99' 



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 IS 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 2S 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 improve. 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. 



