Concrete, heal thyself

Materials World magazine
,
3 Aug 2015

Rhiannon Garth Jones talks to Tanvir Qureshi, a PhD student in the Department of Engineering, University of Cambridge, about smart minerals for self-healing concrete.

Ancient Roman structures still survive because the volcanic ash and lime based mortar they used has self-healing characteristics. Cementitious materials have always had a significant impact on the civil engineering construction industry and, over the past century, traditional Portland Cement (PC)-based concrete has once again become a popular choice in construction. However, it is becoming increasingly expensive to maintain and repair concrete structures, such as bridges and tunnels. Around 50% of the average annual construction budget in Europe is spent on their repair and maintenance. This trend is also evident in the UK, where the tally stands at around 45%. This challenge can be mitigated with the adaptation of self-healing technology in concrete structures. With such large amounts of money being spent, it is no surprise that a great deal of research is now focusing on this area.

Doing the research

Tanvir Qureshi is a lead PhD researcher working on the development of self-healing concrete using expansive smart minerals, particularly reactive magnesium oxide, bentonite clay and quicklime. According to Qureshi, damages and undesirable cracks in the concrete can cause water leakage and reduction in mechanical strength. Deeper fractures further allow the water to reach into the reinforcing bars, thus causing corrosion that ultimately results in the failure of the structure. Improvements in self-healing serve two principal purposes, he claims – mechanical strength recovery and efficient crack sealing capacity.

Tanvir has found that the self-healing capability of traditional PC-based concrete can be improved by adding optimum proportions of expansive minerals, such as magnesia, bentonite and quicklime. Those minerals have expansion properties that, when concrete cracks, will react with ingress air and water to expand within the crack and heal it. Through laboratory experiments, he has confirmed that expansive minerals have improved the self-healing efficiently in the nano-micro scale.

The minerals used in Tanvir’s experiment have intelligent properties as well as strength and durability. The optimum combinations of minerals with PC results in a self-controlled ability where minerals are not only able to sense various external stimuli and expand through cracks, but exhibit this response in a controlled manner. 

His laboratory experiments suggest that microcracks in the range of 100–400µm in the self-healing cement samples containing minerals had promising healing performance. Crack sealing was monitored under a digital microscope and, finally, a sophisticated gas permeability experiment was conducted, using liquid methanol, to physically estimate the sealing efficiency. Most cracks in the self-healing cement samples were found to be close to 100% sealed within two weeks and successful sealing was confirmed by gas permeability tests after 28 days. Smaller cracks were found healed and micro-analysis on the healed zones confirmed the formation of minerals expansion products that bridge the crack.

Mechanical strength recoveries were investigated using a three-point bend test in the prism samples. Similar to the improvement in the crack sealing capacity, mechanical strength was recovered up to 68% in the self-healing cement samples compared with 10% for traditional PC. Although the addition of expansive minerals reduced mechanical strength performance of PC by a minor amount, self-healing strength recovery was found to have improved almost seven times.

The fundamentals

Tanvir is trying to further understand how self-healing materials form and behave structurally at a fundamental level, which requires testing at an extremely small scale. In this part of his research, Tanvir is using scanning electron microscopy (SEM) to identify the self-healing materials formed in the crack zone, which captures high definition topographical images of the self-healing bridges.

The most common cement used in the concrete is PC, as it has faster hydrating and hardening properties. This means the expansive minerals (magnesia, bentonite, quicklime) remained mostly un-hydrated within the quick hydrating PC matrix and, when cracks formed, they become available for reacting and bridging the cracks. 

The expansive minerals that have been used in the different cement mixes for self-healing had formed different kinds of healing compounds, based on their mix proportions. This was because of the difference in functionality of those minerals in self-healing properties. Magnesia serves expansion and bonding, bentonite serves expansion and scaffolding, and quicklime serves the functionality of crystallisation, which enhances the healing performance. 

Higher magnification SEM image of the healed sample with highly reactive magnesia showed that flower-like crack bridging network structures had formed. This had effectively expanded within the crack, which had sealed and healed the cracks. SEM images of the healing materials presented have confirmed the formation of micro-scale scaffolding healing bridge structures. Other healing compounds such as calcite, portlandite, magnesite were found expanded within this network, which had filled and bridged the cracks. 

Tanvir is currently exploring efficient self-healing techniques in concrete and implementation in industrial trials. 

He is working with glass tube encapsulated systems, as such expansive minerals were encapsulated within the glass tube capsules and embedded into the mortar and concrete matrix. He is expecting expansive minerals to come in to action as soon as cracks form in the matrix and heal the cracks. 

Tanvir’s group in Cambridge is also developing a microcapsule that contains mineral-based healing cargo materials. Their concept is to imbed those microcapsules into the concrete matrix, which will work like a first-aid kit in a bubble.They are now scaling up different self-healing concrete in field trials, with the help of industrial collaborators. 

This work will be beneficial in the coming years as the civil construction practice fully integrate self-healing concrete. In the future, we can hope to have concretes that no longer require the levels of maintenance we see today.