Self-curing concrete is a new technology used by the construction sector to overcome the natural curing practice issues of traditional one. Traditional water-based curing with external application of water using ponding, spraying, or covering with wet burlap has several limitations—particularly when controlling concrete hydration in severe or dry conditions. One of the solutions is the self-curing concrete where internal curing agents are included in the mixture which will provide additional moisture to the mixture from the inside and promote continuous curing process. This not only minimizes shrinkage but also increases the long-term performance and durability of concrete structures. This post will explore the science and working as well as applications of self-curing concrete, its process, benefits and the materials that enable it.
What is Self-Curing Concrete?
Self-curing concrete is a type of concrete that minimizes or eliminates the need for external curing by using internal curing agents. These agents are water-retaining polymers, lightweight aggregates (LWA) and superabsorbent polymers (SAPs), holding water within the concrete as if it were a water tank. The purpose of these internal curing compounds is to gradually release water during the hydration process, maintaining adequate internal humidity throughout the curing period. This internal moisture ensures the continued progress of cement hydration, which is essential for the formation of calcium silicate hydrate (C–S–H) gel—the primary binding compound responsible for the strength and durability of concrete.
Traditional concrete laying process, in particular, concrete with a low water to cement (w/c) ratio, has hydration difficulties because the water is quickly used up by the cement. With the lack of sufficient moisture, hydration slows or ceases, which can cause problems such as cracking, reduced strength, and decreased durability. Self-curing concrete overcomes this issue by retaining water available for cement hydration during a longer time period, which permits the more complete hydration of cement, lessens the shrinkage, and avoids the formation of early age cracks.
How Does Self-Curing Concrete Work?
The self-curing ability of concrete is dependent on two principles, absorption and controlled release of moisture from internal curing agents. These agents provide a constant water source to the concrete preserving sufficient hydration characteristics even when the water quantity in the mix is low or the curing conditions are limited.
In self-compacting concrete, researchers used lightweight aggregates like expanded shale/clay and perlite and superabsorbent polymers (SAP) / super absorbent polymer (SAPs) as water reservoirs. These substances can absorb large amount of water, when it is mixed. “These internal curing mediates take up the water in the concrete mixture and retain it in their capillary-porous structure.” As the cement sets and the surface water is gone, these reservoirs of water are slowly bled off so that the mix is still wet for the cement to hydrate completely.
Chemicals and Materials Used in Self-Curing Concrete
Several chemicals are added to self-curing concrete mixes to enhance the internal curing process and ensure continuous hydration. These chemicals, including superabsorbent polymers (SAPs), polyethylene glycol (PEG), and polyacrylamide (PAM), play a crucial role in maintaining the necessary moisture levels for hydration.
Polyethylene Glycol (PEG)
PEG is hydrophilic and absorbs water to hydrogen bond with it. PEG is a water-retaining substance, which is essential for maintaining moisture to support hydration of self-curing concrete. When included in the concrete mix, it inhibits the evaporation of water and keeps the moisture for a long time. This produces a more thorough hydration and better strength, especially in low W/C ratio mixtures. Furthermore PEG has been observed to reduce permeability, increasing the concrete resistance to water and chemicals.
Super Absorbent Polymers (SAPs)
Superabsorbent polymers (SAPs) are materials capable of absorbing and retaining water in quantities of 1,000 times their weight. In self-curing concrete SAPs are used to simulate internal curing by the provision of a reservoir of water supplied slowly to the concrete. SAPs provide an excellent performance, especially in low w/c ratio mixes, for which traditional external curing methods may not be practical. They are also capable of maintaining hydration in the absence of surface moisture loss, which helps to enhance the strength and durability of the concrete.
Lightweight Aggregates (LWA)
Expanded clay, shale, and perlite are some of the usual LWA used in self-curing concrete systems because of their ability to absorb and retain water. They have an open pore structure which enables the aggregates to absorb water during the mixing. Upon addition to the concrete the water is released from these particles over time and contributes to the hydration of the cement matrix. LWA is particularly effective in mass concrete (large volumes of concrete) applications. Large volumes are prone to generate negative post Effects that can cause cracking.
Admixtures Used in Self-Curing Concrete
Besides the internal curing agents, self-curing concrete commonly employs a number of admixtures for enhancing its performance, workability and shrinkage control. These are the shrinkage reducing admixtures (SRAs), the water retaining admixtures (WRAs) and the viscosity modifying agents (VMAs).
Shrinkage-Reducing Admixtures (SRAs)
Shrinkage-reducing admixtures (SRAs) are generally added to mitigate internal stresses generated by drying shrinkage. These admixtures reduce the surface tension of the water in the concrete; therefore, the capillary forces that cause shrinkage. SRAs, when incorporated with internal curing agents, can further contribute to the self-curing performance of the concrete by reducing shrinkage cracking and increasing dimensional stability.
Water-Retaining Admixtures (WRAs)
Water retaining admixtures (WRAs) also contribute to retaining the water within the concrete mix, which delays its evaporation. Such admixtures are particularly of importance in hot and/or windy climes where a high moisture loss is more likely. WRAs function primarily to help reduce the water content of a concrete mix and to maintain the workability of the concrete at the same time as reducing the water content. This phenomenon is particularly beneficial to the production of concrete.
Viscosity Modifying Agents (VMAs)
Viscosity-modifying agents (VMAs) are used to improve the workability and stability of the concrete mix. VMAs help control the viscosity of the mix, preventing segregation and ensuring that the internal curing agents, such as SAPs and LWAs, are evenly distributed throughout the concrete. VMAs are especially beneficial when using self-curing agents like SAPs, as they help maintain consistency and improve the overall performance of the concrete.
Advantages of Using Self-Curing Concrete
There are many benefits to using self curing concrete including: increased strength, improved durability and ease of use:
Reduced Shrinkage and Cracking
Self-curing concrete helps retain internal moisture, minimizing the risk of shrinkage cracks that typically occur due to rapid water evaporation in conventional mixes. By maintaining sufficient internal hydration, self-curing effectively controls autogenous shrinkage, especially in mixes with a low water-cement ratio. Research indicates that autogenous shrinkage can be reduced by up to 30% in some cases, leading to stronger, more durable, and crack-resistant concrete.
Improved Strength and Durability
Higher compressive strength is achieved by improving hydration, which leads to a denser microstructure. The unceasing water will lead to the generation of calcium silicate hydrate (C–S–H), and the latter will provide the strength and incentive for the concrete to resist from being eroded. Therefore, self-curing concrete is more suitable for adverse environments as subjected to the freezing and thawing cycles and chemical attacks.
Reduced Monitoring Work
Self-curing concrete eliminates the need for traditional curing methods, saving both time and cost. The moisture required for curing is provided by internal agents or from the surrounding environment, reducing the need for constant monitoring and external curing. This is particularly advantageous in remote or arid regions where external curing is difficult or impossible.
Sustainability Benefits
Self-curing concrete also offers sustainability benefits by reducing water usage and waste. The internal curing agents help conserve water, which is particularly important in areas where water resources are scarce. Additionally, self-curing concrete’s improved durability and reduced maintenance requirements contribute to the overall longevity of structures, reducing the need for repairs and replacements.
Disadvantages of Using Self-Curing Concrete
While self-curing concrete has numerous advantages, it also comes with some challenges:
Higher Material Costs
The addition of internal curing agents, such as SAPs, LWAs, and PEG, increases the cost of the concrete mix. However, this additional cost is often offset by the long-term benefits, such as improved durability and reduced maintenance.
Workability Challenges
When excessive amounts of internal curing agents like SAPs (Super Absorbent Polymers) and PAM (Polyacrylamide) are used, they can cause the concrete mix to become thicker and more viscous, leading to challenges with placement and finishing. To maintain optimal workability, it’s often necessary to adjust the mix design by adding plasticizers or viscosity-modifying agents (VMAs).
Limited Control Over Dosage
The effectiveness of internal curing agents depends on proper dosing. Overdosing can lead to excessive voids in the concrete matrix, which can reduce strength and durability. Conversely, underdosing may result in insufficient hydration and increased shrinkage. Careful monitoring and testing are required to optimize the mix.
Applications of Self-Curing Concrete
Self-curing concrete is employed in different applications, especially when the external curing becomes difficult or not feasible:
- Precast and Prestressed Concrete: Self-curing concrete is very useful for precast and prestressed elements where early removal of formwork and loading is required.
- Rigid Pavements: Rigid pavements are made with self-curing concrete, which is commonly used for roads, airports, and other infrastructure in arid environments or where water is scarce.
- Mass Concrete Structures: Self-curing concrete enables to have heat and shrinkage control in large concrete populations, including dams, foundations and retaining walls.
- Water Retaining Structures: In tanks, reservoirs and sewage treatment works where reduced permeability and resistance to cracking are required, self-cured concrete is frequently used.
Conclusion
Self-curing concrete is an innovative solution that addresses many of the limitations of traditional curing methods, offering enhanced strength, durability, and reduced shrinkage. By utilizing internal curing agents, it ensures consistent hydration, even in challenging environments. Although it presents some challenges such as higher material costs and workability concerns, the long-term benefits far outweigh these drawbacks.
FAQ
Yes, in many cases, self-curing concrete can significantly reduce or even eliminate the need for external curing, especially in low-w/c ratio mixes or challenging environments.
SAPs are typically dosed at 0.1% to 0.6% by cement weight, depending on the mix design and performance requirements.
Properly dosed internal curing agents improve hydration, resulting in stronger concrete. However, overdosing SAPs or LWAs can lead to voids, which may reduce strength.
Yes, self-curing concrete reduces water usage, saves labor, and enhances long-term durability, making it an environmentally friendly option for many projects.
Apoorva H P Achar