- Email: [email protected]
Influence of the degree of pore saturation on the resistivity of concrete and the corrosion rate of steel reinforcement
CEMENT and CONCRETE RESEARCH. Vol. 23, pp. 368-376, 1993. Printed in the USA. 0008-8846/93. $6.00+00. Copyright © 1993 Pergamon Press Ltd.
INFLUENCE OF CONCRETE
THE DEGREE OF PORE SATURATION ON THE RESISTIVITY AND THE CORROSION RATE OF STEEL REINFORCEMENT
OF
W. L 6 p e z U n i v e r s i d a d N a c i o n a l A u t d n o m a de M6xico. F a c u l t a d de Qu£mica, E d i f i c i o D. C i u d a d U n i v e r s i t a r i a . M 4 x i c o 04510, D F
J.A. G o n z ~ l e z C e n t r o N a c i o n a l de I n v e s t i g a c i o n e s M e t a l d r g i c a s (CENIM). Avda. G r e g o r i o del Amo 8, E - 2 8 0 4 0 Madrid, S p a i n (Communicatedby C.D. Pomeroy) (Received March 16, 1992)
ABSTRACT Quantitative relations between the corrosion rate of reinforcements (i^r)t)(p a n d the d e g r e e of p o r e s a t u r a t i o n (PS) and resistivity of m o r t a r s w i t h o u t CI- a n d w i t h 2% CI" w e r e o b t a i n e d . T h e m o r t a r s p e c i m e n s w e r e c u r e d in a w a t e r fog c h a m b e r before the f i n a l exposure at 50o_c a n d 50% relative humidity (RH). The e l e c t r o l y t e supply was found to d e t e r m i n e the ~mortar ? r e s i s t i v i t y w h i c h v a r i e s o v e r a w i d e r a n g e (5 x 10J--5 x i0 ~cm) a n d t h i s in t u r n i n f l u e n c e s the i of the reinforcements. There is a critical corr practical PS va~ue (PS) that results in a mortar resistivity of 10" flcm, ~ e l o w w h i c h i values are too s m a l l to p o s e a n y d u r a b i l i t y p r o b l e m s . B ~ r o w the PS c v a l u e , the resistivity of the mortar prevents activ~ state c o r r o s i o n of r e i n f o r c e m e n t s as e f f e c t i v e l y as p a s s i v a t i n g l a y e r s of s t e e l in m o r t a r s w i t h o u t CI'. _
INTRODUCTION
T h e res.istivity of c o n c r e t e v a r i e s o v e r v e r y broadA r a n g e s , viz. f r o m i0 H ~ c m for o v e n - d r i e d m a t e r i a l to l e s s t h a n I0 J ~ c m for v e r y wet concrete (i). G j o r v (2) r e p o r t e d r e s i s t i v i t y v a l u e s of 7 × i0 ~ a n d 6 x l0 b ~ c m for 1 0 0 % a n d 20% s a t u r a t e d c o n c r e t e , respectively, which are perfectly possible in n a t u r a l e n v i r o n m e n t s . Thus, the r e s i s t i v i t y of c o n c r e t e is m a r k e d l y d e p e n d e n t on t h e d e g r e e of p o r e saturation (PS) a n d also, to a l e s s e r e x t e n t , on the d e g r e e of paste hydration ( c u r i n g ) a n d the p r e s e n c e of d i s s o l v e d s a l t s in the a q u e o u s p h a s e of the c o n c r e t e (3). O n the o t h e r hand, humidity is a l s o t h e f a c t o r most strongly a f f e c t i n g the a c t i v e c o r r o s i o n r a t e of r e i n f o r c e m e n t s . Accordingly, 368
VOI. 23, No. 2
PORE SATURATION, RESISTIVITY, CORROSION, STEEL REINFORCEMENT
369
the r e l a t i o n s h i p b e t w e e n c o n c r e t e r e s i s t i v i t y and r e i n f o r c e m e n t c o r r o s i o n rate has b e e n s t u d i e d by some a u t h o r s (4-6) and it was found that t h e s e two p a r a m e t e r s are i n v e r s e l y p r o p o r t i o n a l o v e r a w i d e r e s i s t i v i t y range. H o w e v e r , there is not a g e n e r a l a g r e e m e n t about the r e s i s t i v i t y level a b o v e w h i c h c o r r o s i o n risks will be n e g l i g i b l e , p r o b a b l y as a r e s u l t of the lack of a c l e a r idea of w h a t is a c c e p t a b l e and u n a c c e p t a b l e in terms of r e i n f o r c e m e n t c o r r o s i o n rate. For example, a c c o r d i n g to H o p e et el.(7) r e i n f o r c e m e n t s e m b e d d e d in c o n c r e t e with a resistivity higher than 104 ~cm will hardly develop c o r r o s i o n , not even in the p r e s e n c e of el-, ~ g r CO~. S t r a t f u l l (3) r e p o r t e d a s o m e w h a t h i g h e r limit (6.5 x ~ ~cm) for c o n c r e t e structures. The aim of this w o r k was to o b t a i n q u a n t i t a t i v e r e l a t i o n s b e t w e e n the r e i n f o r c e m e n t c o r r o s i o n rate and the r e s i s t i v i t y and d e g r e e of pore s a t u r a t i o n of m o r t a r s w i t h and w i t h o u t CI-. This is the first time that the a s s o c i a t i o n b e t w e e n these t h r e e v a r i a b l e s has b e e n investigated.
EXPERIMENTAL
Two s e r i e s of m o r t a r s s p e c i m e n s (Fig i) w e r e p r e p a r e d , w i t h o u t Cl a d d i t i o n s and c o n t a i n i n g 2 %CI- (as NaCI) by c e m e n t weight, b o t h s e r i e s w i t h a c e m e n t ~ s a n d ~ w a t e r ratio of 1:3:0.5. All m o r t a r s were d e m o u l d e d 24 h a f t e r h a r d e n i n g and t h e i r a v e r a g e p o r o s i t y was 16.11 ± 0.8%. Two t y p e s
of
mortar
specimens were
used,
one
of
8x2x2
cm
for
resistivity measurements and other of 8xS.Sx2 cm for electrochemical corrosion rate determinations (Fig. I). The r e s i s t i v i t y s p e c i m e n s w e r e m e c h a n i c a l l y ground, a f t e r d e m o u l d i n g , in o r d e r to e q u a l i z e t h e i r d i m e n s i o n s , and the o p p o s i n g 2x2 cm faces w e r e s u b s e q u e n t l y c o a t e d w i t h g r a p h i t e c o n d u c t i n g p a i n t in o r d e r to f a c i l i t a t e electrical contact during the r e s i s t i v i t y measurements.
Figure 1 Appearance of the specimens fabricated
mortar
LEFT:
for r e s i s t i v i t y measurements
RIGHT:
for e l e c t r o c h e m i c a l measurements
370
W. I ~
and J.A. Gonzfdez
Vol. 23, No. 2
Two c a r b o n steel rods were e m b e d d e d at s y m m e t r i c a l p o s i t i o n s into the m o r t a r s s p e c i m e n s for a c t i n g as w o r k i n g e l e c t r o d e s (WE) d u r i n g the e l e c t r o c h e m i c a l m e a s u r e m e n t s , w h i l e a s t a i n l e s s steel rod was e m p l o y e d as c o u n t e r e l e c t r o d e (CE). An e x t e r n a l s a t u r a t e d c a l o m e l e l e c t r o d e (SCE) was used as r e f e r e n c e e l e c t r o d e (RE) t h r o u g h o u t . T h e s e s p e c i m e n s a l l o w m o n i t o r i n g of the v a r i a t i o n of E~rr, ic0rr, PS and the o h m i c r e s i s t a n c e b e t w e e n the two w o r k i n g e l e c £ r o d e s . All e l e c t r o c h e m i c a l d e t e r m i n a t i o n s w e r e c a r r i e d out in d u p l i c a t e . The m o r t a r s w e r e c u r e d in a s t a n d a r d w a t e r fog c h a m b e r at 23"C for 40 d a y s and s u b s e q u e n t l y w e r e t r a n s f e r r e d to an a i r - t i g h t c h a m b e r at a r e l a t i v e h u m i d i t y (RH) > 90% and a m b i e n t t e m p e r a t u r e , w h e r e they were a l l o w e d to stand for 60 f u r t h e r days b e f o r e t h e y w e r e f i n a l l y e x p o s e d to 50"C and 50% RH for 500 days. The d e g r e e of pore s a t u r a t i o n of the m o r t a r s was c a l c u l a t e d from t h e i r w e i g h t by u s i n g the f o l l o w i n g e q u a t i o n :
W t
-
W0
W s
-
W0
PS =
wherecompleteW,d e n o t e s the m o r t a r weight, at time t, 2 W the w e i g h t u p o n s a t u r a t i o n by b o i l i n g in w a t e r for ~, and W 0 the new w e i g h t o b t a i n e d a f t e r d r y i n g the m o r t a r by h e a t i n g in a s t o v e at I05"C u n t i l w e i g h t c o n s t a n c y . The resistivity of the mortars specimens used for the e l e c t r o c h e m i c a l m e a s u r e m e n t s was d e t e r m i n e d by m u l t i p l y i n g the R~ v a l u e b e t w e e n the two WE by the cell c o n s t a n t for this g e o m e t r y , w h i c h was in t u r n o b t a i n e d f r o m the slope of the p l o t of the r e s i s t i v i t i e s of the m o r t a r s s p e c i m e n s of k n o w n g e o m e t r y v e r s u s the wR~ v a l u e s of those used in the e l e c t r o c h e m i c a l studies, b o t h of ich w e r e m a d e w i t h the same c o m p o s i t i o n and e x p o s e d to the same t e m p e r a t u r e and RH c o n d i t i o n s (Fig 2). The R~ v a l u e s of the m o r t a r s were determined by u s i n g a S o l a r t r o n 1251 f r e q u e n c y response a n a l y s e r at a c o n s t a n t f r e q u e n c y of 1 kHz. 1.0E,06
1.0E-05 0
Figure 2 Resistivity vs ohmic resistance plot to determine the cell c o n s t a n t of the m o r t a r s for electrochemical measurements
rr,u
r,~
Z
.<
1.0E*04
[n
• •
I.|NI:AL
Hi:QHI8810N
r,n
r~
1,0E*03
1.0E'02 %0E*03
. ~
,
",,4 ~ d ~5~"~' /
I
J
[
oorr
•
• 0,1897.
b • - 6 a l . E (ohm) n • ,nSO II/m) K CELL • $,415 (oal)
I IIIIA
1.0E*04
Resistivity
I
I
,
t [ii11
1.0E-05
(ohm.---)
t
i
,
i
ii
1.0E*06
Vol. 23, No. 2
PORE SATURATION,RESISTIVITY,CORROSION, STEEL REINFORCEMENT
371
RESULTS
Figures 3a--3d show the changes with time in the monitored p a r a m e t e r s : ic0r~ Ec0rr, r e s i s t i v i t y and w a t e r loss of the m o r t a r s , r e s p e c t i v e l y . T h e z o n e s l a b e l l e d w i t h 0, I and 2 c o r r e s p o n d to the c u r i n g in the w a t e r fog a t m o s p h e r e , the s t a b i l i z a t i o n p e r i o d at RH > 90% and a m b i e n t t e m p e r a t u r e , and the e x p o s u r e to RH = 50% and 50°C, r e s p e c t i v e l y . Figure 4 shows the relationship b e t w e e n ic0rr and the r e s i s t i v i t y of 1 2 A the m o r t a r s , b o t h c o n t a i n i n g 2% C1 and w i t h o u t CI', f r o m the b e g i n n i n g of the s t a b i l i z a t i o n p e r i o d at RH > " k 90% and a m b i e n t t e m p e r a t u r e to a f t e r 500 d a y s of e x p o s u r e to RH = 50% and 50°C. The p o i n t s w i t h i n the c i r c l e c o r r e s p o n d to the i .... and r e s i s t i v i t y v a l u e s of the m o r t ~ s at the end of curing, while those inside the r e c t a n g l e c o r r e s p o n d to t h o s e of the B~ J m o r t a r s c o n t a i n i n g 2% Cl- d u r i n g the s t a b i l i z a t i o n period. R a t h e r t h a n to s ignificant changes in the ° / resistivity of c o n c r e t e (Fig 3c), t h e s e l a t t e r p o i n t s are r e l a t e d to the small w a t e r loss from the m o r t a r s in passing from the curing fog c h a m b e r to the a t m o s p h e r e w i t h a J RH > 90% (Fig 3d). c
Figure 5 shows the relationship b e t w e e n the d e g r e e of s a t u r a t i o n of the c o n c r e t e p o r e s (PS) and i ..... As can be seen, the m a x i m u m ic tr ~ l u e s c o r r e s p o n d to PS v a l u e s o~ 60--70%. Also, at l o w e r PS values, the i... v a l u e s of the m o r t a r s c o n t a i n l n g ~ CI- are s m a l l e r by a b o u t two o r d e r s of m a g n i t u d e , consistent with the inverse proportionality between • " Fig 4. the r e s i s t i v i t y and ic0rr in F i n a l l y , as can also be s e e n in the figure 5, the ~ values of the m o r t a r s w i t h o u t ~ x ~ are i n d e p e n d e n t of PS b e t w e e n 100% and 50%, but the ie0rr d e c r e a s e for PS less t h a n 50%.
o
el
| t4 m W
o
¢D
t
o
Ioo
:too
3o0
40o
ooo
eoo
TINR ( , I - y l )
a.
2 % CI-
0% CI-
TOO
Figure 3 V a r i a t i o n of the d i f f e r e n t m o n i t o r e d v a r i a b l e s w i t h time since the p e r i o d of c u r i n g u n t i l e x p o s u r e to 50:C and RH = 50%
372
W.I..,6pez and J.A. Go'nzile,z
1.0E-O6
-
Vol. 23, No. 2
1.OE-O5 "
-F +
O"
GI-
2 " Cl"
/
\
+ ++
1.0E-Oe
\
~
1.0E-Oe
•
+
il U .<
+
i
N
+
+4-
E
0 < v
+ 1.0E-07 •
+ 1.0E-07
.+.
.+
•
.
•
o +
+
+
+
+
1.0E-08
1.0E-08
I 1.OE-O9
, ,,,,,., , 1.0E+03 1.0E+04
,,,,,,,, laE~8
, , ,,,,., 1.0E*06
Resistivity
,
,,,,l., tOE+07
(ohm.
o.
GI-
-~-
2.
GI-I
i
i
i
i
i
i
i
30
40
80
60
70
80
90
100
Pore
cm)
Saturation
Figure 5
Figure 4
Influence of the d e g r e e of m o r t a r p o r e s a t u r a t i o n on the corrosion rate of the reinforcement
Relationship between mortar r e s i s t i v i t y and the c o r r o s i o n rate of the r e i n f o r c e m e n t
F i g u r e s 6a and 6b s h o w the relationship between the w a t e r loss from the m o r t a r s and PS with the mortar resistivity, respectively. As can be seen, the r e s i s t i v i t y of a w a t e r - s a t u r a t e d mortar (i.e. c u r e d in a w a t e r - f o g atmosphere that p r o d u c e s a visible water l a y e r on the m o r t a r ) s c a r c e l y c h a n g e s on exchanging water with the e n v i r o n m e n t u n t i l the m o r t a r weight decreases by ca. 2% (Fig 6), which corresponds to PS m 70% (Fig 6b). Further water releases to the environment result in gradually larger increases in resistivity and progressively smaller decreases i n PS ( F i g s 6a and 6b).
-
i
1.0E-09
, ,,,,,,, 1.0E*08
I .Ol~O
1
++++
++..
• 4-..
-t-k.
..+
t.
+
m~
1.0E+OO
÷
.
¢ ,;a e~ 3~
+
.........
, ,, ,,,,.
100.000"
80.000-
÷ +
00.000-
..........................
[ ....
~--o-ls
• +°
TO.O00"
460.000"
+.
80.000-
40.000"
+
Figure
6
Effect of the water loss and p o r e s a t u r a t i o n on the mortar resistivity
30.000" 20.000
• • i i .............
1.0E*03
+
LOEb04
".......................... 1-01=*08 1 . 0 E * 0 8 1.0E*07
Resistivity (oha-ca)
1.0E408
Vol, 23, No. 2
PORE SATURATION,RESISTIVITY,CORROSION,STEEL REINFORCEMENT
373
DISCUSSION
On e x p o s u r e to an a t m o s p h e r e at a g i v e n r e l a t i v e h u m i d i t y and t e m p e r a t u r e , the h u m i d i t y c o n t a i n e d in the p o r o u s of the c o n c r e t e e q u i l i b r a t e s w i t h the a m b i e n t humidity (Fig 3c). The p r o c e s s t h r o u g h w h i c h such an e q u i l i b r i u m is e s t a b l i s h e d d e t e r m i n e s the d e g r e e of s a t u r a t i o n of the c o n c r e t e pores, PS, and a n u m b e r of the p h y s i c o - c h e m i c a l p r o p e r t i e s of c o n c r e t e i n c l u d i n g gas d i f f u s i o n t h r o u g h the p o r e s (8,9) and e l e c t r i c a l r e s i s t i v i t y (1,2), both of w h i c h are m a r k e d l y i n f l u e n t i a l on the c o r r o s i o n k i n e t i c s at the s t e e l / c o n c r e t e interface. The w a t e r c o n d e n s a t i o n - - e v a p o r a t i o n e q u i l i b r i u m o c c u r r i n g in m o r t a r p o r e s can be d e s c r i b e d by m e a n s of the K e l v i n e q u a t i o n (I0):
p In
2V¥
m
Po
rRT
A c c o r d i n g l y , in the e v e n t of w a t e r c o n d e n s i n g on the m o r t a r s u r f a c e - - a s o b s e r v e d d u r i n g the c u r i n g p r o c e s s in a w a t e r o v e r s a t u r a t e d a t m o s p h e r e - - , p o r e s of all d i a m e t e r s will be fully s a t u r a t e d w i t h water. T h e r e f o r e , u n d e r these h u m i d i t y c o n d i t i o n s , the t r a n s f e r of 02 t h r o u g h the c o n c r e t e p o r e s to the r e i n f o r c e m e n t will take p l a c e e k c l u s i v e l y t h r o u g h liquld-phase" d i f f u s i o n . In fact, ic0 was 0.3 N A / c m 2 for the m o r t a r s c o n t a i n i n g 2% CI- and 0.15 ~ A / c ~~ for the m o r t a r s w i t h o u t CI- (points inside the c i r c l e in Fig 4). On the o t h e r hand, a c c o r d i n g to the K e l v i n e q u a t i o n , d u r i n g the s t a b i l i z a t i o n p e r i o d at 23"C and RH > 90% ( p / ~ > 0.90), for a p a r t i a l w a t e r v a p o u r p r e s s u r e in the a t m o s p h e r e o f 0.90, w a t e r will e v a p o r a t e from all p o r e s w i t h a radius l a r g e r t h a n 116 A. U n d e r these c o n d i t i o n s , no w a t e r will c o n d e n s e on the m o r t a r surface, but only in those p o r e s w i t h a radius s m a l l e r t h a n 116 A. E a c h RH v a l u e will thus have a m a t c h i n g c r i t i c a l pore r a d i u s b e l o w w h i c h p o r e s will remain water-saturated and above w h i c h w a t e r will e v a p o r a t e , t h e r e b y e s t a b l i s h i n g p r e f e r e n t i a l a e r a t i o n c h a n n e l s for p e n e t r a t i o n of O I t h r o u g h v a p o u r - p h a s e d i f f u s i o n w i t h a c o e f f i c i e n t D02 c a . I0" t i m e ~ g r e a t e r than in a l i q u i d p h a s e (8). As 02 r e a c h e s the r e i n f o r c e m e n t t h r o u g h the p r e f e r e n t i a l a e r a t i o n channels, it creates differential aeration cells and hence i n c r e a s e s E .... by a b o u t i00 mV in m o r t a r s c o n t a i n i n g no Cl" (Fig 3b), and i m p r o v e s the steel p a s s i v i t y (Fig. 3a). On the o t h e r hand, the m o r t a r s c o n t a i n i n g 2% CI- u n d e r g o a v e r y sharp initial rise in Ec0rr of c a . 300--400 mV (Fig 3b), c o n s i s t e n t w i t h a slight d e c r e a s e in ~c0rr' f o l l o w e d by a d r o p to a c t i v e p o t e n t i a l s and a c o n c o m i t a n t i n c r e a s e in i.... ( F i g 3 a ~ On e x p o s u r e t ~ % 0 * C a n d R H = 50%, the m o r t a r s r e l e a s e w a t e r up to ca. 4% of t h e i r w e i g h t (Fig 3d). This r e s u l t s in a ~ r a ~ a t i c i n c r e a s e in the m o r t a r r e s i s t i v i t y from 3 x l0 w ~cm to I0"--I0 ~cm (Fig 3c). Under these conditions, the i values of the r e i n f o r c e m e p t e m b e d d e d in m o r t a r s w i t h 2% CI" C ~ r s t l y i n c r e a s e up to 3 B A / c m ~ due to the a c c e s s of 02 by p r e f e r e n t i a l a e r a t i o n channels, and l a t e r d e c r e a s e to 0.02 ~ A / c m 2 by e f f e c t of the m o r t a r r e s i s t i v i t y (Fig 3a). •
~U[[
374
W. L6pez and J.A. Gonz~ez
Vol. 23, No. 2
At 5 0 ° C a n d RH { 50%, r e s i s t a n c e c o n t r o l e n c o m p a s s e s resistivities between 7 x i0 ~ c m a n d the u p p e r limit, i~ ~cm (Fig 4). T h e o c c u r r e n c e of an i n v e r s e p r o p o r t i o n a l i t y relationship between the r e s i s t i v i t y a n d i cr in t h i s p o r t i o n of the plot, w i t h a s l o p e c l o s e to -i, u n e q u i v o c a ~ l y r e f l e c t s the c o n t r o l l i n g r o l e of the m o r t a r resistivity in the c o r r o s i o n k i n e t i c s in the a c t i v e state. H o w e v e r , in the p a s s i v e s t a t e ( m o r t a r s w i t h o u t CI'), the c o r r o s i o n r a t e is i n d e p e n d e n t of the r e s i s t i v i t y and anodic control takes o v e r on a c c o u n t of the p r o t e c t i v e f e a t u r e ~ of the p a s s i v a t i n g layer of s t e e l up to r e s i s t i v i t i e s of 3-4 x i ~ ~cm. T h e r e s i s t i v i t y of d r i e r m o r t a r s is an e v e n g r e a t e r h i n d r a n c e to c o r r o s i o n t h a n is the passivating l a y e r (Fig 4). Corrosion of r e i n f o r c e d concrete thus s e e m s to a r i s e f r o m two opposing effects, namely: - - T h e i n c r e a s e d ~2 s u p p l y t h r o u g h the l a r g e r p o r e s r e s u l t i n g from a decrease In t h e d e g r e e of s a t u r a t i o n d o w n to 60--70%, w h i c h r e s u l t s in i n c r e a s e d i c (Fig 5) t h r o u g h t r i g g e r i n g of oil . the passivity--activity transltlon in the steel and local acidification at the steel/concrete interface (ii). The resistivity of c o n c r e t e s c a r c e l y v a r i e s o v e r t h i s PS r a n g e (Fig 6b). - - A sharp, g r a d u a l l y m o r e r a p i d i n c r e a s e in r e s i s t i v i t y at PS v a l u e s b e l o w 70% (Fig 6b), w h i c h b e c o m e s the f a c t o r c o n t r o l l i n g the c o r r o s i o n kinetics at the s t e e l / c o n c r e t e interface and r e s u l t s in a m a r k e d d e c r e a s e in ic0tt (Fig 4). T h e c o r r o s i o n p r o c e s s of a c t i v e c o r r o d i n g s t e e l in c o n c r e t e a n d the atmospheric corrosion of m e t a l s are s i m i l a r in t h a t t h e y " s t o p " b e l o w a RH c r i t i c a l v a l u e (RHc). R a t h e r t h a n a c r i t i c a l relative humidity, concrete structures f e a t u r e a c r i t i c a l d e g r e e of p o r e saturation, PS , w h i c h can be ~ssumed to be ca. 35% and corresponding to ic0rr - 0.02 D A / c m (Fig 5), w h i c h r e s u l t s in a penetration of 0.2 D m / y e a r t h a t is c l e a r l y n e g l i g i b l e . F r o m F i g s 4 a n d 5, o n e m a y d e f i n e t h r e e t y p e s of c r i t i c a l PS v a l u e s allowing the b e s t c h a r a c t e r i z a t i o n of t h e c o r r o s i o n process of s t e e l in c o n c r e t e , n a m e l y : - - T h e u p p e r c r i t i c a l limit, P S c , w h i c h s i g n a l s the b e g i n n i n g of r e s i s t a n c e control a n d w o u ~ d be 3of ca. 70% (Fig 5) a n d c o r r e s p o n d to a r e s i s t i v i t y of 7 x i0 ~ c m (Figs 4 a n d 6b) for the c o n d i t i o n s t e s t e d h e r e i n (50°C a n d RH = 50%). - - T h e l o w e r c r i t i c a l limit, PSlc , viz. 35% as n o t e d e a r l i e r , below which corrosion "stops". -- The practical critical limit, PS , b e l o w w h i c h c o r r o s i o n risks are negligible for p r a c t i c a l ~ u r p o s e s . S u c h a l i m i t i~ 45--50%, w h i c h c o r r e s p o n d s to an ic0rr b e t w e e n 0.2 a n d 0.I D A / c m (Fig 5 ) . T h e p r o p o s e d PS v a l u e r e s u l t s in a r e s i s t i v i t y of ca. 105 ~ c m (Fig 6b), i.e. ver] ¢ close to the 6.5 x I0 ~ ~ c m l i m i t above which Stratfull ceased t? d e t e c t visible evidence of c o r r o s i o n in reinforced concrete-. At this resistivity values, the results obtained with active and passive structures (i.e. with and without chlorides) are very similar: i in the r a n g e b e t w e e n 0.i a n d 0.2 o cort D A / c m ~ (Fig 4), w h i c h c o r r e s p o n d to p e n e t r a t i o n s of 1--2 D m / y e a r . Such small penetrations will hardly pose durability problems t h r o u g h o u t the s e r v i c e l i f e of a r e i n f o r c e d c o n c r e t e s t r u c t u r e a n d c o n f i r m s the v i s u a l o b s e r v a t i o n s r e p o r t e d by S t r a t f u l l .
Vot. 23, No. 2
PORE SATURATION, RESISTIVITY, CORROSION, STEEL REINFORCEMENT
375
CONCLUSIONS
The r e s u l t s o b t a i n e d to be drawn:
in this
work allow
the
following
conclusions
I. The d e g r e e of pore s a t u r a t i o n (PS) of the c o n c r e t e has a v e r y strong effect on the c o r r o s i o n k i n e t i c s and d e t e r m i n e s the active state corrosion mechanism at the steel/concrete interface. 2. One can distinguish three d i f f e r e n t c r i t i c a l PS values: an upper limit (PS.~) c o i n c i d i n g w i t h the m a x i m u m c o r r o s i o n rate and below whic~ resistance control begins; a lower limit (PSIc) s i m i l a r to the c r i t i c a l r e l a t i v e h u m i d i t y in a t m o s p h e r i c corrosion and b e l o w w h i c h c o r r o s i o n d e v e l o p s to a n e g l i g i b l e extent; and a p r a c t i c a l limit ( P S c) that s i g n a l s the start of u n a c c e p t a b l e c o r r o s i o n rates and ~ e n c e of p o t e n t i a l d u r a b i l i t y problems. 3.
The resistivity of c o n c r e t e r e m a i n s v i r t u a l l y u n c h a n g e d at PS > PS and corrosion decreases g r a d u a l l y as a r e s u l t of diffusion control in an i n c r e a s i n g l y s a t u r a t e d pore network. On the other hand, at PS < PSuc, the r e s i s t i v i t y i n c r e a s e s e x p o n e n t i a l l y and i i n v e r s e l y p r o p o r t i o n a l to i .... over a v e r y broad range (7 x 1 0 t l x l0 b flcm). ~u,,
4. The corrosion rate in the passive state depends on the properties of the passivating layer of steel in c o n c r e t e r a t h e r than on c o n c r e t e r e s i s t i v i t y . However, below PSp , the concrete resistivity b e c o m e s an e v e n more p o w e r f u l h i n d r a n c e to c o r r o s i o n t h a n is the p r o t e c t i v e e f f e c t of the p a s s i v a t i n g layers. ACKNOWLEDGEMENTS
This w o r k was s u p p o r t e d by the C o m i s i 6 n I n t e r m i n i s t e r i a l de C i e n c i a y T e c n o l o g ~ a (CICYT) of the M i n i s t r y of E d u c a t i o n and S c i e n c e of Spain. One of the a u t h o r s (WL) a c k n o w l e d g e s the f i n a n c i a l s u p p o r t p r o v i d e d by the U n i v e r s i d a d N a c i o n a l A u t 6 n o m a de M 4 x i c o and the A g e n c i a Espafiola de C o o p e r a c i 6 n I n t e r n a c i o n a l . REFERENCES
1.
G. E. Monfore. (Portland Cem. (1968).
2.
O.E. Gjorv, O. V e n n e s l a n d , A.H.S. E i - B u s a i d y . Proc.O f f s h o r e Technol. Conf., 9 No. I, 5 8 1 - 5 8 8 (1977).
3.
R. F. S t r a t f u l l . How Chlorides affect concrete used with R e i n f o r c i n g Steel. M a t e r i a l s P r o t e c t i o n , ! No.3, 29-34 (1968).
Electrical Ass.) Res.
resistivity of Develop. Lab.,
concrete. J. P C A i O No. 2, 35-48
Annu.
376
W. L6pez and J.A. Gonz~lez
Vol. 23, No. 2
4.
S. Felid, J.A. GonzAlez, S. Felid Jr. and C. Andrade. R e l a t i o n s h i p b e t w e e n c o n d u c t i v i t y of c o n c r e t e and c o r r o s i o n of r e i n f o r c i n g bars. B r i t i s h C o r r o s i o n Journal, 24 No. 3, 1 9 5 - 1 9 8 (1989).
5.
A. Molina, C. Andrade, C. A l o n s o controlantes de la v e l o c i d a d de e m b e h i d a s en m o r t e r o s de cemento. No. 2, 9-15 (1985).
6.
C. Alonso, C. A n d r a d e and J.A. Gonzglez. Relation between r e s i s t i v i t y and c o r r o s i o n rate of r e i n f o r c e m e n t s in c a r b o n a t e d m o r t a r made w i t h s e v e r a l C e m e n t types. C e m e n t and C o n c r e t e Research, 8 No. 5, 6 8 7 - 6 9 8 (1988).
7.
B. B. Hope, A. K. Ip and D.G. Manning. Corrosion and e l e c t r i c a l I m p e d a n c e in concrete. C e m e n t and C o n c r e t e R e s e a r c h ,
y J. A. GonzAlez. Factores c o r r o s i 6 n de las a r m a d u r a s Rev. T4c. Ing., Univ. Zulia,
i__55 No. 3, 525-534 (1985). 8.
E. Levin. Corrosion of underground acidification: laboratory investigation.
J o u r n a l , 2__66 No. I , 9.
structures due to British Corrosion
63-66 (1991).
E. J. Garboczi. P e r m e a b i l i t y , D i f f u s i v i t y and m i c r o e s t r u c t u r a l Parameters: A c r i t i c a l Review. C e m e n t and C o n c r e t e R e s e a r c h , 2__00 No. 4, 591-601 (1990).
I0. S. J. G r e g g and K.S.W. Adsorption, Surface A c a d e m i c Press, London, p 162 (1967).
a r e a and P o r o s i t y .
II. J.A. GonzAlez, A. Molina, E. O t e r o and W. Ldpez. On the m e c h a n i s m of steel c o r r o s i o n in concrete: the role of o x y g e n d i f f u s i o n . M a g a z i n e of C o n c r e t e R e s e a r c h , 42 No. 150, 23-27 (1990).
INFLUENCE OF CONCRETE
THE DEGREE OF PORE SATURATION ON THE RESISTIVITY AND THE CORROSION RATE OF STEEL REINFORCEMENT
OF
W. L 6 p e z U n i v e r s i d a d N a c i o n a l A u t d n o m a de M6xico. F a c u l t a d de Qu£mica, E d i f i c i o D. C i u d a d U n i v e r s i t a r i a . M 4 x i c o 04510, D F
J.A. G o n z ~ l e z C e n t r o N a c i o n a l de I n v e s t i g a c i o n e s M e t a l d r g i c a s (CENIM). Avda. G r e g o r i o del Amo 8, E - 2 8 0 4 0 Madrid, S p a i n (Communicatedby C.D. Pomeroy) (Received March 16, 1992)
ABSTRACT Quantitative relations between the corrosion rate of reinforcements (i^r)t)(p a n d the d e g r e e of p o r e s a t u r a t i o n (PS) and resistivity of m o r t a r s w i t h o u t CI- a n d w i t h 2% CI" w e r e o b t a i n e d . T h e m o r t a r s p e c i m e n s w e r e c u r e d in a w a t e r fog c h a m b e r before the f i n a l exposure at 50o_c a n d 50% relative humidity (RH). The e l e c t r o l y t e supply was found to d e t e r m i n e the ~mortar ? r e s i s t i v i t y w h i c h v a r i e s o v e r a w i d e r a n g e (5 x 10J--5 x i0 ~cm) a n d t h i s in t u r n i n f l u e n c e s the i of the reinforcements. There is a critical corr practical PS va~ue (PS) that results in a mortar resistivity of 10" flcm, ~ e l o w w h i c h i values are too s m a l l to p o s e a n y d u r a b i l i t y p r o b l e m s . B ~ r o w the PS c v a l u e , the resistivity of the mortar prevents activ~ state c o r r o s i o n of r e i n f o r c e m e n t s as e f f e c t i v e l y as p a s s i v a t i n g l a y e r s of s t e e l in m o r t a r s w i t h o u t CI'. _
INTRODUCTION
T h e res.istivity of c o n c r e t e v a r i e s o v e r v e r y broadA r a n g e s , viz. f r o m i0 H ~ c m for o v e n - d r i e d m a t e r i a l to l e s s t h a n I0 J ~ c m for v e r y wet concrete (i). G j o r v (2) r e p o r t e d r e s i s t i v i t y v a l u e s of 7 × i0 ~ a n d 6 x l0 b ~ c m for 1 0 0 % a n d 20% s a t u r a t e d c o n c r e t e , respectively, which are perfectly possible in n a t u r a l e n v i r o n m e n t s . Thus, the r e s i s t i v i t y of c o n c r e t e is m a r k e d l y d e p e n d e n t on t h e d e g r e e of p o r e saturation (PS) a n d also, to a l e s s e r e x t e n t , on the d e g r e e of paste hydration ( c u r i n g ) a n d the p r e s e n c e of d i s s o l v e d s a l t s in the a q u e o u s p h a s e of the c o n c r e t e (3). O n the o t h e r hand, humidity is a l s o t h e f a c t o r most strongly a f f e c t i n g the a c t i v e c o r r o s i o n r a t e of r e i n f o r c e m e n t s . Accordingly, 368
VOI. 23, No. 2
PORE SATURATION, RESISTIVITY, CORROSION, STEEL REINFORCEMENT
369
the r e l a t i o n s h i p b e t w e e n c o n c r e t e r e s i s t i v i t y and r e i n f o r c e m e n t c o r r o s i o n rate has b e e n s t u d i e d by some a u t h o r s (4-6) and it was found that t h e s e two p a r a m e t e r s are i n v e r s e l y p r o p o r t i o n a l o v e r a w i d e r e s i s t i v i t y range. H o w e v e r , there is not a g e n e r a l a g r e e m e n t about the r e s i s t i v i t y level a b o v e w h i c h c o r r o s i o n risks will be n e g l i g i b l e , p r o b a b l y as a r e s u l t of the lack of a c l e a r idea of w h a t is a c c e p t a b l e and u n a c c e p t a b l e in terms of r e i n f o r c e m e n t c o r r o s i o n rate. For example, a c c o r d i n g to H o p e et el.(7) r e i n f o r c e m e n t s e m b e d d e d in c o n c r e t e with a resistivity higher than 104 ~cm will hardly develop c o r r o s i o n , not even in the p r e s e n c e of el-, ~ g r CO~. S t r a t f u l l (3) r e p o r t e d a s o m e w h a t h i g h e r limit (6.5 x ~ ~cm) for c o n c r e t e structures. The aim of this w o r k was to o b t a i n q u a n t i t a t i v e r e l a t i o n s b e t w e e n the r e i n f o r c e m e n t c o r r o s i o n rate and the r e s i s t i v i t y and d e g r e e of pore s a t u r a t i o n of m o r t a r s w i t h and w i t h o u t CI-. This is the first time that the a s s o c i a t i o n b e t w e e n these t h r e e v a r i a b l e s has b e e n investigated.
EXPERIMENTAL
Two s e r i e s of m o r t a r s s p e c i m e n s (Fig i) w e r e p r e p a r e d , w i t h o u t Cl a d d i t i o n s and c o n t a i n i n g 2 %CI- (as NaCI) by c e m e n t weight, b o t h s e r i e s w i t h a c e m e n t ~ s a n d ~ w a t e r ratio of 1:3:0.5. All m o r t a r s were d e m o u l d e d 24 h a f t e r h a r d e n i n g and t h e i r a v e r a g e p o r o s i t y was 16.11 ± 0.8%. Two t y p e s
of
mortar
specimens were
used,
one
of
8x2x2
cm
for
resistivity measurements and other of 8xS.Sx2 cm for electrochemical corrosion rate determinations (Fig. I). The r e s i s t i v i t y s p e c i m e n s w e r e m e c h a n i c a l l y ground, a f t e r d e m o u l d i n g , in o r d e r to e q u a l i z e t h e i r d i m e n s i o n s , and the o p p o s i n g 2x2 cm faces w e r e s u b s e q u e n t l y c o a t e d w i t h g r a p h i t e c o n d u c t i n g p a i n t in o r d e r to f a c i l i t a t e electrical contact during the r e s i s t i v i t y measurements.
Figure 1 Appearance of the specimens fabricated
mortar
LEFT:
for r e s i s t i v i t y measurements
RIGHT:
for e l e c t r o c h e m i c a l measurements
370
W. I ~
and J.A. Gonzfdez
Vol. 23, No. 2
Two c a r b o n steel rods were e m b e d d e d at s y m m e t r i c a l p o s i t i o n s into the m o r t a r s s p e c i m e n s for a c t i n g as w o r k i n g e l e c t r o d e s (WE) d u r i n g the e l e c t r o c h e m i c a l m e a s u r e m e n t s , w h i l e a s t a i n l e s s steel rod was e m p l o y e d as c o u n t e r e l e c t r o d e (CE). An e x t e r n a l s a t u r a t e d c a l o m e l e l e c t r o d e (SCE) was used as r e f e r e n c e e l e c t r o d e (RE) t h r o u g h o u t . T h e s e s p e c i m e n s a l l o w m o n i t o r i n g of the v a r i a t i o n of E~rr, ic0rr, PS and the o h m i c r e s i s t a n c e b e t w e e n the two w o r k i n g e l e c £ r o d e s . All e l e c t r o c h e m i c a l d e t e r m i n a t i o n s w e r e c a r r i e d out in d u p l i c a t e . The m o r t a r s w e r e c u r e d in a s t a n d a r d w a t e r fog c h a m b e r at 23"C for 40 d a y s and s u b s e q u e n t l y w e r e t r a n s f e r r e d to an a i r - t i g h t c h a m b e r at a r e l a t i v e h u m i d i t y (RH) > 90% and a m b i e n t t e m p e r a t u r e , w h e r e they were a l l o w e d to stand for 60 f u r t h e r days b e f o r e t h e y w e r e f i n a l l y e x p o s e d to 50"C and 50% RH for 500 days. The d e g r e e of pore s a t u r a t i o n of the m o r t a r s was c a l c u l a t e d from t h e i r w e i g h t by u s i n g the f o l l o w i n g e q u a t i o n :
W t
-
W0
W s
-
W0
PS =
wherecompleteW,d e n o t e s the m o r t a r weight, at time t, 2 W the w e i g h t u p o n s a t u r a t i o n by b o i l i n g in w a t e r for ~, and W 0 the new w e i g h t o b t a i n e d a f t e r d r y i n g the m o r t a r by h e a t i n g in a s t o v e at I05"C u n t i l w e i g h t c o n s t a n c y . The resistivity of the mortars specimens used for the e l e c t r o c h e m i c a l m e a s u r e m e n t s was d e t e r m i n e d by m u l t i p l y i n g the R~ v a l u e b e t w e e n the two WE by the cell c o n s t a n t for this g e o m e t r y , w h i c h was in t u r n o b t a i n e d f r o m the slope of the p l o t of the r e s i s t i v i t i e s of the m o r t a r s s p e c i m e n s of k n o w n g e o m e t r y v e r s u s the wR~ v a l u e s of those used in the e l e c t r o c h e m i c a l studies, b o t h of ich w e r e m a d e w i t h the same c o m p o s i t i o n and e x p o s e d to the same t e m p e r a t u r e and RH c o n d i t i o n s (Fig 2). The R~ v a l u e s of the m o r t a r s were determined by u s i n g a S o l a r t r o n 1251 f r e q u e n c y response a n a l y s e r at a c o n s t a n t f r e q u e n c y of 1 kHz. 1.0E,06
1.0E-05 0
Figure 2 Resistivity vs ohmic resistance plot to determine the cell c o n s t a n t of the m o r t a r s for electrochemical measurements
rr,u
r,~
Z
.<
1.0E*04
[n
• •
I.|NI:AL
Hi:QHI8810N
r,n
r~
1,0E*03
1.0E'02 %0E*03
. ~
,
",,4 ~ d ~5~"~' /
I
J
[
oorr
•
• 0,1897.
b • - 6 a l . E (ohm) n • ,nSO II/m) K CELL • $,415 (oal)
I IIIIA
1.0E*04
Resistivity
I
I
,
t [ii11
1.0E-05
(ohm.---)
t
i
,
i
ii
1.0E*06
Vol. 23, No. 2
PORE SATURATION,RESISTIVITY,CORROSION, STEEL REINFORCEMENT
371
RESULTS
Figures 3a--3d show the changes with time in the monitored p a r a m e t e r s : ic0r~ Ec0rr, r e s i s t i v i t y and w a t e r loss of the m o r t a r s , r e s p e c t i v e l y . T h e z o n e s l a b e l l e d w i t h 0, I and 2 c o r r e s p o n d to the c u r i n g in the w a t e r fog a t m o s p h e r e , the s t a b i l i z a t i o n p e r i o d at RH > 90% and a m b i e n t t e m p e r a t u r e , and the e x p o s u r e to RH = 50% and 50°C, r e s p e c t i v e l y . Figure 4 shows the relationship b e t w e e n ic0rr and the r e s i s t i v i t y of 1 2 A the m o r t a r s , b o t h c o n t a i n i n g 2% C1 and w i t h o u t CI', f r o m the b e g i n n i n g of the s t a b i l i z a t i o n p e r i o d at RH > " k 90% and a m b i e n t t e m p e r a t u r e to a f t e r 500 d a y s of e x p o s u r e to RH = 50% and 50°C. The p o i n t s w i t h i n the c i r c l e c o r r e s p o n d to the i .... and r e s i s t i v i t y v a l u e s of the m o r t ~ s at the end of curing, while those inside the r e c t a n g l e c o r r e s p o n d to t h o s e of the B~ J m o r t a r s c o n t a i n i n g 2% Cl- d u r i n g the s t a b i l i z a t i o n period. R a t h e r t h a n to s ignificant changes in the ° / resistivity of c o n c r e t e (Fig 3c), t h e s e l a t t e r p o i n t s are r e l a t e d to the small w a t e r loss from the m o r t a r s in passing from the curing fog c h a m b e r to the a t m o s p h e r e w i t h a J RH > 90% (Fig 3d). c
Figure 5 shows the relationship b e t w e e n the d e g r e e of s a t u r a t i o n of the c o n c r e t e p o r e s (PS) and i ..... As can be seen, the m a x i m u m ic tr ~ l u e s c o r r e s p o n d to PS v a l u e s o~ 60--70%. Also, at l o w e r PS values, the i... v a l u e s of the m o r t a r s c o n t a i n l n g ~ CI- are s m a l l e r by a b o u t two o r d e r s of m a g n i t u d e , consistent with the inverse proportionality between • " Fig 4. the r e s i s t i v i t y and ic0rr in F i n a l l y , as can also be s e e n in the figure 5, the ~ values of the m o r t a r s w i t h o u t ~ x ~ are i n d e p e n d e n t of PS b e t w e e n 100% and 50%, but the ie0rr d e c r e a s e for PS less t h a n 50%.
o
el
| t4 m W
o
¢D
t
o
Ioo
:too
3o0
40o
ooo
eoo
TINR ( , I - y l )
a.
2 % CI-
0% CI-
TOO
Figure 3 V a r i a t i o n of the d i f f e r e n t m o n i t o r e d v a r i a b l e s w i t h time since the p e r i o d of c u r i n g u n t i l e x p o s u r e to 50:C and RH = 50%
372
W.I..,6pez and J.A. Go'nzile,z
1.0E-O6
-
Vol. 23, No. 2
1.OE-O5 "
-F +
O"
GI-
2 " Cl"
/
\
+ ++
1.0E-Oe
\
~
1.0E-Oe
•
+
il U .<
+
i
N
+
+4-
E
0 < v
+ 1.0E-07 •
+ 1.0E-07
.+.
.+
•
.
•
o +
+
+
+
+
1.0E-08
1.0E-08
I 1.OE-O9
, ,,,,,., , 1.0E+03 1.0E+04
,,,,,,,, laE~8
, , ,,,,., 1.0E*06
Resistivity
,
,,,,l., tOE+07
(ohm.
o.
GI-
-~-
2.
GI-I
i
i
i
i
i
i
i
30
40
80
60
70
80
90
100
Pore
cm)
Saturation
Figure 5
Figure 4
Influence of the d e g r e e of m o r t a r p o r e s a t u r a t i o n on the corrosion rate of the reinforcement
Relationship between mortar r e s i s t i v i t y and the c o r r o s i o n rate of the r e i n f o r c e m e n t
F i g u r e s 6a and 6b s h o w the relationship between the w a t e r loss from the m o r t a r s and PS with the mortar resistivity, respectively. As can be seen, the r e s i s t i v i t y of a w a t e r - s a t u r a t e d mortar (i.e. c u r e d in a w a t e r - f o g atmosphere that p r o d u c e s a visible water l a y e r on the m o r t a r ) s c a r c e l y c h a n g e s on exchanging water with the e n v i r o n m e n t u n t i l the m o r t a r weight decreases by ca. 2% (Fig 6), which corresponds to PS m 70% (Fig 6b). Further water releases to the environment result in gradually larger increases in resistivity and progressively smaller decreases i n PS ( F i g s 6a and 6b).
-
i
1.0E-09
, ,,,,,,, 1.0E*08
I .Ol~O
1
++++
++..
• 4-..
-t-k.
..+
t.
+
m~
1.0E+OO
÷
.
¢ ,;a e~ 3~
+
.........
, ,, ,,,,.
100.000"
80.000-
÷ +
00.000-
..........................
[ ....
~--o-ls
• +°
TO.O00"
460.000"
+.
80.000-
40.000"
+
Figure
6
Effect of the water loss and p o r e s a t u r a t i o n on the mortar resistivity
30.000" 20.000
• • i i .............
1.0E*03
+
LOEb04
".......................... 1-01=*08 1 . 0 E * 0 8 1.0E*07
Resistivity (oha-ca)
1.0E408
Vol, 23, No. 2
PORE SATURATION,RESISTIVITY,CORROSION,STEEL REINFORCEMENT
373
DISCUSSION
On e x p o s u r e to an a t m o s p h e r e at a g i v e n r e l a t i v e h u m i d i t y and t e m p e r a t u r e , the h u m i d i t y c o n t a i n e d in the p o r o u s of the c o n c r e t e e q u i l i b r a t e s w i t h the a m b i e n t humidity (Fig 3c). The p r o c e s s t h r o u g h w h i c h such an e q u i l i b r i u m is e s t a b l i s h e d d e t e r m i n e s the d e g r e e of s a t u r a t i o n of the c o n c r e t e pores, PS, and a n u m b e r of the p h y s i c o - c h e m i c a l p r o p e r t i e s of c o n c r e t e i n c l u d i n g gas d i f f u s i o n t h r o u g h the p o r e s (8,9) and e l e c t r i c a l r e s i s t i v i t y (1,2), both of w h i c h are m a r k e d l y i n f l u e n t i a l on the c o r r o s i o n k i n e t i c s at the s t e e l / c o n c r e t e interface. The w a t e r c o n d e n s a t i o n - - e v a p o r a t i o n e q u i l i b r i u m o c c u r r i n g in m o r t a r p o r e s can be d e s c r i b e d by m e a n s of the K e l v i n e q u a t i o n (I0):
p In
2V¥
m
Po
rRT
A c c o r d i n g l y , in the e v e n t of w a t e r c o n d e n s i n g on the m o r t a r s u r f a c e - - a s o b s e r v e d d u r i n g the c u r i n g p r o c e s s in a w a t e r o v e r s a t u r a t e d a t m o s p h e r e - - , p o r e s of all d i a m e t e r s will be fully s a t u r a t e d w i t h water. T h e r e f o r e , u n d e r these h u m i d i t y c o n d i t i o n s , the t r a n s f e r of 02 t h r o u g h the c o n c r e t e p o r e s to the r e i n f o r c e m e n t will take p l a c e e k c l u s i v e l y t h r o u g h liquld-phase" d i f f u s i o n . In fact, ic0 was 0.3 N A / c m 2 for the m o r t a r s c o n t a i n i n g 2% CI- and 0.15 ~ A / c ~~ for the m o r t a r s w i t h o u t CI- (points inside the c i r c l e in Fig 4). On the o t h e r hand, a c c o r d i n g to the K e l v i n e q u a t i o n , d u r i n g the s t a b i l i z a t i o n p e r i o d at 23"C and RH > 90% ( p / ~ > 0.90), for a p a r t i a l w a t e r v a p o u r p r e s s u r e in the a t m o s p h e r e o f 0.90, w a t e r will e v a p o r a t e from all p o r e s w i t h a radius l a r g e r t h a n 116 A. U n d e r these c o n d i t i o n s , no w a t e r will c o n d e n s e on the m o r t a r surface, but only in those p o r e s w i t h a radius s m a l l e r t h a n 116 A. E a c h RH v a l u e will thus have a m a t c h i n g c r i t i c a l pore r a d i u s b e l o w w h i c h p o r e s will remain water-saturated and above w h i c h w a t e r will e v a p o r a t e , t h e r e b y e s t a b l i s h i n g p r e f e r e n t i a l a e r a t i o n c h a n n e l s for p e n e t r a t i o n of O I t h r o u g h v a p o u r - p h a s e d i f f u s i o n w i t h a c o e f f i c i e n t D02 c a . I0" t i m e ~ g r e a t e r than in a l i q u i d p h a s e (8). As 02 r e a c h e s the r e i n f o r c e m e n t t h r o u g h the p r e f e r e n t i a l a e r a t i o n channels, it creates differential aeration cells and hence i n c r e a s e s E .... by a b o u t i00 mV in m o r t a r s c o n t a i n i n g no Cl" (Fig 3b), and i m p r o v e s the steel p a s s i v i t y (Fig. 3a). On the o t h e r hand, the m o r t a r s c o n t a i n i n g 2% CI- u n d e r g o a v e r y sharp initial rise in Ec0rr of c a . 300--400 mV (Fig 3b), c o n s i s t e n t w i t h a slight d e c r e a s e in ~c0rr' f o l l o w e d by a d r o p to a c t i v e p o t e n t i a l s and a c o n c o m i t a n t i n c r e a s e in i.... ( F i g 3 a ~ On e x p o s u r e t ~ % 0 * C a n d R H = 50%, the m o r t a r s r e l e a s e w a t e r up to ca. 4% of t h e i r w e i g h t (Fig 3d). This r e s u l t s in a ~ r a ~ a t i c i n c r e a s e in the m o r t a r r e s i s t i v i t y from 3 x l0 w ~cm to I0"--I0 ~cm (Fig 3c). Under these conditions, the i values of the r e i n f o r c e m e p t e m b e d d e d in m o r t a r s w i t h 2% CI" C ~ r s t l y i n c r e a s e up to 3 B A / c m ~ due to the a c c e s s of 02 by p r e f e r e n t i a l a e r a t i o n channels, and l a t e r d e c r e a s e to 0.02 ~ A / c m 2 by e f f e c t of the m o r t a r r e s i s t i v i t y (Fig 3a). •
~U[[
374
W. L6pez and J.A. Gonz~ez
Vol. 23, No. 2
At 5 0 ° C a n d RH { 50%, r e s i s t a n c e c o n t r o l e n c o m p a s s e s resistivities between 7 x i0 ~ c m a n d the u p p e r limit, i~ ~cm (Fig 4). T h e o c c u r r e n c e of an i n v e r s e p r o p o r t i o n a l i t y relationship between the r e s i s t i v i t y a n d i cr in t h i s p o r t i o n of the plot, w i t h a s l o p e c l o s e to -i, u n e q u i v o c a ~ l y r e f l e c t s the c o n t r o l l i n g r o l e of the m o r t a r resistivity in the c o r r o s i o n k i n e t i c s in the a c t i v e state. H o w e v e r , in the p a s s i v e s t a t e ( m o r t a r s w i t h o u t CI'), the c o r r o s i o n r a t e is i n d e p e n d e n t of the r e s i s t i v i t y and anodic control takes o v e r on a c c o u n t of the p r o t e c t i v e f e a t u r e ~ of the p a s s i v a t i n g layer of s t e e l up to r e s i s t i v i t i e s of 3-4 x i ~ ~cm. T h e r e s i s t i v i t y of d r i e r m o r t a r s is an e v e n g r e a t e r h i n d r a n c e to c o r r o s i o n t h a n is the passivating l a y e r (Fig 4). Corrosion of r e i n f o r c e d concrete thus s e e m s to a r i s e f r o m two opposing effects, namely: - - T h e i n c r e a s e d ~2 s u p p l y t h r o u g h the l a r g e r p o r e s r e s u l t i n g from a decrease In t h e d e g r e e of s a t u r a t i o n d o w n to 60--70%, w h i c h r e s u l t s in i n c r e a s e d i c (Fig 5) t h r o u g h t r i g g e r i n g of oil . the passivity--activity transltlon in the steel and local acidification at the steel/concrete interface (ii). The resistivity of c o n c r e t e s c a r c e l y v a r i e s o v e r t h i s PS r a n g e (Fig 6b). - - A sharp, g r a d u a l l y m o r e r a p i d i n c r e a s e in r e s i s t i v i t y at PS v a l u e s b e l o w 70% (Fig 6b), w h i c h b e c o m e s the f a c t o r c o n t r o l l i n g the c o r r o s i o n kinetics at the s t e e l / c o n c r e t e interface and r e s u l t s in a m a r k e d d e c r e a s e in ic0tt (Fig 4). T h e c o r r o s i o n p r o c e s s of a c t i v e c o r r o d i n g s t e e l in c o n c r e t e a n d the atmospheric corrosion of m e t a l s are s i m i l a r in t h a t t h e y " s t o p " b e l o w a RH c r i t i c a l v a l u e (RHc). R a t h e r t h a n a c r i t i c a l relative humidity, concrete structures f e a t u r e a c r i t i c a l d e g r e e of p o r e saturation, PS , w h i c h can be ~ssumed to be ca. 35% and corresponding to ic0rr - 0.02 D A / c m (Fig 5), w h i c h r e s u l t s in a penetration of 0.2 D m / y e a r t h a t is c l e a r l y n e g l i g i b l e . F r o m F i g s 4 a n d 5, o n e m a y d e f i n e t h r e e t y p e s of c r i t i c a l PS v a l u e s allowing the b e s t c h a r a c t e r i z a t i o n of t h e c o r r o s i o n process of s t e e l in c o n c r e t e , n a m e l y : - - T h e u p p e r c r i t i c a l limit, P S c , w h i c h s i g n a l s the b e g i n n i n g of r e s i s t a n c e control a n d w o u ~ d be 3of ca. 70% (Fig 5) a n d c o r r e s p o n d to a r e s i s t i v i t y of 7 x i0 ~ c m (Figs 4 a n d 6b) for the c o n d i t i o n s t e s t e d h e r e i n (50°C a n d RH = 50%). - - T h e l o w e r c r i t i c a l limit, PSlc , viz. 35% as n o t e d e a r l i e r , below which corrosion "stops". -- The practical critical limit, PS , b e l o w w h i c h c o r r o s i o n risks are negligible for p r a c t i c a l ~ u r p o s e s . S u c h a l i m i t i~ 45--50%, w h i c h c o r r e s p o n d s to an ic0rr b e t w e e n 0.2 a n d 0.I D A / c m (Fig 5 ) . T h e p r o p o s e d PS v a l u e r e s u l t s in a r e s i s t i v i t y of ca. 105 ~ c m (Fig 6b), i.e. ver] ¢ close to the 6.5 x I0 ~ ~ c m l i m i t above which Stratfull ceased t? d e t e c t visible evidence of c o r r o s i o n in reinforced concrete-. At this resistivity values, the results obtained with active and passive structures (i.e. with and without chlorides) are very similar: i in the r a n g e b e t w e e n 0.i a n d 0.2 o cort D A / c m ~ (Fig 4), w h i c h c o r r e s p o n d to p e n e t r a t i o n s of 1--2 D m / y e a r . Such small penetrations will hardly pose durability problems t h r o u g h o u t the s e r v i c e l i f e of a r e i n f o r c e d c o n c r e t e s t r u c t u r e a n d c o n f i r m s the v i s u a l o b s e r v a t i o n s r e p o r t e d by S t r a t f u l l .
Vot. 23, No. 2
PORE SATURATION, RESISTIVITY, CORROSION, STEEL REINFORCEMENT
375
CONCLUSIONS
The r e s u l t s o b t a i n e d to be drawn:
in this
work allow
the
following
conclusions
I. The d e g r e e of pore s a t u r a t i o n (PS) of the c o n c r e t e has a v e r y strong effect on the c o r r o s i o n k i n e t i c s and d e t e r m i n e s the active state corrosion mechanism at the steel/concrete interface. 2. One can distinguish three d i f f e r e n t c r i t i c a l PS values: an upper limit (PS.~) c o i n c i d i n g w i t h the m a x i m u m c o r r o s i o n rate and below whic~ resistance control begins; a lower limit (PSIc) s i m i l a r to the c r i t i c a l r e l a t i v e h u m i d i t y in a t m o s p h e r i c corrosion and b e l o w w h i c h c o r r o s i o n d e v e l o p s to a n e g l i g i b l e extent; and a p r a c t i c a l limit ( P S c) that s i g n a l s the start of u n a c c e p t a b l e c o r r o s i o n rates and ~ e n c e of p o t e n t i a l d u r a b i l i t y problems. 3.
The resistivity of c o n c r e t e r e m a i n s v i r t u a l l y u n c h a n g e d at PS > PS and corrosion decreases g r a d u a l l y as a r e s u l t of diffusion control in an i n c r e a s i n g l y s a t u r a t e d pore network. On the other hand, at PS < PSuc, the r e s i s t i v i t y i n c r e a s e s e x p o n e n t i a l l y and i i n v e r s e l y p r o p o r t i o n a l to i .... over a v e r y broad range (7 x 1 0 t l x l0 b flcm). ~u,,
4. The corrosion rate in the passive state depends on the properties of the passivating layer of steel in c o n c r e t e r a t h e r than on c o n c r e t e r e s i s t i v i t y . However, below PSp , the concrete resistivity b e c o m e s an e v e n more p o w e r f u l h i n d r a n c e to c o r r o s i o n t h a n is the p r o t e c t i v e e f f e c t of the p a s s i v a t i n g layers. ACKNOWLEDGEMENTS
This w o r k was s u p p o r t e d by the C o m i s i 6 n I n t e r m i n i s t e r i a l de C i e n c i a y T e c n o l o g ~ a (CICYT) of the M i n i s t r y of E d u c a t i o n and S c i e n c e of Spain. One of the a u t h o r s (WL) a c k n o w l e d g e s the f i n a n c i a l s u p p o r t p r o v i d e d by the U n i v e r s i d a d N a c i o n a l A u t 6 n o m a de M 4 x i c o and the A g e n c i a Espafiola de C o o p e r a c i 6 n I n t e r n a c i o n a l . REFERENCES
1.
G. E. Monfore. (Portland Cem. (1968).
2.
O.E. Gjorv, O. V e n n e s l a n d , A.H.S. E i - B u s a i d y . Proc.O f f s h o r e Technol. Conf., 9 No. I, 5 8 1 - 5 8 8 (1977).
3.
R. F. S t r a t f u l l . How Chlorides affect concrete used with R e i n f o r c i n g Steel. M a t e r i a l s P r o t e c t i o n , ! No.3, 29-34 (1968).
Electrical Ass.) Res.
resistivity of Develop. Lab.,
concrete. J. P C A i O No. 2, 35-48
Annu.
376
W. L6pez and J.A. Gonz~lez
Vol. 23, No. 2
4.
S. Felid, J.A. GonzAlez, S. Felid Jr. and C. Andrade. R e l a t i o n s h i p b e t w e e n c o n d u c t i v i t y of c o n c r e t e and c o r r o s i o n of r e i n f o r c i n g bars. B r i t i s h C o r r o s i o n Journal, 24 No. 3, 1 9 5 - 1 9 8 (1989).
5.
A. Molina, C. Andrade, C. A l o n s o controlantes de la v e l o c i d a d de e m b e h i d a s en m o r t e r o s de cemento. No. 2, 9-15 (1985).
6.
C. Alonso, C. A n d r a d e and J.A. Gonzglez. Relation between r e s i s t i v i t y and c o r r o s i o n rate of r e i n f o r c e m e n t s in c a r b o n a t e d m o r t a r made w i t h s e v e r a l C e m e n t types. C e m e n t and C o n c r e t e Research, 8 No. 5, 6 8 7 - 6 9 8 (1988).
7.
B. B. Hope, A. K. Ip and D.G. Manning. Corrosion and e l e c t r i c a l I m p e d a n c e in concrete. C e m e n t and C o n c r e t e R e s e a r c h ,
y J. A. GonzAlez. Factores c o r r o s i 6 n de las a r m a d u r a s Rev. T4c. Ing., Univ. Zulia,
i__55 No. 3, 525-534 (1985). 8.
E. Levin. Corrosion of underground acidification: laboratory investigation.
J o u r n a l , 2__66 No. I , 9.
structures due to British Corrosion
63-66 (1991).
E. J. Garboczi. P e r m e a b i l i t y , D i f f u s i v i t y and m i c r o e s t r u c t u r a l Parameters: A c r i t i c a l Review. C e m e n t and C o n c r e t e R e s e a r c h , 2__00 No. 4, 591-601 (1990).
I0. S. J. G r e g g and K.S.W. Adsorption, Surface A c a d e m i c Press, London, p 162 (1967).
a r e a and P o r o s i t y .
II. J.A. GonzAlez, A. Molina, E. O t e r o and W. Ldpez. On the m e c h a n i s m of steel c o r r o s i o n in concrete: the role of o x y g e n d i f f u s i o n . M a g a z i n e of C o n c r e t e R e s e a r c h , 42 No. 150, 23-27 (1990).