibers, and for both 0.5% and 1% steel fibers. The overall flexural performance of polyester, steel wool, and carbon fibers reinforced geopolymer mortars was dismal. Increasing the duration of heat curing from 4 h to 24 had little change in total porosity and compressive strength. In flexural behavior however, the effect of prolonged heat curing was more noticeable.

Development and application of an environmentally friendly ductile alkali-activated composite

This paper presents a development of a ductile alkali-activated fly ash (FA) and ground granulated blast furnace slag (GBFS) based composite as an environmentally friendly material for structural concrete application. For this purpose, polyvinyl alcohol (PVA) fibres and sand aggregate were combined with alkali-activated paste. Workability, setting time, mechanical properties and failure mode of PVA fibres in the mixture were studied by slump test, Vicat needle test, flexural and compression tests, and Scanning Electron Microscopy (SEM) imaging, respectively. Although the mixture sets in a short period of time (less than 30 min), the workability was good and the developed fibre reinforced composite was used for a large scale application in a canoe. Casting a large volume (45 l compared to 3 l, as initially designed) did not affect the workability and the setting time of the mixture. Mechanical properties of specimens coming from “small” (3 l) and “large” (45 l) batches were tested at different ages (up to 120 days) and compared. It was shown that their flexural and compressive strength are similar, i.e. not affected by the upscaling. Furthermore, it was shown that the mixture with PVA fibres exhibits deflection hardening behaviour even with aggregate particles as large as 4 mm, although single crack localization led to failure. The SEM images of fractured surfaces indicated that combined fibre pull-out and fibre rupture occurred, with the latter one causing the final failure. The developed mixture, additionally reinforced with the plastic fiberglass mesh, was used in the 5.8 m long and 16 mm thick canoe for the student competition, which for the very first time, was constructed without the use of Ordinary Portland cement (OPC). The upscaling was successful and the results show the potential of fibre-reinforced alkali-activated FA and GBFS composite to be used as a durable and resistant material suitable for the structural application in thin shell elements, exemplified by the canoe. Such an application and a low risk project was suitable to gain the necessary experience and confidence with this innovative, “concrete like”, material for which no codes or regulations are available. Furthermore, similar applications are the first step for larger scale structural applications, like structural elements in the building industry, bridges and other civil engineering structures.

Comparative strain and deflection hardening behaviour of polyethylene fibre reinforced ambient air and heat cured geopolymer composites

This paper compares strain hardening and deflection hardening behaviour of polyethylene (PE) fibre reinforced two types of geopolymer composites. The first composite is heat cured fly ash based geopolymer composite while the other is ambient air cured fly ash and slag blended geopolymer composite. Comparison is also made with counterpart ordinary Portland cement (OPC) based composite. Effect of different volume fractions of PE fibre on compressive strength, strain hardening and deflection hardening behaviour of above three composites is evaluated and a critical volume fraction of PE fibre is identified. Results show that both heat and ambient cured geopolymer composites exhibited better strain hardening and deflection hardening behaviour than their counterpart OPC based composite containing same volume fraction of PE fibre. Results also show that the geopolymer composites and cement composite containing PE fibre volume fractions of 0.75–1.0% exhibit the highest ultimate tensile strain, deflection at peak load and maximum number of multiple cracks than other fibre contents. Compressive strength of OPC composite is higher than that of both geopolymer composites. Among geopolymer composites, the ambient cured geopolymer (AGP) composite exhibited much higher deflection capacity at peak load than heat cured geopolymer (HGP) under three-point load. Similar behaviour is also observed in uni-axial tension. Scanning electron microscopic analysis shows adherence of higher amount of cement matrix and geopolymer gel on PE fibre in cement and heat cured geopolymer composite that that on PF fibre in ambient cured geopolymer composite. This indicates the likely hood of higher frictional bond of PE fibre with matrix in cement and heat cured geopolymer than in ambient cured geopolymer.

Development of sandwich panels combining Sisal Fiber-Cement Composites and Fiber-Reinforced Lightweight Concrete

This research proposes the development of an innovative structural panels based on the use of thin outer layers of Sisal Fiber-Cement Composites (SiFCC) together with a core layer of Polypropylene Fiber-Reinforced Lightweight Concrete (PFRLC).The influence of sisal fibers was studied in two different ways, short sisal fibers (50 mm) randomly distributed in the matrix, and long unidirectional aligned sisal fibers (700 mm) applied by a cast hand layup technique. Lightweight aggregates and polypropylene fibers were used in the concrete layer forming the panel's core in order to reduce its density and improve its post-cracking tensile strength and energy absorption capacity.The behavior of the sandwich panels in four-point bending test is described, and the various failure mechanisms are reported. Mechanical properties of both SiFCC and PFRLC were obtained, which were also used in the numerical simulations. Pull-off tests were performed to evaluate the bond strength between the outer SiFCC layers and the core PFRLC. The results revealed that the long sisal fibers were more effective in terms of providing to the panel higher flexural capacity than when using short sisal fibers, long fibers ensured the development of a deflection hardening behavior followed by the formation of multiple cracks, while short sisal fibers promoted a softening response after cracking.

Mechanical properties of ambient cured high strength hybrid steel and synthetic fibers reinforced geopolymer composites

Ambient cured geopolymer offers significant promise to the construction world as a possible alternative to ordinary Portland cement (OPC). However, as a member of the ceramic family, geopolymers exhibit extremely brittle behaviour. The inclusion of short discrete fibers is an effective way to enhance their ductility. In this research, a series of fiber combinations and volume fractions between steel fibers with end-hooked or spiraled and synthetic fibers (made of high strength polyethylene (HSPE)) were incorporated in a high strength ambient cured geopolymer matrix. The performance of synthesized geopolymer composites was compared in terms of fresh and hardened state properties, such as workability, uniaxial compressive strength, modulus of elasticity, Poisson's ratio, flexural tensile strength, energy absorption capacity and post-peak residual strength etc. The interfacial bond between the spiral steel fiber and the geopolymer matrix as well as fiber distribution in the composites were assessed through individual fiber-pull out tests and physical examination of the cast samples, respectively. The test results show that the addition of fibers significantly improved the load carrying capacity of the composites under flexure load, i.e. increased from 3.89 MPa to 11.30 MPa together with an improved behaviour in compression. In general, all fiber reinforced composites displayed a stable deflection hardening response and multiple-cracking failure mode. Moreover, among composites with different fiber volume fractions, the composite having 1.60% steel+0.40% HSPE showed the highest ultimate flexure strength, correspondingly the highest energy absorption capacity. The individual fiber pull-out test curves ascertained a strong bonding between the geopolymer mortar and spiral-steel fiber.

Increasing the flexural capacity of geopolymer concrete beams using partially deflection hardening cement-based layers: Numerical study

Experimental research has demonstrated the great flexural performance of deflection hardening cement based composites for strengthening the beams. This paper numerically investigates the feasibility of adding thin deflection hardening fiber reinforced layers to the bottom of geopolymer concrete beams to enhance the flexural performance. To properly predict the structural behavior and crack patterns of beams, the smeared crack approach was adopted to simulate the beams. The numerical simulations executed in the FEM-based computer program. To validate the developed numerical models, the tested experimental tests beams with two layers cross section were numerically simulated. Regarding the results obtained , the models would well predict the structural behavior and crack patterns of beams. Ensuring the efficiency and accuracy of the adopted constitutive model to predict the structural behavior and crack patterns of beams, the numerical FE models used to simulate the added hardening deflection fiber reinforced layer to the bottom of geopolymer concrete beams. The numerical results revealed that adding a thin fiber reinforced layer to geopolymer concrete beams results in increasing ultimate load capacity, ultimate deflection, and ductility. The greatest enhancement in the flexural performance of the strengthened beams was found in the ultimate load capacity of the strengthened beams.

Experimental investigation on the behaviour of reinforced concrete slabs strengthened with ultra-high performance concrete

Nine rectangular specimens were tested to investigate the behaviour of composite reinforced concrete (RC) slabs strengthened with ultra-high performance concrete (UHPC). The specimens were two series with various UHPC strengthening configurations. The first, a rehabilitation series, tested UHPC as patch material for repairing deteriorated concrete structures. The second, a UHPC overlay series, was used to retrofit soffits of RC members. The results showed that using the rehabilitation series, the UHPC reduced diagonal cracking and developed more flexural cracks as compared to RC slabs with no UHPC strengthening. The UHPC exhibited excellent energy absorption with extensive deflection hardening and ductility during the post cracking range. In the UHPC overlay series, each slab showed diagonal shear cracks and debonding modes. The UHPC overlay delayed the development of shear cracking. As the overlay thickness increased, the ultimate load increased; but the tendency for the UHPC to undergo fracture failure also increased.

Residual flexural strengths of steel fiber reinforced concrete with multiple hooked‐end fibers

This paper presents the results of 3‐point bending tests on 69 prenotched steel fiber reinforced concrete (SFRC) prisms conducted in accordance with EN 14651 to obtain the stress versus crack‐mouth‐opening displacement curves. SFRC prisms with fiber content greater than 20 kg/m3 and containing multiple hooked‐end fibers were found to exhibit deflection hardening behavior in bending after first crack. Based on the test results, empirical models to determine the residual flexural stresses are established through multiple regression analysis and further validated with test results available in the literature. The residual flexural strengths were found to be proportional to the reinforcing index, the square root of concrete compressive strength, and the square of the number of hooks at the fiber ends. Size effect is accounted for by introducing a factor which is a function of the fiber length. The empirical equations for the residual flexural strengths enable the use of Model Code 2010 constitutive relation for SFRC in tension without resorting to prism tests.

Development of deflection hardening cementitious composites using glass fibres for flexural repairing/strengthening concrete beams: experimental and numerical studies

This paper presents experimental and numerical programmes employed to strengthen concrete beams using glass fibre-reinforced concrete in order to achieve deflection hardening behaviour. Numerous tests were carried out on 270 specimens in the experimental phase in order to characterise mechanical properties of beams reinforced with glass fibre. Three beams were then cast with plain concrete and experimentally tested under flexural load conditions. Based on the experimental results, numerical simulations were conducted and the results were calibrated to the experimental results. Then, the robust-able FE models were used to strengthen concrete beams through numerical simulations. Furthermore, section enlargement method was also used to strengthen the concrete beams. The experimental and numerical results showed that using glass fibre-reinforced concrete layer to strengthen the concrete beams through the section enlargement method significantly improves the load carrying capacity and ultimate deflection.

Ductile behavior of polyethylene fibre reinforced geopolymer composite

This paper presents the effects of various volume fractions of polyethylene (PE) fibres of 0.5%, 1%, 1.5% and 2% on tensile, flexure and compressive behavior of PE fibres reinforced geopolymer composites (PE-FRGC). Results show that the 1% PE fibre by volume is the optimum fibre volume fraction for fly ash geopolymer composite, which exhibit superior strain and deflection hardening behavior in uni-axial tension and three-point bending, respectively. Results also show that the compressive strength of the composites decreases with increase in volume fractions of PE fibre. The fibre surface examination of PE fibre in the gopolymer matrix using scanning electron microscope (SEM) and energy dispersive spectroscopy (EDS) revealed no significant damage of PE fibre in the alkaline geopolymer matrix.