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Meniscus effects and fracture in Portland cement paste
Beaudoin, J. J.
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N21dno. 1436
National Research
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Meniscus Effects and Fmcture in
Portland Cement Paste
by J.J. Beaudoin
Reprinted from
Journal of Materials Science Letters
Vol. 5, No. 11, November 1986
p. 1107-1108
(IRC Paper No. 1436)
Price $2.00
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ABSTRACT On t h e b a s i s of r e s u l t s of f r a c t u r e mechanics e x p e r i m e n t s a t d i f f e r e n t h u m i d i t i e s , i t i s s u g g e s t e d t h a t meniscus f o r c e s g e n e r a t e d by d r y i n g of p o r t l a n d cement p a s t e c o n t r i b u t e t o c r a c k growth p r o c e s s e s . Se b a s a n t s u r l e s r g s u l t a t s d ' e x p g r i e n c e s d e mgcanique d e l a r u p t u r e r g a l i s g e s 3 d i f f g r e n t s d e g r g s d ' h u m i d i t g , l ' a u t e u r a v a n c e q u e l ' a c t i o n d e s m6nisques due a u s g c h a g e d e l a p a t e d e ciment P o r t l a n d c o n r r i b u e aux p r o c e s s u s i n t e r v e n a n t d a n s l ' a l l o n g e m e n t d e s f i s s u r e s .
J O U R N A L O F M A T E R I A L S S C I E N C E L E T T E R S 5 ( 1 9 8 6 ) 1107-1108
Meniscus effects and fracture in Portland cement paste
J . J . B E A U D O I N
Institute of Research in Construction, National Research Council of Canada, Ottawa, Canada K7A OR6
Recent studies have indicated that relative humidity, temperature, and stress corrosion in crack tip regions are among the factors that influence crack growth of Portland cement paste [l-31. Several suggestions regarding crack growth mechanisms have been advanced, including one that meniscus forces have a significant effect. This letter presents the results of fracture mechanics experiments designed to clarify the
1 role of meniscus effects in the fracture process.
Crack growth experiments were carried out on both cement paste and porous glass systems (Vycor glass
i was supplied by the Dow Corning Co, USA; porosity
was 28% and surface area 175 x 103m2kg-I). Two test geometries - simply supported, centrally loaded
notched beams, and double-torsion (DT) specimens
- were used to obtain critical stress-intensity factor
(Kc) data and crack velocity against stress-intensity factor (V-K,) curves. Test beams were 12mm x
1.27 mm x 75 mm long for the paste and 6.4 mm x
6.4mm x 75 mm long for porous glass. The mid- span notch was 0.25mm wide by half the specimen depth. DT specimens for paste were 38 mm x 76 mm x
1 mm thick, with a 1.5 x 0.5 mm deep centrally located guide groove along the length. The glass specimens were 6 and 8 mm thick and the guide groove was half the specimen depth. The test procedures have been described previously [4-61. All tests were performed at 22°C in an environment chamber mounted in an Instron machine. Care was taken to avoid carbon- ation of the cement samples.
Stress-intensity factor is plotted against relative #
humidity for cement paste in Fig. 1. As drying pro- ceeds from the saturated condition, Kc and I?, (median ;I value of K taken from each V-K, curve, not presented) increase in both curves to a maximum at 32% r.h. This is followed by a decrease in K as humidity decreases
R E L A T I V E H U M I D I T Y
1%)
Figure I Stress intensity factor-relative humidity curves for Port- land cement paste [I, 51. W/C = 0.35. (---) Double torsion speci- men, ( )notched beam specimen.
further to 1 1 % r.h. Finally, K increases from 11 % r.h. to the dry state.
Qualitatively, both test geometries provided similar results for the porous glass system (Fig. 2); each plot point represents the average obtained from at least three specimens. The dry state for the double torsion specimens was difficult to achieve because the majority of the samples cracked. Two were obtained with no apparent crack, however, and the plot point repre- sents the results from these samples. K-adsorption data were obtained by drying to 32% r.h. and rewet- ting to the desired humidity. On desorption, the curves exhibit maxima at about 75% r.h. K decreases as humidity is lowered to about 60 to 70% r.h. It then increases until the dry state is reached. No maxima can be observed on the adsorption cycle. Non-pourous glass does not exhibit a maximum on desorption [7]. That between 60 and 85% r.h. in the compressive strength-relative humidity curve for porous glass has already been reported by Hiller [8].
0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0 1 0 0
R E L A T I V E H U M I D I T Y (70)
Figure 2 Stress intensity factor-relative humidity curves for porous glass. Curve 1, notched beam specimens: ( 0 ) desorption, (e)
adsorption. Curve 2, double torsion specimens: desorption ( 0 ) 8 mm specimen, (0) 6 mm specimen; adsorption (M) 6 mm specimen.
Figure 3 Weight change isotherms for (1) porous glass and (2) cement paste (reversible isotherm) [9, 101. W/C = 0.50.
Weight change and length change isotherms for porous glass and cement paste are sketched in Figs 3 and 4 [9, 101. The cement paste isotherm has been separated by Feldman into "reversible" and "irre- versible" components [lo]. The reversible isotherm sketched in Figs 3 and 4 does not include the effects of interlayer water. The primary hysteresis loops (weight change) clob at about 60% r.h. for glass and at 32% r.h. for cement paste. These relative humidities are close to those for which maxima occur in the K-r.h. curves for glass and cement paste.
Length change on desorption exceeds that for adsorption (in the region of primary hysteresis); con- traction changes to expansion at 70% and 40% r.h. for glass and paste, respectively. Length change reversal on drying of paste has also been observed [ll]. On further drying, contraction resumes at 60% and 32% r.h.
Length change reversal on drying of porous glass has been associated with meniscus rupture [9]. The meniscus effects are associated primarily with the desorption branch of these hysteresis loops. On adsorption (60 to 80% r.h.) there is only a small increase in the length of the porous glass owing to a compensation of forces arising from both capillary
R E L A T I V E H U M I D I T Y ( % )
Figure 4 Length change isotherms for ( )porous glass and (---)
cement paste [9, 101.
condensation and continued adsorption. On desorp- tion, it is thought that the two processes can be separated and that between 60 and 70% r.h. pores empty by evaporation from concave menisci [9] so that expansion results.
On the basis of such arguments and noted simi- larities between porous glass and cement paste, it is suggested that meniscus forces generated by the dry- ing of Portland cement paste contribute to crack growth processes in this system.
References
1. 1. J . B E A U D O I N , Cem. Concr. Res. 15 (1985) 871.
2. Idem, ibid. 15 (1985) 988. 3. Idem, I1 Cemento 82 (1985) 195.
4. D. P. W I L L I A M S and A . G. E V A N S , J. Test. Eval. 1 (1973) 264.
5. J . J . B E A U D O I N , Cem. Concr. Res. 12 (1982) 705. 6. Idem, ibid. 12 (1982) 289.
7. S. M. W I E D E R H O R N , S. W . F R I E M A N , E. R.
F U L L E R J r and C . J . S I M M O N S , J. Mater. Sci. 17
(1982) 3460.
8. K . H. H I L L E R , J . Appl. Phys. 35(1964) 1622.
9. C . H. A M B E R G and R . M c I N T O S H , Can. J . Chem. 30
(1952) 1012.
10. R. F . F E L D M A N , in "Proceedings of 5th International Symposium on the Chemistry of Cement", Tokyo, 1968, Part 111, Vol. 111, p. 53.
1 1 . R . H . M I L L S , Highways Research Board, Special Report 90 (1966) p. 84.
Received 4 April
T h i s p a p e r i s b e i r g 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 ~ u b l i c a t f 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 R e s e a r c h 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 - 2 - p a r t p a r l t 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 . On 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 d e 1' I n s t i t u t p o r t a n t s u r l e s t e c h n i q u e s ou les r e c h e r c h e s e n r n a t i z r e d e b 2 t i m e n t e n G c r i v a n t 3 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 d e 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 ) , K I A OR6.
