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*RESPONSE OF CONCRETE BEAMS REINFORCED WITH PERFORATED STEEL PLATES

1 Ali R. Khaloo, 2 Peyman Askarinejad
1 Professor of Civil Eng., Sharif University of Technology, P.O.Box 11365-9313 Tehran, E-mail: khaloo@sharif.edu
2 PhD in structures, Sharif University of Technology, P.O.Box 1933813177, E-mail: peyman as2003@yahoo.com

Abstract. Behavior of flexural members reinforced with high-stiffness perforated steel plates as tension reinforcement is investigated. An experimental program was carried out on beams reinforced with one, two, and four layers of perforated steel plates. Test result on beams indicated the capability of plates as longitudinal reinforcement in tension to acquire considerable strength at levels of cracking, yielding of the steel plate, and ultimate, and also to provide considerable ductility. Tests on beams reinforced with splice perforated steel plates showed a short splice length is required for transferring force in splice region as compared to that recommended by ACI code for ordinary reinforcing steel bars.

1 Introduction

Concrete is a brittle material that possesses little strength and ductility under tensile loading. A system capable of preventing crack growth and providing integrity can improve strength and ductility of concrete. In concrete under compression, a lateral confining system in form of perforated plate makes these improvements possible (1,2).

The perforated plates in tension region of concrete members, provide a uniform reinforcing system in the transverse direction and an integral reinforcing system along the transverse and longitudinal directions of the member. The bonding of perforated plates with concrete, is expected to require less development and splice lengths than ordinary reinforcement. Reliable performance of perforated plates in tension region of concrete members should be based on experimental verifications.

In this study, behavior of flexural members reinforced with perforated steel plates as tension reinforcement is investigated. An experimental program was carried out on beams reinforced with one, two and four layers of perforated steel plates. Test was also perforated on beams reinforced with splice perforated plates in order to estimate the required development length of perforated plates.

An experimental program was carried out to investigate the influence of perforated steel plates as tensile reinforcement on strength and ductility of concrete flexural members.

2.1 Flexural test

Beam specimens with dimensions of 18x20x110cm were tested under flexure. The thickness of plate and size of plate holes were similar in all the beams. Percentage of tensile reinforcement was changed by using different number of plate layers. Influence of spacing between plate layers was also investigated. The beams reinforced with two different splice lengths of perforated plates also tested. Details of flexural test program are given in table1. In total, six beams and four compressive cylinders for each beam were cast.

2.2 Perforated plates and concrete

Steel plate with a thickness of 3 mm was used to make 13 pieces of 110x17.4 cm rectangular perforated plates for beam specimens. The diameter of holes in beam plates was set to 48 mm (figure 1). Perforation of the plates was carried out using drill to avoid possible development of fine cracks that may occur if punching method was used. The maximum aggregate size of concrete was 12.7 mm for beam specimens. The concrete mixture was designed for compressive strength of 35 MPa and adequate workability to assure placement of concrete inside the holes and under the perforated plates.

2.3 Design, fabrication and testing of beams

Shear reinforcements were designed to prevent shear failure of beams reinforced with perforated steel plates (3). Variations in the cross-sectional area of perforated steel plate along the beam is shown in figure 2. Based on loading configuration presented in figure 3 and ACI design code, the flexural and shear strength of beams were calculated. The beams were reinforced with one plate layer (beam IL), two plate layers (beam 2L), two plate layers spaced at 2.5 cm (beam 2L-S) and four plate layers (beam 4L). The cover concrete was 2.5 cm. Two steel bars (?6) were used in the compression region of all the beams. Two beams were reinforced with splice perforated steel plates, one with lap length of 6 cm (2 hole-lapped) and the other with lap length of 12 cm (4 hole-lapped) (figure 4). These specimens were designed to fail in a bond splitting mode. The tensile test on steel plates resulted in yield and ultimate strength of 300 MPa and 400 MPa, respectively. Vertical deflection at center of beams and strains in compressive and tensile regions of beams were also measured.

3 Experimental results and discussions
3.1 Flexural strength

The cracking strength (Mc?), yield strength (M?) and ultimate strength (Mu) of beams are given in table 2. The load-deflection curves of beams in figure 5 show that increase in steel ratio (p) increased ultimate strength and decreased ductility of beams similar to conventrently reinforced beams. The ultimate strength of beams are calculated according to ACI design code (3) using the minimum and average values of steel ratio (i.e., A, (min)=1.7 cm and A, (av)=2.5 cm for one steel plate layer). The experimental ultimate strength is higher than that according to ACI code using the minimum steel ratio (figure 6a). Basically, the ultimate strength should be equivalent to the strength at the weakest section of beam. This difference is related to strain hardening action and special shape of steel, which shares the contribution of concrete in carrying the load. This is the more pronounced in beams IL and 2L-S, in which the plates in single layer interact with concrete. Non-uniformity of stress in different elements of plate distributed the stress and enhanced the strength in weaker sections.

The yield strength (My) is calculated (4) and compared with experimental date in Figure 6b. The experimental yield strengths are closer to the calculated values than the case of ultimate strength, since stress distribution and strain- hardening did not start up to this load stage. The cracking strength, based on transformed section and the average area of perforated steel is calculated according to ACI design Code. The cracking strength is up to 2.35 times (for specimen 4L) of that based on ACI prediction. The concrete within holes of plates is tensioned along the beam length and is under compressive perpendicular to the beam length. The perforation relates cracking strength of beams reinforced with perforated steel plates to beam reinforced conventionally.

Mcr / Mcr (ACI)=1+K (Pv) ²

In which Pv is volumetric steel percentage (Aav/bh) and K equals to 2000. Comparison of test results with prediction of equation 2 is shown in figure 6c.

3.2 Bond of perforated steel plates

The ultimate strength of beams using 6 cm tension splice (2 hole lap) and using 12-tension splice (4 hole lap) are 55% and75% of beam reinforced with continuous plate, respectively (Table 2). The ratios corresponding to yield strength are 83%, respectively. The relatively short length of tension splicing of perforated steel plates, compared to the design length of ordinary reinforcement (ranging between 50 cm to 77 cm for class A splicing (3) has provided considerable strength. The Design splice length should not be less than 30 cm according to ACI Code. The development length of perforated steel plate depends mainly on the concrete area passing through the steel plate perforations, while the bond between plain surface of steel plate and concrete is significant.

3.3 Ductility

The ultimate curvature and deflection correspond to ultimate strength of the beams, at which the concrete compression region is near crushing. The yield strain of steel (Esy), corresponds to the point with sudden reduction in the slope of load-strain curve. The change in the slope occurred when the edge elements of steel plate yielded and the inclined elements of steel plate were near yielding (Figure 2). The curvature ductility (µ?=?u/?y) and deflection ductility (µd=du/d?) are calculated and given in table 3. Increase in volumetric steel content reduced the ductility. The ductility in beam 2L-S due to better bond between the spaced steel plates and concrete is higher than that in beam 2L. The deflection ductility of all the beams is grater than the commonly required ductility of 3 to 5 for seismic loads and moment-redistribution (4.5). The beam 4L, with Pav=0.031 and Pav/Pb=0.57 provided deflection ductility of 5.5. The ultimate deflection to span length ratio is 1.8%, 2%, 2.4% and 1.5% for beams 1L, 2L, 2L-S and 4L, respectively.

In the beam reinforced with 6 cm lap-splice steel plate, the cover concrete under the splice region was broken and delaminated resulting in sudden drop in the beam strength. However, increase in splice length to 12 cm, increased the deflection corresponding to delamination of cover concrete by more than 150%. The deflection ductility in beams 2H-L and 4H-L with respect to the beam 1L are 19% and 28%, respectively.

3.4 Mode of failure

The interaction between the perforated plates and concrete resulting in multiaxial stress state in the neighbouring concrete did not cause premature failure in the beams. The confined concrete in the plate holes resisted the shear forces, and the steel plate yielded in all directions (Figure 7). The steel plate yield first in the edge elements parallel to the beam span then inclined elements of the plate yielded and finally the elements perpendicular to the beam span yielded near the ultimate strength. This resulted in a gradually reducing slope in the load-deflection curve at yielding. The failure of the beams was gradual and was accompanied by rupturing of different elements of the steel plate at different load levels. The steel plates did not fail completely, perpendicular to the longitudinal direction in any of the beams.

4 Conclusions

1. Perforated steel plates as tensile reinforcements in beams increase the cracking strength up to 135%. The amount of increase depended on the volumetric steel percentage.
2. The shape of the plate and passage of concrete through the holes, provide the bond between concrete and steel plates. The ultimate strength of beams is higher than that based on minimum cross-sectional area of steel plate. Spacing between the perforated plates in the tension region of beams increased the strength and ductility compared to the beam reinforced with perforated plate layers placed on each other.
3. The ductility of beams was considerable, and for span length of 95 cm, about 2 cm deflection at ultimate strength was achieved. Ductility of beams reduced with increase in steel ratio, however for steel ratio of 0.031 (i.e., 0.57Pb), deflection ductility of 5.5 was obtained.
4. Yielding and rupturing of plates, and failure of beams were gradual. Different elements of the plate yielded at various load levels. Rupturing of plate elements was gradual and up to the final stage of beam failure, integrity of the plates was maintained.
5. The test results on beams with splice plates indicated that a relatively short splice length provides the required strength and ductility. Bond between plates is achieved by a few holes overlap to transfer the force.
6. The perforated plates may be made of materials other than steel, such as composite of glass fiber. This can increase the useful life of RC structures in saline corrosive environments.

Fig.1-Dimensions of plates used in beam specimens

Fig.2-Variation in steel area along the lengthof plate used in beam specimens & location
of strain gages on inclined elements(1),edge elements (2)& perpendicular elements(3)

Fig.3-Test setup of beams

Fig.4-Configuration of beam reinforced with perforated steel plates,(a)plates with 2-hole lapped,
(b) plates with 4-hole lapped

Midspan deflection (mm)
Fig.5-Load-deflection curves of beams reinforced with perforatec

Avg. reinforcement ratio (a)

Avg. reinforcement ratio (b)

Vol. steel ratio(r)
fig.6-(a) Ultimate strength, (b) yield strengt, (c) cracking Strength

Fig.7-Results of strain gages mounted on beam 1L

Acknowledgments

The support of research committee of Sharif University of Technology in conducting this study is greatly appreciated.

6 References
1. Khaloo, A.R., “compressive behavior of concrete confined with high-stiffness perforated steel plates” Asian Journal of Civil Engineering, V. 3, NO. 4,2002, PP. 57-73.
2. Local-Global Confinement of RC Columns, patent No. 26562, June 16, 1993.
3. Building Code Requirements for Structural Concrete (ACI 318-99) and commentary (ACI 318R-99), American Concrete Institute, Farmington Hills, 1999, 391p.
4. Park, R., and Paulay, T., Reinforced Concrete structures, John Wiley and Sons Inc., 1975,p. 769.
5. Recommended Lateral Forces Requirements and Commentary, Structural Engineers Association of California (SEAOC), 1996, p. 7.

Table 1. Experimental program

Beam Reinforcement	Specimen Designation	Effective Depth (d) (cm)	Pav	Pb (ACI)
One steel layer	1L	17.3	0.008	0.054
Two steel layer	2L	17.0	0.016	0.054
Two steel layers, spaced at 2.5cm	2L-S	16.0	0.0174	0.054
Four steel layers	4L	17.5	0.031	0.054
One Spliced steel layer (2 hole overlapped)	2H-Lap	17.2	0.008	0.054
One Spliced steel layer (4 hole overlapped)	4H-Lap	17.2	0.008	0.054

Table 2. Flexural strength at cracking yield and ultimate

Beam	Mċr (KN-m)	My (KN-m)	Mu (KN-m)
1L	5.6	9.6	12.0
2L	8.0	16.5	19.2
2L-S	8.0	15.5	19.3
4L	16.0	32.0	39.0
2H-Lap	4.0	6.4	6.6
4H-Lap	5.0	8.0	9.0

Table 3. Curvature and deflection ductility of beams

Specimen	d (cm)	ξsy	ξau	Φγ (1/m)	Φu (1/m)	δy (mm)	δu (mm)	µ	μ
1L	17.3	0.0035	0.053	0.027	0.33	1.4	17	12	12
2L	17.0	0.0030	0.035	0.025	0.23	2.0	19	9.2	9.5
2L-S	16.0	0.0033	0.044	0.031	0.30	2.0	23	9.7	11.5
4L	17.5	0.0035	0.042	0.034	0.26	2.5	14	7.7	5.5
2H-Lap	17.2	0.0020	0.006	0.015	0.05	0.8	1.8	3.3	2.3
4H-Lap	17.2	0.0025	0.01	0.019	0.08	1.4	1.4	4.2	3.3