Hollow-core are prestressed floor elements with voids in the cross-section. Structural efficiency and excellent flexural capacity of prestressed hollow core [PHC] slabs allow larger spans to be built leading to cost-saving. Increased dead or live loads, material degradation, architectural modifications, installation of heavy machinery, code revision and opening in slab pose challenges to the structural integrity of PHC slabs. Possibility of large shear forces on PHC slabs in high in buildings like warehouses, parking garages and where masonry partition walls are supported directly on the slab. Test results showed increased in the flexural capacity due to GFRP strengthening ratio plays a major role in changing the failure modes and the efficiency of GFRP strengthening.
The usage of precast concrete elements in construction is growing around the globe. Prestressed Hollow Core slabs Strengthening of PHC slabs may be required due to increased dead or live loads, material degradation, architectural modifications, unexpected installation of heavy machinery, reinforcement corrosion, and due to the above changes must be carefully addressed to ensure safety and integrity. The use of fiber Reinforced polymers [FRPs] has emerged as one of the most promising technologies in the field of structural strengthening. FRP has excellent mechanical with high strength to weight ratio, directional strength, corrosion resistance, weather resistance, non-magnetic characteristics and dimensional stability with low thermal conductivity. The objective of this study is to understand the behavior of FRP strengthened PHC slabs and study the changes in failure modes.
Three precast prestressed hollow core concrete slabs are tested until failure under a four-pointed bending configuration. Test specimens have a constant thickness of 250 mm, a width of 600 mm, and a length of 3500 mm. Each slab has a total of 44% of voids and is reinforced with high-strength low-relaxation steel tendons. All specimens were tested at a shear span-to depth ratio of 5.4.
All specimens were cast using normal weight, ready-mix concrete with a target compressive strength of 50Mpa at 28 days. The unit weight of concrete was taken as 2400kg/m3. The tested concrete strength for the slabs produced by the manufactures was 45 MPa at 7 days and 53.1 MPa at 28 days.
The type of standard used in the tested specimen were seven-wire low-relaxation strands with an ultimate value of tensile strength of 1860 MPa and modulus of elasticity of196.5GPa. 12.7-mm and 9.5 mm diameter strands were used at bottom and top respectively with an effective prestress of 1215 MPa in the bottom.
External Reinforcement- Glass fiber Reinforced polymer [GFRP]
The unidirectional GFRP laminate used in this research program was SIKA WRAP 930G. It was supplied in a roll package of the fabric having 500 mm width, 0.385 mm thickness and 100 meter length. Sika Wrap 930G is a unidirectional glass fiber. The epoxy resin used to bond the GFRP sheets to the concrete, masonry, or timber; for bonding steel or FRP to concrete.
All specimen has similar instrumentation details. Deflections were recorded using linear variable differential transducers[LVDT]. TML strain gauges with a gauge length of 120 mm were used to measure the strains in the concrete across the depth. Specific locations of LVDTs were chosen to capture the entire curvature profile during testing. Surface was thoroughly cleaned and strain gauges were installed at top and bottom at the centerline on the concrete slab to capture the strain profile. HBM data acquisition system was used to capture the data.
Test Setup And Loading Procedure
Slabs were tested in a four-points loading configuration by which a constant moment region along the mid-span was obtained. A 250kN MTS hydraulic actuator was utilized to apply two concentrated loads centered over the mid-span of each test specimen. The loads were transferred to the concrete specimen via a single longitudinal rigid steel spreader beam, stiffened with web stiffeners for high rigidity. Loading was paused in displacement control intermittently to observe the failure progression.
Behaviour Of HCS-250-5.4-FO-SO.As aforementioned this specimen was the control slabs. The total span of this slab was 3,500 mm; the slab was loaded by two-line loads 600 mm apart as shown schematically. During the application of loading, the flexural cracks first appeared when the load reached 129kN. Yielding of the prestressing tendons started when the load reached 160kN. The peak load measured was 187kN and corresponding mid-span deflection was 38.2 mm. Failure progression involved: Flexural cracking at the bottom over the constant moment region; Crack propagation and distribution of cracks velopment of diagonal cracks as extensions of previously existing flexural cracks; sudden compression failure below the loading points occurred.