CFRP STRENGTHENING OF RC STRUCTURES USING THE NSM TECHNIQUE

CFRP composite materials can be applied to reinforced concrete (RC) structures using two different techniques: near surface mounting (NSM) or externally bonded reinforcement (EBR).
NSM is a recently adopted strengthening technique based on applying an appropriate adhesive and laying CFRP bars, rods or strips into pre-cut grooves on the concrete cover of the elements to strengthen them. In the EBR method, after surface preparation, CFRP sheets adhesively bond to the tension face of the concrete member using epoxy adhesive.
Figure-1 shows cross-section details of NSM & EBR strengthening techniques using CFRP rods and strips.

Figure 1: CFRP strengthening techniques

Advantages of the NSM strengthening technique over the EBR technique

When strengthening RC elements using the EBR technique, these factors should be taken into account:

The NSM technique is easier and faster to apply than the EBR technique.
For strengthening RC beams in shear, using the NSM method is much more effective; it will increase the beam’s load carrying capacity, deformability and ductility at its failure more than using the EBR technique.
CFRP applied using the EBR technique is less effective than the NSM technique because of various environmental reasons, such as:
- In the EBR technique, CFRP sheets are directly exposed to weathering conditions
- Negative influence of freeze/thaw cycles
- The effect of high and low temperatures

Strengthening RC columns using the NSM technique

RC columns in concrete structures are the most vulnerable elements since their failure leads to the collapse of the whole structure. These are the advantages of using the NSM technique to strengthen them:

The load-bearing capacity of RC columns strengthened by the NSM CFRP technique increases by about 34%~92% which depends on the existing cross-sectional area of longitudinal steel rebar in the column as well as the amount of cross-sectional area to be used for CFRP application. ¹
Repairing and strengthening RC columns with the NSM CFRP technique is more efficient than the EBR CFRP technique. ²
By increasing the CFRP cord spaces, the load resistance, axial deformation capacity, ductility and toughness of the RC column will decrease. ² The volume of CFRP in use, as well as the free spacing between CFRP cords, have a significant effect on stress distribution.
Groove dimensions greatly affect the column’s load-carrying capacity; the larger width to depth ratio, the higher the load-carrying capacity of the column. ² Note that by increasing the width to depth ratio of grooves, the confinement area of CFRP in the NSM technique increases and the free spacing between the CFRP rods/laminates will decrease. Therefore, by increasing the confinement area of the CFRP, the confinement efficiency will be higher and the load-bearing capacity of the column will be increased.
Combining the NSM CFRP technique with EBR CFRP sheets located in between existing steel hoops is a highly effective strengthening technique. Applying this combined technique not only significantly increases the flexural resistance of the column, but also the CFRP sheets contribute to avoid the buckling of the NSM CFRP and will increase the shear resistance of the RC columns (Figure-2). ³

Figure 2: NSM CFRP strengthening of RC columns (increases the flexural & shear resistance and confinement of RC columns)

Strengthening concrete beams using the NSM CFRP technique

In RC beams strengthened with the NSM CFRP technique for shear, the maximum load increases by 9% and the corresponding deflection is 16% less than the comparable values of the beam reinforced with steel stirrups in the equivalent shear reinforcement ratio. ¹
For flexural NSM CFRP strengthening of RC beams, the following results have been registered: ¹
- The average increase in the load corresponding to concrete cracking is 51%.
- The average increase in the load corresponding to the deflection of the serviceability limit state is 32% and the rigidity at this load level increases by 28%.
- The average increase in the load at the beginning of yielding the steel reinforcement is 39%.
- The maximum strain values (between 62%~91%) of the CFRP show that by using the NSM technique, we can increase the stress levels close to the tensile strength of CFRP materials.
The flexural capacity of a NSM-strengthened beam increases by about 25.7%~44.3% over the control beam. ⁴
It is worth mentioning that the ‘bond’ is of high importance for the effectiveness of NSM CFRP strengthening. Whenever there is not a perfect bond, increasing the amount of the NSM reinforcement does not have a significant effect on the load-bearing capacity the RC beams. In the case of poor bonding of NSM CFRP, even by using twice the area of NSM reinforcement, the flexural capacity of the beam increases by only 11%. ⁴

Figure 3: NSM- CFRP strengthening of beam- For Flexure

Figure 4: NSM CFRP strengthening of RC beams for shear

Authors
Parastoo Azad and Dr. Mehrtash Soltani (June 22, 2021)
References

Joaquim A.O. Barros, D. R. (2005). Assessing the effectiveness of embedding CFRP laminates in the near surface for structural strengthening. Construction and Building Materials.
Yasmeen Taleb Obaidat, A. M. (2020). A new technique for repairing reinforced concrete columns. Journal of Building Engineering.
Joaquim A.O. Barros, A. A. (2006). HYBRID CFRP-BASED STRENGTHENING TECHNIQUE TO INCREASE THE FLEXURAL RESISTANCE AND CONCRETE CONFINEMENT OF RC COLUMNS SUBMITTED TO AXIAL AND CYCLIC LATERAL LOADING. Challenges for Civil Construction.
Laura De Lorenzis, A. N. (2000). Strengthening of Reinforced Concrete Structures with Near Surface Mounted FRP Rods. International Meeting on Composite Materials.