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Bond strength development between latex-modified cement paste and
steel
-- - - - - 7
S e r
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National Research
Conseil national
N21d
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council Canada
de recherches Canada
no.
1474
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2
Institute for
lnstitut de
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Research in
recherche en
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Construction
construction
Bond Strength Development
between Latex-Modified
Cement Paste and Steel
by M. Nakayama and J.J. Beaudoin
Canad3
774/
702
Reprinted from
Cement and Concrete Research
Vol. 17, No. 4, 1987
p. 562
-
572
(IRC Paper No. 1476)
L I B R A R Y
Price $3.00
NRCC 28321
DEC
4 1367
BIBLIOTHSQUE
I R C
I
RSSUMB
Les auteurs ont dgterming le processus de formation de
l'adhsrence de l'acier
3 la pate de ciment modifise au latex
1grace
2 une nouvelle technique, soit la msthode de la poutre en
porte-&faux.
11s ont $tudi& six types de latex.
En gsngral,
l'ajout de latex amEliore 11adh5rence, qui dfipend des facteurs
suivants
:la teneur en eau non Gvaporable, la chaleur
d'hydratation et la teneur en CH.
11s ont ggalement 6valug le
r81e que joue la zone d'interface sur 11adh6rence.
- -
-
CEMENT and CONCRETE
RESEARCH.
Vol. 17, pp. 562-572, 1987. Printed in the USA. 0008-8846187 $3.00+00. Copyright ( c ) 1987 Pergamon Journals, Ltd.BOND STRENGTH DEVELOPMENT BETWEEN LATEX-MODIFIED CEMENT PASTE AND STEEL
M.
Nakayamal and J. J. Beaudoin Institute for Research in ConstructionNational Research Council of Canada Ottawa, Canada, K1A OR6
(communicated by
M.
Daimon) (Received Feb. 19, 1987)ABSTRACT
Bond strength development between latex-modified cement paste and steel was determined using a new technique, the overhanging beam method. Six latexes were studied. In general bond strength is improved with latex addition. Factors affecting bond strength include non-evaporable water content, heat of hydration, and CH content. The role of the interface zone on bond strength has been evaluated.
Introduction
Active interest in cement systems containing water dispersible polymers on latexes began more than 50 years ago (1). The most commonly used polymer latexes include: styrene-butadiene rubber, poly(acry1ic ester),
poly(viny1idene chloride-vinyl chloride), poly(ethy1ene-vinyl acetate) and poly(viny1 acetate). Latex-modified portland cement mortar and concrete have increased tensile strength, reduced drying shrinkage, increased
, durability, and improved adhesion or bond strength over conventional mortar and concrete (1).
There is a substantial amount of published data on the bond strength of latex-modified cements applied to mortar and concrete substrates. Howe~rer, only a limited amount of data is avsilable on the bond strength between polymer-modified cements and steel surfaces. Increased bond strength
I between cementitious materials and steel is of practical importance in the
application of repair materials to damaged surfaces, and in the design and
I
use of steel fiber-reinforced cement composites and grouts for structurali
purposes.I
This paper discusses the development of bond strength betweeni
lpresent address: Kajima Institute of Construction Technology, Kajima Corporation, Tokyo-182, Japan.
Vol. 1 7 , No. 4 563
BOND STRENGTH, LATEX, CEMENT PASTE, STEEL
l a t e x - m o d i f i e d cement p a s t e and s t e e l employing a new p r o c e d u r e (overhanging beam method) developed by t h e a u t h o r s ( 2 ) . The t e c h n i q u e p e r m i t s e s t i m a t e s of b o t h 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 of cement) and bond s t r e n g t h a t e a r l y ages. 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. 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 o r cement p a s t e modified w i t h s i x d i f f e r e n t
l a t e x e s . 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 u s i n g SEM and EDX a n a l y s e s of s e l e c t e d debonded s u r f a c e s .
Experimental
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%); A120 (6.20%); Fe O 3 (2.23%); CaO (64.83%); MgO (1.84%); SO3 (3.13%); Na20
(0.0!?%); K20 (0.42). B l a i n e f i n e n e s s was 300 m2/kg.
The f o l l o w i n g s i x l a t e x e s were used: two poly(ethy1ene-vinyl a c e t a t e ) copolymers {EVA(I) and EVA(II)}; s t y r e n e b u t a d i e n e r u b b e r (SBR); p o l y ( v i n y 1 a c e t a t e ) (PVA); p o l y ( a c r y 1 i c - v i n y l a c e t a t e ) copolymer (AVA); and
poly ( v i n y l i d e n e c h l o r i d e
-
v i n y l c h l o r i d e ) copolymer (V-V).
Some p r o p e r t i e s of t h e s e l a t e x e s a r e g i v e n i n Table 1. A defoaming a g e n t was added t o t h eAVA b e f o r e i t s u s e w i t h cement p a s t e .
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.
Cement P a s t e Mixes
L a t e x - m o d i f i e d cement p a s t e s were produced w i t h e a c h of t h e s i x l a t e x e s . The p a s t e s were made a t t h r e e waterlcement r a t i o s : w/c = 0.25, 0.30, and 0.35. Three l a t e x / c e m e n t r a t i o s , R/c = 0.04, 0.08, and 0.15 were used a t e a c h w/c r a t i o . Water-cement r a t i o = 0.40 was adopted f o r V-V modified cement p a s t e because of t h e p a s t e ' s low w o r k a b i l i t y a t w/c = 0.25. Cement p a s t e s c o n t a i n i n g no l a t e x were a l s o made.
Specimen P r e p a r a t i o n
Composite beam specimens f o r u s e w i t h t h e overhanging beam method were made by a p p l y i n g latex-modified cement p a s t e t o t h e s u r f a c e of s t e e l f e e l e r gauge 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. Four t e s t specimens were made f o r e a c h t e s t c o n d i t i o n . The t h i c k n e s s of t h e p a s t e (approximately 0.75 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 . 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 t h e a p p l i c a t i o n of t h e p a s t e . ~ lspecimens were p r e p a r e d a t a t e m p e r a t u r e of l 20f 2Oc.
Some specimens were a l s o p r e p a r e d f o r a d i r e c t - t e n s i o n bond t e s t . Two procedures were used. I n t h e f i r s t , p a s t e s were c a s t a g a i n s t a 25 mm s q u a r e s t e e l f e e l e r - g a u g e s u r f a c e epoxied t o a b r a s s t e s t block. A f t e r a
predetermined c u r i n g p e r i o d a t 100% r e l a t i v e humidity (RH) a n o t h e r b l o c k was epoxied t o t h e f r e e cement p a s t e s u r f a c e . I n t h e second, p a s t e was c a s t between t h e two b r a s s blocks. Some specimens were allowed t o d r y a t 0% and 55%
RH
b e f o r e t e s t i n g .Vol. 1 7 , No. 4
M. Nakayama and J.J. Beaudoin
TABLE 1
P r o p e r t i e s of L a t e x e s L a t e x
- --
P r o p e r t i e s EVA ( I ) EVA (11) SBR PVA AVA V-V N o n - v o l a t i l e s ( % ) * 55.5 46.0 48.4 55.1 55.4 56.2 PH 4.5 6.0k1.0 4.0k0.5 5.3k0.7 8.3 V i s c o s i t y ( c p s ) B r o o k f i e l d LVT 6 0 rpm 1700 100Ok500 RVT 20 rpm 2300 RVT 25 rpm 1050k150 S p e c i f i c g r a v i t y 1.07 1.06 1.10k0.01 1.07f0.01 1.26 P a r t i c l e s i z e - range (pm) 2.5k1.5 0.35k0.15
P a r t i c l e Charge Non-ionic A n i o n i c Anionic, *measured by a u t h o r s ; o t h e r v a l u e s of p r o p e r t i e s were p r o v i d e d by
m a n u f a c t u r e r s .
Bond S t r e n g t h D e t e r m i n a t i o n
A n o v e l t e c h n i q u e employing a n overhanging beam method (OBM) was u s e d t o e s t i m a t e t h e bond s t r e n g t h between t h e l a t e x - m o d i f i e d cement p a s t e and s t e e l . The method h a s been d e s c r i b e d i n d e t a i l i n p r e v i o u s p a p e r s (2-4). The p r o c e d u r e i s b r i e f l y a s f o l l o w s . Specimens a r e p r e p a r e d by a p p l y i n g a
cement c o a t i n g t o a s t e e l s u b s t r a t e . The geometry of t h e beam (overhang = 0.46 x midspan) i s chosen t o n u l l o u t t h e e f f e c t of any u n i f o r m l y d i s t r i b u t e d w e i g h t changes i n t h e cement c o a t i n g . S t r e s s e s developed i n t h e cement p a s t e c o a t i n g c a n t h e n be c a l c u l a t e d from measurements of midspan d e f l e c t i o n (2). Measurements 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 specimens a r e p l a c e d i n a chamber c o n d i t i o n e d a t
100% RH. D e f l e c t i o n s of t h e overhanging beam due t o h y d r a t i o n of cement p a s t e a r e c o n t i n u o u s l y 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 chamber a t 0% RH. S h r i n k a g e s t r e s s e s i n t h e cement p a s t e due t o d r y i n g r e s u l t i n a d d i t i o n a l d e f l e c t i o n of t h e beam. The bond s t r e n g t h a t a p a r t i c u l a r t i m e i s t h e r e s u l t a n t of t h e 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 OX RH. Bond s t r e n g t h s were d e t e r m i n e d f o r p e r i o d s up t o 168 h.
Bond specimens were a l s o t e s t e d w i t h a d i r e c t t e n s i o n method u s i n g a
T i n i u s Olsen t e s t i n g machine. The l o a d i n g r a t e was 0.50 mm/min. Non-evaporable Water-content D e t e r m i n a t i o n
Non-evaporable w a t e r c o n t e n t was determined f o r cement p a s t e and l a t e x - m o d i f i e d p a s t e specimens a t w/c = 0.30 and R/c = 0.08. The p a s t e samples were removed from t h e s t e e l s u b s t r a t e a t v a r i o u s times, and h y d r a t i o n was s t o p p e d by pumping specimens i n a vacuum d e s i c c a t o r u n t i l ready f o r t e s t i n g . Non-evaporable w a t e r c o n t e n t s were d e t e r m i n e d by TGA i n a n i t r o g e n atmosphere u s i n g a 1090 Dupont Thermal Analyzer w i t h a 951 TGA a c c e s s o r y . The h e a t i n g r a t e was 20°c/min, and t h e maximum t e m p e r a t u r e was 1000°C. Non-evaporable w a t e r c a l c u l a t i o n s i n c l u d e d a c o r r e c t i o n f o r w e i g h t
Vol. 1 7 , No. 4 565
BOND STRENGTH, LATEX, CEMENT PASTE, STEEL
l o s s due t o decomposition of t h e l a t e x . It i s assumed t h a t no c h e m i c a l l y bound w a t e r i s a s s o c i a t e d w i t h t h e f o r m a t i o n of new complexes due t o any latex-cement m i n e r a l i n t e r a c t i o n . No such complexes have been i d e n t i f i e d . Conduction C a l o r i m e t r y
The r a t e and amount of h e a t development i n h y d r a t i n g cement and l a t e x - m o d i f i e d cement systems were measured u s i n g a conduction c a l o r i m e t e r . Only specimens p r e p a r e d a t w/c = 0.30 and R/c = 0.08 were i n v e s t i g a t e d . For d a t a a c q u i s i t i o n and p r o c e s s i n g , t h e c a l o r i m e t e r was i n t e r f a c e d w i t h a n Apple I I e microcomputer u s i n g a Taurus One 12-bit a n a l o g - t o - d i g i t a l c o n v e r t o r and a n Apple s u p e r - s e r i a l i n t e r f a c e card.
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 used f o r examination of debonded s u r f a c e s of s e l e c t e d specimens h y d r a t e d f o r 24 and 72 h. Latex-modified cement specimens made a t w/c = 0.30 and R/c = 0.08 were examined. Specimens were o b t a i n e d by c u t t i n g a t e s t p i e c e from t h e
overhanging beam.
R e s u l t s and d i s c u s s i o n H y d r a t i o n Parameters
Hydration p a r a m e t e r s such a s non-evaporable water c o n t e n t , r a t e of h e a t development, and CH c o n t e n t a r e u s e f u l i n f o l l o w i n g t h e m i c r o s t r u c t u r a l development of cement systems.
Non-evaporable w a t e r c o n t e n t (wn) d e t e r m i n a t i o n s f o r t h e l a t e x - m o d i f i e d cement p a s t e systems (w/c = 0.30, R/c = 0.08) a r e p l o t t e d i n F i g . 1. I n t h e f i r s t 7 h , wn v a l u e s f o r t h e AVA, PVA, V-V, and EVA(1) systems a r e
I s i g n i f i c a n t l y g r e a t e r t h a n t h o s e f o r t h e c o n t r o l , SBR and EVA(I1). A t 24 h , PVA and EVA(II), have lower w v a l u e s t h a n t h e c o n t r o l , and V-V h a s t h e
i
h i g h e s t wn value. A t 168 h t E e wn v a l u e s a r e s i m i l a r f o r a l l systems e x c e p t1
f o r V-V and PVA, which have h i g h e r and lower v a l u e s r e s p e c t i v e l y .I n g e n e r a l t h e s e f i n d i n g s a r e i n agreement w i t h t h o s e of o t h e r workers. L a t e x e s a r e known t o a c c e l e r a t e o r r e t a r d h y d r a t i o n of cement depending on l a t e x type. Those l a t e x e s known t o a c c e l e r a t e t h e h y d r a t i o n of cement, a t
least i n i t i a l l y , i n c l u d e V-V and a c r y l i c s (5). PVA h a s been r e p o r t e d t o r e t a r d h y d r a t i o n (6). It i s s u b j e c t t o h y d r o l y s i s and i n t e r a c t s w i t h CH. High w v a l u e s f o r V-V, AVA, and PVA a t 3 h may i n v o l v e a c c e l e r a t e d
h y d r a d o n of t h e a l u m i n a t e p h a s e s a s a s i g n i f i c a n t amount of C3S h y d r a t i o n h a s n o t occurred. D e h y d r o c h l o r i n a t i o n of V-V i s known t o o c c u r i n a h i g h l y a l k a l i n e cement s y s t e m ( 5 ) . P r e s e n c e of c h l o r i d e i o n i s known t o a c c e l e r a t e h y d r a t i o n of cement. Release of c h l o r i d e i o n i n t h e V-V s y s t e m may
c o n t r i b u t e t o i n c r e a s e d wn v a l u e s a t l a t e r ages.
Rate of h e a t development v e r s u s time c u r v e s were o b t a i n e d f o r t h e s e systems u s i n g c o n d u c t i o n c a l o r i m e t r y (Fig. 2). The h e a t developed i n t h e f i r s t 2 h and 3.5 h i s g r e a t e r f o r AVA and PVA r e s p e c t i v e l y compared t o t h a t w i t h t h e c o n t r o l . T h i s i s i n c o n c e r t w i t h t h e h i g h e r wn v a l u e s observed d u r i n g t h i s period. The i n i t i a l r a t e of h e a t development f o r V-V was much lower t h a n expected. A f t e r t h e f i r s t few h o u r s t h e t o t a l h e a t developed i s
lower t h a n t h e c o n t r o l f o r a l l latex-cement systems. The r a n k i n g of t h e s e systems f o r t o t a l h e a t developed i s a s f o l l o w s : cement p a s t e > EVA(1) > EVA(I1) > SBR > V-V > AVA > PVA.
Vol. 1 7 , No. 4
M. Nakayama and J . J . Beaudoin
0 3 7 16 24 48 72 168 H Y D R A T I O N T I M E , h v-- ---* 0 C E M E N T FIG. 1
Non-evaporable w a t e r c o n t e n t v e r s u s h y d r a t i o n time f o r l a t e x - m o d i f i e d cement p a s t e s
CH e s t i m a t i o n was c a r r i e d o u t by t h e r m o g r a v i m e t r i c a n a l y s i s (TGA) and t h e Franke Method ( 7 ) . R e s u l t s a r e n o t r e p o r t e d because of p o s s i b l e i n t e r f e r e n c e e f f e c t s due t o t h e p r e s e n c e of t h e l a t e x .
Bond S t r e n g t h
E s t i m a t e s of peak bond s t r e s s (bond s t r e n g t h ) between l a t e x - m o d i f i e d cement p a s t e and s t e e l were made a t v a r i o u s h y d r a t i o n t i m e s from
e x p e r i m e n t a l r e s u l t s of i n t e r n a l s t r e s s development w i t h time. Maximum v a l u e s of peak bond s t r e s s and peak bond s t r e s s v a l u e s a t 5 h h y d r a t i o n f o r a l l mixes a r e p r e s e n t e d i n Table 2. The maximum v a l u e s f o r t h e s e systems i n c r e a s e i n t h e f o l l o w i n g o r d e r : V-V < AVA < c o n t r o l < PVA < EVA(1) < SBR < EVA(I1). The i n c r e a s e i n maximum peak bond s t r e s s compared t o t h e c o n t r o l v a r i e s from 15.7% f o r PVA t o 73.2% f o r EVA(I1). Values of bond-strength i n c r e a s e f o r PVA-modified mortar and s t e e l may v a r y , and up t o 59% f o r 20% PVA h a s been r e p o r t e d ( 8 , 9 ) . There i s a p a u c i t y of d a t a a v a i l a b l e f o r o t h e r l a t e x e s .
I n a l l c a s e s b u t one t h e maximum v a l u e s of peak s t r e s s o c c u r r e d a t R/c = 0.08. For EVA(II), w/c = 0.30 t h e maximum v a l u e i s t h e same a t R/c = 0.08 ( 2 4 h) and R/c = 0.15 ( 7 2 h ) . L a t e x cements a r e n o t normally cured a t 100% RH f o r p e r i o d s l o n g e r t h a n 24 h t o f a c i l i t a t e f i l m formation. Prolonged moist c u r i n g u s u a l l y i s d e l e t e r i o u s . The s t a b l e v a l u e of bond s t r e n g t h f o r most l a t e x cements would a p p e a r t o be t h e 24 h value. A t 72 h t h e bond s t r e n g t h f o r R/c = 0.15 i s s t i l l i n c r e a s i n g . EVA(I1) a p p e a r s t o have s u p e r i o r p r o p e r t i e s w a r r a n t i n g f u r t h e r i n v e s t i g a t i o n . The w/c r a t i o a t which t h e maximum peak s t r e s s o c c u r s was dependent on l a t e x type. Except
f o r t h e SBR system t h e w/c r a t i o v a l u e s f o r which t h e l a r g e s t v a l u e of maxirtium peak s t r e s s was o b t a i n e d a r e l e s s t h a n f o r t h e unmodified cement
Vol. 1 7 , No. 4 567
BOND STRENGTH, LATEX, CEMENT PASTE, STEEL
0
0 8 16 2 4 32 40 48
TIME, h
FIG. 2
Conduction c a l o r i m e t r i c c u r v e s f o r latex-modified cement p a s t e s
p a s t e , i . e . , w/c = 0.35. It was found t h a t t h e non-evaporable w a t e r v e r s u s time c u r v e f o r t h e SBR system i s i n c l o s e proximity t o t h e c o n t r o l p a s t e .
The 5 h peak bond s t r e s s v a l u e s i n c r e a s e i n t h e f o l l o w i n g o r d e r : AVA < SBR < c o n t r o l < V-V < EVA(I1) < EVA(1) < PVA. The o r d e r v a r i e s from t h a t of t h e maximum peak s t r e s s v a l u e s . S t r e s s v a l u e s f o r EVA(1) and (11) and PVA remain s i g n i f i c a n t l y g r e a t e r t h a n t h e c o n t r o l . Most of t h e maxirrmm
5 h peak bond s t r e s s v a l u e s occur a t t h e lowest w/c r a t i o and R/c r a t i o .
P o s s i b l y t h e l a t e x c o n t i g u i t y has n o t occurred.
An example of bond s t r e n g t h development i s g i v e n by t h e c u r v e s f o r p a s t e modified w i t h EVA(I1) i n Fig. 3. Each d a t a p o i n t i s t h e average o f f o u r t e s t r e s u l t s . V a r i a t i o n i n r e s u l t s i s dependent on h y d r a t i o n time and w/c r a t i o . V a r i a t i o n from t h e mean ranges from a b o u t 4% a t e a r l y a g e s t o
18% a t l a t e r ages. The bond s t r e n g t h f o r t h e EVA(I1) system i s
s i g n i f i c a n t l y improved a f t e r 15 h f o r a l l l a t e x c o n t e n t s . A t R/c = 0.08 t h e
maximum bond s t r e n g t h i s reached i n 24 h. S t r e n g t h r e t r o g r e s s i o n of l a t e x cement w i t h prolonged moist c u r i n g i s w e l l known! The r e t r o g r e s s i o n f o r EVA(I1) (R/c = 0.04 and 0.08 a t 24 h ) i s , however, much l e s s t h a n f o r t h e
o t h e r l a t e x systems. The PVA-cement systems f o r example l o s e a l l s t r e n g t h a t 72 h. S t r e n g t h r e t r o g r e s s i o n f o r unmodified cement p a s t e h a s been a t t r i b u t e d t o
CH
c r y s t a l growth i n a w e l l d e f i n e d t r a n s i t i o n zone 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 (2,lO). T h i s mechanism may a l s o c o n t r i b u t e t o s t r e n g t h r e t r o g r e s s i o n i n t h e l a t e x - m o d i f i e d cement systems.Vol. 1 7 , No. 4 M. Nakayama and J.J. Beaudoin
I
TABLE 2Maximum and 5h Bond S t r e n g t h Values f o r Latex Modified Cement P a s t e
Maximum Bond S t r e n g t h 5h Bond S t r e n g t h (MPa)
(MPa) R/c A/ c Latex w/c 0.04 0.08 0.15 0.04 0.08 0.15 0.25 2.20 0.91 0.52 0.45 0.24 0.58 AVA 0.30 1.36 2.29 0.78 0.58 0.58 0.58 0.35 1.33 1.87
-
0.69 0.45-
0.25 1.29 3.04 1.52 0.86 0.17 0.52 SBR 0.30 1.20 1.94 1.45 0.14 0.14 0.14 0.35 1.13 4.43-
0.79 0.34-
0.25 1.91 2.42 2.81 1.91 2.42 2.81 PVA 0.30 3.23 3.24 3.07 1.55 2.03 2.20 0.35 1.91 3.23-
1.58 2.30-
0.30 1.58 1.84-
1.17 0.21-
V-V 0.35 1.84 1.29 0.97 0.48 0.38 0.34 0.40 1.26 1.45 1.33 0.38 0.27 0.38 0.25 1.24 CONTROL 0.30 2.06 (no l a t e x ) 0.35 2.88 0.40 1.82 Bond s t r e n g t h v a l u e s f o r t h e l a t e x - m o d i f i e d p a s t e s (w/c = 0 . 3 0 )determined by d i r e c t t e n s i o n t e s t s were dependent on t h e t e s t p r o c e d u r e used. The maximum v a l u e s o b t a i n e d i n t h e t e s t where t h e p a s t e was c a s t between t h e two end b l o c k s v a r i e d from 0.96 t o 3.02 MPa a t 7 2 h f o r PVa and SBR r e s p e c t i v e l y (R/c = 0.08). The c o r r e s p o n d i n g bond s t r e n g t h of
unmodified cement p a s t e was 2.74 MPa. SBR was t h e o n l y l a t e x f o r which t h e bond s t r e n g t h exceeded t h e c o n t r o l p a s t e v a l u e . Optimum d r y i n g c o n d i t i o n s f o r l a t e x f i l m f o r m a t i o n were n o t provided i n t h i s t y p e of t e s t . It i s n o t known why SBR l a t e x performed b e t t e r t h a n o t h e r l a t e x e s i n t h i s t e s t .
Drying specimens a t 0% and 55% RH f o r v a r i o u s times d i d n o t improve t h e bond s t r e n g t h , a s m o i s t u r e l o s s due t o d r y i n g and subsequent f i l m f o r m a t i o n was g r e a t l y h i n d e r e d by specimen geometry.
The maximum bond s t r e n g t h v a l u e s i n t h e a l t e r n a t e t y p e of d i r e c t t e n s i o n t e s t , where t h e l a t e l r c e m e n t s u r f a c e was allowed t o d r y a t 55% RH b e f o r e a t t a c h i n g t h e b r a s s end b l o c k , were o b t a i n e d f o r t h e EVA(I1) system. These v a l u e s i n c r e a s e d from 1.03 t o 1.71 MPa a s d r y i n g time i n c r e a s e d t o
Vol. 1 7 , No. 4 569 BOND STRENGTH, LATEX, CEMENT PASTE, STEEL
0
0 5 16 2 4 7 2
H Y D R A T I O N TIME, h
FIG. 3
Bond s t r e n g t h v e r s u s time f o r poly(ethylene-viny1acetate) copolymer (EVA(I1)). Specimens c u r e d a t 100% RH u n t i l t e s t e d by overhanging beam method
16 h. No a p p r e c i a b l e bond s t r e n g t h was r e c o r d e d f o r unmodified cement p a s t e t e s t e d i n t h i s manner. I n t e r n a l s t r e s s developed d u r i n g d r y i n g was
s u f f i c i e n t t o weaken t h e bond i n t h e s e specimens. Bond S t r e n g t h and Hydration Parameters
There i s no s i n g l e r e l a t i o n s h i p between bond s t r e n g t h and wn f o r a l l t h e systems. I f complexes form due t o latex-cement m i n e r a l i n t e r a c t i o n t h e s i g n i f i c a n c e of w v a l u e s may be q u e s t i o n e d . The g e n e r a l tendency i s , however, t h a t bona s t r e n g t h i n c r e a s e s w i t h wn f o r e a c h system.
There i s a l s o no unique r e l a t i o n s h i p between t h e amount of h e a t developed a t a g i v e n h y d r a t i o n time and bond s t r e n g t h f o r a l l t h e systems. The s i g n i f i c a n c e of t h e t o t a l h e a t v a l u e s would a l s o depend on t h e
p o s s i b i l i t y of complex f o r m a t i o n due t o latex-cement m i n e r a l i n t e r a 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
-
approximately 50 pm t h i c k-
a t t h e i n t e r f a c e between cement p a s t e and v a r i o u s s u b s t r a t e m a t e r i a l s h a s been i d e n t i f i e d by s e v e r a l workers (11). The zone i s porous and r i c h i n CH. A d e t a i l e d d e s c r i p t i o n of I t h i s i n t e r f a c e r e g i o n i s g i v e n e l s e w h e r e ( 1 0 , l Z ) . It i s g e n e r a l l y f e l t t h a tt h e p r o p e r t i e s of t h i s zone i n f l u e n c e t h e behaviour of cement composites. S t e e l and cement p a s t e s u r f a c e s were examined i n t h e SEM a f t e r t h e cement p a s t e c o a t i n g had r e l e a s e d from t h e cement s u r f a c e . F a i l u r e o c c u r r e d w i t h i n 5-10 pm of t h e s t e e l s u r f a c e i n a l l c a s e s , i . e . , i n t h e i n t e r f a c e zone. The 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 a t 24 h h y d r a t i o n appeared
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M. Nakayama and J . J . Beaudoin
very f i n e , w i t h n e e d l e - l i k e c r y s t a l l i n e m a t e r i a l c o n s i s t i n g p r i n c i p a l l y of C-S-H and CH. Photomicrographs of t h e SBR-cement system, (Fig. 4 a ) r e v e a l needle-shaped c r y s t a l 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 . A small amount of e t t r i n g i t e was d e t e c t e d i n some of t h e specimens. L a t e x f i l m was found a d h e r i n g t o b o t h t h e s t e e l s u r f a c e and h y d r a t i o n p r o d u c t s a t t a c h e d t o t h e s t e e l i n EVA(1) and (111, SBR, and PVA systems.
L a t e x - m o d i f i e d cement s u r f a c e s were g e n e r a l l y covered by a t h i n l a y e r of CH. T h i s i s shown i n Fig. 4b f o r t h e EVA(1)-cement system. Large a r e a s of f i n e m a t e r i a l have been i d e n t i f i e d a s CH. EDX a n a l y s i s of t h e s e a r e a s g i v e s v e r y h i g h Ca/Si r a t i o s . C l u s t e r s of n e e d l e - l i k e C-S-H p a r t i c l e s were observed i n t h o s e a r e a s where t h e CH l a y e r was removed. These a r e a s appear a s a network of m i c r o c a v i t i e s between l a r g e CH masses. L a t e x f i l m was found around t h e C-S-H c r y s t a l s , e s p e c i a l l y i n EVA(1) and AVA-cements.
The 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 a f t e r 72 h h y d r a t i o n were g e n e r a l l y C-S-H and CH, s i m i l a r t o t h e p r o d u c t s p r e s e n t a t 24 h. Large CH c r y s t a l s (15-20 pm) were found i n t h e PVA system (Fig. 4c).
Latex f i l m was a l s o found on t h e s t e e l s u r f a c e and around t h e s o l i d s a t t a c h e d t o t h e s t e e l i n t h e EVA(I1)-cement system (Fig. 4d). T h i s may account f o r t h e r e l a t i v e l y h i g h bond s t r e n g t h i n t h i s system. It seems t h e l a t e x f i l m may a c t b o t h a s a n a d h e s i v e and r e i n f o r c e m e n t of t h e porous i n t e r f a c e l a y e r .
Latex-cement s u r f a c e s were a l s o covered by a l a y e r c o n t a i n i n g CH and C-S-H. Numerous l a r g e CH c r y s t a l s (5-10 ym) and p l a t e - l i k e CH c r y s t a l s
(10-100 pm) were found on PVA and AVA-cement s u r f a c e s r e s p e c t i v e l y . Photomicrographs of t h e AVA-cement system (Fig. 4e and 4f ), d e p i c t t h e growth of l a r g e CH c r y s t a l s i n t h e i n t e r f a c e zone. CH c r y s t a l growth may be r e s p o n s i b l e f o r lower bond s t r e n g t h a t 72 h.
D i f f e r e n c e s were observed i n t h e morphology of t h e i n t e r f a c e between unmodified cement p a s t e and l a t e x - m o d i f i e d cement p a s t e . A l a r g e number of n e e d l e - l i k e c r y s t a l s (C-S-H and a small amount of e t t r i n g i t e ) were observed i n t h e i n t e r f a c e zone i n unmodified cement p a s t e s a t b o t h 24 and 72 h. Needle-like c r y s t a l s were s p a r s e and i n d i s t i n c t a t 24 h i n t h e
l a t e x - m o d i f i e d systems (EVA(1) and ( 1 1 ) , PVA, and AVA). Needle-like c r y s t a l s (C-S-H and a small amount of e t t r i n g i t e ) were abundant i n t h e EVA(1) and ( 1 1 ) systems a t 72 h (Fig. 4g) and n o t a p p a r e n t i n t h e PVA and
AVA cement system. Needle-like morphology may be a s s o c i a t e d w i t h h i g h e r bond s t r e n g t h of EVA(1) and (11). These c r y s t a l s were a l s o observed i n t h e SBR and V-V modified systems a t 24 h. The morphology changed, however, a t 72 h.
Conclusions
1. A new t e c h n i q u e
-
t h e overhanging beam method-
f o r d e t e r m i n i n g t h e development of bond s t r e n g t h a t e a r l y a g e s between latex-modified cement and s t e e l h a s been developed. The method a c c o u n t s f o r t h e development o f 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 of cement.2. Latex a d d i t i o n g e n e r a l l y i n c r e a s e s 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 . A l l l a t e x e s t e s t e d e x c e p t p o l y ( v i n y 1 i d e n e c h l o r i d e
-
v i n y l c h l o r i d e ) and p o l y ( a c r y l i c u i n y 1 a c e t a t e ) copolymers i n c r e a s e d t h e bond s t r e n g t h .3. The maximum bond s t r e n g t h of l a t e x modified cement was g e n e r a l l y
V o l . 1 7 , N o . 4 571 BOND STRENGTH, LATEX, CEMENT PASTE, STEEL
FIG. 4
SEM micrographs of debonded surfaces:
( a ) s t e e l s u r f a c e , SBR, 24 h hydration
( b ) cement surface, EVA(I), 2 4 h hydration
( c ) s t e e l s u r f a c e , PVA, 7 2 h hydration
( d ) s t e e l s u r f a c e , EVA(II), 7 2 h hydration
( e ) cement s u r f a c e , AVA, 7 2 h hydration ( f ) d i f f e r e n t magnification of ( e )
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M. Nakayama and J . J . Beaudoin
t o h y d r a t i o n of cement h a s b e e n produced a t t h i s time.
4. Bond s t r e n g t h g e n e r a l l y i n c r e a s e s w i t h non-evaporable w a t e r c o n t e n t i n t h e f i r s t 24 he
5. Bond f a i l u r e of l a t e x - m o d i f i e d cements g e n e r a l l y o c c u r s i n a zone o r i n t e r f a c e l a y e r 5-10 pm from t h e s t e e l s u b s t r a t e s u r f a c e .
6. The i n t e r f a c e l a y e r c o n s i s t s mainly o f C-S-H and CH. L a t e x f i l m i n some l a t e x cements c a n be d e t e c t e d i n t h i s l a y e r . T h i s f i l m may c o n t r i b u t e t o i n c r e a s e d bond s t r e n g t h .
7. There
i r
a t e a d e n c y f o r l a r g e amounts o f CH t o form i n t h e i n t e r f a c e zone r a t h e r t h a n i n t h e b u l k of t h e cement system. 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 d u e t o growth o f CH c r y e t a l s p r e s e n t i n t h e i n t e r f a c e zone.8. C a u t i o n s h o u l d b e e x e r c i s e d i n u s i n g t h e e s t i m a t i o n o f t o t a l a w u n t of CH a s a n i n d i c a t o r ~f t h e d e g r e e of h y d r a t i o n i n latex-cement systems.
Acknowledgementr
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 EIG. Quinn w i t h t h e e x p e r i m e n t a l r o r k .
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