INTERNATIONAL JOURNAL OF CIVIL ENGINEERING AND TECHNOLOGY (IJCIET) STRENGTHENING REINFORCED CONCRETE ONE WAY SLABS BY OVERLAY TECHNIQUE & USING CFRP

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1 INTERNATIONAL JOURNAL OF CIVIL ENGINEERING AND TECHNOLOGY (IJCIET) ISSN (Print) ISSN (Online) Volume 5, Issue 2, February (214), pp. 52- IAEME: Journal Impact Factor (214): (Calculated by GISI) IJCIET IAEME STRENGTHENING REINFORCED CONCRETE ONE WAY SLABS BY OVERLAY TECHNIQUE & USING CFRP Samir A.AL Mashhadi University of Babylon, College of Engineering Ahmed Hassan Hadi University of Babylon, College of Engineering Abstract The objective of this research is to study the behavior of reinforced concrete one way slabs, that strengthened with concrete overlay with and without externally bonded Carbon Fiber Reinforced Polymer (CFRP) sheets. The experimental results showed that increasing in concrete overlay thickness from 25mm to mm caused an increase in the first cracking load value, this increasing ranged about (33% to 1%) while for ultimate load was about (31% to 88%) in compared with control slab specimens. The increasing in compressive strength of the concrete overlay did not give a considerable increase in the first crack loading and ultimate loading. It was obtained that using the bonding materials gave higher ultimate loading, low deflection and low concrete strain in compared with specimens without bonding materials. From results the bonding materials can be arranged according to bonding. The specimens strengthened with suggested combined strengthening system (concrete overlay plus CFRP sheets) showed the best results which gave an increase 164% in ultimate load and 167% in first crack loading when compared with the corresponding control specimens. 1. INTRODUCTION Reinforced concrete is the most frequently applied structural material. That is explainable with its easy use, its advantageous mechanical properties which develop after hardening. The commonly held view, that concrete is a durable, maintenance-free construction material has been changed in recent years. Several examples can be shown where concrete did not perform as well as it was expected. These are the results of the insufficient consideration of durability during the design process, the inadequate execution and the maintenance. Therefore, repair or strengthening of a concrete structure may become necessary: 52

2 To increase live-load capacity, example of a bridge subject to increased vehicle loads or a building the use of which is to change from residential to commercial. To add reinforcement to a member that has been under designed or wrongly constructed. To improve seismic resistance, either by providing more confinement to increase the strain capacity of the concrete, or by improving continuity between members. To replace or supplement reinforcement, example damaged by impact or lost due to corrosion. To improve continuity, example across joints between precast members (Ibrahim and Mahmood, 29). In the past, reinforced concrete slabs were strengthened by conventional methods such as concrete overlay as shown in figure 1, span shortening and externally bonded steel reinforcement. Today there are several types of Carbon Fiber Reinforcement Polymer (CFRP) strengthening systems and techniques available to strengthen reinforced concrete slabs. The suitability of each system depends on the type of structure that shall be strengthened. Therefore, it is essential for engineers to understand the consequences of the design choice in terms of efficiency and failure mechanism for different systems before further attempts are carried out (Tan, 23). Concrete Bonding Material Existing concrete slab Figure 1: Conventional concrete overlay 2. SPECIMEN DETAILS The experimental study consisted of testing twenty-one reinforced concrete one way slabs under one point loading. All slabs were 12mm long, and had 2mm wide, 1mm high cross section. The flexural reinforcement of these slabs consisted of 3-Ø8mm in the main direction and Ø4mm@1mm in the transverse direction. The flexural reinforcement ratio is.68 % which is below the maximum reinforcement ratio allowed under the current ACI Code. The flexural reinforcement of overlay slabs consisted of 3-Ø4mm in the main direction which represent the minimum steel reinforcement and Ø4mm@1mm in the transverse direction. The concrete overlay used was 25 or mm thickness. Figure 2 shows the cross section and the reinforcement details for the bare slab and concrete overlay. 53

3 3. SPECIMENS IDENTIFICATION AND STRENGTHENING SCHEMES In order to identify the test specimens with different variables and strengthening schemes, the following designation system is used: Type of loading: P: Positive bending (i.e. placed concrete overlay in compression zone) and N: Negative bending (i.e. placed concrete overlay in tension zone). Type of bonding materials between the hard concrete (bare slab) and the fresh (concrete overlay): QM: Quickmast 18, SBR: SBR, OPC: Ordinary Portland Cement and WT: Without bonding material. Thickness of concrete overlay: 25: Thickness of overlay 25mm and : thickness of overlay mm. Reinforcement of concrete overlay: R: reinforced concrete overlay and P: Plain concrete overlay. Nominal compressive strength of concrete overlay: C25: 25MPa, C4: 4MPa and C: MPa. Number of CFRP sheets: : No CFRP, 1: one sheet and 2: two sheets. Table 1 illustrates the specimen identification system used based on the specimen identification pattern described above. 4. STRENGTHENING SCHEMES Figure 2: Cross section of test specimens Six of the reinforced concrete slabs were strengthening by externally bonded CFRP placed in the tension zone. The slab specimens P-QM 25 R-C25-1 and P-QM R-C25-1 were strengthened with one CFRP sheets bonded on bare slabs while the slab specimens P-QM 25 R-C25-2 and P-QM R-C25-2 were strengthened with two CFRP sheets bonded on bare slab and the slab specimens N- 54

4 QM 25 R-C25-2 and N-QM R-C25-2 were strengthened with two CFRP sheets bonded on concrete overlay slab, as shown in Figure 3. Specimen Table 1: Specimen Identification and Strengthening Schemes Overlay Type of Thickness Overlay Type of reinforcement bonding of overlay f c loading description material (mm) (MPa) CFRP amount Nil Nil Nil Nil Nil Nil Nil Nil Nil Nil P-QM 25 R-C25- Positive Quickmast 25 Reinforced 25 P-QM R-C25- Positive Quickmast Reinforced 25 P-QM 25 R-C4- Positive Quickmast 25 Reinforced 4 P-QM R-C4- Positive Quickmast Reinforced 4 P-QM 25 R-C- Positive Quickmast 25 Reinforced P-QM R-C- Positive Quickmast Reinforced P-WT R-C25- Positive Without Reinforced 25 P-SBR R-C25- Positive SBR Reinforced 25 P-OPC R-C25- Positive Cement Reinforced 25 P-QM 25 P-C25- Positive Quickmast 25 Plain 25 P-QM P-C25- Positive Quickmast Plain 25 N-QM 25 R-C25- Negative Quickmast 25 Reinforced 25 N-QM R-C25- Negative Quickmast Reinforced 25 P-QM 25 R-C25-1 Positive Quickmast 25 Reinforced 25 1 P-QM R-C25-1 Positive Quickmast Reinforced 25 1 P-QM 25 R-C25-2 Positive Quickmast 25 Reinforced 25 2 P-QM R-C25-2 Positive Quickmast Reinforced 25 2 N-QM 25 R-C25-2 Negative Quickmast 25 Reinforced 25 2 N-QM R-C25-2 Negative Quickmast Reinforced

5 Figure 3: Schematic representation of CFRP strengthening schemes All stirrups of CFRP of length 12 mm, width 8 mm and thickness.131mm comprising an equivalent steel area 75mm MATERIALS 5.1 Cement Ordinary Portland Cement (Type I) of Tasluja-Bazian mark was used in casting all the specimens. Test results indicate that the adopted cement conforms to Iraqi specifications (I.O.S. NO.5/1984).OPC was used as bonding additive (bonding old concrete with new concrete) by scattering water on the surface of the bare slab and scattering dry OPC as a slurry coat Fine Aggregate (S and) Natural sand from Al-Akhaider region was used as a fine aggregate in this research. Results indicate that the fine aggregate grading and the sulphate content are within the requirements of the Iraqi specification No.45/l Coarse Aggregate (Gravel) Locally available rounded gravel of (19mm) and (1mm) maximum size were used. For high strength concrete, it was used crushed gravel with a maximum size of (1mm). Results indicate that the Coarse Aggregate grading and the sulphate content are within the requirements of the Iraqi specification No.45/l

6 5.4 High Range Water Reducing Admixture (HRWRA) A chemical admixture based on modified polycarboxylic ether, which is known commercially (Glenium 51) was used in producing HSC as a high range water-reducing admixture. 5.5 Mixing Water Ordinary clean tap water was used for casting and curing all the specimens. 5.6 Carbon Fiber Reinforced Polymer (CFRP) The mechanical properties of CFRP sheets used here is taken from manufacturing specifications (Sika, 25). 5.7 Steel Reinforcing Bars For all slabs specimens, two size of steel reinforcing bars were used (Ø8mm and Ø4mm). 5.8 Impregnation resin Sikadur-33 (impregnation resin) was used in this work for the bonding of CFRP sheet. 5.9 Quickmast 18 Quickmast 18 is a two components, solvent free epoxy resin bonding agent, supplied in preweighed packs containing base and hardener. The surface of the bare slab was grinded using an electrical hand grinder to expose the aggregate and to obtain a clean sound surface and painted by Quickmast Styrene- Butadiene- Rubber (SBR) admixture SBR is used as bonding additive (bonding old concrete with new concrete) when used as a slurry coats in the following proportions: 1 SBR: 1 clean water: 3 OPC (by volume) Concrete Mix All bare slabs that used with control mix proportions of materials; 1:1.8:2.5 (cement: sand: gravel), and water/cement ratio is.5 (by weight). Concrete overlay that used with three types of compressive strength: 25 MPa cylinder compressive strength, the mix proportions were: 1:1.91:2.8 (cement: sand: gravel), and water/cement ratio is.48 (by weight). Concrete mix was designed according to the American mix design method ACI 211 (Neville, 1995) to have compressive strength 25 MPa at age of 28 days with maximum rounded aggregate size (1mm) and the slump is mm (from seven experimental mixes). 4 MPa cylinder compressive strength, the mix proportions were: 1:1.34:1.61 (cement: sand: gravel), water/cement ratio was.35 (by weight) and superplasticizer of.44% quantity by weight of cement was used. MPa cylinder compressive strength, the mix proportions were: 1:1.134:1.93 (cement: sand: gravel), water/cement ratio is.28 (all by weight) and superplasticizer of 1.4 % quantity by weight of cement was used. Mix proportions and details for all mixes are shown in Table 2. 57

7 Slab group Bare slab Cement content kg/m 3 Table 2: Details of Mixes Used in Slabs S.P, % Sand Gravel w/c kg/m 3 kg/m 3 weight of ratio cement Slump (mm) f c MPa f c f c f c * where: f c : Average cylinder 1 3mm compressive strength. * Average cube 1 1 1mm compressive strength Test Setup After curing, the slabs specimens were transported to the Structural Laboratory of Babylon Technical Institute to test them under one point loading up to failure. All slab specimens were tested in one point loading. slab specimens were tested as simply supported over 1mm span in 1 kn capacity hydraulic machine. Each slab specimen was supported and loaded by rollers. Forces were distributed through steel bearing plate 2mm in length to cover the entire slab width. To observe crack development, slab specimens were painted white with emulsion paint before testing. Cracks were traced by pencil. Figure 4 shows the test setup. Deflection of the slab specimens was measured at mid-span using a dial gage with travel distance of 3mm and accuracy of.1mm. An ELE mechanical strain gage having an accuracy of.2mm was used to measure the concrete strains. Demec discs were calibrated using an accompanying special ruler. Arrangement and distribution of these Demec discs are shown in Figure EXPERIMENTAL RESULTS 6.1 Compressive Strength The compressive strength test results are obtained from the average of three concrete cubes cast with two concrete slab specimens except slab specimen P-WT R-C25- cast with three cubes and tested at the same age of the slab specimens, as presented in Table Flexural Strength (Modules of Rupture) The results of flexural tensile strength test of the concrete specimens (prisms 1 1 4mm) were tested at the same age of the slab are given in Table First Cracking Loads In specimens series P-QM 25 R-C25- to P-WT R-C25- strengthening of RC slab by adding concrete overlay showed better enhancement in first cracking loads when compared with control slab specimens 1 and 2 as shown in Table 5. 58

8 Figure 4: Test setup for slab specimens Figure 5: Arrangement of fixed demec steel disks 59

9 Table 3: Measured Values of Compressive Strength at the same age of the slab Slab specimen Bare slab compressive strength Overlay MPa compressive strength MPa Cube Cylinder Cube Cylinder P-QM 25 R-C25- P-QM R-C P-QM 25 R-C4- P-QM R-C P-QM 25 R-C- P-QM R-C P-WT R-C P-SBR R-C25- P-OPC R-C P-QM 25 P-C25- P-QM P-C N-QM 25 R-C25- N-QM R-C P-QM 25 R-C25-1 P-QM R-C P-QM 25 R-C25-2 P-QM R-C N-QM 25 R-C25-2 N-QM R-C Table 4: The Flexure Strength Results for Bare and Overlay Slabs Slab specimen Measured fr MPa Measured fr MPa for bare slab for overlay P-QM 25 R-C25- P-QM R-C P-QM 25 R-C4- P-QM R-C P-QM 25 R-C- P-QM R-C P-WT R-C P-SBR R-C25- P-OPC R-C P-QM 25 P-C25- P-QM P-C N-QM 25 R-C25- N-QM R-C P-QM 25 R-C25-1 P-QM R-C P-QM 25 R-C25-2 P-QM R-C N-QM 25 R-C25-2 N-QM R-C

10 Table 5: First Cracking Load for The Tested Slabs Specimen First Increase in cracking cracking load, kn load,% 7.5 N / A 7.5 N / A P-QM 25 R-C P-QM R-C P-QM 25 R-C P-QM R-C P-QM 25 R-C P-QM R-C P-QM P-C N-QM R-C P-SBR R-C P-OPC R-C P-WT R-C N-QM 25 R-C N / A P-QM 25 P-C N / A P-QM 25 R-C P-QM 25 R-C P-QM R-C P-QM R-C N-QM 25 R-C Cracking Patterns The control slab specimens 1 and 2 were tested in order to compare between them and the other that strengthened. The appearance of flexural cracks was first at 7.5 kn under the point load region. Flexural cracks formed and widened as loading proceeded. At 12.5 kn loading, new flexural cracks formed and wide speared along the mid-span. Further flexural cracks occurred near the quarter-span of the specimens as the load increased to 2 kn. At 27.5 kn loading, no new cracks are appeared and that means the specimens is controlled by yielding of steel, as shown in Plate 1. The first crack in strengthened slab specimens P-QM 25 R-C25-, (which was strengthened by 28.6 MPa compressive strength reinforced concrete overlay of 25mm thickness bonded by quickmast 18) was observed in mid-span region at loading of 1 kn. As the load was increased to 17.5 kn new cracks formed within the quarter-span of the specimens. The cracks stretched to the overlay at the loading of 25 kn and no separation had been occurred between the overlay and the bare slab. Finally, typical flexural failure occurred at loading of 38.4 kn as shown in plate 2. The slab specimens P-SBR R-C25- was strengthened by adding 28.8 MPa compressive strength reinforced concrete mm overlay thickness and the bonding material SBR placed between bar slab and the concrete overlay. The first cracking was observed at the load of 15 kn within midspan region. As the loading increased the cracks increased and widened, the separation had occurred between the concrete overlay and the slab at the load of 4 kn as shown in Plate 3 The first crack in strengthened slab specimens P-QM 25 R-C25-1, (which was strengthened by adding 32.5 MPa compressive strength reinforced concrete overlay of 25mm thickness and strengthened with one strip of CERP) was observed at load 12.5 kn under point load region. The formation of flexural cracks (that occurred as a result of the yielding of embedded steel 61

11 reinforcement) was at loading 45 kn. The spacemen failed by debonding failure which occerred at load 52.5 kn as shown in Plate 4. (a): bottom view (b): side view Plate 1: Cracking pattern for slab control 1 (a): bottom view (b): side view Plate 2: Cracking pattern for slab P-QM 25 R-C25-62

12 (a): bottom view (b): side view Plate 3: Cracking pattern for slab P-SBR R-C25- (a): bottom view (b): side view Plate 4: Cracking pattern for slab P-QM 25 R-C

13 6.5 Load-Deflection Curves The structural behavior of tested slab specimens are represented here by their load versus mid-span deflection as shown in Figures 6 to 14 For the unstrengthened control specimens (control 1 and 2) a large increase in deflection was noticed in the third stage, while the applied load changed little, as shown in Figure 6. Figures 7 to 8 show the behavior of the flexural slab specimens with different thickness of concrete overlay. It is shown from these Figures that the values of the maximum deflection of the specimens with concrete overlay (25 or mm) are smaller than the deflection for control slabs 1 and 2. The specimens with concrete overlay (25 or mm) have greater stiffness to resist the applied load and as the thickness of the concrete overlay increased the resistance of the section will increase accordingly. 9 8 Applied load (kn) Midspan deflection (mm) Figure 6: Load vs. deflection for control slabs 1 and 2 Applied load (kn) Midspan deflection (mm) P-QM 25 R-C25- P-QM R-C25- Figure 7: Load vs. deflection for slabs with different thickness concrete overlay and compressive strength 25 Mpa 64

14 Applied load (kn) Midspan deflection (mm) P-QM 25 R-C4- P-QM R-C4- Figure 8: Load vs. deflection for slabs with different thickness concrete overlay and compressive strength 4 MPa Applied load (kn) Midspan deflection (mm) P-QM 25 R-C- P-QM R-C- Figure 9: Load vs. deflection for slabs with different thickness concrete overlay and compressive strength MPa The specimens P-WT R-C25-, P-OPC R-C25- and P-SBR R-C25- failed suddenly and did not reach the third stage because these specimens failed in shear interface between the bare slab and concrete overlay as show in figure 1, while P-QM R-C25- showed high ductility and did not fail in shear interface. Figures 11 to 14 show the effect of CFRP strengthening on the flexural behavior of the slab specimens. The deflection of the slabs that strengthened with CFRP sheets was lower than the deflection of the corresponding control slabs 1 and 2. That is due to increasing the reinforcement by adding CFRP sheets. 65

15 Applied load (kn) Control2 P-QM R-C25- P-SBR R-C25- P-OPC R-C25- P- WT R-C Midspane deflection (mm) Figure 1: Load vs. deflection for slabs with different bonding Materials Applied load (kn) P-QM 25 R-C25- P-QM 25 R-C25-1 P-QM 25 R-C Midspane deflection (mm) Figure 11: Load vs. deflection for slabs with positive loading, different CFRP amount and concrete overlay thickness 25mm Applied load (kn) P-QM R-C25- P-QM R-C25-1 P-QM R-C Midspane deflection (mm) Figure 12: Load vs. deflection for slabs with positive loading, different CFRP amount and concrete overlay thickness mm 66

16 Applied load (kn) N-QM 25 R-C25- N-QM 25 R-C Midspane deflection (mm) Figure 13: Load vs. deflection for slabs with negative loading, different CFRP amount and concrete overlay thickness 25mm Applied load (kn) Midspane deflection (mm) N-QM R-C25- N-QM R-C25-2 Figure 14: Load vs. deflection for slabs with negative loading, different CFRP amount and concrete overlay thickness mm 6. CONCRETE STRAINS The distribution of concrete strains at mid-span section of the tested slab specimens was measured by using ten, twelve and fourteen Demec discs over the depth (1, 125 and 1mm) respectively of each slab. The concrete strain distribution over the depth of all the tested slabs at different load levels as shown in Figures 15 to

17 Distance from soffit (mm) Load 5 kn Load 1 kn Load 17.5 kn Load 22.5 kn Load 27.5 kn Strain (mm/mm) Figure 15: Concrete strain distribution for control slab 1 Distance from soffit (mm) Load 5 kn Load 1 kn Load 15 kn Load 2 kn Load 3 KN Strain (mm/mm) Figure 16: Concrete strain distribution for slab P-QM R-C25- Distance from soffit (mm) Load 17.5 KN Load 27.5 KN Load 4 KN Load 55 KN Load KN Strain (mm/mm) Figure 17: Concrete strain distribution for slab P-QM R-C

18 7. CONCLUSIONS AND RECOMMENDATIONS Based on the results of this work, the following conclusions are obtained: 1. Increasing the thickness of concrete overlay that placed in compression zone (positive bending) (25 or mm) caused an increase in the first cracking load about (33% to 1%) and (31% to 88%) in ultimate load when compared with the control specimens. 2. Placing the concrete overlay with mm thickness at the tension zone (negative bending) shows a small increase in ultimate load when compared with the of 25mm thickness, while a considerable increase in first crack loading appear. 3. No considerable effect on the first cracking load and ultimate load when increasing the compressive strength of concrete overlay, except improving the durability. 4. The bonding materials that used between fresh concrete overlay and bare slab do not affect on the first crack loading, but there is a considerable increase in the ultimate load capacity. The materials that give increasing in ultimate load arranged downward as follows: Quickmast 18, SBR and Ordinary Portland Cement. 5. For the cased study it was observed that the deflection values decrease by using the bonding materials between the bare slab and the concrete overlay, these values are arranged upward as follows: Quickmast 18, SBR and Ordinary Portland Cement. 6. The slab specimens strengthened by adding plain concrete overlay failed suddenly, exhibited no ductility and also gave lower ultimate load capacity when compare with the corresponding slabs strengthened with reinforced concrete overlay. 7. The slabs strengthened with suggested combined (concrete overlay plus CFRP sheets) showed the best results which gave an increase in the first cracking load and ultimate load 167% and 164% respectively when compared with the corresponding control specimens. 8. Further studies focus on repairing of reinforced concrete one way slabs by adding concrete overlay and/or CFRP can be investigated with different predamaged ratios. 8. REFERENCES [1] American Concrete Institute, ACI Committee 318, (25), "Building Code Requirements for Structural Concrete (ACI 318-5) and Commentary (ACI 318R-5)", American Concrete Institute, Farmington Hills, MI. [2] ACI Committee 441 Report, (1997), "High Strength Concrete Columns State-of-the-Art", ACI Structural Journal, Vol.94, No.3, pp [3] British standards Institution, (1989), "Method for Determination of Compressive Strength of Concrete Cubes", B.S.1881:Part 116:, pp.3. [4] Ibrahim and Mohamed Sh.Mahmood, (29), "Finite Element Modling of Reinforced Concrete Beams with FRP Laminates", European Journal of Scientific Research ISSN X Vol. NO. 4, pp [5] Iraqi Specification, No.5/1984, "Portland cement". [6] Iraqi Specification, No.45/1984, "Aggregate From Natural Sources for Concrete and Construction". [7] Iraqi Building Code Requirement for R.C., Code 1, 1987, Building Research Center, pp.7. [8] Neville, A. M., Properties of Concrete, 4th ed., John Wiley and Sons, Inc., New York, [9] Dr. Salim T. Yousif, New Model of CFRP-Confined Circular Concrete Columns: Ann Approach, International Journal of Civil Engineering & Technology (IJCIET), Volume 4, Issue 3, 213, pp , ISSN Print: , ISSN Online:

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