Shear Strength of Prestressed Concrete T-Beams with Steel Fibers Over Partial/Full Depth

ACI Structural Journal, May/Jun 2006 by Thomas, Job, Ramaswamy, Ananth

Figure 3 presents the effect of addition of 1.5% steel fibers over partial or full depth on the load-deflection response of partially prestressed concrete T-beams of various strength grades. The load-deflection response indicates that the stiffness of the beams having fibers over the web and over the entire section is nearly the same for the same concrete grade. In the postpeak region, the reduction in the load was obtained as the crushing of concrete progressed. It was observed that the rate of widening of the inclined crack was at slower for the fiber-reinforced concrete beams when compared with the corresponding control beams having no fibers.

PREDICTIONS AND EXPERIMENTAL RESULTS

The proposed model presented in Eq. (7) for the prediction of shear resistance for partially prestressed beams was obtained by multiple linear regression analysis of 518 test data, including the various data compiled from the literature and the present study, as detailed in Table 3. The proposed model uses the size effect factor proposed by Bazant and Sun24 that accounts for the effect of aggregate size and amount of transverse steel. The predicted shear stress at ultimate v^sub up^, based on the earlier and present models, was compared with the present T-beams and are given in Table 5. The average ratio of the predicted strength based on the proposed model to the observed strength was found to be 0.97 with a coefficient of variation of 0.06. The details of the test data compiled from the literature used for calibrating the proposed model are given in Table 3. The average value of the ratio of the predicted shear stress to the experimentally observed shear stress v^sub up1^/v^sub uo^ has been found to be varying from 0.94 to 1.52. This variation in the prediction of the test data from various laboratories is attributed to the variation in the tensile strength of the fiber-reinforced concrete due to the variation in fiber length scales, variation in constituent materials (for example, aggregate size) and differences in the experimental setup. The shear strength of the test beam specimens was computed for varying the parameters such as compressive strength f'^sub cu^ , partial depth fiber parameter ψ^sub pF^, fiber factor F, and shear span to depth ratio (a/d).

Figure 4 shows the variation of the shear strength of a prestressed concrete beam with respect to concrete strength for various values of fiber factor F, and shear span to depth ratio (a/d). The shear strength of the beam due to the addition of fibers at various depths corresponding to the partial depth fiber parameter ψ^sub pF^ equal to 0.00, 0.50, 0.75, and 1.00 has been represented by different curves in the plots given in Fig. 4. A comparison of plots corresponding to the various values of partial depth fiber parameter ψ^sub pF^ indicates that the contribution of fibers over partial depth increases with increase in compressive strength. Most of the empirical models reported were designed to predict the inclusion of fibers over full depth (ψ^sub pF^ = 1.00) and assumed constant contribution from fiber pullout mechanism v^sub F^ for all concrete strength grades. Analytical models (for example, Cho and Kim12), which were originally developed for predicting the shear strength of nonprestressed reinforced beams having steel fibers, are amenable for the prediction of shear strength of prestressed T-beams having steel fibers over partial or full depth by including the effect of prestress separately (Eq. (10)). The formula for estimating shear resistance proposed by Cho and Kim12 has been based on the plasticity theory considering fibrous concrete in tension as ties. The yield lines for the different failure conditions, namely bottom bar yielding, top bar yielding, and no bar yielding, have been separately examined by Cho and Kim.12 Moreover, the cited analytical model 12 considers the pullout strength of hooked-end steel fibers as function of matrix strength. The semi-empirical analytical model for predicting the shear resistance proposed in the present study considers yielding of tension steel, matrix strength, and fiber matrix interaction effects including fiber pullout.