Behavior of reinforced concrete corbels … of reinforced concrete corbels strengthened with ferrocement sheets 94 carrying capacity with an acceptable amount of deformation. Different

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Behavior of reinforced concrete corbels strengthened with

ferrocement sheets

Azad A. Mohammed

Civil Engineering, Faculty of Engineering, University of Sulaimani, New Campus, Sulaimani,

Iraq. e-mail: [email protected]

Dunyazad K Assi

Civil Engineering, Faculty of Engineering, University of Sulaimani, New Campus, Sulaimani,

Iraq. e-mail: [email protected]

ABSTRACT

The present research outlines behavior of reinforced concrete corbels strengthened with precast ferrocement sheets made from high strength mortar contain wire meshes. Such type of sheets was tested to be an alternative for those strengthening materials having high costs per unit. For this purpose nine reinforced concrete corbels were cast and tested for ultimate load capacity and deformations. Test results indicate that there is a load enhancement over that of control reinforced concrete corbels as a result of strengthening with HSF sheets, regardless the amount of steel reinforcement provided to the corbel. Percentage of ultimate load varies from 132% to 207%. The load enhancement was found to be higher for those corbels lightly reinforced for flexure. Test results also indicate that there is a proportionality between load enhancement and thickness of HSF sheets provided to the corbel. An equation obtained from regression analysis was proposed to calculate the ultimate load capacity of reinforced concrete corbel strengthened with HSF sheets and can be used for the design of reinforced concrete corbels strengthened with HSF sheets.

Keywords: bonding, corbel, ferrocement sheets, strengthening

1. INTRODUCTION

Reinforced concrete corbels likewise other structural members may subject to damage of

cracking. As a result the efficiency of such structural member may reduce. For this purpose there is a need for strengthening to retrofit the corbel structural efficiency represented by load

Behavior of reinforced concrete corbels strengthened with ferrocement sheets

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carrying capacity with an acceptable amount of deformation. Different methods of strengthening technique were followed, different strengthening materials and bonding agents were attempted[1-3]. Fiber reinforced polymers were found to be active materials for strengthening structural members. Test results[4] indicate that concrete corbel reinforced with sufficient amount of flexural and shear reinforcements damaged by cracking can be repaired with CFRP sheets for a sufficient value of load capacity close to the load capacity of true corbel. However; the load enhancement of true reinforced concrete corbel after strengthening was found to be 28.3% using best CFRP sheet strengthening configuration provided to flexural and shear zones. It is worthy to note that fiber reinforced polymers are organic materials of high cost and using such materials for strengthening lead to the total high cost of the process. For this purpose there is good chance to develop other methods for strengthening provided that they become efficient and economic.

In this paper a new method of strengthening is presented and the efficiency of the method is checked via testing reinforced concrete corbels. Some of tested corbels are deliberately reinforced with low amount of flexural or shear reinforcement to investigate the efficiency of strengthening to regain the load capacity which is expected to be damaged due to the shortcomings in the reinforcement amount. The strengthening material attempted in this research is high strength ferrocement ( HSF ) sheets of precast units locally prepared and bonded to the corbel surface using epoxy resin commercially available.

2. EXPERIMENTAL

2.1 Materials

Ordinary Portland cement was used in concrete mixes for casting corbels and in mortar for

ferrocement sheets. Locally available rounded river coarse aggregate of maximum size equal to 12.7 mm and medium graded sand passed by 100% on 4.75mm sieve were used for corbels. For HSF sheets river sand passed by 100% on 2.36 mm sieve was used as fine aggregate. High range water reducer of Glenum type (GLENUM ACE 30) was used for such sheets. The ferrocement mesh was steel wires of 0.6 mm diameter spaced at 12.7mm. Based on test results the yield stress of wire mesh steel was found to be 460 N/mm2. Two types of steel reinforcement were used for corbels. For flexure, column reinforcement and framing bars deformed steel bars of 12.7 mm diameter was used. The yield stress of this reinforcement was found to be 600 N/mm2 and tensile strength was 773.24 N/mm2 . For shear reinforcement and column ties deformed steel bars of 8 mm diameter were used. Based on test results the yield stress of this type of reinforcement was found to be 725.28 N/mm2 and tensile strength was 810.28 N/mm2 .Locations of these reinforcements are shown in Fig.1.

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For bonding HSF sheets an epoxy resin of Turkey origin commercially available was used. The Sikadur 32 epoxy is a solvent-free, two component bonding agent. Sikadur 32, provides a bond of far greater strength than the tensile strength of the concrete itself.

2.2 Mix Proportions and Preparation of Specimens

Concrete corbels were cast from a concrete with a constant mix proportion of 1:2:2 ( cement:

sand: gravel ), and water / cement ratio of 0.5. HSF sheets made from a mortar mix proportion of 1:1 ( cement: fine aggregate). Water / cement ratio for mortar mix was 0.34 and the high range water reducer ( superplasticizer) equal to 1% of cement weight was used. Corbels were cast in steel moulds in three groups with each group three concrete cylinders ( 150 x 300mm ) were cast for measuring concrete compressive strength. HSF sheets were cast having the same dimensions as those of corbels except the thickness which was variable. Two different thickness sheets were cast of thicknesses 25mm and 50mm. For 25mm sheets four layers of wire mesh were used per sheet while for 50mm sheets eight mesh layers were used and accordingly the ratio of wire mesh for sheets was constant and equal to 0.356% at both directions. Pertaining the high strength mortar mixes, three cubes dimensioned 70 mm aside were cast for measuring mortar compressive strength. After casting, the corbels were left in the mould for 24 hrs. They have demolded and cured for 28 days, and left in the laboratory for another 28 before strengthening.

2.3 Strengthening and Testing Technique

Epoxy resin was prepared according to the manufacturer recommendation. One of the corbel sides of those corbel to be strengthened was wrapped with the epoxy and left for about one hour and the HSF sheet was put on the corbel surface as shown in Fig.2 and pressed to bond well with the corbel. A moderate weight was put on the sheet to fix it and left to bond to the corbel. After 24 hrs the process is repeated for other side of the corbel. Seven days after strengthening the corbel was tested for the complete load- deflection response. Fig.3 shows one of the control corbels ready for testing. Corbel C8 which was strengthened with 50 mm HSF sheets was bolted with steel bolts of 6 mm diameter by making eight holes in both sheets and the corbel. The bolts were provided at the time of connecting sheets to the corbel with the epoxy. The extra hole spaces were filled with the epoxy and the bolts were connected with the screws tightly. The aim of connecting the sheets in this corbel by bolts was to know the efficiency of strengthening the corbel with sheets using bolts in addition to the epoxy. Table 1 shows reinforcement details and strengthening configuration for all corbels. All corbels were tested using steel frame hydraulic jack machine of 500 kN load capacity. For


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deflection measurement a dial gage was fixed at the bottom surface of the column. At testing, load was increased and the corresponding central deflection was measured for each 5 kN loading value till the failure load is reached. After testing, photographs were taking to show the cracking pattern of tested corbels.

3. RESULTS AND DISCUSSION

Results of concrete compressive strength and ultimate load capacity of control and strengthened corbels in addition to the percentages of ultimate load are shown in Table 2. Average compressive strength of HSF cubes was found to be 72.45 N/mm2. Based on direct tension test on a special fabricated HSF specimen the tensile strength of HSF sheet was measured and found to be 5.31 N/mm2. Fig.4 shows the percentage of load capacity of strengthened corbels over that of control corbels. One can observe that there is a linear relationship between load percentage and HSF sheet thickness for all corbels except corbel C8. It is appear that using bolts have no useful effect and creates weak zones inside the corbel lead to lower load enhancement compared with the other corbels strengthened with 50 mm thick HSF sheets. It is expected that if corbel C8 contained no bolts the load capacity become higher and a linear relationship similar to other corbels will occur.

Fig.1 Corbel dimensions and reinforcement details

Fig.2 Bonding HSF sheet to the corbel


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Table 1 Details of corbels reinforcement and strengthening configuration

Group

Corbel

Reinforcement Detail

Strengthening Condition

(1)

C1

C2

C3

3 bars for flexure+2 closed stirrups for shear


3 bars for flexure+1 closed stirrups for

shear

-

-

-

(2)

C4

C5

C6




25mm thick HSF sheets



(3)

C7

C8

C9





50mm thick HSF sheets bonded with epoxy and steel bolts.



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Fig.3 One of control corbels ready for testing Fig.4 Variation of ultimate load percentage

with HSF sheet thickness

Comparison between ultimate load corbels C5 and C1 indicates the role of strengthening with 25 mm HSF sheets to regain corbel strength when there is a shortcoming in flexural reinforcement. Ultimate load capacity of corbel C5 is higher than that of C1 indicating the useful role of strengthening with 25 mm HSF sheets to regain corbel strength when there is a shortcoming in flexural reinforcement. Comparison between ultimate load corbels C6 and C1 indicates the role of strengthening with 25 mm HSF sheets to regain corbel strength when there is a shortcoming in shear reinforcement. Ultimate load capacity of corbel C6 is higher than that of C1 indicating the useful effect of strengthening with HSF sheets for shear. Comparison between ultimate load capacity of corbel C4 and corbel C1 indicates the role of strengthening those corbels, sufficiently reinforced with flexure and shear reinforcement, using HSF sheets, for increasing load capacity.

Results show that the load capacity is increased by 32% and by 64% for 25 mm and 50 mm sheets, respectively. Therefore there is a chance to increase load capacity of reinforced corbels using HSF sheets and the load enhancement is considerably higher after strengthening. Results also indicate that the performance of HSF sheets for strengthening is better for flexure compared with that for shear, as observed from the ultimate load capacity of corbels C5 and C6. The same observation can be found in Fig.4 in which the level of the upper relationship is considerably higher compared with the others. Fig.5 shows the load- deflection relationships for Group (1) corbels. Fig.6 shows the load- deflection relationships for Group (2) corbels and Fig.7 shows the load- deflection relationships for Group (3) corbels.


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With regard the deflection of corbels, strengthening corbel with HSF sheets lead to a lower deflection for any load value smaller than the ultimate load. Therefore, the stiffness of the corbel is increased as a result of strengthening. From Fig.7 one can find that deflection of corbel C8 is larger than that for other corbels, because corbel C8 contain holes and these holes are the source of large deformations and cracks extension. Finally, the corbel failed at a lower load level before that the true strength of corbel will reach. Fig.8 shows corbel C8 after testing. Due to high bond between the corbel and HSF sheet the crack extended to the bonded sheet. Some of tested corbels was failed in debonding HSF sheets but after that the sheet suffered from high damage by cracking before collapse. Percentage of load capacity

(Pu) increase for 50 mm sheet corbel over that of 25 mm sheet corbel is shown in the last column of Table 2. The ratio is more than twice times except that of corbel C8, indicating the positive role of ferrocement sheet thickness. However; using too thick sheets may cause difficulties in application at the time of strengthening. Choosing the desired thickness remains for the designer and depending on corbel properties and the nature of loading.

4. PREDICTION OF ULTIMATE LOAD CAPACITY

From the forgoing knowledge presented in this study it is obvious that load capacity of corbels made from normal strength concrete reinforced with different amount of flexural and shear reinforcements depends on HSF sheet thickness. For calculating load capacity of corbels strengthened with HSF sheets, it is necessary to develop equations depending on the corbel type and HSF sheet thickness. Based on the obtained test data presented in this study a regression analysis was carried out and the following equation was obtained for calculating load capacity of reinforced concrete corbels strengthened with HSF sheets

Pus = Puc ( α + β tf ) (1)

In which Pus is the ultimate load capacity of strengthened corbel, Puc is the ultimate load of corbel to be strengthened, tf is the HSF sheet thickness ( mm ), α and β are constants of regression analysis.

For those corbels reinforced sufficiently with flexure and shear reinforcements α is equal to 1 and β is equal to 0.0128. For those corbels reinforced sufficiently with flexural reinforcement and there is a shortcoming in shear reinforcements α is equal to 0.9833 and β is equal to


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0.0168. For those corbels reinforced sufficiently with shear reinforcement and there is a shortcoming in flexural reinforcements α is equal to 1 and β is equal to 0.0412.

Calculated ultimate load capacity of corbels in addition of the test ultimate load are shown in Table 2. It should be noted that the ratio of ultimate load capacity of corbel C8 is equal to 303% based on the proposed equation for calculation. The test ultimate load ratio is only 207%. It is expected that the test ratio is close to 303% if the corbel is fabricated similar to C8 without using steel bolts.

Table 2 Test results of ultimate load capacity of corbels

Corbel Concrete compressive

strength (N/mm2)

Ultimate Load,Pu

( kN)

Percentage of Pu

Percentage Pu increase for 50mm sheet corbel over

25mm sheet corbel

Calculated ultimate load,

Pus

C1

C2

C3

32.54

32.54

32.54

220

140

190

100

100

100

-

-

-

220

140

186.83

C4

C5

C6

33.48

33.48

33.48

290

282

260

132

203

137

-

-

-

290

282

266.32

C7

C8

C9

24.71

24.71

24.71

360

290

350

164

207

184

200%

104%

227%

164

424.49

346.82


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Fig.5 Load- deflection relationship of Group(1)corbels Fig.6 Load- deflection relationship

of Group(2)corbels

Fig.7 Load- deflection relationship of Group(3) corbels Fig.8 View of corbel C8 after testing

It is obvious that there is a good agreement between calculated and test ultimate loads. Therefore one can decide that the proposed equation for calculating ultimate load capacity can be used for designing reinforced concrete corbels for strengthening using HSF sheets. To obtain more accurate knowledge on strengthening corbels using HSF sheets, other researches in this context should be carried out. It is important to study the roles of wire mesh ratio and type on the behavior of strengthened corbels if this technique of strengthening is used by structural engineers.


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5. CONCLUSIONS

The following conclusions can be drawn from the present study

1) Using HSF sheets bonded with epoxy, reinforced concrete corbels can be strengthened successfully for increasing load capacity and regain the true load capacity when there is shortcoming in flexure and shear reinforcements. 2) Using steel bolts beside epoxy for strengthening was found to be not useful for further load enhancement, due to the creation weak zones inside the corbels, and epoxy material is better to be used alone for bonding.

6. REFERENCES

[1] K. Nagrodzka-Godycka, “Behavior of corbels with external prestressing bars–

experimental Study,” in ACI Structural Journal, vol. 96, no. 6, November- December, 1999, pp.1033-1040.

[2] G. Campione, La. L. Mendola, and M. Papia, ” Flexural behavior of concrete corbels containing steel fiber or wrapped with FRP sheets” , in Material and Structures, vol. 38, July 2005, pp. 617-625.

[3] A. Heidayat, A. Ramadhan, and O. Q. Aziz , “ Repairing of damaged reinforced concrete corbels by externally bonded steel plate,” in The Scientific Journal of Salahaddin University- Erbil, Iraq, vol. 16, no. 1, 2004.

[4] A. A. Mohammed and G. B. Hassan , “Behavior of reinforced concrete corbels wrapped

with CFRP sheets, Lambert Academic Publishing, 2013, pp.135.

Documents

Behavior of reinforced concrete corbels … of reinforced concrete corbels strengthened with ferrocement sheets 94 carrying capacity with an acceptable amount of deformation. Different