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International Journal of Civil Engineering and Technology (IJCIET) Volume 8, Issue 5, May 2017, pp. 819–831, Article ID: IJCIET_08_05_091 Available online at http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=8&IType=5 ISSN Print: 0976-6308 and ISSN Online: 0976-6316 © IAEME Publication Scopus Indexed

EFFECT OF ELEVATED TEMPERATURE ON RC PRECRACKED BEAMS REPAIRED AND

STRENGTHENED USING JACKETS OF CEMENTITIOUS MATERIALS

Yehia Abdel Zaher Department of Structural Engineering, Faculty of Engineering, Ain Shams University, El Sarayat Street, Abbasia, Cairo, Egypt

Yasmin Hefni Abdel Aziz

Talaat Mostafa Construction Company, 36 Msadak Street, Dokki, Giza, Egypt

ABSTRACT Due to various causes of deterioration during the service life of concrete

structures, retrofitting is a major need aimed to improve their future performance. Fire attack is one of the important causes which may lead to serious symptoms of distress and in many cases to total collapse of the structure. Jacketing has been considered as one of the most widely important methods of rehabilitation of deteriorated beams. An experimental study, including testing of 16 reinforced concrete beams strengthened with normal and reinforced concrete jackets containing activated fly ash and glass fibers and loaded in four point loading test, was carried out. The parameters of the study included level of damage induced before jacketing, effect of exposure to fire and effect of reinforcing the jackets. Beams were of dimensions 100x200x2250 mm and main reinforcement of 2T12mm. The jackets were three sided with thickness 50 mm. Measurements and observations included the cracking load, crack propagation, deflection, ultimate flexural capacity, failure mode and beam ductility. The experimental study showed that, incorporating chemically activated fly ash with ratio of 40% and glass fiber of 0.7 % by weight of cement in the concrete jackets enhanced load carrying capacity, cracking load and ductility at normal temperature and after exposed to elevated temperature.

Key words: RC Precracked, Beam, concrete structures. Cite this Article: Yehia Abdel Zaher and Yasmin Hefni Abdel Aziz, Effect of Elevated Temperature on RC Precracked Beams Repaired and Strengthened Using Jackets of Cementitious Materials. International Journal of Civil Engineering and Technology, 8(5), 2017, pp. 819–831. http://www.iaeme.com/IJCIET/issues.asp?JType=IJCIET&VType=8&IType=5

Yehia Abdel Zaher and Yasmin Hefni Abdel Aziz


1. INTRODUCTION One popular solution to the problem of strengthening old reinforced concrete (RC) structures is to place jackets around the structural elements. Jackets have been constructed using traditional or precast concrete, steel and FRP wrapping 1,2,3. Despite jacketing increases the member size significantly, it increases the stiffness, enhances the load carrying capacity 3 and improves the fire resistance. Several investigations were performed to determine the efficiency of beam jacketing at normal and elevated temperatures by using cementitious materials 4,5,6,7,8,9,10,11. Plain and reinforces jackets have been used in strengthening of RC beams 7. Incorporating fly ash in jackets may show economical and environmental benefits. Potha R.M. et al 12 and Azhar et al 13 concluded that adding fly ash in concrete by large doses improved the concrete strength at late ages at normal and elevated temperatures. Strength improvement at elevated temperature may be attributed to the relatively porous structure and the reduction in thermal conductivity. The obstacles of using fly ash in jackets are the extended setting time and the slow strength development. Different methods of activation have been developed to overcome these disadvantages and to enhance the reactivity. Chemical activation of fly ash is one of the major solutions to this problem. It was superior to grinding or thermal curing conditions 14. Alkali activation using Ca(OH)2 or NaOH and sulfate activation using CaSO4·2H2O or Na2SO4

15 are the most common chemical activation techniques. Other types of activation include CaCl2, Na2SiO3 and CaO15,16,17. Gopalsamy et al16. studied the effect of using a mixture of CaO and Na2SiO3 in the ratio of 1: 8, respectively, for the activation of fly ash.

On the other hand, the use of glass fibers improves the tensile strength, imparts the concrete ductility and better crack arrest and propagation. Reddy and Vijayan 18, Sudheer et al 19 and Ravikumar and Thandavamoorthy 20 concluded that glass fibers improve the concrete fire resistance. This may be attributed to the lower thermal resistance of glass fibers ( 0.05 W.m.Co ) which is lower than concrete (from 1.2 to 1.4 W.m.Co). This could explain the better fire resistance of fiber reinforced concrete as fibers isolate the inner matrix of concrete and reduce crack propagation and fire intrusions through cracks.

2. RESEARCH SIGNIFICANCE The objective of this study is to experimentally investigate the flexural resistance of RC beams strengthened using jackets containing activated fly ash as 40% replacement of cement weight and glass fiber as 0.7% of cement weight at normal temperature and after exposure to elevated temperature according to ASTM E119 fire rating curve. Also, the isolation efficiency of the jacket was investigated.

3. EXPERIMENTAL STUDY Eighteen RC beams were cast. One of them was control beam (B1a) tested to failure to find out the load carrying capacity and the corresponding deflection. Another beam was exposed to elevated temperature according to ASTME119 fire rating curve before testing and evaluating the load carrying capacity and the corresponding deflection after exposure to fire. The remaining 14 beams were strengthened using reinforced or unreinforced jackets. Some beams were preloaded to 65% of the ultimate load of control beam B1a before jacketing. The material used for the jackets was either control mix of fly ash mix. Figure 1 shows the details of the experimental study.

Effect of Elevated Temperature on RC Precracked Beams Repaired and Strengthened Using Jackets of Cementitious Materials


Figure 1 Details of the experimental study

Materials Cement: Ordinary Portland cement (CEM I 42.5N) provided by Tourah Cement Company complying with ES 4756-1/ 2007 21 was used in the current research. Aggregates: natural siliceous sand with fineness modulus 2.85 was used as fine aggregate and crushed stone in two sizes; S1 (5 to 20 mm particle size) and S2 (10 to 25 mm particle size) was used as coarse aggregate. Water: Tap water was used as mixing water. Admixtures: Sikament FF complying with ASTM C 494 Type F was used as high range water reducing admixture. Fly ash: Type F fly ash with light grey spherical particles, less than 10% retained on sieve 45 micron, and specific gravity of 2.0. The chemical oxide composition (SiO2 + Al2O3 + Fe2O3) of the used fly ash was 92.47 % and the Loss of Ignition value was 1.1% was used in the current research. Glass fibers: E-glass single filament fibers provided from Sika Egypt Company with diameter of 13 micron, and two lengths of 6 mm and 18 mm in the ratio of 1:1 was used in the current research. Sodium silicate manufactured directly in a solution from by "Wet Process" where the silica leached out under pressure by concentrated caustic soda solution. The percentage of Na2O and that of SiO2 was 14.7 and 29.4 and density of the activator was 1.5 g/cm3. Reinforcement: Reinforcement was used in the study complying with the Egyptian Standard Specification (E.S.S. 262/2011)22. Longitudinal ribbed bottom reinforcement consisted of 12 mm nominal diameters with yield strength of 420 MPa and 370 MPa were used for beams and jackets, respectively. The longitudinal compression reinforcement at the top of the beam and jackets consisted of ribbed bars of 10 mm diameter with yield strength of 410 MPa and 360 MPa for beams and jackets, respectively. Plain round bars of 8 mm diameter with average yield strength of 300 MPa were used as stirrups for beams and jackets.



Concrete Mixes The proportions for the mixes for original beams and jackets are illustrated in table 1. In the mixing process, the chemical activator was added to the water to form a solution before adding to the dry materials. The total mixing time for all mixes was 120 second.

Table 1 Mix proportions (Kg/m3) for beam and jackets

Mix Ceme

nt Fly ash Sand Crushe

d stone

Glass fiber

s

Water

Activator

Na2SiO3

Super plasticiz

er

Original beams 375 - 604 1250 - 155.0 - 7.2

Jackets

Control 350 - 540 1107 - 157.5 - 8.4

FA 210 140 540 1107 2.4 151.5 9.33 8.8

Beam Specimens Reinforced concrete beams of span 2050mm between supports, and the cross-section of 100mm x 200mm were used. The beams were reinforced with 2T12mm in tension and 2T10mm in compression and tided with stirrups R8@200 at the middle third and R8 @ 100 near supports.

The RC jackets were three sides with 50mm thickness for each side. The final dimensions of the jacketed beams were 200x250x2250 mm. 5R6/m shear connectors with total length 130mm were planted in the RC beam before jacketing. Reinforced Jackets had 2T10mm and 2T12mm top and bottom reinforcement, respectively, and tided with planted stirrups R8@200 at the middle third and R8@100 near the supports. The surface of the original beams was chipped, roughened and cleaned before casting the concrete jacket. Figure 2 shows the details of test specimens.

Dimensions (cm) and details of reinforcement for original beam

Figure 2 Beam specimens

Original Beam X-sec

Rein. Jacket X-sec

Unrein. Jacket X-sec



The designed slump of concrete used for casting beams and jackets was 100 mm. Cubic samples (100x100x100 mm) were used to determine the compressive strength according to the E.S.S 1658. Cylinders (100x200 mm) were used to determine the splitting tensile strength according to ASTM C 496-90. Cylinders (150x300 mm) were used to determine the Young,s Modulus according to ASTM C469. Concrete mixes were cast and kept in the moulds for 24 hours at room temperature, and then samples were de-moulded and kept in water of temperature 20±2oC until the time for testing. Three replicates were cast for each test and the average value was used throughout the study. Test results of concrete samples are shown in table 2.

Table 2 Properties of concrete mixes

Mix type Beams mix (180 days) Jackets control mix (56 days)

Jackets FA mix (56 days)

Tempo C 20 o 400 o 600 o 800 o 20 o 400 o 600 o 800 o 20 o 400 o 600 o 800 o Compressive strength (N/mm2)

30 21.5 18.5 7.0 25 21.0 15.6

8 5.0 35 29.75

25.6

12.0

Tensile strength (N/mm2)

3.3 2.4 1.75

0.70 2.6 2.1 1.35 0.6

5 3.3 2.8 2.66

0.90

Young,s Modulus (N/mm2)x 103

24.00

12.10

6.20

2.20

23.00

11.24 6.15 2.2

0 23.0

0 10.8

0 5.00

2.19

Firing Scheme

Concrete Mix Samples Samples were heated in an electric oven with heating rate of 5oC/min until reaching the target temperature (up to 800°C) and sustained for a total heating duration of 3 hrs. The specimens were allowed to cool to room temperature before testing.

Beams and Jackets RC beams and jackets were exposed to elevated temperature following ASTME119 fire regime for 180 minutes in a firing furnace. The firing furnace is a chamber of dimensions 5.0x1.5x5.0m with side door of dimensions 5.0x5.0m. Six fire channels, three at each side, the furnace channels are connected to gas tanks outside the furnace. The furnace includes nine temperature sensors to determine the temperature rise through the furnace and 8.0m chimney to exclude excess vapor. The body of the furnace is made of metal frame with thermal brick and glassy wool isolation.

The top surface of the beams and jackets were isolated by Steel Guard TM 581. The temperatures were measured at distances 0.0 mm, 25 mm, 50 mm and 75 mm from the bottom of the specimens by using thermocouples connected to 16 channel standard thermocouples monitor model SR 630. Figure 3 shows thermocouples locations.



Figure 3 Thermocouples locations

Testing Four points loading over a simply supported span with length of 2050 mm was carried out for all test specimens. The load was applied gradually till failure. During the loading, observations and measurements included crack pattern, central deflection and the failure modes. Dial gages of sensitivity 0.002 mm were used for recording the central deflection. Dial gages were also installed on the concrete top surface directly over the supports to record any settlement of the supports during testing. Figure 4 shows the loading scheme for test specimens.

Figure 4 Loading scheme

4. RESULTS AND DISCUSSION The results of the experimental program are presented and discussed in this section. Cracking load (Pcr), ultimate loads (Pult), displacement ductility index (Di) and post elastic strength enhancement factor (V) as expressed by Demir A. and Tekin M.23 are illustrated in Table 4. Load deflection curves for test beams are shown in from Figures 5 to 8. Failure modes and crack patterns are shown in Figures 9 and 10.



Table 4 Summery of test results

Beam No. Preloading Elevated

Temp. Pult Pcr Δult Δy Di V KN KN mm mm

B1a - - 71 35 35 9.2 3.80 1.48 B1b - √ Spalled B2 - - 142 38.5 34 7.0 4.86 1.45 B3 - - 165 48 50 8.0 6.25 1.54 B4 - √ 77 18 40 8.8 4.55 1.64 B5 - √ 95 25 46 8.4 5.48 1.79 B6 √ - 134 33 31 11.0 2.82 1.72 B7 √ - 155 40.5 36 7.3 4.93 1.87 B8 √ √ 75 15 38 10.1 3.76 1.49 B9 √ √ 88 20 42 10.9 3.85 1.45 B10 - - 79 36 35 8.0 4.38 1.52 B11 - - 83 40 37 7.3 5.07 1.54 B12 - √ 74 20.9 34 7.0 4.86 1.82 B13 - √ 81 28 36 6.8 5.29 1.80 B14 √ - 75 27.5 36 8.5 4.24 1.67 B15 √ - 79 38 38 7 5.43 1.65 B16 √ √ 72 15 33 6 5.50 1.89 B17 √ √ 78 25 37 6.2 5.97 1.86

As illustrated in Table 4, use of FA mix in jackets exhibits superior performance compared to control mix at normal temperature and after exposure to elevated temperatures. Reinforced jackets are also superior to unreinforced ones. Use of reinforced Jackets with uncracked beams increased the load carrying capacity at normal temperature compared with the control beam B1a by 132.4% and 100.0 % for FA mix and control mix, respectively. These ratios were limited when using unreinforced jackets to 16.9% and 11.3 %, respectively. Similarly, cracking load was increased by 37.1 % and 10.0% when strengthened with reinforced jackets made of FA mix and control mix, respectively. These ratios were reduced when using unreinforced jackets to be 14.3% and 2.8 %.

Preloading of beams to 65% of the ultimate load of the control beam B1a before strengthening with reinforced jackets had a slight effect on the behavior of repaired beams. This may be attributed to the fact that the rigidity of the jacket, regardless the type of mix used, makes it the significant part in carrying the load compared with the original beam.

When using unreinforced jackets, preloading showed a marginal weakening effect for the repaired beams in case of control mix while FA mix did not show the same effect. This is attributed to the inclusion of glass fibers which delayed the propagations of cracks.

Ductility index was affected by the type of mix used in jackets. Use of FA mix for jacketing enhanced the ductility index at normal temperature compared to control mix. Use of reinforced Jackets with uncracked beams increased ductility index by 64.5% and 17.9 % when using FA mix and control mix, respectively. These ratios were limited when using unreinforced jackets to 33.4% and 15.3%, respectively.

Use of FA mix in the strengthening jackets of the preloaded beams increased the ductility index by 29.7% and 42.6%, for reinforced and unreinforced jackets, respectively. On the

Δult: deflection at ultimate load Δy: deflection at yield load Di :

(Δult/ Δy) V: the ratio between the ultimate loads, (Pult), and the load at yield, Py (Pult/Py)



other hand, the ratios for control mix reduction by 25.6% and improving by 11.5%, respectively. From these results, it is obvious that inclusion of glass fibers in FA mix increased the ductility index of jackets when compared to control mix.

The effect of elevated temperature was tremendous on test beams. Control beam B1b was spalled while the ultimate and cracking loads were dramatically reduced for other beams. The load carrying capacity for beams strengthened by reinforced jackets made of FA and control mixes was reduced by 42.4% and 45.8%, respectively. These values slightly reduced for preloaded beams to 42.2% and 44.0%, respectively. For unreinforced preloaded beams, the ultimate load carrying capacity was reduced by only 1.2% and 4.0% for FA and control mixes, respectively. This is due to the significant effect on steel bars in case of reinforced jackets.

The cracking load was also reduced after exposure to elevated temperature due to thermal damage of concrete cover. Better performance was observed for jackets made with FA mix where the lower thermal conductivity of glass fibers and fly ash retards the heat transmission to the core concrete. Initial cracking loads were reduced for strengthened beams with reinforced jackets made of FA and control mixes by 47.9% and 53.2%, respectively. These values were limited for unreinforced preloaded jackets to 34.2% and 45.4%, respectively.

The ductility index after exposure to elevated temperature for strengthened beams with reinforced jackets made of FA and control mix were 5.48 and 4.55, respectively. These values were reduced for preloaded beams to 3.85 and 3.76, respectively. Unreinforced jackets impart the strengthened beam higher ductility index.

Figure 5 shows load deflection curves for strengthened uncracked beams. The trend was bilinear for all beams. Using reinforced jackets enhanced both stiffness and ultimate load. The stiffness of beams with reinforced jackets (B2 and B3) was comparable till a load value of 130 KN (first phase of the behavior). After this load, where the repaired beams suffered from excess cracking, the stiffness was higher for FA mix compared with control one due to the contribution of glass fibers and fly ash.

Similar trend was obtained for preloaded strengthened beams as shown in Figure 6. Precracking showed a significant effect in decreasing the stiffness when using control mix jackets.

Figure 5 Load deflection curve for uncracked strengthened beams

Figure 6 Load deflection curve for preloaded strengthened beams

Figures 7 shows load deflection curves for strengthened uncracked beams after exposure to elevated temperature, whereas Figures 8 shows those for preloaded beams. The significant distortion due to exposure to elevated temperature minimized the difference in the load carrying capacity between reinforced and unreinforced jackets. However, it was surprising for



preloaded beams that use of unreinforced jacket made of FA mix showed identical behavior to reinforced jacket made of control mix. The credit goes to glass fibers and fly ash in enhancing both stiffness and ultimate capacity.

Figure 7 Load deflection curve for uncracked strengthened beams (After exposure to elevated temperature)

Figure 8 Load deflection curve for preloaded (After exposure to elevated temperature)

Figure 9 shows failure modes and crack patterns for test beams at normal temperatures. Most cracks are flexural cracks appeared in the middle third portion of the beam. Number of cracks were increased in beams strengthened with jackets made of FA mix due to the fiber action that bridge the stress through more cracks. Preloading the beams did not show different behavior for the mode of failure. However, wider cracks generated for beams strengthened with jackets made of control mix.

B

B

B

B



Figure 9 Failure mode and crack pattern for test beams

Table 5 illustrates temperature values at six locations in strengthened beams. The temperature at the beam surface was almost the furnace temperature, as the heating source was near the beam surface. All beam surfaces were heavily damaged and ground after exposed to elevated temperatures. The upper isolated surface of the beams was slightly affected by elevated temperature. Thermal cracks and cover spalling occurs extensively for beams strengthened with jackets made of control mix compared to those of FA mixes due to good fire resistance and thermal properties of the FA mixes. Figure 10 shows Failure modes and crack patterns for beams loaded after exposure to elevated temperatures.

Table 5 Temperature at different locations in beams

Beam No. Mix Avg. Temp. at locations

1 2 3 4 5 6

B5, B9, B15 FA 1040

530 790 165 139 160

B4, B8, B14 C 1040

750 900 530 330 400

Figure 10 shows failure modes and crack patterns for test beams after exposure to elevated temperatures. Shear and flexural cracks appeared in beams strengthened with reinforced jackets whereas only flexural cracks generated in beams strengthened by with unreinforced jackets. Spalling and wider cracks at concrete surface were extensive in case of control mix compared with FA mix. Preloading of beams before strengthening did not significantly affect the failure modes and crack patterns.

B

B

B

B



Figure 10 Failure modes and crack patterns for beams loaded after exposure to Elevated

temperatures

In a previous experimental study, Topcu I. B. and Karakurt C.24 concluded that reinforcement lost almost its strength when the temperature at the reinforcement location exceeds 500oC. The yield strength loss of S420 steel rebars was 84% for 800◦C exposure temperature. Further increase of temperature to 950◦C reduces the yield strength to 89%24.

Based on the conclusions of this study, the reduction in yield strength for rebars of jackets made of FA mix was marginally lower than that of control mix. The reinforcement of original beams was not affected since the temperature at its location did not exceed the critical one of 500◦C with marginal efficiency in case of FA mix.

The behavior of concrete is different, the concrete surface temperatures were almost 1129◦C for all beams exposed to elevated temperature, and this caused total damage and grinding of concrete at the surface. The temperature at locations 1 (25 mm) for beams

B

B

B

B

B

B



strengthened by using FA mix and control mix were 440◦C and 750◦C, respectively, from table 2 the degradation of concrete compressive strength by using FA mix and control mix were 30% and 80%, respectively. No strength loses for concrete were observed at locations 4 (75 mm), The temperature for strengthened beams by using FA mix and control mix were 60◦C and 330◦C, respectively.

5. CONCLUSIONS Use of FA mix in jackets exhibits superior performance compared to control mix at normal

temperature and after exposure to elevated temperatures.

Reinforced jackets are superior to unreinforced ones with respect to load carrying capacity, cracking load, stiffness and ductility.

Preloading of beams to 65% of the ultimate load of the control beam before strengthening with reinforced jackets had a slight effect on the behavior of repaired beams.

inclusion of glass fibers in FA mix increased the ductility index of jackets when compared by control mix

Elevated temperature was tremendous on test beams. Control beam was spalled while the ultimate and cracking loads were dramatically reduced for other beams.

Preloaded beams that use of unreinforced jacket made of FA mix showed identical behavior to reinforced jacket made of control mix after exposure to elevated temperatures. The credit goes to glass fibers and fly ash in enhancing both stiffness and ultimate capacity

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