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Title:
NATIONAL RESEARCH COUNCIL OF CANADA
Technical Translation TT-574
The pathology of construction: the causes of cracking of concrete,
( ~ a t h o l o g i e de la construction: les causes de la fissuration d u beton),
Author: Jean-Pierre L6vy.
Reference: Bgtir,
42:
32-37, 1954.
Translator: D. A. Sinclair, Translations Section, N.R. C. Library,
The Division of Building Research is vitally con- cerned with dimensional change in construction materials and particularly in the mechanisms responsible in various types of mterials, This translation was requested for the Materials Section of the Division as supplementary inf'ormtion in this
field with reference to studies on concrete, Although of a
general nature, this paper swnmarizes the causes of cracking in concrete and makes specific reference to more detailed studies,
The Division is grateful to blr. D.A. Sinclair and the
Translations Section of the National Research Council Library for preparing this translation.
Robert F. Legget,
THE PATHOLOGY OF CONSTRUCT102 THE CAUSES OF CRACKING OF C O N C R E B
I n our 1 0 t h i s s u e i ~ a n u a r y 1951), we publishes a s t u f i j of cracks due t o shrinkage of concrete, I t was
b a s i c a l l y a question t h e r e sf the e f f e c t of the ri?ater:.a.ls e n t e r i n g i n t o the composition of t h e concrete acCi t h z effec'; of the form g5ven t o the poured u n i t s . I-Ioweve:~, there a r e o t h e r causes of cracking ~vhflch may be fo.l;,r,r'i exds- tine; by themszlves, 3 u t the e f f e c t s of which a r s o f t e x sc.per- imposed on those of shrinkage, -;;he l a t + , e r rsmainir~g ';;?is mc.sL c h a r a c t e r i s t i c because they depend on intrillsic: p r o p e r t l v ? ~ o f the m a t e r i a l , C e r t a i n I3rit)ish engineers have v;ldertalr,e;.l t o p r e s e n t t h e i r i d e a o f the problem of crackfng ir, a -;ra~-L
t a b l e s m a r i z i n g the various causes and showing the p r i n c i - p a l means of preventing them, ':Je thought t h a t i n view of
i t s extremely graphic foYm the p u b l i c a t i o n of t h i s docwx:eii$ vrould be of i n t e r e s t t c ~ o w r e a d e r s , and ~?l;re have asked i;h.z
author of the 1951 study mentioned a b o ~ e , kin3iy - f ; ~ afid h i s
comments here,
I n t h i s d l s e ~ ~ s s i o r i w e s h a l l r e t a i r * the p l a n Fcllov;ed i n the t a b l e given, Cracks a:re very I.ogiss?_ly diT,i;.-7.er!, in.-;:=. Lwcl
C
c l a s s e s : those which occur befora tke ha~de,r:!ng o? &--,s? c a : c s ~ ~ e + u ~ and those whicI?. f ollov: ~ e . i . t ? - : ~ and harrlenirGe S o ~ 2 J? t h e m - h w e
3
..
7 bee3 explained a t lel--gtIi .1.r t:?~ z ~ t i c ~ e publisheg- 5.2 11.952.~ vr-?~..-eo t h e r s s t i l l need t c l b e c%a?r'.?:'.sd,
Cracks tn?hich ax2& i c d e n e q c , ~ $ - ~ ~ - ~ h ~ = ~ ~ c r e t z zrs
---
.------ -
easyt o d e a l with, Thzgr iaesuL+, f'';>-,1~ .t'T?.y~?; ::Ln the ~ Q % s - ~ I ~ ~ c ~ ~ ~ J : ? c ~ j ?
f r m e , Tnus i t vj;r".l:i. 5 c the s-k.-;e o 3 h e su-rfacca o~;. n2l:Ls??- Lkte concrete i s t:, be powed which w i l l he dez?.sLvc=
:
s.ubs:-knl:5 c l rd e f ' o m t i o n s due tc :Load o r hwnLdfty;, or e l s a the r;~a:'!-L?r ;"--;F.
f r a a l n g and i.ts assemb?-!.y [sl:r:rel'r.2n.g 02 the wcrL->d, p o i n t s ycr,:?l;ir
secured, poor asaer~~blagea, weak formsj
.
I n t h i s comec-bioii ;e-5u s r e c a l l t h a t f ~ e s h c o m r e t e e x z t s a pressuxae on a f'as~n v,nich f i r s t i n c r e a s e s 7:rith the p,mrzd h3fghkp passes th.?:ailgh a nm;~5-mu.: a t a c e r t a i n thicl:iless, and theii clec;?eases, The mnar-.2.~,"ivrn i.3 a L s c
a f - m e t i o n of the r a t e of pi?oa:?b~~g, Thus, i f the rs->e 3.i i:r:~ict; the concrete i s poured i n t o thz fo!-c;1 i s 0,33 in, p e r I10v2, i;!:.e
maximum pressure vri.11 be 2,000 1cg,/mo2
,
reached a t a depth of 0.90 m. If the r a t e o f pouring i s increased, e.g. up t o 3 m./hr., then a t 0.90 m. the prbessui7c m i l l be 2,000 ke;m./rn.*, b u t the ~mxirmun w i l l be a t t a i n e d a t a p r e s s u r e of 3,800 kgnl./m.* a t2,40 m. The q u z l i t y of the wood used f o r frcming and t h e con- d i t i o n s under which i L i s employed would j u s t i f y a study i n it-
s e l f .
The other causes of craclriw p r i o r t o hardening a r i s e out of the n a t u r e of the concrete o r i t s c o n s t i t u e n t s and a r e u9timmtely due t o shrinkage phenomena,
The wall effee-t, a s i s broadly kno~vn, o r i g i n a t e s
b a s i c a l l y f r o m the d i f f i c u l t g of p l a c i n g l a r g e aggregates between the f r a n i n g and the reinforcements. The r e s u l t i s , of course, the n e c e s s i t y oi' 8ra.duatLn.g fhe aggregates as a f u n c t i o n o f the i r o n ~ ~ r o r k di:nellsions; on the other hand, i t may be necessary t o place a snoother mortzr f n contact with t h e walls f o r the purpose of reducing the hollow a;?pearanze of the surface,
I n t h i s case w e a r e concerned n i t h a heterogeneity o f s u r f a c e and hence t o dlrf'erznces of pomed q u a n t i t i e s , vihick pro- Cuce d i f f e r e w e s i n r e s i s t a n c e r e s u l t i n g i n i n t e r n a l s t r e s s e s and c r a c l i ~ .
Crackinn due t o s e ~ r e g a - ~ i a n i s a l s o caused by a hetero- g e n e i t y v ~ l ~ i z h r e s u l t s i n i n t 3 m a l tensions. During the pouring
of the concrete and before i t s a t s , t h e r e may b e segregation, i. e,
,
s e p a r a t i o n due $0 g~avity of elements of diPference size. The re-s u l t i s a raising o f moptar and g r ~ n t , Generally speaking, *he wall e f f e c t and segregation a r e very s i m i l a r , although the causes a r e not q u i t e the same, I n the f i r s t case i t i s necessary t o take i n t o accoun-i; tile e f f e c t of the wall i n order t o calcu1ai;e the g r a i n s i z e distribution, b u t i n e i t h e r case an indispensable n e c e s s i t y i s the production of a compac-t concrbete n i t h a c a r e f u l l y c o n t r o l l e d g r a i n s i z e d i s t r i b u t i o s ,
The advice sometines given not t r ~ complete the coating u n t i l a f t e r the segrega-tion craclrs have appeared i s c l e a r l y wise,
s i n c e otherwise the flaws w i l l be camouflaged, b u t i t i s b e t t e r t o s t r i v e t o avoid t h e i r appearance i n the f i r s t place,
Cracks around l a r a e aggregates t o which a s i m i l a r
-
remew i s t o be applied, a r e the r e s u l t o f a d i f f e r e n t evol-ution. I n t h i s case the l a r g e aggregate i s not an obstacle. It i s a case o f a shrinkage of the p a s t e deposited i n too t h i c k a coat around the g r a i n s o f the aggregzte. I n essence, such a. phenom- enon i s t h e r e s u l t o f i n s u f ' f i c i e n t homogeneity o f the mixture: cement p a s t e a n d f i n e s on the one hand, n o r t a r and l a r g e aggpe- g ~ . t e s on the other, The remedjr i s t o be found not only i n the adequate compaction of a c a r e f u l l y proportioned concrete, b u t a l s o i n the q u a l i t y o f the mixing process, a matter of prime irii- portance f o r the production o f good concrete,
17e now ccrne t o s h r i n k a p , including t h a t which takes p l a c e i n the course o f setting and t h a t o c c w r i n g during the
subsequent hardening. l'ie s h a l l not discuss t h i s problem again i n d e t ~ i l , a s i t w a s deal5 with tllorowllly i n i s s u e No. 10 of " B B t i r " and continues $ 0 b e the p o i n t of departure f o r any study
of cracks i n concrete. L e t us h e a r i n rriind, however, t h a t the shrinkage phenomena a r e s~perimposed on the e f f e c t s due t o ab- normal s t r a i n s , d e a l t with s e p a r a t e l y under No. 7 i n the t a b l e ; w e s h a l l yeturn $c t h i s l a t e ? ,
We s h a l l t a k e a somewhat c l o s e r loolr a t the cases of f a l s e s e t t i u o f cement and t h e l f s m a e r e s i s " ,
The phenomenon of " f a l s e s e t t i @ , or of " s t i f f e n i n g " manifests i t s e l f i-n a poorer ivorlcability o f t h e concrete, during o r a f t e r mixing and before o r during the pouring of the concrete.
Classification of the Causes of Cracking
I
Before hardening After hardening
I
I
I
(1)
I
(il
(3)I
(4) (5) ( 6)Movements Shrinkage due Shrinkage Physical
I
Chemical Thermal Abnomal (r Con-I)
Acc dentsof the con- to wall effect during effects effects effects concen- ception
st tion and segregation setting tration of of the
stresses con-
A)
(1A) hlovement Movement of sub- of forms 8 tructure L - ( 2 ~ ) ( 2 8 ) ( 2 ~Wall effect Shrinkage Shrinkage Ragid Slow Composition Reactive ( 5 ~ ) ( 5 ~ ) Foreign (5C) (7~) ( 7 B ) (7~) at right at right evolution evolution of cement aggregates bodies rein- nature of Iron Particular Extrusion
angle to angles to and carbon- and corrosion force- the con-
reinforce- grains of ation rnents s truc tion
ments and eggregates obstacles
A
A '
( 3 4 (3) ( 4 4
(a)
Plastic Drying Drying LIovelilent
shrinkage shrinkage shrinkage due to
A
- , humidityI
struction r (6A) thermal Internal stressesq
Var at ons of external tempeTature ( 6B) ( 6D) T h e m 1 Action proper- of gelties of and ice
Accor3iw to the morks of Lhopitallier, Stiglitz and
Vidal (~erilh, h-L:3?., Techn. No, 53), false setting is due to the
hydration and independent crystallization of a part of the semi-
hydrated calcium s~~lphate present in the cement (derived from the
gypsum added to the cenent in order to control the setting),
The stiffening of the ccncrete during its placing rnus-b be attrib- uted to the skelzton of the bihydrate crysxals then formed, False
se+,ting, as opposed to rap13 setting, is not accompanied by any
distinct liberation of heat.
If
the phenomenon of false settingis discovered, $he only solution to adopt consists in increasing the time of mixing, since false setting will not resist this
gentle, ~upplementa~y mixing process, One must resist the temp-
tation to combat this by adding more water, as this will com-
pletely alter the character of the concrete, making it softer, and will cause shrinlcage cracks, Filling in by supplementary tamping
of 'the concrete already in piace is merely a palliative and should
not be generally applied, Indeed, as soon as the possibilizy of false setting has been recognized, steps m s t be taken to change
the mixing process, but net, of COT-wse, to prolong it so as to
produce conditions conciuc2ve to the phenomenon of "repeafing"
(a condition which is brought about9 as is known, by too pr~longed
a rnix2m.g rebtive tc the setking time),
As for-^.aeresisw it comprises a shrinkage of the
.---
cement gel formed d u r i ~ ~ the settirig, folloviing an inopportune
and too rapid e ~ u l - s i o n cf the water contained in the gel during
+,he course of hyika-kion, The phenorrleno3 may be prod.uced either
as a resu2t of %e great dryness o f the concrete, or because the
excessively porous an5 unhunlidified supports at first have
absorbed too much of the mixing water, Shrinkage of the cement gel then takes plase with irrrmned5ate cracking, This phenomenon occurs when mortars or sealTng coats are applied under poor con-
Alternate swellily?: and shrinkix due to wetting and subsequent drying are general phenomena which occur in all
materials, As far as normal, rf?ell compacted concretes are con-
cerned, after their initial shrinkage variations of this sort
are comparatively slight and will not have serious consequences,
since the resulting variations in volume do not produce any strains greater than the tensile stress of the concrete, This problem will require greater attention, however, in the case of
light- eight concretes or ceLlulara concretes hardened in air, It is no longer a questioc then of products which are as compact
as heavy concretes, hov~ever homogeneous they may be, and their
tensile strength is comparatively small, Means must be found to compensate for this lack of strength, Thus, when partitions
are being assemb?-ed, coatiws may be applied v~ithout danger,
provided that a metal lattlce work is incorporated in them,
Cracks due -Lo chem.i.cal effects require little comment, They are due either to the presence of expansion agents in the
materials, or t o pclssible reaction between certain aggregates and
the alkaiines in the cement; the latter phenomenon rarely occurs
in this country, Xore s e ~ i o u s are the expansion phenomena due to
corrosion of the reinf'orcements; in order to avoid these, a very
compact concrete should b e used and the iron reinforcement rods
shoulrZ be adequately erxl~sed. These precautions are still not adequate in themselves where the concrete must resist the action
of acids or fats, or the action o f aggressive in?aters,
Thermal effe- are also summarized in an explicit
P..
e r a As Tar as t h e m 1 strains resulting from the heat of
..qrdral;ion are concerned, it must be borne in mind that when a
considerable amount of concrete f s being poured, cooling will be
:,lore rap5d. near the walls than at the centre of the mass, a fact
< ,
IE connection with variations of external temperature we have had occasion to note an extreme case of cracking: this mas a case of a concrete mass supporting the hearth of a blast furnace, The differences of temperature betneen the extreme points of the mass (several hundreds of degrees) resulted in
stresses of the order of several tons per square metre. In very
warm countries the variations betaem the day-time and night-time temperatures may also resul5 in considerable variations (in this
connection see Qur issue of June
1953,
the study on the deterior-ation of concrete fapades). Similarly, special concretes must be
studied for constructions in which they must resist high temper- atures very rapidly applied (concretes for deep well drilling).
Abnormal concentrations of stresses;. and particularly
---.
cracks due to reinforcements deserve attention,
The cracking of reinforced concrete in zones subjected
to tensile forces is a we1.i-knovm phenomenon v~hich results from
differences in elasticity between the iron of the reinforcements and the concrete which surrounds them, The case is fully pro-
vided for in rules BA 45 (Sec-:, 2, 3 1 and 2, 12) where a diszlnc-
tion is made between cases where the cracking is harmful and
cases where il is II~~X. In particuiar, it is indicated (~ec, 2,
111) that the concrats employed will have to have an admissible
conrpression stress, expressed in ~ t ~ . / r r ~ ~ equal to at least
L/20th of the admissible tensi?-z. stress for steel, expressed fn
kgm,/cm.". The ahissible tensile stress for steel cannot
exceed 18 kzln/crn.' excapt for special reasons,
Differer-t rules my be applied in cases where notched
rods [deformed Sars] or Tor s-keel rods are employed (1948 re-
commendations, rods nte(?) 40-60) and new ones are foreseen in
It may easily be shown, indeed, that the width of the
cracks decreases increasing perimeter of the steels and
increzsed adhesive force, But the increase of these two para-
meters results in a decrease of the distance bet~veen successive
craclrs, Thus, notched or twisted steel rods produce more numerous, but snmller cracks than smooth rods, Thus a grezter admissible
tension may be accepted for them than for ordinary round rods, without producing harmful cracks,
It shou2d be n ~ t e d that these cracks do not detract
from the strength of reinforced concrete, since in calculating
this strength no account is talcen of the tensile strength of the concrete itself, Experiments elsevrhere have shorn that these are
inoffensive capillary cracks as far as penetration of moistu~e to
the reiflorcement is conce~ned, provided the rules referred to
above have been colnrectly applietl. P r o ~ n all this it folloi:rs that
for every grade of steel, above a certain percentage of rsinf'orce-
inent, c m c k s are inev3table si3c.e they arise from the very
princi2le of the process, Never-theless, they are pre judiclal to
the qunlif,y of the construction, These cracks are sufficiently
characte-r.ist.ic ir, appearance Cullat %?ley cannot bs conrused ~ri$h
those of other o~5.gin,
Tq proverlt the appearance 0% fine, hair-line zj?ackr;
when the concret13 is brough7h t.3 its elastic limit, the appli-
cation of pre-stressing or post-slesslng has been rec.o~x.~ended,
But the fol1onin.g must be clearly ~ u ~ d e r s t o ~ ~ d : pre-stressed con-
crete is no ioLzer the same matepis1 as ordinary concrete, and
under cur~ent. conditions of practice it is not clear how pre-
stressing could 'be appiled t s a concrete not especially tor-
ceived for this appli~at~l~n,
The other causes of' crackiag vrhich have been inentioned
do n ~ t requj-re special ne:? obscrvntions, Let us recall., as ne
being superilnposed and those due t o abnormal concentrations o f t e n s i l e s t r e s s e s (whether or not t h e r e are reinforceiizents). T h i s coul8 happen when a construction subject t o t e n s i l e f o r c e s i s poured i n s e v e r a l stagl2s a'; wi5ely d i f f e r e n t times s o t h a t the
concrete a1~~r;aw- laid has completed i t s shrinkage by tlje time FL
I
-
BEFORE HARDE2TINGNo. Nature of the cause Explanation Recommended precautions
1, I~Tovements of the construction
A) lv!ovements of the 1) Deformations due to swelling Checking means suitable to substructure (fig. 1) of the soil due to humidity substructure
2) Inadequate compaction B) llovements of the 1) Expansion of forms due to
forms (fig. 2) swelling of wood
2) Expulsion of points at time Suitable design and assem- I of damage or vibration bly of forrns I-'
N
3)
Frames not resistingpressure of fresh concrete
2. Shrinkape due to wall
effect and seg-tion A) Wall effect
B) Shrinkage at right angles to reinforce- ments and similar obstacles (fig. 3 )
C) Shrinkage at right argles to aggregate grains (fig.
4)
Heterogeneity of surface due to Good graduation of aggre- difficulty of laying aggregates gates
The concrete begins setting on 1) Compact concrete with the surface while segregation carefully controlled continues in the interior grain sizes; low water
content and effective means of compaction Sediccntation of paste around 2) Delay coating or cover- aggregate grains ing until after some
--
149. Xature of the cause Explanation Recomnended precautions
3.
Shrinkee during setting A) Plastic shrinkage B) Drying shrinkage (fig,5)
1) Rapid evolution 1) False setting 2) Synaeresis1) Paring d o ~ m of films of air and water separating the particles, with consequent mutual attraction of solids by surface tension
2) Loss of vrater in the sub-
structures or through the
f orrns
Carry out supplementary mixing of concrete
Avoid concretion on supports that are too porous and not hunidif ied
Vigorous ramming (or tamp- ing) of concrete during setting is sometimes recom- mended for prevention of I false setting or synaeresis, but it is not a rule to '-W I
apply generally I
Above all it is necessary to make a good concrete without an excess of water and using good cements. Avoid additives. Retard evaporxtioll by spraying, cover .cvi th wet bags, varnish temporarily, etc,
Humidify the su~~ports. Use water-tight forrns (especially when ceixents are employed with a large proportion of grout)
No. Nature of the cause Explanation Rezorimlended precautions 2) Slow evolution Shrinkage of cement gel with
loss of water
Reduce the water content of the concrete and the cement content, and ensure proper drying so as to permit n o r m 1 hydration of
the cement
I1
-
AFTER wDEITINGNo. Nature of the cause Explanation Recomrnended precautions 4, Physical effects A) Drying shrinkage B) Movements due to rio i s ture See
3B
(2) above 1) Alternating humidification and drying2) Porous aggregates or aggre- gates of inf'erior quality
5.
Chemical effectsA) Composi3ion of cement 1) Excess of' lime, inagnesi~un and carbonation or sulphates
(fig. 6)
See
3B
(2) aboveLittle to fear in the case of a well executed, cornpact concrete
Mdce sure that the aggre- gates are of high quality
Keep to standard proportions (to be verified by tests) 2) Exposure of the cement to
NO. Nature of the cause Explanation Recommended precautions
B ) Reactive aggregates Aggregates such a s p n i t e s , and Keep cement down t o 0.5- very a l k a l i n e cements 0.6 $ sodium o r potassium
hydroxide content
C ) Foreign bodies and Oxidation of mild s t e e l rein- Cover r e i n f orcenlents
corrosion ( f i g .
7)
forcements s u i t a b l y , make concrete a silrpermeable EIE p o s s i b l e
6. Thermal e f f e c t s
A ) I n t e r n a l thermal
s t r e s s e s Temperature d i f f e r e n c e s due t o t h e heat of h y b a t i o n of t h e 1) Pour under a height srna.11
cement 2 ) Leave time betvreen ttr~o pourings
3 ) Use a cement of small h e a t of h ~ d r a t i o n
4)
Cool the concreteB ) Thermal p r o p e r t i e s of Aggregates showing d i f f e r e n t the aggregates thermal expansions
C ) Variations of ex- Bearing on the rxixtu-re and the Provide expansion j o i n t s t e r n a l tel-qerature m a t e r i a l s used capable o f absorbing struc-
t u r a l expansions varying from 5 t o
7.5
m i l l i o n t h sD ) Action of f r o s t and P e n e t r a t i o n of water i n t o the Nake t h e concrete as : com-
i c e pores, follov~red by f r e e z i n g p a c t and impermeable a s p o s s i b l e o r employ an a i r occlusion agent ( a f t e r con- s u l t z t i o n with s p e c i a l i s t s )
c m a, a, E 0 0 a O r i k Ill 0 q i a, d r l Q 0 a LI-P .rl
No. Nature of the cause Explanation Recornended precautions 8. Conception of the
construction The study o f the loads and s t r e s s e s may be made, generally spezlring, f o r s t r u c t u r e s under {;he r e a l load conditions borne, taking i n t o nccomlt the mater- i a l s used and the q u a l i t y of the worlrmanship
However, i t i s d i f f i c u l t t o foresee the e f f e c t s of s e t t l e - ment o f the foundations and t o
l o c a l i z e them
1) Accidental overloads, ex- cessive vibrations not fore- seen i n the preliminary study
2 ) Effects of f i r e , s t o r m s and Structures erected i n earthquakes regions subject t o earth-
quakes must be b u i l t accor- ding t o c e r t a i n special r u l e s
Fig. 1
Crack due to a
novement of the substructure.
Fig. 2 Fie;. 3
Crack due to a Shrinkage at right
movement of the angles to a rein-
form. f orcement rod.
Pig.
4
Pig. 5 Fig. 6Shrinkage at right Large, three-branch Network of fine cracks
angles to the aggre- crack due to shrink- due to carbonztion of
gate grains. age d u r i ~ ~ drying. the cement,
Fig.
7
Fig, 8 Fig. 9Cracks due to ex- Here the cracks are Concentration of
pansion of an oxi- caused by the stresses tensile stresses at
dized reinforcement to which a bent rein- a square opening.
rod. forcernent rod is sub-
