REINFORCED CONCRETE. 51 



verse plane section before flexure remains a plane section 

 after flexure. On this assumption the curves of stress on 

 each side of the neutral axis have been derived. The stress 

 strain curve obtained from testing plain concrete prisms 

 in compression under gradually applied loads, in which the 

 abscissae represent the strains and the ordinates the loads 

 producing them are of approximate parabolic form, 1 and 

 this form is usually assumed for the curve representing the 

 compressive stress from the neutral axis to the extreme 

 fibre, where the maximum ordinate represents the intensity 

 of compressive stress at the extreme fibre. 



In order to test the accuracy of this assumption, ten 

 beams were made 72 inches long, of square cross section 

 10 by 10 inches, one beam was of plain concrete, the others 

 were reinforced each with three rods, varying in diameter 

 from f of an inch to 1 inch. The beams were supported at 

 points 40 inches apart and loaded at each extremity, so 

 that the bending moments and corresponding stresses 

 between these points of support were nearly constant. 

 Four sets of Martens' mirror extensometers 2 were arranged 

 on each side of the beam to be tested, at equal distances 

 from the centre of the beam, and Martens' sectors were 

 arranged at the top and bottom of the beam in order to 

 determine the strains produced by the loads applied, not 

 only at the extreme fibres, but at four other points in the 

 depth of the beam on each side. Martens' sectors and 

 dials were also attached to the beam in order to determine 

 the end and centre deflections. The loads were applied at 

 the ends of the beam by means of two hydraulic presses, 

 and two rolled steel beams, resting upon the table of a 



1 Further Experiments on the Strength and Elasticity of Reinforced 

 Concrete.— Proc. Eoy. Soc. N.S. Wales, Sept. 7, 1904. 



a Apparatus for ascertaining the minute strains which occur in materials 

 when stressed within the elastic limit, by Prof. Warren. The theory of 

 the Reflecting Extensometer of Prof. Martens, by G. H. Knibbs— Proc. 

 Roy. Soc. N.S. Wales, July and August 1897. 



