Failure of white coat plasters: cause and remedy

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Failure of white coat plasters: cause and remedy

FAILURE

OF

WHITE COAT PLASTERS

-

CAUSE AND REMEDY by

V.

S.

Ramachandran and

P.

_J.

_Sereda

I

I I D l V l S f O H OF BUILDING R E S E A R C H

-

N A T I O N A L R E S E A R C H COUPPClL

.

O T T A W A

-

C A N A D A September

1974

FAILURE

OF

WHITE COAT PLASTERS -

CAUSE

A N D REh4EDY

by

V.

S. Ramachandran and P.

J.

Sereda

The Division of Building Research has investigated plaster failures occurring in various types of buildings such as offices, hospitals and apartment buildings. In almost a11 cases studied the failure of white coat plaster cockld be attributed mainly to the u s e of Type N hydrated lime. The work also resulted in the development of a new t e s t f o r determining the potential soundness of limes and of a method of estimating the various constituents in the w h i t e c o a t plaster ( 1 , 2 , 3). This note d e s c r i b e s w h i t e coat p l a s t e r , the types

and c a u s e s of failure, and remedial measures. l t does not cover all types of failure.

W h i t e Coat Plaster

The final coat of plaster comprises lime, plaster of Paris and

w a t e r . Plaster may be applied in three coats, v i z , 'scratch\ 'brownt

and 'finish'. The backing for plaster consists of concrete or a dense

type a£ masonry. The plaster is generally coated with paint or sealed with o t h e r coatings a few weeks after it has been applied. Specifications

f o r l i m e and gypsum plaster a r e covered by Canadian Standards A s s o c i a - tion and American Society f o r Testing and Materials 34, 5).

Description of the failure

The t e r m s 'blistering1, 'blowingf, tpoppingt and 'pittingr are used to _{describe plaster f a i l u r e s .}

The blisters or bulges may be observed 5

-

15 years after the

laster

is applied, increasing in number and severity with the age

of

the building. L a r g e blisters o r bulges occur as a separation of the

smooth or o u t e r coat f r o m the underlying layer of w h i t e coat, or as a separation of the white coat f r o m the sanded brown coat or bond

plaster b a s e . Occasionally t h e r e is a l s o a weakening of the bond b e t w e e n the sand-lime-plaster and the concrete b a s e . These types of failure are described in greater d e t a i l in another publication ( 6 ) .

U n d e r w a r m weather and high humidity conditions, the white c o a t plaster is particularly susceptible to early failure.

Cause of f a i l u r e

Some of the causes leading t o failure are d e f e c t s in the materials,

which may originate i n t h e i r physical oz chemical nature, exposure condi-

tions and/or mistakes in the execution of _the job. _{A cammon cause} of f a i l u r e is the presence of par'iicles of hard-burned quick-lime or

m a g n e s i u m oxide in the lime used i n the plaster mix. These oxides

a r e resistant to easy hydratian and hydrate gradually over the years

by the moisture provid2d by high humidity conditions. W a t e r f o r

hydration is also provided by the reaction of atmospheric carbon

d i o x i d e with lime in plaster mix. The r a t e of hydration of MgO or

CaO depends, among other factors, upon the temperature at which the

lime was originally burnt before i t w a s mixed w i t h the p l a s t e r .

F i g u r e

P

indicates the effect of calcination temperature on the rate

o f hydration of MgO in calcined dolomite (7). The hydration was

c a r r i e d out in water in this case but when the material is e x p o s e d t o

ambient conditions of temperature and humidity the r a t e of hydration

of _{MgO is decreased severalfold.}

The magnitude of the s o l i d volume change of calcium oxide and magnesium o x i d e , when they are converted t o hydroxides, i s

r e s p e c t i v e l y 91% and 117%. These f i g u r e s , h o w e v e r , d o not represent

actual expansions in practice because the material is a porous s y s t e m

which tends to a c c m m d a t e most of these expansions. But these

figures do provide a b a s i s to predict the potential, o v e r

-

a l l expansioa, i n qualitative terms.

Patching of the walls

The patching or the replas tering work after

t

k

_{plaster finish c o a t} h a s f a i l e d , shonld take into account the severity and extent of failure. If f a i l u r e is not s e v e r e and widespread, trimming should be done around the edges so as to reach the firmly bonded area. Patching should then

be done with a well-gauged putty to _obtain a hard, well-bonded patch. One of the most common problems connected w i t h the patching o f _{plaster w a l l s is that w a t e r u s e d in the patching mix soaks into the}

plaster adjacent to the s p o t being patched. This r e s u l t s in the weakening of _{the bond a t} the interface of the white coat plaster and the base and

f u r t h e r expansion due to hydration

of

the

MgO.

Excess water t h u s tends t o hasten the f a i l u r e of the apparently sound plaster.

A technique t o prevent water in the patching mix f r o m spreading

t o adjacent areas has been developed. It involves the application of a

bonding compound. The liquid bonding agent is a film-forming, non-

oxidizing cornpasition, that can be applied by brush, roller or sprayer,

The bonding agent seals off the pores and l a r g e r crevices in the sound plaster adjacent to the Role. Thus w a t e r ingress into the base of the

sound plaster is prevented. It also allows for some expansion without excessive stress. Liquid bonding agents should satisfy the requirements of the CSA standard (8).

The patching of a failed spat involves the following steps:

( 1 )

Remove all loose plaster f r o m in and around the area that has

been affected.

( 2 ) Apply the bonding compound on the failed area ensuring that it i a covered w e l l . Pay particular attention t o the edges and corners. ( 3 ) Let i t dry f o r about 1 to 3 days or according to the m a n u f a c t u r e r ' s

recommendations.

(4) Patch the spot w i t h w e l l mixed, white coat plaster containing,

preferably, a pressure hydrated lime; in other words the plaster, before its application on the w a l l should contain minimal amounts

of unhydrated MgO and

CaO.

( 5 ) Allow two w e e k s or mare before subjecting the w a l l t o painting or

other decorative finish.

( 6 ) If blistering or failure o c c u r s over a large area i t should be

serously considered whether it would not be economical t o remove the entire finish coat and refinish the w a l l with a new putty coat. ( 7 ) Failure of the brown coat is also possible. This can be checked

for firmness by scratching it with a trowel or any sharp tool. A

hard, sound brown coat would need considerable pressure for an

impression to be made. H the b r o w n coat shows signs of deterioration

it may require removal down to lathing before the r e f i l l i n g operation. Conclusions

A f e w precautions taken before the application of white coat plaster

ensure good Long-term performance.

The lime used in the plaster mix should preferably contain minimal

amounts of free calcium oxide and magnesium oxide. Type S hydrated Pime satisfies this r e q u i r e ment.

The plaster mix should contain higher proportions of plaster of P a r is.

Plaster that gives a good bond s t r e n g t h should be used.

After application of white coat adequate curing should be allowed

before paint is applied.

If a p l a s t e r has failed locally a bonding campound should be used before patching.

If a plaster has failed, i t is e a s y to t e s t whether i t is d u e to h y d r a t i o n of f r e e MgO by carrying out comparative t e s t s u s i n g compacts of the plaster that is still sound and located elsewhere o n the s a m e w a l l (1, 2, 3).

If the plaster f a i l u r e spreads o v e r a _{large a r e a i t may be judged}

m o r e e c o n o m i c a l to p e e l off the entire coat and replaster w i t h a s o u n d plaster

.

R E F E R E N C E S

V.

S . Ramachandran, P. J . Sereda and R. F. Feldman. "Delayed

hydration of white c o a t plaster", Materials Res. Standards, Vol. 4,

No.

12, 1964, p. 663-666.

V. S . Ramachandran,

R.

F.

_{Feldrnan and P.}

_J.

_{Sereda. "An un-}

soundness t e s t for limes without cement", M a t e r i a l s Res. Standards,

Vol. 5 ,

NO.

10, 1 9 6 5 , p. 510-515.

V. S . Ramachandran, P.

J.

S e r e d a and R. F. Feldrnan. " D e l a y e d hydration in white c o a t plaster: c o m p a r i s o n of different methods of analysis", Materials Res. Standards, Vol. 8, No. 1, 1968,

p. 24-28.

Gypsum Plasters - CSA. 8 8 2 . 2 2 - 1 9 6 3 ( S e e also A82.20-1964; A82. 3 0 - 1 9 6 5 ; A82.41-1950; A82.42-1950; A82.44-1950; A82.46

-19621.

llPhysical testing of quick l i m e and hydrated lime".

A S T M

C110-71:

Anncral ASTM Standards Part 9 , 1 9 7 3 . ( S e e a l s o C6-49; C22-50;

C25-72; C 2 8 - 6 8 ; C206-49; C207-49; C471-72; C472-73).

L.S. W e l l s , W.

F.

Clarke, E . S . Newrnan and D. L. Bishop. 'qnvestigation of failures of white c o a t plaster", U. S. Dept.

Commerce, National B u r e a u of Standards, Building M a t e r i a l s

7. _{T . L. W e b b . "Chemical a s p e c t s of unsoundness and plasticity in}

building limes", South African Industrial C h e m i s t ,

-

6 ,

p. 2 9 0 - 2 9 4 , 1952.

8. "Liquid banding agents f o r i n t e r i o r plaster s". CSA Standard

A261-1970.

800

1 0 6 0 1 2 0 0 1 4 0 0 T E M P E R A T U R E ,

"C

FIGURE 1 H Y D R A T I O N OF M A G N E S I A

IN