CONCRETE CONSTRUCTION IN THE PHILIPPINE ISLANDS. 139 



The stones were submitted to Mr. E. D. Merrill, botanist of the Bureau 

 of Science, for miscroscopic examination. His report is as follows : 



Examination of the pebbles shows that they are more or less covered with a 

 very thin, greenish or yellowish film, which is formed by minute, unicellular 

 green algse, apparently Protococcus. 



The pebbles were undoubtedly secured from the upper strata of some stream 

 bed, for these algse are abundant in streams throughout the Philippines. 



Gravel should never be taken from the surface layers in the stream bed, for 

 this organism is always present in greater or less qualities, both in still and 

 swift water. The upper few inches should be discarded, and the gravel of lower 

 layers used. 



If surface gravel is used, the film of algae should be removed by heating the 

 gravel sufficiently to destroy all vegetable matter on the surface of the pebbles. 

 It is possible that the exposure of the gravel to the sun and air for a few week3 

 would cause the disappearance of the algse, but this is uncertain, especially in 

 wet weather, for these low forms of plant life are very resistant. 



The simplest method of avoiding difficulties in connection with the use of 

 gravel that is more or less covered with algse is that noted above : the removal and 

 discarding of the surface layers. 



From the experience of Spackman and Lesley, 10 and of S. B. Thomp- 

 son, 11 we find that vegetable matter has also a marked effect on the 

 strength of mortars. It is less readily detected in sand, but when present 

 gives the moist silt a greasy nature which interferes with the hardening 

 of the cement. In the Philippines and other tropical countries, where 

 in rainy seasons, especially, low forms of plant life flourish so vigorously, 

 the concrete workers should exert extra precaution against this source of 

 danger to the strength of their structures. 



CONCLUSION. 



The discussion and data recorded above lead to the following con- 

 clusions : 



1. Adequate consideration is not at present given to the fact that the 

 nature of the aggregate is fully as important as the quality of the cement 

 itself. 



2. It is a mistake to use good Portland cement with poor aggregate. 



3. The efficiency of concrete is limited to that of its most inferior 

 constituent. In a normal mixture the strength and durability of con- 

 crete can not be insured to exceed that of either its gravel, stone, or 

 mortar; and for any given mixture the mortar efficiency depends as 

 much upon the nature of the sand as upon the quality of the cement. 



4. Concrete specifications must include standard requirements for 



10 Sands : Their Relation to Mortar and Concrete. Proc. Amer. Soc. for Treat- 

 ing. Materials, (1908) 8, 451. 



11 Hid., 449. 



