Internal curing is a technique that extends the hydration of Portland cement without increasing the initial water-cementitious material ratio.
The technique involves employing water stored in the saturated lightweight aggregate or superabsorbent polymer to cure concrete and help further with the hydration of cement.
The lightweight aggregate is saturated before batching to absorb water in its pores, and the superabsorbent polymer absorbs a high quantity of water and releases it once the polymer contacts ionic materials.
Internal curing improves various aspects of concrete, for instance, reduction of chloride penetration, early-age cracking, curling, and improves durability, increasing the structure's service life.
How Does Internal Curing Affect Concrete Properties?
Early-Age Shrinkage Reduction
Internal curing is very effective in reducing autogenous shrinkage and subsequent crack development. Figure-1 represents the effect of internal curing on the reduction of early-age shrinkage.
The mortar specimen made using normal weight aggregate and 8% cement replaced with fine silica fumes (FSF) underwent the highest autogenous shrinkage.
Two mortar samples named LWA20 (light-weight aggregate) and LWA08 as shown in the Figure-1, made by replacing 20% and 08% of normal weight with prewetted lightweight aggregate respectively were cured internally. Autogenous shrinkage was almost eliminated in the LWA08 mortar specimen.
A 0.4% addition by the mass fraction of cement of superabsorbent polymer (SAP) particles was used in the mixture SAP. The effect of the SAP can be observed in Figure-1. Figure-2 shows the optimum amount of sand that needs to be replaced by prewetted lightweight sand to minimize the likelihood of crack development.
It may be argued that several factors contribute to cracking in concrete, but it is worth mentioning that the major factors are volume changes and restraints. So, controlling volume changes by minimizing autogenous shrinkage leads to a considerable reduction in potential cracking.
The internal curing increases the compressive strength of concrete noticeability, especially in the case of high-performance concrete. The effect of internal curing using prewetted lightweight aggregate is more paramount at later ages of concrete when moist curing is finished and the concrete surface is dried.
For optimum strength development, the prewetted lightweight aggregate should be conditioned properly. For instance, prewetted aggregate performs better internal curing than air-dried lightweight aggregate.
Nonetheless, the over-wetting of the lightweight aggregate should be avoided; otherwise, the mechanical properties of concrete will be compromised. Therefore, careful mix proportion is a must to make internal curing feasible.
Internal curing is an excellent method for producing high-early flexural strength concrete. This is specifically beneficial for concrete pavement, where it helps put the pavement into service at an early age.
It is demonstrated that prewetted lightweight aggregate in the concrete pavement achieved around 70% of the 28-day flexural strength in three days. So, the internal curing improved flexural strength by 7%, and the enhancement was around 8% at 28 days.
Modulus of Elasticity
Replacing natural aggregate with prewetted lightweight aggregate for internal curing purposes can reduce the modulus of elasticity of concrete, but the change is not always proportional.
For instance, replacing a small quantity of natural sand with prewetted lightweight sand can improve the modulus of elasticity. In contrast, a large replacement quantity decreases the modulus of elasticity of concrete.
A low modulus of elasticity is desirable in certain cases because it decreases tensile stresses and reduces the likelihood of crack development. Therefore, the designer should look for changes in the modulus of elasticity when modifications in concrete compressive and tensile strength are needed.
Permeability and Resistance to Deicing Chemicals
Commonly, low permeable concrete has a satisfactory resistance against deicing chemicals. High-durable concrete can be produced using a low water-cementitious material ratio, supplementary cementitious materials, or a combination thereof.
Internal curing can also help decrease concrete permeability and its resistance to deicing chemicals by encouraging better cement hydration. The effect of internal curing is more effective in the longer term.
Replacing around 10% of natural sand with prewetted lightweight fine aggregate can reduce the rapid chloride permeability test by 25%.
Warping Mitigation and Dimensional Stability
It is demonstrated that internal curing through prewetted lightweight fine aggregate reduces moisture warping effectively. This is due to the release of water from prewetted lightweight fine aggregate when surface drying occurs, and because of increased cement hydration.
Higher cement hydration leads to a denser concrete surface, lower concrete permeability, and ultimately water loss reduction. The reduction in water loss and provision of internal moisture keeps a uniform humidity in concrete i.e. prevent moisture gradient, which is the major cause of warping.
Freezing and Thawing Resistance
The freezing and thawing resistance is the ability of concrete to contain expansive pressures generated by freezing water in the capillary pores. So, when the water leaves pores of lightweight aggregate due to internal curing, the void in the aggregate will help concrete to withstand freezing and thawing cycles.
Creep occurs due to moisture movement from concrete to the environment. So, when internal curing is considered, the internal relative humidity of concrete is kept high, resulting in creep reduction.
Internal curing increases cement hydration, improves concrete strength, and reduces permeability, resulting in decreased creep.
No, water in lightweight aggregate (LWA) or superabsorbent polymer (SAP) does not count as part of the water-cementitious material ratio of the concrete mixture because the water is hidden in an inclusion (LWA, SAP) and does not comes out before concrete sets.
Higher cement hydration due to internal curing leads to a denser concrete surface, lower concrete permeability, and ultimately water loss reduction. The reduction in water loss and provision of internal moisture keeps a uniform humidity in concrete, i.e., prevents moisture gradient, which is the major cause of warping.
Internal curing improves various aspects of concrete, for instance, reduction of chloride penetration, early-age cracking, curling, and improves durability, leading to an increase in the structure's service life.
Internal curing can also help decrease concrete permeability and its resistance to deicing chemicals by encouraging better cement hydration. Internal curing is more effective in the longer term. Replacing around 10% of natural sand with prewetted lightweight fine aggregate can reduce the rapid chloride permeability test by 25%.
Internal curing increases cement hydration, increasing concrete strength and reducing permeability, resulting in decreased creep.