NSM technique to increase the load carrying capacity of continuous RC slabs

Abstract This work presents and analysis the results of an ongoing research project on the use of the Near Surface Mounted (NSM) carbon fiber-reinforced polymer (CFRP) laminates for the increase of the load carrying capacity of statically indeterminate (two spans) reinforced concrete (RC) slabs. The test program consisted of seventeen slab strips, grouped in two series that are different on the adopted strengthening scheme: H series, where H is the notation to identify the slabs strengthened with NSM CFRP laminates exclusively applied in the hogging region; HS series, where HS is the notation to identify the slabs strengthened with NSM CFRP laminates applied in both hogging and sagging regions. The program includes six unstrengthened reference RC slab strips, and eleven strengthened with CFRP NSM configurations designed to increase in 25% or 50% the load carrying capacity of the corresponding reference RC slabs. An extensive monitoring system was applied in the constituent materials, in the critical regions of the tested slabs, in order to collect information for the discussion about the effectiveness of NSM technique in terms of load carrying and moment redistribution capacity. The strengthening procedures adopted in the laboratory tests followed, as much as possible, the real strengthening practice for this type of interventions. The obtained results show that the proposed technique is able to increase significantly the load carrying capacity of statically indeterminate RC slabs, even for those with relatively high steel reinforcement ratios. The load carrying capacity of the strengthened slabs was limited by its shear capacity or by the detachment of the strengthened concrete cover layer. At failure of the strengthened slabs, the longitudinal steel bars at intermediate support (hogging region) and at loaded sections (sagging regions) have already yielded, and the deflection was quite large. However, for some strengthening configurations, the CFRP laminates led to a decrease of the moment redistribution capacity of the slabs.

[1]  A. K. M. Anwarul Islam,et al.  Effective methods of using CFRP bars in shear strengthening of concrete girders , 2009 .

[2]  Paulo B. Lourenço,et al.  Efficient Strengthening Technique to Increase the Flexural Resistance of Existing RC Slabs , 2006 .

[3]  K. Zilch,et al.  Beiträge im fib-Bulletin 14: Externally bonded FRP Reinforcement for RC Structures , 2001 .

[4]  Matthew B. Blaschko,et al.  Externally bonded FRP reinforcement for RC structures , 2001 .

[5]  J. Barros,et al.  Near surface mounted CFRP laminates for shear strengthening of concrete beams , 2006 .

[6]  H. Akbarzadeh Bengar,et al.  Experimental investigations and verification of debonding strain of RHSC continuous beams strengthened in flexure with externally bonded FRPs , 2010 .

[7]  Joaquim A. O. Barros,et al.  Shear Strengthening of T Cross Section Reinforced Concrete Beams by Near-Surface Mounted Technique , 2008 .

[8]  Ashraf F. Ashour,et al.  Flexural strengthening of RC continuous beams using CFRP laminates , 2004 .

[9]  Stephanie L. Walkup,et al.  Guide for the Design and Construction of Externally Bonded FRP Systems for Strengthening Concrete Structures (ACI 440.2R-02) , 2005 .

[10]  Ashraf F. Ashour,et al.  Sagging and Hogging Strengthening of Continuous Reinforced Concrete Beams Using CarbonFiber-Reinforced Polymer Sheets , 2003 .

[11]  M. Aiello,et al.  Moment redistribution in continuous reinforced concrete beams strengthened with carbon-fiber-reinforced polymer laminates , 2007 .

[12]  Joaquim A. O. Barros,et al.  Possibilities and challenges of NSM for the flexural strengthening of RC structures , 2008 .

[13]  S. Rizkalla,et al.  Near-Surface-Mounted Fiber-Reinforced Polymer Reinforcements for Flexural Strengthening of Concrete Structures , 2004 .

[14]  Comite Euro-International du Beton,et al.  CEB-FIP Model Code 1990 , 1993 .

[15]  J. Barros,et al.  Efficacy of CFRP-based techniques for the flexural and shear strengthening of concrete beams , 2007 .

[16]  V. Bianco,et al.  New Approach for Modeling the Contribution of NSM FRP Strips for Shear Strengthening of RC Beams , 2010 .

[17]  Ashraf F. Ashour,et al.  Sagging and hogging strengthening of continuous reinforced concrete beams using CFRP sheets. , 2003 .

[18]  Anders Carolin,et al.  Carbon Fibre Reinforced Polymers for Strengthening of Structural Elements , 2003 .

[19]  Irene S. T. Liu.  Intermediate crack debonding of plated reinforced concrete beams , 2006 .

[20]  Everaldo Bonaldo.  Composite materials and discrete steel fibres for the strengthening of thin concrete structures , 2008 .

[21]  Nabil F. Grace,et al.  Strengthening of Cantilever and Continuous Beams Using New Triaxially Braided Ductile Fabric , 2004 .

[22]  Deric J. Oehlers,et al.  Tests on the Ductility of Reinforced Concrete Beams Retrofitted with FRP and Steel Near-Surface Mounted Plates , 2006 .

[23]  Joaquim A. O. Barros,et al.  Flexural Strengthening of RC Continuous Slab Strips Using NSM CFRP Laminates , 2011 .

[24]  J. Barros,et al.  Flexural strengthening of concrete beams with CFRP laminates bonded into slits , 2005 .

[25]  Joaquim A. O. Barros,et al.  Performance of reinforced concrete T beams strengthened in shear with NSM CFRP laminates , 2010 .