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News Article | March 4, 2016
Site: news.mit.edu

Concrete is the world’s most widely used construction material, so abundant that its production is one of the leading sources of greenhouse gas emissions. Yet answers to some fundamental questions about the microscopic structure and behavior of this ubiquitous material have remained elusive. Concrete forms through the solidification of a mixture of water, gravel, sand, and cement powder. Is the resulting glue material — known as cement hydrate, or calcium silicate hydrate (CSH) — a continuous solid, like metal or stone, or is it an aggregate of small particles? As basic as that question is, it had never been definitively answered. In a paper published this week in the Proceedings of the National Academy of Sciences, a team of researchers at MIT, Georgetown University, and France’s CNRS (together with other universities in the U.S., France, and U.K.) say they have solved that riddle and identified key factors in the structure of CSH that could help researchers work out better formulations for producing more durable concrete. Roland Pellenq, a senior research scientist in MIT’s department of civil and environmental engineering, director of the MIT-CNRS lab 2 hosted by the MIT Energy Initiative, and a co-author of the new paper, says the work builds on previous research he conducted with others at the Concrete Sustainability Hub (CSHub) through a collaboration between MIT and the CNRS. “We did the first atomic-scale model” of the structure of concrete, he says, but questions still remained about the larger, mesoscale structure, on scales of a few hundred nanometers. The new work addresses some of those remaining uncertainties, he says. One key question was whether the solidified CSH material, which is composed of particles of many different sizes, should be considered a continuous matrix or an assembly of discrete particles. The answer turned out to be that it is a bit of both — the particle distribution is such that almost every space between grains is filled by yet smaller grains, to the point that it does approximate a continuous solid. “Those grains are in a very strong interaction at the mesoscale,” he says. “You can always find a smaller grain to fit in between” the larger grains, Pellenq says, and thus “you can see it as a continuous material.” But the grains within the CSH “are not able to get to equilibrium,” or a state of minimum energy, over length scales involving many grains, and this makes the material vulnerable to changes over time, he says. That can lead to “creep” of the solid concrete, and eventually cracking and degradation. “So both views are correct, in some sense,” he explains. The analysis of the structure of hardened concrete found that pores of different sizes play important roles in determining the material’s characteristics. While smaller, nanoscale pores had been previously studied, mesoscale pores, ranging from 15 to 20 nanometers on up, had been more difficult to study and not well-characterized, Pellenq says. These pore spaces can play a major role in determining how susceptible the material is to water that can enter the material and cause cracking, eventually leading to structural failure. (This cracking, perhaps surprisingly, has nothing to do with the expansion of the water when it freezes, however). The new mesoscale simulations are the first that can adequately match the sometimes conflicting and confusing results seen in experiments measuring the CSH texture, Pellenq says. The new simulations make it possible to match the values of key characteristics such as stiffness, elasticity, and hardness, which are seen in real concrete samples. That shows that the modeling is useful, he says, and might help guide research on developing improved formulas, for example ones that reduce the required amount of water in the initial mix with cement powder. It is the manufacturing of the cement powder, a process that requires cooking limestone (with clays) at very high temperatures, that makes concrete production one of the leading sources of human-caused greenhouse gas emissions. Fine-tuning the amount of water needed for a given application could also improve the material’s durability, the researchers found. The amount of water used in the original mixture can make a big difference in concrete’s longevity, even though most of that evaporates away during the setting process. While water is needed in order to make the slurry flow so that it can be poured in place, too much water leads to much bigger pore spaces and more loose, “fluffy” regions in the set concrete, the team found. Such regions might leave the material more vulnerable to later degradation, or could even be designed to improve its durability. “This is a quintessential step towards the provision of a seamless atom-to-structure understanding of concrete, with huge mid-term practical impact in terms of material design and optimization,” says Christian Hellmich, director of the Institute for Mechanics of Materials and Structures at the Vienna University of Technology, who was not involved in this research. He adds, “this research helps to promote concrete research as a cutting-edge scientific discipline, where the cooperation of engineers and physicists emerges as a driving force for the reunification of natural sciences across the often too-tightly set boundaries of sub-disciplines.” The first contributor of this work is MIT postdoc Katerina Ioannidou. The team also included other researchers at MIT; the University of California at Los Angeles; Newcastle University in the U.K.; and Sorbonne University, Aix-Marseille University, and CNRS, in France. The work was supported by Schlumberger, the French National Science Foundation (ANR) through the Labex ICoME2, and the CSHub at MIT.


News Article | February 9, 2016
Site: www.techtimes.com

While most people know the old addage "don't step on a crack or you'll break your mother's back" in reference to the fissures found in your everyday sidewalk concrete, scientists at MIT have discovered the chemical makeup of one of the most widely-used substances on the planet — including why these "unlucky" cracks show up at all, and what binds the material in the first place. In a statement released on Feb. 8, a research team at MIT in collaboration with Georgetown University, the French National Centre for Scientific Research (CNRS) and other participants revealed that concrete, which is so commonly used that the fumes the material emits during production serves as the third-largest contributor of greenhouse gas emissions, is not made from either an amalgamation of grain-like particles or continuous matter akin to plastic. In the end, it turns out that either answer is partially right — all because the material, made out of an admixture of water, gravel, sand and cement powder, also known as cement hydrate (CSH), is made of both. In a paper published in Proceedings of the National Academy of Sciences, the group of researchers explained that their results were gleaned from constructing an "atomic-scale model" of a concrete structure, a first for the scientific community. The structure revealed that concrete contains mesoscale and nanoscale pores, which account for the mercurial nature of the material — namely, its porous nature and its propensity for structural disintegration, like forming cracks after a certain period of time. The discovery of the varied pores proved that the multi-size grains that make up the material are what allow them to exist — and depending on the amount of pores present in any given chunk of cement, the more prone it is to allowing water to affect its composition, leading to — you guessed it — the appearance of cracks. "You can always find a smaller grain to fit in between," summarized Roland Pellenq, one of the researchers who worked on the study. "[Y]ou can see it as a continuous material ... [but grains found in CSH] are not able to get to equilibrium," which accounts for a lack of a particular stasis of minimum energy. So, why exactly is this discovery important? Considering the frequency of which CSH is utilized, understanding what keeps concrete together can help us reinforce it to last longer and be stronger, vastly improving the upkeep of every edifice and road for which we use concrete. "This is a quintessential step towards the provision of a seamless atom-to-structure understanding of concrete, with huge mid-term practical impact in terms of material design and optimization," says Christian Hellmich, Vienna University of Technology's director of the Institute for Mechanics of Materials and Structures. "[T]his research helps to promote concrete research as a cutting-edge scientific discipline, where the cooperation of engineers and physicists emerges as a driving force for the reunification of natural sciences across the often too-tightly set boundaries of sub-disciplines," added Hellmich, who is unaffiliated with the MIT study. If you're itching to see how construction and science's wonder material is made, check out the video below.


Kandler G.,Institute for Mechanics of Materials and Structures | Fussl J.,Institute for Mechanics of Materials and Structures | Serrano E.,Linnaeus University | Eberhardsteiner J.,Institute for Mechanics of Materials and Structures
Wood Science and Technology | Year: 2015

The mechanical properties of structural timber—particularly in terms of stiffness and strength—are subject to high variability, which also affects the properties of timber products made from structural timber, e.g., glued laminated timber (GLT). In this paper, the influence of the longitudinal stiffness variability of wooden lamellas on the effective stiffness of GLT is investigated. In a first step, the local fiber orientation on the surfaces of 350 lamellas of Norway spruce was determined by an optical scanning device. This fiber angle information in combination with a micromechanical model for wood was used for the generation of a longitudinal stiffness profile of each lamella. Recording the position and orientation of each lamella, a total number of 50 GLT beams were assembled (with 4, 7, and 10 laminations) and tested under four-point bending. Knowing the stiffness profile of each board and its location within the GLT beam allowed for an accurate numerical finite element model, which is able to predict the effective GLT stiffness with high accuracy. Interesting insights into the relation between the stiffness of lamellas and the resulting GLT beams could be gained, and finally, a numerical simulation tool which is able to reproduce the experimental results appropriately was obtained. © 2015 Springer-Verlag Berlin Heidelberg


Murin J.,Slovak University of Technology in Bratislava | Aminbaghai M.,Institute for Mechanics of Materials and Structures | Hrabovsky J.,Slovak University of Technology in Bratislava | Kutis V.,Slovak University of Technology in Bratislava | And 2 more authors.
11th World Congress on Computational Mechanics, WCCM 2014, 5th European Conference on Computational Mechanics, ECCM 2014 and 6th European Conference on Computational Fluid Dynamics, ECFD 2014 | Year: 2014

In this contribution, a new 3D-beam finite element of double symmetric cross-sectional area, made of a Functionally Graded Material (FGM) is presented, which can be used in modal, elastostatic and buckling analysis of single beams and beam structures. There, the material properties vary continuously in longitudinal direction while the variation with respect the transversal and lateral directions is assumed to be symmetric in a continuous or discontinuous manner (Figure 1). The shear force deformation effect and the effect of consistent mass distribution and mass moment of inertia are taken into account. Additionally, the Winkler elastic foundation and the effect of axial force are included by the finite element equation derivation as well. Homogenization of the spatially varying material properties to the effective material properties with longitudinal variation is done by multilayer method. For the homogenized beam the finite element matrix, consisting of the stiffness and mass inertia terms, is established. Numerical experiment is made to show the accuracy and effectiveness of the new 3D FGM beam element.


Murin J.,Slovak University of Technology in Bratislava | Kugler S.,University of Applied Sciences Wiener Neustadt | Aminbaghai M.,Institute for Mechanics of Materials and Structures | Hrabovsky J.,Slovak University of Technology in Bratislava | And 2 more authors.
11th World Congress on Computational Mechanics, WCCM 2014, 5th European Conference on Computational Mechanics, ECCM 2014 and 6th European Conference on Computational Fluid Dynamics, ECFD 2014 | Year: 2014

In the contribution, the homogenization techniques of spatial varying (continuously or discontinuously) material properties for the Functionally Graded Material (FGM) beams and shells are presented. The expressions are proposed for derivation of the effective elastic, thermal and electrical material properties by the extended mixture rules and laminate theory, and by the direct integration method. The results of numerical experiments are evaluated and disscussed.

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