A dirty gravel or one that contains impurities should be washed. This 
will not only improve the strength of the concrete, but will make a more 
uniform and desirable color for the finished block. 
It is now agreed that cement hardens by a process of crystallization 
of the active elements. Water must be present for the crystallizing to go 
on. Therefore it is necessary that the proper amount of water be used 
in mixing the concrete. This, by some authorities, is from 8 to 18 per 
cent. Also it is necessary that after moulding, the block must not be 
allowed to dry out, as no subsequent addition of water will give per- 
fect crystallization. Some makers cure their blocks in a steam bath, there- 
by insuring constant moisture. The economical value of steaming con- 
crete blocks is a subject for experiment as yet. Most specifications limit 
the time after making at which blocks may be used in the wall, so that the 
increased speed of hardening by the steam process is not so important. 
The specification for crushing strength of concrete blocks, in most 
cases, is 1,000 pounds per square inch of gross area, no allowance being 
made for the hollow spaces. The block must reach this strength in 28 or 
30 days after making. 
The city of Indianapolis has recently adopted specifications for con- 
crete building blocks, and the results of the first series of tests for the 
block makers of that city by the Laboratory for Testing Materials of Pur- 
due University, indicate a chance for improvement. 
Of 75 tests of blocks, supposed to have been made under the specifica- 
tions, only 28 per cent. passed the specification for crushing strength, and 
the average age of these was 41 days instead of 30. Similar results have 
no doubt been found in all cities which have adopted a building block 
ordinance. However, under the influence of these somewhat rigorous 
specifications, it is to be expected that the quality of the product on the 
market will greatly impreve. This in itself will strengthen the industry 
for those makers who are content to manufacture good blocks at a rea- 
sonable profit. 
Purdue University, 
Lafayette, Ind. 
