Comparative Performance of Prestressed Concrete Beam Using FRP Wrapping

Abstract

Use of prestressed concrete construction is becoming increasingly more common and popular today in India starting with general building construction to a bridge construction. In past the use of prestress concrete was mostly restricted to big structures i.e. tall buildings and bridges with large span girders. The use of prestressed concrete in small scale is thus a less practiced and hence less known method of construction.

The Behavior of Prestressed concrete beams subjected to two point loading of different types is examined in this report. Effect of strengthening using Glass fiber reinforced polymer (GFRP) laminates before & after first cracking has been evaluated on various parameters pertaining to prestressed beams. The experimental program includes testing of twelve simply supported prestressed concrete beams and four reinforced concrete beams having cross-section 150 mm x 200 mm with effective span of 2.7 meter in case of all the beams. Four PSC beams are initially loaded upto first cracking. In addition to these four prestressed beams also are different strengthened using GFRP laminates and then are tested upto failures. Effects of different strengthening patterns on deflection, failure load, failure mode, strain and moment-curvature relationship has been evaluated and discussed in detail in this report.

Four different cases are undertaken in the experimental program. For (2L/7) span loading, wrapping of full length at bottom and upto 1/3 of depth is provided, forming a U-shape around the beam cross section. For (2L/6) span loading similar wrapping explained as is been provided. For (2L/4) span loading, wrapping of full length at bottom and upto 1/3 of vertical depth is provided. In addition to this extra wrapping is also provided near the supports in this case.

For (2L/3) span loading, for the full depth, U shape wrapping is provided near the supports. It is found from study that in (2L/7) & (2L/6) span loadings, the FRP wrapping on beam along longitudinal direction, significantly reduces beam deflections and increases the load carrying capacity of the beam respectively. In (2L/4) span loading, combination of vertical and horizontal GFRP sheets, together with a proper epoxy adhesion, has led to about more difference in the ultimate load carrying capacity of the beam. Lastly, in (2L/3) span loading, presence of vertical GFRP sheets near support reduces the shear effects considerably.

For these tests, two point loading is applied with variation of 2 kN axial load with simply supported boundary conditions. The structural response is measured in terms of displacement, loading position on the beam and strain at center of bottom side of beam. LVDT and load cell are used to measure displacement and load respectively. Electronic and mechanical strain gauges are used to evaluate strain induced at different locations for the prestressed beam.

By compiling the results in different loading positions, few of very important parameters i.e. deflection, electronic strain, mechanical strain, momentcurvature relationship etc. are obtained. Improvement in behavior of specimens wrapped before and after first crack also is evaluated and discussed.

Analytical models of prestressed concrete and reinforced concrete beams are prepared in ANSYS considering similar boundary conditions as used in the experimentation. Considering behavior of specimens up to the linear stage only, load is applied to the model. Theoretical values of failure load are calculated for control as well as strengthened specimens. To calculate failure load, Limit State method is used for control specimens, whereas maximum strain observed in GFRP during testing is utilized for the strengthened specimens.

Experimental results in terms of displacement and strain are compared with ANSYS results. Model is found stiffer compared to specimen tested in the experiment due to only linear material property assigned to the model. This comparison encourages need of learning and incorporating material non-linearity in the model. Also analytical and experimental failure loads are compared here.

This comparison has elaborated the need of finding contribution of concrete, steel and FRP in resisting the total load by obtaining strains individually in concrete, steel and FRP respectively using electronic strain gauges.