THE DURABILITY OF BRICKS.
This is one of the most important parts of our subject, and it may
be approached from several points of view. When a brick decays, its
structure, for the most part, is responsible therefor. A great deal
depends on whether the ingredients forming the brick are merely baked
in the process of manufacture, or whether they are wholly or in part
agglutinated by igneous fusion. A rough and ready plan of determining
this point, in the absence of experience, is by ascertaining the
porosity of the brick. Other things being equal, the absorption test
is undoubtedly the best all-round method of gauging the weathering
qualities of a brick. But there are certain kinds of bricks which defy
that method; an imperfectly burnt one with a vitreous exterior is
especially treacherous in that respect, and, indeed all “vitrified”
bricks are difficult to deal with by the “absorption process.” Again,
a brick cracked all over, not with superficial cracks only, but with
those which go far into the interior, will not yield its quality by
mere immersion in water. The water, it is true, finds its way right
into the brick, but, as often as not, the sides of the cracks are
perfectly vitrified and almost damp proof, so that on lifting the brick
out of the water the latter rolls off as though it were on “a duck’s
back.” Yet such a brick, yielding but the merest fraction as a result
of the immersion, may be utterly worthless when put into a building,
because it would not be strong enough.
Then we have those bricks which are seriously affected chemically, but
which seem fairly good in other respects. They also, in many cases,
defy the efforts of the experimenter in regard to absorption; though
they are nevertheless easily detected as being of bad quality, by
other methods. Such bricks often resist great “crushing weights,” and
generally bear a good character, their subsequent behaviour when put in
the building to the contrary notwithstanding.
In determining the weather-resisting qualities of a brick we have the
following things to consider:--
1. The chemical composition of the brick.
2. Its absorptive capacity.
3. Its minute structure.
4. Its specific gravity.
5. Its strength.
The last-mentioned property can often be inferred from a knowledge of
the three preceding ones, and need not, therefore, form the subject
of direct experiment. In spite of that, however, we find that the
“crushing strength” is much more popular than the others. The reason,
so far as brick manufacturers are concerned, is not far to seek.
Architects demand that especial quality. “What is the ‘crushing
strength’ of your bricks?” enquires the architect. And if the maker
does not know, he stands a good chance of losing the order. Figures
are demanded, and if the maker cannot produce a higher figure than his
neighbour, woe betide him. But statistics are ever deceptive, and as
applied to bricks in regard to their strength especially so.
In general, we have to consider whether the brick is strong enough
for the purpose to which it is to be applied; and that depends much
more on the manner in which it is built up, than on the strength of
the individual brick. For ordinary building purposes almost any
kind of brick is, _per se_, strong enough, and a mere inspection of
the specimen suffices to carry conviction as to its suitability or
otherwise in that respect. For certain structures, such as buildings
to carry heavy weights--especially moving weight--for engineering
purposes, and the like, we ought, it is true, to know a little more.
Yet the engineer would be a very poor one who could not tell at sight
whether a brick submitted to him was fit or not for the purpose he has
in view, from the point of view of its weight-carrying properties. In
any case, however, fashion demands the “crushing weight” in figures,
and although such figures are in general of but little practical value,
they must be given.
The principal difficulty the architect and engineer have to contend
with is not lack of strength, but the setting in of decay, and that
even in bricks sometimes of the strongest description. Unless the
strength is going to be maintained, it is of no use whatever, in a
scientific sense, to give it in the first instance.
After these few preliminary observations, it will be well to treat the
subject more systematically.
THE EFFECT OF THE ATMOSPHERE ON BRICKS.
Air is a mixture of gases; dry air consists of at least four of them,
namely, nitrogen, oxygen, carbonic acid, and argon. Of these, by far
the most abundant is nitrogen, present to the extent of about 78 per
cent., then oxygen, 20.96 per cent., argon about 1 per cent., and
carbonic acid 0.04 per cent. Extremely minute quantities of ammonia and
ozone, though practically always present, have been omitted from the
preceding results of analysis of air.
We have been speaking of pure dry air; but the atmosphere is hardly
ever of precisely the same chemical composition in two different
places. By the seaside it has more ozone, and chloride of sodium
is found in particular abundance. In cities, especially where
large factories exist, nitric acid and sulphuric acid appear most
conspicuously, and the proportion of ammonia becomes larger. In the
air of streets and houses, the proportion of oxygen diminishes, whilst
that of carbonic acid increases. Dr. Angus Smith has shown that very
pure air should contain not less than 20.99 per cent. of oxygen, with
0.030 of carbonic acid; but he found impure air in Manchester to have
only 20.21 of oxygen, whilst the proportion of carbonic acid in that
city during fogs was ascertained to rise sometimes to 0.0679, and in
the pit of a theatre to the very large amount of 0.2734. Although these
may seem to be very small percentages, yet the total amount of carbonic
acid in the atmosphere is enormous, and plays a conspicuous part in the
decay of certain kinds of bricks.
Sulphuric acid is found in the air of large cities principally as
a product of combustion, and is, of course, a distinct impurity. A
portion of this acid is free, and a larger quantity is combined. Free
sulphuric acid is very destructive to clay goods in the open; and it
should be remembered that the relative abundance of this impurity
depends on the precise _locale_ in the city. A great deal has been said
and written about the decomposition of the stone of which the Houses of
Parliament are built. The air in the immediate vicinity must be highly
charged with both sulphuric and nitric acid from the proximity of the
busy factories on the opposite banks of the Thames in Lambeth. Had the
Houses of Parliament been erected, say, in Kensington, where but few
factories exist, it is conceivable that the stone would have behaved
much better.
Air in itself, however, has no power to destroy bricks--the various
gases, acids, chlorides, salts, solid carbon, inorganic and organic
dust can do nothing by themselves. But the air is always laden with
vapour, the most important of which is water vapour, which condenses
into rain, hail, snow, and dew. When rain is formed, the drops of water
take up minute quantities of air with its proportion of carbonic acid,
sulphuric acid, or what not, and it is these acids, applied to the
surface of bricks through the medium of rain and moisture generally,
that are liable to do the damage if the nature and composition of the
brick are favourable.
Let us assume that we have a brick composed of a goodly percentage of
carbonate of lime. The carbonic acid in the rain reduces this to a
bi-carbonate, which is soluble in water, and hence the surface of the
brick decays, the rain water washing it away. Other things being equal,
it follows that the same brick will decay most rapidly in a district
where the rainfall is very great and where there is the largest
proportion of these deleterious acids in the air.
Whilst speaking of the various acids which attack and destroy bricks,
we must not forget those formed by the decomposition of organic matter
on the surface of bricks which “vegetate.” The lichens, mosses,
and so forth, growing from cracks in the wall, or spread over on
to the brick from the mortar, yield, on decomposition, some of the
most powerful acids in existence. A brick with a “crumbly” surface
affords good foothold for these plants, and when they die they give
rise to the so-called humus acids--crenic and apocrenic acid--which
undoubtedly do an immense amount of damage. By keeping the surface
of the brick moist, the plants permit the ordinary acids in rain to
do more execution than they otherwise would. Taking two bricks, one
which “vegetates” and one that does not, and exposing them in the same
situation, it will be found that after a smart shower of rain the
surface of the former has become thoroughly soaked, and the vegetation
keeps it so, completely rotting it in time; whereas the surface of the
latter, exposed to the same shower, may be quite dry within an hour or
two after the rain has fallen.
Returning to the subject of rainfall, which exercises such material
influence on the durability of bricks, we may give a few particulars
concerning the distribution of rain in this country. Speaking
generally, the east coast of England is the driest part of the country,
the west coast having the greatest rainfall. The annual quantity at
sea-level ranges from 60 to 80 inches on the west coasts of Ireland and
Scotland, to about 20 inches on the east coast of England.[10] In some
localities, however, the fall is much greater, amounting to 154 inches
on the average of six years at Seathwaite, in Borrowdale, at the height
of 422 feet above the sea.
The quantities which fall in particular showers are often very great,
and this aspect of rainfall also has its interest for us. About London
a fall exceeding an inch in 24 hours is comparatively rare, although
on August 1, 1846, 3.12 inches were collected in St. Paul’s Churchyard
in two hours and seventeen minutes.[11] On our west coasts this amount
is often exceeded. On October 24, 1849, 4.37 inches were collected at
Wastdale Head; June 30, 1881, 4.80 inches at Seathwaite; on April 13,
1878, 4.6 inches fell at Haverstock Hill, London; and a fall of 5.36
inches was recorded from Monmouthshire on the 14th July, 1875.
Taking averages of districts, we may give the following statistics,
referring, of course, to annual rainfall:--
Less than 25 inches = Essex, Suffolk, Norfolk, Cambridgeshire,
Huntingdonshire, Rutland, Middlesex, and parts of Surrey, Oxfordshire,
Buckinghamshire, Bedfordshire, Northamptonshire, Leicestershire,
Nottinghamshire, Lincolnshire, Yorkshire, and Durham. In other words,
with the exception of parts of the North and East Ridings of Yorkshire
and parts of Herts. and Bucks., which have a rainfall of from 25 to 30
inches, the eastern half of England, to the east of a line drawn from
Sunderland to Reading, and then eastwards to the mouth of the Thames,
has only a rainfall of 25 inches, or slightly less, per annum.
Between 30 and 40 inches = Practically the whole of the south coast
from Kent to Devonshire, the whole of Somerset, Wilts., and the west of
England generally, with the exceptions about to be noticed.
Between 40 and 50 inches = A great part of Devon and Cornwall, the
western half of Wales, with the exceptions presently to be given, a
great part of Lancs., and Cumberland.
Between 50 and 75 inches = A small patch in the centre of Devon,
a large strip in West Wales, and an enormous tract of country in
Cumberland, Westmorland, with Lancs. and north-west Yorks.
Above 75 inches = The wettest parts of the country. A small part of
Dartmoor, a region in Wales in the vicinity and to the south-east of
Snowdon, and the Lake District.
With reference to statistics concerning rainfall, it should be borne
in mind that those relating to special districts, especially to hilly
parts of the country, are often very deceptive, and require careful
local study. A slight difference in the physical features of a locality
is often sufficient to lead to considerable variation--the proximity
of a conical hill rising from the plain, the sudden convergence of the
two sides of a valley, or, conversely, the widening of a valley into a
flat stretch of land, all materially affect the local distribution of
rain. A clump of trees situated in proximity to a house will frequently
be the means of a downpour that would otherwise have passed over.
With winding valleys great latitude must be allowed. Then, again,
the geological structure of the locality is an important factor in
determining the amount of moisture delivered at a given spot. Where we
find a thick clay cropping out in the bottom of a valley, with more
or less porous rocks rising on either side of it, we soon ascertain
that the houses on the clay receive more moisture (or the latter is
distributed over a longer period) than those edifices on the hill sides
in the same district.
Our readers could no doubt give us plenty of instances where in a
circumscribed area their bricks have behaved very erratically--the
bricks of a house in one part of the district weathering well, and
in another badly. That may often be due, not only to the actual
distribution of the rain, but to the manner in which the rain or dew
has fallen. If an inch of rain falls in the neighbourhood in one day,
that would not tend to weather the bricks so vigorously as though the
fall had been spread over, say, a week.
A very important aspect of the subject is that which deals with the
“efflorescence” on bricks. This appears to be greatly misunderstood,
being commonly assumed to be due to one set of circumstances
rather than to the conspiracy of several. There are many kinds of
efflorescence, and an explanation of one of them obviously will not
apply to all. The “scum” that appears on the surface of bricks is,
however, to some extent bound up in the composition of the rain in the
particular locality where it occurs. Examined attentively, the commoner
kinds of efflorescence are seen to be minute white and yellowish-white
crystals. The substance of which these are formed has been drawn out
of the brick, or the mortar, or both, and rain has been the principal
agent in accomplishing this work, though its power in that respect must
necessarily vary according to the chemical composition and structure
of the brick or mortar, as compared with the nature of impurities in
the rain. If some substance were present in the rain that could readily
form an alliance with an ingredient of the brick, and the union was
capable of crystallising out, the surface of the brick would naturally
form a convenient spot for the crystallisation to take place. To
prevent it, we ought to know the composition of the air at the spot
where the house is to be erected, and also the chemical and physical
structure of the brick to be employed. That is rather too much to
expect from the manufacturer and architect; but there is a method--we
will not say an infallible one--which may be adopted to get rid of that
particular kind of scum. That method could not always be adopted, as
will be seen. The bricks must be burned more thoroughly, and at a high
temperature; that would lead in most cases to the active employment of
practically all the ingredients of which the bricks are composed, and
the impurities in the rain would, in consequence, stand less chance
of successfully inducing some of them to break their allegiance.
In practice, however, we believe it would be found that the high
temperature requisite to bring about the result just stated would
either tend to spoil the colour of the brick or partially melt it. The
latter could be prevented with due care, but we are afraid the former
could not be so easily dealt with, with the majority of brick-earths.
And if the brick is to be permanently discoloured to prevent
efflorescence, it is better to permit the latter to manifest itself.
The life of the “scum” is very variable; sometimes, after having once
appeared and disappeared, it will never come again. The passing shower
may wash it off (though it is not always so easily removed), and it may
come again and again for years. It behaves very erratically. The amount
of the efflorescence may be such as, in course of time, to lead to the
surface of the brick “bursting” and peeling off, or, on the other hand,
it may be a mere film.
There is one thing in connexion with efflorescence which cannot be
overlooked in regarding its practical effects in the building. In ever
so many cases we find that the scum, or the major part of it, is only
to be found in the neighbourhood of the mortar joints. That is a matter
of direct observation, and we have taken some considerable trouble to
verify it, as it has always been regarded as a point whereon to hinge
a debate. We do not say that in all cases the efflorescence appears
only in the position on the brick just indicated; but it unquestionably
does so in too many instances to enable us to regard its occurrence as
mere accident. Taking a large surface of brickwork just commencing to
show efflorescence, we find that the vicinity of the mortar joints are
the first places, in very many instances, where the nuisance begins to
manifest itself. From thence it spreads over the surface of the brick
until the whole is more or less discoloured.
It seems impossible to deny that the mortar is guilty, to some extent,
in such cases. At the same time, we must confess that we have never
seen the efflorescence spreading over the mortar. It would appear that
something in the mortar enters into chemical alliance with certain
ingredients of the brick, and that neither without the other could
produce the phenomenon alluded to. The remedy suggesting itself most
readily is to chemically analyse the efflorescence, the brick, and the
mortar; supplementing the experiments with a micro-examination to see
how far it is possible to locate the deleterious substances found to
exist, so that they may be removed in the manufacture of the materials,
if that is possible. But information on that head is of the scantiest
description, and much more will have to be done before the question is
definitely settled.
Another kind of “efflorescence” that often appears on bricks in
damp situations is mere vegetable growth, which bears a superficial
resemblance to the crystalline “scum” just described, though it can,
of course, be easily differentiated on examination with a lens. The
damp atmosphere is no doubt largely responsible for this, though
ineffectual damp-courses are contributors. The remedy lies in having a
less absorbent brick--one that will not afford ready foothold to the
vegetation.
The influence of rain on the weathering of bricks may be considered
from yet another standpoint. Where the brick is fairly porous, its
durability is liable to be materially influenced through the agency
of successive frosts. The water finds its way a short distance into
the brick and saturates it. During frost the water is turned into ice
at and near the surface of the brick. In forming, the ice exerts
considerable expansive force, which forces asunder the particles
(sand-grains and the like) of which the brick is composed--that is
to say, near the surface of the brick. The accumulated effects of
successive frosts in this way tends to weather the brick by breaking
up its exposed surfaces. To be materially affected, however, the brick
would have to be of very poor quality, and it will be seen that the
presence of cracks would much facilitate the operation.
The style of a building, the manner of its construction, and especially
the class of metals used for exterior decoration, all assist rain in
its work. A projecting course will have its upper surface washed clean,
whilst the underside remains very dirty--in cities, becoming quite
black. The limit of this dark discolouration is often frayed out by
the irregular action of the rain dripping from the projecting ledge,
assisted by the wind. Where the projection is so designed that the
rain is induced to drain to one point, and then to fall over on to the
wall, an unsightly streak down the latter is the result. The free use
of metal ornaments, railings, for supporting signs, for down-pipes,
&c., is unfortunate in not a few instances. At the point of junction
between the metallic substance and the brick into which it is inserted,
or in the immediate neighbourhood above which it is fastened, the
brickwork is sure to be discoloured. This may arise from the dripping
of rain-water from the metal, or it may be from the decomposition of
the latter, or from both. Iron rust leads to brown streaks, zinc-compo.
to dirty red, and so on.
The action of the wind as affecting the durability of bricks is
sufficiently important to warrant passing allusion. It drives rain and
its deleterious acids farther into the brick than the moisture would
soak in the ordinary way. It leads to wet walls interiorly, unless the
latter are so constructed as to overcome the effects. On the other
hand, a gentle breeze dries moisture on the face of the brickwork. In
cities, wind indirectly assists rain and its impurities by blowing
organic matter from the streets into niches and corners, where it
lodges, and, decomposing, provides powerful acids capable of doing much
work. Discolouration is the chief effect produced on the average brick
through this medium. In certain countries, wind, by driving dust, sand,
&c., acts as a species of sand blast.
Considerable diurnal variations in temperature are known to be
peculiarly destructive to certain kinds of brick and terra-cotta work.
Very porous bricks are not much affected, but the more compact kinds,
and especially terra-cotta blocks, often suffer. These observations
do not so much apply to our own country as to warmer climates; though
we are not altogether without experience here. On being heated these
materials expand; when made loosely, as in rubbers and the like, the
effect of the expansion is not very manifest, because the motion
is absorbed, so to speak, by the brick itself. On the other hand,
increased compactness of the particles leads to a perceptible increase
in the size of the bricks, and when the sun has gone down contraction
takes place as the bricks are cooling. It often happens in hot climates
that the brick or terra-cotta block is unable to part with its heat as
rapidly as the surrounding air becomes cooler, although it tries hard
to do so, and this leads to corners of the brick being broken off, the
physical forces exerted during the struggle doing the damage.
A highly interesting case of the effects of temperature on terra-cotta
was detailed by Mr. T. Mellard Reade, C.E., F.G.S., a few years
ago.[12] He shews that the cumulative effect of small, but repeated
changes of temperature is very striking, and describes the lengthening
of a terra-cotta coping in that connexion. The coping in question,
which was freely exposed to the direct rays of the sun, consisted of
two courses of red Ruabon terra-cotta bricks set in cement upon a
fence wall, built with common bricks in mortar, a brick and a half
in thickness. The courses were level, but, in consequence of the
inclination of the road, the coping stepped down at intervals, so that
the undercourse of bricks of one length was just gripped and held in
position by the top course of the next length of coping. It will be
observed that that form of construction constituted, by liability
to lifting, a more delicate test than ordinarily of any increase of
length, that might take place in the coping. On subsequent examination
of the coping, the end position of one length, abutting against the
next length at the drop in the level, was found to be thrown up into an
arch-shape bend of about 6 feet span; the coping bricks being lifted
in the highest part one inch from their bed. There was a fracture at
the crown of the arch, and another at the foot or springing, but for
a distance of 30 feet the coping was practically one solid continuous
bar. A careful examination shewed that the coping had “grown” about a
quarter of an inch longer than when it was first set, and that this
lengthening, as shewn by movement on the corbel bricks which occur at
intervals, was evenly distributed along a length of 30 feet.
Mr. Mellard Reade tells us that this is by no means an isolated case.
In the neighbourhood of Blundellsands inspection of brick copings
shewed that it was quite a common feature, and he has noted several
instances in which the end brickwork and piers have been badly
fractured by the force of expansion. In a case where the coping was
of blue Staffordshire bricks, the top course in cement and the under
course in mortar, a change in length was clearly shewn by the coping
being lifted off the wall at each of the two ramps which exist in its
length, and the movement was readily measured on the corbel bricks as
in the case previously detailed. In this case the lengthening was also
a quarter of an inch, and was evenly distributed over a considerable
length of coping.
Whilst speaking of changes of temperature in their effect on bricks, we
may allude to the behaviour of the material in severe conflagrations.
A general rule cannot be laid down, because it is customary now-a-days
to use fire-bricks for ordinary building purposes which will withstand
practically any heat to which they may be subjected. Leaving them out
of the question, and referring to ordinary bricks, it may be said
that those of an inferior class frequently become cracked all over
during a fire, or, it may be, by the sudden cooling after the fire has
been put out, or by the sudden lowering of the temperature in them
by the continuous action of the fireman’s hose. All the same, the
average brick withstands heat far better than any kind of granite, or
similar igneous holo-crystalline rock; loosely compacted sandstones
and limestones crumble up on the surface, or flake, or may be utterly
destroyed when subjected to a conflagration that would not have the
slightest effect on bricks.