No. 20, taken from a paper by Prof. J. M. Porter of Lafayette College. Prof. Porter had ten samples taken from the same number of barrels of Portland cement, thoroughly mixed, and then divided into ten smaller portions which were sent to ten different. persons with a request that a seven-day tensile test, one cement to three sand, be made according to the standard of the American. Society of Civil Engineers.

These results would seem to indicate that such tests are of little value when a report from one laboratory would cause the cement to be rejected without hesitation under ordinary specifications, and as unhesitatingly accepted according to the report of another equally reliable, and when a special effort has been made to have all conditions as nearly alike as possible. This is hard to explain. But on account of these variations tests of cement must not be given up, but continued with more care, and perhaps on different lines.

It is rarely possible to give the cement used in any large and important work sufficient tests to demonstrate its absolute fitness. It must be done analogically. It is necessary, however, to find a brand of cement before the work is begun that either by experience or long-time tests has been proved to be all that is required. If the former, a series of tests should be made extending over a sufficient

period of time and comprising enough individual samples of the cement to establish a rigid standard for that particular brand. It should include neat tests and also those mixed with every proportion of sand that is liable to be used on the work, to ascertain as well what mixture of sand will produce the requisite strength. During construction work cement is liable to be delivered in such quantities that it is not possible to make long-time tests without working a hardship on the contractor. If, however, a standard has been established, and it is definitely known that a certain strength. neat in seven days will develop into a certain other strength in thirty or ninety days mixed with the specified amount of sand, a very accurate and satisfactory conclusion can be arrived at. Each cement, however, must have its own standard, and the operator who makes the original tests should be retained to carry them on during the prosecution of the work.

No new cement should be accepted on short-time tests. They are often very deceptive. Unless it has been used and gained a reputation, careful and elaborate tests should be made as detailed above. The briquettes should be mixed neat and with the proportions of sand determined upon, the same day and by the same person, using the sime sample of cement for both neat and sand briquettes, so that the loss of strength occasioned by the added sand can be accurately determined. Long-time tests are absolutely nec essary, as a few cements with a moderate amount of sand will give practically as great a strength as when tested neat. As it is longtime results that are desired in construction, the importance of this can be readily seen. Table No. 21 clearly illustrates this.

The above is the average of five briquettes, and the natural product well known in the New York market. cent of water was used in the neat mixture and 14 per san 1.

cement is a

Thirty per

cent in the

Mr. E. B. Noyes, in Journal of Engineering Societies for June 1896, gives a case in point when a good cement was rejected and a poorer one accepted on comparatively short-time tests without apparently any previous knowledge. Table No. 22 gives his

The cement was an American brand, and the briquettes were mixed one part cement to one part of sand. Nos. 1 and 2 were not used on account of their poor showing in their first tests, while at the end of the year their superiority was clearly demonstrated.. No. 2 was certainly a remarkable specimen, and any engineer would be justified in rejecting it upon the six months' test without having had any previous knowledge of its wonderful recuperative powers. In many works, too, it could not be used notwithstanding its great strength in one year, as its development during the first six months. is very slow. Sample No. 4 actually receded in strength, though so little that it might have been caused by some individual briquette. It would seem to be a fair inference that it had practically reached its limit in six months.

The author several years ago had some tests made of the principal American cements tributary to the city where he was then located, practically on the lines as indicated above. The results were very satisfactory, demonstrating the necessity of such action, an in this particular case bearing out some action that had been taken in rejecting certain cements. Table No. 23 gives the results attained.

TABLE NO. 23.

CEMENT MIXED NEAT.

Briquettes 2 hours in air, remainder in water.

24 Hours. 7 Days. 15 Days. 30 Days. 90 Days. 6 Months. 9 Months. 1 Year.

CEMENT 1 PART, SAND 2 PARTS, REMAINDER IN WATER.

This shows that No. 1, which was the weakest at the end of a year neat, was the strongest when mixed as it is generally used; and that Nos. 3 and 5, which were two of the highest neat, were but one-half the average strength of the other at the end of the year when mixed with sand.

Some engineers in making cement specifications go very elaborately into the component parts of the material, exacting a certain percentage of one substance and ruling out more than a certain amount of another. This practice is dangerous, unless one is perfectly sure of his standing, or the limits are so elastic as to be of no value. It is really encroaching on the prerogative of the manufacturer. The engineer wishes results, and it is the maker's business to produce a cement that will give them. The manufacturer will have no difficulty in meeting any requirements, but at what cost to the long-time test he alone might be able to tell. Then the products of different mills differ so that a slight excess of one ingredient might be neutralized by that of another. It is well known that many excellent brands of cement are made. It is better to obtain a perfect knowledge of the peculiarities of each

and, after specifying certain of these, make sure that each delivery is kept up to the standard.

In the case of an excessive demand when the output is small, manufacturers are liable to put on the market a product that in the ru-h has not received sufficient attention, and which ordinarily would not be sent forth-or it may happen without their knowledge. It is the object of the tests to detect this or similar defects in standard brands.

In the paper by Mr. Lesley previously referred to, he gives the requirements for tensile strength as found in different specifications and shown in Table No. 24.

Cement specifications generally specify a time within certain limits for the initial and final sets. When this is done, and in fact the time of setting is generally noted in all tests, it is necessary to define what is meant by these terms. A standard was first adopted by General Totten at his work at Fort Adams, R. I., previous to 1830. This was that when the mortar would sustain a wire