Fig. 91 is a sketch taken from the illustrated advertisement of the machine.
The handle H is then turned; it is fitted with a pinion which works in the rack R. The end of the rack being drawn down by the motion of the pinion, draws down tliu clip L, and brings a stress upon the briquette, which in its turn draws down V and the short arm S of a bent lever.
Fig. 91. Michele's Cement Testing Machine.
While they rise, the leverage with which they act increases with their horizontal distance from the fulcrum F. When the stress produced is sufficient to overcome the resistance of the briquette it breaks across.
The nuts n which secure the clips prevent the weights WW from falling back more than about half an inch.
The stress applied is measured along the graduated arc A. The pointer p is carried up with the long arm of the lever as it rises, but remains when the weights fall, to show the point to which they rise.
These machines are made to test up to 1500 lbs. on the briquette.
Faija's Testing Machine is shown in Fig. 92, from the patentee's circular.
The machine is 14 inches high, 14 inches long, by 3 inches wide, and weighs less than 80 lbs. A special gearing prevents the strain from being put on too quickly.
On receiving the machine, clean off all old oil and relubricate, attach the balance weight W to the short end of the lever.
See that the quadrant A is in the position shown in sketch, so that the chain B to the dial C is slack, and the lever D free and balanced.
Turn the wheel E from right to left, until the lower clip F can be raised into contact with the upper clip G.
Insert the briquette to be tested in the clips, taking care that it is put in true and evenly, and so that the pull on it and the clips is true and vertical; then turn the wheel E from left to right, which will bring down the lower clip F, and secure the briquette firmly in the clips. (It is generally advisable to put such a strain on the briquette by turning wheel E that about 100 lbs. is indicated on the dial.) When in this position there should be about half an inch between the under side of knife edge H, and the buffer or recoil spring I. Having seen that the pinion K is in gear with the wheel L, turn the handle M until the briquette breaks. The loose pointer will show on the dial the strain in lbs. at which the briquette broke. To Eeturn to Zero. - Throw the pinion K out of gear with the wheel L by removing the pin and pushing it to the left; turn the wheel L from left to right until the quadrant A has returned to its normal position with the chain B slack; put the loose pointer back to zero; release the lower clip F by turning wheel E from right to left; remove the broken briquette, and insert the next that is to be broken.
Beid and Bailey's Cement>Tester is shown in elevation in Fig. 93, which is taken from the makers' circular.1
The briquette having been inserted in the clips c c holds down the short arm of a straight lever.
The long arm has a graduated measure attached to its end; this is gradually weighted by water running from the cistern above.
When the briquette breaks, the fall of the long arm of the lever draws down h, and shuts off the supply cock. The weight required to rupture the briquette is indicated by the amount of water in the measure. Mr. Reid states that this machine is reliable and accurate. The weight is applied very gradually and without tremulous vibration, and is recorded automatically by the machine itself.
Fig. 92. Faija's Testing Machine.
Fig. 93. Reid and Bailey's Cement Tester.
1 Messrs. Bailey and Co., Salford.
Bailey's Table Pattern Cement Tester. - Another of Messrs. Bailey and Co.'s cement testers is shown in Fig. 93a, which explains itself. It takes sections of 1 inch square, and is sometimes fitted with an automatic arrangement for poxiring the shot into the can.
In order to avoid the difficulty of getting the stress fairly distributed over the area to be fractured, which always occurs in tensile tests, cement has been tested by twisting.
Professor Thurston's machine used for testing metals by torsion has, in America, been applied to cements.
This machine has not, however, been adapted in this country, and it need not therefore be described.
A tank of water suspended from the specimen may be used as a good simple method of testing the tensile strength. The weight in lbs. at different depths can be marked inside the tank.
The following specification has been used where there is no testing machine available.
"The cement is to be made into small blocks 1 inch square, and 8 inches long, after being made, these blocks are to be immersed in water for seven days, and then tested by being placed on two supports 6 inches apart, when they must stand the transverse strain produced by a weight of 75 lbs. placed in the centre."1
The importance of having a chemical test for Portland cement, in addition to the tests already mentioned, has lately been strongly urged, in consequence of failures arising from an excess of magnesia which has slaked and expanded in the work, causing rupture; an excess of lime may have the same effect, or cause weakness;2 more than 2 per cent of magnesia, or 11/2 per cent of sulphuric acid, is said to be injurious to Portland cement.
It is stated in the circulars of some cement manufacturers that iron slag is used for the adulteration of Portland cement. If this is suspected, the only way to avoid it is to refuse to take cement from any manufacturer who has slag on his premises.
Slag has been refused both for cement making and as an aggregate for concrete, for fear that the lime that it contains should disintegrate after use in the work.
This is doubtless a wise precaution. Slag properly treated by being burnt with lime is, however, sometimes used as the basis of a good Portland cement, as described at p. 161.
1 Messrs. D. B. Stevenson. M.P.I.C.E., vol. lxxxvii. p. 229. 2 M.P.I.C.E, vol. lxxxvii. p. 163.