Ch. 24] APPENDIX 461 



Surface Texture 



The roughness and pore characteristics of the surface and near-surface por- 

 tions of aggregate particles create a surface texture which in large part deter- 

 mines the integrity of the bond established between the particle and the cement 

 paste. The surface texture reflects the original internal structure and com- 

 position of the particles as well as the natural and artificial processes of im- 

 pact and abrasion to which they have been subjected. 



Specific Gravity 



Specific gravity of aggregate influences the unit weight of concrete but is 

 of direct importance only where design or structural considerations require 

 that concrete have an unusually high or low unit weight. Although low specific 

 gravity of an aggregate is frequently considered an indication of unsoundness, 

 numerous exceptions to this criterion prevent its application without confirma- 

 tion by other means. 



Porosity 



A rock or mineral particle can be penetrated by water to the extent that 

 interconnected voids or fractures are present. The amount of water absorbed 

 depends on the abundance and size of the internal voids and fractures, and 

 the rate of penetration depends on their size and continuity. Porosity, per- 

 meability, and absorption strongly influence chemical stability, abrasion re- 

 sistance, elasticity, and apparent specific gravity of particles, as well as the 

 degree of bond between particle and cement. 



Volume Change 



Volume change in aggregate resulting from changes in moisture content and 

 temperature is a common source of injury to concrete. Shales, clays, and some 

 rock materials expand by absorption of water and shrink on dehydration. In 

 designing structural elements, allowance must be made for thermal volume 

 changes of concrete, which are influenced by the average thermal expansivity 

 of the aggregate. Damaging internal stresses may develop when the change 

 in volume of an aggregate due to temperature is different from the change pro- 

 duced in the cement paste. 



Coatings 



Coatings form on the surfaces of natural aggregates through deposition of 

 mineral substances from ground water. Coatings usually are composed of 

 clay, silt, or calcium carbonate; but opal, iron oxides, gypsum, manganiferous 

 substances, and soluble phosphates can occur as coatings. Particles with sur- 

 face coatings are generally undesirable for use as concrete aggregates. The 

 bond between particle and coating may be weak and thus will decrease strength 

 of aggregate-cement bond. Many natural coatings contain substances (such 

 as opal) susceptible to reaction with alkalies in cement. Soluble alkali salts 



