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.