The toughness indices based on plain concrete first crack deflections and first crack areas were higher than those based on S.F.R.C. first crack deflections and first crack areas.The first crack load defined as the load level at which load deflection diagram deviates from linearity can be misleading since in the case of high fiber coment the presence of fibers causes the crack to extend very slowly and thus forced the load deflection diagram to maintain linearity beyond the actual first cracking load. Accordingly, the first crack areas of S.F.R.C. were larger than those of plain concrete due to the influence of fibers in increasing the first crack loads and first crack deflection. The average increase in first crack loads of S. F. R. C. to those of plain concrete were 3, 11,8 and 20 percent for using 0.75%, 1.50% by volume of straight and deformed
fibers respectivelyl6J. Similar results were given by Snyder and Lankardf11J , ACI
Commitee 544[1J and Mangatl121 .