Page 13 - June-Month
P. 13

TECHNICAL PAPER


               4000                                                 4000
                       24 kN                           196 kN
               3500                                                 3500
                        60 kN
               3000    84 kN                                        3000                                        196 kN
                                                                                                                184 kN
            Strain (microstrain)  2500  116 kN                     Strain (microstrain)  2500                   136 kN
                                                                                                                160 kN
                                                                                                                116 kN
                                                                    2000
               2000
                       136 kN
               1500
                                                                    1500
                       184 kN
               1000    160 kN                                       1000                                        84 kN
                                                                                                                60 kN
                       196 kN
                500                                                  500
                                                    24 kN                                                       24 kN
                 0                                                    0
                    G     F       E      D       C   B      A           1B  2B   3B   4B   5B   6B   7B  8B   9B
                                   Gauge Location                                   Strain Gage Position
           Figure 11: Strain profile along the central FRP strip. Location A is closest   Figure 12: Strain along line B (483 mm from the edge of the overhang)
                        to the edge as shown in Figure 5(b)


           This can be further elucidated through the strain measurements   4.  SUMMARY AND CONCLUSIONS
           from the NSM FRP strips. Figure 11 shows strain development
           along the central NSM FRP strip at the same load levels as in    The use of NSM FRP strips is seen to more than adequately
           Figure 9(b). These strain profiles indicate that the maximum   strengthen the deck overhang to enable the addition of the
           strain in the FRP strips occurs directly above the edge of the   sound barrier without premature failure. Failure was seen to be
           stem wall adjacent to the deck slab overhang, referenced as   non-catastrophic within the concrete and at levels of moment
           location “B” in the figure [details of location are shown in Figure   capacity higher than required. The NSM FRP-rehabilitated
           5(b)]. At ultimate capacity, the maximum strain measured across   specimen reached ultimate capacity under an applied load
           all strips was 3846 microstrain at a location along this line.   of 196 kN per hydraulic jack which is equivalent to an applied
           The strains are noted to drop off sharply for distances further   moment of 189.2 kN-m. This corresponds to a 72% increase in
           away from the end of the overhang, with the majority of the   ultimate capacity over the as-built unstrengthened specimen,
           strain gages on the inner side of the stem wall (location “C”)   which failed under an applied load of 114 kN corresponding
           exhibiting less than a third of the strain values shown at location   to an applied moment of 110.0 kN-m. In comparison with the
           “B”. The sharp drop in strain values at distances away from the   unstrengthened specimen which showed significant drop in
           stem wall and the minimal strains further away indicate that   stiffness at the higher load levels after initial cracking, the NSM
           significantly shorter lengths of FRP strips could have been used   FRP-strengthened specimen showed better resilience and a
           to optimize material usage and improve constructability without   progressive mode of failure based on separation of isolated
           affecting load transfer and overall system response.   individual strips within the concrete/adhesive. No fracture of the
                                                                  FRP strips themselves was seen.
           Details for strain development along line B, the location where
           the maximum strains occur in the specimen, are shown in Figure   The tests clearly show the viability of strengthening deck
           12, wherein it can be seen that the distribution of strains was   overhangs to allow for placement of additional weight on
           even along the specimen until a load level of 116 kN per jack   the edge such as would be required by sound/noise walls
           was reached. At this level, cracking was first observed in the   placed at the very edge of the overhang region due to space
           specimen and higher loading levels resulted in less uniform   considerations. The ability to conduct rapid strengthening
           strains along the specimen, as a result of local cracking and   through the use of NSM FRP strips bonded to the concrete as
           redistribution. The average strain along line B in the specimen   an alternative to substantial removal of concrete and placement
           at ultimate capacity was 3423 microstrain, whereas the minimum   of additional reinforcing steel which would not require
           and maximum strains along line B were 2943 microstrain and   significant traffic disruption and time intensive construction is
           3846 microstrain, respectively. Using the strain data throughout   a major advantage. Since the FRP strips are prefabricated in
           the specimen and following the procedure described by Siem   long lengths which can easily be cut to size on site and then
           et al.  [21] , the shear stress between the concrete and the CFRP   bonded into easily cut grooves with adhesives already used
           strips was determined to be 1.75 MPa in comparison to a listed   in the construction industry, the risk of the newer technique
           shear strength of 24.8 MPa  [22]  again indicating the effectiveness   is extremely low since no special equipment or methods are
           of the NSM FRP and its ability to ensure a non-brittle failure.  needed. Rapid cure of the adhesive also results in faster opening


                                                                               THE INDIAN CONCRETE JOURNAL | JUNE 2021  17
   8   9   10   11   12   13   14   15   16