Self Curing Concrete An Introduction
Ambily P.S, Scientist, andRajamane N P, Deputy Director and
Head, Concrete Composites Lab Structural Engineering Research
Centre, CSIR, ChennaiExcessive 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 Selfcuring.Need for
SelfcuringWhen 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 ICThe
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 powderChemicals to Achieve SelfcuringSome
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
ICThe common SAPs are added at rate of 00.6 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 125250 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 Selfcuring Using LWAWater/moisture required for internal
curing can be supplied by incorporation of saturated-surfacedry
(SSD) lightweight fine aggregates (LWA).Water Available from LWA
for SelfcuringIt 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 CuringAbout 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 24 mm are found to be beneficial.Utility of LWA Near
Surface of ConcreteAt 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 ShrinkageAs 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
CuringCrushed 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 SelfcuringIt depends
upon chemical and autogenous shrinkages expected during hydration
reactions.Types of Shrinkage DryingShrinkages 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 ShrinkageChemical
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
ShrinkagePortland 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,
C1608Autogenous ShrinkageIt 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-desiccationIt 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/ HSCThe 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. Selfdesiccation 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
ShrinkagesChemical shrinkage creates empty pores within hydrating
paste and stress generated is stimated by equation:
cap= 2 * / r = - In (RH) * R * T / Vm
where ,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:
= ( S * cap/ 3 ) * [ (1/K) (1/Ks)]
where = 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 HPCReduction 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 curingThis can be done by:i.
Measuring weight-lossii. X-Ray powder diffractioniii. X-Ray
microchromatographyiv. Thermogravimetry (TGA) measurementsv.
Initial surface absorption tests (ISAT)vi. Compressive strengthvii.
Scanning electron microscope (SEM)viii. Change internal RH with
timeix. Water permeabilityx. NMR spectroscopyAdvantages of Internal
Curinga. 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.b. IC provides water to keep the relative humidity
(RH) high, keeping self-desiccation from occurring.c. IC eliminates
largely autogenous shrinkage.d. 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.e. 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:i. Thirst for water by the hydrating
cement particles is very intense,ii. Capillary action of the pores
in the concrete is very strong, andiii. Water in the properly
distributed particles of LWA (fine) is very fluid.Concrete
Deficiencies that IC can AddressThe 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 LWAInternal 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 24 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 pasteaggregate
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-curingFor 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 CuringThe 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 LWAEstimates 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 m)(Early and middle hydration estimates in agreement with
x-ray absorption-based observations on mortars during curing).Size
of pores for Internal Water StorageWater 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 PavementsThe major
problem of cracking in pavements may be alleviated by internal
curing, besides imparting many potential benefits.Usefulness of IC
for Early-Age CrackingThe 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 PoresIC 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 ICIC 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).ConclusionThe 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.AcknowledgmentThe authors thank Dr. N. Lakshmanan,
Director, SERC, Chennai, for permitting to publish this
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