Concrete is composed of cement, sand, stone and water. An appropriate amount of admixtures are often added to improve some properties of concrete sometimes.
Advantages: huge amount of raw materials and low cost; Good plasticity; High strength; Good durability.
General working process: In concrete, sand and stone play the role of the skeleton; Cement and water mix into the cement slurry which wraps around the skeleton and fills the gaps. Before hardening, the cement slurry is used for lubrication, giving the mixture a certain workability. After the cement slurry hardens, the mixture is cemented into a solid whole.
Sand and stone are actually a kind of ground rock, presented in different sizes. The tiny sand can fill the gaps between the stone and make the whole structure more solid.
However, not all kinds of rocks could be used as a material for making concrete. The rock must contain calcium carbonate, which is the main component of limestone. Limestone is the material that organisms are buried in the ground layer by layer and fused by high temperature and high pressure after millions of years of crustal movement. Besides, it also needs a type of rock which contains silicate.
In order to make it possible to react with water, the chemical bond between calcium carbonate and silicate inside the rock must be broken first. It’s not that easy to do this. The chemical bond between calcium carbonate and silicate is very stable. Thus, the key point is heating, and the temperature needs to be up to 1450 ℃. Rocks will begin to split and reorganize in the high temperatures, produce a group of substances called Calcium silicate.
The accretive fibrils will interlock and form bonds to lock more water until the cement becomes solid.
There are two ways to break the material.
The first one is “elastic-plastic fracture”, such as pulling a chewing gum. When the material is pulled, it will produce lattice rearrangement and lead to an extension. The middle gets thinner and finally is divided into two parts. Most metals can be broken in this way, but it will take a lot of effort to do so because many dislocations must be moved. This is also the reason why metals have high strength and toughness.
Another fracture method is “brittle fracture”, such as the fracture of glass. As long as there is a crack in the material, it will destroy the whole structure.
The fracture of concrete belongs to the second method.
These are some samples I made to study the material properties of concrete:
I build a box first and put different materials inside the box, then pour the liquid concrete into the box and wait until it dries.
Later, I test the concrete by following a unique pottery-making process. I make a bucket and glue some textured cardboard inside. Then I pour some liquid concrete, slosh it to attach to the bucket wall and wait till the first layer is dry. Then repeat the process. Thus, a free-standing model is built with concrete.
To strengthen the structure of concrete, steel bars are added inside the concrete like a skeleton. The expansion and contraction rates of steel and concrete are almost the same. Thus, the bending stress will be absorbed by the steel bars in the concrete without creating any large cracks.
However, the shortage of adding the steel bar is that the water could erode the reinforcement, which will cause the diffusion of rust in the concrete and damage the whole reinforcement structure.
Self-healing Concrete contains a bacterium called B.pasteurii, which is very tenacious and can hibernate in rocks for decades. Scientists found that these bacteria secrete Calcite which is one of the ingredients of concrete.
These bacilli are usually dormant and surrounded by aqueous calcium silicate fibrils. They will regain their freedom, wake up when they encounter water and start looking for food when cracks appear in the concrete.
So, we add some starch in the concrete. The bacilli will eat the starch, grow and reproduce, and secrete calcite.
Calcite is a kind of calcium carbonate. it will bond with concrete and form a mineral structure to fill the crack and make the crack no longer expand.
I suggest you not to google the self-healing concrete image is you have a Trypophobia (dense phobia)
Another type of concrete containing biological components is called Permeable Concrete. This concrete is very porous and can be occupied by natural bacteria. These pores can also let water penetrate, which means it doesn’t need a drainage system.
The bacteria in the concrete can decompose oil and other pollutants, so it has the function of water purification.
There is another kind called Concrete Cloth.
This material can be rolled and fixed into the shape you want as long as you add some water.
Although Concrete Cloth is very suitable for sculpturing, its largest application is disaster relief. The Concrete Cloth could build temporary shelters in the disaster area in a short time.
- Author: Mark Andrew Miodownik. 《Stuff Matters: Exploring the Marvelous Materials That Shape Our Man-Made World》. Published by Beijing United Publishing Company, 2015.
- Author: Xia Zhengnong, Chen Zhili. 《Da Ci Hai: materials and science volume》, published by Shanghai Dictionary Publishing House on December, 2015.