Hardening of Reinforced Concrete Columns Against Blast Loading by Prestressing and ECC Jacketing

Abstract

The design of blast-resistant civilian structures is not a common practice in the construction industry. However, the increasing threat of terrorist attacks using vehicle bombs targeting civilian facilities has necessitated the development of innovative solutions. A key solution to prevent existing buildings from damage due to blast is to mitigate risk by providing a secured distance around the perimeter of buildings or hardening building components. Columns, being critical elements of buildings, are responsible for the overall strength and stability of the entire structure. Columns are primarily designed to carry vertical loads, with some also offering resistance to lateral loads caused by earthquakes and wind. However, their lateral capacity is limited in the event of a nearby explosion. Columns' lateral deflections must be limited to avoid localized column failure under such loadings. The loss of a column can also trigger progressive collapse, leading to partial or complete building collapse. Protecting existing building columns through blast strengthening/hardening can reduce blast risk and improve structural performance under such extreme events. This research aims to address blast risk to reinforced concrete (RC) columns and explores potential improvements in their response by developing innovative hardening techniques. It focuses on ground-floor columns, which are susceptible to external surface blasts typically triggered by vehicular or hand-carried bombs. The proposed hardening technique primarily consists of lateral support(s) provided by external prestressing strands against blast loads, which is at the forefront of innovation. The concept of prestressing against blast loading has also been extended to new columns designed to incorporate internal prestressing, thereby stabilizing column performance under blast loads. Column hardening has also been extended to include jacketing of columns by Engineered Cementitious Composites (ECC). Under blast loads, ECC's high tensile strain capacity allows it to absorb energy and reduce damage propagation in hardened columns. The research consists of extensive experimental and analytical components. The experimental program involved designing, constructing, and testing 15 RC columns under simulated blast shock waves. Twelve of these columns were hardened either by external or internal prestressing or by ECC jacketing. The remaining three columns represent as-built columns without the implementation of any strengthening/hardening. The columns were tested against shock waves (high transient air pressures) generated by a blast simulator in the form of a shock tube while also subjected to vertical gravity loads, also known as axial loads. The test parameters included the location of the prestressing strands (external or internal), the prestressing force and strand longitudinal profile, the amount and arrangement of column transverse reinforcement, the dosage of fibers in the ECC mix, the thickness of the ECC jacket, and the level of reflected pressure-impulse combinations. Analysis of the results obtained in the experimental phase revealed that the proposed strengthening and hardening techniques significantly enhanced the behavior of the columns subjected to combined axial and blast loading. Deployment of external post-tensioned strands noticeably reduced the displacement at the critical section created by elastic and plastic deformations, thus increasing the resistance of the column. Posttensioned columns using internal prestressing also exhibited improved blast resistance response due to additional stability and moment resistance compared to their companion reference column. Similarly, ECC jacketing proved to be an effective method to improve column resistance, resulting in reduced deformations in critical sections. ECC jacket, with its superior ductility and crack control, prevented fragmentation and spalling of concrete at the critical section. In general, the hardened columns resisted between 20% and 40 % higher blast loads as compared to their companion reference columns. When subjected to a similar blast load of their companion reference columns, the maximum and residual deflection of strengthened columns were reduced by 40 to 80 % and 76 to 100 %, respectively. The analytical phase of the research consisted of developing resistance functions for the tested columns, which were then used to conduct nonlinear single-degree-of-freedom (SDOF) dynamic analyses. Resistance functions were developed by considering the strain rate effect (i.e., dynamic increase factor, DIF), material non-linearity, and the contributions of prestressing and ECC jacketing. The SDOF analysis also considered the effect of the P-delta moment (i.e., secondary moment). Comparisons of analytical and experimental column behavior showed that the inelastic SDOF analysis provides reasonably accurate predictions of column behavior under blast loads, making it an effective design tool. In addition, support reactions were also calculated using equations known as dynamic reactions. The research findings were used to develop economically viable and structurally sound column hardening techniques for use in engineering practice.