Evaluation of the Effects of Orientation and Coverage Areas of FRP Lamination Bonded with Two-Way RC Slabs – A Modular Approach

Many researchers have proposed different schemes of fiber reinforced polymer (FRP) wrapping to strengthen the Reinforced Concrete (RC) slab. However, the ideal system for FRP laminations is not readily available. A modular approach for strengthening of RC slabs (especially two-way slab) needs to be developed with respect to orientations and coverage area of FRP sheets in order to figure out the effective strengthening system. This paper describes the experimental study carried out to explore the effects of fiber reinforced polymers in different orientation and coverage areas to strengthen the structural members. Three systems of laminates have been used to strengthen the RC slabs using two types of fibers, namely carbon fiber reinforced polymer (CFRP) and glass fiber reinforced polymer (GFRP) composite. The systems are classified with respect to the orientation of FRP sheets such as edge, corner and center wrapping. Each system has three proportions of FRP coverage area in the order of 25%, 35%, and 45%. A set of fifty-four slabs were fabricated and tested at the rate of three specimens for each variant. The performance of the flexural strength of two-way RC slabs with different strengthening systems has been evaluated and compared. It shows that all the systems of strengthening are effectively enhancing the flexural strength of two-way RC slabs. The extent depends on the system orientation and coverage area of the FRP sheet. The higher-order performance is noticed when the FRP lamination has higher contact with the slab area and with closer proximity to the loaded area.


Introduction
Structures, especially RC -Reinforced Concrete structural members are always facing new challenges during their life span, in terms of loading, structural behavior and durability. The core problem is to provide sustainable durability for the structure and structural members in an effective manner, by adopting economical and reliable repair and strengthening techniques when structures suffer some distress. The readily available strengthening techniques based on lamination with fiber-reinforced polymers ( Many researchers have proposed different schemes of FRP wrapping to strengthen the RC slab; Hussein et al. (2010) proposed the strengthening system for RC slabs with CFRP strips. Use of FRP for strengthening results in the effective upgrade of structural capacity of slabs up to 500% for un-reinforced specimens and 200% for steel-reinforced specimens (Mosallam et al. 2003). Aljazaeri et al. (2018), evaluated the flexure performance of RC slabs with composite materials, and the composites increased the ultimate load to about 1.3 to 2 times than that of un-strengthened slabs. Gherdaoui et al. (2018) used the following orientations of the CFRP laminates used to strengthen the slabs such as 0 o , 45 o , 0 0 /90 0 and 45 0 /135 o . As of the available literature reviews, it is clear that the existing strengthening systems need to be developed further for RC slab with the concern on the effective orientation and sufficient coverage area of FRPs.
From the previous research, it is understood that the ideal approach of FRP laminations is not readily available, or ideal approaches are not considered in FRP bonded RC slabs like the beams strengthening systems of U, L, and bottom FRP lamination or wrapping (ACI440-R8, 2008). It is also learnt from the literature review that all strengthening systems used in previous researches have the consent only on the enhancement of performance factors Civil Engineering and Architecture 8(4): 706-713, 2020 707 instead of an effective enhancement.
A modular approach for strengthening of RC slabs (especially two-way slab) needs to be developed with respect to orientation and coverage area of FRP sheets in order to figure out the effective strengthening system. This paper aims to study the effectiveness of orientation with a satisfactory coverage area of FRPs on two-way RC slabs and corresponding performance factors such as flexural strength and mode of failure, ductility, and stiffness. CFRP and GFRP sheets have been used as a single ply on the two-way RC slabs for this study.
Three types of orientation have been adopted in this study, namely, Corner, Edge and Centre FRP wrapping with different coverage areas that are 25%, 35%, and 45 in each orientation type. In all the three orientations and for all the coverage areas, FRP laminates have been used only as uni-directional sheet wraps. Bi-directional sheet wraps have not been considered since bi-directional wrapping is a significant factor only if FRP Strips are used. Since, in this study, FRP Sheets are used, instead of Strips, bi-directional effect is automatically accommodated.
These coverage areas were chosen to explore the enhancement rate of flexural strength of two-way RC slabs with minimum amount of FRP wrapping. Although considerable research has been focused on bi-directional FRP sheets or strips with regards to its suitability as a repair and strengthening material, unidirectional FRP sheets or strips have received considerably less attention. In this study, unidirectional FRP sheets were used instead of bi-directional FRP sheets.

Strengthening System and FRP Properties
Three FRP laminated bonded strengthening systems were used to strengthen the RC slabs with carbon fiber reinforced polymer (CFRP) and glass fiber reinforced polymer (GFRP) composite. The systems are classified with respect to the orientations of FRP sheet such as edge (0, e yi ), corner (e xi , e yi ), and center (0, 0) wrapping. Each system has three variants of FRP coverage area which include 25%, 35%, and 45%. The Strengthening Systems are named System-I, System-II and System-III for edge, corner and center FRP wrapping, respectively. Unidirectional CFRP and GFRP sheets have been used to strengthen the RC slabs. The CFRP sheet has nominal thickness of 0.

Peak Load and Failure Pattern
The peak load and failure mode of the slabs are summarized based on the experimental result in Tables 1  and 2. Both the two FRP materials show higher-order flexural strength than the control slabs without any FRP bonding. The flexural strength factor was determined as the ratio of peak load and yielding load of specimens (Mufi et al. 1996), as shown in Table 1 (Column 8). Specimens with higher coverage area of FRP showed an increase in peak load for all three systems of orientation.
In comparison, it is found that all systems with GFRP sheets exhibited increased flexural strength effectively with higher deflection than systems with CFRP sheets. System -III specimens both with CFRP and GFRP exhibited better performance as compared to System -I and II. In System-III, there was an increase in peak load in CFRP strengthened slabs, to the extent of 85%, 87%, and 97% with 25%, 35%, and 45% coverage area. Whereas in the system III, the slab with the GFRP sheet, the increase in peak load was to the extent of 99%, 96%, and 109% with 25%, 35% and 45% coverage area, respectively. The observation shows that the FRP wrapping is effectively increasing the flexural strength when it is closer to the load point. It is evidenced in System-I and II slabs with 35%, and 45 % of FRP coverage area from the higher peak load values, the superior performance due to the FRP lamination has higher contact with the slab area and with closer proximity to the loaded area than 25 % coverage area. Note: a e xi = ratio of distance from center of the slab to center of FRPs sheet and length of the slab along x -direction; e yi = ratio of distance from center of the slab to center of FRPs sheet and breath of the slab along y -direction; b a i = percentage of coverage area of FRPs on slab; c P y = yielding load of slab; P u = Peak load.
The influence and the effects of FRP type, orientation, and coverage area are reflected through the different failure patterns of the strengthened slabs ( Fig. 2-4). The details of the failure modes are presented in Table 2 (Column 6). Partial delamination or rupture or both was observed in System-I and II with both FRPs when the coverage area of FRP reaches beyond the benchmark coverage area of 25%. In System-III, with both FRPs, irrespective of coverage area, full delamination is observed initially by the yielding of steel, then crack propagation is extended towards the edge of the slab. In all the strengthened slabs, non-orthogonal flexural cracks are observed, which is different from the conventional orthogonal flexural crack of control slab.
The different trends of delamination were observed with respect to the coverage area and orientation of FRP sheet. In System-I (edge) and System-II (corner), partial delamination or rupture or no delamination was observed. The degree of delamination in System-I and II depends upon the distance between the FRP location and load point. All the 3 types of coverage area of FRP which were used in System-III (center wrapping) show the full delamination irrespective of percentage of coverage and type of FRP composite, which is due to the higher contact with the load area. It is evident that the level of delamination depends on the level of contact of FRP sheet with load point.

Ductility
As shown in Table 2 (Column 5), the ductility index of slabs was calculated as the ratio of ultimate displacement and yielding displacement of specimens (Aljiazaeri et al. and Mufi et al. 1996). The ductility index values show that the ductility of System-I, with both FRPs up to 35 % coverage area, is more or less equal to control slab ductility. In this System -I, the reduction of ductility is noticed in 45% coverage area specimens, CEC-45, and GEC-45. In System-II, the CFRP strengthened slabs have a higher deflection ductility than the control slab. Whereas for the GFRP strengthened slabs, GCO-25 and GCO -35, have higher ductility, but GCO-45 has its ductility reduced to about 10%. In the case of System-III, ductility increased in both FRP strengthened slabs. The maximum ductility is noticed in System-III, which is about 30% higher compared to the control slab. Observation shows, the maximum reduction in ductility of all systems is about 10 % with CFRP and GFRP sheet, which remained lesser than 30% (acceptable limit). It is confirming that the similarity in the ductility trend of Attari et al.

Load-deflection Behavior
The load-deflection curves in Fig.5-Fig.7 show the different behaviors of various systems. The behavior differences are not only because of systems FRP sheet orientation but also due to the various amounts of coverage area. All the systems provide a significant improvement in the peak load and linear and nonlinear stiffness of the slabs. The linear and nonlinear performance was affected because of FRP types, the orientation of FRP, and the coverage area of FRP. However, the differences in load-deflection behavior have not affected the load enhancement / Load Capacity Increase of the two-way RC slabs. The load-deflection curves of various systems with different coverage areas of FRPs show that the increase of the coverage area of FRPs leads to higher-order peak loads along with higher-order displacement in two-way RC slabs.

Conclusions
Based on the results of the experimental investigation presented in this paper, the following conclusions can be drawn.  All the systems of strengthening enhanced the flexural strength of RC slabs. The extent of enhancement depends on the systems and their coverage area of FRP. The performance in the enhancement of flexural strength with GFRP sheet is superior to the CFRP sheet.  The increase of ultimate load in CFRP strengthened slabs is to the extent of about 85%, 87%, and 97% with 25%, 35%, and 45% coverage area respectively. Whereas in the slabs with the GFRP sheet, the load capacity was increased by about 99%, 96%, and 109% with 25%, 35%, and 45% coverage areas, respectively.


The maximum ductility is noticed in center wrapping, which is about 30% higher compared to the control slab.  Both FRPs are effectively increasing the flexural strength when the wrapping is closer to the load area. It is evidenced in edge and corner wrapping with 35%, and 45 % of FRP coverage area, which shows the superior performance due to the higher contact with load point than 25 % coverage area.  Partial delamination or rupture or both are observed in edge and corner FRP wrapping with 35% and 45% of FRP Coverage area.  In all the strengthened slabs, a non-orthogonal flexural crack pattern is observed, which is different from the conventional orthogonal flexural crack observed in control slab.  Further extensive research on the effect of coverage area and orientation of the FRP laminates is required to explore the mechanism and additional factors to strengthen the RC slabs.  A mathematical model with FEM analysis may be helpful in validating the increase in load capacity and ductility and their relationship with the percentage coverage of FRP contact area.