USE OF ACM IN REHABILITATION PROJECTS IN EGYPT Amr Abdelrahman, Mohamed Mohamadien, Sami Rizkalla, and Gamil Tadros SYNOPSIS Use of ACM in the form of FRP laminates in rehabilitation of concrete structures is the prime application of ACM in Egypt. FRP laminates are applied for strengthening reinforced concrete slabs or beams in flexure and shear as well as for confinement of reinforced concrete columns. This paper briefly introduces selected projects to demonstrate the current practice of FRP in Egypt. In the first application, carbon FRP (CFRP) laminates in the form of strips and sheets were applied to strengthen a public building suffering from differential settlement of the foundation. In a different application, CFRP laminates were used to upgrade a residential building to be used for commercial purpose. The paper summarizes the design aspects, construction details and recommendations for future application of ACM. Keywords: Application, Beam, Carbon, Construction, Design, Slab, and Rehabilitation.
Dr. Amr Abdelrahman, MACI, is Associate Professor in the Structural Engineering Department at Ain Shams University, Abasia, Cairo, Egypt. He is a member in the National Egyptian Code for Design and Construction of Reinforced Concrete Structures and Design of Structures with Fiber Reinforced Polymers. He is also an Associate member in the ACI Committee 440, Fiber Reinforced Polymer Reinforcement. Dr. Mohamed Mohamadien, is Professor in the Structural Engineering Department, Suez Canal University, Port Said, Egypt. He is a member in the National Egyptian Code for Design of Structures with Fiber Reinforced Polymers. Dr. Sami Rizkalla, FACI, is Distinguished Professor of Civil Engineering and Construction and Director of the Constructed Facilities Laboratory (CFL) at North Carolina State University. He is a chairman of ACI Committee 440, Fiber Reinforced Polymer Reinforcement, and is a member of Joint ACI-ASCE Committee 550, Precast Concrete Structures. Dr. Gamil Tadros, MACI, is Technical Application Consultant in the Canadian Network of Centers of Excellence on Intelligent Sensing for Innovative Structures ISIS Canada, Winnipeg, Manitoba, Canada. He is an advanced engineer with Strait Crossing Inc. with responsibilities in bridge design and construction.
INTRODUCTION The Middle East is challenged by the rapid deterioration of concrete structures due to corrosion of steel reinforcement. The long coasts on the Mediterranean, the Red Sea and the Arab Gulf cause an adverse environment that accelerates corrosion of steel. Use of advanced composite materials, (ACM), proved to be a competitive solution to extend the service life of structures. This is not only due to the non-corrosive properties of the material but also for the large improvement in mechanical properties that allows the structural engineer to choose the best material for application. ACM have been used in civil engineering applications in Egypt for over a decade. Research started in the use of Fiber Reinforced Polymers (FRP) bars as internal reinforcement, followed by externally bonded FRP laminates for strengthening concrete structures (1). As experience was gained in the design and application of FRP, several rehabilitation projects using FRP laminates were successfully completed. In this paper, an overview on the application of FRP materials in civil engineering in Egypt, is presented. Research in this field started in Egypt in the early 1990 s following the initial international footsteps. Engineering awareness about the new materials started in 1996 through a series of specialized conferences on application of structural composites in the Middle East (1-3). Since then, the increase in number of research papers and applications of ACM in Egypt is quite evident. As a result, a national Egyptian committee on Design and application of FRP in the construction fields was formed in 2002. The committee task is to introduce guidelines for design, testing, application and maintenance of structures utilizing FRP. Application of ACM in Egypt so far is limited to strengthening of existing structures using externally bonded FRP reinforcement. This could be attributed to the high cost of other products such as reinforcing and prestressing FRP bars or pultruded sections. In this respect, it should be mentioned that internationally most of the FRP applications are related to strengthening of existing structures. This paper introduces two case studies where carbon FRP (CFRP) laminates were used to strengthen concrete structures. In the first case study, concrete slabs and beams of a hotel suffered from differential settlement in the foundations and needed to be rehabilitated. The owner required the civil work to be performed without interrupting the use of the hotel. Rehabilitation of the structure was successfully completed in 1998. In the second case study, CFRP laminates were used to upgrade a residential building to be used for commercial purposes. It was the owner s requirement to complete the design, application and handover of all the work in only three weeks. Choice of FRP material in the two case studies was decided after considering other strengthening techniques. FIRST CASE STUDY: THE PANORAMA BUILDING The Panorama building in one of the hotels in the city of Sharm-El-Shaikh, Egypt, is 11.0 m wide and 50.0 m long (2) with a triangular shape, as shown in Figure 1. The one story
building is located close to the edge of a hill. Villas are located at the ground level, while the roof was not used. It was the owner s desire to use the roof as an open-air café having a view of the city from the top of the hill. The building consists of reinforced concrete slabs supported on rectangular beams and reinforced concrete columns. After two years of construction, both the slabs and beams were severely cracked, as shown in Figure 1. The crack width was up to 1.0 mm in some locations. The cracks were observed on the entire width of the slab, at the fourth bay away from the hill, crossing all beams and walls. Flexural and shear cracks were also observed at some of the beams. The Parapet of the roof, which is made of brick, was also cracked. The cracks were wide at the top of the parapet, reducing in width towards the bottom. The location of these cracks coincided with the cracks in the concrete slab. No cracks were observed in the columns. Diagnosis of the Problem The crack pattern indicated that differential settlement of the foundations of the building had occurred. The columns close to the edge of the hill settled more than the interior columns causing a rigid body movement of that part of the structure and cracks across the entire building. Soil pits were carried out to determine the properties of the soil. The investigations showed a layer of fine sand beneath the shallow foundations. The fine sand was washed out of the hill with the water drained from spraying plants close to the building. The washed sand found its way out from the side of the hill. A good soil stratum was located 10.0 meters below the ground level. beams cracked in shear 16.0 beam cracked in torsion Beams cracked in flexure 31.0 Plan Figure 1 Crack pattern of the Panorama building Proposed Strengthening Scheme Before starting the strengthening scheme, it was decided to move the plants away from the building and eliminate any source of water close to the foundation of the building. In the first phase of the scheme, it was essential to eliminate the cause of the problem and stop the settlement of the foundations before remedy of the superstructure. A rigid reinforced concrete mat foundation supported on plain concrete caissons was cast to support the columns. The caissons were 1.0 meter diameter and 10 meters long, bearing
on the good soil stratum. Two caissons were cast around each existing column. In order to ensure that the deformation of the structure was stopped after casting the new foundation, the cracks were monitored to record any changes. Several gypsum marks were made crossing the existing cracks in the slabs and beams. No further development of the cracks was observed. The second phase of the strengthening scheme was to restore the building and increase the structural capacity of both the slabs and beams to resist higher live loads. It was the owner s demand to minimize the time and the working space during this phase of work. Two alternatives were studied to strengthen the superstructure; enlarging the concrete section of beams and slabs, and use of CFRP laminates. Based on cost benefit, it was decided to use the CFRP laminates to strengthen the slabs in flexure and the beams in flexure and shear. Material Selection CFRP strips of 1.2 mm thickness and 50 mm width were used to strengthen the slabs and beams in flexure, as shown in Figure 2. The strips have a fiber volume content of 68% and epoxy resin (4). The tensile strength and modulus of the strips were 2800 MPa and 165 GPa, respectively. The strips were bonded to the concrete surface using epoxy-based two-component adhesive mortar. The adhesive strength of the mortar to the concrete surface was 4 MPa. CFRP laminates with a thickness of 0.13 mm were also used to strengthen the beams in shear as shown in Figures 3 and 4. The tensile strength and modulus of the laminates were 3500 MPa and 230 GPa, respectively. Design Criterion It was important to estimate the stress in the steel reinforcement caused by the additional straining actions resulting from the excessive settlement of the foundations. The measured crack width of the slabs and beams were used to estimate the tensile stress in the steel reinforcement. The required area of CFRP was calculated to allow for double the live load, accounting for the increase in the stress of the steel reinforcement. CFRP strips, 11.0 meters long and spaced every 500 mm were used on top of the slabs, as shown in Figure 4. CFRP strips were used on the bottom surface of two cracked beams to increase its flexural capacity. One layer of CFRP laminates was applied on the sides of the beams in a U shape to strengthen the beams in shear, as shown in Figure 4. The laminates were 300 mm wide and spaced every 50 mm.
Figure 2 Flexural strengthening of slabs Figure 3 Surface preparation of beams Figure 4 Shear strengthening of beams Figure 5 The Panorama building after retrofit Application of CFRP The tensile strength of the concrete was measured and found to have an average value of 2.0 MPa. Before application of the laminates, the humidity of the concrete was measured to ensure the dryness of the concrete surface. Concrete surface of the beams was prepared and leveled to ensure that the unevenness of the surface did not exceed 10 mm in 2.0 meters length. The surface was cleaned and the blowholes were filled with epoxy mortar. The strips were cut to the specified length and the adhesive epoxy mortar was applied with a roof shaped spatula onto the strips to a thickness of 1 to 2 mm. The adhesive epoxy mortar was also applied with 1 mm thickness to the prepared surface of the concrete. The strips were carefully applied to the concrete surface and pressed with a rubber roller. CFRP laminates were also applied to the concrete surface in a similar fashion as the CFRP strips. The edges of the concrete beams were rounded to prevent any stress concentration at the corners. The Panorama building is shown in Figure 5 after completion of the retrofit work. SECOND CASE STUDY: FLAT SLAB BUILDING The second case study is a reinforced concrete multi-story building constructed ten years ago using a flat slab system in Heliopolis, Cairo. The building is 30 m wide and 80 m
long divided into three parts with two expansion joints across the entire width of the building. In addition to the basement, the first and second parts of the building are 12 stories high, while the third part is only seven stories high. The thickness of the solid slab is 220 mm in the first and second parts and 240 mm in the third part. The flat slab is supported on rectangular columns spaced every 4 to 6 m. The columns are supported on 1.0-meter deep mat foundation. A general layout of parts 1 and 2 of the building showing the expansion joint and distribution of columns is shown in Figure 6. 50.0 23 Part 1 Part 2 25 Expansion joint 40.0 Figure 6 Layout of parts (1) and (2) of the building The different floors of the residential building were designed to carry a live load of 2.0 kn/m 2. The owner of the building requested to use the ground and first floors in the first and second parts for commercial purpose. This required retrofit for both slabs to carry a live load of 5.0 kn/m 2. It was also the request of the owner to finish the strengthening procedure in three weeks period including design and construction. Finite element analysis was carried out to calculate the induced bending moments in the flat slab due to self-weight, superimposed dead, and live loads independently. The maximum calculated service moment was 88 kn-meter, while the resistance of the critical section of the slab was only 65 kn-meter. This indicated that an increase in the flexural capacity of the slab by 35 percent was required. The punching shear capacity of the slab was checked and found to be sufficient without strengthening the slab in shear. Proposed Strengthening Schemes Two strengthening schemes were proposed to upgrade the slab and allow for 250 percent increase in the live load. The first proposed strengthening scheme was to increase the thickness of the slab by casting 80-mm reinforced concrete layer on the top of the existing slab. This required adding steel anchors to connect the old with the new concrete. It also required closing the area where the construction was to be carried out to allow for shuttering for at least one month. The second alternative was to use CFRP
plates to strengthen the slab in flexure at the mid-span and around the columns. After careful consideration of the technical aspects, cost and time limitation, it was the choice to use the CFRP system. CFRP plates were designed to carry the superimposed dead load, weight of partitions and live loads, while the self-weight of the slab was carried by the internal steel reinforcement only. The estimated time of CFRP application was only ten days. Material Properties CFRP plate bonding system requires use of unidirectional carbon fibers attached with resin to the surface of the concrete. The system utilized high strength CFRP plates with an ultimate tensile strength of 2800 MPa and elastic modulus of 165 GPa. The plates were 50-mm wide and 1.2-mm thick with a fiber volume content of 68 percent. The plates were supplied in 50-m length and rolled with 1.5-m diameter as shown in Figure 7. The resin was a two-component structural epoxy paste adhesive with tensile strength and elastic modulus of 25 MPa and 4.5 GPa, respectively. Epoxy mortar with higher tensile strength and modulus was used for leveling the concrete surface before applying the plates. Pull-off tests were carried out on the concrete slab, as shown in Figure 8 to ensure enough tensile strength before proceeding with the application of CFRP. The tensile strength of concrete was found to be 2.5 MPa, which is higher than the minimum value recommended by the manufacturing company of 2.0 MPa. The high tensile strength of concrete ensured good bond between the CFRP plates and concrete. Figure 7 Shipment of CFRP strips Figure 8 Pull-off tests of the concrete
Design Philosophy Design of CFRP plates was carried out using the requirements of both the ACI Code 318 (5) and the recommendations of the ACI Committee 440 (6) for design of FRP systems used for strengthening concrete structures. The design criteria covered the strength, serviceability, and ductility requirements as well as the risk of loss of CFRP plates in the event of fire. Strength Requirements The nominal strength of the slab was calculated based on the flexural, shear, and bond capacity of the section. The design criteria of the section included each of the following: Compression crushing of concrete; where concrete strains reaches a value of 0.003. Rupture of CFRP plates; where the ultimate strain in the plates reaches 1.7 percent. Delamination of CFRP plates. Anchorage failure or local failure of concrete layer between the plate and longitudinal tension reinforcement (7). Strength reduction factors, φ, was taken equal to 0.7 for both flexure and shear, and equal to 0.6 for delamination of plates. Mechanical anchorage was used in case that anchorage length of CFRP plates was not satisfied. Serviceability Adding externally bonded CFRP plates to the existing section of the concrete slab have a minor effect on the flexural stiffness of the section due to the small area of the plates. However, the thickness of the concrete slab was sufficient to satisfy the serviceability requirement even with the increase of the applied live load. The span-to-depth ratio of the slab was 1/28, which is higher than that recommended by the ACI Code, which specifies a minimum value of 1/32. Ductility The strain in the steel at the onset of maximum capacity of the concrete sections was maintained to be at least 0.005. This was achieved by limiting the maximum percentage of CFRP plates in RC sections so that the strain in the steel reinforcement exceeds 0.005. This strain value ensures yield of the steel and enough deformation to satisfy the ductility requirement. The ACI Committee 440 (6) recommends a minimum of 0.005 tensile strain to design tension-controlled sections. At this strain level, high deflection and large crack width will guarantee enough warning signs before failure. Fire Risk of loss of CFRP plates in the unlikely event of fire was taken into design consideration. High temperatures due to fire will cause a plastic flow of the epoxy resulting in a loss of the load transfer to the CFRP plates. Typically, the critical temperatures for epoxy are in the range of 50 o C to 90 o C (6). In recognition of the
temperature risks, the unstrengthened slab was checked to ensure an ultimate capacity that provides a positive factor of safety against collapse. The unstrengthened capacity was capable of resisting the service loads without yielding of steel reinforcement. The ultimate strength of the unstrengthened system exceeded the service loads by a factor, φ T of 1.2 according to equation 1. φ T S n 1.2 (S D + S L ) (1) Where S n, S D, and S L are the nominal strength of the unstrengthened section and induced straining actions due to dead and live loads, respectively. Construction Details Application of CFRP plates started with preparation of the concrete surface, followed by applying the epoxy mortar and finally attachment of the plates. The purpose of surface preparation was to remove the outer, weak and potentially contaminated concrete skin together with poorly bound material, in order to expose small- to medium-sized pieces of aggregate. Mechanical steel hammers were used to prepare the surface. This was achieved without causing micro-cracks or other damage that may reduce bond. The concrete surface was leveled and epoxy mortar was used prior to the application of the plates to fill large voids and blowholes, as shown in Figure 7. Extra care was taken during the application of epoxy mortar to ensure that it produced full bond between CFRP and concrete and that composite action was developed by the transfer of shear stress across the thickness of the adhesive layer. CFRP plates were cleaned and attached to the concrete surface using epoxy mortar. The maximum spacing between plates was kept five times the slab thickness, while the minimum spacing was 100 mm, as shown in Figures 9 and 10. Installation of the plates was carried out on the top and bottom surface (overhead) of the concrete using the same procedure. Both the piping system and light fixtures were not removed during application of the plates as shown in Figure 9. Figure 9 CFRP strips attached to the bottom surface of concrete
Figure 10 CFRP strips attached to the top surface of concrete CFRP plates bonded to the bottom surface of the slab had enough development length; while the plates bonded perpendicular to the edges of the top surface of the slab did not have enough length to transfer forces to the concrete. Steel angles anchored to the concrete slab with steel bolts were used as mechanical anchorage for the CFRP plates, as shown in Figure 11. It was reported that the mechanical anchorage can transfer 60 percent of the maximum force of the plates (8). The maximum tensile force in the mechanically-anchored CFRP plates was designed to be 30 percent of the maximum force to allow for a factor of safety of 2 against anchorage failure. Another detailing problem was encountered during placing the CFRP plates on the top surface of the slab. This was due to the fact that the plates did not intersect with the columns, but were placed around the columns, as shown in Figure 12. The dimensions of the columns varied from 0.6 x 0.6 m to 0.30 x 1.50 m, which means that there were no bonded CFRP plates on a 1.5-m segment of the maximum negative bending moment zone. In order to evaluate the forces in the slab around the columns, finite element analysis was conducted. Both steel reinforcement and the CFRP plates were modeled as frame elements, while the concrete was modeled as shell elements. Distribution of the stresses in the steel bars perpendicular to the long side of the columns is given in Figure 12. It can be seen that the maximum stresses occur in the steel bars located on the sides of the column, reducing towards the centerline of the column. The results of the FEA showed that adding CFRP plates only in the perpendicular direction to the long side of the column reduced the stresses in the steel bars by 5 to 28 percent. Bonding CFRP plates in the perpendicular and transverse directions to the long side of the column reduced the stresses in the steel bars by 9 to 28 percent. Therefore, it was decided to bond CFRP plates around the columns in both directions to reduce the stresses in the steel bars. The calculated maximum stresses in the steel bars with and without strengthening were 194 and 214 MPa respectively, as shown in Figure 12.
Figure 11 Mechanical Anchorage of CFRP plates A total of 2200 linear meters of CFRP plates were used in strengthening the concrete slab. Strengthening procedure did not stop the other civil or the electrical work in the building. The Application of the plates was successfully completed in only ten days. 220 200 180 Stress in steel bars (MPa) 160 140 120 Longitudinal CFRP Elevation Column width -1.5-1.0-0.5 0 0.5 1.0 1.5 Distance from the middle of the column (m) Unstrengthened Strengthened with transverse CFRP Strengthened without transverse CFRP Longitudinal CFRP strips Transverse CFRP strips Figure 12 Distribution of the stresses in steel bars Loading Test After application of the CFRP plates, loading tests were performed on the largest two bays of the slab, according to the Egyptian Code of Practice for Design and Construction Plan
of Concrete Structures (9). Sand packages were placed uniformly on the top surface of the slab to produce an equivalent load of 9.0 kn/m 2, which is equal to one and half the value of expected live load in addition to the weight of the superimposed dead load on the slab. The maximum deflection was found to be 6 mm at the mid-span of the slab after 24 hours of application. Most of the deflection was rebound after release of the load and the residual deflection was less than 0.4 mm. CONCLUSIONS This paper introduces design concept and construction details of two successful applications of CFRP laminates in strengthening RC structures in Egypt. Analysis of the strengthened concrete slab with CFRP strips using finite element analysis is also introduced. CFRP strips were attached to the concrete surface by epoxy paste as well as mechanical anchorages using steel angles. Application of CFRP was completed in short time compared to other conventional techniques in strengthening. REFERENCES 1. A. Hosny and A. Abdelrahman (2002), Proceeding of The Third Middle East Symposium on Structural Composites for Infrastructure Applications, Aswan, Egypt, 14-17 December. 2. Hosny A., Mahfouz I., and Sarkani S., (1999) Proceeding of The Second Middle East Symposium on Structural Composites for Infrastructure Applications, April 26-29, Hurghada, Egypt, 482p. 3. Hosny A., Rizkalla S., Mahfouz I., and Mosallam A., (1996) Proceeding of The First Middle East Workshop on Structural Composites, June 14-15, Sharm El-sheikh, Egypt, 486p. 4. Sika Egypt (1998), Strengthening of Structures with CFRP Laminates: The Sika Carbodur System, Technical Report, July. 5. ACI Committee 318 (1995), Building Code Requirements for Structural Concrete (ACI 318-95) and Commentary (ACI 318R-95), American Concrete Institute, Framington Hills, Mich., 391p. 6. ACI 440.2R-02, Emerging Technology Series (2002), Guide for the Design and Construction of Externally Bonded FRP Systems for Strengthening Concrete Structures, October, 45p. 7. Ehsani M., and Saadatmanesh H. (1998), Design and Retrofit of structures with fiber composites, Lecture Notes for a Short Course, Second International Conference on Composites in Infrastructure (ICCI 98), Tucson, Arizona, 200p. 8. Sika Egypt (1999), Tensile Strength of CFRP Laminates with Mechanical Anchorage: The Sika Carbodur System, Personal Communications. 9. Ministry of Housing, Utilities, and Urban Communities (2001), Egyptian Code of Practice for Design and Construction of Reinforced Concrete Structures.