THE CEMENTING POWER OF ROAD MATERIALS. 



INTRODUCTION. 



For several years the road material laboratory, now included in 

 the Division of Tests, has been desirous of impressing upon engineers 

 and others interested in road building the importance of that property 

 of road materials known as cementing power and its relation to the 

 selection of the best materials. Everyone who has given careful 

 attention to this subject is aware that different rocks vary widely in 

 their binding power. While some broken stones will easily and rap- 

 idly consolidate under the roller or traffic, others will do so only 

 slowly and with difficulty, while again some totally lack any binding 

 power whatever. Furthermore, as pointed out in previous publica- 

 tions from this laboratory, the species of the rock is practically no 

 guide as to its road-making quality, and a" study of Table III (p. 17) 

 clearly shows this to be true in regard to cementing value. This 

 cementing power is caused by a hydrated condition of the particles, 

 which is the result of certain secondary changes produced in the min- 

 erals composing the rock structure. Whether these changes take 

 place depends upon circumstances, and not, broadly speaking, upon 

 the rock species. This subject, however, will be more fully discussed 

 later on. 



The statement is frequently seen in road-building literature that 

 so-called trap rocks invariably bind well and make the best road 

 material. While this is perfectly true of the traps in certain locali- 

 ties, it is by no means true of those in others. In the same way a 

 widespread impression prevails that sandstones are, as a rule, useless 

 as road-building material. This* again is by no means invariably true, 

 and tests made in the laboratory, which were afterwards substantiated 

 in practice, have shown that some sandstones are eminently suited for 

 building good roads. 



The question of the geographical distribution of road materials is one 

 in which this laboratory is much interested. The accumulation of suf- 

 ficient data from which to prepare and issue maps is only a question of 

 time, and already the data obtained from the tests on about a thousand 

 samples in this laboratory, while not sufficiently complete to warrant 

 publication, point none the less to some very interesting facts. 



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