Search

Nowadays, hybrid fiber reinforced concrete is being considered for structural applications due to its enhanced mechanical properties compared to concrete without fibers/plain concrete. In this work, the mechanical properties of new kind of hybrid fiber reinforced concrete, i.e. CaCO3 whisker-steel fiber-basalt fiber reinforced concrete (CSBFRC) with various basalt fibers percentages are studied. All the fiber-reinforced concretes are compared with that of plain concrete (PC). The CaCO3 whisker, steel and basalt fiber lengths are 20–30 μm, 35 mm and 12 mm, respectively. Steel fibers and CaCO3 whiskers contents are 0.32% and 0.9%, by volume, respectively. Various basalt fiber contents of 0.34%, 0.68%, 1.02% and 1.36%, by volume, are added. For each batch, cylinders and beam-lets are cast and tested under respective compressive, splitting tensile and flexural load as per ASTM standards. Stress-strain curves and load-deflection curves are obtained. Strengths, energy absorptions and toughness indices are determined against for each type of loading. The scanning electron microscopy (SEM) analysis is performed to reveal the behavior (interfacial bonding) of CaCO3 whiskers, basalt fibers and steel fibers. It is concluded that, with increasing content of basalt fibers up to 0.68%, there is an increase in the mechanical properties of hybrid fiber reinforced concrete and CSBFRC4 is found to be an optimum. However, beyond 0.68%, the mechanical properties of CSBFRC decrease with an increase in the basalt fiber content. The resistance against cracking provided by hybrid fibers is observed by SEM images.

Inclusion of hybrid fiber in cementitious composites could restrict the growth of cracks at different stages and improve the mechanical properties of concrete, and adding calcium carbonate (CaCO3) whisker would further improve the mechanical properties of cementitious composites. Nevertheless, no attention has been paid to the effects of steel- polyvinyl alcohol (PVA) hybrid fiber and whisker on ultrasonic pulse velocity (UPV) and resonant frequency (RF) of hybrid fiber reinforced cementitious composites (HyFRCC), although they have been used widely in quality assessment of concrete. In this research, 114 specimens have been used to explore the influences of steel-PVA hybrid fiber and whisker on the UPV, RF parameters and the mechanical properties. The results of this research present that there may exists “fiber gradation” in HyFRCC, because there is always a best fiber combination to reach the greatest compactness and highest non-destructive testing results. In addition, the partial substitution of PVA fiber by whisker decreased UPV and RF and increased density loss rate and compressive strength except for one mixture. Therefore, UPV, RF can reflect the effect of whisker on the inner structure and detect the variation, and have potential use to evaluate HyFRCC in engineering applications.

Fiber distribution is an important factor which affects the fresh and hardened properties of composites. A variety of methods are available for studying the fiber distribution but image analysis is more widely used for fiber reinforced cement-based composites (FRCs). In this paper, different fiber distribution methods are discussed in detail including optical microscope, scanning electron microscopy, fluorescence microscopy and X-ray computed tomography. It is found that fluorescence microscopy is more widely used for studying fiber dispersion in FRCs and can be applicable to the synthetic fibers.

The new models using reinforcing index to predict workability and toughness of hybrid fiber reinforced cementitious composites (HyFRCC) were systematically studied in this paper. The existing models are discussed and new quadratic function models are proposed. A RIV considering elastic modulus of fibers is employed to calculate the workability parameter of fresh HyFRCC, and the analysis indicates that the slump cone, V funnel, J-Ring and L-Box test results can all be predicted by this workability model. In addition, a RIV considering tensile strength of fibers is employed to calculate the toughness of harden HyFRCC. The analysis indicates that this toughness model can be used to predict the compressive toughness, flexural toughness, equivalent flexural strength (according to JCI-SF4), I5, I10 and I20 (according to ASTM C1018 ) of HyFRCC, without considering the matrix (mortar or concrete) and fiber combination (e.g., steel-polyvinyl alcohol fiber-whisker, steel-cellulose fiber, steel-polypropylene fiber and so on). The new models have extensive applicability and good correlation with experimental results, which can help offering good behaviors to HyFRCC fresh mixtures, even hardened states, in design and construction practice via fiber mix optimization.

A new hybrid fiber system containing steel fiber, polyvinyl alcohol fiber (PVA fiber) and calcium carbonate (CaCO3) whisker was employed to further improve hardened properties and reduce engineering cost of cement mortar. In this paper, the rheological properties, flow rate and flow spread were measured to evaluate the workability of hybrid fiber reinforced mortar in fresh state. Mechanical strength and flexural toughness of mortars were tested in hardened state. The results indicated that, flowability of fresh hybrid fiber reinforced mortar decreased after substituting PVA fiber for steel fiber and further CaCO3 whisker for PVA fiber in mortar. This is mainly due to the flexibility of PVA fiber and large specific surface area of CaCO3 whisker. The hybrid fiber reinforced mortar shows promising mechanical strength and flexural toughness due to the multi-scale crack resistance formed by micro- and macro-fibers. M5 specimen (1.5% steel fiber + 0.4%PVA fiber + 1.0% CaCO3 whisker) exerts the best flexural performance and relatively higher compressive strength, exhibiting an obviously positive hybrid effect of fibers.

The effects of freeze–thaw (F-T) damage on fracture properties and microstructure of steel-PVA hybrid fibers reinforced cementitious composites containing calcium carbonate whisker (CW-SPFRCC) were investigated in this paper. Three-point bending tests were carried out to study the fracture properties of CW-SPFRCC after different F-T cycles based on Double K fracture criterion. Compared with SPFRCC, the relative fracture parameters in CW-SPFRCC dropped less within 0 to 50F-T cycles, while rapid deterioration was observed in 50 to 100F-T cycles, indicating that the presence of CW could effectively delay F-T damage in SPFRCC. Moreover, fracture parameters were estimated to quickly predict the fracture behavior of CW-SPFRCC subject to F-T cycles. The microstructures of CW-SPFRCC was analyzed using scanning electron microscope (SEM), vacuum epoxy impregnation (VEI), mercury intrusion porosimetry (MIP) and optical microscope observation (OM), respectively. SEM results showed that PVA fiber and CW maintained intact morphologies subject to F-T cycles, but the surface of steel fibers was severely corroded by F-T actions. The results of VEI and MIP demonstrated that better frost resistance of SPFRCC was related to the improved pore structure because of the presence of CW. Furthermore, F-T damage was more likely to occur on the interfacial transition zone (ITZ) of steel fibers or aggregates. Finally, a parabolic model of F-T damage was developed to predict the service life on-site of CW-SPFRCC.

The bond at fiber–matrix interface is considered as one of the most important factors influencing the performance of fiber reinforced cementitious composites (FRCC). To improve such bond, adding silica fume is considered as an effective and efficient method. In modern FRCC, CaCO3 (calcium carbonate) whisker and basalt fibers witnessed increasing popularity due to their effectiveness in improving mechanical performance at multi-scales. However, systematic research still lacks for the bond improvement at their fiber–matrix interface. In this regard, silica fume of various contents was added to plain concrete (PC), CaCO3 whisker reinforced cementitious composites (CWRCC) and basalt fiber reinforced cementitious composites (BFRC). The bond strength at the fiber–matrix interface and mechanical properties of PC, CWRCC and BFRC were evaluated. Test results show that the addition of silica-fume up to 10% could enhance the interface bond and mechanical properties of CWRCC and BFRC up to 30% and 34%, respectively. SEM analysis revealed improved bond at the fiber–matrix interface with addition of silica fume as evidenced by the fact that the whisker after pulled out from the matrix was encapsulated by cement paste, as compared to that of plain composite without silica fume. Moreover, EDS energy spectrum of whisker and basalt fiber surface show that the Si content increased noticeably as a result of the increased hydration products adhering to the whisker and basalt fiber surface, thus improving the bond at the fiber–matrix interface. The incorporation of silica-fume in BFRC lead to a large number of attached hydration products to the surface of fiber after pulling out. At the initial stage of cement hydration, dispersed silica-fume particles play a role of “crystal nucleus”, which can absorb Ca + and OH– ions, and increase the number of CH nuclei. Furthermore, silica-fume continuously reacts with CH and procedure C-S-H gel which decreases the porosity of the interface layer, makes the structure more compact and improves the interfacial adhesion. The microstructure of the interface layer is enhanced by C-S-H which is generated by “secondary reaction” and increase the interfacial bond strength of fiber and matrix. Lastly, a microstructure schematic representation for the fiber–matrix interface was developed to understand the mechanism between the fiber and the cement matrix.

Abrasion resistance is a critical parameter in the durability of cement-based materials. Calcium carbonate (CaCO3) whiskers are added into the mortar to enhance the abrasion resistance. Herein, two standards (GB/T 16925-1997 and JC/T 421-2004) were employed to evaluate the abrasion resistance of the mortar modified with CaCO3 whiskers (MMCW). The mortar abrasion resistance was characterized using the abrasion parameters such as abrasion resistance depth (D), abrasion loss ( G ), mass loss (ML), and abrasion resistance indices ( I α ). These results indicate that the abrasion resistance of MMCW was markedly improved, and the abrasion resistance of MMCW increased with the rise in CaCO3 whiskers content. By adding different content of CaCO3 whiskers into mortar, the compressive strength (fcu) enhanced from 2.4% to 10.2%, as compared to that of the plain mortar. The bridging effect of CaCO3 whiskers prevented the cracks development, while the filling effect of CaCO3 whiskers refined the large pores. The calcium hydroxide (CH) orientation improvement effect and the dilution effect of CaCO3 whiskers limited the growth of CH hydration product of mortar, enhanced the compactness of mortar, and ultimately, improved the abrasion resistance of MMCW. An excellent linear relationship was established between the abrasion resistance and compressive strength. By comparing the experimental data with the data from the literature, the prediction model of the relative abrasion resistance parameters was obtained, which can provide a reference for studying the abrasion resistance of different cementitious materials.

Fiber reinforced cementitious composites (FRCC) have been widely used in the field of civil engineering in terms of high-performance building material. The fracture mechanics has been generally considered as an effective way to perform the stability analyses of cracks and safety assessment of concrete structures. This review article first introduces several theoretical models of fracture mechanics and different testing methods of fracture parameters for FRCC. Then the factors affecting fracture properties of FRCC are reviewed respectively, including fiber type, fiber content (Vf), water-to-cement ratio (w/c), maximum aggregate particle size (dmax), aggregate type, initial crack length (a0), high temperature and freeze–thaw cycle times. Furthermore, the reinforcement mechanism of fiber to improve fracture properties of FRCC is discussed, and future work is recommended. This review provides fundamentals on measuring fracture properties, also is to understand the fracture behaviors of FRCC.

A testing program was undertaken to evaluate the effects of fibers on properties of expansive self-consolidating concrete (ESCC). Hooked end steel fibers and monofilament polypropylene fibers were used in the tests with three selected volume fractions (0.25%, 0.50% and 0.75% of the total volume of concrete) for steel fibers and one volume fraction (0.10%) for polypropylene fibers. Workability of fresh concrete, mechanical properties and shrinkage of hardened concrete were investigated. Slump flow, J-ring and V funnel tests were carried out to evaluate the filling ability, passing ability, and viscosity of the fresh concrete. Mechanical properties including compressive strength, splitting tensile strength and flexural strength of hardened concrete were studied. Test results indicate that workability of fresh concrete decreases with increased volume fraction of fibers. The compressive strength of ESCC is improved at 7 days with added expansive admixture. Combined addition of expansive admixture and fibers reduces the concrete strength at 7 days, while it does not influence the 28 days strength noticeably. For flexural performance, steel fiber improves the linear load-deflection relationship of ESCC beams. Steel fiber reinforced beam specimens with fiber content higher than 0.50% show deflection-hardening behavior. It is also found that free expansive rate of ESCC reduces with the increase of steel fibers content. Overall, fiber reinforced ESCC both at 0.25% and 0.50% volume content satisfy the target performance criteria.

A novel kind of cement blend with high temperature resistance and favorable sustainability was developed in this paper. The physical and chemical changes due to high temperature exposure were examined in this sustainable cement paste containing fly ash (FA) and aragonite calcium carbonate whiskers (CW) as supplementary cementitious materials. High flexural strength is demonstrated before 400 °C because FA has pozzolanic effect, self-cementitious properties and potentials to react with CW under hydrothermal condition. When the exposure temperature is above 400 °C, especially above 800 °C, high residual properties and less deterioration are observed in flexural and compressive strengths and elastic modulus, mainly because FA is involved in the sintering process and formation of ceramic phases. In summary, the sustainable cementitious composites studied in this paper are very promising to have high temperature resistance to reduce fire damage on constructions and at the same time lower carbon dioxide footprint of cement manufacture.

Calcium carbonate whisker (CW) is used to modify the steel-PVA hybrid fibers reinforced cementitious composites (SPFRCC). The addition of CW result in excellent mechanical properties and considerable economic benefit owing to its microscopic reinforcement effect and low cost. In this study, the flexural behavior, fiber distribution characteristics and compression response of CW modified steel-PVA hybrid fibers reinforced cementitious composites (CW-SPFRCC) are investigated by three-point bending test, image processing technique and uniaxial compression test, respectively. The results indicate that the addition of CW can effectively improve the flexural properties of SPFRCC because of the crack resistance and filling effect at microscale. The best fiber distribution characteristics are observed in CW10-S15P05 due to the presence of CW that modify the adhesion and flowability of cement matrix. The uniaxial compression strength, peak strain, elastic modulus and Poisson's ratio of CW-SPFRCC are increased by 37.1%, 40.2%, 3.7% and 30.7%, respectively. Meanwhile, the uniaxial compression constitutive model is proposed to describe the compression stress–strain relationship of CW-SPFRCC based on the experimental results. Moreover, the microstructure analysis demonstrates that the modification mechanism of CW in SPFRCC is attributed to the interface improvement, CW pull-out, CW slippage, and CW rupture.

By experimental research on the steel- polyvinyl alcohol (PVA) fibers- calcium carbonate whisker (CW) multi scale fiber-reinforced cement-based composites (MSFRC) after exposure of temperatures up to 900 °C, the flexural, compressive strength, and microstructures of multi scale fibers were investigated in this paper. The incorporation of steel-PVA fibers-CW can effectively improve the flexural and compressive strength of mortar after elevated temperatures. Compared with normal concrete, hybrid fiber reactive powder concrete (RPC) and engineered cement-based composites (ECC), the MSFRC present better capacity of high temperature resistance. The flexural and compressive strength increase first and then decrease with the increasing temperature, and the critical temperatures are 200 °C and 400 °C respectively. Models for describing the relationship between strength, CW content and temperature are proposed. The good high temperature resistance of MSFRC relates to the hybrid effect of melt of PVA fiber, good heat conduction of steel, and the phase transformation from aragonite to calcite of CW. Comprehensive morphology observations by digital camera and optical microscope are simple and useful way to evaluate the changes of fibers and CW at high temperature. This research is very beneficial for the applications of MSFRC in construction project with fire risk.

Building fire disasters may cause large economic loss and casualties. To improve the fire resistance ability of concrete and repair the fire-damaged concrete structures using an in-situ repairing technique (such as post-fire-curing) can greatly offset these adverse effects. Thus, the present study employed fly ash (FA) to form a CW-FA-cement system which was proved to enhance the high-temperature resistance of cement and CW blended cement and was also favorable for recovery of compressive strength and microstructure induced by water re-curing. In details, incorporation of FA was effective to reduce the deterioration of cement and CW blended cement and was favorable for recovery of compressive strength and microstructure after water re-curing. Incorporation of CW was favorable to increase the restorability of compressive strength re-cured after 400 °C and performed a combined effect with FA to improve the recovery of compressive strength re-cured after 800 °C. Through the microstructural tests, the deterioration of compressive strength was due to the decomposition of hydration products. The recovery of compressive strength was due to the rehydration of dehydrated cement, further hydration of unhydrated cement particles, pozzolanic effect of FA and combined effect of CW and FA. Furthermore, the cement blends containing CW and FA had a good high temperature resistance and restorability.

Calcium carbonate whisker is an emerging class of inorganic fiber having tremendous properties and vast applications. Calcium carbonate whisker is extensively used as a filler due to its tremendous properties including low price, simple preparation requirements and ease of availability. As a filler, it increases the physical and chemical properties such as heat resistance, strength and modulus. Its unique properties are seeking the attention of researchers for utilizing it as a filler in various applications. In this paper, different basic preparation techniques of calcium carbonate whisker such as metathesis reaction, sol-gel, carbonation, gravity crystallization and urea hydrolysis are described briefly. In addition to this, the review focus on research advancements achieved in the utilization of calcium carbonate whisker in different applications such as friction materials, paper making, reinforcing composite materials, cement, mortar and concrete. Calcium carbonate whisker is mostly used for enhancing the properties of cementitious materials. In this review, a special emphasis is given to the role of whisker in cementitious materials such as cement, concrete and mortar.

In order to reinforce cement mortar, a new kind of micro-fibrous material, calcium carbonate whisker (CaCO3 whisker), was incorporated in this study. Microstructure, mechanical properties and reinforcing mechanism of this composite were characterized. It was found that the addition of CaCO3 whisker improved not only the compressive and flexural strength of cement mortar, but also the load–deflection curves and work of fracture. Further work using mercury intrusion porosimetry tests confirmed the filler effect and the refining of the pore distribution of whiskers in cement mortar. Scanning electron microscopy showed that the microscopic mechanism primarily consists of whisker pullout, crack deflection, whisker-cement coalition pullout, whisker bridging and whisker breakage. These mechanisms are related to the matrix strength. As compared to the strong matrix, the weak matrix that was modified with CaCO3 whisker achieved the highest increase in strength and toughness of the cement mortar. This is likely attributed to the crack deflection mechanism, which is weakened by the strong interfacial bonding between the CaCO3 whisker and cement matrix in the stronger mortar matrix.

A new kind of fiber hybridization containing steel fibers, polyvinyl alcohol (PVA) fibers and cheap calcium carbonate (CaCO 3 ) whiskers (approximately $230 per ton) was designed to improve the mechanical response and shrinkage performance as well as reduce the production cost of fiber reinforced cementitious composites. Compressive response, flexural response, drying shrinkage and plastic shrinkage of this designed hybrid fiber reinforced composites were presented. The results indicated that the designed hybrid fiber shows a significant positive hybrid effect on mechanical response and plastic shrinkage of cementitious composites. Steel fibers are more effective in restricting drying shrinkage. PVA fibers and CaCO 3 whiskers are more effective in restraining plastic shrinkage. Good mechanical response and satisfactory shrinkage performance of the designed hybrid fiber reinforced cementitious composites make it possible to partly replace the steel fibers and PVA fibers by using CaCO 3 whisker, thus helping to decrease the production cost of fiber reinforced cementitious composites for large scale construction project applications in the future.

Calcium carbonate whisker (CW) could improve mechanical properties of cementitious composites at room temperature. Aiming at using CW as high-performance and low-cost microfiber at high temperature, the microstructural changes of cementitious composites with CW exposed to high temperature are clarified in this research. From room temperature to 500 °C, additional hydration of unhydrated binder grains in steam environment occur (“internal autoclaving”). Moreover, CW transferred from aragonite to calcite in cement paste at about 375 °C (“phase transformation”). Under these coupling effects, calcite CW forms stronger bond with these rehydration products than that at room temperature, also improving the pore distribution and compressive strength. At 800 to 1000 °C, CW increases the pore size and decreases the compressive strength of cement paste due to CW decomposition. From 1000 to 1100 °C, the residual compressive strengths of CW reinforced cement pastes increase slightly because of the forming of calcium silicate and Ca(OH)2.

Restriction of propagation and coalescence of cracks at microscopic level is one of the most effective means to achieve high properties of cement based composite. In order to improve the properties of cement based composite microcosmically, calcium carbonate whisker was introduced as a kind of microreinforcer. The effects of whisker on macro and micromechanical properties, fracture morphology, microstructures, and crack resistance of the composite with different water to binder ratios, as well as the reinforcing mechanisms were studied. The results indicated that whiskers are effective in delaying the formation and propagation of microcracks. The micromechanical mechanism is relative to the bond properties between whiskers and cement matrix. An appropriate interfacial bond strength is advantageous to optimize the reinforcing effect of whisker in cement based composite.