Tests on fibrous plaster sheet, reinforced with sisal and glass fibres in the bulk density range 56 to 72.5 Ib/ft 3 (W/P : 0.9 to 0.6), show that sisal fibre decreases the strength of the gypsum composite by approximately 30 to 50% and glass fibre by approximately 0 to 30%. The effect on compressive strength shown by the two fibres can be largely explained on the basis of differing volumes and stiffness. Variation of the fibre content in the plaster sheet increases the tensile strength with increasing fibre content much as would be expected.
A theory based on a non-linear stress-strain curve is proposed and the predictions of this theory are found to be in reasonable agreement with the test results. However, it is concluded that variations in the ultimate flexural strength, where sisal is used as the reinforcing fibre, are of such magnitude that it is impossible to predict flexural strengths with any degree of accuracy. Until the source of this variability can be found it is not possible to make any design improvements to the strength of the fibrous plaster sheet. The outlook is more hopeful for the use of glass fibre as a reinforcing material but further work is required to confirm this.