Ch. 24] APPENDIX . 459 



Appendix 



THE INFLUENCE ON CONCRETE OF VARIOUS PROPERTIES 

 OF AGGREGATE 



Strength 



The average crushing strength of rock types which would commonly be used 

 as aggregate ranges from 10,000 to 30,000 pounds per square inch. Thus al- 

 most any rock possessing the other characteristics necessary for an aggregate 

 will be far stronger than concrete we expect to make. High strength in rock 

 can be beneficial to concrete subjected to stress, but the degree to which the 

 strength and elasticity of the aggregate contribute those qualities to concrete 

 is controlled largely by the integrity of the bond between cement and aggregate. 



Hardness 



In the mineralogic sense, hardness of particles depends wholly on the hard- 

 ness of the constituents, not on the firmness with which the constituents are 

 knit together. When used this way, a convenient distinction can be made 

 between "weak" particles, which may be composed of either hard or soft 

 grains that are weakly joined, and "soft" particles, which abrade easily how- 

 ever strongly the component grains may be joined. "Weakness" is never 

 tolerable in concrete aggregate, but sometimes "softness," if not extreme, may 

 be admissible. Weakness and softness are frequently associated, and a safe 

 rule is to keep both weak and soft particles at a minimum; in special-purpose 

 concrete (for example, abrasion-resistant concrete) aggregate must be both 

 hard and strong. 



Compressibility 



The compressibility of aggregate particles strongly influences the drying 

 shrinkage of concrete. A compressible particle is one that is reduced in 

 volume by contraction of the surrounding cement paste as the concrete dries. 

 As the cement paste shrinks, aggregate particles are thrown into compression; 

 if the particle is strong and the cement-to-aggregate bond is good, shrinkage 

 of the cement paste is restrained, and, consequently, the volume change of 

 the concrete is inhibited. On the other hand, if the particle can be com- 

 pressed, the drying shrinkage of the concrete will be high. Drying shrinkage 

 of concrete will be increased greatly by particles that expand and contract 

 with wetting and drying. 



Durability 



Durability of concrete is its resistance to disintegration, volume change, or 

 loss of strength and elasticity under conditions to which it is subjected. 



