Strengthening of RC slabs with cut-out openings using FRP composite materials preliminary aspects of research program

Transcription

1 Strengthening o RC slabs with cut-out openings using FRP composite materials preliminary aspects o research program FLORUT SORIN-CODRUT*, STOIAN VALERIU** *,**Department o Civil Engineering Politehnica University o Timisoara 2 nd T. Lalescu, Timisoara, ROMANIA codrut.lorut@ct.upt.ro, valeriu.stoian@ct.upt.ro Abstract: - This paper presents some preliminary aspects regarding a research program that is in progress at the Politehnica University o Timisoara. The program deals with FRP composite based solutions or strengthening o reinorced concrete slabs with cut-out openings. Theoretical and experimental researches have been perormed in order to determine the eectiveness o these strengthening solutions in the particular case o cut-outs created in the corners and on the edges o the slabs. The experimental program involves tests on our large scale elements. The irst element will be a ull slab and will serve as reerence, while in each o the other three elements a dierent shape o cut-out will be created. A series o analytical and numerical calculations have been perormed in order to study the behavior o the slabs in elastic and post elastic range, and to design the strengthening proposals. Key-Words: - reinorced concrete, slab, cut-out, opening, strengthening, FRP 1 Introduction Nowadays, building engineering considers more and more the composite materials, especially the Fiber Reinorced Polymer (FRP). The composite materials properties have made their use to prove a real success in a series o applications rom local strengthening to highly complex works. The FRP composites are light, very highly resistant, corrosion proo and nonmagnetic. The basic principles in using FRP are grounded on the principles o using steel strengthening bars in reinorced concrete building structures, but with some changes made, in order to consider the composites` physicmechanical characteristics. Preerring composites in some applications instead o traditional steel or reinorced concrete based strengthening solutions is grounded on many reasons. The composites` very high corrosion resistance along with the short amount o needed construction time is probably the most important o all. One o the experimental programs that are in progress at the Politehnica University o Timisoara concerns the study o strengthening solutions that involve the use o FRP or reinorced concrete slabs with cut-outs. In many situations, openings are needed in slabs, in places that were not considered during the structural design o a building. This need emerges mostly due to a series o changes in unctionality. There is also the case in which some openings were considered in the design process but due to changes in unctionality or in destination, the loads to which the slabs are subjected become much higher. In either one o these previously mentioned situations, the slab s overall behavior becomes deicient, both as stiness and strength. The area in which these eects are o most importance is the area around the cutout, where stresses are highly concentrated. Any situation rom the two previously mentioned, does not only lead to alteration o stiness and strength but also to an important change in the overall ailure mode, leading to new and unexpected ones. 2 Similar studies perormed previously worldwide The interest in inding new solutions or strengthening reinorced concrete slabs using FRP composites was quite important or a series o researchers, experimental and theoretical programs being developed all around the world. Studies were made on one-way and two-way slabs with or without cut-out openings. Most o the researches on the reinorced concrete slabs strengthening were made on one-way ones. In some o those studies, the reinorced concrete slabs conduct was very similar to that o beams. The usual strengthening method presumes the disposal o the lamellas or sheets bonded on the tensioned side using resins. The sheets are mounted parallel to the long edge o the slabs, the same way as lexural strengthening o beams. There is also the possibility to prestress the sheets, complementarily to the simple procedure o bonding them. The main advantage o using the prestressed sheets consists o delaying the ormation and propagation o cracks. ISSN: ISBN:

2 Generally speaking, the two-way reinorced concrete slabs are elements subjected to lexure with irrelevant shear eect. That is why they are susceptible rather to lexural than shear ailure. This particularity makes the use o composite materials in two-way reinorced concrete slabs strengthening to be considered as an optimal solution. In addition to the important increase in serviceability and lexural capacity, using o FRP strengthening methods is justiied by its unlaborious appliance. The reserve in using o FRP materials in strengthening o lexural structural members is the brittleness o such materials that can cause a decrease in their stiness. The strengthening method, same as oneway reinorced concrete slabs, presumes the disposal o the lamellas or textures bonded on the tensioned side by using resins. In two way reinorced concrete slabs, there is o course a dierence, the composite material being mounted parallel to both length and width o the slabs. This method increases the capacity in both directions o the element. For slabs with cut-out openings strengthened using FRP, the available research is not as extended, only several research programs being reported in literature, the work conducted by Tan & Zhao [16] and Vasquez & Karbhari [17], Enochsson [7] or Smith [14] being o high importance. The solution applied by all o the researches consisted in laying up CFRP or GFRP strips or sheets o abrics around the cut-out and bonding them to the concrete surace using epoxy based resins on the tensioned side. Dierent conigurations or the lay-out o the strengthening materials were used, the most common being the one in which the material is placed parallel to the edges o the cut-out. Fig.1. Some o the conigurations used by Tan & Zhao In all o the previous research programs, the cut-outs were created in the center o the tested slabs and none o the programs dealt with circular cut-outs. Considering these two main aspects, the research program conducted at the Politehnica University o Timisoara is innovative, covering a research area that has not yet been approached. 3 Experimental elements The experimental program consists in testing our reinorced concrete slabs up to ailure. The elements are large scale ones, having dimensions o 2750x3950x120 mm. The irst element will be a ull slab and will serve as reerence, while in each o the other three elements a dierent shape o cut-out will be created. The purpose o the program is to design strengthening solutions that will restore the stiness and strength or the elements with cut-out up to the level o the ull slab. The irst element will be a ull slab that will be tested unstrengthened and will serve as reerence, being named RCS-FS-01. The second element will have a large rectangular cut-out with dimensions o 1060x2750 mm, being created on one o the short edges o the slab. This slab was named RCS-RLC-01. In the third element a smaller cut-out will be sawn, having dimensions o 1060x1540 mm and being created at one corner o the slab, with the larger dimension oriented parallel with the longer edges. This slab was named RCS-RSC-01. The ourth element will have a circular cut-out opening with the diameter o 1280 mm and it will also be placed at one o the corners o the slab. The ourth slab was named RCS-CC-01. The slabs were cast using C30/37 concrete and were reinorced with steel wire meshes at the inerior side (4 mm in diameter placed at every 100 mm) and with steel rebar at the superior one (6 mm and 10 mm bars). The inerior reinorcement was laid on the entire surace o the slab, while the superior one was placed only along the edges. Since the reproduced situations involved simple supported slabs, the superior reinorcement was designed mainly due to constructive reasons. The coniguration o the reinorcement was designed in order to simulate real situations that exist in a lot o buildings erected in Romania rom large precast reinorced concrete panels. A statistical study was previously conducted in order to determine which type o elements were used on a larger scale, and then the ones that would be suited in real situations or inserting cut-outs were chosen. Several papers that reer to large panels precast structures were studied, including some that deal with the behavior o panels with cut-outs subjected to inplane loads [4], [5], [6], [13]. In Fig. 2 the geometry, position and size o inerior reinorcement is presented or the ull slab, the reinorcement being identical or the ISSN: ISBN:

3 other three slabs, o course with the exception o the cut-outs, where reinorcement is missing. In Fig. 3 the geometry o the three elements in which cut-outs have been created are presented. Fig. 2. Geometry and inerior reinorcement o ull slab 4 Loading and testing strategy The loading and testing strategy was considered in such a manner so it would be in accordance with the real situation o the slabs. Taking this into account, the slabs would have to be simply supported on all o the our horizontal edges and loaded gravitationally. In order to match the loads considered or the design, the slabs should be subjected to uniormly distributed loads. The applied loads, even i they are not uniormly distributed on the entire surace o the slab, they try to simulate as much as possible this kind o action. Using this system, the inerior side becomes the tensioned one, the steel wire mesh reinorcement becoming the tensioned reinorcement. The existence o the superior reinorcement can be neglected in analytical and numerical models, since its presence does not inluence in a major way the overall behavior and capacity o the slab. Several recommended test procedures and loading strategies were studied in order to determine the most appropriate one [2], [3]. In order to properly investigate and observe the cracks distribution and propagation on the inerior side, the slabs will have to be mounted at a certain height above the loor o the laboratory. A series o supporting elements were created rom reinorced concrete beams and brick masonry. On top o these elements a steel beam will be mounted. Each edge o the experimental elements will rest on the steel beam on a width o 100 mm. The test setup can be observed in Fig. 4. Fig. 4. Coniguration o the test setup Fig. 3. Geometry o the three slabs with cut-outs ISSN: ISBN:

4 5 Analytic design o FRP strengthening system The FRP strengthening material will be placed around the cut-out on the tensioned side o the element. For this particular experimental program, the tensioned side is the inerior one. The amount o FRP that will be placed around the cut-out is to be determined analytically by equalizing the traction orce that would have been undertook by the steel reinorcement eliminated by creating the cut-out, and the traction orce that will be undertook by the FRP. F s = F (1) yd A = As E ε (2) Inside ormula (2), the strain in FRP composite is limited to 0.8%, being the accepted limit or elements subjected to lexure. This value is much lower than the ultimate strain provided by the producers, being considered the value at which debonding occurs. The design yield strength o reinorcement was assumed in computation with a value o 370 MPa, corresponding to the data provided by the producer or the STNB type o reinorcement. A series o dierent types o FRP materials with dierent characteristics can be used in the experimental program. It was decided to use CFRP lamellas that have a modulus o elasticity o MPa and a thickness o 1.2 mm. As an exempliication, the way the quantity o strengthening material was determined or slab RCS- RSC-01 is presented below. When creating the cut-out, iteen transversal reinorcing steel bars and eleven longitudinal ones were eliminated. Since the diameter o one bar is 4 mm, the total eliminated reinorcement area is mm 2 on the transversal direction and mm 2 on the longitudinal direction. The above calculated area is needed around the cutout, the length o the CFRP outside the cut-out being designed so that it will provide the necessary anchorage length. No other special anchorages will be used, the needed anchorage being provided solely by the extra length o the CFRP. The necessary anchorage length was determined according to the procedure described in ib Bulletin 14 [5]. Thus, the maximum orce that can be anchored (N a,max ) and the maximum anchorage length (l b,max ) are given by the equations below (5) and (6). N a, max = α c1 kc kb b E ctm (5) E t b = (6), max c2 ctm l N a, max = N a, max = 1137 kn l b, max = = 185mm The anchorage length will be grater than the 185 mm resulted rom calculation, considering at least 500 mm or the strengthening solution. This extra length will be provided since the steel reinorcement in the immediate vicinity o the cut-out is practically unanchored, the FRP composite material being the one that will provide the required strength in that area. The way the CFRP is laid around the cut-out is presented in Fig. 5. A 370 MPa MPa = 188.5mm (3) A 370 MPa MPa = 138.3mm (4) The area o CFRP that is required is approximately 55 mm 2 on the transversal direction and 39 mm 2 in the longitudinal direction. For a thickness o the lamella o 1.2 mm, the required width is 45 mm respectively 32 mm on the two directions. It was decided to use one lamella or each o the longitudinal and transversal direction, the width being 50 mm. Fig. 5. Display o the FRP on the inerior side o slab RCS- RSC-01 ISSN: ISBN:

5 In the same manner, the slabs RCS-RLC-01 and RCS-CC-01 are to be strengthened. 6 Numerical models or unstrengthened slabs A series o numerical models by means o inite element analysis were created. Using AXISVM and ABAQUS sotware, analyses in elastic range were perormed or the our unstrengthened slabs. The models revealed the necessity o a strengthening solution, proving the act that the behavior o the slabs becomes deicient in case o sawing in a cut-out opening. The lexural capacity o the slab determined analytically or the both directions at the inerior side is m rd,x =4.75 knm/m respectively m rd,y =4.55 knm/m. This capacity is reached or a ull slab with the proposed geometry and simply supported along the our edges at a level o the uniormly distributed load o p=6.60 kn/m 2, plus the sel weight o the slab. The sel weight o the slab is g=3.00 kn/m 2. For the RCS-RSC-01 slab, considering a uniormly distributed load o p=6.60 kn/m 2 the maximum bending moment has a value o m Ed,x =9.85 knm/m, more than twice the value o the lexural capacity. For the RCS- CC-01 slab, considering a uniormly distributed load o p=6.60 kn/m 2 the maximum bending moment has a value o m Ed,x =6.92 knm/m, higher than the value o the lexural capacity. For the RCS-RLC-01 slab, considering a uniormly distributed load o p=6.60 kn/m 2 the maximum bending moment has a value o m Ed,x =7.00 knm/m, also higher than the value o the lexural capacity. All o the maximum values o the bending moment or the slabs with cut-outs are reached in areas around the cut-out. Fig. 6. Distribution o m x bending moment in elements RCS-FS-01 and RCS-RSC-01 Fig. 7. Distribution o m x bending moment in elements RCS-CC-01 and RCS-RLC-01 7 Conclusions Considering the characteristics o the experimental elements, the research program is quite innovative. It covers a gap in the world wide knowledge o strengthening o reinorced concrete slabs with cut-out using FRP materials. 8 Acknowledgments The research work was granted to some extent by the Ministry o Education and Research through the CEEX and CNCSIS programs o the National University Research Council o Romania. Reerences: [1] Arduini M., Nanni A., Romagnolo M., Perormance o one-way reinorced concrete slabs with externally bonded iber-reinorced polymer strengthening, ACI Structural Journal, V. 101, No. 2, 2004, pp [2] Dan D., Stoian V., Nagy T., Theoretical and experimental studies concerning the load bearing capacity o steel and composite joints, Proceedings o International Conerence Steel - New and traditional material or building, Brasov, Romania 2006, pp [3] Dan D., Stoian V., Nagy T. C. Daescu, Composite joint or buildings placed in seismic areas theoretical and experimental studies, 9-th International conerence on Steel, Space & Composite Structures, Yantai Beijing, China 2007, pp [4] Demeter I., Nagy-György T., Stoian V., Dan D., Quasi-static Loading Strategy or Earthquake Simulation on Precast RC Shear Walls, 12th WSEAS International Conerence on SYSTEMS, Greece, ISSN: ISBN:

6 2008, ISBN , ISSN , pp [5] Demeter I., Nagy-György T., Stoian V., Axial loading strategy or experimental tests on precast RC walls subjected to in-plane seismic actions, Buletinul Stiintiic al UPT, Tomul 52(66), 2007, ISSN , pg [6] Demeter I., Nagy-György T., Stoian V., Analytical and numerical models or predicting the behaviour o in-plane loaded precast RC wall panel experimental elements, Acta Technica Napocensis, 51/II-2008, Civil Engineering, ISSN , pp [7] Enochsson O., CFRP strengthening o concrete slabs, with and without openings. Experiment, analysis, design and ield application. Licentiate thesis, Luleå University o Technology, [8] Floruț S. C., Nagy-György T., Stoian V., RC slabs strengthened with externally bonded FRP composite materials - Literature review, Composite Materials Elements and Structures or Construction, Timisoara, Romania, may 2007, pp [9] Limam O., Foret G., Ehrlacher A., RC two-way slabs strengthened with CFRP strips: experimental study and a limit analysis approach, Composite Structures 60 (2003), pp [10] Mosallam, A. S., Mosalam, K. M., Strengthening o two-way concrete slabs with FRP composite laminates, Construction and Building Materials 17 (2003), pp [11] Mosallam A., Kreiner J., Lancey T., Haroun M., Elsanadedy H., Experimental and numerical analysis o two-way concrete slabs repaired with polymer composites, Center or Advanced Composites & Smart Systems or Repair and Rehabilitation. [12] Nagy-György T., FRP composite materials or strengthening masonry and concrete elements (in Romanian), Ed. Politehnica, Timişoara, ISBN , [13] Sas G., Demeter I., Carolin A., Nagy-György T., Stoian V, Täljsten B., FRP strengthened RC panels with cut-out openings, Challenges or Civil Construction (CCC 2008), Porto, Portugalia, apr 2008, ISBN , pp (+8 pg). [14] Smith S.T., Kim S.J., Strengthening o one-way spanning RC slabs with cutouts using FRP composites, Construction and Building Materials, Volume 23, Issue 4, 2009, pp [15] Stoian V., Nagy-György T., Dan D., Gergely J., Dăescu C., Composite materials or construction (in Romanian), Editura Politehnica Timişoara, [16] Tan K. H., Zhao H., Strengthening o openings in one-way reinorced-concrete slabs using carbon iber-reinorced polymer systems, J. Compos. or Constr., Volume 8, Issue 5, pp (September/October 2004). [17] Vasquez A., Karbhari V. M., Fiber-reinorced polymer composite strengthening o concrete slabs with cutouts, ACI Structural Journal, V. 100,No. 5, September-October 2003, pp [18] Guide or the Design and Construction o Externally Bonded FRP Systems or Strengthening Concrete Structures, ACI 440.2R 02, 2002 [19] Technical Report on the Design and Use o Externally Bonded FRP Reinorcement or Reinorced Concrete Structures, ib Bulletin 14, TG 9.3, ISSN: ISBN: