786 EXPERIMENT STATION RECORD. 



The final couclusious drawn from this investigation are (a) that where the 

 support furnished to the surface is unyielding the wear of bridge surfaces of 

 wood blocks, sheet asphalt, stone blocks, bricks, and other materials is no 

 different from what it is under the same conditions on highways; and (b) 

 that the construction of plank surfaces is rarely economical and its use should 

 be generally abolished. 



The determination of internal temperature range in concrete arch, bridges, 

 C. S. Nichols and C. B. McCullough {loiva Engin. Sta. Bui. SO, 1913, pp. 101, 

 pis. 10, figs. 62). — The results are given of a large number of experiments made 

 on concrete arch highway bridges in Iowa with the object in view of determining 

 the actual yearly range in temperature of reenforced concrete arch structures 

 typical of the highway arch construction in that State and to investigate the 

 comparative values of the factors affecting this range, such as prevailing 

 winds, sun, shade, etc. The following conclusions are stated from the results 

 of the experiments: 



(1) The yearly range in temperature in a typical reenforced concrete arch 

 structure is in that latitude not far from 80° F. (2) The relation between 

 the depth of concrete covering at any point and the yearly temperature range 



53 



may be obtained from the curve formula j/=90— — x, in which y equals the 



yearly temperature range in degrees F. and x equals the distance from the 

 nearest exposed surface in inches. (3) The amount of direct sunlight modifies 

 somewhat the actual temperature in the concrete for a considerable distance 

 into the interior of the mass. (4) The data on the rapidity with which the 

 different portions of the structure respond to the external temperature changes 

 show that in structures of this type the minimum temperatures are attained 

 in time intervals from less than 1 day to 4 days after the atmospheric minimum. 

 The interval depends upon the position of the portion of the structure con- 

 sidered, and is roughly proportional to the distance from the nearest exposed 

 face. (5) Because of the high temperature in the concrete when it attains 

 its set and the effect of atmospheric temperature on this maximum, other con- 

 ditions being equal, the pouring of an arch ring at a temperature near the 

 atmospheric mean annual temi)erature tends to lower materially the stresses in 

 the ring induced by temperature variation. (6) When uninfluenced by other 

 factors than atmospheric variation the rise and fall of an arch ring agree quite 

 closely with theory. (7) The shrinkage of concrete, if unrestrained by reen- 

 forcing. amounts in 100 days after placing to 0.004 per cent. This induces bend- 

 ing stresses analogous to those produced by a temperature drop, but these are 

 so modified by the initial stresses, due to shrinkage, that the chief effect is to 

 cause a high compression in the steel on the compression side of the bending. 

 When, due to other forces acting on a structure, a high compressive stress in 

 the steel is encountered, the effect of this shrinkage should be carefully studied. 

 (8) To render an arch ring structurally safe, provision should be made in that 

 latitude for stresses induced by a temperature variation of at least 40° F. each 

 way from an assumed temperature of no stress. Particular circumstances 

 may demand that a greater variation be used, this remaining largely a matter 

 of judgment with the designing engineer. 



Comparative mortar tests with mixtures of normal consistency and work 

 consistency (Engin. and Contract., liO (1013), No. 3, pp. 58. 59). — In a paper by 

 W. B. Reinke, presented before the American Society for Testing Materials, the 

 results of comparative tests of mortars using both standard sand and natural 

 sand, and in mixtures of the ordinary dry consistency of the laboratory and of 

 the wet or fluid consistency common in actual construction are given. It is 



