STRESS ANALYSIS OF CFRP STRENGTHENED SLABS SUBJECTED TO TEMPERATUREE CHANGE

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1 International Journal of Civil Engineering and Technology (IJCIET) Volume 8, Issue 1, January 2017, pp Article ID: IJCIET_08_01_079 Available online at &IType=1 ISSN Print: and ISSN Online: IAEME Publication Scopus Indexed STRESS ANALYSIS OF CFRP STRENGTHENED SLABS SUBJECTED TO TEMPERATUREE CHANGE Abdul Ridah Saleh Al-Fatlawi Assistant Professor, College of Engineering, University of Babylon, Iraq Dhoha Saad Hanoon College of Engineering, University of Babylon, Iraq ABSTRACT This paper includes an experimental and analytical investigation of flexural behavior of reinforced concrete two-way slabs strengthened with CFRP sheets, CFRP bar due to temperature change. Normal concrete was used to cast the slabs. The experimental work includes testing of twelvee reinforced concrete slab specimens with dimensions (900mmx900mmx70mm). These slabs can be divided according to temperature change to three groups at 23,45 and 80 each one content four specimens. Also slabs can be divided according to strengthening to four groups each group contain three specimens, first group was tested without strengthening acts as reference slabs (control), second group was reinforced with (CFRP) bar, third group was strengthening with (CFRP) sheet 5cm each 10 cm and the last group was strengthening with (CFRP) sheet 10cm each 20cm. The use of CFRP sheets delays the appearance of the cracks by (60%-74%) compared with slabs without strengthening. The experimental results showed that the slabs reinforcement with CFRP bar reduces the ultimate load carrying capacity by (0.61%-15.67%) compared with reinforcement concrete slab. Also the experimental results show that the ultimate loads are increased by about (0.61%-83.03%) for the slabs strengthened with CFRP sheets with respect to the unstrengthened reinforced concrete slab (control slab). The increase in load carrying capacity of slab due to temperature change was (36.36%-51.5%) compared with the same slabs without strengthening at 23. The optimum a result was in the slabs strengthen by CFRP sheets 5cm each 10cm at temperature 80 by increasing percentage (83.03%) compared without strengthen slabs. The numerical analyses present three-dimension nonlinear model by using computer program (ABAQUS 6.13). Key words: Temperature change, CFRP bar, CFRP sheet, Uniformly Distributed Load and Strengthening Cite this Article: Abdul Ridah Saleh Al-Fatlawi and Dhoha Saad Hanoon, Stress Analysis of CFRP Strengthened Slabs Subjected to Temperature Change. International Journal of Civil Engineering and Technology, 8(1), 2017, pp /issues.asp?jtype=ijciet&vtype=8&itype= = editor@iaeme.com

2 Stress Analysis of CFRP Strengthened Slabs Subjected to Temperature Change 1. INTRODUCTION The use of FRP to strengthen concrete structures has been a serious problem since the early 1990s. The implementation of research activities to study the effect of temperature on CFRP reinforced concrete structures. Aiello, et al. (1999)[1] studied the effects of thermal loads on the structural performance of FRP reinforced concrete elements. The difference in transverse coefficient of thermal expansion (CTE) between FRP and concrete causes the presence of tensile stresses within the concrete and, eventually, the formation of cracks when the temperature increases. They address the evaluation of temperature variations on concrete elements reinforced with AFRP and GFRP rebar, varying the thickness of the concrete cover and the shape of the cross-section, in absence of transverse reinforcement. Results obtained confirm the influence of temperature variations on the state of strain and stress within FRP reinforced concrete elements and the necessity of a minimum concrete cover to be provided in order to avoid the formation of through cracks. While Masmoudi, et al. (2005) [2] presents the results of an experimental investigation to analyze the effect of the ratio of concrete cover thickness to FRP bar diameter c/db on the strain distributions in concrete and FRP bars, using concrete cylindrical specimens reinforced with a glass FRP bar and subjected to thermal loading from 30 to +80 C. The experimental result shows that The minimum and maximum transverse strains for the five FRP bar diameters tested in this study vary between 1,000 and 2,000 microstrain at 30 C and between +1,000 and 2,500 microstrain at 80 C, respectively. This observation seems to be independent of the concrete cover thickness, and as expected, the transverse strain of FRP bars is more dependent on the temperature value at the interface of FRP bar/ concrete, since the test procedure guarantees a stabilized temperature. 2. OBJECTIVE OF THIS STUDY The objectives of the present work are:- Investigating experimentally the effect of temperature variation on the flexural capacity of two way RC slabs. Investigating. Experimentally, the behavior of T.W RC slabs with strengthened with (CFRP sheet) subjected to thermal load and static uniform loading. Investigating the effect of the different distribution of strengthening Finite element analysis by using ABAQUS program and comparing the results with those obtained experimentally 3. MATERIAL PROPERTIES 3.1. Cement Ordinary Portland cement was used in this study (Iraqi cement) This cement satisfies Iraqi specifications No.5 (1984)[3] Fine Aggregate (Sand) Normal-weight natural sand obtained from Al-Ukhaider region was used as fine aggregate for concrete mixes in this study. The obtained results indicated that the fine aggregate grading and the sulfate content were within the limits of Iraqi specifications (No.45/1984)[4] Coarse Aggregate (Gravel) Natural crushed gravel from Al-Badra-wa-Jasan region with maximum size (17mm), was used in this work. The crushed gravel was washed and cleaned by water for several times until the out washing editor@iaeme.com

3 Abdul Ridah Saleh Al-Fatlawi and Dhoha Saad Hanoon water clean then stored in air for surface to dry, and then stored in a saturated dry surface condition before using Water Using ordinary clean tap water for casting and curing all the slab specimens as well as for washing the fine and coarse aggregate Steel Reinforcement According to (ASTM 996 M-05) (2005) [5], deformed steel bars ( 8 mm ) in diameter were used as reinforcement to test slab specimens obtained from China production Carbon Fiber Reinforced Polymer (CFRP) and Sikadur-330 The CFRP sheet used in the strengthening application was SikaWarp Hex-230C unidirectional flexible sheets. The structural adhesive paste used for bonding the SikaWarp Hex-230C sheets to the concrete substrate was Sikasdure-330 which is high-modulus high-strength two component (A and B) products CFRP Bar The CFRP bar used in reinforcing of concrete. 4. CONCRETE MIX In this work use Normal Concrete 4.1. Electrical -Thermal Oven It is an electric oven consists of a total of heater in different special lengths. And the temperature would be raising gradually from Also there are temperature indicatore that are showing degree of heat inside the oven. There are also thermal sensors placed inside the holes to discover the extent of temperature model. Where the holes are placed on special divided aspects and mid model. As show in Figure (1). 5. SPECIMENS DESCRIPTION Figure 1 Electrical -Thermal oven 5.1. The Molds Preparation A total of twelve two-way RC square slabs were cast and cured under laboratory conditions. All slabs were casting in plywood mold to give two-way slab specimens with dimension (900 X 900) mm and thickness (70 mm), and aspects of the mold is made of a plate thick (20mm) to insure that the two way slab had a smooth surface editor@iaeme.com

4 Stress Analysis of CFRP Strengthened Slabs Subjected to Temperature Change 5.2. Specimens Design The specimens were designed to study the effect of temperature change (from23 to 80) on the flexural response of strengthened RC two-way slabs by (CFRP bar, CFRP sheet). All of the test slabs were of the same dimensions (900 mm * 900 mm * 70 mm), and had the same flexural reinforcement with (5 8mm in diameter) spaced at 15 cm c/c in x- and y-directions equivalent to reinforcement ratio of (ρ= ) and similar arrangement (square mesh) Supporting and Loading System All slab specimens were tested in a universal testing machine, with maximum capacity of (2000 kn). The slabs were simply supported over four (800mm) lines on all four sides (knife edge) resting on stiff steel frame and subjected to a uniform load applied to the top face of slabs, Before the testing of all slabs exposed to temperatures required by placing them inside the oven was manufactured for this purpose as show in Figure (2).All four support lines were 50 mm from the slab edges, so the effective span of the slab in both directions was 800mm. Figure 2 Supporting base and load system 6. SPECIMEN IDENTIFICATION AND STRENGTHENING SCHEMES The present study deals with twelve RC two way slab specimens were investigated in this research. They were subjected to both mechanical loading and temperature variation from (23 to 80 ). the specimens can be divided into groups according to: 6.1. Temperature change ( ) Four specimens subjected to Four specimens subjected to Four specimens subjected to 6.2. Strengthening Schemes Three slabs without strengthening (control) Three slabs reinforced with CFRP bars Three slabs externally strengthening with CFRP sheets (5cm each 10cm) Three slabs externally strengthening with CFRP sheets (10cm each 20cm). Specimen shapes and Strengthening Schemes are shown in Figure (3) editor@iaeme.com

5 Abdul Ridah Saleh Al-Fatlawi and Dhoha Saad Hanoon Figure 3 Specimen shapes and Strengthening 7. CONCRETE CASTING AND CURING The internal surfaces of cube, cylinder, prism, T.W molds are well cleaned and oiled to avoid adhesion with concrete after hardening. Afterwards, the reinforcement is placed in the right position for all slab molds. 8. THE MECHANICAL PROPERTIES OF HARDENED CONCRETE TESTS In the hardened phase, only destructive test is done. Destructive testing are compressive strength (cube & cylinder), splitting tensile strength, modulus of rupture, and stress-strain relationship (static modulus of elasticity).all the hardened properties of concrete for all specimens at 28 days are listed in Table (1) Group Table 1 Hardened Properties for Specimens Specimen ' f symbol cu (Mpa) f c (Mpa) f ct (Mpa) (E c ) (Mpa) * S S1c S1s-5cm S1s-10cm S S2c S2s-5cm S2s-10cm S S3c S3s-5cm S3s-10cm editor@iaeme.com

6 Stress Analysis of CFRP Strengthened Slabs Subjected to Temperature Change 9. EXPERIMENTAL RESULT OF SLAB MODELS 9.1. General Behavior All specimens were designed with a flexural reinforcement ratio of (0.499%) with a clear cover to the reinforcement of 15mm, which is higher than the minimum reinforcement ratio required (0.0018) by ACI building code First Cracking Loads The first cracking loads (Wcr) which were obtained from. Generally, the visible first crack load of all the specimens varied from (18%) to (43.85 %) of the experimental average ultimate loads Ultimate Load and Failure Modes Table (2) provides a comparison for slabs with effect of temperature variation, effects of the strengthening by using CFRP sheets on increasing the ultimate loads and failure modes with respect to unstrengthened slab (control slab). The comparison is based on the ratio of the measured ultimate load for each slab with respect to the ultimate measured load of the control slab. Specimen Table 2 Ultimate load capacity in groups Ultimate load (kn/m 2 ) Increase in ultimate load (%) S S1c S1s-5cm S1s-10cm S S2c S2s-5cm S2s-10cm S S3c S3s-5cm S3s-10cm Cracking Patterns The cracking behavior of each group slab specimen was discussed in the following: Group 1: Contains four specimens subjected to temperature (23) editor@iaeme.com

7 Abdul Ridah Saleh Al-Fatlawi and Dhoha Saad Hanoon Group 2: Contains four specimens subjected to temperature(45) Group 3: Contains four specimens subjected to temperature(80)

8 Stress Analysis of CFRP Strengthened Slabs Subjected to Temperature Change 10. FINITE ELEMENT METHOD RESULTS Ultimate Load and Ultimate Deflection A comparison between the theoretical and experimental values of the first crack load, ultimate load and ultimate deflection for all slabs models as shown in Table (3). The table shows a good agreement between the theoretical and the experimental results. Table 3 Experimental and theoretical, ultimate load and max. deflection. Slab Symbol Temp. Ultimate Load (kn/m2) F.E./EXP. (%) Max. Deflection (mm) Exp. F.E Exp. F.E F.E./EXP. (%) S S1C S1S-5cm S1S-10cm S S2C S2S-5cm S2S-10cm S S3C * 6.332* S3S-5cm ** 3.917** S3S-10cm *** 3.01*** *Deflection at load 130 kn/m 2 *** Deflection at load 180 kn/m 2 ***Deflection at load 140 kn/m CONCLUSION The experimental test results confirmed that the strengthening technique of (CFRP sheet) system is applicable and can increase the ultimate capacity for strengthened of R.C two way slab. In this study, the ultimate load capacity of the strengthened slabs ranged between about (24.24% for S1S-5cm to 83.03% for S3S-5cm) over the ultimate load capacity of the reference (unstrengthen) slab. The use of CFRP bar showed a decrease in stiffness. In this study the slab reinforced with CFRP bar gave the minimum results in the ultimate load (-9.09%,-0.61% and 15.76%) for (S1C,S2C,S3C) respectively compared with the control solid slab S1. In this study, use CFRP sheet as strengthening the slabs, same amount of material strengthening but different distribution. The group strengthening with CFRP sheet 5cm each 10cm is more stiffer than the group strengthening with CFRP sheet 10cm each 20cm. But these two groups were affected by temperature change, the ultimate load was increase when the temperature increased, this increasing is about (24.24% at 23 to 83.03% at 80) for slab strengthening by CFRP sheet 5cm each 10cm and (- 0.61% at 23 to 57.58% at 80) for slab strengthening by CFRP sheet 10cm each 20cm compared with the control slab (without strengthening and at 23 ) The presence of CFRP sheet at the bottom tension zone surface reduced the tensile concrete strains, and this reduction was reflected to strains in the bottom tension steel bar reinforcement (i.e., reducing the editor@iaeme.com

9 Abdul Ridah Saleh Al-Fatlawi and Dhoha Saad Hanoon tension steel bar strains), and this means increasing the tension strength and some tensile stresses would be carried out by CFRP sheets. Using CFRP sheet as external strengthening had a significant effect on crack appearance and pattern of the reinforced concrete two way slabs by delaying the crack appearance. The increase in cracking load was about (62.5%) for slabs strengthening compared with the control slab in the same group. The three-dimensional finite element model used in the present study was able to simulate the strengthened reinforced concrete two way slabs with (CFRP bar and CFRP sheet) with variable temperature. The cracking loads, ultimate deflection and predicted ultimate loads were close to that measured during the experimental testing with the maximum difference ratio in the ultimate load was lower than (12%) for all the tested and analyzed slabs. REFRENCES [1] Aiello, M., Focacci, F., Huang, P.C., and Nanni, A. (1999), "Cracking of Concrete Cover in FRP Reinforced Concrete Elements under Thermal Loads," Selected Presentation Proc., 4th International Symposium on FRP for Reinforcement of Concrete Structures (FRPRCS4), Baltimore, MD, Nov. 1999, pp [2] Masmoudi, R., Zaidi, A., and Ge rard, (2005), Transverse thermal expansion of FRP bars embedded in Concrete. Journal of Com- posites for Construction, 9(5): pp [3] Iraqi Specification No. 5,, (1984), Portland Cement, Baghdad. [4] Iraqi Specification No. 45, "Natural Sources for Gravel that is Used in Concrete and Construction", Baghdad, [5] ASTM 996M, (2005), Rail-Steel and Axle-Steel Deformed Bars for Concrete Reinforcement, Annual book of ASTM standards, Vol , pp. 5. [6] Javaid Ahmad and Dr. Javed Ahmad Bhat, Ductility of Timber Beams Strengthened Using CFRP Plates, International Journal of Civil Engineering and Technology, 4 (5), 2013, pp [7] Dr.Salim T.Yousif, New Model of CFRP-Confined Circular Concrete Columns: An Approach, International Journal of Civil Engineering and Technology (IJCIET), 4 (3), 2013, pp editor@iaeme.com