Microsoft Word - 29-2733_s_ETASR_V9_N3_pp4225-4229 Engineering, Technology & Applied Science Research Vol. 9, No. 3, 2019, 4225-4229 4225 www.etasr.com Buller et al.: Flexural Behavior of Reinforced RAC Beams Exposed to 1000°C Fire for 18 Hours Flexural Behavior of Reinforced RAC Beams Exposed to 1000°C Fire for 18 Hours Abdul Hafeez Buller Department of Civil Engineering, Quaid-e-Awam University of Engineering, Science & Technology, Nawabshah, Pakistan ah.buller@quest.edu.pk Mahboob Oad Department of Civil Engineering, Quaid-e-Awam University of Engineering, Science & Technology, Nawabshah, Pakistan mahbooboad04@gmail.com Bashir Ahmed Memon Department of Civil Engineering, Quaid-e-Awam University of Engineering, Science & Technology, Nawabshah, Pakistan basher_m@hotmail.com Abstract—In order to meet the socio-economic demands around the globe, construction industry not only consumes concrete at a very fast pace but also yields huge amounts of construction and demolishing waste. The phenomenon gives rise to environmental issues due to production of concrete ingredients and due to dumping of the waste. Therefore, one of the solutions is the production of green concrete utilizing demolished waste. This research work studies the effect of prolonged fire (18 hours) on the flexural behavior of reinforced concrete–recycled aggregate beams. The beams were using 50% replacement of natural coarse aggregates with demolished concrete. The beam samples were cast as both normal and rich mix concrete and were cured for 28 days. After curing, the beams were exposed to fire at 1000°C in a purpose made oven, followed by testing in a universal load testing machine under central point load. The test results show that the proposed beams (cast with rich mix) exhibited about 22% reduction in flexural strength. The failure mode of the beams was observed as shear failure. Keywords-green concrete; fire effect; flexural behavior; demolished waste; recyclable aggregates I. INTRODUCTION Fire is one hazard a structure may face during its lifetime. It not only affects a structure’s appearance, but also deteriorates its strength. Therefore, a fire damaged structure needs careful examination and proper retrofitting. This requires investigation of the effects of fire, fire duration, temperature gain and behavior of reinforced concrete members during and after fire. On the other hand, to meet the socio-economic development, old or short height structures are demolished to build new, higher, structures, particularly in city centers. The demolishing of the structures generates massive waste, raising, among others, environmental issues. Green concrete, the concrete made with demolishing waste, is one of the solutions. This research work studies the use of demolished concrete as coarse aggregates in new concrete and the effect of prolonged fire (18- hour duration) on it. Author in [1] highlights the challenges and issues regarding aggregates along with possible solutions. The author also summarized different aspects of recycled aggregates and investigations on concrete made with these aggregates. Similar work is also done in [1, 2] but for composite members. Authors in [4, 13] used different dosages of demolishing waste in making concrete cubes for strength checking after exposing to fire at 1000°C for different durations. They observed that at 50% replacement of the natural coarse aggregates, the compressive strength of the concrete is within acceptable limits. Authors in [5, 6] used 50% replacement of natural coarse aggregates with demolished concrete to check the flexural strength vs strain of reinforced concrete beams. In depth experimental investigations showed comparable performance with specimens made from conventional concrete. Combined use of wollastonite and recycled waste as partial replacement of cement and coarse aggregates is reported in [7]. Additionally, silica was used in 10% by weight to check performance under elevated temperatures (20°C-800°C) among other properties. The results showed that the wallastonite had adverse effects on strength and durability but positive effect on flexural and tensile strength, whereas, the recycled waste as coarse aggregates performed well. Authors in [8] used ilmenite and baryte concretes for the study of the effects of fire on heavy weight concrete used in nuclear structures as shield, and investigated the strength after exposing the samples in fire for 1 to 3 hours at 250°C to 950°C. The results showed that ilmenite concrete was more fire resistant than baryte and normal concrete. Further testing revealed that addition of foam or increased air entrapped percentage led the concrete with better performance than other types as nuclear radiation shield. Water throwing on burning objects is the most common firefighting method, however other methods may also be used. To this end, authors in [9] studied different cooling methods and their effect on the residual strength of concrete. The authors exposed concrete members to fire from 300°C to 900°C. They used water throwing, immersion in water, covering by plastic and slow cooling (up to 224 days) and tested strength properties. Based on the obtained results they outlined strength changes and related matters related to the subject matter. Ramp slab of a shopping mall damaged due to accidental fire was investigated in [10] from the retrofitting point of view. Various research groups have also studied the effects of fire on the thickness of concrete cover in residential structures [14], the effects of high temperature on physical and Corresponding author: Abdul Hafeez Buller Engineering, Technology & Applied Science Research Vol. 9, No. 3, 2019, 4225-4229 4226 www.etasr.com Buller et al.: Flexural Behavior of Reinforced RAC Beams Exposed to 1000°C Fire for 18 Hours mechanical properties [15], and reinforced concrete structural material [16]. The discussion above shows the importance of the subject and the variation in results and exploration of the parameters for different condition, which motivated this project on the evaluation of the effects of fire on reinforced concrete beams made with partial replacement of natural coarse aggregates with demolished concrete exposed to prolonged 18- hour fire. II. MATERIALS AND TESTING The demolished concrete used in this research work was obtained from a demolished 50 year old school building in the shape of large blocks (Figure 1). These blocks were manually hammered to obtain aggregates of 25mm in size (Figure 2). The cracked particles were screened and separated carefully (Figure 3). Sieve analysis of both natural and recyclable aggregates was done to have well graded aggregates. Fig. 1. Demolished concrete Fig. 2. Hammering process Fig. 3. Cracked particles As old mortar is attached with the demolished concrete and requires more water in the concrete mix to ensure workability, water absorption test of recyclable and natural coarse aggregates was done in standard fashion. The water absorption of natural coarse aggregates was obtained equal to 1.80% and that of recyclable aggregates was equal to 3.92%. Accordingly, the water demand in concrete mix was adjusted. Twenty four reinforced concrete beams were prepared using 1:2:4 (normal mix) and 1:1.5:3 (rich mix). In each concrete mix 50% beams were cast with all-natural coarse aggregates and 50% with equal proportion of the natural and recyclable aggregates following the conclusions of [3, 12]. To reinforce the beams, 2#4 deformed steel bars were used both in tension and compression zones along with #3 bars as shear reinforcement at 150mm center to center along the beam length. The size of all beams was taken equal to 900mm×150mm×150mm. The prepared beams (Figure 4) were cured for 28 days by fully immersing in water, and were air-dried for 24 hours. All the beams were then exposed to fire for 18 hours in a purpose made oven (Figure 5) at 1000°C. Burnable wood was used as the source of fire. It was observed that 1 hour was required to reach the temperature of 1000°C, which was then maintained for the required duration of time. After the elapse of fire duration, the fire source was cut off and beams were left in the oven for 24 hours to avoid sudden attack of atmospheric moisture. Fig. 4. Beam samples Fig. 5. Oven Engineering, Technology & Applied Science Research Vol. 9, No. 3, 2019, 4225-4229 4227 www.etasr.com Buller et al.: Flexural Behavior of Reinforced RAC Beams Exposed to 1000°C Fire for 18 Hours Fig. 6. Beam testing Finally, all the beams were tested in a universal testing machine under central point load in accordance with [11] (Figure 6) until failure. During the testing load, deflection and cracking were monitored at regular intervals. The flexural capacity of the beams was then computed using the numerical expression given in [11]. The obtained results for the beams cast with all-natural coarse aggregates (G1) and 50% recyclable aggregates (G2), with normal mix are given in Tables I and II respectively. Similarly, Table III lists the test results of reinforced concrete beams cast with rich mix and all-natural coarse aggregates (G3) while the results of reinforced concrete beams cast with rich mix and equal proportion of natural and recyclable aggregates (G4) are given in Table IV. All beams were exposed to fire for 18 hours. TABLE I. TEST RESULTS-GROUP G1 S. No Beam No Load (N) Deflection (mm) Flexural Strength N/mm² Psi 1 19 45279 8.40 18.11 2626.18 2 20 41360 8.05 16.54 2398.88 3 21 45836 8.30 18.33 2658.49 4 22 45570 7.90 18.23 2643.06 5 23 45365 8.10 18.15 2631.17 6 24 45697 8.30 18.28 2650.43 Average 44851.17 8.18 17.94 2601.37 TABLE II. TEST RESULTS-GROUP G2 S. No Beam No Load (N) Deflection (mm) Flexural Strength N/mm² Psi 1 49 36650 9.05 14.66 2125.70 2 50 37235 9.15 14.89 2159.63 3 51 36870 9.2 14.75 2138.46 4 52 36272 9.0 14.5 2103.8 5 53 36560 9.15 14.62 2120.48 6 54 36980 9.0 14.792 2144.84 Average 36761.17 9.13 14.70 2132.15 TABLE III. TEST RESULTS-GROUP G3 S. No Beam No Load (N) Deflection (mm) Flexural Strength N/mm² Psi 1 109 45145 9.50 18.06 2618.41 2 110 42120 9.05 16.85 2442.96 3 111 42730 9.10 17.09 2478.34 4 112 45216 9.65 18.09 2622.53 5 113 43976 9.45 17.59 2550.61 6 114 45225 9.55 18.09 2623.05 Average 44068.67 9.38 17.63 2555.98 TABLE IV. TEST RESULTS-GROUP G4 S. No Beam No Load (N) Deflection (mm) Flexural Strength N/mm² Psi 1 79 35125 10.10 14.05 2037.25 2 80 35070 10.25 14.03 2034.06 3 81 35235 10.35 14.09 2043.63 4 82 33840 10.20 13.54 1962.72 5 83 32480 10.25 12.99 1883.84 6 84 35350 10.45 14.14 2050.30 Average 34516.67 10.27 13.81 2001.97 III. RESULTS AND DISCUSSION The load and deflection recorded are graphically shown in Figures 7-10 for groups G1-G4 respectively. It may be observed that the load-displacement pattern of the beams within and across the group is similar except peak values. Fig. 7. Load vs displacement – G1 beams Fig. 8. Load vs displacement – G2 beams Fig. 9. Load vs displacement – G3 beams Engineering, Technology & Applied Science Research Vol. 9, No. 3, 2019, 4225-4229 4228 www.etasr.com Buller et al.: Flexural Behavior of Reinforced RAC Beams Exposed to 1000°C Fire for 18 Hours Fig. 10. Load vs displacement – G4 beams The comparison of maximum load attained is shown in Figure 11. The same relationship between G3 and G4 beams is shown in Figure 12. It may be observed from these graphs that due to induction of recyclable aggregates, normal mix beams exhibit 19.06% reduction in maximum load, whereas, the same for rich mix concrete beams is equal to 21.82% with respect to its counterpart. It may be noted that the rich mix concrete beams exhibited more reduction in maximum load capacity (14.4%) than normal mix beams due to fire exposure. This is mainly attributed to the larger cement content in these beams. Fire loss of moisture from the body of concrete weakens the bond leading to reduction of load carrying capacity. Therefore, more care should be taken while considering rich mix concrete with recycled aggregates and fire exposure. Fig. 11. Maximum load – G2, G1, G4 beams Fig. 12. Maximum load – G3 and G4 beams All the beams containing recycled aggregates showed reduction in flexural strength with the same percentage decrease as peak load. The results of the deflection showed increase for both normal and rich mix concrete beams. Normal mix beams with recycled aggregates exhibited 17.61% increase in comparison to their counterpart cast with all-natural aggregates. Whereas, mix concrete beams with recycled aggregates recorded equal increase to 9.48%. In comparison to ACI-318 specified approximate allowable deflection values, the deflection at failure in normal mix beams tripled and in rich mix beams doubled. Therefore, rich mix concrete beams showed better performance regarding deflection in comparison to normal mix reinforced concrete beams. Cracks and cracking pattern at failure were also observed during testing. Due to fire exposure, several surface cracks were presented even before the start of the test, therefore it was difficult to monitor the first crack due to loading. Crack pattern after failure was observed as diagonal cracks from center towards supports or near to supports, with arching action in some of the specimens. Therefore, the failure mode of the beams was identified as shear failure (Figure 13). Fig. 13. Cracking in beams IV. CONCLUSION The presented experimental study aimed to evaluate the effects of 18-hour fire on reinforced concrete beams cast with 50% dosage of both natural and recycled aggregates. Normal and rich mix concrete were used in reinforced concrete beams. Control specimens with all-natural aggregates were cast to compare the results. After exposure to fire, all beams were tested for peak load, deflection and cracking. The obtained flexural strength of the proposed beams was 19.06% and 21.82% reduced for normal and rich mix beams respectively. At failure, increase in deflection up to three times than approximate allowable deflection by ACI-318 and shear failure were observed. Although 18 hours of fire is a prolonged duration, the loss of flexural strength or peak load is about 22%. This reduction is due to the combined effects of recycled aggregates and exposure to fire, therefore may be considered a reasonably low value. However, proper retrofitting decision should be taken before putting the structure back in service. REFERENCES [1] B. A. 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