Is polyurea creating an unknown hazard? Polyurea coatings on blast-resistant modular buildings (BRMBs) may cause more damage from a blast event. Recent research into the anti-corrosion coating polyurea indicates it may decrease the blast resistance when applied to steel structures such as BRMBs. This realization has raised concerns in the blast-resistant engineering community about polyurea coatings.
Polyurea has been marketed as a cost-effective blast mitigation coating technology, but experimental data indicates its use with steel structures may be overstated. Research has shown that polyurea coatings potentially increase the deflection of steel members, particularly when applied to the exterior side facing the blast.
Polyurea is a polymer commonly used in moisture-resistant coating systems. It is derived from step-growth polymerization, which involves combining a synthetic resin with isocyante reactive materials. The result is a fast-curing synthetic polymer that is highly resistant to corrosion and abrasion, has a characteristically high elongation, and has a high bond strength with concrete and metal substrates. Due to these physical characteristics, polyurea is the chief component in many coating and lining products designed to protect against corrosion and abrasion.
Polyurea coatings have also been utilized to enhance the blast resistance of existing masonry walls. The coating is applied to the inside, nonload face of the wall. In the event of a blast, the polymeric coating essentially holds the components together, minimizing spalling and fragmentation. In many cases, the coating is also applied to the exterior face. Several physical tests have demonstrated that, with the proper installation, the polyurea retrofit significantly decreases the damage to masonry components exposed to impulse loads.
The next step in the evolution of blast-retrofit polyurea coatings was their utilization for steel-clad structures. Due to its corrosion resistance and apparent ability to increase blast resistance, polyurea quickly became a highly touted coating for steel BRMBs.
However, the success experienced with masonry components did not transfer to the application with steel elements. Several studies conducted over the past decade have concluded that polyurea coatings applied to the loaded face of the steel component increase the level of damage from impulse loading.
Experiments published by Amini et al. examined the deformation and failure modes of circular steel plates and steel-polyurea bilayer specimens using the direct pressure-pulse loading technique. The experiment included three test groups. The baseline steel specimens were uncoated. The polyurea-coated specimens were tested in two distinct geometries: one with the coating on the side facing the impulse load, and one on the back side not facing the impulse load. The results of the experiment indicated the presence of polyurea on the loading side of the specimen magnified the initial shock effect and further promoted failure of the specimen. Numerical modeling of the experiment produced similar results. The publication concludes: "The most significant result of our study is that, when a polyurea layer is cast onto front face (the impact side), its presence promotes failure during the initial shock effect. Under pressure, the stiffness of the polyurea layer increases substantially, attaining a better impedance match with the steel and thereby increasing the energy that is transferred to the plate."
Similar results were obtained by Ackland et. al. Square steel plates, both coated and uncoated, were tested in flexure by being subjected to localized blast loads. Two different thicknesses of polyurea coatings were evaluated. The post-event measurements of the specimens indicated the plastic deformation increased with the thickness of the polyurea coating. The polyurea de-bonded radially outward from the center, resulting in a hyperelastic extension of the coating. The maximum residual deflection for the polyurea-coated plates was approximately twice that of the uncoated steel. Figure 1 shows the experimental results as published. The data suggests a linear relationship between the thickness of the polyurea coating and the residual deformation.
Similar results were obtained from the 2014 publication by Chen and Linzell. The trend in the experimental data supports the conclusion that polyurea coatings potentially increase the deflection of steel plates subjected to impulse loading. The experimental procedure utilized a pendulum impact device to test four steel specimens with different thicknesses of polyurea coatings. Finite element analyses were also conducted in an effort to validate the numerical calculations with the experimental results.
The results of these tests raise serious concerns for polyurea-coated steel structures. The experimental data indicates the use of polyurea on the exterior of the building increases the deflection of the structural member under blast loading. This increase could potentially raise the response level from medium to high, or even from high to the failure level.
Polyurea can be a positive tool in blast mitigation and corrosion resistance when utilized in the proper application. To gain the benefits of polyurea, it should be applied on the interior, nonloaded surface of the steel member. The current state of blast-resistant engineering and design does not account for the adverse effects of polyurea as demonstrated in these tests. More research into the structural interactions on an aggregate level is needed in order to accurately account for the increased deflection potential and reduced blast-resistance rating of steel structures with polyurea coatings.
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