Publisher’s version / Version de l'éditeur:
Vous avez des questions? Nous pouvons vous aider. Pour communiquer directement avec un auteur, consultez la première page de la revue dans laquelle son article a été publié afin de trouver ses coordonnées. Si vous n’arrivez pas à les repérer, communiquez avec nous à PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca.
Questions? Contact the NRC Publications Archive team at
PublicationsArchive-ArchivesPublications@nrc-cnrc.gc.ca. If you wish to email the authors directly, please see the first page of the publication for their contact information.
https://publications-cnrc.canada.ca/fra/droits
L’accès à ce site Web et l’utilisation de son contenu sont assujettis aux conditions présentées dans le site
LISEZ CES CONDITIONS ATTENTIVEMENT AVANT D’UTILISER CE SITE WEB.
Cement and Concrete Research, 17, May 3, pp. 478-488, 1987-05-01
READ THESE TERMS AND CONDITIONS CAREFULLY BEFORE USING THIS WEBSITE. https://nrc-publications.canada.ca/eng/copyright
NRC Publications Archive Record / Notice des Archives des publications du CNRC :
https://nrc-publications.canada.ca/eng/view/object/?id=9bf77222-b4d0-4b87-82e0-3f6fb63b02fa
https://publications-cnrc.canada.ca/fra/voir/objet/?id=9bf77222-b4d0-4b87-82e0-3f6fb63b02fa
NRC Publications Archive
Archives des publications du CNRC
This publication could be one of several versions: author’s original, accepted manuscript or the publisher’s version. / La version de cette publication peut être l’une des suivantes : la version prépublication de l’auteur, la version acceptée du manuscrit ou la version de l’éditeur.
For the publisher’s version, please access the DOI link below./ Pour consulter la version de l’éditeur, utilisez le lien DOI ci-dessous.
https://doi.org/10.1016/0008-8846(87)90011-1
Access and use of this website and the material on it are subject to the Terms and Conditions set forth at
A novel technique for determining bond strength development between
cement paste and steel
Ser
TH1
National Research
Conseil national
11
co
uncil Canada
de recherches Canada
no.
1477
c .
2
Institute for
lnstitut de
B
LDG
Research in
-
Construction
construction
recherche en
A Novel Technique for
Determining Bond Strength
Development between Cement
Paste and Steel
by M.
Nakayama and
J.J. Beaudoin
j ? ? + l r \ ? ! ? ' ~ ~ ~
Reprinted from
Cement and Concrete Research
Vol. 17, No.
3,
1987
p. 478
-
488
(IRC Paper No. 1477)
Price $3.00
NRCC 28322
Les a u t e u r s d g c r i v e n t une n o u v e l l e t e c h n i q u e s e r v a n t b d g t e r m i n e r l ' a d h e r e n c e d e l a p h t e d e ciment 1 l ' a c i e r . C e t t e t e c h n i q u e t i e n t compte d e l a c o n t r a i n t e i n t e r n e c a u s e e p a r l e p r o c e s s u s d ' h y d r a t a t i o n . Les a u t e u r s p r e s e n t e n t l e s r g s u l t a t s d e s e x p g r i e n c e s r g a l i s g e s d a n s l e domaine d e l ' a d h s r e n c e e t e x p l i q u e n t l e r a l e j o u 6 p a r l a zone d ' i n t e r f a c e d a n s l a d e t e r m i n a t i o n d e 1' a d h g r e n c e e n u t i l i s a n t l e s i n f o r m a t i o n s o b t e n u e s p a r u n examen, a u microscope 1 b a l a y a g e , d e s s u r f a c e s d8tachSes.
CENXNT and CONCRETE RESEARCH. Vol. 17, pp. 478-488, 1987. Printed in the USA. 0008-8846187 $3.00+00. Copyright (c) 1987 Pergamon Journals, Ltd.
A NOVEL TECHNIQUE FOR DETERMINING BOND STRENGTH DEVELOPMENT BETWEEN CEMENT PASTE AND STEEL
M. ~ a k a ~ a m a l and J.J. Beaudoin Institute for Research in Construction
National Research Council of Canada Ottawa, Canada, K1A
OR6
(Communicated by
M.
Daimon) (Received Feb. 19, 1987)ABSTRACT
A new technique for determining bond strength between cement paste and steel is described. Internal stess resulting from the hydration process is taken into account. Experimental results of bond
strength development are presented. The role of the interface zone in determining bond strength is discussed with reference to
information provided by SEM examination of debonded surfaces.
Introduction
The bond strength between cement systems and metallic substrates such as steel is an important factor in determing the behaviour of many concrete construction elements. Typical examples where the bonding of steel and concrete is of practical importance include the following: steel fiber- reinforced cement composites, grouting of steel bolts and fastenings in concrete, grouting of post-tensioning tendons in prestressed concrete members, and the fabrication of steel-concrete composite beams.
Estimates of bond strength between steel and cement systems have been obtained by several different methods. These include calculation of average bond strengths From pull-out tests, direct tension tests, slant-shear tests,
and various types of flexural tests (1). All these tests involve
application of external load to the specimen, potential alignment, and reproducibility problems, as well as inherent difficulties in controlling test environments. In addition, measuring the development of bond strength at early ages is not practicable.
In this paper a new method for determining the bond strength between cement paste and steel is described. The method permits estimates of bond
lpresent address: Kajima Institute of Construction Technology, Kajima Corporation, Tokyo-182, Japan.
. -
Vol. 1 7 , No. 3
CEMENT PASTE, STEEL, BOND STRENGTH, TECHNIQUE
s t r e n g t h a t e a r l y a g e s and a c c o u n t s f o r t h e i n t e r n a l s t r e s s developed ( i n t h e cement m a t r i x ) due t o t h e h y d r a t i o n p r o c e s s . It i s a p p l i c a b l e t o t h i n c o a t i n g s e n c o u n t e r e d i n r e p a i r s i t u a t i o n s , does n o t i n v o l v e a p p l i c a t i o n of e x t e r n a l l o a d , and i s e a s i l y a d a p t a b l e t o t e s t i n g i n v a r i o u s c o n t r o l l e d environments. T e s t r e s u l t s o b t a i n e d w i t h t h i s method a r e p r e s e n t e d . F a c t o r s a f f e c t i n g t h e development of bond s t r e n g t h a r e d i s c u s s e d w i t h r e f e r e n c e t o SEM and EDX a n a l y s i s of s e l e c t e d debonded s u r f a c e s .
E x p e r i m e n t a l M a t e r i a l s
The P o r t l a n d cement used had t h e f o l l o w i n g composition: SiO (20.78%); A,t,03 (6.20%); F e f l (2.232); CaO (64.83%); MgO (1.84%); SO3 (3.1?%); Na20 (0.05%); K20 (0.40%j. B l a i n e f i n e n e s s was 300 m2/kg.
The s t e e l s u b s t r a t e c o n s i s t e d of s t a n d a r d s t e e l f e e l e r gauge p i e c e s of dimensions 12.70 mm x 304.80 mm x 0.25 mm.
Mixes and Specimen P r e p a r a t i o n
Cement p a s t e mixes were made a t t h e f o l l o w i n g water-cement r a t i o s : 0.25, 0.30, 0.35, and 0.40. Cement p a s t e was a p p l i e d t o t h e s u r f a c e of t h e s t e e l f e e l e r gauge by s p r e a d i n g t h e p a s t e w i t h a s p a t u l a between g u i d e s s e t a d j a c e n t t o t h e f e e l e r gauge. T h i c k n e s s of t h e p a s t e , g e n e r a l l y
0.51-0.76 mm, was c o n t r o l l e d by t h e t h i c k n e s s of t h e g u i d e s used. The s u r f a c e of t h e f e e l e r gauge was c l e a n e d w i t h a c e t o n e p r i o r t o a p p l i c a t i o n of t h e p a s t e . The cement p a s t e - s t e e l composite beams were used t o d e t e r m i n e t h e development of i n t e r n a l stress i n t h e p a s t e and bond s t r e n g t h between s t e e l and cement p a s t e . Up t o f o u r specimens were used f o r e a c h t e s t . Mean v a l u e s of t e s t r e s u l t s were u s e d i n p l o t t i n g d a t a .
C a l c u l a t i o n of I n t e r n a l S t r e s s
The development of i n t e r n a l s t r e s s i s f o l l o w e d d u r i n g t h e h y d r a t i o n p r o c e s s by a d a p t i n g a n overhanging beam method (OBM) o r i g i n a l l y used f o r measuring i n t e r n a l s t r e s s i n s o l v e n t c a s t c o a t i n g s (2). D e t a i l s of t h e t h e o r y and i t s l i m i t a t i o n s a r e d i s c u s s e d by Corcoran (3). The f o l l o w i n g p r o c e d u r e i s used. A cement p a s t e c o a t i n g of u n i f o r m t h i c k n e s s (measured w i t h a micrometer a t t h e completion of t h e t e s t ) i s a p p l i e d on a f l e x i b l e s t e e l s u b s t r a t e t o form a composite beam. The t e s t c o n f i g u r a t i o n i s t h a t of
L
,
0.44 I ,wLL [CWE PASTE BEARINGSUBSTRATE
SUPPORT1'
WRT, CAPACITATIVETRANSDUCER (MID SPAN)
FIG.
1Vol. 1 7 , No. 3 M. Nakayarna and J . J . Beaudoin
FIG. 2
S e q u e n t i a l p h o t o of overhanging beam method and a p p a r a t u s
1. Cement p a s t e - s t e e l composite beam d e f l e c t e d upwards due t o s w e l l i n g s t r e s s e s developed d u r i n g h y d r a t i o n
2. Beam i n i n t e r m e d i a t e p o s i t i o n d u r i n g d r y i n g
3. Beam d e f l e c t e d due t o s h r i n k a g e s t r e s s e s developed d u r i n g d r y i n g
4 . Cement p a s t e c o a t i n g h a s debonded from s t e e l s u b s t r a t e
a n overhanging beam (Fig. 1) where t h e overhang (1,) i s e q u a l t o 0 . 4 6 t i m e s t h e midspan l e n g t h ( 2 ) . A photograph of t h e a p p a r a t u s i s g i v e n i n F i g . 2. The midspan d e f l e c t i o n f o r t h e overhanging beam geometry i s i n d e p e n d e n t of any u n i f o r m l y d i s t r i b u t e d weight changes i n t h e cement c o a t i n g . Measurement of t h e midspan d e f l e c t i o n w i t h time p r o v i d e s a means of d e t e r m i n i n g t h e development of i n t e r n a l s t r e s s . The s t r e s s i s t r a n s f e r r e d t o t h e s u b s t r a t e by s h e a r developed a t t h e cement p a s t e - s t e e l i n t e r f a c e . A s s t r e s s d e v e l o p s i n t h e cement p a s t e due t o t h e h y d r a t i o n p r o c e s s t h e beam bends. A n a l y s i s ( 3 ) h a s shown t h a t t h e i n t e r n a l s t r e s s , a, c a n be c a l c u l a t e d u s i n g t h e r e l a t i o n s h i p : where: yc = midspan d e f l e c t i o n Es = modulus of e l a s t i c i t y of s t e e l s u b s t r a t e t = t h i c k n e s s of s t e e l s u b s t r a t e t c = t h i c k n e s s of t h e cement p a s t e l a y e r v = P o i s s o n ' s r a t i o of t h e s t e e l s u b s t r a t e R = midspan l e n g t h of t h e overhanging beam
Vol. 1 7 , No. 3
CEMENT PASTE, STEEL, BOND STRENGTH, TECHNIQUE
Bond S t r e n g t h E s t i m a t e s
Measurements f o r t h e e s t i m a t i o n of bond s t r e n g t h a r e t a k e n i n two s t a g e s . I n t h e f i r s t s t a g e t h e overhanging beam specimens a r e p l a c e d f o r a p r e d e t e r m i n e d t i m e i n a chamber c o n d i t i o n e d a t 100% r e l a t i v e humidity (RH). The i n t e r n a l s t r e s s developed d u r i n g h y d r a t i o n i s monitored. I n t h e second s t a g e t h e specimens a r e removed from t h e 100% RH environment and p l a c e d i n a second chamber c o n d i t i o n e d a t 0%
RH.
I n t h i s chamber t h e cement p a s t e w i l l l o s e w a t e r and s h r i n k a g e s t r e s s w i l l be g e n e r a t e d i n t h e m a t e r i a l . When t h e i n t e r n a l s t r e s s imposed on t h e cement p a s t e due t o d r y i n g i . e . , s h r i n k a g e s t r e s s , i n c r e a s e s t o t h e v a l u e of t h e bond s t r e n g t h between cement p a s t e and s t e e l , t h e cement p a s t e s p o n t a n e o u s l y debonds from t h e s t e e l s u b s t r a t e .The bond s t r e n g t h a t a p a r t i c u l a r time i s t h e r e s u l t a n t of i n t e r n a l s t r e s s due t o h y d r a t i o n a t 100% RH and t h e a d d i t i o n a l s h r i n k a g e s t r e s s r e q u i r e d t o debond t h e sample a t 0% RH.
SEM and EDX A n a l y s i s
A Cambridge S t e r e o s c a n 250 w i t h a TN-5500 X-Ray Analyzer was u s e d f o r e x a m i n a t i o n of debonded s u r f a c e s . R e s u l t s and d i s c u s s i o n I n t e r n a l S t r e s s Development I n t e r n a l s t r e s s developed i n t h e cement p a s t e d u r i n g h y d r a t i o n a t 100% RH was c a l c u l a t e d from t h e e x p e r i m e n t a l d a t a f o r p e r i o d s up t o 168 h. R e s u l t s a r e p l o t t e d i n F i g . 3. T e n s i l e s t r e s s e s a r e developed d u r i n g t h e f i r s t 15 h. Maximum t e n s i l e s t r e s s o c c u r s between 5 h and 7 h. They a r e a t t r i b u t e d t o autogenous s h r i n k a g e r e s u l t i n g from a d e c r e a s e i n s p e c i f i c I volume of t h e h y d r a t i o n p r o d u c t s w i t h r e s p e c t t o t h e r e a c t a n t s ( 4 ) . A f t e r
7 h compressive s t r e s s e s a r e g e n e r a t e d because of s w e l l i n g of t h e cement p a s t e . S w e l l i n g o r i n c r e a s e s i n volume of t h e cement p a s t e d u r i n g t h i s
I p e r i o d have been a t t r i b u t e d t o d i f f e r e n t f a c t o r s ( 5 , 6 ) . It h a s been
s u g g e s t e d t h a t e x p a n s i o n i s due t o t h e h e a t evolved d u r i n g t h e h a r d e n i n g of t h e p a s t e (5). An e x p l a n a t i o n based on s t r e s s r e l a x a t i o n r e s u l t i n g from a n a l l e v i a t i o n of s e l f - d e s i c c a t i o n e f f e c t s h a s a l s o been advanced ( 6 ) .
G e n e r a t i o n of i n c r e a s e d amounts of C-S-H and H 0 e n t e r i n g t h e i n t e r l a y e r may a l s o c o n t r i b u t e t o s w e l l i n g . C o n f l i c t i n g r e s u l t s on t h e e f f e c t of
water-cement r a t i o on t h e amount of s w e l l i n g have been r e p o r t e d . The r e s u l t s i n Fig. 3 a r e i n g e n e r a l agreement w i t h t h e volume-change
measurements of d e l Campo, who observed t h e s w e l l i n g e f f e c t t o d e c r e a s e w i t h w a t e r c e m e n t r a t i o ( 5 ) .
Bond S t r e n g t h
Bond s t r e n g t h between cement p a s t e and s t e e l v e r s u s h y d r a t i o n time i s p l o t t e d i n Fig. 4. Each s t r e n g t h v a l u e r e p r e s e n t s t h e a v e r a g e of up t o f o u r t e s t r e s u l t s . The v a r i a b i l i t y of t e s t r e s u l t s i s dependent on h y d r a t i o n t i m e . V a r i a t i o n from t h e mean f o r w/c = 0.25 p a s t e s r a n g e s from 4.2% a t 3 h h y d r a t i o n t o 17.6% a t 168 h. Bond s t r e n g t h i n c r e a s e s t o a maximum a t 24 h f o r water-cement r a t i o w/c = 0.25 and 0.35, a t 15 h f o r w/c = 0.30, and a t 48 h f o r w/c = 0.40. Maximum bond s t r e n g t h s w i t h r e s p e c t t o water-cement r a t i o a r e i n t h e f o l l o w i n g o r d e r : 0.35, 0.30, 0.40, and 0.25. The bond s t r e n g t h a t w/c = 0.40 i s o u t of sequence. P a s t e s w i t h w/c
>
0.40 have h i g h e r p o r o s i t y , h i g h e r d e g r e e of h y d r a t i o n , lower CIS r a t i o of t h e C-S-HM. Nakayama and J . J . Beaudoin Vol. 1 7 , No. 3 0 357 15 24 48 168 H Y D R A T I O N TIME, h FIG. 3 I n t e r n a l s t r e s s developed i n cement p a s t e d u r i n g h y d r a t i o n a t 100% RH. S h r i n k a g e s t r e s s e s a r e p o s i t i v e p r o d u c t and d i f f e r e n t p o r e s t r u c t u r e t h a n p a s t e s w i t h lower w/c r a t i o ( 7 ) . U n l i k e mechanical p r o p e r t i e s s u c h a s compressive s t r e n g t h , bond s t r e n g t h of
cement p a s t e t o s t e e l does n o t i n c r e a s e w i t h a d e c r e a s e i n t h e water-cement r a t i o . T h i s may be due t o d i f f e r e n c e s i n i n t e r f a c e p r o p e r t i e s from bulk p r o p e r t i e s . I n t e r f a c e p r o p e r t i e s of cement composites d e t e r m i n e t h e magnitude of bond and o t h e r p r o p e r t i e s s u c h a s work of f r a c t u r e . The dependence of f r a c t u r e e n e r g y , f o r example, on t h e w/c r a t i o of some f i b e r - r e i n f o r c e d cement composites i s s i m i l a r t o t h a t f o r bond s t r e n g t h i n t h i s i n v e s t i g a t i o n (8).
T y p i c a l bond-strength v a l u e s determined by o t h e r t e s t methods and t h o s e c a l c u l a t e d from t h i s work a r e g i v e n i n T a b l e 1.
T h e - b o n d - s t r e n g t h v a l u e s f o r t h e overhanging beam method a r e of t h e same o r d e r of magnitude a s t h o s e d e t e r m i n e d by o t h e r t e s t methods.
A d e c r e a s e i n bond s t r e n g t h o c c u r s a f t e r t h e maximum v a l u e i s o b t a i n e d . T h i s o c c u r s when a s i g n i f i c a n t p r o p o r t i o n of t h e t o t a l amount of CH h a s formed f o r t h e w/c = 0.40 and 0.25 p a s t e s (12). A t h y d r a t i o n t i m e s g r e a t e r t h a n 168 h t h e bond s t r e n g t h v a l u e s ( n o t r e p o r t e d ) g e n e r a l l y d i d n o t change s i g n i f i c a n t l y . O t h e r workers have o b s e r v e d d e c r e a s e s i n bond s t r e n g t h a t l a t e r a g e s a f t e r a maximum h a s been reached (13-15). Burakiewicz r e p o r t e d t h a t t h e bond s t r e n g t h of s t e e l f i b e r t o cement mortar d e c r e a s e d a f t e r 14 days (13). S c h n i t t g r u n d determined t h a t bond s t r e n g t h of cement p a s t e and a l i t e p a s t e t o p o l i s h e d s t e e l d e c r e a s e d a f t e r 14 and 7 days
Vol. 1 7 , No. 3
CEMENT PASTE, STEEL, BOND STRENGTH, TECHNIQUE
4.0nl I I I I
W I C
-
1H Y D R A T I O N TIME, h
FIG. 4
Bond s t r e n g t h between cement p a s t e and s t e e l
TABLE 1
Maximum Bond S t r e n g t h Values Between Cement Systems and S t e e l Bond S t r e n g t h (MPa) D e s c r i p t i o n T e s t Method Reference
1.52
-
3.17 s t e e 1 beams Pu 11-out French ( 9 ) embedded i n0.61 m of c o n c r e t e
s t e e l f i b e r i n Pull-ou t Gray and
cement p a s t e Johnson (10) s t e e l f i b e r i n Pull-out Maage ( 1 1 ) m o r t a r steel-cement D i r e c t s h e a r Alexander ( 1 2 ) p a s t e cubes w i t h z e r o normal s t r e s s
cement c o a t i n g Overhanging T h i s work on s t e e l beam method
s u b s t r a t e
r e s p e c t i v e l y (14). Laws e t a l . observed a d e c r e a s e i n bond s t r e n g t h between g l a s s f i b e r and cement p a s t e a f t e r s e v e r a l d a y s h y d r a t i o n a t 100%
RH
(15). Zimbelmann found t h a t bond s t r e n g t h between cement s t o n e and a g g r e g a t eM. Nakayama and J.J. Beaudoin
Vol. 1 7 , No. 3
d e c r e a s e d a f t e r 100 d a y s h y d r a t i o n ( 1 6 ) . Decreases i n bond s t r e n g t h a t
l a t e r a g e s have been a t t r i b u t e d t o c r y s t a l growth i n t h e c o n t a c t l a y e r between cement p a s t e and t h e i n c l u s i o n o r s u b s t r a t e m a t e r i a l (16).
D i f f e r e n c e s i n maximum bond s t r e n g t h and t h e i n c i d e n c e of s t r e n g t h d e c r e a s e may a r i s e from time-dependent d i f f e r e n c e s i n t h e p r o p e r t i e s of t h e c o n t a c t
zone. These d i f f e r e n c e s may be a s c r i b e d t o t h e s e n s i t i v i t y of t h e test
method a s w e l l a s t o t h e changes i n t h e m i c r o s t r u c t u r e of t h e matrix. The
r o l e of t h e t n t e r f a c e o r c o n t a c t l a y e r i n d e t e r m i n i n g bond s t r e n g t h is d i s c u s s e d i n t h e f o l l o w i n g s e c t i o n .
Cement P a s t e - S t e e l I n t e r f a c e
A unique zone a t t h e cement-steel o r cement-aggregate i n t e r f a c e h a s
been i d e n t i f i e d by s e v e r a l workers (17). The zone h a s g r e a t e r p o r o s i t y t h a n
t h e bulk m a t r i x and i s t y p i c a l l y about 40-50 +m t h i c k . A duplex f i l m
c o n s i s t i n g of a l a y e r of CH and C-S-H c o v e r s t h e s u r f a c e of t h e s u b s t r a t e
m a t e r i a l . A l a y e r of l a r g e impure CH c r y s t a l s e x t e n d s from t h e duplex f i l m
i n t o t h e b u l k of t h e cement-paste matrix. E t t r i n g i t e h a s a l s o been observed
on t h e s u b s t r a t e s u r f a c e s (17).
There i s no agreement on how t h e mechanical p r o p e r t i e s of t h e zone
a f f e c t s bond s t r e n g t h . SEM and x-ray a n a l y s i s of s e l e c t e d debonded s u r f a c e s
( o b t a i n e d w i t h t h e overhanging beam method) p r o v i d e some i n s i g h t i n t o t h e r o l e of t h e i n t e r f a c e zone i n d e t e r m i n i n g bond s t r e n g t h .
SEM micrographs of t h e debonded s u r f a c e s
-
cement-paste s i d e and s t e e ls i d e
-
were t a k e n a f t e r 24 h and 72 h h y d r a t i o n . They a r e p r e s e n t e d i nFigs. 5 and 6 . I n a l l c a s e s f a i l u r e t a k e s p l a c e w i t h i n 50 pm of t h e s t e e l
s u r f a c e , i . e . , i n t h e i n t e r f a c e zone. A t 24 h h y d r a t i o n t h e s o l i d s a d h e r i n g
t o t h e s t e e l s u r f a c e were mostly n e e d l e - l i k e c r y s t a l s c o n s i s t i n g p r i n c i p a l l y
of CH and C-S-H, t h e r e l a t i v e p r o p o r t i o n s depending on water-cement r a t i o .
The s o l i d s a t w/c = 0.25 have h i g h e r CIS r a t i o (18). T h i s i s c o n s i s t e n t
w i t h t h e o b s e r v a t i o n t h a t t h e Ca/Si peak i n t e n s i t y r a t i o was much g r e a t e r a t
w/c = 0.25. A t h i g h e r water-cement r a t i o s t h e s o l i d s a r e predominantly
C-S-H. There a r e few a r e a s f o r which t h e Ca/Si r a t i o i s v e r y l a r g e .
D i f f e r e n c e s i n C-S-H/CH r a t i o may account f o r lower bond s t r e n g t h a t
w/c = 0.25. Some e t t r i n g i t e was a l s o d e t e c t e d i n a l l t h e p r e p a r a t i o n s .
The s u r f a c e coverage on t h e s t e e l i n c r e a s e d w i t h water-cement r a t i o
(Fig. 5a-5d). E s t i m a t e s of s u r f a c e coverage f o r water-cement r a t i o = 0.25,
0.30, 0.35, and 0.40 a r e 10, 15, 65, and 85% r e s p e c t i v e l y .
Examination of t h e debonded cement s u r f a c e s r e v e a l s a r e a s of p l a t y CH
and c l u s t e r s of n e e d l e - l i k e c r y s t a l s of C-S-H. The n e e d l e - l i k e morphology
was c l e a r on h i g h e r m a g n i f i c a t i o n photomicrographs, which a r e n o t p r e s e n t e d . The d i s t r i b u t i o n of CH c r y s t a l s t h a t have s e p a r a t e d from t h e s t e e l s u r f a c e
i s d e p i c t e d i n t h e micrographs (Fig. 5e-5h). A t w/c = 0.25 v i r t u a l l y a l l
t h e CH f i l m h a s been removed from t h e s t e e l s u r f a c e . A s t h e water-cement
r a t i o i n c r e a s e s l e s s CH i s removed from t h e s t e e l , and f a i l u r e t a k e s p l a c e
through t h e C-S-H behind t h e CH f i l m .
A f t e r 72 h h y d r a t i o n t h e s u r f a c e coverage of s o l i d s on t h e debonded
s t e e l d e c r e a s e d a t h i g h e r water-cement r a t i o s (Fig. 6a-6d), c o r r e s p o n d i n g t o
lower bond s t r e n g t h s f o r t h e s e specimens. Coverage was approximately 20,
5,
30, and 1% f o r water-cement r a t i o s = 0.25, 0.30, 0.35, and 0.40
Vol. 1 7 , N o . 3 455 CEMENT PASTE, STEEL, BOND STRENGTH, TECHNIQUE
FIG. 5
SEM micrographs of debonded s u r f a c e s (24 h h y d r a t i o n , OBM specimens) ( a ) - ( d ) : s t e e l s u r f a c e ( e l - ( h ) : cement s u r f a c e
( a ) , ( e l : ~ 1 ~ 0 . 2 5 ( b ) , ( f ) : w/c=O. 30 ( c ) , ( g ) : w/c=O.35 ( d ) , ( h ) : w/c=0.40
Vol. 1 7 , No. 3 M . Nakayama and J . J . Beaudoin
FIG. 6
SEM micrographs o f debonded s u r f a c e s ( 7 2 h hydration, OBM specimens) ( a ) - ( d ) : s t e e l s u r f a c e ( e ) - ( h ) : cement s u r f a c e
( a ) , ( e ) : w/c=0.25 ( b ) , ( f ) : w/c=O. 30 ( c ) , ( g ) : w/c=O.35 ( d ) , ( h ) : w/c=0.40
Vol. 1 7 , No. 3
CEMENT PASTE, STEEL, BOND STRENGTH, TECHNIQUE
Micrographs of debonded cement s u r f a c e s a r e g i v e n i n Fig. 6e-6h.
Evidence of CH c r y s t a l growth i s p r o v i d e d i n Fig. 6e. A t w/c = 0.25, l a r g e CH c r y s t a l s 20-40 pm i n s i z e were observed. CH c r y s t a l growth may
c o n t r i b u t e t o lower bond s t r e n g t h a t 72 h.
Micrographs of f a i l u r e s u r f a c e s were a l s o o b t a i n e d f o r a few w/c = 0.30 specimens (Fig. 7a-7d), which had been debonded i n a d i r e c t t e n s i o n t e s t a f t e r 24 h h y d r a t i o n . The bond s t r e n g t h was 1.50 MPa, compared t o t h e v a l u e of 1.63 MF'a o b t a i n e d w i t h t h e overhanging beam method. The morphology of t h e f a i l u r e s u r f a c e s i s q u i t e d i f f e r e n t from t h o s e o b t a i n e d w i t h t h e overhanging beam method. A much g r e a t e r p r o p o r t i o n of s o l i d m a t e r i a l remains a t t a c h e d t o t h e s t e e l s u r f a c e . The debonded cement s u r f a c e a p p e a r s t o be much more porous and t h e C-S-H i s n o t masked by p l a t y s h e e t s of CH. A t 72 h t h e i n t e r f a c e m i c r o s t r u c t u r e i s n o t v e r y d i f f e r e n t . There a p p e a r s t o be a s l i g h t d e c r e a s e i n t h e amount of s o l i d s a t t a c h e d t o t h e s t e e l s u r f a c e and l a r g e r CH c r y s t a l s masking t h e C-S-H on t h e debonded cement s u r f a c e s . The b e h a v i o u r of t h e i n t e r f a c e zone i n a bond-strength
d e t e r m i n a t i o n may be dependent on t h e t e s t method
-
a s u b j e c t t h a t needs f u r t h e r i n v e s t i g a t i o n .Concluding remarks
A new method h a s been developed f o r d e t e r m i n i n g t h e bond s t r e n g t h of cement p a s t e t o s t e e l . The development of bond s t r e n g t h a t e a r l y a g e s of cement h y d r a t i o n c a n be followed. Experimental d i f f i c u l t i e s a s s o c i a t e d w i t h
FIG. 7
SEM micrographs of debonded s u r f a c e (24 and 72 h h y d r a t i o n , d i r e c t t e n s i o n ) ( a ) , ( b ) : s t e e l s u r f a c e ( c ) , ( d ) : cement s u r f a c e
( a ) , ( c ) : w/c=0.30, 24 h h y d r a t i o n ( b ) , ( d ) : w/c=0.30, 72 h h y d r a t i o n
M. Nakayama and J.J. Beaudoin
Vol. 1 7 , No. 3
a p p l i c a t i o n of s t r e s s t o bond-strength specimens have been e l i m i n a t e d . I n t e r n a l s t r e s s developed d u r i n g t h e h y d r a t i o n p r o c e s s i s t a k e n i n t o account i n t h e e s t i m a t i o n o f bond s t r e n g t h . The f o l l o w i n g c o n c l u s i o n s a r e drawn:
1. Maximum bond s t r e n g t h i n c r e a s e s w i t h w/c r a t i o when 0.25 < w/c <
0.35.
2. Bond f a i l u r e o c c u r s c o n s i s t e n t l y i n a l a y e r a few micrometers from t h e s t e e l s u r f a c e .
3. The i n t e r f a c e l a y e r c o n s i s t s e s s e n t i a l l y of CH and C-S-H, and i t s composition and m i c r o s t r u c t u r e a r e c o n s i s t e n t w i t h a unique i n t e r f a c e zone d e s c r i b e d by many workers.
4. The amount of CH l a y e r removed from t h e s t e e l s u r f a c e a p p e a r s t o d e c r e a s e w i t h water-cement r a t i o .
5. Bond s t r e n g t h d e c r e a s e s a t l a t e r a g e s , p o s s i b l y due t o c r y s t a l growth of CH i n t h e i n t e r f a c e zone.
Acknowledgements
The a u t h o r s acknowledge t h e a s s i s t a n c e of Messrs. R.E. Myers and E.G. Quinn w i t h t h e e x p e r i m e n t a l work.
R e f e r e n c e s
1. H.R. S a s s e and M. F i e b r i c h , M a t e r i a l s and S t r u c t u r e s ,
16
(941, 293 (1983).S.G. C r o l l , J. O i l and Col. Chem. Assoc.,
63,
271 (1980). E.M. Corcoran, J. P a i n t Tech.,41
(538), 635 (1969).S. Z i e g e l d o r f and H.K. H i l s d o r f , 7 t h I n t . Cong. Chem. Cem. P a r i s . , Vol. I V , 333 (1980).
M. d e l Campo, RILEM B u l l . No. 4, 18 (1959).
J.F. R a f f l e , Proc. B r i t . Ceram. Soc., No. 35, 295 (1984). R.F. Feldman, Cem. Tech.
2,
5 (1972).S. Akihama, T. Suenaga, and T. Banno, Kajima I n s t . of C o n s t r u c t i o n Tech., Report No. 53, 97 (1984).
E.L. French, i n Proc. Conf. on Aspects of Adhesion, U n i v e r s i t y of London P r e s s , London, England, 120 (1963).
R.J. Gray and C.D. J o h n s t o n , RILEM Symp. on T e s t i n g and T e s t Methods of F i b r e Cement Composites, The C o n s t r u c t i o n P r e s s , Hornby, England, Paper 6.1, 317 (1978).
M. Maage, RILEM Symp. on T e s t i n g and T e s t Methods of F i b r e Cement Composites, The C o n s t r u c t i o n P r e s s , Hornby, England, Paper 6.2, 321
(1978).
12. V.S. L m a c h a n d r a n and R.F. Feldman, I1 Cemento
15
( 3 ) 311 (1978).13. A. Burakiewicz, RILEM Symp. on T e s t i n g and T e s t Methods of F i b r e Cement Composites, The C o n s t r u c t i o n P r e s s , Hornby, England, Paper 6.5, 355 (1978).
14. G.D. S c h n i t t g r u n d , Bond S t r e n g t h Between Cement H y d r a t e s and S t e e l , PhD T h e s i s , U n i v e r s i t y of I l l i n o i s , Urbana, I l l i n o i s , U.S.A., 192 (1975). 15. V. Laws, A. Langley, and J. West, J. Mat. S c i . 21 289 (1986).
16. R. Zimbelmann, Cem. Concr. Res.,
15
( 5 ) , 801 (I&).17. P.J.M. Monteiro, J . C . Maso, and J.P. O l l i v i e r , Cem. Concr. Res.,
3
953 (1985).T h i s p a p e r i s being d i s t r i b u t e d i n r e p r i n t f o r m by t h e I n s t i t u t e f o r R e s e a r c h i n C o n s t r u c t i o n . A l i s t of b u i l d i n g p r a c t i c e and r e s e a r c h p u b l i c a t i o n s a v a i l a b l e from t h e I n s t i t u t e may be o b t a i n e d by w r i t i n g t o t h e P u b l i c a t i o n s S e c t i o n , I n s t i t u t e f o r R e s e a r c h i n C o n s t r u c t i o n , N a t i o n a l Research C o u n c i l o f C a n a d a , O t t a w a , O n t a r i o , KIA 0R6. Ce document e s t d i s t r i b u e s o u s forme d e t i r 6 - a - p a r t p a r 1 ' I n s t i t u t de r e c h e r c h e e n c o n s t r u c t i o n . O n p e u t o b t e n i r une l i s t e d e s p u b l i c a t i o n s de 1 ' I n s t i t u t p o r t a n t s u r les t e c h n i q u e s ou l e s r e c h e r c h e s e n matiere d e b l t i m e n t e n 6 c r i v a n t B l a S e c t i o n d e s p u b l i c a t i o n s , I n s t i t u t de r e c h e r c h e e n c o n s t r u c t i o n , C o n s e i l n a t i o n a l d e r e c h e r c h e s du Canada, Ottawa ( O n t a r i o ) , K1A OR6.
