138 



KEIBLING. 



The logical conclusions of the results recorded in this discussion would 

 seem to be a modification of concrete specifications in one of two direc- 

 tions, either they should state that the concrete must show a certain 

 strength in a given time, or they should state the granularmetric com- 

 position of the screened sand and gravel to be used. 



The first method does not insure either the future of the concrete, its 

 surface appearance, or its economical use. Its enforcement would im- 

 mediately result in higher bidding. The testing would also be expensive 

 and might cause much delay. Failure to meet the required tests would 

 result in long discussions and eventually prove ruinous to all interests 

 involved. 



The second method would insure safe building and future durability. 

 The work could be rushed as rapidly as possible, the sieving and screein- 

 ing being done to meet the requirements. It would thus be possible to 

 calculate the mixture more closely and actually to reduce the cost of 

 construction. 



Sand and gravel should always be screened dry. With durable rotary 

 screens the cost should be very low. One screening of the sand and one 

 of the gravel would probably be sufficient for most work. This would 

 insure fixed ratios in the morter and concrete and it would then be 

 necessary to allow only for variations in the percentage of voids. 



Eecently, also, another source of danger to concrete construction became 

 manifest when the following cubes were tested : 



Concrete cubes from Aoatan River, Cortes, Bohol. 



Dimensions, in 

 inches. 



Propor- 

 tions. 



Age. 



Total strength, 

 in pounds. 



Strength, in 

 pounds, per 

 square inch. 



Mark on cubes. 



In 

 water. 



In 

 air. 



First 

 crack. 



Ulti- 

 mate. 



First 

 crack. 



Ulti- 

 mate. 



6X6X6 



1:2:4 



Days. 

 11 



Days. 

 12 



f 12,590 

 1 10,955 

 ] 10,845 

 Aven 



25, 335 

 20, 395 

 26, 420 

 ige = 



349 

 304 

 301 

 318 



704 

 566 

 734 

 668 



CI -A. Floor. 

 Do. 

 Do. 

 Do. 





Examination of the crushed concrete revealed the fact that the stones 

 were covered with green algae, the effect of which is illustrated by Plate 

 III, which is a photograph of the cone resulting from one of these 

 crushed cubes. 



From this photograph, which shows the exposed sockets with their 

 bordering, thin ridges of sound mortar, it is seen how effectually the 

 coating of organic matter prevented adhesion between the mortar and 

 the gravel. 



