18 REINFORCED CONCRETE CONSTRUCTION 



It should be noticed in the table that the stone with the smaller 

 percentage of voids gives the lower strength. This seeming 

 irregularity is due to the fact that this stone measured loose 

 has more solid material per cubic foot than the stone with the 

 higher percentage of voids, and hence with the same proportions 

 by volume, this stone gives a greater bulk of concrete, and less 

 cement, per unit volume. A smaller amount of cement shows 

 here to have more influence in decreasing the strength than the 

 greater density has in increasing it. It must not be inferred 

 from this that the aggregate with the largest percentage of voids is 

 best to use. Such an aggregate requires more .cement to a given 

 volume of concrete, and although the aggregate with fewer voids 

 is sometimes slightly inferior in strength, the latter is the more 

 economical. 



All the preceding results for the crushing strength refer to a 

 compressive force applied over the entire upper surface of the 

 specimen. Experiments show that if the load is applied upon 

 only the central portion of the upper surface, the specimen will 

 bear a greater unit load, due to the fact that the outer portions 

 will aid the interior portion in resisting stress. In connection 

 with the designing of concrete footings for the Boston Elevated 

 Railway, thirty-six 12-in. cubes of 1:0:2 and 1:2:4 concrete 

 were crushed at different ages by applying the load over the 

 entire upper surface of the cube; then the same number of similar 

 cubes were crushed by applying the stress over an area of 10 by 

 10 in. and then a third set by applying the stress over an area 

 of 8 by 8 1/4 in. The third series gave a strength 128 per cent of 

 the first, and the second series gave 112 per cent. Age of the 

 concrete and proportions of ingredients did not seem to have 

 any influence on the above results. 



11. Tensile Strength. Concrete is frequently subjected to 

 tension in reinforced concrete construction; not directly, how- 

 ever, but as the result of bending as in beams, girders, and 

 slabs. The value of this transverse strength is of little impor- 

 tance; because, on account of the brittleness of concrete in tension 

 and its liability to crack from shrinkage or from the shock of 

 some of the applied loads, it is unsafe to depend upon the tensile 

 strength of- concrete in reinforced concrete design. In certain 

 computations, however, the tensile strength must be considered. 



The character of the sand and aggregate, and poor workman- 

 ship, have a greater influence on the tensile strength than upon 



