Modern production of high qualiy concrete is closely connected with wide use of different types of admixtures, which in small doses allow to obtain the required physical, technical, exploitation and economical properties of concrete. Improvement of the mechanical properties of normal strength R.C. beams in flexure by using plasticizing chemical admixtures is becoming on increasing by popular retrofit technique among researchers and engineering world wide. Plasticizing admixtures, which contain in their composition alkali industrial wastes, represent practical interest especially for our local country conditions. Therefore, the main purpose of the project described in this paper is to give better and full understanding of the flexural behaviour and failure mechanism of normal strength R.C. rectangular beams containing in their compositions alkali wastes and secondary products of oil and cellulose paper industries and reinforced with different amounts of steel under static loading. The main variables studied in this research were: three different types of plasticizing admixtures, which contain in their compositions different amounts of alkali wastes from secondary products of oil and cellulose paper industries, and three different types of beams reinforced with different amounts of tension steel bars(under reinforced -type A, balanced reinforced -type B,and over reinforced - type C) . The experimental results showed that, the suggested plasticizing admixtures containing amounts of alkali industrial wastes in their compositions have a good effect on increasing the ultimate capacity of under and balanced R.C. beams by about 27% & 77% respectively. Also, they decrease their maximum central deflection by about 20% & 18% respectively compared to the control tested beams. These admixtures, especially type (SM-S) decrease the cracking capacity of under reinforced beams by about 20% and increased it for balanced reinforced beams by about 40% . They increase factor of safety of tested beams against complete failure by about 90% for under reinforced, 117% for balanced reinforced and 19% for over reinforced beams. The suggested admixtures have a bad influence on the flexural behavior of over reinforced tested beams compared to the control ones.