PORTI^VND CEMENT TESTING. 167 



In this laboratory all sand briquettes are broken in a German machine 

 (HugershofE), invented by Jlichaelis. The Fairbank's roller grips are 

 so heavy and the surface of contact so narrow that they crush through the 

 majority of 7-day mortar briquettes, giving bad In-eaks and figures 

 representing low tensile strength, and this is especially true of cement 

 which does not in itself develop great strength. The German machine 

 offers a wider surface of contact and the grips support their own weight. 

 ComiDarative tests carried on for months in routine work give 10 per cent 

 higher results with the latter, but the variation is greater, as the machine 

 is more delicate; the probability of obtaining bad breaks is also greater, 

 but when our system of tamping is used this jirobability is reduced to 

 a minimum. 



It is especially difficult in this climate to obtain uniformity in the 

 demonstration of tensile strength. The laboratory temperature seldom 

 falls below 26°, and is often as high as 31°. 5. Our own experiments 

 bear out the conclusions derived from all published data on the influence 

 of temperature. High temperature is conducive to slightly greater tensile 

 strength on 7- and 28-day tests, and also to a greater variation between 

 breaks. 



The different tensile strengths secured by different machines, molds, 

 and grips is another reason why there is such a great lack of uniformity 

 between different la1)oratories. Johnson, Sabin, Thompson and Taylor, 

 Spalding, Butler, and in fact ahnost every authority on cement testing, 

 devote considerable space to illustrating the variable results that occur 

 from this source. A specification that allows any form of grip and mold 

 can not hope to accomplish its purpose. The Army siDecifications allow 

 the use of any tensile strength system. The American Society specifica- 

 tions recommend a special form of l^riquette and regulate certain im- 

 portant factors in the grips. To insist upon a certain machine, grip, 

 and mold would be a rather delicate undertaking, liut until this is done 

 there may always be a large difference due to "personal equation" between 

 the tensile strength determinations between dift'erent laboratories. 



The American Society introduces a very good check upon the mixing 

 and molding of briquettes l)y specifying that tliey should be weighed just 

 before immersion and that all which vary more than 3 per cent from the 

 average, should be rejected; in this way greater certainty in results is 

 obtained. It is very easy to work within these limits, and every tester 

 should strive to attain weights which approach eacli other within 1 j)er 

 cent. This determination of weight, in addition to being a check upon 

 the uniformity of mixing and molding, may also disclose the effect of 

 unequal drying and of imperfect molds. Sand briquettes are more liable 

 to variation beyond the limits of 2 per cent than are neat. This dif- 

 ference is due, as is the variation in breaking strength, to the lack of 

 uniformity in the size and shape of the sand particles and the irregularity 

 in voids. 



