Means For Testing Tensile Strength Of Cement

It has already-been mentioned that the tensile strength of Portland cement for important works should always be tested by direct experiment.

There are several different machines by means of which this test can be accurately applied.

A few of these will now be described.

Adie's Testing Machines were among the first adopted for this purpose, and are still widely known and extensively used.

Fig. 88. Adie's No. 1 Cement  Testing Machine.

Fig. 88. Adie's No. 1 Cement- Testing Machine.

Adie's No. 1 Machine. - This machine, by means of a straight lever, applies a known strain to a briquette of cement (see p. 172), until the latter breaks across at the narrow central part, the area of which is accurately known.

Fig. 88 gives an elevation of the machine.

Fig. 88a is a split mould for the briquette. It is arranged to divide longitudinally into two parts, so that the briquette may easily be liberated without the aid of a press. Split moulds are sometimes hinged at one end.

These figures and the following instructions are from the circular of the maker (Mr. Adie, 15 Pall Mall).

Maker's Instructions

To set up the machine, drop the spindle K into its place in the table, then put the pillar G in position and insert the notched plate in groove of spindle, bolting down G so that the beam when strained [by putting a moulded brick of cement into the clips B and C, and then tightening by means of the wheel R] may take its position freely in the centre of fork H. The wire cord passes twice round pulley at H, and once round that at G, and should never be very tight. When the cement is set, open the mould carefully, undoing both screws simultaneously, and treat the briquette as described at p. 172.

Fig. 88a. Split Mould.

Fig. 88a. Split Mould.

Insert the brick in the clips B and C, then turn the wheel R till the beam at H rises to the pulley [i.e. well above the zero line], and roll the weight D gently along until the fracture takes place.

The weight N should not be in its place, except when testing below 300 lbs., when the top row of figures is used.

If the weight D is not sufficient to break the brick, roll it back and hang on the extra weight in a notch in the beam near H.

The speed with which the weight is brought to bear upon the briquette, or at which it is increased, materially affects the result. When the weight is moved rapidly the resulting tensile strength, as shown by the machine, is higher than when the weight is wound slowly.

Adie's Cement Tester with Automatic Kegister. - In order to arrange for a uniform speed, Mr. Adie has added an automatic regulator to his machine, which is shown in Fig. 88b.

Fig. 88b. Cement Tester, with Patent Automatic Regulator.

Fig. 88b. Cement Tester, with Patent Automatic Regulator.

Patentee's Instructions

To make use of this regulator, fix the brass tube to the floor, vertically underneath the pulley marked No. 2 in the drawing above, and fill the tube with clean water. Attach the long cord to the left side of the vernier E, pass it downwards over No. 4 round No. 3 and then upwards round No. 2, and down again to the eye at 3. Attach the short cord to the right side of E, pass it under the pulley at H and twice round it, then along over No. 5 and hang on the weight W.

Put the brick into its place and make all arrangements before actually putting on the strain. Screw up R, lifting the point of the beam well above the zero line on the pillar H, so as to free the check K, and while the weight D is travelling observe that the check is not allowed to touch the rachet wheel; screw up R more if necessary to allow for any slip or springing of the clips. The catch stops the rolling weight when the fracture takes place, and the result is shown in lbs. on the scale.

To commence another test take off the weight W, before gently letting down the piston, at the same time easing back the weight D by the hand.

Adie's No. 2 Testing Machine. - Fig. 89 is a smaller machine by Mr. Adie for testing briquettes having a central section of 1 inch square. It can also be used as a weighing machine.

This figure, slightly modified, and the following instructions, are from the maker's circular.

If the standard A be not in its place when received from the manufacturer, bolt it down to the stand, so that the beam when strained (by putting a moulded brick of cement into the clips B and c and then tightening by means of the wheel underneath at d) may take its position freely in the centre of fork e. Wind the cord once round the pulley at a, and twice round that at E. Mix the cement to be tested with as little water as possible, consistent with perfect homogeneity, and having laid the mould on a flat surface, or on the iron plate supplied for the purpose, fill it with the cement as described, and scrape the top flush. When set, take the briquette out of the mould carefully, and place it on the flat plate in water for seven days, it will then be ready for testing. To effect this, place it in the clips, turn the wheel at D till they clasp the brick with sufficient force to raise the end of the beam nearly up to the pulley above.

(a) For strains from 0 to 130 lbs., using the bottom row of figures, hang the transferable weight F in the notch at the end of the beam (as shown in the drawing), and roll the vernier weight g along, taking about one minute to travel the length of the beam.

(b) From 90 to 200 lbs., using the middle row of figures, remove f from the machine, and roll G forward as before.

(c) Above 150 lbs., using the top row of figures, hang on F in the notch under G, and roll G (carrying f with it) forward as before.

To use as a weighing machine, remove the sliding block carrying D and c; take out the top clip B, and hang on the scale pan instead. A hook passing through a hole in the stand can be supplied with one-tenth bushel measure to weigh the cement if required.

Michaelis's Double Lever Cement Testing Apparatus, Figs. 90 and 90a. - Maker's Directions. - This apparatus consists of a japanned cast iron column, which carries two levers, the combined leverage of which is 1 to 50 : that of the longer, being 1 to 10, and that of the shorter, 1 to 5.

Fig. 89. Adie's No. 2 Cement Testing Machine.

Fig. 89. Adie's No. 2 Cement Testing Machine.

Fig. 90. Fig. 90a. Michaelis's Double Lever Cement Testing Apparatus.

Fig. 90. Fig. 90a. Michaelis's Double Lever Cement Testing Apparatus.

Each lever has three hardened steel knife edges acting upon hardened steel concave bearings, so that an extremely accurate balance is obtained.

The short arm of the upper lever is provided with a movable counterpoise, to secure the correct position of the levers, which is indicated by a mark on the upright catch at the top of the column. At the extremity of the long arm is suspended a small brass frame to carry the shot bucket.

On the lower lever, near its fulcrum, is suspended the upper clamp or clip for holding the briquettes. The lower clamp is fixed to the base of the column and adjusted by means of a screw.

To make a test, the cement briquette is taken out of the water, dried and put into the clamps, which must be accurately applied to the sides of the briquette, and the screw applied until the upper edge of the long lever is opposite the mark on the upright catch. Fine shot is then poured from the self-acting shot-run into the bucket suspended from the long lever until the briquette breaks, when the supply of shot is instantly cut off. The breaking strain per square inch is thus exactly fifty times the weight of the bucket and shot; but to avoid all calculation and possible risk of error, a Salter spring balance with a special dial is supplied, upon which the bucket and shot are weighed and the exact breaking strain of the briquette at once shown.