442 bhoades. CONCRETE AGGREGATE [Ch. 24 



texture largely define the reaction and resistance to the various modi- 

 fying processes and thus influence the kind and extent of the modifica- 

 tion that will occur. 



For example, the shape of a sand or gravel particle is in part the 

 result of attrition during transportation, but it is controlled also by the 

 initial shape which in turn results from the spacing and patterns of 

 joints and fractures in the parent rock. Similarly, the maximum size 

 of particle in a gravel deposit will depend in part on attrition during 

 transportation but can never exceed the dimensions of the particle 

 originally supplied at the source, again a function of the spacing of 

 joints or fractures. The mineralogy and petrography of the parent 

 rock will also determine the initial clay content, which may be related 

 to absorption and volume change under conditions of wetting and dry- 

 ing. The initial porosity of the rock is related to its internal texture. 

 Surface texture may be related directly to the mineralogy and texture 

 of the parent rock, if governed by cleavage planes, as in feldspar, or 

 by the fracture form characteristic of certain minerals as, for instance, 

 the conchoidal fracture of quartz, both of these surface textures, unless 

 subsequently rendered rugose by leaching or attrition, being too smooth 

 for development of optimum bond with cement. 



The chemical characteristics of aggregate, although in part related 

 to subsequent leaching, solution, weathering, and the like, which may 

 attend the processes of sedimentation, are also in part controlled by the 

 initial composition of the source material. Thus the mineralogy of 

 the parent rock will dictate whether or not weakening, softening, in- 

 creased porosity or absorption, or the development of secondary clay 

 will be caused by these processes, and to what degree. 



The most important chemical reaction which occurs between aggre- 

 gate and cement in concrete, the so-called alkali-aggregate reaction, is 

 usually the direct result of the mineralogy or petrography of the parent 

 rock (ASTM, 1948a; Rhoades and Mielenz, 1946; McConnell et al., 

 1948) . The rocks and minerals susceptible to this type of reaction are 

 the glassy volcanic rocks of acid to intermediate composition (rhyolites 

 through andesites), such silica minerals as tridymite, opal, and chal- 

 cedony (and cherty rocks of which they are constituents), and prob- 

 ably a hydromica which occurs in some phyllitic rocks. These rock 

 and mineral types, when exposed to the attack of the excess alkalies 

 contained in a high-alkali cement, yield a silica gel that will imbibe 

 water by osmosis and swell with the development of large, expansive 

 pressures. Concrete so affected expands and cracks in an unsightly 

 manner and not infrequently becomes unsafe or unserviceable. How- 

 ever, the initial reactive potentialities of an aggregate may "be lessened 



