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
Exce Excess ssiv ive e evap evapor orat atio ion n of wate waterr (int (inter erna nall or exte extern rnal al)) from from fres fresh h conc co ncre rete te shou should ld be avo avoided ided;; ot othe herw rwis ise, e, the the degr degree ee of ce ceme ment nt hydr hydrat atio ion n woul would d get get lowe lowere red d and and ther thereb eby y co conc ncre rete te may may deve develo lop p unsa unsati tisf sfac acto tory ry prop proper erti ties es..
Curi Curing ng oper operat atio ions ns shou should ld ensu ensure re that that
adequate amount of water is available for cement hydration to occur. his 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-!" Code states that #internal curing re$ers to the process by %hich the hydration o$ cement occurs because o$ the a&ailability o$ additional internal %ater that that is not not part part o$ the mi'in mi'ing g (ater (ater))* Con& Con&ent entio ional nally ly,, curin curing g concr concrete ete me means ans creating conditions such that %ater is not lost $rom the sur$ace i)e), curing is ta+en to happen $rom the outside to inside) In contrast, internal curing is allo%ing $or curing $rom the inside to outside through the internal reser&oirs .in the $orm o$ saturated light%eight $ine aggregates, superabsorbent superabsorbent polymers, or saturated %ood $ibers/ Created) Internal curing is o$ten also re$erred as Sel$0curing) Need for Self–curing
(hen the mineral admi'tures react completely in a blended cement system, their demand $or curing %ater .e'ternal or internal/ can be much greater than that in a
con&entional ordinary 1ortland cement concrete) (hen this %ater is not readily a&ailable, due to depercolation o$ the capillary porosity, $or e'ample, signi$icant autogenous de$ormation and .early-age/ crac+ing may result) Due to the chemical shrin+age occurring during cement hydration, empty pores are created %ithin the cement paste, leading to a reduction in its internal relati&e humidity and also to shrin+age %hich may cause early-age crac+ing) This situation is intensi$ied in H1C .compared to con&entional concrete/ due to its generally higher cement content, reduced %ater2cement .%2 c/ ratio and the po33olanic mineral admi'tures .$ly ash, silica $ume/) The empty pores created during sel$-desiccation induce shrin+age stresses and also in$luence the +inetics o$ cement hydration process, limiting the $inal degree o$ hydration) The strength achie&ed by IC could be more than that possible under saturated curing conditions) 4$ten specially in H1C, it is not easily possible to pro&ide curing %ater $rom the top sur$ace at the rate re5uired to satis$y the ongoing chemical shrin+age, due to the e'tremely lo% permeabilities o$ten achie&ed) Potential Material for IC
The $ollo%ing materials can pro&ide internal %ater reser&oirs6 •
Light%eight Aggregate .natural and synthetic, e'panded shale/,
•
L(S Sand .(ater absorption 789 :/
•
L(A 8;mm Coarse .(ater absorption 7
•
Super-absorbent 1olymers .SA1/ .=!-!! mm si3e/
•
•
SRA .Shrin+age Reducing polyethylene-glycol/
Admi'ture/
.propylene
glycol
type
i)e)
(ood po%der
C!emical to Ac!ie"e Self–curing
Some speci$ic %ater-soluble chemicals added during the mi'ing can reduce %ater e&aporation $rom and %ithin the set concrete, ma+ing it sel$-curing) The chemicals should ha&e abilities to reduce e&aporation $rom solution and to impro&e %ater retention in ordinary 1ortland cement matri') Su#er$aborbent Polymer (SAP) for IC
The common SA1s are added at rate o$ !0!)= %t : o$ cement) The SA1s are co&alently cross-lin+ed) They are Acrylamide2acrylic acid copolymers) 4ne type o$ SA1s are suspension polymeri3ed, spherical particles %ith an a&erage particle si3e o$ appro'imately another type o$ SA1 is solutionpolymeri3ed and then crushed and sie&ed to particle si3es in the range o$ 8 about ;?: o$ the SA1 %orld production is used as a urine absorber in disposable diapers) SA1s can be produced %ith %ater absorption o$ up to ?!!! times their o%n %eight) Ho%e&er, in dilute salt solutions, the absorbency o$ commercially produced SA1s is around ?! g2g) They can be produced by either solution or suspension polymeri3ation, and the particles may be prepared in di$$erent si3es and shapes including spherical particles) The commercially important SA1s are co&alently cross-lin+ed polyacrylates and copolymeri3ed polyacrylamides2 polyacrylates) @ecause o$ their ionic nature and interconnected structure, they can absorb large 5uantities o$ %ater %ithout dissol&ing) rom a chemical point o$ &ie%, all the %ater inside a SA1 can essentially be considered as bul+ %ater) SA1s e'ist in t%o distinct phase states, collapsed and s%ollen) The phase transition is a result o$ a competiti&e balance bet%een repulsi&e $orces that act to e'pand the polymer net%or+ and attracti&e $orces that act to shrin+ the net%or+) The macromolecular matri' o$ a SA1 is a polyelectrolyte, i)e), a polymer %ith ionisable groups that can dissociate in solution, lea&ing ions o$ one sign bound to the chain and counter-ions in solution) or this reason, a high concentration o$ ions e'ists inside the SA1 leading to a %ater $lo% into the SA1 due to osmosis) Another $actor contributing to increase the s%elling is %ater sol&ation o$ hydrophilic groups present along the polymer chain) Elastic $ree energy opposes s%elling o$ the SA1 by a retracti&e $orce) SA1s e'ist in t%o distinct phase states, collapsed and s%ollen) The phase transition is a result o$ a competiti&e balance bet%een repulsi&e $orces that act to e'pand the polymer net%or+ and attracti&e $orces that act to shrin+ the net%or+) Mean of Pro"iding %ater for Self–curing &ing '%A
(ater2moisture re5uired $or internal curing can be supplied by incorporation o$ saturated-sur$acedry .SSD/ light%eight $ine aggregates .L(A/)
%ater A"ailable from '%A for Self–curing
It is estimated by measuring desorption o$ the L(A in SSD condition a$ter e'posed to a salt solution o$ potassium nitrate .e5uilibrium RH o$ ;:/) The total absorption capacity o$ the L(A can be measured by drying a Saturated Sur$ace Dry .SSD/ sample in a dessicator) %ater in '%A for Internal Curing
About =9: o$ the %ater absorbed in the L(A can get transported to sel$-desiccating paste) Some %ater remains al%ays in the L(A in the high RH range and it becomes use$ul %hen the o&erall RH humidity in concrete is signi$icantly reduced) The %ater retained in L(A in air-dry condition may not be enough to pre&ent autogenous shrin+age %hose magnitude, ho%e&er, may be reduced signi$icantly) The $ine light%eight aggregate, in saturated condition, produce a more uni$orm distribution o$ the %ater needed $or curing throughout the microstructure) The grain si3e o$ the L(A used as curing agent should be less in order to minimise the paste0 aggregate pro'imity, i)e) the distance to %hich the internal curing %ater could di$$use) The grain si3e o$ do%n to <0B mm are $ound to be bene$icial) &tility of '%A Near Surface of Concrete
At the sur$ace o$ the concrete, as the %ater e&aporates $rom the concrete sur$ace, a humidity gradient de&elops) This accelerates the appearance o$ the locali3ed humidity gradients) The %ater $rom the L(A near the sur$ace is then used up $aster than in the interior o$ the concrete thus causing the near-sur$ace layer o$ the concrete to become denser in a shorter time) This helps reduce the amount o$ %ater that %ould normally e&aporate and contributes to impro&e internal curing o$ the concrete) It also leads to reduced or no stresses due to drying helping in eliminating the sur$ace crac+ing) Potential of '%A for Reducing Autogenou S!rinage
As the cement hydrates, the %ater %ill be dra%n $rom the relati&ely #large* pores in the L(A into the much smaller ones in the cement paste) This %ill minimise the de&elopment o$ autogenous shrin+age as the shrin+age stress is controlled by the si3e o$ the empty pores, &ia the el&inLaplace e5uation) The radii o$ capillary pores $ormed during hydration in the cement paste are smaller than the pores o$ the L(A) (hen the RH decreases .due to hydration and drying/, a humidity gradient de&elops> %ith the L(A acting as a %ater reser&oir, the pores o$ the cement paste absorb %ater $rom the L(A by capillary suction) The unhydrated
cement particles $rom the cement paste no% ha&e more $ree-%ater a&ailable $or hydration and ne% hydration products gro% in the pores o$ the cement paste thus causing them to become smaller) The capillary suction, %hich is the in&erse to the s5uare o$ the pore radius, increases as the radius becomes smaller and thus enabling the pores to continue to absorb %ater $rom the L(A) This continues until most o$ the %ater $rom the L(A has been transported to the cement paste) Cru!ed '%A for Internal Curing
Crushed L(A could pro&ide a better sur$ace $or binder interaction as the pelletising process o$ten produces L(As %ith sealed sur$ace) The &esicular sur$ace resulting $rom the crushing operation allo%s paste penetration and pro&ides more sur$ace area $or reaction bet%een the aggregate and paste) The transition 3one associated %ith a crushed aggregate has ad&antages o&er a more smooth and sealed sur$ace) %ater Reuired for Self–curing
It depends upon chemical and autogenous shrin+ages e'pected during hydration reactions) *y#e of S!rinage Drying
Shrin+ages may occur at earlyages or at later ages o&er a longer period> di$$erent types o$ shrin+ages may be identi$ied as 6 Drying shrin+age, autogenous shrin+age, thermal shrin+age, and carbonation shrin+age) Reaon for C!emical S!rinage
Chemical shrin+age is an internal &olume reduction due to the absolute &olume o$ the hydration 1roducts being less than that o$ the reactants .cement and %ater/) or e'ample6 Hydration o$ tricalcium silicate6 CS
?)
H
-
C8)9SHB
8)
Folar 98)8
CH &olumes
;?)"
-
8!9)"
B
i)e,
8==);
-
8?!)"
There$ore, Chemical shrin+age 7 .8?!)" 08==);/ 2 8==); 7 -!)!;= mL2mL 7 -!)!9!B mL2g cement or complete reaction o$ each gram o$ tricalcium silicate, there is a need to supply !)!9 gram o$ e'tra curing %ater to maintain saturated conditions) .A &alue o$ !)!? $or 9?: hydration at <" day %as e'perimentally obser&ed by 1o%ers in 8;?/) +uantity of C!emical S!rinage
1ortland cement hydration is typically accompanied by a chemical shrin+age on the order o$ !)!9 mass o$ %ater per mass o$ cement $or complete hydration6 $or silica $ume, slag, and $ly ash, these coe$$icients are about !)<<, !)8", and !)8! to !)8=, respecti&ely) It can be measured by ASTF standard test method, C8=!" Autogenou S!rinage
It is as a &olume change in concrete occurring %ithout moisture trans$er $rom the en&ironment intoconcrete) It is due to the internal chemical and structural reactions o$ the concrete) Autogenous shrin+age is prominent in H1Cs due to the reduced amount o$ %ater and increased amount o$ &arious binders used) At early ages .the $irst $e% hours/, be$ore the concrete has $ormed a hardened s+eleton, autogenous shrin+age is o$ten due to only chemical shrin+age) At later ages .8days/, the autogenous shrin+age can also result $rom sel$-desiccation since the hardened s+eleton resists the chemical shrin+age) The e'ternal .macroscopic/ dimensional reduction o$ the cementitious system under isothermal sealed curing conditions> can be 8!! to 8!!! micro strains) Self$deiccation
It is the locali3ed drying resulting $rom a decreasing relati&e humidity .RH/ %hich could be the result o$ the cement re5uiring e'tra %ater $or hydration) It is the reduction in the internal relati&e humidity o$ a sealed system %hen empty pores are generated) Potential of Selfdeiccation Prominent in ,PC- ,SC
The $iner porosity o$ HSC2H1C .%ith a lo% %2c/, causes the %ater meniscus to ha&e a greater radius o$ cur&ature, causing large compressi&e stress on the pore %alls,
leading to greater autogenous shrin+age as the paste is pulled in%ards) Sel$0 desiccation is only a ris+ %hen there is not enough locali3ed %ater in the paste $or the cement to hydrate and it occurs the %ater is dra%n out o$ the capillary pore spaces bet%een the solid particles) At later ages, a strong correlation e'ists bet%een internal relati&e humidity and $ree autogenous shrin+age) Fineral admi'tures, such as $ly ash and silica $ume, in concrete tend to re$ine the pore structure to%ards a $iner microstructure thereby %ater consumption %ill be increased and the autogenous shrin+age due to sel$-desiccation %ill be increased) Inter$de#endance of Autogenou C!emical S!rinage
Chemical shrin+age creates empty pores %ithin hydrating paste and stress generated is stimated by e5uation6 Gcap 7
<
2
r
7
-
In
.RH/
R
T
2
Jm
%here ,Jm 7 Sur$ace tension and molar &olume o$ the pore solution, r 7 the radius o$ the largest %ater-$illed pore .or the smallest empty pore/, R
7
the
uni&ersal
gas
constant,
and
T
is
the
absolute
temperature
The si3es o$ empty pores regulate both internal RH and capillary stresses) These stresses cause a physical autogenous de$ormation .shrin+age strain/ gi&en by6 K
7
.
S
G cap 2
/
.82/
0
.82s/M
%here K 7 shrin+age .negati&e strain/, S 7 degree o$ saturation .! to 8/ or &olume $raction o$ %ater$illed pores, 7 bul+ modulus o$ elasticity o$ the porous material, and s 7 bul+ modulus o$ the solid $rame%or+ %ithin the porous material) The abo&e e5uation is only appro'imate $or a partiallysaturated &isco-elastic material such as hydrating cement paste, but still pro&ides insight into the physical mechanism o$ autogenous shrin+age and the importance o$ &arious physical parameters The internal drying is analogous to e'ternal drying shrin+age) /arly /0ternal %ater Curing and Crac in ,PC
Reduction o$ autogenous shrin+age due to e'ternal curing in H1Cs is possible $or $irst one or t%o days %hen the capillary pores are yet interconnected) Early %ater curing can lead to higher strain gradients %hen the s+in o$ the concrete becomes %ell cured .no shrin+age/ %hereas, autogenous shrin+age, %hich is generally
di$$icult to control, begins at the interior o$ the concrete) These problems can be mitigated by use o$ a pre-soa+ed L(A) Monitoring of Self – curing
This can be done by6 i)
Feasuring %eight-loss
ii)
N-Ray po%der di$$raction
iii)
N-Ray microchromatography
i&)
Thermogra&imetry .TOA/ measurements
&)
Initial sur$ace absorption tests .ISAT/
&i)
Compressi&e strength
&ii)
Scanning electron microscope .SEF/
&iii)
Change internal RH %ith time
i')
(ater permeability
')
PFR spectroscopy
Ad"antage of Internal Curing
a) Internal curing .IC/ is a method to pro&ide the %ater to hydrate all the cement, accomplishing %hat the mi'ing %ater alone cannot do) In lo% %2c ratio mi'es .under !)B and increasingly those belo% !)B!/ absorpti&e light%eight aggregate, replacing some o$ the sand, pro&ides %ater that is desorbed into the mortar $raction .paste/ to be used as additional curing %ater) The cement, not hydrated by lo% amount o$ mi'ing %ater, %ill ha&e more %ater a&ailable to it) b) IC pro&ides %ater to +eep the relati&e humidity .RH/ high, +eeping sel$desiccation $rom occurring) c) IC eliminates largely autogenous shrin+age)
d) IC maintains the strengths o$ mortar2concrete at the early age .8< to 9< hrs)/ abo&e the le&el %here internally Q e'ternally induced strains can cause crac+ing) e) IC can ma+e up $or some o$ the de$iciencies o$ e'ternal curing, both human related .critical period %hen curing is re5uired is the $irst 8< to 9< hours/ and hydration related .because hydration products clog the passage%ays needed $or the $luid curing %ater to tra&el to the cement particles thirsting $or %ater/) ollo%ing $actors establish the dynamics o$ %ater mo&ement to the unhydrated cement particles6 i)
Thirst $or %ater by the hydrating cement particles is &ery intense,
ii)
Capillary action o$ the pores in the concrete is &ery strong, and
iii)
(ater in the properly distributed particles o$ L(A .$ine/ is &ery $luid)
Concrete Deficiencie t!at IC can Addre
The bene$it $rom IC can be e'pected %hen •
Crac+ing o$ concrete pro&ides passage%ays resulting in deterioration o$ rein$orcing steel,
•
lo% early-age strength is a problem,
•
permeability or durability must be impro&ed,
•
•
rheology o$ concrete mi'ture, modulus o$ elasticity o$ the $inished product or durability o$ high $ly-ash concretes are considerations) Peed $or6 reduced construction time, 5uic+er turnaround time in precast plants, lo%er maintenance cost, greater per$ormance and predictability)
Im#ro"ement to Concrete due to Internal Curing •
Reduces autogenous crac+ing,
•
largely eliminates autogenous shrin+age,
•
Reduces permeability,
•
1rotects rein$orcing steel,
•
Increases mortar strength,
•
Increases early age strength su$$icient to %ithstand strain,
•
1ro&ides greater durability,
•
Higher early age .say day/ $le'ural strength
•
Higher early age .say day/ compressi&e strength,
•
Lo%er turnaround time,
•
Impro&ed rheology
•
Oreater utili3ation o$ cement,
•
Lo%er maintenance,
•
use o$ higher le&els o$ $ly ash,
•
higher modulus o$ elasticity, or
•
through mi'ture designs, lo%er modulus
•
sharper edges,
•
greater curing predictability,
•
higher per$ormance,
•
impro&es contact 3one,
•
does not ad&ersely a$$ect $inishability,
•
does not ad&ersely a$$ect pumpability,
•
reduces e$$ect o$ insu$$icient e'ternal curing)
/ffect of Particle Si1e and Content of '%A
Internal curing by saturated light%eight aggregate can eliminate autogenous shrin+age %ith the smallest possible amount o$ light%eight aggregate) The grain si3e o$ the L(A used as curing agent needs to be reduced in order to minimi3e the paste0 aggregate pro'imity, i)e) the distance to %hich the internal curing %ater
should di$$use) The reduction o$ the grain si3e .do%n to <0B mm/, is sho%n to be bene$icial) Ho%e&er, the $urther reduction o$ grain si3e could result in a decrease o$ curing e$$iciency) The e$$ecti&eness o$ internal curing depends not only on %hether there is su$$icient %ater in the L(A, but also on %hether it is readily a&ailable to the surrounding cement paste as %ell) Hence, i$ the distance $rom some location in the cement paste to the nearest L(A sur$ace is too great, %ater cannot permeate $ully %ithin an acceptable time inter&al) This distance can be called the paste0 aggregate pro'imity) Alternati&ely, aggregate distribution can be described by means o$ aggregate0 aggregate pro'imity, %hich is the distance bet%een t%o nearest L(A sur$aces, o$ten called spacing) or a gi&en amount o$ aggregate, the paste0 aggregate pro'imity can be adusted by the si3e o$ the aggregate) The $iner the aggregate si3e, the closer %ill be the paste0 aggregate pro'imity) The L(A can be used $or internal curing %ithout considerable detrimental e$$ects on strength %hen added in the amounts ust re5uired to eliminate sel$-desiccation) 2Protected Pate 3olume4 Conce#t in Self$curing
or sel$-curing, besides pro&iding necessary 5uantity o$ %ater inside the matri', it is essential to ensure the pro'imity o$ the cement paste to the sur$aces o$ the source o$ %ater so that re5uired high RH is generated around the cement grains $or hydration reaction) In this regard, the #protected paste &olume* concept is use$ul to recognise the e$$ecti&e &olume o$ cement paste) or this, the aggregates are represented by impenetrable spherical or ellipsoidal particles and each aggregate particle is surrounded by a so$t penetrable shell representing the inter$acial transition 3one) Instead o$ the inter$acial transition 3ones, the saturated L(A .$ine aggregate/ particles surrounded by a shell o$ &ariable thic+ness can be assumed $or e&aluation) Then, by systematic point sampling, one can determine the &olume $raction o$ paste contained %ithin these shells and hence the relati&e pro'imity o$ the cement paste to the additional %ater) Ditribution of Internal %ater Reer"oir for Curing
The transport distance o$ %ater %ithin the concrete is limited by depercolation o$ the capillary pores in lo% %2c ratio pastes) (ith %ater-reser&oirs %ell distributed %ithin the matri', shorter distances ha&e to be co&ered by the curing %ater and the e$$iciency o$ the internal-curing process is conse5uently impro&ed) The concept o$ internal curing %as established, based on dispersion o$ &ery small, saturated L(A throughout the concrete, %hich ser&e as tiny reser&oirs %ith su$$icient %ater to compensate $or sel$-desiccation) The spacing bet%een the L(A particles is con&eniently small so that the %ater tra&els smaller distances to counteract sel$-
desiccation) The amount o$ %ater in the L(A can there$ore be minimi3ed, thus economising on the content o$ the L(A) *ra"el of %ater from Surface of '%A
Estimates o$ tra&el o$ internal %ater $rom the sur$ace o$ %ater reser&oir in the concrete matri' are6 •
early hydration
•
middle hydration ? mm
•
late hydration 8 mm or less
•
#%orst case* !)
.Early and middle hydration estimates in agreement %ith '-ray absorption-based obser&ations on mortars during curing/) Si1e of #ore for Internal %ater Storage
(ater is held in pores primarily by capillary $orces) 4nly pore si3es abo&e appro'imately 8!! nm are use$ul $or storage o$ internal curing %ater) In smaller pores the %ater is held so tightly that it is not a&ailable $or the cementitious reactions) Since some o$ the %ater absorbed by the L(A in the smaller pores %ill not be released to the hardening cement paste, an amount o$ %ater more than su$$icient to counteract sel$desiccation should be absorbed in the L(A) A great 5uantity o$ %ater is in $act entrapped in the internal porosity o$ the larger particles> one should consider that only about hal$ o$ it is a&ailable $or internal curing) In case o$ smaller $raction, the opposite seems to hold6 the absorption is lo%er, but almost "! : o$ the %ater is lost by "?: RH) &efulne of IC in Pa"ement
The maor problem o$ crac+ing in pa&ements may be alle&iated by internal curing, besides imparting many potential bene$its) &efulne of IC for /arly$Age Cracing
The IC can in$luence the Early- Age Crac+ing Contributors %hich are mainly thermal e$$ects and autogenous shrin+age) During initial ages o$ concrete, hydration heat can raise concrete temperature signi$icantly .causing e'pansion/, subse5uent thermal contraction during cooling can lead to early-age .global or local/ crac+ing i$
restrained .globally or locally/) Another prominent e$$ect %ould be autogenous shrin+age, especially in concretes %ith lo%er %ater-binder ratios %here su$$icient curing %ater cannot be supplied e'ternally, the chemical shrin+age accompanying the hydration reactions %ill lead to sel$-desiccation and signi$icant autogenous shrin+age .and possibly crac+ing/) Pore Si1e in Internal Reer"oir Ca#illary Pore
IC distributes the e'tra curing %ater throughout the -D concrete microstructure so that it is more readily a&ailable to maintain saturation o$ the cement paste during hydration, a&oiding sel$desiccation .in the paste/ and reducing autogenous shrin+age) @ecause the autogenous stresses are in&ersely proportional to the diameter o$ the pores being emptied, $or IC to do its ob, the indi&idual pores in the internal reser&oirs should be much larger than the typical si3es o$ the capillary pores .micrometers/ in hydrating cement paste) +uantifying /ffecti"ene of IC
IC can be e'perimentally measured by6 •
Internal RH
•
Autogenous de$ormation
•
Compressi&e strength de&elopment
•
Degree o$ hydration
•
Restrained shrin+age or ring tests
•
-D N-ray microtomography .Direct obser&ation o$ e -D microstructure o$ cement-based materials/)
Concluion
The internal curing .IC/ by the addition o$ saturated light%eight $ine aggregates is an e$$ecti&e means o$ drastically reducing autogenous shrin+age) Since autogenous shrin+age is a main contributor to early-age crac+ing, it is e'pected that IC %ould also reduce such crac+ing) An additional bene$it o$ IC beyond autogenous shrin+age reduction is increase in compressi&e strength) As internal curing maintains saturated conditions %ithin the hydrating cement paste, the magnitude o$ internal
sel$-desiccation stresses are reduced and long term hydration is increased) IC is particularly e$$ecti&e $or the highper$ormance concretes containing silica $ume and OO@S) In cement mortar containing a Type $ly ash, the $ly ash $unctions mainly as a dilutent at early ages, and higher and coarser porosity at early ages result in less autogenous shrin+age) The sel$-desiccation is the reduction in internal relati&e humidity o$ a sealed hydrating cement system %hen empty pores are generated) This occurs %hen chemical shrin+age ta+es place at the stage %here the paste matri' has de&eloped a sel$-supporti&e s+eleton, and the chemical shrin+age is larger than the autogenous shrin+age) E$$ects o$ sel$-desiccation depend on the si3es o$ the generated empty pores) These pore si3es in turn are dependent on the initial %aterto- binder ratio .%2b/, the particle si3e distributions o$ the binder components, and their achie&ed degree o$ hydration) The continuing trends to%ards $iner cements and much lo%er %2b ha&e signi$icantly reduced the capillary pore #diameters* .spacing/ in the paste component o$ the $resh concrete, and ha&e o$ten resulted in materials and structures %here the e$$ects o$ sel$-desiccation are all to &isible as early-age crac+ing) Fany strategies $or minimi3ing the detrimental e$$ects o$ sel$desiccation .mainly the high internal stresses and strains that may lead to early-age crac+ing/, such as internal curing, rely on pro&iding a #sacri$icial* set o$ larger %ater-$illed pores %ithin the concrete microstructure that %ill empty $irst %hile the smaller pores in the hydrating binder paste %ill remain saturated) It may be noted that the e$$ects o$ sel$-desiccation are not al%ays detrimental, as e'empli$ied by the bene$its o$$ered by sel$-desiccation in terms o$ an earlier RH reduction $or $looring applications and an increased resistance to $rost damage) IC is use$ul %hen per$ormance speci$ications are important than prescripti&e speci$ications $or concrete) 1rime applications o$ IC could be6 concrete pa&ements) precast concrete operations, par+ing structures, bridges, H1C proects, and architectural concretes) Concrete, in the <8st century, needs to be more controlled by the choice o$ ingredients rather than by the uncertainties o$ construction practices and the %eather) Instead o$ curing through e'ternal applications o$ %ater, concrete 5uality %ill be engineered through the incorporation o$ %ater absorbed %ithin the internal curing agent) Acno5ledgment
The authors than+ Dr) P) La+shmanan, Director, SERC, Chennai, $or permitting to publish this paper) 6ibliogra#!y on Selfcuring (Internal Curing)
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88)
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