Carbon Fibre Reinforced Polymers (CFRP) strengthening – NSMR


The utilization of Near Surface mounted Reinforcement for solid and concrete structures are not another development. A kind of NSMR has been utilized since the 1940s, where steel reinforcement is set in spaces in the solid cover or also solid cover that is moulded onto the structure. Here steel bars are put in spaces in the cement structure and after that the openings are grouted. It has additionally been very basic to utilize steel bars, affixed to the outside of the structure secured with shot Crete. However in these applications it is frequently hard to get a decent attach to the first structure and now and again, it is not generally simple to cast the solid around the entire steel strengthening bars. From 1960s the improvement of solid glues and adhesives, for example, epoxies, for the construction industry propelled the technique facilitate by holding the steel bars in sawed openings in the solid cover. Be that as it may, because of the consumption affectability of steel bars an extra solid cover is still required. For these applications, epoxy covered steel bars are not generally consumption safe for different reasons that won’t be examined here. The utilization of steel NSMR can’t be said to have demonstrated incredible achievement. In any case, by utilizing CFRP NSMR some of these downsides that steel NSMR gangs can be overcome.

Firstly, CFRP NSMR does not oxidize, so thick concrete spreads are not required. Furthermore, the CFRP cover can be carefully fit for close surface applications and additionally the lightweight of the CFRP overlays makes them simple to mount. At last, contingent upon the type of the type of the cover air voids behind the behind can be kept away from. Both epoxies and frameworks utilizing top notch bond mortar can be utilized. Notwithstanding, before continuing, a short portrayal of how to attempt a reinforcing work with NSMR will be given. In viable execution the accompanying strides should by and large be performed amid fortifying:

  • Sawing openings in the solid concrete cover, with the profundity relying on item utilized and the profundity of solid concrete cover.
  • Watchful cleaning of the spaces in the wake of sawing utilizing high-pressurized water, roughly 100-150 bars is suggested. No saw mud is permitted in the space.
  • On the off chance that a cement system is utilized, the opening must be dry before holding. On the off chance that a bond framework is utilized it is for the most part prescribed that the current surfaces are wet at the duration of solid mortar molding.
  • Cement is connected in the opening, chiefly with an adhesive, concrete mortar is connected in the space.

Table:  Aspects and characteristics of externally bonded FRP reinforcement

Properties NSMR
Shape Rectangular strips or laminates
Dimensions:

Thickness

Width

Simple bonding of factory made profiles with adhesive or cement mortar in presawed slots in the concrete cover
Application aspects For usage in flat surfaces

Based on the distance to steel reinforcement

A slot has to be sewn up in the concrete cover

The slot has to be intricately cleaned ahead of bonding

Bonded with a thixotropic adhesive.

Can use cement mortar for bonding

Appropriate for strengthening in bending

Slight protection against fire

 

Assumptions in NSMR Strengthening

In the outline for reinforcing with NSMR the accompanying suppositions are made:

Bernoulli’s theory applies, i.e.; straight strain over the segment differs rectilinearly. This suggests the linear strain in the concrete, steel support and lamination that happening at a similar level is of a similar size. Composite activity applies between every one of the materials included.

Concrete stresses are acquired from the materials characteristic bend. Concrete pressure strain is constrained to an affirmed failure strain of €=3.5%.

For a broke cross segment, the concrete’s lingering ensile quality is disregarded.

The stress in ductile and compaction steel support is taken from the reinforcement’s attributes bend relating to the aggregate strain. The aggregate strain in cover may not surpass the failure strain.

The lamination is thought to be linearly flexible until breakage i.e.; Hooke’s law applies.

Notwithstanding this it is essential to realize that if there exists a strain field on the structure, due to for instance the dead load, this must be considered in design outline.

Strengthening techniques

The NSMR strengthening is dependent on fixing,by epoxy adhesive, Carbon Fibre Reinforced Polymers into precut slits exposed in the concrete cover of lateral surfaces of beams. The EBR and NSM strengthening methodoliges are shown in the figures below:

  1. External bounded
  2. Near surface mounted

Itemizing of the close surface mounted reinforcement is an essential issue; we have to choose the most appropriate FRP cross-section and adhesive. In configuration there ought to be viewed as the base distance between contiguous reinforcement to keep away from level propagation of the splitting breaks, and the minimum separation from the edge of the part to maintain a strategic distance from edge partition impact.

Utilization of near surface mounted FRP fortification comprises of the accompanying working strides:

In the initial step a groove is cut utilizing a saw with maybe a couple of diamond blades edges or a processor with measurements in capacity of the reinforcement size and sort. Additional arrangement of the section comprises of cleaning the surface from tidy and lose parts utilizing vacuum or compacted air, then the groove is filled most of the way with cement, a short time later the FRP bar/strip is embedded and delicately squeezed to let the cement stream around the FRP. At long last, the depression is loaded with more glue and the surface is leveled .The base measurement of the grooves ought to be taken at least 1.5 circumstances the distance across of the FRP bar. At the point when a   (strip) with expansive angle proportion is utilized, the base dimensions has to  be 3 times the bar width and 1.5 circumstances the bar stature. In different examples, the base section measurement could be the consequence of establishment necessities rather than engineering. For instance the depression width might be constrained by the least sharp edge measure and the profundity by the solid cover. We ought to dependably abstain from cutting of the current steel reinforcement. Optimal measurements of the notch may rely on upon characteristics of the cement, surface treatment of FRP, and cement elasticity, surface totals.

Spacing of FRP shear reinforcement cannot go beyond lnet /2 or 600 mm

Advantages of NSMR

  • A low mass of the fiber makes it simple to hold exclusive of elating tools at the site
  • Insignificant alterations of cross-section, self mass and free elevation of a construction.
  • Easy to apply

Disadvantages of NSMR

  • Exclusive of any kind of protection the reinforcement is fragile to exposure of fire and impact.
  • Design clients, contractors and consultants have limited familiarity.