Autogenous healing concrete

What is “Autogenous Healing”?

“Autogenous Healing” is the term used to describe the natural occurring process in which concrete, in the presence of moisture, can self-repair cracks. The phenomena of concrete being able to repair itself is not new. The first documented examples where conducted by the French Academy of Science in 1836. Over the years, many examples of autogenous healing have been demonstrated and documented.

How does Autogenous healing repair the crack?

The generally accepted belief is that autogenous healing occurs through a combination of:

  1. The further hydration of the cement in the concrete
  2. The carbonization of the calcium hydroxide (free lime) by carbon dioxide found in the surrounding air and water

Normal Portland cement consists of approximately 50% Tricalcium silicate and 25% Dicalcium silicate and in the presence of moisture reacts to becomes calcium silicate hydrate releasing calcium hydroxide. The presence of carbon dioxide then hardens the calcium hydroxide into crystals that become trapped within the crack and accumulate and eventually the crack will become blocked.

In the following quotation extracted from the Concrete Pipe Association of Australasia´s Technical Brief on Autogenous Healing,

“In the presence of moisture the cement in a concrete structure continues to hydrate, enhancing the strength and durability of the concrete. Autogenous healing is an extension of this life-like quality.”

The Concrete Pipe Association of Australasia points out that autogenous healing capabilities of concrete are rooted in the ability of the concrete to hydrate and that the self-healing capabilities are contributable to the hydration process.

This is evidenced in the report by D. Roberts, “Autogenous Healing – The Self-Healing of Fine Cracks” Concrete Advise Nº09, The Concrete Society, July 2003. A conclusion can be made that cement hydration is a primary mechanism while carbonization of the calcium hydroxide is a distinct secondary mechanism. In the testing conducted by Roberts, he noted unabated autogenous healing in blended concretes that did not contain “free lime”. Given his confirmation that the types of water and the cementitious materials have a minimal effect on autogenous healing, one can safely conclude that the hydration of the cement in the concrete is the significant factor to the self-healing process of concrete cracks.

This idea can be further supported through the research of Adam Neville (Autogenous Healing – A Concrete Miracle, Concrete International, Nov 2002), in which Neville determines, that

“at an early age, continuing cement hydration, in which calcium silicates in the cement are converted to calcium silicate hydrate, can play a direct role in the healing process.”

What are the limitations of Autogenous Healing?

There are many variables involved in the determination of autogenous healing in regards to width of cracks and time. Test conditions seem to vary and it seems that every test achieves differing overall results. As published by The Concrete Society both BS 8007, the design of concrete structures for retaining aqueous liquids, and the Water Services Association’s Specification, it is implied that cracks up to 0.2 mm wide will autogenously seal within 28 days; cracks up to 0.1 mm will seal within 14 days.  Keep in mind that the time (and even the ability to actually autogenously heal the cracks) may vary subject to how the cracks are caused, applications of tension, static water or flowing water, variations in hydrostatic pressure, fresh water or salt water, the thickness of the structure, the shape or form of the crack, the depth of the crack and the external conditions surrounding the crack. One must consider that every factor surrounding the crack and the concrete will impact positively or negatively the performance of the concrete to autogenously heal.

How does Krystaline Technology work?

Krystaline´s crystalline technology, both surface and admix applications, work as a catalyst between the unhydrated cement particles in the concrete and any moisture that may be present (or become present in the future) causing the concrete to further hydrate. The hydration promoted by the catalytic reaction is enhanced resulting in additional calcium silicate hydrate crystals within the concrete filling the pores, capillaries and even the micro-cracks within the concrete.  The catalytic nature of the technology results in an ability to be constantly present within the concrete even years later.

Will Krystaline Technology affect the autogenous healing process of concrete?

Given that the hydration process can be considered a critical component in the development of autogenous healing, any factors that influence the hydration capabilities of the concrete also influence the capability of the concrete to autogenously heal. An enhanced hydration results in an enhanced ability to autogenously heal concrete. Krystaline Technology enhances the hydration process there by enhancing the autogenous healing process within the concrete.

What are the performance capabilities when using Krystaline Technology?

The implied limits found within the BS 8007 and the Water Services Association’s Specification (see above) discuss a limitation of autogenously healing of cracks “up to” 0.2 mm wide”. Krystaline Technology, under identical conditions, increases this upper limit and may even exceed 0.4 mm when optimal conditions are present.

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