PORTLAND CEMENT CONCRETE ROADS. 7 



The theory ^ has recently been advanced that the strength of the 

 concrete depends entirely on the ratio of the amount of water to the 

 amount of cement so long as the mix is workable. According to this 

 theory variation in the grading of the aggregates affects the strength 

 of the concrete made with a given amount of cement merely because 

 it affects the amount of water that is required to produce a work- 

 able consistency. If the proportion of cement be varied to main- 

 tain a constant ratio of cement to water, any reasonable grading of 

 aggregates can be made to yield a concrete of approximately any 

 desired strength. An arbitrary quantity known as the "fineness 

 modulus " is determined by sieve analysis of the aggregates and this 

 quantity together with the maximum size of the aggregate determines 

 the amount of cement to be used. The strength of the concrete made 

 from the cement and aggregate in the determined proportions will 

 depend upon the amount of water used. Tables and charts based 

 upon experimental data supply the means for the practical applica- 

 tion of the theory. 



Another theory * is based on the assumption that to produce con- 

 crete of a given strength a certain amount of cement is required for 

 each unit of surface area of the aggregate, taking into account the 

 amount of water used in mixing. The particles are assumed to be 

 spheres, and tables have been worked out from which the surface 

 area of a given amount of the aggregate can be determined from the 

 sieve analysis. 



It is worthy of note that each of these theories tends to the use 

 of well-graded aggregates and rich mixes where strong concrete is de- 

 sired. They have all been evolved in the attempt to design concrete 

 of high strength, which is needed in pavement concrete to enable 

 the pavement to resist temperature and impact stresses without 

 excessive cracking. That concrete high in compressive strength is 

 also highly resistant to abrasion is the conclusion drawn from tests 

 conducted by Prof. Duff A. Abrams, Lewis Institute, Chicago. It 

 was observed in these tests that the resistance to abrasion fell off 

 sharply when the compressive strength dropped below 3,000 pounds 

 per square inch. The tests conducted by the Bureau of Public Roads 

 do not support this conclusion, but indicate, rather, that the amount 

 of wear of the concrete depends upon the character of the coarse 

 aggregate. It should be noted that in the tests conducted by Pro- 

 fessor Abrams only two kinds of coarse aggregate were used. For 

 any given coarse aggregate, however, it is likely that increase in 

 compressive strength will result in corresponding decrease in wear. 

 From experience it has been found that pavement concrete should 

 be proportioned to have a compressive strength of not less than 3,000 



s Bulletin 1, Structural Materials Research Laboratory, Lewis Institute, Chicago. 

 * Proceedings of the A. S. T. M. for 1918, pt. 2, p. 236. 



