STRENGTH OF BRICKS.
A very great deal is known concerning the strength of bricks. In
addition to the innumerable experiments carried out by public bodies,
we have the results of painstaking investigation by professors in
universities and colleges, and the results carried out for and
published by brickmakers themselves. Yet another large series of
results have been published from time to time by professional
journals, and it is, indeed, to these that we must look (at any
rate in Britain) for anything like detailed work. The “Minutes of
Proceedings of the Institution of Civil Engineers,” the “Transactions
of the Royal Institute of British Architects,” the “Proceedings” of
several allied provincial architectural societies, the “Builder,” the
“British Clayworker,” builders’ “Price Books,” and several engineering
“Handbooks,” have all contributed to our knowledge in regard to the
strength of bricks. Of works consecrated entirely to the subject there
are none--applied to British materials; but we have that excellent
text-book by Professor Unwin, F.R.S., “The Testing of Materials of
Construction,” and the important work by Mr. David Kirkaldy, both of
the greatest possible value as being the results, largely, of original
work. The experiments of recent years have been made almost exclusively
by Mr. David Kirkaldy at his works in Southwark; by Professor W. C.
Unwin at the Central Institution of the City and Guilds of London
Institute; and by the Yorkshire College, Leeds.
With such a wealth of information a whole treatise might profitably
be written, but it will be understood that in a small work like the
present we can only give a comparatively few results, prefaced by
observations to impart a general idea.
With the strength of brickwork, it is different, and it would seem
rather remarkable, at first sight, that architects and engineers, who
are every day using thousands of bricks, should have been at little
pains to ascertain the “safe load” which this or that brick pier or
wall would carry. Experience is, of course, of great value in all work
of that description; but there is always the lurking suspicion that the
engineer is making his piers too big, and that the architect is by no
means running the thing close. The real reason why so little has been
done to test the strength of brickwork is the difficulty in getting
machines of such capacity as would crush sufficiently large masses.
Small piers have been built from time to time, and bricks embedded in
putty for mortar have served their purpose, but practically nothing
of a really serious nature was carried out in Britain until a few
months ago. The science committee of the Institute of Architects, well
knowing the advantage of information as to the strength of brickwork,
have partially carried out a most elaborate series of experiments,
the first fruits of which have already been published, but it would
be out of place to allude to them here. When the remaining brickwork
shall have been built long enough at the experimental station, the
final experiments will be made, and the results will, we have no doubt,
be the most important contribution to our knowledge concerning the
strength of brickwork that has ever been published in the kingdom.
But we must give our attention solely to the strength of bricks. To
begin with, we must deprecate the idea that experiments as at present
carried out give anything like the actual strength of bricks--the
results are generally either too high or too low. Neither are the
results comparative, except to a limited extent. One kind of brick has
a “frog” on one side, another is recessed on both sides, a third is
stamped with the maker’s name, or some device by way of trade mark,
a fourth is as flat on all sides as may be, a fifth is pressed, a
sixth is hand made, and a seventh wire-cut, and there are many other
varieties of make. With such different kinds it is next to impossible
to arrive at comparative data that shall be of much use for working
purposes. Again, the whole brick may be subject to the experiment, or
only the half-brick. The faces placed between the dies of the crushing
machine may not be flat, and they are most frequently irregular. If
the dies are applied to such bricks it is evident that corners will
be broken off before the brick has really suffered much, and that to
get the best result the faces must either be made perfectly true and
parallel to each other, or some other method adopted to put matters
right. That commonly employed is to place some yielding substance
between the faces and the surface of the dies. Sometimes thin sheets
of lead or pine wood are inserted. Professor Unwin has the faces of
the brick made smooth and parallel by means of plaster of Paris, and
the brick is then crushed between two pieces of millboard or between
the iron pressure-plates, one plate having an arrangement to allow for
any slight want of parallelism between the two surfaces of the brick
applied to the plates.
Now it will be obvious, what with the difference in the shape and the
various modes of experimenting, that the results are by no means
comparative unless the precise facts are given; and when they are, it
is but rarely that you can find more than half-a-dozen or so kinds
of bricks of each category that offer all the elements necessary for
comparison. So that, with all the wealth of information, we are by no
means laden with much that is of actual comparative value, and if the
experiments and their results are not comparative, of what use are
they? So long as experimenters are each allowed a different method of
research, and so long as makers will have partial or whole “frogs,”
will stamp their names or initials, or will produce plain bricks only,
so long will it be impossible to arrive at the best results that are
really attainable. What we want is a government testing station as
they have in Germany; or, at least, the mode of experimenting should
be under some central control. The experimenter, further, should
select the samples to be crushed, and should be at liberty to publish
all results obtained. At present, if the brickmaker does not like
the results arrived at, he, of course, does not publish them. And,
if he has had a number of experiments carried out from time to time,
he will, usually, quote only the highest results on his bricks. That
is perfectly natural, and would be understood as “business.” All
brickmakers may not do that, and a few may publish every or average
results (we do not mean of one set of experiments, on say six bricks)
of different experiments, but we fancy they are very rare. Therefore,
in a matter so important to the architect and the engineer, and indeed
to the general public, from the point of view of safety, we maintain
that the whole thing should be carried out under some central control,
as on the continent.
And now to proceed with the description of results on a few typical
bricks. Glancing at table I, we may say that the strength of bricks
as a whole is often quoted as here given, and has done duty for many
years as the average strength of bricks. These bricks were crushed in a
Clayton machine, and all were bedded upon a thickness of felt and laid
upon an iron faced plate, and the experiments were conducted by the
Metropolitan Board of Works.
STRENGTH OF BRICKS.--I.
-------------------------------------------------
Description. | Pressure in tons to
+----------+---------
| Crack. | Crush.
----------------------------+----------+---------
Four white bricks, each | 16.25 | 41.00
Three „ „ „ | 17.05 | 41.05
Red bricks, ordinary | 13.00 | 26.25
Red bricks, not well burned | 13.75 | 25.05
Best Paviours | 14.00 | 23.00
Grey Stocks, London | 12.00 | 14.00
----------------------------+----------+---------
Turning to the second table, compiled for the most part from
brickmakers’ circulars, and from the original results obtained for
the late Building Exhibition, at the Agricultural Hall, all the
experiments, we believe, having been carried out by Mr. David Kirkaldy,
it will be noted that great variation in strength is apparent,
following the different kinds of bricks. The highest result, 1064.2
tons per square foot, was obtained on a blue Staffordshire brick,
though that is very closely run by bricks made from slate débris
(1056.2 tons) from South Wales. The lowest result, 139.5 tons per
square foot, was from a Worcester brick.
STRENGTH OF BRICKS.--II.
-------------------+----------------+--------------------+------------------
| | | Mean stress of
| | Dimensions, | six samples in
Locality. | Description. | Inches. |tons per square ft
| | +---------+--------
| | | Cracked | Crushed
-------------------+----------------+--------------------+---------+--------
West Bromwich | Blue | 2.74, 9.03 × 4.36 | 548.6 | 1064.2
| | | |
„ „ | Blue (another |} 2.80, 8.75 × 4.12 | 260.7 | 651.0
| make) |} | |
| | | |
„ „ | White glazed, |} | |
| “Terra |} | |
| Metallic,” |} 3.10, 8.80 × 4.22}| 225.0 | 273.7
| recessed |} 3.16, 8.70 × 4.34}| |
| both sides |} | |
| | | |
„ „ | Blue vitrified | 2.55, 9.03 × 4.30 | 245.1 | 654.9
| | | |
{| “Pressed,” |} | |
Worcester {| recessed top |} 3.20, 9.14 × 4.50 | 65.0 | 139.5
{| and bottom |} | |
| | | |
„ | “Builders.” |} | |
| recessed top |} 3.20, 9.30 × 4.50 | 56.1 | 155.5
| and bottom |} | |
| | | |
Saltley, | Red, recessed |} 3.20, 8.90 × 4.35}| 138.7 | 180.5
Birmingham | one side |} 3.25, 8.95 × 4.40}| |
| | | |
Rowley Regis, | Blue vitrified |} 2.85, 8.75 × 4.20 | 385.6 | 722.7
Staffs. | no recess |} | |
| | | |
Leicester | Red, recessed |} 2.65, 8.90 × 4.25}| 105.9 | 150.6
| both sides |} 2.75, 9.10 × 4.36}| |
| | | |
Napton-on-the-Hill,| Light brown, |} 2.85, 8.92 × 4.20}| 131.6 | 303.9
Rugby | wire cut |} 2.90, 9.10 × 4.25}| |
| | | |
Ruabon | Red, no |} 3.10, 8.75 × 4.28}| 439.2 | 676.8
| recess |} 3.15, 8.73 × 4.29}| |
| | | |
„ | Blue, no |} 3.02, 8.99 × 4.37}| 358.9 | 561.2
| recess |} 3.01, 8.95 × 4.36}| |
| | | |
Glogue, Whitland, | Slate débris | 2.33, 8.70 × 4.25 | 556.4 | 1056.2
S. Wales | | | |
| | | |
Ravenhead, St. | Red, brown |} 2.90, 9.00 × 4.20}| 215.8 | 354.7
Helens, Lancs. | wire cut |} 2.90, 8.90 × 4.27}| |
| | | |
Earith, St. Ives, | Yellow, wire |} 2.50, 8.70 × 4.10}| 135.9 | 178.8
Hunts. | cut |} 2.50, 8.80 × 4.20}| |
| | | |
Gillingham, Dorset | Red, wire cut |} 2.60, 8.90 × 4.30}| 159.5 | 261.7
| |} 2.60, 8.90 × 4.25}| |
| | | |
Newton Abbot, | Vitrified |} 2.80, 8.90 × 4.35}| -- | 445.2
Devon | “granite” |} 2.80, 9.10 × 4.55}| |
-------------------+----------------+--------------------+---------+--------
Table III. is by Professor Unwin,[18] and records the strength of
several well-known bricks. Professor Unwin’s mode of experimenting we
have already alluded to.
STRENGTH OF BRICKS.--III.
---------------+----------------+-----------+----------+--------+------------
| |Cracked, at|Crushed at| |
Description. | Dimensions. | tons per | tons per | Colour.| Remarks.
| Inches. | sq. ft. | sq. ft. | |
---------------+----------------+-----------+----------+--------+------------
London stock |4.6 × 4.1 × 2.4 | 128 | 177 | Yellow |Half brick
„ „ |4.6 × 4.0 × 2.45| 133 | 181 | „ | „
„ „ |9.2 × 4.1 × 2.8 | -- | 129 | „ |
„ „ |8.9 × 4.2 × 2.3 | -- | 113 | „ |
„ „ |8.9 × 4.25 × 2.5| -- | 103 | „ |
Aylesford, |8.9 × 4.4 × 2.7 | 48 | 183 | Pink |
common | | | | |
„ „ |8.9 × 4.4 × 2.7 | 111 | 228 | „ |
„ pressed|9.1 × 4.3 × 2.7 | 71 | 141 | Red |Deep frog
Rugby, common |9.5 × 4.2 × 2.9 | 158 | 190 | „ |{ Between }
„ „ |9.0 × 4.2 × 3.0 | -- | 120 | „ |{ pine bds.}
Lodge Colliery,| | | | |
Notts |9.0 × 4.2 × 3.4 | 127 | 159 | „ |
„ „ |9.0 × 4.2 × 3.25| 55 | 122 | „ |
Digby Colliery,| | | | |
Notts |9.3 × 4.1 × 3.25| 248 | [353] | „ |Not crushed
„ „ |4.6 × 4.2 × 3.2 | 414 | 414 | „ |Half brick
Ruabon, pressed|8.8 × 4.3 × 2.7 | 361 | [361] | „ |Not crushed
Grantham, wire |9.2 × 4.4 × 3.2 | -- | 83 | „ |
cut | | | | |
Leicester, |4.4 × 4.1 × 2.6 | 251 | 337 |Pale red|Half brick
„ „ „ |4.3 × 4.1 × 2.6 | 109 | 308 | „ | „
„ „ „ |9.06 × 4.2 × 2.8| 115 | 229 | „ |
Cranleigh, |4.7 × 4.6 × 2.5 | 149 | 181 | „ |Half brick
pressed | | | | | frog.
„ „ |4.6 × 4.6 × 2.5 | 165 | 237 | „ |„ „ „
Candy, pressed |8.8 × 4.3 × 2.8 | 80 | 381 | -- |
Gault, wire cut|8.7 × 4.1 × 3.0 | 111 | 173 | White |
„ „ |4.4 × 4.2 × 2.5 | 119 | 145 | „ |Half brick
„ „ |8.7 × 4.1 × 2.9 | -- | 169 | „ |
Staffordshire | | | | |
blue, common |4.5 × 4.3 × 3.0 | 216 | 464 | Blue | „
„ „ „ |4.3 × 4.2 × 3.0 | 152 | 386 | „ | „
„ „ „ |8.9 × 4.3 × 3.1 | 240 | [353] | „ |Not crushed
Staffordshire | | | | |
blue, pressed|9.0 × 4.3 × 3.1 | -- | 275 | „ |
Glazed brick |8.8 × 4.4 × 3.3 | 69 | 166 | -- |Frog.
„ „ |8.9 × 4.4 × 2.9 | 166 | 174 | -- |
---------------+----------------+-----------+----------+--------+------------
Table No. III. is specially instructive as indicating the relative
strength of several well-known bricks, the experiments being carried
out solely for scientific purposes. Yet the figures must not be taken
too seriously. Glancing at those relating to “London Stocks,” we find
the strength varied from 103 tons per square foot to 181 tons. But
more recent experiments made by Professor Unwin[19] on some London
Stocks from Sittingbourne, in Kent, shewed that with four samples one
crushed at 60.76 tons per square foot and another gave out 94.6 tons,
the mean strength of the four yielding 84.27 tons per square foot.
With such heterogeneous materials as London Stocks, we ought not to be
surprised at these results, but they form a striking commentary on the
value of general statements concerning the strength of bricks of varied
character going by the same name in the market.
When we consider the strength of homogeneous bricks, and especially
where these latter are made of thick marine clays, or where the
relative proportions of earths employed are carefully attended
to in the raw material, the results appear to be more generally
applicable--as far as they go.
With ordinary Gault bricks we find a range in strength from 145 tons to
173 tons per square foot; but Professor Unwin,[20] in his more recent
experiments, finds that of four Gault bricks, one reached as high as
197.6 tons per square foot, and he gives 182.2 tons as the average
strength.
To shew the absurdity of alluding to the strength of “blue
Staffordshire” bricks, without also giving the precise locale of the
samples dealt with, the reader is requested to refer to Table III.,
where the figures indicate a range from 275 tons to 464 tons per square
foot, and to compare them with the results on Staffordshire bricks as
stated in Table II., where we find a range from 651 tons to 1,064.2
tons per square foot. Of what value can a single formula be which gives
the strength of Staffordshire bricks as a whole as based on such widely
divergent figures as these? Professor Unwin, in his recent series of
experiments alluded to, finds that with four Staffordshire blue bricks,
the weakest gave a result of 564.8 tons per square foot, and the
strongest 788 tons; the mean of the four being 701.1 tons per square
foot.
The results on the Leicester “reds” are no more encouraging; the
figures in the foregoing tables are 150.6 tons, 229 tons, 308 tons, and
337 tons per square foot. Similarly, Professor Unwin has more recently
found that the Leicester “reds” from Elliston, near Leicester, bear a
crushing strain varying from 311.4 tons to 591.4 tons per square foot
in four samples.
From the foregoing it will appear to the reader that average results
are of very little value to the architect or engineer, unless--(1)
the brickyard is mentioned from which the bricks experimented with
came; (2) the particular class of brick from that yard; (3) the method
of experimenting, as to whether any substance was placed between the
dies of the press and the brick to be crushed, and if so, what; (4) if
recessed or initialled; (5) whether machine or hand made, and (6) as to
whether the surfaces of the bricks were concave, convex, or flat.
Results on bricks not localised are not of much value, and it is
absolutely useless for working purposes to give in one figure the
strength of “London Stocks,” “Staffordshire blues,” “Leicestershire
reds,” and the like. In a general way, of course, it will be admitted
that the “Staffordshire blue” is a stronger brick than the “London
Stock,” and so forth; but that is as much as can be permitted--it is of
no practical use to give relative figures in general terms.
It frequently happens that the capacity of the machine used for testing
the strength of bricks is not enough for those bricks having a very
high resistance to crushing. In the recent experiments by Professor
Unwin, more than once alluded to in this article, it was found
necessary to experiment with half-bricks only, and he ascertained that
bricks tested as half-bricks shew about 25 per cent. less resistance
per square foot than when tested as whole bricks.
Further observations on strength are made under the next heading in
connexion with other forms of testing the value and physical properties
of bricks.