Behavior Of Cementitious Composites Research Articles

Cementitious composites modified by nanocarbon fillers with cooperation effect possessing excellent self-sensing properties

Abstract The safety and durability of concrete structures are prone to damage and result in significant harm to human society. Hence, monitoring and estimating the states of concrete structures is of great significance to protecting human safety. Graphene nanoplatelets (GNPs) and carbon nanotubes (CNTs) are promising candidates to endow cementitious composites with piezoresistivity for the health monitoring of concrete structures. Therefore, the aim of this article is to explore the effect of the hybrid GNPs and CNTs on behavior of cementitious composites with cooperation effects. The cementitious composites containing the hybrid GNPs and CNTs with simplified manufacturing methods are developed first. And then, the mechanical behaviors, electrical conductivities, and piezoresistive performances of the composites are investigated systematically. Finally, the modification mechanisms are also discussed. The results proved that the electrical resistivity of the composites with the hybrid GNPs and CNT concentration of 5.0 wt% is reduced by three orders of magnitudes, and the FCR and sensitivities are reached at 36.0% and 1.1% MPa−1/177.9, respectively. Its compressive strength/elastic modulus is achieved at 73.3 MPa/16.9 GPa. Therefore, the hybrid GNPs and CNTs modified cementitious composite present great potential application in monitoring and evaluating service states of civil infrastructures.

AFt/AFm distribution and microstructural properties of thermally cured concretes containing rice husk ash

The behaviour of concretes with and without rice husk ash was experimentally investigated with respect to the susceptibility to delayed ettringite formation. Concrete specimens have previously undergone a specific initial thermal curing up to 85°C, which was designed to trigger delayed ettringite formation. Specimens were subsequently exposed to a specific exposure environment by water immersion at 38°C, over 1 year. Expansion measurements and microstructural analyses were performed to evaluate the level of attack, as well as the integrity of concretes and chemical characteristics of compounds by means of SEM-EDS. Mechanical properties were also assessed over one year to detect the level of damage from delayed ettringite formation. The data has indicated that the incorporation of 8% of rice husk ash could partially reduce the level of DEF expansion and damage. The microstructure of cement matrices from both the reference and RHA concrete indicated different features, sulfate phase compositions and degradation stages. The reference concrete contained radial cracking from filled pores and nearby interfacial transition zone, whereas RHA concrete presented denser features, with microcracking scattered at random across cement matrix and minor neoformations.

Preface

FIBRE CONCRETE 201910th international conference on fibre reinforced concretes (FRC), textile reinforced concretes (TRC) and ultra-high performance cementitious composites (UHPC)The Fibre Concrete Conference series is held biennially to provide a platform to share knowledge on fibre reinforced concretes, textile concretes and ultra-high performance cementitious composites regarding material properties and behaviour, technology procedures, topics of long-term behaviour, creep, durability; sustainable aspects of concrete including utilisation of waste materials in concrete production and recycling of concrete. The tradition of Fibre Concrete Conferences started in eighties of the last century. Nowadays the conference is organized by the Department of Concrete and Masonry Structures of the Czech Technical University in Prague, Faculty of Civil Engineering.The 10th International Conference Fibre Concrete 2019 provides opportunity for presenting of 50 papers for participants from 14 countries all over the world. The keynote lectures are given by Professor Bažant and Professor Akira Hosoda.The conference program covers wide range of topics from scientific research to practical applications. The presented contributions relate to performance and behaviour of cementitious composites, their long-term behaviour and durability, sustainable aspects, advanced analyses of structures from these composites and successful applications.This conference was organized also to honour Professor Vladimír Křístek on the occasion of his jubilee and to appreciate his merits and discoveries in the field of concrete and cement based composites, structural mechanics and engineering.

Creep and relaxation of cement paste caused by stress‐induced dissolution of hydrated solid components

AbstractThe creep and relaxation of cement paste caused by dissolving solid hydration products is evaluated in this work. According to the second law of thermodynamics, dissolution or precipitation of solid constituents may be altered by the change in stress/strain fields inside cement paste via alteration of the stress power or strain energy. Thus, it is hypothesized that stress‐induced dissolution can affect the overall creep/relaxation behavior of cement composites. A novel, fully coupled thermodynamic, mechanical, and microstructural model (TM2) that uses the finite element method was developed to predict the time‐evolving properties of cement paste under prescribed strains and to test the hypothesis. In the model, the strain energy was incorporated to accurately predict the effect of stress and strain fields on cement microstructure change. From the simulation results, depending on the stress/strain levels and the choice of the domain (over which the thermodynamic equilibrium is enforced), stress‐induced dissolution of solid constituents can lead to significant creep/relaxation.

Debonding mechanism of polymeric fibers in reinforced cementitious composites as a function of crystallinity degree of fibers

Engineered cementitious composites containing polymeric fibers such as poly(vinyl alcohol) show high tensile ductility and toughness. The aim of the current study was to evaluate the effect of crystallinity degree of poly(vinyl alcohol) fibers on fiber debonding from cementitious matrix in the bending test. In this work, two types of poly(vinyl alcohol) fibers and one grade of polypropylene fiber were separately incorporated in cementitious composites. The chemical structure, surface chemistry, roughness, and microstructure of fibers were examined by Fourier transform infrared, attenuated total reflection, atomic force microscopy, and wide-angle X-ray diffraction tests, respectively. The compression and flexural behaviors of cementitious composites were also assessed. Attenuated total reflection results were indicative of similar surface chemistry for both poly(vinyl alcohol) fibers, while the main difference was observed in the case of degree of crystallinity, which plays an important role in the Poisson’s ratio. Finally, the way in which the degree of crystallinity and Poisson’s ratio of fibers can lead to premature debonding was described and confirmed by scanning electron microscopic images.

Применение комплексной методики анализа поведения цементных композитов с силикатными добавками при повышенных температурах

Применение комплексной методики анализа поведения цементных композитов с силикатными добавками при повышенных температурах

Behaviour of cement composites with lightweight and heavyweight aggregates at high temperatures

This paper presents new results of the research of the influenceof individual input components done on the behaviour of cementbased composite materials exposed to high temperatures. Anew approach of this research lies in combining a fraction ofbasalt fine aggregate and an aggregate base on expanded clayfor a composite material with lower volume weight and higherstrengths and a resistance to high temperatures. In the field ofdispersed reinforcement, new polypropylene fibers treated bylow temperature plasma were tested. Degradation of the structureof designed mixtures was evaluated by comparing densitychanges, mechanical and acoustic properties before and afterexposure to high temperatures. An optical microscope was usedfor determining the width of cracks caused by a thermal load.The contribution of this research was the use of non-destructivemethods for diagnostics of quality and properties of designedconcrete mixtures exposed to high temperatures.

Behavior of Cement Composites Loaded by High Temperature

In this paper, there are summarized the results of an experimental program focused on basic, mechanical and thermal properties of cement composites according to the high – temperature loading. Four different materials were studied, which differed in used kind of cement and amount of fibers. As a matrix for studied composites the aluminous cement was chosen because of its resistance in high temperature. For a comparison the Portland cement was also tested. The second main ingredient used to provide better resistance in high temperatures - the basalt aggregate, was mixed in every specimen. The basalt fibers were chosen for two of the measured samples, remaining two ones were tested without fibers. The obtained data in this presented analyses show that the application of the aluminous cement leads to increase (depending on temperature) of porosity, which is the cause of decreasing of the coefficient of thermal conductivity. It can seems, that these cement composites will have low mechanical strength in high temperatures, but because of better sintering, the aluminous cement keeps its strength in high temperatures better than Portland cement.

Effect of Metakaolin Sand on Properties of Cement Composites

The Slovak natural raw material kaolin sand containing 36 wt.% of kaolinite from Vyšný Petrovec deposit was thermally transformed at 650 °C for 1 hour to the metakaolin sand with relevant content of metakaolinite. Behaviour of cement composites having replacement of Portland cement with metakaolin sand including 0; 5; 10 and 15 wt.% of metakaolinite and water to solids ratio of 0.5 cured in water for 28 days and 90 days was studied by thermal analysis, X-ray diffraction analysis and mercury intrusion porosimetry analysis. The study concerned calciumsilica hydrate and calcium aluminate hydrate formation, portlandite dehydroxylation and calcite decarbonation. The influence of curing time and metakaolinite content were estimated. The replacement of Portland cement by metakaolin sand led to positive effect on relevant compressive strengths. The changes in microstructure involved especially reduction in portlandite content and pore structure refinement.

Tensile Response and Fracture Process of High-Performance Hybrid Fiber-Reinforced Cement Composites

Five twisted steel fibers (steel cord, SC) are used to improve the post-cracking behavior of cement composites. In addition to these steel cord macrofibers, microfibers are used to enhance the pre- and post-cracking strength and behavior. The three specific hybrid fiber blends pursued in this investigation are SC and polyethylene (PE), SC and polypropylene (PP) as well as SC and polyvinyl alcohol (PVA) fibers. The research results show that SC macrofibers with high ultimate strain, strength and bond stress can bridge the macrocracks in the matrix. Therefore, cement-based composites reinforced with SC fibers exhibit a pseudo-strain hardening in tension as a result of the formation of multiple cracks. The microfibers improved pre-peak mechanical performance, strength, and fracture process by delaying the formation of a macrocrack. The tensile strength, energy consumption capacity and ductility of SC macrofibers and high elastic modulus microfiber PE reinforced cement-based composites are tremendously improved compared to SC macrofibers with lower elastic modulus microfibers PVA and PP.

Experimental investigation of the strain rate dependent impact behaviour of cementitious composites

As for most construction materials, cementitious composites exhibit a strain rate dependent mechanical behaviour. The frequent occurrence of impact loads during the lifetime of civil engineering constructions makes the experimental research into the impact properties of construction materials sensible. To this purpose split Hopkinson bar tests are often used. However, due to the specific behaviour of plain cementitious materials—low strains and softening—special attention has to be paid to some aspects of the experimental setup, the treatment of the recorded signals and the interpretation of the final results. Some results of impact tensile tests on microconcrete are presented. These results show that the strength, the strain at maximum stress, the ultimate deformation and the energy absorbing capacity are higher than in static test conditions.

Fracture mechanics of cementitious materials

The concepts of classical fracture mechanics were first applied to cementitious materials by Kaplan in 1961; since then, an enormous literature on this topic has emerged, the more recent publications emphasizing the nonlinear fracture mechanics approach. Unfortunately, this body of literature is highly fragmented. It appears in a bewildering number of journals and conference proceedings, and is often highly contradictory in its conclusions. This is at least one reason why the fracture mechanics approach has not been much used in the analysis and design of concrete structures. Thus, the present book by two widely respected researchers, Brian Cotterell and Yiu-Wing Mai, is particularly welcome at this time. It brings together in one place a detailed discussion of well over 400 studies of the fracture of cementitious composites, supplemented by the considerable experience of the two authors. The book is aimed primarily at graduate students, and at structural engineers who may be interested in finding out what advantages the fracture mechanics approach may bring. The coverage of the book is very broad. Since fracture mechanics is little taught to civil engineers at the undergraduate (or even the graduate) level, it begins with a brief but lucid introduction to fracture mechanics itself. This is followed by a description of the nature of the fracture process in cementitious materials, with particular emphasis on the fracture process zone and on the fibre bridging zone in fibre reinforced cements and concretes. Chapter 3 deals with the experimental determination of the fracture parameters, which will be of particular value to graduate students embarking on an experimental study of the fracture of concrete. The next two chapters deal, in considerable detail, with the theoretical models for the fracture of cementitious materials and fibre reinforced cementitious composites, respectively. The statistical aspects of the fracture process are then discussed, with some emphasis on size effects. Chapter 7 deals with the timedependency of the fracture process, devoted primarily to slow crack growth and creep problems. A notable omission, unfortunately, is the almost complete lack of any discussion concerning the applications of fracture mechanics to dynamic (blast or impact) loading of concrete, since this is an area of great current interest. Finally, the book concludes with an all-too-brief discussion of the application of fracture mechanics to the design of concrete structures, probably because there have been relatively few such applications in practice. While the authors do not present any new theories regarding the applications of fracture mechanics, they do present a balanced and comprehensive review of what is now known. Moreover, they do this in a way that makes the fracture mechanics approach understandable even to those with little previous exposure to this method of analysis. This book should help to overcome the mistrust with which most stmctural engineers view fracture mechanics. It is highly recommended for graduate students, for structural engineers, and indeed for anyone concerned with the strength and mechanical behaviour of cementitious composites.

Tensile stress-strain behaviour of cementitious composites at high loading rates

The influence of loading rate on the tensile stress-strain behaviour of cementitious composites was studied experimentally. The project was undertaken to obtain an insight into the possible relation between internal structure parameters of composites and their loading-rate sensitivity. Five different types of cementitious composites were applied. Composite structure data were obtained by testing porosity and by quantitative observation of fracture surfaces. Direct tensile tests were performed at four different loading rates within the range 0.001–1000 MPa s−1. The tensile stress-strain behaviour was significantly influenced by the loading rate and structure parameters of composites. The relative tensile strength increase due to an increase of loading rate was found to be higher for composites with higher total porosity. Recorded stress-strain diagrams obtained at various loading rates are presented and discussed with the aid of continuous damage mechanics.

Slow Crack Growth in Cement Composites

The resistance to crack growth in mortar, concrete and fiber reinforced concrete was studied by testing double torsion and double cantilever speciments. R-curves (plots of strain energy release rate vs. crack extension) were obtained from the test data. To include the inelastic strain energy absorbed during the crack extension, the definition of the strain energy release rate was modified. It was found that R-curves for mortar and concrete were similar for both double torsion and double cantilever speciments. R-curves were sensitive to the volume and size of aggregates and to the presence of fibers. It appears that R-curves can be a useful specimen-geometry-independent parameter for chracterizing the fracture behavior of cementitious composites. However, the effects of differing crack-opening displacement on the strain energy release rate must be considered; especially for fiber reinforced concrete.