Strengthening of Reinforced Concrete Slab-Column Connection Subjected to Punching Shear with FRP Systems

Transcription

1 IACSIT International Journal o Engineering and Tehnology, Vol. 5, No., Deember 2013 Strengning o Reinored Conrete Slab-Column Connetion Subjeted to Punhing Shear with FRP Systems G. I. Khaleel, I. G. Shaaban, K. M. Elsayedand, and M. H. Makhlou Abstrat This study aims to determine eiieny o using Fiber Reinored Polymers (FRP) systems to strengn slab olumn onnetions subjeted to punhing shear. The used strengning systems onsisted o external FRP stirrups made rom glass and arbon ibers. The stirrups were installed around olumn. Also, external steel links were used as a onventional strengning method or omparison. Over last ew years, use o FRP or strengning o onrete strutures has been investigated by many researhers, whihonerning with strengning o reinored onrete slabs, beams and olumns. The use o FRP in strengning onrete slabs in lexure is done by bonding it to tension ae o slabs. The use o FRP or strengning lat slabs against punhing shear an be onsidered as a new appliation. This researh shows results obtained rom an experimental investigation o 4 hal-sale two-way slab-olumn interior onnetions, whih were onstruted and tested under punhing shear aused by entri vertial load. The researh inluded one unstrengned speimen, whih onsidered as ontrol speimen, one speimen strengned with steel links, one speimen strengned with external stirrups made rom Glass Fiber Reinored Polymer (GFRP), and one speimen strengned with external stirrups made rom Carbon Fiber Reinored Polymer (CFRP). So, type o strengning material is basi parameter in this study. The experimental results showed a notieable inrease in punhing shear resistane and lexural stiness or strengned speimens ompared to ontrol speimen. Also, strengned tested slabs showed a relative dutility enhanement. Finally, equations or punhing shear strength predition o slab-olumn onnetions strengned using dierent materials (Steel, GFRP & CFRP) were applied and ompared with experimental results. Index Terms Fiber reinored polymer, punhing shear, reinored onrete slab-olumn onnetion, Strengning and repair. I. INTRODUCTION Punhing shear ailures are usually sudden and atastrophi. Suh ailure is undesirable as y do not allow an overall yield mehanism to develop beore punhing [1]. Many ailures o lat slab strutures were reported during onstrution [2], and in many ases punhing shear ailures initiated and led to progressive ollapse, speially, when lower loors have ailed to support impat o above ones. This brittle ailure ours without muh warning. There are mainly two ways to inrease punhing shear Manusript reeived Marh 15, 2013; revised June 29, The authors are with Civil Engineering Department, Faulty o Engineering in Benha, Benha University, Egypt ( gamal22_2@yahoo.om, igshaaban@mailer.eun.eg, khelsayed@hotmail.om; mohamedmakhlou83@yahoo.om). strength o onrete slabs: 1- inreasing slab thikness in viinity o olumn by providing a drop panel or a olumn head; 2- Providing shear reinorement. Sometimes, ater onstrution o some building, inrease o punhing shear resistane or reinored onrete slab-olumn onnetion may be needed. The strengning o slab-olumn onnetion against punhing shear resistane by using traditional methods (steel plates, steel stirrups, steel studs, or inreasing onrete dimensions) was studied [3-5]. Few studies onerned with using FRP strengning systems or lat slabs []. The present study aims to evaluate using o FRP materials to inrease punhing shear resistane o onrete slab-olumn. The values o punhing shear strength were predited taking into aount ontribution o applied strengning systems. The alulated values were ompared with orresponding experimental results in order to evaluate used equations. II. EXPERIMENTAL PROGRAM All speimens were tested under punhing shear aused by entri vertial load till ailure. The experimental results inluded maximum load whih presents punhing shear strength o tested speimens, load deletion urves, and rak patterns o eah tested speimen. III. TESTED SPECIMENS The tested speimens were hosen as retangular slabs o m dimensions with an average eetive depth o 8 m. Stub olumn having m ross setion. The olumn with 80m total height was ast monolithially at enter o slab and extended outside both sides o slab as shown in Fig. 1. Fig. 1. Details o tested RC speimen DOI: /IJET.2013.V5.3 57

2 IACSIT International Journal o Engineering and Tehnology, Vol. 5, No., Deember 2013 The tested speimens were designed to be simply supported along our edges with lear spans o 100 m in both diretions, and orners were ree to rise. This test arrangement is onvenient to simulate atual boundary onditions in prototype. High tensile steel bars o 10 mm diameters were used as top and bottom reinorement. The tension reinorement was 11 Φ 10 mm and ompression reinorement was 7 Φ 10 mm in both diretions. The onrete olumns were reinored with 4 Φ 12 longitudinal high tensile steel bars and with normal mild steel mm every 10 m as stirrups. Full details o slab-olumn onnetion, are shown in Fig. 1. The properties o our tested speimens are shown in Table I, strengned speimens were divided into three speimens in addition to ontrol speimen. TABLE I: THE PROPERTIES OF TESTED SPECIMENS Notation Speimen state Strengning type SPH Control none SPS Strengned Steel Links SPG Strengned GFRP stirrups SPC Strengned CFRP stirrups IV. MATERIALS PROPERTIES The speimens were ast using normal strength onrete o 30 MPaharateristi strength. The onrete mix was prepared in onrete Lab o Benha aulty o Engineering using Ordinary Portland ement, lean silieous sand, rushed lime stone, and potable water. Tests were arried out aording to ECP and Egyptian standards to deine properties or onrete onstituents, onrete mix and reinorement steel. Table II gives properties o GFRP WR and CFRP wraps used in this work, given by manuaturer. TABLE II: MECHANICAL PROPERTIES OF GFRP AND CFRP WRAPS [7] Property Glass Fiber Woven Roving Wraps (WR) CFRP (Sika Wrap Hex-230C) Fabri thikness, mm Tensile strength, MPa Modulus o elastiity, MPa Elongation at ailure, % V. TEST SET-UP The slab speimens were mounted in a horizontal position inside steel rame, and load was applied to stub-olumn in a vertial diretion using a hydrauli jak o 50 ton apaity, as shown in Fig. 3. The development o raks was observed during loading on both aes o slab. Dial gauges o 0.01 mm auray and 50 mm total range were used to measure deletion at bottom ae o slab (tension side). The dial gauges were ixed at enter o slab to reord maximum deletion, and at supports whih onsidered a datum level to get net deletion values o slab. Load was applied on suessive inrements. Slab deletion, and raking onditions were reorded ater eah inrement till slab ailure. Fig. 3. Test setup and instrumentations VI. STRENGTHENING SYSTEMS Fig. 2. The shape and dimensions o dierent strengning systems A. Using Steel Links The proposed onventional strengning system onsisted o steel links whih were loally abriated using normal tensile steel bars o mm diameter ixed at both ends by two steel plates o 5 mm thikness and 25 mm width. Steel links 58

3 IACSIT International Journal o Engineering and Tehnology, Vol. 5, No., Deember 2013 were installed through holes whih penetrated total slab thikness. The learane between steel links and holes was illed by polyester to ensure good bond between onrete and steel links. B. Using FRP Stirrups Speimens were strengned by using intertwined FRP rods whih were manually manuatured rom iber wraps o mm diameter stirrups whih prepared rom 4 m. Vertial holes with 10 mm diameter were drilled in slabs. The FRP wraps were saturated with polyester or GFRP and epoxy resin (Sikadur-330) or CFRP, and n intertwined rods were ormed and stithed through holes along thikness o slab. 24 hours later, learane between GFRP or CFRP stirrups and holes was illed by polyester or Sikadur-330, respetively, to ensure good bond between FRP stirrups and onrete. loading level, lower deletion values were reorded or strengned speimens, eir with steel links, GFRP or CFRP stirrups, in omparison with ontrol speimen, as shown in Fig. 5. Craking pattern o reerene speimen SPH VII. EXPERIMENTAL RESULTS AND DISCUSSION For eah tested speimen, omplete load deletion urve was plotted and raks propagation was reorded. The obtained results were analyzed and disussed in order to study eet o strengning material type on mehanial behavior o slab-olumn onnetions under punhing shear. A. Craking Behavior and Failure Mode All tested speimens were loaded under entri vertial load and ailed due to shear punhing. The raks appeared irstly at tension side o slab. For eah tested speimen, load value orresponding to raking initiation (Vr) is shown in Table III. The strengning o speimen aused a signiiant inrease o irst rak load. Using steel links, GFRP and CFRP stirrups led to improve its value by 29%, 35% and 47%, respetively, in omparison with ontrol speimen. As applied load inreased, raks propagated and beame wider and new raks appeared as well. The radial raks propagated in several diretions towards slab edges orming a van shape. The ailure surae interseted with slab bottom surae orming a irumerential rak o nearly square shape. The distane between olumn ae and surrounding ailure rak was larger in ase o strengned speimens. Also, a higher number o radial raks were observed or strengned speimens in omparison with ontrol speimen, as shown in Fig. 4. The number o radial raks was 11 or ontrol speimen; however, it inreased to 20 & 23 when steel links and FRP stirrups were applied, respetively. B. Load-Deletion Plots For all tested speimens, deletion value, measured at entral point, was reorded at eah load inrement, and load-deletion relationship was plotted. Fig. 5 shows a omparison between dierent strengned speimens, also load-deletion plot o ontrol speimen was presented in same igure or omparison. Although ultimate load o strengned speimens was dierent, load-deletion relationships o se speimens were moderately similar. All strengning materials led to a signiiant inrease o rigidity and resistane o tested speimens against shear punhing. At same Craking pattern o speimen SPS Craking pattern o speimen SPG Craking pattern o speimen SPC Fig. 4. Craking patterns 59

4 IACSIT International Journal o Engineering and Tehnology, Vol. 5, No., Deember 2013 Load (ton) Fig. 5. Comparison between load-deletion plots o tested speimens C. Ultimatepunhing Shear Strength Table III shows ultimate load (V u ) reorded experimentally or tested speimens. The use o steel links gave an inrease o ultimate load by 0% o that reorded or ontrol speimen. The use o GFRP and CFRP stirrups with same number o rows and same diameter as steel links inreased o ultimate load by 0% and 73%, respetively. Fig. shows a omparison between tested speimens with respet to raking and ultimate loads. TABLE III: EXPERIMENTAL RESULTS Slab Code V u, exp (ton) V u, al (ton) V u, exp. / V u, al. SPH SPS SPG SPC Load (ton) Craking Load Ultimate Load Central deletion (mm) Steel links, SPS Fig.. Eet o strengning types on raking and ultimate Load VIII. ANALYTICAL MODELING GFRP stirrups,spg CFRP stirrups, SPC Control, SPH SPH SPS SPG SPC All tested speimens ailed as a result o onrete exhaustion under punhing shear stress at ritial setion loated at a distane d/2 rom outermost row o punhing shear reinorement. For predition o ultimate test loads, ollowing equation an be used to alulate values o onrete nominal punhing shear strength (V ) [8]; α 1 v = 0.33(1 ) ' MPa (1) α : ratio o ritial setion distane rom olumn ae to slab eetive depth 4 α 1; ': onrete ylinder ompressive strength; For speimens reinored with shear reinorement or strengned with steel links nominal punhing shear strength may be expressed as: V + V : shear resisted by onrete; V s : shear resisted by reinorement; n = V Vs (2) V = ( A d) S (3) s yv / A v : area o vertial legs orming shear reinorement units in one row; yv : yield stress o used steel or shear reinorement units; S: spaing between rows; The shear ore resisted by onrete at any ritial setion an be alulated rom ollowing equation; V = v b d (4) v : given by (1); b : perimeter o ritial setion (at a distane d/2 rom outermost row o punhing shear reinorement); In speimens strengned with FRP, nominal punhing shear strength may be expressed as: n V = V + V (5) V : is shear resisted by iber reinorement; The shear strength provided by iber reinorement (V ) an be determined by alulating ore resulting rom eetive tensile stress in iber ( e ) whih depends on its eetive strain (Є e ). v e V = ( A d ) / S () A v s v = n n t w (7) e e = ε E (8) Є e = (or ompletely wrapping all 4 sides) [9]; S : spaing between iber rows; t :iber reinorement thikness; w : width o iber strip; n v : number o side row stirrups; n s : number o vertial legs in one side o row; A v : area o iber in one row; d : depth o iber stirrups. The above equations were applied to predit ultimate punhing shear load o tested speimens. Table IV shows a omparison between alulated values o ultimate load (V u, al. ) and orresponding experimental values (V u, exp. ). The equations used to predit ultimate loads are moderately onservative, where experimental values are higher than alulated ones by 12% to 29%. TABLE IV: COMPARISON OF BETWEEN THE CALCULATED AND EXPERIMENTAL ULTIMATE LOADS 0

5 IACSIT International Journal o Engineering and Tehnology, Vol. 5, No., Deember 2013 IX. CO ONCLUSION Based on researh work, ollowing onlusions and reommendation an be summarized: 1) All speimens ailed due to punhing shear. 2) All materials used in this researh to strengn slab-olumn onnetionn were eetive and improved signiiantly shear punhing behavior in terms o lexural rigidity, initial raking load and ultimate arrying apaity. Using steel links, GFRP and CFRP stirrups o same area inreased initial raking load by 29%, 35% and 47%, respetively, and ultimate apaity by 0% %, 0% and 73%, respetively. 3) The CFRP intertwined stirrups was best strengning material, whih led to highest improvement in rigidity and ultimate apaity o tested speimens. 4) The strengned speimens, eir with steel links or FRP stirrups, inreased number o radial raks. 5) The surae ailure o tested speimens orms a nearly square shape with tension slab side. ) The distane between olumn ae and surrounding ailure rak was larger in ase o strengned speimens. 7) The equations used to predit ultimate apaity o tested speimens gave moderate onservative values, where experimental values are higher than alulated ones by 12% to 29%. REFERENCES [1] F. I. Ehab, Eet o well-anhored stirrups on punhing shear resistane in R.C. slab olumn onnetion, M.S. sis, Cairo university, Faulty o engineering, Department o ivil engineering, [2] A. M. T. Said, Strengning o R.C Slab-Column Connetion to Resist Punhing Shear, Ph.D. sis, Faulty o Engineering, Cairo University, [3] A. Sheri, Behavior o Reinored Conrete Flat Slabs, Ph.D. sis, University o Calgary, Canada, 199. [4] A. Moussa, G. Ghanem, and A. Tarkhan, Inluene o lexural reinorements on punhing shear apaity o reinored onrete lat slabs, in Pro. o 5th Alexandria International onerene in strutural& Geotehnial Eng., AICSCES, Egypt, De [5] A. Moussa, Some additional arrangements to inrease punhing shear apaity o reinored onrete slab-olumn onnetions, in Pro. o 2nd Intr. Speialty onerene on oneptual approah to strutural design, Milan, Italy, vol. 2, July, [] G. Ismail, A. A. Karem, A. Debeiky, and M. Makhlou, Strenghtening and Repair o Reinorement Conrete Slab-olumn Connetion Subjeted to Punhing Shear using (CFRP - GFRP - Steel) Stirrups, in Pro. o Fourth International Conerene on Strutural Stability and Dynamis, India, [7] A. M. M. Ramadan, Repair and Strengning o Slab-Column Connetions with openings, Ph.D. sis, Faulty o Engineering, Minouiya University, [8] Authorization, stud rails as shear reinorement in support zones o slabs with point supports (zulassungsbesheid Nr , kopbolzen-dubelleisten als shubbewehrung im stuzenbereih punktorming gestutzter platten), Institute ur Baulehnik, Berlin, July [9] ACI Committee, Guide or design and onstrution o externally bonded FRP systems or strengning onrete strutures, January Gamal Ismail Khaleel was born in Damietta on 222 o February o 1958, PhD in Civil Engineering (eet o axial ores on shear strength o RC element ) rom Department o Civil Engineering, Faulty o Engineering, Cairo University, Egypt, He is Proessor o Reinored Conrete Strutures, Faulty o Engineering in Benha and Vie President, Benha University, Egypt. His publiations and researh interests onern with ields o strengning/repairr o RC strutures using dierent tehniques. Dr. Khaleel is a member o Egypt Syndiate o Engineers. Ibrahim Galal Ibrahim Shaaban was born in Cairo on 190, PhD in Civil Engineering rom Department o Civil Engineering, Dundee University, Dundee, U.K., He is Proessor o Reinored Conrete Strutures, Faulty o Engineering at Shoubra, Benha University, Egypt,and Exeutive Diretor o projet Management Unit Development o Higher Eduation, Ministry o Higher Eduation. His publiations and researh interests onerning with ields o strengning o RC strutures using High Strength Conrete. Dr. Shaabanis a member o Egypt Syndiate o Engineers, Egyptian Soiety o Engineers and Member o Soiety or Promoting use o Briks and Stones in Buildings. Khaled M. Elsayed was born in Cairo on 15 o Deember o 19, PhD in Civil Engineering (strutural advaned materials program) rom Materials Sienes Department o EsuelaTénia Superior de Caminos, Canales y Puertos, UniversisdadPoliténia de Madrid, Spain, He is Assoiate Proessor and HEAD o Civil Engineering Department, Faulty o Engineering inbenha, Benha University, Egypt. His publiations and researh interests are onerning with ields o rature mehanis o onrete and strengning/retroitting o RC strutures using iber reinored polymer materials. Dr. Elsayed is a member o Egypt Syndiate o Engineers and Egyptian Soiety o Engineers. Mohamed Hussein Ali Makhlou was born in Cairo on 5 o November o 1983, M.SC. in Civil Engineering rom Department o Civil Engineering, Faulty o Engineering inbenha, Benha University, Benha, Egypt, He is Assistant Leturer o Strutural Engineering, Faulty o Engineering in Benha, Benha University, Egypt. His publiations and researh interests onernwith ield o strengning and repairr o RC slab-olumn onnetion using dierent materials and tehnique. Eng. Makhlou is a member o Egypt Syndiate o Engineers. 1