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.
137 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 strength138 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,139 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 on140 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.
141
strength of bricks.—ii.
locality. description. dimensions,
inches. mean stress of
six samples in
tons per square ft
cracked crushed
west bromwich blue 2.74, 9.03 × 4.36 548.6 1064.2
? ? blue (another make) 2.80, 8.75 × 4.12 260.7 651.0
? ? white glazed, “terra
metallic,” recessed
both sides 3.10, 8.80 × 4.22
3.16, 8.70 × 4.34 } 225.0 273.7
? ? blue vitrified 2.55, 9.03 × 4.30 245.1 654.9
worcester “pressed,” recessed
top and bottom 3.20, 9.14 × 4.50 65.0 139.5
? “builders.”
recessed top
and bottom 3.20, 9.30 × 4.50 56.1 155.5
saltley, birmingham red, recessed
one side 3.20, 8.90 × 4.35
3.25, 8.95 × 4.40 } 138.7 180.5
rowley regis, staffs. blue vitrified
no recess 2.85, 8.75 × 4.20 385.6 722.7
leicester red, recessed
both sides 2.65, 8.90 × 4.25
2.75, 9.10 × 4.36 } 105.9 150.6
napton-on-the-hill,
rugby light brown,
wire cut 2.85, 8.92 × 4.20
2.90, 9.10 × 4.25 } 131.6 303.9
ruabon red, no recess 3.10, 8.75 × 4.28
3.15, 8.73 × 4.29 } 439.2 676.8
? blue, no recess 3.02, 8.99 × 4.37
3.01, 8.95 × 4.36 } 358.9 561.2
glogue, whitland,
s. wales slate débris 2.33, 8.70 × 4.25 556.4 1056.2
ravenhead, st.
helens, lancs. red, brown
wire cut 2.90, 9.00 × 4.20
2.90, 8.90 × 4.27 } 215.8 354.7
earith, st. ives,
hunts. yellow, wire cut 2.50, 8.70 × 4.10
2.50, 8.80 × 4.20 } 135.9 178.8
gillingham, dorset red, wire cut 2.60, 8.90 × 4.30
2.60, 8.90 × 4.25 } 159.5 261.7
newton abbot, devon vitrified “granite” 2.80, 8.90 × 4.35
2.80, 9.10 × 4.55 } — 445.2
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.
142
strength of bricks.—iii.
description. dimensions.
inches. cracked,
at tons
per sq. ft. crushed
at tons
per sq. ft. colour. remarks.
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, common 8.9 × 4.4 × 2.7 48 183 pink
? ? 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 cut 9.2 × 4.4 × 3.2 — 83 ?
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, pressed 4.7 × 4.6 × 2.5 149 181 ? half brick 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 unwin19 on some london stocks from sittingbourne, in kent, shewed that with four samples one crushed at143 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, the144 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, more145 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.
