It is a curious thing that though many engineers specify the number of meshes to the square inch in the sieves to be used, very few mention the gauge of the wire of which the sieves are to be made, although it is manifest that the size of the orifice of the mesh in the sieve must depend upon the thickness of the wire; two sieves with the same number of meshes to the inch, but of different gauges of wire, must pass cements quite different as to fineness.
It is therefore necessary to state the gauge of the wire to be used, and as uniformity in this is desirable, the author has obtained from Messrs. Currie and Co. of Leith a list of gauges appropriate for sieves of the meshes stated below.
1 M.P.I.C.E 1880, vol. lxii. p. 242.
Meshes per sq. inch.
Gauge of Wire. BWG.
Gauge of Wire BWG.
This particular is generally carefully ascertained. It used to be considered that a good weight per bushel was a sign of thorough burning, but it is now realised that the weight is greatly influenced by the degree of fineness to which the cement is ground, upon the degree to which it has been aerated, and upon the way in which the measure is filled.
The weight is, however, generally specified in connection with the degree of fineness required.
The weight of the Portland cement in the market varies from 95 lbs. to about 120 lbs. per striked bushel.
A heavy cement is likely to be thoroughly burnt throughout, but care must be taken to ascertain that its weight is not caused by its containing a large proportion of coarse unground particles.
In some cases a heavy cement contains a large proportion of over-burnt particles, and unless these are most carefully ground to a fine powder, they slake very slowly, frequently not till they have been used in the work, in which case they cause serious injury.
In very heavy cements there is some danger of an excess of lime, which, if left in a free state, that is uncombined with the silicic acid of the clay, is liable to cause disintegration in the work. It also renders the cement unfit for the joints of sewers,2 or for any position where it would be liable to the attacks of chemical agents, which would destroy the carbonate of lime.
As above mentioned, the weight of a given bulk of cement depends to a great extent upon the fineness of its grit; a coarse cement is heavier than one equally well burnt which is finely ground.
1 The experiments of Messrs. Grant, Colson, and Mann show that this rule does not always hold good.
2 Mr. Baldwin Latham, Min. Proc. Inst. Civ. Eng., vol. xxxii. p. 63.
The weight depends also upon the amount of aeration the cement has received, sometimes after weighing one bushel it will be found that the next bushel weighs 1 lb. or 11/2 lbs. less. In testing the weight of large quantities, therefore, the sample bushels should be taken from different parts of the heap.
Lastly, the weight depends upon the method in which the measure is filled - one sample must not be more tightly packed than another.
To ensure this the cement should be poured into the measure as described at p. 168.
The effect of fine grinding upon weight is shown in the following results, obtained by Messrs. Currie and Co. of Leith. The figures will, however, vary considerably with different cements.
Meshes per square inch of sieve.
Percentage retained by sieve.
Weight of Cement per bushel in lbs.
Weight of Cement per cubic foot in lbs.
It is evident from the above that the weight test ought never to be used without the sieve test or it would be a direct incentive to coarse grinding.
On the other hand, to use the sieve test only would lead to being supplied with light easily ground cements of no great tensile strength.
The practical difficulty, however, of accurately comparing the weights of cements makes the weight test unreliable, and engineers therefore sometimes require the cement to be of a given specific gravity, which cannot vary with the different degrees of fineness of grit.
The weight of the Portland cement originally used on the Metropolitan Main Drainage Works was specified to be at least 110 lbs. per striked bushel.
The cement actually supplied averaged 114-15 lbs. per bushel in weight.
The cement for the later series of experiments by Mr. Grant was specified to weigh. 112 lbs. per striked bushel. It weighed 113-2 lbs., including the coarser particles, but only that portion of it was used which passed through a mesh of 400 holes to the inch, and weighed 110-56 lbs. per bushel, as above stated.
In a few cases a cement weighing 123 lbs. per bushel was experimented upon. It does not seem desirable, as a rule, to specify a weight for cement of more than from 110 to 115 lbs. per bushel. Mr. Grant recommends that when a weight is specified it should not be more than 112 lbs. a bushel.
The weight is generally stated in lbs. per bushel; 21 bushels (each containing 1.283 cubic foot) make a cubic yard. Sometimes it is stated in lbs. per cubic foot.
A very heavy and strong cement is required in important engineering works; but for ordinary purposes of building great tensile strength is not of the first importance, and in some cases, e.g. for rendering walls, a lighter and more quickly-setting cement may be used with advantage.