Evaluating Interply Bitumen in Bituminous Membrane Samples for Hail Impact Distress

This paper was published at the Forensic Engineering 9th Congress, held November 4 - 7, 2022 in Denver, Colorado. The published version of this paper is available for purchase and download at: https://ascelibrary.org/doi/book/10.1061/forensicengineering2022 Benefits of Evaluating Interply Bitumen in Bituminous Roofing Membrane Samples in Assessments of Hail Impact Distress Justin Donaldson, P.E., M. ASCE 1 Marco DeLeon, M.E., P.E., M. ASCE2 Garrett Simpson, E.I.T.3 Technical Director and Discovery Lab Director, Nelson Forensics, 2740 Dallas Parkway, Suite 220, Plano, Texas 75093; email: [email protected]; phone: 877-850-8765 Vice President – Special Projects, Nelson Forensics, 2740 Dallas Parkway, Suite 220, Plano, Texas 75093; email: [email protected]; phone: 877-850-8765 c/o Nelson Forensics, 2740 Dallas Parkway, Suite 220, Plano, Texas 75093; email: [email protected]; phone: 877-850-8765 ABSTRACT Hail impact evaluations of multi-ply bituminous roofing membranes such as built-up roofing (BUR) membranes, often include laboratory testing of field-procured membrane samples. Such testing is performed to further evaluate the samples for distress present within the membrane cross section. Popular tests performed by forensic laboratories in the United States include delamination, wherein a sample's individual plies are separated for observation with interply bitumen still present, and desaturation, wherein all bitumen is dissolved from a sample so the bare felt reinforcement mat can be directly evaluated. This paper addresses the potential shortcomings of using desaturation testing alone when evaluating roof membrane samples for hail impact distress, as distress or other conditions can be found in the bitumen that is not identifiable after the bitumen has dissolved. As the bitumen is the waterproofing element of the membrane, identification of bitumen damages and conditions is an important part of any evaluation that is missed from desaturation alone. This paper presents the findings from multiple roof membrane samples that were subjected to discrete impact forces, similar to hailstone impacts, and then examined by both delamination and desaturation. It is shown that distress to the bitumen may exist at impacted locations, without distress being identifiable on the reinforcing mats after desaturation. This testing is also supplemented with discussion of actual conditions found in delaminated membrane samples from actual hail-damage investigations. The potential for the mistaken identification of "false positives" for impact distress when evaluating desaturated felts is also discussed. INTRODUCTION BUR membranes are composed of alternating layers of asphalt-impregnated reinforcing felt plies and hot-mopped bitumen (typically asphalt, but sometimes coal tar). The felts themselves can be composed of organic fibers or inorganic fibers. The felt ply reinforcement provides dimensional stability and strength to the membrane, while the bituminous component provides waterproofing and adhesive properties. Laboratory testing of BUR membrane samples can be an important part of a forensic evaluation aiming to determine if a roof has been damaged by hail impact. Such laboratory testing can confirm the presence of, or lack of, conditions consistent with an impact force within the layered membrane composition, providing information beyond what is obtainable during a field evaluation. The most common types of laboratory testing used for this purpose are delamination and desaturation. There are distinct differences between these two types of tests. The authors of this paper have performed both types of tests and have reviewed similar laboratory work performed by others. Collectively, the authors have directly tested and evaluated over 2,000 BUR membrane samples through delamination and/or desaturation, in addition to similar testing that has been performed on polymer-modified bitumen membrane samples. This experience, in conjunction with reviews of the testing performed by others, has provided ample evidence of significant benefits to using delamination testing rather than desaturation. Such benefits are discussed herein. The basic process and characteristics of the two types of tests are described below. Delamination Testing Delamination of a BUR membrane sample consists of separating it into its individual plies, typically after freezing the sample. Using temperature manipulation to achieve delamination of a BUR membrane sample is briefly described in item 6.8 of ASTM D2829/D2829M – 07 (Reapproved 2019) "Standard Practice for Sampling and Analysis of Existing Built-up Roof Systems". While this item directly mentions dry ice as an aid for freezing the sample, samples are typically easiest to separate by hand after having been frozen with liquid nitrogen, in the authors' experience. After the plies have been delaminated, interply bitumen remains adhered to the topside and underside of each ply, allowing for direct visual evaluation of the bitumen. Additionally, the plies themselves can be visually and/or tactilely evaluated in this delaminated state. Desaturation Testing Desaturation of a BUR membrane sample is the process of dissolving all of the bitumen contained within the sample through use of a chemical solvent. The plies may or may not be delaminated prior to the desaturation, and often an entire sample is submerged at once in the solvent. The desaturating solvent is undiscerning and dissolves all of the bitumen in the sample, including both the mopped bitumen and bitumen impregnated into the felt plies themselves. This process leaves behind just the felt reinforcement fibers to be visually and/or tactilely evaluated. Solvent desaturation of a BUR membrane sample is briefly described in item 10.7.3 of ASTM D3746/D3746M – 85 (2015) "Standard Test Method for Impact Resistance of Bituminous Roofing Systems."