Shear Capacity of Hollow-Core Slabs with Concrete Filled Cores

Abstract

Precast, prestressed, extruded hollow-core slabs are commonly used as floor and roof systems in concrete structures. This member is an economical and efficient cross section because it is fabricated with continuous voids along the length that reduce weight while maintaining strength. The extrusion manufacturing method is not conducive to using transverse shear reinforcement, so concrete failure generally governs the shear capacity. Adding slab cross-sectional area is the intuitive way to increase the available shear capacity, but this increases the unit weight, may require a deeper section, and can raise material costs. This project investigated an alternative method to add shear capacity by filling the cores with concrete where the expected shear demand was greater than the capacity. By targeting the local regions requiring increased shear capacity, this method was a practical means to economically add shear capacity. While this practice is already used in the industry, the core fill performance has not been studied thoroughly. Core fill concrete was added to cured slabs and added immediately following extrusion of new slabs to characterize how the core fill timing affected the shear strength. One slab that was immediately core-filled was aged for 209 days to study the long-term shear strength of core-filled slabs. Additionally, several novel core fill modifications, such as including welded wire reinforcement in the core fill, roughening the slab walls prior to core filling, using steel fiber-reinforced core fill concrete, and placing longitudinal rebar in the core fill concrete, were tested to evaluate their potential for improving the hollow-core slab shear capacity. Where core fill concrete was added to cured slabs, the core fill concrete behaved as unreinforced concrete, and slabs with core fill added immediately following extrusion behaved as if the core fill concrete was fully prestressed. The 209-day test of an immediately core-filled slab had a shear strength lower than that of the identically fabricated slab tested about a month after fabrication. Additionally, the core fill concrete of the 209-day specimen behaved as unreinforced concrete. Of the core fill modifications, only the slab wall roughening provided slightly improved shear performance compared to the typical immediate filled specimen. Specimens where the core fill concrete remained composite with the slab concrete following web-shear failure had the highest web-shear strengths, which indicated that bond between the core fill concrete and slab concrete is a critical factor in the shear strength of hollow-core slabs with filled cores. All test results were compared to an assortment of Code and Specification predictions for web-shear capacity. Using the ACI 318 (2018) web-shear strength equation for the slab with the core fill treated as unreinforced concrete provided consistently conservative estimates of capacity for all of the specimens.