This image of a simulated concrete sample shows the "packing fraction" which describes the fraction of the volume that is filled with solid material. In this case, the average packing fraction is 0.52. Colors indicate the variations within the sample, ranging from less than 0.4 to more than 0.64. The size of the cube is 0.6 microns. (Image courtesy of the researchers/MIT.)

Understanding concrete’s structure on a microscopic scale is of great interest. Formed from a mixture of water, gravel, sand, and cement powder, concrete is a glue-like material known as cement hydrate (CSH). In order to improve understanding of the internal structure of concrete, researchers from MIT, Georgetown University and France’s SNRS recently collaborated to determine whether cement is a continuous solid or composed of a collection of small particles.

Their goal is to identify ways to improve concrete durability by manipulating key factors in the CSH structure and to clearly define how the material behaves under various conditions.

Roland Pellenq from MIT’s department of civil and environmental engineering said, “We did the first atomic-scale model of the structure of concrete, 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.”

A Continuous Matrix or an Assembly of Particles?

Once the researchers’ CHS solidified, they studied the particle distribution and determined that within the spaces there were smaller and smaller grains, causing the CHS to approximate a continuous material.

Dense packing of real sand and rock particles for simulating the rheology of a concrete. (Image courtesy of the National Institute of Standards Technology Material Measurement Laboratory.)

“Those grains are in a very strong interaction at the mesoscale…you can always find a smaller grain to fit in between the larger grains,” Pellenq said. “You can see it as a continuous material.”

However, because these grains cannot reach a state of minimum energy, solid concrete material eventually loses integrity, cracks and degrades. Because of these characteristics, CHS is also considered an assembly of particles.

Material properties depend on hardened concrete pore sizes, and the researchers’ studies at the nanoscale successfully measured key characteristics. At the mesoscale distance of 10-15 nanometers between pores, however, these characteristics become difficult to measure.

For the first time, Pellenq and his team achieved accurate results. Pore spacing determines the amount of water the material can absorb and cause structural failure; this research provides concrete results that will lead to processing improvements.

Mesoscale Measurements

Stiffness, elasticity, and hardness of concrete are recorded using the new mesoscale simulations. These are characteristics that are also seen in actual concrete samples. The modeling is useful and will guide research on developing improved formulas.

Manufacturing cement powder creates the highest amount of greenhouse gas emissions out of the entire concrete making process. By reducing initial water needed in the mixture, the amount of cement powder needed can decrease. Reducing the amount of water improves durability, as water causes larger pore spacing and poor concrete setting.

“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,” said Christian Hellmich, director of the Institute for Mechanics of Materials and Structures at the Vienna University of Technology.

For more information on MIT’s research into concrete, the researchers’ paper is available through the Proceedings of the National Academy of Sciences.