In the contemporary world, natural fibers reinforced polymer composite (NFRPC) materials are of great interest owing to their eco-friendly nature, lightweight, life-cycle superiority, biodegradability, low cost, noble mechanical properties. NFRPCs are widely applied in various engineering applications and this research field is continuously developing. However, the researchers are facing numerous challenges regarding the developments and applications of NFPRCs due to the inherent characteristics of natural fibers (NFs). These challenges include quality of the fiber, thermal stability, water absorption capacity, and incompatibility with the polymer matrices. Ecological and economic concerns are animating new research in the field of NFRPCs. Furthermore, considerable research is carried out to improve the performance of NFRPCs in recent years. This review highlights some of the important breakthroughs associated with the NFRPCs in terms of sustainability, eco-friendliness, and economic perspective. It also includes hybridization of NFs with synthetic fibers which is a highly effective way of improving the mechanical properties of NFRPCs along with some chemical treatment procedures. This review also elucidates the significance of using numerical models for NFRPCs. Finally, conclusions and recommendations are drawn to assist the researchers with future research directions.

Natural fiber reinforced composites: Sustainable materials for emerging applications

This special issue of Sādhanāis rightly dedicated to the fracture mechanics of concrete. In particular, the size effect is highlighted. As appropriately pointed out in the first international conference on fracture mechanics of concrete structures, FraMCos-I, organized by Z P Băzant, at Breckenridge, Colorado in 1992, fracture mechanics of concrete can be called the 3rd phase in the evolution of concrete structures. Since then the number of published papers on the topic have increased several-fold. Going back in the memory pipeline, it was M F Kaplan1 (in 1961) who tried to obtain the fracture toughness of concrete. It was observed later that there was no consistent value of fracture toughness of concrete. Therefore, towards the early eighties, nonlinear fracture theories were proposed. The process zone ahead of the crack tip was identified as having an important role to play in the nonlinear fracture mechanics of concrete. A little later, it led to the size effect, which as proposed in fracture mechanics is different from Weibull’s hypothesis for size effect. Later, there was a workshop on the size effect in concrete structures, organized by H Mihashi, H Okamura and Z P Bă zant, at Sendai, Japan in 1993. Various international committees were formed, the earliest one being the RILEM Committee chaired b y F H Wittmann. Several other important meetings took place at different places like the international workshop at Locarno, Switzerland in September, 1990 organised by Wittmann and Dungar, and the international workshop on concrete fracture, organised by A Carpinteri, at Torino in October 1991. In recent years, as high performance concrete is gaining importance, its fracture behaviour is being studied with great seriousness. High strength concrete is nearer to linear theories of fracture and is relatively more brittle. The challenge is whether one can make high strength concrete relatively more ductile by improving the cohesiveness of cracks. The next question is how to bring the size effect into codes of practice on the design of reinforced concrete structures, since large structures like dams, nuclear reactors, very tall towers, do contain large sized members. The issue to be addressed is whether it is in order to assume the same tensile strength as obtained in the laboratory for the full scale structure also. This special issue is intended to focus on the above features. Fracture mechanics of concrete has yet to go a long way. One may say that a complete textbook in the strict sense of the term came out only as recently as in 1995, written by B L Karihaloo. The contributors to this special issue , F H Wittmann, A Carpinteri and B Chiaia, V E Saouma and D Natekar, B Karihaloo and Q Z Xiao, Jin Keun Kim and Seong Tae Yi, and Vladimir Cervenkaet al, are all leading researchers in the field of fracture mechanics of concrete. This special issue will certainly be an important document relating to the latest research on the subject. It will also help active researchers to pursue further research on the topic.

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Fracture mechanics of concrete - Foreword

Fly ash (FA) is a residual from thermal industries that has been effectively utilized in the production of FA-based geopolymer concrete (FGPC). To avoid time-consuming and costly experimental procedures, soft computing techniques, namely, random forest regression (RFR) and gene expression programming (GEP), are used in this study to develop an empirical model for the prediction of compressive strength of FGPC. A widespread, reliable, and consistent database of compressive strength of FGPC is set up via a comprehensive literature review. The database consists of 298 compressive strength data points. The influential parameters that are considered as input variables for modelling are curing temperature , curing time , age of the specimen , the molarity of NaOH solution , percent SiO2 solids to water ratio in sodium silicate (Na2SiO3) solution, percent volume of total aggregate (), fine aggregate to the total aggregate ratio , sodium oxide (Na2O) to water ratio in Na2SiO3 solution, alkali or activator to the FA ratio , Na2SiO3 to NaOH ratio , percent plasticizer (), and extra water added as percent FA . RFR is an ensemble algorithm and gives outburst performance as compared to GEP. However, GEP proposed an empirical expression that can be used to estimate the compressive strength of FGPC. The accuracy and performance of both models are evaluated via statistical error checks, and external validation is considered. The proposed GEP equation is used for sensitivity analysis and parametric study and then compared with nonlinear and linear regression expressions.

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Compressive Strength of Fly-Ash-Based Geopolymer Concrete by Gene Expression Programming and Random Forest

Effect of elevated temperatures on mechanical properties of lightweight geopolymer concrete

Evaluation of fracture processes under shear with the use of DIC technique in fly ash concrete and accurate measurement of crack path lengths with the use of a new crack tip tracking method

Properties of fresh and hardened fly ash/slag based geopolymer concrete: A review

Mechanical properties tests and multiscale numerical simulations for basalt fiber reinforced concrete

Fracture performance and numerical simulation of basalt fiber concrete using three-point bending test on notched beam

Numerical modeling of rebar-matrix bond behaviors of nano-SiO2 and PVA fiber reinforced geopolymer composites

Bonding behavior of concrete matrix and alkali-activated mortar incorporating nano-SiO2 and polyvinyl alcohol fiber: Theoretical analysis and prediction model

Zhen Gao

Papers

Properties of fresh and hardened fly ash/slag based geopolymer concrete: A review

Mechanical properties tests and multiscale numerical simulations for basalt fiber reinforced concrete

Fracture performance and numerical simulation of basalt fiber concrete using three-point bending test on notched beam

Numerical modeling of rebar-matrix bond behaviors of nano-SiO2 and PVA fiber reinforced geopolymer composites

Bonding behavior of concrete matrix and alkali-activated mortar incorporating nano-SiO2 and polyvinyl alcohol fiber: Theoretical analysis and prediction model