320 REPORTS ON THE STATE OF SCIENCE, ETC. 
to the edge, has two pronounced peaks as indicated, while in A there is a somewhat 
similar distribution, which occurs in a more or less degree within the zone marked by 
the dotted curve F G H. When the materials pressed together are not alike, the effect 
is still more pronounced, and it cannot be got rid of when the faces are initially plane 
and are of different areas. It is, however, possible to obtain perfectly uniform com- 
pression stress well away from the end plates, and this suggests a means of obtaining 
perfect compression stress in a rectangular block by the interposition of short com- 
pression members of the same cross-section (fig. 6B) as the block under test, so that all 
the inequalities of stress distribution are kept within these, and do not penetrate into 
the test portion. Photo-elastic experiments show that this does actually give an 
extremely close approximation to pure compression stress when all the contact areas 
of the blocks are scraped surfaces and exactly parallel, and although the modified test 
is more difficult to make, no simpler way of getting rid of these complex end effects has 
proved of value. In general, it is also seen to be inaccurate to determine elastic proper- 
ties of the compression block A (fig. 64) by strain measurements between the faces 
of the blocks B and C. 
The Testing of Cement. 
Although cement, whether used alone or as part of an aggregate, is employed as a 
material to resist compression stresses, its mechanical properties are often deter- 
mined by tension tests alone, but happily American and Canadian procedure 
recognises compression tests as an important element of this branch of testing. 
As regards tension tests of cement, it may be truly said that the shapes of all 
standard forms are such as to produce throughout the briquette a highly complex 
stress system, as may easily be seen when a transparent model of any form is loaded 
and viewed in a circularly polarised field of light. 
This is, perhaps, even more noticeable in that part of the briquette between the 
grips than at the waist, for here the relatively great contact stresses imposed by 
the grips produce compression stresses of much greater intensity than the tension 
system in the waist, and since the minor principal stresses are also small, the phote- 
elastic effect is great, whereas at the waist of the briquette the tensional stress system 
across the minimum cross-section is accompanied by cross stress also tensional 
throughout and of considerable magnitude, so that the differential photo-elastic 
effect is relatively much smaller, although the stresses when separated are found to be 
great. In an early attempt‘ to determine the stress system in transparent models 
of briquettes, advantage was taken of the fact that the maximum stress at the waist 
is at the contour, where there is no cross stress, and it was then found that in the 
British Standard this maximum stress was 1°75 times the mean average stress across 
the section, while in the American and Canadian forms this ratio was 1-70. Lately 
the former standard has been investigated more elaborately and the stress system 
has been measured for the whole of the briquette. It is therefore possible to indicate 
the chief results for the waist, which is the important part, as, although the stress 
system across the grips occasionally causes failure due to compression stress, it is a 
comparatively rare occurrence, owing to the great ability of cement to resist that type 
of stress. On a transparent briquette of full size, the measurements of (p + q) at 
the waist, with a load giving a mean average stress of 500 lbs. persq. in., are plotted 
on the accompanying fig. 7, from which it will be seen that the normal tension p varies 
from 1-74 to 0:8 times the mean average stress, confirming the earlier value for the 
maximum stress here within one per cent., but the q curve of minor principal stress is 
comparatively large and for the greater part of the section is only slightly less than 
50 per cent. of the mean stress applied. 
Such a system of stresses acting on a specimen of material obviously requires very 
careful interpretation if it is to be useful in giving information regarding its tensional 
properties, and it may be added that lateral measurements on cement briquettes, 
when a month or so old, indicate that the stress system produced in them under load 
is much the same. Unfortunately, standard briquettes, like standard test-bars, 
differ from country to country, and there is much need of co-ordination, so that a test 
of a material in one country can be directly compared with another without the 
necessity of applying a correction factor, even if this latter is feasible, to take into 
account the peculiarities of form of one standard as compared with another. 
Standard cement briquettes, as they exist to-day, seem particularly difficult to 
bring into some degree of unity in this respect ; but as cement, like steel, is a material 
4 Appendix 8. 
