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RESTORATION OF MASONRY FACADES. RENDERS AND FINAL COATS IN A SEVERE CLlMATE

A.M . Waldum Norwegian Building Research Institute

Trondheim, Norway

ABSTRACT

For the last 10-20 years, considerable attention has been paid in Norway to the subject of "conservation and restoration of old masonry buildings". This paper concentrates on problems connected with protective coatings for facades in a severe climate. The composition and durability of old rendering mortars are described. Results from laboratory studies ( including accelerated tests) and field exposure of renders and final coats for restoration are briefly presented and discussed . Suggestions are made for the composition of the coating system for old masonry walls in severe climate.

Finally the restoration of the externai walls of the city campus buildings of the University of Oslo, is presented as a case study. These consist of three large brick buildings built in the period 1870-1880, and the restoration took place in 1987-88. The "history" of the buildings and ali the steps in the restoration work, including the up­todate evaluation, are described .

INTRODUCTION

Today rehabilitation-related jobs represent a large portion of the overall construction activity in Norway, and the restoration of relatively old masonry buildings has here a central position. The masonry buildings in question date from the period 1870-1920 and are principally located in the 4-5 largest cities. These buildings represent above

100 000 flats, offices and industrial buildings etc. In the last 10-15 years, considerable attention has been paid to the restoration of these masonry buildings. Several factors can explain this activity. Unsuccessful restoration work in the 1960's and, the increasing effects of air pollution and acid rain are two important factors. The grovvth in standard of living in general in the 80's is a third one. The unsuccessful jobs done in the 60's were often caused by lack of knowledge about old mortars, and led to selection of incorrect materiais. In some cases facades exposed for 70-80 years without significant problemsshowed substantial damage 1-3 years after rehabilitation . The deterioration of the masonry facades is principally caused by the outdoor climate, and driving rain and frost are the key factors . Figure 1 indicates the amount of driving rain per year in various parts of the country. Another climate parameter of interest

, __ ;.:.:~_= regarding deterioration of masonry facades is the - - number of days with both precipitation and

Figure 1 Driving rain in Norway and the main direction ofthe rain

freezing/thawing-conditions.

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In figure 2 this number is presented for the last 30 years for a meteorological station just outside Oslo.

NO.OF OAYS 120,-------------------------------------------,

AVERAGE

110

60~~4-~~+-~~4_~~+_~~~~~~_r4-~~

1960 1965 1970 1975 1980 1985 1990

Figure 2 Number of days with both precipitation and freezing/thawing conditions. Gardermoen, Oslo

MAlERIALS FOR RENDERING AND FINAL COAllNG

Up to 1910-20, lime was the only binder in rendering mortars in NorWay. lhe use of portland cement together with lime was in the parts of the country with the most severe climate and was founded on the need for mortars with better frost resistance than a pure lime mortar normally has. After 1930, portland cement was a component in rendering mortar ali over the country. For about 40 years, a traditional rendering mortar was equivalent to a LCmortar. Around 1970, masonry cement beca me the dominating cementitous binder in mortars and for some years after 1970, M mortars were used without hesitation for repairing lime based renders. lhese strong mortars are inefficient at following the deformation movements of the underlayer, and failures arise. Today, however, there seems to be agreement on the following requirements for mortars for rendering restoration: Strong adhesion to a non­homogeneous underlayer and good frost resistance are of prime importance. lheir reaction on temperature and moisture must be closely similar to that of other materiais forming the structure. lhe drying of the structure should not be inhibited in any way by the restoration mortars used. Finally, the mortar should be easy to replace, give the building a pleasing appearance, and maintain the same style .

From about 1920 to 1955 cement paints were the dominating type of product for decorative finishes for cementitious rendered facades. lhese paints are still an important final coating for masonry buildings despite having been largely superceded by emulsion paints and other organic-based coatings the last 30-35 years.

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The cement paints are used as a low cost application to previously cement painted or unpainted surfaces since they cannot be applied over coats of organic paint. Where a specially final coating with high water vapour permeability is demanded, a cementpaint or some other sort of mineral paint will be the only alternatives.

After being more or less offthe Norwegian market for more than 50 years, silicate paint was reintroduced in 1985 and is today an interesting alternative.

Emulsion paints and solvent borne masonry paints, however, represent the most widely used treatment for rendered surfaces in recent decades. Oil based, solvent­bearing paints are little used today, masonry paints based on synthetic resin having won a growing share of the market in recent years.

INVESTIGATION OF THE CONDITION OF SOME OLD MASONRY BUILDINGS

Failure investigations have been carried out on 20 masonry buildings from different parts of Norway, built during the period 1870-1930. The renders on these buildings had normally been repaired; Usually with a harder mortar than the original. Several layers of paint were also normal.

During the field investigations samples of bricks, rendering mortars and final coatings were gathered for examination in the laboratory. The main conclusions regarding the rendering and final coating were :

The type of decorative coating seemed to have a great influence on the condition of lime- and LC-based rendering .

The combination of a low-strength rendering mortar (L-LC) and an organic-based paint had very often led to disturbance in the moisture equilibrium of the wall with severe failures in the rendering and sometimes in the substrate as the result.

Blisters in the paint were pointed out only on facades with organic paints. Efflorescence (salt crystals) occured in some cases under the blisters (Figure 3). Cracks in the paint films appeared also more often with this type of final coat. The south- and westoriented walls were most exposed to these types of failures.

A coarse rendering surface texture will in general mean a better durability compared with a smooth finish. The coarse texture, however, can lead to an unfavourable difference in the paint film thickness.

Facades which had undergone extensive render repair had more failures in the paint film than corresponding walls with a more homogeneous render. An inadequate composition of the repairing render was here an important factor.

When lack of bond between render and substrate or other render failures occur on more than 25-30 % of a wall surface, a new render on the whole wall can be recommended.

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Some facades with an inorganic (cement) paint suffered from discolouring.

Poor flashing and/or insufficient maintenance due to the absence of gutters. downpipes. etc. was often the decisive factor for the development of fa ilures.

Figure 3 A rendered surface with blisters and cracks in the paint film

Laboratory research

Figure 4 Failure due to poorly detailed footing

Rendering mortars with different types of masonry paints have been tested in the laboratory. The test programme included weather resistance determined in a cyclic short time testo

The samples to be tested are mounted in a special apparatus for accelerated weather resistance testing and exposed to an artificial weathering cycle for an agreed period of time. The weather resistance is found from the detectable changes in the visual characteristics and measurable properties.

One test cycle consists of :

A Radiation from Osram Ultra Vitalux GUR 53 300 W or similar sun lights lamps and simultaneous heating to an elevated temperature. Radiation intensity is 1900 W/m2 (input) . .

B Wetting with a spray of demineralized water. 15 ± 2 litres per square metre and hour at a temperature of 291 ± 5 K (18 ± 5 °C) .

C Cooling and freezing to a temperature of 253 ± 5 K (-20 ± 5 °C) .

D Thawing at room temperature 296 ± 2 K (23 ± 2°C) and 40 ± 10 % RH. with possibilities for inspecting and changing the samples without stopping the test apparatus.

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After the exposure period, some render/paint combinations were totally destroyed while others had practically no changes compared with unexposed samples. (Exposed area per sample, 1/8 m2) . On a LC 50/50/610 render silicate paint gave the best results. Of the organic paints, a copolymer solvent borne product had, however, only minor failures (small blisters). Samples with emulsion based thick film coating (1 .0 mm film) had cracks and blisters in the final coating and frost damage of the render.

Higher strength renders given the same exposure had fewer failures both in the paint film and render.

Since ali kinds of accelerated ageing tests involve a certain degree of uncertainty, the results have to be considered with care. However, when the results were compared with the water vapour permeability of the same coating systems, a quite c1ear correlation was found in that systems with a high water vapour permeability gave the best results in the aging testo

REHABILlTATION OF THE UNIVERSITY OF OSLO, ClTY CAMPUS

Short history of the buildings

THE ClTY CAMPUS OF THE UNIVERSITY OF OSLO consists of 3 masonry buildings built, except for an addition, in 1870-80. Ali exterior walls are of brick, and are on average 900-1000 mm thick. The facades have a lime mortar render with a smooth surface and a final coat (Figure 5) and have had several minor repairs during their service time.

/ I

Figure 5 A section of the university campus

In the first decades the final coat was based on lime. Later, a cement-based product was used. In 1955, the walls were for the first time painted with an organic coating, and were repainted in 1970. Parts ofthe walls had at then developed a great number of fissures. These areas were reinforced with fibernetting embedded

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in the first layer of final coat. In 1986, it was decided that the facades had to undergo complete rehabilitation.

Inspection

Already at the first inspection, it beca me guite clear that the facades had suffered substantial deteriorations. The failures can briefly be described as blisters and cracks in the coating; also the areas with fiber netting had fissures. The rendering had areas where its strength had completely deteriorated (broken up into a fine powder under the effects of salt and frost) . The original render was, however, found to be in good condition in some areas.

The rendering had, as mentioned above, undergone several repairs, normally with a harder mortar than the original; in some places pure portland cement mortar had been used. The result of this difference in mortar composition was cracks between the original mortar and that applied during repairs. The degree of damage was in general influenced by the roof gutters and downpipes partly having been out of order for a long time. The thickness of the rendering system was found to be up to 60 mm with an average of 45-50 mm.

Alterna tive principies for restoration

A basic aim in planning the rehabilitation work was that at least one alternative should meet the requirements set for restoration of historical buildings. Laboratory tests of the original rendering showed a relatively small content of aggregate, 2,5-3 times the lime contento The principal average size ofthe aggregate measured 0,2-1,0 mm with maximum sizes up to 4 mm.

The compressive strength of "good specimens" was 2,0 MPa. 8y impregnating the mortar specimens with pure potassium silicate the strength increased to about 3,0 MPa. Weather resistance tests showed that improved durability was also obtained by this treatment.

As mentioned previously, experience using organic coatings on L renderings or low­strenght LC renderings has not been good . The condition of these facades supported this observation. Df greater interest were the coatings employing mineral binders like lime, cement and silicate. In 1986, silicate paint had practically not been in use in Norway since the beginning of this century. Experiences from Germany and Sweden however, made this product very interesting . After studying the weather resistance of mortas with pure silicate paint by accelerated testing in a special apparatus in the laboratory, the paint became even more interesting.

The preliminary research led to a description of two recommended alternatives :

A. Remove the excisting coating and low-strength render. Treat the fracture surface of the remaining render with apure potassium silicate. Use a factory made repairing mortar for repair (based on hydraulic lime and air hardening lime) . Use apure silicate paint for the final coat.

8. Remove the whole coating system down to the brick surface. Applya "modern" multiple rendering system based on masonry cement as a binder. The final coating can be based on organic or inorganic binders.

The rehabilitation started in 1987 and alternative A was adopted.

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The implementation of the rehabilitation

The rehabilitation work started with removal of the excisting paint. A paint stripper based on organic solvents was used. The final cleaning operation was carried out using hot water from a high pressure cleaner and apressure up to 150 bar. During the cleaning operation, most of the deteriorated rendering mortar was washed off the wall.

The mortar used for restoration had a composition ULH 40/60 with 500 kg aggregate per 100 kg binder. The maximum size of aggregate was 3.0 mm. Laboratory tests indicated that fresh mortar had an air content of 16-18 %, satisfactory weather resistance and a compressive strength 2.5-3.0 MPa under "good" curing conditions. After a mixing time of 7-8 minutes, the workability of the mortar was classified as "very good" . The old morta r, however, absorbed water rapidly from the fresh repair morta r, and premoistening and good curing conditions were important to avoid cracking (3 days with periods of water spraying) . Furthermore the maximum mortar thickness applied in one operation should not exceed 10 mm. This leads to 4-5 separate operations, with one day between each operation .

The completed rendered surface had some small fissures and pores. This was taken into account when deciding the composition of the silicate paint. Fine grained quartz aggregate was used as filler when applying the 1st and in some cases in the 2nd coat, whereas the 3rd coat always consisted of potassium silicate and pigmento

Lack of experience with the silicate paint led to some starting-problems. Among the most important requirements when the paint is being applied are use of a suitable brush, painting the whole height of a continuing wall without long breaks and keeping the curing temperature above + 5 0c. To meet the last mentioned requirement in winter, the scaffolding had to be covered with a plastic sheet and the air heated. The required temperature could be reached by using 2 diesel driven heaters with an effect of 20 kW each and working with areas from 300-500 m2 at a time. The whole rehabilitation job was completed at the end of 1988. More than 10000 m2 had been restored in about 20 months.

Evaluation of the result

2-3 years after completion ofthe job, the overall expectations are satisfied . A slight tendency for discolouring has been observed . Some people, however, think this produces the patina typical for old mineral facades. The bond between the substrate and the silicate paint is broken in certain places. These are, however, located close to downpipes which had been out of order before the restoration (Figure 6).

The masonry was very wet there when rehabilitation started and too little drying time was allowed before painting . The moisture prevented the coating material from penetrating well, and chrystallization of salts will break a poor bond. When the bond is good, however, the salts deposit on the surface without causing any damage.

Summary

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Figure 6 Blisters In the paint due to high moisture content in the substrate

lhe evaluation of old masonry facades and the laboratory tests described in this paper show that an inorganic paint should be chosen for decorative coatings on low-strength (L and LC) renders. lhe general impression also supports this conclusion . lhese coatings have a high water vapour permeability and more important they do not, unlike organic products, form a separate film on the surface . lhis means, for example, that moisture can be transported out to the wall surface as liquid. lhe buildings in question very often have cracks or poor details where water can penetrate into the wall. lhe substrate may also contain water-soluble salts. lhe properties of an inorganic coating are therefore necessary in order not to disturb the moisture equilibrium of the substrate .

For facades with renders that have a higher frost resistance and hopefully lack cracks or water-soluble salts in the substrate, both inorganic- and organic-based coatings can be used with good results.

REFERENCE

lhorborg Perander, luula Raman: ANCIENl AND MODERN MORlARS IN lHE RESlORAllON OF HISlORICAL BUILDINGS (Espoo, May 1985).