100

11-12.

Vth International Brick Masonry Conference

Evaluation of Corrosion Resistance of Reinforcement Embedded in Masonry J oints

J. Kropp, H .K. Hilsdorf Institut für Baustofftechnologie University of Karlsruhe, Germany

ABSTRACT

Similar to reinforced concrete corrosion protection of reinforcement embedded in mortar joints has to be taken in to account in the construction details of reinforced masonry.

i n this context the decrease of the alkalinity of the mortar joints due to carbonation is of particular significance. The extent to which C0 2 from the surrounding atmosphere penetrates the mortar joints depends on the porosity of the mortar. In b1°ick masonry CO2 may be transported also through the bricks and along the interfaces between joints and bricks.

[n an experimental study six types of bricks and four types of mortars have been investigated in order to develop some insight into the corrosion properties of steel reinfoTCement embedded in masonry.

in addition to observations of the extent of corrosion of reinforcement, also diffusion coefJicients for CO 2

transport through various types of bricks and mortar joints were determined. The moisture content of these materiais was introduced as an additional parameter. It was found that depth of carbonation of mortar joints is not only a function of mortar porosity but also of brick porosity. It can be related to the measured diffusion coefJicients. For most practical cases additional measures such as coatings or special bricks will be required to ascertain suJJicient corrosion protection of the reinforcement.

De même que pour le béton armé, il faut également dans la fabrication de maçonnerie armée tenir compte de la protection contre la corrosion des armatures de maçonnerie.

La réduction du milieu alcalin protecteur au moyen de la carbonatation est d'une importance décisive pour la corrosion de I' acier dans les materiaux mélangés de cimento Le degré de pénétration de CO 2 dans le mortier déPend de sa porosité. Ce CO 2 issu de l'atmosphere ne peut pourtant pénétrer l'intérieur de la maçonnerie non seulement par le mortier, mais encore par la brique et les interfaces mortier-briques.

Au cours d'une étude expérimentale six types de briques et quatre types de mortier ont été combinés afin d'obtenir une meilleure connaissance de la corrosion des armatures dans la maçonnerie.

En plus des observations relevées sur le degré de corrosion des armatures, on put également déterminer les coefJicients de diffusion de CO 2 sur différents types de briques et de mortier. L'état d 'humidité de ces matériaux fut incorporé comme parametre additionnel. Il résulta de ces expériences que l'étendue de la carbonation du joint n'est pas seulement une fonction de la porosité du mortier mais aussi une fonction de la porosité des briques. Celle-ci peut étre reliée aux coefficients de diffusion déterminés au cours de ces mêmes expériences. Dans un but pratique on aura besoin de mesures additionnelles telles que couvertures ou briques spéciales afin de garantir une protection suffisante de l'armement contre la corrosion.

Beurteilung des Korrosionsschutzes einer Fugenbewehrung bei bewehrtem Ziegelmauerwerk Ahnlich wie im Stahlbetonbau muB auch bei der Planung und Herstellung eines bewehrten Ziegelmauerwerkes

dem Korrosionsschutz der in eine Fuge eingelegten Bewehrung Rechnung getragen werden. Von ausschlaggebender Bedeutung für die Korrosion von Stahl in zementgebundenen WerkstoJJen wie Beton

oder Mortel ist die Aujhebung des schützenden alkalischen Milieus durch die Karbonatisierung. in welchem MaBe Kohlendioxid in Beton oder Mortel eindringen kann und somit zur Karbonatisierungführt, ist abhangig von der Porositat des durchdrungenen Mediums.

Im Mauerwerk kann Kohlendioxid aus der Atmosphare jedoch nicht nur durch den Mortel, sondem auch durch den Stein und durch die Kontaktzone Ziegel-Mortel in das Mauerwerksinnere eindringen.

In einer experimentellen Untersuchung wurden 6 verschiedene Ziegel mit 4 unterschiedlichen Morteln kom­biniert, um AufschluB über das Karbonatisierungsverhalten der Morteifuge und die Korrosion der eingelegten Stahleinlagen zu erhalten.

Da das Eindringen des Kohlendioxids in das Mauerwerk nach den Gesetzen der Diffusion verlauft, wurden an den entsprechenden Mauerwerkskomponenten sowie an der Kontaktzone Ziegel-Mortel Diffusions-koeffizienten für CO 2 bestimmt, um eine quantitative Aussage über den CO 2-Transport in das Mauerwerksinnere trefJen zu konnen. Da Mauerwerk stets eine gewisse Eigenfeuchte hat, die einen Gastransport beeiriflussen kann, muBte die Eigenfeuchte der untersuchten Materialien ais Variable berücksichtigt werden.

Die gewonnen Ergebnisse sollen zur Festlegung von Mindestanforderungen an die Qualitat der BaustoJJe sowie deren geometrischen Abmessungen im bewehrten Ziegelmauerwerksbau beitragen.

Session lI, Paper 12, Evaluation of COTrosion R esistance of Reinforcement Embedded in Masomy J oinls 101

Valutazione delta protezione contro coTrosione di una giuntura armata in una muratura m'mata di mattoni Come nel caso delte costruzioni di cemento annato, anche nel caso dei progetto e costruzione di una muratura

di mattoni armata bisogna dare importanza alta protezione contro corrosione dell'annatura collocata in una giuntura.

Per la corrosione dell'acciaio in materiali uniti ai cemento, come per esempio calcestruzzo o malta, acquista grande importanza l'eliminazione dell'ambiente alcalino protettivo per effetto delta carbonatizzazione. Fino a qual punto il biossido di carbonio riesca a penetrare nel calcestruzzo o malta provocando in questo modo la carbonatizzazione, diPende dalla porosità del mezzo attraversato.

Nella muratura il biossido di carbonio pua penetrare dall'atmosfera pera non soltanto attraverso ls malta, bensí anche attraverso la pietra e la zona di contatto fra mattoni e malta nell'interno della muratura.

In una ricerca sperimentale sono stati combinati sei difJerenti mattoni con quattro malte differenti, con lo scopo di otteneTe un 'infurrnazione sul comportamento delta carbonatizzazione delta giuntura di malta e della coTrosione delt' acciaio che vi viene frapposto.

Dato che l'introduzione dei biossido di carbonio nelta muratura si esegue in base ai principi della diffusione, nei corrispondenti componenti della muratura e cosí pure nella zona di contatto tra mattoni e malta, sono stati detenninati i coeJJicienti di diffusione per il CO 2. Cosí pTOcedendo si ottiene un 'informazione quantitativa sul trasporto del CO z all'interno delta muratura. Dato che la muratura sempre possiede una cCTta umidità propria, che pua influire sul trasporto dei gas, abbiamo dovuto consideraTe l'urnidità propria dei materiali esaminati come una variabile.

I nsultati ottenuti serviranno per fissare delte esigenze minime nei riguardi delta qualità dei rnateriali da costTuzione e cosí pure delte dimensioni geometriche nella costruzione di murature di mattoni aTmate.

PROBLEMSTATEMENT

Conventional masonry can under normal circumstances not be subjected to tensile or flexural stresses thus seri­ously limiting its range of applicability. Furthermore, con­ventional masonry structures show in most cases an extremely brittle behavior.

umerous experimental investigations as well as prac­tical experience have shown that mason ry may exhibit ductile properties and may be subjected to tensile stresses if steel reinforcement is placed in the mortar joints. Under such circumstances masonry behavior may approach that of reinforced concrete structures.!

Most of the research work on reinforced masonry has been directed to the determination of mechanical prop­erties of such units in order to establish the basic infor­mation required for their design o Little attention has been given to the problems related to the corrosion protection of the reinforcement. Despite the favorable mechanical properties of the reinforced masonry its practical appli­cation necessitates the guarantee of sufficient durability of the reinforcement even after prolonged periods of time.

The experiments described in the following deal with this particular questiono

MECHANISM OF CORROSION OF STEEL REINFORCEMENT IN HYDRAULlC CEMENT MORTARS

Because of dissolved Ca(OHh the pore water in a hydraulic cement matrix is alkaline and has a pH-value of 12.6. In this environment a passive surface layer consisting of Fe:J04 and FezOa is formed on the surface of a steel embedded in such a matrix. Thus decomposition of iron is prevented and the steel is protected against corrosion. This surface layer may be dissolved if the high pH-value of the pore water drops below a value of 9 e.g. by car-

bonation of the hydraulic cement matrix. Then the steel becomes active and iron ions may be formed which together with oxygen and water result in the formation of corrosion products. Carbonation may occur if COz from the surrounding atmosphere penetrates the hydraulic cement matrix. This may occur along different paths depending upon the effective porosity i.e. the volume of continuous pores in the system. In good quality concrete the depth of carbonation is limited to approx. ) 5 mm so that embedded reinforcing bars are protected against cor­rosion if the concrete cover is sufficiently thick. Because of the high water cement ratios which are required for the manufacturing of masonry mortars larger depths of car­bonation should be expected in such systems. Thus the experiences gained from reinforced concrete construction with respect to corrosion protection cannot be directly applied to reinforced masonry.2

EXPERIMENTAL INVESTIGA TION

Test Program

The objective of the experimental investigation was to determine the corrosion properties of reinforcing bars embedded in masonry mortar joints. From the obtained results requirements regarding the properties of the masonry materiais as well as required minimum dimen­sions for sufficient corrosion protection should be deduced. Small masonry units consisting of two bricks and a rein­forced mortar joint have been studied (Fig. I) . The major parameters investigated have been type and quality of bricks, composition of joint mortar, thickness of mortar joint and mortar cover. In additional test series the cor­rosion behavior of galvanized reinforcing bars has been investigated. The galvanized bars have been bent prior to placing them into the mortar joints. The composition of the various types of mortars is given in Table I. The brick

102

properties are shown in Table 2. In Table 3 the test program is summarized.

Experimental Procedures

Curing and Storage of Specimens

Immediately following their manufacturing the test specimens have been stored in air of 60% RH at a tem­perature of 20°C. The laboratory air had a natural CO2

content of 0.03% by volume. Subsequently the specimens have been stored for an additional period of two months at 60% RH, 20°C and a COz content of 2% by volume in order ro accelerate carbonation or the mortar joints. Fol­lowing this period the specimens have been stored in air with 90% RH, 20°C and a natural CO2 content of 0.03% by volume.

Observation of Depths of Carbonation

During storage in a 60% RH environment specimens without reinforcement have been opened after various exposure times in order to determine depths of carbona­tion by means of phenolphtalein as an indicaror.

Determination of Diffusion Coefficients for CO z through Bricks and Masonry Mortar

In order to describe COz transport through masonry, diffusion coefficients for diffusion of COz through the brick material and through the masonry mortar have been determined. Since masonry, when used in 's tructures will have a moisture content which is in equilibrium with the surrounding environment the diffusion experiments have been conducted on specimens with various moisture con­tents . The method used to determine the diffusion coef­ricients is similar to that described in DIN 526 15 for the measurement or water vapor diffusion.'l In the experi­ments the moisture concentration has been kept constant whereas there existed a controlled CO2 concentration gra­dient across the thickness or the specimen. The diffusion coeHicient has been deduced from gravimetric determi­nation of COz absorption . In Fig. 2 the principies of the experimental set-up are described .

Determination of Extent of Corrosion

Arter srorage over a period of 6 months at 90% RH the reinforcement has been removed from the mortar joints, corrosion products have been cleaned off and the extent of corrosion of the reinforcing bars has been determined by weighing the bars.

Test Results

Development of Carbonation

Rapid development or carbonation has been observed already during the initial period of storage in air with a normal content of COzo Arter 5 months depths of carbon­ation up ro II mm have been observed. These experi­ments indicated that also the type of brick has a pro­nounced effecl upon carbonation. In specimens made of bricks with a large porosity carbonation cou ld be observed along the brick-mortar interface (Fig. 3) . From this it may be concluded that transport of CO2 does not occur only

Vth International Brick Masonry Conference

through the mortar but also through the brick if the porosity of the brick is sufficiently high .

Subsequent storage in air with a COz content of 2% by volume over a period of 2 months caused complete car­bonation of the morta r joints in masonry made of bricks with a high porosity . When low porosity bricks have been used a small region in the center of the mortar joint remained not carbonized. No significant difference in the carbonation behavior of mortars of different composition has been found.

Diffusion Coefficients

The diffusion coefficients for the various types of bricks investigated varied by a facror of 10. High strength engi­neering bricks show a diffusion coefficient for CO2 trans­port of 5.8 x 10- 4 cm2/sec, whereas high porosity bricks exhibited diffusion coefficients as high as 55 x 10- 4 cm z/ seCo The diffusion coefficients of the various types of mor­tar varied between 20.0 x 10- 4 and 33.7 x 10-4 cm2/sec. No pronounced effect of the moisture content on diffu­sion coefficients could be found. The individual values of the diffusion coefficients are summarized in Table 4. Com· parison of the diffusion coefficients for bricks and for mortar confirmed the observation of depths of carbona­tion. Bricks with high porosity are more permeable to COz then the mortars so that COz can be transported through the bricks to the mortar joints resulting in additional ver­tical carbonation development in the joints. There is little transport of COz through the dense high strength engi­neering bricks. Only horizontal carbonation through the mortar joints takes place.

Extent of Corrosion

Plain R einforcing Bars

Visual inspection of corrosion showed that the corrosion products have been distributed uniformly over the entire steel surface (Fig. 4a). A maximum weight loss of 128.8 g per m2 of surface area of the reinforcing bar has been observed for test series AI , which consisted of bricks with high poros­ity,joint thickness of 15 mm and a mortar cover of 10 mm (Table 5).

Galvanized Reinforcing Ban

The straight sections of the galvanized reinforcing bars showed no corrosion whereas in bent sections the brittle iron-zinc alloy of the hot dipped galvanized bars has cracked or spalled locally. Corrosion has been observed at these locations after removal form the mortar joints (Fig. 4b).

CONCLUSIONS

When bricks of high porosity are used COz transport takes place also through the bricks resuIting in additional ver­tical carbonation developme nt in the mortarjoint.

Because of their high water-cement ratio development of carbonation in mortar joints occurs at a faster rate than in conventional reinforced concrete. Carbonation does not seem ro approach a limiting final value .

Session lI, Paper 12, Evaluation ofCorrosion Resistance of Reinforcement Embedded in MasonryJoints 103

Rapid carbonation causes a decrease of the pH-value of the mortal' joints resulting in insufficient corrosion pro­tection of reinforcing bars embedded in such joints. AU reinforcing bars investigated showed corrosion attack.

Galvan izing of the reinforcing bars is effective only if the entire surface of the reinforcing bars is protected. Because of the brittle nature of the iron-zinc alloy the surface may crack or spall in bent sections. At such loca­tions the reinforcing bars may be corroded when embed­ded in carbonized mortar joints.

In order to ensure sufficient a nd durable corrosion pro­lection of the t·einforcement cmbedded in masonry joints

Test specimen

,.

_'r'''~~~ '''~~.ó't-. 'j.' ç

Figure 1.

sealed surfaces

brick

mortar joints with 2 reinforcing bars

brick

sealed surfaces

additional measures for corrosion protection are required.

REFERENCES

I. H. K. Hilsdorf, Bewehrtes Ziegelmauerwerk- Literatursi­chtung "Materialprüfanstalt für das Bauwesen der Technischen Hochschule München, Bericht Nr. 22, 1962 2. H .C. MoI!, Über die Korrosion von Stahl im Beton DAfStb, Heft 1969 3. DIN 526 15, Bestimmung der Wasserdampfdurchlassigkeit von Bau- und Dammstoffen , 1973.

~2LZZ2LZ2ZZ;~~:2~--- specimen

11----- glass container

--tt---- salt solution

I.!:::===º=~~~~~~==:lr--- CO2-absorbent

Figure 2. Test Set-Up for Diffusion Experiments

I c~~::5t5a~~i;S:/; ~ I specimen 03 20 .0mm

+--+ +---+ 70 mm 7.0 mm I

115 mm

20,0 mm

I k; ; ; ;~ ;Çf0 ; ; J I specimen 06

I I I I 11.0 mm 11.0 mm I I

"5 mm

Figure 3. Extent of carbonation in mortar joinls arter 5 monlhs slOrage at 60% reI. humidilY, 20°C and 0.03 Vol. 7c CO2

104 Vth lnternational Brick Mason ry Conference

C ]11

C 312.

C 321

C322

C 331

Figure 4. Corrosion of reinforcing bars

T ABLE 1-Types of Mortar

Mortar No. Mix Proportions

PC Sand W/C Additives

I I 4 l.l -2 I 4 1.2 hydrated lime 20% by weight of cement content

3 I 5 1.4 -

TABLE 2-Types of Bricks

Compressive Open Effective Strength Coring Density Porosity Porosity

Brick No. N/mm2 % g/cm3 % %

I 12 38 1.60 36 3.9 2 12 31 1.59 32.4 3.5 3 20 42 1.89 26.2 2.0 4 >28 20 2.15 6.1 0.42

Session ll, Paper 12, Evaluation of C01Tosion Resistance of Reinforcement Embedded in Masonry J oints 105

TABLE 3-Test Program

Mortar Cover Joint

(mm) Thiekness Briek (No.) Mortar (No.) Reinforeement

(mm) Test 10 20 30 15 20 25 I 2 3 4 I 2 3 Plain Calvan.

5eries 5teel 5teel

A I x x x x x A2 x x x x x A3 x x x x x A4 x x x x x A5 x x x x x A6 x x x x x A7 x x x x x A8 x x x x x A9 x x x x x B I x x x x x B2 x x x x x B3 x x x x x

C I x x x x x C2 x x x x x C3 x x x x x

D I x x x x x D2 x x x x x D3 x x x x x D4 x x x x x

O I x x x 02 x x x 03 x x x 04 x x x 05 x x x 06 x x x

51 x x x x 52 x x x x 53 x x x x

T ABLE 4-C02-Diffusion Coefficients for Bricks and Masonry Mortar

Briek Mortal'

I 2 3 4 I 2 3

em' .10-4 see 55.0 49.2 28.57 5.8 23.6 20.0 33.7

TABLE 5-Average Weight Loss (g/m 2) of Reinforcing Bars Due to Corrosion

Test series A I A3 A5 A7 A8 A9 B I

A verage weight loss 128.8 87.26 60.34 78.32 61.29 82. 11 30.55

Test series B2 B 3 CI C2 C3 D I 02

Average weight loss 26.13 33.79 29.47 25.87 34.24 28.13 27.71

Test series D 3 D4 5 I 52 5 3

Average weight loss 29. 34 20.76 32.76 29.74 3 1.95