CHAPTER XV 

 REINFORCING CONCRETE; CEMENT- WORKING TOOLS 



134. The Principle of Reinforcing. Plain concrete is 

 strong in compression, but will not resist a very great load 

 when in tension. Steel is a material that has a great tensile 

 strength, as well as compressive strength, so, by combining 

 the two, we have a resultant material which is strong in both 

 tension and compression, and can be adapted to most any 

 use. 



The design of reinforced concrete structures is quite tech- 

 nical and has no place in a text of this character. For simple 

 types of construction, such as reinforcing for a silo, water 

 tank, retaining wall, fence posts and well tops, the student 

 can refer to tables in hand-books, or use his best judgment, 

 bearing in mind that the amount of reinforcing will vary from 

 3/4 to 1-1/2 per cent of the cross-section of the member being 

 reinforced. 



135. Compression and Tension in Beams. A con- 

 sideration of the basic principles underlying simple reinforced 

 concrete construction may be of interest. Consider a simple 

 beam of uniform cross-section like a 2" x 4", supported at each 

 end, with a load applied at the center (Fig. 144). It will be 

 found that the upper part of the beam will be in compression, 

 or tending to crush together, and the lower part will be tearing 

 apart, or in tension. It will be noted that there is a plane 

 perpendicular to the force applied and cutting the beam in 

 half where there is neither tension nor compression. This is 

 called the neutral plane or neutral axis. 



Now, since the lower part of the beam is in tension, and 



135 



