Search

The present work investigates the shear performance of concrete beams reinforced longitudinally with steel rebars incorporating randomly distributed stiff elements known as “needles”. Compared to commonly used fibers, needles function differently in concrete members that carry increasing internal stresses. To measure the effect of needles on the mechanical performance of concrete, 32 longitudinally reinforced concrete (RC) beams and 96 non-reinforced concrete prisms were tested. Among numerous possibilities, the dimensions of the needles used in the present study were selected based on a number of practical considerations. The results showed that the needles increased the peak and residual shear capacity of the reinforced beams significantly. However, the improvements were not greater than those achieved by using the same volumetric content of fibers made of the same material as that the needles were produced from. Based on the measured results and the observation of cracking patterns, guidance for optimizing the geometry of needles and test specimens are provided for future research.

Many studies have been conducted on the mechanical properties of concrete incorporating recycled tire aggregate. The present work has the same theme with two clear distinctions: First, a vast majority of past experiments were conducted on concretes with compressive strengths in the range between 30 and 60 MPa. In the present work, concretes with compressive strength values below 20 MPa were designed and prepared in the laboratory by replacing conventional aggregates by recycled tire aggregates with an amount of 10, 20 and 30% by volume. Compressive strength was measured up till 90 days and results showed that replacing a small portion of natural fine aggregate with a finer rubber aggregate increased the 90 days compressive strength of concrete by 12%, possibly due to the resulted densification of the fine aggregate. All the past studies concluded that using rubber aggregate reduces the compressive strength of concrete. Based on the statistical analyses of the results from the past studies and the present work, a relationship with high accuracy was developed to predict the compressive strength of concrete incorporating fine rubber aggregate. Secondly, the environmental performance of rubberized concrete was also evaluated by assessing two environmental impact indicators; CO2 footprints and volume utilization of raw materials. Finally, an analytical framework was developed for the sustainable selection of rubberized concrete by incorporating mechanical and environmental performance. Two multi-criteria decision-making techniques were used to select the best ideal solution in favor of rubberized concrete as a structural material and medium to low strength material. The great potential of using rubber content in concrete as a medium to low strength material was observed.

Many life cycle assessment (LCA) studies have been conducted in the past to measure the environmental impacts of producing concrete with coarse recycled aggregate (CRA) obtained from processing construction and demolition waste (CDW) and compare them with those of concrete with coarse natural aggregate (CNA). The present work studies the influence of the choice of geographic boundaries on the results of regional LCA studies in a large and dense metropolitan area. Specifically, the study incorporates an LCA on the CDW produced, processed, and used in concrete in the five boroughs of New Yok City. The results were compared to those from a similar past study with geographic boundaries far beyond the five boroughs forming an area over 10 times larger. The comparison revealed that when the boundary is limited to the borders of New York City, the environmental benefits of replacing CNA with CRA in concrete caused by the avoided transportation of CDW to landfills are significantly (often over 100%) higher. In addition, it was found that when concrete with CNA is produced in the five boroughs the overall environmental burden of transporting raw materials to concrete plants is approximately 25% higher.

Fiber-reinforced polymer (FRP) composite materials have a number of outstanding mechanical properties; some surpass those of commonly used structural materials such as steel. Nevertheless, developing techniques for efficient recycling of FRP has proven to be challenging, leading to a number of negative long-term economic consequences. The existing techniques either have significant environmental footprints or turn FRP waste into products with functions markedly inferior to those of the original products that were recycled. This chapter focuses on a specific method of recycling FRP—mechanically processing FRP waste into particles or pieces that replace aggregates in concrete. Studies on this topic reveal that aggregates obtained by processing FRP waste tend to impair the mechanical performance of concrete. Large fiber-like cut pieces of FRP, known as “needles,” have been shown to improve the tensile and shear load-carrying capacities of concrete. However, from both technical and economic perspectives, further evidence is needed to justify recycling FRP into needles for use in concrete.

Several pioneering life cycle assessment (LCA) studies have been conducted in the past to assess the environmental impact of specific methods for managing mineral construction and demolition waste (MCDW), such as recycling the waste for use in concrete. Those studies focus on comparing the use of recycled MCDW and that of virgin components to produce materials or systems that serve specified functions. Often, the approaches adopted by the studies do not account for the potential environmental consequence of avoiding the existing or alternative waste management practices. The present work focuses on how product systems need to be defined in recycling LCA studies and what processes need to be within the system boundaries. A bi-level LCA framework is presented for modelling alternative waste management approaches in which the impacts are measured and compared at two scales of strategy and decision-making. Different functional units are defined for each level, all of which correspond to the same flow of MCDW in a cascade of product systems. For the sole purpose of demonstrating how the framework is implemented an illustrative example is presented, based on real data and a number of simplifying assumptions, which compares the impacts of a number of potential MCDW management strategies in New York City.

A wind turbine blade shell made of glass fiber reinforced polymer (GFRP) composite materials was mechanically processed into slender elements referred to as “Needles”. The Needles were used to replace 5% and 10% of the coarse aggregate, by volume, in concrete mixtures that were tested to investigate a number of important properties of fresh and hardened concrete. It was found that the Needles did not affect the stability and workability of fresh concrete negatively. Although the incorporation of the Needles did not have a notable effect on compressive, tensile, and flexural strength of concrete, it resulted in a significant increase in energy absorption capacity (toughness) from 1.2 J in control specimens up to 33.3 J in those with 10% Needle replacement. A polymer burn-off test revealed that, due to the directions in which the shell was cut, in most of the Needles glass fibers were perpendicular to the Needle axis, and therefore to the tensile stress carried by the Needles. Although the Needles with transversely-aligned fibers improve the mechanical performance of concrete, if the cutting directions of wind blades can be optimized so that in the majority of the Needles the fibers are primarily aligned longitudinally, improvements are expected to be more significant.

The purpose of the present study is to compare the environmental impacts of using coarse natural aggregate (NA) and coarse recycled concrete aggregate (RCA) to produce concrete in the New York City area, by means of a unique LCA framework that incorporates comprehensive regional data. A comparative environmental impact assessment study was performed on the critical processes of the life cycles of NA and RCA concretes. For this purpose, concrete ready-mix plants, construction and demolition waste (CDW) recycling plants, NA quarries, and other producers and distributers of concrete raw materials, in addition to CDW landfills in the New York City area, were located. NA and RCA concrete mix proportions that result in the same compressive strength of concrete were used. Also, the environmental impact that would be caused if CDW was landfilled rather than processed into RCA was measured. In the New York City area, replacing NA with RCA as a concrete aggregate does not affect the environmental impact of concrete production significantly. However, if CDW is recycled only for the purpose of producing concrete aggregate, the avoided landfilling of the CDW will be a result of producing RCA concrete. When avoided landfilling is accounted for, the magnitude of some of the environmental impact indicators for RCA concrete is significantly lower than those of NA concrete (16 and 17% for acidification and smog formation, respectively). In addition, it was found that the impact from transporting RCA to ready-mix plants is on average 37% less than that caused by transporting NA to the plants. Sensitivity analyses and normalization of the results revealed that the environmental impact of changing the type of concrete aggregate from NA to RCA is negligible compared to the total environmental burden of New York City. If RCA concrete is used for all types of construction projects in the NYC area, achieving a significant reduction in the environmental impacts is unlikely. Future work is needed to study specific projects in the region that are categorized based on demand for transportation and cement (the largest environmental stressors of concrete production) to determine for which type of project the use of RCA concrete has the highest environmental benefits.

The production of fiber reinforced polymer (FRP) composite materials is a major source of non-degradable waste. The use of FRP bars for reinforcing concrete is increasing in construction. This study investigates the impact of replacing coarse natural aggregate (NA) in concrete with cut FRP bar waste (FRP-RA) on the compressive and tensile strength of both high strength and normal strength concretes. Concrete cylinders without any FRP-RA, with 40% of NA (the large-grade particles) replaced with FRP-RA, and with 100% of NA replaced with equivalently graded FRP-RA were tested. The results show that the effect of the FRP-RA on the mentioned mechanical properties depends on the concrete strength, replacement ratio and the gradation-size of the NAs replaced. High-strength concrete performed better than low strength concrete and replacement of larger sized NA with FRP-RA resulted in better performance compared to full replacement. The test results show that existing empirical models can sometimes overestimate the tensile strength of lower-strength FRP-RA concretes. The findings show that although the use of FRP waste in concrete reduces the strength, concrete with structural grade mechanical properties can be produced with FRP-RA. The potential durability issues of FRP-RA concrete are discussed and future studies on this topic is proposed.

Shear failure of reinforced concrete members is brittle and catastrophic, and should be avoided. This work presents a comparative study of concrete beams with natural crushed stone and those incorporating recycled concrete aggregate (RCA). The study indicates that the equation presented in ACI 318-14 Code requirements for predicting the shear strength of concrete is less conservative for RCA concrete beams. The presented experimental investigations show that strengthening with fiber reinforced polymer (FRP) fabrics can be designed so that the shear capacity of the beams with weaker (RCA) concrete is higher than that of the control beams with natural aggregate concrete. The design of the FRP strengthening system and the reasons for the effectiveness of the design are explained.

This paper presents a new type of discrete reinforcing element for concrete produced from either waste or new pultruded fiber-reinforced polymer (FRP) composite materials. These elements, r...

For the last three decades, fiber reinforced polymer (FRP) composite materials have been widely used in major engineering industries. Managing FRP waste is becoming an important issue due to the growth in the production of FRP composite materials. In this article, the issue of FRP waste management is discussed and the commonly used methods for the handling of FRP waste are reviewed. One potentially viable use of FRP waste is in the partial replacement of fillers or aggregates in cementitious materials (particularly portland cement mortar and concrete). A number of important prior investigations performed on the use of FRP waste in concrete and mortar are reviewed. The results from most of those investigations suggest that FRP aggregates significantly reduce the strength of cementitious materials with little significant effect on durability. Recommendations for future research in this area are provided for producing stronger mortars and concretes incorporating FRP production and end-of-life waste.

A major challenge of successfully incorporating nanometric inclusions (nanoinclusions) within cement paste is achieving a uniform distribution of the nanoinclusions. Cement particles have a larger diameter than the average spacing between nanoinclusions when the nanoinclusions are fully dispersed, which means that the presence of cement particles in the fresh paste degrades the maximum achievable dispersion of the nanoinclusions in the hardened paste. To determine the significance of this effect, a novel method for dispersion quantification was implemented to calculate the theoretical maximum achievable dispersion of nanoinclusions in fresh cement paste. Three-dimensional simulations were performed for cement pastes with common values of water to cement ratio, nanoinclusion to cement ratio, and cement fineness. The results show that for cementitious nanocomposites simulated in this study, degradation of the maximum theoretical achievable dispersion of nanoinclusions due to the presence of cement particles is negligible as long as the cement particles are not agglomerated.

To study the effects of functionalized carbon nanotubes (CNTs) and carbon nanofibers (CNFs) on the mechanical properties of cement composites, both untreated and treated CNFs and CNTs were added to cement paste in concentrations of 0.1% and 0.2% by weight of cement. The surface-treated nanofilaments were functionalized in a solution of sulfuric acid (H2SO4) and nitric acid (HNO3). The nano- filaments were dispersed by using an ultrasonic mixer and were then cast into molds. Each specimen was tested in a custom-made three-point flexural test fixture to record the mechanical properties (i.e., the Young's modulus, flexural strength, ductility, and modulus of toughness) at the age of 7, 14, and 28 days. The microstructure was analyzed by using a scanning electron microscope. Untreated CNTs and CNFs were found to enhance the mechanical properties of cementitious materials, whereas the acid-treated CNTs and CNFs degraded the mechanical properties. DOI: 10.1061/(ASCE)NM.2153-5477.0000041. © 2012 American Society of Civil Engineers. CE Database subject headings: Nanotechnology; Cement; Mechanical properties; Carbon. Author keywords: Carbon nanotubes; Carbon nanofibers; Cement; Surface treatment; Mechanical properties.

A novel and robust method for quantifying the quality of dispersion of discrete inclusions in a composite was recently introduced. The method quantifies dispersion based on the minimum amount of work required to move the inclusions such that they are in a fully uniform state. As the new dispersion quantification method is founded on a physical thermodynamic framework, it is distinct from previously developed methods that have strictly a mathematical and statistical basis. However, implementation of the new method is computationally expensive for domains consisting of many inclusions. In this paper, the novel dispersion quantification method is recast in terms of continuum mechanics and thermodynamics in order to make use of diffusion models, which makes possible the quantification of dispersion considering domains with many inclusions. To demonstrate the practicality of the new approach, the dispersion of carbon nanofibers in an aqueous solution is analyzed.

The distribution of particles (e.g. micro and nanoinclusions, carbon nanotubes, carbon nanofibers) within a composite material plays a vital role in dictating many important constitutive properties. In this study, a general method to quantify the dispersion and agglomeration of nano-particles in composite materials has been developed. The dispersion quantity, D, is measured by the free-path spacing between particles; whereas, the agglomeration quantity, A, is measured by the particle size. In both cases, a quantifiable percentage is calculated based on the statistical probability that either the free-path spacing or particle size falls within a certain percentage above and below μ, where μ is either the mean spacing or particle size. The proposed method is used to quantify the dispersion and agglomeration of both carbon nanofibers (CNFs) and carbon nanotubes (CNTs) within aqueous solutions.

Carbon nanotubes (CNTs) and carbon nanofibers (CNFs) are quickly becoming two of the most promising nanomaterials because of their unique mechanical properties. The size and aspect ratio of CNFs and CNTs mean that they can be distributed on a much finer scale than commonly used microreinforcing fibers. As a result, microcracks are interrupted much more quickly during propagation in a nano- reinforced matrix, producing much smaller crack widths at the point of first contact between the moving crack front and the reinforcement. In this study, untreated CNTs and CNFs are added to cement matrix composites in concentrations of 0.1 and 0.2% by weight of cement. The nanofilaments are dispersed by using an ultrasonic mixer and then cast into molds. Each specimen is tested in a custom-made three-point flexural test fixture to record its mechanical properties; namely, the Young's modulus, flexural strength, ultimate strain capacity, and fracture toughness, at 7, 14, and 28 days. A scanning electron microscope (SEM) is used to discern the difference between crack bridging and fiber pullout. Test results show that the strength, ductility, and fracture toughness can be improved with the addition of low concentrations of either CNTs or CNFs. DOI: 10.1061/(ASCE)MT.1943-5533.0000266. © 2011 American Society of Civil Engineers.

Distribution of constituents within a composite material dictates important constitutive properties and is therefore of interest for all multiphase materials including nano-, micro-, and macro-composites. In the first part of this paper, previously proposed methods for quantifying dispersion are reviewed and their applications and possible shortcomings are discussed. In the second part, we propose a novel definition for dispersion based on the thermodynamic concept of work; dispersion is measured based on the amount of work required to translate inclusions so they form the state of maximum uniformity. The method quantifies dispersion with a single parameter. Although multiple parameter methods can provide more information about the spatial distribution of inclusions, the proposed method is particularly useful when comparing overall dispersion quality of different domains. As an example, the dispersion of carbon nanotubes in an Al coating is quantified to demonstrate the robustness and practicality of the novel dispersion quantification method.

Carbon nanofibers (CNFs) and nanotubes (CNTs) are known to be extremely strong and stiff, and their potential as reinforcement has been of interest to many investigators in the past decade. One of the most important keys for fully harnessing the properties of any type of fiber is to control the distribution in the material matrix. As far as CNFs–CNTs are concerned, the strong attraction among nanoscale fibers due to van der Waals forces makes this task difficult. This study focuses on some of the problems that prevent a uniform distribution of CNFs–CNTs in cement paste and the methods used in the past to enhance dispersion. The first phase of the experimental program investigates the effect of using superplasticizers accompanied by sonication on the dispersion of CNFs in water and paste. The second phase focuses on the problem of cement grain size and limitations that the use of fine grain cement causes. Finally, on the basis of results and past studies, suggestions are made for achieving enhanced dispersion of CNFs–CNTs in cement paste.

This work reports on an experimental investigation about the effect of carbon nanofibers (CNFs) on the compressive strength of: (1) hardened portland cement paste and (2) two types of concrete; with natural and recycled coarse aggregates. It was observed that the incorporation of nanofibers caused a slight increase in the compressive strength of cement paste. However, for each type of aggregate, the strength of CNF-incorporated concrete was lower than that of control plain concrete. Based on the SEM studies and visual observations of the hardened paste specimens, the reason for these different behaviors was found to be the different dominating mechanisms by which bleeding is controlled in paste and concrete mixes; in paste CNF clumps partially block the pores and bleed water channels, while in concrete a portion of free water is adsorbed on the surfaces introduced by aggregates.

The incorporation of carbon nanofilaments in cementitious materials has been researched extensively in the past decade. However, the poor dispersibility of these filaments remains a hurdle to harnessing their exceptional properties. This work presents a method for enhancing the dispersion of carbon nanofibers (CNFs) in cementitious matrices. The method incorporates silica fume to stabilize the distribution of CNFs in fresh cement paste. The investigations show that this method is significantly more effective than similar approaches. To examine the effectiveness of the method, specimens of CNF incorporated cement paste with different concentrations of silica fume were produced and imaged by optical microscopy. Using a novel dispersion quantification method, the dispersion values of CNFs in the specimens were measured and compared. The results show that silica fume, if used in sufficient proportions, can largely prevent the reagglomeration of CNFs in fresh cement paste, and thereby stabilize their d...

Carbon nanofibers/tubes (CNF/Ts) are very strong and stiff and as a result, are expected to be capable of enhancing the mechanical properties of cementitious materials significantly. Yet there are practical issues concerning the utilization of CNF/Ts in cementitious materials. This study summarizes some of the past efforts made by different investigators for utilizing carbon nanofilaments in cementitious materials and also reports recent experimental research performed by the authors on the mechanical properties of CNF-reinforced hardened cement paste. The major difficulties concerning the utilization of CNF/Ts in cementitious materials are introduced and discussed. Most of these difficulties are related to the poor dispersibility of CNF/Ts. However, the findings from the research presented in this work indicate that, despite these difficulties, carbon nanofilaments can significantly improve the mechanical properties of cementitious materials. The results show that CNFs, even when poorly dispersed within the cementitious matrix, can remarkably increase the flexural strength and cracking resistance of concrete subjected to drying conditions.

Twenty-seven large-scale beams were instrumented and tested under monotonic centerpoint loading to determine the effect of a newly developed high-modulus macrosynthetic fiber on the shear strength and failure behavior of longitudinally reinforced concrete (RC) beams without stirrups. Slender and short beams with respective shear span-depth ratios (a/d) of 3.5 and 2.3 were tested. The length of the beams varied between 1.9 and 3.2 m (75 and 126 in.), and the macrosynthetic fibers were added at volume fractions of 0.50, 0.75, and 1.0%. Deflection of the beam, strain in the concrete and in the flexural reinforcing bars, and the cracking pattern were monitored during the test at different stages of the monotonic loading until failure. The results showed that the addition of macrosynthetic fibers significantly improved the shear strength and ductility of the RC beams and modified the cracking and failure behavior. The ultimate shear strength of slender and short beams was increased up to 30% compared to the control RC beams.