Blast Resistance Research
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Can Insulated Cement-Based Wall Systems be Blast Resistant?

With increased attention being paid to terrorist bombings the market for blast-resistant structures is growing. The most cost-effective method is to keep the terrorist and his explosive device away from the structure, commonly referred to as the “stand-off distance.” The “stand-off distance” is determined by the size of the explosive to be protected against; i.e. the larger the explosive the greater the standoff distance required. However, not all structures, planned or existing, have the luxury of the required surrounding real-estate to provide the necessary stand-off distance.

Existing structures without surrounding land face difficult challenges. They must be retrofitted/hardened to withstand a blast if the structure is not able to withstand it. This can be costly. For new construction, the hardening must be part of the design considerations, leaving building owners and designers with several questions. What cost-effective solutions are available to create a blast-resistant building? Are there commercial-off-the–shelf (COTS) building systems that can meet the owner’s needs either as currently configured or through economic modifications? Must all solutions require a “fortress like” architectural solution? The portland cement industry is working with other industry trade associations and the U.S. Air Force to research the response of insulated cement-based wall systems to blast pressures.

At the last two Force Protection Equipment Demonstrations, held at the Marine Corps Base Quantico Va., the Insulating Concrete Form Association (ICFA) held non-instrumented demonstrations on the effectiveness of an insulated concrete form (ICF) wall to withstand a 50-pound TNT equivalent explosive charge at less than 1-foot stand-off distance with minimal damage. The wall tested was an insulated wall with 2.5 inches of expanded polystyrene foam on the inside and outside face of a 4-inch concrete panel. That demonstration resulted in several briefings, conducted by PCA’s Washington, DC office, to Department of Defense personnel on the viability of ICF and other insulated cement based wall systems to provide an economical building envelope with improved blast protection. The U.S. Army Corps of Engineers was the first to express an interest in investigating these systems. However, the demands of the Iraq conflict prevented the opportunity of commencing a study. The Air Force then stepped forward to conduct the research.

PCA met with its allied trade associations, the National Concrete Masonry Association (NCMA), National Ready Mixed Concrete Association (NRMCA), ICFA, Tilt-up Concrete Association (TCA), Concrete Home Council (CHC), Precast/prestressed Concrete Institute (PCI), and the Concrete Reinforcing Steel Institute (CRSI) to determine the level of industry participation. All parties agreed to the merits of the research and PCA entered into negotiations with the Air Force to develop the Cooperative Research and Development Agreement (CRADA) that formed the framework for the research. PCA then entered in a joint Memorandum of Understanding (MOU) with the allied trade associations. Because of these efforts, the industry has agreed to provide insulated cement-based wall panels for research at the U.S. Air Force Air Force Research Laboratory (AFRL), at Tyndall Air Force Base, Florida.

Included in this research scope are several double wythe insulated masonry wall systems (NCMA and Brick Industry Association - BIA), two precast/prestressed sandwich panel systems (PCI), two tilt-up concrete sandwich panel (TCA), three different insulating form wall systems (ICFA) and an exterior insulated cast-in-place wall (CHC). Predictive analytical models, laboratory testing and full-scale explosive experiments are the primary research components.

The desired results are engineering level tools that can be used to predict the blast response of these COTS wall systems for both new construction and retrofits to existing structures. An example of a solution for a situation with stand-off distance limitations might be using a precast or tilt-up insulated sandwich panel, with the appropriate architectural treatments already cast-in, as an “armored girdle” around the building’s exterior two or three stories high or adding another masonry wythe with insulation in the cavity to an existing building.

The research program will comprise three phases. The first phase will examine existing COTS insulated cement-based wall systems, designed with standard mix concrete to meet a specified wind load and seismic design. The second phase will examine several unique concrete wall systems. The third phase will look at any modifications that could significantly enhance the blast performance of phase one and two products.

The research began in 2006 with the evaluation of 30-ft precast/prestressed insulated sandwich panels. While the blast effects are still being analyzed, the damage observed appeared minimal. The next wall assemblies scheduled in 2007 will be the reinforced masonry systems. A third wall assembly planned later in 2007 will be either an insulated tilt-up sandwich panel wall or one of the three ICF wall systems.