pumice, volcanic cinders, scoria, and diatomite rocks. Pumice is fre- 

 quently used in structural concrete. For example, parts of the large 

 concrete counterweights on a bascule bridge (Fig. 15], were composed of 

 normal structural concrete (weight about 23.6 kilonewtons per cubic meter or 

 150 pounds per cubic foot), lightweight pumice concrete of 15.4 kilonewtons 

 per cubic meter (98 pounds per cubic foot), and heavyweight concrete of 35.3 

 kilonewtons per cubic meter (225 pounds per cubic foot). Concrete made with 

 pumice weighs from 14.1 to 15.7 kilonewtons per cubic meter (90 to 100 pounds 

 per cubic foot) . 



In the artificial lightweight aggregate family is perlite. This produces 

 a poor grade of concrete weighing from 7.9 to 12.6 kilonewtons per cubic 

 meter (50 to 80 pounds per cubic foot) , which is often used as an underlayer 

 for built-up roof decks. It will not produce structural grade concrete. 

 Expanded clay aggregates produce a lightweight, structural quality concrete 

 with densities ranging from 14.1 to 17.3 kilonewtons per cubic meter. 

 Although there are other manufactured lightweight aggregates for making 

 concrete, this report will conclude with vermiculite. Concrete made with 

 the vermiculite (not used for structural concrete) is used extensively as 

 an insulating material and weighs from 5.5 to 11.8 kilonewtons per cubic 

 meter (35 to 75 pounds per cubic foot) . 



(3) Heavy Aggregate . Heavyweight concrete is made with normal 

 coarse aggregates (ASTM Standard C33) and heavy natural or manufactured 

 aggregates such as magnetite (specific gravity 4.2 to 4.4), limonite 

 (specific gravity 5.0 to 5.5), and barite (specific gravity 2.5 to 3.5). 

 Some of these minerals could contain pyrite, which can decompose on weather- 

 ing, and should not be used in concrete. Magnetite and limonite should be 

 tested for the presence of pyrite before using in concrete. 



Manufactured heavy aggregates are usually iron and steel products. 

 Concrete from these products can reach more than 47.1 kilonewtons per cubic 

 meter (300 pounds per cubic foot) . More information on heavy aggregates is 

 provided in "Design and Control of Concrete Mixtures" (Portland Cement 

 Association, 1979) . 



(4) Regional Aggregates . For the sake of economy, it may be 

 desireable to use aggregates from the nearest source, unless they contain 

 harmful minerals such as pyrite or chalcedony, even if they do not fully 

 meet with the requirements of ASTM Standard C33. If it is essential to use 

 aggregates containing reactive minerals, pozzolon admixtures are added to 

 reduce or eliminate potential expansion from alkali reactive aggregates. 



(5) Coral . Coral deposits are found in many oceans of the world. 

 When mined and prepared for use as an aggregate for making concrete, the 

 physical and chemical properties of coral may vary widely. When coral 

 aggregates are used to produce structural concrete, a strength factor is 

 established by using trial mixes until the proper strength has been attained. 

 Cores were taken in 1972 from coral aggregate concrete (made by the Japanese 

 many years prior to 1941], on the island of Kwajalein. The average com- 

 pressive strength of the cores was 13.8 kilopascals (2 000 pounds per 

 square inch] . 



(6) Chemical Reactions of Aggregates . 



(a) Types of Reactions . Chemical reactions of aggregates in 

 concrete can affect the performance of concrete. Some reactions may be 



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