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Durability of concrete structures in acidic water
CEMENT and CONCRETE RESEARCH. Vol. 19, pp. 783-792, 1989. Printed in the USA. 0008-8846/89 $3.00+00 Copyright (c) 1989 Pergamon Press plc
DURABILITY OF CONCRETE STRUCTURES IN ACIDIC WATER
H. Grube and W. Rechenberg Forschungsinstitut der Zementindustrie Postfach 30 I0 63, D-4OOO Dusseldorf 30, F.R. Germany
(Communicated by F.W. Locher) (Received Dec. 29, 1988) ABSTRACT When p l a n n i n g and e r e c t i n g c o n c r e t e s t r u c t u r e s , one of the f a c t o r s to be c o n s i d e r e d is the a t t a c k on the concrete by c h e m i c a l s u b s t a n c e s . The d e g r e e of a s o l v e n t a t t a c k is g e n e r a l l y e x p r e s s e d in terms of concentration (mg/l and pH value) of a t t a c k i n g acid. An e v a l u a tion of the l i t e r a t u r e and our own experiments have shown that the conditions of transport of the a t t a c k i n g and the d i s s o l v e d substances often have a much greater influence on the loss of mass than the concentration of the attacking substances. This phenomenon is due to a p r o t e c t i v e layer of SiO 2 gel. As a result of experiments in connection wlth a calculation model the loss of mass to be e x p e c t e d u n d e r p r a c t i c a l c o n d i t i o n s can be e s t i m a t e d in advance, in terms of q u a n t i t y , as a f u n c t i o n of the type and c o n c e n t r a t i o n of the a t t a c k i n g acid solution, the composition of the c o n c r e t e and the state of the prot e c t i v e layer. Introduction Water which contains diluted acid, e.g. lime-attacking carbonic acid, will affect c o n c r e t e . The acid d i s s o l v e s the c a l c i u m from its bonds. As a result, the c o n c r e t e is progressively destroyed from the s u r f a c e to the inside. The rate at w h i c h the h a r d e n e d c e m e n t p a s t e and p o s s i b l y the a g g r e g a t e are dissolved m a i n l y d e p e n d s on the c o n c e n t r a t i o n of the a t t a c k i n g acid in the w a t e r and on the c h e m i c a l r e s i s t a n c e of the concrete. M a n y n a t i o n a l s t a n d a r d s (I - 3) d e t e r m i n e the d e g r e e of a t t a c k p r i m a r i l y on the b a s i s of the c o n c e n t r a t i o n of the aggressive substances, and the same a p p l i e s to the d r a f t of an i n t e r n a t i o n a l ISO s t a n d a r d (4). T e s t r e s u l t s have shown that the chemical r e s i s t a n c e of the c o n c r e t e a g a i n s t a s o l v e n t attack can be e n h a n c e d by u s i n g i n s o l u b l e a g g r e g a t e s and cement 783
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with a h i g h e r SiO 2 c o n t e n t (8) and by k e e p i n g the w a t e r cement ratio low (w/c < 0 . 6 0 ) . Test r e s u l t s show (5,6,9) that only a very small proportion of the aggressive s u b s t a n c e s in the r u n n i n g w a t e r reacts with the h a r d e n e d c e m e n t paste. Also the flow rate seems to have very little i n f l u e n c e (i0) as long as we stay w i t h i n the g r o u n d w a t e r range of flow rates. This behaviour is to be e x p l a i n e d by the f o r m a t i o n of a p r o t e c t i v e layer of SiO 2 gel which, with i n c r e a s i n g duration of attack, acts as a P a r t i e r to the t r a n s p o r t of a g g r e s s i v e s u b s t a n c e to and r e m o v a l of d i s s o l v e d c o n s t i t u e n t s from the concrete. The chemical r e s i s t a n c e of the c o n c r e t e a g a i n s t s o l v e n t a t t a c k s is thus e s s e n t i a l l y d e p e n d i n g on the e x i s t e n c e of that protective layer. Reference to these correlations was made by several a u t h o r s (ii - 14). H o w e v e r , in p l a n n i n g and d e s i g n i n g concrete structures they are not yet generally considered. It was t h e r e f o r e the o b j e c t i v e of our s t u d y to d e v e l o p a mathematical model for estimating in advance the loss of c o n c r e t e as a f u n c t i o n of the c o n c r e t e c o m p o s i t i o n , the c o n c e n t r a t i o n of the aggressive s o l u t i o n and the s t a b i l i t y of the p r o t e c t i v e layer. For b u i l d i n g c o n c r e t e s t r u c t u r e s a reliable estimate is of great importance. If only the c o n c e n t r a t i o n of the a g g r e s s i v e s o l u t i o n is used for the estimate, the loss of mass of structural components in less p e r m e a b l e soils will u s u a l l y be o v e r e s t i m a t e d even at high c o n c e n t r a t i o n s . In c o n t a i n e r s whose walls are regularly c l e a n e d the loss of c o n c r e t e even at low c o n c e n t r a t i o n s (e.g. 30 mg/l) of l i m e - a t t a c k i n g carbonic acid may be c o n s i d e r a b l y u n d e r e s t i m a t e d .
Course
of r e a c t i o n
Tests have shown (14,15) that c a r b o n d i o x i d e (COp) d i s s o l ved in w a t e r first leads to the f o r m a t i o n of a thin -layer of calcium carbonate (CaCO~) in the areas of m o r t a r or c o n c r e t e w h i c h are c l o s e to the surface. Then a d d i t i o n a l CO 2 d i s s o l v e s the CaCOq to form Ca(HCO3) 2 w h i c h is r e m o v e d by the water. Next in t~e same p r o c e s s c a l c i u m from the c a l c i u m s i l i c a t e s is dissolved and removed. What remains is a gel-like layer c o n s i s t i n g of h y d r o u s s i l i c o n d i o x i d e w h i c h also c o n t a i n s iron and aluminium oxide (14,16). The gel layer b e c o m e s t h i c k e r with p r o g r e s s i n g attack. The p r o c e s s by w h i c h the concrete is dissolved from its s u r f a c e is i l l u s t r a t e d s c h e m a t i c a l l y by the three r e p r e s e n t a t i o n s of Figure i. The gel layer is not particularly strong. Aggregate particles having lost their bond with the h a r d e n e d c e m e n t paste can no l o n g e r be retained and, e s p e c i a l l y upon an a d d i t i o n a l m e c h a n i c a l impact, s e p a r a t e from the s t r u c t u r e . As a result, the thickness of the gel layer is reduced and the rate of c o n c r e t e r e m o v a l increased. However, the rate of c o n c r e t e r e m o v a l does not i n c r e a s e if the aggregate particles with the gel layer r e m a i n in place b e i n g held, for instance, by the s u r r o u n d i n g soil. Rate It is usually instantaneously and
of d i s s o l u t i o n
assumed that inorganic reactions c o m p l e t e l y . This, however, a p p l i e s
occur only
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785 CONCRETE, DURABILITY, ACID, SOLVENT
F Io °;l CIA,FIH
Ca(OH)2//~, i," Z
.~Z
-- Co (HCO3)z COz
CSH
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xxC {A.F) H ~
-
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Ca
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~ FezO3"z HzO
FIG.
1
R e a c t i o n of h a r d e n e d c e m e n t paste in c o n c r e t e with a g g r e s s i v e c a r b o n d i o x i d e ( s c h e m a t i c ) . F o r m a t i o n of c a l c i u m c a r b o n a t e . H y d r a t e p h a s e s forming a p r o t e c t i v e layer w i t h i n c r e a s e d d i f f u s i o n r e s i s t a n c e . G r o w t h of the p r o t e c t i v e layer c o n s i s t i n g of SiO 2, A I 2 0 3 and F e 2 0 3 if the r e a c t i o n p a r t n e r s are p r e s e n t in d i s s o l v e d form. If one of the r e a c t i o n p a r t n e r s is a solid s u b s t a n c e , the rate of d i s solution is d e l a y e d . Thus, for instance, sugar d i s s o l v e s slowly in s t a t i o n a r y water. In the c o u r s e of time the dissolution rate will even decrease because a non-stationary diffusion will form. The rate of d i s s o l u t i o n can be increased by stirring the water, as the t h i c k n e s s of the water layer w i t h i n w h i c h the s a t u r a t i o n c o n c e n t r a t i o n on the s u r f a c e of the solid substance declines to the c o n c e n t r a t i o n in the s o l u t i o n dec r e a s e s with r i s i n g speed of stirring, so that the mass d i s s o l ved per unit of time and area i n c r e a s e s a c c o r d i n g to the first law of Fick. In this c o n n e c t i o n it is important to remember that there is a m a x i m u m rate of d i s s o l u t i o n , i.e. the s p e c i f i c d i s s o l u t i o n rate, w h i c h cannot be e x c e e d e d (17). If, however, the solid m a t e r i a l forms a p r o t e c t i v e layer, the rate of diss o l u t i o n will b e c o m e e s s e n t i a l l y i n d e p e n d e n t of the flow rate of the solution. In that case it will only be d e t e r m i n e d by the t h i c k n e s s and the d i f f u s i o n r e s i s t a n c e of the protective layer. This d i s s o l u t i o n p r o c e s s can be slowed down further by r e d u c i n g the flow rate of the s u r r o u n d i n g water towards zero or by increasing the d i f f u s i o n r e s i s t a n c e in the e n v i r o n m e n t of the c o n c r e t e by a tight soil. Determinin@
the CaD
removal
from m o r t a r
and c o n c r e t e
Koelliker measured the rate at w h i c h CaD is r e m o v e d by d i s s o l u t i o n from 4 x 4 x 16 cm' P o r t l a n d c e m e n t mortar prisms
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containing a b o u t 500 kg of c e m e n t per cubic meter and having a w a t e r - c e m e n t ratio of 0 . 5 0 (14,15). Lime-dissolving carbonic acid was dissolved in the a t t a c k i n g water. The r e s u l t i n g pH value was about 4.4. The p r i s m s were stored in w a t e r for 1 to 2 m o n t h s and 1 year, r e s p e c t i v e l y , b e f o r e they were e x p o s e d to the l i m e - d i s s o l v i n g c a r b o n i c acid. F i g u r e 2 shows how the surface-related rate of d i s s o l u t i o n d e c l i n e s ( f o l l o w i n g approx, a I/~-~law) w i t h i n c r e a s i n g e x p o s u r e time. This i n d i c a t e s that, due to the e f f e c t of the l i m e - d i s s o l v i n g c a r b o n i c acid, a prot e c t i v e layer forms w h i c h g a i n s in t h i c k n e s s in the course of time and thus i n c r e a s i n g l y b l o c k s the d i f f u s i o n of the d i s s o l ved CaO.
,~
1Oh
d
7d
90d
10-7
? E u
~
10-8
p 10.9 10 5
10 *
10 6
10 7
duration of aggression (s) FIG.
2
D i s s o l u t i o n rate of c a l c i u m oxide from m o r t a r p r i s m s ( c o m p o s i t i o n a c c o r d i n g to DIN 1164) by a g g r e s s i o n of c a r b o n d i o x i d e s o l u t i o n with pH ~ 4.4 ( a c c o r d i n g to K o e l l i k e r (14)) By means of a model b a s e d on F i c k ' s first law e q u a t i o n 1 was developed (18) w h i c h can be used for c a l c u l a t i n g the t h i c k n e s s d of the d e s t r o y e d layer a f t e r any given e x p o s u r e time t. The influencing variables are the d i f f u s i o n c o e f f i c i e n t , the mass of s o l u b l e m a t t e r per v o l u m e unit, the area percent of the soluble matter in the area of a t t a c k and the d i f f e r e n c e in Ca(HCO3) 2 c o n c e n t r a t i o n :
2
d =
•
mL •
D
°
Ages
A L
(c*
-
c[ )
.
t
s
111
whereby d D
is the t h i c k n e s s of d e s t r o y e d c o n c r e t e layer Ca(HCO3) 2 d i f f u s i o n c o e f f i c i e n t in cm2/s
in cm
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CONCRETE, DURABILITY, ACID, SOLVENT of the p r o t e c t i v e layer ~gel layer) (D ~ 1.8 • I0 -v cm 2 • sfor SiO 2 gel of h a r d e n e d P o r t l a n d c e m e n t paste) mass of s o l u b l e m a t t e r in g of CaO per cm 3 of c o n c r e t e
mt • Cs ct A[/Ages VL/Vges A[/Ages
Ca(HCO~) 2 c o n c e n t r a t i o n in g / c m 3 in the s o l u t i o n s u r r o u ~ d z n g intact c o n c r e t e C a ( H C O ~ ) ~ c o n c e n t r a t i o n in g / c m ~ in the s o l u t i o n w h i c h i s - u n a f f e c t e d by the c o n c r e t e ratio of s o l u b l e m a t t e r area to total area r a t i o of the v o l u m e of s o l u b l e c o n s t i t u e n t s (if i n s o l u b l e a g g r e g a t e is used ---~ volume of h a r d e n e d c e m e n t paste) to total volume = VL / V g e s
E x a m p l e s of c o m p u t a t i o n i n c l u d i n g the d e t e r m i n a t i o n of c o n c e n t r a t i o n d i f f e r e n c e (c: - c L) are given in (18).
the
The first root in e q u a t i o n 1 r e p r e s e n t s a constant valid the r e s p e c t i v e case. E q u a t i o n 1 can t h e r e f o r e be s i m p l i f i e d form e q u a t i o n 2. d = a • f'~ /2/
FIG.
3
in to
L,. u ¢0
Depth of dissolved material in r e l a t i o n to time - w i t h o u t b r u s h i n g off the gel - with f r e q u e n t (line I) and rare b r u s h i n g off (line II)
0 °__ ,4--
2 /
durntion of oggression t The loss of mass w h i c h o c c u r s w i t h an intact protective layer and follows equation 2 is shown by the full bold line in F i g u r e 3. It had been a s s u m e d e a r l i e r that the loss of mass might f o l l o w a . ~'6~law (9,11-13). However, in the past it was not possible to describe different test p e r i o d s by one function. This was due to the fact that the test specimens were dabbed with a moist cloth b e f o r e b e i n g w e i g h e d . In this way part of the gellike p r o t e c t i v e layer was removed so that the subsequent reaction occurred much faster than it w o u l d have done with an intact p r o t e c t i v e layer. If the protective layer is removed
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regularly it is to be e x p e c t e d that the p a r a b o l i c function turns to a linear one as shown by the s t r a i g h t lines I and II in F i g u r e 3. The "linear loss of mass" will be the s t e e p e r the more f r e q u e n t l y the protective layer is removed (straight line I). It will be the flatter the less f r e q u e n t l y the p r o t e c t i v e layer is r e m o v e d (straight line If). This "linear loss of mass" has already been o b s e r v e d in c o n n e c t i o n with l o n g - t e r m tests (6). In order to v e r i f y this b e h a v i o u r e x p e r i m e n t a l l y 4 x 4 x 16 cm p r i s m s a c c o r d i n g to DIN 1164 were made of a 35 F Portland cement and a 35 L blast furnace slag cement. They c o n t a i n e d 550 kg/m 3 of c e m e n t and had a w a t e r - c e m e n t ratio of 0.50. The p r i s m s were p r o d u c e d and s t o r e d in w a t e r by the standard procedure. At the age of 7 days they were p l a c e d in tap water w h i c h was c o n t i n u o u s l y e n r i c h e d with CO and renewed every week. The lime-attacking carbonic aci~ content was about ii0 mg CO2/I, the pH was 6.15 (mean values). The water flow rate was about 4.2 • i0 -z cm/s. The experimental s e t u p is d e s c r i b e d in d e t a i l in (5). The prisms were weighed every week. On the basis of the bulk d e n s i t y of the p r i s m s the w e i g h t loss was c o n v e r t e d to the d e p t h d of concrete removal which was compared with the d e p t h of r e m o v a l o b t a i n e d with e q u a t i o n i. In an i n i t i a l s e r i e s of tests the p r i s m s were only d a b b e d once a week. After 9 w e e k s some of the p r i s m s were b r u s h e d once a week, the o t h e r s were not s u b j e c t e d to any further mechanical treatment. The prisms of the s e c o n d test s e r i e s were b r u s h e d three times per day t h r o u g h o u t the test period. The s t o r a g e w i t h o u t any further m e c h a n i c a l t r e a t m e n t describes the chemical attack in slowly running water a c c o r d i n g to DIN 4030 (3) w i t h the d i f f u s i o n b e i n g c o n t r o l l e d by the p r o t e c tive layer. Dabbing presents a mild and b r u s h i n g a v i g o r o u s a d d i t i o n a l m e c h a n i c a l t r e a t m e n t (M) of the protective layer. The results for Portland cement m o r t a r in F i g u r e 4 and for blast f u r n a c e slag c e m e n t m o r t a r in Figure 5 show that the weight loss resulting from the uniform c h e m i c a l a t t a c k by ii0 mg CO9/I d e p e n d s very much on the type and intensity of the a d d i t i o n a l m e c h a n i c a l t r e a t m e n t . Thus, the p u r e l y c h e m i c a l a t t a c k (CA) c a u s e s the s m a l l e s t w e i g h t loss in time. The rate of c o n c r e t e r e m o v a l is i n c r e a s e d by d a b b i n g (CMA i) and more so by brushing (CMA 2 and CMA 3). For P o r t l a n d cement m o r t a r there is not much difference between the loss of concrete (Figure 4) resulting from b r u s h i n g once a week (CMA 2) and 3 times per w o r k d a y (CMA 3). The rate of concrete removal was approx. 1 mm in 18 w e e k s or about 3 mm/a (in the 4 x 4 x 16 cm prisms a w e i g h t loss of 60 g c o r r e s p o n d s to about 1 mm of loss of c o n c r e t e ) . The rate of concrete loss was lower for the blast f u r n a c e slag c e m e n t m o r t a r s than it was for the P o r t l a n d c e m e n t m o r t a r s . This leads to the a s s u m p t i o n that the higher content of silicon oxide and the r e s u l t i n g lower c o n t e n t of calcium oxide in blast furnace slag cement might be an explanation for this b e h a v i o u r . The q u e s t i o n , however, was not s t u d i e d in g r e a t e r detail.
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789 CONCRETE, DURABILITY, ACID, SOLVENT
-10n
- 100
-8
-80
-6
~
-Z'
-60
-20
0
3
6 9 12 15 18 21 Z& duration of aggression (weeks)
FIG.
Z7
0
3
4
6 9 12 15 18 21 2z. duration 0, aggression ( weeks ]
FIG.
5
W e i g h t loss of W e i g h t loss of PC-mortar prisms.., bfs-cement mortar prisms... by a g g r e s s i v e c a r b o n dioxide. The c o m p o s i t i o n of the p r i s m s was a c c o r d i n g to DIN 1164. C A - c h e m i c a l a t t a c k only, CMA 1 - a t t a c k i n c r e a s e d m e c h a n i c a l l y by d a b b i n g b e f o r e w e i g h i n g , CMA 2 - b r u s h i n g off o n c e per week, CMA 3 - b r u s h i n g off three times per day. D o t t e d line ( c a l c u l a t i o n ) and first b r u s h off at 26 w e e k s ( e x p e r i m e n t a l ) The v a r i o u s l o s s e s of mass o b t a i n e d in the tests are b e l o w the ones calculated using e q u a t i o n /i/ for, e.g., w e e k l y losses, b e c a u s e b r u s h i n g did not r e m o v e all of the SiO~ gel from the pores. The longer the time p e r i o d of an u n d i s t u r b e d gel layer was the b e t t e r was the a g r e e m e n t between the calculated and the experimental results (18). This can be seen from the d o t t e d line in Figure 4, w h i c h s h o w s the calculated depth of dissolved concrete according to e q u a t i o n I. The first b r u s h off at the age of 26 w e e k s shows the marked value slightly b e l o w the c a l c u l a t e d value. Practical
application
In addition to the c o n s t r u c t i o n cost the d e s i g n e n g i n e e r should i n c l u d e in his c o n s i d e r a t i o n s the s e r v i c e life and the repair and m a i n t e n a n c e cost of a s t r u c t u r e . If c o n c r e t e is exposed to a c i d s the r e s u l t i n g loss of mass may be i r r e l e v a n t to
27
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its use. This often is the case for s t r u c t u r e s like f o u n d a tions or d r i l l e d f o u n d a t i o n piles. In c o n t r a s t , the practical use of a c o n t a i n e r or a w a t e r duct were the p r o t e c t i v e layer may be r e m o v e d r e g u l a r l y can be considerably impaired by a constant loss of e.g. 0.5 mm of c o n c r e t e per year. T h e r e f o r e the d e s i g n e n g i n e e r , b e s i d e s c o n s i d e r i n g the acid concentration in the a t t a c k i n g water, should also pay a t t e n t i o n to the e x t e r n a l c o n d i t i o n s on w h i c h the c o n c r e t e is exposed to the water. For illustration F i g u r e 6 shows the d e p t h of c o n c r e t e loss in mm a f t e r 20 y e a r s for d i f f e r e n t conditions of transport and different c o n t e n t of a g g r e s s i v e CO 2. The c o n d i t i o n s of t r a n s p o r t are d e s c r i b e d in Table i. The d i f f e r e n t c u r v e s r e p r e s e n t the loss of mass after 20 years of s e r v i c e life w h e n a c o n c r e t e is used c o n s i s t i n g of a c i d - i n s o l u b l e a g g r e g a t e w i t h 350 kg of Portland cement per cubic meter of concrete and a w a t e r - c e m e n t ratio of 0.50. If the w a t e r - c e m e n t ratio is i n c r e a s e d or if acid-soluble aggregate is used a b i g g e r loss of mass has to be e x p e c t e d . If the c o n c r e t e is made of b l a s t f u r n a c e slag c e m e n t i n s t e a d of Portland cement, the loss will be smaller. The c u r v e s s h o w t~at in a c o m p a c t soil with a p e r m e a b i l i t y coe f f i c i e n t of k<10 cm/s the chemical attack is negligible (TC I) if the water flows very slowly below a hydraulic gradient. In a more p e r m e a b l e soil w i t h a r i s i n g flow rate the loss of mass i n c r e a s e s o n l y s l o w l y (TC 2). The flow rate has a g r e a t e r i n f l u e n c e than the acid c o n c e n t r a t i o n . In freely running water with u n d i s t u r b e d or d i s t u r b e d , i.e. r e g u l a r l y re-
¢-
~k E
> 2 (} u~
,4-
brush off l~/week L_ -'-
~5
5
corDon dioxide (mg/[) /100 ~ ~'~, 60
350 kg PClm3 concrete
"~....j ~ . . ~ . . .
"/c=0"50 I iI
30 F----~
e-
0 0,1 1 5 10
50 100
200 (sco[e:-~/-) 500
d depth of dissolved concrete o~er 20 years (mm) FIG.
6
D e p t h of d i s s o l v e d (and r e m o v e d ) c o n c r e t e a f t e r 20 y e a r s in d i f f e r e n t c o n c e n t r a t i o n s of a g g r e s s i v e c a r b o n d i o x i d e at d i f f e r e n t c o n d i t i o n s of t r a n s p o r t (see below) for the p r o d u c t s of the r e a c t i o n
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791 CONCRETE, DURABILITY, ACID, SOLVENT
moved, p r o t e c t i v e layer (TC 3 and 4), the t r a n s p o r t conditions obviously have a much g r e a t e r e f f e c t than the acid c o n c e n t r a tion. The dots in the f i g u r e r e p r e s e n t the r e s u l t s of experiments and c a l c u l a t i o n s . The d o t t e d parts of the c u r v e s in the areas of TC 5 and 6 are e x t r a p o l a t i o n s that can at present not be v a l i d a t e d by test results. TABLE Conditions
of t r a n s p o r t
TC 1
1
according
to F i g u r e
6
Conditions C o m p a c t d e p o s i t e d soil, p e r m e a b i l i t y k < 10 -4 cm/s, very s l o w l y m o v i n g or s t a g n a n t g r o u n d water, protective layer, F i c k ' s 2. l~w. P e r m e a b l e soil with k > i0cm/s, g r o u n d water m o v i n g slowly, p r o t e c t i v e layer, F i c k ' s I. law. F l o w i n g free water, p r o t e c t i v e layer n e a r l y u n d i s t u r bed, F i c k ' s i. law. F l o w i n g free water, p r o t e c t i v e layer r e m o v e d p e r i o d i cally, F i c k ' s I. law. F l o w i n g free water, high v e l o c i t y of flow, no prot e c t i v e layer, a p p r o x i m a t i o n to s p e c i f i c v e l o c i t y of dissolution. F l o w i n g free water, very high v e l o c i t y of t u r b u l e n t flow, c a v i t a t i o n . Conclusions
- The d i f f u s i o n model p r e s e n t e d here and the tests described permit to arrive at a q u a n t i t a t i v e e s t i m a t e of the loss of m o r t a r and c o n c r e t e to be e x p e c t e d in case of a chemical a t t a c k by l i m e - d i s s o l v i n g c a r b o n i c acid. -
The exposure to l i m e - d i s s o l v i n g c a r b o n i c acid leads to the formation of a protective layer consisting of gel-like silicon dioxide.
- In the presence mass f o l l o w s a • ~ - If the concrete -
protective is r e m o v e d
of an intact law.
protective
layer is mechanically at an a c c e l e r a t e d rate.
layer
the
loss of
destroyed
the
A disturbance of the p r o t e c t i v e layer, even if it occurs at r e l a t i v e l y long i n t e r v a l s , can have a much more damaging effect than an increase in the concentration of the a g g r e s s i v e acid.
- The tests d e s c r i b e d , i.e. the stripping of the protective layer at regular intervals, and the mathematical model p e r m i t a l o n g - t e r m e s t i m a t e of the loss of mass also caused by other than lime-dissolving carbonic acid. E x p e r i m e n t s w i t h other a c i d s can be m a d e in about 3 months. - It a p p e a r s
possible
to c o n s i d e r
more
completely
in
future
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H. Grube and W. Rechenberg
standards the f a c t o r s w h i c h r e d u c e a n d i n c r e a s e the l o s s of mass from concrete e x p o s e d to attacking acids. This would be to the b e n e f i t of c o n c r e t e structures. It is s u g g e s t e d to d e f i n e t h e d e g r e e s of a t t a c k by l o s s of m a s s .
References I. CSN 73 2020 (1954): Vodostavebn~ Bet6ny-Defincia, roztriedenie a technick~ podmienky. Vydavate1' Avo Uradupre Normaliz~cin, Praha. 2. TGL 33 408: Betonbau, Korrosion und Korrosionsschutz. Berlin 1980. 3. DIN 4030 (1969): Beurteilung b e t o n a n g r e i f e n d e r W~sser, B~den und Gase. Beuth-Verlag, Berlin und KSln. 4. ISO-Draft DP 9690 (1987): Classification of environmental exposure c o n d i t i o n s for concrete and reinforced concrete s t r u c t u r e s . ISO-TC 71/33. 5 Locher, F.W., und S. Sprung: beton 25, 241 (1975). 6 Locher, F.W., W. Rechenberg und S. Sprung: beton 34, 193, (1975). 7 Friede, H., P. Schubert und H.P. L~hr: beton 29, 250, (1979) 8 Efes, Y., und H.P. L~hr: TIZ-Fachber., 104, 153 (1980). 9 Friede, H., Dissert. RWTH Aachen 1983. 10 Gille, F: beton 12, 467 (1962). 11 Tremper, B.: Amer. Concr. Inst., Proc. 28, 1 (1932). 12 Romb~n, L.: CBI forskning research 1:78 und 9:79, Cement- och betong instituter, Stockholm. 13. Polak, A.F.: Beton; Zhelezobeton, Nr. 8, 5 (1978). 14. Koelliker, E.: Intern. Kolloquium "Werkstoffwissenschaften und Bausanierung", B e r i c h t s b a n d TA Esslingen, 1963, 195. E. M o e l l e r GmbH, Filderstadt 1983. 15. Koelliker, E. : Cem. Concr. Res. 15, i00 (1965). 16. Gr~n, R., und K. Obenauer: Zement 33, i0 (1944). 17. Becket, H.: Z e m e n t - K a l k - G i p s 39, 273 (1986). 18. Grube, H., und W. Rechenberg: beton 37, 446 (1987).
DURABILITY OF CONCRETE STRUCTURES IN ACIDIC WATER
H. Grube and W. Rechenberg Forschungsinstitut der Zementindustrie Postfach 30 I0 63, D-4OOO Dusseldorf 30, F.R. Germany
(Communicated by F.W. Locher) (Received Dec. 29, 1988) ABSTRACT When p l a n n i n g and e r e c t i n g c o n c r e t e s t r u c t u r e s , one of the f a c t o r s to be c o n s i d e r e d is the a t t a c k on the concrete by c h e m i c a l s u b s t a n c e s . The d e g r e e of a s o l v e n t a t t a c k is g e n e r a l l y e x p r e s s e d in terms of concentration (mg/l and pH value) of a t t a c k i n g acid. An e v a l u a tion of the l i t e r a t u r e and our own experiments have shown that the conditions of transport of the a t t a c k i n g and the d i s s o l v e d substances often have a much greater influence on the loss of mass than the concentration of the attacking substances. This phenomenon is due to a p r o t e c t i v e layer of SiO 2 gel. As a result of experiments in connection wlth a calculation model the loss of mass to be e x p e c t e d u n d e r p r a c t i c a l c o n d i t i o n s can be e s t i m a t e d in advance, in terms of q u a n t i t y , as a f u n c t i o n of the type and c o n c e n t r a t i o n of the a t t a c k i n g acid solution, the composition of the c o n c r e t e and the state of the prot e c t i v e layer. Introduction Water which contains diluted acid, e.g. lime-attacking carbonic acid, will affect c o n c r e t e . The acid d i s s o l v e s the c a l c i u m from its bonds. As a result, the c o n c r e t e is progressively destroyed from the s u r f a c e to the inside. The rate at w h i c h the h a r d e n e d c e m e n t p a s t e and p o s s i b l y the a g g r e g a t e are dissolved m a i n l y d e p e n d s on the c o n c e n t r a t i o n of the a t t a c k i n g acid in the w a t e r and on the c h e m i c a l r e s i s t a n c e of the concrete. M a n y n a t i o n a l s t a n d a r d s (I - 3) d e t e r m i n e the d e g r e e of a t t a c k p r i m a r i l y on the b a s i s of the c o n c e n t r a t i o n of the aggressive substances, and the same a p p l i e s to the d r a f t of an i n t e r n a t i o n a l ISO s t a n d a r d (4). T e s t r e s u l t s have shown that the chemical r e s i s t a n c e of the c o n c r e t e a g a i n s t a s o l v e n t attack can be e n h a n c e d by u s i n g i n s o l u b l e a g g r e g a t e s and cement 783
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H. Grube and W. Rechenberg
with a h i g h e r SiO 2 c o n t e n t (8) and by k e e p i n g the w a t e r cement ratio low (w/c < 0 . 6 0 ) . Test r e s u l t s show (5,6,9) that only a very small proportion of the aggressive s u b s t a n c e s in the r u n n i n g w a t e r reacts with the h a r d e n e d c e m e n t paste. Also the flow rate seems to have very little i n f l u e n c e (i0) as long as we stay w i t h i n the g r o u n d w a t e r range of flow rates. This behaviour is to be e x p l a i n e d by the f o r m a t i o n of a p r o t e c t i v e layer of SiO 2 gel which, with i n c r e a s i n g duration of attack, acts as a P a r t i e r to the t r a n s p o r t of a g g r e s s i v e s u b s t a n c e to and r e m o v a l of d i s s o l v e d c o n s t i t u e n t s from the concrete. The chemical r e s i s t a n c e of the c o n c r e t e a g a i n s t s o l v e n t a t t a c k s is thus e s s e n t i a l l y d e p e n d i n g on the e x i s t e n c e of that protective layer. Reference to these correlations was made by several a u t h o r s (ii - 14). H o w e v e r , in p l a n n i n g and d e s i g n i n g concrete structures they are not yet generally considered. It was t h e r e f o r e the o b j e c t i v e of our s t u d y to d e v e l o p a mathematical model for estimating in advance the loss of c o n c r e t e as a f u n c t i o n of the c o n c r e t e c o m p o s i t i o n , the c o n c e n t r a t i o n of the aggressive s o l u t i o n and the s t a b i l i t y of the p r o t e c t i v e layer. For b u i l d i n g c o n c r e t e s t r u c t u r e s a reliable estimate is of great importance. If only the c o n c e n t r a t i o n of the a g g r e s s i v e s o l u t i o n is used for the estimate, the loss of mass of structural components in less p e r m e a b l e soils will u s u a l l y be o v e r e s t i m a t e d even at high c o n c e n t r a t i o n s . In c o n t a i n e r s whose walls are regularly c l e a n e d the loss of c o n c r e t e even at low c o n c e n t r a t i o n s (e.g. 30 mg/l) of l i m e - a t t a c k i n g carbonic acid may be c o n s i d e r a b l y u n d e r e s t i m a t e d .
Course
of r e a c t i o n
Tests have shown (14,15) that c a r b o n d i o x i d e (COp) d i s s o l ved in w a t e r first leads to the f o r m a t i o n of a thin -layer of calcium carbonate (CaCO~) in the areas of m o r t a r or c o n c r e t e w h i c h are c l o s e to the surface. Then a d d i t i o n a l CO 2 d i s s o l v e s the CaCOq to form Ca(HCO3) 2 w h i c h is r e m o v e d by the water. Next in t~e same p r o c e s s c a l c i u m from the c a l c i u m s i l i c a t e s is dissolved and removed. What remains is a gel-like layer c o n s i s t i n g of h y d r o u s s i l i c o n d i o x i d e w h i c h also c o n t a i n s iron and aluminium oxide (14,16). The gel layer b e c o m e s t h i c k e r with p r o g r e s s i n g attack. The p r o c e s s by w h i c h the concrete is dissolved from its s u r f a c e is i l l u s t r a t e d s c h e m a t i c a l l y by the three r e p r e s e n t a t i o n s of Figure i. The gel layer is not particularly strong. Aggregate particles having lost their bond with the h a r d e n e d c e m e n t paste can no l o n g e r be retained and, e s p e c i a l l y upon an a d d i t i o n a l m e c h a n i c a l impact, s e p a r a t e from the s t r u c t u r e . As a result, the thickness of the gel layer is reduced and the rate of c o n c r e t e r e m o v a l increased. However, the rate of c o n c r e t e r e m o v a l does not i n c r e a s e if the aggregate particles with the gel layer r e m a i n in place b e i n g held, for instance, by the s u r r o u n d i n g soil. Rate It is usually instantaneously and
of d i s s o l u t i o n
assumed that inorganic reactions c o m p l e t e l y . This, however, a p p l i e s
occur only
Vol.
19, No. 5
785 CONCRETE, DURABILITY, ACID, SOLVENT
F Io °;l CIA,FIH
Ca(OH)2//~, i," Z
.~Z
-- Co (HCO3)z COz
CSH
( A F) H',////////t2
C ( A . ) H,~, ~
K///////~////////Ix.
,L\ <.\ \ \ W'- u u z
~.~/,/./,,/,///~/bb\bbb~.,~"q ,/xL,O( UH J2~/ ,
_..,.Ca(OH)Z
Ca(tO3) ,~,\\
CSH I////////C
--,"
ca (co3!,%. sioz- x HzO
xxC {A.F) H ~
-
- -~
Ca
( HC0 3 ) Z
~ FezO3"z HzO
FIG.
1
R e a c t i o n of h a r d e n e d c e m e n t paste in c o n c r e t e with a g g r e s s i v e c a r b o n d i o x i d e ( s c h e m a t i c ) . F o r m a t i o n of c a l c i u m c a r b o n a t e . H y d r a t e p h a s e s forming a p r o t e c t i v e layer w i t h i n c r e a s e d d i f f u s i o n r e s i s t a n c e . G r o w t h of the p r o t e c t i v e layer c o n s i s t i n g of SiO 2, A I 2 0 3 and F e 2 0 3 if the r e a c t i o n p a r t n e r s are p r e s e n t in d i s s o l v e d form. If one of the r e a c t i o n p a r t n e r s is a solid s u b s t a n c e , the rate of d i s solution is d e l a y e d . Thus, for instance, sugar d i s s o l v e s slowly in s t a t i o n a r y water. In the c o u r s e of time the dissolution rate will even decrease because a non-stationary diffusion will form. The rate of d i s s o l u t i o n can be increased by stirring the water, as the t h i c k n e s s of the water layer w i t h i n w h i c h the s a t u r a t i o n c o n c e n t r a t i o n on the s u r f a c e of the solid substance declines to the c o n c e n t r a t i o n in the s o l u t i o n dec r e a s e s with r i s i n g speed of stirring, so that the mass d i s s o l ved per unit of time and area i n c r e a s e s a c c o r d i n g to the first law of Fick. In this c o n n e c t i o n it is important to remember that there is a m a x i m u m rate of d i s s o l u t i o n , i.e. the s p e c i f i c d i s s o l u t i o n rate, w h i c h cannot be e x c e e d e d (17). If, however, the solid m a t e r i a l forms a p r o t e c t i v e layer, the rate of diss o l u t i o n will b e c o m e e s s e n t i a l l y i n d e p e n d e n t of the flow rate of the solution. In that case it will only be d e t e r m i n e d by the t h i c k n e s s and the d i f f u s i o n r e s i s t a n c e of the protective layer. This d i s s o l u t i o n p r o c e s s can be slowed down further by r e d u c i n g the flow rate of the s u r r o u n d i n g water towards zero or by increasing the d i f f u s i o n r e s i s t a n c e in the e n v i r o n m e n t of the c o n c r e t e by a tight soil. Determinin@
the CaD
removal
from m o r t a r
and c o n c r e t e
Koelliker measured the rate at w h i c h CaD is r e m o v e d by d i s s o l u t i o n from 4 x 4 x 16 cm' P o r t l a n d c e m e n t mortar prisms
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Vol. 19, No. 5 H. Grube and W. Rechenberg
containing a b o u t 500 kg of c e m e n t per cubic meter and having a w a t e r - c e m e n t ratio of 0 . 5 0 (14,15). Lime-dissolving carbonic acid was dissolved in the a t t a c k i n g water. The r e s u l t i n g pH value was about 4.4. The p r i s m s were stored in w a t e r for 1 to 2 m o n t h s and 1 year, r e s p e c t i v e l y , b e f o r e they were e x p o s e d to the l i m e - d i s s o l v i n g c a r b o n i c acid. F i g u r e 2 shows how the surface-related rate of d i s s o l u t i o n d e c l i n e s ( f o l l o w i n g approx, a I/~-~law) w i t h i n c r e a s i n g e x p o s u r e time. This i n d i c a t e s that, due to the e f f e c t of the l i m e - d i s s o l v i n g c a r b o n i c acid, a prot e c t i v e layer forms w h i c h g a i n s in t h i c k n e s s in the course of time and thus i n c r e a s i n g l y b l o c k s the d i f f u s i o n of the d i s s o l ved CaO.
,~
1Oh
d
7d
90d
10-7
? E u
~
10-8
p 10.9 10 5
10 *
10 6
10 7
duration of aggression (s) FIG.
2
D i s s o l u t i o n rate of c a l c i u m oxide from m o r t a r p r i s m s ( c o m p o s i t i o n a c c o r d i n g to DIN 1164) by a g g r e s s i o n of c a r b o n d i o x i d e s o l u t i o n with pH ~ 4.4 ( a c c o r d i n g to K o e l l i k e r (14)) By means of a model b a s e d on F i c k ' s first law e q u a t i o n 1 was developed (18) w h i c h can be used for c a l c u l a t i n g the t h i c k n e s s d of the d e s t r o y e d layer a f t e r any given e x p o s u r e time t. The influencing variables are the d i f f u s i o n c o e f f i c i e n t , the mass of s o l u b l e m a t t e r per v o l u m e unit, the area percent of the soluble matter in the area of a t t a c k and the d i f f e r e n c e in Ca(HCO3) 2 c o n c e n t r a t i o n :
2
d =
•
mL •
D
°
Ages
A L
(c*
-
c[ )
.
t
s
111
whereby d D
is the t h i c k n e s s of d e s t r o y e d c o n c r e t e layer Ca(HCO3) 2 d i f f u s i o n c o e f f i c i e n t in cm2/s
in cm
Vol. 19, No. 5
787
CONCRETE, DURABILITY, ACID, SOLVENT of the p r o t e c t i v e layer ~gel layer) (D ~ 1.8 • I0 -v cm 2 • sfor SiO 2 gel of h a r d e n e d P o r t l a n d c e m e n t paste) mass of s o l u b l e m a t t e r in g of CaO per cm 3 of c o n c r e t e
mt • Cs ct A[/Ages VL/Vges A[/Ages
Ca(HCO~) 2 c o n c e n t r a t i o n in g / c m 3 in the s o l u t i o n s u r r o u ~ d z n g intact c o n c r e t e C a ( H C O ~ ) ~ c o n c e n t r a t i o n in g / c m ~ in the s o l u t i o n w h i c h i s - u n a f f e c t e d by the c o n c r e t e ratio of s o l u b l e m a t t e r area to total area r a t i o of the v o l u m e of s o l u b l e c o n s t i t u e n t s (if i n s o l u b l e a g g r e g a t e is used ---~ volume of h a r d e n e d c e m e n t paste) to total volume = VL / V g e s
E x a m p l e s of c o m p u t a t i o n i n c l u d i n g the d e t e r m i n a t i o n of c o n c e n t r a t i o n d i f f e r e n c e (c: - c L) are given in (18).
the
The first root in e q u a t i o n 1 r e p r e s e n t s a constant valid the r e s p e c t i v e case. E q u a t i o n 1 can t h e r e f o r e be s i m p l i f i e d form e q u a t i o n 2. d = a • f'~ /2/
FIG.
3
in to
L,. u ¢0
Depth of dissolved material in r e l a t i o n to time - w i t h o u t b r u s h i n g off the gel - with f r e q u e n t (line I) and rare b r u s h i n g off (line II)
0 °__ ,4--
2 /
durntion of oggression t The loss of mass w h i c h o c c u r s w i t h an intact protective layer and follows equation 2 is shown by the full bold line in F i g u r e 3. It had been a s s u m e d e a r l i e r that the loss of mass might f o l l o w a . ~'6~law (9,11-13). However, in the past it was not possible to describe different test p e r i o d s by one function. This was due to the fact that the test specimens were dabbed with a moist cloth b e f o r e b e i n g w e i g h e d . In this way part of the gellike p r o t e c t i v e layer was removed so that the subsequent reaction occurred much faster than it w o u l d have done with an intact p r o t e c t i v e layer. If the protective layer is removed
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regularly it is to be e x p e c t e d that the p a r a b o l i c function turns to a linear one as shown by the s t r a i g h t lines I and II in F i g u r e 3. The "linear loss of mass" will be the s t e e p e r the more f r e q u e n t l y the protective layer is removed (straight line I). It will be the flatter the less f r e q u e n t l y the p r o t e c t i v e layer is r e m o v e d (straight line If). This "linear loss of mass" has already been o b s e r v e d in c o n n e c t i o n with l o n g - t e r m tests (6). In order to v e r i f y this b e h a v i o u r e x p e r i m e n t a l l y 4 x 4 x 16 cm p r i s m s a c c o r d i n g to DIN 1164 were made of a 35 F Portland cement and a 35 L blast furnace slag cement. They c o n t a i n e d 550 kg/m 3 of c e m e n t and had a w a t e r - c e m e n t ratio of 0.50. The p r i s m s were p r o d u c e d and s t o r e d in w a t e r by the standard procedure. At the age of 7 days they were p l a c e d in tap water w h i c h was c o n t i n u o u s l y e n r i c h e d with CO and renewed every week. The lime-attacking carbonic aci~ content was about ii0 mg CO2/I, the pH was 6.15 (mean values). The water flow rate was about 4.2 • i0 -z cm/s. The experimental s e t u p is d e s c r i b e d in d e t a i l in (5). The prisms were weighed every week. On the basis of the bulk d e n s i t y of the p r i s m s the w e i g h t loss was c o n v e r t e d to the d e p t h d of concrete removal which was compared with the d e p t h of r e m o v a l o b t a i n e d with e q u a t i o n i. In an i n i t i a l s e r i e s of tests the p r i s m s were only d a b b e d once a week. After 9 w e e k s some of the p r i s m s were b r u s h e d once a week, the o t h e r s were not s u b j e c t e d to any further mechanical treatment. The prisms of the s e c o n d test s e r i e s were b r u s h e d three times per day t h r o u g h o u t the test period. The s t o r a g e w i t h o u t any further m e c h a n i c a l t r e a t m e n t describes the chemical attack in slowly running water a c c o r d i n g to DIN 4030 (3) w i t h the d i f f u s i o n b e i n g c o n t r o l l e d by the p r o t e c tive layer. Dabbing presents a mild and b r u s h i n g a v i g o r o u s a d d i t i o n a l m e c h a n i c a l t r e a t m e n t (M) of the protective layer. The results for Portland cement m o r t a r in F i g u r e 4 and for blast f u r n a c e slag c e m e n t m o r t a r in Figure 5 show that the weight loss resulting from the uniform c h e m i c a l a t t a c k by ii0 mg CO9/I d e p e n d s very much on the type and intensity of the a d d i t i o n a l m e c h a n i c a l t r e a t m e n t . Thus, the p u r e l y c h e m i c a l a t t a c k (CA) c a u s e s the s m a l l e s t w e i g h t loss in time. The rate of c o n c r e t e r e m o v a l is i n c r e a s e d by d a b b i n g (CMA i) and more so by brushing (CMA 2 and CMA 3). For P o r t l a n d cement m o r t a r there is not much difference between the loss of concrete (Figure 4) resulting from b r u s h i n g once a week (CMA 2) and 3 times per w o r k d a y (CMA 3). The rate of concrete removal was approx. 1 mm in 18 w e e k s or about 3 mm/a (in the 4 x 4 x 16 cm prisms a w e i g h t loss of 60 g c o r r e s p o n d s to about 1 mm of loss of c o n c r e t e ) . The rate of concrete loss was lower for the blast f u r n a c e slag c e m e n t m o r t a r s than it was for the P o r t l a n d c e m e n t m o r t a r s . This leads to the a s s u m p t i o n that the higher content of silicon oxide and the r e s u l t i n g lower c o n t e n t of calcium oxide in blast furnace slag cement might be an explanation for this b e h a v i o u r . The q u e s t i o n , however, was not s t u d i e d in g r e a t e r detail.
Vol.
19, No. 5
789 CONCRETE, DURABILITY, ACID, SOLVENT
-10n
- 100
-8
-80
-6
~
-Z'
-60
-20
0
3
6 9 12 15 18 21 Z& duration of aggression (weeks)
FIG.
Z7
0
3
4
6 9 12 15 18 21 2z. duration 0, aggression ( weeks ]
FIG.
5
W e i g h t loss of W e i g h t loss of PC-mortar prisms.., bfs-cement mortar prisms... by a g g r e s s i v e c a r b o n dioxide. The c o m p o s i t i o n of the p r i s m s was a c c o r d i n g to DIN 1164. C A - c h e m i c a l a t t a c k only, CMA 1 - a t t a c k i n c r e a s e d m e c h a n i c a l l y by d a b b i n g b e f o r e w e i g h i n g , CMA 2 - b r u s h i n g off o n c e per week, CMA 3 - b r u s h i n g off three times per day. D o t t e d line ( c a l c u l a t i o n ) and first b r u s h off at 26 w e e k s ( e x p e r i m e n t a l ) The v a r i o u s l o s s e s of mass o b t a i n e d in the tests are b e l o w the ones calculated using e q u a t i o n /i/ for, e.g., w e e k l y losses, b e c a u s e b r u s h i n g did not r e m o v e all of the SiO~ gel from the pores. The longer the time p e r i o d of an u n d i s t u r b e d gel layer was the b e t t e r was the a g r e e m e n t between the calculated and the experimental results (18). This can be seen from the d o t t e d line in Figure 4, w h i c h s h o w s the calculated depth of dissolved concrete according to e q u a t i o n I. The first b r u s h off at the age of 26 w e e k s shows the marked value slightly b e l o w the c a l c u l a t e d value. Practical
application
In addition to the c o n s t r u c t i o n cost the d e s i g n e n g i n e e r should i n c l u d e in his c o n s i d e r a t i o n s the s e r v i c e life and the repair and m a i n t e n a n c e cost of a s t r u c t u r e . If c o n c r e t e is exposed to a c i d s the r e s u l t i n g loss of mass may be i r r e l e v a n t to
27
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Vol. 19, No. 5 H. Grube and W. Rechenberg
its use. This often is the case for s t r u c t u r e s like f o u n d a tions or d r i l l e d f o u n d a t i o n piles. In c o n t r a s t , the practical use of a c o n t a i n e r or a w a t e r duct were the p r o t e c t i v e layer may be r e m o v e d r e g u l a r l y can be considerably impaired by a constant loss of e.g. 0.5 mm of c o n c r e t e per year. T h e r e f o r e the d e s i g n e n g i n e e r , b e s i d e s c o n s i d e r i n g the acid concentration in the a t t a c k i n g water, should also pay a t t e n t i o n to the e x t e r n a l c o n d i t i o n s on w h i c h the c o n c r e t e is exposed to the water. For illustration F i g u r e 6 shows the d e p t h of c o n c r e t e loss in mm a f t e r 20 y e a r s for d i f f e r e n t conditions of transport and different c o n t e n t of a g g r e s s i v e CO 2. The c o n d i t i o n s of t r a n s p o r t are d e s c r i b e d in Table i. The d i f f e r e n t c u r v e s r e p r e s e n t the loss of mass after 20 years of s e r v i c e life w h e n a c o n c r e t e is used c o n s i s t i n g of a c i d - i n s o l u b l e a g g r e g a t e w i t h 350 kg of Portland cement per cubic meter of concrete and a w a t e r - c e m e n t ratio of 0.50. If the w a t e r - c e m e n t ratio is i n c r e a s e d or if acid-soluble aggregate is used a b i g g e r loss of mass has to be e x p e c t e d . If the c o n c r e t e is made of b l a s t f u r n a c e slag c e m e n t i n s t e a d of Portland cement, the loss will be smaller. The c u r v e s s h o w t~at in a c o m p a c t soil with a p e r m e a b i l i t y coe f f i c i e n t of k<10 cm/s the chemical attack is negligible (TC I) if the water flows very slowly below a hydraulic gradient. In a more p e r m e a b l e soil w i t h a r i s i n g flow rate the loss of mass i n c r e a s e s o n l y s l o w l y (TC 2). The flow rate has a g r e a t e r i n f l u e n c e than the acid c o n c e n t r a t i o n . In freely running water with u n d i s t u r b e d or d i s t u r b e d , i.e. r e g u l a r l y re-
¢-
~k E
> 2 (} u~
,4-
brush off l~/week L_ -'-
~5
5
corDon dioxide (mg/[) /100 ~ ~'~, 60
350 kg PClm3 concrete
"~....j ~ . . ~ . . .
"/c=0"50 I iI
30 F----~
e-
0 0,1 1 5 10
50 100
200 (sco[e:-~/-) 500
d depth of dissolved concrete o~er 20 years (mm) FIG.
6
D e p t h of d i s s o l v e d (and r e m o v e d ) c o n c r e t e a f t e r 20 y e a r s in d i f f e r e n t c o n c e n t r a t i o n s of a g g r e s s i v e c a r b o n d i o x i d e at d i f f e r e n t c o n d i t i o n s of t r a n s p o r t (see below) for the p r o d u c t s of the r e a c t i o n
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791 CONCRETE, DURABILITY, ACID, SOLVENT
moved, p r o t e c t i v e layer (TC 3 and 4), the t r a n s p o r t conditions obviously have a much g r e a t e r e f f e c t than the acid c o n c e n t r a tion. The dots in the f i g u r e r e p r e s e n t the r e s u l t s of experiments and c a l c u l a t i o n s . The d o t t e d parts of the c u r v e s in the areas of TC 5 and 6 are e x t r a p o l a t i o n s that can at present not be v a l i d a t e d by test results. TABLE Conditions
of t r a n s p o r t
TC 1
1
according
to F i g u r e
6
Conditions C o m p a c t d e p o s i t e d soil, p e r m e a b i l i t y k < 10 -4 cm/s, very s l o w l y m o v i n g or s t a g n a n t g r o u n d water, protective layer, F i c k ' s 2. l~w. P e r m e a b l e soil with k > i0cm/s, g r o u n d water m o v i n g slowly, p r o t e c t i v e layer, F i c k ' s I. law. F l o w i n g free water, p r o t e c t i v e layer n e a r l y u n d i s t u r bed, F i c k ' s i. law. F l o w i n g free water, p r o t e c t i v e layer r e m o v e d p e r i o d i cally, F i c k ' s I. law. F l o w i n g free water, high v e l o c i t y of flow, no prot e c t i v e layer, a p p r o x i m a t i o n to s p e c i f i c v e l o c i t y of dissolution. F l o w i n g free water, very high v e l o c i t y of t u r b u l e n t flow, c a v i t a t i o n . Conclusions
- The d i f f u s i o n model p r e s e n t e d here and the tests described permit to arrive at a q u a n t i t a t i v e e s t i m a t e of the loss of m o r t a r and c o n c r e t e to be e x p e c t e d in case of a chemical a t t a c k by l i m e - d i s s o l v i n g c a r b o n i c acid. -
The exposure to l i m e - d i s s o l v i n g c a r b o n i c acid leads to the formation of a protective layer consisting of gel-like silicon dioxide.
- In the presence mass f o l l o w s a • ~ - If the concrete -
protective is r e m o v e d
of an intact law.
protective
layer is mechanically at an a c c e l e r a t e d rate.
layer
the
loss of
destroyed
the
A disturbance of the p r o t e c t i v e layer, even if it occurs at r e l a t i v e l y long i n t e r v a l s , can have a much more damaging effect than an increase in the concentration of the a g g r e s s i v e acid.
- The tests d e s c r i b e d , i.e. the stripping of the protective layer at regular intervals, and the mathematical model p e r m i t a l o n g - t e r m e s t i m a t e of the loss of mass also caused by other than lime-dissolving carbonic acid. E x p e r i m e n t s w i t h other a c i d s can be m a d e in about 3 months. - It a p p e a r s
possible
to c o n s i d e r
more
completely
in
future
792
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H. Grube and W. Rechenberg
standards the f a c t o r s w h i c h r e d u c e a n d i n c r e a s e the l o s s of mass from concrete e x p o s e d to attacking acids. This would be to the b e n e f i t of c o n c r e t e structures. It is s u g g e s t e d to d e f i n e t h e d e g r e e s of a t t a c k by l o s s of m a s s .
References I. CSN 73 2020 (1954): Vodostavebn~ Bet6ny-Defincia, roztriedenie a technick~ podmienky. Vydavate1' Avo Uradupre Normaliz~cin, Praha. 2. TGL 33 408: Betonbau, Korrosion und Korrosionsschutz. Berlin 1980. 3. DIN 4030 (1969): Beurteilung b e t o n a n g r e i f e n d e r W~sser, B~den und Gase. Beuth-Verlag, Berlin und KSln. 4. ISO-Draft DP 9690 (1987): Classification of environmental exposure c o n d i t i o n s for concrete and reinforced concrete s t r u c t u r e s . ISO-TC 71/33. 5 Locher, F.W., und S. Sprung: beton 25, 241 (1975). 6 Locher, F.W., W. Rechenberg und S. Sprung: beton 34, 193, (1975). 7 Friede, H., P. Schubert und H.P. L~hr: beton 29, 250, (1979) 8 Efes, Y., und H.P. L~hr: TIZ-Fachber., 104, 153 (1980). 9 Friede, H., Dissert. RWTH Aachen 1983. 10 Gille, F: beton 12, 467 (1962). 11 Tremper, B.: Amer. Concr. Inst., Proc. 28, 1 (1932). 12 Romb~n, L.: CBI forskning research 1:78 und 9:79, Cement- och betong instituter, Stockholm. 13. Polak, A.F.: Beton; Zhelezobeton, Nr. 8, 5 (1978). 14. Koelliker, E.: Intern. Kolloquium "Werkstoffwissenschaften und Bausanierung", B e r i c h t s b a n d TA Esslingen, 1963, 195. E. M o e l l e r GmbH, Filderstadt 1983. 15. Koelliker, E. : Cem. Concr. Res. 15, i00 (1965). 16. Gr~n, R., und K. Obenauer: Zement 33, i0 (1944). 17. Becket, H.: Z e m e n t - K a l k - G i p s 39, 273 (1986). 18. Grube, H., und W. Rechenberg: beton 37, 446 (1987).