prefabricated reinforced concrete (RC) plates

Reinforced concrete (RC) beams which are insufficient in terms of shear and flexural capacities are strengthened by various methods. Steel and fibre reinforced polymer (FRP) plate bonding methods are very widely used in strengthening of beams. Strengthening methods such as bonding steel and FRP plates have deficiencies as corrosion, fire and buckling. In this work, it is aimed strengthening of damaged RC beams using prefabricated RC rectangular and U cross-sectional plates. The rectangular cross-sectional plates were bonded to the bottom sides of the beams by rods and epoxy. The plates having U cross-section were bonded to the three sides of the beams. The strengthened beams were incrementally loaded up to maximum load capacities. The load carrying capacity of the beams increased 41% with the strengthening rectangular cross-sectional plates, however when the U cross-sectional plates were used, the capacity increased up to 76%. The experimental results were compared with the theoretical values. In addition, post-elastic strength enhancement and displacement ductility of beams were investigated. The advantages of this method do not require shuttering, concrete and steel workmanships in situ. Also, the application of this method is very easy and economic. For these reasons, this method should be preferred against other methods. 
 
   
 
 Key words: Prefabricated reinforced concrete (RC) plate, strengthening, experimental, theoretical, ductility.


INTRODUCTION
The existing concrete structures and/or elements maybe damaged by chemical processes due to aggressive environment, excessive loading and poor initial design.It becomes both environmentally and economically preferable to repair or strengthen rather than rebuilt them.The choice between these and rebuilt is based on specific factors of each individual case, but certain issues are considered in every case.The strengthening of these beams would be desirable if rapid, economic, effective and simple strengthening techniques are available.Different methods are available for strengthening of existing concrete structures and/or elements.These are bonding with steel plates (Swamy et al., 1987(Swamy et al., , 1989;;Barnes et al., 2001;Sevuk et al., 2005;Adhikary and Mutsuyoshi, 2006;Arslan et al., 2008;Su et al., 2010), fibre reinforced polymer (FRP) sheets (El-Mihilmy and Tedesco, 2000;Buyukozturk and Karaca, 2002;Eshwar et al., 2004;Lu et al., 2005;Pham et al., 2006;De Lorenzis and Teng, 2007;Ozcan et al., 2009), external pre-stressing, external post tensioning and additional concreting (Diab 1998).The plate bonding technique is becoming preferable for strengthening due to several advantages, such as easy construction work, and minimum change in the overall size of the structure after plate bonding.The disadvantage of this method, however, is the danger of corrosion at the adhesive-steel interface, which adversely affects the bond strength (Sevuk et al., 2005).Swamy et al. (1987) researched the effect of glued steel plates on the first cracking load, cracking behavior, deformation, serviceability, and ultimate strength of RC beams.Adhikary and Mutsuyoshi (2006) presented the results of a parametric study accounting for the effects of plate depth/beam depth ratio, plate thickness, concrete strength and internal shear reinforcement ratio.The effects of each parameter on shear strength of beams with web-bonded steel plates were discussed.Finally, a design formula to compute the shear strength of beams with web-bonded continuous steel plates was presented.
A comparison between the shear strengths computed using the proposed formula and FEM as well as the experimental results was made.The objectives of this work are to investigate the effectiveness of flexural and shear strengthening with prefabricated RC plates having three different crosssections and load-displacement behavior of RC beams after strengthening.The first and second type prefabricated rectangular cross-sectional plates were used to increased flexural capacity of beams.The third type prefabricated U cross-sectional plate was to increased flexural and shear capacity of beams.Experimental results were compared with theoretical results obtained according to ACI 318 and TS-500 (2002).
It is thought that the strengthening method proposed can be useful, practical and reliable for a building or a bridge where similar beam sizes exist.

Experimental study
Two types of RC beam with the 150x250 mm dimensions were produced in the laboratory.The first type beams named as "A" had stirrups with 8 mm diameter and 150 mm interval.The second type beams named as "B" had stirrups with 8 mm diameter and 250 mm interval.From each type, three beams were produced.The first type beams were named as A1, A2 and A3.The second type beams were named as B1, B2 and B3.The beams were reinforced with two Ø10 bars (10 mm in diameter) in the compression zone, two Ø12 bars (12 mm in diameter) in the tension zone, as shown in Figure 1.Two type rectangular cross-sectional prefabricated RC strengthening plates were shown in Figure 1.The plates with 80 and 120 mm in thickness were reinforced with two Ø12 bars (12 mm in diameter) in the tension zone.Stirrups of 8 mm in diameter and 100 mm in interval were applied (Figure 1).The first type plate named as "a" was bonded to the bottom face of the beam by epoxy called "HILTI HIT-RE 500" and anchorage rods.
Before the rods of 10 mm in diameter were applied, the holes of 12 mm in diameter on the bottom faces of the beams and strengthening plates were drilled.These holes were filled with "Hilti".To each hole, a rod was driven about 150 mm into the beam (Figure 2).The second type plate named as "b" is bonded to the beams as "a" plate.The first type strengthening plate was used once; the second type strengthening plate was used twice in the experiments.The loading of the strengthened beam was as shown in Figure 3.
The third type of strengthening application was with U crosssectional prefabricated RC strengthening plates (Figure 4).The plates with 80 mm in thickness were reinforced with two Ø12 bars (12 mm in diameter) in the tension and compression zones.Stirrups of 8 mm in diameter and 150 mm in interval were applied (Figure 4).The strengthening plates were bonded to the three faces of the beams by epoxy based glue called "HILTI HIT-RE 500" and anchorage rods.Before the rods of 10 mm in diameter were applied, the holes of 12 mm in diameter on the two sides and bottom faces of the beams and strengthening plates were drilled (Figure 4).These holes were filled with "Hilti".To each hole, a rod was driven about 150 mm into the beam.The loading of the strengthened beam is shown in Figure 5.

Materials
Table 1 shows the properties of the beams and plates.Three samples were taken from each type of reinforcement.The tensile tests were carried out, and the yield strength and Young's modulus of these samples summarized (Table 2).The technical properties of epoxy were detailed in Table 3.

Finding of theoretical failure loads for strengthened and unstrengthened beams
The flexural demand should be computed with the load factors according to ACI 318 and TS500 (2002).The moment capacity for a rectangular RC member strengthened with prefabricated plate is given by Equation 2, where the moments of the internal beam forces are summed about the neutral axis.The equilibrium of internal and external moments is shown in Figures 6 and 7.
Where, As is the cross-sectional area of tensile reinforcement; fs yield strength in tensile reinforcement; fc compression strength of concrete cube specimens.In the strengthened beams, As is the total tension area of the beam and the strengthening plate.k1 is the ratio of the depth of the equivalent rectangular stress block to the depth of the neutral axis and expressed in ACI 318 and TS500 (2002).Mr is the moment carrying capacity; Md is the moment due to loads.PF, PG, Ln are the theoretical failure load, uniform distributed load due to weight of beam and clear span of beam, respectively.

EXPERIMENTAL AND THEORETICAL RESULTS
The beams A1, A2, A3, B1, B2, and B3 were loaded until   flexural cracks started.These cracks were repaired with epoxy called "Hilti".The strengthened beams were loaded until they failed.The failure loads and increase in load carrying capacities were given in Tables 4 and 5.The present experimental results indicated that the load-displacement curves of the beams (Figures 8 and 9) can be idealized by a bi-linear curve (Figure 10).The displacement ductility factor µ ∆ , which is defined as the ratio between the displacement at peak load ∆ u and the notional yield displacement ∆ y is adopted to measure the ductility performance of the strengthened beams (Su et al., 2010).The displacement ductility factors of all beams were calculated using the above definitions and the results were shown in Tables 6 and 7. Substantial post-    elastic strength enhancement can be found in Figures 8  and 9.The post-elastic strength enhancement factor ν is defined as the ratio between the peak strength, P u and the yield strength, P y , (Figure 10) (Su et al., 2010).Tables 6 and 7, shows the post-elastic strength enhancement factors of the beams.

Conclusions
There are several strengthening methods found in the literatures which use FRP plates or steel plates.But, corrosion is an important problem for steel plates.
Although, FRP plates are safe and light in weight, fire and freeze-thaw are important problems for them.Moreover, in strengthening with FRP and steel plates, the structure does not get extra strength against lateral loads.Therefore a new strengthening method for structural beams is proposed in this study.
In this method, the lateral load carrying capacity increases since the depth of the beams increase.In addition, the moment and shear capacities of beams are increased.Strengthening with rectangular cross-sectional plates, the load carrying capacity of the beams increased to 41% and those strengthened with U cross-sectional plates increased to 76%.
Since the behavior of epoxy and anchorage rods are not taken into account in equations, experimental and theoretical results of strengthened beams are incompatible.It is more compatible for un-strengthened beams according to others.
The post-elastic strength enhancement and displacement ductility are identified as two important structural performance criteria for structures predominantly subjected to gravity loads.These two criteria were greatly influenced by the prefabricated strengthening plates.It was observed that sufficient ductility and strength enhancement could be achieved by the rectangular and U cross-sectional plates.
Depending on the reasons mentioned above it can be said that the strengthening method examined both experimentally and numerically is practical, reliable and economic.Since the strengthening plate is made of the same material as the beams, it is more aesthetic and economic.The strengthening method proposed is a good alternative to strengthening with FRP and steel plates.More experimental and theoretical studies are recommended with reverse cycling loading for the better determination behavior of strengthened beams with prefabricated RC plates.

Figure 1 .
Figure 1.The strengthening with "a and b" plates of the first type beams named "A".

Figure 2 .
Figure 2. The implementing of Hilti and anchorage rods.

Figure 3 .
Figure 3.The loading of the strengthened beam.

Figure 4 .
Figure 4.The strengthening with "c" plates of the second type beams named "B".

Figure 5 .
Figure 5.The loading of the strengthened beam.

SciFigure 6 .Figure 7 .
Figure 6.Finding of theoretical failure load for un-strengthened and strengthened beam with rectangular cross-sectional plates.

Figure 8 .
Figure 8. Experimental load-displacement curves for un-strengthened and strengthened beams with plates rectangular crosssection.

Table 1 .
Material, steel, cross-section properties of elements.
*30 mm concrete with fine aggregate.

Table 3 .
Technical properties of HILTI HIT-RE 500.

Table 4 .
Failure Loads for Beams (with Rectangular Cross-section).

Table 5 .
Failure Loads for Beams (with U Cross-section).

Table 6 .
Comparison of displacement ductility factors and post-elastic strength enhancement factors.

Table 7 .
Comparison of displacement ductility factors and post-elastic strength enhancement factors.