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Self Curing Concrete - A Futuristic Technology

Bharathi Ganesh, Dr. H. Sharada Bai

Curing of concrete is one major area that lacks due attention and due importance in the construction field, especially in India. Durability, quality, performance of concrete structures and its longevity is at stake due to lack of awareness of the importance of curing of concrete. New developments in curing of concrete are on the horizon throughout the world. In the next century, mechanization of the placement, maintenance and removal of curing mats and covers will advance as performance-based specifications quantify curing for acceptance and payment. In addition, effective sealants and compounds that prevent the loss of water and promote moist curing conditions of concrete will be in high demand. Self-curing technology should become available commonly in the not-too-distant future.

Consumption of Water for construction

With the current debate about global warming in the world, the interest to save water and reduce water footprint for goods and services is a critical issue that requires more attention and investigation. Freshwater shortage is becoming more and more a serious problem for the world. Construction Industry is one of the major consumers of world's water resources.  Can the construction industry look at using the available water more judiciously? Or from another angle, can lesser water be used for construction work, without sacrificing design specifications and quality?

The construction sector is very demanding in terms of water use, it was noted that the construction of a 100,000 sq. ft. multi-storey structure can require about 10 million litres of water for production, curing and site development activity. A double lane flyover can consume 70 million litres of water on the same scale. (Ref: How much water should buildings consume- Online edition of India's National Newspaper 'The Hindu' Saturday, Feb 07, 2004). So it is the time now, we need to adapt and adopt newer and newer technologies, that contribute to the sustainability in construction. Internal/self curing of concrete is one such technology.

Curing is the process of controlling the rate and extent of moisture loss from concrete during cement hydration. It may be either after concrete has been placed in position (or during the manufacture of concrete products), thereby providing time for the hydration of cement to occur. Since the hydration of cement does take time  days and even weeks rather than hours, curing must be under taken for a reasonable period of time if the concrete is to achieve its potential strength and durability. Curing may also encompass the control of temperature since this affects the rate at which cement hydrates. The curing period may depend on the properties required of the concrete, the purpose for which concrete is to be used and the ambient conditions, ie the temperature and relative humidity of the surrounding atmosphere.

As may be seen that concrete allowed to dry out without curing achieves only 40% of the strength of the same concrete water cured for the full period of 180 days. Even three days water curing increases this figure to 60%, whilst 28 days water curing increases it to 95%. Keeping concrete moist is therefore, a most effective way of increasing its ultimate strength.

Concrete that is allowed to dry out quickly also undergoes considerable early age drying shrinkage. Inadequate or insufficient curing is one of main factors contributing to weak, powdery surfaces with low abrasion resistance. The durability of concrete is affected by a number of factors including its permeability and absorptivity, result of lack of curing. These are related to the porosity of the concrete and whether the pores and capillaries are discrete or interconnected. Whilst the number and size of the pores and capillaries in cement paste are related directly to its water- cement ratio, they are also related indirectly to the extent of water curing. Long time water curing cause hydration products to fill the pores and capillaries present either partially or completely in turn to reduce the porosity of the paste.

Curing is designed primarily to keep the concrete moist by preventing the loss of moisture from the concrete during the period in which it is gaining strength. Curing may be applied in a number of ways using traditional methods or unconventional methods.

Conventional Curing Methods

The traditional methods of curing generally adopted in the field are: 

• Ponding method - Ponding is a quick, inexpensive and effective form of curing when there is a ready supply of good ‘dam’ material (eg. clay soil), a supply of water. Ponding does not interfere with subsequent building operations.
• Formwork - Leaving formwork in place is often an efficient and effective method of curing concrete.
• Plastic sheeting - Plastic sheets or other similar materials form an effective barrier against water loss provided they are kept securely in place and are protected from damage. Their effectiveness is very much reduced if they are not kept securely in place. The movement of forced draughts under the sheeting must be prevented when plastic sheets are used.
• Water Curing - General Water curing is carried out by supplying water to the surface of concrete in a way that ensures that it is kept continuously moist. (Spraying or sprinkling water).
• Fog curing - Fine spray or fog of water can be used an efficient method of supplying additional moisture for curing during hot weather, helps to reduce the temperature of the concrete. 

Sprinkling of water at regular intervals for vertical Surfaces

Curing by Ponding

Ponding method of water curing

Wet coverings - Fabrics such as hessian or materials such as sand can be used like a 'mulch' to maintain water on the surface of the concrete. On flat areas, fabrics may need to be weighed down. Also, it is important to see that the whole area is covered. Wet coverings should be placed as soon as the concrete has hardened sufficiently to prevent surface damage.

Wet coverings & Formwork

The most appropriate means of curing may be dictated by the site conditions or the construction method. The choice of method depends on the structural member being cured, as each method offers its own advantages. However, Curing is one major area that lacks due attention and due importance in the field, especially in India! An alternative solution to overcome this is to adopt of unconventional methods for curing of concrete.

Plastic sheeting

Membrane Forming Curing Compounds

Unconventional Curing Methods

These are long awaited engineering techniques that make the post construction processes and maintenance much simpler and easier. They are much needed techniques and to be adopted in field, when,

• The  quality water resource available for construction is less
• Cost of obtaining water with desired quality is expensive
• Hot weather regions where large quantity of water is needed for curing
• At all the sites where curing is given least importance and curing is ignored which make the structure suffer in its quality and strength parameters.
• Membrane Forming Curing Compounds - Curing membrane is an isolating barrier for reducing the water evaporation from the exposed surface of concrete, which inhibits and delays water evaporation from the surface. Curing compounds are liquids which are usually sprayed directly onto concrete surfaces and which then dry to form a relatively impermeable membrane that retards the loss of moisture from the concrete. Their properties and use are described in AS 3799 Liquid Membrane forming curing compounds for concrete.
• Self Curing of Concrete - Self curing is a curing system, introduced during 1990’s with the introduction of chemical admixture which can reduce water evaporation in the set concrete, providing water internally resulting in 'self curing'. Hence the name Self Curing Concrete'. In essence, the existing curing techniques require 'external actions' involving curing concrete from 'outside in'. On the other hand 'internal curing' is the one where curing is from 'inside out' (C. S. Viswanatha 2008).

Of the two methods, the former has been in practice for over a decade because of its simplicity of implementation. However Self Curing Concrete is an interesting concept and is yet to be successfully adopted in the field. Here it is intended to present few aspects of Self curing concrete.

Self Curing or Internal Curing

Self curing of concrete takes place by means of internal curing where "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." - Definition as per ACI-308 Code.

Hydration of cement is aided by additional internal water that is not part of the mixing water, supplied within the concrete matrix through Super Absorbent Polymers (SAP) or Partial replacement of fine aggregate with structural lightweight aggregate (fine and coarse) of high absorption capacity that are incorporated into the concrete while mixing as an admixture. They inhibit moisture loss and thereby improve long term strength and reduce drying shrinkage. They have been used in tunnel linings and underground mines to provide at least partial curing when traditional methods are difficult or even impossible to employ. Internal curing compounds are technology yet to get popularized and care should be taken when utilized.

Materials used for self curing effect

Internal water curing or self curing could be achieved using one of the following materials or one or two in combination

• Super Absorbent Polymers (SAP) (60-300 mm size) - Sodium salts of poly-acrylic acid, polyacrylamide copol., ethylene maleic anhydride copol., cross-linked carboxy- methyl-cellulose, polyvinyl alcohol copol, etc.,
• Natural or Synthetic Light Weight Aggregate Fines, expanded shale with higher water absorption capacity.
• Coarse aggregate of nominal maximum size and gradation
• Shrinkage Reducing Admixture - SRA polyethylene-glycol.
• Saturated Wood powder / fibres

Super Absorbent Polymers (SAP) - Process of water evaporation from within and also through the surface of concrete can be reduced using some specific water-soluble polymers added during mixing of concrete, which can also improve water retention in ordinary Portland cement matrix. Various types of polymers are used in the construction industry depending on the requirement of the properties in the finished application, for E.g. alteration of the initial setting time (delaying setting time or rapid setting time), increased tensile adhesion strength, higher pot life or alteration of compressive strength among others, which can be introduced with different polymer composition. These polymers, depending on the chemistry can be water soluble polymers or semi soluble. All water soluble polymers are not water retentive polymers.

Polymers used as self curing chemical are cross-linked polymers, made by solution or by suspension polymerization (cost effective) which can absorb and retain large amount of water called Super Absorbent Polymers (SAP). They act as water reservoirs to release water gradually, supplement the water loss and increase the water retention of concrete compared to conventional concrete.

SAPs are a group of polymeric materials that have the ability to absorb a significant amount of liquid from the surroundings (water uptake) through hydrogen bonding with the water molecule and to retain the liquid within their structure without dissolving. SAPs can be produced with water absorption of up to 5000 times their own weight. They can be produced by either solution or suspension polymerization and the particles may be prepared in different sizes and shapes including spherical particles.

Cross - linked Polyelectrolytes

Because of their ionic nature and interconnected structure, they can absorb large quantities of water without dissolving. The commercially important SAPs are covalently cross-linked polyacrylates and copolymerized polyacrylamides / polyacrylates. They are Acrylamide/acrylic acid copolymers

Rheometer and Continuous Shear Rate Test results.

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.

Mechanism of SAP swelling - Following schematic diagram explains Mechanism of SAP swelling.

Hydrophilic water soluble polymers absorb water and keep surrounding medium moist. The thickening effect caused by SAP can be used with advantages for some practical situations like pumping of concrete, particularly for shotcreting of wet mix. The yield stress and plastic viscosity of cementitious system can be studied if the Rheometer is used to measure the rheology of concrete.

Mechanism of SAP swelling

Super Absorbent Polymers - before water absorption and after water absorption

Collapsed Phase and Swollen phase are the two distinct states of SAPs. Repulsive force helps in expansion of SAP and elastic free energy opposes swelling of the SAP by a retractive force. There is a competitive balance between the repulsive forces that act to expand the polymer network and attractive forces that act to shrink the network which results in phase transition. Observation of SAP swelling in concrete indicates that total absorption is about half the absorption of synthetic fluid which in turn is several times the absorption of SAP in water.

One type of SAPs are suspension polymerized, spherical particles with an average particle size of approximately 200 µm, where as another type of SAP is solution polymerized and then crushed and sieved to particle sizes in the range of 125 - 250 µm.  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.

Effect of time - process of Absorption and release of Water in SAP

Dosage of SAP in concrete

Max. water absorption capacity of SAP is 1500 g/g , however commercial SAP used in construction industry are of  the capacity of the capacity of 50 - 300 g/g , generally SAPs are added at rate of (0 - 0.6) % wt of cement. Large particles may be broken by mixing and filling with hydration products of small SAP, as large particles take more time for uptake of water.

Super absorbent polymer Particle - before & after water absorption

Super absorbent polymer Particle - Different Size as per requirement in concrete

Quantifying Effectiveness of SAP of Internal Curing Agents

Various technologies are used to study the effectiveness of using internal curing materials, admixtures / SAP in fresh and hardened state of concrete. They are measured experimentally using the following methods.

• Internal RH - Change internal RH with time
• Degree of hydration
• Measuring weight-loss
• Autogenous deformation
• Scanning electron microscope (SEM)
• Compressive strength development
• Restrained shrinkage or ring tests
• 3-D X-ray micro tomography (Direct observation of e 3-D microstructure of cement-based materials).
• X-Ray powder diffraction
• Thermogravimetry (TGA) measurements
• Initial surface absorption tests (ISAT)
• Water permeability
• NMR spectroscopy

Hydration and Water transport

Water released from the SAP has function to continue hydration of cement further in a sealed condition. The rate of change of degree of hydration depends on several material parameters such as size and amount of SAP in the mixtures, original water cement ratio, procedure of how the SAP is added during mixing of mortar and concrete.

source: Jensen and Hansen CCR 2001, 2002

Pore Structure (STAR 225- SAP)

Compared to normal concrete, the presence of SAP certainly changes the microstructure of concrete. The main reasons for SAP to change the microstructure of concrete especially the pore structure of concrete containing SAP are likely to be,

• When SAP is fully filled with water it act as a soft aggregate and when it is empty it acts as an air void in the concrete.
• Non uniformity of dispersion of SAP during mixing.
• The mechanism of water uptake of SAP and release of water at later stages changes the effective w/c ratio in the early stages of hydration and water release from SAP affects further hydration process.
• Interface between SAP and cement paste matrix may further induce some additional pores.

Suspension-polymerized SAP

The effect of SAP on the pore structure of concrete is not only on its total porosity but also on its pore size and pore size distribution. The increased hydration degree of cement paste results in reduction of capillary porosity in the matrix. And also void introduced by the SAP in the mixture is homogeneously distributed, when SAP is distributed evenly throughout the concrete mixture.

Closed Vapour Transport System	Open Vapour Transport System

Vapour Transport Mechanism (R. K. Dhir 1994)

When a pure liquid A is held in a container a certain quantity of molecules of A will evaporate until the vapour pressure is such that the liquid and vapour phases are in thermodynamic equilibrium. Equilibrium is achieved when the chemical potentials (free energy) of these two phases are equal. Where as when the closed system is opened, the vapour disperses. Continuous evaporation takes place in an attempt to reach equilibrium.

Microstructure of concrete with SAP (R. K. Dhir 1996)

Calcium hydroxide at cement-aggregate interface in control mortar: (a) viewed side-on; (b) viewed from above.

Calcium hydroxide at cement-aggregate interface in mortar containing 0.10M of chemical: (a) viewed side-on; (b) viewed from above.

 (a) and (b) - CSH in 28-day mortar specimen cured in air (control).

 (a) and (b) - CSH in 28-day mortar specimen cured in air (0.10M chemical addition).

Influence of microstructure on the physical properties of Self Curing Concrete is presented in this study. During the development of self-curing concrete it was found that one particular self-curing admixture used produces a number of effects with respect to particular physical properties and powder x-ray diffraction characteristics.

The paper attempts to explain these observations at a micro structural level. Two computer models were used to illustrate the influence the admixture was thought to have on hydrated cement microstructure. At low dosages, good strength and improved permeability characteristics were observed. At high dosages, it appeared that the admixture has a detrimental effect on the concrete’s compressive strength due to an alteration of the nature of calcium hydroxide at the cement-aggregate interface. However, it seemed that at the same dosages the CSH gel structure was altered beneficially, producing a highly impermeable concrete. It is suggested that although a lowering of strength does occur at high dosage, a much lower permeability for a given strength could be obtained.

This paper concludes that there is evidence that the chemical is altering CSH gel morphology also, possibly by growth inhibition. This appears to enhance the nature of the CSH gel, leading to better permeability characteristics. Despite a reduction in strength at high dosages, it can be concluded that a lower concrete permeability for a given strength can be obtained using this chemical. The chemical dealt with in this report has the effect of inhibiting growth of calcium hydroxide crystals. The change in CH morphology that results can be detrimental to concrete strengths at high admixture concentrations.

Effect of SAP on strength characteristics of concrete and mortar

Introduction of SAP in to concrete mixture results often in certain strength reduction at early ages. Slight strength reduction is observed at the early age of few days of curing. However the correct dosage of SAP and the internal curing water and also optimum technological procedure of mixing casting and curing help to overcome the strength reduction.

However some studies report that after 28 days the strength may be even exceed that of reference concrete mixture. One of the possible reasons of the enhancement of concrete containing SAP with age may be the improvement of degree of hydration of cement grains, which is achieved by internal curing. In addition, cement hydration product at later stages grow inside the original grain boundaries of SAP particles contributing to the strength of mature system. In case of HPC, usage of SAP is mainly to mitigate self desiccation and autogenous shrinkage without impairing the concrete strength substantially.

In case of mortar, introduction of SAP as internal curing agent has a little influence on the strength of high performance mortars, while the strength of cement paste was reduced more severely. The different effect of SAP introduction in cement paste and mortar can be explained by different largest defect size present in the solid mortar of these materials. The tensile strength and Elastic modulus of concrete with SAP are expected to increase as well when care is taken to mix the SAP properly in concrete, however it does not happen always.

For Mortar

Equivalent Hydrostatic Pressure development in Mortar

Figure shows the equivalent hydrostatic pressure developed in the ASG by mortars with different amounts of SAP type A and consequently, different amount of entrained water. SAP additions are given by weight of cement, where o.6% corresponds to an entrained w/c ratio of 0.075. Basic w/c ratio is 0.3 for all mixes. Temperature is 20⁰C.

Different Curing Conditions - There are not many publications where different curing conditions like air curing, moisture curing and water curing are used to compare the behaviour / properties of same mix of concrete. Sealed curing is used for autogenous shrinkage where as air curing or moist curing is adopted for strength studies. The table summarizes the effect on strength of concrete with SAP addition, determined under different curing conditions (S, M and D ) at 28 days only.

Table 1 - Effect of SAP on concrete strength at 28 days under different curing conditions (Summary from different papers - Source - STAR 225 - SAP)

Sl. No	Moist Curing (M)	Dry Curing(D)	Sealed Curing(C)
1	Compressive strength decreased by (16 - 31)%, depending on the SAP content
2	Compressive strength decreased by (28 - 35)%,	Compressive strength decreased by (26 - 28)%,
3			Compressive strength decreased by (3-22)%, Tensile strength decreased by (11-33)%, depending on the SAP content
4	Compressive strength decreased by (8 -14)%,
5			10% reduction of compressive strength in pastes, no influence of SAP addition in mortars
6		Compressive strength decreased by 2% (cubes) 13% (prisms), uniaxial strength by 19% and flexural strength by 25%	Compressive strength decreased by 10% (cubes) 13% (prisms), uniaxial strength by 7% and flexural strength by 31%
7	Compressive strength decreased by 30%	Compressive strength decreased by 20%
8			Compressive strength decreased by 10% (cubes) 13% (prisms), uniaxial strength by 7% and flexural strength by 31%
9			Compressive strength decreased by 36% in cubes (12-30)% in prisms, depending on the SAP content
10			Almost no compressive strength reduction and slight increase in flexural strength in SAP mixes Slight increase (3-4)% in compressive strength

Durability of Self Cured Concrete

Effect of internal curing in concrete on its durability parameters - Dhir et al in their paper reported the results of several durability tests conducted on self-cure concrete specimens. It was found that initial surface absorption (ISAT), chloride ingress, carbonation, corrosion potential and freeze/thaw resistance characteristics were all better in air cured self- cure concrete than in the air cured control concrete. This improvement appears to be dependent on the admixture dosage, although the durability properties obtained in the study were not as good as the film cured concrete. It may be possible to achieve such properties with higher quantities of self-cure chemical. Concrete that is capable of retaining greater quantities of water than ordinary concrete when cured in air has been developed by means of addition of a self curing chemical which was a water-soluble polymeric glycol identified as the chemical. The water retention leads to a greater degree of cement hydration and hence improved properties of concrete in comparison to control test specimens. One particular feature of self-cure concrete is its good durability properties.

Shrinkage of Concrete

Volume changes in concrete due to shrinkage are main sources of Eigenstresses and stresses due to the restraint in concrete structure that leads to many instances of cracking. Four types of shrinkages generally considered as a result of this are

1. Plastic Shrinkage occurring during first few hours after mixing when concrete still behaves as a formable mass.
2. Autogenous shrinkage   of hardening concrete as a result of hardening process.
3. Drying Shrinkage due to loss of water caused by surrounding atmosphere.
4. Carbonation shrinkage resulting from carbonation of concrete.

Autogenous shrinkage typically defined as bulk deformation of closed (sealed) isothermal cementitious material system not subjected to external curing. Autogenous shrinkage of normal cured concrete is slightly smaller when compared to drying shrinkage.

SAP for Mitigation of Shrinkage

Addition of SAP often in conjunction with extra water influence not only autogenous shrinkage but also other types of volumetric changes and development of stresses due to restrained shrinkage. Application of SAP as a means of internal curing agent mitigates autogenous shrinkage. Advantages of SAP in concrete is that they can be in principle engineered for special purpose of internal curing by determining the necessary particle size and shape, water absorption capacity and other properties.

Literatures on chemical shrinkage are scanty. 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.

Restrained and Free Autogenous Shrinkage Measurements

Figure suggested schematic effect of SAP particle size on reduction of autogenous shrinkage. The suggested optimum size, about 100µm, refers to the swollen state.

Effect of Particle Size on reduction in Autogenous shrinkage

Light Weight Aggregates (LWA) for Mitigation of Shrinkage

The application for the concept of internal curing by means of saturated lightweight aggregate was studied and applied in HPC which proves to be effective in eliminating autogenous shrinkage. High performance concretes (HPC) with extremely low water to binder (w/b) ratios are characterized by high-cracking sensitivity, which is a consequence of increased autogenous shrinkage. The major reason for autogenous shrinkage - self-desiccation cannot be eliminated by traditional curing methods. Zhutovsky et al. in their paper described an approach to optimize the size and porosity of the lightweight aggregate to obtain effective internal curing with a minimum content of such aggregate. In this study the primary emphasis was placed upon an investigation of the effects of the replacement level of normal weight coarse aggregates by saturated lightweight ones, and the degree of water saturation of lightweight aggregate. These parameters provided the means to control the effectiveness of autogenous curing.

Several factors affect to counteract self-desiccation and therefore, autogenous shrinkage are (i) aggregate pore size: If it is very fine, water may not migrate readily into the surrounding paste. (ii) The spacing between the aggregate particles: if it is too large, the paste surrounding the aggregates may not be accessible to the water in the aggregate within a reasonable time.

Crushed pumice	Expanded shale

Initial Surface Absorption Test (ISAT)

ISAT values obtained from concrete test specimens were compared with those of air cured control specimens (Fig. 20). The surface of the air cured control specimens absorbs water at a highest rate and the least permeable surfaces are those of the time cured specimens. The higher dosage of the self-cure chemical provided a greater improvement in surface characteristics, but at both concentrations the chemical decreases the rate of absorption at the surface.

10 Minute Initial Surface Absorption Test Results

Literatures also summarize that the  surface quality, chloride diffusion, carbonation, corrosion potential and freeze thaw resistance of self-cure concrete provides improved performance when compared to air cured specimens. The improvements in concrete durability properties are dependent on dosage of SAP in it. It is conceivable that higher dosages could produce air-cured concrete with properties rivalling those achieved in the film-cured situation.

Measurement of RH of concrete specimen

Sorptivity Measurement of concrete specimen

Internal Curing - Merits

Internal curing  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 (fine)LWA is very fluid.

Benefits of using SAP in concrete

SAP in concrete is advantageous, as it

• Reduces autogenous cracking resulting from  autogenous shrinkage
• Protects concrete and reinforcing steel through reduced permeability
• Reduces post construction process of curing and provides higher performance and greater durability.
• Improves rheology without affecting Pumpability and finishability adversely
• Higher degree of finishing of structural elements and less maintenance requirement.
• Lower turnaround time and labour requirement at site.
• Improved rheology improves contact zone, and results in lower maintenance,
• Greater utilization of cement and use of higher levels of fly ash.

Potential benefits of using LWA in concrete

• Better thermal properties;
• Better fire resistance;
• Improved skid-resistance;
• Reduced autogenous shrinkage;
• Reduced chloride ion penetrability;
• Improved freezing and thawing durability;
• An improved contact zone between aggregate and cement matrix; and,
• Less micro-cracking as a result of better elastic compatibility.

First SAP application in practice

Usage of internal curing material refers back to Roman times. Pantheon, Rome, 126 A.D. was constructed with pozzolanic cement and pumice stone for internal curing. First SAP application was in 2006 used for the construction of Pavilion of Olympic stadium in Kaiserslautern for FIFA World Cup (Ref: Mechtcherine et al. 2006)

Early example of internal curing - Pantheon, Rome, 126 A.D. pozzolanic cement and pumice stone

Application Internal Curing or Self Curing materials / compounds- From the literatures, it is clear that the internal curing is an effective means for drastically reducing autogenous shrinkage. Internal curing is useful when ‘performance specifications' are important than 'prescriptive specifications' for concrete. Current application of using SAP/LWA in concrete structures have been included for shrinkage reduction, frost protection, rheology modification, water proofing and fire protection to name a few. Hence self curing or internal curing concept is a futuristic technology, used for technology modifications and large applications like shortcreting, backfilling, self compaction, low autogenous shrinkage, thin walled sections and enhanced ductility of HPC members and for high durability.

FIFA World Cup Pavilion in Kaiserslautern( Ref:Mechtcherine et al. 2006)

The areas of prime applications of internal curing may 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. (Rajamane, N. P 2006).

Other uses SAP are in diapers, for blocking water penetration in cables, water retention for plants, control of spill and waste fluids, artificial snow, candles, etc.,

Conclusion

Challenge of effective application of this technology lies in selection of right type of curing agent/ absorbent / light weight aggregate fines suitable to site conditions and type of concrete and other factors. The desired result of internal curing can be achieved through

• Proper mix design procedure / mix proportioning technique for internal curing (with polymer).
• Its effectiveness in tropical atmosphere condition
• Its effectiveness for ordinary strength concrete with high w/c ratio

There are clearly many opportunities to use this technology in various functions to improve the performance and durability of the built environment. It is expected that future applications will increasingly move towards the use of self curing technology in the concrete sector construction, as this technology becomes better known through good practice and evidence of good performance records.

Until behaviour of admixtures and their effects on other properties of concrete are readily understood, use of self curing agents remains relatively sparse. With significant result yielding research contributions in to this area and reduction in the cost involved in using these self curing compounds, self curing is an assured futuristic technology which is going to gain popularity and to continue to prevail in the industry for long.

Acknowledgment

Authors wish to acknowledge that materials are drawn from the following references mainly from the papers of Prof. Ravindra K. Dhir, Dr. C. S. Viswanatha and Dr. Rajamane N P along with other references.

References

1. Ambily P.S, Scientist, and Rajamane N P, Self Curing Concrete An Introduction, RILEM State of art Reports, http://nbmcw.com
2. Curing of Concrete – Cement Concrete and Aggregate Australia, 2006.
3. Dhir R K, Hewlett P C and Dyer T D, "Influence of microstructure on the physical properties of self curing concrete, ACI material Journal, Vol 93, No.5, 1996, pp 465-471.
4. Dhir R K, Hewlett P C and Dyer T D, "Durability of Self cure concrete, Cement and Concrete Research, Vol 25, No.6, 1995, pp 1153-1158.
5. Dhir, R. K.; Hewlett, P. C.; Lota, J. S.; and Dyer, T. D., "Investigation into the Feasibility of Formulating ‘Self-Curing’ Concrete," Materials and Structures, V. 27, 1994, pp. 606-615.
6. Dhir, R.K. Hewlett, P.C. Dyer, T.D., "Mechanisms of water retention in cement pastes containing a self-curing agent," Magazine of Concrete Research, Vol No 50, Issue No 1, 1998, pp. 85-90.
7. Self-curing concrete: Water retention, hydration and moisture transportConstruction and Building Materials, Volume 21, Issue 6, June 2007, Pages 1282-1287
8. Lecture - 6B, Internal Curing, EMPA, Shrinkage and Cracking of Concrete: Mechanisms and Impact on Durability, ETHZ, 26.10.11.
9. Ravindra K. Dhir, Peter C. Hewlett, and Thomas D. Dyer , Influence of Microstructure on the Physical Properties of Self-Curing Concrete, ACI Materials Journal, V. 93, No. 5, September-October 1996.
10. Kung-Chung Hsu and Jing-Siang Jhuang, Effect of a Hydrogel on the self Curing Properties of Mortars.
11. STAR 225 - SAP, Application of SAP in concrete Constructions, published by Springer, http://www.springer.com .
12. Tarun R. Naik, Fethullah Canpolat, SELF-CURING CONCRETE, Report No. CBU-2006-11 REP-610, April 2006, A CBU Report.
13. Viswanatha C S. ‘Self Curing Concrete - Recent Developments, Proceedings of National Seminar on Engineered Concrete, 2008.
14. Chella Gifta C., Prabavathy S. and Yuvaraj Kumar G. Study on internal curing of High Performance concrete using super absorbent polymers and light weight aggregates, Asian journal of Civil Engineering. Vol.No.. 14, NO. 5 (2013) pp. 773-781

Authors

Bharathi Ganesh is an Associate Professor in the Department of Civil Engineering, Global Academy of Technology, Bangalore, and Karnataka, India. She has 2 years of industrial experience and nearly 20 years of Teaching experience. Her research interest includes Self Compacting Concrete, Fresh and Hardened Properties of Concrete, Fly Ash and Pond Ash in Concrete. She is a Life Member of profession bodies ICI, ACCE, INSDAG, IASE, IWSF and INSTRUCT.

Dr. H. Sharada Bai is a Professor in the Department Civil Engineering, UVCE, Bangalore University, Bangalore, India. She has more than 30 years of Teaching Experience and is also a Design and Proof Checking Consultant. Her areas of interest include self compacting concrete, use of alternative materials in concrete, analysis and Design of Structures, and evaluation of structural systems. She is a Life Member of profession bodies- ICI, ACCE, INSDAG, and ISTE.