اعضای بتن آرمه تحت ضربه افتان Reinforced concrete members under drop-weight impacts

Reinforced concrete structures might be exposed sometime in their lives to some extreme dynamic loading conditions owing to impacts. Typical examples include transportation structures subjected to vehicle crash impact, marine and offshore structures exposed to ice impact or dropped object impact from passing ships, protective structures under projectile or aircraft impact and civil structures impacted by tornado-borne debris. In recent years the assessment of the performance and vulnerability of concrete reinforced structures under an impact load has become more important.1 Increasingly engineers are resorting to numerical models to carry out designs, assessments and safety checks, and there is a requirement for high-quality data from physical tests to assist in validation of these models. When subjected to an impact load, structural members can behave differently compared with those under a static load, owing to the transient and usually localised pattern of impact loading. The dynamic properties of materials can also be different to those under static loading. Investigations2–۴ have shown that both concrete and steel are stress/strain rate sensitive: both the tensile and compressive strengths and Young’s modulus can increase if there is an increase in the stress/strain rate. There have been a number of studies of impact on reinforced concrete members, in particular drop-weight impact tests on reinforced and prestressed concrete beams. Kishi and his coworkers conducted drop-weight impact tests in which a beam failed in either a bending or shear failure mode.5,6 The impact forces, reactions and deflections of the beams were measured, and the absorbed energy of a beam was determined using the reaction forces at the supports and residual deflection of the beam at mid-span. Empirical design formulae were then proposed based on the static bending or shear capacity of a beam and required residual displacement. Given the dynamic nature of impact-loaded reinforced concrete structures, a degree of scatter was found between the test results and the predictions from the proposed design formulae. A comprehensive series of tests on single-span reinforced concrete beams, using a drop-weight impact facility, was conducted by Hughes and Speirs.7 The impacts were imposed to the top of the beams at mid-span through a plywood or steel pad, in order that the stiffness of the impact zone could be varied. In the majority of the tests the beam failed in a flexural mode, with flexural cracks at the bottom of the beam concentrated towards the mid-span, and at the top of the beam at 1/5 and 5/6 span. Diagonal cracks were observed in the impact zone and shear cracks at 1/3 and 2/3 span on the beam in some of the tests. It was found that the stiffness of the impact zone, which was a function of the impactor, the pad and the local stiffness of the beam, had a more significant influence than the support conditions on the response of the beam and that high vibration modes were more likely to be excited as the impact zone became stiffer. A theoretical analysis, based on an elastic vibration model of beams, was used to determine impact force–time history and limits of beam displacement. While the analysis gave a reasonable predication of impact forces and extents of beam displacement, it was concluded that a more accurate analysis would be beneficial in order to account for cracking, crushing, yielding of the reinforcement and strain rate effects, if there are any. Agardh et al. 8 investigated the impact resistance of highstrength concrete reinforced concrete beams. Strain gauges attached to the surface of reinforcing bars were used to measure the reinforcement strains. A strain rate of 102/s in the reinforcement was reported to have been recorded. Strain gauges installed in this way are considered to affect the bond between the bars and surrounding concrete. It is therefore difficult for the gauges to provide accurate results where a crack occurs and they are easily damaged, as occurred with some of the gauges in Agardh’s tests. A vast number of investigations into reinforced concrete slabs under impacts have also been carried out.9,10 The effects of various parameters were studied, including size of slabs, ratio of reinforcement, shape of impactors and impact velocities. Both drop-weight and hydraulic testing machines were used to test slabs under low-velocity impacts (up to 10 m/s).11,12 The failure patterns found in the tests included spalling, penetration, scabbing, perforation and shear plugs.13 Owing to the complexity of the impact behaviour of reinforced concrete structures, impacted structural members have been generally evaluated using empirical formulae to provide estimates on the extent of any impact damage, such as penetration depth, the possibility of scabbing or perforation for slabs14 and the load capacity for beam.5,6 With the advancement of modern computing facilities and finite-element theories, more details of the dynamic response of structural members can be accounted for.15–۱۸ Recently, computational techniques based on combined continuum/discontinuum (finite/discrete element) methods have been developed for simulating the impact behaviour of reinforced concrete beams,19 especially within the nonlinear range when extensive cracking of the concrete can take place in conjunction with yielding of the steel reinforcement. The experimental work described in the current paper has been motivated by a lack of high-quality test results that has hampered the development and appraisal of such computational simulation techniques as mentioned above. The objective was to provide both the necessary input data for computation and results with which to appraise the numerical predictions. The strategy has been devised as that under the assumption that, by undertaking a controlled series of experimental studies, the final extent of the impact damage to the specimens together with the development of the failure mechanism in time can be recorded. The latter included monitoring the transient impact force, deformations/ accelerations, strains and the progress of cracking, spalling and scabbing during the tests. The findings from the study should also help to gain further insights into the impact behaviour of reinforced concrete structures.