CFRP prestressed concrete prisms as reinforcement in continuous concrete T-beams

Today, use of both fibre reinforced polymer (FRP) materials and high strength concrete has become a more common practice in construction. FRP materials have unique mechanical properties such as high tensile strength, light weight, corrosion resistance, and magnetic neutrality. However, most FRP materials have relatively low modulus of elasticity compared to steel. Due to this low modulus and the relatively small cross sectional area of FRP bars, when FRP reinforced concrete flexural members crack, the neutral axis considerably shifts up in the section causing a significant decrease in flexural stiffness. This is accompanied by increased deflections and crack widths under service load condition if not enough reinforcement is provided. However, placing additional FRP bars in the section in order to satisfy serviceability criteria increases the initial cost. In addition, by using higher reinforcement ratio in the section, high ultimate tensile strength of these materials cannot be utilized in design.;This thesis investigates the behaviour of two-span continuous concrete T-beams reinforced with carbon fibre reinforced polymer (CFRP) prestressed concrete prisms at service and ultimate load conditions. Companion steel and CFRP reinforced beams were tested for comparison and to evaluate the effect of reinforcement on beam behaviour. High strength concrete prisms were concentrically prestressed by a single CFRP bar and used as main reinforcement in these beams. Theoretical analysis showed superior performance of PCP reinforced beams before cracking of prisms and higher ductility than CFRP reinforced beams at ultimate. Test results confirmed that prior to cracking of the prisms, PCP reinforced beams had comparable deflection with steel reinforced beams whereas deflection in CFRP reinforced beam was 2.5 times larger. PCP reinforced beams exhibited the narrowest crack widths prior to cracking of the prisms. At ultimate, steel reinforced beams showed the highest ductility and moment redistribution followed by PCP and CFRP reinforced beams.;Bond characteristics between the precast CFRP reinforced prisms and cast in place concrete and development length of the prisms were investigated through conducting pull-out tests and results revealed that there is enough bond between all reinforcements to develop the ultimate tensile capacity of the CFRP bar with no evident slip.;As part of this study, the behaviour of CFRP prestressed high strength concrete prisms under direct tension was also investigated. Seven prestressed concrete prisms with different levels of prestressing were cast and tested. Prisms were 50 x 50 mm in cross section and their lengths varied between 1400 and 2000 mm. Concrete compressive strength was as high as 147 MPa. Tension stiffening, crack width and crack spacing in prisms were investigated. Concrete properties such as stress-strain relationship under direct tension and bond strength were also determined. Test results revealed that tension stiffening in CFRP prestressed high strength concrete is significant when higher concrete strength and higher prestressing level is applied. Tension stiffening factors were proposed based on post cracking behaviour of concrete. Experimental results also showed that increasing the prestressing level increases the amount of tension stiffening and reduces the number of cracks while delays their appearance. However, cracks widened at a faster rate in the prisms with higher prestressing level. Experimental results were compared with CEB-FIP and ACI proposed equations. Modifications were suggested for the above equations to account for use of CFRP bars in prestressed sections.;FRP materials have high ultimate tensile strength and this advantage can be greatly utilized if FRP is used for prestressing. FRP prestressed concrete prisms (PCP) are bars of small cross section made with high strength concrete and concentrically pre-tensioned by a single FRP bar. These bars can be used as reinforcement in concrete structures. Due to the effect of prestressing, when PCP is used as reinforcement, improvements in service load behaviour of PCP reinforced concrete flexural elements are expected.;Mechanical properties of ultra high strength concrete (UHSC) with compressive strength in excess of 160 MPa as part of materials study for this research was investigated in both compression and tension. Experimental results showed that mechanical properties of UHSC are slightly different from those suggested by codes and other research on high strength concrete (HSC). Consequently, some modifications were suggested to existing provisions and new equations were developed wherever it was necessary.