Phenomenological investigation of concrete deterioration in a median barrier

CEMENT and CONCRETERESEARCH. Vol.21, pp. 917-927, 1991. Printedin the USA. 0008-8846/91. $3.00+00. Copyright(c)1991 PergamonPressplc.

PHENOMENOLOGICAL

INVESTIGATION OF CONCRETE MEDIAN BARRIER

DETERIORATION

IN A

Shondeep L. Sarkar and Pierre-Claude Ai'tcin Faculty of Applied Sciences, University of Sherbrooke Sherbrooke (Quebec), Canada J1K 2R1 (Refereed) (ReceivedFeb.25; in finalform June 6, 1991)

ABSTRACT This paper discusses the findings from an investigation of a deteriorated reinforced concrete median barrier in Sherbrooke, Quebec, Canada. The barrier has been subjected to repeated freezing and thawing, deicer salts and exhaust fumes for a number of years. The study based on field samples, focuses attention on the combined effect of physical and chemical deterioration in terms of the long-term performance and durability of a reinforced concrete element under severe conditions. Various types of deterioration were revealed using techniques such as SEM, XRDA, CI- ion permeability and mercury porosimetry. The most prevalent signs of deterioration were map cracking, longitudinal cracking, rebar corrosion, scaling, and spalling, which are related to microstructural and physicochemical phenomena.

INTRODUCTION While the AASTHO Task Force has undertaken the important study on how to improve the resistance of concrete bridges to chloride attack from deicer salts (1), and the Lehigh Portland Cement Co. has reported significant improvements achieved by constructing median barriers with white cement (2), the authors have been investigating the deterioration of a grey cement concrete median barrier constructed in 1972 in the city of Sherbrooke, Province of Quebec, Canada. The study of reinforced concrete d e t e r i o r a t i o n under different e n v i r o n m e n t a l conditions has been a subject of extensive research (3,4). The results of these investigations have brought into focus the deteriorative problems generated by poor concrete quality, improper placing and inadequate curing (5). While a number of these investigations have been confined to laboratory conditions, studying actual field samples provides true insight in to the deteriorative mechanisms at work. Concrete as such is not immune to deterioration. Additionally, it must be noted that median barriers for harsh climatic conditions such as in Quebec must be well constructed in order to withstand repeated freezing and thawing action as well as frequent application of deicer salts throughout the winter. Moreover, median barriers must also bear the brunt of pollutants from vehicles. These combined factors obviously place a serious demand on the performance and long-term durability of such structures. 917

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S.L. Sarkar and P.-C. AYtcin

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Site investigation of the subject median barrier, 1.5 km long, revealed several types of deterioration, some of them quite severe. Map cracking and longitudinal cracks were frequently occurring features. In some sections, spalling was observed, while scaling was noted in others. There were distinct signs of severe rebar corrosion in different parts of the concrete. Though the median barrier had not yet reached catastrophic failure, the severity of deterioration called for immediate attention. At the time of writing, several parts of the median barrier were undergoing extensive repairs. To ensure longevity and structural safety, the removal of damaged concrete and its replacement with cast-inplace concrete was in progress. It is the purpose of this paper to address the problem from a microstructural point of view, that is, to present the cause and effects of physicochemical reactions resulting in such serious damage. METHODS Microstructural investigative techniques prove ideal for the study of this type of problem. Hence, core samples of concrete from the affected parts of the median barrier were first subjected to SEM/EDXA for mineralogical and morphological analysis of deteriorated concrete. XRDA was performed to identify the products of deterioration; petrographic examination was used to evaluate the state and condition of the aggregates. Other laboratory testing methods included chloride-ion permeability, mercury porosimetry, and bubble spacing. RESULTS The visual findings concur with those of O'Connor and Olson (6), who recently reported similar types of concrete failure in a parking garage. However, to approach the problem from a more fundamental standpoint, the petrographic examination of aggregates showed that the metamorphic coarse aggregates, mostly constituted of "green stone" (green-schist facies), were not affected adversely, though there is minor evidence of polygonal cracking of quartz fine aggregates (Fig. 1).

Fig. 1

The composition of original concrete (Table standard mix proportioning.

A coarse aggregate (A) with no visible signs of deterioration and a quartz (fine aggregate) showing polygonal fracturing

1) shows a relatively low W/C ratio and

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REINFORCEDCONCRETE.DETER/ORATION,MEDIANBARRIER

TABLE 1:

919

Composition of original concrete

W/C: Water: Cement: Fine aggregate Coarse aggregate: Air entraining agent: Dispersant:

167 363 863 952 67 1050

0.46 kg/m 3 kg/m 3 kg/m 3 kg/m 3 mL/m 3 mL/m 3

The air content of this concrete (Table 2) was within the specifications (5-7% air content, measured according to ASTM C457 method). The strength tested at 28 day was 28 MPa. The bubble spacing factor calculated on this concrete gives a spacing factor (L-) of 270 /.tm, whereas the normal acceptable limit is 250 p.m (7). The specific surface or the average bubble size (a) is 17.5 mm-1; the air content is 7.1%. It must be noted that in a concrete with an average air content of 5-6%, a sould be much higher. TABLE 2:

Contrete properties

Fresh stat~ (1972) Air content: Slump: Temperature:

5.8% 89 mm 23°C

Hardened c o n c r e t e (1972) Compressive strength at at

7d: 28d:

average: (1991) Spacing factor {L--): Specific surface (ct): Air: Paste:

(i) (ii)

25,6 27.6 28.2 28

MPa MPa MPa Mpa

270 lain 17.5 mm "1 7.1% 34.8%

From XRDA of the deteriorated concrete (Fig. 2a), an elevated level of chloroaluminate is clearly identifiable. A significant amount of CH is present. Ettringite, monosulfoaluminate, and a l u m i n o h y d r a t e also appear in the diffraction pattern. Though calcite is not an aggregate constituent, its level is fairly high, indicating carbonation in the concrete. Higher amount of carbonation is also evident in the corroded region from the calcite peak at 29.4°20 (Fig. 2b), together with magnetite and goethite, the former being more prevelant. These two minerals represent products of corrosion (8,9). CH and chloroaluminate are notably absent in the corroded section. SEM/EDXA of the concrete and the corroded part reveal several important features that further help to elucidate the reaction paths. Profusion of ettringite (Fig. 3) and well-formed platy crystals of chloroaluminate (Fig. 4) feature in the concrete microstructure. Calcium aluminohydrate, though detectable, is not as well crystallized. While CH is present as oriented crystals in some of the transition zones, a number o f them have developed the characteristic debonding with ettringite concentrated at the paste-aggregate interface (Fig. 5). Leaching of concrete constituents is characterized by vacant spaces and sockets, originally occupied by CH

920

S.L. Sarkar and P.-C. Aitcin

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Vol. 21, No. 5

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XRD pattern of d e t e r i o r a t e d concrete and rust in the corroded part. E = ettringite, MS = monosulfate, CL = chloro-aluminate, P = CH (portlandite), AL = aluminohydrate, Q = quartz A = albite, MI - microcline, AN = anorthrite, CC = calcite, M = magnetite, and G = geothite

Fig. 3

Profusion of ettringite (E) developed over a fissure (F)

and fine aggregates (Fig. 5). Fissures in the paste are a b u n d a n t (Figs. 3 carbonation is clearly visible in the form of prismatic calcite crystals (Fig. 6).

and

5);

The interface of the corroded rebar and cement paste displays several types of morphology of F e - b e a r i n g crystals, the most p r e d o m i n a n t b e i n g the smooth lamellar crystals (Fig. 7). In some parts, their surfaces are covered with a multitude of short accicular crystals (Fig. 8). Some of the globular features were found to have developed extremely fine whiskery growths (Fig. 9), whereas others were composed of tiny rhombic crystals. Some of the globules appear to have burst open to display a floral morphology in the i n t e r i o r part (Fig. 10), s u g g e s t i n g that the s t r u c t u r e is h o l l o w . A distinct preponderance of C1 (associated with Fe) was detected in these globular masses, and the

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Fig. 4

Fig. 5

REINFORCEDCONCRE'I~ DETERIORATION,MEDIANBARRIER

921

Chloroaluminate crystals (+) in the paste together with its EDX spectrum

Debonding of paste-aggregate. Transition zone filled with ettringite. A fissure (F) in the paste is also visible. A aggregate, P - paste

Fig. 6 Calcite crystals in the paste due to c a r b o n a t i o n

l a m e l l a e with a c i c u l a r c r y s t a l l i t e were totally devoid o f C1.

deposition,

whereas

the

smooth

lamellar

crystals

Though the amount o f CI is variable in the globular masses it is a clear indication that some o f them contain Fe-chloro complexes (9) that may have formed at a later stage of corrosion. These, however, could not be distinguished in the X-ray pattern. Marchese (10) also o b s e r v e d s i m i l a r l a m e l l a r and g l o b u l a r c o r r o s i o n products. Multilayer

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S.L. Sarkar madP.-C. Aitcin

Vol. 21, No. 5

Fig. 7 Smooth lamellar Fe-rich crystals in the corroded zone

Fig. 8

Fig. 9

Short accicular crystals seen covering these lamellae

Fine whiskery growth on globular Fe and Cl-rich masses

(*)

corrosion reported by Glasser (11) was recognizable; Figure 11 shows distinct morphological zoning. Stacked Ca-rich crystals predominate (Fig. 12) in close proximity to the rebar. From XRDA, these can be surmised to be calcite crystals. Chloride induced corrosion is evidenced by the strong CI signals obtained from a typical elemental X-ray mapping (Fig. 13) of the interfacial region. Chloride-ion permeability of this concrete (marked A410 in Fig. 14) when compared with another surface deteriorated concrete (A55) recently reported by the authors (12) shows a peculiar curve comprising an extremely high initial current, which descends with time. Whiting (13) characterizes this type of curve for porous or high W/C

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Fig. 10

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923

The hollow inner part of these globules

Fig. 11 Multilayer corrosion at the rebar- paste interface

Fig. 12 Stacked Ca-rich crystals, probably calcite

concrete. However, the gradual drop in current remains unexplained. The cumulative charge on this concrete was calculated to be high: 3500 Coulombs versus 1200 coulombs for the A55 concrete which according to Whiting (12) corresponds to low W/C concrete. Mercury porosimetry results (Fig. 15) also demonstrate this concrete to be extremely porous. Selective

chemical

some

the

analysis of the deteriorated and corroded parts (Table 3) confirm findings, s u c h as lower amounts of ettringite in the interracial region. Though there is distinctly lower amount of Si02 in the corroded part, its Ca0 content is not low, suggesting the local presence of Ca-rich hydration products, most of

other

924

S.L. Sarkarand P.-C. Aitcin

Vo. 21, No. 5

Fig. 13

Elemental X-ray maps of Ca, Si, Fe, and CI at the rebar-paste interface

Chloride Ion Permeability 11.4

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Fig. 14 Charge versus time for two deteriorated concretes, where curve A410 is representative of the concrete under study, and A55 represents another bridge parapet concrete (12)

likely CaC03. Higher Mg0, Na20, and K20 in the deteriorated section is attributable to aggregate composition. Slightly higher H20 in the corroded part is a likely component of Fe hydroxide. Water soluble C1- ion determination (14) reveals nearly as high level of CI- at the interface as within the concrete. This confirms the contamination of concrete by CI" ions up to the rebar level.

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REINFORCEDCONCRETE, DETERIORATION,MEDIANBARRIER

925

43.0

344 .J

o > ¢-~ 25.8 LId

Fig. 15 Mercury porosimetry curve o f the deteriorated concrete, where A represents intrusion of mercury, and B is the extrusion

~) nl-- 172 Z 8.6

0.0 0.001

0.01

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TABLE 3:

I0.0

1.0

RADIUS

(IJm)

Chemical analysis of deteriorated concrete and rust

Sample

Si02

A1203

Concrete

44.4

6.0

Rust

15.2

1.8

Fe0

Mg0

Ca0

Na20

K20

S

H20

Sol. CI*

2.6

2.0

23.3

1.0

0.9

0.3

3.2

184.1

38.9

0.6

15.7

0.5

0.3

--

1.6

163.1

* mg/L DISCUSSION In discussing the state of this concrete, it canl be postulated that the observed surface d a m a g e is c h a r a c t e r i s t i c o f repeated f r e e z e - t h a w action (5,6,12). The p r i n c i p a l d e s t r u c t i v e effect o f enhanced f r e e z e - t h a w is s c a l i n g and s p a l l i n g , a g g r a v a t e d by salting. Though it is a physical process, represented by a cycle o f ice melting, salt dissolution, absorption by the cement pore system, then freezing, the complete process creates expansion stresses inside the concrete (9). This, in turn, can create severe damage, appearing as cracks and fissures, not to mention spalling and scaling. To arrive at a b a s i c understanding o f - the mechanism underlying chemical deterioration, one must first consider the effective role of CI- ions. Once the physical process o f breakdown is set in motion, C1- ions permeate the concrete. Some complex with the aluminate ions i n the cement to form chloroaluminate, whereas the rest is attracted towards the steel rebar by virtue of the localized acid environment and the positively charged Fe ions. According to the findings of Rosenberg (15), the formation o f C1 e q u i l i b r i u m c o m p l e x , however, does not m i t i g a t e the p r o b l e m o f corrosion. Regourd's study (16) shows the chloroaluminate to be an expansive mineral. The profusion of ettringite in the paste, particularly on the surface concrete, can be due to the supply o f sulfate ions from extrinsic sources such as vehicle exhaust fumes to which m e d i a n barriers are amply e x p o s e d (17,18). A l t e r n a t i v e l y , it is mainly because o f the unstable nature of chloroaluminate in a sulfate environment, and thus

926

converts to ettringite (19). c o n f i r m s that though CIettringite does not.

S.L. Sarkar and P.-C. Aitcin

The authors' ions c o n t i n u e

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SEM e x a m i n a t i o n and XRD a n a l y s i s also to p e n e t r a t e d e e p e r i n s i d e the concrete,

The original concrete composition (Table 1) suggests that the d e t e r i o r a t i v e processes such as freeze-thaw action f o l l o w e d by leaching of constituents led to a significant increase in porosity. Chloride ion p e r m e a b i l i t y and m e r c u r y p o r o s i m e t r y results confirm that the concrete in its present state is highly porous. A c c o r d i n g to Mehta (5), CH, which is the seed of potential deterioration, is either removed due to leaching, or forms deleterious products such as CaC03. There is sufficient evidence to support that. both these phenomena occurred in this concrete. This o b v i o u s l y reduces the concrete's ability to prevent water from p e r c o l a t i n g into its interior. Given the volume of research reported in the literature on rebar corrosion, it can be briefly stated that there is strong evidence of Cl-induced corrosion. Corrosion due to carbonation, which lowers the alkanity of the m e d i u m , that is, reduces the pH by removal of Ca ions from the pore solution (20), also appears to be present. Though less frequent, since it basically represents attack by weak carbonic acid, it nevertheless exists, as can be seen from the accumulation of CaC03 crystals. Bentur and Diamond (21) submit that a CH-rich l a y e r m o v e s t o w a r d s the h y d r o p h y l l i c surface o f steel during hydration. However, due to the porous nature o f the concrete, C02 ingress converts these to CaC03. Hime and Erlin (22) also reported that depassivation can occur due to neutralization of the cement by atmospheric C02. Figg (8) claims that F e - c h l o r o c o m p l e x e s possess increased solubility that enables them to move further away from the anodic region to produce more voluminous rust. It is interpretated in terms of j u x t a p o s i t i o n of bulky C1- ions with Fe, whereas at lower porosity these C1- ions are squeezed out, making way for magnetite formation. The present study, however, indicates these g l o b u l a r structures, or at least some of them, to be hollow, and therefore less likely to cause damage, since they break up, p o s s i b l y under pressure. Besides, it is doubtful if the delicate whiskers on these g l o b u l e s w o u l d create any e x p a n s i v e force. The authors are in a g r e e m e n t with M a r c h e s e (9) who proposes that the e x f o l i a t e d or l a m e l l a r c r y s t a l s favour a large volume, representative both of high specific surface and an e x p a n s i v e product.

CONCLUSIONS

Microstructural techniques (SEM/EDXA and X R D A ) and o t h e r l a b o r a t o r y testing methods can be used to detect and analyze deterioration of concrete. This study reveals that a c o m b i n a t i o n of factors can cause concrete deterioration. Median barriers are p a r t i c u l a r l y v u l n e r a b l e to d e g r a d a t i o n in c o l d c l i m a t e s because of generous use of deicer salts during winter. Physical processes, such as repeated freezing and thawing, a s s o c i a t e d with c h e m i c a l reactions b e t w e e n h y d r a t e d cement p a s t e c o n s t i t u e n t s and d e i c e r salt, can result in severe damage. Surface spalling, scaling and fissuration, f o l l o w e d by leaching o f concrete constituents m a k e s the concrete porous, and thus vulnerable to subsequent chemical attack. The e m b e d d e d metal can deteriorate due to deep penetration of CI- ions, together with carbonation, b o t h of which are responsible for depassivation of the steel. The corrosion products are mainly Fe oxide, hydroxide, and Fe c h l o r o - c o m p l e x e s , arranged in multilayers, whereas the degradation products of the c e m e n t paste are c o n s t i t u t e d m a i n l y o f c h l o r o a l u m i n a t e and ettringite. The l a m e l l a r F e bearing c r y s t a l s rather than the g l o b u l a r ones appear to cause expansion at the rebar interface.

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ACKNOWLEDGEMENTS The project was jointly funded by the Qu6bec provincial Ministry of Transport and the Centres of Excellence on High-performance Concrete. The authors wish to acknowledge the helpful discussions with Professor Adam Neville.

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