What is Internal Curing of Concrete?

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“Internal curing refers to the process by which the hydration of cement occurs because of the availability of additional internal water that is not part of the mixing water”, as per ACI concrete terminology.

Internal curing helps the concrete to achieve its full potential in an economic and sustainable manner.

Internally cured concrete utilizes pre-wetted absorbent materials that contain moisture. This moisture is dispersed throughout the concrete when the internal humidity of the concrete drops below 100% to keep cement paste saturated, which leads to greater hydration of cement. As a result, the strength and durability of concrete are improved.

What is Internal Curing?

Internal curing means introducing a component to a concrete mixture that acts as a curing agent. The curing agent can be saturated porous aggregate or superabsorbent polymers (SAPs).

Expanded shale, slag, pumice, perlite, and clay are examples of lightweight aggregate that can be used instead of normal strength aggregate for the purpose of internal curing.

The porous lightweight aggregates work as an internal reservoir that provides an internal source of water which is required to replace the moisture lost through chemical shrinkage and self-desiccation.

The internal water promotes hydration of various cementitious materials. As a result, it minimizes the development of autogenous shrinkage and subsequent cracking. 

Internal curing is specifically beneficial for low water-cementitious material ratio because it has low permeability and external curing has a marginal effect on hydration in the internal part of the concrete.

Internal curing does not replace surface curing of concrete but acts with it to produce better concrete. It helps compensate for poor, ordinary concrete that frequently occurs on the construction site, and for poor weather conditions that may be detrimental to the strength development of concrete.

How does Internal Curing Work?

Internal curing of concrete is controlled by three major parameters, including the volume of water required from the lightweight aggregate, the desorption properties of lightweight aggregate, and the spacing of lightweight aggregate within the mixture.

Volume of Water Required from Lightweight Aggregate

The volume of aggregate required from lightweight aggregate is governed by chemical shrinkage and autogenous shrinkage. These two elements are measurable, and the difference between them equals the amount of water required from the lightweight aggregate.

Desorption Capability of Lightweight Aggregate

For internal curing to occur properly, the internal reservoir (saturated aggregate) should have much larger individual pores than typical pores created in the cement paste and be well-connected.

During cement hydration, the capillary pores in the cement paste are refined, and the radii of these pores will be smaller than the pores in lightweight aggregate. This facilitates the movement of water from aggregate to cement paste.

As soon as the relative humidity reduces due to chemical shrinkage and self-desiccation, humidity gradient and capillary pressure develop. This generates suction that withdraws water from the lightweight aggregates into the hydrating or drying cement paste, which helps cement hydration continuation.

When further hydration occurs, the pores in the cement paste get reduced in size and create greater suction force, pulling more water from the reservoir. Water movement will not stop unless the entire cement is hydrated or relative humidity within the internal reservoir of lightweight aggregate is equal to that in the hydrating cement paste, which eliminates capillary pressure. 

As shown in Figure-2, a large quantity of water is pulled out from the saturated lightweight aggregate when relative humidity reaches 96%. Almost all the water is withdrawn when the relative humidity drops to 92%.

Spacing of Lightweight Aggregate

The moisture stored in saturated lightweight aggregate should be distributed properly throughout the concrete mixture to protect all of the paste. The required spacing of lightweight aggregate is determined by the ability of water to move through the hydrating mixture.

If available water in lightweight aggregate can travel 1 mm, most of the cement paste can be protected. The amount of lightweight aggregate that replaces normal weight aggregate affects the ability of moisture to move within the mixture.

If the mixture is highly impermeable, a higher quantity of lightweight aggregate should be used to achieve the necessary spacing and provide sufficient water.

Superabsorbent Polymers

The use of superabsorbent polymers (SAP) is another form of moisture provision for the internal curing of concrete. SAP swells into a gel-like substance once in contact with water. 

The swelling of SAP ranges from 20 to 2000 times its own mass in water. The absorbed water is released when the SAP comes into contact with other ionic substances such as salt.

Advantages

Internal curing improves internal cement hydration, which leads to the following advantages:

1. Increased concrete strength
2. Decreased permeability
3. Reduced warping
4. Increased resistance to early age cracking
5. Improved dimensional stability
6. Enhanced resistance to freezing and thawing cycles, deicing chemicals, and chemical attacks
7. Improved concrete resistance against creep

Applications

Internal curing is found to be highly beneficial and practical for the construction of bridges, pavements, and similar transportation infrastructures. 

It is potentially applicable for constructing continuously reinforced concrete pavement, white topping, ultra-thin white topping, and jointed plain concrete pavement.

FAQs

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