PSI - Issue 6

Vimal Kumar et al. / Procedia Structural Integrity 6 (2017) 11–18 Author name / Structural Integrity Procedia 00 (2017) 000 – 000

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Keywords: Prestressed Concrete; Multiple Impacts; Damage; Impact Force; Energy Absorption;

1. Introduction Structures are subject to high intensity shock loads originated from earthquake, tsunami, vehicle accidents, ballistic attacks. These natural and manmade disasters are always accompanied with loss of money and human lives. Attempts have been made in the past to study the impact resistance behavior of different types of concrete and concrete structural members under low to high rate of loading conditions. The performance of fully and partially restrained girder has been investigated by Kishi et al. (2002) under free fall of heavy drop mass of capacity 3 and 5 Ton. The fully restrained girder had the impact resistance capacity about 1.7 times of partially restrained girder. The damage resistance of rock shed structures without protective layers was investigated by Delhomme et al. (2005), Mougin et al. (2005); and with protective layers was investigated by Schellenberg et al. (2008), Bhatti et al. (2011) , under impulse loading conditions. The claddings (protective layers) were found to be an effective technique by Schellenberg et al. (2008), Bhatti et al. (2011), Starr and Krauthammer (2005), to reduce the damage to underlying structural member under impact loading. Similarly, studies are available on plain concrete, reinforced concrete, steel-concrete composites but the literature is very limited where the damage resistance capacity of prestressed concrete structural members were studied, see Kumar et al. (2016, 2017 and 2017) and Iqbal et al. (2017). The impact resistance capacity of the prestressed concrete plates under single impact has been found to have increased by inducing the prestress in the concrete. However, the performance of prestressed concrete under multiple impacts has not been explored. In present study, the drop weight impact tests were performed to study damage resistance of prestressed concrete (PC) and reinforced concrete (RC) slabs. The influence of induced prestress on the impact-force and deflection response of the target slabs was investigated and compared under multiple impacts. The crack propagation and progressive damage in PC and RC slabs under multiple impact were discussed with the help of post-impact pictures of damaged slab specimens. 2. Material and test specimens The drop weight impact experiments were performed on square shaped concrete slabs. The prestressed and reinforced concrete slab specimens were identical in size; 800 × 800 × 100 mm. The concrete mix was designed to achieve unconfined compressive strength, 40 N/mm 2 . The strength of concrete was assured by simultaneously casting the concrete cubes (150 mm) from the same concrete batch mix used for slabs, and also by testing under a Compression Testing Machine (CTM). The testing was done after a water curing period of 28 days, at the loading rate, 140 kg/sq.cm/min. Thus, the average compressive strength of concrete cubes obtained was 48 N/mm 2 . Both the prestressed and reinforced concrete target slabs were casted using the same concrete batch mix and also cured in water in identical ambient conditions for 28 days. The prestressed concrete slabs had similar reinforcement pattern as provided in reinforced concrete slabs with additional prestressing force. Thus, the only difference between PC and RC slab was the applied pre-tensioning force that was studied under multiple impacts. The initial stress induced in concrete was approximately 20% of its compressive strength. 3. Experimental approach The prestressed and reinforce concrete slabs ware impacted by freely dropping a heavy steel mass (242.85 kg) at the center of the span. Both the slabs were fixed in the setup by clamping their all four edges. The hammer was then lifted up to a predefined fixed height 500 mm and dropped under gravity from rest, see Table 1. The suffix 0, 1, 2 and 3 with slab-ID RC or PC represents the number of repeated drop of steel mass, see column b and d of see Table 2. The generated impact response was recorded in a data acquisitions system at a time interval of 1 × 10 -4 second. The load cell was mounted on dropping mass to measure impact force and displacement sensors were attached at the center and quarter span to measure displacement response. The post-impact damage under each repeated fall of impactor was captured using a digital camera to study the crack pattern.