What is Self-healing Concrete?

Self-healing Concrete is defined as a type of concrete that exhibits self-healing properties. Self-healing or self-repairing of concrete is that property by which concrete undergoes crack repair and rectification, either by autonomous or by autogenous methods.

The characteristics of concrete that defines all of these activities is it’s durability. Durability is the materialistic property which helps the concrete shield itself from attacks that can be of physical or chemical nature. With lessening durability, there is more chances of development of cracks.

Some of the major types of cracks in concrete are: Settlement cracks, Expansion cracks, Plastic shrinkage cracks, Shear and flexural cracks, Cracks resulting from corrosion of reinforcement, Cracks due to weathering, due to thermal stresses and faulty designing.

The reasons behind crack formations can be many. But to state a few:

• Improper mixing, vibration and curing.
• Application of heavy loads.
• Usage of inferior quality materials (Like reinforcements – easy corrosion)
• Adverse temperature influences
• Poor mix design
• Excessive water in concrete, Non- maintenance of proper water- cement ratio.

So, what can be done to help solve the issue? Let’s have a brief exploration on the same!

Methods of Self-healing in Concrete

There are various types of self-healing methods of concrete. Some of them are :

Autogenous Self-healing Concrete

Concrete in itself processes naturally certain autogenous healing properties.  In this method unhydrated cement particles work as a healing material. The hydration of clinkers minerals present in it initiates the healing activity.

This reaction with water of cement helps rectify hair line cracks by the innate properties of concrete and dissolution of calcium.

Autonomous Self-healing Concrete

While autogenous self-healing method is intrinsic, it can only prove effective to tackle small cracks. This is where autonomous self-healing methods hold ground. To improve performance and for concrete repair, here additional external self-healing provisions are catered.

Vascular self-healing method

Vascular self – healing is a multiple healing technique where concrete is embedded with a hollow tube network bearing a healing agent inside it.  While choosing the tubes one should note that they are chemically inert. Also, these long parallel tubes should form a good bonding with concrete.

Inorganic Phosphate Cement (IPC) is highly preferable along with clay tubes. On encountering a crack in the concrete, the healing agent leaks due to hydrostatic pressures along with capillary or gravitational forces to seal the crack.

Capsule based Self-healing method

The micro capsule used in this technique can be of cylindrical or spherical shape. The shell material can be of silica, ceramics, glass, polystyrene or urea-formaldehyde. The healing efficiency of this method depends on the cementitious matrix and properties of embedded capsules.

Even, multi-capsule system can be relied on. When these capsules come into contact with one another and breaks open, healing components are released and crack gets cured. The healing agents can be epoxy resins, poly urethane or Methyl methacrylate monomers (MMA).

Embedding shape memory alloys (SMAs) method

Shape Memory Alloys are a type of smart material that can be used in concrete since it has a unique property of determining the previous shape even after undergoing deformation. Basically they serve the objective of self-centering.

Researchers have investigated the effects of using electrical currents to heat the Shape Memory Alloy wires and there after administering the same into concrete beams. The Shape Memory Alloys were used for the purpose of retrofitting and a pre-tension was given to them prior to testing. During the test cycles, loading was provisioned at the mid-span and deformation and cracking was analysed. The heated wires proved to be effective in crack repairing and closed the cracks.

Microbial or Bacterial Self-healing Concrete

Calcoum Carbonate (CaCO₃ ) precipitation help repair the micro-cracks in concrete under the method of microbial/bacterial self – healing. Microbial spores and calcium nutrients containing healing agents are first produced in batches and later incorporated into concrete. Researchers have trailed with microbial concrete intrusion in concrete, for example in the side walls of a ship lock and observed the cracks go healed within a time frame of 60 days.

An Experimental Investigation on Bacterial Concrete

Here’s a brief summary on the insights of addition of bacteria to concrete in comparative conditions of normal as well as saline water.

Choice of bacterial species

Bacillius Subtilis or BKH2, a genetically modified thermophilic anaerobic bacterium (A genetically-enriched microbe).

Reason for the choice

• Ideal for self-healing concrete with the provision of an interface which is consumer friendly.
• Bacteria that is stable and able to thrive in alkaline conditions.
• Also aiding in the repair of cracks in concrete, by production of calcium carbonate crystals. 
• Bacteria helps reduction of pH  to the range of about 10 or 11 when concrete cracking commences.
• An environment-friendly as well as cost effective option.

Summary of process

Quantity of bacteria administration: 45-60 ml

Direct application of bacteria – Addition of spores of bacteria and a calcium nutrient like calcium lactate while concrete mixing.

Now, once the concrete breaks, bacteria activates (starts germinating by the breaking of spores) and ingests calcium lactate to form limestone which further helps in the filling up of cracks in concrete.

Curing can be either by normal water or saline water.

Result analysis

Compressive strength inference on Self-healing Concrete

Normal water curing – There is significant increase in compressive strength with curing with normal water of concrete administered with bacteria. For 7 days, strength for the concrete increases by the range of 40% and for 28 days, approximately by 45%.

Saline water curing – Experimental investigation for the first week of curing suggests that salt crystallization helps increase the strength by about 50%. This is suggestively because of optimized content of calcium chloride. However, considering 28 days strength, strength in saline water curing decreases progressively due to the hydration of lime by about 40%.

So, that was a little insight on the self- healing methods of concrete and experimental studies on bacterial self-healing to support the case.

It is valid and of prime importance to treat the cracks, the sooner the better. Why?

A reason being, if the cracks are left open they can widen without proper treatment and lead to ultimate failure of the structure and it’s elements!

Today, lot of investigations are happening in the domain of the healing properties of concrete. One needs to give attention to these as – more the information, the easier is it to deal with the problems. Another important activity related to concreting is estimation concrete takeoff service will help minimizing these efforts.

Hope this article helped you get a brief understanding on the matter!