1917] RURAL ENGINEERING. 885 



mix of concrete seemed to exert a small influence on tlie contraction ; the richer 

 the mix tl'.e greater was the change in length. 



Concrete alternately wetted and dried was made to expand and contract 

 owing to these causes. The expansion due to wetting was more rapid than the 

 contraction on drying. The thoroughly dried specimens of concrete did not 

 recover their original wet length when immersed. Concrete stored in the outer 

 air and exposed to the weather did not contract to the same extent as the ahove- 

 described specimens, except under very dry conditions. A waterproof covering, 

 such as coal tar, prevented the rapid change in moisture content and greatly 

 retarded the expansion and contraction. Reinforcement decreased but did not 

 prevent the shrinkage and expansion of concrete due to drying, and had no 

 effect on temperature changes. 



" Reinforcement can not therefore entirely prevent cracks, but seems to dis- 

 tribute them and keep them small. Concrete roads are affected by both tem- 

 perature and moisture. VN^ien the drainage is good and the sub-base not wet, 

 the temperature effects seem to be most important. A wet sub-base may add to 

 the temperature expansion by about 0.01 to 0.02 per cent. The restraining effect 

 of friction at the ba.se seems to be almost negligible when figuring temperature 

 and moisture expansion and contraction. In very dry climates shrinkage due 

 to drying must be added to contraction due to fall in temperature. A shrinkage 

 of 0.04 per cent (0.25 in. in 50 ft.) is a safe allowance due to drying. 



" Temperature at time of construction of road should be considered in design- 

 ing joints. Cold-weather construction requires a full allowance for temperature 

 expansion, and on wet sub-bases for moisture expansion also. Hot-weather con- 

 struction theoretically requires no joints at all, even in wet sub-ba.ses, as the tem- 

 perature contraction exceeds the moisture expansion. However, the difficulty 

 of keeping the cracks clear probably renders joints imperative." 



Toughness of bituminous aggregates, C. S. Reeve and R. H. Lewis (U. S. 

 Dept. Affr., Jour. Agr. Research, 10 {1911), No. 7, pp. S19-330, pis. 2).— Tests of 

 the toughness of several representative samples of various types of rock when 

 used as aggregates in bituminous mixtures are reported, it being concluded that 

 the toughness of bituminous aggregates in which a given bituminous material 

 is tested will not be the same for every type of rock. 



" Tests of laboratory specimens can be directly correlated with results in 

 service. The difference in behavior of the various rock types can not be directly 

 attributed to any of the routine physical test values of the rock, but appears 

 to be due largely to differences in the surface character of the rock particles. 

 While relatively soft or fluid bitumens may yield satisfactory results in bitumi- 

 nous concrete with some types of rock, their use with other types will lead to 

 failure of the road surface. The impact or toughness test of bituminous aggi'e- 

 gates offers possibilities as a means of determining in advance the relative 

 behavior in service of bituminous concretes. While the authors at this time 

 have no definite recommendations to offer with regard to their last conclusion, 

 it may be stated that further experiments will be made with that end in view." 



Concrete culverts [Cement and Engin. News, 29 {1917), No. 4, pp. 109, 110, 

 figs. 4)- — Comparative cost data on cast-iron and reinforced concrete culvert 

 pipe for road use are given, shov.'ing in general the economy of the latter type 

 of construction. 



Mechanical properties of woods grown in the United States, J. A. Newlin 



and T. R. C. Wilson {U. S. Dept. Agr. Bui. 556 {1917), pp. 47, pis. 3).— This 



bulletin reports the results of about 130,000 tests on the mechanical properties 



of woods, including data on both green and air-dry timber. A glossary of terms 



32950°— IS— No. 9 7 



