What is Autogenous Self- Healing of Concrete ?

Autogenous self-healing concrete possesses the natural ability to repair or heal the cracks in them in the presence of moisture. Self-healing in concrete can be autogenous, which is intrinsic healing, or can be autonomous, which is engineered healing.

Autogenous healing is a process recognized early in the development of concrete structures, and it is the main reason for the substantial life extension of ancient structures and buildings. 

Autogenous Self- Healing of Concrete

As per experimental studies and research in the 1900s, autogenous self-healing is most likely due to physical, mechanical, and chemical processes. Some dormant cracks observed on concrete slabs, patios, water retaining, or watertight structures do not require healing as they self-heal with time. Nevertheless, it can be wise to reach out to a concrete patio contractor or engineer to further analyze the severity of cracks.

Self-healing in concrete can occur autogenously or autonomously. However, studies show that autogenous self-healing is more efficient, cost-effective, safer, and easier to implement in full-scale applications.

This article explains the basic principle and process of autogenous self-healing of concrete.

Autogenous Self-Healing of Concrete

Autogenous healing of concrete occurs when the continuity between two sides of a crack is restored without repair work. According to the Concrete Society, based in the UK, "Autogenous healing is the natural process of crack repair that can occur in the presence of moisture and the absence of tensile stress."

The process mends the fissures or cracks in the concrete mainly by the continued hydration of cement clinker minerals or carbonation of calcium hydroxide. The healing product that fills the crack is calcium carbonate. The process demands the availability of free water to perform desired chemical reactions and heal the cracks.

 

Autogenous healing has practical applications for closing dormant cracks in a moist environment, such as in mass structures and water-retaining or watertight structures.

Causes of Autogenous Healing in Concrete

Autogenous healing in concrete can occur due to the following:

1. Physical Cause
2. Chemical Cause
3. Mechanical

Causes of Autogenous Self-Healing in Concrete

Chemical Cause

Autogenous healing in concrete can either occur due to the following: (Figure-2)

Precipitation of Calcium Carbonate (Calcite): The primary healing process in autogenous self-healing of a matured concrete is the crystallization of calcium carbonate within the crack. The process can be explained as follows;

Once the crack is induced, "surface-controlled crystal development" occurs. During the process, the calcium ions present on the fracture surfaces are easily accessible and form crystals. This reaction creates an initial layer of calcite on the crack wall surface. Now, the amount of calcium ions is less rich on the cracked surface. 

Now, the reaction transits to "diffusion-controlled crystal growth," where the calcium ions diffuse through the concrete and the calcite layer formed to reach the crack surface to undergo further precipitation of the healing product. 

When carbon-di-oxide present in the atmosphere dissolves in water, carbonates are obtained. 

                                  H2O + CO2 ↔ H2CO3 ↔ H+ + HCO3− ↔ 2H+ +CO32−

These carbonates react with the calcium in the concrete to form calcium carbonates. There are several stages for generating calcium carbonate in different water pH levels, as shown in the chemical equations above.

Ca2+ + CO32− ↔ CaCO3 (pH water ˃ 8)

(2)

Ca2+ + HCO32+ ↔ CaCO3 + H+ (7.5 < pH water < 8)

Continued Hydration: This reaction results in hydration products like C-S-H that help in the strength and density of concrete. The hydration form CaCO3 facilitates the filling of the cracks. This will result in a delayed and weaker precipitation of calcium carbonate.

Also Read: Polymer Composites in Construction

Physical Cause

The moisture absorption by concrete results in a volume increase that help in bringing small crack surface together and healing it. 

Mechanical Cause

• The cracks are filled with fine particles that are present in the water.
• The loose particles resulting from cracking close the cracks

Factors Affecting Autogenous Healing of Concrete

The necessary conditions for autogenous healing in concrete are:

1. Age of Concrete: At an early stage of concrete, the hydration products formed by the hydration of calcium silicates into C-S-H calcium silicate hydrate take the role of a self-healing process.    In the later stage, the main products involved in self-healing are calcium carbonates formed by carbon-di-oxide in water and calcium in concrete, as explained in the above equations. 
2. Internal Stress: Impelledd compressive stress makes the cracked face come into contact. Studies have shown that the concrete specimens cured under some amount of compressive stress healed much better than those cured under no compressive stress.
3. Curing Period: The process of curing can recover the strength of concrete. But, highly humid conditions are not enough to trigger the healing process in concrete.
4. Moisture Content: Water accelerates the hydration of unhydrated cement particles and increases the dissolution of calcium hydroxide from the concrete matrix near the crack surface. 
5. Crack Width: Spontaneous or autogenous healing is the most efficient for tiny cracks of less than 0.3 mm in width. We need to enhance the process for more significant cracks using additional materials or autonomous healing. 

Read More: Hydration reaction and hydration products of Cement.

How to Improve Autogenous Healing in Concrete?

Autogenous healing in concrete can be enhanced and improved in the following ways:

1. The fiber in Autogenous-Healing of Concrete
2. Shrinkable Polymers in Autogenous-Healing of Concrete
3. Mineral Admixture in Autogenous-Healing of Concrete

1. Fiber in Autogenous-Healing of Concrete

The incorporation of fibers in concrete helps to enhance the autogenous self-healing property of concrete. Fibre is an important material that is used for Fibre-Reinforced Concrete (FRCC) as well as in Engineered Cementitious Composites (ECC). 

Fig.1. Fiber in Autogenous Healing of Concrete

The arrangement of fibers in a randomly distributed manner helps to bridge the cracks. This bridging helps to decrease the crack width and hence prevent the migration of aggressive agents like chloride ions and carbon-di-oxide. Hence, these activities improve the autogenous self-healing capacity of the concrete. 

On the other side, the design of FRCC, ECC, and HFRCC is very costly and it is a great challenge to maintain the homogeneity of fibers in the matrix in a consistent manner to facilitate self-healing properties.  

2. Shrinkable Polymers in Autogenous-Healing of Concrete

Shrinkable Polymers are polymers that shrink when they are activated by heating in a specific condition. One of the examples of shrinkable polymers is polyethylene terephthalate (PET) tendons. The stress created using during this shrinking is used to bring the crack tip closure for efficient healing.

Fig.2.  Shrinkable Polymers in Autogenous-Healing of Concrete

As per the study conducted by Cardiff University Self-healing research team, the restrained polymer tendons that are used in concrete are activated thermally once the initial curing of the concrete is done. Once the tendon gets activated, the used polymer tendon closes the performed macrocracks and imparts significant stress across the crack faces. This would enhance the autogenous self-healing process in concrete.

3. Mineral Admixture in Autogenous Healing of Concrete

The self-healing capacity of concrete can be improved by introducing mineral admixture and supplementary cementitious materials (SCMs). The minerals introduced in concrete can help the self-healing process in the below ways:

1. To remain unhydrated after the initial mixing stage of the concrete.
2. To undergo reaction and produce expansive hydrated products that would heal the cracks.

Fig.3. Mineral Admixture in Autogenous Healing of Concrete

The different types of SCMs that can be used to auto-heal the concrete cracks through autogenous self-healing are silica fumes, fly ash, and blast-furnace slag, and expansive minerals like lime, bentonite clay, crystalline additive (CA), calcium sulphoaluminate, magnesium oxide.

Examples of Autogenous Self-Healing in Concrete

Cracked Concrete Structures

Autogenous self-healing has been observed in various concrete structures, such as buildings, bridges, and tunnels, where small cracks formed naturally over time have closed and healed without external repair work. This phenomenon has contributed to the longevity and durability of ancient structures.

Water-Retaining Structures

Concrete structures that come into contact with water, such as water tanks or reservoirs, have shown autogenous self-healing capabilities. The presence of moisture in these structures provides the necessary conditions for the chemical reactions that lead to healing, resulting in the closure of small cracks.

Limitations and Challenges of Autogenous Self-Healing in Concrete

Crack Size and Width: Autogenous self-healing is most efficient for small cracks with widths below 0.3 mm. As the crack width increases, the effectiveness of autogenous healing decreases. Larger cracks may require additional measures or materials to facilitate healing, such as incorporating autonomous healing agents or engineered repair methods.

Time Required for Healing: The healing process in concrete occurs gradually over time, and the rate of healing depends on factors such as crack width, moisture availability, and the presence of reactive minerals. Complete healing may take weeks or months, and in some cases, extensive or complex cracks may not fully heal through autogenous self-healing alone.

Dependence on Moisture: Autogenous self-healing requires the presence of moisture to facilitate the chemical reactions responsible for crack closure. In dry or arid environments, the availability of sufficient moisture may be limited, hindering the effectiveness of autogenous self-healing. Additionally, excessive moisture or water ingress can dilute the healing agents or cause other issues, affecting the healing process.

Limitations in Structural Integrity: While autogenous self-healing can effectively repair small cracks and restore the integrity of concrete structures, it may not address more significant structural issues or damage. In cases where the cracks are extensive, deep, or result from significant structural stress, additional measures such as external repairs or reinforcement may be necessary to ensure adequate structural stability.

Long-Term Durability and Predictability: The long-term durability and predictability of autogenous self-healing in concrete are still subjects of ongoing research and development. Factors such as material composition, environmental conditions, and maintenance practices can influence the effectiveness and longevity of autogenous self-healing, and further studies are needed to optimize its application in various scenarios.

It is important to note that the effectiveness of autogenous self-healing and its limitations may vary depending on specific project conditions and materials used. Consulting with concrete experts, engineers, or contractors can provide more detailed insights tailored to specific situations.

Also Read: What is Self-Healing Concrete ?