Hollow-core slabs (HCS) are precast slabs of prestressed concrete widely used in the construction of residential parking and industrial buildings floors credit to the benefits they present: i.e. high-quality control, ease installation and reduced construction time – among others. Typically, they are manufactured by extrusion or slip-formworks using concrete with very low workability. With current modifications, HCS are capable of spanning long distances and offer low self-weight hence overall reduction in total self-weight of the structure. HCS are usually simply supported at their ends. This makes their end zones very critical regions with regard to shear force. To be precise, the end zones are disturbed regions mainly stressed in tension by shear forces (in a zone where the beneficial effects of prestressing are not totally active) and splitting actions. Consequently, it is vital that these zones be thoroughly studied, particularly under shear loading, for new reinforcement solutions to be developed. Fortunately, a solution is within reach if Fiber Reinforced Concrete (FRC), which has been proven to be very effective in enhancing shear strength of Reinforced Concrete (RC) structures and prestressed elements, was to be employed.
Ideally, fibers can be used to replace the conventional web reinforcement required in these elements for both minimum shear reinforcement and equilibrium. A review of existing literatures reveals that significant reduction in end slip of tendons can be achieved with increase in the amount of fibers. Nonetheless, it is evident that there lacks adequate knowledge concerning the shear behavior of HCS reinforced by macro-synthetic fibers. In light of this, researchers from the University of Brescia in Italy: Dr. Antonio Conforti, Eng. Alan Piemonti and Professor Giovanni A. Plizzari together with Dr. Francisco Ortiz-Navas at the Concrete Science and Technology Institute of Universitat Politecnica de Valencia in Spain investigated the possibility of using macro-synthetic fibers as reinforcement of HCS end zones. Their work was motivated by promising results presented in prior research. Their current work is published in the research journal, Engineering Structures.
In their approach, an experimental campaign on five full-scale HCS (420 mm deep, 1200 mm wide and 6000 mm long) was carried out. Two different reinforcement solutions were considered: typical conventional reinforcement generally adopted in practice (reference samples, RC) and Polypropylene Fibre Reinforced Concrete (PFRC specimens). Specimens were tested under shear at end zones by considering two different loading configurations: a/d = 3.5 and a/d = 2.8 according to EN1168.
The authors reported that the tested macro-synthetic fibers were able to improve the shear strength of hollow-core slabs by about 25%. The researchers also noted that the tests according to EN1168 were more affected by arch actions as compared to a/d = 3.5. In addition, all samples showed a web-shear cracking starting from the external webs, since those webs were generally characterized by the highest tendon slip.
In summary, the study evaluated the possibility of enhancing the shear strength of Hollow-Core Slabs (HCS) by using Polypropylene Fiber Reinforced Concrete (PFRC). The team found that PFRC enhanced the shear strength of hollow-core slab end zones mainly by improving the bond between tendons and concrete with consequence reduction of tendon slip. In a statement to Advances in Engineering, Professor Giovanni A. Plizzari, the lead author pointed out that the comparison between experimental results and predictions of four international codes (Eurocode 2, ACI 318-14, Model Code 2010 and EN1168), further revealed the need of improving the actual shear formulations.
Antonio Conforti, Francisco Ortiz-Navas, Alan Piemonti, Giovanni A. Plizzari. Enhancing the shear strength of hollow-core slabs by using polypropylene fibres. Engineering Structures, volume 207 (2020) 110172.