An experimental study of synthetic fibre reinforced cementitious composites

Fibre reinforcement is one of the effective ways of improving the properties of concrete. However, current studios on fibre -reinforced concrete (FRC) have focused mainly on reinforcements with steel and glass fibres. Thin paper reports on an experimental programme on the properties of various synthetic fibre reinforced cementitious composites and the properties of the reinforcing fibres. Acrylic, polyester, and aramid fibres were tested in uniaxial tension, both in their original state as we!! as after ageing in nerO*nL Samples of these fibres were found to lose varying amounts of strength with time, depending on the ageing temperature. Two different test methods were used to measure the fibre-cement interfacial bond strength. The tensile properties of concrete reinforced with acrylic, nylon, and aramid fibres, in the form of random distribution or unioxial alignment, were studied by means of three different tests: compact tension, flexural, and splitting tensile tests. The properties of concrete, particularly that of apparent ductility, were found to be greatly improved by the inclusion of such fibre reinforcement.     

Investigation of mixed-mode fracture of aligned steel fibre reinforced cementitious composites

International Journal of Fracture - Mixed-mode fracture experiments were conducted on aligned steel fibre reinforced cementitious composite (ASFRC) and ordinary steel fibre reinforced cementitious...     

Effect of interfacial thickness and stiffness on the stress distributions in fibre reinforced cementitious composites

The different microstructure of the fibre–cement interface might result in different failure mechanisms. It is expected that improvement of strength and toughness in fibre-reinforced cementitious composites will depend on their interfacial thickness and stiffness. A three-phase model, subject to a transversely uniform tensile stress, was utilized to investigate the effect of interfacial thickness and stiffness on the stress distributions near the fibre–cement interface and the corresponding failure mechanism. The results suggest that optimum interfacial microstructure of fibre-reinforced cementitious composites can be tailored to obtain a higher strength and toughness. Optimum interfacial thickness and stiffness was evaluated for various reinforcements, including steel, carbon, glass and polypropylene fibres. This revised version was published online in November 2006 with corrections to the Cover Date.     

On the equivalent order of crack-tip singularity defined by caustics

The problem of a transverse Griffith crack in an infinite plate submitted to simple tension at infinity was studied by using its closed form solution described by the elastic potential function (z). The exact form of the caustic and its generatrix curve formed around the crack tips was exactly described by using the (z)-function. These exact forms were compared with the respective forms given either by the singular one-term solution of the problem and accepting that the order of singularity at the crack tip is (1/2), or by a solution defining the order of singularity and the respective stress intensity factor by taking into consideration the influence of the distance from the crack tip where these quantities are evaluated. It was shown by comparing the first stress invariant I ₁, whose gradient defines the respective caustic, that the differences between the exact values and the values of I ₁ derived by the proposed method with variable order of singularity is much smaller than the differences between the exact solution and the singular solution. The singular solution is based on the assumption of a constant value of the order of singularity.

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Résumé On a étudié le problème d'une fissure de Griffith transversale dans une plaque infinite soumise à traction simple à l'infini en utilisant une solution de forme fermée décrite par la fonction des potentiels élastiques (z). La forme exacte de la caustique et de sa génératrice au voisinage de l'extrémité de la fissure a été décrite avec exactitude en recourant à la fonction (z). On a comparé ces formes exactes avec les formes données respectivement par une solution singulière à un terme du problème, en acceptant que la singularité à l'extrémité de la fissure soit d'ordre 1/2, ou par une solution dans laquelle l'ordre de la singularité et les facteurs d'intensité de contraintes sont définis en tenant compte de l'influence de la distance qui sépare l'extrémité de la fissure dulieu où ces quantités sont évaluées. On montre que, pour le premier invariant de contrainte I ₁, les différences entre valeur exacte et valeur déduite de la méthode proposée, avec un ordre de singularité variable, sont plus faibles que les différences entre la solution exacte et la solution singulière, laquelle est basée sur l'hypothèse de valeurs constantes de l'ordre de singularité.

     

Research on production,performance and fibre dispersion of PVA engineering cementitious composites

Abstract

Polyvinyl alcohol (PVA) fibre is considered as one of the most suitable polymeric fibres to be used as the reinforcement of engineered cementitious composites (ECCs). Research and application have shown that PVA–ECC can significantly counteract the deficiency of ordinary concrete. In the present paper, micromechanics based design theory and fracture mechanics formulation leading to energy and strength criteria to achieve strain hardening and multiple cracking are described. Engineered cementitious composites showing pseudo strain hardening behaviour with over 6% of strain capacity under tension is produced. Uniaxial tensile tests of PVA–ECC are conducted and the results support the validity of the proposed theory. Also viscosity modifying agent plays an important role in the dispersion of the fibres in the matrix. It is shown that a uniform distribution of fibres throughout the bulk of the composite material is crucial to its excellent workability, tight crack width and reduction in the autogenous and drying shrinkage strains.     

Printability and fire performance of a developed 3D printable fibre reinforced cementitious composites under elevated temperatures

ABSTRACT

To demonstrate printability and fire performance of 3D printable fibre reinforced cementitious materials at elevated temperatures, large-scaling printing and fire performance testing are required for engineering applications. In this work, a mixture design of 3D printable fibre reinforced cementitious composite (3DPFRCC) for large-scale printing was developed. A structure with dimensions of 78 × 60 × 90?cm (L × W × H) was printed by a gantry printer in 150?min, which demonstrates that the developed 3DPFRCC mixture possesses good buildability. The rheological property, setting-time, and mechanical properties under normal and elevated temperatures of the developed 3DPFRCC were then characterised. Final results indicate that the developed 3DPFRCC is suitable for engineering applications due to its good printability and mechanical properties under normal and elevated temperatures.     

In situ fibre strength determination in metal matrix composites

Single fibre fragmentation tests were performed at room temperature on SiC/Ti-6242 specimens in order to estimate the in situ fibre strength. Tensile specimens were instrumented with two acoustic emission transducers and an extensometer in order to monitor the strain at which fibre breaks occurred. Data analysis utilized Monte Carlo simulations of fibre fragmentation. The fibre/matrix stress transfer profile near a fibre break was derived using a finite element analysis. Cohesive zone model is used to describe damage of the interfacial zone. Thermally induced residual stresses and matrix plastic deformations were accounted for. The results presented in this paper show that the in situ Weibull parameters of the fibre are smaller than the reference obtained on as received fibres. Analysis of data raised questions about the validity of the Monte Carlo simulation method.     

Design and structural applications of stress-crack width relations in fibre reinforced concrete

The stress-crack width relationship has been shown to be the key to an understanding of fracture propagation in and mechanical behaviour in tension of fibre reinforced concrete materials and structures. A model is derived for the stress-crack width relationship for randomly oriented short fibre composites which takes hybrid fibre systems and possible fibre rupture into account. It is shown how this stress-crack width relationship can be included in a structural model for the prediction of crack widths in reinforced concrete structures. With this combination of models a rational design tool for the design of composite materials and structures has been established. It is shown how different fibre systems can be tested for structural applicability and how combined material and structural optimization can take place.     

A review on durability properties of strain hardening fibre reinforced cementitious composites (SHFRCC)

This paper reviews and presents various durability properties of strain hardening fibre reinforced cementitious composites (SHFRCC). Published research results show that, due to its tight crack width properties compared to ordinary concrete and ordinary fibre reinforced concrete, SHFRCC significantly resists the migration of aggressive substances in to the concrete and improves the durability of reinforced concrete (RC). It is also reported that, due to the strain hardening and multiple cracking behaviours, SHFRCC meets the tight crack width limits for durability of RC structures proposed by different design codes. Based on the reviewed durability properties it is argued that SHFRCC materials can be used in selected locations of RC structural members to improve their overall durability performances.     

Durability modelling of glass fibre reinforcement in cementitious environment

A main topic of consideration for glass-fibre reinforced cementitious composites is their durability. Recent developments of new cementitious matrices and advances in chemical composition and sizing of glass fibres lead to increased durability of cementitious composites with glass fibre reinforcement. Still, the relative importance of the main degradation mechanisms is not fully understood. A joint experimental program at RWTH-Aachen (Germany) and VUB-Brussels (Belgium) shows that the pH of the matrix has a considerable influence on the durability of the studied composite. Recent investigations on single filaments in high alkalinity solutions and on rovings in concrete show that filaments lose strength mainly due to chemical attack in local weak points of the glass structure. Once this chemical attack becomes diffusion controlled, further corrosion slows down to a considerably lower rate. The aim of these investigations is to finally build a model, allowing prediction of the long-term behaviour of textiles—made of glass filaments—in concrete, which is based on the physico-chemical background of degradation.     

Derivation of applied stress-crack opening displacement relationships for the evaluation of effective stress intensity factor range

Linear elastic fracture mechanics (LEFM) integrated with the interference of fracture surface asperities has been formulated. The asperities are considered to simulate the influence of the microstructures and possibly oxide debris. The applied stress/load-crack opening displacement (COD) relationships in several cases have been derived. In the original LEFM, the stress-COD relationship is represented by a straight line passing through the origin of the stress-COD plot. The insert of one asperity results in a deviation of the stress-COD response from the LEFM relationship, leading to the exhibition of an inflection point (first contact point, σ_op), a larger slope, and a residual COD. In the case of two asperities, the slope and the residual COD of the stress-COD relationship become further larger, and two inflection points emerge. A general stress-COD expression in the case of multiple asperities has been derived. The slope of the stress-COD equation, the residual COD, and the minimum COD all increase with increasing number of asperities for a given loading condition, resulting in a smaller ΔCOD and Δσ_eff. The number of the inflection points is the same as that of the asperities. To the authors' knowledge, this paper is the first to derive analytically an applied stress-COD curve with a gradual variation below σ_op, caused by the asperity-/roughness-, or oxide-induced crack closure.     

Evaluation of the consistency of fiber reinforced cementitious composites

The rheological properties of fresh concrete, mortar or cement paste are among the most important parameters when cementitious building materials are placed. New material designs, like high or ultrahigh performance concretes, include the addition of a high volume of fibers to the fresh mix influencing its workability properties. However, the analysis of the rheological properties of fiber reinforced cementitious composites is difficult. Conventional methods mostly do not apply, especially when a high fiber content and relatively stiff mixtures are used. For this reason, a new method was developed to evaluate the workability of fiber reinforced composites. This method was applied to carbon and PVA fiber reinforced high performance composites and was used to optimize the rheological properties of these composites for an application in a centrifugation casting process.     

Experimental evaluation of residual stresses in single fibre composites by means of the fragmentation test

The residual stresses in both thermosetting and thermoplastic single-fibre composites have been experimentally evaluated by means of an original technique based on the continuous monitoring of the fragmentation test performed at various temperatures. The difference between the strain at the break of a single fibre in air and one embedded in a polymeric matrix has been measured as a function of temperature. By considering the compressive fibre modulus this strain difference has been converted into fibre compressive stresses related to the matrix thermal shrinkage after curing of the samples. In fact, as the test temperature increased, the thermal compressive stresses decreased until a zero value was obtained, corresponding to a so called stress free temperature, equal to the curing temperature for amorphous thermosetting matrix composites or equal to the matrix melting temperature for semicrystalline-thermoplastic matrix composites. The experimental results have been compared with data obtained from a theoretical model and a good agreement was found especially if the temperature dependence of the matrix Young's modulus and matrix thermal expansion coefficient are accounted for in the computation.     

Experimental characterisation of fibre failure and its influence on crack growth resistance in fibre reinforced titanium metal matrix composites

Abstract

The present paper addresses the effects of fibre failure on the fatigue crack growth resistance of a Ti-6AI-4V (wt-%) alloy matrix unidirectionally reinforced with continuous Sigma (SM1240) SiC fibres. Fibre fracture was monitored in situ using a PAC Locan acoustic emission (AE) analyser, and the exact spatial locations of the individual fibre failure events were identified using novel experimental techniques. A fibre probe technique has been illustrated to be a viable method with which to identify whether a fibre is broken or remains intact within a testpiece. Examination of exact spatial locations of fibres is possible, and evidence suggests that individual fibre failure is of ten followed by another fibre failure within the same row of a single mat lay up. Experimental observations and AE data reveal that crack arrest occurs if relatively few fibres fail in the crack wake as they are breached by matrix fatigue crack growth, and that fibre failure occurs only in the crack wake and behind the growing fatigue crack tip.     

Behavior of unreinforced masonry prisms and beams retrofitted with engineered cementitious composites

The impact of a thin layer of a ductile fiber-reinforced concrete referred to as engineered cementitious composites (ECC) on unreinforced masonry (URM) prisms and beams has been evaluated. The objective of the research was to characterize the performance and potential benefits of using ECC to retrofit URM with eventual application to masonry infill walls in non-ductile reinforced concrete frames. Compression tests of masonry prisms and flexural tests of masonry beams with different ECC retrofit schemes were conducted. The variables studied were the use of wall anchors to improve the ECC-masonry bond and alternate steel reinforcement ratios within the ECC layer in the form of welded wire fabric. The ECC retrofit was found to increase the strength and stiffness of URM prisms by 45 and 53 %, respectively compared to those of a plain specimen. When wall anchors were installed on the masonry specimens, the bond between the ECC layer and the masonry surface was improved. Four-point bending tests indicated that the strength and more importantly the ductility of an ECC retrofitted brick beam are increased significantly, especially when light reinforcement is added to the ECC layer, relative to an URM beam. Analytical models for estimating the strength and stiffness of ECC retrofitted masonry specimens are proposed and evaluated.     

Development of engineered cementitious composites with limestone powder and blast furnace slag

Nowadays limestone powder and blast furnace slag (BFS) are widely used in concrete as blended materials in cement. The replacement of Portland cement by limestone powder and BFS can lower the cost and enhance the greenness of concrete, since the production of these two materials needs less energy and causes less CO₂ emission than Portland cement. Moreover, the use of limestone powder and BFS improves the properties of fresh and hardened concrete, such as workability and durability. Engineered cementitious composites (ECC) is a class of ultra ductile fiber reinforced cementitious composites, characterized by high ductility, tight crack width control and relatively low fiber content. The limestone powder and BFS are used to produce ECC in this research. The mix proportion is designed experimentally by adjusting the amount of limestone powder and BFS, accompanied by four-point bending test and uniaxial tensile test. This study results in an ECC mix proportion with the Portland cement content as low as 15% of powder by weight. This mixture, at 28 days, exhibits a high tensile strain capacity of 3.3%, a tight crack width of 57 μm and a moderate compressive strength of 38 MPa. In order to promote a wide use of ECC, it was tried to simplify the mixing of ECC with only two matrix materials, i.e. BFS cement and limestone powder, instead of three matrix materials. By replacing Portland cement and BFS in the aforementioned ECC mixture with BFS cement, the ECC with BFS cement and limestone powder exhibits a tensile strain capacity of 3.1%, a crack width of 76 μm and a compressive strength of 40 MPa after 28 days of curing.