Mustafa Kareem Moosa Alhassnawi B.Sc civil engineering 2016, faculty of engineering, University of, Babylon, Babel, Iraq

Transcription

1 xinternational Journal of Civil Engineering and Technology (IJCIET) Volume 9, Issue 10, October 2018, pp , Article ID: IJCIET_09_10_092 Available online at ISSN Print: and ISSN Online: IAEME Publication Scopus Indexed EXPERIMENTAL INVESTIGATION ON THE INFLUENCE OF ELEVATED TEMPERATURE ON REINFORCED CONCRETE COLUMNS STRENGTHENED WITH CARBON FIBER REINFORCED POLYMER PRODUCTS Mustafa Kareem Moosa Alhassnawi B.Sc civil engineering 2016, faculty of engineering, University of, Babylon, Babel, Iraq Abd alridah Saleh Alfatlawi Assist. Prof. Structural engineering, faculty of engineering, University of Babylon, Babel, Iraq ABSTRACT This paper has shown an experimental work on the rehabilitations of reinforced concrete columns exposed to fire by CFRP and subjected to eccentric load. The experimental program consists of fifteen small scale RC column as two group according to main reinforcement. The layout of building is taken in this paper (the number of column side that exposed to fire 2-, 3-, and 4-side). All columns tested eccentric load 40 mm and the burning columns exposed to fire with 500 C o for time period 1.5 hour. The result conclude that the RC columns due to exposed to fire flame lost from their load capacity by about ( ) % for 2-side exposure, ( ) %for 3-side exposure, and ( ) % for critical damage column 4-side exposure for group A and B respectively. The strengthening by CFRP increased the ultimate load of fire damaged columns by about ( ) %. These percentages of improvement can be consider a few because of weakly bond between fire exposed concrete surface and CFRP. Depend on the experimental results, the load-mid height lateral displacement curves, load-axial deformation curves, load-crack width curves, mode of failure, and ductility of specimens were recorded, showed, and discussed. Using of CFRP in strengthening of fire exposed columns enhance most of these properties. Keywords: CFRP, column, exposed to fire, and eccentric load editor@iaeme.com

2 Mustafa Kareem Moosa Alhassnawi and Abd alridah Saleh Alfatlawi Cite this Article: Mustafa Kareem Moosa Alhassnawi and Abd alridah Saleh Alfatlawi, Experimental Investigation on the Influence of Elevated Temperature on Reinforced Concrete Columns Strengthened with Carbon Fiber Reinforced Polymer products, International Journal of Civil Engineering and Technology, 9(10), 2018, pp INTRODUCTION One of the problems facing the buildings is the possibility of rising temperatures above normal conditions as happens during the fire as an example. Therefore, a good thermal insulation should be provided to the structural members such as columns, beams, slabs, shear walls, etc One of the benefits of concrete is its considered a fire resistance material due to the good mechanical and thermal properties of its constituents, but concrete structures must still endurance fire by using proper design criteria. Structural components still must be able to withstand dead and live loads without collapse during the fire, which causes a decrease in strength and modulus of elasticity for concrete and steel reinforcement (Chen, et al., 2009). In order to resist high temperature without collapse or failure. It is also necessary to study ways and the possibility of rehabilitation of those members who were exposed to fire with construction materials such as fiber reinforced polymer FRP products. (Raut and Kodur, 2011) study the response of columns under fire induced biaxial bending on HSC and NSC. The results of the test show that the significant factors that influence fire resistance of RC columns are fire scenario, concrete strength (permeability), biaxial bending arising from 1-, 2-, or 3-face exposure, load eccentricity, or uneven spalling, load ratio, and slenderness ratio. For the same surface area of exposure, two adjacent sides exposed to fire have a higher reduction in strength compared to two opposite sides exposed (Balaji, et al., 2016). (Al-Kamaki, et al., 2014) the confinement of concrete with fiber-reinforced polymer (FRP) composites and provide can significantly enhance its strength and ductility. Previously damaged by elevated temperatures and retrofitted externally using CFRP fabrics and then tested under axial compression. The columns were subjected to 30 % of maximum load at ambient temperatures during heating (up to 1000 C) and cooling before being treated with CFRP fabrics and subsequently tested to failure. The results show that CFRP fabric can considered an excellent technique for strengthening heat-damaged RC columns. The aim of this research is to investigate experimentally the feasibility and effectiveness of wrapping the fire exposed columns with CFRP by comparing the resulting strength with that of unwrapped damaged and undamaged RC columns. 2. EXPERIMENTAL PROGRAM 2.1. Material The materials that used in this research are: Concrete Normal strength concrete used in casting of all specimens and the compressive strength is 31 Mpa. The mix design shown in Table (1) editor@iaeme.com

3 Experimental Investigation on the Influence of Elevated Temperature on Reinforced Concrete Columns Strengthened with Carbon Fiber Reinforced Polymer products Table 1 Mix proportion. Cement Water Sand Gravel W/C ratio (Kg/m3) (Kg/m3) (Kg/m3) (Kg/m3) Steel Two types of steel reinforcing bars were used in the tested columns of this study: first, deformed steel bar nominal diameter Ø10mm was used as corbel reinforcement in all columns and as main reinforcement in group A of specimens (Ukrainian production). Second, deformed steel bars nominal diameter Ø8 mm was used as transverse ties in all columns and longitudinal reinforcement in group B of specimens, obtained (Turkish production) Carbon Fiber Reinforced Polymer The kind of CFRP Sheet used in this study is (Sika Wrap Hex- 230C), it has width 50 cm Specimens details All columns are identical in size and the nominal dimensions. The model dimensions selected in the present investigation was a square section of mm and a total length of 900mm. The length between corbels (middle portion) is 500mm, as shown in Figure (1). Figure 1 The dimension details of columns specimens. The columns specimens can be arranged in two groups (A & B) according to main reinforcement, and these groups of columns can be dividing into subgroup according to type of exposure to fire and type of strengthening as shown in Table (2). Table 2 Details of the tested columns specimens. Specimen symbol Reinforcement Notes WU4 4 Ø 10 mm Without fire and unstrengthening by CFRP WU6 6 Ø 8 mm Without fire and unstrengthening by CFRP FAU 4 4 Ø 10 mm Fire on all faces and unstrengthening by CFRP FASC4 4 Ø 10mm Fire on all faces and strengthening by CFRP. Complete editor@iaeme.com

4 Mustafa Kareem Moosa Alhassnawi and Abd alridah Saleh Alfatlawi warping FAU6 6 Ø 8 mm Fire on all faces and unstrengthening by CFRP FASC6 6 Ø 8 mm Fire on all faces and strengthening by CFRP. Complete warping FHU4 4 Ø 10 mm Fire on half of column and unstrengthening by CFRP FHSC4 4 Ø 10 mm Fire on half of column and strengthening by CFRP. Complete warping FHSS4 4 Ø 10 mm Fire on half of column and strengthening by CFRP. Strip warping FHU6 6 Ø 8 mm Fire on half of column and unstrengthening by CFRP FHSC6 6 Ø 8 mm Fire on half of column and strengthening by CFRP. Complete warping FTU 4 4 Ø 10 mm Fire on three faces and unstrengthening by CFRP FTSC4 4 Ø 10mm Fire on three faces and strengthening by CFRP. Complete Warping FTU6 6 Ø 8 mm Fire on three faces and unstrengthening by CFRP FTSC6 6 Ø 8 mm Fire on three faces and strengthening by CFRP. Complete Warping 2.3. Fire test The fire exposure experiments were carried out in the laboratory of concrete research and material properties in the faculty of engineering at University of Babylon. The furnace is manufactured with dimensions of (illustrate in Figure2), the main structure is made of refractory brick and refractory mortar with a small opening to provide the sufficient air for the burners. The main purpose of the furnace compartment is to elevate the temperature levels of the fire exposure to the target temperature and to keep the temperature constant for a required duration. In order to control the fire temperature, the heating process consists of the equipment shown in Figure Application of CFRP-Sheets To apply CFRP sheet on fire exposed columns, the following steps were followed: 1. Cutting CFRP-sheet in required length and concrete surface must be cleaned to remove any contaminations. 2. Two types of epoxy (A and B) adhesive (Sikadur-330) mixed together in the proportion (4:1) respectively till the color be homogenous. 3. Applying epoxy on column and CFRP-sheet with thickness about 1.5mm. 4. After that, setting CFRP-sheet on columns surface on the coated region by epoxy and pressure applying by a rubber roller to seat the sheet by that cause epoxy forced out from both sides of the sheet. And excess epoxy removed from the sides of CFRP-sheet Test setup All columns were tested up to failure using a calibrated electro hydraulic testing machine with a maximum range capacity of 1000 KN in the structures laboratory at University of Babylon. The load was applied, using load control, at a loading rate of 1 kn/s. Two dial gauges having accuracy of mm per deviation and capacity of 50 mm were used to measure the lateral deformation at the mid-height and axial displacement for each load increment. These dial gauges were mounted at the mid-height along the vertical center line of the column specimens and on the piston of electro hydraulic testing machine. Testing continued until the reinforced concrete column shows a drop in load capacity with increasing deformation editor@iaeme.com

5 First crack load Pcr (KN) Load Carrying Capacity Pu (kn) Percentage Decreasing Load Carrying Capacity (%) Ultimate axial deformation (mm) Ultimate mid-height lateral deflection (mm) Crack width (first crack at ultimate load ) (mm) Experimental Investigation on the Influence of Elevated Temperature on Reinforced Concrete Columns Strengthened with Carbon Fiber Reinforced Polymer products Figure 2 All details of the stove and equipment, strengthening, and electro hydraulic testing machine. 3. RESULTS AND DISCUSSION In order to evaluate the structural behavior of the tested column specimens the main structural characteristics observed and recorded during the test of each column specimens and at each stage of loading. Table 3 Experimental test results of column specimens. Columns Symbol WU FAU4 Pre-cracked FTU4 Pre-cracked FHU4 Pre-cracked FASC4 Pre-cracked FTSC4 Pre-cracked FHSC4 Pre-cracked FHSS4 Pre-cracked WU FAU6 Pre-cracked FTU6 Pre-cracked FHU6 Pre-cracked FASC6 Pre-cracked FTSC6 Pre-cracked FHSC6 Pre-cracked editor@iaeme.com

6 Mustafa Kareem Moosa Alhassnawi and Abd alridah Saleh Alfatlawi 3.1. Load Carrying Capacity of Reinforced Concrete Columns The load carrying capacity reflected the ultimate applied load that can be subjected to the tested column specimens, after that a drop in machine reading appeared with a rapid deformation on column, which termed as failure. The results showed that the non-exposed control columns of group B were more load capacity than the control column of group A. This can be considered normal and expected due to the distribution of the bars and also the greater compressive strength for group B. As for the columns exposed to fire, the decrease in the load carrying capacity of the columns exposed from four sides was the largest ( ) %. The decrease in the columns exposed fire on three sides ranged from ( ) %. Two sides exposed columns were the least damage, with the descent rate ( ) % as shown in figure (3). These results were also confirmed by other research such as (Raut, 2011 and Balaji, et.al. 2015). Those results were for the two groups (A and B) respectively, i.e., the ratio of the descent of the group B to more than the other group A. This is due to the increase in the ratio of reinforcement and the number of reinforcement bars. Also, there is a slight increase in the coefficient of thermal expansion of the iron on the concrete, Generate internal stresses due to the large areas of different expansion. As well as increasing the compressive strength of the group B negatively affecting on the load capacity of columns when burning. This is confirmed in many researches such as (Balaji, et.al, 2015). The main objective of the research is to study the effect of CFRP and its degree of improvement of the load carrying capacity of fire exposed columns. The use of CFRP to treat the fired columns by full warping of the column, increased the load capacity about ( ) % relative to those exposed to burning. As for the use of strip of CFRP in the treatment as in specimen (FHSS4), the results were not good as the load capacity of the burned model increased by 15.77% as shown in figure (3). Figure 3 The percentage of load carrying capacity of columns editor@iaeme.com

7 Axial load (kn) Axial load (kn) Axial load (kn) Axial load (kn) Experimental Investigation on the Influence of Elevated Temperature on Reinforced Concrete Columns Strengthened with Carbon Fiber Reinforced Polymer products 3.2. load-displacements relationship of reinforced concrete columns There is a convergence between the values of lateral and axial displacements, both of which indicate an increase in the values of ultimate displacement when the burnt side increases. CFRP strengthening has improved the behavior of columns with respect to displacement values with loading. Figure (4, 5, 6, and 7) shown some of load-displacement curves. The load-deflection curves for group B are more sensitive to high temperatures compared with group A. The use of CFRP by strip shape to strengthening did not give any improvement in displacement FASC4 FTSC4 FHSC4 FHSS FASC6 FTSC6 FHSC Mid-hight lateral deflection (mm) Mid-hight lateral deflection (mm) Figure 4 Load deflection of exposed and strengthened column of group A Figure 5 Load deflection of exposed and strengthened column of group B FASC4 FTSC4 FHSC4 FHSS FASC6 FTSC6 FHSC Axial deformation (mm) Axial deformation (mm) Figure 6 Load axial deformation of exposed and strengthened column of group A. Figure 7 Load axial deformation of exposed and strengthened column of group B First Crack Load and Crack Width of RC Columns The width of crack was recorded by crack meter and the first crack known by the normal viewing and then recorded the corresponding load. The flexure transverse cracks are usually originated at the tension zone and propagate toward the compression zone for column specimens eccentrically loaded, while for the longitudinal cracks, they initiated at the corbel as a shear crack. The models exposed to burning were cracked by burning with so-called editor@iaeme.com

8 Mustafa Kareem Moosa Alhassnawi and Abd alridah Saleh Alfatlawi (Hairline cracks) so they are pre-cracked, plate (1) shown the cracks that appear due to burning. Figures (8 and 9) represent the applied load versus the maximum crack width relationship for the control and fire exposed column specimens. Form these figures and Table (3) can be noted that the maximum crack width a few affected by the scenario of burning because the tension and compression face were exposure to fire in each case of burning. But, can noted the specimens that exposed to fire from two side the lowest among the burned models in terms of crack width due to difficulty continuing the path of crack a cross the unburned side compared to the burn side. Plate 1 The crack that appear due to fire and Spread of crack in specimens with different burning scenario. 4. MODE OF FAILURE In general, the columns were tested with eccentric load by 40 mm. the columns tested with such type of loading often have compression failure. The failure began gradually with presence cracks in tension side and progressive spread to compression side passing through the other sides. In unburning columns, the presence of cracks and spread it is slow and difficult by compare with burning columns. In other hand, the exposed columns which have pre-cracked due to fire. Subsequently, the transition and expansion of cracks were faster under the influence of lower load, as shown in plate (2). The easy of initiated cracks increase with increased number of fire exposed side. The failures occur by spalling of the concrete cover at compression zone in the middle third of column. Then the cover was started in editor@iaeme.com

9 Experimental Investigation on the Influence of Elevated Temperature on Reinforced Concrete Columns Strengthened with Carbon Fiber Reinforced Polymer products crushed at centerline of column and developed longitudinal and transverse until completely removed as shown in plates (1, and 2).The burned columns from 3-side were the failure as in the previous exposed columns, but, the crushed of cover started close to the edge of unburned face as shown in plate (1). This due to move the neutral axis from its position (caused biaxial bending) because the un-symmetrical burning make some difference in modulus of elasticity of un-loading exposed concrete columns, (Raut and Kodur, 2011). The columns that strengthened by CFRP, at failure appear rupture in CFRP sheet in tension face and spalling in compression face as shown in plate (2). This confirms the exposed strengthening columns take the same behavior in the form of failure of the unstrengthening columns. The warping by CFRP provide confinement for specimen so its increase the difficulty of crushing under the same load. Plate 2 Column specimen's failure 5. Ductility of Reinforced Concrete Column In this paper the method of (energy absorption capacity) is adopted in the study of ductility. The energy absorption capacity of the concrete column defined as the area enclosed by the load-displacement curve until the maximum load was reached. The result of calculation area under curves (ductility) list in Figure (10). From these results and figures turns out the strengthening by CFRP provide a significant enhancement for the ductility of exposed columns. Strengthening by CFRP increased ductility by % for columns exposed from 4-side, % for column exposed from 3- side, and % for 2-oppsite side exposed column. These results were confirmed by (Toutanji, 1999), the confinement of concrete by FRP sheets can significantly improve the strength, ductility, and energy absorption capacity of the concrete specimens editor@iaeme.com

10 Mustafa Kareem Moosa Alhassnawi and Abd alridah Saleh Alfatlawi Figure 10 The ductility of specimens. 6. CONCLUSION From an experimental results work, conclusion can be added in this chapter on the behavior of these columns as well as some suggestions for future works are presented. After fire exposure at ( ºC) during the burning, the moisture in concrete is transformed to vapor and appears on the surface of concrete in the form of patches. From the load carrying capacity column test, the specimens of group A were stiffer than group B after the burn. Due to the increase in the number of reinforcement bars, the thermal expansion places are increased, causing internal stresses. By tracking the first crack width verses the applied load, cracks width becomes more difficult to spread and expand as the numbers of burning sides are less. The specimens that strengthened by CFRP, group A regained its original load capacity, conversely, group B not restored its original load capacity because of the significant decrease in its resistance. The use of CFRP to treat the fire exposed columns by full warping of the column, increased the load capacity about ( ) % relative to those exposed to burning. And it is increase the ductility by about ( ) % compared to controls specimens. Strengthening by strips in FHSS4 specimens, was useless even though they applied to the least affected models and did not achieve good result for strengthening. Due to damage to burn, failure occurs in any available (unstrengthen) area between CFRP strips editor@iaeme.com

11 Experimental Investigation on the Influence of Elevated Temperature on Reinforced Concrete Columns Strengthened with Carbon Fiber Reinforced Polymer products REFERENCE [1] Chen, Y.-H., Chang, Y.-F., Yao, G. C., & Sheu, M.-S. (2009). Experimental research on post-fire behavior of reinforced concrete columns. Fire Safety Journal, 44(5), [2] Raut, N., & Kodur, V. (2011). Response of reinforced concrete columns under fireinduced biaxial bending. ACI Structural Journal, 108(5), 610. doi: /j.engstruct [3] Balaji, A., Luquman K, M., Nagarajan, P., & Madhavan Pillai, T. M. (2016). Studies on the behavior of Reinforced Concrete Short Column subjected to fire. Alexandria Engineering Journal, 55(1), [4] Al-Kamaki, Y. S. S., Al-Mahaidi, R., & Bennetts, I. D. (2014). Confinement of heatdamaged RC circular columns using CFRP fabrics. 23rd Australasian Conference on the Mechanics of Structures and Materials (ACMSM23), I, Retrieved from ticle=1082&context=acmsm23 [5] Toutanji, H. A. (1999). Stress-strain characteristics of concrete columns externally confined with advanced fiber composite sheets. ACI Materials Journal, 96(3),