EXPANSION AND CONTRACTION OF CONCRETE. 3 



According to these figures, neat cement shrinks about 2| inches in 

 100 feet when kept in air six months, and in four years the shrinkage, 

 due merely to the drying out of the moisture, amounts to almost 4 

 inches in 100 feet. Mortar mixed in the proportions of 1 part of 

 cement to 3 parts of sand, when allowed to harden in the air, shrinks, 

 according to White's results, from 0.08 per cent to 0.1 per cent. 

 These figures confirm in a general way the previously published 

 figures of Bauschinger and others. 



As neat cement and mortar were known to have this physical 

 characteristic of expanding and contracting, depending upon their 

 moisture content, it was considered reasonable to suppose that con- 

 crete, too, would show the same general behavior. The following 

 laboratory tests were made on concrete to determine the amount of 

 movements produced by the drying out and absorption of water. 

 These movements, combined with temperature movements, probably 

 account for most of the expansion and contraction of concrete pave- 

 ments, and a knowledge of their values is of assistance in the design 

 of expansion joints. 



DESCRIPTION OF EXPANSION AND CONTRACTION SPECIMENS. 



White and the earlier investigators made tests on the change in 

 length of neat cement and mortar with specially designed micro- 

 meters on very small specimens, White's being only 4 inches in length. 

 Obviously it would be impossible to use such small test pieces as these 

 to determine the expansion and contraction of concrete containing 

 coarse aggregates, and the type adopted was a column 8 inches square 

 and 5 feet high. The column form of specimen was used to permit 

 the free expansion of one end, in this way preventing friction from 

 affecting the accuracy of the measurements; 



PREPARATION OF SPECIMENS. 



All concrete mixtures were proportioned by weight with a stand- 

 ard brand of Portland cement, Potomac River sand, and crushed 

 gneiss or gravel. The cement was normal and passed the specifica- 

 tions of the American Society for Testing Materials. The sand was 

 coarse and clean and considered a good grade of concrete sand. All 

 concrete was hand mixed and was made of various consistencies. The 

 specimens of very dry consistency required hard tamping to consoli- 

 date the concrete in the mold, while those of wet consistency were 

 made simply by puddling the mixture in place. The amount of 

 water used for the dry mixtures was about 8.5 per cent of the weight 

 of dry materials. For the wet mixtures from 10 to 12 per cent was 

 used. 



