Structure and properties of aerated concrete: a review

Aerated concrete is relatively homogeneous when compared to normal concrete, as it does not contain coarse aggregate phase, yet shows vast variation in its properties. The properties of aerated concrete depend on its microstructure (void–paste system) and composition, which are influenced by the type of binder used, methods of pore-formation and curing. Although aerated concrete was initially envisaged as a good insulation material, there has been renewed interest in its structural characteristics in view of its lighter weight, savings in material and potential for large scale utilisation of wastes like pulverised fuel ash. The focus of this paper is to classify the investigations on the properties of aerated concrete in terms of physical (microstructure, density), chemical, mechanical (compressive and tensile strengths, modulus of elasticity, drying shrinkage) and functional (thermal insulation, moisture transport, durability, fire resistance and acoustic insulation) characteristics.     

Mechanical properties of ultra-high-performance fiber-reinforced concrete: A review

A comprehensive investigation into the mechanical properties of ultra-high-performance fiber-reinforced concrete (UHPFRC), considering various influential factors, is imperative in order to obtain fundamental information for its practical utilization. Therefore, this paper reviewed the early-age strength (or setting) development and mechanical properties of hardened UHPFRC. In connection with the latter, the effects of the curing conditions, coarse aggregate, mineral admixtures, fiber properties, specimen size, and strain-rate on the mechanical performance of UHPFRC were specifically investigated. It was obvious that (1) heat treatment accelerates the hydration process, leading to higher strength; (2) a portion of the silica fume can be replaced by fly ash, slag, and rice husk ash in mechanical perspective; (3) the use of deformed (hooked and twisted) or long straight steel fibers improves the mechanical properties at a static rate; and (4) high rate loading provides a noticeable increase in the mechanical properties. Alternatively, there are some disagreements between the results from various ‘size effect’ tests and the effectiveness of using twisted steel fibers at static and high rate loadings. Further research to reduce the production cost of UHPFRC is also addressed in an attempt to make its widespread use more practical.     

Ecofriendly geopolymer concrete: a comprehensive review

Clean Technologies and Environmental Policy - After ordinary Portland cement (OPC) concrete, geopolymer concrete (GPC) is the most advanced form of concrete. GPC has many advantages including...     

Bamboo reinforced concrete: a critical review

The use of small diameter whole-culm (bars) and/or split bamboo (a.k.a. splints or round strips) has often been proposed as an alternative to relatively expensive reinforcing steel in reinforced concrete. The motivation for such replacement is typically cost—bamboo is readily available in many tropical and sub-tropical locations, whereas steel reinforcement is relatively more expensive—and more recently, the drive to find more sustainable alternatives in the construction industry. This review addresses such ‘bamboo-reinforced concrete’ and assesses its structural and environmental performance as an alternative to steel reinforced concrete. A prototype three bay portal frame, that would not be uncommon in regions of the world where bamboo-reinforced concrete may be considered, is used to illustrate bamboo reinforced concrete design and as a basis for a life cycle assessment of the same. The authors conclude that, although bamboo is a material with extraordinary mechanical properties, its use in bamboo-reinforced concrete is an ill-considered concept, having significant durability, strength and stiffness issues, and does not meet the environmentally friendly credentials often attributed to it.     

Effects of electronic stability control (ESC) on accidents: a review of empirical evidence

This study summarizes evidence from empirical studies on the effects of electronic stability control (ESC) on accidents in a meta-analysis. Large reductions of single vehicle accidents have been found (-49%; 95% confidence interval: [-55%; -42%]), and smaller but still significant reductions of head-on collisions (-13%; 95% confidence interval: [-17%; -8%]). Multi-vehicle fatal accidents are also significantly reduced (-32%; 95% confidence interval: [-43%; -20%]). The effects can be explained with the potential of ESC to improve driving dynamics and to reduce the probability of loss of control. However, there are significant amounts of heterogeneity in the results, especially for single vehicle accidents, and a sensitivity analysis shows that the results for single vehicle accidents are likely to be affected by publication bias. The results for single vehicle accidents are in excess of what might be expected based on studies that have estimated the total amount of accidents that may be affected by ESC. Consequently, the proportions of accidents that can be avoided by ESC is assumed to be somewhat smaller than suggested by most empirical studies. Properties of the vehicles, time trends, and driver behaviour may have contributed to the large empirical effects.     

Influence of mixing procedure and mixer type on fresh and hardened properties of concrete: a review

Mixing concrete is not yet a fully understood issue, with many parameters having an influence on the resulting fresh and hardened concrete properties. Even for the same composition, a somewhat different microstructure can be obtained by changing the mixing procedure and the mixer type. A mixing procedure can differ in mixing time, mixing speed, air pressure in the mixing pan, addition time of the superplasticizer, temperature, etc. The concrete industry shows a great interest in controlling these influences in order to produce a concrete of which the mechanical, rheological and durability properties are well known. In this overview, different concrete mixers, mixing times, mixing speeds, different addition times of the superplasticizer and a different air pressure in the mixing pan will be examined. A review of existing literature is presented, as well as some new experimental results.     

A review of the hardened mechanical properties of self-compacting concrete

Data from more than 70 recent studies on the hardened mechanical properties of self-compacting concrete (SCC) have been analysed and correlated to produce comparisons with the properties of equivalent strength normally vibrated concrete (NVC).The significant scatter obtained in much of the data is a consequence of the wide range of materials and mixes used for SCC, but clear relationships have been obtained between cylinder and cube compressive strength, tensile and compressive strengths, and elastic modulus and compressive strength. It is also clear that limestone powder, a common addition to SCC mixes, makes a substantial contribution to strength gain.Bond strength of SCC to reinforcing and prestressing steel is similar to or higher than that of normally vibrated concrete. Variation of in situ properties in structural elements cast with SCC is similar to that with NVC, and the performance of the structural elements is largely as predicted by the measured material properties.The analysis has shown that sufficient data have been obtained to give confidence in the general behaviour of SCC, and future studies need only be focused on specific or confirmatory data for particular applications.     

Role of graphene-based materials (GO) in improving physicochemical properties of cementitious nano-composites: a review

Journal of Materials Science - This work deals with the review of graphene-based cementitious composites, which is primarily focused on the effect on microstructure, hydration mechanism, and...     

The durability characteristics of high performance concrete: a review

Durability problems of ordinary concrete can be associated with the severity of the environment and the use of inappropriate high water/binder ratios. High-performance concrete that have a water/binder ratio between 0.30 and 0.40 are usually more durable than ordinary concrete not only because they are less porous, but also because their capillary and pore networks are somewhat disconnected due to the development of self-desiccation. In high-performance concrete (HPC), the penetration of aggressive agents is quite difficult and only superficial. However, self-desiccation can be very harmful if it is not controlled during the early phase of the development of hydration reaction, therefore, HPC must be cured quite differently from ordinary concrete. Field experience in the North Sea and in Canada has shown that HPCs, when they are properly designed and cured, perform satisfactorily in very harsh environments. However, the fire resistance of HPC is not as good as that of ordinary concrete but not as bad as is sometimes written in a few pessimistic reports. Concrete, whatever its type, remains a safe material, from a fire resistance point of view, when compared to other building materials.     

Testing superabsorbent polymer (SAP) sorption properties prior to implementation in concrete: results of a RILEM Round-Robin Test

This article presents the results of a round-robin test performed by 13 international research groups in the framework of the activities of the RILEM Technical Committee 260 RSC “Recommendations for use of superabsorbent polymers in concrete construction”. Two commercially available superabsorbent polymers (SAP) with different chemical compositions and gradings were tested in terms of their kinetics of absorption in different media; demineralized water, cement filtrate solution with a particular cement distributed to every participant and a local cement chosen by the participant. Two absorption test methods were considered; the tea-bag method and the filtration method. The absorption capacity was evaluated as a function of time. The results showed correspondence in behaviour of the SAPs among all participants, but also between the two test methods, even though high scatter was observed at early minutes of testing after immersion. The tea-bag method proved to be more practical in terms of time dependent study, whereby the filtration method showed less variation in the absorption capacity after 24 h. However, absorption followed by intrinsic, ion-mediated desorption of a specific SAP sample in the course of time was not detected by the filtration method. This SAP-specific characteristic was only displayed by the tea-bag method. This demonstrates the practical applicability of both test methods, each one having their own strengths and weaknesses at distinct testing times.     

Recycled concrete as an aggregate for concrete—a review

The present state of knowledge on the use of recycled concrete as an aggregate in new concrete is reviewed and suggestions made as to what further work is necessary before a proper assessment of the material can be made. Where crushed uncontaminated concrete is used the properties of the material as an aggregate and the basic engineering characteristics of the concrete made with it are well established. Much less is known about the type and quantity of impurities which could occur in crushed concrete from general building rubble and the effect these would have on concrete made using such crushed concrete as aggregate.

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Résumé On fait le point sur l'état présent des connaissances relatives à l'utilisation du béton recyclé comme granulat du béton nouveau. Et l'on présente quelques suggestions sur les travaux à accomplir avant qu'une évaluation correcte de ce matériau, puisse être faite. Lorsqu'il est utilisé sans impuretés, les propriétés du béton concassé en tant que granulat et les caractéristiques techniques de base du béton confectionné avec ce matériau sont bien établies. Mais on en sait beaucoup moins sur les types et quantités d'impuretés qui se rencontrent dans le béton concassé à partir d'un matériau de démolition global, et sur les effets qui peuvent en résulter pour le béton confectionné avec ces débris.

     

Metakaolin and calcined clays as pozzolans for concrete: a review

The utilisation of calcined clay, in the form of metakaolin (MK), as a pozzolanic material for mortar and concrete has received considerable attention in recent years. This interest is part of the widely spread attention directed towards the utilisation of wastes and industrial by-products in order to minimise Portland cement (PC) consumption, the manufacture of which being environmentally damaging. Another reason is that mortar and concrete, which contain pozzolanic materials, exhibit considerable enhancement in durability properties. This paper reviews work carried out on the use of MK as a partial pozzolanic replacement for cement in mortar and concrete and in the containment of hazardous wastes. The literature demonstrates that MK is an effective pozzolan which causes great improvement in the pore structure and hence the resistance of the concrete to the action of harmful solutions.     

Air tornado-like vortices: Mathematical modeling (a review)

Computational and theoretical works devoted to studying the generation and dynamics of air tornado-like vortices are reviewed. Currently developed approaches to mathematical modeling of the main characteristics of vertical atmospheric vortices are presented.     

Strength of glass (a review)

After defining the termglass, brief consideration is given to the theories of glass structure which have been propounded. That glass, in practice, is very much weaker than its theoretical strength predicts is established, and the prime factor for this is shown to be the surface condition of the glass. Surface defects which are accentuated by heat treatment or other processes are considered, and the effects of the structural state, loading, and water and other liquids are dealt with. Methods of measuring the strength of glass are discussed and techniques of strength reinforcement, particularly in respect of producing a flaw-free surface and protecting it against subsequent damage, in the light of current knowledge are indicated. Many possible techniques for achieving strengthening have been postulated in the literature and some of these have now been commercially applied. Particularly, thermal and chemical toughening processes — the latter including ion-diffusion techniques recently developed — are reviewed. Where possible, references to the original papers and patents are provided. The references given, whilst not comprehensive, will permit a wider study of the subject.     

Active control of fluid equilibrium in a thermosyphon

We have theoretically and experimentally studied the problem of automated control over the mechanical equilibrium of an inhomogeneously heated fluid in a rectangular convection loop (thermosyphon), which comprises two coupled vertical channels with rectangular cross sections arranged in a unit made of a heat-conducting material. The equilibrium is maintained using a control feedback subsystem (controller) capable of responding to the onset of a convective motion by introducing small changes in the spatial orientation of the thermosyphon in the gravitational field. The effect of the dynamic stabilization of the equilibrium, which is unstable in the absence of control, is achieved in a broad range of parameters of the system. It is established that excess feedback leads to the excitation of oscillations in the system. An analysis of the theoretical model showed that these oscillations are related to a delay in the correction introduced by the controller into the state of the system under control. The experimental data are in perfect agreement with the results of analysis of the theoretical model.     

The effect of a superplasticizer (from cashew nut shell liquid) on the properties of concrete and mortar

The paper describes a new, cheaper water-reducing compound obtained from cashew nut shell liquid (CNSL). The properties of mortars, namely flow, setting time and compressive strength, were determined in the presence of 0.1 to 1.0% superplasticizer. The compressive strength of mortar containing the superplasticizer from CNSL was higher than that of the control     

棉籽蛋白接枝共聚丙烯酸-丙烯酰胺高吸水性树脂的制备及吸液性能

以棉籽蛋白、丙烯酸和丙烯酰胺为主要原料,N,N’-亚甲基双丙烯酰胺为交联剂,过硫酸钾-亚硫酸氢钠为引发剂,采用溶液聚合法制备了棉籽蛋白接枝共聚丙烯酸-丙烯酰胺高吸水性树脂[P(CP-g-AA/AM)],并对其较佳工艺条件以及综合吸水性能进行了考察。结果表明,制备P(CP-g-AA/AM)的适宜条件为:wCP∶wAA+AM=10%、w交联剂∶wAA+AM=0.1%、w引发剂∶wAA+AM=1.0%、wAA∶wAM=75∶25、AA中和度为80%、反应温度60℃、反应时间2h。在此条件下合成的P(CP-g-AA/AM)树脂在去离子水中的饱和吸水倍率为1130g/g。