New Concrete Can Heal Millimeter Cracks in 24 Hours for Long Lasting Infrastructure

Researchers at Worcester Polytechnic Institute (WPI) have developed a new self-healing concrete that could multiply structures’ lifespans and slash CO2 emissions.

Most infrastructure is composed of concrete, and indeed, in its many forms, it is the single most used material in the world. However, the use of concrete comes at an environmental cost. While the production of concrete materials does not produce a large volume of carbon emissions by itself, its sheer volume is responsible for almost 8% of human made global carbon emissions and 3% of global energy demand. Therefore, healing rather than replacing concrete offers a significant benefit to the environment. Here, we present a new paradigm by introducing a novel mechanism to naturally heal cement paste that actively consumes rather than generate it.

This will eliminate the need for expensive repairs or replacements. The work, published in the peer-reviewed journal Applied Materials Today, uses an enzyme that automatically reacts with atmospheric carbon dioxide (CO2) to create calcium carbonate crystals, which mimic concrete in structure, strength, and other properties, and can fill cracks before they cause structural problems.

“We looked to nature to find what triggers the fastest CO2 transfer, and that’s the CA enzyme,” said Rahbar, who has been researching self-healing concrete for five years. “Since enzymes in our bodies react amazingly quickly, they can be used as an efficient mechanism to repair and strengthen concrete structures.”

The process, which Rahbar has patented, can heal millimeter-scale cracks within 24 hours.

Inherent brittleness of concrete leads to damage through several mechanisms such as freeze-thaw cycles pervasive in our environment. Current repair processes for cracked and damaged concrete rely on matching dissimilar materials, such as the inorganic C-S-H of concrete with organic petroleum-derived epoxies. The success of repairing concrete by patching and resurfacing processes relies upon the removal of the damaged material, which can cause further damage. Universally, rehabilitating concrete with mismatched materials creates additional flaws in the repairs, undermining the process. The repair process can take several forms, but the general first step is to chip away until only sound concrete remains, often exposing the reinforcing steel bar. One study found that only around 50% of repairs are durable while around 25% failed. Furthermore, only after 5–7 years, most repairs failed. The main mechanisms of failure here are the bond breakage between materials due to chemical attack, thermal fluctuation, and inadequate preparation or application.

Applied Materials Today – An enzymatic self-healing cementitious material

Highlights
• Inspired by the extremely efficient process of CO2 transport in cells, a self-activated healing mechanism for a cementitious matrix is proposed using Carbonic Anhydrase (CA) enzyme.

• The CA Enzyme, a protein, is used here as a catalyst; hence it is not consumed in the process.

• The rate of crystal precipitation in the proposed enzymatic mechanism can be can be up to four orders of magnitude higher than bacterial concrete.

• Comparing to bacterial concrete the process is entirely safe and odorless. It removes the use of bacteria/microbes in civil infrastructure.

• The healed crack sizes (larger than 1 mm) are significantly larger than bacterial concrete, due to the enhanced crystal precipitation rate.

Abstract
Concrete is the most widely used material in the world and is responsible for 8% of global carbon emissions. It is inherently brittle, and it requires frequent repair or replacement, which are expensive and generate large volumes of CO2. Current methods of repair by agents such as mortar and epoxies result in structures with reduced strength and resiliency due to material mismatch, therefore, a self-healing cement paste (concrete’s main matrix) is needed to overcome this problem. The leading self-healing mechanism is based on the use of bacteria and microbes, which are slow and have limited applications, as well as unknown health effects. Inspired by the extremely efficient process of CO2 transfer in biological cells, this study introduces a method to develop a self-healing mechanism in a cementitious matrix using trace amounts of the enzyme Carbonic Anhydrase (CA). CA catalyzes the reaction between Ca2+ ions and atmospheric CO2 to create calcium carbonate crystals with similar thermomechanical properties as the cementitious matrix. The crystal growth rate using this method is orders of magnitude faster and more efficient than bacterial methods, resulting in the healing of large flaws on timescale orders of magnitude shorter. This method is capable of self-healing samples with millimeter-scale flaws within 24-hours and is significantly faster than all current methods that need a minimum of 28-days for strength recovery of microscale cracks. This inexpensive method is biologically safe, actively consumes CO2, and avoids using unhealthy reagents. It can be an efficient mechanism to repair and strengthen the existing concrete structures.

SOURCES – Applied Materials Today, Worcester Polytechnic Institute
Written By Brian Wang, Nextbigfuture.com