2o8 Journal of Agricultural Research voi. vi, no. e 



Deformation-measuring apparatus. — Deformations of the top of 

 the slab were measured at right angles to the supports, and also, in the 

 case of one slab, parallel to the supports, with a Berry strain gauge of 

 2o-inch gauge length. The degree of accuracy attained was probably 

 within 0.0002 inch in that gauge length. Short brass plugs drilled at one 

 end with a No. 55 drill were embedded in the concrete, or in some cases 

 cemented in holes drilled for the purpose; and the movements of these 

 plugs as measured with the strain gauge were considered the fiber defor- 

 mations. • 



In the last slab tested (No. 934) deformation readings were also taken 

 of the steel reinforcement, and for this purpose holes were drilled in the 

 steel bars 20 inches apart to accommodate the points of the strain 

 gauge. Although readings were not taken on all of the bars, a sufficient 

 number were measured to determine the distribution of the steel stresses 

 throughout the slab. The layout of strain-gauge points between which 

 readings were made is shown in figures 2, 3, and 4. The arrowheads 

 mark the position of the points on the top of the slab and in the case 

 of slab 934 (fig. 4) the gauge points in the steel are marked by small 

 circles. 



Deflection-measuring apparatus. — The deflection measurements 

 were made in somewhat different ways during these tests, and the ap- 

 paratus was improved as the tests progressed. In its final form in slab 

 934, the deflection-measuring equipment consisted of a network of piano 

 wires stretched tightly at a fixed distance above the concrete supports, 

 and being entirely independent of the slab. At the points where measure- 

 ments were taken, steel plates were set in plaster of Paris on top of the 

 slab. Readings were then made between these plates and the wires by 

 means of a specially designed instrument consisting of a brass stand 

 carrying a bell-crank lever, one end of which touched on the piano wire 

 above and the other end bore on the plunger of an Ames dial. By means of 

 a slow-motion screw the end of the bell-crank lever was adjusted to touch 

 the wire as indicated by an electric buzzer. The dial readings taken at 

 different loads then indicated the deflections at the various points on 

 the slab. This instrument is probably a more convenient form of measur- 

 ing device than the ordinary inside micrometer and is accurate to 



0,004 inch. 



DESCRIPTION OF SPECIMENS 



All three specimens were 32 feet wide, 16 feet span, and were made 

 of machine-mixed concrete in the proportions i to 2 to 4. Potomac 

 River sand and gravel were used as the aggregates, mixed with Port- 

 land cement. A rather wet mix was used, and the work of molding was 

 done by laborers at the ArHngton Farm who were experienced in work 

 of this character. There was no attempt to make the concrete any 

 better than it would ordinarily be made in the field, but efforts were 



