262 W. S. DE LISLE KOBEETS. 



qualities of sand obtained about Sydney, as compared with the 

 standard. In ordinary concrete work therefore the calculations 

 for computing the strength necessary to stand a given load, these 

 points should always be taken into consideration. 



For instance, eight blocks of cement mortar 4" diameter (made 

 with 15% of water, or about the proportion of water usually used 

 for concrete work when three of sand to one of cement is used), 

 gave after seven days an average crushing strain of 36 libs, per 

 square inch, using standard sand ; by substituting an average 

 good sand for standard sand the average strength was reduced to 

 233Ibs sand coefficient of 73*5, the average crushing strain of the 

 same cement, using standard sand with 9*7% of water; was 

 1042Hbs. per square inch, so that by using an ordinary good sand, 

 the actual coefficient of strength of cement when mixed with 15% 

 of water would only be 25*5, or about a quarter of the strength 

 the cement was shown to have when tested. This would naturally 

 upset all calculations when computing the size necessary to with- 

 stand a given load, based on the strength of cement as tested in 

 the ordinary way. 



For such work as the Monier system, where cement mortar is 

 used without stone, great compression is much easier than with 

 concrete, and a much smaller percentage of water can be used. 



As the most common defect in cement is too large a proportion 

 of free or uncombined lime, the greatest attention should be given 

 to its detection especially as the strength of cement with an over- 

 dose of free lime for the first month or so, is greater than a normal 

 cement. The free lime may either be in a state of hydrate, or it 

 may be unslaked, if unslaked the first effect will be an increase 

 in bulk due to slaking with overburnt cement, or with a silicate 

 of lime, this process of slaking may not be completed for several 

 months ; after slaking the formation of carbonate of lime com- 

 mences, this means another increase in bulk, which continues 

 more or less, according to the situation, and the facilities afforded 

 it to take up C0 2 from the atmosphere. 



