BLAST HARDENING: EXAMPLES OF RETROFIT SOLUTIONS

By Eamonn Ryan, derived from an ACI podcast

A March 2024 American Concrete Institute (ACI) podcast by Marlon Bazan, a principal at Protection Engineering Consultants, shared insights on effective blast hardening retrofit concepts covered in the ACI 370R report. This is Part 2 of a three-part series

The presentation provided several illustrative examples covered in the ACI 370 report on blast retrofitting. For columns, solutions ranged from FRP wrapping to steel jackets, each tailored to address specific blast scenarios—from far-range detonations to closer, more severe blast impacts. Similarly, retrofit solutions for beams and slabs included FRP steel plates and additional concrete, aimed at bolstering structural integrity without compromising existing functionalities.

Looking ahead, Bazan highlighted ongoing advancements in retrofit technologies, particularly in the application of FRP and composite materials. These materials offer high tensile strength and corrosion resistance, making them ideal for reinforcing vulnerable structural components. However, ongoing research and development are essential to standardise design methodologies and ensure consistent performance across diverse retrofit applications.

Bazan distinguishes between two primary types of detonations: far-range and close-range. A far-range detonation, Bazan explains, generally poses a lower risk of overloading structural columns due to the distributed nature of the blast wave. In contrast, a close-range detonation exerts localised pressures that can lead to significant damage, including spalling and concrete pulverisation.

To mitigate the effects of blast detonations on columns, Bazan advocates for several retrofitting techniques. FRP wrapping emerges as a viable solution, as illustrated by examples from the ACL 370 report. Bazan highlights the effectiveness of rounding corners on rectangular columns before applying FRP wrapping, a simple yet crucial detailing technique that enhances performance.

Steel jackets present another option, particularly suitable for mitigating localised damage from higher-intensity detonations. However, Bazan cautions about the potential for direct shear failure where the jacket ends. He emphasises the importance of extending jackets and securing them with anchors to the ground to prevent such failures and discusses the broader implications of column strengthening on the overall structural behavior under seismic loads.

Moving beyond columns, Bazan explores retrofit strategies for beams and slabs. He explains that retrofitting typically targets the tension side of beams, where the addition of FRP or steel plates can significantly increase flexural capacity. Moreover, the reinforcement of beams with additional concrete or formwork helps manage increased shear demands resulting from enhanced flexural strength.

Conversely, retrofitting slabs proves more challenging due to the complexities involved in augmenting both flexural and shear capacities simultaneously. Bazan underscores the need for careful assessment to determine the feasibility and effectiveness of retrofit solutions specific to slab configurations.

Composite backing wall systems

Bazan concludes by discussing composite backing wall systems, which entail adding layers of concrete with reinforcement to existing concrete or masonry walls. While referencing UFC document 402302, he notes that although these systems are effective, the lack of active maintenance and updates necessitates careful consideration of their applicability and modern standards.

Shifting focus to CMU (concrete masonry unit) or masonry walls, Bazan highlights bonded polymer as a popular retrofitting option. This method involves applying polymer either through spray-on or trowel-on applications to strengthen walls against blast loads. The polymer significantly improves ductility compared to FRP, reducing the likelihood of sudden failures and acting almost like a tension membrane after reaching threshold capacity. However, Bazan notes that bonded polymers are best suited for weaker walls like unreinforced CMU, as they may not provide substantial benefits for reinforced CMU due to their already enhanced strength. For non-load-bearing walls subjected to relatively low blast loads, bonded polymer retrofits can effectively mitigate damage, although deflections tend to be higher compared to FRP applications.

Bazan emphasises several critical design considerations applicable across these retrofit methods. For FRP and bonded polymers, specific proprietary methods and characteristics drive analysis and design, limiting universal applicability across products. Unlike FRP, which has seen efforts towards standardisation, each bonded polymer product requires distinct design approaches tailored to its properties and behavior.

Localised strain concentrations pose another significant challenge. Bazan illustrates this with examples where bonded polymers showed unpredictable crack formations—either multiple cracks distributed along the wall length or concentrated cracks at specific points. This variability complicates prediction and underscores the need for rigorous testing and tailored design solutions to ensure reliable performance under blast conditions.

Continued in Part 3…