SPADE GHOLOGIST: 349 
Sand.—In mortar tests, both for crushing and tensile strength, the 
kind of sand employed is of vital importance. Probably the most important 
work on this point has been done by Feret, who has carried out an enor- 
mous number of carefully made tests endeavoring to determine the rela- 
tion between the volume of sand and cement and the resulting strength. 
His results are expressed in the simple formula: 
Pak = f a) 
in which P is the crushing strength in kilograms per square centimeter, 
KX a coefficient, depending on the kind of cement used, c, w and h are 
the volumes of cement, water, and pore space expressed in terms of 
the absolute volume, weight divided by specific gravity. Water plus pore 
space may also be expressed by: volume, I, minus cement, plus sand, or 
Rak 
c 
(es) | 
What remains is*water and pore space, assuming always that the mortar 
is plastic. Feret finally arrived at the formula: 
Raat (—) ; 
In these formulae s=sand. 
By using a number of mixtures under exactly determined conditions 
and employing an inert sand, not a material containing clay or pozzuolane, 
the value of K for any cement may be readily determined. Its approxi- 
inate value is 1965. It is thus seen that the kind of sand is one of the 
principal functions of the strength of Portland cement mortars. Fret, 
himself, maintaining the same volume of cement, but changing the sizes 
of sand, was able to produce fluctuations in strength of over 150 per cent. 
Modes of Making Briquettes.—Since the mixing of the sand and 
cement is done in the dry condition, it does not matter very much what 
means are employed for this purpose. The blending of the cement with 
water, however, is subject to variations by different manipulators, and hence 
for producing standard conditions various mortar mixers have been sug- 
gested and used. One of the best known is probably the Steinbrueck mixer 
(see figure 67). It is still more important, however, that the briquette be 
put into the mold under as uniform conditions as possible and for this pur- 
pose several devices have been used. The Boehme hammer is largely em- 
ployed in Europe and consists principally of a balanced steel hammer, 
weighing 4.4 pounds, which is allowed to drop onto a metal part fitting the 
briquette mold. By means of a cam wheel the hammer is allowed to drop 
about 10 inches. In 15 revolutions the hammer makes 150 blows and the 
apparatus is stopped automatically at this point. 
In regard to the errors of the tensile and crushing strengths it has 
been found by Professor Martens that under the most favorable condi- 
tions possible the mean error of the tensile strength test is from +3 to 15 
per cent., that of the crushing strength tests from +1 to +2 per cent. — 
