Self Curing Concrete An Introduction

Ambily P.S, Scientist, and Rajamane N P, Deputy Director and Head, Concrete Composites Lab Structural Engineering Research Centre, CSIR, Chennai
Excessive evaporation of water (internal or external) from fresh concrete should be avoided; otherwise, the degree of cement hydration would get lowered and thereby concrete may develop unsatisfactory properties. Curing operations should ensure that adequate amount of water is available for cement hydration to occur. This paper discusses different aspects of achieving optimum cure of concrete without the need for applying external curing methods.

Definition of Internal Curing (IC)

The ACI-308 Code states that “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.” Conventionally, curing concrete means creating conditions such that water is not lost from the surface i.e., curing is taken to happen ‘from the outside to inside’. In contrast, ‘internal curing’ is allowing for curing ‘from the inside to outside’ through the internal reservoirs (in the form of saturated lightweight fine aggregates, superabsorbent polymers, or saturated wood fibers) Created. ‘Internal curing’ is often also referred as ‘Self–curing.’

Need for Self–curing

When the mineral admixtures react completely in a blended cement system, their demand for curing water (external or internal) can be much greater than that in a conventional ordinary Portland cement concrete. When this water is not readily available, due to depercolation of the capillary porosity, for example, significant autogenous deformation and (early-age) cracking may result.

Due to the chemical shrinkage occurring during cement hydration, empty pores are created within the cement paste, leading to a reduction in its internal relative humidity and also to shrinkage which may cause early-age cracking. This situation is intensified in HPC (compared to conventional concrete) due to its generally higher cement content, reduced water/cement (w/ c) ratio and the pozzolanic mineral admixtures (fly ash, silica fume). The empty pores created during self-desiccation induce shrinkage stresses and also influence the kinetics of cement hydration process, limiting the final degree of hydration. The strength achieved by IC could be more than that possible under saturated curing conditions.

Often specially in HPC, it is not easily possible to provide curing water from the top surface at the rate required to satisfy the ongoing chemical shrinkage, due to the extremely low permeabilities often achieved.

Potential Materials for IC

The following materials can provide internal water reservoirs:
  • Lightweight Aggregate (natural and synthetic, expanded shale),
  • LWS Sand (Water absorption =17 %)
  • LWA 19mm Coarse (Water absorption = 20%)
  • Super-absorbent Polymers (SAP) (60-300 mm size)
  • SRA (Shrinkage Reducing Admixture) (propylene glycol type i.e. polyethylene-glycol)
  • Wood powder

Chemicals to Achieve Self–curing

Some specific water-soluble chemicals added during the mixing can reduce water evaporation from and within the set concrete, making it ‘self-curing.’ The chemicals should have abilities to reduce evaporation from solution and to improve water retention in ordinary Portland cement matrix.

Super-absorbent Polymer (SAP) for IC

The common SAPs are added at rate of 0–0.6 wt % of cement. The SAPs are covalently cross-linked. They are Acrylamide/acrylic acid copolymers. One type of SAPs are suspension polymerized, spherical particles with an average particle size of approximately 200 mm; another type of SAP is solutionpolymerized and then crushed and sieved to particle sizes in the range of 125–250 mm. The size of the swollen SAP particles in the cement pastes and mortars is about three times larger due to pore fluid absorption. The swelling time depends especially on the particle size distribution of the SAP. It is seen that more than 50% swelling occurs within the first 5 min after water addition. The water content in SAP at reduced RH is indicated by the sorption isotherm.

SAPs are a group of polymeric materials that have the ability to absorb a significant amount of liquid from the surroundings and to retain the liquid within their structure without dissolving. SAPs are principally used for absorbing water and aqueous solutions; about 95% of the SAP world production is used as a urine absorber in disposable diapers. SAPs can be produced with water absorption of up to 5000 times their own weight. However, in dilute salt solutions, the absorbency of commercially produced SAPs is around 50 g/g. They can be produced by either solution or suspension polymerization, and the particles may be prepared in different sizes and shapes including spherical particles. The commercially important SAPs are covalently cross-linked polyacrylates and copolymerized polyacrylamides/ polyacrylates. Because of their ionic nature and interconnected structure, they can absorb large quantities of water without dissolving. From a chemical point of view, all the water inside a SAP can essentially be considered as bulk water. SAPs exist in two distinct phase states, collapsed and swollen. The phase transition is a result of a competitive balance between repulsive forces that act to expand the polymer network and attractive forces that act to shrink the network. The macromolecular matrix of a SAP is a polyelectrolyte, i.e., a polymer with ionisable groups that can dissociate in solution, leaving ions of one sign bound to the chain and counter-ions in solution. For this reason, a high concentration of ions exists inside the SAP leading to a water flow into the SAP due to osmosis. Another factor contributing to increase the swelling is water solvation of hydrophilic groups present along the polymer chain. Elastic free energy opposes swelling of the SAP by a retractive force.

SAPs exist in two distinct phase states, collapsed and swollen. The phase transition is a result of a competitive balance between repulsive forces that act to expand the polymer network and attractive forces that act to shrink the network.

Means of Providing Water for Self–curing Using LWA

Water/moisture required for internal curing can be supplied by incorporation of saturated-surfacedry (SSD) lightweight fine aggregates (LWA).

Water Available from LWA for Self–curing

It is estimated by measuring desorption of the LWA in SSD condition after exposed to a salt solution of potassium nitrate (equilibrium RH of 93%). The total absorption capacity of the LWA can be measured by drying a Saturated Surface Dry (SSD) sample in a dessicator.

Water in LWA for Internal Curing

About 67% of the water absorbed in the LWA can get transported to self-desiccating paste. Some water remains always in the LWA in the high RH range and it becomes useful when the overall RH humidity in concrete is significantly reduced. The water retained in LWA in air-dry condition may not be enough to prevent autogenous shrinkage whose magnitude, however, may be reduced significantly. The fine lightweight aggregate, in saturated condition, produce a more uniform distribution of the water needed for curing throughout the microstructure.

The grain size of the LWA used as curing agent should be less in order to minimise the paste– aggregate proximity, i.e. the distance to which the internal curing water could diffuse. The grain size of down to 2–4 mm are found to be beneficial.

Utility of LWA Near Surface of Concrete

At the surface of the concrete, as the water evaporates from the concrete surface, a humidity gradient develops. This accelerates the appearance of the localized humidity gradients. The water from the LWA near the surface is then used up faster than in the interior of the concrete thus causing the near-surface layer of the concrete to become denser in a shorter time. This helps reduce the amount of water that would normally evaporate and contributes to improve internal curing of the concrete. It also leads to reduced or no stresses due to drying helping in eliminating the surface cracking.

Potential of LWA for Reducing Autogenous Shrinkage

As the cement hydrates, the water will be drawn from the relatively “large” pores in the LWA into the much smaller ones in the cement paste. This will minimise the development of autogenous shrinkage as the shrinkage stress is controlled by the size of the empty pores, via the Kelvin- Laplace equation.

The radii of capillary pores formed during hydration in the cement paste are smaller than the pores of the LWA. When the RH decreases (due to hydration and drying), a humidity gradient develops; with the LWA acting as a water reservoir, the pores of the cement paste absorb water from the LWA by capillary suction. The unhydrated cement particles from the cement paste now have more free-water available for hydration and new hydration products grow in the pores of the cement paste thus causing them to become smaller. The capillary suction, which is the inverse to the square of the pore radius, increases as the radius becomes smaller and thus enabling the pores to continue to absorb water from the LWA. This continues until most of the water from the LWA has been transported to the cement paste.

Crushed LWA for Internal Curing

Crushed LWA could provide a better surface for binder interaction as the pelletising process often produces LWAs with sealed surface. The vesicular surface resulting from the crushing operation allows paste penetration and provides more surface area for reaction between the aggregate and paste. The transition zone associated with a crushed aggregate has advantages over a more smooth and sealed surface.

Water Required for Self–curing

It depends upon chemical and autogenous shrinkages expected during hydration reactions.

Types of Shrinkage Drying

Shrinkages may occur at earlyages or at later ages over a longer period; different types of shrinkages may be identified as :

Drying shrinkage, autogenous shrinkage, thermal shrinkage, and carbonation shrinkage.

Reason for Chemical Shrinkage

Chemical shrinkage is an internal volume reduction due to the absolute volume of the hydration

Products being less than that of the reactants (cement and water). For example: Hydration of tricalcium silicate:

C3S + 5.3 H -> C1.7SH4 + 1.3 CH

Molar volumes

71.1 + 95.8 -> 107.8 + 43 i.e, 166.9 -> 150.8

Therefore,

Chemical shrinkage = (150.8 –166.9) / 166.9 = -0.096 mL/mL = -0.0704 mL/g cement

For complete reaction of each gram of tricalcium silicate, there is a need to supply 0.07 gram of extra curing water to maintain saturated conditions. (A value of 0.053 for 75% hydration at 28 day was experimentally observed by Powers in 1935).

Quantity of Chemical Shrinkage

Portland cement hydration is typically accompanied by a chemical shrinkage on the order of 0.07 mass of water per mass of cement for complete hydration: for silica fume, slag, and fly ash, these coefficients are about 0.22, 0.18, and 0.10 to 0.16, respectively. It can be measured by ASTM standard test method, C1608

Autogenous Shrinkage

It is as a volume change in concrete occurring without moisture transfer from the environment intoconcrete. It is due to the internal chemical and structural reactions of the concrete. Autogenous shrinkage is prominent in HPCs due to the reduced amount of water and increased amount of various binders used.

At early ages (the first few hours), before the concrete has formed a hardened skeleton, autogenous shrinkage is often due to only chemical shrinkage. At later ages (>1+days), the autogenous shrinkage can also result from self-desiccation since the hardened skeleton resists the chemical shrinkage.

The external (macroscopic) dimensional reduction of the cementitious system under isothermal sealed curing conditions; can be 100 to 1000 micro strains.

Self-desiccation

It is the localized drying resulting from a decreasing relative humidity (RH) which could be the result of the cement requiring extra water for hydration. It is the reduction in the internal relative humidity of a sealed system when empty pores are generated.

Potential of Selfdesiccation Prominent in HPC/ HSC

The finer porosity of HSC/HPC (with a low w/c), causes the water meniscus to have a greater radius of curvature, causing large compressive stress on the pore walls, leading to greater autogenous shrinkage as the paste is pulled inwards. Self–desiccation is only a risk when there is not enough localized water in the paste for the cement to hydrate and it occurs the water is drawn out of the capillary pore spaces between the solid particles. At later ages, a strong correlation exists between internal relative humidity and free autogenous shrinkage.

Mineral admixtures, such as fly ash and silica fume, in concrete tend to refine the pore structure towards a finer microstructure thereby water consumption will be increased and the autogenous shrinkage due to self-desiccation will be increased.

Inter-dependance of Autogenous & Chemical Shrinkages

Chemical shrinkage creates empty pores within hydrating paste and stress generated is stimated by equation:

σcap = 2 *γ / r = - In (RH) * R * T / Vm

where γ,Vm = Surface tension and molar volume of the pore solution,

r = the radius of the largest water-filled pore (or the smallest empty pore),

R = the universal gas constant, and T is the absolute temperature

The sizes of empty pores regulate both internal RH and capillary stresses. These stresses cause a physical autogenous deformation (shrinkage strain) given by:

ε = ( S * σcap/ 3 ) * [ (1/K) – (1/Ks)]

where ε = shrinkage (negative strain), S = degree of saturation (0 to 1) or volume fraction of waterfilled pores, K = bulk modulus of elasticity of the porous material, and Ks = bulk modulus of the solid framework within the porous material.

The above equation is only approximate for a partiallysaturated visco-elastic material such as hydrating cement paste, but still provides insight into the physical mechanism of autogenous shrinkage and the importance of various physical parameters The internal drying is analogous to external drying shrinkage.

Early External Water Curing and Cracks in HPC

Reduction of autogenous shrinkage due to external curing in HPCs is possible for first one or two days when the capillary pores are yet interconnected. Early water curing can lead to higher strain gradients when the skin of the concrete becomes well cured (no shrinkage) whereas, autogenous shrinkage, which is generally difficult to control, begins at the interior of the concrete. These problems can be mitigated by use of a pre-soaked LWA.

Monitoring of Self – curing

This can be done by:
  1. Measuring weight-loss
  2. X-Ray powder diffraction
  3. X-Ray microchromatography
  4. Thermogravimetry (TGA) measurements
  5. Initial surface absorption tests (ISAT)
  6. Compressive strength
  7. Scanning electron microscope (SEM)
  8. Change internal RH with time
  9. Water permeability
  10. NMR spectroscopy
Advantages of Internal Curing
  1. Internal curing (IC) is a method to provide the water to hydrate all the cement, accomplishing what the mixing water alone cannot do. In low w/c ratio mixes (under 0.43 and increasingly those below 0.40) absorptive lightweight aggregate, replacing some of the sand, provides water that is desorbed into the mortar fraction (paste) to be used as additional curing water. The cement, not hydrated by low amount of mixing water, will have more water available to it.
  2. IC provides water to keep the relative humidity (RH) high, keeping self-desiccation from occurring.
  3. IC eliminates largely autogenous shrinkage.
  4. IC maintains the strengths of mortar/concrete at the early age (12 to 72 hrs.) above the level where internally & externally induced strains can cause cracking.
  5. IC can make up for some of the deficiencies of external curing, both human related (critical period when curing is required is the first 12 to 72 hours) and hydration related (because hydration products clog the passageways needed for the fluid curing water to travel to the cement particles thirsting for water). Following factors establish the dynamics of water movement to the unhydrated cement particles:
    1. Thirst for water by the hydrating cement particles is very intense,
    2. Capillary action of the pores in the concrete is very strong, and
    3. Water in the properly distributed particles of LWA (fine) is very fluid.

Concrete Deficiencies that IC can Address

The benefit from IC can be expected when
  • Cracking of concrete provides passageways resulting in deterioration of reinforcing steel,
  • low early-age strength is a problem,
  • permeability or durability must be improved,
  • rheology of concrete mixture, modulus of elasticity of the finished product or durability of high fly-ash concretes are considerations.
  • Need for: reduced construction time, quicker turnaround time in precast plants, lower maintenance cost, greater performance and predictability.
Improvements to Concrete due to Internal Curing
  • Reduces autogenous cracking,
  • largely eliminates autogenous shrinkage,
  • Reduces permeability,
  • Protects reinforcing steel,
  • Increases mortar strength,
  • Increases early age strength sufficient to withstand strain,
  • Provides greater durability,
  • Higher early age (say 3 day) flexural strength
  • Higher early age (say 3 day) compressive strength,
  • Lower turnaround time,
  • Improved rheology
  • Greater utilization of cement,
  • Lower maintenance,
  • use of higher levels of fly ash,
  • higher modulus of elasticity, or
  • through mixture designs, lower modulus
  • sharper edges,
  • greater curing predictability,
  • higher performance,
  • improves contact zone,
  • does not adversely affect finishability,
  • does not adversely affect pumpability,
  • reduces effect of insufficient external curing.

Effect of Particle Size and Content of LWA

Internal curing by saturated lightweight aggregate can eliminate autogenous shrinkage with the smallest possible amount of lightweight aggregate. The grain size of the LWA used as curing agent needs to be reduced in order to minimize the paste– aggregate proximity, i.e. the distance to which the internal curing water should diffuse. The reduction of the grain size (down to 2–4 mm), is shown to be beneficial. However, the further reduction of grain size could result in a decrease of curing efficiency.

The effectiveness of internal curing depends not only on whether there is sufficient water in the LWA, but also on whether it is readily available to the surrounding cement paste as well. Hence, if the distance from some location in the cement paste to the nearest LWA surface is too great, water cannot permeate fully within an acceptable time interval. This distance can be called the paste– aggregate proximity. Alternatively, aggregate distribution can be described by means of aggregate– aggregate proximity, which is the distance between two nearest LWA surfaces, often called spacing. For a given amount of aggregate, the paste–aggregate proximity can be adjusted by the size of the aggregate. The finer the aggregate size, the closer will be the paste– aggregate proximity.

The LWA can be used for internal curing without considerable detrimental effects on strength when added in the amounts just required to eliminate self-desiccation.

“Protected Paste Volume” Concept in Self-curing

For self-curing, besides providing necessary quantity of water inside the matrix, it is essential to ensure the proximity of the cement paste to the surfaces of the source of water so that required high RH is generated around the cement grains for hydration reaction. In this regard, the “protected paste volume” concept is useful to recognise the effective volume of cement paste. For this, the aggregates are represented by impenetrable spherical or ellipsoidal particles and each aggregate particle is surrounded by a soft penetrable shell representing the interfacial transition zone. Instead of the interfacial transition zones, the saturated LWA (fine aggregate) particles surrounded by a shell of variable thickness can be assumed for evaluation. Then, by systematic point sampling, one can determine the volume fraction of paste contained within these shells and hence the relative proximity of the cement paste to the additional water.

Distribution of Internal Water Reservoirs for Curing

The transport distance of water within the concrete is limited by depercolation of the capillary pores in low w/c ratio pastes. With water-reservoirs well distributed within the matrix, shorter distances have to be covered by the curing water and the efficiency of the internal-curing process is consequently improved. The concept of internal curing was established, based on dispersion of very small, saturated LWA throughout the concrete, which serve as tiny reservoirs with sufficient water to compensate for self-desiccation. The spacing between the LWA particles is conveniently small so that the water travels smaller distances to counteract self-desiccation. The amount of water in the LWA can therefore be minimized, thus economising on the content of the LWA.

Travel of Water from Surfaces of LWA

Estimates of travel of internal water from the surface of water reservoir in the concrete matrix are:
  • early hydration — 20 mm
  • middle hydration — 5 mm
  • late hydration — 1 mm or less
  • “worst case” — 0.25 mm (250 ìm)
(Early and middle hydration estimates in agreement with x-ray absorption-based observations on mortars during curing).

Size of pores for Internal Water Storage

Water is held in pores primarily by capillary forces. Only pore sizes above approximately 100 nm are useful for storage of internal curing water. In smaller pores the water is held so tightly that it is not available for the cementitious reactions. Since some of the water absorbed by the LWA in the smaller pores will not be released to the hardening cement paste, an amount of water more than sufficient to counteract selfdesiccation should be absorbed in the LWA. A great quantity of water is in fact entrapped in the internal porosity of the larger particles; one should consider that only about half of it is available for internal curing. In case of smaller fraction, the opposite seems to hold: the absorption is lower, but almost 80 % of the water is lost by 85% RH.

Usefulness of IC in Pavements

The major problem of cracking in pavements may be alleviated by internal curing, besides imparting many potential benefits.

Usefulness of IC for Early-Age Cracking

The IC can influence the ‘Early- Age Cracking Contributors’ which are mainly thermal effects and autogenous shrinkage. During initial ages of concrete, hydration heat can raise concrete temperature significantly (causing expansion), subsequent thermal contraction during cooling can lead to early-age (global or local) cracking if restrained (globally or locally). Another prominent effect would be autogenous shrinkage, especially in concretes with lower water-binder ratios where sufficient curing water cannot be supplied externally, the chemical shrinkage accompanying the hydration reactions will lead to self-desiccation and significant autogenous shrinkage (and possibly cracking).

Pore Sizes in Internal Reservoirs & Capillary Pores

IC distributes the extra curing water throughout the 3-D concrete microstructure so that it is more

readily available to maintain saturation of the cement paste during hydration, avoiding selfdesiccation (in the paste) and reducing autogenous shrinkage. Because the autogenous stresses are inversely proportional to the diameter of the pores being emptied, for IC to do its job, the individual pores in the internal reservoirs should be much larger than the typical sizes of the capillary pores (micrometers) in hydrating cement paste.

Quantifying Effectiveness of IC

IC can be experimentally measured by:
  • Internal RH
  • Autogenous deformation
  • Compressive strength development
  • Degree of hydration
  • Restrained shrinkage or ring tests
  • 3-D X-ray microtomography (Direct observation of e 3-D microstructure of cement-based materials).

Conclusion

The internal curing (IC) by the addition of saturated lightweight fine aggregates is an effective means of drastically reducing autogenous shrinkage. Since autogenous shrinkage is a main contributor to early-age cracking, it is expected that IC would also reduce such cracking. An additional benefit of IC beyond autogenous shrinkage reduction is increase in compressive strength. As internal curing maintains saturated conditions within the hydrating cement paste, the magnitude of internal self-desiccation stresses are reduced and long term hydration is increased. IC is particularly effective for the highperformance concretes containing silica fume and GGBS. In cement mortar containing a Type F fly ash, the fly ash functions mainly as a dilutent at early ages, and higher and coarser porosity at early ages result in less autogenous shrinkage.

The self-desiccation is the reduction in internal relative humidity of a sealed hydrating cement system when empty pores are generated. This occurs when chemical shrinkage takes place at the stage where the paste matrix has developed a self-supportive skeleton, and the chemical shrinkage is larger than the autogenous shrinkage. Effects of self-desiccation depend on the sizes of the generated empty pores. These pore sizes in turn are dependent on the initial waterto- binder ratio (w/b), the particle size distributions of the binder components, and their achieved degree of hydration. The continuing trends towards finer cements and much lower w/b have significantly reduced the capillary pore “diameters” (spacing) in the paste component of the fresh concrete, and have often resulted in materials and structures where the effects of self-desiccation are all to visible as early-age cracking. Many strategies for minimizing the detrimental effects of selfdesiccation (mainly the high internal stresses and strains that may lead to early-age cracking), such as internal curing, rely on providing a “sacrificial” set of larger water-filled pores within the concrete microstructure that will empty first while the smaller pores in the hydrating binder paste will remain saturated. It may be noted that the effects of self-desiccation are not always detrimental, as exemplified by the benefits offered by self-desiccation in terms of an earlier RH reduction for flooring applications and an increased resistance to frost damage.

IC is useful when ‘performance specifications’ are important than ‘prescriptive specifications’ for concrete. Prime applications of IC could be: concrete pavements. precast concrete operations, parking structures, bridges, HPC projects, and architectural concretes. Concrete, in the 21st century, needs to be more controlled by the choice of ingredients rather than by the uncertainties of construction practices and the weather. Instead of curing through external applications of water, concrete quality will be engineered through the incorporation of water absorbed within the internal curing agent.

Acknowledgment

The authors thank Dr. N. Lakshmanan, Director, SERC, Chennai, for permitting to publish this paper.
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NBMCW July 2007